Method for producing a silicon epitaxial wafer

Abstract

A method for producing a silicon epitaxial wafer (10) in which a silicon epitaxial layer (12) is formed on a surface of a boron-doped silicon substrate (11), the method comprising: Pulling a single-crystalline silicon ingot coated with a boron concentration of 2.7 10 17 atoms / cm 3 or more and 1.3·10 19 atoms / cm 3 or less doped, using a Czochralski process; Producing a silicon substrate (11) by processing the single-crystal silicon ingot; and Forming a silicon epitaxial layer (12) on a surface of the silicon substrate (11), wherein pulling the single-crystal silicon ingot comprises controlling the single-crystal pulling conditions of the single-crystal silicon ingot such that the boron concentration Y (atoms / cm 3 ) and the initial oxygen concentration X (·10 17 atoms / cm 3) a relationship expression of X ≤ -4.3·10 -19 Y+16.3 fulfill, wherein pulling the single-crystal silicon ingot further comprises decreasing the initial oxygen concentration X in the ingot pulling direction as pulling of the single-crystal silicon ingot proceeds in accordance with the boron concentration Y becoming larger in the ingot pulling direction by controlling the single-crystal pulling conditions of the single-crystal silicon ingot.

Description

Technical area

[0001] The present invention relates to a silicon epitaxial wafer, and more particularly to a silicon epitaxial wafer having an epitaxial layer formed on the surface of a boron-doped p-type silicon substrate. background

[0002] Silicon epitaxial wafers are widely used as a substrate material for semiconductor devices. A silicon epitaxial wafer has an epitaxial layer formed on the surface of a bulk silicon substrate. Because silicon epitaxial wafers exhibit a high degree of crystal perfection, they can be used to manufacture highly reliable, high-quality semiconductor devices.

[0003] Silicon substrates containing a high concentration of p-type or n-type impurities are used for silicon epitaxial wafers, whose applications include solid-state imaging elements and power semiconductor devices. For example, JP 2011-228459 A describes a silicon epitaxial wafer containing a wafer containing a nitrogen concentration of 1 10 12 atoms / cm 3 or more and whose specific electrical resistance is set to 20 mΩ·cm or less by boron doping, and comprising an epitaxial layer arranged on the surface of the wafer, wherein the initial oxygen concentration of the silicon epitaxial wafer is 14·10 17 atoms / cm 3 or less.

[0004] Compared with conventional wafers, silicon wafers heavily doped with nitrogen or boron can easily produce oxygen precipitates during device manufacturing processes because nitrogen and boron enhance the stability of oxygen precipitate nuclei. Therefore, if laser spike annealing (LSA) is performed in a state where plate-like oxygen precipitates have grown to a large size, dislocations originating from an oxygen precipitate nucleus can easily occur. However, a silicon epitaxial wafer as described above can prevent dislocations originating from an oxygen precipitate nucleus from occurring even when LSA is performed during the device manufacturing process.

[0005] JP 2012 - 138 576 A describes an epitaxial wafer to be used for a solid-state imaging element of a back-illumination type, wherein the silicon epitaxial wafer comprises a p-type silicon substrate containing carbon and nitrogen added thereto and having an electrical resistivity of less than 100 mΩ cm, a first p-type epitaxial layer formed on the p-type silicon substrate, and a second p-type or n-type epitaxial layer formed on the first p-type epitaxial layer, wherein the interstitial oxygen concentration in the p-type silicon substrate is between 10 10 17 atoms / cm 3 and 20·10 17 atoms / cm 3 wherein the precipitate density in a central part of the p-type silicon substrate when viewed in the depth direction thereof is 5·10 5 cm 2 or more and 5·10 7 cm 2or less. According to this epitaxial wafer, back-illumination type solid-state imaging elements can be manufactured with high yield.

[0006] US 2014 / 0361 408 A1 describes a boron-doped single-crystalline silicon wafer whose boron concentration is at least approximately 1×10 19 cm -3 The oxygen content is between 4.5×10 17 cm -3 and 5.5×10 17 cm -3 Epitaxial layers are also deposited on this wafer. A similar wafer is also known from US 2012 / 0 043 644 A1.

