JP7477433B2 - Polishing Method - Google Patents

Description

The present invention relates to a method for polishing a workpiece such as a wafer, substrate, or panel, and in particular to a technique for determining the thickness of a layer to be polished based on the spectrum of reflected light from the workpiece.

The manufacturing process of semiconductor devices includes various steps such as polishing insulating films such as _SiO2 and polishing metal films such as copper and tungsten. The manufacturing process of back-illuminated CMOS sensors includes a step of polishing a silicon layer (silicon wafer) in addition to the step of polishing insulating films and metal films.

Polishing of the layer to be polished (insulating film, metal film, silicon layer, etc.) constituting the exposed surface of the workpiece is terminated when the thickness of the layer to be polished reaches a predetermined target value. An example of a method for measuring the thickness of the layer to be polished during polishing is an optical monitoring method in which light is irradiated onto the surface of the workpiece, a Fourier transform is performed on the spectral waveform of the light reflected from the workpiece, and the thickness is determined based on the peak of the obtained frequency spectrum (see, for example, Patent Document 1). The peak of the frequency spectrum changes depending on the thickness of the layer to be polished. Therefore, the thickness of the layer to be polished can be monitored by tracking the peak of the frequency spectrum while the workpiece is being polished.

JP 2013-110390 A

However, false peaks can appear in the frequency spectrum due to the polishing environment, such as the slurry, or due to a base layer that exists below the layer to be polished. Conventional optical monitoring methods can erroneously track such false peaks, resulting in an inability to determine the accurate thickness.

Furthermore, conventional optical monitoring methods can fail to determine the thickness of the layer being polished when false peaks caused by noise overlap with the target peaks corresponding to the thickness of the layer being polished.

The present invention aims to provide a polishing method that can accurately determine the thickness of the layer to be polished without being affected by noise.

In one reference example , there is provided a polishing method for polishing a layer to be polished of a workpiece, the polishing method including: rotating a polishing table supporting a polishing pad; pressing the workpiece against the polishing pad to polish the layer to be polished; irradiating the workpiece with light; receiving reflected light from the workpiece; measuring the intensity of the reflected light for each wavelength; generating a spectroscopic waveform showing the relationship between the intensity and the wavelength of the reflected light; performing Fourier transform processing on the spectroscopic waveform to generate a frequency spectrum; moving a peak search range of the frequency spectrum in accordance with polishing time; determining a peak of the frequency spectrum that is within the peak search range; and determining a thickness of the layer to be polished that corresponds to the determined peak.

In one reference example , the peak search range is a range that includes a value calculated based on the previously determined thickness of the layer to be polished and the polishing rate of the workpiece.
In one reference example , the polishing rate is a preset polishing rate.
In one embodiment , the polishing rate is a polishing rate calculated during polishing of the workpiece.

In one aspect, a polishing method for polishing a layer to be polished of a workpiece is provided, which includes rotating a polishing table supporting a polishing pad, pressing the workpiece against the polishing pad to polish the layer to be polished, irradiating the workpiece with light, receiving reflected light from the workpiece, measuring the intensity of the reflected light for each wavelength, generating a spectral waveform showing the relationship between the intensity and the wavelength of the reflected light, removing noise from the spectral waveform using a filter, performing a Fourier transform process on the spectral waveform from which the noise has been removed to generate a frequency spectrum, determining the thickness of the layer to be polished based on the peak of the frequency spectrum, and supplementing the thickness of the layer to be polished corresponding to the peak of the frequency spectrum that has disappeared due to the noise removal by extrapolation using multiple values of the thickness of the layer to be polished obtained during polishing of the workpiece.

In one aspect, the filter is configured to remove noise from the spectral waveform having a frequency of a peak in the frequency spectrum that does not move with the time that the workpiece is polished.
In one embodiment, the noise is caused by light reflected from an underlying layer of the layer to be polished.
In one embodiment, the filter is a bandstop filter.
In one embodiment, the polishing method further includes, before polishing the workpiece, polishing another workpiece having the same pattern as the workpiece, and creating the filter for removing noise contained in the spectral waveform of reflected light from the other workpiece.
In one embodiment, the polishing method further includes the step of creating the filter for removing noise contained in a spectral waveform of reflected light from the workpiece while the workpiece is being polished.

In one reference example, a polishing apparatus for polishing a layer to be polished of a workpiece is provided, the polishing apparatus comprising: a rotatable polishing table supporting a polishing pad; a polishing head pressing the workpiece against the polishing pad on the polishing table; an optical sensor head irradiating light onto the workpiece held by the polishing head and receiving reflected light from the workpiece; a spectrometer measuring the intensity of the reflected light for each wavelength; and a polishing control unit determining a thickness of the layer to be polished from the intensity of the reflected light, the polishing control unit being configured to generate a spectroscopic waveform showing the relationship between the intensity and the wavelength of the reflected light, perform Fourier transform processing on the spectroscopic waveform to generate a frequency spectrum, move a peak search range of the frequency spectrum in accordance with polishing time, determine a peak of the frequency spectrum within the peak search range, and determine a thickness of the layer to be polished corresponding to the determined peak.

