JP2005064392A - METHOD OF MANUFACTURING SiC SINGLE-CRYSTAL SUBSTRATE - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a SiC substrate having a flat surface. <P>SOLUTION: The method of manufacturing a SiC single-crystal substrate includes steps: (a) preparing a SiC single-crystal substrate 10 having a surface to which a mirror polishing has been applied; (b) forming an oxide layer 12 on the surface of the SiC single-crystal substrate 10 by oxidizing the surface of the SiC single-crystal substrate by plasma, and (c) removing at least a part of the oxide layer 12 by reactive ion etching, wherein preferably the surface is smoothed by repeating the steps (b) and (c). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a SiC single crystal substrate.

  The SiC single crystal has a wider band gap, a higher dielectric breakdown electric field and higher thermal conductivity than the Si single crystal and GaAs single crystal. Such characteristics are suitable for semiconductor elements operating at high temperatures and power semiconductor elements having a high withstand voltage, and SiC single crystals are used as semiconductor elements having characteristics that cannot be obtained with conventional Si semiconductors. Research on semiconductor devices is underway. Since the SiC single crystal produced by the prior art is not high in crystal quality, a semiconductor element is produced by forming a SiC epitaxial layer on a substrate using the SiC single crystal. However, in recent years, SiC single crystals having good characteristics have been obtained, and the production of semiconductor elements in SiC single crystal substrates has also been studied.

  In recent years, research has been conducted on GaN-based semiconductor elements that emit ultraviolet light and blue light as light sources for recording / reproducing information at a high recording density using optical recording media and light sources for image display and illumination. It is being advanced. Since a GaN-based semiconductor is generally difficult to grow into a large single-crystal ingot shape with few crystal defects, a technique for epitaxially growing a GaN-based semiconductor layer on a SiC single crystal substrate has attracted attention.

  For this reason, a SiC single crystal substrate having a smooth surface free from warpage and the like is desired. The inventor of the present application has proposed a technique for obtaining a flat SiC substrate by removing processing stress in an unpublished patent application.

  On the other hand, CMP (Chemical Mechanical Polishing) has been generally used as a technique for smoothing the surface of the SiC substrate. However, since SiC has hardness next to diamond, CMP cannot provide a sufficient polishing rate, and processing efficiency is very poor. In order to increase the polishing rate, a method of performing CMP while applying a high pressure to the SiC substrate is known, but in this case, there is a problem that the work-affected layer easily enters deep inside the SiC substrate.

  In order to solve these problems, Patent Document 1 discloses a method of smoothing the surface of SiC using reactive etching and steam oxidation. Specifically, the surface of the SiC substrate is mechanically mirror-polished, subjected to organic and inorganic cleaning, and then subjected to reactive etching on the surface of the SiC substrate to maintain a uniform surface while maintaining the flatness of the surface. Remove the damage layer. Then, after oxidizing the surface of the substrate with water vapor, the oxide layer is removed with hydrofluoric acid.

In Patent Document 2, the work-affected layer on the surface of the SiC substrate is removed by first reactive etching using Ar or the like, and an ion irradiation damaged layer generated in the surface region of the substrate by the first reactive etching is CF _4. And a technique of removing by reactive etching using O ₂ or the like.

  Non-Patent Document 1 reports that the processing efficiency can be improved by subjecting the surface of the SiC substrate to steam oxidation and then performing CMP.

  However, in reactive ion etching used in these prior art methods, etching is performed almost reflecting the shape of the surface. For this reason, when the surface of the substrate is scratched, the scratch cannot be completely removed even if reactive etching is performed. In reactive ion etching, radical species are accelerated and collide with the substrate, so that it is difficult to completely remove the damage caused on the substrate. The method of forming an oxide layer by steam oxidation and removing the oxide layer is not practical because it requires exposure to steam for a long time while maintaining the SiC substrate at a high temperature.

