Abstract:
A method of forming an epitaxial layer on a substrate such as a sapphire wafer that does not readily absorb thermal radiation. The method includes coating a first side surface of the substrate with an energy-absorbing opaque material. The opaque material forms a thermally absorptive coating on the substrate. The coated substrate may be heated to remove contaminants from the thermally absorptive coating. The coated substrate is positioned in a vacuum deposition chamber and heated by directing radiative energy onto the thermally absorptive coating. An epitaxial layer such as GaN or SiGe is formed on a second side surface of the substrate opposite the thermally absorptive coating.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/315,383 filed on Mar. 30, 2016, titled “THERMAL ABSORPTION COATING (TAC) ON SAPPHIRE FOR EPITAXIAL PROCESS,” the contents of which is hereby incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The invention described herein was made in the performance of work under a NASA contract and by an employee of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. §202, the contractor elected not to retain title. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Epitaxy is a process by which one or more crystalline overlayers are deposited on a crystalline substrate (e.g. sapphire). Various processes for fabricating sapphire substrates (wafers) and related devices have been developed, as described in U.S. Pat. Nos. 9,449,818, 9,455,374, 7,558,371, 8,226, 767 and U.S. Patent Publication No. 20140264459. Epitaxy processes involving semiconductor materials such as cubic or zinc blende on a sapphire substrate (wafer) typically require the sapphire substrate/wafer to be at a high temperature in the range of about 600° C. to about 1000° C. For example, the growth of a GaN epitaxial layer on a sapphire substrate for LED fabrication may require a sapphire substrate temperature of about 850° C. Forming an SiGe epitaxial layer on a sapphire substrate may require a sapphire substrate temperature of about 890° C. Known methods to heat sapphire substrates may suffer from problems such as low yield and/or high production cost. For example, if the heat absorption rate of a sapphire substrate is low, heating the substrate to the desired temperature may be slow, leading to increased expense. 
         [0004]    Because heating of sapphire substrates/wafers generally occurs in a vacuum environment, the sapphire substrates or wafers cannot be heated utilizing conductive or convective heat transfer. Thus, radiative heat sources/transfer must be utilized. However, because the sapphire substrates/wafers may be optically transparent, the sapphire substrates/wafers do not readily absorb radiative energy. 
         [0005]    There are at least two known processes for heating sapphire substrates for epitaxial processes. First, the sapphire substrates may be positioned in a heated nitrogen or argon charged vacuum chamber for several hours to raise the temperature of the sapphire substrates. A plurality of the sapphire substrates may be positioned in the chamber to provide a batch process. Another known method of heating sapphire substrates involves coating a sapphire substrate/wafer with a metallic or carbon film utilizing a sputtering process. The coating may be deposited on a back surface of a sapphire substrate/wafer. The coating may be light-absorbing, and the coating can therefore absorb thermal radiation from a heat source to heat the sapphire substrate/wafer. 
         [0006]    Because sapphire is optically thin and transparent, it may be difficult to raise the temperature of a sapphire substrate/wafer in a vacuum environment to a desired (uniform) level over the entire area of the sapphire substrate. Although thermal energy can be transferred from a heating element to a sapphire substrate/wafer by conduction through direct contact, direct conduction may cause uneven heating. Thus, conductive heating is prone to hot spots, thermal stresses, and subsequent breaking of the sapphire substrates/wafers. Convection is impossible in vacuum. Conduction and convection are therefore not viable methods for heating sapphire substrates/wafers in a vacuum for epitaxial processes. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    One aspect of the present disclosure is a method of forming an epitaxial layer on a sapphire wafer. The sapphire wafer has first and second opposite side surfaces. The method includes coating the first opposite side surface of the sapphire wafer with an energy-absorbing opaque material that is stable at high temperatures to thereby form a thermal absorption coating on the first side of the sapphire wafer. The coated sapphire wafer is incrementally heated to remove contaminants from the thermal absorption coating. The coated sapphire wafer is then cooled, and the coated sapphire wafer is positioned in a vacuum deposition chamber. The coated sapphire wafer is then heated in the vacuum deposition chamber by directing radiative energy onto the thermal absorption coating. An epitaxial layer is then formed on the second side of the coated sapphire wafer opposite the thermal absorption coating. 
