WO2013035691A1 - SiCエピタキシャルウェハ及びその製造方法 - Google Patents
SiCエピタキシャルウェハ及びその製造方法 Download PDFInfo
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- WO2013035691A1 WO2013035691A1 PCT/JP2012/072454 JP2012072454W WO2013035691A1 WO 2013035691 A1 WO2013035691 A1 WO 2013035691A1 JP 2012072454 W JP2012072454 W JP 2012072454W WO 2013035691 A1 WO2013035691 A1 WO 2013035691A1
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Definitions
- the present invention relates to a SiC epitaxial wafer and a method for manufacturing the same.
- Silicon carbide (SiC) has superior characteristics such as about 10 times larger dielectric breakdown electric field and about 3 times larger band gap than silicon (Si), so it can be applied to power devices, high temperature operation devices, etc. Is expected.
- Such a SiC device is formed on a SiC single crystal substrate obtained by processing from a SiC bulk single crystal grown by a sublimation recrystallization method or the like by a chemical vapor deposition (CVD) method or the like.
- CVD chemical vapor deposition
- it is manufactured using a SiC epitaxial wafer on which a SiC epitaxial film serving as an active region is grown.
- Non-Patent Documents 1 and 2 As a technique for nondestructively detecting crystal defects such as dislocations and stacking faults contained in a SiC single crystal substrate or a SiC epitaxial wafer having an epitaxial film formed thereon, an X-ray topography method (Non-Patent Documents 1 and 2). ) And the photoluminescence method (Patent Document 1) are known.
- the threading screw dislocation (Threading Screw Dislocation: TSD) is a dislocation whose Burgers vector propagating in the c-axis direction is ⁇ 0001> or twice that.
- threading edge dislocation (TED) is a dislocation whose Burgers vector propagating in the c-axis direction is 1/3 ⁇ 11-20>.
- basal plane dislocation (BPD) is a dislocation whose Burgers vector existing in the c-plane is 1/3 ⁇ 11-20>.
- the SiC epitaxial film is a step in which an SiC single crystal substrate is inclined from the (0001) plane (c-plane) in the ⁇ 11-20> direction with an off angle of 10 ° or less and the step density is intentionally increased as a growth plane.
- the crystal is generally formed by lateral crystal growth (step flow growth).
- basal plane dislocations (BPD) existing in the c-plane are also exposed to the growth surface.
- threading screw dislocations (TSD) and threading edge dislocations (TED) extending in the c-axis direction are also exposed on the growth surface.
- the basal plane dislocation (BPD) propagated to the epitaxial film is not stable in the epitaxial film, but is easily decomposed into two energy-favorable Shockley dislocations and stacked between the two Shockley partial dislocations.
- a defect is made. Since the stacking fault acts as a carrier lifetime killer, the current is concentrated in a region where the stacking fault does not exist, and the area where the current flows is reduced. As a result, the on-resistance is increased.
- a bipolar device such as a pn diode, one of the above two partial dislocations has Si as a core and the other has C as a core, and only partial dislocations having a Si core are electrons and holes. The area of the stacking fault is enlarged by moving by the recombination energy (Non-patent Document 3).
- carrot defects in the epitaxial film are formed by the interaction between the basal plane dislocation (BPD) and the threading screw dislocation (TSD) of the SiC single crystal substrate.
- the basal plane dislocation (BPD) existing on the growth surface of the SiC single crystal substrate becomes a stacking fault in the SiC epitaxial film.
- the carrot defect is formed by the interaction between the basal plane dislocation (BPD) and the threading screw dislocation (TSD) of the SiC single crystal substrate.
- the present inventors form the SiC epitaxial film in the SiC epitaxial film. It has been found that the surface density of stacking faults due to basal plane dislocations (BPD) in the substrate has a regularity that is substantially proportional to the surface density of basal plane dislocations (BPD) in the SiC single crystal substrate.
- the surface density of carrot defects in the SiC epitaxial film is equal to the basal plane dislocations in the SiC single crystal substrate ( BPD) was found to have a correlation with the surface density.
- a SiC epitaxial film having a predetermined film thickness is formed under a predetermined epitaxial film growth condition using a SiC single crystal substrate having a predetermined off angle, basal plane dislocations on the growth surface of the SiC single crystal substrate are formed. If the surface density of (BPD) and threading screw dislocation (TSD) is known, the surface density of carrot defects in the formed SiC epitaxial film can be predicted, and the present invention has been conceived.
- An object of the present invention is to provide a SiC epitaxial wafer having a reduced surface density of stacking faults originating from basal plane dislocations on the growth surface of a SiC single crystal substrate in a SiC epitaxial film, and a method for manufacturing the same.
- Another object of the present invention is to provide a SiC epitaxial wafer having a reduced surface density of carrot defects in a SiC epitaxial film, and a method for manufacturing the same.
- the present invention employs the following configuration.
- a method for producing a SiC epitaxial wafer comprising a SiC epitaxial layer on a SiC single crystal substrate having an off angle, wherein the basal plane dislocation (BPD) is present on the growth surface of the SiC single crystal substrate having the off angle.
- the SiC single crystal substrate is formed on the SiC single crystal substrate under the same conditions as the epitaxial film growth conditions used in the step of determining the upper limit of the surface density and the SiC single crystal substrate equal to or less than the upper limit. And a step of forming an epitaxial film.
- a method of manufacturing an SiC epitaxial wafer comprising: (2) When determining the ratio, the surface density of BPD on the growth surface and the surface density of stacking faults in the SiC epitaxial film due to BPD on the growth surface are determined by X-ray topography or photoluminescence. 2. The method for producing an SiC epitaxial wafer according to claim 1, wherein the measurement is performed by any one of sense methods.
- FIG. 2 is a PL image of a SiC epitaxial wafer using the SiC single crystal substrate shown in FIG. 1, wherein (a) the stacking fault (SF) density is 2.5 ⁇ 10 2 / cm 2 , (b) 9.2 Pieces / cm 2 . It is a graph which shows the correlation of the basal plane dislocation (BPD) density of a SiC single crystal substrate, and the stacking fault (SF) density in an epitaxial film about the data shown in Table 1.
- [SiC epitaxial wafer] 1A and 1B show topographic images of a SiC single crystal substrate obtained by an X-ray topography method.
- 1A and 1B show basal plane dislocation (BPD) surface densities of 6.5 ⁇ 10 4 pieces / cm 2 and 5.5 ⁇ 10 3 pieces / cm 2 , respectively.
- the arrow in the image indicates a part of the basal plane dislocation (BPD).
- crystal defects in the wafer plane can be detected by measuring X-ray diffracted light from a silicon carbide single crystal wafer using reflection X-ray topography.
- reflection X-ray topography there is an advantage that the position of the crystal defect can be detected nondestructively without using a destructive technique such as etching.
- synchrotron radiation was used to separately detect basal plane dislocations, threading edge dislocations, and threading screw dislocations in the crystal.
- the data shown in this specification is based on synchrotron radiation from Spring-8.
