SILICON WAFER AND MANUFACTURING METHOD FOR IT

Abstract

[Task] To provide a silicon wafer capable of generating a high density of thermally stable oxygen precipitate cores in a bulk part, unaffected by customer heat treatment, while minimizing oxygen precipitates in a surface layer. [Solution] In a silicon wafer (50), a density of oxygen precipitates (BMD) generated in a surface layer (53) up to 30 µm deep from a surface (50a) by an initial evaluation heat treatment, in which a visualization heat treatment at 950°C to 1000°C for 16 hours is performed after a heat treatment at 780°C for 3 hours, is 1 × 10⁷ cm⁻³ to 1 × 10⁸ cm⁻³, and a density of BMD generated in a bulk part (54) deeper than the surface layer (53) by the The first evaluation heat treatment is generated from 1 × 109cm-3 to 7 × 109cm-3.If a mean density of BMD produced in the bulk part (54) by the first evaluation heat treatment is defined as a first bulk density d1, and a mean density of BMD produced in the bulk part (54) by a second evaluation heat treatment, in which the visualization heat treatment is performed after a heat treatment at 1150°C for 2 minutes, is defined as a second bulk density d2, then d2 / d10.74 to 1.02.

Description

Silicon wafer and its manufacturing method

[0001] The present invention relates to a silicon wafer and a method for manufacturing the same, and more particularly to a method for heat-treating silicon wafers produced by slicing a silicon single crystal ingot manufactured by the Czochralski method (CZ method), and also to silicon wafers heat-treated by such a heat-treating method.

[0002] Most silicon wafers, which are used as substrate materials for semiconductor devices, are produced using silicon single crystal ingots produced by the CZ method. The CZ method is a method for growing a single crystal larger than the seed crystal by gradually lifting a seed crystal that is in contact with molten silicon in a quartz crucible while rotating it relative to the molten silicon. The CZ method can increase the production yield of large-diameter silicon single crystals.

[0003] It is known that when growing silicon single crystals by the CZ method, oxygen dissolved from the surface of a quartz crucible is incorporated into the silicon melt. The oxygen in the silicon melt becomes supersaturated during the cooling process of the silicon single crystal, and the oxygen aggregates to form oxygen precipitation nuclei.

[0004] The density of oxygen precipitates in bulk silicon wafers immediately after being sliced ​​from silicon single crystal ingots is very low, and the impact of low-density oxygen precipitates on semiconductor device characteristics is minimal. However, various heat treatments are repeatedly performed during the semiconductor device manufacturing process, which can result in an increased density of oxygen precipitates. Oxygen precipitates present in the surface layer of the silicon wafer, which is the device active region, can cause deterioration of device characteristics such as junction leakage. On the other hand, oxygen precipitates present in the bulk portion other than the device active region effectively function as gettering sites that capture metal impurities that degrade device characteristics. Therefore, it is desirable to maintain a low density of oxygen precipitates in the surface layer of the silicon wafer and a high density of oxygen precipitates in regions deeper than the surface (inside the wafer).

[0005] To obtain such a silicon wafer, for example, Patent Document 1 describes a method for manufacturing a silicon wafer, which includes a first heat treatment step in which a silicon wafer is heated at 1100 to 1200°C for 1 to 30 seconds in a furnace in a non-oxidizing atmosphere, a second heat treatment step in which the silicon wafer is heated at 800 to 975°C for 2 to 10 minutes after the first heat treatment step, and a third heat treatment step in which the silicon wafer is heated at 1000 to 1200°C for 1 to 10 minutes after the second heat treatment step.

[0006] JP 2021-168382 A

[0007] In recent years, the BCD (Bipolar-CMOS-DMOS) process, which fabricates bipolar, CMOS, and DMOS on the same substrate, has attracted attention as a manufacturing process for power management semiconductor devices. The BCD process involves high-temperature heat treatment, which makes wafers prone to slip dislocations. To improve not only the gettering ability but also the slip resistance of silicon wafers, it is necessary to increase the oxygen precipitate density. Furthermore, since the BCD process requires a deep denuded zone (DZ) of several tens of microns, an epitaxial film may be formed on the surface of the silicon wafer beforehand. However, in addition to the slip problem associated with high-temperature heat treatment, the epitaxial film formation process also faces the thermal stability issue of oxygen precipitates, which are prone to disappearance. Thus, increasing the density and stabilization of oxygen precipitates are important issues for silicon wafers used in the BCD process.

[0008] However, in the silicon wafer manufacturing method described in Patent Document 1, for example, 8×10 １７ atoms / cm ３ When using a bulk silicon wafer with a low oxygen concentration of about 11×10, the oxygen precipitate nuclei cannot be grown sufficiently by the first to third heat treatment steps, and the oxygen precipitate nuclei disappear in the subsequent heat treatment by the customer, making it difficult to increase the oxygen precipitate density in the bulk portion. １７ atoms / cm ３When a bulk silicon wafer having a relatively high oxygen concentration is used, oxygen precipitates are likely to occur not only in the bulk portion of the wafer but also in the surface layer portion, which may make it unable to accommodate future BCD devices.

[0009] Therefore, an object of the present invention is to provide a silicon wafer and a manufacturing method thereof that can generate thermally stable oxygen precipitation nuclei in a bulk portion at a high density, which are not affected by the customer's heat treatment, while minimizing oxygen precipitation in the surface layer portion.

[0010] In order to solve the above problems, a silicon wafer according to the present invention has a surface layer portion extending from the surface to a depth of 30 μm and a bulk portion deeper than the surface layer portion, and the density of oxygen precipitates generated in the surface layer portion by a first evaluation heat treatment is 1.0×10 ７ ~1.0 x 10 ８ cm －３ and the density of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment is 1.0×10 ９ ~7.0 x 10 ９ cm －３ The average density of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment is defined as a first bulk density d １ The average density of oxygen precipitates generated in the bulk portion by the second evaluation heat treatment is defined as a second bulk density d ２ When the first bulk density d １ the second bulk density d ２ The ratio (d ２ / d １ ) is in the range of 0.74 to 1.02, the first evaluation heat treatment is a two-stage heat treatment in which a heat treatment is performed at 780°C for 3 hours followed by a visualization heat treatment, the second evaluation heat treatment is a two-stage heat treatment in which a heat treatment is performed at 1150°C for 2 minutes followed by the visualization heat treatment, and the visualization heat treatment is a heat treatment at 950 to 1000°C for 16 hours.

[0011] According to the present invention, the density of oxygen precipitates in the surface layer after the evaluation heat treatment is 1.0 × 10 ８ cm －３It is possible to provide a silicon wafer having a low oxygen precipitate density of 10 times or less in the bulk portion than in the surface layer portion, and which is thermally stable. Therefore, it is possible to improve the yield and reliability of semiconductor devices such as BCDs manufactured using the silicon wafer.