         [0008]    The thermal absorption coating may comprise paint that is applied using a room temperature spray process. The process may include allowing the paint to dry at room temperature before removing contaminants from the thermal absorption coating by incrementally heating the coated sapphire wafer. The paint may include copper and/or chromium and/or carbon black pigments. The sapphire wafer may comprise a C-plane sapphire material, and the method may include polishing the first and second opposite sides of the sapphire wafer. The method may also include removing the energy-absorbing opaque material from the first opposite side surface, applying an energy-absorbing opaque material onto the epitaxial layer on the second opposite side surface, and forming an epitaxial layer on the first opposite side surface. Incrementally heating the coated sapphire wafer may include heating the coated sapphire wafer to about 650° C. The coated sapphire wafer may be baked at about 400° C. for about 15 minutes, followed by baking at about 500° C. for about 15 minutes, followed by baking at about 650° C. for at least about 20 minutes. The method may include allowing the coated sapphire wafer to cool at room temperature to about 450° C. or less after baking the coated sapphire wafer at about 650° C. The coated sapphire wafer may be heated in the vacuum deposition chamber to a temperature of at least about 850° C., and an epitaxial layer of GaN may be formed on the second opposite side surface of the coated sapphire wafer. The coated sapphire wafer may be heated in the vacuum deposition chamber to a temperature of at least about 890° C. and the method may include forming an epitaxial layer of SiGe on the second opposite side surface of the coated sapphire wafer. 
         [0009]    These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic cross sectional view of an uncoated sapphire substrate/wafer in a vacuum environment showing incident thermal energy, transmitted thermal energy, and emitted thermal energy; 
           [0011]      FIG. 2  is a schematic cross sectional view showing a sapphire wafer with a thermal absorption coating and incident and emitted thermal energy; 
           [0012]      FIG. 3  is a flow diagram showing a process according to one aspect of the present disclosure; 
           [0013]      FIG. 4  is a photographic image of sapphire substrates/wafers that have been coated with a thermal absorption coating, wherein the top left wafer is coated but not yet baked, the top right wafer has been baked but has not yet been coated in an epitaxial process, the bottom left wafer has been coated in an epitaxial process but the thermal absorption coating has not yet been removed, and the bottom right wafer has had the thermal absorption coating removed; 
           [0014]      FIG. 4A  is an optical micrograph of a blank sapphire wafer; 
           [0015]      FIG. 4B  is an optical micrograph of a sapphire wafer after a thermal absorption coating has been applied; 
           [0016]      FIG. 4C  is an optical micrograph of a sapphire wafer after the thermal absorption coating has been removed utilizing sonication; and 
           [0017]      FIG. 5  is a schematic view showing removal of a thermal absorption coating utilizing vibrating water molecules (sonication). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the components as oriented in  FIGS. 1 and 2 . However, it is to be understood that the components may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0019]    With reference to  FIG. 1 , a heating element  20  may be used to generate radiation  12  that is incident on a surface  8  of a conventional uncoated sapphire substrate/wafer  1  disposed in a vacuum chamber  5 . The sapphire substrate  1  is an optically transparent and thin material that does not readily absorb heat from incident thermal radiation  12 . Sapphire substrate  1  may be in the form of a wafer (e.g. a thin, flat disk-shaped object). However, the sapphire substrate  1  may have virtually any size or shape (e.g. square, rectangular, etc.) as required for a particular application. Thus, the term “substrate/wafer” as used herein is not limited to any particular geometry, size, shape, thickness, etc. A high percentage of the photons simply pass through uncoated sapphire substrate/wafer  1  and exit surface  9  as radiative heat  18 . A small portion  14  of the incident thermal radiation  12  is typically reflected at an uncoated surface  8  of the sapphire substrate/wafer  1 . Only a small portion of the incident thermal wave  12  is absorbed while it passes through the sapphire substrate/wafer  1 . A portion  16  of the absorbed energy is lost by re-emission process when the temperature of the sapphire substrate/wafer  1  is increased. It will be understood that sapphire substrate/wafer  1  may comprise other optically transparent materials (e.g. ceramics) other than sapphire that are optically transmissive or transparent and do not readily absorb thermal radiation. Thus, the present disclosure is not limited to sapphire substrates. Rather, the materials and processes of the present disclosure may be utilized in connection with virtually any substrate material used in an epitaxial process that is optically transparent or at least partially transmissive of thermal radiation. 
