WO2008115606A2

WO2008115606A2 - Method and apparatus for reducing the effects of window clouding on a viewport window in a reactive environment

Info

Publication number: WO2008115606A2
Application number: PCT/US2008/005544
Authority: WO
Inventors: Kenneth C. Harvery
Original assignee: Verity Instruments, Inc.
Priority date: 2007-03-21
Filing date: 2008-04-29
Publication date: 2008-09-25
Also published as: WO2008115606A3; CN101681802A

Abstract

Multichannel array for establishing viscous flow for preventing the flow of particulates causing window clouding. Process chamber for confining process pressure within a process volume with a viewport window along the chamber for viewing portion of process volume. Ingress port disposed in the process chamber for receiving flow of process gas and egress port for extracting flow rate of gas from the process volume. Multichannel array (MCA) disposed between viewport window and process volume of the process chamber. Window chamber separate viewport window from MCA with chamber window port for receiving gas. Viscous flow formed at window side of channels, prevents material from entering window chamber and adhering to window. Viscous flow established by increasing pressure in window chamber via chamber window port. Viscous flow rate substantially lower than rate of process gas flow into process volume.

Description

METHOD AND APPARATUS FOR REDUCING THE EFFECTS

OF WINDOW CLOUDING ON A VIEWPORT WINDOW IN A

REACTIVE ENVIRONMENT

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates generally to a device for reducing the effects of clouding of an optical sensor window in a plasma environment. More particularly, the present invention relates to a system and method for implementing a mass flow in a multichannel array for reducing window clouding.

2. Description of Related Art

[0002] In the art of semiconductor processing, in order to form integrated circuit structures from wafers, selectively removing or depositing materials on a semiconductor wafer is well known. Removal of material from a semiconductor wafer is accomplished by employing some type of etching process, for instance and including reactive ion etching, deep-ion etching, sputtering etching and plasma etching. Depositing material on a wafer may involve chemical and physical vapor depositions, evaporative deposition, electron beam physical vapor deposition, sputtering deposition, pulsed laser deposition, molecular beam epitaxy and high velocity oxygen deposition. Other removal and deposition processes are known. Such processes are tightly controlled and are done in a sealed process chamber. Because exact amounts of material are deposited onto or removed from the substrate wafer, its progress must be continually and accurately monitored to precisely determine the stopping time or endpoint of a particular process. Optically monitoring the chamber processes is one very useful tool for determining the stage or endpoint for an ongoing process. For instance, the interior of the chamber may be optically monitored for certain known emission lines by spectrally analyzing predetermined wavelengths of light emitted or reflected from the target in the chamber. Typical methods are optical emission spectroscopy (OES), absorption spectroscopy, reflectometry, etc. Typically, an optical sensor or source is positioned on the exterior of the chamber and adjacent to a viewport or window, with a vantage point to the target area in the chamber to be observed.

[0003] One problem with optical monitoring chamber processes is that during many of these processes, the interior of the chamber contains alloys, polymers and reactive gases that result in deposits on the interior surfaces of the chamber, including the viewport window. Additionally, the window is subject to etching, and further degradation, by the reactive gases in the chamber. As the window becomes clouded, its optical properties are altered, which may effect the measurements by the optical sensor. While it is expected that the entire interior surface of the chamber must be cleaned of deposits from time to time, and the chamber recertified, the window must be cleaned or replaced much more frequently for maintaining consistently accurate optical measurements. Under certain conditions, the viewport window must be cleaned ten or twenty times, and the optical sensor recalibrated, between chamber cleanings. Maintaining the chamber window is time consuming, expensive and decreases the available runtime of the chamber.

[0004] Typically, the prior art handles window clouding in one of three ways to reduce the frequency between window maintenance between chamber cleaning cycles: adjusting the optical measurements to account for window clouding; in situ cleaning of the window; and preventing the optical degradation of the window. There is no single method for adjusting the optical measurements to suit all situations and processes. The success of these methods varies on a case by case basis, by the particular process, and even by the spectral wavelength being monitored for a process. In situ cleaning typically involves some mechanism for cleaning the viewport window without removing the window and with little interruption to the process schedule. One method is to direct an inert gas toward the exterior surface of the window to remove contaminants from the window. Gases such as helium and nitrogen are often used, but other, non-inert gases, may also aid in cleaning the viewport window, such as O₂. However, the use of an inert gas on a window (or any non-process gas) that is exposed to the interior of the chamber and mixes with the process gas may adversely affect the process. U.S. Patent No. 6,052,176 to Ni, et al., entitled "Processing Chamber With Optical Window Cleaned Using Process Gas" discloses using a process gas to remove contaminants from the window. A port for the process gas is oriented parallel to the exterior window surface. The process gas flow dislodges any by-products from the surface of the window and then directs the same onto the processing chamber. U.S. Patent No. 6,344,151 to Chen, et al., entitled "Gas Purge Protection of Sensors and Windows in a Gas Phase Processing Reactor," discloses a gas purged viewport for endpoint detection in a gas phase processing chamber which prevents contamination of an optical monitoring window by use of a purge gas flow. The gas purge viewport includes a prechamber between the optically transparent window and the process chamber. The purge gas is passed through the prechamber and into the processing chamber to purge the window. Chen, et al. discuss using the gas purge system to purge other parts of the system, including sensors exposed to the chamber. U.S. Patent Nos. 6,390,019 and 6,712,927 to Grimbergen, et al., entitled "Chamber Having Improved Process Monitoring Window," disclose using energized process gas ions to energetically bombard the window and remove process residues deposited thereon. An electric field source comprises an electrode with one or more apertures which is disposed between a window and light source to provide an electric field that is perpendicular to the plane of the window and accelerate process gas ions toward the window.

[0005] The use of purge gas, even process gases, may reduce the flow of process gas to the shower and result in a detrimental effect on the process. U.S. Patent No. 6,301 ,434 to McDiarmid, et al., entitled "Apparatus and Method for CVD and Thermal Processing of Semiconductor Substrates," discloses a dual gas injection manifold which is used in a thermal processing system, which has a purge gas showerhead on its top surface and a process gas showerhead on its bottom surface. The manifold prevents unwanted deposition on the underside of the window, as well as injects the reactant gas for deposition and etching. [0006] Preventing window clouding before it influences the optical properties of the window would seem to be the most viable solution to clouding, yet, heretofore has not yielded complete success. Preventing contaminants from reaching the viewport window often involves restricting the size of the passage(s) to the window. U.S. Patent No. 6,762,849 to Rulkens entitled "Method for In-Situ Film Thickness Measurement and Its Use for In-Situ Control of Deposited Film Thickness," discloses installing a fine metal mesh screen or bundle of small diameter tubes over the internal surface of the optical port entry for protecting the window. U.S. Patent No. 4,407,709 to Enjouji, et al. entitled "Method and Apparatus for Forming Oxide Coating by Reactive Sputtering Technique," discloses a window with slits for preventing clouding of the viewport window of a sputtering apparatus.

[0007] Another technique is to place a restrictor plate between the window and chamber that inhibits the passage of contaminants to the window. U.S. Patent No. 6,170,431 to DeOrnellas, et al., entitled "Plasma Reactor with a Deposition Shield" discloses a reactor that includes a shield that prevents the deposition of materials along a line-of-sight path from a wafer toward and onto a window. The shield is comprised of a plurality of louvers or slats which are positioned at a skewed angle with respect to the wafer. However, this particular configuration would also inhibit line-of-sight optical measurements. Other restrictor devices include protruding shield designs, such as taught by Nakata, et al., in U.S. Patent No. 6,576,559 to Nakata, et al., "Semiconductor Manufacturing Methods, Plasma Processing Methods and Plasma Processing Apparatuses." There, the protruding shield has an angular cylindrical shape and is disposed between a laser source and window to prevent reaction generated material from intruding into the inner surface of the window as much as possible. The magnitude of gaps between shields is determined by properties of the laser beam and the scanning operation to be carried out by the laser galvano mirror. Brcka discloses, in U.S. Patent No. 6,666,982 entitled "Protection of Dielectric Window in Inductively Coupled Plasma Generation," protecting a dielectric window in an inductively coupled plasma reactor from depositions of coating or etched material with a slotted shield, however the slots permit some material to pass toward the window.

[0008] Other prior art window clouding restrictors include the notion of the mean free path of the molecules to be restricted. U.S. Patent No. 5,145,493 to Nguyen, et al., entitled "Molecular Restricter," discloses a restricter plate with cell dimensions based on mean free path of the molecules to be restricted. The molecular restricter comprises a plate with at least one elongated cell with parallel walls and open ends, wherein the cell has a width and length. Optimally, Nguyen, et al. report that the width should be less than one mean free path and the length of the cells should be greater that ten times the mean free path. Nguyen, et al. further assert that for an aspect ratio of 2/1 (length/width), the molecular transmission is about half of that where it is 1/1. At a ratio of 5/1 , only about 9% is transmitted, on down to about 1% transmitted at a ratio of only about 12.5/1. Aqui, et al. also disclose, in U.S. Patent No. 5,347,138 entitled "In Situ Real Time Particle Monitor for a Sputter Coater Chamber," the use of mean free path to determine the dimensions of shield tubes open to a chamber, but for use on metal atoms dislodged from a target by a laser beam. Aqui, et al. state that the optimal width of the shield tubes is equivalent to less than one mean free path and their length are three times the mean free path or greater.

[0009] Still other attempts at preventing window clouding employ both a restrictor and the use of purge gas. U.S. Patent No. 5,681 ,394 to Suzuki entitled "Photo- Excited Processing Apparatus and Method for Manufacturing a Semiconductor Device by Using the Same," discloses a photo-excited processing apparatus that includes a reaction chamber filled with reaction gas, photo-excitation irradiating light source and a light transmissive window between the light source and chamber. A multi-holed transparent diffusion plate is arranged between the light transmissive window and a substrate in the chamber. However, the thickness of this diffusion plate is not discussed. Purge gas, either N₂ or O₂, enters between the transmissive window and the transparent diffusion plate. The combination of the diffusion plate and purge gas suppresses depositions to the surface of the light transmissive window. U.S. Patent No. 6,110,291 to Haruta, et al. entitled "Thin Film Forming Apparatus Using Laser," discloses introducing a clean purge gas, such as oxygen, through a pipe directly at the window (either the laser window or a sensor window) in order to clean the window. Additionally, Haruta, et al. teach the placement of an aperture and, alternatively, an elongated grid between the chamber and window so that the solid angle between the laser window and target is smaller in order to reduce the amount of dust that accumulates on the window.