[0012] In the present invention, the minimum density d of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) and the minimum density d of oxygen precipitates generated in the bulk portion by the second evaluation heat treatment ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) are preferably both 2 or less. In this case, the minimum density d of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment is ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ It is more preferable that the minimum density d of oxygen precipitates generated in the bulk portion by the second evaluation heat treatment is 1.30 or less. ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) is more preferably 1.32 or less. This allows thermally stable oxygen precipitates that are not affected by the customer's heat treatment to be generated uniformly and at a high density in the bulk portion.

[0013] In the present invention, the average density of oxygen precipitates formed in the surface layer portion by the first evaluation heat treatment and the average density of oxygen precipitates formed in the surface layer portion by the second evaluation heat treatment are both 2.1 × 10 ７ cm －３ This makes it possible to provide silicon wafers in which the density of oxygen precipitates in the surface layer is sufficiently reduced without depending on the customer's heat treatment.

[0014] Further, a silicon wafer according to the present invention comprises a silicon substrate and an epitaxial silicon film formed on a surface of the silicon substrate, the silicon substrate having a surface layer portion extending from the surface to a depth of 30 μm and a bulk portion deeper than the surface layer portion, and a density of oxygen precipitates generated in the surface layer portion by a first evaluation heat treatment is 1.0×10 ７ ~1.0 x 10 ８ cm －３ and the density of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment is 1.0×10 ９ ~7.0 x 10 ９ cm －３ wherein, when an average density of oxygen precipitates formed in the bulk portion by the first evaluation heat treatment is defined as a first bulk density and an average density of oxygen precipitates formed in the bulk portion by the second evaluation heat treatment is defined as a second bulk density, a ratio of the second bulk density to the first bulk density is within a range of 0.98 to 1.02, the first evaluation heat treatment is a two-stage heat treatment in which a heat treatment is performed at 780°C for 3 hours followed by a visualization heat treatment, and the second evaluation heat treatment is the visualization heat treatment, which is a heat treatment at 950 to 1000°C for 16 hours.

[0015] According to the present invention, the density of oxygen precipitates in the surface layer after the evaluation heat treatment is 1.0 × 10 ８ cm －３ It is possible to provide an epitaxial silicon wafer having a low oxygen precipitate density of 10 times or less in the bulk portion than in the surface layer portion, and which is thermally stable. Therefore, it is possible to improve the yield and reliability of semiconductor devices such as BCDs manufactured using the epitaxial silicon wafer.

[0016] In the present invention, the minimum density d of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) and the minimum density d of oxygen precipitates generated in the bulk portion by the second evaluation heat treatment ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ) are preferably both 2 or less. In this case, the minimum density d of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment is ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ It is more preferable that the minimum density d of oxygen precipitates generated in the bulk portion by the second evaluation heat treatment is 1.29 or less. ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) is more preferably 1.35 or less. This allows thermally stable oxygen precipitates that are not affected by the customer's heat treatment to be generated uniformly and at a high density in the bulk portion.

[0017] Furthermore, in the method for producing a silicon wafer according to the present invention, the oxygen concentration is 7×10 １７ ~10 x 10 １７ atoms / cm ３ The method includes a first heat treatment step of heating a silicon wafer conforming to ASTM F-121 (1979) at a first temperature, a second heat treatment step of heating the silicon wafer at a second temperature lower than the first temperature after the first heat treatment step, and a third heat treatment step of heating the silicon wafer at a third temperature higher than the second temperature after the second heat treatment step, wherein the first temperature is 1210 to 1250°C, and the holding time at the first temperature is 10 to 60 seconds, the second temperature is 800 to 975°C, and the holding time at the second temperature is 2 to 10 minutes, and the third temperature is 1150 to 1250°C, and the holding time at the third temperature is 5 to 15 minutes.

[0018] According to the present invention, a first heat treatment step at a high temperature and in a relatively short time, a second heat treatment step at a low temperature and in a relatively long time, and a third heat treatment step at a higher temperature than the second heat treatment step are performed to generate thermally stable oxygen precipitate nuclei at a high density inside the silicon wafer while reducing the oxygen precipitate nuclei in the wafer surface layer portion. Therefore, it is possible to manufacture a silicon wafer having a high density of thermally stable oxygen precipitate nuclei in the bulk portion that are not affected by the customer's heat treatment, and a low density of oxygen precipitate nuclei in the device formation region.

[0019] Preferably, the first heat treatment step is performed in a non-oxidizing atmosphere containing ammonia or nitrogen, and the second and third heat treatment steps are performed in a non-oxidizing atmosphere not containing ammonia or nitrogen. By performing the first heat treatment step in a non-oxidizing atmosphere containing ammonia or nitrogen, a nitride film is formed on the wafer surface and vacancies are introduced into the wafer through the nitride film, thereby increasing the density of oxygen precipitate nuclei inside the wafer.

[0020] In the present invention, the temperature increase rate to the first temperature and the temperature increase rate from the second temperature to the third temperature are preferably 10 to 50°C / sec. Also, the temperature decrease rate from the first temperature to the second temperature is preferably 20 to 120°C / sec. This allows thermally stable oxygen precipitate nuclei to be generated at a high density.

[0021] In the present invention, the silicon wafer before being heat-treated in the first heat treatment step is preferably one cut from a defect-free region of a silicon single crystal ingot that is free of agglomerates of interstitial silicon point defects and agglomerates of vacancy point defects. This makes it possible to produce a silicon wafer that has a low density of oxygen precipitate nuclei in the surface layer, a high density of oxygen precipitate nuclei in the bulk layer, and is thermally stable. Therefore, the yield and reliability of semiconductor devices such as BCDs manufactured using the silicon wafer can be improved.

[0022] According to the present invention, it is possible to provide a silicon wafer and a manufacturing method thereof that can generate thermally stable oxygen precipitates in a bulk portion at a high density, which are not affected by the customer's heat treatment, while minimizing oxygen precipitation in the surface layer portion.

[0023] FIG. 1 is a flowchart illustrating a method for manufacturing a silicon single crystal according to an embodiment of the present invention. FIG. 2 is a flowchart illustrating a process for heat-treating a silicon wafer. FIG. 3 is a graph showing temperature changes during heat treatment, with the horizontal axis representing time and the vertical axis representing heating temperature. FIGS. 4(A) to 4(I) are schematic diagrams illustrating changes in a silicon wafer occurring during the first to third heat treatments. FIG. 5 is a schematic diagram illustrating a method for measuring the oxygen precipitate density of a silicon wafer using light scattering tomography. FIG. 6 is a schematic diagram of an evaluation procedure for determining the stability and uniformity of manufactured silicon wafers.

[0024] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0025] FIG. 1 is a flow chart that schematically shows a method for producing a silicon single crystal according to an embodiment of the present invention.

[0026] As shown in FIG. 1 , the method for manufacturing a silicon wafer according to this embodiment includes a step S11 of manufacturing a silicon single crystal ingot by the Czochralski (CZ) method, a step S12 of processing the silicon single crystal ingot to produce a silicon wafer, and a step S13 of heat-treating the silicon wafer.

