USRE34861E - Sublimation of silicon carbide to produce large, device quality single crystals of silicon carbide - Google Patents

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Classifications

• • H01L21/205—
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/01—Manufacture or treatment
  • H10H20/011—Manufacture or treatment of bodies, e.g. forming semiconductor layers
  • H10H20/014—Manufacture or treatment of bodies, e.g. forming semiconductor layers having light-emitting regions comprising only Group IV materials
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B23/00—Single-crystal growth by condensing evaporated or sublimed materials
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/36—Carbides
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S148/00—Metal treatment
  • Y10S148/021—Continuous process
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S148/00—Metal treatment
  • Y10S148/148—Silicon carbide

Definitions

• the present invention is a method for controlling the sublimation growth of silicon carbide to produce high quality single crystals.
• Silicon carbide is a perennial candidate for use as a semiconductor material. Silicon carbide has a wide bandgap (2.2 electron volts in the beta polytype, 2.8 in the 6H alpha), a high thermal coefficient, a low dielectric constant, and is stable at temperatures far higher than those at which other semiconductor materials such as silicon remain stable. These characteristics give silicon carbide excellent semiconducting properties, and electronic devices made from silicon carbide can be expected to perform at higher temperatures, and at higher radiation densities, than devices made from the presently most commonly used semiconductor materials such as silicon. Silicon carbide also has a high saturated electron drift velocity which raises the potential for devices which will perform at high speeds, at high power levels, and its high thermal conductivity permits high density device integration.
• the basic semiconductor material in order to be useful as a material from which useful electrical devices can be manufactured, the basic semiconductor material must have certain characteristics. In many applications, a single crystal is required, with very low levels of defects in the crystal lattice, along with very low levels of unwanted impurities. Even in a pure material, a defective lattice structure can prevent the material from being useful for electrical devices, and the impurities in any such crystal are preferably carefully controlled to give certain electrical characteristics. If the impurities cannot be controlled, the material is generally unsatisfactory for use in electrical devices.
• silicon carbide is a fundamental requirement for the successful manufacture of devices from silicon carbide which would have the desirable properties described above.
• a sample should be of a single desired crystal polytype (silicon carbide can form in at least 150 types of crystal lattices), must be of a sufficiently regular crystal structure of the desired polytype, and must be either substantially free of impurities, or must contain only those impurities selectively added to give the silicon carbide any desired n or p character.
• CVD chemical vapor deposition
• sublimation technique The other main technique for growing silicon carbide crystals is generally referred to as the sublimation technique.
• sublimation techniques generally use some type of solid silicon carbide material other than a desired single crystal of a particular polytype, as a starting material, and then heat the starting material until solid silicon carbide sublimes. The vaporized material is then encouraged to condense, with the condensation intended to produce the desired crystals.
• Lely's technique lines the interior of a carbon vessel with a silicon carbide source material. By heating the vessel to temperatures at which silicon carbide sublimes, and then allowing it to condense, recrystallized silicon carbide is encouraged to redeposit itself along the lining of the vessel.
• the Lely process can generally improve upon the quality of the source material, it has to date failed to produce on a consistant or repeatable basis, single crystals of silicon carbide suitable for electrical devices.
• Hergenrother U.S. Pat. No. 3,228,756, discusses another sublimation growth technique which utilizes a seed crystal of silicon carbide upon which other silicon carbide can condense to form the crystal growth.
• Hergenrother suggests that in order to promote proper growth, the seed crystal must be heated to an appropriate temperature, generally over 2000° centigrade, in such a manner that the time period during which the seed crystal is at temperatures between 1800° C. and 2000° C. is minimized.
• Knippenberg U.S. Pat. No. 3,615,930 and 3,962,406, discuss alternative attempts at growing silicon carbide in a desired fashion.
• the '930 patent discusses a method of growing p-n junctions in silicon carbide as a crystal grows by sublimation. According to the discussion in this patent, silicon carbide is heated in an enclosed space in the presence of an inert gas containing a donor-type dopant atom, following which the dopant material is evacuated from the vessel and the vessel is reheated in the presence of an acceptor dopant. This technique is intended to result in adjacent crystal portions having opposite conductivity types and forming a p-n junction.
• Knippenberg discusses a three-step process for forming silicon carbide in which a silicon dioxide core is packed entirely within a surrounding mass of either granular silicon carbide or materials which will form silicon carbide when heated.