[0007] A relationship between doping concentration and resistivity for boron-doped silicon wafers, among others, is known from WR Thurber et al., “The Relationship Between Resistivity and Dopant Density for Phosphorus- and Boron-Doped Silicon”, NBS Special Publication 400-64, 1981.

[0008] US 2008 / 0 038 526 A1, US 2007 / 0 089 666 A1, US 6 222 252 B1 and US 5 951 755 A describe other wafers with similar boron and oxygen concentrations. Summary of the invention[Problems to be solved by the invention]

[0009] With regard to epitaxial wafers to be used for solid-state imaging elements and other applications, boron contained in the silicon substrate is indispensable for ensuring the gettering capability and reducing the electrical resistance of the silicon substrate. On the other hand, when boron is diffused to a large extent from the silicon substrate to the epitaxial layer side, a problem arises that the impurity profile of the epitaxial layer is changed, deteriorating the in-plane resistivity uniformity of the wafer. Furthermore, the width of the boron concentration transition region (resistance-varying layer) in the epitaxial layer at and near the boundary between the silicon substrate and the epitaxial layer is increased, reducing the effective thickness of the epitaxial layer, and in turn, deteriorating the characteristics of the semiconductor device.For this reason, it is necessary to suppress boron diffusion in the silicon substrate as much as possible.

[0010] It is known that the diffusion of boron atoms contained in a silicon substrate is enhanced when the silicon substrate is subjected to heat treatment in an oxidizing atmosphere. When a thermal oxide film forms on the surface of a silicon substrate, some of the Si atoms in the silicon substrate are displaced from their original positions because their original positions are occupied by SiO2 molecules, and are then pushed out of the crystal lattice. This increases the number of interstitial silicon atoms. On the other hand, when boron atoms are replaced by interstitial silicon atoms, they diffuse through kick-out diffusion. Accordingly, the diffusion of boron atoms is enhanced as a result of the increase in the number of interstitial silicon atoms.Therefore, one technique for suppressing enhanced diffusion of boron atoms is to avoid performing any heat treatment in an oxidizing atmosphere as much as possible.

[0011] However, there are cases where device properties deteriorate due to boron diffusion, even when the device fabrication process does not involve any heat treatment in an oxidizing atmosphere. Therefore, it is highly desirable to improve existing techniques for suppressing enhanced diffusion of boron atoms. [Means of solving the problems]

[0012] In view of the technical problems identified above, it is therefore an object of the present invention to provide a method for producing a silicon epitaxial wafer in which enhanced boron diffusion in the silicon substrate is suppressed.

[0013] To solve the above problems, the present inventors intensively studied the mechanism of boron diffusion in silicon wafers. As a result of intensive research, the present inventors discovered that not only during heat treatments in an oxidizing atmosphere but also during any heat treatment, interstitial silicon atoms are released when oxygen precipitates grow in the silicon substrate, and kick-out diffusion of boron atoms is promoted by interstitial silicon atoms. In particular, boron diffusion attributable to oxygen precipitates progresses rapidly when the density of oxygen precipitates in the silicon substrate exceeds a threshold level. The present invention is achieved by the present inventors paying attention to this fact.

[0014] Accordingly, the present invention is based on the technical knowledge described above.

[0015] According to the invention, a method for producing a silicon epitaxial wafer is provided according to claim 1. The subclaims define further embodiments.

[0016] Thus, a method for producing a silicon epitaxial wafer is provided in which a silicon epitaxial layer is formed on a surface of a boron-doped silicon substrate, the method comprising: Pulling a single-crystalline silicon ingot coated with a boron concentration of 2.7 10 17 atoms / cm 3 or more and 1.3·10 19 atoms / cm 3 or less doped, using a Czochralski process; Producing a silicon substrate by processing the single-crystal silicon ingot; and Forming a silicon epitaxial layer on a surface of the silicon substrate, wherein pulling the single-crystal silicon ingot comprises controlling the single-crystal pulling conditions of the single-crystal silicon ingot such that the boron concentration Y (atoms / cm 3 ) and the initial oxygen concentration X (·10 17 atoms / cm 3 ) a relationship expression of X ≤ -4.3·10 -19 Y+16.3 fulfill, wherein pulling the single-crystal silicon ingot further comprises decreasing the initial oxygen concentration X in the ingot pulling direction as pulling of the single-crystal silicon ingot proceeds in accordance with the boron concentration Y becoming larger in the ingot pulling direction by controlling the single-crystal pulling conditions of the single-crystal silicon ingot.

