Method for evaluating gettering ability of silicon wafers

Abstract

To provide a method of evaluating the gettering ability of a silicon wafer, capable of improving the accuracy of evaluation of inferior gettering ability even if diffusion heat treatment for metal impurities is performed at low temperatures and a simple evaluation method in which only part of the front surface of a silicon wafer is observed is adopted.SOLUTION: Gettering ability of oxygen precipitates to metal impurities in a silicon wafer that has oxygen precipitates formed at the inside thereof is evaluated by subjecting the silicon wafer to an intentionally contaminating process with the metal impurities, diffusion heat treatment at a temperature of 850°C or below, a selective etching process, and an observation process with a microscope. The intentionally contaminating process is performed under such conditions that a coverage rate α of a shallow pit aggregation according to a predetermined definition on the front surface of a silicon wafer is 1.5×10-2 or more when gettering ability of the silicon wafer is inferior.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for evaluating the gettering ability of a silicon wafer, which includes a step of intentionally contaminating the silicon wafer with metal impurities and a step of diffusing and heat-treating the metal impurities, and which evaluates the gettering ability of oxygen precipitates (BMD: Bulk Micro Defects) in the silicon wafer against metal impurities. [Background technology]

[0002] Semiconductor devices are manufactured through a device formation process in which device structures such as integrated circuit elements are formed on the surface of a silicon wafer. In particular, epitaxial silicon wafers, in which a silicon epitaxial layer made of single crystal silicon is formed on a bulk silicon wafer, are suitable for use as substrates for manufacturing various semiconductor devices. In the device formation process, there is concern about contamination of the silicon wafer with metal impurities, which can cause defective characteristics of semiconductor devices. One of the techniques for reducing metal impurities in silicon wafers is gettering technology. One of the gettering techniques is a method called intrinsic gettering (IG), in which metal impurities are captured by forming oxide precipitates (BMD) in the silicon wafer.

[0003] In order to avoid metal contamination of silicon wafers in device formation processes, it is important to evaluate the gettering ability of BMD in silicon wafers against metal impurities. Patent Document 1 describes a method for evaluating the gettering ability of silicon wafers, which includes a step of intentionally contaminating a silicon wafer with metal impurities formed therein, a step of subjecting the silicon wafer to heat treatment, a step of subjecting the surface of the silicon wafer to selective etching, and a step of evaluating the gettering ability of the BMD in silicon wafer against metal impurities based on whether dislocations are detected when the selectively etched surface is observed with an optical microscope. In the intentional contamination step, a contaminating liquid containing metal impurities is applied to the back surface of the silicon wafer by spin coating. The metal impurities introduced into the silicon wafer by intentional contamination are thermally diffused in the silicon wafer by heat treatment and captured by the BMD. At this time, if there are metal impurities that have not been captured by the oxide precipitates (BMD), such metal impurities form metal impurity silicide on the front surface of the silicon wafer. Therefore, when selective etching is performed on the front surface of the silicon wafer, this silicide becomes apparent as shallow pits (dislocations). Therefore, if shallow pits are not observed with an optical microscope, the gettering ability can be evaluated as good, and if shallow pits are observed, the gettering ability can be evaluated as poor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-149471 A Summary of the Invention [Problem to be solved by the invention]

[0005] The present inventors have recognized the following new problem. Conventionally, heat treatment for diffusing metal impurities after intentional contamination (hereinafter also simply referred to as "diffusion heat treatment") has generally been performed at a high temperature of 900°C or higher. Here, in recent years, device heat treatment (heat treatment performed in the device formation process) has been performed at lower temperatures. In order to simulate this device heat treatment, the diffusion heat treatment must also be performed at a low temperature. However, it has been found that the following problems arise when the diffusion heat treatment is performed at a low temperature of 850°C or lower.

[0006] That is, when diffusion heat treatment is performed at a high temperature of 900°C or higher, in silicon wafers with poor gettering ability, shallow pits are distributed uniformly over the entire front surface of the silicon wafer after selective etching.In contrast, when diffusion heat treatment is performed at a low temperature of 850°C or lower, as shown in Figure 2, after selective etching, the shallow pits show a mottled non-uniform distribution on the front surface of the silicon wafer, consisting of areas where there are no shallow pits and areas where shallow pits (white bright spots on the right side of Figure 2) are densely concentrated (shallow pit clusters: gray spots on the left side of Figure 2).

[0007] Even when shallow pits are distributed unevenly in this way, if the entire front surface of the silicon wafer is thoroughly observed with an optical microscope, the shallow pits will not be overlooked, and the silicon wafer with poor gettering ability can be correctly evaluated as having "poor gettering ability". However, it is inefficient to thoroughly observe the entire front surface of the silicon wafer with an optical microscope. Considering the efficiency of the evaluation, a simple observation method is desired in which only a part of the front surface of the silicon wafer (for example, multiple fields of view at predetermined intervals on the diameter) is observed. When shallow pits are distributed unevenly, there is a risk that shallow pits will be overlooked when such a simple observation method is used. If shallow pits are overlooked, a silicon wafer with poor gettering ability that should be evaluated as having "poor gettering ability" will be erroneously evaluated as having "good gettering ability". In other words, it was found that when a simple gettering ability evaluation method in which a diffusion heat treatment of metal impurities is performed at a low temperature and only a portion of the front surface of the silicon wafer is observed is adopted, there is a problem in that it is not possible to ensure the accuracy of the evaluation that the gettering ability is inferior.

