WO2002000963A1

WO2002000963A1 - Selective beam deposition

Info

Publication number: WO2002000963A1
Application number: PCT/US2001/020067
Authority: WO
Inventors: Steven John Ouderkirk
Original assignee: Steven John Ouderkirk
Priority date: 2000-06-23
Filing date: 2001-06-22
Publication date: 2002-01-03

Abstract

An apparatus and process for selectively depositing material from a gas phase to produce a solid three-dimensional object. The apparatus includes one or more energy beams, such as that provided by a semiconductor diode laser, which are focused to points within a reaction chamber that contains a reactant gas. The focused light from the lasers is intense enough to cause the substrate or previously deposited material to heat to a temperature sufficient to thermally decompose the reactant gas to form a deposit material onto a substrate or previously deposited material. Translation and rotation of the focal point or supporting substrate are controlled with respect to each other to allow formation of three-dimensional structures. One or more types of optical sensors are confocally aligned with the laser to monitor temperature of the material deposited near the focal point of the laser, the lateral geometry of the material deposited near the focal point of the laser, or the distance between the focal point and the material deposited near the focal point of the laser. The data provided by the optical sensors can be used in a feedback mode to control system operating parameters to generate structures with precise geometry and morphology.

Description

TITLE: SELECTIVE BEAM DEPOSITION

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

FIELD OF INVENTION

The invention relates to an apparatus for producing a three-dimensional part by

utilizing a directed beam from an energy source, such as a laser, to selectively deposit

material from a gas. BACKGROUND

Chemical vapor deposition (CVD) uses gaseous compounds that decompose at elevated temperatures to form deposits of solid material. CVD is a widely used technique

for the production of thin films of metals, semiconductors, and insulators for the

fabrication of integrated circuits, solar cells, flat panel displays and coatings of diamondlike carbon. Contact heating or resistance heating of the substrate usually serves as the source of the heat to drive the CVD reaction.

The use of lasers to drive a CVD reaction provides a method for providing the

heat needed to decompose the reactant gases. This technique dates from at least 1972 as

described in an article by L. S. Nelson and N. L. Richardson entitled "Formation of ThinRods of Pyrolytic Carbon by Heating with a Focused Carbon Dioxide Laser". Since

then, laser chemical vapor deposition (LCVD), also called selective area laser deposition (SALD), has been used to produce deposits in the form of films, coatings, fibers, rods, ribbons, and three-dimensional structures. U.S. Pat. No. 5,611,883 describes the

production of fasteners by this process. A wide range of materials can be deposited by the

LCVD process, including metals, semiconductors, and ceramics. Richard W. Thissell

described the use of this technology in his article "Design of a Solid Freeform Fabrication

Diamond Reactor".

The use of lasers for the creation of three-dimensional structures has been

described by D. Bauerle in an article entitled "Laser Induced Chemical Vapor Deposition,"

published in 1983 in the Springer Series in Chemical Physics 33. Bauerle described the

device and methods he used to create lines and rods of nickel and silicon. Improvements

in the use of LCVD for the production of fibers were provided by P. Nordine in U.S. Pat.

Nos. 5,126,200; 5,336,360; 5,339,430; and 5,549,971. These patents describe the growth of high strength fibers and ribbons of various materials, including amorphous boron. In U.S. Pat. No. 5,017,317, Ff. L. Marcus describes the growth of arbitrary three- dimensional shapes. In U.S. Patent No. 5,135,695, Marcus describes the use of non-

intrusive methods for determination of the position of the deposited material. In U.S.

Patent No. 5,169,579, Marcus describes the use of catalysts to increase deposit nucleation for improved materials properties and growth rates. In the article "New Developments in Processing and Control of Selected Area Laser Deposition of Carbon," R. Thissell and H.

L. Marcus describe the production of carbon stmctures. Nonlinear growth processes were

encountered during the manufacture of these parts that caused the layers to become rough

as each layer was added.

The chemistry and physics involved in chemical vapor deposition is complex and not well understood. The most significant parameters involved in the decomposition are

the temperature at the deposition surface, the gas composition, and gas pressure. The factors that affect the temperature at the deposition zone are the power of the heat source, the emissivity of the deposition surface, and the geometry and thermal conductivity of the

substrate and of the deposited material.

Production of three-dimensional parts by LCVD, as described in the above

references, suffers several problems. Previous approaches do not address high reactant

consumption rates and often suffer contamination of the reactants and/or deposition surface by byproducts and smoke. The geometry of structures that can be produced, and

the efficiency of producing the structure, is limited in conventional LCVD. Three-

dimensional structures are limited to those supported by removable structures or those

produced in a layering process. The geometry of the underlying stmcture can also interact

with the focus of the energy beam and produce unintended stmctures. Unintended stmctures can include not only variations in the physical geometry, but the morphology of

the deposits as well. Traditional LCVD is also characterized by slow deposition rates compared to conventional manufacturing processes, particularly when the size of the laser

and support equipment is compared to the size of parts being generated.

Conventional LCVD uses lasers with relatively high optical quality, including

single and multimode Nd: YAG lasers. These lasers can use lenses with a small numerical

aperture to produce sufficient beam quality to sustain thermal deposition. High power

semiconducting laser diodes are characterized by poor beam quality, being typically multimode, astigmatic, and having asymmetric divergence. As a consequence, the most common application of high power semiconducting laser diodes is for applications requiring relatively low power density, including soldering, sintering, and pumping solid

state lasers. High numerical aperture lenses may be used to increase laser intensity at a

focal point, but this brings the deposition process close to the lens and window, leading to optical contamination issues. Similar issues exist for laser beams delivered by optical

fibers, since the optical fiber output is preferably focused with a high numerical aperture

lens.

Purging systems using a gas directing plenum are used in laser systems that

produce contaminates that can deposit on optical elements. This includes laser cutting and

perforation systems. Cutting and perforation has a relatively wide operating window to

gas flow. LCVD stmctures may have aspect ratios of over 100 to 1, and can be moved under gas flow conditions. This movement is extremely undesirable for stable LCVD

growth.

