JP7819626B2 - Single-sided polishing method for SOI wafer - Google Patents

Description

The present invention relates to a method for single-sided polishing of SOI wafers.

In recent years, SOI wafers with 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.

An SOI wafer generally has a structure in which an insulating layer such as silicon oxide (SiO ₂ ) and a semiconductor layer such as a single-crystal silicon layer used as a device active layer are sequentially formed on a support substrate wafer made of a single-crystal silicon wafer. This semiconductor layer is also called an active layer or SOI layer, and will be referred to as the "active layer" hereinafter in this specification. 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 provided on an insulating layer, which significantly reduces the parasitic capacitance. Therefore, SOI wafers are advantageous in terms of increasing device speed, increasing breakdown voltage, and reducing power consumption.

As semiconductor devices become increasingly highly integrated, there is a demand for improvements in the surface quality of semiconductor wafers, such as flatness. To meet this demand, improvements are being made to semiconductor wafer polishing equipment and methods.

For example, in the semiconductor wafer polishing apparatus described in Patent Document 1, a polishing head having a plurality of pressure chambers is used to prevent deformation of the semiconductor wafer due to the polishing pressure.

Furthermore, in Patent Document 2, in order to achieve a uniform film thickness distribution in the active layer, the radial film thickness distribution of the active layer is measured prior to polishing, focusing on the radial film thickness distribution of the active layer rather than the thickness distribution of the entire SOI wafer. Patent Document 2 then appropriately controls the pressure distribution in the pressure control chamber in the polishing head based on the measurement results.

Patent Document 2 also describes a polishing apparatus with an optical film thickness measurement unit that evaluates film thickness during polishing. This optical film thickness measurement unit has a light source that can emit measurement light and a light-receiving sensor that can receive the reflected light of this measurement light. The incident measurement light from the light source is then collected by a lens via an optical fiber, and the measurement light is then 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 transmitted via the lens and optical fiber to the light-receiving sensor, where it is received. In this way, the single-sided polishing apparatus of Patent Document 2 measures the film thickness of the SOI layer (active layer) during polishing.

JP 2009-131920 A Japanese Patent Application Laid-Open No. 2021-106193

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 "film 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 fall within the range of [target thickness] ± a. In other words, the difference between the maximum and minimum film thickness values must fall within 2a. The film thickness range refers to the difference between this maximum and minimum film thickness values. There is a need for a single-sided polishing method for SOI wafers that can further narrow the film thickness range compared to conventional techniques.

SUMMARY OF THE INVENTION It is therefore an object of the present invention 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.

The inventors conducted extensive research to solve the above problem. They came up with the idea of using an index known as the 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 absolute value of the roughness index, and then reducing the overall thickness of the entire active layer after the roughness index during polishing satisfies certain conditions, they can achieve a uniform film thickness distribution in the active layer. The gist of the present invention, which was completed based on the above findings, is as follows:

(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 the steps of:
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;
The method for polishing a single side of an SOI wafer, wherein the unevenness index is a difference between the average thicknesses of the central portion and the outer periphery of the active layer.

(2) A 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 peripheral portions is the number of times the film thickness is measured until the polishing head completes one oscillation cycle.

(3) A single-sided polishing method for an SOI wafer according to (1) or (2), in which 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.

The present invention provides a method for polishing one side of an SOI wafer that can achieve a more uniform film thickness distribution in the active layer.

FIG. 10 is a schematic diagram illustrating an example of a method for calculating a point-average unevenness index. FIG. 10 is a schematic diagram illustrating an example of a method for calculating a region unevenness index. FIG. 10 is a diagram showing an example of the relationship between the unevenness index and the film thickness range. FIG. 10 is a diagram showing an example of an index correction relationship in a wafer with a concave film thickness. FIG. 10 is a diagram showing an example of an index correction relationship in a wafer with a convex film thickness. 1 is a diagram showing a portion of a single-sided polishing apparatus for an SOI wafer according to an embodiment of the present invention. 1 shows the radial film thickness distribution measured before and after polishing in Example 1. 1 shows the radial film thickness distribution measured before and after polishing in Comparative Example 1. 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. 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.

Prior to explaining the embodiments, the following preliminary experiment was conducted to confirm the relationship between the evaluation method for film thickness distribution during polishing and the film thickness distribution actually measured.

