JPH0757464B2 - Method for polishing thin film on substrate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for polishing a thin film sample in which a thin film having a constant film thickness such as SOI (Silicon on insulator) is formed on a substrate.

[Conventional technology]

Conventionally, polishing of a semiconductor substrate or the like has been performed by pressing the substrate against a rotating platen having a diameter larger than that of the semiconductor substrate and rotating the platen. In this case, the accuracy of the polishing process is greatly affected by the flatness of the rotary platen and the uniformity of the pressure load on the substrate. In recent years, the parallelism of the entire semiconductor substrate including the thin film is 0.5 μm due to the improvement of the machining precision. Can be finished to a degree. Also, the so-called
When the SOI substrate is manufactured by the attachment method, the thin film formed on the semiconductor substrate must be further polished so as to have a uniform film thickness of about 1 μm.

However, in the above-described conventional polishing method, uniform planarization of the entire substrate including the thin film can be performed, but the flatness of the substrate itself is particularly poor, and the thickness of the thin film formed on the substrate is not uniform. In such a case, it was impossible to finish the thin film itself to a constant and uniform film thickness. The uniformity of the film thickness of this thin film has been a problem because it significantly affects the performance of such semiconductor devices.

In view of such circumstances, it is an object of the present invention to provide a polishing method capable of forming a thin film on a substrate to a uniform film thickness with extremely high accuracy.

[Means for Solving the Problems]

The method for polishing a thin film on a substrate according to the present invention is such that a plurality of measurement points for measuring the thickness of the thin film are set in advance on the substrate on which the thin film to be polished is formed, and the measurement points are used as the centers of the respective measurement points. A processing area having a certain area is defined, and a polishing pad whose contact surface diameter is set to be smaller than the processing area is moved along the zigzag path in the processing area. At the same time, the contact pressure of the contact surface of the pad against the thin film is changed according to the film thickness measurement value at the measurement point located at the center of the processing area, and the polishing processing is sequentially performed for each processing area.

In this case, the contact surface of the polishing pad, the diameter is preferably at least 1/2 of the length of one side of the processing region, and when formed into a convex curved surface with respect to the thin film surface Good. Further, the shape of the contact surface may be polygonal, and the angular velocity of rotation of the pad and the angular velocity of revolution of the pad should be in opposite directions and of the same size, and the radii of revolution and revolution should be equal. It can also be set.

[Action]

In each processing area, the amount of processing at the center is larger than that at the periphery, so that the thin film on the substrate can be locally polished around the desired measurement point and the film thickness at each measurement point is measured. Since the amount of polishing is adjusted according to the value, it is possible to perform polishing so that the film thickness of the thin film is uniform over the entire region of the substrate.

Further, by uniformly specifying the diameter and shape of the contact surface of the polishing pad, and further appropriately selecting the rotational and revolution angular velocities of the pad, the above-mentioned uniformization of the film thickness can be effectively achieved. .