The present invention addresses the shortcomings of traditional LCVD. SUMMARY OF THE INVENTION

In one aspect, the present invention includes a chemical vapor deposition (CVD) system that incorporates at least one semiconductor diode laser to provide a directed

energy beam that decomposes a reactant gas in a deposition zone. In one embodiment, an

array of independently operable diode lasers may be configured to provide multiple

deposition zones. In a preferred embodiment, the diode laser includes a heat exchanger configured to pass a reactant gas through the heat exchanger to pre-heat the reactant gas while providing cooling to the diode laser. In another embodiment, the energy from the diode laser is conveyed to the deposition zone through an optical fiber, or through an array of optical fibers. In yet another embodiment, the laser output is controlled in a

feedback loop using data from one or more optical sensors used to monitor the reaction in

the deposition zone.

In another aspect, the present invention includes a miniature optics CVD system

that incorporates an optical element between the energy source and the deposition zone,

including an objective lens that focuses the energy beam into a deposition zone and

optionally a window positioned between the objective lens and the deposition zone, where

the optical element is within 10 mm of a deposit created in the deposition zone.

In yet another aspect, the present invention includes a CVD system that

incorporates an objective lens configured to confocally focus an energy beam from an energy source onto a focal point in a deposition zone, while simultaneously collecting light

emanating from the deposition zone and conveying the collected light to one or more

optical sensors. The energy beam may include infrared light beams, visible light beams,

ultraviolet light beams, ion beams, electron beams, and focused plasma beams, but is preferably a light beam from a visible or infrared semiconductor diode laser. In various embodiments, the optical sensor(s) may determine the temperature of the deposition zone by detecting thermal emission from the deposition zone; the position of a deposition

surface in the deposition zone relative to the focal point of the energy beam by detecting

thermal emission or reflected laser light; and/or the lateral geometry of a deposition surface in the deposition zone by detecting thermal emission. The lateral geometry is the shape and size of the deposition zone in the plane perpendicular to the optic axis of the

energy beam. In another embodiment, the CVD system is used to provide more efficient

utilization of laser power. In a preferred embodiment, the optical sensor(s) are used in a

feedback loop to control the output of the energy source and/or to control the position of the deposition surface relative to the focal point.

In yet another aspect, the present invention includes a CVD system having a small-

volume gas plenum that is positioned to direct a reactant gas into a deposition zone. The

effective volume of said gas plenum is less than 10 cm³, preferably less than 1 cm³, and

most preferably less than 0.1 cm^J. In one embodiment, the CVD system includes an

objective lens, a deposition zone, optionally a window positioned between the objective

lens and the deposition zone, and a gas plenum that is positioned to direct a reactant gas

into the deposition zone and is configured so that the reactant gas sweeps contaminants

from a surface of the objective lens or window prior to entering the deposition zone. The

reactant gas is preferably pre-heated prior to entering the deposition zone.

In yet another aspect, the present invention includes a method of recycling reactant

gases in a chemical vapor deposition system, including the steps of: (a) controlling the

temperature, pressure, and composition of a first reactant gas in a reactant gas

conditioning system; (b) introducing the first reactant gas into a reaction chamber through a gas plenum configured to direct the reactant gas into a deposition zone where it is thermally decomposed by an energy beam to form a deposit of solid material; (c)

optionally diluting waste gas in the reaction chamber with an inert gas and purging the

waste gas out of the reaction chamber into a gas scrubbing system; (d) scmbbing solid particulate materials and gaseous waste products from the waste gas to give a second reactant gas having desirable gas components; (e) performing on-line analysis of the composition of the second reactant gas and providing feedback to a gas conditioning

system to adjust the composition of the second reactant gas to form a recirculated reactant

gas having the same temperature, pressure and composition as that of the first reactant gas; and (f) introducing the recirculated reactant gas back into the reaction chamber

through the gas plenum.

BRLEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of the major elements of the device of this invention

and the detail of the confocal sensor system.

Figure 2 is a schematic view of the detail of a reactant gas plenum that will minimize reactant gas mixing.

Figure 3 is a schematic view of the detail of a reactant gas plenum that provides

reduced obstruction.

Figure 4 is a schematic view of the detail of how a multi-element diode laser can be

focused to multiple points through a single lens.

These figures are not to scale, and are intended to be merely illustrative and non-limiting. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the production of stmctures composed of

metals, alloys, ceramics, semiconductors, and other solid elements or chemical compounds by means of selective beam deposition. Fig. 1 schematically shows the major elements of a chemical vapor deposition system of the present invention, including an optical assembly 10, mounted on a single axis vertical translation stage 12, a reaction cell 11 mounted on a dual axis horizontal translation stage 13, a reactant gas processing system 20, and a

computer control system 21. The optical assembly 10 provides an energy source 5,

producing an energy beam 8 focused by objective lens 3, through window 19 to focal point 7 on substrate 1 within reaction cell 11. The energy beam 8 is, for example, an infrared light beam, visible light beam, ultraviolet light beam, ion beam, electron beam, or

a focused plasma beams, but is preferably a light beam from a visible or infrared laser such

as that provided by a semiconductor diode laser. The focused energy beam 8 causes the

deposition zone 2 to be brought to a temperature sufficient to decompose the reactant gas provided by the reactant gas processing system 20, resulting in deposition of a solid

product or a liquid product that will solidify upon cooling.

Semiconductor diode laser

The preferred energy source of the present invention is a semiconductor diode

laser. With proper control of the laser power and processing conditions, it has been found

that semiconductor diode lasers can have sufficient power to be used in a chemical vapor deposition system to create high strength pyrolytic deposits. Since the power density of a laser diode is limited, it is preferred that the laser power be maintained at a relatively constant power, and that the nature of the deposit be controlled through translation rate or

gas parameters as will be described below. In certain cases, power requirements may

exceed the output of the laser diode, such as, for example, where the geometry or

composition of the substrate causes substantial thermal conductivity from the deposition zone.

