Patent Application
20100028240
The present invention relates to a method for producing a single crystal of silicon carbide (SiC), and in particular, relates to a method for producing an SiC single crystal characterized in that the growth of crystal is attained by a crystal growth method for growing a high-quality SiC single crystal.
SiC is a material which is physically and chemically stable, as demonstrated by having a high thermal conductivity, a superior thermal resistance and mechanical strength, and a high radiation resistance, and also having large band gap energy. Thus, SiC can be utilized, for example, as a material for an environmentally resistant device that can be used even under high temperature conditions, a radiation-resistant device, a power device for power control, and a short-wave light-emitting device, and the like. In addition, in recent years, SiC has in particular attracted attention as a power device for power control, and intense development thereof is progressing.
In order to promote widespread usage of SiC single crystals as a material for power devices for power control, crystals without defects, such as dislocations and the like, have been necessary.
A method of producing an SiC single crystal is known in which a crystal growth crucible is provided with a low temperature region and a high temperature region, a seed crystal substrate formed of an SiC single crystal is arranged in the low temperature region of the crystal growth crucible, the SiC raw material is arranged in the high temperature region, and a sublimation gas that is sublimed from the SiC raw material is deposited on the seed crystal substrate. In this case, there are problems in that defects occur in the seed crystal due to the stress when attaching the SiC seed crystal to the crucible and heat distortion during growth, and these defects cause the deterioration in the crystallinity of the growing crystal.
In order to solve these problems, a method has been proposed (refer, for example, to Japanese Patent Application Laid-Open No. 2004-269297) in which a stress absorbing material for relieving the thermal stress that acts between the SiC seed crystal and the platform which holds the SiC seed crystal is arranged therebetween, and the tensile strength of this stress absorbing material is 10 MPa or less. Another method has been proposed (Japanese Patent Application Laid-Open No. 2004-338971) in which, while growing the single crystal, an absorbing member is interposed between the SiC seed crystal and a lid member that supports this SiC seed crystal, the seed crystal is adhered to one surface of a plate portion in this absorbing member, a projection that has a pin pole is formed in the other surface thereof, and the absorbing member is coupled to the lid by this pin hole.
Even when using such growth methods, however, multiple defects still occur in the growing crystal. In particular, dislocations and micropipes that are produced at the interface between the seed crystal and the growing crystal reduce the quality of the SiC single crystal.
It is an object of the present invention to provide a method that solves the problems described above and that produces a high quality SiC single crystal having few crystal defects.
The present invention has been made to attain the objects described above and comprises the following aspects. (1) The first aspect of the invention provides a method for producing an SiC single crystal comprising: providing a low temperature region and a high temperature region in a crystal growth crucible; disposing a seed crystal substrate formed of an SiC single crystal in the low temperature region of the crystal growth crucible; disposing an SiC raw material in the high temperature region, and depositing a sublimation gas that sublimes from the SiC raw material on the seed crystal substrate to grow the SiC single crystal, wherein a material used in the crucible member where the seed crystal is disposed is a material having a room-temperature linear expansion coefficient that differs from that of SiC by 1.0×10−6/K or less.
By using the method for producing an SiC single crystal of the present invention, it is possible to produce an SiC single crystal wafer having few crystal defects, and thus, it is possible to produce high-performance semiconductors, and in particular, inverters, with a high yield.
The present invention will now be explained in detail.
The method for producing an SiC single crystal of the present invention is a method in which, fundamentally, a sublimation gas sublimed from SiC raw material at a temperature of 2000° C. or more is introduced to a seed crystal formed of an SiC single crystal, and the SiC single crystal is grown on the seed crystal.
As shown, for example, in
During this growth, generally, an inert gas such as argon is fed into the crucible 6 through a feed pipe 7, the inert gas in the crucible 6 is discharged through the gas exhaust port 8, and the pressure inside the reacting furnace is reduced to about 10 Torr (about 1.3 kPa). Note that the heating device may also be a resistance heating type heater. In addition, using these heating means, the temperature of the crystal growth region where the seed crystal is arranged is set to 1800° C. to 2300° C., and the coil position and the like are adjusted such that the temperature of the high temperature region where the SiC raw material is arranged becomes 2000° C. to 2400° C., which is higher than the temperature of the crystal growth region.
