JP2012004276A - Polishing method and polishing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polishing method and a polishing device, capable of accurately detecting a polishing end point of a semiconductor layer such as a silicon layer, using a visible light ray.SOLUTION: The polishing method includes: while polishing the semiconductor layer, irradiating the semiconductor layer with visible light; receiving reflection light from the semiconductor layer; measuring intensity of the reflection light in a predetermined wavelength range; calculating a relative reflectivity by dividing the measured intensity value by predetermined reference intensity; generating a spectrum indicative of relationship between the relative reflectivity and the reflection light wavelength; from the spectrum, obtaining a polishing index that varies with the thickness of the semiconductor layer; and based on a time point when the polishing index reaches a predetermined threshold, completing polishing of the semiconductor layer.

Description

  The present invention relates to a method and an apparatus for polishing a semiconductor layer made of a semiconductor material such as silicon, and more particularly to a polishing method and an apparatus for detecting a polishing end point of a semiconductor layer based on a spectrum of reflected light from the semiconductor layer.

The semiconductor device manufacturing process includes various steps such as a step of polishing an insulating film such as SiO ₂ and a step of polishing a metal film such as copper and tungsten. The manufacturing process of the back-illuminated CMOS sensor includes a process of polishing a silicon layer (silicon substrate) that forms a light receiving surface, in addition to a process of polishing an insulating film or a metal film. A back-illuminated CMOS sensor is a kind of image sensor, and a photodiode is embedded in a silicon layer. Then, the distance between the light receiving surface and the photodiode is adjusted by polishing the silicon layer.

FIG. 1 is a cross-sectional view schematically showing a back-illuminated CMOS sensor. As shown in FIG. 1, a photodiode 2 and a silicon layer 1 are formed on a wiring structure 6 including a metal wiring (for example, Cu wiring) 4 and an insulating film (for example, SiO ₂ film) 5. . The silicon layer 1 is typically made of silicon containing a trace amount of impurities such as boron and phosphorus. The photodiode 2 is embedded in the silicon layer 1, and the exposed surface of the silicon layer 1 becomes the light receiving surface 1a. As can be seen from FIG. 1, the back-illuminated CMOS sensor has a feature that the photodiode 2 can receive light efficiently because the wiring structure 6 does not exist between the light receiving surface 1a and the photodiode 2.

  In the back-illuminated CMOS sensor, the distance from the light receiving surface 1a of the silicon layer 1 to the photodiode 2 is important. Typically, after the silicon layer 1 is mechanically ground, the silicon layer 1 is further polished using CMP (Chemical Mechanical Polishing) to adjust the distance between the light receiving surface 1 a and the photodiode 2. Therefore, in CMP, it is important to finish polishing when the silicon layer 1 reaches a predetermined thickness.

  In CMP of a silicon layer, polishing of the silicon layer is generally terminated when a predetermined polishing time has elapsed. This polishing time is determined by measuring the time required for the silicon wafer to reach a predetermined thickness by polishing a sample wafer of the same kind in advance. However, this method has a problem that the thickness of the silicon layer after polishing is not stable because it is affected by a change in polishing rate over time due to abrasion of the polishing pad.

  As another polishing end point detection method, as shown in Patent Document 1, a method of optically detecting a polishing end point based on reflected light from a substrate is known. According to this method, light is applied to the silicon layer during polishing of the silicon layer, a spectrum is generated from the reflected light from the silicon layer, and the polishing end point is detected based on the change in the spectrum.

However, the silicon layer has a higher refractive index than an insulating film such as SiO ₂ (about _2.5 to 4 times that of SiO ₂ depending on the wavelength band of light), and is difficult to transmit light. have. Although it is possible to use light having a long wavelength with good transparency, for example, infrared rays, a polishing end point detection device newly designed for infrared rays is required. For example, it is necessary to use a spectroscope diffraction grating and a filter designed for infrared rays, and further, an infrared optical fiber and a photodetector are required, resulting in an increase in apparatus cost. Further, such a polishing end point detection device dedicated to the silicon layer is not suitable for detecting the polishing end point of an insulating layer having a small refractive index or film thickness.

Japanese Patent Laid-Open No. 2004-154928

  The present invention has been made in view of the above-described conventional problems, and provides a polishing method and a polishing apparatus capable of accurately detecting the polishing end point of a semiconductor layer such as a silicon layer using visible light. Objective.

