WO2012177663A2

WO2012177663A2 - Autofocus system with reference configuration

Info

Publication number: WO2012177663A2
Application number: PCT/US2012/043186
Authority: WO
Inventors: Michael R. Sogard; Daniel Gene Smith; Eric Peter Goodwin
Original assignee: Nikon Corporation
Priority date: 2011-06-23
Filing date: 2012-06-19
Publication date: 2012-12-27
Also published as: WO2012177663A3

Abstract

A system (222) for measuring the position of a work piece (200) includes a redirector assembly (244), a light source assembly (240), a detector assembly (242), and a control system (224). The redirector assembly (244) is positioned near and spaced apart from the working surface (200A). The light source assembly (240) directs a measurement beam (246A) and a reference beam (246B) at the redirector assembly (244). The detector assembly (242) detects the measurement beam (246A) reflected off of the work piece (200) and generates a measurement signal, and detects the reference beam (246B) reflected off of the redirector assembly (244) and generates a reference signal. The control system (24) uses the measurement signal and the reference signal to determine the position of the work piece (200).

Description

AUTOFOCUS SYSTEM WITH REFERENCE CONFIGURATION

for

ERIC PETER GOODWIN, MICHAEL R. SOGARD and DANIEL GENE SMITH

of

PCT PATENT APPLICATION

RELATED INVENTION

This application claims priority on U.S. Provisional Application Serial No. 61/500,521 , filed June 23, 201 1 and entitled "AUTOFOCUS SYSTEM WITH REFERENCE CONFIGURATION". As far as is permitted, the contents of U.S. Provisional Application Serial No. 61 /500,521 is incorporated herein by reference.

BACKGROUND

[0001] Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly having an optical axis, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. The measurement system constantly monitors the position of the reticle and the wafer, and the control system controls each stage assembly to constantly adjust the position of the reticle and the wafer. The features of the images transferred from the reticle onto the wafer are extremely small. Accordingly, the precise positioning of the wafer and the reticle is critical to the manufacturing of high quality wafers.

[0002] In certain designs, the measurement system includes an autofocus system that is used to map the topography of the wafer surface along the optical axis prior to exposing the wafer. Subsequently, with information regarding the position along the optical axis, the wafer stage assembly can be controlled to properly position the wafer along the optical axis.

[0003] One type of autofocus system includes a slit light source that illuminates a set of slits and an imaging system that projects the set of slits onto the wafer at a glancing angle of incidence. The imaging system can include a vibrating mirror that translates the slit image(s) across a small portion of the wafer. The light reflected from the wafer is then directed into a receiving imaging system that projects an image of the slits onto a second set of slits. The light that passes through the second set of slits is subsequently measured by a slit detector assembly. With information from the slit detector assembly, the position of the wafer along the optical axis can be determined. Further, the wafer can be moved in a plane perpendicular to the optical axis to map the position along the optical axis of the entire wafer.

[0004] The autofocus system discussed above utilizes the plurality of slits of light reflected off of the wafer to determine the position of the wafer along the optical axis. Unfortunately, instabilities in the autofocus system and environmental conditions can adversely influence the accuracy of the autofocus system and ultimately the accuracy in which the wafer is positioned along the optical axis. For example, instabilities in the vibrating mirrors, optics, and/or other components of the autofocus system can adversely influence the accuracy of the autofocus system. Further, environmental effects such as the refractive index changes of air due to temperature, atmospheric pressure, and/or humidity changes or gradients can adversely influence the accuracy of the autofocus system.

[0005] Moreover, in certain designs of the autofocus system, a high angle of incidence on the slits of light relative to the wafer surface normal is required to achieve a high sensitivity level to height changes of the wafer. The high angle of incidence can lead to a longer beam path length in the air, with a corresponding increase in sensitivity to environmental changes. In addition, the size of future wafers is expected to be increased. As a result thereof, the beam may be required to travel through an even longer region of air.

[0006] Environmental control of the air in which the slits of light of the autofocus system travel can only be achieved to a certain level. For example, in immersion lithography systems, the evaporation of the immersion fluid contributes to temperature and humidity changes of the air near the autofocus system.

[0007] Accordingly, there is a need for an autofocus system that is less influenced by instabilities in the autofocus system and environmental conditions. SUMMARY

[0008] The embodiment of the present invention is directed to a system for measuring the position of a work piece along a first axis that is orthogonal to a working surface of the work piece. In one embodiment, the system includes a redirector assembly, a light source assembly, a detector assembly, and a control system. The redirector assembly is positioned near and spaced apart from the working surface along the first axis. The light source assembly directs a measurement beam at a grazing angle of incidence at the working surface and a reference beam at a grazing angle of incidence at the redirecter assembly. Further, the measurement beam is spaced apart from the reference beam along a second axis that is orthogonal to the first axis prior to the working surface and the redirector assembly. The detector assembly (i) detects the measurement beam reflected off of the work piece and generates a measurement signal, and (ii) detects the reference beam reflected off of the redirector assembly and generates a reference signal. The control system uses the measurement signal and the reference signal to determine the position of the work piece along the first axis.

[0009] With this design, the system can compensate for instabilities in the components of the system and can compensate for environmental effects such as the refractive index changes of air. As a result thereof, measurements taken with the system are more accurate and the work piece can be positioned with improved accuracy. Further, the system can be used with an exposure apparatus to manufacture higher density wafers.

[0010] In one embodiment, the redirector assembly includes (i) a first redirector that redirects the reference beam from the light source assembly to be approximately parallel to and spaced apart from the working surface, and (ii) a second redirector that receives the reference beam redirected by the first redirector and redirects the reference beam back at the first redirector. In this embodiment, the first redirector also receives the reference beam redirected from the second redirector and redirects the reference beam at the detector assembly. As provided herein, the first redirector can be a fold mirror and the second redirector can be a reference mirror that is optically coincident with the working surface.

[0011] As provided herein, the reference beam is incident on the first redirector at a first redirector area, and the reference beam is incident on the second redirector at a second redirector area. Further, the first redirector area is positioned a first separation distance from the working surface along the first axis and the first director location is spaced apart from the second redirector area a second separation distance along the second axis. In certain embodiments, the second separation distance is approximately equal to the first separation distance. As a result thereof, the redirected beam has approximately the same distance of travel as the measurement beam. This can be important for maintaining good optical imaging simultaneously for both the second redirector and the working surface.

[0012] In one embodiment, the measurement beam is simultaneously imaged onto the working surface at a plurality of measurement areas positioned along a third axis that is orthogonal to the first axis and the second axis, and the reference beam is simultaneously imaged onto the first redirector at a plurality of reference areas positioned along the third axis. In this embodiment, the detector assembly detects the measurement beam reflected off of the work piece and generates a measurement signal for each of the measurement areas, and the detector assembly detects the reference beam reflected off of the first redirector and generates a reference signal for each of the reference areas. As is known, the working surface has a surface length measured along the third axis. In certain embodiments, the plurality measurement areas are spaced apart along substantially the entire surface length, and the reference areas are spaced apart along a reference length that is approximately equal to the surface length. With this design, the reference areas are near the measurement areas, and in certain embodiments, there is a unique reference area corresponding to each measurement area

[0013] In one embodiment, the light source assembly generates a plurality of spaced apart slits of light. In this embodiment, a first portion of the slits of light are concurrently directed at and reflected off of the working surface, and a second portion of the slits of light are concurrently directed at and reflected off of the redirector assembly. Alternatively, the light source assembly generates a continuous fringe pattern of light. In this embodiment, a first portion of the fringes of light are concurrently directed at and reflected off of the working surface, and a second portion of the fringes of light are concurrently directed at and reflected off of the redirector assembly.

[0014] The embodiment of the present invention is also directed to a stage assembly that moves a work piece, the system described herein, and the stage assembly including a stage that retains the work piece. In yet another embodiment, the embodiment of the present invention is directed to an exposure apparatus that includes an illumination system and a stage assembly that moves the stage relative to the illumination system. In still another embodiment, the embodiment of the present invention is directed to a process for manufacturing a device that includes the steps of providing a substrate and forming an image to the substrate with the exposure apparatus.

