A DEVICE FOR HEAT TREATMENT OF OBJECTS AND A METHOD FOR PRODUCING A SUSCEPTOR

Abstract: A device for heat treatment of objects (11) comprises a susceptor (7) adapted to receive an object and means (9) for heating the susceptor and thereby said object. The susceptor comprises an inner (17) and an outer (18) circumferential wall forming a closed space (19) therebetween and the inner of which defines a room (13) for receiving said object. Said space (19) is filled with a powder.

Description: A device for heat treatment of objects and a method for producing a susceptor TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a device for heat treatment of ob¬ ject comprising a susceptor adapted to receive an object and means for heating the susceptor and thereby said object as well as a method for producing a susceptor according to the preambles of the corresponding appended independent claims. Accordingly, the invention relates to all kinds of heat treatment of objects in which an object is received in a susceptor and heated by heating said susceptor. Such devices presently known for such heat treatments involves a number of problems, which may by way of an non-limitative example be illustrated in the case of epitaxially growing crystals, i.e. single crystals, by Chemical Vapour Deposi¬ tion (CVD). Furthermore, said problems being in common for grow¬ ing crystals by CVD will below be discussed for the particular case of growing crystals of SiC, but similar reflections are valid for such growth of crystals of other materials, such as in particular Group Ill- nitrides and all types of alloys of such nitrides and/or SiC. This growth is mostly carried out while at the same time intentional dope the object grown. SiC single crystals are in particular grown for being used in different types of semiconductor devices, such as for example different types of diodes, transistors and thyristors, which are intended for applica¬ tions in which it is possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to function well under extreme conditions. The large bandgap between the valance band and the conduction band of SiC makes devices fabricated from said material able to operate at high tem¬ peratures, namely up to 1000K. However, high temperatures are needed for obtaining a good or- dered growth thereof. The epitaxial growth of silicon carbide by Chemical Vapour Deposition is therefore carried out in a tempera¬ ture regime in excess of 1400°C. These high temperatures are needed both to obtain decomposition by cracking of Si- and C-con- taining precursor gases of the gas mixture used for the CVD and to ensure that the atoms are deposited on the surface of the substrate on which said crystal is grown in an ordered manner. However, high temperatures also mean problems with impurities coming out of different types of material, so that the temperature could until now for the growth of SiC not be raised above a temperature interval of 1400-1700°C, resulting in a rather low growth rate (some μm per hour) that it is out of the question to grow boules for forming i.a. substrates by using CVD, so this method is only used for growing objects in the form of layers. However, it is not possible to even grow layers of SiC by CVD through the devices already known at such a high growth rate that a commercial production thereof will be commercially interesting. A raise of the temperature for increasing said growth rate could until now not be tried, since this would have resulted in rapid degrada- tion of the walls of the susceptor due to an increased etching of hot spots thereof leading to an unacceptable incorporation of impurities therefrom into the layers grown. As already mentioned, it is due to said low growth rates impossible to grow boules, which require growth rates in the order of millime¬ tres per hour, by CVD, so that the seeded sublimation technique is presently used for growing boules, which may then be sliced to substrates. However, the crystalline quality of the boules grown by this technique is low in comparison with that of the SiC layers epi- taxially grown by CVD. The substrates produced in this way are perforated by small holes called micropipes or pinholes, which limit the device area considerably which consequently make high-power devices of SiC not commercially interesting yet. In the seeded sub¬ limation technique the source is a SiC powder that sublimes, whereupon the gas species are transported by a temperature gradi¬ ent to the seed crystal where the growth occurs. The conditions in the gas phase are governed by thermodynamics only, which makes it difficult to keep the C/Si ratio constant due to Si vapour leakage out of the system. Furthermore, the purity of the gases used for Chemical Vapour Deposition are several orders of