  In order to achieve the above-described object, one embodiment of the present invention is a method for polishing a substrate having a semiconductor layer, in which the semiconductor layer is irradiated with visible light during polishing of the semiconductor layer, and the semiconductor layer is irradiated with visible light. The reflected light is received, the intensity of the reflected light in a predetermined wavelength range is measured, the relative reflectance is calculated by dividing the measured value of the intensity by the predetermined reference intensity, and the relative reflectance and the reflected light. A spectrum indicating a relationship with the wavelength of the semiconductor layer is generated, and a polishing index that varies according to the thickness of the semiconductor layer is obtained from the spectrum, and the semiconductor layer is measured based on a point in time when the polishing index reaches a predetermined threshold value. Polishing is terminated.

In a preferred aspect of the present invention, an upper limit value of the predetermined wavelength range is 800 nm or less.
In a preferred aspect of the present invention, the polishing index is a relative reflectance at a predetermined wavelength in the range of 650 nm to 800 nm.
In a preferred aspect of the present invention, the polishing index is a moving average of relative reflectance at the predetermined wavelength.

In a preferred aspect of the present invention, the polishing index is such that the first relative reflectance at the first wavelength in the range of 650 nm to 800 nm and the second relative at the second wavelength in the range of 300 nm to 550 nm. A characteristic value calculated from a plurality of relative reflectances including at least the reflectance, wherein the characteristic value is a sum of the plurality of relative reflectances obtained by selecting one relative reflectance selected from the plurality of relative reflectances. It is obtained by dividing by.
In a preferred aspect of the present invention, the polishing index is a moving average of the characteristic values.

In a preferred aspect of the present invention, the method further includes a step of smoothing the spectrum, and the polishing index is obtained from the smoothed spectrum.
In a preferred aspect of the present invention, the semiconductor layer is a silicon layer constituting a light receiving surface of a backside illumination type CMOS sensor.

  Another aspect of the present invention is an apparatus for polishing a substrate having a semiconductor layer, wherein a light projecting unit that irradiates the semiconductor layer with visible light during polishing of the semiconductor layer, and reflected light from the semiconductor layer. A light receiving unit that receives light, a spectroscope that measures the intensity of the reflected light in a predetermined wavelength range, calculates a relative reflectance by dividing the measured value of the intensity by a predetermined reference intensity, and the relative reflectance and the A processing device that generates a spectrum indicating a relationship with the wavelength of the reflected light, the processing device obtains a polishing index that varies according to the thickness of the semiconductor layer from the spectrum, and the polishing index is a predetermined threshold. A polishing end point of the semiconductor layer is determined based on a time point when the value is reached.

  According to the present invention, since the polishing end point can be detected using visible light, the polishing end point detection device used for polishing a metal film or an insulating film can be used as it is. In addition, according to the present invention, since the polishing end point is detected based on the spectrum of reflected light from the semiconductor layer, not the polishing time, accurate polishing end point detection can be performed without being affected by abrasion of the polishing pad. Realized.

It is sectional drawing which shows a back surface illumination type CMOS sensor typically. FIG. 2A is a schematic diagram for explaining the principle of polishing end point detection according to an embodiment of the present invention, and FIG. 2B is a plan view showing the positional relationship between the substrate and the polishing table. . FIG. 3A to FIG. 3C are graphs showing the spectrum of reflected light obtained by polishing the substrate having the structure shown in FIG. It is a graph which shows the time change during grinding | polishing of the relative reflectance in wavelength 600nm shown in Fig.3 (a) thru | or FIG.3 (c). It is a graph which shows the time change during grinding | polishing of the relative reflectance in wavelength 750nm shown in Fig.3 (a) thru | or FIG.3 (c). It is a graph which shows the change along the time-axis of the characteristic value calculated using the some relative reflectance in the spectrum shown to Fig.3 (a) thru | or FIG.3 (c). It is a graph which shows the spectrum obtained by applying a numerical filter to the spectrum shown in FIG.3 (c). It is a graph which shows the time change during grinding | polishing of the relative reflectance in wavelength 750nm on the spectrum after the numerical filter application shown in FIG. It is a graph which shows the time change of the relative reflectance in wavelength 750nm when the board | substrate which has the dispersion | variation in the thickness of a silicon layer in the range of +/- 700nm is grind | polished. It is a graph which shows the time change during grinding | polishing of a relative reflectance when using the spectroscope whose wavelength resolution is about 10 nm. It is sectional drawing which shows typically the grinding | polishing apparatus provided with the grinding | polishing end point detection apparatus. It is sectional drawing which shows the modification of the grinding | polishing apparatus shown in FIG.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 2A is a schematic diagram for explaining the principle of polishing end point detection according to an embodiment of the present invention, and FIG. 2B is a plan view showing the positional relationship between the substrate and the polishing table. . The substrate W to be polished is a substrate on which the back-illuminated CMOS sensor shown in FIG. 1 is formed, and the silicon layer 1 is an object to be polished. The thickness of the silicon layer 1 before polishing is several μm and is usually 5 μm or more. The surface of the substrate W (that is, the light receiving surface 1a of the silicon layer 1) is pressed against the polishing pad 22 on the rotating polishing table 20, and the silicon layer 1 of the substrate W is polished by sliding contact with the polishing pad 22. During polishing of the substrate W, a polishing liquid (slurry) is supplied onto the polishing pad 22.