[0027] In step S11 of producing a silicon single crystal ingot, polycrystalline silicon filled in a quartz crucible is heated in a CZ furnace to produce a silicon melt. Next, a seed crystal is brought into contact with the silicon melt, and the seed crystal and the quartz crucible are rotated while the seed crystal is gradually pulled up, thereby growing a large single crystal at the bottom of the seed crystal.

[0028] Next, in the silicon wafer production step S12, the silicon single crystal ingot is sliced ​​using a wire saw or the like, and then subjected to lapping, etching, mirror polishing, cleaning, etc. to complete bulk silicon wafers (polished wafers) as intermediate products. The oxygen concentration of the CZ silicon wafers produced in this way is 7×10 １７ ~10 x 10 １７ atoms / cm ３(ASTM F-121, 1979). １７ atoms / cm ３ If the concentration is lower than this, stable oxygen precipitates cannot be generated in the bulk at a high density. １７ atoms / cm ３ If the concentration is higher, the oxygen precipitates in the surface layer cannot be reduced sufficiently.

[0029] Here, the silicon wafer is preferably a so-called COP-free wafer, substantially free of crystal-origin particle (COP) defects. That is, the silicon wafer is preferably cut from a defect-free region of a silicon single crystal ingot, free of agglomerates of interstitial silicon point defects and vacancy point defects. COPs (crystal-originating particles) are crystallographically perfectly oriented octahedral cavities, the inner walls of which are typically covered with an oxide film 1 to 4 nm thick. Vacancy-related crystal defects, such as COP defects, can cause problems in semiconductor devices, similar to surface oxygen precipitates. Examples of device problems include reduced gate oxide integrity (GOI) and current leakage at p-n junctions. To address these issues, low-defect crystal growth methods can be applied to reduce the number of vacancy defects in the device region near the surface in some device applications. Vacancy defect levels can be reduced by changing the crystal pulling speed and crystal cooling rate. This allows for the recombination of vacancies and interstitial silicon atoms, the aggregation of vacancies, and the control of oxygen concentration, thereby reducing surface defects. In the COP-free wafer, "substantially free of COPs" means that the density of COPs consisting of aggregates of vacancy-type point defects is 1×10 ５ cm －３ This means that:

[0030] In step S13 of heat-treating the silicon wafer, the wafer is heat-treated in a rapid thermal annealing (RTA) furnace over a three-stage temperature range to generate thermally stable oxygen precipitate nuclei at a high density. Here, the expression "thermally stable" means that the wafer has a density sufficient to getter metal impurities and maintain wafer strength as shipped, and that the density is not affected by subsequent heat treatment in a customer's device. Also, "high density" means a density of at least 1×10 ９ / cm ３ or more, preferably about 5×10 ９ / cm ３ This refers to a density of 1000 or more.

[0031] Fig. 2 is a flow chart illustrating the step S13 of heat-treating the silicon wafer. Fig. 3 is a graph showing the temperature change during the heat treatment, with the horizontal axis representing time and the vertical axis representing the heating temperature.

[0032] As shown in FIGS. 2 and 3, the silicon wafer heat treatment method according to the embodiment of the present invention involves heating the silicon wafer in an RTA furnace to a first temperature T １ a first heat treatment step S21 in which the substrate is heated at a first temperature T １ A second temperature T ２ a second heat treatment step S22 in which the silicon wafer is heated at a second temperature T ２ A third temperature T ３ The method includes a third heat treatment step S23 in which the silicon wafer is heated at a temperature of 1000 K. In this embodiment, the first to third heat treatment steps S21 to S23 are preferably performed consecutively in the same RTA furnace. However, after the first heat treatment step S21 is performed in the RTA furnace, the wafer may be removed from the RTA furnace and the second heat treatment step S22 and the third heat treatment step S23 may be performed in a different heat treatment device.

[0033] The first heat treatment step S21 is a rapid thermal annealing (RTA) performed in an RTA furnace in a non-oxidizing atmosphere. The non-oxidizing atmosphere is preferably an inert gas containing ammonia or nitrogen, and the inert gas is preferably Ar gas. High-temperature heat treatment in a non-oxidizing atmosphere can introduce a large number of vacancies into the wafer, thereby increasing the density of oxygen precipitate nuclei within the wafer. Furthermore, by using Ar gas containing ammonia or nitrogen, a nitride film can be formed on the wafer surface, and vacancies can be introduced into the wafer through the nitride film, thereby increasing the density of oxygen precipitate nuclei within the wafer. Furthermore, silicon wafers have minute oxygen precipitate nuclei generated during crystal growth, but the rapid thermal annealing described above can reduce the number of oxygen precipitate nuclei in the surface layer of the wafer.

[0034] The first temperature T in the first heat treatment step S21 １ The first temperature T １ If the first temperature T is lower than about 1180° C., the oxygen precipitation nuclei in the surface layer cannot be reduced sufficiently. １ This is because if the standby temperature T is higher than about 1250°C, the probability of slip dislocation occurring in the silicon wafer increases. ０ (30) to the first temperature T １ The temperature rise rate (32) when switching to is preferably about 10 to 50° C. / sec.

[0035] The first temperature T in the first heat treatment step S21 １ Retention time H １ The first temperature T １ Retention time H １ If the holding time H is shorter than about 10 seconds, the density of oxygen precipitate nuclei in the surface layer cannot be sufficiently reduced. １ This is because, even if the time exceeds about 60 seconds, not only is an increase in the number of vacancies not observed, but the probability of slip dislocations occurring increases. The first heat treatment step S21 can eliminate oxygen precipitate nuclei in the surface layer and introduce a large number of vacancies into the silicon wafer.

[0036] The second heat treatment step S22 is a step of heating the silicon wafer heat-treated in the first heat treatment step S21 at a first temperature T １ A second temperature T ２ Unlike the first heat treatment step S21, the second heat treatment step S22 is preferably performed in a non-oxidizing atmosphere that does not contain ammonia or nitrogen. Therefore, after the first heat treatment step S21 is completed, the atmospheric gas in the RTA furnace is replaced.

[0037] The second temperature T in the second heat treatment step S22 ２ The second temperature T is preferably about 800 to 975°C. ２ If the second temperature T is less than about 800° C., thermally stable oxygen precipitation nuclei cannot be generated. ２ This is because if the first temperature T exceeds about 975°C, oxygen precipitate nuclei cannot be generated at a high density. １ to the second temperature T ２ The temperature lowering rate (34) when switching to is preferably about 20 to 120° C. / sec.

[0038] The second temperature T in the second heat treatment step S22 ２ Retention time H ２ The second temperature T ２ Retention time H ２ If the holding time H is shorter than about 2 minutes, oxygen precipitate nuclei cannot be generated at a high density. ２ This is because even if the second heat treatment time exceeds about 10 minutes, the density of oxygen precipitate nuclei does not increase and the cost only increases. The second heat treatment step S22 can stably generate oxygen precipitate nuclei in the silicon wafer at a high density.

[0039] The third heat treatment step S23 is a step of heating the silicon wafer heat-treated in the second heat treatment step S22 at a second temperature T ２ A third temperature T ３ The third heat treatment step S23 is preferably carried out in a non-oxidizing atmosphere that does not contain ammonia or nitrogen, similar to the second heat treatment step S22.