[0017] A silicon epitaxial wafer manufactured according to the present invention is characterized in that an epitaxial layer is formed on a surface of a boron-doped silicon substrate, wherein the boron concentration in the silicon substrate is 2.7 10 17 atoms / cm 3 or more and 1.3·10 19 atoms / cm 3 or less, an initial oxygen concentration in the silicon substrate 11·10 17 atoms / cm 3 or less and, when an oxygen precipitate evaluation heat treatment is performed on the silicon epitaxial wafer, the oxygen precipitate density in the silicon substrate is 1·10 10 / cm 3 or less.

[0018] Accordingly, the oxygen precipitate density in the silicon substrate according to the present invention is 1·10 10 / cm 3or less, and therefore, even if oxygen precipitates grow in the silicon substrate due to any heat treatment in the device manufacturing process, it is possible to suppress the increase in the number of interstitial silicon atoms that results from an increase in the oxygen precipitate density. Then, it is also possible to suppress the kick-out diffusion of boron atoms in the silicon substrate toward the epitaxial layer side due to the interstitial silicon atoms, so that consequently, diffusion of boron atoms can be suppressed to a level practically equal to the level of boron diffusion observed when the oxygen precipitate density is substantially zero.

[0019] In the present invention, the boron concentration in the silicon substrate is preferably 2.7 10 17 atoms / cm 3 or more and 1.3·10 19 atoms / cm 3or less and the initial oxygen concentration in the silicon substrate is preferably 11·10 17 atoms / cm 3 or less. If the initial oxygen concentration in the silicon substrate is 11·10 17 atoms / cm 3 or less, the oxygen precipitate density in the silicon substrate can be 1·10 10 / cm 3 or less even if oxygen precipitates grow in the silicon substrate due to any heat treatment in the device manufacturing process.

[0020] In the present invention, the boron concentration Y (atoms / cm 3 ) and the initial oxygen concentration X (·10 17 atoms / cm 3 ) prefers a relationship expression of X ≤ -4.3·10 -19Y+16.3. As long as the boron concentration and the initial oxygen concentration in the silicon substrate meet the requirement defined above, the oxygen precipitate density can be maintained at 1·10 10 / cm 3 or less, regardless of the boron concentration in the silicon substrate. Therefore, it is possible to suppress enhanced boron diffusion attributable to oxygen precipitates. [Advantages of the invention]

[0021] Accordingly, as described above, the present invention can provide a silicon epitaxial wafer that can suppress increased boron diffusion in the silicon substrate even when oxygen precipitates grow due to any heat treatment in the device manufacturing process. Short description of the drawings Fig. 1 is a schematic cross-sectional view illustrating a structure of a silicon epitaxial wafer manufacturable according to an embodiment of the present invention; Fig. 2 is a flowchart illustrating a process for manufacturing the silicon epitaxial wafer; Fig. 3 is a graph illustrating the depth profile of boron concentration of each of the silicon epitaxial wafer samples #1 to 4# before and after the oxygen precipitate evaluation heat treatment; and Fig. Figure 4 is a graph illustrating the relationship between oxygen precipitate density, initial oxygen concentration, and boron concentration. Detailed description of the embodiments

[0022] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0023] Fig. 1 is a schematic cross-sectional view illustrating a structure of a silicon epitaxial wafer manufacturable according to an embodiment of the present invention.

[0024] As in Fig. As shown in Figure 1, a silicon epitaxial wafer 10 in this case includes a silicon substrate 11 and an epitaxial layer 12 formed on the surface of the silicon substrate 11. The silicon substrate 11 is a polished wafer cut from a silicon monocrystal ingot grown by the Czochralski (CZ) method and has a mirror-polished surface. The silicon substrate 11 plays a role of ensuring the mechanical strength of the silicon epitaxial wafer 10 and, at the same time, acting as a getter sink for capturing heavy metals. While the thickness of the silicon substrate 11 is not specifically defined, as long as it can ensure the mechanical strength of the silicon epitaxial wafer 10, it can typically be made equal to 725 µm.