magnitude higher than that of the source material used for seeded sublimation growth. The prior art includes devices using susceptors of graphite, which contain major amounts of unwanted impurities with a tendency to be released and deposited on the crystal grown when the susceptor is heated. This problem has been partially solved by applying a SiC- coating on the susceptors and additionally a plate of SiC can be placed underneath the substrate. However, after a while the SiC- coating of the susceptor may be removed, especially by etching, and impurities are introduced into the gas-phase. Cracks in the SiC- coating may also be produced due to stresses arising between the different materials, namely SiC and the graphite, as a result of tem¬ perature changes. In most cases when other crystals than SiC is grown the tempera¬ ture is lower and therefore the impurity problems due to high tem- peratures is not as accentuated as for SiC, but the problems with etching of the susceptor or protective coating thereof, cracks of such a coating and so on remain. As already mentioned the invention is applicable to all types of heat treatments of this kind, and the problem that the susceptor sets an upper limit for the temperature that may be used for the heat treat¬ ment as well as the problem with impurities coming out from the susceptor are in common therefore. SUMMARY OF THE INVENTION The object of the present invention is to provide devices of the type defined in the introduction finding remedies to the inconveniences discussed above, so that such heat treatments may be carried out at higher temperatures and/or with less impurity problems. This object is in accordande with the invention obtained by provid¬ ing such a device with a susceptor that comprises an inner and an outer circumferential wall forming a closed space therebetween and the inner of which defines a room for receiving said object, and that said space is filled with a powder. Thanks to the arrangement of said powder within said space there will in the case discussed above of growing SiC by CVD, and for the rest in no other heat treatment either, be any risk of occurance of any cracks at the inner surface of the susceptor as in the case of graphite coated by SiC. Furthermore, when a Rf-field radiating means is used for heating the susceptor, which is usually the case, the Rf-field will couple very well to that powder and an efficient heating will be ac- complished. But the major advantage of this device is the possibility of raising the temperature without formations of irregularities of the temperature distribution of the walls of the susceptor independent of the cross section shape thereof if a powder with sublimation capac¬ ity is used and said heat treatment is carried out by heating the susceptor at a temperature above the sublimation temperature of said powder. In such cases, if there is a hot-spot on the susceptor for example due to a thinner thickness of the susceptor wall there, the powder will sublime and be deposited at a colder spot. Due to the smaller amount of powder at the hot-spot the Rf-coupling will decrease at this point and the temperature will be lowered until the temperature is uniform again. In this way very uniform heating can be accomplished which adapts itself to external circumstances (gas flow, external cooling etc). This means that it will be possible to raise the temperature without any further measures in comparison to already known susceptors, so that the temperature for the entire susceptor may be raised to and above a level which said hot-spots had in the susceptors according to the prior art without giving rise to additional problems with incorporation of impurities from the sus¬ ceptor walls in the object grown, which in the case of growing crys¬ tals of SiC by Chemical Vapour Deposition means a possibility to grow such crystals of high quality with a considerably higher growth rate than before, so that it will even be possible and thereby very interesting to grow boules of SiC by using the CVD-technique and the advantages thereof in comparison to the seeded sublimation technique. According to a preferred embodiment of the invention the device is adapted for heating an object in the form of a substrate and a gas mixture fed to the substrate for epitaxial growth of a crystal on said substrate by Chemical Vapour Deposition. This embodiment has the advantages discussed above, and these are especially present when crystals of SiC, a Group Ill-nitride or alloys thereof are grown. According to a further preferred embodiment of the invention said walls on the susceptor are made of SiC, an alloy of SiC and the material grown or the material grown. This means that the walls of the susceptor may not release any material which may detrimentally