[0040] The third temperature T in the third heat treatment step S23 ３The third temperature T ３ If the third temperature T is lower than about 1150° C., the oxygen precipitation nuclei cannot be thermally stabilized. ３ This is because if the second temperature T is higher than about 1250°C, the probability of slip dislocations occurring increases. ２ to the third temperature T ３ The temperature rise rate (36) when switching to is preferably about 10 to 50° C. / sec. This makes it possible to increase the density of oxygen precipitate nuclei and make the nuclei more thermally stable.

[0041] The third temperature T in the third heat treatment step S23 ３ Retention time H ３ The third temperature T ３ Retention time H ３ If the holding time H is shorter than about 5 minutes, high density oxygen precipitate nuclei cannot be established. ３ This is because if the time exceeds about 15 minutes, the effect of stabilizing oxygen precipitate nuclei will not be particularly increased and the cost will increase.

[0042] The third heat treatment step S23 stabilizes oxygen precipitate nuclei formed in the silicon wafer and outwardly diffuses excess vacancies inside the wafer, thereby suppressing the generation of excess oxygen precipitates in the customer's subsequent heat treatment.Furthermore, the oxygen precipitate nuclei newly formed in the surface layer of the wafer in the second heat treatment step S22 can be eliminated, thereby reducing the density of oxygen precipitates generated in the surface layer up to 30 μm from the wafer surface to 1 / 100 or less of that in the bulk region.

[0043] 4A to 4I are schematic diagrams showing the changes that occur in the silicon wafer 40 during the first to third heat treatment steps S21 to S23. As shown in FIG. 4A, a large number of minute oxygen precipitate nuclei 41 that are generated during crystal growth are present in the silicon wafer 40. As shown in FIG. 4B, the holding time H １ It is understood that during this time, minute oxygen precipitate nuclei 41 disappear and simultaneously Frenkel pairs 42 of vacancies 44 and interstitial silicon atoms 45 are generated.３ N ４ The oxygen precipitation nuclei 41 generated during the crystal growth are eliminated by this heat treatment, and therefore the oxygen precipitation nuclei in the DZ 46 formed in a later step can be sufficiently reduced.

[0044] Next, as shown in FIG. 4C, at time t ３ and ４ During the temperature-lowering period between the first and second zones, out-diffusion of interstitial silicon atoms 45 and some of the vacancies 44a occurs, and some of the vacancies 44b move from the upper zone 40a to the lower zone 40b of the wafer, forming a DZ 46 with a low oxygen precipitate nucleus density, as shown in FIG. 4(D).

[0045] Next, as shown in FIG. 4E, the holding time H ２ During the holding time H of the third heat treatment step S23, oxygen precipitate nuclei 47, 47a are formed from the combination of vacancies 44, and the nuclei reach a size large enough for stabilization. However, some vacancies 44 remain. As shown in FIG. 4(F), the holding time H of the third heat treatment step S23 ３ Between the nuclei 47a, the remaining vacancies 44 and small oxygen precipitate nuclei 47a further recombine to form larger and more stable oxygen precipitate nuclei 47. As shown in FIG. 4(G), large and stable oxygen precipitate nuclei 47 are formed and a DZ 46 having a desired width is formed, which ultimately reduces the oxygen precipitate density in the surface layer within 30 μm from the wafer surface, and allows stable oxygen precipitates to be generated at a high density in the bulk portion deeper than 30 μm. ３ N ４ Layer 43 has been removed by etching or polishing, showing the eventual formation of DZ 46. As shown in Figure 4(I), even though the wafer has been processed to have epitaxial layer 48, DZ 46 remains and the density of oxygen precipitate nuclei 47 is not reduced.

[0046] FIG. 5 is a schematic diagram illustrating a method for measuring the oxygen precipitate density of a silicon wafer using light scattering tomography.

[0047] As shown in FIG. 5 , oxygen precipitates in a silicon wafer 50 can be observed as BMDs (bulk micro defects). The silicon wafer 50 is cleaved, and an infrared laser beam 51 is incident on its surface (main surface) 50 a. The infrared laser beam 51 is then moved along the cleavage plane 50 b to scan the BMDs in the cleavage direction. Because the material being inspected is primarily silicon, Rayleigh scattered light can be collected by focusing an appropriate infrared laser beam on the sample. Tiny dots that appear in a captured image of the wafer's cleavage plane 50 b correspond to BMDs 52. By counting the number of BMDs 52 within a given depth region, the BMD density within that depth region can be calculated. The wafer surface 50a is considered to have a depth of zero, and the BMD density in a surface layer 53 within 30 μm from the wafer surface 50a is evaluated as the surface BMD density, while the BMD density in a bulk layer 54 deeper than 30 μm, for example, 50 to 300 μm from the wafer surface, is evaluated as the bulk BMD density.

[0048] The density of the BMDs 52 is calculated by dividing the number of BMDs 52 contained in a rectangular parallelepiped formed by a scan width (standard condition: 125 μm) corresponding to the horizontal width of the photographed image of the cleavage plane 50 b, a depth corresponding to the spot diameter of the infrared laser light (standard condition: 8 μm), and an arbitrary depth direction distance by the volume of the rectangular parallelepiped. ３ ) corresponds to the number of BMDs 52 per wafer. By increasing the scan width, for example to 398 μm, the accuracy of the BMD density measurement can be improved. Because the BMD density measurement involves cleaving and destroying the wafer, characteristics associated with testing one wafer from a wafer batch are assumed to apply to the entire wafer batch.

[0049] The silicon wafers heat-treated as described above are taken out of the RTA furnace and put on the market as so-called annealed silicon wafers. The density of oxygen precipitates generated in the surface layer up to 30 μm from the surface of the silicon wafer according to this embodiment is 1.0×10 ７ ~1.0 x 10 ８ cm －３The BMD layer, which refers to a layer of oxygen precipitates, is robust. The robustness here takes into account the change in the density of oxygen precipitates (BMDs) from a lower heat treatment temperature of less than about 1000° C. to a higher heat treatment temperature of about 1000° C. or higher, which is the range of heat treatments in the manufacturing process of semiconductor integrated circuits. That is, the average density of oxygen precipitates (first bulk density d １ ) the average density of oxygen precipitates generated in the bulk by high-temperature heat treatment (second bulk density d ２ ) ratio (d ２ / d １ ) is 0.74 to 1.02, and the change in oxygen precipitate density due to heat treatment is within 30%. Even after the silicon wafer has been subjected to the desired heat treatment in the manufacturing process of a semiconductor device, the average density of oxygen precipitates in the wafer is about 4 × 10 ８ ~1 x 10 １０ / cm ３ and the fluctuation rate of this range remains within ±30%, more preferably ±15%, even more preferably ±10%, and even more preferably ±5%. In this way, the silicon wafer according to the present embodiment contains a high density of thermally stable oxygen precipitate nuclei that are not affected by the customer's heat treatment, thereby improving the quality and reliability of semiconductor devices such as BCD devices.