- reflection X-ray topography was measured using X-rays having a wavelength of 1.54 nm as incident light.
- the diffraction vector (g-vector) for diffracting X-rays is not particularly limited as long as the object of the present invention can be achieved, but 11-28 or 1-108 is used for 4H—SiC crystal. It is normal. In this specification, a topographic image using 11-28 is shown.
- a topographic image can be obtained by irradiating the sample with X-rays and detecting the diffracted light reflected from the sample.
- a recording medium such as a high-resolution X-ray film or a nuclear dry plate is used in order to obtain a sufficient resolution for determining the defect type. This time, a nuclear plate was used. From the images, the number of basal plane dislocations, threading edge dislocations and threading screw dislocations was counted.
- FIGS. 2 (a) and 2 (b) show a photoluminescence (PL) imaging method for a SiC epitaxial wafer in which a SiC epitaxial film having a thickness of 10 ⁇ m is formed on the SiC single crystal substrate shown in FIGS. 1 (a) and 1 (b).
- the PL image obtained by is shown.
- the SiC epitaxial wafers of FIGS. 2A and 2B are formed simultaneously in the same production lot.
- 2A and 2B respectively show the stacking faults (SF) in the epitaxial film having a surface density of 2.5 ⁇ 10 2 / cm 2 and 9.2 / cm 2 .
- the arrow in the image indicates a part of the basal plane dislocation (BPD) or a part of the stacking fault (SF).
- Table 1 shows the surface density of four basal plane dislocations (BPD) for a SiC single crystal substrate and the epitaxial film in the SiC epitaxial wafer in which a SiC epitaxial film having a thickness of 10 ⁇ m is formed on the SiC single crystal substrate having the BPD surface density. It shows the surface density of stacking faults.
- the surface density of the four basal plane dislocations (BPD) is 3.2 ⁇ 10 4 pieces / cm 2 and 3.0 ⁇ 10 2 in addition to the case of the SiC single crystal substrate sample shown in FIG. In the case of the piece / cm 2 .
- FIG. 3 shows a graph of the data shown in Table 1 with the horizontal axis representing the basal plane dislocation (BPD) surface density of the SiC single crystal substrate and the vertical axis representing the stacking fault (SF) surface density in the epitaxial film. . It can be seen that the BPD surface density and the SF surface density have a substantially proportional relationship. Since it has such a relationship, the upper limit of the BPD surface density of the SiC single crystal substrate required for producing a SiC epitaxial wafer having a desired SF surface density can be determined.
- BPD basal plane dislocation
- SF stacking fault
- 4A and 4B show topographic images of a SiC single crystal substrate obtained by a synchrotron radiation topography method.
- the surface density of basal plane dislocations (BPD) is 5.0 ⁇ 10 4 pieces / cm 2 and the surface density of threading screw dislocations (TSD) is 2.8 ⁇ 10 4 pieces.
- the surface density of basal plane dislocations (BPD) is 2.0 ⁇ 10 4 pieces / cm 2 and the surface density of threading screw dislocations (TSD) is 5.4 ⁇ 10 3 pieces / cm 2.
- Typical basal plane dislocations (BPD) and threading screw dislocations (TSD) are shown in the image of FIG.
- FIGS. 5 (a) and 5 (b) show an optical surface inspection apparatus (Candela) for a SiC epitaxial wafer in which a SiC epitaxial film having a thickness of 10 ⁇ m is formed on the SiC single crystal substrate shown in FIGS. 4 (a) and 4 (b). ) Shows the candela image obtained. Note that the SiC epitaxial wafers of FIGS. 5A and 5B are formed simultaneously in the same production lot. In each of FIGS. 5A and 5B, the left image shows a defect map, and the right image is a candela image indicated by an arrow in the left defect map. FIGS. 5A and 5B show that the carrot defect density in the epitaxial film is 20.2 / cm 2 and 0.2 / cm 2 , respectively.
- Candela optical surface inspection apparatus
- a method for manufacturing a SiC epitaxial wafer according to a first embodiment of the present invention is a method for manufacturing a SiC epitaxial wafer having a SiC epitaxial layer on a SiC single crystal substrate having an off angle, and includes a SiC single wafer having an off angle.
- a step of determining a ratio of stacking faults in the SiC epitaxial film having a predetermined thickness formed on the SiC single crystal substrate, and based on the ratio The same conditions as the epitaxial film growth conditions used in the step of determining the upper limit of the BPD surface density on the growth surface of the SiC single crystal substrate to be used and the step of determining the ratio using the SiC single crystal substrate below the upper limit And a step of forming a SiC epitaxial film on the SiC single crystal substrate.
- SiC single crystal substrate Any polytype substrate can be used as the SiC single crystal substrate, and 4H—SiC, which is mainly used for producing a practical SiC device, can be used.
- a SiC single crystal substrate processed from a bulk crystal produced by a sublimation method or the like is used as the substrate of the SiC device, and an SiC epitaxial film that becomes an active region of the SiC device is usually formed thereon by chemical vapor deposition (CVD). Form.
- CVD chemical vapor deposition
- the off-angle of the SiC single crystal substrate can be any off-angle, and there is no limitation, but from the viewpoint of cost reduction, the off-angle is small, for example, 0.4 ° to 5 ° Is preferred. 0.4 ° is the lower limit of the off-angle at which step flow growth is possible.
- the off angle of the SiC single crystal substrate has been mainly 8 °.
- the terrace width of the wafer surface is small and step flow growth can be easily obtained, but the larger the off-angle, the smaller the number of wafers obtained from the SiC ingot. Those with an off angle of about 4 ° are mainly used.
- a 0.4 ° off-angle substrate has a terrace width 10 times that of a 4 ° off-angle substrate, and the step flow growth length is an order of magnitude longer. It should be noted that it is necessary to adjust the conditions of the step flow growth that has been used in the substrate.
- the SiC single crystal substrate a substrate obtained by processing the growth surface of the SiC epitaxial layer into a convex shape can be used.
- the back surface of the SiC single crystal substrate is directly heated from a heated susceptor, but the front surface (formation surface of the SiC epitaxial layer) is a vacuum. Exposed to the space and not heated directly.
- the carrier gas hydrogen flows over the front surface, heat is taken away. For these reasons, the front surface during epitaxial growth is at a lower temperature than the back surface.
- the SiC single crystal substrate Due to this temperature difference, the magnitude of thermal expansion is smaller on the front surface than on the rear surface, and the SiC single crystal substrate is deformed so that the front surface is recessed during epitaxial growth. Therefore, by using a SiC single crystal substrate with a growth surface of the SiC epitaxial layer processed into a convex shape, the SiC single crystal substrate can be epitaxially grown in a state in which the dent (warp) of the substrate during epitaxial growth is eliminated. It becomes possible.
- the thickness of the SiC epitaxial layer is not particularly limited. For example, if the film is formed for 2.5 hours at a typical growth rate of 4 ⁇ m / h, the thickness becomes 10 ⁇ m.