influence the growth of said crystal. According to a preferred embodiment of the invention at least the inner wall of the susceptor is made of SiC. This constitutes a great advantage, since SiC is a material forming an excellent diffusion barrier, so that it is very interesting as a material for said susceptor wall also when other crystals than SiC is grown, for instance crys¬ tals of AIN. According to another preferred embodiment of the invention said powder is made of SiC, an alloy of SiC and the material grown or the material grown, which means that it would not be harmful even if there would be a diffusion of material through the susceptor walls or a crack therein, which in any way will not result. According to another preferred embodiment of the invention said powder is selected from a group of materials having sublimation ca- pacity and a sublimation temperature being lower than the tempera¬ ture at which the susceptor walls are intended to be heated by said heating means due to said growth. This will as discussed further above result in a uniform temperature of the susceptor walls making it possible to use higher temperatures and thereby achieving higher growth rates and even the possibility to use the Chemical Vapour Deposition technique to grow boules, in particular of SiC. The invention also comprises a device for heat treatment of an ob- ject comprising a susceptor adapted to receive an object and means for heating the susceptor and thereby said object, which is charac¬ terized in that the susceptor is made of SiC powder heat treated to the shape of the susceptor. Such a susceptor will due to the nature of SiC have no tendency to release any impurities even at very high temperatures, so that it will be especially well suited for such heat treatment in the form of growing crystals of in particular SiC by Chemical Vapour Deposition at high temperatures, so that such growth may take place at higher temperatures than used before and thereby higher growth rates may be achieved, so that even the CVD technique may be used for growing boules. According to a preferred embodiment of the device last mentioned according to the invention the susceptor is made of SiC powder ex¬ certed to hot isostatic pressing to the shape of the susceptor. A susceptor manufactured in this way will be compact and strong with no blisters therein. This manufacturing technique also makes it possible to produce the susceptor in one step without the need of any further treatment afterwards. Furthermore, the invention also comprises a method for producing a susceptor in this advantageous way according to the appended independent method claim. According to a preferred embodiment of the invention the device is adapted for growing layers, which may be grown at a higher tem¬ perature and thereby with a higher growth rate than before or at the same temperature as before but with less impurity problems. According to a still further preferred embodiment of the invention the device is adapted for growing boules and said substrate is a seed crystal. This is as mentioned above possible due to the higher temperatures possible as a consequence of the construction of the susceptor, so that the boules having a superior crystalline quality to that obtained by using the seeded sublimation technique may be obtained at commercially interesting growth rate through the High Temperature Chemical Vapour Deposition (HTCVD) according to the invention. Further preferred features and advantages of the device and the method according to the invention will appear from the following de¬ scription of the other dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a specific description of a preferred embodiment of the invention cited as an example. In the drawings: Fig 1 is a longitudinal cross-section view showing a device for epi- taxially growing crystals, especially of SiC, by Chemical Vapour Deposition, in which a device according to the invention may be in¬ corporated, Fig 2 is a longitudinal cross-section view of a device according to a first preferred embodiment of the invention, Fig 3 is an end view of the device according to Fig 2, Fig 4 is a perspective view of a device for heat treatment of objects according to a second preferred embodiment of the invention, and Fig 5 is an end view of a device according to a third preferred em¬ bodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig 1 shows schematically a device for epitaxially growing crystals, especially of SiC, by Chemical Vapour Deposition on a SiC-sub- strate in a simplified manner, and it is obvious that this device also comprises other means, such as pumps. The device according to the invention may be incorporated in a device of this kind and this device will therefore be described for better understanding