         [0020]    With further reference to  FIG. 2 , the temperature of a sapphire substrate/wafer  1 A in a vacuum chamber  5  may be raised more readily by reducing transmission loss. Specifically, a thermally absorptive coating  10  according to the present disclosure facilitates effective heating of a sapphire substrate/wafer  1 A in a vacuum chamber  5 . The thermally absorptive coating  10  absorbs the radiative thermal energy  12  from heating element  20 , and the thermal energy  22 , is then conducted from thermally absorptive coating  10  into the sapphire substrate/wafer  1 A through surface  8 A as shown in  FIG. 2 . Heating element  20  may be actuated to generate thermal radiation before and/or during and/or after layer  50  is deposited utilizing an epitaxial process. As discussed below, layer  50  may comprise a semiconductor material such as GaN or SiGe. As discussed in more detail below, thermally absorptive coating  10  may comprise high temperature black paint that is applied to surface  8 A of the sapphire substrate/wafer  1 A via a spraying process. 
         [0021]    A process  60  according to one aspect of the present disclosure is shown in  FIG. 3 . One or more suitable substrates such as sapphire substrate/wafers  1 A are provided at step  25 . Sapphire substrate/wafers  1 A may comprise c-plane sapphire or other materials. For example, sapphire substrate  1  may comprise sapphire having other orientations or other materials (e.g. ceramics) that are optically transparent or at least partially optically transmissive. One side (e.g. side  9 A,  FIG. 2 ) of the sapphire substrates/wafers  1 A may be polished for epitaxy, and the other side  8 A may be rough. The rough side  8 A may be coated with thermally absorptive coating  10  at step  26 . It will be understood that both sides  8 A,  9 A of the sapphire substrate/wafer  1 A may be polished to reduce scatter from laser imaging devices that may be utilized to scan for defects, and/or to permit development of epitaxial domains (films) on both sides  8 A and  9 A. Surface  8 A may be polished, and thermally absorptive coating  10  may be applied to the polished surface  8 A. The thermally absorptive coating  10  can be removed from a polished surface  8 A without damaging surface  8 A utilizing the sonication process described below in connection with  FIG. 5 . If epitaxial films are to be deposited on polished surface  8 A, solvents and/or additional polishing may be utilized to ensure that polished surface  8 A is sufficiently clean/smooth to permit further epitaxial processing. 
         [0022]    After the thermally absorptive coating  10  is applied to surface  8 A of sapphire wafer  1 A, the thermally absorptive coating  10  is allowed to dry at step  28  before baking. Drying may be accomplished in air at room temperature for 30 minutes. This gives the thermally absorption coating  10  time to adhere and bond to the surface  8 A tightly for effective conduction heat transfer to the sapphire substrate/wafer  1 A. Completion of step  28  results in a dried (but not baked) wafer as shown upper-left in  FIG. 4 . 
         [0023]    As shown at step  30 , the sapphire substrate/wafer  1 A with thermally conductive coating  10  may then be incrementally heated (baked) in a furnace before exposing the sapphire substrate/wafer  1 A to the high temperatures and vacuum environment during epitaxy. A baking process according to the present disclosure may start at about 400° C. (for about 15 minutes), increase to about 500° C. (for about an additional 15 minutes), then reach a maximum of about 650° C. (for about an additional 20 minutes). It will be understood that these times and temperatures are examples of suitable times and temperatures, but the present invention is not limited to these specific times/temperatures. For example, the times could be reduced to provide higher production rates. Completion of baking (step  30 ) results in a wafer as shown upper-right in  FIG. 4 . Baking removes acetone, solvents, any polymer binders, and any other organic material that could otherwise contaminate vacuum deposition chambers and interfere with film deposition during epitaxy. Also, baking provides better adhesion of the thermally absorptive coating  10  to the surface  8 A of the sapphire substrate/wafer  1 A. 
         [0024]    The sapphire substrate/wafer  1 A and thermally absorptive coating  10  are then slowly cooled at step  32 . This annealing involves slow cooling of back-side coated sapphire substrate/wafer  1 A from about 650° C. to about room temperature. Slow cooling avoids the potential risks of thermal shock and cracking of the sapphire substrate/wafer  1 A. The sapphire substrate/wafer  1 A is allowed to cool for a few minutes to let the temperature decrease by at least about 200-300° C. before removing the sapphire substrate/wafer  1 A from the furnace. 