- the SiC single crystal substrate is first polished.
- polishing is performed with reference to the extent to which the lattice disorder layer on the surface thereof is 3 nm or less, as described in JP 2011-49496 A.
- the “lattice disordered layer” is an atomic layer (lattice) of a SiC single crystal in a TEM lattice image (image in which the lattice can be confirmed), as shown in FIGS. 7 and 8 of Japanese Patent Application Laid-Open No. 2011-49496. Is a layered structure in which a stripe structure or a part of the stripe is not clear.
- the surface density of basal plane dislocations (BPD) existing on the growth surface of the SiC single crystal substrate is mainly determined by the finishing degree of this ⁇ polishing step>.
- the surface density of the basal plane dislocations (BPD) existing on the growth surface of the SiC single crystal substrate is the upper limit determined in the ⁇ upper limit determination step>. Polishing is performed using the polishing method described below until the surface density is equal to or lower than the surface density.
- the finishing degree of the ⁇ polishing step> required for manufacturing a SiC epitaxial wafer having a stacking fault surface density of 0.1 / cm 2 or less corresponds to the degree that the lattice disorder layer is 3 nm or less.
- the polishing process includes a plurality of polishing processes such as rough polishing usually called lapping, precision polishing called polishing, and chemical mechanical polishing (hereinafter referred to as CMP) which is ultra-precision polishing.
- the polishing process is often performed in a wet manner, but the common process in this process is to apply a polishing head to which a silicon carbide substrate is bonded while supplying polishing slurry to a rotating surface plate to which a polishing cloth is attached. Is to be done.
- the polishing slurry used in the present invention is basically used in such a form, but the form is not limited as long as it is wet polishing using the polishing slurry.
- the particles used as the abrasive grains may be particles that do not dissolve and disperse in this pH range.
- the pH of the polishing liquid is preferably less than 2.
- diamond, silicon carbide, aluminum oxide, titanium oxide, silicon oxide, or the like can be used as the abrasive particles.
- abrasive particles having an average diameter of 1 to 400 nm, preferably 10 to 200 nm, more preferably 10 to 150 nm are used as abrasive grains.
- silica is preferred in that small particles are commercially available at low cost. More preferred is colloidal silica.
- the particle size of an abrasive such as colloidal silica can be appropriately selected depending on processing characteristics such as processing speed and surface roughness. When a higher polishing rate is required, an abrasive having a large particle size can be used. When the surface roughness is small, that is, when a highly smooth surface is required, an abrasive having a small particle diameter can be used. Those having an average particle diameter exceeding 400 nm are expensive because they are expensive and the polishing rate is not high. When the particle diameter is extremely small such as less than 1 nm, the polishing rate is remarkably reduced.
- the addition amount of abrasive particles is 1% by mass to 30% by mass, preferably 1.5% by mass to 15% by mass. If it exceeds 30% by mass, the drying speed of the abrasive particles becomes high, which increases the risk of causing scratches, and is uneconomical. Further, if the abrasive particles are less than 1% by mass, the processing speed becomes too low, which is not preferable.
- the polishing slurry in the present invention is a water-based polishing slurry, and the pH at 20 ° C. is less than 2.0, desirably less than 1.5, and more desirably less than 1.2. In the region where the pH is 2.0 or more, a sufficient polishing rate cannot be obtained.
- the slurry less than pH 2 the chemical reactivity with respect to silicon carbide is remarkably increased even under a normal indoor environment, and ultraprecision polishing becomes possible.
- the silicon carbide is not directly removed by the mechanical action of the oxide particles in the polishing slurry, but the polishing liquid causes the silicon carbide single crystal surface to chemically react with the silicon oxide, and the silicon oxide is mechanically treated by the abrasive grains.
- polishing composition liquid so that silicon carbide can easily react, that is, setting the pH to less than 2, and selecting oxide particles having an appropriate hardness as abrasive grains can cause scratches and scratches. It is very important to obtain a smooth surface without an altered layer.
- the polishing slurry is adjusted to have a pH of less than 2 using at least one, preferably two or more of hydrochloric acid, nitric acid, phosphoric acid, and sulfuric acid.
- the reason why it is effective to use a plurality of acids is unknown, but it has been confirmed by experiments, and there is a possibility that a plurality of acids interact with each other and enhance the effect.
- the amount of acid added is, for example, within the range of 0.5 to 5% by mass of sulfuric acid, 0.5 to 5% by mass of phosphoric acid, 0.5 to 5% by mass of nitric acid, and 0.5 to 5% by mass of hydrochloric acid.
- the type and amount are selected so that the pH is less than 2.
- the inorganic acid is effective because it is a stronger acid than the organic acid and is extremely convenient for adjusting to a predetermined strongly acidic polishing liquid. If an organic acid is used, it is difficult to adjust the strongly acidic polishing liquid.
- the polishing of silicon carbide is performed by removing the oxide layer with oxide particles due to the reactivity to the oxide film generated on the surface of silicon carbide by the strongly acidic polishing liquid. In order to accelerate this surface oxidation, polishing is performed.
- an oxidizing agent is added to the slurry, a further excellent effect is recognized.
- the oxidizing agent include hydrogen peroxide, perchloric acid, potassium dichromate, ammonium persulfate sulfate, and the like.
- the polishing rate is improved by adding 0.5 to 5% by mass, preferably 1.5 to 4% by mass, but the oxidizing agent is not limited to hydrogen peroxide solution. .
- An anti-gelling agent can be added to the polishing slurry to suppress gelation of the abrasive.
- phosphate ester type chelating agents such as 1-hydroxyethylidene-1,1-diphosphonic acid and aminotriethylenephosphonic acid are preferably used.
- the anti-gelling agent is added in the range of 0.01 to 6% by mass, preferably 0.05 to 2% by mass.
- the damage pressure is reduced to 50 nm by using a polishing pressure of 350 g / cm 2 or less and using abrasive grains having a diameter of 5 ⁇ m or less in mechanical polishing before CMP.
- the polishing slurry contains abrasive particles having an average particle diameter of 10 nm to 150 nm and an inorganic acid, and preferably has a pH of less than 2 at 20 ° C.
- Silica more preferably 1 to 30% by mass, and more preferably at least one of inorganic acid, hydrochloric acid, nitric acid, phosphoric acid and sulfuric acid.
- FIG. 6A shows a SiC epitaxial wafer in which an SiC epitaxial film having a thickness of 10 nm is formed on a SiC single crystal substrate that has been polished so that a lattice disorder layer of 10 nm or more remains, and is obtained by a photoluminescence (PL) imaging method.
- the obtained PL image is shown.
- the upper PL image in the figure is an infrared wavelength with a detection wavelength of 750 nm or more, and the portion observed darker than the surroundings in the image indicates a stacking fault that becomes a carrier trap (the arrow indicates a part thereof). Show).
- the lower PL image has a detection wavelength of 450 ⁇ 10 nm detected using a bandpass filter, and the portion observed whiter than the surrounding in the image indicates a stacking fault having an 8H structure (the arrow indicates Some of them).