(Preliminary experiment)
Although the aforementioned film thickness range can accurately account for errors in the active layer film 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, if measurements are performed during polishing, polishing inevitably progresses even 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 linearly decreases (the center is convex) or linearly increases (the center is concave) from the center to the periphery. Therefore, to easily grasp the state of the film thickness range during polishing, the inventors considered introducing an index called a "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.

<Roughness 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 (referring to the arithmetic average value, hereinafter simply referred to as the "average") of the active layer film thickness at the center point 101 of the SOI wafer 100 and the active layer film thickness on a circumference 111 that is 10% of the wafer radius from the center of the SOI wafer 100 is used as a representative value of the central portion. Similarly, the average value of the active layer film thickness on a circumference 191 that is 90% of the wafer radius from the center of the SOI wafer 100 is 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 central film thickness] - [average peripheral film thickness]) is specifically referred to as the point-average roughness index, and is distinguished from the area-average roughness index described later with reference to FIG. 1B.

Figure 2 shows a graph comparing the point-average irregularity index with the film thickness range determined by actual film thickness measurements across the entire surface of a 200 mm diameter SOI wafer. These results demonstrate a good correlation between the irregularity index and the film thickness range. In other words, by determining the irregularity index of the active layer film thickness of an SOI wafer, the film thickness range can be accurately predicted. For example, if the irregularity index is a positive value (in this case, the central portion is convex), polishing conditions are adopted in the first stage of polishing in which the polishing allowance at the periphery is relatively small and the polishing allowance at the central portion is relatively large. By completing the first stage of polishing after minimizing the absolute value of the irregularity index in the first stage of polishing, and then simply polishing the active layer to the target thickness in the second stage of polishing, an SOI wafer with a substantially targeted film thickness range can be obtained. Conversely, if the irregularity index is a negative value (in this case, the central portion is concave), polishing conditions are adopted in the first stage of polishing in which the polishing allowance at the periphery is relatively large and the polishing allowance at the central portion is relatively small. Then, by making the absolute value of the unevenness index as small as possible in the first stage of polishing and then similarly performing the second stage of polishing, an SOI wafer having a film thickness substantially within the target range can be obtained.

(Method for polishing one side of SOI wafer)
That is, 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 the film thickness distribution of the active layer of the SOI wafer. It includes the following steps: a first step of measuring the 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 roughness index before the start of polishing; a third step of measuring the film 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 the 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 film thickness of the central and peripheral portions of the active layer reaches the target thickness. Here, the roughness index is the difference between the average thicknesses of the central and peripheral portions of the active layer. Details of each step are explained below.

<First step>
In the first step, the thickness distribution of the active layer before the start of polishing is measured, and a roughness index is calculated based on the thickness distribution of the active layer before the start of polishing. The method for measuring the 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 device may be used, or an optical film thickness measurement mechanism incorporated in the polishing device, such as that described in Patent Document 2, may be used.

＜＜Irregularity index＞＞
As described above, the roughness index is an index introduced as an indicator of the film thickness distribution of the active layer and is defined as the difference in the average thickness between the central and peripheral portions 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 active layer's film thickness distribution and determine the film thickness range. This allows for a simple evaluation of the active layer's roughness shape. While the specific ranges of the central and peripheral portions are not limited when calculating the roughness index, the central portion refers to the range from the wafer center to approximately 30% of the wafer radius, and the peripheral portion 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 film thickness range can be easily predicted from the roughness index obtained by any calculation method. The point-average roughness index described with reference to Figure 1A may also be used as the roughness index. Note that Figure 1A uses the central film thickness of the SOI wafer, the film thickness value on a circle 10% of the wafer radius from the center, and the film thickness value on a circle 90% of the wafer radius from the center, but this is merely an example.

-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.

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 can be calculated using the measurement results for the entire wafer, or it can be calculated from several to several tens of measurement points.

<Second process>
In the second step, initial polishing conditions are determined based on the roughness index before polishing determined in the first step. The initial polishing conditions can be appropriately selected so that the absolute value of the roughness index of the polished SOI wafer is small. Specifically, a radial pressure distribution of the wafer can 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 to the center of the wafer by the member holding the wafer can 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 to the center of the wafer by the member holding the wafer can be relatively small.

<3rd process>
In the third step, the active layer is polished based on the initial polishing conditions determined in the second step, and the 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.