[0050] An epitaxial silicon film may be formed on the surface of the silicon wafer that has been subjected to the first to third heat treatment steps S21 to S23. When forming an epitaxial silicon film, the silicon wafer (silicon substrate) is exposed to a high temperature of approximately 1150°C. If the oxygen precipitate nuclei in the silicon wafer are thermally unstable, the oxygen precipitate nuclei may disappear after the device heat treatment, resulting in a significant decrease in the oxygen precipitate density. However, according to this embodiment, the oxygen precipitate nuclei are thermally stable, so the decrease in the oxygen precipitate density can be suppressed, and a decrease in the gettering ability and the wafer strength can be prevented.

[0051] Silicon wafers for manufacturing power semiconductor devices such as BCD devices are required to have both gettering ability and slip resistance. To satisfy such wafer characteristics, at least about 4×10８ / cm ３ , preferably about 1×10 ９ / cm ３ It is believed that oxygen precipitates of about 4 × 10 are required in a silicon wafer after device heat treatment. For example, in a conventional annealed silicon wafer manufactured by the technology of JP 2021-168382 A, even if a high-temperature heat treatment such as an epitaxial growth process is performed in the initial stage of device processing, ８ / cm ３ However, it was not possible to reduce the oxygen precipitate density in the surface layer to a level that would allow a sufficient device formation region to be secured.

[0052] However, the silicon wafer manufacturing method according to this embodiment eliminates the oxygen precipitate nuclei (as grown) during crystal growth by rapid heating and cooling at approximately 1210 to 1250°C, and then generates and grows new minute oxygen precipitate nuclei within the wafer by a relatively long heat treatment at approximately 800 to 975°C for approximately 2 to 10 minutes. The oxygen precipitate nuclei generated and grown within the wafer are thermally stable, and can generate high-density oxygen precipitates regardless of the type of heat treatment performed by the customer. Furthermore, by continuously performing high-temperature heat treatment at approximately 1150 to 1250°C for approximately 5 to 15 minutes, the minute oxygen precipitate nuclei are further stabilized, and excess vacancies within the wafer are diffused outward, thereby achieving both further stabilization of the oxygen precipitate nuclei density and a reduction in the oxygen precipitate nuclei density in the surface layer.

[0053] The above describes a preferred embodiment of the present invention, but the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the present invention, and it goes without saying that these modifications are also included in the scope of the present invention.

[0054] <Preliminary Observation of Three-Stage Heat Treatment> A p-type silicon single crystal ingot with a diameter of 300 mm and a (100) crystal orientation was grown by the CZ method. The silicon single crystal ingot was sliced ​​to produce CZ silicon wafers. The CZ silicon wafer was then heat-treated to produce two annealed silicon wafer samples according to Example A1 and Comparative Examples A1 to A3.

[0055] In the production of the annealed silicon wafer according to Example A1, the oxygen concentration was 8×10 １７ atom / cm ３ A silicon wafer (CZ silicon wafer) conforming to ASTM F-121, 1979 was used, and a three-stage heat treatment process was carried out using an RTA apparatus, in which the first heat treatment step (high temperature 1), the second heat treatment step (low temperature), and the third heat treatment step (high temperature 2) were performed in that order. Specifically, the process was as follows: room temperature loading → temperature increase at 50°C / sec → 1250°C (held for 10 seconds) → temperature decrease at 70°C / sec → 900°C (held for 5 minutes) → temperature increase at 50°C / sec → 1200°C (held for 5 minutes) → temperature decrease at 10°C / sec → removal to room temperature. Ammonia-containing Ar gas was used as the atmospheric gas during the first heat treatment step, and ammonia-free Ar gas was used as the atmospheric gas during the second and third heat treatment steps. Thus, a sample of the annealed silicon wafer according to Example A1 was obtained.

[0056] In producing the annealed silicon wafer according to Comparative Example A1, the heat treatment was carried out under the same conditions as in Example A1, except that the temperature in the first heat treatment step was set to 1150° C. In this way, a sample of the annealed silicon wafer according to Comparative Example A1 was obtained.

[0057] In producing the annealed silicon wafer according to Comparative Example A2, the heat treatment was carried out under the same conditions as in Example A1, except that the temperature in the third heat treatment step was 1000° C. and the holding time was 1 minute.

[0058] In the production of the annealed silicon wafer according to Comparative Example A3, the oxygen concentration was 11×10 １７ atom / cm ３A silicon wafer (ASTM F-121, 1979) was used, and heat treatment was carried out under the same conditions as in Example A1, except that the temperature in the first heat treatment step was 1150°C, the temperature in the third heat treatment step was 1000°C, and the holding time was 1 minute.

[0059] Table 1 shows a summary of the heat treatment conditions for Example A1 and Comparative Examples A1 to A3.

[0060]

[0061] Next, one of the two samples of each annealed silicon wafer was subjected to a combination of a heat treatment simulating the thermal history at the beginning of the device process and a heat treatment for making oxygen precipitate nuclei visible (first evaluation heat treatment), and the other sample was subjected to a combination of a heat treatment simulating the epitaxial film formation process and a heat treatment for making oxygen precipitate nuclei visible (second evaluation heat treatment). The first evaluation heat treatment was a two-stage heat treatment consisting of a low-temperature heat treatment at 780°C for 3 hours and a visualization heat treatment at 950°C for 16 hours, in sequence. The second evaluation heat treatment was a two-stage heat treatment consisting of a high-temperature heat treatment at 1150°C for 2 minutes and a visualization heat treatment at 1000°C for 16 hours, in sequence.

[0062] FIG. 6 is a schematic diagram of the evaluation procedure for determining the stability and uniformity of manufactured silicon wafers.

[0063] As shown in FIG. 6 , a silicon wafer 71 to be evaluated is cleaved and divided into two portions. Portion A 72 is sequentially subjected to a low-temperature heat treatment 74 and a precipitate visualization heat treatment 76, and portion B 73 is sequentially subjected to a high-temperature heat treatment 75 and a precipitate visualization heat treatment 76. Alternatively, two wafers from a prepared wafer batch can be used, as the two wafers represent properties relevant to the entire batch. Each wafer portion is then subjected to an HF treatment 77 to remove oxide from its surface, and then optical scattering tomography 78 is used to determine their oxygen precipitate density distribution. In this manner, the oxygen precipitates in each of the two portions can be evaluated to determine the stability of the wafer, i.e., the ratio of the resulting BMD densities in each portion subjected to evaluation heat treatments at different temperatures and hold times. Because the evaluation procedure is destructive, properties relevant to testing one wafer from a wafer batch can be attributed to the entire wafer batch.

[0064] Next, for each sample after the evaluation heat treatment, the BMD density in the surface layer within 30 μm from the wafer surface and the BMD density in the bulk layer deeper than the surface layer were measured using an infrared scattering tomography device at approximately 5 mm intervals (30 measurement points) in any radial direction from the center to the edge of the wafer, and the average values ​​were calculated. The diameter of the infrared laser light of the infrared scattering tomography device was 8 mm, which is the standard condition, and the measurement range (scan width) per measurement point was 398 μm, which is wider than the standard condition, in order to measure the surface layer BMD density as accurately as possible. In addition, the bulk BMD density after the first evaluation heat treatment (first bulk density d １ ) after the second evaluation heat treatment (second bulk density d ２ ) ratio (BMD density ratio d ２ / d １ Furthermore, the minimum bulk BMD density d ｍｉｎ Maximum value d for ｍａｘ The ratio d ｍａｘ / d ｍｉｎ The evaluation results are shown in Table 2.