In one reference example, there is provided a polishing apparatus for polishing a layer to be polished of a workpiece, the polishing apparatus comprising: a rotatable polishing table supporting a polishing pad; a polishing head pressing the workpiece against the polishing pad on the polishing table; an optical sensor head irradiating light onto the workpiece held by the polishing head and receiving reflected light from the workpiece; a spectrometer measuring the intensity of the reflected light for each wavelength; and a polishing control unit determining a thickness of the layer to be polished from the intensity of the reflected light, the polishing control unit being configured to generate a spectral waveform showing the relationship between the intensity and the wavelength of the reflected light, remove noise from the spectral waveform using a filter, perform Fourier transform processing on the spectral waveform from which the noise has been removed to generate a frequency spectrum, determine the thickness of the layer to be polished based on a peak in the frequency spectrum, and complement a thickness of the layer to be polished corresponding to a peak of the frequency spectrum that has disappeared due to the removal of the noise by extrapolation using a plurality of values of the thickness of the layer to be polished obtained during polishing of the workpiece.

In one reference example, a computer-readable recording medium is provided that has recorded thereon a program for causing a computer to execute the steps of issuing a command to a table motor to rotate a polishing table that supports a polishing pad, issuing a command to a polishing head to press a workpiece against the polishing pad to polish a layer of the workpiece to be polished, issuing a command to a light source to irradiate the workpiece with light, generating a spectral waveform that indicates the relationship between the intensity of reflected light from the workpiece and the wavelength of the reflected light, performing a Fourier transform process on the spectral waveform to generate a frequency spectrum, moving a peak search range of the frequency spectrum according to polishing time, determining a peak of the frequency spectrum that is within the peak search range, and determining the thickness of the layer to be polished that corresponds to the determined peak.

In one reference example, a computer-readable recording medium having a program recorded thereon is provided for causing a computer to execute the steps of: issuing a command to a table motor to rotate a polishing table supporting a polishing pad; issuing a command to a polishing head to press a workpiece against the polishing pad to polish a layer of the workpiece to be polished; issuing a command to a light source to irradiate the workpiece with light; generating a spectral waveform showing the relationship between the intensity of reflected light from the workpiece and the wavelength of the reflected light; removing noise from the spectral waveform using a filter; performing a Fourier transform process on the spectral waveform from which the noise has been removed to generate a frequency spectrum; determining the thickness of the layer to be polished based on the peak of the frequency spectrum; and supplementing the thickness of the layer to be polished corresponding to the peak of the frequency spectrum that has disappeared due to the noise removal by extrapolation using multiple values of the thickness of the layer to be polished obtained during polishing of the workpiece.

According to the present invention, the polishing method can determine the exact thickness of the layer to be polished without tracking false peaks caused by noise, by moving the search range for the peak in the frequency spectrum according to the polishing time.
Additionally, the polishing method can determine the exact thickness of the layer being polished by filtering the spectral waveform generated from the reflected light from the workpiece to remove noise.

FIG. 1 is a schematic diagram illustrating an embodiment of a polishing apparatus. FIG. 1 is a schematic diagram for explaining the principle of an optical film thickness measurement device. FIG. 2 is a plan view showing the positional relationship between the workpiece and the polishing table. FIG. 13 is a diagram showing an example of a spectral waveform generated by a polishing control unit. FIG. 13 is a diagram showing an example of a frequency spectrum generated by a dressing control unit. FIG. 13 is a diagram for explaining a peak search range in the Nth measurement. FIG. 13 is a diagram for explaining a peak search range in the (N+1)th measurement. FIG. 13 is a diagram for explaining how the peak search range is shifted in accordance with the polishing time. 11 is a flowchart illustrating an example of a process for determining the thickness of a layer to be polished by moving a peak search range. FIG. 13 is a diagram showing a frequency spectrum before filtering. FIG. 13 is a diagram showing a frequency spectrum after filtering. FIG. 13 is a diagram for explaining the extrapolation of a disappeared peak. 11 is a flowchart illustrating an example of a process for removing noise using a filter.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Fig. 1 is a schematic diagram showing one embodiment of a polishing apparatus. As shown in Fig. 1, the polishing apparatus includes a polishing table 3 supporting a polishing pad 2, a polishing head 1 pressing a workpiece W having a layer to be polished against the polishing pad 2, a table motor 6 rotating the polishing table 3, a polishing liquid supply nozzle 5 for supplying a polishing liquid such as a slurry onto the polishing pad 2, and a polishing control unit 49 for controlling the operation of the polishing apparatus. The upper surface of the polishing pad 2 constitutes a polishing surface 2a for polishing the workpiece W. Examples of the workpiece W include a wafer, a substrate, and a panel.

The polishing head 1 is connected to a head shaft 10, which is connected to a polishing head motor (not shown). The polishing head motor rotates the polishing head 1 together with the head shaft 10 in the direction indicated by the arrow. The polishing table 3 is connected to a table motor 6, which is configured to rotate the polishing table 3 and the polishing pad 2 in the direction indicated by the arrow. The polishing head 1, the polishing head motor, and the table motor 6 are connected to a polishing control unit 49.