[0025] The silicon substrate 11 is a boron-doped p-type silicon substrate. The boron concentration in the silicon substrate 11 is preferably 2.7 10 17 atoms / cm 3 or more and 1.3·10 19 atoms / cm 3 or less, while the electrical resistivity of the silicon substrate 11 is preferably 20 mΩ cm or less. The electrical resistivity of the silicon substrate 11 can be reduced to such a low level, and the silicon substrate 11 can be provided with a sufficiently high gettering ability by using a silicon substrate 11 doped with boron to such a high concentration level.

[0026] An epitaxial layer 12 is formed on the surface of the silicon substrate 11. A semiconductor device, such as a MOS transistor, can typically be formed in the epitaxial layer 12. The thickness of the epitaxial layer 12 is preferably between 1 and 10 µm. The epitaxial layer 12 may have a multilayer structure formed by stacking multiple epitaxial layers with different properties. Typically, the electrical resistivity of the epitaxial layer 12 is made higher than the electrical resistivity of the silicon substrate 11, and the silicon substrate is made to contain a p-type dopant (boron) or an n-type dopant (phosphorus, arsenic, or antimony) added thereto.

[0027] When an oxygen precipitate evaluation heat treatment is performed on the silicon epitaxial wafer 10, the oxygen precipitate density in the silicon substrate 11 is 1·10 10 / cm 3 or less. As will be discussed in more detail below, the oxygen precipitates in the silicon substrate 11 will only minimally affect boron diffusion when the oxygen precipitate density is 1·10 10 / cm 3 or less. Then, boron diffusion can be maintained at a level practically equal to the diffusion level observed when the oxygen precipitate density is essentially zero.

[0028] An oxygen precipitate extraction heat treatment is a two-step heat treatment in which a first heat treatment (nucleation step) and a second heat treatment (nucleation growth step) are typically performed sequentially at 700°C for 3 hours and 1000°C for 16 hours, respectively. These are heat treatments that simulate those of a device manufacturing process. The heat treatments are performed in a nitrogen atmosphere rather than an oxidizing atmosphere, so no thermal oxide film is formed there, and therefore no problem of enhanced diffusion of boron atoms attributable to thermal oxide film production arises. However, as the oxygen precipitate nucleus grows in the silicon substrate 11, the oxygen precipitate density increases, giving rise to an additional cause of boron diffusion.Furthermore, it is known that boron in the silicon substrate 11 has an effect of promoting oxygen precipitation, and therefore, as the boron concentration in the silicon substrate 11 increases, the oxygen precipitate density in the silicon substrate 11 also increases. While the existence of oxygen precipitates is necessary to a certain extent to ensure gettering capability, enhanced boron diffusion occurs as the volume of the oxygen precipitate increases.

[0029] In order to make the oxygen precipitate density in the silicon substrate 11 1·10 10 / cm 3 or less, it is necessary to make the initial interstitial oxygen concentration of the silicon substrate 11 11·10 17 atoms / cm 3 or less. If the initial oxygen concentration is higher than 11·10 17 atoms / cm 3The number of interstitial silicon atoms increases as the oxygen precipitate density increases, thus allowing enhanced boron diffusion due to interstitial silicon atoms to occur. The lower the initial oxygen density, the better it is to keep the oxygen precipitate density below the limit level defined above. While no lower limit for the initial oxygen concentration is defined, it is currently impossible from the perspective of actual manufacturing to produce silicon single crystals with an initial oxygen concentration lower than 11 10 17 atoms / cm 3 to grow. It is noted that all oxygen concentrations mentioned in this patent specification are measured values ​​obtained by Fourier transform infrared spectrometry (FT-IR), as defined in ASTM F-121 (1979).