[0015] Additionally, the embodiment of the present invention is directed to a method for measuring the position of a work piece along a first axis that is orthogonal to a working surface of the work piece, the method comprising the steps of: positioning a redirector assembly near and spaced apart from the working surface along the first axis; directing a measurement beam at a grazing angle of incidence at the working surface; directing a reference beam at a grazing angle of incidence at the redirecter assembly, the reference beam being spaced apart from the measurement beam along a second axis that is orthogonal to the first axis prior to the working surface and the redirector assembly; detecting the measurement beam reflected off of the work piece and generating a measurement signal with a measurement system; detecting the reference beam reflected off of the redirector assembly and generating a reference signal with the measurement system; and determining the position of the work piece along the first axis utilizing the measurement signal and the reference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The novel features of embodiments of this invention as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:

[0017] Figure 1 is a schematic illustration of an exposure apparatus having features of the embodiment of the present invention;

[0018] Figure 2 is a simplified side view of an autofocus system having features of the embodiment of the present invention and a work piece;

[0019] Figure 3A is a simplified side view of another embodiment of an autofocus system having features of the embodiment of the present invention and a work piece; [0020] Figure 3B is a simplified illustration of an aperture that controls which orders are directed at the working surface to create the continuous fringe pattern of light from Figure 3A;

[0021] Figure 4A is a simplified side illustration of the work piece, a measurement beam, a reference beam, and a redirector assembly having features of the embodiment of the present invention;

[0022] Figure 4B is a simplified top illustration of the work piece, the measurement beam, the reference beam, and the redirector assembly of Figure 4A;

[0023] Figure 4C is a simplified top perspective illustration of the work piece, the measurement beam, the reference beam, and the redirector assembly of Figure 4A;

[0024] Figure 5A is a simplified top illustration of the work piece, a first embodiment of the measurement beam, a first embodiment of the reference beam, and a portion of the redirector assembly;

[0025] Figure 5B is a simplified side view of a second redirector of Figure 5A;

[0026] Figure 5C is a simplified top illustration of the work piece, a second embodiment of the measurement beam, a second embodiment of the reference beam, and a portion of the redirector assembly;

[0027] Figure 5D is a simplified side view of a second redirector of Figure 5C;

[0028] Figure 5E is a simplified illustration of a portion of a detector assembly having features of the embodiment of the present invention;

[0029] Figures 5F and 5G are simplified illustrations of a portion of another embodiment a detector assembly having features of the embodiment of the present invention;

[0030] Figure 6A is a simplified top plan illustration of the work piece, a portion of the reference beam, an another embodiment of a reference system having features of the embodiment of the present invention;

[0031] Figure 6B is a simplified top illustration of a detector from Figure 6A;

[0032] Figure 6C is a simplified side illustration of the work piece, the reference beam, and the reference system of Figure 6A; [0033] Figure 7A is a simplified side illustration of the work piece, the measurement beam, the reference beam, the redirector assembly, and an auxiliary measurement system having features of the embodiment of the present invention;

[0034] Figure 7B is a simplified top illustration of the work piece, the measurement beam, the reference beam, the redirector assembly, and a pair of auxiliary beams;

[0035] Figure 7C is a simplified side illustration of the work piece, the measurement beam, the reference beam, the redirector assembly, and another auxiliary measurement system having features of the embodiment of the present invention;

[0036] Figure 8A is a simplified side view of still another embodiment of an autofocus system having features of the embodiment of the present invention and a work piece;

[0037] Figure 8B is a cut-away view taken on line 8B-8B in Figure 8A;

[0038] Figure 8C is another cut-away view of a portion of the autofocus system on Figure 8A;

[0039] Figure 9A is a simplified illustration of an environmental chamber having features of the embodiment of the present invention;

[0040] Figure 9B is an end view of the environmental chamber of Figure 9A, and a work piece;

[0041] Figure 10A is a flow chart that outlines a process for manufacturing a device in accordance with the embodiment of the present invention; and

[0042] Figure 10B is a flow chart that outlines device processing in more detail.

DESCRIPTION

[0043] Figure 1 is a schematic illustration of a precision assembly, namely an exposure apparatus 10 having features of the embodiment of the present invention. The exposure apparatus 10 includes an apparatus frame 12, an illumination system 14 (irradiation apparatus), an optical assembly 16, a reticle stage assembly 18, a wafer stage assembly 20, a position system 22, and a control system 24. The design of the components of the exposure apparatus 10 can be varied to suit the design requirements of the exposure apparatus 10. [0044] The exposure apparatus 10 is particularly useful as a lithographic device that transfers a pattern (not shown) of an integrated circuit from a reticle 26 onto a semiconductor wafer 28. The exposure apparatus 10 mounts to a mounting base 30, e.g., the ground, a base, or floor or some other supporting structure.

[0045] As an overview, the position system 22 includes an autofocus system 22A that measures the position of a work piece, e.g. the wafer 28, along a Z axis with improved accuracy. More specifically, in certain embodiments, the autofocus system 22A is uniquely designed to reduce the influence of instabilities in the components of autofocus system 22A, and/or reduce the influence of environmental conditions on the accuracy of the autofocus system 22A. As a result thereof, the wafer 28 can be positioned with improved accuracy, and the exposure apparatus 10 can be used to manufacture higher density wafers 28.

[0046] A number of Figures include an orientation system that illustrates an X axis, a Y axis that is orthogonal to the X axis, and the Z axis that is orthogonal to the X and Y axes. It should be noted that any of these axes can also be referred to as the first, second, and/or third axes. The autofocus system 22A can be fixedly secured to the optical assembly 16 or a support frame which supports the optical assembly 16.

[0047] There are a number of different types of lithographic devices. For example, the exposure apparatus 10 can be used as a scanning type photolithography system that exposes the pattern from the reticle 26 onto the wafer 28 with the reticle 26 and the wafer 28 moving synchronously. Alternatively, the exposure apparatus 10 can be a step-and-repeat type photolithography system that exposes the reticle 26 while the reticle 26 and the wafer 28 are stationary. However, the use of the exposure apparatus 10 provided herein is not limited to a photolithography system for semiconductor manufacturing. The exposure apparatus 10, for example, can be used as an LCD photolithography system that exposes a liquid crystal display device pattern onto a rectangular glass plate or a photolithography system for manufacturing a thin film magnetic head.

[0048] The apparatus frame 12 is rigid and supports the components of the exposure apparatus 10. The apparatus frame 12 illustrated in Figure 1 supports the reticle stage assembly 18, the optical assembly 16, the wafer stage assembly 20, and the illumination system 14 above the mounting base 30. [0049] The illumination system 14 includes an illumination source 32 and an illumination optical assembly 34. The illumination source 32 emits a beam (irradiation) of light energy. The illumination optical assembly 34 guides the beam of light energy from the illumination source 32 to the optical assembly 16. The beam illuminates selectively different portions of the reticle 26 and exposes the wafer 28.

[0050] The illumination source 32 can be a g-line source (436 nm), an i-line source (365 nm), a KrF excimer laser (248 nm), an ArF excimer laser (193 nm), a F₂ laser (157 nm), or an EUV source (13.5 nm). Alternatively, the illumination source 32 can generate charged particle beams such as an x-ray or an electron beam. For instance, in the case where an electron beam is used, thermionic emission type lanthanum hexaboride (LaB₆) or tantalum (Ta) can be used as a cathode for an electron gun. Furthermore, in the case where an electron beam is used, the structure could be such that either a mask is used or a pattern can be directly formed on a substrate without the use of a mask.

[0051] The optical assembly 16 projects and/or focuses the light passing through the reticle 26 to the wafer 28. Depending upon the design of the exposure apparatus 10, the optical assembly 16 can magnify or reduce the image illuminated on the reticle 26. The optical assembly 16 need not be limited to a reduction system. It could also be a 1 x (unit magnification) or magnification system (enlargement system).

[0052] The reticle stage assembly 18 holds and positions the reticle 26 relative to the optical assembly 16 and the wafer 28. In Figure 1 , the reticle stage assembly 18 includes a reticle stage 18A that retains the reticle 26, and a reticle stage mover assembly 18B that positions the reticle stage 18A and the reticle 26. The reticle stage mover assembly 18B can be designed to move the reticle 26 along the X, Y and Z axes, and about X, Y and Z axes.

[0053] Somewhat similarly, the wafer stage assembly 20 holds and positions the wafer 28 with respect to the projected image of the illuminated portions of the reticle 26. In Figure 1 , the wafer stage assembly 20 includes a wafer stage 20A that retains the wafer 28, and a wafer stage mover assembly 20B that positions the wafer stage 20A and the wafer 28. The wafer stage mover assembly 20B can be designed to move the wafer 28 along the X, Y and Z axes, and about X, Y and Z axes. In this embodiment, the wafer 28 can be scanned while the wafer stage assembly 20 moves the wafer 28 along the Y axis. [0054] The position system 22 monitors movement of the reticle 26 and the wafer 28 relative to the optical assembly 16 or some other reference. With this information, the control system 24 can control the reticle stage assembly 18 to precisely position the reticle 26 and the wafer stage assembly 20 to precisely position the wafer 28. For example, the position system 22 can utilize multiple laser interferometers, encoders, autofocus systems, and/or other measuring devices.