The workpiece W is polished as follows. While the polishing table 3 and polishing head 1 are rotated in the direction shown by the arrow in Figure 1, polishing liquid is supplied from the polishing liquid supply nozzle 5 to the polishing surface 2a of the polishing pad 2 on the polishing table 3. While the workpiece W is rotated by the polishing head 1, the polishing head 1 presses the workpiece W against the polishing surface 2a of the polishing pad 2 with the polishing liquid present on the polishing pad 2. The surface of the workpiece W is polished by the chemical action of the polishing liquid and the mechanical action of the abrasive grains contained in the polishing liquid and the polishing pad 2.

The polishing apparatus is equipped with an optical film thickness measuring device 40 that measures the thickness of the layer to be polished of the workpiece W. The optical film thickness measuring device 40 is equipped with a light source 44 that emits light, a spectroscope 47, and an optical sensor head 7 connected to the light source 44 and the spectroscope 47. The light source 44 and the spectroscope 47 are connected to a polishing control unit 49. The optical sensor head 7, the light source 44, and the spectroscope 47 are attached to the polishing table 3 and rotate together with the polishing table 3 and the polishing pad 2. The position of the optical sensor head 7 is a position where it crosses the surface of the workpiece W on the polishing pad 2 every time the polishing table 3 and the polishing pad 2 rotate once.

The light emitted from the light source 44 is transmitted to the optical sensor head 7, and is irradiated from the optical sensor head 7 onto the surface of the workpiece W. The light is reflected by the workpiece W, and the reflected light from the workpiece W is received by the optical sensor head 7 and sent to the spectrometer 47. The spectrometer 47 breaks down the reflected light according to wavelength and measures the intensity of the reflected light at each wavelength. The reflected light intensity measurement data is sent to the polishing control unit 49.

The polishing control unit 49 is configured to generate a spectrum of the reflected light from the reflected light intensity measurement data. The spectrum of the reflected light is represented as a line graph (i.e., a spectral waveform) showing the relationship between the wavelength and intensity of the reflected light. The intensity of the reflected light can also be represented as a relative value such as reflectance or relative reflectance.

Figure 2 is a schematic diagram for explaining the principle of the optical film thickness measurement device 40, and Figure 3 is a plan view showing the positional relationship between the workpiece W and the polishing table 3. In the example shown in Figure 2, the workpiece W has a base layer and a layer to be polished formed thereon. The layer to be polished is, for example, a silicon layer or an insulating film.

The optical sensor head 7 is composed of the ends of a light-projecting optical fiber cable 31 and a light-receiving optical fiber cable 32, and is positioned opposite the surface of the workpiece W. The optical sensor head 7 irradiates light onto the workpiece W and receives reflected light from the workpiece W every time the polishing table 3 rotates once.

The light irradiated to the workpiece W is reflected at the interface between the medium (water in the example of FIG. 2) and the layer to be polished, and at the interface between the layer to be polished and the base layer, and the waves of light reflected at these interfaces interfere with each other. The manner in which the waves of light interfere varies depending on the thickness of the layer to be polished (i.e., the optical path length). Therefore, the spectrum generated from the reflected light from the workpiece W varies according to the thickness of the layer to be polished. The polishing control unit 49 performs Fourier transform processing (or fast Fourier transform processing) on the spectral waveform to generate a frequency spectrum, and determines the thickness of the layer to be polished based on the peak of the frequency spectrum. When the layer to be polished is a silicon layer and the medium is water as shown in FIG. 2, it is preferable to use light with a wavelength of 1100 nm or less to prevent the light from being absorbed by water.

During polishing of the workpiece W, the optical sensor head 7 moves across the workpiece W with each rotation of the polishing table 3. When the optical sensor head 7 is below the workpiece W, the light source 44 emits light. The light is transmitted through the light-projecting optical fiber cable 31 and is irradiated from the optical sensor head 7 onto the surface (surface to be polished) of the workpiece W. The reflected light from the workpiece W is received by the optical sensor head 7 and sent to the spectrometer 47 through the light-receiving optical fiber cable 32. The spectrometer 47 measures the intensity of the reflected light at each wavelength over a predetermined wavelength range and sends the reflected light intensity measurement data to the polishing control unit 49. The polishing control unit 49 generates a spectrum of the reflected light (i.e., a spectral waveform) representing the light intensity for each wavelength from the intensity measurement data.

Figure 4 shows an example of a spectral waveform generated by the polishing control unit 49. In Figure 4, the horizontal axis represents the wavelength of the reflected light from the workpiece W, and the vertical axis represents the relative reflectance derived from the intensity of the reflected light. Relative reflectance is an index that indicates the intensity of the reflected light, and is the ratio of the light intensity to a predetermined reference intensity. By dividing the light intensity (actual intensity) at each wavelength by the predetermined reference intensity, unnecessary noise such as variations in intensity inherent to the optical system of the device and the light source can be removed from the actual intensity.