• the system is heated to a temperature at which a silicon carbide shell forms around the silicon dioxide core, and then further heated to vaporize the silicon dioxide from within the silicon carbide shell. Finally, the system is heated even further to encourage additional silicon carbide to continue to grow within the silicon carbide shell.
• Vodadkof U.S. Pat. No. 4,147,572 discusses a geometry-oriented sublimation technique in which solid silicon carbide source material and seed crystals are arranged in parallel close proximity relationship to one another.
• Addamiano U.S. Pat. No. 4,556,436, discusses a Lely-type furnace system for forming thin films of beta silicon carbide on alpha silicon carbide which is characterized by a rapid cooling from sublimation temperatures of between 2300° centigrade and 2700° centigrade to another temperature of less than 1800° centigrade. Addamiano notes that large single crystals of cubic (beta) silicon carbide are simply not available and that growth of silicon carbide on other materials such as silicon or diamond is rather difficult.
• German (Federal Republic) Patent No. 3,230,727 to Siemens Corporation discusses a silicon carbide sublimation technique in which the emphasis of the dicussion is the minimization of the thermal gradient between silicon carbide seed crystal and silicon carbide source material.
• This patent suggests limiting the thermal gradient to no more than 20° centigrade per centimeter of distance between source and seed in the reaction vessel.
• This patent also suggests that the overall vapor pressure in the sublimation system be kept in the range of between 1 and 5 millibar and preferably around 1.5 to 2.5 millibar.
• Tairov suggests use of a seed as a method of improving the Lely process.
• Tairov suggests controlling the polytype growth of the silicon carbide crystal by selecting seed crystals of the desired polytype or by growing the recondensed crystals on silicon carbide faces worked at an angle to the 0001 face of the hexagonal lattice.
• Tairov suggests axial temperature gradients for growth of between approximately 30° and 40° centigrade per centimeter.
• Tairov investigated the effects of adjusting various parameters on the resulting growth of silicon carbide, while noting that particular conclusions are difficult to draw.
• Tairov studied the process temperatures and concluded that growth process temperature was of relatively smaller importance than had been considered by investigators such as Knippenberg.
• Tairov likewise was unable to draw a conclusion as to the effect of growth rate on the formation of particular polytypic crystals, concluding only that an increase in crystal growth rate statistically corresponds to an increase in the percentage of disordered structured crystals.
• Tairov was similarly unable to draw any conclusions between vapor phase stoichiometry and crystal growth, but pointed out that certain impurities will favor the growth of particular silicon carbide polytype crystals. For example, high nitrogen concentrations favor cubic polytype silicon carbide crystals, aluminum and some other materials favor the growth of hexagonal 4H polytype, and oxygen contributes to the 2H polytype.
• Tairov concluded that no understanding of the mechanisms leading to these effects had yet been demonstrated.
• FIG. 1 is a cross-sectional diagram of a sublimation crucible used in accordance with the method of the present invention
• FIG. 2 is an enlarged view of the seed crystal holder of the crucible of FIG. 1;
• FIG. 3 is a cross-sectional diagram of a sublimation furnace used in accordance with the method of the present invention.
• FIG. 4 is a diagram of a sublimation system illustrating a screw type mechanism for continuously introducing silicon carbide source powder into a system
• FIG. 5 is a diagram of a sublimation system showing a gas feed mechanism for introducing silicon carbide precursor materials into the sublimation system
• FIG. 6 is a diagram of a sublimation system illustrating independent heating elements used in accordance with the method of the present invention.
• FIG. 1 illustrates a cross-sectional view of a sublimation crucible used in accordance with the method of the present invention.
• the crucible is broadly designated at 10 and is typically formed of graphite.
• Crucible 10 is generally cylindrical in shape and includes a porous graphite liner 11, a lid 12, and a seed holder 13, an enlarged view of which is illustrated in FIG. 2.
• the remainder of the crucible is defined by the walls 14 and the floor 15.
• the porous graphite liner 11 is formed in such a manner as to provide an annular chamber 16 between lower portions of the porous graphite liner 11, the crucible walls 14 and the crucible lid 12.
• a central sublimation chamber is illustrated at 20.