[0030] The preferred range of an initial oxygen concentration in the silicon substrate 11 changes as a function of the boron concentration in the substrate. When the boron concentration in the silicon substrate 11 is low, no problems arise if the initial oxygen concentration is high to some extent. However, when the boron concentration is high, an oxygen precipitate is excessively produced unless the initial oxygen concentration is kept low, because an oxygen precipitate can easily grow in such an environment, so it may not be possible to cause the oxygen precipitate density to exceed 1 10 10 / cm 3 or less. When the initial oxygen concentration in the silicon substrate 11 is given by X (·10 17 atoms / cm 3 ) and the boron concentration is expressed by Y (atoms / cm 3), they preferentially satisfy the requirement of the relationship expression of X ≤ -4.3·10 -19 Y+16.3. As long as they satisfy the requirement of the relational expression defined above, the oxygen precipitate density in the silicon substrate 11 can be maintained at 1·10 10 / cm 3 or less.

[0031] Fig. 2 is a flowchart illustrating a process for manufacturing the silicon epitaxial wafer 10.

[0032] As in Fig. As shown in Figure 2, to produce the silicon epitaxial wafer 10, a boron-doped silicon single-crystal ingot is first grown using the Cz process (step S1). In this step, the silicon single crystal is doped with boron to a concentration level between 2.7 10 17 atoms / cm 3 and 1.3·10 19 atoms / cm 3doped. Although the silicon single crystal contains oxygen to the level of supersaturation because oxygen is eluted from the quartz crucible used for the manufacturing process, the oxygen concentration in the silicon single crystal can be controlled by controlling the single crystal pulling conditions. More specifically, the single crystal pulling conditions are controlled so that the initial oxygen concentration X (·10 17 atoms / cm 3 ) and the boron concentration Y (atoms / cm 3 ) in the silicon single crystal, the above-described requirement of the relational expression of X ≤ -4.3·10 -19 Y+16.3 fulfill.

[0033] When silicon is placed as the starting material in a quartz crucible, a predetermined amount of boron is added to the raw material to cause the single crystal pulled from the quartz crucible to contain boron. More specifically, boron is added in an amount that causes the single crystal to exhibit the desired electrical resistivity at the top of the single crystal. When the added boron is melted with the starting material of silicon, a boron-containing silicon melt is produced. While the single crystal pulled from the silicon melt contains boron in a given proportion, the boron concentration in the ingot pulling direction increases due to segregation as the crystal growth process progresses. Therefore, the oxygen concentration in the ingot pulling direction must be reduced to cause the ingot to meet the requirement of the relationship expression defined above.

[0034] The oxygen concentration in the single crystal can be controlled by adjusting the rotation speed of the quartz crucible and / or the power supplied to the heating element. To reduce the oxygen concentration in the single crystal, it is sufficient to select a low rotation speed for the quartz crucible and / or a low power output for the heating element. Accordingly, the oxygen concentration in the single crystal can be kept low by controlling the conditions under which the single crystal is pulled in the manner described above.

[0035] The MCZ method of pulling the single crystal while applying a magnetic field to the silicon melt is very effective in reducing the oxygen concentration in the pulled single crystal. The MCZ method suppresses the convection of the silicon melt under the influence of the magnetic field, thus preventing the elution of oxygen from the quartz crucible into the silicon melt, and thus keeping the oxygen concentration in the single crystal pulled from the silicon melt at a low level.

[0036] Then, the silicon single-crystal ingot is processed to produce the silicon substrate 11 (step S2). As described above, the silicon substrate 11 is a polished wafer cut from the silicon single-crystal ingot, the surface of which is mirror-polished. The boron concentration of the silicon substrate 11 is 2.7 10 17 atoms / cm3 or more and 1.3·10 19 atoms / cm 3 or less and the initial oxygen concentration in the silicon substrate 11 is 11·10 17 atoms / cm 3 or less.

[0037] Next, an epitaxial layer 12 is formed on the surface of the silicon substrate 11 by a well-known method (step S3). As a result of performing the above steps, a finished epitaxial wafer 10 is produced.

[0038] A silicon epitaxial wafer 10 manufactured in the manner described above is then used as a substrate material for semiconductor devices. Various semiconductor devices can then be produced by using such a silicon epitaxial wafer through various processing steps. Such processing steps include various heat treatment steps, and as a result, an oxygen precipitation nucleus is formed in the silicon substrate 11, wherein the oxygen precipitation nucleus grows, so that the oxygen precipitate density in the silicon substrate is increased. However, since the oxygen precipitate density in the silicon substrate is 1 10 10 / cm 3 or less, any enhanced boron diffusion attributable to oxygen precipitate can be prevented from taking place.