The reference intensity is the intensity of light measured in advance for each wavelength, and the relative reflectance is calculated for each wavelength. Specifically, the relative reflectance is calculated by dividing the light intensity (actual intensity) at each wavelength by the corresponding reference intensity. The reference intensity is obtained, for example, by directly measuring the intensity of light irradiated from the optical sensor head 7, or by irradiating light from the optical sensor head 7 onto a mirror and measuring the intensity of the light reflected from the mirror. Alternatively, the reference intensity may be the intensity of reflected light from a silicon substrate measured by the spectroscope 47 when a silicon substrate (bare substrate) on which no film is formed is being polished in the presence of water on the polishing pad 2, or when the silicon substrate (bare substrate) is placed on the polishing pad 2.

ここで、λはワークピースＷから反射した光の波長であり、Ｅ（λ）は波長λでの強度であり、Ｂ（λ）は波長λでの基準強度であり、Ｄ（λ）は光を遮断した条件下で測定された波長λでの背景強度（ダークレベル）である。 In actual polishing, the corrected actual intensity is obtained by subtracting the dark level (background intensity obtained under conditions where light is blocked) from the actual intensity, and the corrected standard intensity is obtained by subtracting the dark level from the standard intensity, and the corrected actual intensity is divided by the corrected standard intensity to obtain the relative reflectance. Specifically, the relative reflectance R(λ) can be obtained using the following formula (1).

where λ is the wavelength of light reflected from the workpiece W, E(λ) is the intensity at wavelength λ, B(λ) is the reference intensity at wavelength λ, and D(λ) is the background intensity (dark level) at wavelength λ measured under light-blocking conditions.

The polishing control unit 49 issues a command to the light source 44 to generate light every time the polishing table 3 rotates. The optical sensor head 7 optically connected to the light source 44 irradiates light onto the surface (surface to be polished) of the workpiece W, and the optical sensor head 7 receives reflected light from the workpiece W. The reflected light is sent to a spectrometer 47 optically connected to the optical sensor head 7. The spectrometer 47 resolves the reflected light according to wavelength and measures the intensity of the reflected light at each wavelength. The reflected light intensity measurement data is sent to the polishing control unit 49, which generates a spectrum as shown in FIG. 4 from the reflected light intensity measurement data. In the example shown in FIG. 4, the spectrum of the reflected light is a spectral waveform showing the relationship between the relative reflectance and the wavelength of the reflected light, but the spectrum of the reflected light may also be a spectral waveform showing the relationship between the intensity of the reflected light itself and the wavelength of the reflected light.

The polishing control unit 49 performs Fourier transform processing (typically fast Fourier transform processing) on the obtained spectral waveform to analyze the spectral waveform. More specifically, the polishing control unit 49 performs Fourier transform processing (or fast Fourier transform processing) on the spectral waveform to extract the frequency components and their intensities contained in the spectral waveform, converts the obtained frequency components to the thickness of the layer to be polished using a predetermined relational expression, and generates a frequency spectrum that shows the relationship between the thickness of the layer to be polished and the intensity of the frequency components. The above-mentioned predetermined relational expression is a function that represents the thickness of the layer to be polished with the frequency components as variables, and can be obtained from actual measurement results, optical film thickness measurement simulations, theoretical expressions, etc.

Figure 5 is a diagram showing a frequency spectrum generated by the polishing control unit 49. In Figure 5, the vertical axis represents the intensity of the frequency components contained in the spectral waveform, and the horizontal axis represents the thickness of the layer to be polished. As can be seen from Figure 5, the frequency spectrum has a peak at thickness t1. In other words, this frequency spectrum indicates that the thickness of the layer to be polished is t1. In this way, the thickness of the layer to be polished is determined from the peak of the frequency spectrum.

The polishing control unit 49 includes a storage device 49a that stores a program for determining the thickness of the layer to be polished, and a processing device 49b that executes calculations according to instructions included in the program. The polishing control unit 49 is composed of at least one computer. The storage device 49a includes a main storage device such as a RAM, and an auxiliary storage device such as a hard disk drive (HDD) or a solid state drive (SSD). Examples of the processing device 49b include a CPU (central processing unit) and a GPU (graphic processing unit). However, the specific configuration of the polishing control unit 49 is not limited to these examples.

The polishing control unit 49 determines the polishing end point based on the determined thickness of the layer to be polished, and controls the operation of the polishing device. For example, the polishing control unit 49 determines the polishing end point, which is the point at which the determined thickness of the layer to be polished reaches a target value. In one embodiment, the polishing end point may be determined by measuring the combined thickness of the layer to be polished and the thickness of the undercoat layer. The polishing control unit for determining the thickness of the layer to be polished and the control unit for controlling the polishing operation of the workpiece W may be configured separately.

