JP5241321B2 - Wafer polishing state monitoring method and polishing state monitoring device - Google Patents

Description

  The present invention relates to a wafer end point detection method and a polishing end point detection apparatus in CMP (Chemical Mechanical Polishing), and more particularly to a wafer end point detection method and a polishing end point detection apparatus that improve accuracy. .

  As miniaturization and multilayering of wiring patterns in VLSI and the like progress, in these semiconductor manufacturing processes, in the CMP process that performs wiring pattern formation, shallow groove type element separation, etc., on a film-formed wafer, The importance of precisely controlling the final film thickness after finishing polishing at a proper polishing end point is increasing.

  Here, even if the end point detection of normal polishing is detected, it is often managed by time. Even if it is managed by time, that is, when polishing is finished by polishing for a predetermined time, if it is polished under the same conditions, it will not change greatly from the estimate, roughly speaking, about 10% at most A stable film thickness can be obtained at a stable polishing rate that does not change.

  However, in the case of polishing, if polishing is continued for a long time, the polishing pad gradually clogs or is smoothed, and the polishing rate changes slightly. In particular, it is very difficult to stabilize the surface state of the polishing pad, and even if polishing is performed for a predetermined time due to a minute polishing rate drift, the resulting film thickness amount slightly changes. Become.

  In the case of an SOI wafer handled in the present application, the control of the film thickness for detecting the end point of polishing is an important factor for determining the thickness of the element formation layer in the SOI wafer. A change in film thickness is not acceptable. Even when polishing under these same polishing conditions, the end point of polishing is detected in real time for the film thickness that varies slightly depending on the polishing condition, and control is required to keep the resulting film thickness constant. It is done.

  Previously, when determining the polishing end point, the CMP apparatus was stopped during polishing, the wafer was taken out of the CMP apparatus, and the completion of polishing, resumption of polishing, and approximate repolishing time were visually determined from the state of the film surface of the wafer. Was. However, in this method, the polishing must be interrupted frequently during the polishing of one wafer, and the productivity is not good. Therefore, a technique is used in which the state of the polished surface can be grasped in real time during polishing. (Patent Documents 1, 2, 3, etc.).

  In each of these techniques, a platen that is a wafer mounting table and a polishing pad attached on the platen are formed with a hole penetrating vertically, and a transparent resin body window is provided in the platen hole or the polishing pad hole. The polishing surface of the wafer on the polishing pad is viewed from the lower surface side of the wafer.

In the case of an SOI wafer handled in the present application, a typical structure is a film structure of Si / SiO _x / Si. In the case of such a film configuration, the reflected light includes light reflected by Si on the surface, light reflected by the Si / SiO _X interface, and light reflected by the underlying SiO _X / Si. There are three. In particular, Si / SiO _X and SiO _X / Si have two members with different refractive indexes in opposite directions, so that one of the phases remains unchanged during reflection and the other has a phase when reflected. It will be reversed. As a result, a typical waveform such as a wave as shown in FIG. It is important to analyze such a waveform to detect the polishing end point.

  In the case of such a waveform, it is extremely difficult to determine the actual end point from the reflectance waveform. One reason is that the refractive index of Si is very high as 3.42, and a short period wave is included in the spectral waveform. In the case of having a dense wave with such a short period, it is very difficult to accurately recognize the waveform. In such a waveform, in particular, it is important to accurately count the positions of the peaks and troughs without making a mistake, rather than the tendency of the shape of the entire waveform.

  If the positions of the peaks and valleys are wrongly counted, it is because the film thickness of Si itself is estimated to be excessive or small. For example, when the position of the valley / valley is wrongly estimated for one period, d for one period is expressed by d = λ / 2n, and when n = 3.42 and λ: 1100 nm, d = 160 nm. Since the amount of film thickness to be controlled at the actual end of polishing is desired to be about ± 20 nm, it is fatal if the estimation is wrong even about 160 nm.

  In Patent Document 1, light is irradiated from the lower surface side of the platen toward the window, and the light intensity of a specific frequency in the reflected light (for example, the reflectance of the Si film has peaks at wavelengths of 700 nm and 770 nm). A technique for determining a polishing state and a technique for photographing a polished surface of a wafer with an imaging device and determining the polishing state by image analysis are described. However, in this method, it may be possible to judge the polishing state from the perspective of the rough spectral reflectance, but the numerical value accurately monitors how much the thickness of the Si layer is polished. Difficult to do.