[0055] In Figure 1 , the position system 22 includes (i) a reticle measurement system 22B (only a portion is illustrated in Figure 1 ) that monitors the position of the reticle stage 18B and the reticle 26, (ii) a wafer measurement system 22C (only a portion is illustrated in Figure 1 ) that monitors the position of the wafer stage 20A along the X and Y axes, and about the Z axis, and (iii) the autofocus system 22A that maps the topography of the wafer 28 relative to the optical assembly 16 along the Z axis (an optical axis 16A), about the X axis, and about the Y axis prior to exposure with improved accuracy. As a result thereof, the wafer stage assembly 20 can be controlled to position the wafer 28 with improved accuracy.

[0056] As provided herein, the autofocus system 22A includes a reference system 36 that provides a reference signal that relates to the measurement of everything that is changing in the autofocus system 22A except for the position of the wafer 28 along the optical axis 16A, and a measurement system 38 that provides a measurement signal that relates to the measurement of everything changing in the autofocus system 22A plus the position of the wafer 28 along the optical axis 16A. By subtracting the reference signal from the measurement signal, the position of the wafer 28 is measured, thereby reducing the stability requirements on much of the components of the autofocus system 22A.

[0057] In this embodiment, the reference system 36 and the measurement system 38 are secured to and monitor the position relative to the optical assembly 16. Alternatively, these system 36, 38 can be secured to and monitor the position relative to another reference.

[0058] The control system 24 is connected to the reticle stage assembly 18, the wafer stage assembly 20, and the position system 22. The control system 24 receives information from the position system 22 and controls the stage assemblies 18, 20 to precisely position the reticle 26 and the wafer 28. The control system 24 can include one or more processors and circuits. [0059] Figure 2 is a simplified illustration of a work piece 200, a control system 224, and one embodiment of an autofocus system 222 that measures the position of a working surface 200A of the work piece 200 along the Z axis, about the X axis and about the Y axis. In this embodiment, for example, the work piece 200 can be the wafer 28 (illustrated in Figure 1 ) and the working surface 200A is the surface where the features are transferred. Alternatively, the autofocus system 222 can be used to monitor the position of other types of work pieces 200 (e.g. the reticle 26 illustrated in Figure 1 ) during manufacturing and/or inspection.

[0060] In certain embodiments, the position of the work piece 200 along the Z axis for each X, Y position can be premapped prior to exposing the work piece 200. With this design, for each X, Y position of the work piece 200, the stage control system 224 can be controlled to make the appropriate adjustment to the Z position of the work piece 200 based on the premapped information. Alternatively, for example, the autofocus system 222 is continuously measuring the position of the work piece 200 along the Z axis. With this design, the stage mover assembly 20 (illustrated in Figure 1 ) can be controlled to make real time adjustments to the Z position of the work piece 200 based on the measurements from the autofocus system 222.

[0061] In Figure 2, the autofocus system 222 is a slit type system that includes a reference system 236 and a measurement system 238. The design and positioning of each of these components can be varied to achieve the measurement requirements of the autofocus system 222. In Figure 2, the reference system 236 and the measurement system 238 share a common light source assembly 240, and a common detector assembly 242. The common detector assembly 242 can be preferred since it reduces errors due to any instabilities of the detector assembly 242. Additionally, in this embodiment, the reference system 236 includes a redirector assembly 244 that is used to provide the reference signal. Alternatively, for example, the reference system 236 and the measurement system 238 can have independent light source assemblies, and detector assemblies.

[0062] The light source assembly 240 generates one or more beams of light that are directed at the workpiece 200 and the redirector assembly 244. Further, the detector assembly 242 measures the light reflected off of the workpiece 200 and the redirector assembly 244. In Figure 2, the light source assembly 240 generates an array of slits of light 246 that are imaged onto the work piece 200 and the redirector assembly 244. In non-exclusive examples, the light source assembly 240 can direct one, five, ten, fifteen, twenty, twenty-five, thirty, or more spaced apart slits of light 246 at the work piece 200 and the redirector assembly 244.

[0063] In Figure 2, the light source assembly 240 includes (i) a broadband light source 248A that generates light 248B, (ii) a lens 248C that collimates the light 248B from the light source 248A, (iii) a slit mask 248D that shapes the slits of light 246, (iv) a lens 248E that collimates the light from the slit mask 248D, (v) a vibrating mirror assembly 248F that causes the slits of light 246 to move back and forth on the work piece 200 and the redirector assembly 244, and (iv) a lens 248G that focuses the slits of light 246 onto the work piece 200 and the redirector assembly 244.

[0064] In one embodiment, (i) a first portion of the slits of light 246, hereinafter referred to as measurement light 246A or measurement beam, are imaged onto the work piece 200; and (ii) a second portion of the slits of light 246, hereinafter referred to as reference light 246B or reference beam, are projected onto the redirector assembly 244. In Figure 2, the slits of light 246 that are farthest out of the page along the Y axis is the reference light 246B, while the slits of light 246 that are into the page along the Y axis is the measurement light 246A. With this design, the redirector assembly 244 picks off and redirects the second portion of the slits of light 246 before they are imaged onto the work piece 200.

[0065] It should be noted that the measurement light 246A is projected onto the work piece 200 at a glancing angle of incidence. Somewhat similarly, the reference light 246B is projected onto the redirector assembly 244 at a glancing angle of incidence. As non-exclusive examples, the angle of incidence can be between approximately sixty-two (62) and eighty-nine (89) degrees. Subsequently, the measurement light 246A is reflected off of the work piece 200 and the reference light 246B is reflected off of the redirector assembly 244 and both are re-imaged onto the detector assembly 242.

[0066] The detector assembly 242 detects the measurement light 246A that is reflected off of the work piece 200 and the reference light 246B light that is reflected off of the redirector assembly 244. In one embodiment, the detector assembly 242 includes (i) a lens system represented by 250A and 250B that image the light reflected off of the work piece 200 onto a slit mask 250C, and (ii) a detector 250D (e.g. a charge-coupled device "CCD"). With this design, the light that passes through the slit mask 250C is subsequently measured by the detector 250D. As a result thereof, in this embodiment, the same detector 250D is used to measure both the measurement light 246A that is reflected off of the work piece 200 and the reference light 246B reflected off of the redirector assembly 244. Thus, the detector 250D can provide to the control system 224 both a measurement signal that relates to the measurement light 246A reflected off of the work piece 200 and a reference signal that relates to the reference light 246B reflected off of the reference system 244.

[0067] As provided herein, the reference signal relates to the measurement of everything that is changing the autofocus system 222 except for the position of the work piece 200 along the Z axis, and the measurement signal relates to the measurement of everything changing in the autofocus system 222 plus the position of the work piece 200 along the Z axis. With this design, the control system 224 can subtract the reference signal from the measurement signal to determine the position of the work piece 200. As a result thereof, the autofocus system 222 compensates for instability in the system components and/or environmental factors that can cause erroneous information to be produced at the detector 250D. Stated in another fashion, with the present design, the control system 224 can determine if the work piece 200 height information produced from the detector 250D is due to changes in the height of the work piece 200, which is what an AF system and method are normally seeking to determine, or whether the information may be erroneous, due to instability in the system components (e.g. the vibrating mirror assembly, optics, and/or other components) and/or environmental factors (e.g. changes in the refractive index of air).

[0068] A further discussion of a slit array autofocus system that does not include reference system 236 is contained in United States Patent No. 5,602,399. As far as permitted, the contents of United States Patent No. 5,602,399 are incorporated herein by reference.

[0069] In one embodiment, the redirector assembly 244 includes a first redirector 252 and a second redirector 254. These components are described in more detail below in reference to Figures 4A-4C. In one non-exclusive embodiment, the first redirector 252 and the second redirector 254 can be fixedly secured to the optical assembly 16 (illustrated in Figure 1 ) with a redirector bracket 55 (illustrated in Figure 1 ). The first redirector 252 and the second redirector 254 may be fixedly secured to the support frame which supports the optical assembly 16.

[0070] Figure 3A is a simplified illustration of a work piece 300, a control system 324, and another embodiment of an autofocus system 322 that measures the position of the work piece 300 along the Z axis, about the X axis and about the Y axis. In this embodiment, the autofocus system 322 can be used to premap the position of the work piece 300 along the Z axis for each X, Y. With this design, for each X, Y position of the work piece 300, the stage control system 324 can be controlled to make the appropriate adjustment to the Z position of the work piece 300 based on the premapped information.

[0071] In Figure 3A, the autofocus system 322 is a fringe projection type system and again includes a reference system 336 and a measurement system 338 that share a common light source assembly 340, and a common detector assembly 342. A discussion of a sinusoidal irradiance fringe pattern autofocus system is contained in U.S. Application Serial No. 13/006,741 , filed on April 22, 201 1 . As far as permitted, the contents of U.S. Application Serial No. 13/006,741 are incorporated herein by reference.

