Patent Application
20130221377
The present invention relates to heterogrowth particularly, but not exclusively to a silicon/silicon carbide heterogrowth.
Silicon carbide is a promising material for future power electronics applications because it can sustain much higher voltages than silicon and has a thermal conductivity similar to copper.
Silicon carbide exists in several different crystal forms (or “polytypes”) depending on the sequence in which bilayers of silicon and carbon stack.
The most commonly-used polytype of silicon carbide is four-step hexagonal stacking sequence silicon carbide (4H-SiC) because it is possible to grow this in single crystal form and produce wafers of the semiconductor material. However, these crystals are produced by physical vapour transport (PVT) process in which a powder of silicon carbide is sublimed at about 2,200° C. producing a vapour which travels and then condenses on a seed crystal. This process is very energy intensive and so silicon carbide wafers are much more expensive to produce than silicon wafers.
Another polytype of silicon carbide, 3-step cubic silicon carbide (3C-SiC), can in principle be grown epitaxially on silicon wafers because they share a cubic crystal form. In this case, a layer of silicon carbide for device fabrication could be realised more cheaply than fabricating a 4H-SiC wafer. However, there are two significant challenges to growing a layer of 3-step cubic silicon carbide on silicon, i.e. 3C-SiC/Si heterogrowth.
Firstly, there is a lattice mismatch between 3-step cubic silicon carbide and the silicon wafer seed.
Secondly, silicon carbide and silicon have different coefficients of thermal expansion. When a layer of silicon carbide is grown on silicon at elevated temperatures and then cooled to room temperature, the silicon carbide contracts at a faster rate than silicon, thus the resulting structure bows.
Attempts have been made to address the problem of cracking in layers of silicon carbide deposited on single-crystal (or “monocrystalline”) silicon wafers. WO 03069657 A describes growing silicon carbide on a single-crystal silicon-germanium substrate having a germanium content of between 5 and 20%.
According to a first aspect of the present invention there is provided a method comprising bonding a wafer and a wafer-like carrier so as to form a composite wafer having a surface, wherein the wafer comprises silicon and has a monocrystalline silicon surface region and the carrier comprises silicon carbide and wherein the wafer and carrier are bonded such that the monocrystalline silicon surface region is at the surface of the composite wafer.
Thus, when the composite wafer is heated to a suitably high temperature for silicon carbide heterogrowth (but below the melting point of silicon), the silicon carbide carrier stretches the silicon in the wafer. When a layer of monocrystalline or polycrystalline silicon carbide semiconductor is grown on the monocrystalline silicon surface region and the resulting heterostructure is cooled, then the difference in rate of contraction between the silicon carbide layer and silicon is reduced which can help avoid or at least reduce wafer bow.
The method may further comprise heating the composite wafer and growing a layer of silicon carbide on the monocrystalline silicon surface region of the wafer.
According to a second aspect of the present invention there is provided a method comprising heating a composite wafer which comprises a wafer comprising silicon and having a monocrystalline silicon surface region and a wafer-like carrier comprising silicon carbide and growing a silicon carbide layer on the monocrystalline silicon surface region of the wafer.
Growing the silicon carbide layer may comprise growing an epitaxial layer of monocrystalline silicon carbide or a layer of polycrystalline silicon carbide.
The layer may comprise a layer of 3-step cubic silicon carbide (3H-SiC). The layer may have a thickness of at least 0.5 μm.
The wafer may comprise a monocrystalline silicon wafer, i.e. not just the surface region is monocrystalline, but the wafer is monocrystalline throughout. However, the wafer may comprise silicon-on-insulator wafer comprising a monocrystalline layer of silicon disposed on a silicon dioxide layer which is turn is disposed on a (monocrystalline) silicon substrate. The monocrystalline silicon surface region may be patterned. The monocrystalline silicon surface region may be an epitaxial layer.