For this reason, it is difficult for the methods disclosed in Patent Document 1 and Patent Document 2 to completely smooth the surface of the substrate and completely remove the damaged layer generated on the substrate surface. Further, the method according to Non-Patent Document 1 completes the polishing in a shorter time than the method of planarizing the surface of the SiC substrate only by CMP. However, since the oxidation process still takes about 3 hours and the polishing process takes about 2 hours, the method according to Non-Patent Document 1 is not practical. Moreover, it is difficult to completely remove scratches on the substrate surface.
JP-A-6-188163 JP-A-9-183700 Nippon Steel Technical Report No. 374, pages 32-36

  The object of the present invention is to solve the above-mentioned problems of the prior art and to provide a practical method for producing a SiC substrate having a smooth surface.

  The method for producing a SiC single crystal substrate according to the present invention includes a step (a) of preparing a SiC single crystal substrate having a mirror-polished surface, oxidizing the surface of the SiC single crystal substrate with plasma, and forming an oxide layer. A step (b) of forming on the surface of the SiC single crystal substrate; and a step (c) of removing at least a part of the oxide layer by reactive ion etching.

  In a preferred embodiment, the steps (b) and (c) are each repeated a plurality of times.

  In a preferred embodiment, in steps (b) and (c), the oxidation and etching are performed for a time period of 1 to 10 minutes, respectively.

  In a preferred embodiment, the steps (b) and (c) are repeated a plurality of times, and after the step (b) is performed, the surface of the SiC single crystal substrate is polished by a chemical mechanical polishing method (d) ).

  In a preferred embodiment, oxygen or a mixed gas of oxygen and an inert gas is used in the step (b).

  In a preferred embodiment, a gas containing F is used in the step (c).

  In a preferred embodiment, in the step (c), the reactive etching conditions are set so that the etching rates of SiC and the oxide layer are equal.

  In a preferred embodiment, in the step (a), an offset angle with respect to the C axis of the SiC single crystal substrate is substantially zero.

  In a preferred embodiment, the steps (b) and (c) are performed by replacing a gas in the same apparatus.

  In a preferred embodiment, the surface of the SiC single crystal substrate is smoothed by repeating the step of oxidizing the surface of the SiC single crystal substrate and removing the oxide layer formed by the oxidation by etching a plurality of times.

  In a preferred embodiment, the number of repetitions is 5 or more.

  The SiC single crystal substrate of the present invention is manufactured by any of the above methods.

  The SiC single crystal substrate of the present invention has a surface roughness Ra of 0.2 nm or less and has a step structure on the surface.

  According to the present invention, plasma oxidation of the SiC surface and removal of the oxide layer generated by oxidation by reactive etching enables formation and removal of the oxide layer at a practical speed, and scratches present on the surface are eliminated. The removed smooth SiC single crystal substrate can be obtained.

  Further, by repeating the oxidation of the SiC surface and the removal of the oxide layer generated by the oxidation, a smooth SiC single crystal substrate can be obtained.

  CMP is widely used for polishing a semiconductor substrate, planarizing a formed semiconductor structure, and the like, and is one of excellent polishing methods. However, when the present inventor polished the mirror-polished SiC single crystal substrate by CMP while applying pressure, a locally smooth surface was obtained, but at the same time, a scratch having a depth of about 10 to 20 nm was formed on the entire substrate. Occurred.

  This is considered to be derived from the fact that SiC has high physical properties and high chemical resistance, but is brittle and easily damaged. Therefore, as long as polishing is performed by CMP, scratches on the surface of the SiC substrate are removed by polishing, but new scratches are generated on the surface after cutting.

  Therefore, when the smoothing of the surface of the SiC substrate was examined by a method other than polishing, it was found that the surface of the SiC substrate could be smoothed by oxidizing the surface of the SiC substrate and then removing the oxidized layer formed by oxidation. I found it. As will be described in detail below, in particular, by repeating oxidation and removal of the oxide layer, the profile of the surface shape is dulled and the grooves due to scratches and scratches are gradually shallowed, and the surface of the substrate becomes smooth.