[0008] In view of the above problems, the present invention has an object to provide a method for evaluating the gettering capability of a silicon wafer, which can improve the accuracy of the evaluation that the gettering capability is inferior, even when a simple gettering capability evaluation method is adopted in which a diffusion heat treatment of metal impurities is performed at a low temperature and only a portion of the front surface of the silicon wafer is observed. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors have conducted extensive research and have obtained the following findings. That is, when a diffusion heat treatment of metal impurities is performed at a low temperature, in a silicon wafer having a poor gettering ability, the size of the shallow pit cluster (unit: cm 2 ) and the in-plane density of the shallow pit population (unit: cm -2) is an index that guarantees accurate evaluation of gettering ability without overlooking shallow pits. Specifically, when the coverage rate of the shallow pit population α is 1.5×10 -2 It was found that if the value is equal to or greater than the above, it is possible to evaluate the gettering ability as being inferior with sufficient accuracy. It was also found that the above-mentioned predetermined coverage rate α can be realized in silicon wafers with inferior gettering ability by devising the conditions of the intentional contamination step of metal impurities. Specifically, it was found that in an intentional contamination method in which a contaminating liquid containing metal impurities is dropped onto the front or back surface of a silicon wafer and spin-coated using a spin coater, applying ultrasonic waves to the contaminating liquid in advance contributes to an increase in the coverage rate α. It was also found that the coverage rate α can be increased by increasing the maximum rotation speed of the spin coater and by shortening the time required to reach the maximum rotation speed.

[0010] The present invention, which has been completed based on the above findings, has the following essential features. [1] preparing a silicon wafer having oxygen precipitates formed therein; An intentional contamination step of dropping a contaminating liquid containing metal impurities onto the front or back surface of the silicon wafer, and rotating the silicon wafer with a spin coater to spread the contaminating liquid onto the front or back surface of the silicon wafer; A step of performing a heat treatment for holding the silicon wafer at a temperature of 850° C. or less; a step of selectively etching the front surface of the silicon wafer to expose a silicide of the metal impurity as a shallow pit if the silicide is present on the front surface of the silicon wafer after the heat treatment; a step of magnifying and observing the front surface of the silicon wafer with a microscope, and evaluating a gettering ability of the oxygen precipitates in the silicon wafer for the metal impurities based on the presence or absence of the shallow pits; having The intentional contamination step is performed by: when the shallow pits are present on the front surface of the silicon wafer, the coverage α of the shallow pit group on the front surface of the silicon wafer, which is defined as follows, is 1.5×10 -2 The method for evaluating the gettering ability of a silicon wafer is carried out under the above-mentioned conditions. The first region is defined as one "shallow pit group" when three or more shallow pits are present in a first region that is a square with a side length L of 10 μm or more, and no shallow pits are present in a second region that is outside the first region and inside the square formed by the four sides that are L / 2 away from each side of the first region.

[0011] [2] The method for evaluating the gettering capability of a silicon wafer according to the above-mentioned [1], wherein the condition of the intentional contamination step is at least one of whether or not ultrasonic waves are applied to the contaminating liquid in advance, the maximum rotation speed of the spin coater, and the time required to reach the maximum rotation speed.

[0012] [3] The method for evaluating gettering capability of a silicon wafer according to the above [2], wherein ultrasonic waves are not applied to the contaminating liquid in advance in the intentional contamination step.

[0013] [4] The method for evaluating gettering capability of a silicon wafer according to the above [3], wherein a maximum rotation speed R (rpm) of the spin coater and a time t (seconds) required to reach the maximum rotation speed satisfy the following formula (1): R ≥ 1.58 × 10 3 ×t 0.408 (1)

[0014] [5] The maximum rotation speed of the spin coater is R (rpm) and the time required to reach the maximum rotation speed is t (seconds), 3000≦R≦6000 and t≦4 4000≦R≦6000, and 4≦t≦9 5000≦R≦6000 and 9≦t≦15 The method for evaluating gettering capability of a silicon wafer according to the above [3],

[0015] [6] The method for evaluating gettering capability of a silicon wafer according to the above-mentioned [2], wherein ultrasonic waves are applied to the contaminating liquid in advance in the intentional contamination step.

[0016] [7] The method for evaluating gettering capability of a silicon wafer according to the above item [6], wherein a maximum rotation speed R (rpm) of the spin coater and a time t (seconds) required to reach the maximum rotation speed satisfy the following formula (2): R ≧ 5.96 ×10 2 ×t 0.408 (2)

[0017] [8] The method for evaluating gettering capability of a silicon wafer according to the above item [6], wherein R (rpm) is a maximum rotation speed of the spin coater and t (seconds) is a time required to reach said maximum rotation speed, and 2000≦R≦6000 and t≦15 are satisfied.