of the invention and the problems solved thereby. The device shown in Fig 1 comprises a vacuum casing 1 constituted by a tube 2 of quartz extending substantially vertically and two opposite end flanges 3 and 4. A conduit 5 for supplying a stream of a gas mixture intended for the growth of a crystal is introduced through the lower end flange 3. The conduit 5 is connected to separate conduits leading to sources for the respective components of said gas mixture and these conduits are provided with flow regulating means not shown for regulating the content of each component in said gas mixture as desired. Furthermore, the device comprises a funnel 6 for concentrating the gas flow from the conduit 5 into a susceptor 7. The space surround¬ ing the susceptor is enclosed and filled by graphite foam 8 for ther- mal insulation for protecting the surrounding quartz tube 2. Rf-field radiating means 9 in the form of a Rf-coil surrounds the tube 2 along the longitudinal extension of the susceptor 7. This heating means 9 is arranged to radiate a Rf-field uniformly heating the sus¬ ceptor and thereby the gas mixture introduced into the susceptor. The susceptor 7 comprises a lid 10, on the lower side of which a SiC substrate 1 1 is arranged and which may be removed from the rest of the susceptor so as to remove the substrate after a crystal has been grown thereon. The lid 10 is provided with peripheral gas outlet holes 12, so that a preferable laminar gas flow will enter the susceptor room 13 through the lower inlet 14 and flow close to the substrate and leave the susceptor through the upper outlets 12 and then the device through a conduit 15 connected to a pump not shown. Due to the fact that the gas outlets are in the same end of the susceptor as the substrate, this susceptor will be most suited for growing layers, but it could also be used for growing boules as long as the dimensions thereof are not so great that they obstruct said outlets. For growing boules the outlets are preferably made at the opposite end of the susceptor. The temperature inside the susceptor 7 may be checked pyrometri- cally by looking into the susceptor 7 through a window indicated at 16. The general function of this device is as follows: a gas mixture in¬ cluding Si- and C-containing precursor gases and one or more car- rier gases is led through the conduit 5 in the direction of the susceptor inlet 14. The Si-containing precursor gas is preferably silane, while the C-containing precursor gas is propane or eventu¬ ally methane. The carrier gas may be H2, but there may also be a content of a non-etching carrier gas, such as Ar therein. The heat- ing means 9 will heat the susceptor walls through which the precur¬ sor gases entering the susceptor will be heated and cracked into Si- and C-atoms, which will be transported to the substrate 1 1 and de¬ posited thereon for epitaxially growing layers of SiC thereon. The present invention resides in the construction of a susceptor suitable for use in a device of the type described above and shown in Fig 1 , although the invention also comprises susceptors for other types, in principle all types, of heat treatment. A susceptor accord¬ ing to a preferred embodiment of the invention is shown in Figs 2 and 3. This has an inner 17 and an outer 18 circumferential wall forming a closed space 19 therebetween, wherein the inner wall 17 defines a room 13 for receiving a substrate 1 1 to be heated by heating of the susceptor walls. The space 19 is sealed at the end by end walls 20. The inner and outer walls 17 and 18, respectively, are preferably formed by a tube, and the space 19 is sealed at the ends by circular plates having a recess, in this case substantially rectan¬ gular, corresponding to the cross section of the tube forming the in- ner wall 17. Said plates 20 may be glued to the tubes with for in¬ stance molten sugar. The space 19 is filled with a powder 21 . The walls of the susceptor are thin with respect to the distance therebetween, and typical dimensions are when the susceptor has an internal diameter of 30 mm, a wall thickness of 0,3-2 mm and a distance between the walls of 15 mm. In the case of growing a crystal of SiC, a Group Ill-nitride or alloys thereof, the walls 17 and 18 are preferably made of SiC, an alloy of SiC and the material grown or the material grown. The end walls are preferably of the same material as the walls. When a crystal of SiC is grown it is most preferred that said walls 17, 18 and 20 are made of SiC. The powder 21 may be made of SiC, an alloy of SiC and the material grown, when a crystal of SiC, a Group Ill-nitride or alloys thereof is grown by Chemical Vapour Deposition, and it is most preferable of SiC when SiC is grown. The function of the device shown in Figs 2 and 3 is the following when