WO1996023912A1

WO1996023912A1 - DEVICE FOR EPITAXIALLY GROWING SiC BY CVD

Info

Publication number: WO1996023912A1
Application number: PCT/SE1996/000071
Authority: WO
Inventors: Nils Nordell; Gunnar Andersson
Original assignee: Abb Research Ltd.
Priority date: 1995-01-31
Filing date: 1996-01-24
Publication date: 1996-08-08
Also published as: SE9500327D0

Abstract

A device for epitaxially growing SiC by Chemical Vapour Deposition on a substrate (9) comprising a susceptor (8) adapted to receive the substrate thereon, a tube (6) arranged to lead a gas mixture containing precursors for said growth closely over and past the substrate and means (10) for heating the susceptor and thereby the substrate and said gas mixture for said growth. Making the tube (6), at least close to the susceptor, of graphite improves the uniformity of the epitaxial growth of SiC on the substrate. Cooling of the walls of the tube close to the susceptor is not necessary.

Description

Device for epitaxially growing SiC by CVD

TECHNICAL FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a device for epitaxially growing SiC by Chemical Vapour Deposition on a substrate comprising a sus¬ ceptor adapted to receive the substrate thereon, a tube arranged to lead a gas mixture containing precursors for said growth closely over and past the substrate and means for heating the susceptor and thereby the substrate and said gas mixture for said growth.

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. SiC has a high thermal stability due to a large band gap between the valence band and the conduction band, such that devices fabricated from said material are able to operate at high temperatures, namely up to 1000K.

However, the high thermal stability of SiC also means that high temperatures are needed for obtaining a good ordered growth thereof. The epitaxial growth of silicon carbide by Chemical Vapour Deposition is therefore carried out in a temperature regime in ex¬ cess of 1400°C. These high temperatures are needed both to obtain decomposition by cracking of the Si- and C-containing precursor gases of said gas mixture and to ensure that the atoms are depos¬ ited on the substrate surface in a good order. The requirement of these high temperatures close to the substrate will also give rise to a high degree of heat deriving to the walls of said tube used to lead said gas mixture closely over the substrate for obtaining a good uniformity of the epitaxial layers grown on the substrate. In already known reactors for epitaxially growing SiC by Chemical Vapour Deposition this tube has been made of quartz, which however may not withstand the high temperatures close to the substrate and which therefor had to be cooled by cooling water, which means that the ceiling of said tube will get very cold, so that there will be a high temperature gradient between the susceptor and the ceiling of the tube. This will give rise to vortex formations in said gas mixture, the flow of which should be perfect laminar and uniform so as to obtain a good uniformity of the deposition of the Si- and C-atoms resulting from the cracking of the precursor gases on the substrate. Thus, the vortex formations in the gas will result in a deteriorated morphology of the SiC-layers grown, and when dopants are switched in into the gas mixture a small concentration thereof may be retained in the vortices also after the supply of dopants has been switched off and be incorporated into layers of the SiC crystal grown which have to be free from dopants or have dopants of another type. Thus, the vortex formations give rise to so called "memory effects". Further¬ more, the quartz of the tube will upon the high temperatures release impurities in the form of oxygen and metals degrading the quality of the crystal grown.

SUMMARY OF THE INVENTION

The object of the present invention is to find a remedy to the incon¬ veniences mentioned above by providing a device making it possi¬ ble to improve the uniformity of the epitaxial growth of SiC on the substrate.

This object is in accordance with the invention obtained by making said tube at least close to the susceptor of graphite. The graphite can take high temperatures, so that no cooling of the walls of the tube close to the susceptor is necessary. Thus, the temperature gradient between the susceptor and the ceiling of the tube will be within an acceptable limit, so that no detrimental vortex formation will take place and the uniformity of the deposition of the Si- and C- atoms on the substrate will be improved. Furthermore, the tube of graphite does also protect the substrate from surrounding quartz, which is mostly used as casing material of the reactor and is a source of impurities.