Suitable semiconductor laser diodes include index guided cavity lasers, gain guided

cavity lasers and vertical cavity surface emitting lasers. One or two dimensional laser

diode cavity arrays may be used to create a corresponding array of deposition zones, where the array of deposition zones can be used to create a combination of separate or connected stmctures through controlled laser deposition. Use of arrays of deposition

zones allows the simultaneous production of identical parts within a single device to improve part production. An example of how a one-dimensional laser diode cavity array may be used to create a corresponding array of deposition zones is shown in Fig. 4. Laser

diode 30 having a one-dimensional diode laser cavity array can utilize objective lens 3 to

focus multiple beam elements 31 to provide an array of deposition zones 32 on substrate

1. A similar array may be achieved through an array of individual diode lasers. The laser

power from each cavity of the laser diode array may be independently controlled and conveyed to the deposition zones 32 through a transfer lens, an optical magnification or

reduction lens system, or through an array of optical fibers. Because of the high power

requirements, it is important that the coupling of the laser diode cavity to the optical fiber

is efficient, and the minimum fiber core diameter be used to maintain high power density.

The optical fibers may be grouped in a number of configurations, depending on the specific application. The output of the optical fibers may be focused on multiple

deposition zones, or concentrated on one deposition zone. Focusing of the output of the optical fibers may be accomplished through a lens or lens assembly positioned on each fiber, or through a lens or lens assembly on an array of optical fibers.

Semiconducting laser diodes have limited output power, and in high power laser

diodes, multimode output with astigmatism and asymmetric divergence. Laser CVD

requires high beam intensity, particularly for small stmctures. Additionally, the amount of laser power required to grow a stmcture can vary by over a factor of about four, depending on the effective thermal conductivity from the deposition zone to the substrate. For example, considerably more power is required to achieve a give growth rate on a

planar substrate compared to on the tip of a fiber.

Index or gain-guided semiconducting lasers are best corrected for astigmatism and divergence by use of a circularizing fiber which is a cylindrical lens placed proximate to the emitting facet of the laser diode so as to reduce the divergence of the laser in the fast

axis. An example of a circularizing fiber lens is described by Snyder, et al. in US Pat. No.

5,963,577. Alternatively astigmatism may be corrected with a tilted plate as described by

Lee in US Pat. No. 5,050,153. Other alternatives include the used of a cylindrical lens, or

anamorphic prisms, coupling the laser output into a length of a fiber optic, or various combinations of these corrective optics. A circularizing fiber lens is the preferred method

for circularizing the laser diode output due to the capability of this correction system for

producing a high quality laser beam in a compact system. Circularizing fiber lenses are the

most preferred correction method for high power multimode laser diodes.

Another approach to improve beam quality is to use vertical cavity surface

emitting lasers (VCSEL) as a laser source. In addition to providing a higher beam quality, VCSELs may be easily incorporated into a one or two-dimensional array for parallel

growth of ceramic stmctures.

The most efficient utilization of most beam sources is where they operate near 100% of rated power. This is particularly true with semiconductor lasers where a higher power beam is split into several low power beams to make stmctures in parallel.

Cooling of the laser diode may be achieved through heat transfer to the reactant

gases. Mounting the laser diode on a small heat exchanger, and passing the reactant gases tlirough the exchanger, then to the reaction zone can produce a highly compact system. In this case, it may be desired to cool the reactant gases before entering the heat exchanger. A significant amount of heat may be removed by supplying at least one component of the reactant gas in a liquefied state, and allowing vaporization of the liquid either before entering or within the heat exchanger.

Optical assembly

The optical assembly 10 of Fig. 1 includes an objective lens 3, a beam splitter 4, a energy source 5 such as a laser, and an optical sensor 6. An energy beam 8 from energy

source 5 is brought to a focal point 7 by the objective lens 3. The objective lens 3 is

preferably a lens capable of confocally collecting light emitted from deposition zone 2 to

form a beam of light emission 9 which is directed to optical sensor 6 by beam splitter 4. Beam splitter 4 preferably selectively directs the majority of energy beam 8 to the focal point 7 and directs the majority of light emission 9 from deposition zone 2 to optical

sensor 6. Beam splitter 4 may divert a small fraction of all wavelengths to sensor 6 or may

selectively remove a specific wavelength of light for optimal performance of sensor 6. Fig. 1 is for illustrative purposes only; it is within the scope of this invention to include multiple

beam splitters, sensors, and lasers within a single optical assembly to allow measurement of all aspects of temperature, lateral geometry and distance from the focal point in any combination. The focal length of objective lens 3 depends on the degree of collimation of

the energy beam 8, the shape and materials of construction of objective lens 3, and on the wavelength of the energy source. Due to these factors, there may be a difference in ray

paths between the energy beam and emission or reflected light from the deposition zone.

This difference will not substantially affect the ability to determine physical displacements, lateral geometry, and temperatures. The difference in ray paths may be estimated through

commercially available numerical ray tracing programs. The optical assembly is typically mounted on a vertical translation stage, which allows positioning of the focal point of the objective lens 3 relative to a substrate 1 within deposition zone 2.

Preferably, the CVD system of the present invention includes a miniature optical

assembly between the energy source and the deposition zone, with the optical element

positioned within 10 mm of a deposit created in the deposition zone. The miniature optics

element includes an objective lens 3 that focuses the energy beam into a deposition zone,

and optionally includes a window 19 positioned between the objective lens and the deposition zone.

The numerical aperture of the final lens or lens assembly focusing the laser on the

deposition zone required for LCVD is dependent on the optical characteristics of the laser

beam and the desired diameter of the deposit. Preferably, the numerical aperture is at least

0.1, more preferably at least 0.25, most preferably 0.5.

Optical sensor(s) One or more optical sensors 6 detect thermal emission or reflected laser light from the deposition zone 2 to determine, for example, the temperature of the deposition zone 2,

the position of a deposition surface in the deposition zone 2 relative to the focal point 7 of the energy beam 8, and/or the lateral geometry of a deposition surface in the deposition zone 2. In a preferred embodiment, the optical sensor(s) 6 are used in a feedback loop to control the output of the energy source 5 and/or to control the position of the deposition

surface relative to the focal point of the energy beam 8. The optical sensor 6 may have a

single sensing element, or may include a number of sensing elements, devices, and optical

elements.

In one aspect, optical sensor 6 can be used to determine the temperature of deposition zone 2 by measuring the intensity of thermal radiation from deposition zone 2. Temperature may be more accurately determined by the ratio of light intensity from deposition zone 2 measured in two different wavelength ranges. The light intensity at

different wavelength ranges may be independently measured by using two photo detectors

and a dichroic beam splitter, an imaging monochrometer on a photosensor array, or a

matrixed color filter on a photodetector array. The latter may be in the form of a common color charged-coupled device (CCD) video detector. These means for measuring temperature are only given as examples, and are not intended to be considered limitations

of the invention. Any method for optical temperature measurement is indirect in nature,

and should preferably be calibrated for absolute temperature determination.

