Single-sided polishing method for SOI wafers

Abstract

To provide a method for polishing one side of a SOI wafer with which it is possible to further uniformize the film thickness distribution of active layers.SOLUTION: The method includes: a first step for measuring the film thickness distribution of an active layer before starting polishing and calculating an irregularity index based on the film thickness distribution of the active layer before starting polishing; a second step for determining an initial polishing condition on the basis of the irregularity index before starting polishing; a third step for polishing the active layer on the basis of the initial polishing condition while measuring the film thickness of central and peripheral sections of the active layer a number of times and calculating an irregularity index in the middle of polishing; a fourth step for terminating polishing under the initial polishing condition when the absolute value of the irregularity index in the middle of polishing drops to or below a desired value; and a fifth step for polishing the active layer on the basis of a finish polishing condition until the average film thickness value of the central and peripheral sections of the active layer reaches the desired value.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for polishing a single side of an SOI wafer. [Background technology]

[0002] In recent years, SOI wafers having an SOI (Silicon on Insulator) structure have been attracting attention for various semiconductor device applications such as highly integrated CMOS elements, high-voltage elements, and image sensors.

[0003] SOI wafers generally have a structure in which an insulating layer such as silicon dioxide (SiO2) and a semiconductor layer such as a single-crystal silicon layer used as the device active layer are sequentially formed on a support substrate wafer made of a single-crystal silicon wafer. This semiconductor layer is also called the active layer or SOI layer, and will be referred to as the "active layer" hereinafter. While bulk single-crystal silicon wafers have a relatively large parasitic capacitance that can occur between the device and the substrate, the active layer is located on an insulating layer, which significantly reduces the parasitic capacitance. Therefore, SOI wafers are advantageous in terms of increasing device speed, increasing voltage resistance, and reducing power consumption.

[0004] The trend toward higher integration of semiconductor devices is accelerating, and this has led to demands for improvements in the surface quality of semiconductor wafers, such as flatness. To meet this demand, improvements have been made to semiconductor wafer polishing equipment and polishing methods.

[0005] For example, in the semiconductor wafer polishing apparatus described in Patent Document 1, multiple pressure chambers are provided to prevent deformation of the semiconductor wafer due to the polishing pressure. Preparation A polishing head that can

[0006] Furthermore, in Patent Document 2, in order to make the film thickness distribution of the active layer uniform, the radial film thickness distribution of the active layer is measured before polishing, focusing on the radial film thickness distribution of the active layer rather than the thickness distribution of the entire SOI wafer. Then, in Patent Document 2, the pressure distribution of the pressure control chamber in the polishing head is appropriately controlled based on the measurement results.

[0007] In addition, Patent Document 2 uses a polishing apparatus having an optical film thickness measurement unit to evaluate film thickness during polishing. This optical film thickness measurement unit has a light source capable of emitting measurement light and a light-receiving sensor capable of receiving reflected light of the measurement light. The incident measurement light from the light source is collected by a lens via an optical fiber, and the measurement light is incident on the exposed surface of the SOI layer (active layer) of the SOI wafer through a window in the surface plate. The measurement light is reflected by the insulating layer and received by the light-receiving sensor via the lens and optical fiber. In this way, the single-sided polishing apparatus of Patent Document 2 measures the film thickness of the SOI layer (active layer) during polishing. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-131920 [Patent Document 2] Patent Publication No. 2021-106193 Summary of the Invention [Problem to be solved by the invention]

[0009] In recent years, there has been an increasing demand for improved flatness in the active layer of SOI wafers. One of the specifications required for the active layer is the "thickness range." When expressing the target thickness of the active layer after polishing, if the allowable thickness error on the thickest and thinnest sides of the active layer's film thickness distribution is denoted as a, then the film thickness distribution of the active layer must be within the range of [target thickness] ± a. In other words, the difference between the maximum and minimum film thickness values ​​must be within 2a. The film thickness range refers to the difference between this maximum and minimum film thickness values. A single-sided polishing method for SOI wafers that can further narrow the film thickness range compared to conventional techniques is required.