  The light projecting unit 11 and the light receiving unit 12 are arranged to face the surface of the substrate W. The light projecting unit 11 emits light substantially perpendicular to the surface of the substrate W, and the light receiving unit 12 receives reflected light from the substrate W. The light emitted from the light projecting unit 11 is visible light. In order to prevent the polishing liquid from entering the light path, a flow of pure water is formed in the space between the light projecting unit 11 and the light receiving unit 12 and the substrate W.

  As shown in FIG. 2B, light is irradiated to a plurality of measurement points including the center of the substrate W every time the polishing table 20 rotates once. A spectrometer 13 is connected to the light receiving unit 12. The spectroscope 13 decomposes the reflected light according to the wavelength, and measures the intensity of the reflected light for each wavelength. The wavelength range that can be measured by the spectroscope 13 is 400 nm to 800 nm, but the present invention is not limited to this, and may be, for example, 300 nm to 800 nm.

  A processing device 15 is connected to the spectrometer 13. The processing device 15 reads the measurement data acquired by the spectroscope 13 and generates an intensity distribution of reflected light from the intensity measurement value. More specifically, the processing device 15 generates a spectrum representing the light intensity for each wavelength. This spectrum can be represented as a line graph showing the relationship between the wavelength and intensity of the reflected light. The processing device 15 is further configured to monitor the progress of polishing from the change in spectrum and determine the polishing end point. As the processing device 15, a general purpose or dedicated computer can be used. The processing device 15 executes predetermined processing steps by a program (or computer software).

  FIG. 3A to FIG. 3C are graphs showing the spectrum of reflected light obtained by polishing the substrate having the structure shown in FIG. 3A to 3C, the horizontal axis represents the wavelength (nm) of light, and the vertical axis represents the relative reflectance (%) calculated from the light intensity. The relative reflectance is an index representing the intensity of light, and specifically, is a ratio of the intensity of reflected light to a predetermined reference intensity (or a ratio between the intensity of reflected light and a predetermined reference intensity). By dividing the intensity of the reflected light (measured intensity) by a predetermined reference intensity, the intensity of the light from which the noise component has been removed can be obtained. The predetermined reference intensity is, for example, the intensity of reflected light obtained when polishing a silicon wafer on which a film is not formed (that is, a bare silicon wafer) while supplying pure water onto the polishing pad. Can do. This bare silicon wafer has a sufficient thickness that does not allow light to pass through. For example, the bare silicon wafer preferably has a thickness of several hundred μm.

The processing device 15 calculates the relative reflectance from the measurement value obtained from the spectroscope 13. Here, an example of how to obtain the relative reflectance will be described. The relative reflectance R (λ) can be obtained using the following equation.
R (λ) = {E (λ) −D (λ)} / {B (λ) −D (λ)} × 100 (1)
Here, λ is the wavelength of light, E (λ) is the intensity (measured value) of the reflected light, B (λ) is the reference intensity, and D (λ) is obtained in the absence of light. Background intensity (dark level). Thus, the relative reflectance is an index indicating the intensity of reflected light using the wavelength as a parameter. This relative reflectance varies according to the thickness of the silicon layer, as will be described below. Therefore, the relative reflectance can also be referred to as an index indicating the progress of polishing, that is, a polishing index. As an example, the intensity of the reflected light itself may be used as a polishing index without using the relative reflectance.