[0018] [9] The method for evaluating a gettering capability of a silicon wafer according to any one of the above [1] to [8], wherein the metal impurity is nickel (Ni).

[0019]

[10] The method for evaluating a gettering capability of a silicon wafer according to any one of the above [1] to [9], wherein the silicon wafer is an epitaxial silicon wafer, and the front surface of the silicon wafer is a surface of an epitaxial layer of the epitaxial silicon wafer. Effect of the Invention

[0020] According to the method for evaluating the gettering ability of a silicon wafer of the present invention, even when a diffusion heat treatment of metal impurities is performed at a low temperature and a simple gettering ability evaluation method in which only a portion of the front surface of the silicon wafer is observed is adopted, it is possible to improve the accuracy of the evaluation that the gettering ability is inferior. [Brief description of the drawings]

[0021] [Figure 1]1 is a flowchart illustrating a method for evaluating the gettering ability of a silicon wafer according to an embodiment of the present invention. [Diagram 2] FIG. 13 is a diagram for explaining the definition of a shallow pit group. [Diagram 3] 1 is a graph showing the relationship between the oxygen concentration and BMD density of a silicon wafer and the evaluation result of the gettering ability. [Figure 4] 1 is a graph showing the influence of the maximum rotation speed of a spin coater and the time required to reach the maximum rotation speed on the coverage α of a shallow pit population in a gettering ability evaluation test including intentional Ni contamination and Ni diffusion heat treatment using a silicon wafer having poor gettering ability, in the case where ultrasonic waves are not applied to the contaminant liquid in advance. [Diagram 5] 5 is a graph showing the relationship between the maximum rotation speed R of the spin coater and the time t required to reach the maximum rotation speed in FIG. 4, and the determination accuracy of the gettering ability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] (Method for evaluating gettering ability of silicon wafers) Referring to FIG. 1, a method for evaluating the gettering ability of a silicon wafer according to an embodiment of the present invention includes a step of preparing a silicon wafer having a BMD formed therein (step S1), a step of intentionally contaminating the silicon wafer with metal impurities (step S2), and a diffusion heat treatment of the metal impurities at a low temperature. process (Step S3), a selective etching step (Step S4), and a magnified observation step using a microscope (Step S5).

[0023] [Step S1] In step S1, a silicon wafer having oxygen precipitates (BMD) formed therein is prepared. The silicon wafer is not particularly limited, and may be a bulk silicon wafer cut from a single crystal silicon ingot grown by the Czochralski method, or an epitaxial silicon wafer in which a silicon epitaxial layer made of single crystal silicon is formed on the bulk silicon wafer. The silicon epitaxial layer may be formed under general conditions by a CVD method. By subjecting these silicon wafers to a heat treatment, oxygen precipitate nuclei introduced when pulling up the single crystal silicon ingot grow to form oxygen precipitates (BMD). The higher the density of the oxygen precipitates (BMD), the higher the gettering ability, and the density of the oxygen precipitates (BMD) depends on the oxygen concentration of the bulk silicon wafer and the conditions of the heat treatment.

[0024] In this specification, of the two main surfaces of a silicon wafer, the flat surface on which semiconductor devices are fabricated is referred to as the "front surface", and the flat surface opposite the front surface is referred to as the "back surface". An identifier (laser mark) recording product information is engraved on the front or back surface of the silicon wafer. For this reason, the front and back surfaces of the silicon wafer are clearly distinguished. When the silicon wafer is an epitaxial silicon wafer, semiconductor devices are fabricated in the epitaxial layer, and therefore the "front surface" of the silicon wafer is the surface of the epitaxial layer of the epitaxial silicon wafer.

[0025] The silicon wafer to be evaluated in this embodiment has an oxygen concentration of 8×10 17 ~16×10 17 atoms / cm 3 It can be assumed that:

[0026] The silicon wafer to be evaluated in this embodiment is 1×10 7 ~4×10 10 / cm 3 The BMD density may be

[0027] In this specification, the "BMD density" of a silicon wafer is defined as follows: After a silicon wafer is heat-treated in an oxygen gas atmosphere at an atmospheric temperature of 900°C for 16 hours, the silicon wafer (in-plane central position) is cleaved in the thickness direction and the cleaved surface is selectively etched to a depth of 2 μm using a Wright Etching solution. The cleaved surface at the center of the silicon wafer's thickness is then observed with an optical microscope to determine the density of etch pits within an area of ​​a specified size, which is defined as the "BMD density." The specified size is the area of ​​the silicon wafer. of Set it appropriately depending on the BMD density. If it is high density, count BMD in a small area, if it is low density, count BMD in a large area.