In another aspect, optical sensor 6 can be used to determine the physical

displacement between the focal point 7 and deposition zone 2 by measuring the optical

properties of light emission 9 from deposition zone 2. Specifically, optical sensor 6 can measure the divergence or convergence of light emission 9 relative to energy beam 8. If light emission 9 is diverging, the focus of objective lens 3 is effectively below the

deposition zone 2, and if it is converging, the focus of objective lens 3 is above the

deposition zone 2. The degree of divergence or convergence can be related to the physical displacement between the focal point of objective lens 3 and deposition zone 2.

Suitable methods for determining the optical properties of light emission 9 include systems commonly used for automatically measuring the focal point of an optical train, such as, for

example, using split prism designs and focusing the light emission on a pinhole, and then measuring the fraction of light transmitted through the pinhole. Other optical methods for

determining the focus of a confocal optical system will be apparent to one skilled in the

art.

In yet another aspect, optical sensor 6 can be used to determine lateral geometry by imaging the reflected or emitted light 9 from the deposition zone 2 onto a multi-pixel optical sensor such as a charged-coupled device (CCD). The pattern of light detected by

the sensor can be used to determine the actual shape and size of the deposition zone 2 by

such methods as mapping to a known standard and generation of test samples for

calibration. The degree of precision in lateral geometry measurements is a function of the light and optical system properties, but can typically resolve features as small as about 100

nm. In addition to measuring the geometry of the deposition zone, the light emitted by the

deposition zone can be used to determine the localized thermal gradient.

Feedback loop The growth of material by selective beam deposition may occur over a range of temperatures. These temperatures are a function of the decomposition temperature of the reactant gas, the intended morphology of the deposit, the deposition rate, and the degree

of soot formation. Normally there is an optimum temperature that will produce a deposit

with a specific morphology, minimal soot formation, and a sufficient deposition rate. For

optimal deposit rate and accuracy, the deposition zone temperature can be monitored and

controlled in a feedback loop using the computer control system 21 of Fig. 1 to monitor the temperature measured by optical sensor 6, compare the measured temperature to a

target temperature, and alter the temperature of the deposition zone as needed by modifying the power or position of the laser beam, the photon flux at the deposition zone,

the gas flow, translation rate, and/or the gas composition.

One method of controlling the deposition zone temperature is by controlling the

photon flux. As described above, the photon flux from a semiconductor laser diode may be affected by varying the laser drive current. Modulation of the laser power provides a

broader range of photon flux production and provides a highly linear method of varying

the average photon flux. The modulation frequency is preferably sufficient to prevent the

deposition zone temperature from oscillating outside the range of temperature that will

provide the morphology intended for the part. The photon flux delivered by the laser can

alternately be modified by intermittently blocking the beam through use of a filter or by

diverting the beam from the optic axis by means of devices such as micro-mirror arrays that can modulate the beam at a high frequency. Another method of reducing the

temperature at the deposition zone is to spread the laser beam over a larger area, such as

by using a movable focusing lens in the optical path or by changing the filling of the

focusing lens by blocking part of the coUimated beam. Yet another way of controlling the deposition zone temperature is by altering the translation speed of the focal point of the laser beam relative to a region on a substrate so that the substrate will act as a variable heat sink. The translation rate is controlled by the computer to allow a feedback loop to

be established. Focusing the beam at the deposition zone produces the highest temperatures. Moving the focal point of the beam above or below the deposition zone will

reduce peak temperatures.

The growth of material by selective beam deposition is unstable due to the fact that

material will deposit over a certain area above and below the focal point, where the photon flux is sufficiently concentrated to cause decomposition of the reactant gas. For example, even when the focal point is not moving, material will continue to deposit in the direction of the focusing lens along the optic axis until the laser light is not concentrated

enough to heat the substrate above the decomposition temperature. Normally a part is

formed to a specific geometry and deviations from the intended geometry will cause the

part to be rejected.

The geometry of a deposited part can be controlled in a feedback loop using a

computer control system 21 to monitor the position of the surface of the deposited

material as a vector along the optic axis and a distance to the focal point as measured by

optical sensor 6. The measured position is combined with the coordinates of the focal

point as measured from the translation stages 12 and 13 to determine the tme coordinates of the deposition surface. The tme position of the deposition surface is compared with the

target position and, when the difference is within tolerance, deposition at that location is halted by removing the laser light, shutting down the reactant gas, or moving the focal

point to a new location. The growth rate of deposited material can affect important characteristics related to the growth of geometric stmctures. For example, if the growth rate is significantly

increased or decreased, the morphology of the deposit may change. Changes in growth

rate may also affect changes in morphology in the cross-section of the growth, and may change stress forces within the deposit. The growth rate of deposited materials may be predicted from the temperature of the deposition zone, or from physically measuring the

growth rate by optical measurements. The growth rate is determined by several factors, including gas composition, the effective thermal conductivity from deposition zone 2 to

substrate 1, the gas pressure, the gas temperature, and the laser wavelength, intensity and power. If the laser power is modulated, the laser peak power, modulation frequency, and duty cycle also affect growth rate. The deposition growth rate is also determined by the

numerical aperture of the laser beam focused on the deposition zone, the rate of

translation along the axis of the laser beam, and the rate of translation perpendicular to the laser beam. Other factors that may affect growth rate include vibration of the stmcture

and contamination of the optical components

To maintain a desired growth rate, one can measure or calculate the rate of growth

of the deposits and then change one or more of the parameters affecting growth rate in a

feedback loop. For example, if the rate of growth is too high, laser power may be reduced

or the rate of translation of the deposition zone may be increased to maintain a constant

cross-section of the deposit. Measuring the temperature of the deposition zone can allow

control of growth rate, the dimensions of the deposits formed at the deposition zone, or

the morphology of the deposits. The temperature of the deposition zone may be

controlled through changes in translation rate, through changes in the laser power heating

the deposition zone, and through changes in reactant gas pressure, flow rate, or gas composition. The approach used to control growth rate will depend on the specific

requirements of the stmcture being grown. For example, if a portion of a stmcture being grown does not require a critical strength, the deposition rate may be increased.