Patent Document 2 is a method of irradiating a substrate surface with light and measuring the surface state of the substrate surface based on the degree of coincidence between a signal waveform obtained from the reflected signal light or transmitted signal light and a reference waveform. The degree of coincidence is calculated using cross-correlation. For example, as a waveform for determining the degree of coincidence, a TaN layer as a barrier layer on a SiO ₂ film and a Cu layer formed thereon are used.

  As a method for evaluating the degree of coincidence between the actually measured waveform and the reference waveform after polishing, a method of viewing the cross-correlation between the two is disclosed, and is shown in FIG. When the overall trends of the waveforms match, the correlation coefficient becomes 0.752, which is regarded as the end point. Here, the fluctuation of a small period is ignored from the correlation coefficient and is regarded as noise. Similarly, FIG. 5 is regarded as an end point based on the correlation coefficient in a wide wavelength region.

  In the case of the waveform analysis on the SOI wafer handled in the present application, such a method mistakes the film thickness as described above. This is because, in the case of SOI, it is required to accurately count a small period variation with a short period.

  Here, the case where the waveform of this application is handled by the method which exists in patent document 2 is assumed. For example, even if there are 10 reference waveforms in the predetermined wavelength region and 11 actual waveforms, there is clearly no single peak, so the waveforms are not accurately verified. However, from the viewpoint of cross-correlation, as shown in FIG. 4 of Patent Document 2, while small periodic fluctuations are ignored, it is considered that the tendency of the overall waveform matches, and the correlation coefficient Becomes larger and is regarded as the end point. As a result, a fatal film thickness estimation error occurs.

  Furthermore, even in the method shown in Patent Document 2, even when the peak position and the valley position of the actually measured reflectance waveform can be clearly distinguished, the cross-correlation occurs when the amplitude is partially smaller than the reference waveform. In some cases, a high correlation coefficient cannot be obtained. As a result, if the position of the mountain and valley is recognized and counted, what is considered to be coincident is not necessarily a large value and may be overlooked when the cross-correlation coefficient is calculated. In such a case, the polishing end point will be missed and cannot be used practically.

  For this reason, in the case of a waveform having a gradual change in reflectance waveform with respect to the wavelength, such as a metal film, it may be applicable in the correlation evaluation of the global shape of the reflectance. In the case where it is required to count the wave number without error in waveform analysis having a short period periodic waveform, the method using the cross-correlation determined by the tendency of the entire waveform is not applicable.

  Patent Document 3 discloses a film thickness measurement method for measuring the film thickness of an active layer 6a with respect to an SOI substrate 6 having an oxide film 6c sandwiched between an active layer 6a and a support substrate 6b. The analysis wavelength region is set so as to exclude the wavelength of the “node” where the interference is weakened by the reflected light on both sides of 6c, and the film thickness of the active layer 6a is calculated using the interference information in the analysis wavelength region A method is disclosed.

The film structure to be handled is the same, and the same type of waveform analysis is performed. However, even in this method, since the reflected light is sometimes scattered by the slurry existing between the pad and the wafer, there is a case where a theoretically clean periodic wave cannot be obtained. Depending on the wavelength region, there are cases where the estimation of the wave number of the periodic waveform is wrong, such as reflected light being absorbed by the slurry, and only four of the originally five short-period waves are observed. Therefore, it may be effective when stable reflected light such as slurry is obtained, but when the slurry is present in the optical path during polishing, a stable reflectance is not necessarily obtained in all wavelength regions. There are many things that cannot be done, and it seems that there is a practical problem in performing stable waveform analysis.
JP-A-7-52032. JP 2001-287159 A. Japanese Patent No. 3946470.

Accordingly, the present invention has the following problems.
-During polishing, the wafer surface can be monitored in real time during polishing without checking the surface condition of the wafer every time, and the end point of polishing can be monitored.
-Even for short-cycle dense reflectance waveforms such as SOI, the number of waves in a predetermined wavelength range is counted, and the end point of polishing is stably monitored without making a mistake.
・ Even if it is likely that the cycle will be mistaken due to the presence of slurry, etc., it will be processed properly to enable stable waveform analysis.