Figure 6 is a diagram illustrating the peak search range R1 in the Nth measurement. In Figure 6, the vertical axis represents the intensity of the frequency components contained in the spectral waveform, and the horizontal axis represents the thickness of the layer to be polished. Each of the multiple frequency spectra shown in Figure 6 was generated by the polishing control unit 49 every time the polishing table 3 rotates once during polishing of the workpiece W. Peak P1 is a peak that appears in the Nth measurement (when the polishing table 3 rotates N times), peak P2 is a peak that appears in the N+1th measurement (when the polishing table 3 rotates N+1 times), peak P3 is a peak that appears in the N+2th measurement (when the polishing table 3 rotates N+2 times), and peak P4 is a peak that appears in the N+3rd measurement (when the polishing table 3 rotates N+3 times). In Figure 6, peaks with small frequency component intensity are omitted. The symbol N is a natural number, for example, 1.

As shown in FIG. 6, the peak of the frequency spectrum moves as the polishing of the workpiece W progresses. The layer to be polished of the workpiece W is polished even while the polishing table 3 rotates once. Therefore, the thickness of the layer to be polished decreases with each rotation of the polishing table 3. The relationship between the thickness t1 corresponding to peak P1, the thickness t2 corresponding to peak P2, the thickness t3 corresponding to peak P3, and the thickness t4 corresponding to peak P4 is t1>t2>t3>t4.

The thickness of the layer to be polished is determined by the optical film thickness measuring device 40 based on the peak of the frequency spectrum each time the polishing table 3 rotates. However, due to pseudo peaks caused by noise (caused by the polishing environment such as slurry, caused by the underlayer, etc.) during polishing of the workpiece W, the polishing control unit 49 may erroneously determine the thickness of the layer to be polished. In FIG. 6, pseudo peak Pf1 is a peak caused by noise generated during polishing of the workpiece W, and is a pseudo peak that appears in the Nth measurement. In this case, the thickness tf1 corresponding to pseudo peak Pf1, which has a greater intensity than peak P1 in the Nth measurement, is erroneously determined as the thickness of the layer to be polished.

The polishing control unit 49 is configured to search for a peak in the frequency spectrum within the peak search range R1. In the example shown in FIG. 6, the polishing control unit 49 determines a peak P1 within the peak search range R1, and determines the thickness t1 (i.e., the exact thickness) corresponding to the determined peak P1 as the thickness of the layer to be polished. At this time, since the pseudo peak Pf1 is not within the peak search range R1, the pseudo peak Pf1 is not erroneously determined.

In FIG. 6, the peak search range R1 represents the peak search range in the Nth measurement. In one embodiment, the polishing control unit 49 determines the peak search range R1 based on the value of the initial thickness of the layer to be polished. For example, the peak search range R1 may be a range that includes the value of the initial thickness of the layer to be polished of the workpiece W. The initial thickness of the layer to be polished is the thickness of the layer to be polished before polishing of the workpiece W begins, and is measured in advance by a stand-alone film thickness measuring device (not shown) or is provided as basic information of the workpiece W.

Figure 7 is a diagram illustrating the peak search range R2 in the N+1th measurement. In Figure 7, peaks P1 to P4 are similar to the peaks described with reference to Figure 6. Pseudo peak Pf2 represents a peak caused by noise generated during polishing of the workpiece W, and is a pseudo peak that appeared during the N+1th measurement. Since this pseudo peak Pf2 is within the peak search range R1 in the previous Nth measurement, the polishing control unit 49 erroneously determines the thickness tf2 corresponding to pseudo peak Pf2 as the thickness of the layer to be polished.

Therefore, in this embodiment, the peak search range of the frequency spectrum is configured to move according to the polishing time. As described above, as the polishing of the workpiece W progresses, the thickness of the layer to be polished decreases, i.e., the peak of the frequency spectrum moves. By moving the peak search range to follow the change in the thickness of the layer to be polished, the polishing control unit 49 can accurately determine the thickness of the layer to be polished even if a false peak is present.

In FIG. 7, peak search range R2 represents the peak search range in the N+1th measurement. The polishing control unit 49 moves the peak search range so as to follow the thickness of the layer to be polished, which changes with the polishing time. In the N+1th measurement, the peak search range is moved from peak search range R1 in FIG. 6 to peak search range R2. For example, the peak search range R2 may be calculated based on the previously determined thickness t1 of the layer to be polished and the polishing rate of the workpiece W, as described below.

The polishing control unit 49 determines a peak P2 within the peak search range R2, and determines a thickness t2 (i.e., an accurate thickness) corresponding to the determined peak P2 as the thickness of the layer to be polished. At this time, since the pseudo peak Pf2 is not within the peak search range R2, the pseudo peak Pf2 is not erroneously determined. Similarly, in the (N+2)th and subsequent measurements, the peak search range is moved according to the polishing time, so that the accurate thickness of the layer to be polished can be determined.

Figure 8 is a diagram for explaining how the peak search range is moved according to the polishing time. In Figure 8, peaks P1 to P4, pseudo peaks Pf1 and Pf2, and peak search ranges R1 and R2 correspond to the same symbols as shown in Figures 6 and 7. The graph in Figure 8 shows the relationship between the polishing time and the thickness corresponding to each peak. Pseudo peak Pf3 represents a pseudo peak that appeared during the N+2th measurement. Pseudo peak Pf4 represents a pseudo peak that appeared during the N+3rd measurement. Peak search range R3 represents the peak search range that was moved during the N+2th measurement. Peak search range R4 represents the peak search range that was moved during the N+3rd measurement.