Conversely, if a portion of a stmcture requires a certain level of strength, the laser power may be changed, the translation rate changed, or other parameters affecting growth may

be adjusted to conditions previously determined that maintain the strength of the part.

The deposition zone lateral geometry can also be controlled in a feedback loop using the computer control system 21 of Fig. 1 to monitor the lateral geometry measured

by sensor 6 and compare the measured geometry to a target geometry. If there is a

difference between the target and measured geometry, the shape of the laser beam is altered. The geometry at the deposition zone is a function of the distribution and shape of the photon flux delivered to the deposition zone from the laser and the rate that heat is

conducted away from the deposition zone by the deposited material and reactant gas. The

minimum feature size for lateral geometry is a function of the laser beam spot size at the

deposition zone; normally features less than half the spot size can not be produced reliably.

One method of altering the deposition zone lateral geometry is by forming an

intermediate image plane with a specific shape by means of a mask and a suitable optical

system as known in the art. The shape of the mask may be static or dynamic. A static

mask may be a filter in the shape of the desired geometry. A dynamic mask may consist of

a multi element LCD filter with element shapes that will allow the formation of various

mask geometries. Another way that a dynamic mask may be created is by use of a micro

mirror array where each mirror element is directed along or away from the optic axis to

form a mask pattern. An advantage of a dynamic multi-element mask is the capability to

selectively control the temperature over the area of the deposition surface by selectively modulating individual filter elements such as a single micro mirror element that

corresponds with the location on the deposition surface where the temperature is to be

altered.

Reactant gas delivery system

In order to control the geometric and morphological properties of the part, the gas

properties need to be monitored and controlled. The most significant gas properties that

should be monitored are temperature, pressure, composition, waste product composition, and waste product concentration. These gas properties should reflect the condition of the gas as it enters the deposition zone. Properties that may vary significantly within the reaction cell, for example temperature, should be measured in close proximity to the

deposition zone to reduce errors. The measurements provided by optical sensors 6 are

fed to a general-purpose computer or a dedicated controller used to maintain or modify

the gas properties.

Thermal decomposition of reactant gases often has a high activation energy, and

usually takes place at a high temperature (over 1000 K). When the incoming reactant

gases are near room temperature, a large fraction of the laser power needed to maintain

the temperature of the deposition zone is used to heat the gases to the reaction temperature. Additionally, the reactant gases can cool the surrounding substrate, causing

a larger thermal gradient through the substrate and resulting in additional cooling of the

deposition zone. Preheating the reactant gases can result in reduction of the laser power

required to sustain thermal decomposition at the deposition zone. Additionally, preheating

the reactant gases can reduce convective currents, increase gas velocity at the heating zone for the same plenum geometry without increasing gas consumption rates, and reduce gas-momentum induced motion in the substrate for the same gas velocity and turbulence. The reactant gas may be preheated in several ways, including heating parts of the reactant

gas processing system 20 (including the inlet, manifold, and/or plenum), heating the

optical elements exposed to the gases prior to entry to the deposition zone, or heating a

combination of all or parts of the gas handling system or optical elements. In one embodiment, when a semiconductor diode laser is used as the energy beam source 5, the laser diode may be mounted on a small heat exchanger, and the reactant gases can be passed through the exchanger to preheat the reactant gases while cooling the diode laser.

Suitable reactant gases for use in the CVD system of the present invention include,

but are not limited to, gaseous alkanes (including methane, ethane, propane, butane, and

their isomers), alkenes (including ethylene, propylene, butene, butadiene, and their isomers), alkynes (including acetylene and methylacetylene), organometallics (including trimethyl aluminum, nickel tetracarbonyl, iron pentacarbonyl, tetramethyl silane), hydrides

(including silane and borane), halides (including boron trichloride, titanium tetrachloride,

and tungsten hexafluoride), and various combinations thereof. For some applications, the

gas composition may include a reactant such as hydrogen to scavenge graphitic carbon or

to suppress the formation of soot. Inert gases may also be used to change the

concentration of reactant gases or to alter the thermal conductivity of the gases. Suitable inert gases include helium, neon, argon, nitrogen, and carbon dioxide.

The deposition zone 2 is heated by an energy beam 8 to a temperature that results in decomposition or reaction of a reactant gas. The decomposition or reaction should

yield either a solid, or a liquid that hardens or crystallizes on cooling. The reactant gas

may be directed to the deposition zone 2 by a plenum as shown in Fig. 2. Fig. 2 shows a deposition zone 2, a substrate 1 (which includes all material on which a deposit is formed, including mandrels, supporting stmctures, and previously deposited material) within the deposition zone 2, a focal point 7 of an energy beam 8, an objective lens 3, an inlet for a reactant gas 23, a manifold for distributing gas 24, and a plenum 22. For some

applications, the dimensions of plenum 22 are critical. Increasing the length of plenum 22

in the direction along the laser beam may create physical interference with substrate 1, but

may allow higher gas velocities at the deposition zone 2. Reducing the length of the plenum may reduce the likelihood of interference with the substrate 1, but may result in reduced gas velocity at deposition zone. Other plenum shapes may be used without

departing from the scope of the invention.

The velocity of gas through the plenum should be sufficiently high to prevent deposition of reaction byproducts on the lens or window, but not so high as to cause motion of the deposit being grown.

The reactant gas plenum can serve several functions. In one embodiment of the

present invention, the gas plenum is configured so that the gas flow prevents

contamination of either the objective lens or a window placed between the objective lens

and the deposition zone. The plenum can also increase deposition rates by providing a high velocity gas stream directed at the deposition zone, it can increase the fraction of

reactants consumed in the deposition process by efficiently directing gas into the

deposition zone, and it serves to reduce the turbulence of the gas stream directed at the deposition zone. By proper design of the plenum, the volume of the plenum can be

minimized and the laminar flow within the plenum designed to allow gas mixtures to be

rapidly changed. A reactive gas plenum also allows the reactant gas in the reaction

chamber to be diluted with an inert gas through a separate inlet to the reaction chamber, while having a relatively high concentration of reactant gas at the deposition zone. Diluting the reactant gases in the chamber with an inert gas has the advantage of purging out waste gases and preventing the buildup of mixtures of reactant gases. Mixtures of reactant gases may be undesirable if the mixture results from switched gases at the plenum. Undiluted by inert gas, this mixture may complicate recycling schemes, or may

create a potential for reaction between the reactant gases.