[0039] If the boron concentration and the initial oxygen concentration in the silicon substrate are known, and the heat treatment conditions (heat history) in the device fabrication process are also known, then the oxygen precipitate density and the extent of enhanced boron diffusion in the silicon substrate that will be observed when such a silicon epitaxial wafer is heat treated during the device fabrication process can be predicted by simulation. If, as a result of such a simulation, the width of the transition region broadened by enhanced boron diffusion cannot be limited within the given allowable range, it may only be necessary to adjust the initial oxygen concentration so that the width of the transition region is limited within the given allowable range.Accordingly, the initial oxygen concentration in the silicon substrate necessary to produce a given density of oxygen precipitate can be predicted from the heat treatment conditions in the device fabrication process, so that the enhanced boron diffusion can be confined within a given allowable range.

[0040] As described in detail above, the silicon epitaxial wafer 10 of this embodiment includes a boron-doped silicon substrate 11 and an epitaxial layer 12 formed on the surface of the silicon substrate 11, and when an oxygen precipitate evaluation heat treatment is performed, it was found that the oxygen precipitate density in the silicon substrate 11 is 1 10 10 / cm 3or less. Accordingly, this embodiment can suppress any enhanced diffusion of boron that may occur due to an increase in the oxygen precipitate density from the silicon substrate 11 to the epitaxial layer 12.

[0041] The silicon epitaxial wafer 10 of this embodiment can be advantageously used as a substrate material for a back-illumination type solid-state imaging element. In the process of manufacturing a back-illumination type solid-state imaging element, metal impurities contained in the silicon substrate can increase the dark current of the sensor portion, which in turn causes defects called "white flaws." However, using a p-type silicon substrate doped with boron atoms to a high concentration level can solve the problem of metal impurities because the silicon substrate acts as a getter sink.

[0042] Furthermore, in a back-illumination type solid-state imaging element, the wiring layer and related parts are arranged in layers lower than the sensor portion so that the sensor portion can directly receive external light. As a result, the imaging element can produce a clear image including moving images. It is necessary to perform a process of removing the silicon substrate 11, typically by polishing, to arrange the wiring layer and related parts in layers lower than the sensor portion, leaving only the epitaxial layer 12 undamaged.If the transition region in the epitaxial layer 12 is widened by enhanced boron diffusion, so that the uniformity of an in-plane resistivity of the wafer is deteriorated, it becomes difficult to determine the appropriate extent to which the silicon substrate 11 should be polished, and furthermore, the characteristics of the solid-state imaging element may be deteriorated because the effective thickness of the epitaxial layer 12 may be reduced. However, the problems identified above are solved, making it possible to manufacture a high-quality back-illumination type imaging element when the width of the transition region is satisfactorily narrow and the effective thickness of the epitaxial layer 12 is sufficiently large.

[0043] While preferred embodiments of the present invention have been explained above, the present invention is not limited thereto. Various modifications may be made to the embodiments without departing from the scope of the present invention, and needless to say, such modifications are also encompassed within the scope of the invention. [Example]

[0044] A silicon substrate with a (100) crystal plane orientation was cut from a silicon single-crystal ingot grown by the CZ method, and the surface of the silicon substrate was mirror-polished. The silicon substrate contained boron at a concentration of 1.0 10 19 atoms / cm 3 was added. The initial oxygen concentration of the silicon substrate was 6·10 17 atoms / cm 3An epitaxial layer with a thickness of 5 μm was formed on the surface of the silicon substrate by vapor deposition at a temperature of 1150 °C to obtain a silicon epitaxial wafer sample, hereinafter referred to as silicon epitaxial wafer sample #1. In addition, silicon epitaxial wafer samples of samples #2 to #4, each of which exhibited initial oxygen concentrations different from the initial oxygen concentration of sample #1, were prepared using processes similar to those used to prepare sample #1. The initial oxygen concentrations of sample #2, sample #3, and sample #4 were 10 10 17 atoms / cm 3 , 11·10 17 atoms / cm 3 or 13·10 17 atoms / cm 3

[0045] Then, an oxygen precipitate evaluation heat treatment was performed on each of the silicon epitaxial wafer samples #1 to #4. In each of the oxygen precipitate evaluation heat treatments, a heat treatment was performed at 700 °C in a nitrogen atmosphere for 3 hours, followed by another heat treatment at 1000 °C, also in a nitrogen atmosphere, for 16 hours. Furthermore, a depth profile of the boron concentration of each of the samples #1 to #4 was observed by SIMS (Secondary Ion Mass Spectroscopy) before and after the oxygen precipitate evaluation heat treatment.