[0028] [Step S2] In the intentional contamination process of step S2, a contaminating liquid containing metal impurities is dropped onto the front or back surface of the silicon wafer, and the silicon wafer is rotated by a spin coater to spread the contaminating liquid over the front or back surface of the silicon wafer. As the metal impurity, a metal impurity that may be a source of contamination in the device formation process can be used, and for example, one selected from Ni, Fe, Cu, Mo, and W can be used, but in this embodiment, it is particularly preferable to use nickel (Ni).

[0029] Regardless of the type of metal impurity, the amount of metal impurity contamination is 1×10 11 ~1×10 15 atoms / cm 2 It is preferable that the amount of contamination is within the range of 1×10 11 atoms / cm 2 When the amount of contamination is less than 1×10, even if the silicon wafer is insufficiently contaminated and has poor gettering ability, shallow pits are not observed by selective etching as described below. 15 atoms / cm 2If it exceeds this amount, the metal impurities cannot be diffused into the silicon wafer. In this specification, the "amount of contamination by metal impurities" is determined by inductively coupled plasma mass spectrometry. The amount of the contaminating liquid to be dripped and the concentration of the metal impurities in the contaminating liquid are not particularly limited and may be appropriately determined so as to achieve the above-mentioned amount of contamination. However, the amount of dripping is set to an amount that allows the contaminating liquid to spread over the entire front or back surface of the silicon wafer before the spin coater is started.

[0030] [Step S3] In the diffusion heat treatment process of step S3, a heat treatment is performed in which the silicon wafer is held at a temperature of 850°C or less. Metal impurities introduced into the silicon wafer by intentional contamination are thermally diffused in the silicon wafer by the heat treatment and captured by oxygen precipitates (BMD). At this time, if there are metal impurities that cannot be captured by the oxygen precipitates (BMD), such metal impurities form a silicide of the metal impurities on the front surface of the silicon wafer.

[0031] With the trend toward lower device heat treatment temperatures, in this embodiment, the temperature of the diffusion heat treatment is set to 850° C. or less, preferably 800° C. or less, and more preferably 750° C. or less. The lower limit of the temperature of the diffusion heat treatment is not particularly limited, but considering the current device heat treatment temperatures, the temperature of the diffusion heat treatment is preferably set to 600° C. or more. The "temperature of the diffusion heat treatment" means the atmospheric temperature, and it can be assumed that the silicon wafer is also held at that temperature. The time of the diffusion heat treatment is preferably 1 minute or more, more preferably 2 minutes or more, in order to sufficiently diffuse the metal impurities into the silicon wafer. In addition, the time of the diffusion heat treatment is preferably 120 minutes or less, more preferably 30 minutes or less, from the viewpoint of the throughput of the heat treatment. The heat treatment atmosphere can be, for example, H2, O2, Ar, N2, He, or a mixed atmosphere thereof. The method of these heat treatments is not particularly limited, and examples thereof include a method using a resistance heating heat treatment furnace.

[0032] [Step S4] In step S4, selective etching is performed on the front surface of the silicon wafer. This allows the silicide of metal impurities present on the front surface of the silicon wafer after the heat treatment to be made visible as shallow pits. From the viewpoint of making the shallow pits more visible, the selective etching is preferably performed using a light etching solution containing chromic acid, with an etching depth of 0.5 to 3.0 μm.

[0033] [Step S5] In step S5, the front surface of the silicon wafer is magnified and observed with a microscope, and the gettering ability of the oxygen precipitates (BMD) in the silicon wafer for metal impurities is evaluated based on the presence or absence of shallow pits. An optical microscope can be used as the microscope. Specifically, if no shallow pits are observed with the microscope, the gettering ability of the silicon wafer can be evaluated as good, and if shallow pits are observed, the gettering ability of the silicon wafer can be evaluated as poor.

[0034] The magnification of the microscope is not particularly limited as long as the shallow pits can be observed, but can be in the range of 50 to 1000 times. The observation range is not particularly limited, and the entire front surface of the silicon wafer may be observed thoroughly with the microscope. However, it is inefficient to observe the entire front surface of the silicon wafer thoroughly with the microscope. Considering the efficiency of the evaluation, a simple observation method is desired in which only a part of the front surface of the silicon wafer (for example, multiple fields of view at predetermined intervals on the diameter) is observed. In this embodiment, as described later, even when such a simple observation method is adopted, it is possible to sufficiently suppress the overlooking of shallow pits, and the accuracy of the evaluation of poor gettering ability can be improved.

[0035] [Distribution of shallow pits] As described above, when a diffusion heat treatment is performed at a low temperature of 850°C or less in a silicon wafer with poor gettering ability, the shallow pits on the front surface of the silicon wafer after selective etching show a mottled non-uniform distribution consisting of a region where there are no shallow pits and a region where shallow pits (white bright spots in the right diagram of FIG. 2) are densely gathered (shallow pit clusters: gray spots in the left diagram of FIG. 2) as shown in FIG. 2. Here, referring to FIG. 2, in this specification, a first region is defined as one "shallow pit cluster" when three or more shallow pits are present in a first region that is a square with a side length L of 10 μm or more, and no shallow pits are present in a second region that is outside the first region and inside a square formed by four sides spaced apart by L / 2 from each side of the first region.