[0010] Therefore, the present invention provides: activity The object of the present invention is to provide a method for polishing one side of an SOI wafer, which can make the film thickness distribution of the layer more uniform. [Means for solving the problem]

[0011] The present inventors conducted extensive research to solve the above problems. They came up with the idea of ​​using an index called a roughness index, which is the difference between the average thicknesses of the central and peripheral portions of the active layer of an SOI wafer. They then discovered that by using this roughness index during polishing to first reduce the roughness index value as an absolute value, and then reducing the overall thickness of the entire active layer after the roughness index during polishing satisfies a predetermined condition, the film thickness distribution of the active layer can be made uniform. The gist of the present invention, which was completed based on the above findings, is as follows:

[0012] (1) A method for polishing a single side of an SOI wafer, in which an active layer of the SOI wafer is polished while measuring a film thickness distribution of the active layer, comprising: a first step of measuring a film thickness distribution of the active layer before the start of polishing and calculating a roughness index based on the film thickness distribution of the active layer before the start of polishing; a second step of determining initial polishing conditions based on the unevenness index before the start of polishing; a third step of measuring the film thicknesses of the central and peripheral portions of the active layer multiple times during polishing while polishing the active layer based on the initial polishing conditions, and calculating the unevenness index during polishing; a fourth step of terminating polishing under the initial polishing conditions when the absolute value of the unevenness index during polishing becomes equal to or less than a target value; a fifth step of polishing the active layer under finish polishing conditions until an average film thickness of the central portion and the outer periphery of the active layer reaches a target thickness; A method for polishing a single side of an SOI wafer, wherein the unevenness index is the difference between the average thicknesses of the central and peripheral portions of the active layer.

[0013] (2) The method for polishing a single side of an SOI wafer according to (1), wherein in the third step, the number of times the film thickness is measured at the central and outer peripheral portions is the number of times the film thickness is measured until the polishing head completes one cycle of oscillation.

[0014] (3) A method for polishing a single side of an SOI wafer according to (1) or (2), wherein an exponential correction relationship between the radial pressure distribution received by the active layer during polishing and the correction force for the unevenness index is determined in advance, and in the second step, the initial polishing conditions corresponding to the unevenness index before the start of polishing are determined based on the exponential correction relationship. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a method for polishing one side of an SOI wafer, which can make the film thickness distribution of the active layer more uniform. [Brief explanation of the drawings]

[0016] [Figure 1A] FIG. 10 is a schematic diagram illustrating an example of a method for calculating a point-average unevenness index. [Figure 1B] FIG. 10 is a schematic diagram illustrating an example of a method for calculating a region unevenness index. [Figure 2] FIG. 10 is a diagram showing an example of the relationship between the unevenness index and the film thickness range. [Figure 3A] FIG. 10 is a diagram showing an example of an index correction relationship in a wafer with a concave film thickness. [Figure 3B] FIG. 10 is a diagram showing an example of an index correction relationship in a wafer with a convex film thickness. [Figure 4] 1 is a diagram showing a portion of a single-side polishing apparatus for an SOI wafer according to an embodiment of the present invention. [Figure 5A] 1 shows the radial film thickness distribution measured before and after polishing in Example 1. [Figure 5B] 1 shows the radial film thickness distribution measured before and after polishing in Comparative Example 1. [Figure 6A] 1 is a graph showing the range of variation in film thickness of an active layer measured on a wafer before polishing in Example 1 and Comparative Example 1. [Figure 6B] 1 is a graph showing the range of variation in film thickness of an active layer measured on a wafer after polishing in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0017] Prior to the description of the embodiments, the following preliminary experiment was carried out to confirm the relationship between the method for evaluating the film thickness distribution during polishing and the film thickness distribution that is actually measured.

