JP2006032655A - Manufacturing method of silicon carbide substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a homo-epitaxial growth layer with a small surface defect density on a silicon carbide substrate with a low off-angle. <P>SOLUTION: This manufacturing method comprises the steps of machining the ä0001} plane of silicon carbide, etching the surface of the silicon carbide by plasma of a material containing at least a halogen after the machining step, and exposing the surface of the silicon carbide to a plasma of a material not containing a halogen and containing at least oxygen after the etching step. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a silicon carbide substrate having a homoepitaxial growth layer on the surface.

Silicon carbide (SiC) is a wide band gap semiconductor having excellent characteristics, and is expected as one of the most promising candidates for substrate materials for next-generation power devices.
A power device using an SiC substrate is built into a low-concentration homoepitaxial growth layer (low-doped layer) formed on the surface of the SiC substrate, and the performance of the device depends on the film quality of this low-doped layer, particularly the surface defect density. It depends.

  By the way, in SiC crystals grown homoepitaxially, there may be “heterogeneous polytypes” having the same chemical composition but different stacking order in the stacking direction. Different types of polytypes mixed during growth generate atomic arrangement defects (stacking faults), which together with polishing distortion and foreign matter increase the surface defect density.

On the other hand, the wafer size has been increased due to demands from device manufacturing, 3 inch wafers are in mass production level, and 4 inch wafers are also prototyped at the laboratory level. Moreover, the chip size of SiC for power devices has been increased due to demands from device applications, and has reached the size of 1 cm square (1 cm × 1 cm). For this reason, the required surface defect density of the low-doped layer needs to be smaller than 1 per 1 cm ² .

  As one method of reducing the mixing of different polytypes, a substrate with the crystal plane on the outermost surface inclined several degrees with respect to the crystal axis so as to convey the lamination information of the underlying SiC substrate (this is referred to as “off-angle The method of growing on a substrate with a "." Is widely used. This makes it difficult for foreign polytypes to be mixed even when grown under the same conditions (for example, see Non-Patent Document 1).

  Therefore, the SiC substrate is sliced with an off angle of about 8 degrees from the SiC ingot, and processing for removing polishing scratches and crystal defects on the substrate surface before epitaxial growth, specifically, chemical machinery Polishing scratches and damaged layers on the substrate surface are removed by polishing (CMP) or reactive ion etching (RIE), exposed to hydrogen gas or hydrogen chloride gas, or a combination of these is performed. Ingenuity is being tried.

Applied Physics, Vol.64, No.7, pp.691-694 (1995)

  However, when the substrate is sliced with an off-angle, there is a problem that a large amount of waste is generated in an expensive SiC ingot. For example, in the case of a 4-inch wafer, an ingot length of 14 mm (= 4 × 2.54 × tan 8 °) is consumed only to introduce 8 degrees off. For this reason, in view of industrial implementation, it is desirable that homoepitaxial growth with an extremely small surface defect density can be performed on a SiC substrate having a low off-angle as much as possible.

  However, as described above, if the off-angle is reduced in order to suppress the consumption of ingots, different types of polytypes (3C-) are likely to be mixed, and the surface is more susceptible to polishing distortion and foreign matter, resulting in surface defects. Density increases. In particular, 4H-SiC, which is expected for power devices, is markedly mixed with different polytypes. Conversely, if the off-angle is increased, the ingot is wasted, so that both are in a trade-off relationship.

  In the process of conducting homoepitaxial growth on a SiC substrate using a low off-angle substrate and studying different polytypes mixed during crystal growth, the present inventors include different polytypes and dislocations mixed during growth. It has been found that large surface defects are caused by extremely small and almost undetectable foreign matter on the order of nanometers adhering to the substrate surface before epitaxial growth.

  This invention is made | formed in view of the above, and makes it the main technical subject to form the homoepitaxial growth layer with a small surface defect density on the SiC substrate of a low off angle.

  The method for manufacturing a silicon carbide substrate according to the present invention includes an etching step (SA1) for etching the surface of the silicon carbide substrate with a plasma of a substance containing at least halogen, and a cleaning for removing foreign substances adhering to the surface of the silicon carbide. It includes a step (SA2) and an epitaxial growth step (SA3) for homoepitaxial growth on the surface of the silicon carbide.