[0072] Additionally, in this embodiment, the reference system 336 includes a redirector assembly 344 that is used to provide the reference signal.

[0073] The light source assembly 340 again generates light that is directed at the workpiece 300 and the redirector assembly 344, and the detector assembly 342 measures the light reflected off of the workpiece 300 and the redirector assembly 344. In Figure 3A, the light source assembly 340 projects light that results in a sinusoidal irradiance pattern that includes a plurality of fringes of light 346 on the work piece 300 and the redirector assembly 344. In one embodiment, the light source assembly 340 includes (i) a light source 348A that generates light 348B, (ii) a lens 348C that collimates the light 348B from the light source 348A, (iii) a grating 348D that creates at least three plane waves (-1 , 0, +1 ) for each wavelength, (iv) a lens 348E that focuses the light from the grating 348D, (v) an aperture plate 348F (illustrated in detail in Figure 3B) that blocks the zero order plane wave and all higher orders while allowing the -1 and +1 waves to pass therethrough, and (iv) a lens 348G that directs the -1 and +1 waves onto the work piece 300 and the redirector assembly 344, where they create a sinusoidal irradiance pattern of light 346. In one, non-exclusive embodiment, the light source 348A includes four, separate light emitting diodes ("LEDs") and each LED generates light over a different wavelength spectrum. Thus, in this embodiment, the light 348B includes four different beams with each beam having a different wavelength spectrum.

[0074] In the embodiment illustrated in Figure 3A, the grating 348D is imaged to the work piece 300 and the redirector assembly 244 via an afocal relay, and all wavelengths generated by the light source 348A create a fringe pattern with the same spatial frequency on the work piece 300 and the redirector assembly 344, and the plane waves for each wavelength have different angles relative to the optical axis (e.g. the Z axis) in the optical space near the work piece 300 and the redirector assembly 344. In this design, in the space between the lens 348E and the lens 348G, the beams from the multiple wavelengths are converging, focusing, and then diverging.

[0075] In one embodiment, (i) a first portion of the fringes of light 346, hereinafter referred to as measurement light 346A or measurement beam, is imaged onto the work piece 300; and (ii) a second portion of the fringes of light 346, hereinafter referred to as reference light 346B or reference beam, is projected onto the redirector assembly 344. In Figure 3A, the fringes of the light 346 that are farthest out of the page along the Y axis is the reference light 346B, while the fringes of light 346 that are into the page along the Y axis is the measurement light 346A. With this design, the redirector assembly 344 picks off and redirects some of the fringes of light 346 before they are imaged onto the work piece 300.

[0076] The detector assembly 342 detects the measurement light 346A that is reflected off of the work piece 300 and the reference light 346B that is reflected off of the redirector assembly 344. In one embodiment, the detector assembly 342 includes (i) a lens 350A that focuses the measurement light 346A that is reflected off of the work piece 300 and the reference light 346B reflected off of the redirector assembly 344, (ii) an aperture 350B that blocks light diffracted by the patterns that exist on the wafer, (iii) a lens 350C that focuses the light that passes from the blocker 350B, and (iv) a detector 350D. With this design, the same detector 350D is used to measure both the measurement light 346A that is reflected off of the work piece 300 and the reference light 346B reflected off of the redirector assembly 344. Thus, the detector 350D can provide to the control system 324 a measurement signal that relates to the measurement light 346A reflected off of the work piece 300 and a reference signal that relates to the reference light 346B reflected off of the redirector assembly 344. In one embodiment, the detector 350D is a charge-coupled device.

[0077] Again, the reference signal relates to the measurement of everything that is changing the autofocus system 322 except for the position of the work piece 300 along the Z axis, and the measurement signal relates to the measurement of everything changing in the autofocus system 322 plus the position of the work piece 300 along the Z axis. With this design, the control system 324 can subtract the reference signal from the measurement signal to determine the position of the work piece 300. [0078] In Figure 3A, the redirector assembly 344 again includes a first redirector 352 and a second redirector 354. These components are described in more detail below in reference to Figures 4A-4C.

[0079] Figure 3B is a simplified illustration of the aperture plate 348F and the light diffracted by the grating 348D that has been focused by lens 348E in the plane of the aperture 348F from Figure 3A. This Figure illustrates that four separate wavelength beams 356A, 356B, 356C, 356D, each with a -1 , 0, +1 order plane wave, are focused in the plane of the blocker 348F, and that the blocker 348F blocks the zero order light for each of the beams 356A, 356B, 356C, 356D, while the +1 order light and the -1 order light for each of the beams 356A, 356B, 356C, 356D are allowed to pass the blocker 348D. In this embodiment, two orders (e.g. the +1 , -1 ) for three of the beams 356A, 356B, 356C are directed at the work piece 300, and two orders (e.g. the +1 , -1 ) of the fourth beam 356D are directed at the redirector assembly 344.

[0080] Figure 4A is a simplified side illustration, Figure 4B is a simplified top illustration, and Figure 4C is a simplified top perspective view of the work piece 400, the measurement light 446A directed at the work piece 400, the reference light 446B directed at the redirector assembly 444, and one embodiment of the first redirector 452 and the second redirector 454 of the redirector assembly 444. As illustrated in these Figures, the reference light 446B can be displaced and spaced apart along the Y axis (the wafer scan direction) from the measurement light 446A prior to the measurement light 446A being incident on the work piece 400 and the reference light 446B being incident on the first redirector 452.

[0081] In this embodiment, (i) the reference light 446B from the light source assembly 240, 340 (illustrated in Figures 2 and 3A) is directed at the first redirector 452,

(ii) the first redirector 452 redirects the reference light 446B at the second redirector 454,

(iii) the second redirector 454 redirects the reference light 446B back to the first redirector 452, and (iv) the first redirector 452 redirects the reference light 446B back to the detector assembly 242, 342 (illustrated in Figures 2 and 3A).

[0082] In one embodiment, the first redirector 452 is a fold mirror that redirects the reference light 446B at an angle relative to the input light. With this design, the first redirector 452 folds the reference beam 446B into a beam plane 458 that is approximately parallel to, spaced apart from, and positioned above a working surface plane 400B of the working surface 400A of the wafer 400. In certain embodiments, the first redirector 452 redirects the reference light 446B out of the XZ plane. The XZ plane may be regarded as a first plane in which the measurement beam traveled. For example, the first redirector 452 can be made from glass, in the shape of a long, right triangle prism (e.g. a long prism with a right triangular shaped cross-section). Alternatively, the first redirector 452 can be made from metal, with the hypotenuse highly polished. This would allow for easier mounting of the first redirector 452. Still alternatively, the first redirector 452 can include a mirror mounted to a metal mount, again for easier mounting. In one non-exclusive embodiment, the first redirector 452 has a width of approximately five millimeters.

[0083] As illustrated in Figure 4A, a center of the reference beam 446B is incident on the first redirector 452 at a first redirector area 460, and the first redirector area 460 is positioned a first separation distance 462 away from the working surface 400A along the Z axis. In alternative, non-exclusive embodiments, the first separation distance 462 can be approximately 1 .5, 2, 2.5, or 3 millimeters.

[0084] The second redirector 454 can be a reflector or mirror that is positioned away from the first redirector 452 along the beam plane 458 (e.g. along the Y axis) such that it is optically coincident with the working surface 400A of the work piece 400. With this design, the second redirector 454 reflects the reference beam 446B back to the first redirector 452 along the beam plane 458 that is parallel to, and spaced apart from the working surface plane 400B. Further, the reference beam 446B is incident on the second redirector 454 at a second redirector area 464 positioned on the beam plane 458, and the second redirector area 464 is spaced apart a second separation distance 466 from the first redirector area 460 along the beam plane 458. Moreover, in certain embodiments, the second separation distance 466 is equal to or approximately equal to the first separation distance 462. This is important because the second redirector 454 is not defocused relative to the working surface 400A of the work piece 400. Stated in another fashion, when the second separation distance 466 is equal to the first separation distance 462, the reference beam 446B travels the same distance via the redirectors 452, 454 as it would have if it was instead incident on the working surface 400A. As a result thereof, the same detector assembly 242, 342 (illustrated in Figures 2 and 3A) can be used for both the measurement light 446A and the reference light 446B. This simplifies the receiving side optics.

[0085] In one non-exclusive embodiment, the second redirector 454 can be made from glass, in the shape of a long rectangular bar. Alternatively, the second redirector 454 can be made from metal, with a side that is highly polished. This would allow for easier mounting of the second redirector 454. Still alternatively, the second redirector 454 can include a mirror mounted to a metal mount, again for easier mounting.