Note that, in the present specification, a description is provided by using an example in which a sublimation gas that directly sublimes from an SiC raw material is used for crystal growth. However, the gist of the present invention lies in the structure of the supporting portion for the SiC seed crystal and the structure of the crystal growth crucible. Naturally, various methods that are directed to the growth of a single crystal, such as using a chemical vapor deposition method (CVD method) in which silane, propane or the like is used as a raw material instead of a sublimation gas, or a liquid phase growth method in which growth is effected by using a Si melt or the like as a solvent can be applied to the growth method in the present invention.
The present invention provides a method for producing an SiC single crystal in which a low temperature region and a high temperature region are provided in a crystal growth crucible, a seed crystal substrate formed of an SiC single crystal is disposed in the low temperature region of the crystal growth crucible, the SiC raw material is disposed in the high temperature region thereof, and a sublimation gas that has been sublimed from the SiC raw material is deposited on the seed crystal substrate to grow the SiC single crystal, in which a material that has a room-temperature linear expansion coefficient that differs from that of SiC by 1.0×10−6/K or less, preferably 0.5×10−6/K or less, and more preferably 0.3×10−6/K or less is used in the crucible member where the low temperature region of the crucible is formed.
In the crystal growth crucible, generally, a material such as graphite, platinum, tantalum or the like is used. However, the difference in thermal expansion rate between these materials and SiC is great, and in the case in which the SiC seed crystal is directly adhered to the crystal growth crucible, the difference in these thermal expansion rates causes a strain on the SiC seed crystal when the temperature increases and decreases during single crystal growth. This strain causes defects in the SiC growth crystal.
In the present invention, the SiC seed crystal is held in a crucible member having room-temperature linear expansion coefficient different from that of SiC by 1.0×10−6/K or less, thereby minimizing the strain that acts on the SiC seed crystal when the temperature is increased or decreased during the single crystal growth. The defects in the growing crystal that are caused by this strain can be thereby reduced. Since the difference in the room-temperature linear expansion coefficient between the member that is used in the present invention and the SiC is most preferably zero, the low temperature region of the crucible of the present invention is preferably fabricated from SiC. In addition, graphite, for which the difference in the linear expansion coefficient is adjusted to 1.0×10−6/K or less, can be suitably used.
In addition, the present invention is in which, as shown, for example, in
In the present invention, when the SiC seed crystal is held on the supporting member for the seed crystal or at the low temperature region of the crucible, SiC, which is a material that is identical to the SiC seed crystal, is used to form these locations, and thereby, the strain that acts on the SiC seed crystal when the temperature increases or decreases during single crystal growth is minimized, and defects in the growing crystal that are caused by this strain can be reduced.
In addition, in the method for producing an SiC single crystal of the present invention, preferably, a supporting member that is formed of SiC or a low temperature region of the crucible that is formed of SiC are made from an SiC single crystal. SiC includes polycrystalline SiC that is produced by sintering and the like, and single-crystalline SiC that is produced by sublimation methods and the like. In the supporting member of the present invention, basically either can be used, but the thermal expansion coefficient of polycrystalline SiC differs slightly from that of the single-crystalline SiC that is grown in the present invention. Thus, in the method for producing an SiC single crystal of the present invention, using a single-crystalline SiC in this supporting member can further reduce the strain that acts on the seed crystal more than using the polycrystalline SiC.
In addition, according to experiments carried out by the inventors of the present application, there are cases in which defects are produced during crystal growth at the bonded surface between the polycrystalline SiC supporting member and the SiC seed crystal, and there are cases in which these defects penetrate the seed crystal and propagate to the growth crystal. Thus, by using the structure of the present invention, such a phenomenon can be avoided, and it is possible to produce an SiC single crystal having fewer defects.
In the method for producing an SiC single crystal of the present invention, preferably, the crystal structure of the SiC single crystal supporting member or the low temperature region of the crucible made of an SiC single crystal (hereinafter, referred to simply as the “SiC single crystal supporting portion”), is identical to the crystal structure of the seed crystal, and the crystal face orientations at these locations and the crystal face orientation of the seed crystal are matched. Then the seed crystal is arranged in the crucible. As explained above, in the present invention, the thermal expansion coefficient of the SiC seed crystal and the supporting portion thereof must be minimized, but even for an SiC single crystal, differences of several percent or greater may occur in the thermal expansion rates thereof due to their crystal structure and the crystal orientation. Thus, by making the crystal structure of the SiC single crystal supporting portion and the crystal structure of the seed crystal identical, and matching the crystal face orientation of the SiC single crystal supporting portion and the crystal face orientation of the seed crystal, it is possible to reduce to a minimum the strain that acts on the seed crystal due to the difference in thermal expansion rate. In addition, there are cases in which the above-described defects that are formed at the bonded surface between the SiC supporting portion and the SiC seed crystal are produced due to differences in the crystal structure and the crystal face orientation thereof, and the generation of these defects can also be prevented by the present invention. Note that the expression “matching the crystal face orientations” means that the Si face and the C face of the SiC single crystal are both distinguished, and means that the Si face and the C face are bonded in the case of an identical crystal orientation. Also note that in the present invention, matching the crystal face orientations means that the crystal face orientations are matched within a range from −10° to +10°, more preferably from −5° to +5°, and even more preferably from −1° to +1°.