  In a system that projects light onto the film surface of the wafer being polished and observes the polishing state in real time from the reflected light, light that reflects off the film surface of the wafer and light that passes through the wafer and reflects off the back surface of the wafer And the combined light. As the film thickness of the wafer decreases, the spectrum of the received light also changes due to a change in the interference state between the light reflected by the film surface of the wafer and the light transmitted through the wafer and reflected by the back surface of the wafer. It is possible to detect or predict the polishing end point by grasping the film thickness from the light intensity of a specific frequency.

  However, the slurry supplied onto the pad during polishing is interposed between the window of the transparent material and the polishing surface of the wafer, and the flowing slurry gives disturbances such as attenuation or scattering to the reflected light, and polishing. This is a cause of reducing the state detection accuracy. In addition, even if the polishing is performed with the polishing surface of the wafer almost in contact with the window to reduce the optical effect of the slurry, the window is cleaned by a pad conditioner (dresser) that cleans the pad surface. The surface of the film may be damaged, and the optical performance may be deteriorated to adversely affect the reflected light.

  Therefore, there is a technical problem to be solved to eliminate the influence of disturbance and accurately detect the polishing state of the wafer and improve the control accuracy of the polishing end point, and the present invention solves the above problem. For the purpose.

The present invention is proposed to achieve the above object, and the invention according to claim 1 is a Si / SiOx / Si film in which a SiOx film is formed on Si and a Si film is formed thereon. In a polishing system that irradiates light on a wafer surface having a configuration and monitors a polishing state during polishing based on the reflected light,
A means for irradiating the wafer surface with white light; and a means for spectroscopically reflecting the reflected light from the wafer surface; during the polishing, a Fourier transform of the reflectance waveform with respect to the wavelength or wave number reflected from the wafer surface;
Dividing the Fourier-transformed waveform into a periodic component due to the Si film thickness and a periodic component due to the SiOx film thickness;
Of these, focusing on the periodic component due to the Si film thickness, refining the changing periodic part,
A step of performing inverse Fourier transform on the waveform of the Si film thickness after filtering to restore the reflectance waveform, and the predetermined reflectance waveform obtained at the end of polishing and the reflectance waveform during polishing are predetermined. The present invention provides a polishing state monitoring method characterized by collating the positions of the peaks and valleys in a region and detecting the polishing end point based on the collation result.

In the above configuration, the reflectance waveform with respect to the wavelength is acquired in real time during polishing. The reflectance waveform for the wavelength is Fourier transformed. Here, the reflectance with respect to the wavelength is used, but the inverse of the wavelength, that is, the wave number may be taken on the horizontal axis, and the reflectance waveform with respect to the wave number may be Fourier transformed. Thereby, it is possible to divide into an interference waveform caused by reflected light having an optical path in Si having a high refractive index and an interference waveform caused by reflected light having an optical path in SiO _X having a low refractive index. Also in FIG. 3B described later, the gentle undulation is caused by the interference of SiO _X , and the waveform having a high period is caused by the interference of Si, and these two are overlapped. By performing Fourier transform, the periods of the two interference waveforms can be separated from each other. Among the separated periods, a long wavelength period is a period caused by SiO _X and a short wavelength period is a period caused by Si.

  Here, when paying attention to a waveform with a short period in particular, the periodic waveform may have some spread. This is not necessarily a complete short-cycle wave due to scattering by the slurry, but a part of the crests and other parts are erased, resulting in a periodic distribution with variation. By refining the periodic distribution having this variation, it is possible to correct a portion that has been scattered and lost periodicity by slurry or the like.

Next, an original waveform having a short-period wave within a long period in SiO _X can be restored by performing an inverse Fourier transform based on this refined periodic distribution. This restored waveform can completely restore the peak and valley positions with the previous correction considering the periodicity, even if the peaks and valleys are crushed due to the scattering of the slurry. It becomes.