  Hereinafter, a method for producing a SiC single crystal substrate according to the present invention will be described in detail.

  First, a SiC single crystal substrate made of a SiC single crystal is prepared. Preferably, the SiC single crystal has a hexagonal structure, and is more preferably 4H—SiC or 6H—SiC. As shown in FIG. 1, the surface 10a for smoothing the SiC single crystal substrate 10 is a (0001) plane, and the offset angle of the substrate is approximately zero degrees with respect to the C axis which is the crystal axis (also called a just substrate). It is preferable that In other words, the C axis is preferably perpendicular to the surface 10a. As shown in FIG. 1, when the offset angle is zero degrees, ideally, the C layer and the Si layer are stacked parallel to and intersecting with the surface 10a. In such a substrate, since the entire surface is uniformly composed of Si or C, the physical and chemical stability of the surface is high, and polishing is generally difficult. The method for producing a SiC single crystal substrate of the present invention can suitably smooth such a substrate.

  As shown in FIG. 2, even if the SiC single crystal substrate 50 is made of 4H—SiC or 6H—SiC single crystal, when the offset angle θ with respect to the C axis is other than zero degrees, Si and C always appear. Since such a surface is generally easy to process, a flat surface can be obtained relatively easily by a conventional polishing method or smoothing method. However, even if the method of the present invention is applied to a substrate having an offset angle θ other than zero degrees as shown in FIG. 2, a smooth surface can be obtained efficiently.

  The surface on which the SiC single crystal substrate 10 is smoothed is preliminarily mirror-finished and preferably mirror-finished, and the surface roughness Ra of the surface 10a is 0.2 nm to 2 nm. More preferred. Here, the surface roughness Ra is a value obtained by measuring a 5 μm area of a sample with an atomic force microscope (AFM). When SiC single crystal substrate 10 has a diameter of 2 inches, for example, it is preferable that the warpage of the substrate is adjusted so that the surface to be smoothed has a flatness of about ± 20 μm or less. However, when the flatness is ± 20 μm or more, the warpage of the substrate can be corrected during the manufacturing process of the SiC single crystal substrate described below.

  As shown in FIG. 3, SiC single crystal substrate 10 is cut out from single crystal SiC lump 20 using, for example, a known method. For cutting the SiC mass 20, a cutting blade of an outer peripheral blade or an inner peripheral blade, a wire saw, or the like can be used. The SiC mass 20 may contain an element that becomes a P-type or N-type impurity other than Si and C. Moreover, other IV group elements, such as Ge, may be included as a substitution element. In the present specification, SiC including the impurity element and the substitution element is collectively referred to as SiC. There is no particular limitation on the outer shape of SiC substrate 10, and various sizes, thicknesses, and planar shapes can be used in the present invention. For example, a disc-shaped SiC single crystal substrate 10 having a diameter of 2 inches and a thickness of about 500 μm is prepared.

  The SiC single crystal substrate 10 cut out from the lump 20 is polished by a known procedure until the work-affected layer generated on the surface is removed and the surface roughness of the substrate surface 10a and the back surface 10b becomes a predetermined value. Is done. At this time, if the SiC single crystal substrate 10 is warped, planar processing is performed so as to be equal to or less than a predetermined flatness.