  This etching step (SA1) is a step of removing a damaged layer on the surface that causes mixing of different polytypes and generation of surface defects, and a reactive ion etching (RIE) method or the like can be applied. Although chemical mechanical polishing (CMP) may be performed instead of etching, it is not preferable to perform only CMP because physical damage causes the substrate to be damaged. However, there is no problem in performing RIE after performing CMP.

  Further, the subsequent cleaning step (SA2) is a step for removing a slight residue remaining on the surface of the surface to be etched after the etching step, which is the most essential part of the present invention.

  This cleaning step (SA2) is preferably a step of exposing the surface to be etched to plasma of a substance containing oxygen. Note that the etching process and the cleaning process may be performed by the same apparatus.

  The plasma atmosphere in the cleaning step (SA2) preferably does not contain a halogen element. This is because when the halogen element is contained, carbon in silicon carbide forms a polymer film with a halogen atom, so that the cleaning efficiency is lowered.

  On the other hand, the etching step (SA1) is preferably performed under conditions that do not cause deposits during the etching. The cleaning process (SA2) performed after the etching process is a process of removing foreign substances deposited on the substrate during etching and after the etching is stopped. In order to completely remove foreign substances in the cleaning process, as much as possible during etching. This is because it is preferable to suppress the formation of deposits.

Specifically, the etching gas in the etching step (SA1) contains carbon fluoride and oxygen, and the number of oxygen atoms contained in the plasma is at least three times the number of carbon atoms supplied as the etching gas. It is preferable to make it. In RIE, when the number of oxygen atoms contained in the plasma is more than three times the number of carbon atoms, on average, there is no chemical reactive species that generates a polymer, so the possibility of generating deposits can be sufficiently reduced. Because. In consideration of plasma non-uniformity, it is desirable to set it to 4 times or more with a margin.
Specifically, for example, when a mixed gas of carbon tetrafluoride gas and oxygen gas is used as an etching gas, theoretically, the mixing ratio (flow rate ratio) of carbon tetrafluoride and oxygen is at least 2: 3, More oxygen than is required.

  When the surface of the silicon carbide substrate is a (0001) Si surface, it is preferable that the cleaning step (SA2) does not oxidize the silicon carbide surface and does not cause oxygen atom knock-on. Specifically, for example, as the oxygen plasma treatment time is longer or the plasma power is larger, oxygen atoms are more easily knocked on. Therefore, it is desirable to finish the cleaning process with the minimum necessary processing time and plasma power.

  The method for manufacturing a silicon carbide substrate according to the present invention can be applied to a 4H-SiC substrate, a 6H-SiC substrate, and other various polytypes, but 4H-SiC is an industrially available polytype, This is because the electron mobility is relatively large and the dielectric breakdown electric field is also large, so that a 4H—SiC substrate is preferable when applied to a high voltage power device.

  The silicon carbide substrate in the present invention may be a substrate having a small off angle. According to the present invention, homoepitaxial growth with a low crystal defect density can be performed regardless of the off-angle, but even if the off-angle is small, the crystal defect density can be reduced, and the technical significance in that case is even greater. .

  Specifically, the surface of the silicon carbide substrate is a {0001} plane and has an inclination of 0 ° to 1 °, more preferably 0.5 ° to 1 ° from the <0001> direction, that is, an off angle, It is preferable to have

  The surface of the silicon carbide substrate is preferably a (0001) Si surface. This is because the Si surface has a smaller residual nitrogen donor density than the C surface, and thus is preferable for manufacturing a high voltage device. Conventionally, in crystal growth on a low off-angle substrate, the C plane has a smaller surface defect density than the Si plane. However, in the present invention, the occurrence of surface defects (triangular surface defects) This does not occur because the foreign matter is removed, and a homoepitaxial growth layer having a very low crystal defect density can be formed on the Si surface, which is an ideal surface when applied to a high voltage device.

  According to the present invention, it is possible to form a homoepitaxial growth layer having a low surface defect density on a low off-angle surface of silicon carbide or a substantially non-off surface. In particular, the effect is remarkable in the {0001} plane of silicon carbide, particularly the (0001) Si plane.

(About the problem solving mechanism)
First, the problem solving mechanism of the present invention will be described. In addition, although the surface of silicon carbide is described by adding a bar on a required number, in this specification, it is expressed by adding a-symbol before the required number.