  3A shows a spectrum immediately after the start of polishing, FIG. 3B shows a spectrum at the middle of polishing, and FIG. 3C shows a spectrum immediately before the end of polishing. As can be seen from FIGS. 3A to 3C, the relative reflectivity hardly changes in the region of wavelength of 600 nm or less (more precisely, 550 nm or less), indicating almost 100% during polishing. Yes. This means that light hardly transmits through the silicon layer in the region of wavelength of 600 nm or less. On the other hand, in the region having a wavelength of 650 nm or more, the relative reflectance increases with the progress of polishing (that is, the decrease in the thickness of the silicon layer). This means that as the thickness of the silicon layer decreases, light is transmitted through the silicon layer and reflected light from the wiring structure under the silicon layer increases. This tendency of increasing the relative reflectance becomes more prominent as the wavelength becomes longer.

  In the spectrum, two wavy interference components due to interference between reflected lights appear. One of the two interference components is an interference component having a small waveform, and the interval between adjacent local maximum points (or local minimum points) is 30 nm or less. The other interference components are interference components having a large waveform as shown in FIG. 3C relatively clearly, and the interval between adjacent maximum points (or minimum points) is about 100 nm. The small interference component is mainly caused by the interference between the reflected light from the upper surface of the silicon layer and the reflected light from the lower surface of the silicon layer (the upper surface of the wiring structure), while the large interference component is This is considered to be caused by interference between reflected lights at the upper and lower interfaces of the wiring structure (insulating material + wiring) under the silicon layer.

  FIG. 4 is a graph showing the time change during polishing of the relative reflectance at a wavelength of 600 nm shown in FIGS. 3 (a) to 3 (c). In FIG. 4, the vertical axis represents relative reflectance (%), and the horizontal axis represents polishing time (seconds). As described above, each time the polishing table 20 rotates once, a plurality of measurement values corresponding to a plurality of regions on the substrate W are acquired. In this embodiment, the processing apparatus 15 calculates | requires several relative reflectance from the intensity | strength of the reflected light in the several measurement point (refer FIG.2 (b)) acquired whenever the grinding | polishing table 20 makes one rotation, Calculate the average. Note that an average of measured values (intensities of reflected light) at a plurality of measurement points may be calculated, and the relative reflectance may be obtained from the average. As can be seen from the graphs of FIGS. 3A to 3C, the silicon layer hardly transmits light in the region of wavelength of 600 nm or less, so that the relative reflectance is substantially constant during polishing.

  FIG. 5 is a graph showing the time change during polishing of the relative reflectance at a wavelength of 750 nm shown in FIGS. 3 (a) to 3 (c). In FIG. 5, the vertical axis represents relative reflectance (%), and the horizontal axis represents polishing time (seconds). The dotted line in FIG. 5 indicates the relative reflectance itself, and the solid line indicates a simple moving average of the relative reflectance. This simple moving average is the average of the relative reflectances acquired during the last 10 seconds. In this embodiment, the moving average period is 10 seconds, but can be changed as appropriate.

  As shown by the dotted line in FIG. 5, the relative reflectance periodically varies with the lapse of the polishing time due to the above-described interference component. The moving average process is performed to smooth the fluctuating relative reflectance. As shown by the solid line in FIG. 5, the moving average of the relative reflectance increases monotonically with the polishing time.

The period of the moving average is preferably determined according to the period of variation along the time axis of the relative reflectance, that is, the interval between adjacent local maximum points or local minimum points. Specifically, the moving average period can be determined as follows. The approximate polishing rate (ie, removal rate) of the silicon layer is v, the refractive index of the silicon layer at the wavelength λ is n, the period of variation along the time axis of the relative reflectance is T, and the thickness of the silicon layer corresponding to the period T When the difference is Δθ, the following equations (2) and (3) are obtained.
Δθ = λ / 2n (2)
T = Δθ / v = λ / 2nv (3)

  The moving average period is set based on the period T obtained from the above equation (3). For example, a substantially integer multiple of the period T is determined as the moving average period. As described above, when the moving average period is set to be larger than the period T, and preferably approximately an integral multiple of the period T, the relative reflectance is smoothed as shown by the solid line in FIG. A moving average of monotonically increasing relative reflectance is obtained. Since the moving average of relative reflectance monotonously increases with the polishing time, the moving average of relative reflectance can be used as an index indicating the progress of polishing of the silicon layer, that is, a polishing index.