[0065]

[0066] (Example A1) In Example A1, the bulk BMD density after the first evaluation heat treatment was 6.6×10 ９ cm －３ , the bulk BMD density after the second evaluation heat treatment was 6.5 × 10 ９ cm －３ The bulk BMD density ratio was 0.98. There was almost no difference in the bulk BMD density after the two evaluation heat treatments, and it was confirmed that the density was very stable. In addition, the surface BMD density after the first evaluation heat treatment was 2.1 × 10 ７ cm －３ , and the surface BMD density after the second evaluation heat treatment was also 2.1 × 10 ７ cm －３ It was confirmed that the BMD density is two orders of magnitude lower than the bulk BMD density. ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) was approximately 1 (2 or less), which was confirmed to be good.

[0067] (Comparative Example A1) In Comparative Example A1, the BMD density after the first evaluation heat treatment was 9.3 × 10 ８ cm －３ , and the BMD density after the second evaluation heat treatment was also 9.3 × 10 ８ cm －３ The bulk BMD density ratio was 1.00. As such, it was confirmed that the bulk BMD density after the evaluation heat treatment was very stable. Furthermore, the uniformity of the BMD density was also confirmed by the minimum value d ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) was approximately 1 (2 or less), which was confirmed to be good. However, the surface layer BMD density after the first evaluation heat treatment was 6.1 × 10 ８ cm －３ , the surface BMD density after the second evaluation heat treatment was 5.8 × 10 ８ cm －３ This is thought to be because the temperature of the first heat treatment was too low to sufficiently eliminate the oxygen precipitates that had formed during the crystal growth stage.

[0068] (Comparative Example A2) In Comparative Example A2, the BMD density after the first evaluation heat treatment was 6.5×10 ９ cm －３ , the BMD density after the second evaluation heat treatment was 2.1 × 10 ８ cm －３ The bulk BMD density ratio was 0.03. Thus, it was confirmed that when the temperature of the third heat treatment was low and the time was short, the bonding of oxygen precipitate nuclei was insufficient, resulting in a decrease in the BMD density after the second evaluation heat treatment, which included a high-temperature heat treatment at 1150°C for 2 minutes simulating an epitaxial film formation process. Furthermore, the decrease in BMD density after the second evaluation heat treatment also worsened the uniformity of the BMD density. Regarding the surface BMD density, the surface BMD density after the first evaluation heat treatment and the surface BMD density after the second evaluation heat treatment were both 2.1 × 10 ７ cm －３ This gives 10. ７ cm －３ The density was low.

[0069] (Comparative Example A3) In Comparative Example A3, the oxygen concentration was high, so a stable and uniform bulk BMD density was ensured. On the other hand, the surface BMD density after the first evaluation heat treatment was 7.0 × 10 ８ cm －３ , the surface BMD density after the second evaluation heat treatment was 6.5 × 10 ８ cm －３ and an increase in the surface BMD density was observed.

[0070] <Evaluation of the First Heat Treatment Step> The influence of differences in heating conditions in the first heat treatment step on the stability and uniformity of the BMD density of the silicon wafer after the evaluation heat treatment was evaluated. The oxygen concentration of the silicon wafer used was 8×10 １７ atoms / cm ３ The second and third heat treatment steps were performed under the same conditions. Specifically, the second heat treatment step was performed in an Ar atmosphere at a low temperature of 900°C for 5 minutes. The third heat treatment step was performed in an Ar atmosphere at a high temperature of 1200°C for 5 minutes.

[0071] In Examples B1 and B2 and Comparative Example B1, the temperature of the first heat treatment step was 1210°C, and the holding times were 20 seconds, 60 seconds, and 10 seconds, respectively. In Examples B3 and B4, the temperature of the first heat treatment step was 1250°C, and the holding times were 10 seconds and 60 seconds, respectively. Table 3 summarizes the heat treatment conditions for Examples B1 to B4 and Comparative Example B1. Table 4 also shows the evaluation results.

[0072]

[0073]

[0074] As shown in Table 4, with regard to the stability of bulk BMD density, in Examples B1 to B4 and Comparative Example B1, the bulk BMD density after the first and second evaluation heat treatments was 10 ９ cm －３ The BMD density ratio was also within the range of 0.90 to 1.02. In other words, it was confirmed that the bulk BMD density was stable regardless of the difference between the subsequent first and second evaluation heat treatments.

[0075] On the other hand, with regard to the surface layer BMD density, in Examples B1 to B4, the surface layer BMD density after the first and second evaluation heat treatments was 10 ７ cm －３ In Comparative Example B1, the surface BMD density after the first and second evaluation heat treatments was 10 ８ cm －３ In other words, it was confirmed that when the first heat treatment conditions were insufficient, the outward diffusion effect in the surface layer was insufficient and the surface layer BMD density was not reduced sufficiently.

[0076] Regarding the uniformity of the bulk BMD density, in all of Examples B1 to B4 and Comparative Example B1, the minimum bulk BMD density d ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) was less than 2, and no deterioration in the in-plane uniformity of the bulk BMD density was observed.

[0077] <Evaluation of the Second Heat Treatment Step> The influence of differences in heating conditions in the second heat treatment step on the stability and uniformity of the BMD density of the silicon wafer after the evaluation heat treatment was evaluated. The oxygen concentration of the bulk silicon wafer used was 8×10 １７ atoms / cm ３ The first and third heat treatment steps were performed under the same conditions. ３ The third heat treatment step was a high-temperature RTA at 1250° C. for 10 seconds in an Ar atmosphere containing HCl. The third heat treatment step was a high-temperature holding step at 1200° C. for 5 minutes in an Ar atmosphere.

[0078] In Examples C1 and C2 and Comparative Example C5, the temperature of the second heat treatment step was 800°C, and the holding times were 2 minutes, 10 minutes, and 1 minute, respectively. In Examples C3, C4, and Comparative Example C6, the temperature of the second heat treatment step was 900°C, and the holding times were 2 minutes, 10 minutes, and 1 minute, respectively. In Examples C5 and C6, the temperature of the second heat treatment step was 975°C, and the holding times were 5 minutes and 10 minutes, respectively. In Comparative Examples C1 and C2, the temperature of the second heat treatment step was 775°C, and the holding times were 2 minutes and 10 minutes, respectively. In Comparative Examples C3 and C4, the temperature of the second heat treatment step was 775°C, and the holding times were 2 minutes and 10 minutes, respectively. Table 5 summarizes the heat treatment conditions for Examples C1 to C6 and Comparative Examples C1 to C6. The evaluation results are also shown in Table 6.