FIG. 4A schematically shows the vicinity of the surface 10 a of the SiC single crystal substrate 10. The surface 10a of the SiC single crystal substrate 10 has a surface roughness on the order of nanometers, and the processing flaw 11 is generated on the surface 10a or the work-affected layer 17 remains in the vicinity of the surface. For this reason, it is preferable that the surface 10a of the SiC single crystal substrate 10 thus prepared is first etched to remove the work-affected layer. Etching is preferably reactive ion etching performed in the subsequent oxide layer removing step, and the etching conditions are also in accordance with it. Subsequently, the surface 10a is oxidized. Various known methods can be used for the oxidation. However, when the SiC single crystal substrate 10 having an offset angle of zero degree is used, the generation rate of the oxide layer is slow in the steam oxidation, and the etching is difficult to proceed in the oxidation with the etchant, so the efficiency is low. It is not preferable. This is because Si layers and C layers are alternately laminated in parallel to the surface 10a, and the chemical reactivity of the surface 10a is poor. For this reason, it is preferable to use oxidation by plasma. The oxidation is preferably performed in an oxygen atmosphere or an atmosphere containing an inert gas such as oxygen and Ar. For example, a power of 0.01 to ² W / cm ² is applied at a pressure of about 10 ⁻¹ to 10 ² Pa. Do it. This step is preferably performed in the same apparatus as the subsequent reactive ion etching. This is because there is no need to transfer the SiC single crystal substrate 10 or the like, and the two steps can be performed in succession only by gas exchange. Oxidation forms an oxide layer 12 on the surface as shown in FIG.

Next, the oxide layer 12 is removed. As a method for removing the oxide layer 12, known chemical and mechanical removal methods can be used, but it is not preferable to remove the oxide layer 12 by CMP while applying pressure. This is because new scratches are generated in the process of removing the oxide layer 12 as described above. It is preferable to remove the oxide layer 12 by a chemical method so that a new work-affected layer or scratch is not generated, and it is more preferable to remove the oxide layer 12 by reactive ion etching. The gas used for reactive ion etching is preferably one containing F, and more preferably CF ₄ . It is preferable that the oxide layer 12 is completely removed substantially throughout. In addition, it is preferable to remove oxide layer 12 under reaction conditions such that the etching rates for oxide layer 12 and SiC constituting SiC substrate 10 are equal. As a result, the flat portion of the oxide layer 12 is scraped faster due to the anisotropy of the reactive ion etching than the vicinity of the processing flaw 11, and the SiC single crystal substrate 10 is also exposed after the surface of the SiC single crystal substrate 10 is exposed. Etched. For this reason, as shown in FIG. 4B, the processing flaw 11 ′ remaining in the trace after the oxide layer 12 is removed becomes shallower than the processing flaw 11 before the oxide layer 12 is formed, and the SiC single crystal substrate 10 The flatness of the surface 10a 'is improved.

  The etching time in the step of removing the oxide layer 12 is typically 1 minute to 10 minutes, although it depends on the thickness of the generated oxide layer 12 and the type of gas used for etching.

  In the present invention, it is preferable to repeat the oxidation step and the removal step described above. As shown in FIG. 4D, an oxide layer 12 'is formed on the surface 10a' of the SiC single crystal substrate 10 by the oxidation method described above, and then the oxide layer 12 'is removed by the etching method described above. As a result, as shown in FIG. 4E, the SiC single crystal substrate 10 having the surface 10 a ″ with further improved flatness is obtained. The depth of the processing flaw 11 ″ remaining on the surface 10 a ″ is smaller than the depth of the processing flaw 11 ′ before forming / removing the oxide layer 12 ′.

  Furthermore, by repeating the oxidation step and the removal step a plurality of times, the depth of the processing scratches that have occurred on the surface of SiC single crystal substrate 10 is reduced, and the smoothness is increased. Oxidation and removal are preferably repeated twice or more, more preferably 5 or more times. When the oxidation step and the removal step are repeated about 10 times, almost perfect smoothness is obtained. On the other hand, the smoothness of the surface is sufficiently good even if the number of repetitions is greater than 15. However, the time required to repeat these steps becomes longer and is not efficient. Therefore, it is most preferable to repeat the oxidation step and the removal step 5 to 15 times. According to the method of the present invention, the oxidation and etching of the oxide layer caused by the oxidation can be performed in the same apparatus. Therefore, even if the short-time oxidation and etching are repeated a plurality of times, the SiC single crystal substrate can be applied to different apparatuses. Time and effort required to replace 10 are not required. For this reason, these steps can be remarkably repeated as compared with the conventional method using water vapor oxidation or chemical etching. By repeating the oxidation step and the removal step a plurality of times as described above, SiC single crystal substrate 10 having smooth surface 13a from which processing flaw 11 has been removed is obtained as shown in FIG.