8, an initial value t0 represents the thickness of the layer to be polished before polishing the workpiece W. The initial value t0 is an initial thickness of the layer to be polished that is measured in advance before polishing the workpiece W, or a value given by a user. In one embodiment, a peak search range R1 at the Nth measurement is set based on the initial value t0. Specifically, the peak search range R1 is determined using the following formula (2).
Peak search range R1 = initial value t0 ± first set value X (2)
Here, the first set value X can be freely set by the user.

In one embodiment, the peak search range R2 is set to a range including a value calculated based on the previously determined thickness t1 of the layer to be polished and the polishing rate PR of the workpiece W. Specifically, the peak search range R2 is set using the following formula (3).
Peak search range R2=
(Previously determined thickness t1-(polishing rate PR x time interval dT))
± second set value Y (3)
Here, the polishing rate is also called a removal rate. The time interval dT corresponds to the time required for the polishing table 3 to make one rotation. Usually, the time interval dT is constant during polishing of the workpiece W. The polishing rate PR may be a polishing rate that is set in advance. The second set value Y can be freely set by the user. The second set value Y may be smaller than or equal to the first set value X.

The peak search ranges R3 and R4 are calculated using formula (3) in the same way as the peak search range R2. In this case, in formula (3), the polishing rate PR may be a polishing rate that has been set in advance, or may be a polishing rate calculated during the polishing of the workpiece W. The polishing rate can be calculated from multiple values of the thickness of the layer to be polished obtained during the polishing of the workpiece W and the polishing time required to obtain these multiple values.

FIG. 9 is a flow chart illustrating an example of a process for determining the thickness of a layer to be polished by moving a peak search range.
In step S<b>101 , the table motor 6 rotates the polishing table 3 supporting the polishing pad 2 .
In step S102, the polishing head 1 starts polishing the workpiece W by pressing the workpiece W against the polishing surface 2a of the polishing pad 2. At this time, as described with reference to Fig. 1, the workpiece W is pressed against the polishing surface 2a of the polishing pad 2 by the polishing head 1 while being rotated by the polishing head 1 with a polishing liquid present on the polishing pad 2.

Next, while the workpiece W is being polished, the thickness of the layer being polished is measured by the optical film thickness measuring device 40 .
In step S103, the light source 44 emits light and irradiates the light onto the surface of the workpiece W from the optical sensor head 7.
In step S104, the optical sensor head 7 receives the reflected light from the workpiece W.
In step S105, the spectroscope 47 measures the intensity of the reflected light from the workpiece W for each wavelength.
In step S106, the grinding controller 49 generates a spectral waveform from the intensity measurement data of the reflected light.
In step S107, the grinding controller 49 performs a Fourier transform process on the spectral waveform to generate a frequency spectrum.

In step S108, the dressing controller 49 moves the peak search range in accordance with the dressing time by calculating the peak search range using the method described with reference to FIG.
In step S109, the dressing controller 49 determines a peak of the frequency spectrum within the peak search range.
In step S110, the polishing control unit 49 determines the thickness of the layer to be polished that corresponds to the determined peak.

The polishing control unit 49 operates according to instructions included in a program electrically stored in the storage device 49a. That is, the polishing control unit 49 executes the steps of issuing a command to the table motor 6 to rotate the polishing table 3 supporting the polishing pad 2 (see step S101), issuing a command to the polishing head 1 to start polishing the workpiece W (see step S102), issuing a command to the light source 44 to irradiate the workpiece W with light (see step S103), generating a spectral waveform from the intensity measurement data of the reflected light from the workpiece W (see step S106), performing Fourier transform processing on the spectral waveform to generate a frequency spectrum (see step S107), moving the peak search range according to the polishing time (see step S108), determining the peak of the frequency spectrum within the peak search range (see step S109), and determining the thickness of the layer to be polished corresponding to the determined peak (see step S110).

The program for causing the grinding control unit 49 to execute these steps is recorded on a computer-readable recording medium, which is a non-transitory tangible item, and is provided to the grinding control unit 49 via the recording medium. Alternatively, the program may be input to the grinding control unit 49 via a communication network such as the Internet or a local area network.

Next, an embodiment will be described in which a filter is used to remove noise, thereby accurately determining the thickness of the layer being polished.
FIG. 10 is a diagram showing a frequency spectrum before filtering. In FIG. 10, the vertical axis represents the intensity of the frequency components contained in the spectral waveform, and the horizontal axis represents the thickness of the layer to be polished. Each of the multiple frequency spectra shown in FIG. 10 is generated by the polishing control unit 49 every time the polishing table 3 rotates once during polishing of the workpiece W. Peak P1 is a peak that appears in the Nth measurement (when the polishing table 3 rotates N times), peak P2 is a peak that appears in the N+1th measurement (when the polishing table 3 rotates N+1 times), peak P3 is a peak that appears in the N+2th measurement (when the polishing table 3 rotates N+2 times), and peak P4 is a peak that appears in the N+3rd measurement (when the polishing table 3 rotates N+3 times). In FIG. 10, peaks with low intensity of frequency components are omitted. The symbol N is a natural number, for example, 1.