  The processing device 15 calculates the moving average of the relative reflectance during polishing of the silicon layer, and determines the polishing end point of the silicon layer based on the time when the moving average of the relative reflectance reaches a predetermined threshold value. The polishing end point may be a time when the moving average of the relative reflectance reaches a predetermined threshold value, or a predetermined overpolish time elapses from the time when the moving average of the relative reflectance reaches the predetermined threshold value. It may be the time when This over-polishing time is obtained by polishing a sample substrate of the same type as that of the substrate W, the polishing time of the sample substrate, data of relative reflectance at the end of polishing, and the thickness of the silicon layer before and after polishing. It can be determined from the actual measured value. Specifically, the polishing rate (removal rate) is obtained from the measured value of the thickness of the silicon layer before and after polishing and the polishing time, and the moving average of the relative reflectance at the end of polishing is set as a threshold value. The overpolish time can be determined from the difference between the target thickness and the measured value of the thickness of the polished silicon layer and the polishing rate.

FIG. 6 is a graph showing changes along the time axis of characteristic values calculated using a plurality of relative reflectances in the spectra shown in FIGS. 3 (a) to 3 (c). This characteristic value is also a polishing index indicating the progress of polishing of the silicon layer. This characteristic value S is obtained from the two relative reflectances at different wavelengths using the following equation.
S (λ1, λ2) = R (λ1) / {R (λ1) + R (λ2)} (4)
In the example shown in FIG. 6, the first wavelength λ1 is 750 nm, and the second wavelength λ2 is 500 nm. The dotted line in FIG. 6 shows the characteristic value itself obtained from the above equation (4), and the solid line shows the simple moving average of the characteristic value.

  As shown in FIG. 5, the relative reflectance at a wavelength of 750 nm increases with the polishing time. On the other hand, the relative reflectance at a wavelength of 500 nm is substantially constant during polishing as described above. Therefore, the characteristic value obtained from the combination of the relative reflectance at the wavelength of 750 nm and the relative reflectance at the wavelength of 500 nm fluctuates with the same period as the relative reflectance at the wavelength of 750 nm during the polishing, and has the same increasing tendency. Show. Thus, a characteristic value that changes according to the polishing time (that is, the change in the thickness of the silicon layer) is obtained from the combination of the relative reflectance that is substantially constant during polishing and the relative reflectance that increases during polishing. . Therefore, the processing apparatus 15 may be configured to detect the polishing end point based on this characteristic value. In this case, the polishing end point may be the time when the characteristic value reaches the predetermined threshold value, or the polishing end point is determined when the predetermined overpolish time has elapsed from the time when the characteristic value has reached the predetermined threshold value. It is good.

  As can be seen from the above equation (4), the characteristic value is obtained by dividing the relative reflectance by the relative reflectance. Therefore, the noise component included in the numerator of Expression (4) and the noise component included in the denominator are canceled, and a stable polishing index can be obtained. Examples of the noise component include a sudden decrease in the amount of received light due to bubbles existing between the polishing pad 22 and the substrate W, and an increase in the amount of received light due to wear of the polishing pad 22. Such a noise component is an unnecessary component in monitoring the polishing of the silicon layer. According to the above formula (4), it is possible to obtain a characteristic value from which the noise component is removed by dividing the relative reflectance by the relative reflectance.

The characteristic value represented by Expression (4) is an index indicating the intensity of reflected light using the wavelength of light as a parameter. As shown in FIG. 6, this characteristic value tends to increase with the polishing time. The number of wavelengths as parameters used for calculating the characteristic value is not limited to two, and may be three or more. That is, the characteristic value is calculated from a plurality of relative reflectances at a plurality of wavelengths. When a plurality of relative reflectivities at wavelengths λ1, λ2,... ΛN are used, the above equation (4) is expressed as follows.
S (λ1, λ2, ..., λN)
= R (λ1) / {R (λ1) + R (λ2) +... + R (λN)} (5)

  A plurality of wavelengths corresponding to a plurality of relative reflectances used for calculating the characteristic value are selected as follows. The plurality of wavelengths includes at least one wavelength longer than 600 nm, preferably 650 nm or more, and further includes at least one wavelength of 600 nm or less, preferably 550 nm or less. For example, the plurality of wavelengths includes at least one wavelength selected from the range of 650 nm to 800 nm and at least one wavelength selected from the range of 300 nm to 550 nm. As described above, the plurality of wavelengths to be selected is composed of a combination of a wavelength corresponding to the relative reflectance that changes during polishing and a wavelength that corresponds to the relative reflectance that hardly changes during polishing.