The term “silicon wafer” is intended to exclude a silicon-germanium wafer. The term “silicon-on-insulator wafer” is intended to exclude a silicon-germanium-on-insulator (SGOI) wafer.
The wafer may have a diameter of at least 2 inches (50.8 mm) or of at least 100 mm. The wafer may have a thickness of at least 250 μm for a wafer diameter of about 50 mm or less, a thickness of at least 500 μm for a wafer diameter of about 100 mm or less or a thickness of at least 650 μm for a wafer diameter of about 150 mm or more. Thus, the wafer can be self-supporting if the carrier is removed.
The carrier may be amorphous and/or polycrystalline. Thus, cheap silicon carbide substrates, for example “dummy wafers” or “blanks”, can be used. Such silicon carbide substrates can be made by converting graphite into silicon carbide. The carrier may monocrystalline
Bonding may include directly bonding the wafer and carrier without an intermediate layer or indirectly bonding the wafer and carrier with one or more intermediate layer.
A surface of the carrier may have a surface roughness of less than or equal to 10 Å or less than or equal to 5 Å. Thus, the surface of the carrier may be sufficiently smooth for wafer bonding. A surface of the carrier may be plasma activated. The wafer may have another surface, opposite the surface which may have a surface roughness of less than or equal to 10 Å or less than or equal to 5 Å.
The diameter of the carrier may be greater than or equal to the diameter of the wafer. The thickness of the carrier may at least 0.8 times or 0.9 times the thickness of the wafer. The carrier may be about the same thickness or thicker than the wafer.
The method may further comprise, after growing a layer of silicon carbide on the monocrystalline silicon, delaminating the wafer and the carrier.
According to a third aspect of the present invention there is provided a composite wafer comprising a wafer comprising silicon and having a monocrystalline silicon surface region and a carrier comprising silicon carbide, wherein the composite wafer has a surface and the monocrystalline silicon surface region is at the surface of the composite wafer.
The wafer may comprises a monocrystalline silicon wafer and the carrier may be amorphous and/or polycrystalline.
According to a fourth aspect of the present invention there is provided a semiconductor structure comprising the composite wafer and a layer comprising silicon carbide disposed on the monocrystalline silicon surface region of the wafer.
The layer may comprise a layer of 3 step cubic silicon carbide. The layer may be monocrystalline or polycrystalline. The layer may have a thickness of at least 0.5 μm.
According to fifth aspect of the present invention there is provided a semiconductor heterostructure comprising a wafer comprising silicon and having a monocrystalline silicon surface region and a silicon carbide layer disposed on the monocrystalline silicon surface region of the wafer, wherein the semiconductor structure is not bowed.
The wafer may comprise a silicon wafer. The silicon carbide layer may have a thickness of at least 0.5 μm.
According to a sixth aspect of the present invention there is provided a semiconductor device comprising a region of monocrystalline or polycrystalline silicon carbide disposed on a monocrystalline silicon substrate. The silicon carbide region is preferably not bowed and/or not cracked.
According to a seventh aspect of the present invention there is provided a method comprising bonding a wafer and a wafer-like carrier so as to form a composite wafer have a surface, wherein the wafer comprises a first material which is a semiconductor material and wherein the wafer has a monocrystalline semiconductor surface region and the carrier comprises a second, different material, wherein the wafer and the carrier are bonded such that the monocrystalline semiconductor surface region is at the surface of the composite wafer. The second material may be a semiconductor material.
According to an eighth aspect of the present invention there is provided a method comprising heating a composite wafer which comprises a wafer comprising a first material which is a semiconductor material and having a monocrystalline semiconductor surface region and a carrier comprising a second, different material and which does not have a monocrystalline semiconductor surface region and growing a layer on the monocrystalline semiconductor surface region, the layer comprising a third material which is a semiconductor material and which is different from the first material. The second material may be a semiconductor material. The third material may be the same material as the second material or may be a different material having the same or similar coefficient of thermal expansion. The layer may be monocrystalline or polycrystalline.