a crystal of SiC is grown on the substrate 1 1. The Rf-heat radiating means 9, indicated by the dashed line in Fig 3, surrounding the susceptor heats the susceptor by radiating a Rf-field, and this field will couple very well to the powder 21 of SiC, and due to the fact that there will be no stresses between said powder and the walls of the susceptor there will be no risk of formation of cracks in said walls, so that the qual¬ ity of the powder may be comparatively low, which in its turn makes the Rf-field to even better couple to the powder. Accordingly, the heating of the susceptor will take place in an efficient way, so that high temperatures may easily be achieved. If the susceptor is heated above the temperature at which the SiC powder starts to sublime to a considerable extent, i.e. above approximately 1 800°C, this powder will see to that the susceptor walls will get a uniform temperature. If there is a hot-spot, the powder will sublime and be deposited at a colder spot. Due to the smaller amount of powder at the hot-spot the Rf-coupling will decrease at this point and the tem¬ perature will be lowered until the temperature is uniform again. Temperatures above 1900°C and in particular temperatures used when boules of SiC are produced by a conventional seeding subli¬ mation technique, i.e. temperatures of 2000°C-2500°C, may be ob- tained when using this susceptor without any introduction of exces¬ sive amounts of impurities into the crystal grown. Thus, such high growth rates may be achieved, that it will be commercially interest¬ ing to grow both layers and boules by using this so called High Temperature Chemical Vapour Deposition technique. Fig 4 shows a device according to a second preferred embodiment of the invention in the form of a cylindrical container 22 for heat treatment of objects, which is made of two concentric tubes of the type in the embodiment according to Figs 2 and 3 and a powder in the space confined therebetween. This container 22 has a bottom 23 of SiC glued with molten sugar to the two tubes and at the other end a ring 24 of SiC, which may also be glued with molten sugar to the tubes. Such a container may be used for all types of heat or thermal treatment of samples or anything else. It is emphasized that the powder may be of any material, but that it is particularly advan¬ tageous to use a powder of a material which will sublime at a tem¬ perature lying below the temperature at which said heat treatment is intended to take place, so that a perfect uniformity of the tempera- ture of the susceptor will be obtained. Fig 5 shows a cross section view of a device according to a third preferred embodiment of the invention, and this susceptor has sub¬ stantially the same cross section as the susceptor according to Figs 2 and 3. However, this susceptor is made of a single uniform wall 25 of SiC and has been produced by hot isostatic pressing (HIP) of SiC powder to this shape with appropriate equipment, through which it was possible to bring the individual granules of the powder to be bound strongly to each other without any blisters therebetween, so that a compact and very strong susceptor is obtained in one single step without any need of further treatment afterwards, which in the case of SiC would be very difficult due to the specific characteris¬ tics, such as hardness, of SiC. Accordingly, a susceptor made of SiC may in this way be obtained easily and to low cost, since SiC does not have to be machined. It should be noted, that the size of the individual granules of the susceptor of Fig 5 has been enor¬ mously exaggerated for better explaining the invention. A susceptor of this kind may be heated to very high temperatures without caus¬ ing any impurity problems in the heating process, such as when for instance a SiC crystal is epitaxially grown by Chemical Vapour Deposition. The invention is of course not in any way restricted to the preferred embodiments of the device and method described above, but seve¬ ral possibilities to modifications thereof are apparent for a man skilled in the art without departing from the basic idea of the inven- tion. All definitions concerning the material of course also include inevi¬ table impurities as well as intentional doping. The susceptor may have different shapes and dimensions than as shown in the Figures. The claim definition "a material not influencing the object at the temperature at which the heat treatment is intended to take place" includes a material not influencing the object because it will not it¬ self be deposited on said object or release impurities depositing on said object as well as because it is the same as that of the object that is being treated, so that it will not be harmful if it is deposited on said object or a crystal grown thereon.