[0079]

[0080]

[0081] As shown in Table 6, with regard to the stability of the bulk BMD density, in Examples C1 to C6, the bulk BMD density after the first and second evaluation heat treatments was 10 ９ cm －３ The BMD density ratio was also within the range of 0.74 to 0.95. In other words, it was confirmed that the bulk BMD density was generally stable regardless of the differences in the subsequent heat treatment conditions for evaluation.

[0082] In contrast, in Comparative Examples C1 to C6, the bulk BMD density after the second evaluation heat treatment was smaller than the bulk BMD density after the first evaluation heat treatment, resulting in a bulk BMD density ratio of less than 0.5. If the temperature of the second heat treatment is too low or too high, BMD nuclei in the bulk portion do not grow and are thought to disappear as a result of undergoing the second evaluation heat treatment. Furthermore, even if the temperature of the second heat treatment is appropriate, if the holding time is too short, BMD nuclei in the bulk portion do not grow and are thought to disappear as a result of undergoing the second evaluation heat treatment, which includes heat treatment simulating an epitaxial film formation process.

[0083] Regarding the surface layer BMD density, in all of Examples C1 to C6 and Comparative Examples C1 to C6, the surface layer BMD density after the first and second evaluation heat treatments was 10 ７ cm －３ In other words, it was confirmed that the surface BMD density was stable at a low density, regardless of the conditions of the subsequent heat treatment.

[0084] As with the evaluation of the stability of the bulk BMD density, the uniformity of the bulk BMD density was good for Examples C1 to C6. However, the minimum value d ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) became larger than 2, and the uniformity of the bulk BMD density was deteriorated. It is considered that the oxygen precipitate nuclei were not sufficiently stabilized.

[0085] <Evaluation of the Third Heat Treatment Step> The influence of differences in heating conditions in the third heat treatment step on the stability and uniformity of the BMD density of the silicon wafer after the evaluation heat treatment was evaluated. The oxygen concentration of the silicon wafer used was 8×10 １７ atoms / cm ３ The first and second heat treatment steps were performed under the same conditions. ３ The first heat treatment step was a high-temperature RTA at 1250° C. for 10 seconds in an Ar atmosphere containing HCl. The second heat treatment step was a low-temperature hold at 900° C. for 5 minutes in an Ar atmosphere.

[0086] In Examples D1, D2, and D3, the temperature of the third heat treatment step was 1150°C, and the holding times were 5, 10, and 15 minutes, respectively. In Examples D4, D5, and D6, the temperature of the third heat treatment step was 1200°C, and the holding times were 5, 10, and 15 minutes, respectively. In Examples D7, D8, and D9, the temperature of the third heat treatment step was 1250°C, and the holding times were 5, 10, and 15 minutes, respectively. In Comparative Examples D1, D2, and D3, the temperature of the third heat treatment step was 1140°C, and the holding times were 5, 10, and 15 minutes, respectively. Table 7 summarizes the heat treatment conditions for Examples D1 to D6 and Comparative Examples D1 to D6. Table 8 also shows the evaluation results.

[0087]

[0088]

[0089] As shown in Table 8, with regard to the stability of bulk BMD density, in Examples D1 to D9, the bulk BMD density after the first and second evaluation heat treatments was 10 ９ cm －３ The BMD density ratio was also within the range of 0.94 to 1.00. In other words, it was confirmed that the bulk BMD density was stable regardless of the difference between the subsequent first and second evaluation heat treatments.

[0090] In contrast, in Comparative Examples D1 to D3, the bulk BMD density after the second evaluation heat treatment was smaller than the bulk BMD density after the first evaluation heat treatment, resulting in a bulk BMD density ratio significantly below 0.5. It is believed that if the temperature of the third heat treatment is too low, BMD nuclei in the bulk portion do not grow and are annihilated by the second evaluation heat treatment, which includes heat treatment simulating the epitaxial film formation process.

[0091] Regarding the surface layer BMD density, in all of Examples D1 to D9 and Comparative Examples D1 to D3, the surface layer BMD density after the first and second evaluation heat treatments was 10 ７ cm －３ The density was low.

[0092] Regarding the uniformity of the bulk BMD density, unlike the evaluation of the stability of the bulk BMD density, not only Examples D1 to D9 but also Comparative Examples D1 to D3 showed good results. ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) was 2 or less, and the in-plane uniformity of the bulk BMD density was good.

[0093] <Evaluation of Oxygen Concentration of Silicon Wafer> The influence of differences in oxygen concentration of silicon wafer on the stability and uniformity of BMD density of silicon wafer after three-stage heat treatment and evaluation heat treatment was evaluated. In Example E1, the oxygen concentration was 7×10 １７ atoms / cm ３ In Example E2, a low-oxygen bulk silicon wafer with an oxygen concentration of 10×10 １７ atoms / cm ３ In Comparative Example E1, a bulk silicon wafer having an oxygen concentration of 11×10 １７ atoms / cm ３ In Comparative Example E2, a bulk silicon wafer having an oxygen concentration of 6×10 １７ atoms / cm ３ Low-oxygen bulk silicon wafers of the above were used. In the three-stage heat treatment, each bulk silicon wafer was subjected to a first heat treatment step at 1250°C for 10 seconds, a second heat treatment step at 900°C for 5 minutes, and a third heat treatment step at 1200°C for 5 minutes. Table 9 summarizes the differences in oxygen concentration between Examples E1 and E2 and Comparative Examples E1 and E2. Table 10 also shows the evaluation results.

[0094]

[0095]

[0096] As shown in Table 10, in Examples E1 and E2, the bulk BMD density and surface layer BMD density were good.

[0097] On the other hand, when the oxygen concentration is 11×10 １７ atoms / cm ３In Comparative Example E1, which used a bulk silicon wafer of the above formula (1), the bulk BMD density was good, but the surface BMD density was high. The in-plane uniformity of the bulk BMD density was good.

[0098] Oxygen concentration is 6 x 10 １７ atoms / cm ３ In Comparative Example E2, which used a silicon wafer of 1000 kJ / cm2, the bulk BMD density after the second evaluation heat treatment was smaller than the bulk BMD density after the first evaluation heat treatment, resulting in a bulk BMD density ratio below 0.5. Regarding the uniformity of the bulk BMD density, the minimum value d ｍｉｎ Maximum value d for ｍａｘ The ratio (d ｍａｘ / d ｍｉｎ ) was 13.3, and as with the evaluation of the stability of the bulk BMD density, a significant deterioration in the in-plane uniformity of the bulk BMD density was observed after the second evaluation heat treatment. The in-plane uniformity of the bulk BMD density after the first evaluation heat treatment was good. Regarding the surface BMD density, an extremely good result was obtained, with no BMDs observed in the surface layer after the first and second evaluation heat treatments.

[0099] <Evaluation of the Effect of Epitaxial Growth> Epitaxial films were formed on wafers manufactured by three-stage heat treatment, and the stability of BMD density after epitaxial growth was confirmed. As shown in Table 11, in Examples F1, F2, and F3, the growth temperature was 1050°C and the holding time was 1 minute, 2 minutes, and 5 minutes, respectively. In Examples F4, F5, and F6, the growth temperature was 1150°C and the holding time was 1 minute, 2 minutes, and 5 minutes, respectively. The thickness of the obtained epitaxial film was 2 μm in Examples F1 and F4, 4 μm in Examples F2 and F5, and 10 μm in Examples F3 and F6.