• the crucibles described are preferably formed of graphite and most preferably of a graphite which has approximately the same coefficient of thermal expansion as silicon carbide. Such materials are commercially available. The relative similarities of thermal coefficients of expansion are a particular requirement for materials which are being heated to the extremely high temperatures described herein and at which these processes take place. In this manner, the crucible can be prevented from cracking during the sublimation process and the lifetime of the crucible will generally be increased.
• graphite is one of the few economically viable materials which can both withstand the high temperatures of these processes and avoid introducing undesired impurities into the vapor flux.
• FIG. 2 also shows an optical opening 22, which in preferred embodiments of the invention provides optical access to the seed so that the temperature of the seed can be monitored with an optical pyrometer.
• a sublimation crucible such as illustrated in FIG. 1 is typically used in conjunction with a sublimation furnace broadly designated at 23 in FIG. 3, in which the crucible is again designated 10.
• Furnace 23 is generally cylindrical in shape and includes a cylindrical heating element 24, opposite portions of which are shown in the drawing.
• Furnace 23 is also surrounded by carbon fiber insulation 25 and includes optical ports 26, 27, and 28 through which optical pyrometers can measure the temperature of portions of the interior of the furnace.
• a power feed-through is generally designated at 30 and the outer housing of the furnace at 31.
• a single seed crystal of silicon carbide having a desired polytype and silicon carbide source power are introduced into a system such as the sublimation crucible and furnace illustrated in FIGS. 1-3. Where the crucible is of the type illustrated in FIG. 1, the silicon carbide source powder is positioned in the annular chamber 16.
• the silicon carbide source powder substantially all of which has a constant polytype composition, the production of a desired crystal growth upon the seed crystal can be greatly improved.
• flux refers to the amount of matter or energy passing through a designated plane of a given area during a given period of time. Accordingly, when used to describe the flow of vaporized species, flux can be measured and designated in units of matter, area and time such as grams per square centimeter per second (g/cm 2 /sec).
• the term "constant polytype composition” refers to a source powder or powders which are made up of a constant proportion of certain polytypes, including single polytypes.
• a source powder which was formed substantially entirely of 6H alpha silicon carbide would exhibit a constant polytype composition, as would source powder that was 50 percent alpha polytype and 50 percent beta polytype.
• the composition--whether homogeneous or heterogeneous with respect to polytypes--must be controlled so as to remain the same throughout the sublimation process.
• the source powder is selected and controlled so that substantially it has a constant polytype composition, the relative amounts or ratios of Si, Si 2 C, and SiC 2 which are generated will remain constant and the other parameters of the process can be appropriately controlled to result in the desired single crystal growth upon the seed crystal.
• the source powder is a variable mixture of various proportions of polytypes of silicon carbide, the relative amounts (ratios) of Si, Si 2 C, and SiC 2 which are generated will continually vary and correspondingly continually encourage alternative polytypes to simultaneously grow upon the seed crystal. This results in growth upon the seed crystal of a number of crystals of different polytypes, an undesirable result.
• the temperature of the silicon carbide source powcer is raised to a temperature sufficient for silicon carbide to sublime from the source powder, typically a temperature on the order of 2300° C. While the temperature of the source powder is being raised, the temperature of the growth surface of the seed crystal is likewise raised to a temperature approaching the temperature of the source powder, but lower than the temperature of the source powder and lower than that at which silicon carbide will sublime. Typically, the growth surface of the seed crystal is heated to about 2200° C.
• a powder having the following particle size distribution enhanced the process, the distribution being defined by the weight percentage of a sample which will pass through a designated Tyler mesh screen:
• the exposed surface area of the source powder is proportional to the particle size.
• a consistency in exposed surface area in turn enhances the overall consistency of the vapor flux, so that controlling the size distribution in this manner enhances the consistency of the flux profile.
• the silicon carbide source powder and the growth face of the seed crystal are both heated to respective different temperatures, with the growth face of the seed crystal being somewhat cooler than the source powder so as to encourage condensation of the sublimed species from the source powder onto the seed crystal.
• the thermal gradient can be controlled in a number of ways. For example, under certain circumstances the thermal gradient is controlled so as to remain constant between the growth surface of the seed crystal while under other circumstances, controllably changing the thermal gradient between the source powder and the growth surface of the seed crystal is preferred.
• a thermal gradient is often introduced by physically separating the source powder from the seed crystal while they are being maintained at their respective different temperatures.
• the resulting thermal gradient is thus a function of geometric separation between the source powder and the growth surface of the seed crystal; e.g. 20° C. per centimeter and the like.