[0018] (Preliminary experiment) Although the aforementioned thickness range can accurately account for errors in the active layer thickness, evaluating the film thickness distribution across the entire wafer by measuring the film thickness during polishing is time-consuming due to the number of measurement points. Furthermore, measurements during polishing inevitably progress during film thickness measurement, making it impossible to accurately evaluate the film thickness distribution. Generally, the thickness of the active layer of an SOI wafer decreases linearly (the center is convex) or increases linearly (the center is concave) from the center to the periphery. Therefore, to easily grasp the state of the film thickness range during polishing, we considered introducing an index called the "roughness index," which uses the difference in the average thickness between the center and periphery of the active layer, as described in detail below, and verified its effectiveness.

[0019] <Unevenness index> In this specification, the difference in the average thickness between the central and peripheral portions of the active layer of an SOI wafer is referred to as the "roughness index." First, with reference to FIG. 1A, the specific roughness index (for convenience, referred to as the "point-average roughness index") used in this preliminary experiment will be described. In the point-average roughness index, the average value (arithmetic mean value, hereinafter simply referred to as the "average") of the active layer film thickness at center point 101 of SOI wafer 100 and the active layer film thickness on circumference 111, which is 10% of the wafer radius from the center of SOI wafer 100, was used as a representative value of the central portion. Similarly, the average value of the active layer film thickness on circumference 191, which is 90% of the wafer radius from the center of SOI wafer 100, was used as a representative value of the peripheral portion. The value obtained by subtracting the average film thickness of the peripheral portion from the average film thickness of the central portion ([average film thickness of central portion] - [average film thickness of peripheral portion]) is specifically referred to as the point-average roughness index, which will be distinguished from the area-average roughness index described later with reference to FIG. 1B.

[0020] Figure 2 shows a graph comparing the point-average unevenness index with the film thickness range determined by actually measuring the film thickness over the entire surface of a 200 mm diameter SOI wafer. From these results, it was found that there is a good correlation between the unevenness index and the film thickness range. In other words, it was found that by understanding the unevenness index of the film thickness of the active layer of the SOI wafer, it is possible to accurately predict the film thickness range. For example, if the unevenness index is a positive value (in this case, the center is convex), then in the first stage of polishing, polishing conditions are adopted in which the polishing allowance at the periphery is relatively small and the polishing allowance at the center is relatively large. Floor The first stage of polishing is completed after making the absolute value of the unevenness index as small as possible by polishing the grain. activity By polishing only enough to reduce the layer thickness to the target thickness, an SOI wafer with a film thickness in the target range can be obtained. Conversely, if the roughness index is a negative value (in this case, the center is concave), the first stage of polishing adopts polishing conditions in which the polishing amount at the periphery is relatively large and the polishing amount at the center is relatively small. Floor By performing the second stage of polishing in the same manner after making the absolute value of the unevenness index as small as possible in the first stage of polishing, it is possible to obtain an SOI wafer having a film thickness substantially within the target range.

[0021] (Single-sided polishing method for SOI wafers) The single-sided polishing method for SOI wafers according to the present invention is a method for polishing an active layer of an SOI wafer while measuring its thickness distribution. The method includes the following steps: a first step of measuring the thickness distribution of the active layer before polishing begins and calculating a roughness index based on the thickness distribution of the active layer before polishing begins; a second step of determining initial polishing conditions based on the roughness index before polishing begins; a third step of measuring the thickness of the central and peripheral portions of the active layer multiple times during polishing while polishing the active layer under the initial polishing conditions and calculating a roughness index during polishing; a fourth step of terminating polishing under the initial polishing conditions when the absolute value of the roughness index during polishing falls below a target value; and a fifth step of polishing the active layer under the finish polishing conditions until the average thickness of the central and peripheral portions of the active layer reaches the target thickness. The roughness index is the difference between the average thicknesses of the central and peripheral portions of the active layer. Each step will be described in detail below.