[0100] The first evaluation heat treatment, which was one of the evaluation heat treatments after epitaxial growth, was a two-stage heat treatment consisting of a low-temperature heat treatment at 780°C for 3 hours and a visualization heat treatment at 950°C for 16 hours in that order. The second evaluation heat treatment was a visualization heat treatment at 1000°C for 16 hours only, omitting the high-temperature heat treatment at 1150°C for 2 minutes that simulated the epitaxial film formation process. The evaluation results are shown in Table 12.

[0101]

[0102]

[0103] As shown in Table 12, under all of the epitaxial growth conditions, the stability and uniformity of the bulk BMD density and the uniformity of the surface BMD density were good, and it was confirmed that the bulk BMD density did not decrease even when epitaxial growth was performed on silicon wafers manufactured by carrying out three-stage heat treatment.

[0104] S11: Step of producing a silicon single crystal ingot S12: Step of producing a silicon wafer S13: Step of heat treating the silicon wafer S21: First heat treatment step S22: Second heat treatment step S23: Third heat treatment step 30: Standby temperature 32: Temperature increase 34: Temperature decrease 36: Temperature increase 40: Silicon wafer 40a: Upper zone 40b: Lower zone 41: Minute oxygen precipitate nuclei (generated during crystal growth) 42: Frenkel pairs 43: Si ３ N ４ Layers 44, 44a, 44b Vacancy 45 Interstitial silicon atoms 46 DZ 47 Oxygen precipitate nuclei 47a Small oxygen precipitate nuclei 48 Epitaxial layer 50 Silicon wafer 50a Wafer surface (main surface) 50b Cleaved surface 51 Infrared laser light 52 BMD (oxygen precipitate) 53 Surface layer portion 54 Bulk portion 71 Silicon wafer 72 Part A of wafer 73 Part B of wafer 74 Low-temperature heat treatment 75 High-temperature heat treatment 76 Precipitate visualization heat treatment 77 HF treatment 78 Light scattering tomography

Claims

1. A steel sheet having a surface layer portion extending from the surface to a depth of 30 μm and a bulk portion deeper than the surface layer portion, wherein the density of oxygen precipitates generated in the surface layer portion by a first evaluation heat treatment is 1.0×10 ７ ~1.0 x 10 ８ cm －３ and the density of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment is 1.0×10 ９ ~7.0 x 10 ９ cm －３ wherein, when an average density of oxygen precipitates formed in the bulk portion by the first evaluation heat treatment is defined as a first bulk density and an average density of oxygen precipitates formed in the bulk portion by the second evaluation heat treatment is defined as a second bulk density, a ratio of the second bulk density to the first bulk density is within a range of 0.74 to 1.02; the first evaluation heat treatment is a two-stage heat treatment in which a heat treatment is performed at 780°C for 3 hours followed by a visualization heat treatment; the second evaluation heat treatment is a two-stage heat treatment in which a heat treatment is performed at 1150°C for 2 minutes followed by the visualization heat treatment; and the visualization heat treatment is a heat treatment at 950 to 1000°C for 16 hours.

2. The silicon wafer according to claim 1, wherein the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the first evaluation heat treatment and the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the second evaluation heat treatment are both 2 or less.

3. The silicon wafer according to claim 1, wherein the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the first evaluation heat treatment is 1.30 or less, and the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the second evaluation heat treatment is 1.32 or less.

4. The average density of oxygen precipitates formed in the surface layer portion by the first evaluation heat treatment and the average density of oxygen precipitates formed in the surface layer portion by the second evaluation heat treatment are both 2.1 × 10 ７ cm －３ 4. The silicon wafer according to claim 1, wherein:

5. A silicon substrate comprising: an epitaxial silicon film formed on a surface of the silicon substrate; the silicon substrate having a surface layer portion extending from the surface to a depth of 30 μm and a bulk portion deeper than the surface layer portion; and a density of oxygen precipitates generated in the surface layer portion by a first evaluation heat treatment of 1.0×10 ７ ~1.0 x 10 ８ cm －３ and the density of oxygen precipitates generated in the bulk portion by the first evaluation heat treatment is 1.0×10 ９ ~7.0 x 10 ９ cm －３ wherein, when an average density of oxygen precipitates formed in the bulk portion by the first evaluation heat treatment is defined as a first bulk density and an average density of oxygen precipitates formed in the bulk portion by the second evaluation heat treatment is defined as a second bulk density, a ratio of the second bulk density to the first bulk density is in a range of 0.98 to 1.02, the first evaluation heat treatment is a two-stage heat treatment in which a visualization heat treatment is carried out after a heat treatment at 780°C for 3 hours, the second evaluation heat treatment is the visualization heat treatment, and the visualization heat treatment is a heat treatment at 950 to 1000°C for 16 hours.

6. A silicon wafer according to claim 5, wherein the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the first evaluation heat treatment and the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the second evaluation heat treatment are both 2 or less.

7. A silicon wafer according to claim 6, wherein the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the first evaluation heat treatment is 1.29 or less, and the ratio of the maximum value to the minimum value of the density of oxygen precipitates formed in the bulk portion by the second evaluation heat treatment is 1.35 or less.

8. Oxygen concentration is 7 x 10 １７ ~10 x 10 １７ atoms / cm ３ a first heat treatment step of heating a silicon wafer conforming to ASTM F-121, 1979 at a first temperature; a second heat treatment step of heating the silicon wafer at a second temperature lower than the first temperature after the first heat treatment step; and a third heat treatment step of heating the silicon wafer at a third temperature higher than the second temperature after the second heat treatment step, wherein the first temperature is 1210 to 1250°C, and the holding time at the first temperature is 10 to 60 seconds; the second temperature is 800 to 975°C, and the holding time at the second temperature is 2 to 10 minutes; and the third temperature is 1150 to 1250°C, and the holding time at the third temperature is 5 to 15 minutes.

9. The method for producing a silicon wafer according to claim 8, wherein the first heat treatment step is performed in a non-oxidizing atmosphere containing ammonia or nitrogen, and the second and third heat treatment steps are performed in a non-oxidizing atmosphere not containing ammonia or nitrogen.

10. The method for producing a silicon wafer according to claim 8 or 9, wherein the rate of temperature rise to the first temperature and the rate of temperature rise from the second temperature to the third temperature are 10 to 50°C / second.

11. The method for producing a silicon wafer according to claim 8 or 9, wherein the rate of temperature decrease from the first temperature to the second temperature is 20 to 120°C / second.

12. The method for producing a silicon wafer according to claim 8 or 9, wherein the silicon wafer before being heat-treated in the first heat treatment step is cut from a defect-free region of a silicon single crystal ingot that is free of agglomerates of interstitial silicon point defects and agglomerates of vacancy point defects.

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