• the source powder is initially maintained at a temperature of, for example, 2300° C.
• the growth surface of the seed crystal is maintained at a temperature of, for example, 2200° C. and a distance of 10 centimeters is initially maintained between the source powder and the seed crystal
• a thermal gradient of 100° C. divided by 10 centimeters, i.e. 10° C. per centimeter will be established.
• the invention comprises introducing the seed single crystal of silicon carbide of a desired polytype and a silicon carbide source powder into a sublimation system.
• the temperature of the silicon carbide source powder is raised to a temperature sufficient for the silicon carbide to sublime and a thermal gradient is introduced between the growth surface of the seed crystal and the source powder by elevating the temperature of the seed crystal to a temperature approaching the temperature of the source powder, but lower than the temperature of the source powder and lower than that at which silicon carbide will sublime, under the vapor pressure conditions of the system.
• the thermal gradient between the growth surface of the seed crystal and the source powder is increased to thereby continuously encourage further crystal growth beyond that which would be obtained by maintaining a constant thermal gradient.
• gas species which contain silicon carbide evolve near the hotter top of the crucible and are transported via the thermal gradient to the seed at its respective lower temperature in the cooler lower portion of the crucible.
• the source material is also in the thermal gradient and sublimation of the source material tends to occur at a much faster rate in the upper portion of the source material than in the lower portion.
• the temperature gradient remains constant, a rapid decrease in flux with time occurs as the upper source material is depleted.
• the crystal grows, its growth surface increases in temperature as a result of its change in position with respect to the thermal gradient. This causes a decrease in the sticking coefficient as a function of time and likewise reduces the growth rate.
• the absolute temperature difference between the source and seed can be maintained at an amount which continues to be most favorable for crystal growth.
• control of the thermal gradient comprises the step of increasing the thermal gradient between the growth surface of the seed crystal and the source powder, and the same is accomplished by increasing the temperature of the source powder while maintaining the temperature of the growth surface of the seed crystal at the initial lower temperature than the source powder.
• the invention comprises maintaining a constant thermal gradient as measured between the growth surface of the seed crystal and the source powder as the crystal grows and as the source powder is used up. It will be understood that the temperature of the growth surface is the most critical temperature with respect to the crystal as the growth surface is the surface at which thermodynamic conditions will either favor or disfavor continued desired growtth of the crystal.
• the step of maintaining a fixed thermal gradient between the growth surface of the seed crystal and the source powder comprises providing relative movement between the growth surface of the seed crystal and the source powder as the seed crystal grows while maintaining the source powder and the growth face of the seed crystal at their respective different, but constant, temperatures.
• the step of maintaining a fixed thermal gradient between the growth surface of the seed crystal and the source powder comprises maintaining a fixed geometric distance between the growth surface of the seed crystal and the source powder as the crystal grows.
• the method of maintaining a constant thermal gradient between the growth surface of the seed crystal and the source powder can comprise independently controlling the source powder and seed crystal temperatures by separately monitoring the temperature of the source powder and the temperature of the seed crystal and separately adjusting the temperature of the source powder and the temperature of the seed crystal maintain the desired thermal gradient.
• thermo gradient control and indeed the entire process of controlling and maintaining temperatures can be enhanced by using resistance heating, rather than radio frequency (RF) induction heating in the method of the present invention.
• RF radio frequency
• Resistance heating offers a number of advantages in the overall sublimation process.
• Induction heating techniques have several limitations which prevent any silicon carbide sublimation processes developed using induction techniques from being similarly scaled up to useful commercial scales.
• the induction coil must be positioned outside of the vacuum vessel in which the sublimation takes place in order to prevent ionization of the gas (e.g. argon) present in the vessel.
• the gas e.g. argon
• induction heating requires the use of a glass vacuum vessel to transmit the RF power.
• thermal insulation present must be increased in thickness or the glass must be cooled, typically with water.
• Increasing the amount of thermal insulation reduces the practical size of the crystal that can be grown, and cooling the vessel with water dramatically reduces the energy efficiency of the entire system.
• resistance heating is significantly more energy efficient than induction heating
• resistance heating elements can be present within the vacuum vessel, skin heating or radial thermal gradient effects are almost entirely eliminated, and resistance heating permits improved temperature stability and repeatability of processes and control over the entire thermal gradient.