[0022] <1st process> In the first step, the film thickness distribution of the active layer before the start of polishing is measured, and a roughness index is calculated based on the film thickness distribution of the active layer before the start of polishing. The method for measuring the film thickness distribution before the start of polishing is not particularly limited, and any device using Fourier transform infrared spectroscopy or the like separate from the polishing apparatus may be used, or an optical film thickness measurement mechanism built into the polishing apparatus, such as that described in Patent Document 2, may be used.

[0023] <<Concave / convex index>> As mentioned above, the roughness index is an index introduced as an index of the film thickness distribution of the active layer, and is defined by the difference in the average thickness between the central and peripheral parts of the active layer. The number of measurement points required to calculate the roughness index is fewer than the number of measurement points required to evaluate the entire surface of the film thickness distribution of the active layer and determine the film thickness range. This makes it possible to simply evaluate the roughness shape of the active layer. Although the specific ranges of the central and peripheral parts are not limited when calculating the roughness index, the central part refers to the range from the wafer center to approximately 30% or less of the wafer radius, and the peripheral part refers to the range from approximately 70% to 100% of the wafer radius. As shown in Figure 2, a good linear correlation is observed between the film thickness range and the roughness index, so the roughness obtained by any calculation method can be easily calculated. finger The film thickness range can be easily predicted from the above figure. The point-average roughness index explained with reference to Figure 1A may be used as the roughness index. Note that Figure 1A uses the film thickness at the center of the SOI wafer, the film thickness values ​​on a circle 10% of the wafer radius from the center, and the film thickness values ​​on a circle 90% of the wafer radius from the center, but this is merely an example.

[0024] -Area average unevenness index- 1B, the area-averaged unevenness index (hereinafter referred to as the "area-averaged unevenness index") may be used as the unevenness index. For example, the area-averaged unevenness index may be calculated by subtracting the average thickness of the active layer in an annular region 192 extending from the center of the SOI wafer 100 to 90% to 100% of the wafer radius from the average thickness of the active layer in a circular region 112 extending from the center of the SOI wafer 100 to 10% of the wafer radius.

[0025] In this process, the film thickness distribution is not measured during polishing, so there is no limit to the number of measurement points in the third process, as described below. The unevenness index may be calculated using the measurement results of the entire wafer, or may be calculated from several to several tens of measurement points.

[0026] <Second process> In the second step, initial polishing conditions are determined based on the roughness index determined in the first step before polishing. The initial polishing conditions should be appropriately selected so that the absolute value of the roughness index of the polished SOI wafer is small. Specifically, the pressure distribution in the wafer's radial direction should be set to reduce the roughness of the active layer before polishing. For example, if the roughness index is a positive value (the active layer has a larger thickness at the center than at the periphery, resulting in a convex shape), the pressure applied by the wafer-holding member to the center of the wafer should be relatively large. Conversely, if the roughness index is a negative value (the active layer has a larger thickness at the periphery than at the center, resulting in a concave shape), the pressure applied by the wafer-holding member to the center of the wafer should be relatively small.

[0027] <3rd process> In the third step, the active layer is polished based on the initial polishing conditions determined in the second step, and the film thicknesses of the central and peripheral portions of the active layer are measured multiple times during the polishing process to calculate the roughness index during the polishing process. For example, the measurement mechanism illustrated in Patent Document 2 can be used to perform such measurements during the polishing process.

[0028] From the viewpoint of film thickness measurement accuracy, it is preferable to measure the central and peripheral portions at least twice each to obtain a more accurate average value. On the other hand, since polishing may be carried out simultaneously during film thickness measurement, from the viewpoint of accuracy and speed, it is better to measure the film thickness at each portion of the central and peripheral portions to calculate the unevenness index as few times as possible, and as described below, this number can be set to the number of measurements required for the polishing head to complete one oscillation cycle, which travels back and forth over a distance equal to the diameter of the SOI wafer. Since the central and peripheral portions each pass twice during one oscillation cycle, multiple measurements are taken at each portion.