BRIEF SUMMARY OF THE INVENTION

[0010] The present invention is directed to a method and apparatus for reducing the effects of window clouding on a viewport window in a reactive environment. A multichannel array structure is provided and a mechanism for establishing a gas flow within the multichannel array for preventing the flow of particulates that cause window clouding. A process chamber is provided for confining a process pressure within a process volume with a viewport window along the chamber for viewing at least a portion of the process volume. An ingress port is disposed in the process chamber, and to the process volume, for receiving a flow of process gas in the process volume and an egress port is disposed, and in the process chamber, to the process volume for extracting a flow of gas from the process volume. A multichannel array (MCA) is disposed between the viewport window and the process volume of the process chamber. The MCA has a plurality of channels, each of the channels having a diameter and a length. A window chamber is defined between the viewport window and MCA with a chamber window port for receiving gas into the chamber volume. A flow is formed at the window side of the channels in the MCA that prevents particulates from entering the window chamber and adhering to the window. The flow is established by increasing pressure in the window chamber via the chamber window port, wherein the window chamber pressure exceeds the process pressure, but not enough to substantially increase the flow rate of gas from the process volume. The flow rate is substantially lower than the flow of process gas into the process volume.

[0011] One or more clouded viewport windows are obtained for testing, in which the clouding results from exposure to the reactive environment. The clouding typically appears as a coating film on the test windows. The clouded viewport windows are analyzed for one or more spectral regions having good transmission. The threshold level of light transmission is determined by the particular application in which the window is used. The spectral regions of good transmission are evaluated for their usefulness with a particular algorithm that will use the spectral data in a production environment. Spectral regions that cannot be evaluated using the subject algorithm are eliminated from consideration. Spectral regions that can be evaluated using the subject algorithm and exhibit low absorption are selected for monitoring in the production environment.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0012] The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings wherein:

[0013] FIGs. 1A and 1 B are diagrams of portions of a multichannel array in accordance with an exemplary embodiment of the present invention;

[0014] FIG. 2 is a diagram of a process chamber with a barrier MCA for reducing window clouding in accordance with an exemplary embodiment of the present invention;

[0015] FIG. 3 is a diagram of a process chamber in which a non-process purge gas is used to create a back pressure between the MCA and viewport window in order to create a viscous flow for reducing window clouding in accordance with an exemplary embodiment of the present invention;

[0016] FIG. 4 depicts a diagram of a process chamber that uses a process gas to create a back pressure between the MCA and viewport window for preventing window clouding in accordance with another exemplary embodiment of the present invention;

[0017] FIG. 5 is a flowchart depicting a process for establishing viscous flow into an MCA while maintaining a low flow rate into the chamber from the MCA in accordance with an exemplary embodiment of the present invention;

[0018] FIG. 6 is a diagram of an MCA containing fluid for preventing window clouding in accordance with another exemplary embodiment of the present invention; [0019] FIG. 7 depicts a diagram of an MCA containing fluid for preventing window clouding in which the fluid flow across the surface of the MCA in accordance with another exemplary embodiment of the present invention;

[0020] FIG. 8 depicts a diagram of an MCA with a window chamber containing fluid for preventing window clouding in accordance with another exemplary embodiment of the present invention;

[0021] FIG. 9 depicts a diagram of an MCA with a window chamber containing fluid for preventing window clouding, where the fluid flows through the window chamber in accordance with another exemplary embodiment of the present invention;

[0022] FIG. 10 is a flowchart depicting a method for implementing an MCA to reduce window clouding while maintaining a low flow rate into the chamber from the MCA in accordance with an exemplary embodiment of the present invention;

[0023] FIG. 11 is a diagram of a system for obtaining accurate OES measurements in accordance with an exemplary embodiment of the present invention;

[0024] Fig. 12 is a diagram of the measurement device used to assess the relative transmission h of windows with varying amounts of coating film and clouding in accordance with an exemplary embodiment of the present invention;

[0025] FIG. 13 is a chart showing the transmission response across the near infrared spectral region for three windows with varying amounts of coating film in accordance with an exemplary embodiment of the present invention;

[0026] FIG. 14 is a chart showing the transmission response across the UV, visible and near infrared spectral regions for the highly clouded window 1 ;

[0027] FIG. 15 is a chart depicting a typical endpoint trend for the etch; [0028] FIG. 16 is a diagram depicting endpoint trends for an etch process that is repeated over an extended time period;

[0029] FIG. 17 is a flowchart depicting a method for identifying a spectral region with low absorption that is useful for measurement determinations in accordance with an exemplary embodiment of the present invention; and

[0030] FIG. 18 is a chart showing typical sensor types and their corresponding spectral ranges.

[0031] Other features of the present invention will be apparent from the accompanying drawings and from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

Element Reference Number Designations

100: Multichannel Array (MCA) 700: Multichannel Array (MCA)

102: Substrate 702: Window

104: Channel 703: Optical Sensor

200: Multichannel Array (MCA) 706: Window Chamber

202: Window 708: High Viscosity Fluid

203: Optical Sensor 710: Processing Chamber

206: Window Chamber 712: Interior Of Processing Chamber

210: Processing Chamber 752: Fluid Inlet

212: Interior Of Processing Chamber 754: Fluid Outlet

214: Wafer Support 800: Multichannel Array (MCA)

216: Wafer 802: Window

220: Plasma 803: Optical Sensor

232: Process Gas Inlet (Shower Head) 806: Window Chamber

238: Processing Chamber Gas Outlet 807: Optional Fluid Window

300: Multichannel Array (MCA) 808: Low Viscosity Fluid

302: Window 810: Processing Chamber

303: Optical Sensor 812: Interior Of Processing Chamber

306: Window Chamber 900: Multichannel Array (MCA)

310: Processing Chamber 902: Window

312: Interior Of Processing Chamber 903: Optical Sensor

314: Wafer Support 906: Window Chamber

316: Wafer 908: High Viscosity Fluid

320: Plasma 910: Processing Chamber

332: Process Gas Inlet (Shower Head) 912: Interior Of Processing Chamber

334: Process Chamber Metering Valve 952: Fluid Inlet

338: Processing Chamber Gas Outlet 954: Fluid Outlet

342: Window Chamber Gas Inlet 1100: OES Measurement Apparatus

344: Window Chamber Metering Valve 1102: Window

400: Multichannel Array (MCA) 1104: Collecting Optics

402: Window 1106: Optical Fiber

403: Optical Sensor 1108: Sensor

406: Window Chamber 1109: Spectrograph

410: Processing Chamber 1110: Processing Chamber

412: Interior Of Processing Chamber 1112: Interior Of Processing Chamber

414: Wafer Support 1114: Wafer Support

416: Wafer 1116: Wafer

420: Plasma 1120: Plasma

432: Process Gas Inlet (Shower Head) 1132: Process Gas Inlet (Shower Head)

Processing Chamber Metering 434: Valve 1138: Processing Chamber Gas Outlet Element Reference Number Designations

436: Process Gas Metering Valve 1200: Test Measurement Apparatus

437: Process Gas Source 1202: Window

438: Processing Chamber Gas Outlet 1203: Source Lamp

442: Window Chamber Gas Inlet 1205: Integrating Sphere

444: Window Chamber Metering Valve 1208: Sensor

600: Multichannel Array (MCA) 1209: Spectrograph

602: Window

603: Optical Sensor

606: Window Chamber

608: High Viscosity Fluid

610: Processing Chamber

612: Interior Of Processing Chamber

[0032] A high-quality optical path is a necessity to perform most spectroscopic techniques, such as optical emission spectroscopy (OES) and reflectometry. Any obstruction that affects the intensity of the radiation degrades the accuracy and reliability of the technique. The obstruction may alter the intensity as a function of the wavelength. Typically, an optical sensor is positioned outside a process chamber and adjacent to a viewport window for obtaining optical measurements of a target within a process environment, (the process environment may be a process chamber, or along the up- or downstream piping associated with a processing chamber). Understanding the optical properties of these windows is critical for obtaining accurate measurements through them. As a viewport window becomes clouded, its optical properties change, sometimes in detrimental ways. Deposits must be cleaned from the viewport window, or the window replaced in order to maintain a high-quality optical path.

[0033] The problems associated with window clouding continues to plague the semiconductor industry. Prior art techniques for solving the window clouding problem involve either adjusting the intensity of the light transmitted through the window to compensate for window clouding (for an optical window), altering the optical measurement algorithms to compensate for clouding (for a view port window) or techniques for decreasing the frequency of window maintenance (cleaning or replacing the window and recalibrating the optical sensors at the viewport). Adjustment techniques are complicated and very difficult to implement as they vary with the specific implementation. Prior art techniques to reduce the frequency of window cleaning include disposing a restrictor plate between the window and the chamber in order to reduce the amount of contaminants that reach the window and, alternatively, to clean the exterior surface of the window with a flow of purge gas. Restrictor plates are not completely effective and merely lessen the amount of contaminants that reach the window. The cross-sectional area of the restrictor apertures may be decreased to further reduce the amount of contaminants that make their way to the window, but smaller apertures tend to clog with contaminates more often than larger apertures. However, unlike cleaning or replacing a viewport window, a restrictor plate can be replaced with an identical plate without having to recalibrate the optical sensors to the new plate. Of course, whenever the window does become clouded, the optical sensors should be recalibrated to the replacement window.