  The denominator of equation (5) is the sum of the plurality of relative reflectances used. On the other hand, the relative reflectance used for the numerator of the formula is any one of a plurality of relative reflectances. For example, the relative reflectance corresponding to the longest or shortest wavelength among the selected wavelengths is placed in the numerator of equation (5). By using the equation (5), it is possible to obtain a characteristic value with less noise and changing according to the polishing time. Therefore, highly accurate polishing end point detection can be realized.

  FIG. 7 is a graph showing a spectrum obtained by applying a numerical filter to the spectrum shown in FIG. The numerical filter (digital filter) used is a third-order Butterworth low-pass filter, and a frequency corresponding to a length of 20 nm along the wavelength axis (interval between the maximum or minimum points of interference components) is set as a cutoff frequency. . This numerical filter was applied twice in both directions along the wavelength axis and used as a zero phase filter. As can be seen from FIG. 7, in the spectrum after application of the numerical filter, fine undulations (interference components) of 20 nm or less are removed, and a smoothed spectrum is obtained.

  FIG. 8 is a graph showing a time change during polishing of the relative reflectance at a wavelength of 750 nm on the spectrum after the numerical filter shown in FIG. 7 is applied. In FIG. 8, the vertical axis represents relative reflectance (%), and the horizontal axis represents polishing time (seconds). The dotted line in FIG. 8 indicates the relative reflectance itself, and the solid line indicates the relative reflectance obtained from the spectrum after applying the numerical filter. As can be seen from FIG. 8, the relative reflectance obtained from the smoothed spectrum increases monotonically with the polishing time.

  Thus, instead of the moving average process, a smoothing process using a filter may be performed on the spectrum obtained at each time point during polishing. In either case, since the relative reflectance monotonously increases during polishing, it is possible to monitor the progress of polishing and detect the polishing end point based on the change in relative reflectance. In addition, when smoothing the spectrum, a moving average process or a smoothing process is further performed on the obtained relative reflectance in order to remove noise components other than fluctuations due to light interference in the silicon layer. May be.

  The substrate used in the above-described example is a substrate in which the thickness of the silicon layer is substantially uniform within the surface of the substrate. For example, the variation in the thickness of the silicon layer is within a range of ± 150 nm. On the other hand, when the thickness of the silicon layer varies widely within the surface of the substrate, the relative reflectance may monotonously increase without performing the moving average process or filtering as described above. As shown in FIG. 2B, the intensity of reflected light from a plurality of measurement points on the substrate is measured each time the polishing table 20 makes one rotation, and a plurality of relative reflectivities obtained from the measured values. It is because the average of is calculated | required. That is, the spectrum obtained at each measurement point has a maximum point and a minimum point that appear at different wavelengths due to the influence of variations in the thickness of the silicon layer. Therefore, when the average of a plurality of relative reflectances at a plurality of measurement points is obtained, these local maximum points and local minimum points are canceled, and the waveform of the relative reflectance over time is smoothed.

  FIG. 9 is a graph showing the change in relative reflectance over time at a wavelength of 750 nm when a substrate having a variation in thickness of the silicon layer in the range of ± 700 nm is polished. As shown in this graph, when the variation in the thickness of the silicon layer is large to some extent, the relative reflectance during polishing increases monotonously with the polishing time. Therefore, in this case, it is not necessary to perform a moving average process or a spectrum smoothing process.

  Even when the variation in the thickness of the silicon layer is small, depending on the resolution of the spectroscope 13, it may not be necessary to perform a moving average process or a spectrum smoothing process. FIG. 10 is a graph showing a change in time during polishing of the relative reflectance when a spectroscope having a large wavelength resolution of about 10 nm is used. In this example, a substrate having the same structure as the example shown in FIG. 5 was used. As can be seen from FIG. 10, it is the same as the example of FIG. 5 in that the relative reflectance increases with the polishing time, but the fluctuation of the relative reflectance of FIG. 10 compared to the relative reflectance indicated by the solid line of FIG. Is small. This is considered because the spectroscope 13 measures the intensity of the reflected light for each relatively wide wavelength range, and as a result, the intensity of the reflected light is averaged.

  FIG. 11 is a cross-sectional view schematically showing a polishing apparatus provided with a polishing end point detection device capable of executing the above-described polishing end point detection method. As shown in FIG. 11, the polishing apparatus supplies a polishing table 20 that supports the polishing pad 22, a top ring 24 that holds the substrate W and presses against the polishing pad 22, and supplies a polishing liquid (slurry) to the polishing pad 22. And a polishing liquid supply mechanism 25. The polishing table 20 is connected to a motor (not shown) disposed below the polishing table 20 and is rotatable about an axis. The polishing pad 22 is fixed to the upper surface of the polishing table 20.