For some applications, the gas inlet 23 of Fig. 2 or gas inlet 23 of Fig. 3 may be

connected to an assembly of valves, which allows gas composition to be changed rapidly.

Preferably, the effective volume of gas between the valve assembly and the deposition zone is less than 10 cm³, more preferably, the volume will be less that 1 cm³, most preferably the volume will be less than 0.1 cm³. Effective volume is defined here as the volume of gas that needs to be introduced by a valve of the valve assembly that is

necessary to change the composition of the reactant gas at the deposition zone.

Additionally, the gas plenum can be positioned to utilize the momentum of the

reactant gas to purge the surface of an optical component closest to the deposition zone as

shown in Fig. 3. Gas inlet 23 feeds gas to manifold 24, which symmetrically distributes

the gas around the optical component. The gas is directed along the optical surface 25,

and the gas momentum directs the gas to the focal point 7. Purging the objective lens 3 or

a window placed between the objective lens 3 and the deposition zone 2 is particularly

important for miniaturized optical systems. Laser decomposition of gases can lead to the formation of small amounts of particulate and high molecular weight gas-borne

contaminants, and these contaminants can form light-scattering or absorbing deposits on

optical components near the deposition zone. Miniature optical systems are defined here

as a window or objective lens within about 10 mm of the deposition zone. Adequate purging is particularly important if the deposition zone is located below the window or

objective lens, since convection currents will tend to carry contaminants toward the optical

components.

The plenum design may be altered to meet the specific geometry and configuration of the laser beam or beams. For example, a semiconducting laser diode array may produce

a corresponding array of laser beams, and each beam be circularized by a fiber lens

attached to the emitting face of the laser diode array and the output of the array be

focused by a lens or lens assembly on a substrate to form an array of deposition zones. The plenum for this system may be designed to provide a rectangular profile of gas flow proximate to the deposition zones.

Example 1

A 840 nm laser diode with an optical power of 1.0 Watt emitting from a 100 by 1 μm aperture was coUimated with a 0.25 NA apochromat microscope objective. The laser

was an Optopower model OPC-A001-840-CT/L laser diode with a fiber circularizing lens.

The laser was mounted on a heat sink controlled at 20°C, and the laser drive current was

1.26 A. The coUimated laser beam was passed through an Edmund Scientific cold mirror, model 43959 mounted at 45° relative to the propagation direction axis of the laser beam,

with the laser beam incident on the mirror at a p-polarization. The laser light was focused

with a 0.45 numerical aperture apochromat microscope objective through a 0.25 mm thick

glass window onto a glossy clay-coated paper substrate. In this and all subsequent examples, the size of the coUimated laser beam incident on the focusing beam was such that about 95% of the laser beam was accepted by the objective lens. The paper substrate

was prepared beforehand by depositing a thin layer of carbon on the surface of the paper with a very fuel rich propane flame. The working distance from the window to the

substrate was 5 mm, and the window was in contact with the microscope objective.

200 seem (standard cubic centimeters per minute) of MAPP™ gas (44% methyl acetylene-propadiene, 56% liquefied petroleum gas, available from BOC gases, Murray Hill, NJ) was delivered through a manifold at a pressure of approximately 0.95 bar

measured at the substrate. The manifold uniformly distributed the gas around window placed in front of the objective lens, and through a 1.0 mm annular gap formed by a 0.2 mm thick plate positioned parallel to the window and in a plane perpendicular to the direction axis of the laser beam. The gas flowed along the window through the gap, then

exited through a 4 mm hole in the plate centered on the objective lens. Gas flow exited the 4 mm hole downward toward the substrate. The surface of the carbonized substrate was moved toward the focal point of the laser beam, initiating decomposition of the MAPP™ gas to form a carbon deposit. The optical assembly, including the objective lens,

window, and plate, was moved upward, away from the substrate, at a rate of

0.1 mm second to form a carbon fiber. The fiber produced was approximately 25 mm

long, with a diameter of 70 μm, and had a glassy and smooth surface. There was no visible deposit formed on the window.

Comparative Example 2

The experiment of Example 1 was repeated, except the window position was at a

fixed distance of 5 mm from the substrate, and the gas was introduced through a 3 mm

internal diameter tube placed on the side of the substrate about 10 mm from the focal

point. The flow of the gas was directed toward the point where decomposition was initiated on the substrate. The gas composition, flow rate, pressure, and laser power were identical to Example 1. A carbon fiber 3 mm long was grown by moving the microscope

objective upwards from the window at a rate of 0.1 mm/second. The window showed

heavy deposits in the region that transmitted the laser beam.

Comparative Example 3

The experiment of Example 1 was repeated, except the gas flow was 100 seem. The

optical assembly, including the objective lens, window, and plate, was moved upward,

away from the substrate, at a rate of 0.1 mm/second to form a carbon fiber. The fiber

produced was approximately 25 mm long, with a diameter of 70 μm, and had a glassy and smooth surface. Substantial deposits were visible on the window.

Example 4

The experiment of Example 1 was repeated, but a fiber 50 mm long was grown. The

window was free of deposits, and the dimension of the fiber was unchanged as visible at

200 times optical magnification. This example shows that sufficient gas flow can be

delivered to prevent deposition on the optical components, but not cause motion of the deposit stmcture even with very high aspect ratio stmctures.

Example 5

The experiment of Example 1 was repeated, except the laser used was an

Optopower 1.5 Watt, 60 micron aperture laser diode operating at about 830 nm, model

number SCT060-830-K2-01. The laser beam was coUimated with an aspheric lens

(available from Geltech Corp., Orlando, FL, model 350330), and expanded in the slow axis of the laser diode output 4 fold with an anamorphic prism pair. The laser was focused to a point near the surface of a carbon-coated paper substrate using an aspheric lens (Geltech Corp. model 350240). A fiber was grown to a length of 120 mm by moving the

objective lens upward from the substrate at a rate of 0.05 mm/second. The fiber had a

diameter of 200 microns, and a surface covered with glassy nodular hemispheres roughly 15 microns in diameter.