[0034] Cleaning the window with purge gas presumes that contaminates have, or will reach the window, but these contaminates can be detached with a current of gas. Firstly, this assumption may be incorrect; the contaminants that reach the window may bind to the surface of the window. In any case, directing a purge gas to the window suffers from shortcomings that makes this technique impractical for certain applications. For instance, utilizing the process gas for the purge stream eliminates incompatibility problems that may be associated with using non-process gases. However, often the process gas itself reacts with the window material which causes clouding. Ultimately, it may be necessary to change the window material in order to use the process gas as the purge gas. Using a non-process gas for the purge gas enables the operator to select the optimal window material for the optical measurement to be taken without concern for the window reacting with the purge gas. Another benefit in using a non-process gas as the purge gas is that the purge gas may be selected for its cleaning properties for the particular type of contaminant. The drawback with using non-process gases as the purge gas is twofold. First, the purge gas will not entirely prevent the process gas from reaching and reacting with the window, so in selecting the type of window material, the susceptibility of clouding by the process gas should be considered. More importantly, the non-process gas will often have a detrimental effect on the process. Therefore, the purge flow rate of the non-process gas should be kept to an absolute minimum, which may worsen the clouding rate.

[0035] Furthermore, each of these purge gas techniques require a significant amount of redesign to the area surrounding the window viewport. For instance, the purge gas should have a sufficient flow rate and oriented in a suitable direction to wipe the exterior of the window of any contaminates that adhere to the window. This requires the port (or ports) either be aimed at the window to force a stream of gas directly on the surface of the window or to design a cavity adjacent to the window that facilitates gas flow in lifting contaminates off the surface of the window and propelling them back into the process chamber.

[0036] None of the prior art techniques have had a substantial impact on the problem of window clouding. Many of these techniques are application-specific and require substantial modifications for each unique implementation. Most require substantial modifications be made to the system, usually at considerable expense, with only a marginal reduction in window clouding. What is needed is a method and system for reducing window clouding to the extent that the frequency of window maintenance be reduced to approximately that of cleaning the system.

[0037] Before discussing the proposed solution to window clouding, it is helpful to more fully understand the causes of window clouding. Understanding the causes for window clouding will facilitate determining the best method to prevent or reduce the clouding. It is necessary to consider the creation (origin) mechanism, the transport to the window surface, and the action on the window surface. These will be discussed in reference to a typical processing chamber that is well known in the semiconductor industry, but this is merely exemplary for the purpose of describing certain aspects of the present invention. The present invention is equally useful in upstream or downstream piping for making optical measurements.

[0038] In many environments, the viewport window is not in the line of sight of the wafer. Also, the mean free path of any material in the chamber gas is much smaller than the distance to the window. So, little sputtered material from the wafer usually goes directly to the window.

[0039] The origin of particles may be from reaction products on the chamber wall that flake off. Alternatively, these particles may be formed from the plasma chemistry and might coalesce in the plasma, or be a byproduct of some other high energy reaction, such as from a laser.

[0040] The particles in the chamber may diffuse to the window surface. The equation for Brownian motion is described below.

x* - ^kT - t (1 )

3π η a

where x² is the mean displacement of the particles, a is the radius of particle, t is the time,

T is the temperature of the media, and η is the viscosity.

[0041] This indicates that migration for particles one micron in diameter is extremely slow, 6x10 ¹⁰ cm/sec, for typical conditions. Particulates may migrate to the window by thereto-mechanical effects of turbulence, e.g., movement from turbulence when the chamber is back-filled, etc. Therefore, care should be taken when back-filling a chamber. Additionally, particulates may move toward the window as a result of thermal gradients, that is, they travel to the window by thermomolecular flow (or thermal transpiration) caused by a difference in temperature between the wafer and the window or thermal turbulence from the high temperature of the plasma, etc. Once at the window, the particulates may adhere to the window as a coating, resulting from either electrostatic attraction or chemisorption.

[0042] Reactive gases from the plasma and reaction products from the wafer may be transported to the window surface by diffusion, turbulence, thermal gradients, etc. At the window surface, these gases may change the optical transmission of the window in a number of ways. If reactive gases reach the window surface, they may bond to the surface by chemisorption, electrostatic attraction, etc. and form a film. If some material is being deposited, then the exact composition of the deposited material should be determined. Alternatively, or additionally, the window surface may be etched by the reactive gases. If the window is fused silica or glass, substituting sapphire as a window material may be advantageous as sapphire is more resistant to etching. Still further, it is possible that a change in the bulk composition of the window is caused by material dissolving into the window. For example, an alkali (Na, Cs, etc.) may dissolve in the quartz to produce a brown color. Radiation from the plasma may cause the optical properties of the window to change. Therefore, some of the gas components may photolyze in the window area and coat the window. Also, some of the constituent gases may chemisorb to the window and be transformed by photocatalysis to a material that coats the window.

[0043] Heating the window may reduce or eliminate coating to the window. This may reduce the sticking coefficient so that material does not stick initially to the window. Alternatively, it may help to evaporate or decompose material that is already deposited. It may be necessary to heat the window to as much as 200° C to prevent the window from clouding. For a continuous mechanism, this may be done by adding heating elements to the window. Other methods might be heat lamps or high power lasers. For a pulsed mechanism, ablation of the absorbed material can be done with flash lamps or pulsed lasers.

[0044] Plates with channels through them are known in the prior art. These plates have been put to many uses such as electron multipliers, atomic beam collimators, neutron collimators, windows, etc. These prior art plates have been made of various metals, insulators and glasses. When used for preventing window clouding, the aperture size is sometimes decreased in order to reduce the amount of contaminants that reach the window, alternatively they are sometimes oriented askew of the optical path to the window to inhibit the straight-line path to the window for contaminants. Some prior art references have suggested a relation between the mean free path (MFP) and the aperture dimensions through the plate.

[0045] In accordance with one exemplary embodiment of the present invention, the dimensions of the channels in the MCA may be predicated on the mean free path (MFP) of the molecules that cloud the window. By using MFP as a metric, the MCA can be designed that will act as a barrier to slow the transport to the window and a getter that collects material in the channels.

[0046] The MFP, L₀, is approximately given by,

^L" -***^■■£& ⁽²⁾ where η is the viscosity, P_mm is the pressure, Tis the temperature, and M is the mass of the particle.

[0047] The La MFP for Argon [Ar) at 150 milliTorr is L₀ = 0.4mm. [0048] For optimal barrier results, the length L of the channels should be much greater than the MFP L₀ of th e gas, or particulate, that will cloud the window (L₀ « L ). This will slow the material that passes through the channel along the axis. Additionally, the channel diameter, d, should be less that the MFP (L_a ≥ d). This will enhance sticking to the wall of the MCA and reduce diffusion. However, the channel diameter of the channels should be large enough to avoid frequent blockage.

[0049] Even though the barrier MCA will reduce the rate of clouding, ultimately material will pass through the channels of the MCA and begin to cloud the window. This clouding can be acceptable if the time between cleaning cycles is much less than the time that it takes to cloud the window.

[0050] FIGs. 1A and 1B are diagrams of portions of a barrier multichannel array (MCA) as will be described below with respect to the present invention. MCA 100 is referred to as a barrier MCA because the structure of the MCA itself inhibits window clouding by acting as a barrier to particulates that may cloud the window. MCA 100 comprises body 102 with first and second surfaces (103 and 105) and a plurality of channels 104 traversing body 102 from first surface 103 to second surface 105. Body 102 of MCA 100 in FIG. 1A is depicted as having a generally circular cross- sectional shape, however this is merely exemplary as the shape of body 102 is predicated on the installation implementation to the processing chamber. Typically, one surface of MCA 100 is the interior or window-side surface 103, and the other surface is the exterior or chamber-side surface 105. The designation of interior and exterior is in reference to a window chamber that will be described below. Because one surface, chamber-side surface 105 is exposed to the interior of the processing chamber, the material selected for body 102 should be non-reactive with the internal processes in the chamber. Furthermore, if a non-opaque material is selected for body 102 (i.e., optically transmissive with respect to the optical sensors employed for the measurements), the chamber-side surface 105 may become optically clouded in a similar manner as the window and affects the optical measurements. Therefore, optimally, body 102 should be opaque for the optical wavelengths being measured or coated with a non-reactive and opaque coating in order to maintain uniform transmission through the MCA as outer surface 105 becomes cloudy.

[0051 J With continuing reference to FIGs. 1A and 1 B, MCA 100 is shown installed on chamber 210 as MCA 200 in FIG. 2. Notice that channels 204 traverse body 102 and are in the optical path between optical sensor 203, located adjacent to and outside the viewport window 202, and the target (here the target is depicted as plasma 220). The axes of channels 204 are substantially parallel to the optical path. Thus, each of channels 204 is parallel to every other channel through body 102. The exact cross-sectional shape of channels 204 is not of particular importance to the present invention, although as a practical matter some cross-sectional shapes are much more easily fabricated than others. What is of concern in preventing particulates from reaching the window is the dimensions of the channels.

[0052] As mentioned above, since the MFP is the distance between collisions, the channel diameter, d, for barrier MCA 200 should be one MCA or less. For a barrier MCA, the diameter d is understood as the minimum cross-sectional distance of the channel opening. Thus, for circular channels, it is the diameter at any point across the center point of the circle, but for polygonal cross-sectional shapes, that placement for d varies with the shape (notice in FIG. 1 B, d is taken across parallel sides, however for a pentagon, d is taken from any vertex to the midpoint of an opposite side). It is expected that the channel diameter d will remain constant across the channel length L, but it should be understood that there may be advantages for varying d with L from window-side surface 103 toward chamber-side surface 105. For instance, a conical channel (small end at window-side surface 103) may direct more light to the optical sensor. To prevent molecules from traversing the length of a channel along its axis, the channel length, L, should be substantially larger than the MFP of the contaminant. Length dimensions of between three and twelve MFPs have been discussed in the prior art. [0053] The material of MCA 200 should have a large sticking coefficient for the materials that are diffusing to the window. This may be accomplished by, for example, using the same material for MCA 200 as for window 202, so the sticking coefficient would be the same. Cooling MCA 200 may also increase the sticking coefficient.