- FIG. 6B shows a SiC single crystal substrate obtained by polishing the sample shown in FIG. 6A again until the lattice disorder layer becomes 3 nm or less, and a SiC epitaxial film having a thickness of 10 nm is similarly formed.
- the PL image obtained about the epitaxial wafer is shown. In the PL image of FIG. 6B, almost no stacking faults were observed, and it was found that basal plane dislocations (BPD) converted into stacking faults by polishing can be reduced. In particular, it was found that basal plane dislocation (BPD) can be effectively reduced by polishing until the lattice disorder layer is 3 nm or less.
- the gas etching is performed for 5 to 30 minutes while maintaining the SiC single crystal substrate at 1400 to 1800 ° C., the hydrogen gas flow rate of 40 to 120 slm, and the pressure of 100 to 250 mbar.
- the substrate After cleaning the polished SiC single crystal substrate, the substrate is set in an epitaxial growth apparatus, for example, a mass production type multiple planetary CVD apparatus. After introducing hydrogen gas into the apparatus, the pressure is adjusted to 100 to 250 mbar. Thereafter, the temperature of the apparatus is raised, the substrate temperature is set to 1400 to 1600 ° C., preferably 1480 ° C. or higher, and gas etching of the substrate surface is performed with hydrogen gas for 1 to 30 minutes. When gas etching with hydrogen gas is performed under such conditions, the etching amount is about 0.05 to 0.4 ⁇ m.
- an epitaxial growth apparatus for example, a mass production type multiple planetary CVD apparatus.
- the pressure is adjusted to 100 to 250 mbar.
- the temperature of the apparatus is raised, the substrate temperature is set to 1400 to 1600 ° C., preferably 1480 ° C. or higher, and gas etching of the substrate surface is performed with hydrogen gas for 1 to 30 minutes.
- the etching amount is about 0.05 to 0.4
- the substrate surface is damaged by the polishing process, and it is considered that not only damage that can be detected as a “lattice disorder layer” in the TEM but also distortion of the lattice that cannot be detected by the TEM exists.
- the purpose of gas etching is to remove the damaged layer (hereinafter referred to as “damage layer”).
- damage layer when the gas etching is not sufficient and the damaged layer remains, different types of polytypes and Dislocations, stacking faults, and the like are introduced, and if etching is performed too much, surface reconstruction occurs on the substrate surface, and step bunching occurs before the start of epitaxial growth. For this reason, it is important to optimize the damaged layer and the amount of gas etching.
- the present inventors have found that the substrate surface has a lattice disorder as a sufficient condition in the production of a step bunching-free SiC epitaxial wafer. They found a combination of the damage layer when the layer was thinned to 3 nm or less and the gas etching conditions described above.
- the root mean square roughness Rq of the outermost surface of the epitaxial layer obtained by analyzing an area of 35% or more of the entire wafer surface using an optical surface inspection apparatus is 1.3 nm or less. Can be confirmed.
- the step bunching is 1.0 nm or less at 10 ⁇ m ⁇ , 1.0 nm or less at 200 ⁇ m ⁇ , and a length of 100 to 500 ⁇ m observed at 200 ⁇ m ⁇ . It can be confirmed that the maximum height difference Ry in (short step bunching) is 3.0 nm or less. It can also be confirmed that the linear density of this step is 5 mm ⁇ 1 or less. It is important to maintain the flatness of the substrate surface in the subsequent film forming process and temperature lowering process.
- SiH 4 gas and / or C 3 H 8 gas may be added to the hydrogen gas.
- SiH 4 gas having a concentration of less than 0.009 mol% is added to hydrogen gas to make the environment in the reactor Si-rich.
- the depth of the shallow pit can be reduced, and the occurrence of short step bunching associated with the shallow pit can be suppressed.
- SiH 4 gas and / or C 3 H 8 gas it is preferable to evacuate once to form a hydrogen gas atmosphere before the film formation (epitaxial growth) step.
- ⁇ Film formation (epitaxial growth) process In the film formation (epitaxial growth) step (after the temperature rise when the growth temperature of the epitaxial film is higher than the cleaning (gas etching) temperature), it is required for the epitaxial growth of silicon carbide on the surface of the cleaned substrate.
- a quantity of SiH 4 gas and C 3 H 8 gas are simultaneously supplied at a concentration ratio C / Si of 0.7 to 1.2 to epitaxially grow silicon carbide.
- “Supply at the same time” means that it is not necessary to be completely the same time, but it is within several seconds.
- Hot Wall SiC CVD VP2400HW
- Ixtron shown in the examples described later, if the difference in supply time between SiH 4 gas and C 3 H 8 gas is within 5 seconds, step bunching-free SiC epitaxial wafer could be manufactured.
- Each flow rate, pressure, substrate temperature, and growth temperature of SiH 4 gas and C 3 H 8 gas are 15 to 150 sccm, 3.5 to 60 sccm, 80 to 250 mbar, higher than 1600 ° C. and lower than 1800 ° C., and the growth rate is 1 hour / hour. It is determined within the range of ⁇ 20 ⁇ m while controlling the off angle, film thickness, carrier concentration uniformity, and growth rate.
- the carrier concentration in the epitaxial layer can be controlled.
- the epitaxial layer to be grown usually has a film thickness of about 5 to 20 ⁇ m and a carrier concentration of about 2 to 15 ⁇ 10 15 cm ⁇ 3 .
- the growth temperature and growth rate depend on the off angle of the SiC single crystal substrate.
- the growth rate is 1 to 3 ⁇ m /
- the growth temperature is 1640 to 1700 ° C.
- the growth rate is 3 to 4 ⁇ m / h.
- the growth temperature is 1700 to 1800 ° C.
- the growth rate is 4 to 10 ⁇ m / h.
- the growth rate is set to 2 to 4 ⁇ m / h.
- the growth temperature is 1640 to 1700 ° C.
- the growth rate is 4 to 10 ⁇ m / h.
- the growth temperature is 1700 to 1800 ° C.
- the growth rate is 10 to 20 ⁇ m / h.
- the supply of SiH 4 gas and C 3 H 8 gas and the introduction of nitrogen gas as a doping gas are stopped and the temperature is lowered.
- gas etching occurs on the surface of the SiC epitaxial film, deteriorating the surface morphology. obtain.
- the timing of stopping the supply of the SiH 4 gas and the C 3 H 8 gas and the timing of temperature decrease are important. By simultaneously stopping the supply of SiH 4 gas and C 3 H 8 gas, holding the growth temperature until these supplied gases disappear from the substrate surface, and then lowering the temperature to room temperature at an average rate of about 50 ° C. It was found that deterioration of morphology was suppressed.
- the surface density of the stacking fault (SF) in a SiC epitaxial film is measured using the photo-luminescence (PL) imaging method.
- the SiC single crystal Of the basal plane dislocations (BPD) existing on the growth surface of the substrate is determined.