According to a preferred embodiment of the invention the internal wall of the tube is coated by SiC at least close to the substrate at least in the flowing direction of said gas upstreams of the substrate for preventing impurities that may be contained in the graphite of the tube to be released into the tube and incorporated in the layers grown on the substrate.

According to another preferred embodiment of the invention said tube has a low height with respect to the width thereof so as to lead said gas mixture closely above the susceptor and the substrate and a high flow rate of the gases over the substrate will be created, which will contribute to the improvement of said deposition uniform¬ ity.

According to a further preferred embodiment of the invention said tube has at least in the region adapted to receive the substrate a width well exceeding the width of said substrate, which does also result in an improved deposition uniformity, since the depletion areas of the cross-section of the gas mixture close to the lateral walls of the tube will be located at a distance to the substrate.

According to a still further preferred embodiment of the invention said heating means are arranged to heat the susceptor from below, which results in a comparatively cold ceiling of the tube in spite of a possible low height of the tube and a temperature gradient within desired limits between the susceptor and the substrate and said ceiling, so that the deposition of C- and Si-atoms from the precursor gases will be slow on the ceiling with respect to that on the sub¬ strate and by that the gas will be much less depleted by such depositions than would the ceiling have a higher temperature. This function of the device will according to another embodiment of the invention be promoted by constructing at least the part of the ceiling of the tube arranged to be located above the substrate with a small thickness, so that this ceiling part may not absorb much heat and stay comparatively cold with respect to the substrate.

Further advantages and preferred characteristics of the invention will appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWING

With reference to the appended drawing, below follows a specific description of a preferred embodiment of the invention cited as an example.

In the drawing:

Fig 1 is a simplified longitudinal cross-section view of a reactor for epitaxial growth of SiC by Chemical Vapour Deposition including a device according to a preferred embodiment of the invention and

Fig 2 is a cross-section according to A-A in Fig 1 of the reactor according to Fig 1 , in which some further unimportant details are omitted.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A reactor 1 for epitaxially growing SiC by Chemical Vapour Depo¬ sition is schematically shown in Fig 1 for explaining the principals of the invention. The reactor 1 comprises a casing 2 constituted by an outer tube 3 of quartz and end flanges 4 and 5 of stainless steel. The casing 2 delimits an inner volume in which vacuum is created by pumps not shown. The reactor further comprises an inner cell 6 of graphite in the form of a tube extending in the longitudinal direc¬ tion of the outer tube 3. A gas mixture of C- and Si-containing gases and a H2 carrier gas will be fed to the inner cell 6 through a conduit 7 penetrating the end flange 5. The inner cell 6 has thin walls (ceiling and bottom are included in this definition) with a thickness of about 2 mm and a rectangular cross-section with a low height with respect to the width thereof well exceeding the width of a substrate to be received therein, as seen in Fig 2, and a part of the bottom of the inner cell is formed by a susceptor 8 of graphite hanging in the inner cell. More exactly, in the embodiment shown said height is 2 cm and the width of the inner cell 7 cm. The susceptor 8 is arranged to receive a substrate 9 thereon for epitaxial growth of layers of SiC thereon. The sub- strate 9 may for instance be crystalline SiC, Si or any Group Ill- nitride. The reactor does also comprise a heating means 10 in the form of a Rf-field radiating coil arranged outside the casing 2 in the region of the susceptor and arranged to heat the susceptor by the Rf-field radiated thereby. Thanks to the fact that the casing 2 is made of quartz, it will be permeable to said Rf-field. The susceptor 8 is furthermore preferably coated by SiC, at least close to the substrate 9, for preventing impurities in the graphite of the suscep¬ tor from being released into the inner cell and be incorporated into the epitaxial layers of the substrate. The inner cell surfaces close to the susceptor are preferably also coated by SiC for the same rea¬ son.