Thus, growth of other materials, such as gallium nitride, can benefit from use of a composite wafer.
According to a ninth aspect of the present invention there is provided a semiconductor structure comprising a composite wafer which comprises a wafer comprising a first material which is semiconductor material and having a monocrystalline semiconductor surface region and a carrier comprising a second, different material which does not have a monocrystalline semiconductor surface region and a layer disposed on the monocrystalline surface region, wherein the layer comprises a third material which is a semiconducting material and which is different from the first material. The second material may be a semiconductor material. The third material may be the same as the second material or may be a different material having the same or similar coefficient of thermal expansion. The layer may be monocrystalline or polycrystalline.
According to a tenth aspect of the present invention there is provided a semiconductor heterostructure comprising a wafer comprising a first semiconductor material having a first coefficient of thermal expansion and having a monocrystalline semiconductor surface region and a layer comprising a second, different semiconductor material disposed on the monocrystalline surface region of the first wafer, wherein the second semiconductor material has a second, different coefficient of thermal expansion and a minimum temperature of growth, wherein the difference between the first and second coefficients of thermal expansion and the difference between the minimum temperature of growth, for example for polycrystalline or monocrystalline growth, and room temperature are sufficiently high to cause bowing and wherein the semiconductor structure is not bowed.
According to an eleventh aspect of the present invention there is provided a semiconductor device comprising a monocrystalline substrate comprising first semiconductor material and a monocrystalline or polycrystalline region comprising a second, different semiconductor material disposed on the substrate, wherein first semiconductor material has a first coefficient of thermal expansion and the second semiconductor material has a second, different coefficient of thermal expansion and a minimum temperature of growth and wherein the difference between the first and second coefficients of thermal expansion and the difference between the minimum temperature of growth and room temperature are sufficiently high to cause bowing after a layer of the first material is grown at or above the minimum temperature of growth and then cooled to room temperature.
Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
a to 1e illustrates stages during a first process of heteroepitaxy;
a to 2f illustrate stages during a second process of heteroepitaxy;
Before describing embodiments of the present invention, a silicon carbide/silicon heteroepitaxy process will be described with reference to
a shows a monocrystalline silicon wafer 1 at a room temperature (about 25° C.). The silicon wafer 1 serves as a seed wafer on which a layer of three-step cubic silicon carbide (3C-SiC) can be epitaxially grown. The silicon wafer 1 has a diameter, d.
The silicon wafer 1 is placed in a silicon carbide epitaxial reactor (not shown) and is heated to about 1350° C. As shown (in highly schematic form) in
Referring to
However, as shown in
For a silicon wafer 1 having a diameter of 150 mm, the edges of the wafer can be raised by a distance, s, relative to the centre of the wafer by about 10 mm.
The present invention seeks to address this problem.
Referring to
a shows a monocrystalline silicon wafer 11 and a polycrystalline silicon carbide substrate 12 at a room temperature.
The silicon wafer 11 has an off-axis, [111] crystal orientation and is polished on both sides 13, 14. The sides 13, 14 may also be referred to as “surfaces” or “faces”. Other crystal orientations may be used, such as [100]. The silicon wafer 11 has a thickness, t1, and a diameter, d1. In this example, t1=200 μm and d1=100 mm. The silicon wafer 11 has a wafer bow less than 25 μm and so can be considered to have substantially no bow.
The silicon wafer 11 serves as a seed wafer on which a layer of monocrystalline three-step cubic silicon carbide can be epitaxially grown on a first side 13 of the wafer 11 (hereinafter referred to as the “upper surface”) on a monocrystalline silicon surface region 15. In some embodiments, silicon-on-insulator may be used and so the monocrystalline silicon surface region can take the form of a monocrystalline silicon layer, for example having a thickness of about 50 to 200 nm, disposed on a layer of silicon dioxide. However, other wafers having different layer structures, but which is mostly or predominantly comprised or made up of silicon can be used. For example, a wafer which mostly comprises silicon but which has one or a few layers of non-silicon material embedded in it may be used. Thus, the total thickness (or volume) of non-silicon material used in layers or regions of the wafer may make up no more about 1%, 0.1% or even 0.01% of the wafer.