[0046] Fig. Figure 3 is a graph illustrating a depth profile of the boron concentration of each of the silicon epitaxial wafer samples #1 to 4# before and after the oxygen precipitation evaluation heat treatment. The horizontal axis of the graph indicates the depth (relative value) from the top surface of the wafer, and the vertical axis of the graph indicates the boron concentration (relative value).

[0047] As in Fig. As shown in Figure 3, all of the silicon epitaxial wafer samples #1 to #4 exhibited essentially similar boron concentration profiles before the oxygen precipitate evaluation heat treatment. Each of them exhibited an abrupt change at and near the boundary between the silicon substrate and the epitaxial layer, thus proving that the amount of boron diffused into the epitaxial layer was very small. The dashed line X with long segments generally shows the boron concentration profiles of samples #1 to #4 before the oxygen precipitate evaluation heat treatments.

[0048] On the other hand, all of the boron concentration profiles of samples #1 to #4 changed significantly after the respective oxygen precipitate evaluation heat treatments compared to the concentration profiles observed before the evaluation heat treatments. Specifically, boron diffusion into the epitaxial layer increased significantly after the oxygen precipitate evaluation heat treatment in each of the samples. Accordingly, it is safe to assume that thermal diffusion due to the evaluation heat treatment is primarily responsible for the noticeable change in the boron concentration profile of each of the samples.

[0049] Of the samples, the boron concentration profile (solid line) of "Sample #1" showed that virtually no boron atoms existed at or near the surface of the epitaxial layer. In other words, Sample #1 showed excellent results. The boron concentration increased abruptly at a depth of about 0.7 μm. The boron concentration was 0.015 μm at a depth of 0.8 μm, 0.2 μm at a depth of 0.9 μm, and 0.5 μm at a depth of 1 μm. Both "Sample #2" (dashed line with short segments) and "Sample #3" (dotted line) showed respective boron concentration profiles that were essentially the same as that of Sample #1.

[0050] The boron concentration profile (dashed line) of Sample #4 differed significantly from the boron concentration profiles of Samples #1 to #3. For Sample #4, boron diffusion continued near the surface of the epitaxial layer. Specifically, the boron concentration began to increase at approximately 0.6 depth, and the boron concentrations at 0.7, 0.8, and 0.9 depths were 0.004, 0.07, and 0.25, respectively. The boron concentration at 1 depth was 0.5, which was equal to the boron concentration at 1 depth for each of Samples #1 to #3.

[0051] From the results obtained, as described above, it became clear that boron diffusion was barely observed in samples #1 to #3, but boron diffused significantly in sample #4. It is safe to assume that enhanced boron diffusion was primarily responsible for the particularly striking change in the boron concentration profile of sample #4.

[0052] Then, each of the silicon epitaxial wafer samples #1 to #4 was cleaved in the thickness direction after the oxygen precipitate evaluation heat treatment, and the cleaved cross sections of each were subjected to a selective etching process to a thickness of 2 µm using a Wright etching solution. Subsequently, the central part of the cleaved cross section in the thickness direction of the silicon wafer was observed through an optical microscope, and the number of etch pits within a 100 µm x 100 µm square was measured as the oxygen precipitate density. Table 1 below shows the obtained results. [Table 1] Wafer-Probe Initial oxygen concentration (atoms / cm 3 ) Borkonzentration(Atome / cm 3 ) Oxygen precipitate density ( / cm 3 ) #1 6·10 17 1,0·10 19 <1·10 7 #2 10·10 17 1,0·10 19 1·10 9 #3 11·10 17 1,0·10 19 1·10 10 #4 13·10 17 1,0·10 19 3·10 10

[0053] As can be seen from Table 1, the oxygen precipitate density of Example #1 was below the measurement threshold (less than 1·10 7 / cm 3 ). The oxygen precipitate density of sample #2 and sample #3 was 1·10 9 / cm 2 or 1·1010 / cm 3 , whereas that of sample #4 3·10 10 / cm 3 From the data in Table 1 and the graph from Fig. 3, it became clear that enhanced boron diffusion was hardly observed in each of the samples #1 to #3, whose oxygen precipitate density was 1·10 10 / cm 3 or less.