  When homoepitaxial growth is performed on the hexagonal silicon carbide (6H—SiC) {0001} plane, if the off angle is small, a large amount of different polytypes, particularly cubic (3C—) silicon carbide is mixed. Conventionally, this cause has been considered to be due to surface damage caused by polishing. Therefore, in order to remove these polishing damages, chemical mechanical polishing (CMP), reactive ion etching (RIE), a mixed gas of hydrogen chloride and hydrogen, or high temperature etching with hydrogen gas under reduced pressure is appropriately combined. Used.

  However, with a substrate having a small off angle, the conventional method of removing the damaged layer does not work, and the surface defect density increases. In particular, the increase in the surface defect density in the growth to a low off-angle substrate is a remarkable phenomenon particularly in the (0001) Si plane, and there is a report recommending the use of the C plane rather than the Si plane. However, as described above, a Si surface capable of lowering the impurity concentration is desirable for a high breakdown voltage power device. Therefore, using the C-plane is not the best method.

  By the way, there are various types of surface defects, but a triangular surface defect is likely to occur when the off-angle is small. This defect is likely to occur in inverse proportion to the sine of the off angle θ (sin θ), so that the defect density increases as the off angle decreases, thereby causing stacking faults due to different polytypes.

  The cause of the occurrence of triangular surface defects is thought to be due to the presence of foreign matter in the step flow growth region during epitaxial growth, not the underlying damage layer. In step flow growth, even if there is a foreign substance having a nanometer level, epitaxial growth is inhibited in that region, and surface defects are generated. The mechanism of this surface defect formation is explained by the following model.

4 to 6 are enlarged views at the atomic level for explaining the mechanism of defect formation caused by the foreign matter, in which (a) is a top view and (b) is a cross-sectional view.
FIG. 4 shows a state before growth and shows a stepped atomic surface composed of the terrace F1 and the step S of the SiC substrate 10. Assume that one foreign substance 12 is attached on the surface terrace F1.

FIG. 5 shows a state in which the epitaxial growth has progressed after some time has elapsed from the state of FIG. Homoepitaxial growth is dominated by step flow mode growth in which lateral growth proceeds from atomic steps.
As shown in FIG. 5B, each of the surface terraces F2 normally expands the step flow growth region where the foreign matter 12 is not present, but the step flow growth is inhibited in the step where the foreign matter is present. The When this state is viewed from the upper surface, as shown in FIG. 5A, a triangular surface defect F2 apex at the position where the foreign matter is attached grows.

  FIG. 6 shows the state where the growth has been completed. It can be seen that a larger triangular surface defect F3 is formed from one foreign substance. FIG. 7 shows a bird's eye view of FIG. The greatly grown triangular surface defect F3 is really concave.

  Although the models shown in FIGS. 4 to 6 are simple, only step flow growth in the off direction is considered, but in reality, the final flow is generated due to generation of two-dimensional nuclei in the defect and wraparound of the step flow. May be buried as shown in FIG. 8 instead of the state shown in FIG. As shown in this figure, a region 15 separated by dislocations is formed next to the foreign material 12. In addition, the region 15 itself separated by this dislocation may further include a stacking fault or a different polytype. And the terrace F4 formed on the area | region 15 separated by the dislocation is really recessed. The defect boundary 16 including dislocations is represented by a broken line extending linearly from the end of the terrace F4. That is, dislocations exist on the broken line. The defect boundary 17 does not include dislocations unless there are dissimilar polytypes or stacking faults within the region 15 separated by dislocations.

  Based on the above considerations, the cause of the generation of foreign matters that may cause surface defects was examined. Deposits deposited on the SiC substrate during the RIE process performed before epitaxial growth to remove the damaged layer Was suspected. The present inventors have thought that this may become a foreign substance and induce a triangular surface defect during epitaxial growth.

  The size of the deposit (foreign matter) adhering to the wafer surface in the RIE process is a nano-level size, and is specifically considered to be a fluorocarbon compound such as tetrafluoroethylene resin. Since these are chemically very stable compounds, they cannot be easily removed by ordinary chemical reaction or wet cleaning.