[0036] In the case of a silicon wafer with good gettering ability, shallow pits are not noticeable, so they are not observed regardless of the observation range under a microscope. However, in the case of a silicon wafer with poor gettering ability, shallow pits are distributed unevenly as shown in Figure 2, so there is a risk of missing shallow pits when using the simple observation method described above.

[0037] In a silicon wafer with poor gettering ability, the coverage rate α of the shallow pit population is 1.5×10 -2 If the coverage factor α of the shallow pit group is less than 1.5×10, there is a high risk of overlooking shallow pits using a simple observation method, and it is not possible to obtain sufficient accuracy in the evaluation of poor gettering ability. Therefore, in this embodiment, in a silicon wafer with poor gettering ability, that is, in a case where shallow pits are present on the front surface of the silicon wafer, the coverage factor α of the shallow pit group is set to 1.5×10 -2 It is essential that the coverage rate of the shallow pits is 2.0×10 or more. This makes it possible to sufficiently prevent shallow pits from being overlooked even when a simple observation method is adopted, and improves the accuracy of the evaluation of poor gettering ability. From this point of view, the coverage rate α of the shallow pit population is 2.0×10 -2 More preferably, it is 3.0×10 -2Although there is no particular upper limit to the coverage rate α of the shallow pit population, even if the intentional contamination process is optimized to the maximum extent, the coverage rate α of the shallow pit population is generally 0.3 or less.

[0038] When a diffusion heat treatment is performed at a high temperature of 900° C. or more, in a silicon wafer having poor gettering ability, shallow pits are uniformly distributed over the entire surface after selective etching.

[0039] When performing a diffusion heat treatment at a low temperature of 850° C. or less, in order to realize a coverage rate α of the above-mentioned predetermined value or more in a silicon wafer with poor gettering ability, it is preferable to optimize the intentional contamination step (step S2). That is, in this embodiment, in a silicon wafer with poor gettering ability, i.e., in the case where shallow pits exist on the front surface of the silicon wafer, it is essential to perform the intentional contamination step (step S2) under conditions that result in a coverage rate α of the above-mentioned predetermined value or more. Specifically, as the conditions of the intentional contamination step, it is preferable to optimize at least one of the conditions of whether or not ultrasonic waves are applied to the contaminating liquid in advance, the maximum rotation speed of the spin coater, and the time required to reach the maximum rotation speed.

[0040] In one embodiment, from the viewpoint of increasing the coverage ratio α, it is preferable to apply ultrasonic waves to the contaminant liquid before dripping the contaminant liquid onto the front or back surface of the silicon wafer. It is presumed that the application of ultrasonic waves causes the metal impurities in the contaminant liquid to be agitated and the concentration becomes uniform, which contributes to the increase in the coverage ratio α.

[0041] In another embodiment, even if ultrasonic waves are not applied to the contaminated liquid in advance, the coverage rate α can be increased by optimizing the maximum rotation speed of the spin coater and the time required to reach the maximum rotation speed.

[0042] When ultrasonic waves are not applied to the contaminated liquid in advance, it is preferable that the maximum rotation speed R (rpm) of the spin coater and the time t (seconds) required to reach the maximum rotation speed satisfy the following formula (1). R ≥ 1.58 × 103 ×t 0.408 (1) or, 3000≦R≦6000 and t≦4 4000≦R≦6000, and 4≦t≦9 5000≦R≦6000 and 9≦t≦15 When these conditions are satisfied, in a silicon wafer having poor gettering ability, that is, in a case where shallow pits are present on the front surface of the silicon wafer, the coverage α of the shallow pit group is set to 1.5×10 -2 It can be more than that.

[0043] When ultrasonic waves are applied to the contaminated liquid in advance, it is preferable that the maximum rotation speed R (rpm) of the spin coater and the time t (seconds) required to reach the maximum rotation speed satisfy the following formula (2). R ≧ 5.96 ×10 2 ×t 0.408 (2) Alternatively, it is preferable to satisfy 2000≦R≦6000 and t≦15. When these conditions are satisfied, in a silicon wafer having poor gettering ability, that is, in a case where shallow pits are present on the front surface of the silicon wafer, the coverage α of the shallow pit group is set to 1.5×10 -2 It can be more than that.

[0044] In another embodiment, in a silicon wafer having poor gettering capability, i.e., in a case where shallow pits are present on the surface of the silicon wafer, it is preferable to intentionally control the conditions of the intentional contamination step so that the coverage α is equal to or greater than the above-mentioned predetermined value.

[0045] Specifically, when ultrasonic waves are not applied to the contaminated liquid in advance, it is confirmed whether or not the maximum rotation speed R (rpm) of the spin coater to be adopted and the time t (seconds) required to reach the maximum rotation speed satisfy the above formula (1), and the method for evaluating the gettering capability of a silicon wafer involving steps S1 to S5 can be carried out using R and t that are confirmed to satisfy formula (1).