〔Example〕

A first embodiment of the method for polishing a thin film on a substrate according to the present invention will be described below with reference to FIGS. 1 to 6. 1 to 3 show the structure of an apparatus used for carrying out the method of the present invention, in which 1 is a base plate of the apparatus, 2 is a table fixed on the base plate 1, and 3 is a table. 2 is a sample table which is mounted so as to be movable along two orthogonal axes (X, Y directions) on which the semiconductor substrate P is mounted and fixed. Reference numerals 4,5 denote the sample table 3 in the X direction, respectively. A drive motor for moving in the Y direction, 6 is a polishing tool unit attached to a column 7 standing on the base plate 1, 8
Is suspended from the sample table 3 through bearings 9 and 9 ', and an attachment 8a for mounting a polishing pad, which will be described later, is provided at the lower end thereof along the axial direction from a predetermined position. A rotary shaft mounted so that it can be moved by an appropriate stroke, 10 is a polishing pad attached to the tip of the attachment 8a, and 11 is a drive for rotating the rotary shaft 8 via gears 12 and 12 '. The motor 13 is connected to a pair of guide shafts 14 and 14 'through a ball screw or the like, and the vertical movement of the guide shafts 14 and 14' can adjust the pressing force of the pad 10 against the substrate P via the spring 15. Blocks 16 and 16 'are guide shafts 14,
A drive motor for rotating 14 'and a control circuit unit 17 are provided. Here, the basic operation of such a polishing apparatus and the outline of the polishing method will be described. First, the film thickness of the substrate P before polishing is measured at a plurality of predetermined points. The film thickness measurement value is stored and compared with a desired film thickness target value, thereby determining the polishing amount corresponding to each measurement point. The pad 10 can take all positions along the X direction and the Y direction relative to the substrate P fixed on the sample table 3 by the movement of the sample table 3, but the control circuit unit 17 makes the relative position therebetween. By determining the positional relationship, a command is sent to the pad 10 located on the measurement point of the substrate P so as to perform processing corresponding to the polishing amount of the measurement point. By the way, a processing region having a constant area centered on each measurement point is defined on the substrate P, and the polishing process is sequentially performed in units of this processing region. In this case, the control circuit unit 17 operates. When the sample table 3 is moved by the drive motors 4 and 5 which have received the signal, and the pad 10 is positioned on an arbitrary measurement point on the substrate P, the sample table 3 is temporarily stopped at this position and the drive motor 11 is moved. The pad 10 is rotated via the gears 12, 12 'and the rotary shaft 8. further,
The control circuit unit 17 controls the operation of the drive motors 4 and 5 that move the sample table 3 so that the pad 10 performs a constant zigzag operation within the processing region of this measurement point, and the pad 1
The driving motor is adjusted so that the contact pressure of the pad 10 with respect to the substrate P during the zigzag movement of 0 becomes a pressure adapted to obtain the polishing amount corresponding to this measurement point stored in the control circuit unit 17. 16, 16 'are operated to adjust the stroke of the attachment 8a to which the pad 10 is attached via the adjustment block 13. In this way, in the unit processing area, the polishing processing is performed by a predetermined amount corresponding to the measurement point which is the center position, and the sample table 3 is moved over the entire area of the substrate P according to the arrangement of the measurement points. As a result, the polishing process for each processing region is sequentially performed.

Next, a specific example of the polishing method using the polishing apparatus having the above configuration will be described in more detail. First, as shown in FIG. 4, the substrate P on which the thin film to be polished is formed has a plurality of measurement points a ₁₁ , a ₁₂ , ..., A _nn (hereinafter, arbitrary measurement points). Is collectively referred to as a measurement point a), and the film thickness at each measurement point a is measured in advance by image analysis of the interference fringe pattern. The distribution of the thickness of the thin film on the substrate P can be known from the film thickness measurement value at the measurement point a. For that purpose, it is ideal to continuously measure the entire thickness of the substrate P. Due to restrictions such as
16 points are suitable as measuring points a for about 5 cm, and 36 points are suitable for about 7.5 cm. When the measurement points a are discretely set in this way, the film thickness between the measurement points a is estimated by linearly interpolating, but since the surface of the substrate P is processed sufficiently smoothly, This interpolation is sufficient to know the accurate film thickness distribution over the entire area of the substrate P.

Next, a processing region of a unit having a constant area is defined around each measurement point. For example, when the measurement point a ₂₂ is the center, eight measurement points a ₁₁ , a ₁₂ , a around it are formed. ₁₃ , a ₂₁ , a ₂₃ ,
Processing area A ₂₂ by a ₃₁ , a ₃₂ , a ₃₃ (similarly A ₂₃ , A ₃₂ ,
.., A _mm , hereinafter, any processing area is collectively referred to simply as processing area A). Incidentally, the diameter of the contact surface 10a to be contacted with the substrate P of the pad 10 of the polishing described above, although the area of the contact surfaces is chosen to be smaller than the area of the processing region A _22, in this case, the processing region A The length of one side of ₂₂ (for example, the distance between the measurement points a ₁₁ and a ₃₁ ) is selected to be at least half (see FIG. 5). When the processing area A ₂₂ is polished, the sample table 3 on which the substrate P is placed so that the pad 10 moves along the zigzag path in the processing area A ₂₂ as shown in FIG. It can be moved on the table 3 by 4,5. Where the pad
The contact pressure of the contact surface 10a of the 10 with respect to the processing area A ₂₂ is adjusted by the adjusting block 13 as described later. The processing characteristics of the thin film on the substrate P by the zigzag movement of the pad 10 are shown in FIG. Show. That is, FIG. 6 is represents the polishing amount of the thin film on the substrate P in the processing region A _22, due to the low processing time of the net by the outer peripheral portion about the pad 10 relative to the central portion of the processing area A ₂₂ The polishing amount decreases monotonically toward the outer peripheral portion.
_It has a pyramidal shape with the point corresponding to ₂₂ as the apex. Therefore, by utilizing such processing characteristics, it is possible to locally reduce the film thickness of the thin film around the desired measurement point a on the substrate P. In this case, the processing area A is changed by the measurement point a. Since it is defined and the film thickness between the measurement points a is accurately estimated, the normal state of change of the film thickness between the measurement points a is not impaired.