Example 6

A 840 nm laser diode with an optical power of 1.0 Watt (Optopower model OPC-

A001-840-CT/L) laser diode with a fiber circularizing lens was coUimated with an aspheric lens (available from Geltech Corp., Orlando, FL, model 350330). The laser was mounted

on a heat sink controlled at 20°C, and the laser drive current was 1.32 . The coUimated laser beam was passed through an Edmund Scientific cold mirror, model 43959 mounted at 45° with the laser beam incident on the mirror at a p-polarization. The laser light was

focused with an aspheric lens (Geltech Corp. model 350240) tlirough a 0.17 mm thick

glass window onto a glossy clay-coated paper substrate. The paper substrate was

prepared beforehand by mbbing a thin layer of graphite onto the paper, followed by

blowing off excess loose graphite.

A CCD camera was used to obtain an image of the laser beam incident on the

objective lens by observing light reflected from a white diffuser temporarily placed on the

objective, and viewed in reflection off the cold mirror. At the above drive current and

temperature, the multimode output of the laser diode produced two spots, each about 3

mm in diameter (diameter determined by the region of 50% intensity). The centers of the

two spots were spaced by about 6 mm. 300 seem of MAPP™ gas (44% methyl acetylene-propadiene, 56% liquefied

petroleum gas, available from BOC gases, Murray Hill, NJ) was delivered tlirough a manifold at a pressure of approximately 0.95 Bar measured at the substrate. The manifold

uniformly distributed the gas around window placed in front of the objective lens, and through a 1.0 mm annular gap formed by a 0.5 mm thick plate positioned parallel to the

window and in a plane perpendicular to the direction axis of the laser beam. The gas

flowed along the window through the gap, then exited through a 4 mm hole in the plate

centered on the objective lens. Gas flow exited the 4 mm hole downward toward the substrate. The surface of the graphite-coated substrate was moved toward the focal point of the laser beam, initiating decomposition of the MAPP™ gas to form a carbon deposit.

The optical assembly, including the objective lens, window, and plate, was moved upward,

produced was approximately 25 mm long, with a diameter of 50 μm, and had a glassy and smooth surface. The fiber cross-section was similar to the beam profile, consisting of two

rounded sections with a diameter of about 30 μm joined by a bridging section about 20

μm thick. This example shows that using a beam with a non-gaussian profile can generate

complex deposit cross-sections.

Example 7

The experiment of Example 6 was repeated, but a transparent diffuser was placed between the CCD imaging lens and the cold mirror. The objective lens collected light

emitted from the deposition zone to form a semi-collimated beam of white light directed

towards the cold mirror. The light reflected off the cold mirror and illuminated a portion

of the diffuser. The image on the diffuser was approximately 5 mm in diameter. During fiber growth, the brightness, integrated light intensity measured over a 10 mm diameter circle centered on the spot on the illuminated diffuser, and spot size varied. Brightness and integrated intensity was at a maximum when the focal point of the objective lens was at the fiber tip, and decreased when the focal point was moved either above or below the fiber tip. The spot size was at a minimum when the focal point was at the fiber tip. This

example shows that brightness, integrated light intensity, and projected spot size can be

used to determine the displacement from the focal point and the deposition zone.

Gas recycling system

In conventional CVD systems the reactant gas is often discarded instead of being recycled, because the presence of even minor contaminants can render the part defective.

In an embodiment of this invention, the gas is recycled to reduce consumption of supplies and to reduce handling of waste product.

The reactant gas processing system 20 of Fig. 1 includes a supply of reactant gas 15,

optionally with other gases that can be blended with the reactant gas 15, which is delivered

through manifold 14 to the reactant gas conditioning system 16 where it is delivered to the

reaction cell 11. The manifold 14 is used to control the delivery rate and composition of

the reactant gas. The reactant gas conditioning system 16 is used to control the temperature, pressure, and composition of the reactant gas before introduction to the

reaction cell 11. The reactant gas is thermally decomposed by an energy beam 8 to form a deposit of solid material. The remaining gas in the reaction chamber is optionally diluted

with an inert gas and purged from the reaction chamber into a gas scrubbing system 18.

The waste reactant gas is scrubbed to remove solid (particulate) and gaseous waste products to form a second reactant gas, and any gases that are toxic or harmful to equipment are removed and the non-hazardous components are vented to the ambient environment. The second reactant gas is analyzed in real time by computer control system

21 to determine the composition of the gas so that reactant gas components consumed in the production process can be supplemented. The recirculated reactant gas is conditioned to have a similar temperature, pressure, and composition as the first reactant gas and is reintroduced to the reaction cell 11. A baffle 17 may be positioned between the reactant

gas inlet and the deposition location to reduce gas turbulence that may disturb the

deposited stmcture. Alternatively, a plenum may be used to direct reactant gas to the deposition zone. The reactant gas conditioning system 16 may also be used to purge the reaction cell 11 of reactant gas by delivering an inert gas such as argon from the gas supply 15 to the reaction cell 11,

Scanning mechanism

Formation of objects by means of selective beam deposition requires that the laser

beam focal point be scanned relative to the supporting substrate or mandrel in a manner so as to form an object of the desired geometry. Fig. 1 shows the method for scanning

employed in a preferred embodiment of the invention. Translation of the optical assembly

10 is accomplished by a vertical translation stage 12, with lateral translation of the

substrate along two perpendicular axes provided by a horizontal translation stage 13.

Since the energy beam 8 can be coUimated, it is also possible to translate just the objective lens 3 along the axis of the laser beam, independent of the entire optical assembly. The

axes are normally orthogonal to each other in order to simplify the control algorithms. For precision in translation, a feedback system such as linear encoders is desirable, but is

not always required. Many forms of translation stages are available commercially utilizing

various types of bearing systems and drives, and mechanisms employed for translational

scanning techniques are apparent to one skilled in the art.

In another scanning method, the focusing lens can be rotated around one or two axis perpendicular to the approximate propagation axis of energy beam 8. These axes are

normally orthogonal to each other in order to simplify the control algorithms. Mechanisms

employed for rotational scanning techniques will be apparent to one skilled in the art.