[0086] In certain embodiments, the reference light 446B is incident at a grazing angle of incidence to the first redirector 452. For this reason, the first redirector 452 may be required to be quite long, depending on the size of the first separation distance 462, the numerical aperture of the reference light 446B, and the length of the second redirector 454. Generally, the smaller the first separation distance 462, the smaller the length that the first redirector 452 is required to be. As non-exclusive examples, the first redirector 452 and the second redirector 454 can be approximately 350, 400, 450 or more millimeters in length (L).

[0087] In this embodiment, the reference beam 446B is not perfectly collinear with the measurement beam 446A. However, the reference beam 446B will still provide a high degree of correction, because the lateral displacement (along the Y axis in this example) is not much and the reference light 446B goes through all the same components of the light source assembly 240, 340 and the detector assembly 242, 342.

[0088] Importantly, in the design described above, the reference beam 446B does not travel through one or more prisms (not shown). The problem with using one or more prisms is that it will add a glass path and dispersion that will only be in the path of the reference beam 446B and not in the path of the measurement beam 446A. This makes it difficult for the reference and measurement beams to have equivalent optical paths. Thus, in certain embodiments, the proposed autofocus system 222, 322 has no glass path to introduce a different level of dispersion in the reference light 446B when compared to the measurement light 446A.

[0089] Figure 5A is a simplified top illustration of the work piece 500, the first redirector 552, the second redirector 554 of the redirector assembly 544, a first embodiment of the measurement beam 546AA imaged on the work piece 500, and a first embodiment of the reference beam 546BA directed at the first redirector 552; and Figure 5B is a side view of the second redirector 554 of Figure 5A. In this embodiment, (i) the light source assembly 240 (as illustrated in Figure 2) generates a plurality of spaced apart slits of light, (ii) a first portion of the slits of light (referred to as the measurement beam 546AA) are concurrently directed at and reflected off of the work piece, and (iii) a second portion of the slits of light (referred to as the reference beam 546BA) are concurrently directed at and reflected off of the redirector assembly 544. It should be noted that the reference beam 546BA will actually be significantly defocused on the first redirector 552 and will not appear as a plurality of slits of light on the first redirector 552.

[0090] Moreover, in this embodiment, (i) the measurement beam 546AA is simultaneously imaged onto the work piece 500 at a plurality of spaced apart measurement areas 555A-555L positioned along the X axis (perpendicular to the Y scanning axis), and the detector assembly 242 (illustrated in Figure 2) detects the measurement beam 546AA reflected off of the work piece 500 and generates a measurement signal for each of the measurement areas 555A-555L; and (ii) the reference beam 546BA is simultaneously imaged onto the second redirector 554 at a plurality of spaced apart reference areas 557A-557L positioned along the X axis, and the detector assembly 242 detects the reference beam 546BA reflected off of the second redirector 554 and generates a reference signal for each of the reference areas 557A- 557L. Additionally, in one embodiment, (i) the work piece 500 has a surface length 570 measured along the X axis; (ii) the plurality measurement areas 555A-555L are spaced apart along substantially the entire surface length; and (iii) the reference areas 557A- 557L are spaced apart along a reference length 572 that is approximately equal to the surface length 570. With this design, the reference areas 557A-557L are near the measurement areas 555A-555L, and the many reference areas 557A-557L spread along the surface length 570 correspond, one to one, to the many measurement areas 555A- 555L, such that every measurement area has a corresponding, unique reference area.

[0091] The number of measurement areas 555A-555L and the number of reference areas 557A-557L can be varied. In certain embodiments, the system is designed so that every measurement area 555A-555L has a corresponding reference areas 557A-557L. With this design, a reference signal is generated for each measurement signal, and there is a one to one ratio of measurement signals and reference signals. Stated in another fashion, with this embodiment, there is a separate, independent, corresponding reference signal for each measurement signal. As provided herein, with this design, the control system can utilize simple subtraction between each corresponding reference and measurement signals for processing. For example, the control system can (i) subtract the first reference signal from the first measurement signal to determine the position of the work piece 500 at the first measurement area 555A; (ii) subtract the second reference signal from the second measurement signal to determine the position of the work piece 500 at the second measurement area 555B; and (iii) subtract the third reference signal from the third measurement signal to determine the position of the work piece 500 at the third measurement area 555C. This process is repeated to determine the position of each of the measurement areas 555A- 555L

[0092] It should be noted that (i) any of the measurement areas 555A-555L can be referred to as a first, second, third, etc. measurement area 555A-555L, (ii) the corresponding measurement signals can be referred to as a first, second, third, etc measurement signal, (iii) any of the reference areas 557A-557L can be referred to as a first, second, third, etc. reference areas 557A-557L, and (iv) the corresponding reference signals can be referred to as a first, second, third, etc reference signal. Further, in certain embodiments, each corresponding measurement area and reference area are approximately aligned along the Y axis.

[0093] Figure 5B is a simplified top illustration of the work piece 500, the first redirector 552 and the second redirector 554 of the redirector assembly 544, a second embodiment of the measurement beam 546AB imaged on the work piece 500, and a second embodiment of the reference beam 546BB directed at the first redirector 552; and Figure 5D is a side view of the second redirector 554 of Figure 5C. In this embodiment, (i) the light source assembly 340 (as illustrated in Figure 3A) generates a plurality of fringes of light, (ii) a first portion of the fringes of light (referred to as the measurement beam 546AB) are concurrently directed at and reflected off of the work piece 500, and (iii) a second portion of the fringes of light (referred to as the reference beam 546BB) are concurrently directed at and reflected off of the redirector assembly 544. It should be noted that the reference beam 546BB will actually be significantly defocused on the first redirector 552 and will not appear as a plurality of fringes of light on the first redirector 552.

[0094] Similarly, in this embodiment, (i) the measurement beam 546AB is simultaneously imaged onto the work piece 500 at a plurality of measurement areas 555A-555M positioned along the X axis, and the detector assembly 342 (illustrated in Figure 3A) detects the measurement beam 546AB reflected off of the work piece 500 and generates a measurement signal for each of the measurement areas 555A-555M; and (ii) the reference beam 546BB is simultaneously imaged onto the second redirector 554 at a plurality of reference areas 557A-557M positioned along the X axis, and the detector assembly 342 detects the reference beam 546BB reflected off of the second redirector 554 and generates a reference signal for each of the reference areas 557A- 557M. Additionally, in this embodiment, (i) the work piece 500 has the surface length 570 measured along the X axis; (ii) the plurality measurement areas 555A-555M are positioned along substantially the entire surface length; and (iii) the reference areas 557A-557M are positioned along the reference length 572 that is approximately equal to the surface length 570. With this design, the reference areas 557A-557M are near the measurement areas 555A-555M, and the many reference areas 557A-557M spread along the surface length 570 correspond, one to one, to the many measurement areas 555A-555L, such that every measurement area has a corresponding, unique reference area.

[0095] In this embodiment, the measurement beam 546AB is substantially continuous across the work piece 500, and the reference beam 546BB is substantially continuous along the redirector assembly 544. In this embodiment, the parsing into individual measurement areas 555A-555M and individual reference areas 557A-557M happens with the spatial discretization at the detector (not shown in Figure 5B). Thus, with this system, there is a plurality of discreet measurement areas 555A-555M and a plurality of discreet reference area 557A-557M.

[0096] It should be noted in this embodiments, the system is designed so that every measurement area 555A-555L again has a corresponding reference area 557A- 557L. With this design, a reference signal is generated for each measurement signal, and there is a one to one ratio of measurement signals and reference signals. As provided herein, with this design, the control system can utilize simple subtraction between each corresponding reference and measurement signals for processing.

[0097] Figure 5E is a simplified illustration of a detector 550D, with the reference beam 546BB (illustrated as a box) and the measurement beam 546AB (illustrated as a box) directed thereon. In this embodiment, the detector 550D can be a CCD that measures both beams 546AB, 546BB. In this embodiment, the detector 550D is relatively long along the X axis and relatively narrow along the Y axis (e.g. the scan direction).

[0098] Figures 5F and 5G illustrate another detector 550D. In this embodiment, as illustrated in Figure 5F, the measurement beam 546AB is directed at the detector 550D, and subsequently, as illustrated in Figure 5G, the reference beam 546BB is directed at the same area of the detector 550D. With this design, the measurement beam 546AB, and reference beam 546BB are each multiplexed onto the same place on the detector 550D. In this case, the system can be designed so that the reference beam 546BB has the same width/size as the measurement beam 546AB.

[0099] As provided herein, the timescale at which the reference beam 546BB will change is likely to be slow relative to the sample rate of the detector 550D. In addition, motions of the pixels in the detector 550D (caused, for example, by deformation that results from temperature changes) will look like a change in height of the substrate. However, as illustrated in Figures 5F and 5G, if the same position (pixels) of the detector 550D is used to measure both beams 546AB, 546BB, this will correct for global motions (i.e. shifts) of the pixels of the detector 550D.