- the ratio can be determined from the graph shown in FIG.
- the sample for determining the conversion ratio is preferably a film formed at the same time. This is because variations in production conditions tend to cause variations in growth conditions.
- ⁇ BPD surface density upper limit determination step> the upper limit of the surface density of BPD on the growth surface of the SiC single crystal substrate to be used is determined based on the determined ratio. That is, the upper limit of the surface density of BPD on the growth surface of the SiC single crystal substrate that can be used to determine a SiC epitaxial wafer having a desired stacking fault surface density or less is determined from the determined ratio.
- SiC epitaxial film formation process In this step, an SiC epitaxial film is formed on the SiC single crystal substrate under the same conditions as the growth conditions of the epitaxial film used in the step of determining the ratio using an SiC single crystal substrate that is equal to or less than the determined upper limit.
- a method for manufacturing a SiC epitaxial wafer according to a second embodiment of the present invention is a method for manufacturing a SiC epitaxial wafer having a SiC epitaxial layer on a SiC single crystal substrate having an off angle, and includes a SiC single wafer having an off angle.
- BPD basal plane dislocations
- TSD threading screw dislocations
- ⁇ Conversion ratio determination process> In this step, among the basal plane dislocation (BPD) and threading screw dislocation (TSD) existing on the growth surface of the SiC single crystal substrate having an off angle, a SiC epitaxial film having a predetermined film thickness formed on the SiC single crystal substrate. Determine the ratio of carrot defects in the film.
- BPD basal plane dislocation
- TSD threading screw dislocation
- carrot defects are formed by the interaction between basal plane dislocations (BPD) and threading screw dislocations (TSD), the conversion ratio of carrot defects depends on these two variables (two kinds of dislocation densities). That is, for example, even if the surface density of basal plane dislocations is the same, when the surface density of threading screw dislocations is different, the conversion ratio of carrot defects is different. However, when the surface density of threading screw dislocations is sufficiently high (for example, when the surface density of basal plane dislocations is 10 4 pieces / cm 2 or more, whereas the surface density of threading screw dislocations is 10 3 pieces / cm 2 or more. ), The conversion ratio of carrot defects is about 1/10000 to 1/10000 of the surface density of basal plane dislocations. Therefore, when the surface density of threading screw dislocations is sufficiently high in relation to the surface density of basal plane dislocations, the conversion ratio of carrot defects to the surface density of basal plane dislocations can be used as the ratio.
- the surface density of the carrot defect in a SiC epitaxial film is measured using the photo-luminescence (PL) imaging method.
- the SiC single crystal Of the basal plane dislocations (BPD) existing on the growth surface of the crystal substrate is determined.
- the sample for determining the conversion ratio is preferably a film formed at the same time. This is because variations in production conditions tend to cause variations in growth conditions.
- the upper limit of the surface density of BPD and TSD on the growth surface of the SiC single crystal substrate to be used is determined based on the determined ratio. That is, from the determined ratio, the upper limit of the surface density of BPD and TSD on the growth surface of the SiC single crystal substrate that can be used to determine the SiC epitaxial wafer having a desired stacking fault surface density or less is determined.
- SiC epitaxial film formation process In this step, an SiC epitaxial film is formed on the SiC single crystal substrate under the same conditions as the growth conditions of the epitaxial film used in the step of determining the ratio using an SiC single crystal substrate that is equal to or less than the determined upper limit.
- SiH 4 gas as a silicon-containing gas and C 3 H 8 gas as a carbon-containing gas, N 2 gas as a doping gas, H 2 gas as a carrier gas and an etching gas are used, and a mass production type multiple-planetary is used.
- Example 1 A SiC epitaxial wafer in which a SiC epitaxial layer is formed on the Si surface of a 4H—SiC single crystal substrate tilted at an off angle of 4 ° is manufactured.
- the 4H—SiC single crystal substrate is not subjected to convex processing.
- the surface density of the four basal plane dislocations (BPD) shown in Table 1 corresponds to the surface density of the basal plane dislocations (BPD) of the SiC single crystal substrate polished under these polishing conditions.
- the thing with the lowest BPD density was performed on the following polishing conditions. That is, mechanical polishing before CMP was performed at a processing pressure of 350 g / cm 2 using abrasive grains having a diameter of 5 ⁇ m or less.
- CMP was performed for 30 minutes using silica particles having an average particle diameter of 10 to 150 nm as abrasive particles, a slurry containing sulfuric acid as an inorganic acid and a pH of 1.9 at 20 ° C. Thereby, the lattice disorder layer on the surface was made 3 nm or less.
- the polished substrate was introduced into the growth apparatus after RCA cleaning.
- the RCA cleaning is a wet cleaning method generally used for Si wafers, and a substrate is prepared by using a mixed solution of sulfuric acid / ammonia / hydrochloric acid and hydrogen peroxide solution and a hydrofluoric acid aqueous solution. Organic substances, heavy metals and particles on the surface can be removed.
- the cleaning (gas etching) step was performed at a hydrogen gas flow rate of 100 slm, a reactor internal pressure of 200 mbar, and a substrate temperature of 1500 ° C. for 20 minutes.
- the substrate temperature is set to 1650 ° C., and C 3 H 8 gas 24 sccm and SiH 4 gas 8 sccm are simultaneously started so that SiH 4 gas and C 3 H 8 gas are simultaneously supplied to the main surface of the substrate. went.
- C / Si was selected to be 1.0.
- a SiC epitaxial layer having a thickness of 10 ⁇ m was formed by carrying out a growth process for 2 hours at a growth rate of 5 ⁇ m / h with the pressure in the reactor set to 200 mbar.
- the surface density of stacking faults (SF) in the SiC epitaxial film was measured using a photoluminescence (PL) imaging method.
- the surface density of the four stacking faults (SF) shown in Table 1 is obtained.
- the surface density of BPD on the growth surface of the SiC single crystal substrate to be used is based on the graph shown in FIG.
- the upper limit of 1.0 ⁇ 10 3 pieces / cm 2 was determined.
- the surface disordered layer was adjusted to 2.5 nm or less by adjusting the polishing conditions.
- the density of the basal plane dislocation was measured using reflection X-ray topography, and found to be 0.9 ⁇ 10 3 pieces / cm 2 .
- the substrate was processed and the SiC epitaxial layer was formed under the same conditions as described above, thereby producing a SiC epitaxial wafer.
- the resulting SiC epitaxial wafer was measured for surface density of stacking faults in the photoluminescence (PL) imaging method, it was 0.09 pieces / cm 2.
- the present invention can be applied to a SiC epitaxial wafer and a manufacturing method thereof.