The thickness of the layer to be polished is determined by the optical film thickness measuring device 40 based on the peak of the frequency component every time the polishing table 3 rotates. However, due to the influence of noise caused by the base layer existing under the layer to be polished, the thickness of the layer to be polished may not be determined accurately. In FIG. 10, the base layer peak Pu is a peak due to reflected light from the base layer of the layer to be polished, and is unnecessary noise in determining the thickness of the layer to be polished. Since the base layer is not polished during polishing of the workpiece W, the position of the base layer peak Pu does not change even if the polishing of the layer to be polished progresses. In the example shown in FIG. 10, the base layer peak Pu appears at a position overlapping with peak P3, so the polishing control unit 49 cannot correctly determine the thickness of the layer to be polished corresponding to peak P3.

Therefore, in this embodiment, the polishing control unit 49 uses a filter to remove noise caused by the base layer from the spectral waveform of the reflected light from the workpiece W, and then determines the thickness of the layer to be polished by extrapolating the thickness of the layer to be polished that corresponds to the peak of the frequency spectrum that has disappeared due to the noise removal.

A filter for removing noise from the spectral waveform of the reflected light is created in advance as follows. Before polishing the workpiece W, another workpiece W' having the same pattern as the workpiece W is polished. During polishing of the other workpiece W', the optical film thickness measuring device 40 measures the thickness of the layer to be polished. The polishing control unit 49 identifies the lower peak, which is noise, based on the frequency spectrum generated from the spectral waveform of the reflected light from the other workpiece W', and creates a filter for removing the identified noise. Specifically, the polishing control unit 49 identifies a peak in the frequency spectrum that does not move with the polishing time of the other workpiece W', and creates a filter for removing the component (noise) having the frequency of the identified peak from the spectral waveform of the reflected light. This filter is a digital filter, and as an example, a band-stop filter.

In one embodiment, the polishing control unit 49 may create a filter by identifying a lower peak that is noise in the early stages of polishing the workpiece W, without using another workpiece W' that has the same pattern as the workpiece W. Specifically, during polishing of the workpiece W, the polishing control unit 49 identifies a peak in the frequency spectrum that does not move with polishing time, and creates a filter that removes components (noise) having the frequency of the identified peak from the spectral waveform of the reflected light.

Figure 11 is a diagram showing the frequency spectrum after filtering. The polishing control unit 49 applies a filter created in advance to the spectral waveform of the reflected light from the workpiece W to remove noise. The multiple frequency spectra shown in Figure 11 are frequency spectra generated by the polishing control unit 49 by performing a Fourier transform process (or a fast Fourier transform process) on the spectral waveform after removing noise. By removing the noise from the spectral waveform of the reflected light, the lower layer peak Pu as well as peak P3, which is located at a position overlapping with the lower layer peak Pu, disappear. Therefore, the polishing control unit 49 cannot accurately determine the thickness of the layer to be polished corresponding to peak P3.

The polishing control unit 49 is configured to complement the thickness of the polishing target layer corresponding to the peak P3 by extrapolation. FIG. 12 is a diagram for explaining the extrapolation of the disappeared peak P3. In FIG. 12, the peaks P1 to P4 correspond to the same reference numerals shown in FIG. 10 and FIG. 11, and the graph shown in FIG. 12 shows the relationship between the polishing time and the thickness corresponding to each peak. As described with reference to FIG. 11, when the peak P3 disappears due to the removal of the lower layer peak Pu, which is a noise, the polishing control unit 49 complements the thickness of the polishing target layer corresponding to the disappeared part of the peak P3 by extrapolation from the thickness values of the polishing target layer corresponding to the peaks P1 and P2. Although the case where the peak P3 disappears is described in FIG. 10 to FIG. 12, this is not limited to this. If multiple values of the thickness of the polishing target layer are acquired during polishing of the workpiece W, the thickness of the polishing target layer can be complemented by extrapolation using the acquired multiple values.

In the above-described embodiment, the peak (noise) that does not change with polishing time is due to the underlying layer of the layer to be polished, but the present invention is not limited to this embodiment. For example, the peak (noise) that does not change with polishing time may be noise inherent to the device, such as the light source 44 or the spectrometer 47.

FIG. 13 is a flow chart illustrating an example of a process for removing noise using a filter.
In step S201, the polishing apparatus polishes another workpiece W' having the same pattern as the workpiece W to be polished.
In step S202, the polishing control unit 49 measures the thickness of the layer to be polished by the optical film thickness measurement device 40 while polishing the other workpiece W'. The polishing control unit 49 performs a Fourier transform on the spectral waveform obtained from the reflected light from the other workpiece W' to generate a frequency spectrum. Steps S201 to S202 are similar to steps S101 to S107 in FIG.
In step S203, the dressing controller 49 identifies a peak in the frequency spectrum that does not change with the dressing time.
In step S204, the polishing control unit 49 creates a filter that removes noise having the identified peak frequency from the spectral waveform of the reflected light from the other workpiece W'.