  The upper surface 22a of the polishing pad 22 constitutes a polishing surface for polishing the substrate W. The top ring 24 is connected to a motor and a lifting cylinder (not shown) via a top ring shaft 28. Thereby, the top ring 24 can be raised and lowered and can be rotated around the top ring shaft 28. The substrate W is held on the lower surface of the top ring 24 by vacuum suction or the like.

  The substrate W held on the lower surface of the top ring 24 is pressed by the top ring 24 against the polishing pad 22 on the rotating polishing table 20 while being rotated by the top ring 24. At this time, the polishing liquid is supplied from the polishing liquid supply mechanism 25 to the polishing surface 22 a of the polishing pad 22, and the surface of the substrate W is polished in a state where the polishing liquid exists between the surface of the substrate W and the polishing pad 22. . A relative movement mechanism for bringing the substrate W and the polishing pad 22 into sliding contact is constituted by the polishing table 20 and the top ring 24.

  The polishing table 20 is formed with a hole 30 that opens on the upper surface thereof. The polishing pad 22 has a through hole 31 at a position corresponding to the hole 30. The hole 30 and the through hole 31 communicate with each other, and the through hole 31 opens at the polishing surface 22a. The hole 30 is connected to a liquid supply source 35 via a liquid supply path 33 and a rotary joint 32. During polishing, water (preferably pure water) is supplied from the liquid supply source 35 to the hole 30 as a transparent liquid, fills the space formed by the lower surface of the substrate W and the through hole 31, and the liquid discharge path 34. It is discharged through. The polishing liquid is discharged together with water, thereby securing an optical path. The liquid supply path 33 is provided with a valve (not shown) that operates in synchronization with the rotation of the polishing table 20. This valve operates to stop the flow of water or reduce the flow rate of water when the substrate W is not positioned over the through hole 31.

  The polishing apparatus has a polishing end point detection device that monitors the progress of polishing according to the above-described method and detects the polishing end point. The polishing end point detection device includes a light projecting unit 11 that irradiates light onto a surface to be polished of a substrate W, an optical fiber 12 that receives reflected light returning from the substrate W, and reflected light from the substrate W. A spectroscope 13 that decomposes according to the wavelength and measures the intensity of the reflected light over a predetermined wavelength range, and generates a spectrum from the measurement data acquired by the spectroscope 13, and determines the progress of polishing based on the change in the spectrum. And a processing device 15 to be monitored. The spectrum indicates the intensity of light distributed over a predetermined wavelength range, and is represented as a line graph indicating the relationship between the intensity of light and the wavelength.

  The light projecting unit 11 includes a light source 40 and an optical fiber 41 connected to the light source 40. The optical fiber 41 is an optical transmission unit that guides light from the light source 40 to the surface of the substrate W. The optical fiber 41 extends from the light source 40 through the hole 30 to a position near the surface to be polished of the substrate W. Each tip of the optical fiber 41 and the optical fiber 12 is arranged to face the center of the substrate W held by the top ring 24, and as shown in FIG. 2B, the center of the substrate W is rotated each time the polishing table 20 rotates. The region including the light is irradiated.

  As the light source 40, a light source that emits light having a plurality of wavelengths, such as a light emitting diode (LED), a halogen lamp, or a xenon flash lamp, can be used. The optical fiber 41 and the optical fiber 12 are arranged in parallel with each other. The tips of the optical fiber 41 and the optical fiber 12 are arranged substantially perpendicular to the surface of the substrate W, and the optical fiber 41 irradiates light almost perpendicularly to the surface of the substrate W.

  During polishing of the substrate W, light is irradiated from the light projecting unit 11 to the substrate W, and reflected light from the substrate W is received by the optical fiber 12. While the light is irradiated, water is supplied to the hole 30, whereby the space between the tips of the optical fiber 41 and the optical fiber 12 and the surface of the substrate W is filled with water. The spectroscope 13 measures the intensity of the reflected light for each wavelength, and the processing device 15 generates a spectrum of the reflected light indicating the relationship between the relative reflectance of the reflected light and the wavelength. Further, as described above, the processing device 15 obtains a polishing index indicating the progress of polishing from the spectrum of the reflected light, and determines the polishing end point based on the time when the polishing index reaches a predetermined threshold value. As described above, the polishing index indicating the progress of polishing is a characteristic value calculated from the relative reflectance at a predetermined wavelength or the relative reflectance at a plurality of predetermined wavelengths.