• FIGS. 4, 5 and 6 illustrate some of the apparatus which can be used to accomplish the methods of the present invention.
• FIG. 4 shows a silicon carbide seed crystal 32 upon which a growing crystal 33 has epitaxially attached.
• the respective crystals 32 and 33 are maintained upon a graphite seed holder 34 which in turn is positioned upon a shaft 35.
• the remainder of the crucible is defined by graphite walls 36 and a porous graphite barrier 37.
• the silicon carbide source powder 40 is maintained in a bed 41.
• a rotating shaft 42 which carries a screw lifting mechanism 43 is positioned with a high density graphite cylinder 44. As illustrated in FIG.
• the screw mechanism 43 will lift silicon carbide source powder 40 to the top of the screw mechanism to a position adjacent the porous graphite barrier 37.
• the silicon carbide source powder at the top of the high density graphite cylinder 44 is maintained at a temperature of about 2300° C., while the temperature of the growth surface of the growing crystal 33 is maintained at a somewhat lower temperature, typically 2200° C.
• the continuous supply further ensures that the subliming source powder generates a consistent flux density.
• new source powder is continuously moved into the sublimation area, providing a constant flux as sublimation proceeds.
• An optical sight hole 45 is also illustrated, and can be used to either monitor the temperature of the growing crystal 33 using an optical pyrometer or to determine the exact position of the crystal with respect to the silicon carbide source powder 40 at the top of the high density graphite cylinder 44.
• the shaft 35 can be pulled in a manner which moves the growth face of the growing crystal 33 away from, or if desired towards, the silicon carbide source powder 40.
• the shaft can be rotated to ensure that the temperature profile across the growth face is constant.
• the crystal can be encouraged to grow symmetrically as the effect of flux variations are dampened out and the growing crystal can be prevented from attaching itself to the graphite enclosure.
• FIG. 6 illustrates a number of the same features as FIG. 4, but with the separate and independent heating elements illustrated.
• the separate and independently controlled resistance heating elements are shown at 46 and 47.
• the upper element 46 can be used to control the temperature of the seed crystal 32 and the growing crystal 33, while the lower heating element 47 can be used to control the temperature of the silicon carbide source powder 40 at the top of the high density graphite cylinder 44.
• optical sight holes 50 and 51 are provided to permit optical pyrometers to monitor the temperatures generated.
• FIG. 5 illustrates an apparatus used to carry out yet another embodiment of the invention.
• the silicon carbide which sublimes and then recondenses as the growing crystal is not supplied as a powder, but instead is introduced into the system by providing respective gas feeds of silane (SiH 4 ) and ethylene (C 2 H 4 ) into the system at a temperature at which they will immediately react to form silicon carbide vapors which will then migrate in the manner in which vapors generated from source powders will migrate through the porous graphite barrier and onto the growing crystal.
• silane SiH 4
• ethylene C 2 H 4
• the system includes seed crystal 32, growing crystal 33, graphite seed holder 34, shaft 35, graphite walls 36, porous graphite barrier 37, and the optical sight hole 45.
• the system includes a silane gas feed 52 and an ethylene gas feed 53.
• silane gas feed 52 In order to keep these molecules from dissociating under the high temperatures of the system, they are insulated in a water cooled molybdenum jacket until they reach a point in the sublimation system where the temperature is maintained at approximately 2400° C., and at which the materials are released and immediately react to form silicon carbide.
• silane and ethylene have left the jacket 54 and have reacted to form silicon carbide containing species, they behave in the same manner as would silicon carbide containing species which had sublimed from a source powder. They pass through the porous graphite barrier 37 and lodge upon the growth face of the growing crystal 33.
• a seed was prepared from a 6H alpha polytype silicon carbide.
• the seed crystal was lapped to insure flatness and then polished with progressively smaller sized diamond paste, finishing with a 0.1 micrometer paste.
• the seed was cleaned in hot sulfuric acid (H 2 SO 4 ) for a period of five minutes, in a one-to-one mixture of ammonium hydroxide (NH 4 OH) and hydrogen peroxide (H 2 O 2 ) for five minutes, in hydrofluroic acid (HF) for one minute, and then finally rinsed in deionized water.
• the seed was oxidized in dry oxygen at 1200° C. for 90 minutes to remove residual polishing damage. The oxide was removed by etching with HF.
• the seed and source powder were then loaded into the crucible.