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Abstract
Description
本願は、2011年9月9日に、日本に出願された特願2011-197626号に基づき優先権を主張し、その内容をここに援用する。
このようにc面に対してオフ角を有する面を成長面とするため、c面に存在する基底面転位(BPD)も成長面に露出することになる。また、c軸方向に延在する貫通螺旋転位(TSD)及び貫通刃状転位(TED)も成長面に露出する。
また、キャロット欠陥についても、基底面転位(BPD)及び貫通螺旋転位(TSD)の密度が高い場合には、SiCエピタキシャル膜中のキャロット欠陥の面密度が、SiC単結晶基板中の基底面転位(BPD)の面密度に相関することを有することを見出した。これにより、所定のオフ角のSiC単結晶基板を用い、所定のエピタキシャル膜の成長条件で、所定の膜厚のSiCエピタキシャル膜を形成する場合、そのSiC単結晶基板の成長面での基底面転位(BPD)及び貫通螺旋転位(TSD)の面密度が既知であれば、形成されるSiCエピタキシャル膜中のキャロット欠陥の面密度が予測できることとなり、本発明に想到した。
また、本発明は、SiCエピタキシャル膜における、キャロット欠陥の面密度が低減されたSiCエピタキシャルウェハ及びその製造方法を提供することを目的とする。
(1)オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハの製造方法であって、前記オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD)のうち、前記SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜において積層欠陥になる比率を決定する工程と、前記比率に基づいて、使用するSiC単結晶基板の成長面におけるBPDの面密度の上限を決定する工程と、前記上限以下のSiC単結晶基板を用いて、前記比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、前記SiC単結晶基板上にSiCエピタキシャル膜を形成する工程と、を有することを特徴とするSiCエピタキシャルウェハの製造方法。
(2)前記比率を決定するに際して、前記成長面におけるBPDの面密度、及び、前記成長面のBPD起因の、前記SiCエピタキシャル膜中の積層欠陥の面密度を、X線トポグラフィ、又は、フォトルミネセンスのいずれかの方法で測定することを特徴とする請求項1に記載のSiCエピタキシャルウェハの製造方法。
(3)前記上限が1.0×103個/cm2以下であることを特徴とする請求項1又は2のいずれかに記載のSiCエピタキシャルウェハの製造方法。
(4)オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハの製造方法であって、前記オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD)のうち、前記SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜においてキャロット欠陥になる比率を決定する工程と、前記比率に基づいて、使用するSiC単結晶基板の成長面におけるBPD及びTSDの面密度の上限を決定する工程と、前記上限以下のSiC単結晶基板を用いて、前記比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、前記SiC単結晶基板上にSiCエピタキシャル膜を形成する工程と、を有することを特徴とするSiCエピタキシャルウェハの製造方法。
(5)オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハであって、SiCエピタキシャル膜における、SiC単結晶基板のBPD起因の積層欠陥の面密度が0.1個/cm2以下であることを特徴とするSiCエピタキシャルウェハ。
図1(a)及び(b)は、X線トポグラフィ法で得られたSiC単結晶基板のトポグラフィ像を示す。図1(a)及び(b)はそれぞれ、基底面転位(BPD)の面密度が6.5×104個/cm2、5.5×103個/cm2のものである。
像中の矢印は基底面転位(BPD)の一部を示している。
本発明においては、反射X線トポグラフィを用いて炭化珪素単結晶ウェハからのX線回折光を測定することにより、ウェハ面内における結晶欠陥を検出することができる。反射X線トポグラフィを用いることにより、エッチング等破壊的な手法を併用することなく、結晶欠陥の位置の検出を非破壊的に行うことができるという利点がある。
モノクロメーターを用いて波長を1.54ÅとしたX線を入射光として反射X線トポグラフィの測定を行った。X線を回折させる際の回折ベクトル(g-vector)としては、本発明の目的を果たすことができる限り特に制限はないが、4H-SiC結晶に対しては11-28あるいは1-108を用いるのが通常である。本明細書では11-28を用いたトポグラフィ像を示している。
X線をサンプルに照射し、該サンプルから反射してきた回折光を検出することにより、トポグラフ像を得ることができる。このトポグラフ像の取得には、欠陥種を判定するために十分な解像度を得るために、高解像度のX線フィルム、原子核乾板などの記録媒体を用いる。今回は原子核乾板を用いた。その画像から、基底面転位、貫通刃状転位、貫通らせん転位の数をカウントした。
図2(a)及び(b)はそれぞれ、エピタキシャル膜中の積層欠陥(SF)の面密度が2.5×102個/cm2、9.2個/cm2のものである。
像中の矢印は基底面転位(BPD)の一部、又は、積層欠陥(SF)の一部を示している。
BPD面密度とSF面密度とがほぼ比例関係を有することがわかる。かかる関係を有することから、所望のSF面密度を有するSiCエピタキシャルウェハを作製するために要する、SiC単結晶基板のBPD面密度の上限を決定することができる。
図4(a)及び(b)はそれぞれ、基底面転位(BPD)の面密度が5.0×104個/cm2かつ貫通螺旋転位(TSD)の面密度が2.8×104個/cm2と、基底面転位(BPD)の面密度が2.0×104個/cm2かつ貫通螺旋転位(TSD)の面密度が5.4×103個/cm2のものである。
図4(a)の像中に、典型的な基底面転位(BPD)及び貫通螺旋転位(TSD)を示している。
図5(a)及び(b)のそれぞれにおいて、左側の像は欠陥マップを示すものであり、右側の像は左側の欠陥マップの矢印で示したカンデラ像である。
図5(a)及び(b)はそれぞれ、エピタキシャル膜中のキャロット欠陥密度が20.2個/cm2のもの、0.2個/cm2のものである。
以下、本発明の実施形態であるSiCエピタキシャルウェハの製造方法について詳細に説明する。
本発明の第1の実施形態であるSiCエピタキシャルウェハの製造方法は、オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハの製造方法であって、オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD)のうち、SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜において積層欠陥になる比率を決定する工程と、比率に基づいて、使用するSiC単結晶基板の成長面におけるBPDの面密度の上限を決定する工程と、上限以下のSiC単結晶基板を用いて、比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、SiC単結晶基板上にSiCエピタキシャル膜を形成する工程と、を有することを特徴とする。
SiC単結晶基板としてはいずれのポリタイプのものも用いることができ、実用的なSiCデバイスを作製する為に主に使用されている4H-SiCを用いることができる。SiCデバイスの基板としては昇華法等で作製したバルク結晶から加工したSiC単結晶基板を用い、通常、この上にSiCデバイスの活性領域となるSiCエピタキシャル膜を化学的気相成長法(CVD)によって形成する。