[0029] <4th process> In the fourth step, polishing under the initial polishing conditions determined in the second step is terminated when the absolute value of the roughness index during polishing falls below the target value. The completion of processing when specific conditions are met is also referred to as endpoint detection. The target value in this step, which serves as the criteria for endpoint detection, can be set appropriately. While this will vary depending on the processing precision of the polishing equipment, if the target value is set to an absolute value close to 0 μm, there is a risk that the target value will not be detected during polishing, resulting in over-polishing. As excessive polishing progresses, the film thickness profile will deteriorate and the roughness index will not converge to the target value. Therefore, it is preferable to set the target value to a few hundredths of a μm. As explained using the preliminary experiment above, if the absolute value of the roughness index during polishing falls below the target value, it can be determined that the roughness of the active layer has been sufficiently resolved and the film thickness range has been narrowed.

[0030] <5th process> Therefore, in the subsequent fifth step, the active layer is polished based on the finish polishing conditions until the average film thickness of the central and peripheral parts of the active layer reaches the target thickness. Although it is not necessary to calculate the roughness index in the fifth step, polishing is continued while measuring the film thickness of the active layer.

[0031] Here, the finish polishing conditions are typically different from the initial polishing conditions. The finish polishing conditions are used to polish the active layer of an SOI wafer, after the roughness of the active layer has been removed or at least alleviated by polishing under the initial polishing conditions, until the average thickness of the active layer at the center and periphery reaches the target thickness while roughly maintaining the roughness trend of the film thickness. The pressure distribution can be set according to the characteristics of the polishing equipment. The pressure distribution at the center and periphery of the wafer can be uniform, or it is preferable to adjust it according to the equipment's characteristics. Generally, if the roughness index before the start of polishing is positive (convex center), the initial polishing conditions should apply a relatively high pressure at the wafer center, and the finish polishing conditions should apply a lower pressure at the wafer center than the initial polishing conditions. Conversely, if the roughness index before the start of polishing is negative (concave center), the initial polishing conditions should apply a relatively high pressure at the wafer periphery, and the finish polishing conditions should apply a lower pressure at the wafer periphery than the initial polishing conditions. The polishing amount of the active layer in the fifth step is determined depending on the desired film thickness of the active layer after finish polishing, the film thickness before polishing begins, and the polishing amount from the third step to the fourth step.

[0032] As described above, in the single-side polishing method according to the present invention, the roughness index is calculated during polishing, the target roughness index is used to detect the end point at an early stage, and polishing is then performed under the finish polishing conditions, so that the film thickness range of the active layer can be made smaller than that achievable with conventional technology.

[0033] <Index correction> In the single-sided polishing method for SOI wafers according to the present invention, it is preferable to determine in advance the relationship between the radial pressure distribution of the active layer and the correction force of the unevenness index in order to further uniformize the film thickness distribution of the active layer. This relationship will be referred to as the "exponential correction relationship" below, and an example is shown in FIG. 3. FIG. 3A shows the correction force of the unevenness index when the pressure at the outer periphery of the polishing apparatus is changed for an SOI wafer with a larger film thickness at the periphery (a wafer with a concave film thickness), while FIG. 3B shows the correction force of the unevenness index when the pressure at the center of the polishing apparatus is changed for an SOI wafer with a larger film thickness at the center (a wafer with a convex film thickness). Here, "Zone 1" and "Zone 3" shown in FIG. 3 are names indicating partial regions of the pressure control chamber of the single-sided polishing apparatus, which will be described later with reference to FIG. 4, and correspond to pressure control chambers 45A and 45C, respectively. The correction force has a linear relationship with the pressure. Therefore, in the second step, by determining initial polishing conditions corresponding to the unevenness index before the start of polishing based on the exponential correction relationship, accurate initial polishing conditions for endpoint detection can be set.