• the source powder consisting of 6H silicon carbide grains having the following size distribution:
• the loaded crucible was then placed in the sublimation furnace while a slight overpressure of argon was maintained in the furnace to inhibit water contamination, and thus reducing the furnace pump down time.
• the furnace was evacuated to a base pressure below 5 ⁇ 10 -6 Torr.
• the furnace was heated in a vacuum (5 ⁇ 10 -4 Torr) to 1200° C. for about ten minutes. It will be understood by those familiar with low pressure systems that an absolute vacuum can never be achieved. Therefore, the term "vacuum” as used herein refers to various systems which are at pressures less than atmospheric pressure, and where appropriate, specific pressures will be employed to best describe the particular conditions.
• the furnace was then backfilled with argon to a pressure of 400 Torr.
• the temperature of the system was then increased until the top of the crucible is approximately 2260° C. and the temperature of the seed is approximately 2160° C., which in the particular system used corresponded to a thermal gradient of 31° C. per centimeter (cm).
• the system was then evacuated slowly over a period of 85 minutes from the pressure of 400 Torr to a pressure of about 10 Torr.
• the system was maintained under these conditions for six hours, after which the system was backfilled with argon to 760 Torr and the temperature reduced to 200° C. over a period of 90 minutes.
• a 6H Alpha-SiC seed was prepared by cutting the (0001) plane 3° towards the [1120] direction. The seed was then lapped to assure flatness, polished with progressively smaller diamond paste, cleaned, oxidized and etched, all as described in Example 1.
• the source material was doped with aluminum in a quantity of 0.2 weight percent.
• the seed and source powder having the same powder size distribution as set forth in Example 1.
• the crucible was loaded, the vessel evacuated, initially heated, and backfilled with argon, all as set forth in Example 1.
• the temperature was then increased until the top of the crucible was 2240° C. and the seed was 2135° C., corresponding to a thermal gradient of 32° C./cm.
• the furnace was evacuated from 400 Torr to 10 Torr as described in Example 1 and the sublimation conditions were maintained for a period of four hours.
• the furnace was then backfilled with argon to atmospheric pressure (760 Torr) and the temperature reduced to 200° C. over a period of 90 minutes.

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Abstract

Description

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Tyler Mesh Screen                                                         
               Weight Percent Passed                                      
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20-40          43%                                                        
40-60          19%                                                        
 60-100        17%                                                        
Over 100       21%                                                        
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              Percentage Passing Through                                  
Tyler Mesh Size                                                           
              (By Weight)                                                 
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20-40         43 percent                                                  
40-60         19 percent                                                  
 60-100       17 percent                                                  
Over 100      21 percent                                                  
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Claims (29)

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Tyler Mesh Screen                                                         
               Weight Percent Passed                                      
______________________________________                                    
20-40          43%                                                        
40-60          19%                                                        
 60-100        17%                                                        
Over 100       21%                                                        
______________________________________                                    

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• 1987
  • 1987-10-26 US US07/113,565 patent/US4866005A/en not_active Ceased
• 1988
  • 1988-10-25 CA CA000581145A patent/CA1331730C/en not_active Expired - Lifetime
  • 1988-10-26 DE DE3856514T patent/DE3856514T2/en not_active Expired - Lifetime
  • 1988-10-26 JP JP50938588A patent/JP3165685B2/en not_active Expired - Lifetime
  • 1988-10-26 EP EP95202796A patent/EP0712150B1/en not_active Expired - Lifetime
  • 1988-10-26 DE DE1143493T patent/DE1143493T1/en active Pending
  • 1988-10-26 DE DE3855539T patent/DE3855539T3/en not_active Expired - Lifetime
  • 1988-10-26 KR KR1019890701158A patent/KR970008332B1/en not_active IP Right Cessation
  • 1988-10-26 EP EP01201980A patent/EP1143493A3/en not_active Withdrawn
  • 1988-10-26 EP EP88910210A patent/EP0389533B2/en not_active Expired - Lifetime
  • 1988-10-26 WO PCT/US1988/003794 patent/WO1989004055A1/en active IP Right Grant
• 1990
  • 1990-10-09 US US07/594,856 patent/USRE34861E/en not_active Expired - Lifetime
• 2000
  • 2000-03-14 JP JP2000070469A patent/JP2000302600A/en active Pending

Patent Citations (22)