Thus, the polishing process is performed in the case of the processing region A ₂₂ as the unit processing region A. Further, the polishing process is performed on all the processing regions A by the same method as described above. On this occasion,
According to the measured value of the film thickness of the thin film at the measurement point located at the center of each processing area A (hereinafter referred to as the center side fixed point)
The contact pressure of the contact surface 10a with respect to the thin film surface is set according to the following equation.

p = k (t−t ₀ ) ^x (1) where p is the contact pressure, k is the proportional constant, t is the measured film thickness, and t ₀
Is the target value of the film thickness, and the index x> 1. As can be seen from the above equation, first, the contact pressure p increases as the film thickness measurement value t of the thin film to be polished is larger than the film thickness target value t ₀ . As a result, an appropriate polishing amount is set for each processing region A over the entire area of the substrate P, and the film thickness can be made uniform as a whole. However, if x> 1, further uniformization can be performed with higher accuracy. Can be achieved with. That is, in general, when the contact pressure p of the contact surface 10a is set by a simple proportional relationship (that is, x = 1) with respect to the change state of the thin film to be polished, both of the part having a large film thickness and the part having a small film thickness are Is insufficient to accurately replace the difference in the amount of processing as the processing amount, and the variation in the film thickness cannot be reduced in a short time. For example, when finishing a silicon thin film with a thickness variation of about 2 to 10 μm to a constant thickness of 1 μm, the typical processing speed by mechanochemical polishing is about 5000 Å / min, so the total processing time is 18 However, it is necessary to eliminate the variation in the film thickness during this period. Therefore, the part with the maximum film thickness should be processed at the maximum processing speed, and the difference in the film thickness from the target film thickness t ₀ should be small. Processing must be done to minimize the amount of processing. For this purpose, it is most effective to set the contact pressure p in a non-linear relationship such as a quadratic function relationship with the film thickness measurement value at the central measurement point of each processing region A. In this way, the contact pressure of the pad 10 is set for each processing region A according to the change in the film thickness of the thin film to be polished, so that the film thickness can be made uniform over the entire area of the substrate P. Adjust block 13 by driving drive motors 16 and 16 '
The vertical movement of the attachment 8a for mounting the pad 10 via the.

By the way, when the contact surface 10a of the pad 10 has a circular shape, when it is rotated at a certain position on the substrate P, the processing characteristics for the thin film correspond to the outer peripheral portion of the contact surface 10a as shown in FIG. And has a sharp edge. Furthermore, when the pad 10 is zigzag-moved as described above (see FIG. 5), a zigzag-shaped groove is formed along the movement path of the pad 10, which seriously impairs the microscopic flatness of the thin film and causes a problem. Becomes This problem can be solved by forming the contact surface 10a of the pad 10 into a polygonal shape. That is, FIG. 8 shows the processing characteristics when the pad 10 having the polygonal contact surface 10a is rotated at a fixed position on the substrate P similarly to the above, and the contact surface 10a as shown in FIG.
It can be seen that the edge-like steepness of the outer peripheral portion of is reduced. This is because the net processing time for the thin film in the outer peripheral portion of the contact surface 10a is relatively decreased as compared with the case where the thin film is circular, and thus the flatness of the thin film surface is microscopically improved.