SiC単結晶基板が2インチ程度までのサイズの場合では SiC単結晶基板のオフ角は主に8°が用いられてきた。このオフ角においてはウェハ表面のテラス幅が小さく、容易にステップフロー成長が得られるが、オフ角が大きいほど、SiCインゴットから得られるウェハ枚数が少なくなるため、3インチ以上のSiC基板においては、主に4°程度のオフ角のものが用いられている。
低オフ角になるほど、SiC単結晶基板の表面のテラス幅が大きくなるため、ステップ端に取り込まれるマイグレーション原子の取り込まれ速度、すなわちステップ端の成長速度にバラツキが生じやすく、その結果、遅い成長速度のステップに速い成長速度のステップが追いついて合体し、ステップバンチングが発生しやすい。また、例えば、0.4°のオフ角の基板では4°のオフ角の基板に比べてテラス幅は10倍になり、ステップフロー成長させる長さが一桁長くなるので、4°のオフ角の基板で用いられてきたステップフロー成長の条件を調整する必要がある点に留意する必要がある。
SiCエピタキシャルウェハの製造(SiCエピタキシャル層の形成(成長))の際、SiC単結晶基板の裏面は加熱されたサセプタから直接加熱されるが、おもて面(SiCエピタキシャル層の形成面)は真空空間に剥き出しの状態にあり、直接加熱されない。さらに、キャリアガスである水素がおもて面上を流れるため、熱が持ち去られる。これらの事情から、エピタキシャル成長時のおもて面は裏面に対して低い温度になる。この温度差に起因して熱膨張の大きさがおもて面は裏面よりも小さく、エピタキシャル成長時にはSiC単結晶基板はおもて面が凹むように変形する。そこで、SiC単結晶基板としてSiCエピタキシャル層の成長面が凸状に加工されたものを用いることで、SiC単結晶基板としてエピタキシャル成長時の基板の凹み(反り)を解消した状態でエピタキシャル成長を行うことが可能となる。
SiC単結晶基板上にSiCエピタキシャル層を形成する前にまず、SiC単結晶基板について研磨を行う。
この研磨工程では、特開2011-49496号公報に記載されている、その表面の格子乱れ層が3nm以下となる程度を目安として研磨する。
「格子乱れ層」とは、特開2011-49496号公報の図7及び図8に示されている通り、TEMの格子像(格子が確認できる像)において、SiC単結晶の原子層(格子)に対応する縞状構造又はその縞の一部が明瞭になっていない層をいう。
所望の積層欠陥(SF)の面密度のSiCエピタキシャルウェハを得るためには、SiC単結晶基板の成長面に存在する基底面転位(BPD)の面密度を<上限決定工程>で決定した上限の面密度以下となるまで、以下に記載する研磨方法を用いて研磨する。
例えば、積層欠陥面密度を0.1個/cm2以下のSiCエピタキシャルウェハを製造するために要する<研磨工程>の仕上げ程度が、上述した格子乱れ層が3nm以下となる程度に相当する。
研磨工程は、通常ラップと呼ばれる粗研磨、ポリッシュとよばれる精密研磨、さらに超精密研磨である化学的機械研磨(以下、CMPという)など複数の研磨工程が含まれる。研磨工程は湿式で行われることが多いが、この工程で共通するのは、研磨布を貼付した回転する定盤に、研磨スラリーを供給しつつ、炭化珪素基板を接着した研磨ヘッドを押しあてて行われることである。本発明で用いる研磨スラリーは、基本的にはそれらの形態で用いられるが、研磨スラリーを用いる湿式研磨であれば形態は問わない。
炭化珪素の研磨は、強酸性研磨液によって炭化珪素の表面に生成した酸化膜に対する反応性により、酸化層を酸化物粒子により除去することで行われるが、この表面酸化を加速するために、研磨スラリーに酸化剤を添加すると更に優れた効果が認められる。酸化剤としては過酸化水素、過塩素酸、重クロム酸カリウム、過硫酸アンモニウムサルフェートなどが挙げられる。たとえば、過酸化水素水であれば0.5~5質量%、望ましくは1.5~4質量%加えることにより研磨速度が向上するが、酸化剤は過酸化水素水に限定されるものではない。
図6(b)は、図6(a)に示したサンプルを再度、格子乱れ層が3nm以下となるまで研磨し直したSiC単結晶基板に、同様に10nm厚のSiCエピタキシャル膜を形成したSiCエピタキシャルウェハについて得られたPL像を示す。
図6(b)のPL像では、積層欠陥はほとんど観察されず、研磨によって積層欠陥に変換される基底面転位(BPD)を低減できることがわかった。特に、格子乱れ層が3nm以下となるまで研磨することにより、基底面転位(BPD)を有効に低減することができることがわかった。
研磨後のSiC単結晶基板の成長面について、反射X線トポグラフィを用いてその基底面転位の密度を測定する。
清浄化工程では、水素雰囲気下で、前記研磨(凸状加工した場合は、研磨及び凸状加工)後の基板を1400~1800℃にしてその表面を清浄化(ガスエッチング)する。
ガスエッチングは、SiC単結晶基板を1400~1800℃に保持し、水素ガスの流量を40~120slm、圧力を100~250mbarとして、5~30分間行う。
この後の成膜工程及び降温工程において、この基板表面の平坦性を維持することが重要となる。
SiH4ガス及び/又はC3H8ガスを添加した場合は、成膜(エピタキシャル成長)工程前に、一旦排気を行って水素ガス雰囲気にするのが好ましい。
成膜(エピタキシャル成長)工程では、(エピタキシャル膜の成長温度が清浄化(ガスエッチング)温度よりも高い場合では昇温後に)前記清浄化後の基板の表面に、炭化珪素のエピタキシャル成長に必要とされる量のSiH4ガスとC3H8ガスとを濃度比C/Siが0.7~1.2で同時に供給して炭化珪素をエピタキシャル成長させる。
(1)オフ角が0.4°~2°の4H-SiC単結晶基板を用いる場合は、炭化珪素膜をエピタキシャル成長させる成長温度を1600~1640℃とするときは、成長速度を1~3μm/hとして行い、成長温度を1640~1700℃とするときは、成長速度を3~4μm/hとして行い、成長温度を1700~1800℃とするときは、成長速度を4~10μm/hとして行い、
(2)オフ角が2°~5°の4H-SiC単結晶基板を用いる場合は、炭化珪素膜をエピタキシャル成長させる成長温度を1600~1640℃とするときは、成長速度を2~4μm/hとして行い、成長温度を1640~1700℃とするときは、成長速度を4~10μm/hとして行い、成長温度を1700~1800℃とするときは、成長速度を10~20μm/hとして行う。
降温工程では、SiH4ガスとC3H8ガスの供給を同時に停止し、SiH4ガスとC3H8ガスとを排気するまで基板温度を保持し、その後降温するのが好ましい。
この工程では、オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD)のうち、SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜において積層欠陥になる比率を決定する。
次に、予め測定して得られたSiC単結晶基板の成長面に存在する基底面転位(BPD)の面密度と、ここで得られた積層欠陥(SF)の面密度とから、SiC単結晶基板の成長面に存在する基底面転位(BPD)のうち、SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜において積層欠陥になる比率を決定する。
具体的には例えば、図3に示したグラフからその比率を決定することができる。
変換比率を決定するサンプルは同時に成膜したものを用いることが好ましい。製造ロットが異なると、成長条件にバラツキが出やすいからである。
この工程では、決定した比率に基づいて、使用するSiC単結晶基板の成長面におけるBPDの面密度の上限を決定する。
すなわち、決定した比率から、所望の積層欠陥面密度以下のSiCエピタキシャルウェハを得るために、使用することができるSiC単結晶基板の成長面におけるBPDの面密度の上限を決定する。
この工程では、決定した上限以下のSiC単結晶基板を用いて、比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、SiC単結晶基板上にSiCエピタキシャル膜を形成する。