Claims: Claims 1. A device for heat treatment of objects comprising a susceptor (7) adapted to receive an object (1 1 ) and means (9) for heating the susceptor and by that said object, characterized in that the suscep¬ tor comprises an inner (17) and an outer (18) circumferential wall forming a closed space (19) therebetween and the inner of which defines a room (13) for receiving said object, and that said space is filled with a powder (21 ). 2. A device according to claim 1 , characterized in that said walls (17, 18) are thin and that the distance between said walls is consid¬ erably greater than the thicknesses thereof. 3. A device according to claim 1 or 2, characterized in that each wall is formed by a tube (17, 18) and that said space (19) is closed at the ends by plates (20, 23, 24). 4. A device according to any of claims 1 -3, characterized in that said walls (17, 18) are made of a material not influencing the object at the temperature at which said heat treatment is intended to take place. 5. A device according to any of claims 1 -4, characterized in that it is adapted for heating an object in the form of a substrate (1 1 ) and a gas mixture fed to the substrate for epitaxial growth of a crystal on said substrate by Chemical Vapour Deposition. 6. A device according to claim 5, characterized in that said walls (17, 18) of the susceptor are made of SiC, an alloy of SiC and the material grown or the material grown. 7. A device according to claim 6, characterized in that at least the inner wall (17) of the susceptor (7) is made of SiC. 8. A device according to any of claims 5-7, characterized in that said powder (21 ) is made of SiC, an alloy of SiC and the material grown or the material grown. 9. A device according to any of claims 5-8, characterized in that said walls (17, 18) as well as said powder (21 ) are made of SiC. 10. A device according to any of claims 5-9, characterized in that said powder (21 ) is selected from a group of materials having sub- limation capacity and a sublimation temperature being lower than the temperature at which the susceptor walls (17, 18) are intended to be heated by said heating means (9) during said growth. 1 1 . A device for heat treatment of objects comprising a susceptor (7) adapted to receive an object (1 1 ) and means (9) for heating the susceptor and by that said object characterized in that the suscep¬ tor is made of SiC powder heat treated to the shape of the suscep¬ tor. 12. A device according to claim 1 1 , characterized in that it is adapted for epitaxially growing crystals of SiC, a Group Ill-nitride or alloys thereof by Chemical Vapour Deposition on a substrate re¬ ceived in the susceptor (7), wherein said heating means is arranged to heat a gas mixture fed to the substrate for the growth by feeding means through heating of the susceptor. 13. A device according to claim 1 1 or 12, characterized in that the susceptor (7) is made of SiC powder excerted to hot isostatic pressing (HIP) to the shape of the susceptor. 14. A device according to any of claims 1 1 -13, characterized in that the susceptor walls have a substantial uniform thickness. 15. A device according to any of claims 1 -14, characterized in that said heating means is a Rf-field radiating means (9) surrounding said susceptor. 16. A device according to any of claims 5-10 or 12-15, character¬ ized in that it is adapted to be used for growing crystals of SiC, a Group Ill-nitride or alloys thereof. 17. A device according to any of claims 5-10 or 12-16, characterized in that it is adapted for growing boules and said substrate (1 1 ) is a seed crystal. 18. A device according to claim 17, characterized in that said heat- ing means (9) is arranged to heat the susceptor walls to a tempera¬ ture within the temperature range used when such boules are pro¬ duced by a conventional seeded sublimation technique. 19. A device according to any of claims 5-10 or 12-16, characterized in that it is adapted for growing layers. 20. A device according to any of claims 5-10 or 12-19, characterized in that it is adapted for growing crystals of SiC. 21. A device according to claim 20, characterized in that said heat¬ ing means is arranged to heat the susceptor walls to a temperature above 1 900°C. 22. A device according to any of claims 1 -21 , characterized in that the walls of the susceptor form a substantially cylindrical susceptor. 23. A device according to any of claims 1 -20, characterized in that the room (13) of the susceptor (7) intended for receiving said object has a substantially rectangular cross section. 24. A method for producing a susceptor (7) to be used in epitaxially growing crystals of SiC, a Group Ill-nitride or alloys thereof by Chemical Vapour Deposition on a substrate (1 1 ) received in the susceptor by heating the susceptor walls and by that said substrate and a gas mixture fed to the substrate for said growth, character¬ ized in that SiC powder is excerted to a hot isostatic pressing (HIP) to the shape desired for the susceptor for forming the susceptor.

Inventors: HERMANSSON WILLY, KORDINA OLLE, TUOMINEN MARKO, HERMANSSON, WILLY, KORDINA, OLLE, TUOMINEN, MARKO
Applicants: ABB RESEARCH LTD, HERMANSSON WILLY, KORDINA OLLE, OKMETIC LIMITED, TUOMINEN MARKO, ABB RESEARCH LIMITED, HERMANSSON, WILLY, KORDINA, OLLE, TUOMINEN, MARKO
Application year: 1996
Main classification: C30B29
IPC codes: C30B23, C30B25