Since the foreign matter is a nano-level foreign matter, it cannot be confirmed with an optical microscope or a scanning electron microscope. Further, if the number density is small, there is a high possibility that none exists in the observation range even by an atomic force microscope (AFM), and there is a possibility that it cannot be confirmed. For example, a number density of 10,000 per square centimeter gives, on average, only one particle per 100 microns square. The 100-micron square corresponds to the largest observation range in an atomic force microscope that is currently in circulation.
The inventor of the present case tried to observe the surface of the SiC substrate with an atomic force microscope in advance after the RIE process, but could not catch the foreign matter within the measurement range. Therefore, it is considered that it was difficult to find the cause of crystal defects and the countermeasures without the theoretical consideration as described above.

  However, based on the above theoretical considerations, it is clear that a small amount of foreign matter adhering to the SiC substrate induces and promotes the generation of defects in homoepitaxial growth. Even if the amount of foreign matter attached is small, when epitaxial growth is performed in the step flow mode, large surface defects grow in various places where foreign matter exists, and the defect density on the surface increases.

  The present invention has been made on the basis of the above considerations and experiments based on the above considerations, and the most essential part of the present invention is that a process for removing (cleaning) foreign substances that cause generation of triangular surface defects is introduced. It is. Hereinafter, embodiments will be described.

(First embodiment)
FIG. 1 is a process diagram for explaining an example of a method for manufacturing a silicon carbide substrate (SiC substrate) according to the present invention. First, a wafer cut from a SiC ingot is prepared. Two types of wafers are used: a 4H-SiC (0001) Si surface 0.7 degree off substrate and a 4H-SiC (000-1) C surface 0.7 degree off substrate.
The reason why 4H-SiC is used is that it has a large dielectric breakdown electric field and is suitable for a high voltage power device. However, the present invention is naturally applicable not only to 4H—SiC but also to 6H—SiC. The reason why both the Si surface and the C surface are used is to compare the effects of the present invention.

In this state, the wafer contains a large amount of polishing scratches and foreign matters generated during slicing. Therefore, the surface of the substrate is etched after being mirror-finished by mechanical polishing and then subjected to predetermined wet cleaning and the like. For example, a reactive ion etching (RIE) apparatus may be used for the etching. By this etching step (SA1), the damaged layer on the surface is removed.
As the RIE apparatus, a parallel plate plasma generator having an electrode interval of about 7 cm was used. As the etching gas, a mixed gas in which carbon tetrafluoride (CF ₄ ) gas and oxygen (O ₂ ) gas were mixed at a ratio of 1: 2 was used. The pressure during etching was about 25 Pa, and etching was performed with high frequency plasma of 13.56 MHz for 35 minutes. The plasma power density was 4 W per square centimeter and the SiC etch rate was 150 nm / min.

  Conventionally, an epitaxial growth step (SA3) has been performed thereafter, but in the present invention, it is important to include a next cleaning step (SA2) between the etching step (SA1) and the epitaxial growth step (SA3).

  The cleaning process (SA2) is a process for removing foreign matters generated during the RIE process or after the RIE plasma is stopped. Although various cleaning methods are conceivable, sputtering is not preferable because it causes damage to a portion on the SiC surface where no foreign matter exists. Therefore, it is preferable to use a reaction by a chemical reactive species excited to a thermal equilibrium state or higher. The simplest method is to use plasma excited oxygen.

Since plasma-excited oxygen can decompose the fluorocarbon polymer which is a foreign substance without causing sputtering, it is suitable for the cleaning process in the present invention. Specifically, the surface of the substrate after etching is exposed to oxygen plasma.
In this cleaning process, the same chamber as the RIE apparatus that has performed the etching process (SA1) may be used, or a different chamber may be used.

  However, cleaning with oxygen plasma requires a little care. Although the details will be described later, in short, if it is too strong or too weak, the desired effect cannot be obtained. For example, in the case of a parallel plate plasma generator, the successful example obtained in the experiment may be exposed to plasma applying a high-frequency power of about 0.4 W per square centimeter at a pressure of 10 Pa, a frequency of 13.56 MHz, and about 2 minutes.

  By this cleaning process, foreign matters such as extremely small amounts of etching deposits attached to the SiC surface are completely removed. Thereafter, epitaxial growth is performed by a chemical vapor deposition (CVD) chamber or the like. In addition, before and after each step, a predetermined wet cleaning step, a step of exposing to a mixed gas atmosphere of hydrogen chloride and hydrogen or a mixed gas atmosphere of hydrogen gas or hydrogen gas and propane gas under reduced pressure, and the like may be included. Good.