[0034] Furthermore, even if the same pressure conditions are used, the polishing removal amount obtained can vary greatly depending on whether the polishing equipment is used immediately after the replacement of the components (at the beginning of use) or immediately before the replacement (at the end of use). Therefore, it is preferable to determine in advance the index correction relationship according to the age of the components used, as this allows more accurate initial polishing conditions to be determined based on the unevenness index of the film thickness of the active layer of the SOI wafer before polishing.

[0035] In the above-described embodiment, the initial polishing and the finish polishing two We explained how to improve the film thickness range by changing the polishing conditions at each stage. three The polishing conditions may be changed in multiple stages, including more than one stage. In this case, it is preferable to detect the end point using the unevenness index except for the final stage. It is preferable to perform single-sided polishing so as to reduce the thickness of the entire wafer only in the finishing state.

[0036] Specific embodiments applicable to the present invention will be described in more detail below, although it should be understood that the present invention is not limited to the following specific examples.

[0037] <SOIウェーハ> To explain the SOI wafer to be polished again, the SOI wafer has a support substrate wafer, an insulating layer provided on one surface of the support substrate wafer, and an active layer provided on the surface of the insulating layer. Here, the film thickness distribution of the active layer of the SOI wafer before polishing is not particularly limited, but generally, SOI wafers have a larger radial variation than circumferential variation, and are characterized by a uniform increase or decrease in the radial direction. The total thickness of the SOI wafer before polishing is approximately 400 to 1200 μm.

[0038] <<Support substrate wafer>> A single crystal silicon wafer made of a silicon single crystal can be used as the support substrate wafer. The single crystal silicon wafer can be obtained by slicing a single crystal silicon ingot grown by the Czochralski method (CZ method) or the floating zone melting method (FZ method) with a wire saw or the like. Carbon and / or nitrogen may be added to the single crystal silicon wafer. Furthermore, any impurity may be added to make it n-type or p-type.

[0039] <<Insulating layer>> The insulating layer is usually a silicon oxide film, although films made of other materials may also be used as long as they have insulating properties.

[0040] <<Active layer>> The active layer is made by bonding a single-crystal silicon wafer made of single crystal silicon to a support substrate wafer via an insulating layer, and then thinning it by grinding, etc. The conductivity type (p-type or n-type) of the active layer may be the same as or different from that of the support substrate wafer.

[0041] (Single-sided polishing device) An example of a single-sided polishing apparatus applicable to the above-described SOI wafer polishing method according to the present invention will now be described with reference to Fig. 4, illustrating its essential components. The object to be polished by the single-sided polishing apparatus for SOI wafers in Fig. 4 is the surface of an active layer 13 of an SOI wafer 10, which includes a support substrate wafer 11, an insulating layer 12 provided on one surface of the support substrate wafer 11, and an active layer 13 provided on the surface of the insulating layer 12. This single-sided polishing apparatus for SOI wafers (hereinafter referred to as "single-sided polishing apparatus") includes a surface plate 20, a window 21, a polishing pad 30, a polishing head 40, and an optical film thickness measurement unit 60. The optical film thickness measurement unit 60 further includes a light source 61 and a sensor 62, and measures the film thickness distribution of the active layer 13 at least in the radial direction. Based on this measurement, the surface of the active layer 13 is polished by controlling the pressure distribution applied to the SOI wafer 10 by the polishing head 40. The film thickness distribution can be measured by collecting incident light of measurement light 60L from a light source 61 by a lens 65 via an optical fiber 63, and directing the measurement light 60L onto the exposed surface of the active layer 13 of the SOI wafer 10 through the window 21 of the surface plate 20.

[0042] Here, the surface plate 20 is equipped with a rotation mechanism that rotates the rotation shaft 25, while the polishing head 40 can also be equipped with a rotation mechanism and an oscillation mechanism. In such an apparatus, the film thickness is measured when the surface plate rotates and the position of the light source 61 overlaps with the position of the window 21, so the film thickness is measured once per rotation of the surface plate. The polishing head 40 oscillates within the measurement range of the optical film thickness measurement unit 60, and during one oscillation cycle, the measurement position of the SOI wafer passes through the center and moves back and forth by the length of its diameter. The active layer thickness measured by the optical film thickness measurement unit 60 is recorded and linked to measurement position information obtained by a position sensor built into the polishing head or an external camera 70 shown in FIG. 4.