  Next, whether or not the polishing end point has been reached is monitored by collating the correction waveform of the actually measured reflectance waveform thus obtained with a reference waveform that is a predetermined reflectance waveform at the end point of polishing. In the collation, a predetermined reference wavelength and a reference wave number are used as a base point, and the first, second, and peak positions are found on the long wavelength side, and the wavelengths (wave numbers) corresponding to the peak positions are sequentially read and entered in the table. A deviation amount between the wavelength of the actually measured waveform and the reference waveform is calculated at each peak position, and all the deviation amounts are added. At the end of polishing, the peak position of the waveform obtained by correcting the actually measured reflectance waveform is completely coincident with the reference waveform at the end of polishing. As a result, even if the amount of shift of each mountain position is calculated and added together, the numerical value is almost zero. In this way, it is possible to confirm that the actually measured waveform has been substantially compared with the reference waveform, and to detect the end of polishing.

When the waveform during polishing according to Patent Document 3 was directly analyzed, it was often the case that the number of peaks and valleys was incorrectly counted and the remaining film thickness estimates were counted incorrectly. However, according to the method of the present application, even when there is scattering due to the slurry, the waveform can be completely restored, so that erroneous detection is not performed. Also, when judging by cross-correlation as in Patent Document 2, it is often the case that a high correlation coefficient is obtained even if the positions of the peaks and valleys do not completely match, and the waveforms are regarded as matching and erroneously detected. Though conceivable, the method according to the present invention does not perform such ambiguous detection, and can accurately detect the end point.

  Furthermore, in the waveform as in Patent Document 2, even if the peak and valley positions can be clearly distinguished, if the amplitude of each waveform (absolute value of reflectance) is smaller than the reference waveform, The coefficient of cross-correlation becomes small and may not be determined to match. In such a case, the end point cannot be detected in practice, and in practical terms, it cannot be used.

According to the method according to the invention,
・ In order to accurately detect the end point, pay attention to the waveform with a short period, especially from the unique waveform characteristics.
・ Perform Fourier transform to close up a periodic waveform with a short period and be able to distinguish between periodic regions. As a result, it is possible to subtract the SiO _X component whose film thickness does not change at the time of waveform analysis.
It is possible to restore the periodic waveform of the reflectance with respect to the wavelength of the reflected light scattered by the slurry by inverse Fourier transform.
・ By restoring the position of the valleys and valleys, it is possible to finally compare the reference waveform at the end of polishing with the restored waveform and determine whether or not the polishing is completed by the comparison.

  Thus, it is possible to obtain a remarkable effect that is not found in the past.

  For example, even if any one of the correction step of the actually measured waveform and the comparison step of the corrected waveform and the reference waveform indicating the end time is omitted, this end point detection does not function. In the step of restoring the waveform, a process capable of monitoring the film thickness without error is performed, and in the collating step, it is determined whether the waveform has definitely reached the end point.

  In particular, as a comparison with Patent Document 2, Patent Document 2 calculates a reflectance waveform, and calculates a cross-correlation between a measured waveform and a reference waveform as a continuous waveform with respect to the wavelength, but in the present invention, it is regarded as a waveform. Rather, the reflectance distribution is taken as a collation of discrete mountain valley positions. For this reason, in the present invention, the matching of the waveforms is not strictly observed, but the points of the peaks in the discrete periodicity of the reflectance are collated. Therefore, this is a completely different contrast method different from the degree of coincidence of continuous waveforms, and its operation and effect are also completely different.

  The invention according to claim 2 provides a polishing state monitoring method in which the predetermined reflectance waveform is a theoretical waveform.

  In the above configuration, the theoretical reflectance waveform obtained from the physical properties of the wafer and the characteristics of the optical system is used as a target for collation of the reflectance waveform during polishing. It is possible to grasp the result of the collation as accurate as when using.

  According to a third aspect of the present invention, there is provided a polishing state monitoring method in which the step of refining the changing periodic portion of the Fourier-transformed reflectivity waveform during the polishing is performed based on the predicted polishing rate. It is.

In the above configuration, the periodic components that change as the polishing progresses are compared with the predicted polishing rates that are estimated to be actually changed, while removing the periodic components that are far apart, and based on the predicted polishing rates. Filtering that passes only the estimated periodic component is performed, and the filtered periodic component is subjected to inverse Fourier transform, thereby correcting the portion that is scattered by the slurry and loses the periodicity.

  The invention according to claim 4 is a step of restoring the reflectance waveform by performing an inverse Fourier transform on the periodic component of the reflectance waveform being polished by Fourier transform to only the periodic component whose film thickness varies due to polishing. A polishing state monitoring method is provided.