  The SiC single crystal substrate 10 obtained by the above process has a surface roughness of about Ra <0.4 nm, and has high smoothness. However, damage caused by ions colliding during the reactive etching in the removing process occurs on the outermost surface of the SiC single crystal substrate 10, and it is preferable to remove this damage. For this reason, at the end of the repetition of oxidation and removal, the above-described oxidation step is performed to oxidize the surface 13a of the SiC single crystal substrate 10 to form an oxide layer 14, as shown in FIG. Then, the generated oxide layer 14 is removed by CMP with low pressure. For CMP, for example, colloidal silica and non-woven fabric are used. The oxide layer 14 can be removed at a practical polishing rate even by using general CMP, and since CMP is performed with a low pressure, there is no possibility of generating new scratches or work-affected layers on the surface. As a result, a SiC single crystal substrate 15 having a surface 15a that is smooth and free of scratches, is removed from the damaged layer and the work-affected layer in the vicinity of the surface, and has a uniform lattice arrangement is obtained.

  FIG. 6 schematically shows the surface of the SiC single crystal substrate 15 obtained by the method of the present embodiment. Surface 15a of SiC single crystal substrate 15 has a surface roughness Ra of less than 0.2 nm. However, since it is difficult to set the offset angle when the SiC single crystal substrate 15 is cut out to an ideal zero, the step structure 18 having the height of the monoatomic layer is formed on the surface 15a of the SiC single crystal substrate 15. Is seen.

  Thus, according to the present invention, an SiC single crystal substrate with high surface smoothness can be obtained by repeating oxidation and removal of the oxide layer formed by oxidation a plurality of times. In particular, by using plasma oxidation and reactive etching, the surface can be finished in a practical processing time. In addition, since the damaged layer and the work-affected layer are removed near the surface of the SiC single crystal substrate, the semiconductor characteristics near the surface are also excellent.

Hereinafter, specific experimental examples will be described.
(First Experiment Example)
A 4H (0001) just substrate having a diameter of 2 inches and a thickness of 350 μm was prepared as a SiC single crystal substrate. The surface roughness Ra of the surface of the substrate is finished to 1.0 nm.

This substrate was held in a chamber of a parallel plate type reactive etching apparatus. By introducing oxygen into the chamber at a flow rate of 100 sccm, generating a plasma by applying a power of 0.2 W / cm ² while maintaining a pressure of 0.7 Pa in the chamber, and exposing the substrate to the plasma for 5 minutes The substrate surface was oxidized.

Thereafter, CF4 as a reactive gas was introduced into the chamber at a flow rate of 100 sccm while holding the substrate in the chamber, and a power of 0.2 W / cm ² was applied while maintaining the pressure in the chamber at 0.7 Pa. The substrate surface was etched for a minute.

  Oxidation and etching were alternately performed 10 times, and then oxidation was performed again. Thereafter, the substrate was taken out, and the surface of the SiC single crystal substrate was polished by CMP using colloidal silica.

  The obtained SiC single crystal substrate was evaluated by AFM (atomic force microscope). When the step in the 5 μm × 5 μm region was determined, the surface roughness Ra was 0.17 nm. A linear step structure was observed on the surface.

(Second experiment example)
As the SiC single crystal substrate, a 6H (0001) just substrate having a diameter of 2 inches and a thickness of 350 μm was prepared, and an SiC single crystal substrate was obtained by the same procedure and the same conditions as in the first experimental example.