In addition, in the present invention, preferably, the SiC supporting portion and the SiC seed crystal have a 4H structure and an orientation that has a ±30° offset with respect to the {0001} plane as bonded crystal face orientations. By using this structure and orientation, the strain that acts on the seed crystal is minimized, and in addition, defects that are generated at the bonded interface can be minimized.
In the method for producing an SiC single crystal of the present invention, the thickness of the supporting member that formed of SiC is preferably within a range of 0.7 mm to 10 mm, and more preferably within a range of 5 mm to 10 mm. By maintaining such a range, the effects of preventing the introduction of defects and reducing the density of defects can be obtained.
In the method for producing an SiC single crystal of the present invention, when the seed crystal is arranged in the supporting portion formed of SiC, preferably, the side portion of the seed crystal is held by using a member formed of SiC. In the production method of the present invention, in the case in which the seed crystal is set in a supporting member formed of SiC, bonding by using an adhesive agent having carbon as a main constituent is possible, but, for example, as shown in
In the method for producing an SiC single crystal of the present invention, as shown in
By cutting the crystal that has been produced by the method for producing an SiC single crystal of the present invention in a direction that is parallel to or oblique to the growth direction by using an outer perimeter cutting edge, a wire saw or the like, it is possible to produce an SiC single crystal wafer having few crystal defects. In addition, by using this, it is possible to produce high performance semiconductor devices, and in particular, an inverter element and the like, at a high yield.
Although an example is shown below, the present invention is not limited thereto.
A seed crystal substrate (50 mm in diameter and 0.4 mm in thickness) composed of a 4H SiC single crystal whose a (000-1) face was exposed was washed with a sulfuric acid-hydrogen peroxide mixed solution at 110° C. for 10 minutes, with running ultrapure water for 5 minutes, with an ammonia-hydrogen peroxide mixed solution for 10 minutes, with running ultrapure water for 5 minutes, with a hydrochloric acid-hydrogen peroxide mixed solution for 10 minutes, with running ultrapure water for 5 minutes, and further with an HF solution. Subsequently, after oxidizing the surface at 1200° C., HF washing was carried out again to complete the seed crystal.
In a graphite crucible and having an inner diameter of 70 mm and a depth of 95 mm, SiC raw material powder (made by Showa Denko K. K. and sold under the product code “#240”) was packed to a height of 60 mm. Then, the seed crystal was held at a lower surface of a graphite crucible lid by using an SiC single crystal supporting member having the structure that is shown in
This lid was disposed on the crucible opening portion, the entire graphite crucible was wrapped in a heat insulating material made of carbon fibers and set in a reaction chamber in a high-frequency heating furnace. The pressure in the interior of the reaction tube was reduced to 5×10−5 Torr via a gas exhaust port 8, then the reaction tube was filled to normal pressure with argon gas that was fed through an inert gas inlet 7, and subsequently the pressure was again reduced to 5×10−5 Torr through the gas exhaust port to expel the air from inside the reaction tube. Then, the reaction tube was filled again with the argon gas introduced through the inert gas inlet to 700 Torr, the upper portion of the graphite crucible was heated to 2200° C., and the lower portion thereof was heated to 2250° C. to 230020 C. Thereafter, the gas was evacuated via the gas exhaust port, the pressure of argon atmosphere was reduced to 5.3 kPa, and growth was carried out for 20 hours.
The obtained crystal was cut perpendicularly to the growth direction, subject to mirror polishing, and a semiconductor wafer having a diameter of 50 mm was thereby produced. The characteristics of this semiconductor wafer were superior in being micropipe-free and having a dislocation density that was lower than the conventional semiconductor wafers.
Since SiC has superior thermal conductivity, heat resistance and mechanical strength, this single crystal can be used in various applications, such as a semiconductor device and an inverter that is composed thereof. In particular, one focus is the usage of SiC as a power device for power control.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-273347 | Oct 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/069888 | 10/4/2007 | WO | 00 | 5/22/2009 |