[0054] MCA 200 will have a quantity of Λ/ channels 204 across its body. The quantity, N, and the placement of channels 204 will affect the character of the optical measurement by optical sensor 203. Therefore, the N channels 204 should be distributed uniformly over at least the portion of MCA 200 that is in the optical path of optical sensor 203 and, if possible, across the entire viewport of optical sensor 203. Because barrier MCAs are not totally effective in preventing contaminants from reaching the window, the amount of material that gets by the MCA is proportional to the number of channels, N, therefore N should be kept as low as possible without sacrificing optical quality.

[0055] With further reference to FIG. 2, a diagram of an implementation of a barrier MCA is shown in accordance with an exemplary embodiment of the present invention. There, processing chamber 210 is shown with interior 212 in which plasma 220 is ignited from, for instance, as reaction on wafer 216 which rests on wafer table 214. Process gas enters interior 212 through ingress port, or process gas inlet 232 (typically a shower head) and exits interior 212 through egress port, processing chamber gas outlet 238 (and on to the vacuum pump). Flow into volume 212 of chamber 210 from process gas inlet 232 is shown diagrammatically as an arrow and is represented as Qw. and flow to the vacuum pump (not shown) is also shown diagrammatically as an arrow but is represented as QT • Typically, window 202 is disposed along one surface of the interior of chamber 210, either side, top or bottom surface, in a position and orientation such that optical sensor 203 will have a direct line of sight to the target (here the target is plasma 220). In implementations where line-of-sight measurements are unnecessary, the position and orientation of window 202 may be different. In some applications, multiple windows will be installed at various locations along the interior surface of chamber 212.

[0056] In any case, MCA 200 is disposed between interior 212 of chamber 210 and window 202 such that a volume is created between the window and MCA, represented as window chamber 206. It should be understood that the exact shape, dimensions, and even the existence of window chamber 206 is relatively unimportant for practicing the present barrier MCA of the present invention. There may, however, be only a slight gap between the inner openings of channels 204 and window 202. The pressure within chamber 210 is represented as chamber pressure Pc and the pressure within window chamber 206 is represented as window chamber pressure Pw- In general, chamber pressure Pc is determined by the process and Pw is substantially equivalent to P_c.

[0057] As mentioned above, barrier MCA 200 can be made of any non-reactive material including, glass, sapphire, and other insulators, stainless steel, aluminum, exotic metals and other conductors and semiconductors. The outer surface (chamber side) of MCAs made from materials that are transparent at the wavelength to be measured by optical sensor 203 may be coated with a non-transmissive coating in order to maintain uniform transmission through the MCA as the outer surface becomes cloudy.

[0058] Next, it is desired to approximate values for the amount of material that reaches the window. But primarily, this is useful for insight into how the various parameters affect the flow rate. The diffusion may occur by:

Molecular - mean free path is much larger than the channel diameter {MFP»d)

Viscous - mean free path is much smaller than the channel diameter {MFP«d) [0059] For molecular diffusion through the channel, the conductance is,

where, r = d/2 is the radius of the channel, L is the length of the channel, and v_m is the mean molecular speed.

[0060] The flow rate Q_a through a single channel is,

where Pc is the chamber partial pressure, and Pw is the window partial pressure.

[0061] The total flow rate Q^ through the multichannel array is,

Q_Λ - N ' Q. (5) where Λ/are the number of channels in the array.

[0062] In accordance with still another exemplary embodiment of the present invention, a novel multichannel array approach for preventing window clouding is presented by creating a gas flow through the MCA that acts as a barrier to particulates, atoms, molecules, ions, etc that would cause the window to cloud. The flow is in the direction of the process chamber from the window chamber. The flow could range from molecular diffusion, as described by equations 3, 4 and 5, to viscous flow. The effectiveness, for preventing window clouding, would increase from the molecular diffusion regime to the viscous flow regime. For viscous flow, in principle, no material will pass through the multichannel array to cloud the window. The viscous flow in the channels act as a barrier and sweeps impurities back into the chamber. The viscous flow need not extend the entire length of the channel. The aim is to establish a flow rate, Q_A, at the MCA, that acts as a barrier to contaminants, while simultaneously maintaining the process flow rate, Qc, substantially higher than the viscous flow rate O^ for the MCA. (Qc » Q_A)- Consequently, the amount of gas flowing into the process chamber through MCA, Qw, will not adversely affect the process.

[0063] The viscous flow rate Q_a, through a channel is given by the Poiseuille equation,

where r = d/2 is the radius of the channel,

L is the length of the channel, η is the viscosity,

Pc is the chamber partial pressure,

Piv is the window partial pressure, and .

P_a is the mean pressure ((PW + PW)/2).

[0064] Therefore, the total flow viscous flow rate, Q_A, through the multichannel array is,

Q_A - N- Q. (7)

[0065] Initially, the viscous flow rate, O_A, across an MCA having particular dimensions is determined for a process (viscosity η and chamber partial pressure Pc) at a given window pressure P_w from Equations 6 and 7. Viscous flow rate Q_A is then compared to the flow rate, Qc, for the process. If Qc is not substantially greater than Q_A, the back pressure Pc can be increased or, alternatively the dimensions of the MCA can be altered (decreasing channel diameter d or increasing channel length L or both). Pc, d, Λ/ and L can be adjusted until Q_A is lowered to an acceptable flow rate. [0066] In accordance with one exemplary embodiment, an MCA is designed with generic dimensions in which viscous flow rate Q_A can be established for a wide variety of processes (viscosities η and the associated chamber partial pressures Pc) such that Qc » Qw , merely by adjusting the back pressure P_W- Alternatively, the generic MCA dimensions would allow for a viscous flow rate across a wide range of back pressure values. For example, by selecting representative dimensions for the MCA, the viscous flow rate Q_A can be determined for a process (pressure viscosity η and the associated chamber partial pressure Pc). For instance, L = 2.0cm, d = 0.1 cm, and D =1.0cm (diameter D is the effective diameter for N channels of diameter d). The chamber pressure is set at the working chamber pressure for the process, e.g., Pc = 150 microns. For a back pressure of P_w = 1 -0 Torr, the viscous flow through the MCA is QA = 0.41 seem. For P_w = 10 Torr, the flow through the MCA is 4.41 seem. Both these flow rates are small compared to a typical working flow rate of Ar in a chamber, Q₀ (Ar) ~ 500 seem.

[0067] With reference now to FIG. 3, a diagram of a process chamber in which a non-process purge gas is used to create a back pressure between the MCA and viewport window in order to create a gas flow for reducing window clouding in accordance with an exemplary embodiment of the present invention. Here, processing chamber 310 is shown with interior 312 in with a plasma 320, as discussed above with regard to FIG. 2. Process gas traverse valve 334 at flow rate QG and enters interior 312 through ingress port 332 at a flow rate of Qc and through egress port 338 at flow rate QT- The chamber pressure is represented as Pc-

[0068] MCA 300 is disposed between interior 312 of chamber 310 and window 302 forming window chamber 306. The specific dimensions of window chamber 306 are unimportant because the existence of the window chamber does not prevent clouding. It merely serves as a manifold to distribute Pw across all of the N channels 304 of MCA 300. Furthermore, the gas flow dynamics within window chamber 306 do not assist in cloud prevention because the viscous flow at the window side of channels 304 acts as a complete barrier to materials that might cloud the window. Clouding is prevented by the viscous flow at the widow side of channels 304 and not because of the existence or structure of window chamber 306. Particulates are stopped by the viscous flow barrier with MCA 300, if not before, and swept out of the MCA by the window flow Q_w.

[0069] Window chamber gas inlet 342 permits purge gas to enter window chamber 306 as metered by window chamber metering valve 344. With regard to the exemplary embodiment, the purge gas comprises a non-process gas, such as an inert gas, e.g., n₂, but in accordance with other embodiments, may instead be process gas. The pressure (or back pressure) within window chamber 306 is represented as window chamber pressure P_w- Because gas enters the interior of chamber 310 from both ingress port 332 and across MCA 300 from window chamber gas inlet 342, Q_τ = Qc + Qw- The purpose of metering valve 344 is to independently adjust back pressure Pw of the purge gas in window chamber 306 and resulting window flow rate, Qw-

[0070] A gas barrier that prevents window clouding may be realized by adjusting window back pressure Pw to create a viscous flow (Q_A) in the window side of channels 304. The gas flow entering chamber interior 312 from MCA 300 (Qw) is kept low in comparison to the gas entering the chamber from the inlet (Q_c), Qc » Qw, by adjusting window back pressure Pw just enough to reach viscous flow in the channels, P_w » Pc, but not so high as to flood chamber 310 with purge gas (i.e., Qc » Qw)- An acceptable value for the window flow rate Q_w can be determined from Equations 6 and 7 and that window flow rate Qw should be compared to the chamber flow rate Qc- If window flow rate Qw is too high, P_c can be reduced or the channel dimensions for MCA 300 can be altered.

[0071] It should be appreciated that the dimensions of channels 304 are not strictly related to the MFP of the molecules causing clouding as in the barrier MCA embodiments described above. In fact, channel diameter d may be significantly larger than MFP and/or channel length L may be significantly shorter than 3x - 12x MFP while still preventing window clouding. This is so because a viscous flow can be established by increasing P_w even though the channel dimensions would not support a barrier MCA. However, high back pressure values tend to increase the window flow rate Qw to a point that may be detrimental to the chamber process.

[0072] As mentioned elsewhere above, with some chamber processes the infusion of large quantities of a non-process gas may have a detrimental effect on the process. Therefore, the flow rate of any non-process gas into chamber 310 should be kept low. As described above, the formation of the viscous flow at the window side of channels 304 prevents window clouding while managing P_w simultaneously keeps the flow rate, Q_w, of purge gas into the process chamber low. Thus, the viscous flow barrier technique provides a useful mechanism for using non- process purge gases for preventing window clouding without detrimentally affecting the process in the chamber.