  For the epitaxial growth, a normal pressure horizontal cold wall CVD apparatus was used. The source gas was a mixed gas of monosilane gas and propane gas, and the carrier gas was hydrogen. The mixing ratio of monosilane gas and propane gas is set to a monosilane gas flow rate of 0.50 ml / min (standard state), and propane gas is set to 0.26 ml / min (standard state) for the (0001) Si surface. 000-1) 0.33 ml / min (standard state) for the C-plane. The flow rate of hydrogen of the carrier gas is 3 l / min (standard state). The substrate temperature during growth was about 1500 ° C. and the growth time was 2 hours.

(Measurement of defect density)
FIG. 2A is a table showing the relationship between the oxygen plasma treatment time in the cleaning step (SA2) and the triangular surface defect density. On both the (0001) Si face and the (000-1) C face, the triangular surface defect density rapidly decreased by the oxygen plasma treatment for only 2 minutes, and thereafter, the tendency was saturated at about 10 per square centimeter. This is presumably because the cause of the occurrence of triangular surface defects was removed by the oxygen plasma treatment.
In the case of a low off-angle substrate, the surface defect density is generally high on the Si surface and low on the C surface, but as is clear from the comparative example, according to the present invention, there was almost no difference between the two. .

  The triangular surface defect density is saturated at about 10 per square centimeter because the foreign material generated by the previous CVD growth adheres to the furnace wall in the CVD growth apparatus, and this is caused when the substrate for the growth is introduced. This is thought to be due to the fall on the substrate surface. In fact, the triangular surface defect density tended to increase slightly with the number of CVD growths. Foreign matter generated by CVD growth is generated downstream of the reaction portion with respect to the flow direction of the carrier gas. By introducing the substrate from the upstream side of the reaction portion, foreign matter generated by CVD growth is avoided. be able to.

  In the CVD growth apparatus, it is preferable to provide a plasma processing apparatus on the upstream side of the reaction part because foreign substances can be prevented from adhering during transfer from the plasma processing apparatus to the CVD growth apparatus. This can be realized, for example, by providing a high-frequency coil upstream of the reaction portion in the CVD apparatus, carrying out RIE and oxygen plasma treatment in the vicinity thereof, and then transporting the substrate to the CVD reaction portion.

  When the oxygen plasma treatment time is long, the fact that the triangular surface defect density slightly increases only on the (0001) Si surface is considered to be due to the knock-on effect of oxygen atoms. That is, when oxygen atoms are adsorbed on the surface of silicon carbide, the oxygen atoms are pushed into the silicon carbide when subjected to ion bombardment due to incidence of oxygen ions. In the (000-1) C plane, since the outermost surface is a carbon atom, the carbon atom and the pushed-in oxygen atom are desorbed in the form of carbon monoxide, so that the pushed-in oxygen atom is removed. However, since the outermost surface is a silicon atom in the (0001) Si surface, in order to remove the pushed-in oxygen atoms, the outermost silicon atoms were pushed away and pushed into the carbon atoms present in the next atomic layer. Oxygen atoms need to be desorbed in the form of carbon monoxide, causing distortion on the surface. Therefore, in the case of the (0001) Si surface, it is better not to unnecessarily increase the oxygen plasma treatment time. For the same reason, in the case of the (0001) Si surface, it is better not to unnecessarily increase the oxygen plasma power.

  According to the method for manufacturing a silicon carbide substrate according to the present invention, it is possible to perform homoepitaxial growth on a low off-angle substrate, ideally, a substantially off-off substrate. However, a substrate that is completely just (no off) is difficult to obtain in the first place, and does not grow in the step flow growth mode. Therefore, just (no off) has a substantially off angle of about 0.5 degrees. It should be understood as a broad concept that means small things.

(Comparative example) -Effect of RIE process-
If foreign matter adheres by the RIE process, it may be considered that the defect density is reduced if the RIE process is not performed or if the RIE processing time is shortened. However, as the following experimental results show, it can be seen that the RIE process is a process necessary for reducing the defect density of the homoepitaxial growth layer.