[0046] Specifically, when ultrasonic waves are applied to the contaminated liquid in advance, it is confirmed whether or not the maximum rotation speed R (rpm) of the spin coater to be adopted and the time t (seconds) required to reach the maximum rotation speed satisfy the above formula (2), and the method for evaluating the gettering capability of a silicon wafer involving steps S1 to S5 can be carried out using R and t that have been confirmed to satisfy formula (2).

[0047] In addition, the contamination level of metal impurities is 1×10 11 ~1×10 15 atoms / cm 2 Within this range, the coverage α of the shallow pit population in the silicon wafer with poor gettering ability is almost independent of the amount of contamination by metal impurities. Also, if the temperature of the diffusion heat treatment is 850°C or less and the time of the diffusion heat treatment is 1 minute or more, the coverage α of the shallow pit population in the silicon wafer with poor gettering ability is almost independent of the temperature and time of the diffusion heat treatment. This is because the non-uniform distribution of the contaminating metal is determined at the time of spin coating.

[0048] Moreover, the coverage α of the shallow pit population in a silicon wafer with poor gettering ability does not depend on the BMD density of the silicon wafer. Whether shallow pits are observed or not is determined by whether the contaminant metal gathers on the front surface of the silicon wafer or on the BMD inside the silicon wafer, and most of the contaminant metal gathers on the front surface of the silicon wafer or on the side where the BMD is dominant. In other words, when the gettering ability is poor (when the BMD density is insufficient), most of the metal precipitates as silicide on the front surface of the silicon wafer regardless of the excess of the BMD, so it is considered that the coverage α does not depend on the degree of the BMD density. EXAMPLES

[0049] (Experimental Example 1) Cut from a silicon single crystal ingot grown by the CZ method, diameter: 300 mm, thickness: 770 μm, surface orientation (100), oxygen concentration: 10 × 10 17 ~14×10 17 atoms / cm 3 P-type silicon wafers were prepared under 16 conditions within the range of ASTM F-121, 1979. A silicon epitaxial layer of 3.0 μm thickness was formed on the surface of this bulk silicon wafer to produce an epitaxial silicon wafer. The epitaxial silicon wafer was subjected to a heat treatment at 950°C for 30 minutes. This heat treatment caused oxygen precipitates (BMD) to form inside the bulk silicon wafer of the epitaxial silicon wafer, imparting gettering capability. Next, the epitaxial silicon wafer was cleaned.

[0050] The Ni contamination solution was dropped onto the back surface of the epitaxial silicon wafer (the surface of the silicon wafer on which the epitaxial layer was not formed) and spin-coated. The amount of Ni contamination was 1×10 14 atoms / cm 2After that, a diffusion heat treatment was carried out at 750℃ for 5 minutes in a nitrogen atmosphere. This diffusion heat treatment simulated the temperature in the device fabrication process, so it is expected that the gettering ability in the device fabrication process can be simulated.

[0051] The epitaxial silicon wafer was then cleaved in half, and the surface of one of the wafers was etched with a light etching solution to reveal the Ni silicide on the wafer surface (surface of the epitaxial layer) as shallow pits.Then, the wafer surface was thoroughly observed with an optical microscope to confirm the occurrence of shallow pits.

[0052] In addition, in order to observe the BMD density, the remaining half of the wafer was cleaned and then subjected to a BMD elicitation heat treatment at 900°C for 16 hours. Condition After cleavage, the wafer was etched to a depth of 2 μm with a light etching solution. BMDs that appeared as etch pits on the cleavage surface were observed with an optical microscope and their density was measured.

[0053] Observation example of shallow pits on the wafer surface (oxygen concentration: 9×10 17 atoms / cm 3 , BMD density: 1×10 9 / cm 3 ) is shown in Figure 2. Each of the white bright spots in the optical microscope image corresponds to a shallow pit. As shown in Figure 2, areas where shallow pit clusters (S pit clusters) exist are scattered across the wafer surface, and no S pits were observed around the S pit clusters. The reason why shallow pits are distributed unevenly across the wafer surface in this way is thought to be that Ni is distributed unevenly across the wafer surface at the time of spin-coating, and shallow pits occur only in areas with a high Ni concentration.

[0054] Figure 3 shows the relationship between the oxygen concentration and BMD density of the silicon wafer and the evaluation result of the gettering ability. When shallow pits were not observed, the gettering ability was evaluated as good, and when shallow pits were observed, the gettering ability was evaluated as poor. Since BMDs are oxygen atoms precipitated inside the silicon wafer, the higher the oxygen concentration of the silicon wafer, the higher the BMD density. As shown in Figure 3, the higher the BMD density, the less shallow pits are generated, and the better the gettering ability is evaluated. In other words, it is possible to correctly evaluate whether contaminated Ni is gettered by the BMD. Even if shallow pits are distributed unevenly as S-pit groups in the plane as in Figure 2, there is no problem in evaluating the gettering ability as long as the entire plane of the wafer can be observed.