[0043] The single-sided polishing apparatus also includes a polishing head 40, which can hold the SOI wafer 10. The polishing head 40 has a head main body 41 and a backing plate 43 that is provided at the center of the lower surface of the head main body 41 and can suction the other surface of the support substrate wafer 11 (i.e., the surface opposite to the insulating layer 12). The polishing head 40 may also be provided with a retainer ring 47 to prevent the SOI wafer 10 from jumping out during polishing. The head main body 41 is supported radially via the backing plate 43. Towards The pressure control chamber 45 (45A, 45B, 45 C) of Preparation The regions corresponding to 45A to 45C are sometimes referred to as zones 1 to 3, respectively. Note that FIG. 4 illustrates a case where pressure control chambers 45A, 45B, and 45C arranged symmetrically in the radial direction constitute the pressure control chamber 45, but this is merely an example. If there are two or more pressure control chambers, it is possible to set the pressure distribution. The pressure control chambers 45A, 45B, and 45C can be separated by a partition membrane or the like, and the pressure of each pressure control chamber can be controlled by a fluid such as pressurized air, thereby making it possible to uniformize the film thickness distribution of the active layer. [Example]

[0044] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples. Note that the calculation of the unevenness index in the following Example 1 and Comparative Example 1 was performed using the above-mentioned method for calculating the point-average unevenness index.

[0045] Example 1 Using the single-sided polishing apparatus described with reference to Figure 4, one SOI wafer with a diameter of 200 mm was polished on one side. First, the film thickness distribution over the entire surface of the active layer was measured as the radial film thickness distribution, and the unevenness index was calculated based on the measurement results, resulting in a value of 0.4 µm. The obtained film thickness distribution is shown in Figure 5A. Since the unevenness index was a positive value, oneIn the second stage of polishing, the initial polishing conditions must be set so that the removal rate is greater in the center than in the periphery. The radial pressure distribution for the initial polishing conditions and the finish polishing conditions is as shown in Table 1, and in particular, the pressure in Zone 3 under the initial polishing conditions was set to 5.3 psi. This pressure distribution is determined by the radial pressure distribution of the three pressure control chambers shown in Figure 4. Pressure distribution, zone 3 is at the center of It corresponds to the central pressure control chamber including the

[0046] Next, the target value of the roughness index was set to 0.04 μm, and the active layer was polished under the radial pressure distribution determined as the initial polishing conditions while measuring the roughness index until the target value was reached. To calculate the roughness index for one run, the film thickness at the center and the film thickness at the outer periphery were each measured twice. When the roughness index reached the target value of 0.04 μm or less, polishing under the initial polishing conditions was terminated. The polishing stock removal under the initial polishing conditions was 1.0 μm.

[0047] Furthermore, following the initial polishing that removed the irregularities, polishing was continued under the finish polishing conditions while maintaining this film thickness profile until the average film thickness at the center and periphery of the active layer reached the target thickness. The endpoint was determined by constantly measuring the film thickness during the finish polishing, and using the average film thickness measured at the center and periphery until the polishing head completed one oscillation cycle. The radial pressure distribution under the finish polishing conditions is shown in Table 1. The polishing stock removal under the finish polishing conditions was 1.0 μm. The film thickness distribution after finish polishing is shown in Figure 5A.

[0048] (Comparative Example 1) Next, for an SOI wafer having a diameter of 200 mm, which has the same roughness index as that of Example 1 when measuring the film thickness distribution of the active layer, the same radial pressure distribution as that of Example 1 was adopted as the initial polishing condition, and polishing was continued until the target thickness was reached without measuring the roughness index during polishing. one step Floor The unevenness index before the start of polishing in Comparative Example 1 was the same as that in Example 1. However, the film thickness distribution is different from that of Example 1 shown in FIG. 5A, as shown in FIG. 5B. At this time, the polishing stock removal under the initial polishing conditions was 2.5 μm. The film thickness distribution after this is shown in FIG. 5B.