[0073] Process gas may also be used as window protection with the presently described viscous flow barrier technique with a multichannel array. FIG. 4 depicts a diagram of a process chamber that uses a process gas to create a back pressure between the MCA and viewport window for preventing window clouding in accordance with another exemplary embodiment of the present invention. Here, the configuration is essentially identical to that described above with regard to FIG. 3, with the exception of the process gas manifold connecting process gas inlet 432 with window gas inlet 442 and allowing process gas to flow into window chamber 406. There, the process gas is received at valve 436 as a flow rate of QG, which is diverted to chamber metering valve 434 and window metering valve 444. The purpose of the metering valves is to enable the pressure and flow rate window chamber 406 to be adjusted independently from the pressure and flow rate of chamber 410. The pressure within chamber 410 is represented as chamber pressure Pc and the pressure within window chamber 406 is represented as window chamber pressure Pw- Because gas enters the interior of chamber 410 from both ingress port 432 and across MCA 400 from window chamber gas inlet 442, QT = Qc + Qw- However, the flow rate to the manifold [QG) is used to feed both window chamber 406 and chamber 410, so QG = QT- AS discussed above, gas entering the chamber from the MCA [Qw) is kept low in comparison to the gas entering the chamber from the inlet (Q₀), Qc » Qw, by adjusting window back pressure P_w just enough to reach a viscous flow in the channels, P_w » Pc- An acceptable value for the window flow rate Qw can be determined from the operating flow rate Qc for the process in the chamber, and a value for window back pressure Pw is determined such that a predetermined threshold value for flow rate Qw is not exceeded.

[0074] Alternatively, process gas for purging window chamber 406 may be secured independently from inlet 437. In that case, the manifold discussed above may be omitted and the system will look and operate identically to that described above with regard to FIG. 3, albeit with process gas rather than non-process gas.

[0075] By understanding that the viscous flow rate at the window side of an MCA will effectively block all clouding materials and the flow across the MCA sweep all particulates from the MCA channels into the chamber, a generic MCA can be constructed that will enable viscous flow for a wide variety of process gases, particulates and chamber pressures, while maintaining a relatively low window flow rate (Qw) into the process chamber (thus maintaining Q_c » Qw)- From Equations 6 and 7 above, it is then apparent that the operator need merely adjust the back pressure P_w to achieve Q_A for the particular MCA. FIG. 5 is a flowchart depicting a process for establishing viscous flow into an MCA while maintaining a low flow rate into the chamber from the MCA in accordance with an exemplary embodiment of the present invention. It is expected that the chamber pressure, Pc, and the flow rate into the chamber, Q_c, will be constant and nonadjustable. Initially, the process flow rate into the chamber (O_c) is found (step 502). Next, the viscous flow rate (Q_A) is calculated for the window side of an MCA with a quantity of channels (N), each having a chamber length (L), chamber diameter d, having a back pressure Pw and chamber pressure Pc for a gas viscosity (η) (step 504). Next, Q_A is compared to Q_c (step 506). If Qc » QA , then process ends as QA is established as the back pressure Q_w necessary for establishing viscous flow without a substantial increase in the chamber flow. If Q_A exceeds a maximum threshold amount, one or all of back pressure P_w , channel quantity N, chamber length L and chamber diameter d is adjusted (step 508) and the process reverts to step 504 and continues to iterate through steps 504 through 508 until Q_A is below the maximum threshold amount and Qc » QA- The process then ends as Q_A is established as the back pressure Q_w necessary for establishing viscous flow without a substantial increase in the chamber flow.

[0076] As mentioned above, while establishing a viscous flow at the window side of the MCA may be desirable, window clouding may be reduced or prevented by creating a pressure differential , across the channels of the MCA. FIG. 10 is a flowchart depicting a method for implementing an MCA to reduce window clouding while maintaining a low flow rate into the chamber from the MCA in accordance with an exemplary embodiment of the present invention. It is expected that the chamber pressure [Pc) and the flow rate into the chamber (Q_c) will be constant and nonadjustable. Initially, the process flow rate into the chamber (Q_c) and the chamber pressure [Pc) are found (step 1002). For some applications of the present invention, the implementation of the MCA may be further constrained by optical measurement to be made through it. In those situations, it is expected that the MCA should have an effective diameter (D) and so the channel diameter (d) and quantity of channels N will be determined for the effective diameter D. Hence, a decision is made as to whether there is a requirement of a specific effective diameter D for the optical measurements (step 1004). If the effective diameter D is known, then the channel length (L) for the N channels is determined for a chamber window pressure (Pw ) (or the back pressure at the MCA), where the window chamber pressure (Pw) is greater than the process chamber pressure (Pc ) (Pw > Pc) such that the process flow rate (Qc ) is greater than the flow rate into the chamber through the MCA (Qw ) (Qc » Qw ) (step 1006). With the channel diameter (d) and channel length (L) for the Λ/ channels, an MCA can be fabricated for reducing window clouding with a back pressure of P_w applied to the window side of the channels (step 1010).

[0077] If, on the other hand, the effective diameter D is not known, then all of the dimensions of the MCA may be manipulated for creating a back pressure (Pw ) to reduce window clouding. Thus, the channel length (L), channel diameter (d) and the quantity of channel N may be determined for a chamber window pressure (Pw )• Recall that window chamber (Pw) is greater than the process chamber (Pc ) (Pw > Pc) and the flow rate into the chamber through the MCA (Q_w ) is much lower than the process flow rate (Qc ) (Qc » Qw ) (step 1008). Here again, with the channel length (L) and channel diameter (d) for the N channels, an MCA can be fabricated for reducing window clouding with a back pressure of P_w applied to the window side of the channels (step 1010).

[0078] Multichannel arrays have been used with fluids for various optical devices. In this context, the behavior of the fluid is determined by the relative strength of the attraction of the surface of the solid to the cohesive intermolecular forces inside the liquid.

[0079] In accordance with one exemplary embodiment of the present invention, an MCA contains a fluid, such as high-vacuum pump oil. The fluid has a relatively low liquid-to-solid surface tension and so wets the MCA. The liquid surface has a relatively greater attraction to the MCA surface than to the bulk liquid. The contact angle is less than 90 degrees and has a concave meniscus. The contact angle is the angle of contact of the surface of the liquid with the wall of the channel. FIG. 6 is a diagram of an MCA containing fluid for preventing window clouding in accordance with another exemplary embodiment of the present invention. Here, structure 610 contains volume 612 in which a target (not shown) is optically monitored. Structure 610 may be a process chamber or up-or down stream pipe with a target. Window 602 is disposed in structure 610 and optical sensor 603 is located adjacent to window 602 on the exterior of structure 610. MCA 600 is disposed between window 602 and volume 612. Each of MCA channels 604 contains fluid 608. Fluid 608 prevents particulates from traversing MCA 600 and thereby prevents the clouding of window 602.

[0080] Note that the configuration in FIG. 6 can withstand a large pressure differential between P_w and Pc since the MCA channel has a small diameter. The relation between the pressures is given by Laplace's equation,

where a is the surface tension,

Pi and P₂ are the pressures at theJnterlaces, and

Ri and R₂ are the radii of curvature for the interfaces.

[0081] FIG. 7 depicts a diagram of an MCA containing fluid for preventing window clouding in which the fluid flows across the surface of the MCA in accordance with another exemplary embodiment of the present invention. Here, the elements are identical to those described above with the exception of the inclusion of fluid inlet 752 and fluid outlet 754. With regard to this embodiment, fluid 708 is caused to flow against MCA 700, is drawn into channels 704 by capillary action. Fluid 708 is removed from channels 704 by a partial vacuum at fluid outlet 754 and is filtered and recycled back to fluid inlet 752 (not shown).

[0082] FIG. 8 depicts a diagram of an MCA with a window chamber containing fluid for preventing window clouding in accordance with another exemplary embodiment of the present invention. Here, the elements are identical to those described above, however fluid 808 is contained in window chamber 806. Fluid 808 has a relatively high liquid-to-solid surface tension and so does not wet channels 804 of MCA 800. The liquid surface has a relatively greater attraction to the bulk of the liquid than to the MCA surface. The contact angle is greater than 90 degrees and has a convex meniscus. The contact angle is the angle of contact of the surface of the liquid with the wall of channel 804.

[0083] FIG. 9 depicts a diagram of an MCA with a window chamber containing fluid for preventing window clouding, where the fluid flows through the window chamber in accordance with another exemplary embodiment of the present invention. Here, the elements are identical to those described above in FIG. 8, except that fluid 908 is circulated through window chamber 906 via fluid inlet 952 and fluid outlet 954. Fluid 908 flows through window chamber 906 against MCA 900 and is removed to be filtered and recycled at fluid outlet 954.

[0084] Notwithstanding the discussion . above, a high-quality optical path is a necessity to perform most spectroscopic techniques, such as optical emission spectroscopy (OES) and reflectometry. Any obstruction that affects the intensity of the radiation degrades the accuracy and reliability of the technique. The obstruction may alter the intensity as a function of the wavelength. Typically, an optical sensor is positioned outside a process chamber and adjacent to a viewport window for obtaining optical measurements of a target within a process environment, (the process environment may be a process chamber, or along the up- or downstream piping associated with a processing chamber). Understanding the optical properties of these windows is critical for obtaining accurate measurements through them. As a viewport window becomes clouded, its optical properties change, sometimes in detrimental ways. Deposits must be cleaned from the viewport window, or the window replaced, in order to maintain a high-quality optical path. [0085] The problems associated with window clouding continues to plague the semiconductor industry. The origin of particles may be from reaction products on the chamber wall that flake off. Alternatively, these particles may be formed from the plasma chemistry and might coalesce in the plasma, or be a byproduct of some other high energy reaction, such as from a laser.

[0086] The particles in the chamber may diffuse to the window surface. The equation for Brownian motion is described below.

^~* - -^— t (9)

3π ^• η ^• a

where x² is the mean square displacement of the particles, a is the radius of particle, t is the time,

T is the temperature of the, media, and η is the viscosity.

[0087] Particulates may migrate to the window by thermo-mechanical effects or turbulence, e.g., movement from turbulence when the chamber is back-filled, etc. Therefore, care should be taken when back-filling a chamber. Additionally, particulates may move toward the window as a result of thermal gradients, that is, they travel to the window by thermomolecular flow (or thermal transpiration) caused by a difference in temperature between the wafer and the window or thermal turbulence from the high temperature of the plasma, etc. Once at the window, the particulates may adhere to the window as a coating, resulting from either electrostatic attraction or chemisorption.