[Experiment] The oxygen plasma treatment time was fixed at 20 minutes, and the RIE time was changed from 7 to 70 minutes. The apparatus and other conditions are the same as described above.
FIG. 2B is a table showing the relationship between the RIE processing time and the triangular surface defect density in a state where the oxygen plasma processing time in the cleaning step (SA2) is fixed to 20 minutes.

  As this result shows, the triangular surface defect density initially decreases with an increase in the RIE time, takes a minimum value when the RIE time is 35 minutes, and thereafter increases. The reason for the increase is not clear. Initially, the reason why the triangular surface defect density decreases as the RIE time increases is considered to be because the silicon carbide surface layer damaged by machining is removed by RIE. Since the silicon carbide surface layer is practically not etched only by the oxygen plasma treatment, the triangular surface defects generated due to this damage cannot be avoided.

  In addition, when only CMP was used instead of RIE, a triangular surface defect of 1000 to 2000 was generated even with a 3.5 degree off substrate, and an off angle of 1 degree or less could not be evaluated because there were too many surface defects. . Since CMP inevitably contains a mechanical polishing component, it is considered that the mechanical polishing component causes damage to the inside of the substrate again. From this, it can be seen that the RIE process is an indispensable process in the present invention.

(Second Embodiment)
It is important to avoid deposits as much as possible during RIE. In the RIE process, a mixed gas of carbon tetrafluoride (CF ₄ ) gas and oxygen (O ₂ ) gas is used, and when the influence of the mixing ratio on the generation of triangular surface defect density was investigated, Interesting results were obtained.

  Specifically, the oxygen plasma treatment time was fixed to 2 minutes and the RIE time was fixed to 35 minutes, and the mixing ratio (flow rate ratio) of carbon tetrafluoride and oxygen during RIE was changed. The apparatus and other conditions are the same as described above. Under these conditions, the etching rate of silicon carbide is about 150 to 200 nm / min.

  FIG. 3 shows the relationship between the mixing ratio of carbon tetrafluoride and oxygen and the triangular surface defect density. As the amount of oxygen mixed increases, the triangular surface defect density clearly decreases. When the mixing ratio of carbon tetrafluoride and oxygen was 2: 1 by flow ratio, the surface after etching showed a net-like morphology, but when the mixing ratio was 1: 1 by flow ratio. Some protrusions were observed, and when the mixing ratio was 1: 2, the flow ratio was completely flat within a range that could be confirmed with an optical microscope.

  It is considered that the increase in the mixing ratio of oxygen results in a decrease in deposits, resulting in improved surface flatness. Thus, in RIE, the triangular surface defect density is reduced by selecting conditions that do not cause deposits as much as possible. This is presumably because oxygen plasma treatment alone is insufficient to remove this deposit.

However, since the deposition is not uniform on the surface of silicon carbide, the flatness cannot be recovered simply by increasing the time of the oxygen plasma treatment or increasing the plasma power. Furthermore, as described above, when a portion where no deposit is generated is unnecessarily exposed to oxygen plasma on the (0001) Si surface, a triangular surface defect is generated during CVD growth. Therefore, it is necessary to select conditions that do not cause deposits in RIE. Instead of fluorocarbon, sulfur hexafluoride (SF ₆ ), nitrogen trifluoride (NF ₃ ), or the like that does not generate a deposit itself may be used.

-Foreign matter-
Here, the foreign matter generated in the RIE process is considered. It is well known that in plasma etching of a silicon wafer (Si), carbon fluoride generates a fluorocarbon-based polymerized deposit. In the case of carbon tetrafluoride, for example,

(Chemical formula 1) CF ₄ (gas) + Si (solid) → CF ₂ (polymerized and deposited) + SiF ₂ (gas)

By this reaction, silicon is etched, and at the same time, a fluorocarbon polymer film is deposited. Similar deposits are seen with carbon tetrachloride. At this time, when oxygen is added, the polymer film is sputtered and removed by oxygen, for example,

(Chemical formula 2) 2CF ₄ (gas) + O ₂ (gas) + 2Si (solid) → 2CO (gas) + 2SiF ₂ (gas)

Thus, the generation of the reactive species to be polymerized can be suppressed. As can be seen from the chemical reaction formula of Chemical Formula 2, theoretically, if the mixing ratio of carbon tetrafluoride and oxygen is more than 2: 1 in terms of flow rate, the generation of deposits can be suppressed.