The silicon carbide carrier 12 is wafer-like in size and appearance, and is polished on both sides 16, 17 and is used as a carrier for the silicon wafer 11. The silicon carbide carrier 12 has a thickness, t2, and a diameter, d2. In this example, t2=500 μm and d2=100 mm.
In this example, a polycrystalline carrier 12 is used. However, a monocrystalline carrier can be used. However, unpolished polycrystalline silicon carbide substrates are cheap and are often used as furnace dummy wafers (or “blanks”) in silicon processing. Unpolished polycrystalline silicon carbide substrates can be produced by cutting slices from a bar of graphite, chemically converting the graphite into a porous form of silicon carbide and then depositing a 75 μm layer of silicon carbide on both faces using chemical vapour deposition. Suitable unpolished silicon carbide substrates are marketed under the name SUPERSiC® by Poco Graphite, Inc., Tex., USA. The surface roughness of unpolished silicon carbide substrates is in the region of 1 μm.
The unpolished silicon carbide substrate is polished using a pure mechanical polishing process using a chemically-inert polishing slurry (not shown) so as to provide a sufficiently flat surface 16 for wafer bonding. Some forms of chemical mechanical polishing may not be appropriate for a non-crystalline surface since they may pull grains from the surface of the substrate. The upper surface 16 has a surface roughness (Ra) of less than or equal to 5 Å. The lower surface 17 can also be polished or may be treated in other ways, for example by etching grooves or other structures, so as to avoid wafer bow. In this case, the lower surface has a surface roughness of less than or equal to 100 Å. The silicon carbide wafer 12 has substantially no bow, having wafer bow less than 50 μm. Surface roughness can be tested using atomic force microscopy.
Before heteroepitaxy, the silicon wafer 11 may be processed, for example, by patterning and etching the upper surface 13 and/or the lower surface 14 (step S1).
For example, the lower surface 14 may be patterned using inverted pyramids or other structure so as to help dislocations arising from lattice mismatch to merge, as described, for example, in “3C-SiC Heteroepitaxial Growth on Inverted Silicon Pyramids (ISP)”, by G. D'Arrigo et al., Materials Science Forum, volumes 645 to 648, pages 135-138 (2010).
Referring to
The upper surface 13 of the silicon wafer 11 forms a first (exterior) surface 19 of the composite wafer 13 (herein referred to as the “upper surface” of the composite wafer 13). The lower surface 17 of the carrier 12 forms a second exterior surface 20 of the composite wafer 13 (herein referred to as the “lower surface” of the composite wafer 13). The lower surface 14 of the silicon wafer 11 and the upper surface 16 of the silicon carbide carrier 12 form an (interior) interface 21. In embodiments which use intermediate layers for wafer bonding, more than one interior interface can be formed.
The composite wafer 18 is placed into a silicon carbide epitaxial reactor (not shown), such as an ACiS M10 marketed by LPE S.p.A., Baranzate, Italy. The reactor chamber (not shown) is subjected to a high-vacuum bake out at about 500° C. and is re-filled with hydrogen at about 100 mbar. The composite wafer 18 is heated via inductive heating of a susceptor (not shown). The surface 13 of the silicon wafer 11 is protected by carbonization using C2H4 at 1170° C. As shown (in highly schematic form) in
Referring to
Thus, the composite wafer 18 is heated to a temperature below the melting point of silicon (i.e. 1410° C.) and significantly below the melting point of silicon carbide (i.e. about 2200° C.). Thus, the silicon wafer 11 becomes plastic and so can be stretched more easily by the underlying silicon carbide carrier 12.