[0088] Reactive gases from the plasma and reaction products from the wafer may be transported to the window surface by diffusion, turbulence, thermal gradients, etc. At the window surface, these gases may change the optical transmission of the window in a number of ways. If reactive gases reach the window surface, they may bond to the surface by chemisorption, electrostatic attraction, etc. and form a film. If some material is being deposited, then the exact composition of the deposited material should be determined. Alternatively, or additionally, the window surface may be etched by the reactive gases. If the window is fused silica or glass, substituting sapphire as a window material may be advantageous as sapphire is more resistant to etching. Still further, it is possible that a change in the bulk composition of the window is caused by material dissolving into the window. For example, an alkali (Na, Cs, etc.) may dissolve in the quartz to produce a brown color. Radiation from the plasma may cause the optical properties of the window to change. Therefore, some of the gas components may photolyze in the window area and coat the window. Also, some of the constituent gases may chemisorb to the window and be transformed by photocatalysis to a material that coats the window.

[0089] Prior art techniques for solving the window clouding problem involve either adjusting the intensity of the light transmitted through the window to compensate for window clouding (for an optical window), altering the optical measurement algorithms to compensate for clouding (for a view port window) or techniques for decreasing the frequency of window maintenance (cleaning or replacing the window and recalibrating the optical sensors at the viewport). Techniques for adjusting the intensity of the light transmitted through the window based on the amount of window clouding are very difficult to implement as they vary with the specific implementation. In many cases, the best that can be achieved is merely to monitor the amount of chemical deposition that accumulates on the inner side of the viewport window within the reactor environment, see for instance U.S. Patent No. 5,536,359 entitled "Semiconductor Device Manufacturing Apparatus and Method with Optical Monitoring of State of Processing Chamber," and then change the window when the accumulation on the window reaches a predetermined threshold amount of clouding.

[0090] Heating the window may reduce or eliminate coating to the window. This may reduce the sticking coefficient so that material does not stick initially to the window. Alternatively, it may help to evaporate or decompose material that is already deposited. It may be necessary to heat the window to as much as 200° C to prevent the window from clouding. For a continuous mechanism, this may be done by adding heating elements to the window. Other methods might be heat lamps or high power lasers. For a pulsed mechanism, ablation of the absorbed material can be done with flash lamps or pulsed lasers.

[0091] Other prior art techniques designed to reduce the frequency of window cleaning include disposing a restrictor plate between the window and the chamber in order to reduce the amount of contaminants that reach the window and, alternatively, to clean the exterior surface of the window with a flow of purge gas. Restrictor plates are not completely effective and merely lessen the amount of contaminants that reach the window. The cross-sectional area of the restrictor apertures may be decreased to further reduce the amount of contaminants that make their way to the window, but smaller apertures' tend to clog with contaminates more often than larger apertures. However, unlike cleaning or replacing a viewport window, a restrictor plate can be replaced with an identical plate without having to recalibrate the optical sensors to the new plate. Of course, whenever the window does become clouded, the optical sensors should be recalibrated to the replacement window.

[0092] Cleaning the window with purge gas presumes that contaminates have, or will reach the window, but these contaminates can be detached with a current of gas. Firstly, this assumption may be incorrect; the contaminants that reach the window may bind to the surface of the window. In any case, directing a purge gas to the window suffers from shortcomings that makes this technique impractical for certain applications. For instance, utilizing the process gas for the purge stream eliminates incompatibility problems that may be associated with using non-process gases. However, often the process gas itself reacts with the window material which causes clouding. Ultimately, it may be necessary to change the window material in order to use the process gas as the purge gas. Using a non-process gas for the purge gas enables the operator to select the optimal window material for the optical measurement to be taken without concern for the window reacting with the purge gas. Another benefit in using a non-process gas as the purge gas is that the purge gas may be selected for its cleaning properties for the particular type of contaminant. The drawback with using non-process gases as the purge gas is twofold. First, the purge gas will not entirely prevent the process gas from reaching and reacting with the window, so in selecting the type of window material, the susceptibility of clouding by the process gas should be considered. More importantly, the non-process gas will often have a detrimental effect on the process. Therefore, the purge flow rate of the non-process gas should be kept to an absolute minimum, which may worsen the clouding rate.

[0093] Furthermore, each of these purge gas techniques require a significant amount of redesign to the area surrounding the window viewport. For instance, the purge gas should have a sufficient flow rate and oriented in a suitable direction to wipe the exterior of; the window of any contaminates that adhere to the window. This requires the port (or ports) either be aimed at the window to force a stream of gas directly on the surface of the window or to design a cavity adjacent to the window that facilitates gas flow in lifting contaminates off the surface of the window and propelling them back into the process chamber.

[0094] A multichannel array (MCA) is a plate that has channels in it. They have been put to many uses such as electron multipliers, atomic beam collimators, neutron collimators, windows, etc. These can be made of stainless steel, aluminum, exotic metals, etc. Typically, they are large with the diameter of the channel is d > 0.1 mm. An MCA that is made of glass can have various sizes and some have channel diameters as small as 10 microns. Since glass is transparent at some wavelengths, it may be necessary to coat the outer surface of the multichannel array to maintain uniform transmission through the MCA as the outer surface becomes cloudy (coated). [0095] A multichannel array is a way to prevent clouding. The multichannel array will act as a barrier to slow the transport to the window and a getter that collects material in the channels. Ultimately, material will begin to cloud the window. But this can be acceptable if the time between cleaning cycles is much less than the time that it takes to cloud the window.

1. The length L of channel should be greater than the mean free path L_a of the gas, or particulate, that will cloud the window, La « L This will slow the material that passes through the channel along the axis.

2. The diameter d of the channel should be less than the mean free path, La ≥ d . This will enhance sticking to the wall and reduce diffusion. However, the diameter of the channels should be large enough to avoid frequent blockage.

3. The channels should be cold so the_ material will stick to surfaces, while it is moving through the channel.

[0096] None of. the prior art techniques have had a substantial impact on the problem of window clouding.. Many of these techniques are application-specific and require substantial modifications for each unique implementation. Most require substantial modifications be made to the system, usually at considerable expense, with only a marginal reduction in window clouding.

[0097] Before describing the present invention, a further description into the background of the present invention may be helpful. Window clouding results from contaminants that adhere to the interior of a process chamber, such as chamber 1110 shown in FIG. 11. These contaminants are essentially baked onto every surface of the interior volume 1112 of process chamber 1110, including the interior surface of optical viewport window 1102. This residue creates a visible film that increases in thickness over time. If these contaminants are allowed to build up on surfaces in chamber interior 1112, eventually they will flake off during operation and compromise the process performed therein. The effect of the contaminates on optical viewport window 1102 is even more detrimental to a production process than residue in the chamber interior because the contaminate film on window 1102 reduces the accuracy of the OES measurements long before the production process is affected by contaminate residue on chamber interior 1112. Thus, viewport window 1102 usually requires more frequent maintenance than chamber interior 1112.

[0098] The type of clouding varies with the process performed in process chamber 1110. For example, etch chemistries with CF_x are widely used in the semiconductor industry. For optical measurements, these chemistries create a problem by coating windows with a substance with a polymer that is similar in many respects to PolyTetraFluoroEthylene (PTFE). That coating can, over time, absorb a large amount of light and reduce the amount of light that can be transmitted through the window. This large absorption of light affects the transmission of ultraviolet and visible radiation. Thus, optical emission spectroscopy (OES) and other measurements that utilize wavelengths in these regions will be detrimentally affected by this film coating on the window. Furthermore, since these wavelength regions are predominantly used in the semiconductor industry for process and diagnostic measurements, window clouding is a serious and ongoing problem with CF_x etch chemistries.

[0099] Prior art efforts in overcoming window clouding problems have assumed that the large light absorption results from an opaque film deposited on the viewport window. This assumption may result from the film being opaque in the visible light spectrum (i.e., the film can be easily seen on a window), and is perhaps reinforced because the film is opaque to conventional measurement equipment employed in the semiconductor industry that uses Si-type CCD detectors. In fact, the vast majority of optical measurement equipment employed in the semiconductor industry is designed to operate in the ultraviolet through visible regions of the radiation spectrum. Thus, the problem of window clouding has assumed that the window film coating is opaque and, therefore, the overwhelming impetus of the semiconductor industry has been directed toward solving the problem of opaque window clouding, for environments that operate in ultraviolet-visible regions of the radiation spectrum. [00100] Applicant, on the other hand, has discovered that the absorption of light in coating films due to, at least, CF_x etch chemistries is not constant across the entire radiation spectrum. That is, certain window films that are nearly opaque in the ultraviolet (UV) through visible regions of the radiation spectrum, but exhibit extraordinarily good transmission at lower frequencies, such as in the near infrared (NIR) region of the radiation spectrum. In other words, Applicant understands that a more definitive solution to the window clouding problem is not to assume that the coating film is opaque, but instead to identify the spectral regions with low absorption and then to select sub-regions of the identified regions that are useful for performing a particular measurement. As mentioned above, since the prior art solutions to the window clouding problem have assumed that the coating film always increases the light absorption, these solutions have primarily involved adjusting the intensity of the light, altering the optical measurement algorithms to compensate for clouding or techniques for decreasing the frequency of window maintenance. Since the present invention does not presume that the window coating film is opaque, neither the light intensity or the optical measurement . algorithms need be altered nor are any additional modifications to the window chamber necessary to decrease the frequency of window maintenance (although the benefits of present invention may be optimized when combined with certain modifications to the window chamber).

[00101] Before discussing the problem of light absorption further, a means for quantifying the amount of light transmission is needed, i.e., relative transmission, I₇. l_τ can be determined from the ratio of two measurements, the transmission with a window in the path of the radiation and a second transmission with the window out of the path of the radiation:

where I_1n is the transmission with the window in the path of the radiation, and l_out is the transmission with the window out of the path of the radiation. [00102] Relative transmission l_τ is scaled between 0.0 and 1.0, with 0.0 being completely opaque and 1.0 being completely transparent to the wavelengths of radiation being investigated.