Yet another scanning method involves scanning the laser beam itself by means of a one or two axis galvanometer. Galvanometer scanning is frequently used where a rapid scan rate is desired in the plane normal to the optic axis. Galvanometer scanning has the advantage of being a fast and inexpensive means to accomplish scanning at the cost of less

precision in focal point placement and increased focal point diameter. The focal point placement accuracy is a function of the ability to measure angular deflection and the

working distance between the scanning mirror and the focal point. The preferred

embodiment of this invention will utilize translational and /or rotational scanning to

fabricate objects by selective beam deposition.

Claims

CLAIMSWhat is claimed is:

1. A chemical vapor deposition system, comprising a semiconductor diode laser

energy source.

2. The chemical vapor deposition system of claim 1, wherein said semiconductor diode laser further comprises a heat exchanger configured to pass a reactant gas through said heat exchanger to pre-heat said reactant gas while cooling the diode laser.

3. The chemical vapor deposition system of claim 1, wherein said semiconductor diode laser beam has a non-gaussian symmetry.

4. The chemical vapor deposition system of claim 1, further comprising an array of

independently operable diode lasers configured to provide multiple deposition zones.

5. The chemical vapor deposition system of claim 1, wherein the energy from said

diode laser is conveyed through an optical fiber.

6. The chemical vapor deposition system of claim 4, wherein the energy from said diode laser array is conveyed through an array of optical fibers.

7. The chemical vapor deposition system of claim 1, wherein an energy beam from

said diode laser decomposes a reactant gas to form a deposit of solid material in a deposition zone, light emitted from the deposition zone is detected by at least one optical sensor, and irifbrmation from the optical sensor is used in a feedback loop to control the growth of the deposit by modifying the power of the diode laser.

8. A chemical vapor deposition system, comprising an energy beam capable of decomposing a reactant gas to form a deposit of solid material in a deposition zone, an optical element comprising an objective lens configured to focus said energy beam into

said deposition zone and optionally a window positioned between the objective lens and

the deposition zone, wherein said optical element is within 10 mm of said deposit in said

deposition zone.

9. A chemical vapor deposition system, comprising an objective lens configured to confocallt focus an energy beam from an energy source onto a focal point in a deposition zone while simultaneously collecting light emanating from said deposition zone and

conveying said collected light to an optical sensor.

10. The chemical vapor deposition system of claim 9, wherein said energy beam is

selected from the group consisting of infrared light beams, visible light beams, and ultraviolet light beams..

11. The chemical vapor deposition system of claim 9, wherein said energy beam has a

non-gaussian symmetry.

12. The chemical vapor deposition system of claim 10, wherein said infrared light beam or said visible light beam is provided by a semiconductor diode laser source.

13. The chemical vapor deposition system of claim 10, wherein said infrared beam or

said visible light beam provided by said semiconductor diode laser source is circularized by a fiber lens mounted proximate to said semiconductor diode laser source.

14. The chemical vapor deposition system of claim 9, wherein said optical sensor determines the temperature of the deposition zone by detecting thermal emission.

15. The chemical vapor deposition system of claim 9, wherein said optical sensor determines the position of a deposition surface in the deposition zone relative to the focal point of the energy beam by detecting thermal emission or reflected light from said energy

beam.

16. The chemical vapor deposition system of claim 9, wherein said optical sensor

determines the lateral geometry of a deposition surface in the deposition zone by detecting

thermal emission.

17. The chemical vapor deposition system of claim 9, wherein information from said at

least one optical sensor is used in a feedback loop to control growth of a deposit in said

deposition zone by modifying the power of said energy source; modifying the position of

an optics assembly or the position of said deposit; modifying the composition, flow rate,

or pressure of a reactant gas; or any combination thereof

18. A chemical vapor deposition system, comprising a laser beam focusing lens, a substrate capable of being heated by the laser beam to form a deposition zone, and a gas plenum positioned to direct a reactant gas onto said deposition zone, wherein the effective

volume of said gas plenum is less than 10 cm³.

19. The chemical vapor deposition system of claim 18, wherein the effective volume of said gas plenum is less than 1 cm³.

20. The chemical vapor deposition system of claim 18, wherein the effective volume of

said gas plenum is less than 0.1 cm³.

21. The chemical vapor deposition system of claim 18, further comprising a valve

assembly having at least two valves capable of controlling the flow of at least two gas streams, a device for combining said gas streams and delivering said gas streams to said

plenum, wherein the effective volume between said valves and said deposition zone is less

than 10 cm³.

22. The chemical vapor deposition system of claim 21, wherein said effective

volume is less than 1 cm³.

23. The chemical vapor deposition system of claim 21, wherein the effective volume of said gas plenum is less than 0.1 cm³.

24. A chemical vapor deposition system, comprising an objective lens, a deposition

zone, optionally a window positioned between the objective lens and the deposition zone, and a gas plenum positioned to direct a reactant gas into the deposition zone, wherein the gas plenum is configured so that the reactant gas sweeps contaminants from a surface of the objective lens or window prior to entering the deposition zone.

25. The chemical vapor deposition system of claim 24, wherein the reactant gas is pre¬

heated prior to entering the deposition zone.

26. The chemical vapor deposition system of claim 25, wherein the reactant gas is pre¬

heated by passing through a heat exchanger on a diode laser.

27. A method of recycling reactant gases in a chemical vapor deposition system,

comprising the steps of:

(a) controlling the temperature, pressure, and composition of a first reactant gas in

a reactant gas conditioning system;

(b) introducing said first reactant gas into a reaction chamber through a gas

plenum configured to direct said reactant gas into a deposition zone where it is

thermally decomposed by an energy beam to form a deposit of solid material;

(c) optionally diluting waste gas in the reaction chamber with an inert gas and

purging said waste gas out of the reaction chamber into a gas scrubbing

system;

(d) scrubbing solid particulate materials and gaseous waste products from said waste gas to give a second reactant gas having desirable gas components;

(e) performing on-line analysis of the composition of the second reactant gas and

providing feedback to a gas conditioning system to adjust the composition of the second reactant gas to form a recirculated reactant gas having the same temperature, pressure and composition as that of the first reactant gas; and (f) introducing the recirculated reactant gas back into the reaction chamber

through said gas plenum.