Such flatness can also be obtained by forming the contact surface 10a of the pad 10 on the thin film into a convex curved surface such as a spherical surface, as shown in FIG. With such a curved surface, the contact pressure on the thin film can be made smaller toward the outer peripheral portion of the contact surface 10a, and as a result, the processing characteristics are substantially the same as those shown in FIG.

Further, as shown in FIG. 10 and FIG. 11, the pad 10 is eccentric with respect to the rotary shaft 8 (attachment 8a) and the pad 10 is caused to make a planetary motion, thereby improving the microscopic flatness of the thin film surface. Can be made. In this case, the pad
Although 10 revolves on a fixed revolution circular orbit while rotating, the rotation angular velocity and the revolution angular velocity are equal in size and in opposite directions, and the radius r of the contact surface 10a of the pad and the revolution radius By making it equal to R, the relative velocities at the contact surface 10a and the contact portion of the thin film become equal at arbitrary positions within the contact surface 10a, and the thin film is uniformly processed.

FIG. 12 shows a second embodiment. In this example, the method of selecting the substrate P and the measurement point a is the same as in the first embodiment (fourth example).
Drawing), for example, a unit processing region A _'22 is measured points a _12, a _21,
It is defined by a ₂₃ and a ₃₂ . As is apparent from the figure, the area of the processing area A ′ is the same as the processing area A in the first embodiment.
Smaller than the area of, and the length of one side of the processing area A _'22 is about
It is 0.707 times. In this case, the substrate P has a processing area A ′ _22.
Is fixed on the sample stage 3 of the polishing apparatus so that each side of the sample is aligned with the X direction and the Y direction (see FIG. 2). Therefore, the substrate P is 45 with respect to the sample stage 3 as in the case of the first embodiment. It is in a state of being rotated by an angle of °, and the processing method is basically the same as that of the first embodiment. According to this example, it is particularly effective when the film thickness of the thin film to be polished formed on the substrate P varies significantly. Here, referring to FIG. 4, the processing area
Taking A ₂₂ as an example, the measurement points a ₁₂ , a ₂₁ , a ₂₃ , a around it
₃₂ (referred to as the first adjacent grid point) and measurement points a ₁₁ , a ₁₃ , a ₃₂ , a
₃₃ (referred to as second adjacent grid points) are processing areas A ₁₂ , A ₂₁ , A ₂₃ , A ₃₂ (referred to as first adjacent processing areas) and processing areas A ₁₁ , A ₁₃ , A ₃₂ centered on these points. , A ₃₃ (referred to as a second adjacent processing area) share the processing area with each other, so that the processing amounts of the first adjacent grid point and the second adjacent grid point are the same as those of the first adjacent processing area and the second adjacent processing area. It will be influenced by both parties. Therefore, when the film thickness of the measurement point a ₂₂ is extremely thin compared to the film thickness of the first and second adjacent grid points, the processing area A ₂₂ is processed to form the first and second adjacent grid points. If the processing amount corresponding to the film thickness is removed from the shared portion of the processed region, the film thickness of this shared portion becomes too thin below the target value of the film thickness. According to the present embodiment, it is possible to avoid such overlapping of the processed regions, and it is particularly effective for a thin film having a large variation in film thickness. In the case of this example, four adjacent measurement points (for example, measurement points a ₁₁ , a ₁₂ , a ₂₁ ,
a ₂₂ ) located at the center of the area defined by
In the figure, it is indicated by reference numeral b ₁₁ . Sign similarly
b ₁₂ , b ₁₃ , ..., b ₃₃ , ..., b _nn ) will not be machined at all, so it is necessary to provide a machining area centered on these points, for example centering on point b _22. Point b ₁₂ ,
A processing area B ₂₂ is defined by b ₂₁ , b ₂₃ and b ₃₂ . Since these points b ₁₁ , ..., b _nn are not selected as measurement points,
Although the film thickness is not measured, the contact pressure of the contact surface 10a of the pad 10 when processing these points is set by the average value of the film thickness measurement values at the surrounding measurement points a.