本発明の第2の実施形態であるSiCエピタキシャルウェハの製造方法は、オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハの製造方法であって、オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD)及び貫通螺旋転位(TSD)のうち、SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜においてキャロット欠陥になる比率を決定する工程と、比率に基づいて、使用するSiC単結晶基板の成長面におけるBPDの面密度の上限を決定する工程と、上限以下のSiC単結晶基板を用いて、比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、SiC単結晶基板上にSiCエピタキシャル膜を形成する工程と、を有することを特徴とする。
研磨後のSiC単結晶基板の成長面について、反射X線トポグラフィを用いてその基底面転位の密度及び貫通螺旋転位の密度を測定する。
この工程では、オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD) 及び貫通螺旋転位(TSD)のうち、SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜においてキャロット欠陥になる比率を決定する。
しかしながら、貫通螺旋転位の面密度が十分に高い場合(例えば、基底面転位の面密度が104個/cm2以上に対して、貫通螺旋転位の面密度が103個/cm2以上の場合)には、キャロット欠陥の変換比率は基底面転位の面密度の1/10000~1/100000程度となる。
従って、基底面転位の面密度との関係で貫通螺旋転位の面密度が十分に高い場合には、基底面転位の面密度に対するキャロット欠陥の変換比率を当該比率として用いることができる。
次に、予め測定して得られたSiC単結晶基板の成長面に存在する基底面転位(BPD) 及び貫通螺旋転位(TSD)と、ここで得られたキャロット欠陥の面密度とから、SiC単結晶基板の成長面に存在する基底面転位(BPD)のうち、SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜においてキャロット欠陥になる比率を決定する。
変換比率を決定するサンプルは同時に成膜したものを用いることが好ましい。製造ロットが異なると、成長条件にバラツキが出やすいからである。
この工程では、決定した比率に基づいて、使用するSiC単結晶基板の成長面におけるBPD及びTSDの面密度の上限を決定する。
すなわち、決定した比率から、所望の積層欠陥面密度以下のSiCエピタキシャルウェハを得るために、使用することができるSiC単結晶基板の成長面におけるBPD及びTSDの面密度の上限を決定する。
この工程では、決定した上限以下のSiC単結晶基板を用いて、比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、SiC単結晶基板上にSiCエピタキシャル膜を形成する。
本実施例では、珪素含有ガスとしてSiH4ガスおよび炭素含有ガスとしてC3H8ガス、ドーピングガスとしてN2ガス、キャリアガスおよびエッチングガスとしてH2ガスを使用し、量産型の複数枚プラネタリー(自公転)型CVD装置であるアイクストロン社製Hot Wall SiC CVD(VP2400HW)によって、4H-SiC単結晶の(0001)面に対して<11-20>軸方向へ微傾斜させたSi面又はC面にSiCエピタキシャル膜を成長させた。
4°のオフ角で傾斜させた4H-SiC単結晶基板のSi面上にSiCエピタキシャル層を形成したSiCエピタキシャルウェハを製造する。
本実施例では、4H-SiC単結晶基板については、凸状加工を施していない。
なお、最もBPD密度が低いものは以下の研磨条件で行った。すなわち、CMP前の機械研磨は直径5μm以下の砥粒を用いて、加工圧力を350g/cm2で行った。また、CMPは、研磨材粒子として平均粒子径が10~150nmのシリカ粒子を用い、無機酸として硫酸を含み、20℃におけるpHが1.9の研磨スラリーを用いて、30分間行った。これにより、表面の格子乱れ層を3nm以下とした。
清浄化(ガスエッチング)工程は、水素ガスの流量100slm、リアクタ内圧力を200mbar、基板温度を1500℃で、20分間行った。
SiCエピタキシャル成長工程では、基板温度を1650℃とし、SiH4ガス及びC3H8ガスが基板主面に同時に供給されるように、C3H8ガス24sccm及びSiH4ガス8sccmを同時に供給開始して行った。C/Siは1.0を選択した。リアクタ内圧力を200mbarとし、成長速度5μm/hで2時間成長工程を実施して、厚さ10μmのSiCエピタキシャル層を成膜した。
この上限のBPD面密度のSiC単結晶基板を得るために、研磨条件を調整して表面の格子乱れ層を2.5nm以下とした。
このSiC単結晶基板を用いて、上述の条件と同じ条件で基板の処理及びSiCエピタキシャル層の形成を行うことにより、SiCエピタキシャルウェハを作製した。
得られたSiCエピタキシャルウェハについて、フォトルミネッセンス(PL)イメージング法で積層欠陥の面密度を測定したところ、0.09個/cm2であった。
Claims (5)
- オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハの製造方法であって、
前記オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD)のうち、前記SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜において積層欠陥になる比率を決定する工程と、
前記比率に基づいて、使用するSiC単結晶基板の成長面におけるBPDの面密度の上限を決定する工程と、
前記上限以下のSiC単結晶基板を用いて、前記比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、前記SiC単結晶基板上にSiCエピタキシャル膜を形成する工程と、を有することを特徴とするSiCエピタキシャルウェハの製造方法。 - 前記比率を決定するに際して、前記成長面におけるBPDの面密度、及び、前記成長面のBPD起因の、前記SiCエピタキシャル膜中の積層欠陥の面密度を、X線トポグラフィ、又は、フォトルミネセンスのいずれかの方法で測定することを特徴とする請求項1に記載のSiCエピタキシャルウェハの製造方法。
- 前記上限が1.0×103個/cm2以下であることを特徴とする請求項1又は2のいずれかに記載のSiCエピタキシャルウェハの製造方法。
- オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハの製造方法であって、
前記オフ角を有するSiC単結晶基板の成長面に存在する基底面転位(BPD)及び貫通螺旋転位(TSD)のうち、前記SiC単結晶基板上に形成された、所定膜厚のSiCエピタキシャル膜においてキャロット欠陥になる比率を決定する工程と、
前記比率に基づいて、使用するSiC単結晶基板の成長面におけるBPD及びTSDの面密度の上限を決定する工程と、
前記上限以下のSiC単結晶基板を用いて、前記比率を決定する工程において用いたエピタキシャル膜の成長条件と同じ条件で、前記SiC単結晶基板上にSiCエピタキシャル膜を形成する工程と、を有することを特徴とするSiCエピタキシャルウェハの製造方法。 - オフ角を有するSiC単結晶基板上にSiCエピタキシャル層を備えたSiCエピタキシャルウェハであって、SiCエピタキシャル膜における、SiC単結晶基板のBPD起因の積層欠陥の面密度が0.1個/cm2以下であることを特徴とするSiCエピタキシャルウェハ。
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