[00100] The embodiment illustrated in Figures 5F and 5G can be done by alternating between measurement light and reference light sequentially in time (so the reference and measurement sources need to be strobed on/off depending on which signal will be measured during which camera frame). Further, a small wedge prism can be positioned in the receiving side pupil (near 350B in Figure 3A, for example), that directs the measurement light and the reference light onto the same region of the detector 550D. Alternatively, one or more mirrors (not shown) can be positioned near the receiving side pupil (near 350B in Figure 3A) so that the measurement light and the reference light are directed onto the same region of the detector 550D.

[00101] Figure 6A is a simplified top plan illustration of the work piece 600, the first redirector 652 and the second redirector 654 of the redirector assembly 644, and the reference beam 646B imaged on the redirector assembly 644. Figure 6A also illustrates the detector assembly 642. In this embodiment, the detector assembly 642 includes a first lens 650A, a multi-faceted mirror 650B, a second lens 650C, and a detector 650D.

[00102] Moreover, in this embodiment, the second redirector 654 is a reflective grating rather than a mirror. The grating 654 creates two or more copies of the collimated reference light 646B, with each copy at a slightly different angle relative to the XZ plane after the second reflection by first redirector 646B. As a result thereof, in the pupil plane of the receiving side optics (e.g. in the plane of 350B of Figure 3A), the copies of the reference beam 646B are spatially separated. As illustrated in Figure 6A, the multi-faceted mirror 650B is used to redirect the + 1 , 0, and -1 orders of the reference light 646B at the second lens 650C and subsequently spaced apart on the detector 650 D. [00103] By creating multiple copies of the reference beam 656B with the grating 654, one copy of the reference signal can accompany each colored measurement light through the non-common path optics, effectively following a similar optical path as the measurement light in the receiving side.

[00104] Figure 6B is a simplified top illustration of the detector 650D from Figure 6A. This embodiment also illustrates the + 1 , 0, and -1 orders of the reference light 646B are directed at separate regions on the detector 650D. Alternatively, a separate detector can be used for each order of the reference light 646B.

[00105] Figure 6C is a simplified side illustration of the work piece 600, the first redirector 652 and the second redirector 654 of the redirector assembly 644, and the reference beam 646B imaged on the redirector assembly 644 of Figure 6A. Figure 6C also illustrates the detector assembly 642 including the first lens 650A, the multi-faceted mirror 650B, the second lens 650C, and the detector 650D.

[00106] It should be noted that the multi-faceted mirror 650B can be replaced with another suitable combination for directing the + 1 , 0, and -1 orders of the reference light 646B at the second lens 650C and subsequently spaced apart on the detector 650D. For example, a small-angle prism (not shown) and an arrangement of mirrors (not shown) can be used instead of the multi-faceted mirror 650B so that the multiple reference orders again do not overlap on the detector 650D (assuming the detector 650D is conjugate to the work piece 500, as is usually the case). However, with this design, each color might go through a different set of optics, the relative stability of which is not known.

[00107] Figure 7A is a simplified side illustration and Figure 7B is a simplified top illustration of the work piece 700, the measurement beam 746A (only shown in Figure 7A), the reference beam 746B (only shown in Figure 7A), the first redirector 752 and the second redirector 754 of the redirector assembly 744, and an auxiliary measurement system 790 that monitors the position of the redirector assembly 744 relative to a reference, such as the optical assembly 16 (illustrated in Figure 1 ) at one or more positions. In this embodiment, the auxiliary measurement system 790 directs one or more auxiliary beams 792 (two spaced apart auxiliary beams 792) at the redirector assembly 744. More specifically, in this embodiment, the auxiliary measurement system 790 directs the one or more auxiliary beams 792 at the first redirector 752. These auxiliary beams 792 are redirected by the first redirector 752 to the second redirector 754. Subsequently, the beams 792 are reflected off of the second redirector 754 back to the first redirector 752 which redirects the beams 792 back to the auxiliary measurement system 790. For example, the auxiliary measurement system 790 can be an interferometer system that includes a light source 794 that generates the one or more auxiliary beams 792 and a detector 796 that measures the light redirected from the redirector assembly 744. Alternatively, another type of sensor can be used to monitor the position of the redirector assembly 744.

[00108] It should be noted that with this design, the reference signals can be adjusted based on any measured movement of the redirector assembly 744 by the auxiliary measurement system 790. This can improve the accuracy of the autofocus system. It should also be noted that with the autofocus designs disclosed herein, the position of the redirector assembly 744 can be easily monitored with the auxiliary measurement system 790.

[00109] As illustrated in Figure 7A, the auxiliary measurement system 790 is positioned so that the auxiliary beams 792 are offset from the reference beam 746B along the Y axis. However, in another embodiment, illustrated in Figure 7C, the auxiliary measurement system 790 can be positioned so that the auxiliary beams 792 are not offset from the reference beam 746B along the Y axis. With this design, the auxiliary beams 792 impinge on the first redirector 752 at approximately the same height along the Z axis as the reference beam 746B. The work piece 700, the measurement beam 746A, the second redirector 754 of the redirector assembly 744, the light source 794 and the detector 796 are also illustrated in Figure 7C.

[00110] Figure 8A is a simplified side view of still another embodiment of an autofocus system 822 having features of the embodiment of the present invention and the work piece 800. In this embodiment, the autofocus system 822 is somewhat similar to the autofocus system 322 described above and illustrated in Figure 3A. However, in this embodiment, the autofocus system 822 includes an environmental control system 871 that provides a controlled environment for the measurement light 846A and the reference light 846B to travel for at least a portion of their paths to reduce errors due to air turbulence. It should be noted that the environmental control system 871 provided herein can be used in other autofocus systems such as the autofocus system 222 described above and illustrated in Figure 2 or another type of autofocus system. [00111] In Figure 8A, the environmental control system 871 includes an environmental chamber 873 and a chamber fluid (for example, well controlled air) source 875. Further, in this embodiment, the environmental chamber 873 is positioned adjacent to and spaced apart from the work piece 800, and in between the last lens 848G of the light source assembly 840 and the first lens 850A of the detector assembly 842. With this design, the environmental control system 871 provides a controlled environment for the light between the light source assembly 840 and the detector assembly 842. Further, the chamber fluid source 875 directs a fluid, such as air, into the environmental chamber 873.

[00112] As provided herein, all optics-based autofocus systems must deal with errors due to refractive index changes of the air in the long air path taken by the measurement light 846A and the reference light 846B. As provided herein, reference schemes rely on a high correlation of the air turbulence between the two light 846A, 846B paths, which can be difficult to obtain and is highly dependent on the separation of the two beams 846A, 846B. Even if the temperature is controlled around the exposure apparatus 10 (illustrated in Figure 1 ), variations in temperature can exist along the path of the beams 846A, 846B that are constantly changing with time, and these changes introduce errors in the form of false work piece 800 height variations.

[00113] This problem will be worse in future systems that use larger workpieces 800, for example 450 mm wafer systems because the light 846A, 846B will have to travel longer distances. Stated in another fashion, as the diameter of the work piece 800 increases, the air path for the beams 846A, 846B also increases. Further, as feature size transferred to the work piece 800 decreases, the allowable error for the autofocus system 822 gets smaller. Both of these issues mean the errors due to air temperature variations become more difficult.

[00114] As provided herein, the problem of air temperature fluctuations in path of the measurement light 846A and the reference light 846B is solved by placing a portion (e.g. a large portion) the beam 846A, 846G path in a temperature controlled environment provided by the environmental chamber 873, with a flow of temperature controlled fluid (i.e. air) from the chamber fluid source 875. This can vastly reduce the sensitivity to air temperature changes, by using a unique optical and mechanical arrangement.

[00115] In certain embodiments, with the embodiment of the present invention, much of the beams 846A, 846G (much of the path between the sending side lens 848G and the receiving side lens 850A) are within a local temperature controlled environment. This should reduce much of the air temperature induced fluctuations. Global temperature change of the air can cause an autofocus error because of the temperature induced refractive index change of air. However, the addition of a reference channel that travels through the same cell of air and experiences nearly the same temperature air can remove the effects of a global air temperature change.

[00116] In Figure 8A, the environmental chamber 873 protects the large air path between the sending and receiving sides. Further, the environmental control system 871 provides a good correlation between the reference beam 846B and measurement beam 846A. Stated in another fashion, the environmental control system 871 provides a method for reducing errors due to air temperature variations in an optical autofocus system by immersing much of the beams 846A, 846B path in a flow of temperature controlled air.