  In the above configuration, the period of the periodic component with respect to the film to be polished changes with the progress of polishing. For this reason, correction of the portion that is scattered by slurry by inverse Fourier transform and loses periodicity is performed only on the periodic component for the film to be polished, thereby restoring the reflectance waveform necessary for detecting the polishing end point. The

The invention according to claim 5 irradiates light onto a wafer surface having a Si / SiOx / Si film structure in which a SiOx film is formed on Si and a Si film is formed thereon, and the reflected light is used as a basis. In a polishing system that monitors the polishing state during polishing,
Means for irradiating the wafer surface with white light, means for spectrally reflecting the reflected light from the wafer surface, and means for Fourier transforming the reflectance waveform for the wavelength or wave number reflected from the wafer surface during polishing;
Means for dividing the Fourier-transformed waveform into a periodic component due to the Si film thickness and a periodic component due to the SiOx film thickness;
Among them, paying attention to the periodic component due to the Si film thickness, and means for purifying the changing periodic part,
It has means to restore the reflectance waveform by inverse Fourier transform of the waveform of the Si film thickness after filtering,
There are means to check the position of the peaks and valleys in a predetermined area in the predetermined reflectance waveform obtained at the end of polishing and the reflectance waveform during polishing, and the polishing end point is detected based on the comparison result A polishing state monitoring device is provided.

  In the above configuration, as in the first aspect of the invention, the influence of disturbance is eliminated based on the collation result between a predetermined reflectance waveform set in advance and the corrected waveform of the reflectance waveform during polishing. It becomes possible to accurately detect the polishing end point.

  Since the invention according to claim 1 detects the end point of polishing by comparing not only the reflectance in a specific frequency band but also the overall reflectance waveform with a predetermined reflectance waveform, disturbance due to flowing slurry or chemicals Is reduced, and the end point detection accuracy is improved. In addition, the polishing end point can be detected with high accuracy by the polishing end point detection technique for the film on the surface of the substrate on which a plurality of layers such as SOI are laminated. Even if the reflected light is scattered by slurry or the like, it is possible to restore the actually measured reflectance waveform from the periodicity of the waveform and accurately collate it with a predetermined reflectance waveform. Even if the actually measured reflectance waveform does not have a sufficient amount of light and the gain is small, it can be accurately compared with a predetermined reflectance waveform, and the polishing end point can be detected.

  According to the second aspect of the present invention, since the theoretical reflectance waveform is used as a target for collation of the reflectance waveform during polishing, it is possible to save the trouble of actually measuring the reflectance waveform at the end of polishing prior to polishing.

  According to the third aspect of the present invention, polishing is performed by removing far-off periodic components while comparing with a predicted polishing rate estimated to be actually changed with respect to a Fourier-transformed reflectance waveform during polishing. Thus, it is possible to perform highly accurate purification on the periodic portion that changes as the process proceeds.

The invention according to claim 4 can eliminate a reflectance spectrum caused by SiO _x whose film thickness does not change, and can display a close-up reflectance spectrum of a short period due to a polished Si layer. . As a result, even a Si layer having a short periodicity can be accurately collated with the theoretical reflectance waveform without counting the number of peaks and the polishing end point can be detected with high accuracy. .

  In the fifth aspect of the invention, as in the first aspect of the invention, the polishing end point is detected by comparing not only the reflectance in a specific frequency band but also the entire reflectance waveform with a predetermined reflectance waveform. Therefore, the influence of disturbance due to the flowing slurry or chemical is reduced, and the end point detection accuracy is improved.

  The present invention relates to a polishing system for irradiating light on a wafer surface and monitoring the polishing state during polishing based on the reflected light, and means for irradiating the wafer surface with white light and spectrally reflecting the reflected light from the wafer surface. A Fourier transform of the reflectance waveform for the wavelength or wave number reflected from the wafer surface during polishing, a step of refining the varying periodicity with respect to the Fourier transformed waveform, and filtering. A step of restoring the reflectance waveform by performing an inverse Fourier transform on the later waveform, and in a predetermined region, in the predetermined reflectance waveform obtained at the end of polishing and the reflectance waveform being polished, With the configuration that collates the position and detects the polishing end point based on the collation result, it eliminates the influence of disturbance and accurately detects the polishing state of the wafer. To achieve the objective of improving the control accuracy.