  However, in silicon carbide etching, since silicon carbide also contains carbon, halogen atoms such as fluorine are taken away by the halogen element scavenging effect of carbon atoms, and polymerization is likely to occur. On the other hand, in order to produce a silicon compound having a high vapor pressure, since halogenation is effective, use of a halide is inevitable. It is clear from the following reaction, for example, that silicon carbide promotes polymerization of halogenated carbon.

(Chemical formula 3) CF ₄ (gas) + SiC (solid) → 2CF ₂ (polymerized and deposited) + Si (solid)

  In order to avoid the generation of deposits, for example, the following reaction will be necessary.

(Chemical formula 4) 2CF ₄ (gas) + 3O ₂ (gas) +4 SiC (solid) → 6CO (gas) + 4SiF ₂ (gas)

  According to the example of silicon, theoretically, if the mixing ratio of carbon tetrafluoride and oxygen is more than 2: 3, the generation of deposits can be suppressed.

  The relationship between the mixing ratio of carbon tetrafluoride and oxygen and the surface morphology is explained as follows. When the mixing ratio is 2: 1, oxygen is insufficient, and a polymer film is deposited by the chemical reaction formula of Chemical Formula 3, for example. When the mixing ratio is 1: 1, oxygen is deficient according to the chemical reaction formula of Chemical Formula 4, but actually, oxygen is sufficient on average, and oxygen is locally present. Polymer is deposited at the shortage. When the mixing ratio is 1: 2, oxygen is sufficient over the entire surface and no polymer is deposited.

  However, in place of fluorocarbon, other halogenated carbon, sulfur hexafluoride, nitrogen trifluoride, etc. may be used in the same chemical reaction as in chemical formula 3, that is, carbon in SiC deprives the halogen atom. The polymerization reaction can occur. In this sense, the generation of the nano-level foreign matter is unavoidable after using a plasma containing halogen.

  Thus, when the mixing ratio of carbon tetrafluoride and oxygen is at least 1: 2 or more than that, plasma power is applied and no polymer is deposited while RIE is in progress. . However, when the plasma power is cut off and the RIE is stopped, a polymer may be formed for the following reason. The gas species excited by the plasma diffuses, and a part of it is deactivated on the silicon carbide surface. Since the oxygen molecule is lighter than the fluorocarbon molecule, it diffuses and deactivates first, and finally, only the fluorocarbon molecule is excited. Thereafter, when the fluorocarbon molecules reach the silicon carbide surface, a polymer (polymer) is generated by a chemical reaction such as Chemical Formula 3, for example.

  For this reason, even if the generation of foreign matter is suppressed in the RIE process, deposits (foreign matter) always adhere after the etching is stopped. Therefore, it is necessary to perform the cleaning step (SA3) after the RIE step.

(Third embodiment)-Utilization of (0001) Si surface-
When producing a SiC power device with a high breakdown voltage, the method for manufacturing a silicon carbide substrate according to the present invention may be applied to the (0001) Si surface rather than the (000-1) C surface of SiC. preferable.

  This is because the (0001) Si plane has a residual nitrogen donor density that is about an order of magnitude smaller than the (000-1) C plane. The same applies to the off-plane and the {0001} neighboring surface.

For example, for a 4H—SiC (000-1) C plane, the minimum residual nitrogen donor density is 8 × 10 ¹⁴ and 2 × 10 ¹⁵ / cm ³ for an 8 degree and 0.3 degree off substrate, respectively. is there. Accordingly, the industrially designable donor densities are 8 × 10 ¹⁵ and 2 × 10 ¹⁶ / cm ³ , respectively, and the theoretical maximum withstand voltages are 1.97 and 0.96 kV, respectively.

  However, in order to manufacture a power device industrially, it is necessary to consider the reliability, and it is necessary to provide a manufacturing margin for electric field concentration and other manufacturing processes. It is limited to about 60 to 70% of the theoretical breakdown electric field. Therefore, the practical maximum breakdown voltage is about 0.67 and 0.33 kV, respectively.

  Therefore, in order to realize a breakdown voltage higher than this, the (0001) Si surface must be used at present.