[00103] Fig. 12 is a diagram of the measurement device used to assess the relative transmission h of windows with varying amounts of coating film and clouding in accordance with an exemplary embodiment of the present invention. The device is generally comprised of radiation source (light source) 1203, for generating light having a broad spectral range, or alternatively radiation source 1203 may be comprised of a plurality of light sources that may be substituted for one another for emitting light in various regions of the spectrum. The light source selected for one test was a continuous infrared lamp which is typically used in a laboratory to heat items. This particular lamp has an infrared transmission filter and produced a nearly continuous blackbody spectrum. Test window 1202 is disposed directly between light source 1203 and a: port on integration sphere 1205. The radiation from light source 1203 passed through test window 1202 and entered a port of integration sphere 1205 (a standard integrating sphere available from the LOT-Oriel Nordic Division in Stockholm, Sweden was used). Integrating sphere 1205 creates a uniform distribution of the radiation to fill entrance slit of spectrograph 1209. The spectrograph is located at another port of integrating sphere 1205 at a right angle to the input port. The transmission measurements were made using a SD512NIR spectrometer available form Verity Instrument, Inc., of Carrollton, Texas, USA. Spectrograph 1209 utilizes sensor 1208 for converting the spectral light into a signal.

[00104] Here it should be mentioned, that the sensor selected for evaluating the relative transmission I₇ of window 1202 (and the coating film thereon), should exhibit good quantum efficiencies across the spectral region under investigation, i.e., spectral region under investigation should be within the spectral operating range of the sensor. For the example discussed below, an InGaAs diode array sensor was utilized. [00105] For the purposes of describing the present invention, three test windows with varying amounts of clouding were tested. The three test windows had progressively longer exposure to the plasma chemistry. Window 1 had the shortest exposure and has lightly clouded, pale yellow appearance, i.e., the coating film is slightly visible; window 2 had a longer exposure and has marginally clouded appearance with a darker yellow appearance, i.e., the film is clearly visible with darker appearance than that of window 1 , but not opaque; and window 3 had the longest exposure to the plasma chemistry and has a highly clouded, dark brown appearance, i.e., the film is highly visible with a darkened appearance and approaches opacity.

[00106] FIG. 13 is a chart depicting the results of that investigation. The chart in FIG. 13 shows the transmission response across the near infrared spectral region for three windows with varying amounts of coating film in accordance with an exemplary embodiment of the. present invention. In obtaining the measurements, care was taken that the radiation passed through the darkest portion of the film on the window. Notice that the relative transmission »/r for window 1 is fairly linear and is extremely high, generally above relatively 0.97. The results for window 2 are similar, although slightly lower relative transmission I₇ approximately linear and above 0.96 between 1100nm and 1600nm. Even window 3, which had an almost opaque coating in visible light, exhibited a marked improvement in the NIR region. Notice in the chart in FIG. 13 that for window 3, with the highly clouded, dark brown appearance, the relative transmission I₇ is over 0.85 (that is 85% of all NIR radiation is transmitted across the coating film) and approximately linear in the longer wavelengths of the NIR region.

[00107] From the transmission chart, it can be seen that the transmission of the windows corresponds to their appearance and the length of exposure to the plasma. The decrease in transmission includes absorption by the film on the window and reflection losses at the interfaces. The films absorb most of the visible light. However, they are nearly transparent in the NIR, to varying degrees, but depending on their length of exposure to the plasma.

[00108] FIG. 14 is a chart showing the transmission response across the UV, visible and near infrared spectral regions for the highly clouded window 1. Measured transmission curve 1402 shows the relative transmission for window 3 in the UV and visible regions of the radiation spectrum that is typically used for measurements in the semiconductor industry, by using, for example, a Si-type CCD sensors. An appropriate light source was used in the measurements for producing UV-visible light. Notice that the relative transmission is very low at the higher frequencies and, therefore, the absorption to the UV-visible wavelengths is extremely large. Furthermore, measured transmission curve 1402 demonstrates that the response across the UV-visible regions is highly wavelength-dependent and correspondingly less desirable for obtaining optical measurements. Measured transmission curve 1404, on the other. hand, shows the relative transmission across the NIR region of the radiation spectrum (approximately 900nm - 1700nm). As mentioned above, these measurements were obtained using- an InGaAs diode array sensor. Notice here that the response in the NIR region is much more transmissive, i.e., the coating film absorbs less light in the NIR region. Also notice that, as seen in FIG. 13 above, measured transmission curve 1404 is approximately independent of wavelength and the relative transmission l_τ is over 0.85 across the entire NIR region.

[00109] It should be mentioned that an acceptable value for the relative transmission l_τ of a window depends on the particular application that the measurement is applied. For example, certain applications, such as diagnostic measurements, are less tolerant to light absorption and consequently the relative transmission threshold will be higher for them, perhaps on the order of «0.99. Other applications, for example endpoint measurements may be more tolerant of absorption, for instance with relative transmission threshold of >0.85. [00110] The intent is to identify a spectral region that have low absorption in the coating film and is useful in a particular determination. Merely identifying a region with a high relative transmission for obtaining optimal measurements is not necessarily meaningful unless the optical measurements from that region are compatible with the particular algorithm that is being used. It is possible to identify spectral regions with exceptional transmission, but determinations from these data leads to inconclusive or invalid results. Hence, these regions are of no significance to the particular algorithm being utilized. For instance, in performing endpoint determination the optical measurements obtained from a spectral region may have high transmission, but do not exhibit any character that can be associated with the endpoint of a particular process. Consequently, although the optical measurements are relatively unaffected by window clouding, they would not be useful in detecting a process endpoint.

[00111] Returning to the discussion of the test windows, merely identifying a region with a high relative transmission, h does not necessarily make the region useful for a particular determination; some character in the measurement should be identified as useful for the determination. To verify this approach, a typical etch was measured in the NIR region. The etch chemistry had CF₄, CHF₃, Ar, and O₂ in a plasma. Layers of ARC and silicon nitride (SiN) were etched in the process. The material that was etched had a photoresist mask and a tungsten suicide stop layer. FIG. 15 shows a typical endpoint trend for the etch. There, intensity curve 1502 is the magnitude of the intensity in the NIR region tracked over time during a process. The light with wavelengths from 1000-1550 nm was measured. Notice that the endpoint can be determined as time 55.3 for this process, which validates the NIR region with high transmission as being useful to the particular endpoint determination being utilized.

[00112] Good transmission in the NIR region also means that extended measurements can be made in a production environment. FIG. 16 is a diagram depicting endpoint trends for an etch process that is repeated over an extended time period. The process was run continuously over a fourteen day period. During that time, 47 measurements were made of the process endpoint. The trends are shown in FIG. 16. These measurements show good reproducibility. The endpoints could be reliably determined, even though the transmission in the visible spectral region had become very poor at the end of the fourteen day period.

[00113] FIG. 17 is a flowchart depicting a method for identifying a spectral region with low absorption that is useful for measurement determinations in accordance with an exemplary embodiment of the present invention. This method is application specific and therefore the results from one application can not necessarily be relied on for another type of reactive environment. Initially, several clouded viewport windows with film coatings should be secured from the reactive environment that the invention is to be used. The relative transmission h of these windows will be measured, essentially as discussed above with regard to FIGs. 12 and 13. Test measurement apparatus 1200 may be used for this purpose, but it should be understood that light source 1203 and sensor 1208 should be selected for the wavelength regions under investigation. Therefore, one or more types of sensors may be necessary for obtaining the measurements, depending on the range of the spectral region to be investigated and the spectral range of the sensor (FIG. 18 is a chart showing typical sensor types and their corresponding spectral ranges). Alternatively, a bolometer may be used for obtaining measurement across a wide spectral range rather than several sensors with more narrow spectral ranges. Optimally, a scanning spectrometer may be used for spectrograph 1209.

[00114] The optical transmission of window film is then measured from the test windows from the particular reactive environment under investigation (step 1702). The measurement results are analyzed for a spectral region with relatively low absorption by the film materials (step 1704). More than one region may ultimately be identified as having an acceptable relative transmission. Obviously, if no other spectral regions satisfy the relative transmission threshold, the process ends. Once a region of low absorption is detected, that region should be thoroughly checked for usefulness with the particular determination, i.e., the optical measurements exhibit some character that can be used for making a particular determination (step 1706). The useful character of a particular region may be apparent to operators familiar with the particular environment or it may be necessary to validate the information obtained from a region in a production environment. If the region is not useful, the process reverts to step 1704 and another region with relatively low absorption is identified and then verified as being useful.

[00115] Once one or more regions are identified that exhibit good relative transmission to a particular coating film and are useful in a determination, the region can be used in the processing environment, such as with OES measurement apparatus 1100 depicted in FIG. 11. Of course, the spectral range of sensor 1108 should include the entire range of region identified as having a high relative transmission to the coating film (step 1708). If not, sensor 1108 of OES measurement apparatus 1100 should be replaced with a sensor having an appropriate spectral range (step 1710). In either case, optical measurements can proceed on OES. measurement apparatus 1100 in the identified region. The process then ends.

[00116] Using the present invention, especially with regard to the NIR region, allows measurements to be made in a production environment for an extended time period over that known in the prior art and without suffering the adverse effects of window clouding. As should be apparent from the preceding, the aim of the present invention is to identify spectral regions that can both be evaluated using algorithms used in the reactive production process and exhibits a high relative transmission of the coating films typically associated with the reactive production process. It should be expected that during operation, that the window will continue to become clouded. It should also be recognized that the tolerance to light absorption varies with the particular application. As mentioned above, some applications can tolerate a relative transmission value of 0.85, while others are less tolerant. Therefore, it is sometimes advantageous to include a mechanism for decreasing the frequency of window maintenance, e.g., a protective grid, a gas purged viewport, window heater or the like, to further reduce window clouding and extend the time period between maintenance. One particularly good option for extending the time between maintenance is a multichannel array that uses process gas to create the back pressure as described above.