[0054] Subsequently, a total of 28 samples of a silicon epitaxial wafer, which were distinguished by using the initial oxygen concentration and the boron concentration in the silicon substrate as parameters, were prepared to investigate the joint relationship between the initial oxygen concentration, the boron concentration, and the oxygen precipitate density, and each of the samples was subjected to oxygen precipitate evaluation heat treatment, and then the oxygen precipitate density of them was measured.

[0055] Fig. Figure 4 is a graph illustrating the relationship between the oxygen precipitate density, the initial oxygen concentration, and the boron concentration. The horizontal axis of the graph indicates the oxygen concentration (·10 17 atoms / cm 3 ) and the vertical axis indicates the boron concentration (atoms / cm 3 ). A “◯” mark was drawn for a sample showing an oxygen precipitate density of 1·10 10 / cm 3 or less, whereas an “×” mark was drawn for a sample showing an oxygen precipitate density greater than 1·10 10 / cm 3 fraud.

[0056] How clear from Fig. 4, it was found that the requirement of an oxygen precipitate density of 1·10 10 / cm 3or less cannot be met unless the initial oxygen concentration is made low when the boron concentration is high. For example, if the boron concentration is as low as 4.8 10 18 atoms / cm 3 was the highest value of the initial oxygen concentration, which required an oxygen precipitate density of 1·10 10 / cm 3 or less, about 14·10 17 atoms / cm 3 . On the other hand, if the boron concentration is as high as 1.6·10 19 atoms / cm 3 was the highest value of the initial oxygen concentration, which required an oxygen precipitate density of 1·10 10 / cm 3 or less, about 9·10 17 atoms / cm 3 .

[0057] The boundary line separating the drawn "◯" marks and the drawn "×" marks was expressed using a linear function to define the area of ​​the drawn "◯" marks. From the results obtained as described above, it was clear that the oxygen precipitate density was equal to or less than 1 10 10 / cm 3 can be made if the oxygen concentration X (·10 17 atoms / cm 3 ) and the boron concentration Y (atoms / cm 3 ) meet the following requirement: X≤−4.3⋅10−19Y+16.3. [List of reference symbols] 10 silicon epitaxial wafers 11 Silicon substrate 12 epitaxial layer

Claims

[1] A method for producing a silicon epitaxial wafer (10) in which a silicon epitaxial layer (12) is formed on a surface of a boron-doped silicon substrate (11), the method comprising: Pulling a single-crystalline silicon ingot coated with a boron concentration of 2.7 10 17 atoms / cm 3 or more and 1.3·10 19 atoms / cm 3 or less doped, using a Czochralski process; Producing a silicon substrate (11) by processing the single-crystal silicon ingot; and Forming a silicon epitaxial layer (12) on a surface of the silicon substrate (11), wherein pulling the single-crystal silicon ingot comprises controlling the single-crystal pulling conditions of the single-crystal silicon ingot such that the boron concentration Y (atoms / cm 3 ) and the initial oxygen concentration X (·10 17 atoms / cm 3) a relationship expression of X ≤ -4.3·10 -19 Y+16.3 fulfill, wherein pulling the single-crystal silicon ingot further comprises decreasing the initial oxygen concentration X in the ingot pulling direction as pulling of the single-crystal silicon ingot proceeds in accordance with the boron concentration Y becoming larger in the ingot pulling direction by controlling the single-crystal pulling conditions of the single-crystal silicon ingot. [2] The method of claim 1, wherein reducing the initial oxygen concentration by controlling the single crystal pulling conditions comprises reducing a rotation speed of a quartz crucible from which the single crystal is pulled. [3] The method of claim 1 or 2, wherein reducing the initial oxygen concentration by controlling the single crystal pulling conditions comprises reducing a power output to a heating element.

Images