  FIG. 12 is a cross-sectional view showing a modification of the polishing apparatus shown in FIG. In the example shown in FIG. 12, the liquid supply path, the liquid discharge path, and the liquid supply source are not provided. Instead, a transparent window 45 is formed in the polishing pad 22. The optical fiber 41 of the light projecting unit 11 irradiates light on the surface of the substrate W on the polishing pad 22 through the transparent window 45, and the optical fiber 12 as the light receiving unit receives reflected light from the substrate W through the transparent window 45. . Other configurations are the same as those of the polishing apparatus shown in FIG.

  The present invention can be applied not only to polishing of a back-illuminated CMOS sensor, but also to polishing of a device having a semiconductor layer having a thickness of several μm or more, whose main component is a semiconductor material such as silicon.

  The embodiment described above is described for the purpose of enabling the person having ordinary knowledge in the technical field to which the present invention belongs to implement the present invention. Various modifications of the above embodiment can be naturally made by those skilled in the art, and the technical idea of the present invention can be applied to other embodiments. Therefore, the present invention should not be limited to the described embodiments, but should be the widest scope according to the technical idea defined by the claims.

DESCRIPTION OF SYMBOLS 11 Light projection part 12 Light reception part 13 Spectrometer 15 Processing apparatus 20 Polishing table 22 Polishing pad 24 Top ring 25 Polishing liquid supply mechanism 28 Top ring shaft 30 Hole 31 Through hole 32 Rotary joint 33 Liquid supply path 34 Liquid discharge path 35 Liquid supply Source 40 Light source 41 Optical fiber 45 Transparent window

Claims (9)

A method for polishing a substrate having a semiconductor layer, comprising:
Irradiating the semiconductor layer with visible light during polishing of the semiconductor layer,
Receiving reflected light from the semiconductor layer;
Measuring the intensity of the reflected light in a predetermined wavelength range;
Divide the intensity measurement by a predetermined reference intensity to calculate the relative reflectance,
Generating a spectrum indicating the relationship between the relative reflectance and the wavelength of the reflected light;
From the spectrum, a polishing index that varies according to the thickness of the semiconductor layer is obtained,
A polishing method, comprising: polishing the semiconductor layer based on a point in time when the polishing index reaches a predetermined threshold value.   The polishing method according to claim 1, wherein an upper limit value of the predetermined wavelength range is 800 nm or less.   The polishing method according to claim 1, wherein the polishing index is a relative reflectance at a predetermined wavelength in a range of 650 nm to 800 nm.   The polishing method according to claim 3, wherein the polishing index is a moving average of relative reflectance at the predetermined wavelength. The polishing index includes at least a first relative reflectance at a first wavelength in the range of 650 nm to 800 nm and a second relative reflectance at a second wavelength in the range of 300 nm to 550 nm. Is a characteristic value calculated from the relative reflectance of
The polishing method according to claim 1, wherein the characteristic value is obtained by dividing one relative reflectance selected from the plurality of relative reflectances by a sum of the plurality of relative reflectances. .   The polishing method according to claim 5, wherein the polishing index is a moving average of the characteristic values. Further comprising a step of smoothing the spectrum,
The polishing method according to claim 1, wherein the polishing index is obtained from the smoothed spectrum.   The polishing method according to claim 1, wherein the semiconductor layer is a silicon layer constituting a light receiving surface of a back-illuminated CMOS sensor. An apparatus for polishing a substrate having a semiconductor layer,
A light projecting portion for irradiating the semiconductor layer with visible light during polishing of the semiconductor layer;
A light receiving portion for receiving reflected light from the semiconductor layer;
A spectroscope for measuring the intensity of the reflected light in a predetermined wavelength range;
A processing unit that calculates a relative reflectance by dividing the measured value of the intensity by a predetermined reference intensity, and generates a spectrum indicating a relationship between the relative reflectance and the wavelength of the reflected light; and
The processor is
From the spectrum, a polishing index that varies according to the thickness of the semiconductor layer is obtained,
A polishing apparatus, wherein a polishing end point of the semiconductor layer is determined based on a time point when the polishing index reaches a predetermined threshold value.

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