  The surface roughness Ra of the obtained substrate was 0.13 nm. A linear step structure was observed on the surface.

  From these experimental examples, the damage layer and the work-affected layer can be removed and processed into a very smooth surface with an aligned lattice, regardless of whether it is a 4H-SiC single crystal substrate or 6H-SiC. I understand.

  According to the present invention, a SiC single crystal substrate having an extremely smooth surface can be obtained. It is possible to epitaxially grow a GaN-based semiconductor layer or a SiC-based semiconductor layer having good characteristics on this SiC substrate to produce a GaN-based semiconductor element or a SiC-based semiconductor element having excellent characteristics. In addition, when a semiconductor element is formed in a SiC single crystal substrate, it is possible to manufacture a SiC-based semiconductor element having excellent characteristics because the semiconductor characteristics in the region near the surface are good.

It is sectional drawing explaining the surface orientation and offset angle of a SiC substrate used suitably by this invention. It is sectional drawing explaining the other surface orientation and offset angle of a SiC substrate. It is a figure explaining the process of cutting out a SiC substrate from a lump. (A)-(f) has shown typically the cross section of the surface vicinity of the SiC substrate in each process of the manufacturing method of the SiC substrate by this invention. (A) And (b) has shown typically the cross section of the surface vicinity of the SiC substrate in the other process of the manufacturing method of the SiC substrate by this invention. 1 schematically shows a cross-sectional structure near the surface of a SiC substrate according to the present invention.

Explanation of symbols

10, 15 SiC single crystal substrate 11, 11 ′, 11 ″ processing scratches 12, 12′14 oxide layer

Claims (13)

A step (a) of preparing a SiC single crystal substrate having a mirror-polished surface;
Oxidizing the surface of the SiC single crystal substrate with plasma and forming an oxide layer on the surface of the SiC single crystal substrate;
Removing at least a portion of the oxide layer by reactive ion etching; and
Of manufacturing a SiC single crystal substrate including   The method for producing an SiC single crystal substrate according to claim 1, wherein the steps (b) and (c) are repeated a plurality of times.   The method for producing a SiC single crystal substrate according to claim 2, wherein the oxidation and etching in the steps (b) and (c) are each performed in a time period of 1 minute to 10 minutes.   The method further includes a step (d) of repeating the steps (b) and (c) a plurality of times and performing the step (b), and then polishing the surface of the SiC single crystal substrate by a chemical mechanical polishing method. Item 4. A method for producing an SiC single crystal substrate according to Item 2 or 3.   The method for producing a SiC single crystal substrate according to any one of claims 1 to 4, wherein oxygen or a mixed gas of oxygen and an inert gas is used in the step (b).   The method for producing a SiC single crystal substrate according to claim 1, wherein a gas containing F is used in the step (c).   The method for producing a SiC single crystal substrate according to any one of claims 1 to 6, wherein in the step (c), the conditions for reactive etching are set so that the etching rates of SiC and the oxide layer are equal.   The method for producing a SiC single crystal substrate according to any one of claims 1 to 7, wherein an offset angle with respect to a C axis of the SiC single crystal substrate is substantially zero in the step (a).   The method for producing a SiC single crystal substrate according to any one of claims 1 to 8, wherein the step (b) and the step (c) are performed by replacing a gas in the same apparatus.   A method for producing a SiC single crystal substrate, wherein the surface of the SiC single crystal substrate is smoothed by repeating the step of oxidizing the surface of the SiC single crystal substrate and removing the oxide layer generated by the oxidation by etching a plurality of times.   The method for producing a SiC single crystal substrate according to any one of claims 2, 4, and 10, wherein the number of repetitions is 5 or more.   A SiC single crystal substrate manufactured by the method according to claim 1.   A SiC single crystal substrate having a surface roughness Ra of 0.2 nm or less and having a step structure on the surface.

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