The inner cell 6 is suspended in the casing by rods 1 1 of quartz or aluminium oxide, and it is made of two parts 12, 13 connectable to and removable from each other, so that the part 12 containing the susceptor (to the left in Fig 1 ) may be removed from the casing 2 after removal of the end flange 4 for checking the SiC crystal grown, replacing said SiC crystal by a new substrate, checking the state of the susceptor and so on. A susceptor may then also be removed from the inner cell for replacement or conditioning.

The reactor also comprises a tube 14 of quartz with a somewhat smaller diameter than the outer tube 3 and which in a room 22 defined thereby receives the inner cell and the susceptor. The inner tube 14 bears on the bottom of the outer tube 3. The tube 14 is internally coated by a thin carbon film 15 with a thickness of 2-10 μ m. This carbon film is highly heat-reflecting and have a metal-like lustrous character. Thanks to the fact that the tube is made of quartz and the carbon film is thin, it will be permeable to the Rf-field emitted by the heating means 10.

The reactor also comprises two graphite plates 16 arranged on both sides of the susceptor 8 for reflecting heat radiated by the susceptor back towards the susceptor as well as a graphite plate 17 deviding the inner tube 14 into two parts and arranged to reflect heat coming primarily from the susceptor and the walls of the inner cell back.

The reactor also comprises means indicated at 18 for supplying a flushing stream of Ar-gas to the casing, which is intended to flow through the casing and leave this in a tube 19 connected to a pump not shown. The Ar-gas is intended to flush the walls of the inner 16 and outer 3 tube of quartz for removing impurities thereon. The main reason for choosing quartz as a material for these tubes is exactly that impurities deposited thereon may very easily be flushed away. It has also been found that the Ar-gas has a not neglectible heat shielding effect, since argon has a very low thermal conductivity. More precisely, the argon gas flows in a small gap 23 between the susceptor and the inner tube 14 and reduces the heat transferral from the susceptor to the inner surface of the tube 14 through the gas therebetween. The argon gas also reduces such heat transferral to other parts of the tube 14.

The reactor has also a window 20 making it possible to determine the temperature within the casing 2 by looking at a bore drilled in the susceptor and measuring the radiation from such an "ideal" black body.

The reactor is used to grow films with a thickness of 1-50 μm for the use in primarily high power semiconductor devices and is as fol¬ lows: a gas mixture of C- and Si-containing precursor gases, for instances propane and silane, and a carrier gas, preferably H2, is introduced into the conduit 7 and drawn through the inner cell 6. A diffuser means not shown is arranged in the transition between the conduit 7 and the inner cell 6 so as to prevent the formation of a central jet inside the inner cell 6. The heating means 10 heats the susceptor 8 and by that the substrate 9 and the gas mixture passing closely above the susceptor and the substrate to a temperature of preferably 1500-1600°C, so that the precursor gases are cracked and the Si- and C-atoms so formed are deposited onto the substrate 9 while forming well ordered epitaxial layers thereon. The substrate 9 is placed downstreams of the longitudinal middle of the susceptor 8, so that the precursor gases of the gas mixture will reach a tem¬ perature resulting in cracking thereof before or when they reach the region of the substrate 9. The comparatively low height of the inner cell 6 make the gases pass closely above the susceptor and the substrate and facilitates the heating of the gas mixture, so that the epitaxial growth of SiC on the substrate 9 is promoted.