[00117] In Figure 8A, the environmental chamber 873 includes a first inlet port 875A and a second inlet port 875B that is spaced apart from the first inlet port 875A. For example, one or both ports 875A, 875B can be in fluid communication with the chamber fluid source 875. For example, the chamber fluid source 875 can direct a fluid into each port 875A, 875B with the fluid exiting from the gap between the environmental chamber 873 and the work piece 800. Alternatively, for example, the chamber fluid source 875 can direct a fluid into one of the ports and pull fluid from the other port so that the fluid flows along the beam paths.

[00118] Figure 8B is a cut-away view taken on line 8B-8B in Figure 8A. In this embodiment, the environmental chamber 873 is shaped somewhat similar to an inverted U channel that supports the second redirector 854. Additionally, the environmental chamber 873 can include a flat window 879, or even a lens that is part of the autofocus optical design, at each end that is transparent to the beams 846A, 846B. Further, the beams 846A, 846B are at normal incidence to the windows 879, which are also anti- reflection coated, so there is minimal reflection loss. Moreover, the environmental chamber 873 can include a divider plate 881 that extends downward between the two beams 846A, 846B to support the first redirector 852. Further, the divider plate 881 can include a plurality of apertures 883 that extend through the divider plate 881 to allow for the flow and good mixing of the fluid traversed by measurement light 846A and the reference light 846B. The walls and top surface of environmental chamber 873 may include exhaust ports for the fluid. These ports can provide some means of adjusting the fluid flow within the environmental chamber 873 while limiting the pressure imposed by the fluid on the work piece 900.

[00119] In this embodiment, the cavity formed by the environmental chamber 873 is filled with air or another type of fluid (i.e. Helium) by the chamber fluid source 875 (illustrated in Figure 8A) so that the measurement beams 846A and the reference beams 846B travel through a controlled environment.

[00120] Figure 8C is a cut-away view of the environmental chamber 873 including the windows 879 that are normal to the incidence of the beams and the fluid 885 directed into the environmental chamber 873.

[00121] Figure 9A is a simplified illustration, and Figure 9B is an end view of another embodiment of an environmental chamber 973 having features of the embodiment of the present invention. In this embodiment, the environmental chamber 973 includes a first manifold 987A positioned on one side and a second manifold 987B positioned on the other side. In this embodiment, fluid from the chamber fluid source 875 (illustrated in Figure 8A) flows into each manifold 987A, 987B, and subsequently flows from each manifold 987A, 987B through apertures 989 into the environmental chamber. Moreover, the fluid can mix and flow through apertures 893 in the divider plate 981 . Further, the fluid can exit the gap between the chamber 973 and the work piece 900.

[00122] In this embodiment, temperature controlled fluid (e.g. air) flows in from manifolds 987A, 987B on each side. Since the first redirector 952 creates different fluid flow geometry for the two channels shown, separate fluid supplies on the two sides may allow better fluid flow adjustment.

[00123] Additionally, the system can include a vacuum scavenging system located beneath the manifolds 987A, 987B which could be added to scavenge the exhaust fluid if desired, to minimize any effects on surrounding sensors. The fluid flow may apply a small pressure on the work piece 900. If the magnitude is significant, the vacuum scavenge could also provide a small vacuum preload to cancel the force.

[00124] In certain embodiments, since the reference plane is reset with each autofocus measurement (each wafer), since the wafer height is measured relative to other locations on that wafer surface, the fluid temperature needs to be stable for only a short time. In certain embodiments, it may not be necessary to flow any fluid during the autofocus measurement, but just cycle and clean it between measurements.

[00125] In the above embodiments, the second redirector is not defocused relative to the working surface. However, the second redirector may be defocused relative to the working surface. In the above embodiments, the reference beam does not travel through one or more prisms. However, the reference beam may travel through one or more prisms.

[00126] In the above embodiments, the longitudinal direction of the first and second redirector is set along the X axis direction. However, the longitudinal direction of the first and second redirector may be set a direction cross to the X axis (e.g. rotate about X, Y, or Z axes).

[00127] In the above embodiments, the second redirector is positioned in opposite side of a space where the measurement beam travels. However, the second redirector may be positioned in same side of a space where the measurement beam travels. In this case, the measurement beam travels through a space between the first redirector and the second redirector.

[00128] Figure 10A is a simplified side illustration, Figure 10B is a simplified top illustration, and Figure 10C is a simplified top perspective view of the work piece 1000, the measurement light 1046A directed at the work piece 1000, the reference light 1046B directed at the redirector assembly 1044, and this modification of the first redirector 1052 and the second redirector 1054 of the redirector assembly 1044. As illustrated in these Figures, the reference light 1046B can be displaced and spaced apart along the Y axis (the wafer scan direction) from the measurement light 1046A prior to the measurement light 1046A being incident on the work piece 1000 and the reference light 1046B being incident on the first redirector 1052.

[00129] In this modification, the second redirector 1054 is positioned in same side of an space where the measurement beam 1046A travels. In this modification, the measurement beam 1046A travels a space between the first redirector 1052 and the second redirector 1054. In this modification, the reference light 1046B travels in the neighborhood of the measurement beam path. Note that this design might require a somewhat larger distance between the first redirector and the second redirector along the Y-axis, which in turn would require the length of the first redirector 1052 in the X direction to increase somewhat. [00130] Semiconductor devices can be fabricated using the above described systems, by the process shown generally in Figure 1 1 A. In step 1 101 the device's function and performance characteristics are designed. Next, in step 1 102, a mask (reticle) having a pattern is designed according to the previous designing step, and in a parallel step 1 103 a wafer is made from a silicon material. The mask pattern designed in step 1 102 is exposed onto the wafer from step 1 103 in step 1 104 by a photolithography system described hereinabove in accordance with the embodiment of the present invention. In step 1 105, the semiconductor device is assembled (including the dicing process, bonding process and packaging process), finally, the device is then inspected in step 1 106.

[00131] Figure 1 1 B illustrates a detailed flowchart example of the above- mentioned step 1 104 in the case of fabricating semiconductor devices. In Figure 1 1 B, in step 1 1 1 1 (oxidation step), the wafer surface is oxidized. In step 1 1 12 (CVD step), an insulation film is formed on the wafer surface. In step 1 1 13 (electrode formation step), electrodes are formed on the wafer by vapor deposition. In step 1 1 14 (ion implantation step), ions are implanted in the wafer. The above mentioned steps 1 1 1 1 - 1 1 14 form the preprocessing steps for wafers during wafer processing, and selection is made at each step according to processing requirements.

[00132] At each stage of wafer processing, when the above-mentioned preprocessing steps have been completed, the following post-processing steps are implemented. During post-processing, first, in step 1 1 15 (photoresist formation step), photoresist is applied to a wafer. Next, in step 1 1 16 (exposure step), the above- mentioned exposure device is used to transfer the circuit pattern of a mask (reticle) to a wafer. Then in step 1 1 17 (developing step), the exposed wafer is developed, and in step 1 1 18 (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step 1 1 19 (photoresist removal step), unnecessary photoresist remaining after etching is removed. Multiple circuit patterns are formed by repetition of these preprocessing and post-processing steps.

[00133] It is to be understood that movers disclosed herein are merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.

Claims

What is claimed is:

1 . A system for measuring the position of a work piece along a first axis that is orthogonal to a working surface of the work piece, the system comprising:

a redirector assembly positioned near and spaced apart from the working surface along the first axis;

a light source assembly that directs a measurement beam at the working surface and a reference beam at the redirecter assembly, the measurement beam being spaced apart from the reference beam along a second axis that is substantially orthogonal to the first axis prior to the working surface and the redirector assembly;

a detector assembly that (i) detects the measurement beam reflected at first measurement area off of the work piece and generates a first measurement signal, and

(ii) detects the reference beam reflected at a first reference area off of the redirector assembly and generates a first reference signal; and

a control system that uses the first measurement signal and the first reference signal to determine the position of the first measurement area along the first axis.

2. The system of claim 1 wherein the redirector assembly includes (i) a first redirector that redirects the reference beam from the light source assembly to be approximately parallel to and spaced apart from the working surface, and (ii) a second redirector that receives the reference beam redirected by the first redirector and redirects the reference beam back at the first redirector.

3. The system of claim 2 wherein the first redirector receives the reference beam redirected from the second redirector and redirects the reference beam at the detector assembly.

4. The system of claim 2 or 3, wherein the first redirector is a fold mirror and the second redirector is a reference mirror that is substantially optically coincident with the working surface.

5. The system of any one of claims 2 to 4, wherein the second redirector includes a diffraction grating.

6. The system of any one of claims 2 to 5, wherein the measurement beam is simultaneously imaged onto the working surface at a plurality of measurement areas positioned along a third axis that is orthogonal to the first axis and the second axis, and the detector assembly detects the measurement beam reflected off of the plurality of measurement areas and generates a measurement signal for each of the measurement areas; and wherein the reference beam is simultaneously imaged onto the first redirector at a plurality of reference areas positioned along the third axis; and the detector assembly detects the reference beam reflected off of the first redirector and generates a reference signal for each of the reference areas.