From the perspective of film thickness measurement accuracy, it is preferable to measure the center and outer periphery at least twice each to obtain a more accurate average value. However, since polishing may also be carried out simultaneously when film thickness is measured, from the perspective of accuracy and speed, it is better to measure the film thickness at each of the center and outer periphery the fewer times to calculate the unevenness index. As described below, this can be the number of measurements taken until the polishing head completes one oscillation cycle, traveling a distance equal to the diameter of the SOI wafer. Since the center and outer periphery are each passed over twice during one oscillation cycle, multiple measurements are taken at each.

<4th step>
In the fourth step, when the absolute value of the roughness index during polishing falls below the target value, polishing is terminated under the initial polishing conditions determined in the second step. The completion of processing when specific conditions are satisfied is also referred to as end-point detection. The target value in this step, which serves as the determination condition for end-point detection, can be appropriately determined. Although this will vary depending on the processing accuracy of the polishing machine, if the target value is set to an absolute value close to 0 μm, there is a risk that the target value cannot be detected during polishing and excessive polishing will occur. As excessive polishing progresses, the film thickness shape will deteriorate and the roughness index will not converge to the target value. Therefore, it is preferable to set the target value to approximately a few hundredths of 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 eliminated and the film thickness range has been narrowed.

<5th step>
Therefore, in the subsequent fifth step, the active layer is polished under 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.

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 initial polishing conditions have removed or at least reduced the active layer irregularities, until the average film thickness of the active layer at the center and periphery reaches the target thickness while roughly maintaining the film thickness irregularity trend. 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 make adjustments based on the equipment characteristics. Generally, if the irregularity index before polishing is positive (convex center), the initial polishing conditions should apply a relatively high pressure at the wafer center, while the finish polishing conditions should apply a lower pressure at the wafer center than the initial polishing conditions. Conversely, if the irregularity index before polishing is negative (concave center), the initial polishing conditions should apply a relatively high pressure at the wafer periphery, while 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 based 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.

As described above, the single-sided polishing method according to the present invention calculates the roughness index during polishing, uses the target roughness index to detect the endpoint at an early stage, and then performs polishing under finish polishing conditions, thereby making it possible to narrow the film thickness range of the active layer compared to the level achievable with conventional technology.

<Index correction>
In addition, 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 applied to 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.

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 replacing the components (at the beginning of use) or immediately before replacement (at the end of use). Therefore, it is preferable to determine the index correction relationship in advance according to the time of use of the components, 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.

In the above-described embodiment, the film thickness range is improved by changing the polishing conditions in two stages, the initial polishing and the finish polishing. However, the polishing conditions may be changed in multiple stages, such as three or more stages. 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 only in the finish state so as to reduce the thickness of the entire wafer.

Specific embodiments applicable to the present invention are described in more detail below. However, it should be understood that the present invention is not limited to the following specific examples.

<SOI wafer>
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 feature that the radial variation is larger than the circumferential variation, and the thickness increases or decreases uniformly in the radial direction. The total thickness of the SOI wafer before polishing is approximately 400 to 1200 μm.

<<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) using 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 the wafer n-type or p-type.

<<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.

<<Active layer>>
The active layer is formed by bonding a single crystal silicon wafer made of single crystal silicon to a support substrate wafer via an insulating layer, and then thinning the wafer 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.

(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 . The object to be polished by the single-sided polishing apparatus for SOI wafers shown 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 the "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 the incident light of the measurement light 60L from the light source 61 by a lens 65 via an optical fiber 63, and irradiating the measurement light 60L onto the exposed surface of the active layer 13 of the SOI wafer 10 through the window portion 21 of the surface plate 20.

Here, the surface plate 20 is equipped with a rotation mechanism that rotates the rotary 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 with 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 thickness of the active layer 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 Figure 4.

The single-sided polishing apparatus also includes a polishing head 40, which can hold the SOI wafer 10. The polishing head 40 includes a head main body 41 and a backing plate 43, which is provided in the center of the underside of the head main body 41 and can adsorb the other side of the support substrate wafer 11 (i.e., the side opposite the insulating layer 12). The polishing head 40 may also be provided with a retainer ring 47 to prevent the SOI wafer 10 from popping out during polishing. The head main body 41 includes pressure control chambers 45 (45A, 45B, 45C ) that can set a pressure distribution in the radial direction via the backing plate 43. The regions corresponding to 45A to 45C are sometimes referred to as zones 1 to 3, respectively. Note that FIG. 4 illustrates a case in which the pressure control chamber 45 is made up of pressure control chambers 45A, 45B, and 45C arranged symmetrically in the radial direction, but this is merely an example. It is possible to set a pressure distribution if there are two or more pressure control chambers. The pressure control chambers 45A, 45B, and 45C can be separated by partition membranes or the like, and the film thickness distribution of the active layer can be made uniform by controlling the pressure in each pressure control chamber with a fluid such as pressurized air.