Thanks to the fact that the inner cell 6 is made of graphite with¬ standing high temperatures it has not to be cooled, so that the temperature gradient between the susceptor and the substrate will be that low that no detrimental vortices are formed in the flow of gases passing the susceptor and the substrate. However, the fact that the susceptor is heated from below will result in a temperature gradient of few hundred °C between the substrate and susceptor on one hand and the ceiling 24 of the inner cell 6 on the other. This means that the rate of deposition of gases will be much lower on said ceiling than on the considerably hotter substrate, so that the depletion by such depositions is reduced. The fact that the ceiling of the inner cell 6 is thin is very advantageous for obtaining said tem¬ perature gradient, since a thick ceiling would absorb a lot of heat and get a temperature lying much closer to that of the substrate. It is for that reason desired to make the inner cell and particularly the part of the ceiling over the substrate as thin as possible without jeopardizing the stability of the inner cell. The considerable lateral distance between the substrate 9 and the susceptor walls 25 also contributes to a better morphology of the epitaxial layers grown on the substrate, since the center of the gas flow with a substantially uniform velocity and depletion degree will pass the substrate, whereas the parts of the flow having considerably lower velocities and a higher depletion due to depositions on said walls 25 will pass the substrate at a distance therefrom. The inner cell 6 coated by SiC at least close to the substrate at least in the flowing direction of said gases upstreams of the substrate will prevent impurities from reaching the substrate and be incorporated in the epitaxial layers grown. The impurities possibly released from the walls will only be transported in the direction of the flow due to the laminar character thereof so that a SiC coating downstreams would not be absolutely necessary. Thus, the inner cell of the invention will result in a very high uniformity of the deposition of Si- and C-atoms on the sub- strate with a low degree of incorporation of unwanted impurities in the layer grown, so that a SiC crystal of a very high quality may be grown.

The carrier gas of the gas mixture and remaining parts of the pre- cursor gases and products of the cracking thereof will leave the inner cell through an opening 21 thereof near its end and through the tube 19. There is a small gap, which has been exaggerated in Fig 1 , between the opening 21 and the tube 19, so that the flushing gas introduced through the conduit 18 and possibly other conduits not shown may also leave the casing 2 through the tube 19. Apart from this opening 21 the inner cell 6 will be completely sealed with respect to the rest of the casing.

The invention is of course not in any way restricted to the preferred embodiment described above, but several possibilities to modifica¬ tions thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention.

All parts of the reactor shown in the figures may of course have other geometric forms, and "tube" as defined in the claims has to be interpreted in its broadest sense also including for instance rectan¬ gular (as for the inner cell of the embodiment shown), triangular and other cross-section shapes and even a "tube" tapering in the flowing direction towards the substrate as it is known for increasing the flow and make the gases passing closer to the layers grown on the substrate. Although the entire inner cell of the embodiment shown is made of graphite, it will be within the scope of the invention to make only a part thereof of graphite as long as it comprises the region close to the susceptor.

Claims

1 . A device for epitaxially growing SiC by Chemical Vapour Deposi¬ tion on a substrate (9) comprising a susceptor (8) adapted to re- ceive the substrate thereon, a tube (6) arranged to lead a gas mix¬ ture containing precursors for said growth closely over and past the substrate and means (10) for heating the susceptor and thereby the substrate and said gas mixture for said growth, characterized in that said tube (6) is at least close to the susceptor made of graphite.

2. A device according to claim 1 , characterized in that the internal wall of the tube (6) is coated by SiC at least close to the substrate at least in the flowing direction of said gas upstreams of the substrate.

3. A device according to claim 1 or 2, characterized in that the susceptor (8) forms a part of the bottom of said tube (6).

4. A device according to any of claims 1 -3, characterized in that said tube (6) has a low height with respect to the width thereof so as to lead said gas mixture closely above the susceptor and the substrate.

5. A device according to any of claims 1 -4, characterized in that said tube (6) has a rectangular cross-section.

6. A device according to any of claims 1 -5, characterized in that said tube (6) has at least in the region adapted to receive the substrate a width well exceeding the width of said substrate.

7. A device according to any of claims 1 -6, characterized in that the susceptor (8) is coated by SiC at least close to the substrate.

8. A device according to any of claims 1 -7, characterized in that said heating means (10) are arranged to heat the susceptor (8) from below.

9. A device according to any of claims 1-8, characterized in that said heating means (10) are arranged to heat the susceptor by radiating a Rf-field.

10. A device according to any of claims 1 -9, characterized in that at least the part of the ceiling (24) of the tube (6) arranged to be located over the substrate (9) has a small thick¬ ness.

1 1. A device according to claim 10, characterized in that the walls (24, 25) of substantially the whole tube (6) have a small thickness.

12. A device according to claim 10 or 11 , characterized in that said thickness is less than 3 mm.