(Other embodiments)
The surface step structure is different between the (0001) Si surface of silicon carbide and the (000-1) C surface. In the (000-1) C plane, the steps of one molecular layer (Si—C pair) occupy the majority, whereas in the (0001) Si plane, the unit cell height (4 molecular layers, 6H in 4H—SiC). -In SiC, a step of 6 molecular layers) is formed. The step of the unit cell height on the (0001) Si plane is preferable for growing a high-quality crystal of a group III nitride semiconductor that usually has a 2H structure.

  That is, due to the unit cell height step, the atomic arrangement of the nitride semiconductor matches between adjacent terraces, and the generation of structural defects due to the difference in atomic arrangement can be suppressed. Such a high-quality group III nitride semiconductor / silicon carbide interface, particularly an interface between aluminum nitride or boron nitride or a mixed crystal thereof and silicon carbide, has extremely small lattice mismatch and very few dangling bonds at the interface. . Since these nitride semiconductors have a large forbidden band width, they can be used as an insulating film in place of an oxide film having many interface state problems. Therefore, a high performance insulating film / silicon carbide structure is realized, and a high performance device can be manufactured.

According to the method for manufacturing a silicon carbide substrate according to the present invention, a homoepitaxial growth layer having an extremely small surface defect density can be formed, which can be said to be a basic technique for producing various devices using silicon carbide.
As described above, the industrial applicability of the present invention is extremely large.

FIG. 1 is a process diagram for explaining an example of a method for manufacturing a silicon carbide substrate (SiC substrate) according to the present invention. FIG. 2A is a table showing the relationship between the oxygen plasma treatment time and the triangular surface defect density. (B) is a table | surface which shows the relationship between RIE processing time and the triangular surface defect density in the state which fixed oxygen plasma processing time to 20 minutes. FIG. 3 is a table showing the relationship between the mixing ratio of carbon tetrafluoride and oxygen and the triangular surface defect density. FIG. 4 is an enlarged view at the atomic level for explaining the mechanism of defect formation caused by foreign matter, and shows a state before epitaxial growth. FIG. 5 shows a state in which the epitaxial growth has progressed after some time has elapsed from the state of FIG. FIG. 6 shows a state in which the epitaxial growth is completed after a further time has elapsed from the state of FIG. FIG. 7 shows a bird's eye view of FIG. FIG. 8 shows a state of epitaxial growth around the foreign matter in consideration of the wraparound of the step flow.

Explanation of symbols

10 Silicon carbide (SiC) substrate 12 Foreign material 15 Region separated by dislocation 16 Defect boundary (including dislocation)
17 Defect boundary (no dislocation unless there are different polytypes or stacking faults in 15 regions)
F1-F4 surface terrace (surface defects)

Claims (8)

An etching step (SA1) for etching the surface of the silicon carbide substrate with plasma of a substance containing at least halogen; a cleaning step (SA2) for removing foreign matter adhering to the surface of the silicon carbide; and a surface of the silicon carbide. A method for manufacturing a silicon carbide substrate, comprising an epitaxial growth step (SA3) for homoepitaxial growth. The method for manufacturing a silicon carbide substrate according to claim 1, wherein the cleaning step (SA2) is a step of exposing to a plasma of a substance containing at least oxygen. The method for manufacturing a silicon carbide substrate according to claim 1, wherein the etching step (SA1) is performed under conditions that do not cause deposits during the etching. The etching gas in the etching step (SA1) includes carbon fluoride and oxygen, and the number of oxygen atoms contained in the plasma is at least three times the number of carbon atoms supplied as the etching gas. A method for manufacturing a silicon carbide substrate according to claim 1. 2. When the surface of the silicon carbide substrate is a (0001) Si surface, the cleaning step (SA2) does not oxidize the silicon carbide surface and does not cause oxygen atom knock-on. The manufacturing method of the silicon carbide substrate of description. The method for manufacturing a silicon carbide substrate according to claim 1, wherein the silicon carbide substrate is a 4H—SiC substrate. 2. The silicon carbide substrate according to claim 1, wherein the surface of the silicon carbide substrate is a {0001} plane and has an inclination of greater than 0 degree and less than or equal to 1 degree from a <0001> direction. Method. The method for manufacturing a silicon carbide substrate according to claim 1, wherein the surface of the silicon carbide substrate is a (0001) Si surface.

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