Explaining further a concrete example of the polishing method according to the second embodiment, two silicon single crystal plates each having a diameter of about 5 cm are oxidized and bonded together, and one of the single crystal plates has a thickness of 5 μm by the conventional method. A substrate P formed by polishing is used. This substrate P
The film thickness distribution is 2 to 7μ as the value measured by the interferometric film thickness meter.
It varied in the range of m. The distance between measurement points a is set to 8 mm. On the other hand, a pad 1 formed in a spherical shape with a diameter of 8 mm
The lower end surface of the 0, one side is stuck artificial leather polishing pad square 5.66mm contact surface 10a ungated, moved such pads 10, for example machining area A 'along the zigzag path at _22, the zig-zag Both the amplitude and the length were set to 5.66 mm. Contact surface of pad 10 in each processing area
The contact pressure of 10a is p = k (t-t ₀ ) ² (see the equation (1)).
Film thickness measurement in the above formula the average value of four of the film thickness measurement value of the measurement points a surrounding contact pressure the center point _{b 11, b 12, ‥‥ b} nn for processing region B described above and sets the It is determined by using it as the value t. Thus, when the polishing process is performed over the entire area of the substrate P, there is a maximum film thickness reduction of 1 μm within the processing time of about 20 minutes, and the contact pressure of the pad 10 is reset based on the film thickness distribution in this state. As a result of repeating the operation of deciding and performing polishing 7 times, 1 ± 0.
An SOI layer having a uniform film thickness of 2 μm was obtained.

〔The invention's effect〕

As described above, according to the method of the present invention, it is possible to apply this type of thin film sample polishing to form a thin film having extremely uniform and high flatness. Excellent effect as a thin film polishing method.

[Brief description of drawings]

1 to 3 are a front view, a plan view and a side view, respectively, of a polishing apparatus used for carrying out the method of the present invention, and FIG. 4 shows a processing region on a substrate according to the first embodiment of the present invention. FIG. 5 is a plan view of a substrate for explaining a method for forming the same, FIG. 5 is a partially enlarged view of the substrate showing a movement path of a polishing pad in the processing region, and FIG. 6 is a diagram showing a relationship between polishing amounts in the processing region,
FIG. 7 is a graph showing the relationship of the machining amount with respect to the radial direction of the contact surface of the circular pad, and FIG. 8 is a graph showing the relationship of the machining amount with respect to the radial direction of the contact surface of the polygonal pad.
FIG. 10 is a cross-sectional view showing a modified example of the contact surface of the pad, FIG. 10 is a cross-sectional view of the pad eccentrically attached to the rotating shaft, FIG. 11 is a view showing the relationship between rotation and revolution of the pad, and FIG. FIG. 6A is a plan view of the substrate illustrating a method of defining a processing region on the substrate according to the second embodiment. 1 ... Base plate, 2 ... Table, 3 ... Sample stage, 6 ... Polishing tool unit, 8 ... Rotation axis, 10 ... Pad, 10a ... Contact surface, 13 ... Adjust block, 17
...... Control circuit unit, a, b …… Measurement points, A, B …… Processing area, P …… Board.

Claims (5)

[Claims] 1. A processing region having a plurality of measurement points formed by previously measuring the thickness of the thin film on a substrate on which a thin film to be polished is formed, each having a constant area centered on the measurement point. And a polishing pad having a diameter of a contact surface set to be smaller than the area of the processing region is moved along a zigzag path in the processing region, and On the substrate, the contact pressure of the contact surface against the thin film is changed according to the film thickness measurement value at the measurement point which is the center position of each processing area, and the polishing processing is sequentially performed for each processing area. Method for polishing thin film. 2. The method for polishing a thin film on a substrate according to claim 1, wherein the diameter of the contact surface of the pad is at least half the length of one side of the processing region. 3. The method of polishing a thin film on a substrate according to claim 1, wherein the contact surface of the pad is formed into a convex curved surface with respect to the surface of the thin film. 4. The method for polishing a thin film on a substrate according to claim 1, wherein the contact surface of the pad has a polygonal shape. 5. The pad is set such that the rotation angular velocity and the revolution angular velocity are opposite in direction and equal in size, and the rotation radius and the revolution radius are equal. The method for polishing a thin film on a substrate according to claim (1).