[0055] (Experimental Example 2) If the epitaxial layer surface is observed thoroughly, it is possible to accurately evaluate the gettering ability, as shown in Figure 3. However, in consideration of the efficiency of the evaluation, a simple observation method such as observing only the radial area of ​​the wafer with an optical microscope is desirable. However, if the density or area of ​​the S pit group is small, there is a concern that the S pits may be overlooked and a correct gettering judgment may not be made. Therefore, we calculated the proportion α of the S pit group covering the wafer surface, and investigated the conditions under which the value of α becomes large. Here, α is the average area of ​​the S pit group (unit: cm 2 ) and the density of the S pit population (unit: cm -2 ) The larger the coverage factor α, the lower the possibility of overlooking S pits, which is believed to enable accurate gettering evaluation.

[0056] The occurrence of the in-plane non-uniform S pit clusters is believed to be due to the non-uniform Ni concentration distribution on the wafer surface during spin coating, so the Ni intentional contamination conditions shown in Table 1 were adopted to carry out the gettering ability evaluation experiment in Figure 1. Note that, since this experiment requires the occurrence and observation of S pits, a wafer without gettering ability (oxygen concentration of 12×10 in Figure 3) was used. 17 atoms / cm 3 The wafers were used.

[0057] [No.1] Ultrasonic waves were applied to the container containing the Ni contaminating liquid used for spin-coating, and stirring was performed. The spin-coating was performed at a maximum speed of 3000 rpm, and the time required to reach the maximum speed was 4 seconds.

[0058] [No.2~13] No ultrasonic waves were applied to the Ni-contaminated liquid, and the maximum rotation speed R (rpm) of the spin coater and the time t (seconds) required to reach the maximum rotation speed were set to the values ​​shown in Table 1.

[0059] In Nos. 1 to 13 in Table 1, when the entire surface of the epitaxial layer was thoroughly observed with an optical microscope, it was confirmed that S pit populations were generated at all levels. Table 1 shows the coverage α of the shallow pit population.

[0060] On the other hand, in consideration of the efficiency of the evaluation, a simple observation method such as observing only the radius of the wafer with an optical microscope is desirable. Therefore, as a simple observation method, observation was performed at 30 points (0 mm, 5 mm, 10 mm, ..., 145 mm from the center of the wafer) at 5 mm intervals in the radial direction from the center of the wafer to the outer periphery at a magnification of 400 times. If an S pit group was found in even one of the 30 points of the field of view, it was defined as no oversight, and if an S pit group was not found in any of the fields of view, it was defined as oversight. This observation at 5 mm intervals was repeated 10 times for each wafer, and the oversight rate was defined as the number of times that an S pit group was overseen divided by the number of measurements (10), and is shown in Table 1. In Table 1, if the oversight rate is 20% or less, it is judged as OK as there is no problem with the gettering evaluation, and if it exceeds 20%, it is judged as NG. In this case, from the value of the coverage rate α shown in Table 1, the threshold value of α for judgments OK and NG is 1.5 × 10 -2 It is.

[0061] From the results in Table 1, the higher the value of the coverage rate α of the S pit population, the lower the oversight rate. In other words, it can be seen that the higher the value of the coverage rate α, the more preferable the intentional contamination conditions. Figure 4 shows the relationship between the maximum rotation speed, the time required to reach the maximum rotation speed, and the coverage rate α of the S pit population, based on Nos. 2 to 13, in which ultrasound was not applied to the contaminating liquid in advance. It can be seen from Figure 4 that the higher the maximum rotation speed is and the shorter the time required to reach the maximum rotation speed is, the higher the coverage rate α becomes.

[0062] As a result of multivariate analysis of the results in Figure 4, it was found that the following relationship exists between the coverage rate α, the maximum rotation speed R (rpm), and the time required to reach the maximum rotation speed t (seconds). α = 7.01 × 10 -8 ×R 1.67 ×t -0.68 ...Equation (3)

[0063] In other words, the threshold value α that enables highly accurate evaluation of gettering ability is 1.5×10 -2 Under this condition, R and t satisfy the following relationship: R = 1.58 × 10 3 ×t 0.408

[0064] Figure 5 shows the relationship between the maximum rotation speed R of the spin coater and the time t required to reach the maximum rotation speed, and the accuracy of the determination of the gettering ability, based on Nos. 2 to 13, in which ultrasonic waves were not applied to the contaminated liquid in advance. From Figure 5, it can be seen that the maximum rotation speed R (rpm) and the time t (seconds) required to reach the maximum rotation speed are R≧1.58×10 3 ×t 0.408 It can be seen that if the above relationship is satisfied, it becomes possible to evaluate the gettering ability with high accuracy. In this case, if R and t are selected from the ranges of "3000≦R≦6000 and t≦4", "4000≦R≦6000 and 4≦t≦9", or "5000≦R≦6000 and 9≦t≦15", the above relationship can be satisfied.