7. The system of claim 6 wherein each reference area corresponds to one of the measurement areas, and wherein the control system uses each measurement signal and its corresponding reference signal to determine the position of each measurement area along the first axis

8. The system of claim 7 wherein the working surface has a surface length measured along the third axis; wherein the plurality measurement areas are positioned along substantially the entire surface length; and wherein the reference areas are positioned along a reference length that is approximately equal to the surface length.

9. The system of any one of claims 1 to 8, wherein the measurement beam is simultaneously imaged onto the working surface at a second measurement area, and the detector assembly detects the measurement beam reflected off of the second measurement area and generates a second measurement signal; and wherein the reference beam is simultaneously imaged onto the redirector assembly at a second reference area, and the detector assembly detects the reference beam reflected off of the second reference area and generates a second reference signal; and wherein the control system uses the second measurement signal and the second reference signal to determine the position of the second measurement area along the first axis.

10. The system of any one of claims 1 to 9, wherein the light source assembly generates a plurality of spaced apart slits of light, and wherein a first portion of the slits of light are concurrently directed at and reflected off of the working surface, and wherein a second portion of the slits of light are concurrently directed at and reflected off of the redirector assembly.

1 1 . The system of any one of claims 1 to 10, wherein the light source assembly generates a substantially continuous measurement beam that is directed at and reflected off of the working surface, and a substantially continuous reference beam that is directed at and reflected off of the redirector assembly.

12. The system of any one of claims 1 to 1 1 , further comprising an auxiliary measurement system that measures the position of the redirector assembly.

13. A stage assembly that moves a work piece, the system of any one of claims 1 to 12 that measures the position of the work piece along an axis, and the stage assembly including a stage that retains the work piece.

14. An exposure apparatus including an illumination system and the stage assembly of claim 13 that moves the stage relative to the illumination system.

15. A process for manufacturing a device that includes the steps of providing a substrate and forming an image to the substrate with the exposure apparatus of claim

14.

16. A method for measuring the position of a work piece along a first axis that is orthogonal to a working surface of the work piece, the method comprising the steps of: positioning a redirector assembly near and spaced apart from the working surface along the first axis;

directing a measurement beam at the working surface;

directing a reference beam at the redirecter assembly, the reference beam being spaced apart from the measurement beam along a second axis that is orthogonal to the first axis prior to the working surface and the redirector assembly;

detecting the measurement beam reflected at a first measurement area off of the work piece and generating a first measurement signal with a measurement system; detecting the reference beam reflected at a first reference area off of the redirector assembly and generating a first reference signal with the measurement system; and

determining the position of the first measurement area along the first axis utilizing the first measurement signal and the first reference signal.

17. The method of claim 16 wherein the step of positioning a redirector assembly includes the steps of (i) positioning a first redirector that redirects the reference beam from the light source assembly to be approximately parallel to and spaced apart from the working surface, and (ii) positioning a second redirector that receives the reference beam redirected by the first redirector and redirects the reference beam back at the first redirector.

18. The method of claim 17 wherein the first redirector is a fold mirror and the second redirector is a reference mirror that is optically coincident with the working surface.

19. The method of claim 17 or 18, wherein the reference beam is incident on the first redirector at a first redirector area, and the reference beam is incident on the second redirector at a second redirector area, and wherein step of positioning a first redirector includes positioning the first redirector so that the first redirector area is positioned a first separation distance from the working surface along the first axis, and wherein the step of positioning a second redirector includes positioning the second redirector so that the second redirector area is spaced apart from the first redirector area a second separation distance along the second axis, the second separation distance being approximately equal to the first separation distance.

20. The method of any one of claims 17 to 19, wherein the step of directing a measurement beam includes the measurement beam being simultaneously imaged onto the working surface at a plurality of measurement areas positioned along a third axis that is orthogonal to the first axis and the second axis, and the detector assembly detects the measurement beam reflected off the measurement areas of the work piece and generates a measurement signal for each of the measurement areas; and wherein the step of directing a reference beam includes the reference beam being simultaneously imaged onto the first redirector at a plurality of reference areas positioned along the third axis, and the detector assembly detects the reference beam reflected off of the reference areas and generates a reference signal for each of the reference areas.

21 . A system for measuring the position of a work piece along a first axis that is orthogonal to a working surface of the work piece, the system comprising:

a redirector assembly positioned near and spaced apart from the working surface along the first axis; a light source assembly that directs a measurement beam at a grazing angle of incidence at the working surface and a reference beam at a grazing angle of incidence at the redirecter assembly, the measurement beam being spaced apart from the reference beam along a second axis that is orthogonal to the first axis prior to the working surface and the redirector assembly;

a detector assembly that (i) detects the measurement beam reflected off a first measurement area of the work piece and generates a first measurement signal, and (ii) detects the reference beam reflected off a first reference area of the redirector assembly and generates a first reference signal; and

a control system that uses the first measurement signal and the first reference signal to determine the position of the first measurement area along the first axis;

wherein the redirector assembly includes (i) a fold mirror that redirects the reference beam from the light source assembly to be approximately parallel to and spaced apart from the working surface, and (ii) a reference mirror that receives the reference beam redirected by the first redirector and redirects the reference beam back at the first redirector;

wherein the first redirector receives the reference beam redirected from the second redirector and redirects the reference beam at the detector assembly;

wherein the reference beam is incident on the first redirector at a first redirector area, and the reference beam is incident on the second redirector at a second redirector area, and wherein the first redirector area is positioned a first separation distance from the working surface along the first axis, and

wherein the first director location is spaced apart from the second redirector area a second separation distance along the second axis, the second separation distance being approximately equal to the first separation distance.

22. The system of claim 21 wherein the measurement beam is simultaneously imaged onto the working surface at a plurality of measurement areas positioned along a third axis that is orthogonal to the first axis and the second axis, and the detector assembly detects the measurement beam reflected off of the work piece and generates a measurement signal for each of the measurement areas; and wherein the reference beam is simultaneously imaged onto the first redirector at a plurality of spaced apart reference areas positioned along the third axis, and the detector assembly detects the reference beam reflected off of the first redirector and generates a reference signal for each of the reference areas.

23. The system of claim 22 wherein the working surface has a surface length measured along the third axis; wherein the plurality measurement areas are positioned along substantially the entire surface length; and wherein the reference areas are positioned along a reference length that is approximately equal to the surface length.

24. The system of claim 23 wherein the light source assembly generates a plurality of spaced apart slits of light, and wherein a first portion of the slits of light are concurrently directed at and reflected off of the working surface, and wherein a second portion of the slits of light are concurrently directed at and reflected off of the redirector assembly.

25. The system of claims 23 or 24, wherein the light source assembly generates a substantially continuous measurement beam that is directed at and reflected off of the working surface, and a substantially continuous reference beam that is directed at and reflected off of the redirector assembly.

26. The system of any one of claims 21 to 25, further comprising an auxiliary measurement system that monitors the position of the redirector assembly.

27. An system for measuring the position of a work piece along a first axis that is orthogonal to a working surface of the work piece, the system comprising:

a light source assembly that directs a measurement beam at the working surface; a detector assembly that detects the measurement beam reflected at first measurement area off of the work piece and generates a first measurement signal;

an environmental control system that creates a controlled environment for the measurement beam between the light source assembly and the detector assembly; and a control system that uses the first measurement signal and the first reference signal to determine the position of the first measurement area along the first axis.

28. A system for measuring the position of a work piece along a first axis that is orthogonal to a working surface of the work piece, the system comprising:

a light source assembly that directs a measurement beam at the working surface along a first plane and a reference beam;

a redirector assembly positioned near and spaced apart from the working surface along the first axis, the redirector assembly redirects the reference beam along a second plane crossed to the first plane; a detector assembly that (i) detects the measurement beam reflected at first measurement area off of the work piece and generates a first measurement signal, and (ii) detects the reference beam reflected at a first reference area off of the redirector assembly and generates a first reference signal; and

a control system that uses the first measurement signal and the first reference signal to determine the position of the first measurement area along the first axis,

wherein the measurement beam being spaced apart from the reference beam along a second axis that is crossed to the first axis prior to the working surface and the redirector assembly.

29. The system of claim 29 wherein the redirector assembly includes (i) a first redirector that redirects the reference beam from the light source assembly spaced apart from the working surface, and (ii) a second redirector that receives the reference beam redirected by the first redirector and redirects the reference beam back at the first redirector.

30. The system of claim 29, wherein the second redirector positioned in opposite side of an space where the measurement beam travels.