[00117] The exemplary embodiments described below were selected and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. The particular embodiments described below are in no way intended to limit the scope of the present invention as it may be practiced in a variety of variations and environments without departing from the scope and intent of the invention. Thus, the present invention is not intended to be limited to the embodiment shown, but is to be accorded the widest scope consistent with the principles and features described herein.

[00118] The exemplary embodiments were described with regard to an exemplary spectroscopic technique of optical emission spectroscopy. However, the present invention is also useful and applies to other measurement techniques, for example reflectometry, fault detection and characterization (FDC), process monitoring, etc. that requires stable, transparent optical windows that do not degrade or change with time. Although RIE is discussed, it is merely an exemplary environment for practicing the invention, others include, but are not limited to all forms of wet or dry etch, chemical vapor deposition (CVD), chemical-mechanical polishing (CMP), etc. Additionally, the exemplary plasma chemistry used in describing the present invention comprises CF₄, CHF₃, Ar, and O₂. These are not intended to limit or define the invention but merely used as a means to describe certain aspects of the invention. The present invention is applicable to any chemistries and substances that cover, cloud, or contaminate the viewport window. These may relate to monitoring a reaction chamber as discussed, effluent gas monitoring, or other monitoring other emission types. [00119] Furthermore, although Applicant has discovered that the NIR region is particularly useful in some applications, especially in applications where window clouding is prevalent in the UV-visible region, any spectral region may be utilized to reduce the effects of window clouding on a production process. Any spectral region that is identified as having good transmission to the window film can be utilized (assuming that the region contains useful spectral intensity from the plasma).

[00120] The exemplary embodiments described below were selected and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. The particular embodiments described below are in no way intended to limit the scope of the present invention as it may be practiced in a variety of variations and environments without departing from the scope and intent of the invention. Thus, the present invention is not intended to be limited to the embodiment shown, but is to. be accorded the widest scope consistent with the principles and features described herein;¹ ;

[00121] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Claims

CLAIMSWhat is claimed is:

1. A method for reducing the effects of window clouding on a viewport window in a reactive environment, comprising: evaluating an optical transmission of at least one viewport window, said at least one viewport window having reduced transmission from use in a reactive environment; identifying a spectral region with predetermined optical transmission level; verifying the identified spectral region as compatible with a production algorithm; monitoring emissions associated with a production process at the identified spectral region; and evaluating the production process by applying the production algorithm to data derived from the identified spectral region. .

2. The method in claim 1 further comprises: obtaining the at least one viewport window used in the reactive environment of the production process, said at least one viewport window having reduced transmission from use in the reactive environment.

3. The method in claim 1 , wherein evaluating an optical transmission of the at least one viewport window projecting further comprises: measuring a first intensity in a path from a radiation source; measuring a second intensity in the path from the radiation source and through the at least one viewport window; and finding a relative transmission as a ratio of the second intensity to the first intensity.

4. The method in claim 3, wherein the predetermined optical transmission level is a relative transmission greater than 0.85.

5. The method in claim 3, wherein the predetermined optical transmission level is determined by the production algorithm.

6. The method in claim 1 , wherein the identified spectral region is the near infrared region of the light spectrum.

7. The method in claim 6, wherein the near infrared region of the light spectrum is between 900nm and 1700nm.

8. The method in claim 1 , wherein verifying the identified spectral region as compatible with a production algorithm further comprises: identifying changes in data obtained from the identified spectral region during a production process; correlating the changes in data obtained to events in the production process by applying the production algorithm to data derived from the identified spectral region.

9. The method in claim 1 , wherein monitoring the identified spectral region during a production process further comprises: receiving at an Indium Gallium Arsenide diode array sensor type of near infrared sensor.

10. The method in claim 1 , wherein the identified spectral region is outside the visible light spectrum.

11. The method in claim 1 , wherein the identified spectral region is outside the visible ultraviolet light spectrum.

12. The method in claim 1 , wherein the identified spectral region is outside a spectral range of an Si-type of CCD detector.

13. The method in claim 3, wherein the predetermined optical transmission level is a relative transmission that is greater than a relative transmission for the visible spectral region.

14. The method in claim 1 further comprises: protecting the viewport window from contaminants during the production process.

15. A method for reducing the effects of window clouding on a viewport window in a reactive environment, comprising: evaluating an optical transmission of contaminate residue on a surface of at least one viewport window, said contaminate residue on a surface of at least one viewport window being deposited from use in a reactive environment window; identifying a spectral region with a higher optical transmission level than a second spectral region; , monitoring emissions associated with a production process at the identified spectral region; and evaluating the production process by applying the production algorithm to data derived from the identified spectral region.

16. A device for reducing the effects of window clouding on a viewport window in a reactive environment, comprising: a process chamber comprising: a plurality of walls which at least partially enclose a process volume; a material within the process volume; and a viewport window disposed along one of the walls of the process chamber; a spectrograph, said spectrograph being optically coupled to said viewport window; and a near infrared sensor, said near infrared sensor being optically coupled to said spectrograph.

17. The device recited in claim 16, wherein the near infrared sensor has a spectral range of between 900nm and 1700nm.

18. The device recited in claim 16, wherein the near infrared sensor is an Indium Gallium Arsenide diode array sensor.

19. The device recited in claim 16, wherein the process chamber further comprises one of a multichannel array, a protective viewport window grid, a viewport window gas purge port and a viewport window heater.

20. The device recited in claim 16, wherein the process chamber further comprises a multichannel array, said multichannel array comprising: a body having an interior surface and an exterior surface for pneumatically isolating a window chamber pressure within the window chamber from the confinement pressure; and a predetermined quantity of channels, each of said predetermined quantity of channels having an interior end and an exterior end, a cross- sectional shape with a channel diameter and a channel length between the interior and exterior ends, at least one of said channel diameter, said channel length and said predetermined quantity of channels being related to establishing a flow rate across the predetermined quantity of channels with a pressure differential across the predetermined quantity of channels.

21. A device for reducing window clouding in a viewport window of a process chamber, comprising: a process chamber comprising: a plurality of walls which at least partially enclose a process volume, wherein a process pressure exists within the process volume; at least one ingress port traversing the process chamber to the process volume; and at least one egress port traversing the process chamber to the process volume; a material within the process volume; a viewport window disposed along one of the walls of the process chamber; a window chamber defined by the viewport window, a portion of the one of the walls of the process chamber and a multichannel array; a window chamber ingress port traversing the one of the walls of the process chamber to the window chamber; and the multichannel array comprising: a body having an interior surface and an exterior surface for pneumatically isolating a window chamber pressure within the window chamber from the confinement pressure; and a predetermined quantity of channels, each of said predetermined quantity of channels having an interior end and an exterior end, a cross- sectional shape with a channel diameter and a channel length between the interior and exterior ends, at least one of said channel diameter, said channel length and said predetermined quantity of channels being related to establishing a flow rate across the predetermined quantity of channels with a pressure differential across the predetermined quantity of channels; and a near infrared sensor, said near infrared sensor.

22. The device recited in claim 21 further comprises: a substrate, wherein the material is one of a process gas or a by-product of the substrate.

23. The device recited in claim 22, wherein the process pressure is related to at least one of an ingress flow rate and an ingress pressure of process gas entering the process chamber at the ingress port and the window chamber pressure is related to at least one of a window chamber ingress port flow rate and a window chamber ingress pressure from window chamber gas entering the window chamber at the window chamber ingress port.

24. The device recited in claim 23, wherein the window chamber pressure is greater than the process pressure.

25. The device recited in claim 24, wherein the window chamber ingress pressure is greater than the ingress pressure.

26. The device recited- in claim 25, wherein the window chamber ingress port pressure is less than the ingress flow rate.

27. The device recited in claim 25 further comprises: an optical sensor, said optimal sensor being adjacent to said window.

28. The device recited in claim 27, wherein said predetermined quantity of channels of the multichannel array are aligned in an optical path between the optical sensor and a target.

29. The device recited in claim 28, wherein the target is one of a plasma ignited with said process volume and the substrate.

30. The device recited in claim 27, wherein the window chamber gas is an inert gas.

31. The device recited in claim 7, wherein the window chamber gas is the process gas.

32. The device recited in claim 21 , wherein the cross-sectional shape is symmetrical and the channel diameter is a shortest path across any symmetry axis.

33. The device recited in claim 22, wherein the cross-sectional shape is elliptical.

34. The device recited in claim 23, wherein the cross-sectional shape is circular.

35. The device recited in claim 22, wherein the cross-sectional shape is polygonal.

36. The device recited in claim 26, wherein the cross-sectional shape is one of a triangle, quadrilateral, square, rectangle, pentagon, hexagon, heptagon, octagon, nonagon, decagon or a combination of shapes.

37. A method for reducing window clouding in a viewport window of a process chamber, comprising: providing a process chamber with a process volume operating at a process pressure; providing a viewport window on a wall of the process chamber, providing a multichannel array on the wall of a process chamber and adjacent to the viewport widow; providing a predetermined number of channels in the multichannel array, each of said channels having interior end adjacent to the viewport window, and exterior end, a diameter and a length between the interior and exterior ends; establishing a flow across the predetermined quantity of channels by exerting a gas pressure on the interior end of the predetermined quantity of channels.

38. The method recited in claim 37, wherein the window pressure is greater than the process pressure.

39. The method recited in claim 18, further comprises: providing a process flow rate in the process volume from process gas entering the process chamber at an ingress port and gas exiting the process chamber at an egress port; wherein the viscous flow rate is less than the process flow rate.

40. The method recited in claim 37, wherein viscous flow rate is related to the viscosity of a gas at the open end of the predetermined quantity of channels, the predetermined quantity of channels, the channel diameter of the predetermined quantity of channels, the length of the predetermined quantity of channels and a difference in pressure between the gas pressure on the interior end of the predetermined quantity of channels and the process pressure.

41. The device recited in claim 21 , wherein the flow rate across the predetermined quantity of channels is a viscous flow.

42. The device recited in claim 21 , wherein the window chamber gas is neither an inert gas or a process gas.

43. The method recited in claim 37, wherein the flow across the predetermined quantity of channels is a viscous flow.