[0065] With reference to Table 1, a comparison of No. 1 and No. 5 shows that the coverage rate α can be increased by intentionally contaminating the Ni contaminating liquid with ultrasonic waves applied. Apply When not using the 5.07 Because it is doubled, ultrasonic Apply When the 5.07 It was estimated that the following equation (4), which is twice as large as α = 3.55 ×10 -7 ×R 1.67 ×t -0.68 ...Equation (4)

[0066] In other words, when ultrasonic waves are applied to the contaminated liquid in advance, the condition that enables highly accurate evaluation of the gettering ability is R ≧ 5.96 ×10 2 ×t 0.408 In this case, if R and t are selected within the ranges of 2000≦R≦6000 and t≦15, the above relationship can be satisfied.

[0067] [Table 1] [Industrial Applicability]

[0068] According to the method for evaluating the gettering ability of a silicon wafer of the present invention, even when a Ni diffusion heat treatment is performed at a low temperature and a simple method for evaluating the gettering ability in which only a portion of the surface of the silicon wafer is observed is adopted, it is possible to improve the accuracy of the evaluation that the gettering ability is inferior.

Claims

1. providing a silicon wafer having oxygen precipitates formed therein; An intentional contamination step of dropping a contaminating liquid containing metal impurities onto the front or back surface of the silicon wafer, and rotating the silicon wafer with a spin coater to spread the contaminating liquid onto the front or back surface of the silicon wafer; A step of subjecting the silicon wafer to a heat treatment at a temperature of 850° C. or less; a step of selectively etching the front surface of the silicon wafer to expose a silicide of the metal impurity as a shallow pit if the silicide is present on the front surface of the silicon wafer after the heat treatment; a step of magnifying and observing the front surface of the silicon wafer with a microscope, and evaluating the gettering ability of the oxygen precipitates in the silicon wafer for the metal impurities based on the presence or absence of the shallow pits; having In the intentional contamination step, when the shallow pits are present on the front surface of the silicon wafer, a coverage α of a shallow pit group on the front surface of the silicon wafer according to the following definition is 1.5×10 －２ The method for evaluating the gettering ability of a silicon wafer is carried out under the above-mentioned conditions. When three or more of the shallow pits are present in a first region, which is a square with a side length L of 10 μm or more, and when no shallow pits are present in a second region which is outside the first region and inside the square formed by the four sides spaced L / 2 from each side of the first region, the first region is defined as one "shallow pit group."

2. 2. The method for evaluating the gettering capability of a silicon wafer according to claim 1, wherein the conditions of the intentional contamination process are at least one of whether or not ultrasonic waves are applied to the contaminating liquid in advance, the maximum rotation speed of the spin coater, and the time required to reach the maximum rotation speed.

3. 3. The method for evaluating the gettering capability of a silicon wafer according to claim 2, wherein ultrasonic waves are not applied to the contaminating liquid in advance in the intentional contamination step.

4. 4. The method for evaluating gettering capability of a silicon wafer according to claim 3, wherein a maximum rotation speed R (rpm) of the spin coater and a time t (seconds) required to reach the maximum rotation speed satisfy the following formula (1): R≧1.58×10 ３ ×t ０．４０８ ・・・(1)

5. The maximum rotation speed of the spin coater is R (rpm), and the time required to reach the maximum rotation speed is t (seconds), 3000≦R≦6000, and t≦4 4000≦R≦6000, and 4≦t≦9 5000≦R≦6000, and 9≦t≦15 4. The method for evaluating gettering capability of a silicon wafer according to claim 3, wherein any one of the above is satisfied.

6. 3. The method for evaluating the gettering capability of a silicon wafer according to claim 2, wherein ultrasonic waves are applied to the contaminating liquid in advance in the intentional contamination step.

7. 7. The method for evaluating gettering capability of a silicon wafer according to claim 6, wherein a maximum rotation speed R (rpm) of the spin coater and a time t (seconds) required to reach the maximum rotation speed satisfy the following formula (2): R≧5.96×10 ２ ×t ０．４０８ ・・・(2)

8. 7. The method for evaluating gettering capability of a silicon wafer according to claim 6, wherein R (rpm) is a maximum rotation speed of the spin coater and t (seconds) is a time required to reach said maximum rotation speed, and 2000≦R≦6000 and t≦15 are satisfied.

9. The method for evaluating gettering capability of a silicon wafer according to any one of claims 1 to 8, wherein the metal impurity is nickel (Ni).

10. 9. The method for evaluating gettering capability of a silicon wafer according to claim 1, wherein the silicon wafer is an epitaxial silicon wafer, and the front surface of the silicon wafer is a surface of an epitaxial layer of the epitaxial silicon wafer.

11. 10. The method for evaluating the gettering capability of a silicon wafer according to claim 9, wherein the silicon wafer is an epitaxial silicon wafer, and the front surface of the silicon wafer is a surface of an epitaxial layer of the epitaxial silicon wafer.

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