         [0025]    As shown at step  34 , the coated sapphire substrate/wafer  1 A may then be positioned in a vacuum deposition chamber. A layer  50  of material (see also  FIG. 5 ) such as GaN or SiGe is then deposited on polished surface  9 A utilizing a known epitaxial process. The epitaxial process may comprise a magnetron sputtering process. However, the epitaxial process may comprise virtually any deposition process or system that uses radiative heating on a non-deposition side of a substrate. Testing has shown that the thermally absorptive coating  10  is stable during deposition at about 900° C. for 2 or more hours. A wafer having an epitaxial layer prior to removal of thermally absorptive coating  10  is shown in the bottom left of  FIG. 4 . 
         [0026]    It will be understood that the process  60  of  FIG. 3  may comprise a batch process in which a plurality of substrates such as sapphire substrates/wafers  1 A are simultaneously processed utilizing steps  26 - 36 . 
         [0027]    In an industrial process, the thermally absorptive coating  10  might present a contamination risk, or it may act as a graphitic conductor and produce undesirable parasitic capacitances. Thus, the thermally absorptive coating  10  is preferably removed via sonification (step  36 ) and cleaned (if required). 
         [0028]    With further reference to  FIG. 5 , removal of thermally absorptive coating  10  may be accomplished by immersing a sapphire substrate/wafer  1 A in deionized water  40  in a container  42 . The thickness of the thermally absorptive coating  10  in  FIG. 5  is increased (exaggerated) for purposes of illustration. Accordingly, it will be understood that the thermally absorptive coating  10  may be significantly thinner relative to the sapphire substrate/wafer  1 A and coating  52  than what is shown in  FIG. 5 . The wafer  1 A is preferably positioned with coating side  8 A and the thermally absorptive coating  10  facing upwardly. Water  40  is then sonicated to break up the thermally absorptive coating  10 . On the molecular scale, water molecules  44  are energized by ultrasonic waves and collide more frequently and vigorously, pushing each other into crevices  46  in the thermally absorptive coating  10 . This breaks off particles of the thermally absorptive coating  10  until the underlying surface  8 A of the sapphire substrate/wafer  1 A is exposed. The energetized water molecules  44  continue to break away the edges of the thermally absorptive coating  10  wherever such exposure to the surface  8 A of sapphire substrate/wafer  1 A exists. No chemicals are required for this process, and the surface  8 A is usually cleaned within about 15 minutes of sonication, leaving the front side  52  of the epitaxial film  50  unharmed. A substrate/wafer with thermally absorptive coating  10  removed is shown at the bottom right in  FIG. 4 . Sonication in water  40  dissolves and breaks up the coating  10  into particles. Some of these particles may be loosely deposited on the surface of epitaxial layer  50  when sapphire substrate/wafer  1 A is removed from water  42 . However, any such particles may be rinsed off easily with clean deionized water. 
         [0029]    As discussed above, surfaces  8 A and  9 A may both be polished, and epitaxial layers may optionally be formed on both surfaces  8 A and  9 A. In this case, after removal of the thermally absorptive coating  10  from surface  8 A, the sapphire substrate/wafer  1 A is then cleaned and dried, and a thermally absorptive coating  10  is then applied to surface  52  of epitaxial coating  50  utilizing steps  26 - 32  ( FIG. 3 ), and a film is then deposited on a surface  8 A (step  34 ). As discussed above, surface  8 A may be treated with solvents and/or polished after the thermally absorptive coating  10  is initially removed via sonification (step  36 ) to ensure that an epitaxial layer can be deposited on surface  8 A. The thermally absorptive coating  10  is then removed (step  36 ) and cleaned (step  38 ). 
         [0030]    The selection of a suitable thermally absorptive coating  10  may take into account several factors. For example, the thermally absorptive coating  10  is preferably a readily available and inexpensive coating material. Also, the thermally absorptive coating  10  preferably provides an easy way to make a uniform, even coating on surface  8 A and/or surface  9 A. 
         [0031]    During testing, several types of coating materials (e.g. paints) failed due to break up under the heat of baking and epitaxial processes. These failures led to thermal shadows and non-uniform wafer temperature. The thermally absorptive coating  10  selected for backside coating of the sapphire substrate/wafers  1 A comprises black silicate-containing spray paint. The black pigments (copper chromium and carbon black) and high temperature stability of this coating material ensures high thermal absorption even under typical substrate temperatures (about 400-900° C.) during epitaxial growth (see e.g. top left coated wafer of  FIG. 4 ). This coating maintained adherence to the sapphire substrate/wafer  1 A even after 2 or more hours of epitaxial growth. 