[0049] 5A and 5B show the radial film thickness distributions before and after single-side polishing in Example 1 and Comparative Example 1, respectively, and the respective unevenness indices and film thickness ranges are shown in Table 2. The unevenness index after polishing in Comparative Example 1 was -0.1 μm, while the unevenness index in Example 1 was 0.04 μm, confirming that the film thickness distribution of the active layer after polishing was made uniform in Example 1.

[0050] [Table 1]

[0051] [Table 2]

[0052] The fluctuation range of the film thickness of the active layer before and after polishing measured in Example 1 and Comparative Example 1 is shown in Figures 6A and 6B, respectively. The fluctuation range of the film thickness represents the thickness error between the target thickness and the measured film thickness ([measured film thickness - target thickness]). Note that these graphs are for Example 1 and Comparative Example 1. that The measured film thickness values ​​across the entire surface of each SOI wafer are plotted at each point, and each measurement was obtained from a single SOI wafer. Figure 6A shows the variation range of the film thickness distribution before polishing, and Figure 6B shows the variation range of the film thickness after polishing. Although there was no significant difference in the variation range of the film thickness distribution before polishing, when the variation range of the film thickness distribution of the active layer after polishing was compared between Example 1 and Comparative Example 1, the variation range of the film thickness was smaller in Example 1, confirming a significant improvement in film thickness accuracy. The standard deviation of the variation range of the film thickness of the active layer was 0.04 μm in Example 1 and 0.08 μm in Comparative Example 1. [Industrial Applicability]

[0053] According to the present invention, it is possible to provide a method for polishing one side of an SOI wafer, which can make the film thickness distribution of the active layer more uniform. [Explanation of symbols]

[0054] 10 SOI wafers 20 Surface Plate 21 Window 25 Rotation axis 30 polishing pads 40 Polishing Head 41 Head body 43 Backing Plate 45 Pressurization Control Room 60 Optical film thickness measurement unit 65 Lens 70 External Camera

Claims

1. 1. A method for polishing a single side of an SOI wafer, comprising: polishing an active layer of the SOI wafer while measuring a film thickness distribution of the active layer; a first step of measuring a film thickness distribution of the active layer before the start of polishing, and calculating an unevenness index, which is a difference between the average thicknesses of a central portion and an average thickness of an outer periphery of the active layer, based on the film thickness distribution of the active layer before the start of polishing; a second step of determining a radial pressure distribution of an initial polishing condition based on the unevenness index before the start of polishing; a third step of measuring the film thicknesses of the central and peripheral portions of the active layer multiple times during polishing based on the initial polishing conditions, and calculating the unevenness index during polishing; a fourth step of terminating polishing under the initial polishing conditions when the absolute value of the unevenness index during polishing becomes equal to or less than a target value; a fifth step of polishing the active layer under finish polishing conditions until an average film thickness of the central portion and the outer periphery of the active layer reaches a target thickness; In the fifth step, a radial pressure distribution of the finish polishing conditions is set so as to maintain the film thickness profile of the active layer in the fourth step.

2. 2. The method for polishing a single side of an SOI wafer according to claim 1, wherein in the third step, the number of times the film thickness is measured at the central portion and the outer periphery is the number of times the film thickness is measured until the polishing head completes one cycle of oscillation.

3. 3. The method for single-sided polishing of an SOI wafer according to claim 1, wherein an exponential correction relationship between a radial pressure distribution received by the active layer during polishing and a correction force for the unevenness index is determined in advance, and in the second step, the radial pressure distribution of the initial polishing conditions corresponding to the unevenness index before the start of polishing is determined based on the exponential correction relationship.

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