The present invention will be explained 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 Example 1 and Comparative Example 1 below was performed using the method for calculating the point-average unevenness index described above.

Example 1
Using the single-sided polishing apparatus described with reference to FIG. 4, a single-sided polishing of a 200 mm diameter SOI wafer was performed. First, the film thickness distribution across the entire active layer was measured as the radial film thickness distribution. The unevenness index was calculated based on the measurement results and was found to be 0.4 μm. The resulting film thickness distribution is shown in FIG. 5A. Because the unevenness index was a positive value, the initial polishing conditions must be set so that the removal rate in the central portion is greater than that in the peripheral portion in the first stage of polishing. The radial pressure distribution for the initial polishing conditions and the finish polishing conditions was as shown in Table 1. In particular, the pressure in Zone 3 under the initial polishing conditions was set to 5.3 psi. Note that this pressure distribution is the radial pressure distribution of the three pressure control chambers shown in FIG. 4, and Zone 3 corresponds to the central pressure control chamber including the center position.

Next, the target value for the unevenness index was set to 0.04 μm, and the active layer was polished using the radial pressure distribution determined as the initial polishing conditions while measuring the unevenness index until the target value was reached. To calculate one unevenness index, the film thickness at the center and the film thickness at the outer periphery were each measured twice. When the unevenness 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.

Following the initial polishing to remove any irregularities, polishing was continued under 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 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 during polishing under the finish polishing conditions was 1.0 μm. The film thickness distribution after finish polishing is shown in Figure 5A.

(Comparative Example 1)
Next, for an SOI wafer having a diameter of 200 mm and having the same roughness index as in Example 1 when measuring the film thickness distribution of the active layer, the same radial pressure distribution as in Example 1 was adopted as the initial polishing conditions, and polishing was completed in one step until the target thickness was reached without measuring the roughness index during polishing. Note that the roughness index before polishing in Comparative Example 1 was the same as in Example 1, but the film thickness distribution, as shown in Figure 5B, differs from the film thickness distribution in Example 1 shown in Figure 5A. In this case, the polishing stock removal during polishing under the initial polishing conditions was 2.5 μm. The film thickness distribution after polishing was completed is shown in Figure 5B.

Figures 5A and 5B show the radial film thickness distribution before and after single-sided polishing in Example 1 and Comparative Example 1, respectively, and Table 2 shows the respective unevenness indices and film thickness ranges. While the unevenness indices after polishing in Comparative Example 1 were -0.1 μm, the unevenness indices in Example 1 were 0.04 μm, confirming that Example 1 achieved a uniform film thickness distribution for the active layer after polishing.

The fluctuation range of the active layer film thickness measured in Example 1 and Comparative Example 1 before and after polishing 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]). These graphs plot the measured film thickness values over the entire surface of each SOI wafer in Example 1 and Comparative Example 1 at each point, and each represents a measurement value obtained from a single SOI wafer. Figure 6A shows the fluctuation range of the film thickness distribution before polishing, and Figure 6B shows the fluctuation range of the film thickness after polishing. Although there was no significant difference in the fluctuation range of the film thickness distribution before polishing, when the fluctuation range of the active layer film thickness distribution after polishing was compared between Example 1 and Comparative Example 1, the film thickness fluctuation range was smaller in Example 1, confirming a significant improvement in film thickness accuracy. The standard deviation of the fluctuation range of the active layer film thickness was 0.04 μm in Example 1 and 0.08 μm in Comparative Example 1.

The present invention provides a method for polishing one side of an SOI wafer that can achieve a more uniform film thickness distribution in the active layer.

REFERENCE SIGNS LIST 10 SOI wafer 20 surface plate 21 window portion 25 rotation shaft 30 polishing pad 40 polishing head 41 head main body portion 43 backing plate 45 pressure control chamber 60 optical film thickness measurement portion 65 lens 70 external camera

Claims (3)

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 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;
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 . The method for single-sided polishing of an SOI wafer according to claim 1, wherein in the third step, the film thickness at the central and peripheral portions is measured the number of times until the polishing head completes one oscillation cycle. 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.