         [0032]      FIGS. 4A-4C  are optical micrographs of sapphire wafers.  FIG. 4A  is an optical micrograph of a blank sapphire wafer.  FIG. 4B  shows a sapphire wafer after the thermally absorptive coating  10  is applied, and  FIG. 4C  shows a sapphire wafer after the thermally absorptive coating  10  is removed via sonication. The image of  FIG. 4C  is darker than the image of  FIG. 4A  due to an epitaxial film of SiGe on the polished side of the sapphire wafer of  FIG. 4C . 
         [0033]    As discussed above, the thermally absorptive coating  10  of the present disclosure may be applied to various substrates utilizing the processes described above. For example, the substrate could comprise glass, quartz, or diamond. Similarly, the thermally absorptive coating  10  and processes described above may also be utilized in connection with substrates comprising higher bandgap materials that are at least partially transparent to infrared light such as GaAs, GaN, Silicon, and Germanium. Removal of the thermally absorptive coating  10  from sapphire or other substrates may be accomplished via sonification ( FIG. 5 ). The thermally absorptive coating  10  may also be removed by applying a high speed stream of gas (e.g. compressed air or other suitable gas), a stream of water or other fluid. Solvents such as acetone may also be utilized. 
         [0034]    The use of a spray paint as a thermally absorptive coating for sapphire or other optically transparent substrates has a number of benefits over prior methods (e.g. metal sputtering in a vacuum chamber or thermal soak). First, the thermally absorptive coating  10  described above raises the sapphire temperature more effectively. As discussed above, sapphire substrates have low thermal absorption due to the optical transparency of thin sapphire wafers. However, the application of an opaque thermally absorptive coating  10  allows the process to obtain high wafer temperatures with less input heat, saving energy and enabling high temperature growths. Also, the thermally absorptive coating  10  may be black, which is more thermally absorptive than gray metal backside coatings. The thermal coating process of the present disclosure provides a high yield production of sapphire wafers by reducing the overall processing time and cost. 
         [0035]    Spray application of the thermally absorptive coating  10  does not normally result in significant defects such as breaks, flakes, bubbles, and unevenness in the thermally absorptive coating  10 . These types of coating irregularities can cause issues with thermal shadows and consequent temperature gradients within the sapphire substrate/wafer  1 A and inconsistent thin film quality. 
         [0036]    The thermally absorptive coating process of the present disclosure is also faster than prior methods. Prior methods may be time consuming and problematic. In some prior methods the sapphire substrate/wafer was loaded into the vacuum system, pumped down, and the metal coating was then applied via a sputtering process. These steps typically require approximately 4 hours total to produce a coating thick enough on one wafer for adequate heat absorption. Also, only about 3-4 wafers could be done before having to vent the system, reload the crucible, and pump down the system, a process that took approximately a half of a day. In contrast, the method of the present disclosure is scalable to permit processing of multiple substrates (e.g. wafers) simultaneously. Specifically, the coating, baking, and removal steps may be accomplished with batches of wafers as a separate batch process. Through such processes with multiple sapphire substrates, a relatively large number of substrates (wafers) can be readily provided for epitaxial deposition within a few minutes. 
         [0037]    The thermally absorptive coating  10  and process of the present disclosure also utilizes less advanced/expensive equipment compared to prior methods. Prior methods, such as metal coating or thermal soak, may suffer from expensive operation of sputtering systems for metal coating or time-consuming thermal soaking processes, respectively. These prior processes may induce system failures of vacuum systems due to the strayed metal deposition on windows, sensors, and the inside walls of the vacuum system. This may interfere with various components of the system such as the substrate rotation gears. Furthermore, these prior processes may result in flakes of metal being deposited on chamber viewports. These metal flakes may also contaminate other electron beam evaporator crucibles. 
         [0038]    The thermally absorptive coating  10  of the present disclosure can be applied without the need for high-maintenance vacuum equipment, resulting in a far lower operating costs and a simpler, more robust manufacturing process. 
         [0039]    It is to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present disclosure, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.