The reactor chamber (now shown) is purged and temperature is ramped down (step S5).
As shown in
The silicon carbide layer 23 can be processed to form semiconductor devices, for example, power electronic devices or nano electro-mechanical systems (NEMS) devices.
Processing may include high-temperature processing steps such as gate oxidation, implant annealing etc. (step S6).
After completing the high-temperature processing steps, the silicon wafer 11 and silicon carbide carrier 12 may be delaminated, for example, by cleaving the interface 21 between the wafer 11 and the carrier 12, so as to leave the silicon wafer 11 (step S7).
Processing of the wafer 11 can continue and may include steps such as thin film deposition, lithography, dry etching and so on (step S8).
Referring to
The device 26 has first and second 3-step cubic silicon carbide epitaxial layers 23a, 23b including a heavily-doped p-type layer 23a which is supported on a p-type silicon substrate 11 and which provides a p-type collector, and a lightly-doped n-type layer 23b which provides a drift region and which is supported on the p-type silicon carbide layer 23a. P-type wells 27 at the surface 28 of the epitaxial layer 23 provide body regions 27. N-type wells 29 within the p-type wells 27 provide contact regions. A channel 30 is formed beneath a gate 31 which separated using a gate dielectric layer 32.
The IGBT shown in
In a test sample using a composite wafer as hereinbefore described, a layer of silicon carbide having a thickness of 1.4 μm can be grown and the structure cooled to room temperature without bow. In a comparative sample using a silicon wafer (i.e. without a composite wafer), a layer of silicon carbide having a thickness of 1.4 μm is grown, but on cooling to room temperature, the structure bows by over 300 μm.
In another test sample using a composite wafer as hereinbefore described, a layer of silicon carbide having a thickness, t3, of 4.4 μm is grown and the structure cooled to room temperature without any bow. In a comparative sample using a silicon wafer (i.e. without a composite wafer), the same thickness of silicon carbide is grown, but on cooling to room temperature, the structure bows by several millimetres and later breaks.
Thus, using a composite wafer helps to minimise and even avoid bow and can be used to grow an epitaxial layer of silicon carbide on silicon having a thickness of over 4 μm without any bow in the resulting structure when cooled.
It will be appreciated that many modifications may be made to the embodiments hereinbefore described.
Different wafer diameters and thicknesses can be used. For example, wafers having diameters of 150 mm, 200 mm, 300 mm or more can be used.
The growth conditions, for instance temperature, pressure and/or precursors, can be varied and optimised.
A material other than silicon can be used for the seed wafer. Thus, wafers made up of (or predominantly of) an inorganic semiconductor can be used.
A material other than silicon carbide can be used for the carrier and be grown. For example, other inorganic semiconductor materials can be used. Moreover, different materials having the same or sufficiently similar coefficient of thermal expansion can be used for the carrier and grown layer. For example, the seed wafer can be silicon, the carrier can comprise germanium (Ge) and the grown layer may comprise gallium nitride (GaN). The carrier need not be made from a semiconductor material provided that the material properties (e.g. thermal expansion coefficient and melting point) suitably match those of the layer being grown and can be suitably bonded to wafer. Thus, the carrier may be formed from a metal, metal alloy or dielectric material.
Wafer bonding may be direct, with no intermediate layer, or indirect, using an intermediate layer.
The process need not involve growing a monocrystalline layer of semiconductor material (i.e. heteroepitaxy), but can involve growing non-monocrystalline layers, e.g. polycrystalline layers of semiconductor material.
An upper surface or face of a wafer or carrier may be referred to as a “front surface”, “front face”, “top surface” or “top face” of the wafer or carrier. Likewise, a lower surface or face may be referred to a “back surface”, “rear surface”, “back face”, “rear face”, “bottom surface”, “bottom face” of the wafer or carrier.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1017279.9 | Oct 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB11/51981 | 10/13/2011 | WO | 00 | 5/10/2013 |