In step S205, the polishing apparatus starts polishing the workpiece W to be polished.
In step S206, the optical film thickness measurement device 40 measures the thickness of the layer to be polished while the workpiece W is being polished. The polishing control unit 49 uses the filter created in step S204 to remove noise from the spectral waveform of the reflected light from the workpiece W. The steps from S205 to S206 up to the generation of the spectral waveform are similar to the steps S101 to S106 in FIG.
In step S207, the grinding controller 49 performs Fourier transform processing on the noise-removed spectral waveform to generate a frequency spectrum.
In step S208, the polishing control unit 49 determines the thickness of the layer to be polished based on the peak of the frequency spectrum.
In step S209, the polishing control unit 49 complements, by extrapolation, the thickness of the layer to be polished that corresponds to the peak of the frequency spectrum that has disappeared due to the noise removal.

The polishing control unit 49 operates according to instructions included in a program electrically stored in the storage device 49a. That is, the polishing control unit 49 executes the steps of issuing a command to the table motor 6 to rotate the polishing table 3 supporting the polishing pad 2, issuing a command to the polishing head 1 to start polishing the workpiece W (see step S205), issuing a command to the light source 44 to irradiate the workpiece W with light, generating a spectral waveform from the intensity measurement data of the reflected light from the workpiece W, removing noise from the spectral waveform using a filter (see step S206), performing a Fourier transform process on the noise-removed spectral waveform to generate a frequency spectrum (see step S207), determining the thickness of the layer to be polished from the peak of the frequency spectrum (see step S208), and supplementing the thickness of the layer to be polished corresponding to the peak of the frequency spectrum that has disappeared due to the noise removal by extrapolation (see step S209).

The program for causing the grinding control unit 49 to execute these steps is recorded on a computer-readable recording medium, which is a non-transitory tangible item, and is provided to the grinding control unit 49 via the recording medium. Alternatively, the program may be input to the grinding control unit 49 via a communication network such as the Internet or a local area network.

The embodiment described in Figures 6 to 9 and the embodiment described in Figures 10 to 13 may be combined. That is, a filter may be used to remove noise from the spectral waveform to generate a frequency spectrum, and the peak search range of the frequency spectrum may be moved to determine the thickness of the layer to be polished.

The above-described embodiments have been described for the purpose of enabling a person having ordinary knowledge in the technical field to which the present invention pertains to practice the present invention. Various modifications of the above-described embodiments would naturally be possible for a person skilled in the art, and the technical ideas of the present invention may also be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be interpreted in the broadest scope in accordance with the technical ideas defined by the scope of the claims.

REFERENCE SIGNS LIST 1 Polishing head 2 Polishing pad 2a Polishing surface 3 Polishing table 5 Polishing liquid supply nozzle 6 Table motor 7 Optical sensor head 10 Head shaft 31 Light projecting optical fiber cable 32 Light receiving optical fiber cable 40 Optical film thickness measuring device 44 Light source 47 Spectrometer 49 Polishing control unit 49a Storage device 49b Processing device P1, P2, P3, P4 Peaks Pf1, Pf2, Pf3, Pf4 Pseudo peak Pu Lower layer peaks R1, R2, R3, R4 Peak search range

Claims (6)

1. A polishing method for polishing a layer to be polished of a workpiece, comprising:
Rotating a polishing table supporting the polishing pad;
Pressing the workpiece against the polishing pad to polish the layer to be polished;
Irradiating the workpiece with light;
receiving light reflected from the workpiece;
measuring the intensity of the reflected light for each wavelength;
generating a spectral waveform indicating a relationship between the intensity and the wavelength of the reflected light;
removing noise from the spectral waveform using a filter;
performing a Fourier transform process on the spectral waveform from which the noise has been removed to generate a frequency spectrum;
determining a thickness of the layer being polished based on the peaks in the frequency spectrum;
A polishing method in which the thickness of the layer to be polished corresponding to the peak of the frequency spectrum that has disappeared due to the removal of the noise is supplemented by extrapolation using multiple values of the thickness of the layer to be polished obtained during polishing of the workpiece. The polishing method of claim 1 , wherein the filter is configured to remove noise from the spectral waveform having a frequency of a peak in the frequency spectrum that does not move with polishing time of the workpiece. 3. The polishing method according to claim 1 , wherein the noise is caused by light reflected from an underlying layer of the layer to be polished. The polishing method according to claim 1 , wherein the filter is a band-stop filter. prior to polishing the workpiece, polishing another workpiece having the same pattern as the workpiece;
The polishing method according to claim 1 , further comprising the step of creating the filter for removing noise contained in a spectral waveform of the reflected light from the other workpiece. 5. The polishing method according to claim 1 , further comprising the step of creating the filter for removing noise contained in a spectral waveform of the reflected light from the workpiece while the workpiece is being polished.

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