FIG. 3 shows a configuration example (FIG. 3A) of a wafer W having an SOI structure and a typical reflectance waveform (FIG. 3B). As shown in FIG. 5B, the reflectance waveform is a state in which a long-period wave caused by SiO _X interference and a short-period wave overlap due to interference by the optical path in the Si active layer. Since the film to be polished is Si, whether or not it can be accurately determined without counting or overlooking this short period of waves has a great influence on the accuracy of end point detection.

  FIG. 3B is a particularly theoretical reflectivity waveform, assuming that there is no disturbance or scattered medium on the optical path. However, during normal polishing, a slurry exists between the pad and the wafer, so that sometimes the reflected light cannot be obtained with sufficient reflectivity due to scattering. Moreover, depending on the frequency band of the reflectance, it may be absorbed by the slurry or the like, and it may be assumed that sufficient reflectance cannot be obtained depending on the wavelength. Here, assuming that the reflectance spectrum is not always perfect in such actual polishing, a practical wafer polishing state monitoring method and apparatus are provided.

  FIG. 1 shows a CMP apparatus 1 in which a disk-shaped platen 2 is mounted on a drive mechanism 4 using a motor 3, and a pad 5 is attached to the upper surface of the platen 2. The polishing head 6 disposed above the platen 2 and at a position displaced from the rotation center of the platen 2 has a disk shape smaller in diameter than the platen 2, and a wafer W to be polished is attached to the lower surface thereof. In the polishing process, slurry as a polishing chemical solution is dropped from the slurry supply device (not shown) onto the upper surface of the pad 5, and the polishing head 6 is lowered to a height at which the wafer W comes into contact with the upper surface of the pad 5. 2 and the polishing head 6 are driven to rotate, and the surface to be polished (lower surface) of the wafer W is polished.

  As shown in FIG. 2, the platen 2 and the pad 5 are formed with holes 7 and 8 penetrating vertically, and a transparent window 9 is embedded in the hole 8 of the pad 5. Each time the platen 2 rotates once, the window 9 passes through the lower surface of the wafer W mounted on the polishing head 6. As shown in FIG. 1, an objective lens 10 of a polishing end point detection device is disposed on the lower surface side of the platen 2 and vertically below the polishing head 6. Although not shown, the objective lens 10 is connected to the light source unit and the polychromator through a two-branch light guide made of optical fiber, and the white light emitted from the halogen lamp of the light source unit is vertically upward from the objective lens 10. When the platen 2 and the holes 7 and 8 of the pad 5 pass over the objective lens 10, the light reflected by the wafer W on the lower surface of the polishing head 6 through the window 9 is projected from the objective lens 10 to the two-branch light guide. Through the polychromator.

  The incident light is spectrally divided for each predetermined wavelength by a diffraction grating in the polychromator, converted into an electrical signal corresponding to each of the split light intensity, and output to a computer. The computer compares the reflectance waveform of the light output from the polychromator with a predetermined reflectance waveform that serves as a reference for determining the polishing end point, and detects the polishing end point based on the comparison result to stop the polishing. I have.

  In the control unit of the computer or the CMP apparatus 1, reflectivity waveform data indicating the light intensity at each frequency of the reflected light at the end of polishing is stored in advance as reference reflectivity waveform data at the end of polishing. As the polishing starts, the control unit samples the reflected light of the wafer W acquired through the objective lens 10.

Next, the step of correcting the acquired reflected light will be described with reference to (a), (b), (c), and (d) of FIG. The reflectance waveform (FIG. 4A) of the acquired reflected light is Fourier transformed in real time and divided into each periodic component (FIG. 4C). The waveform divided into periodic components can be divided into two periodic components, a periodic component F ₁ obtained by the Si optical path and a periodic component F ₂ obtained by the SiO _X optical path. Periodic component F ₂ of SiO _X is not a SiO _X itself polished, because not the film thickness by the polishing is small, it will not be focused on this spectrum.

Here particular importance relates periodic components F ₁ of Si. In particular, since the Si film decreases as the Si film is polished, the wavelength period of the interference waveform having the inside of Si as an optical path changes gradually. The rate of change is the rate of polishing. When polishing while performing pad dressing, the polishing rate does not vary greatly, and polishing is performed at a substantially constant rate. That is, since the removal speed of the Si film is removed at a constant rate, it can be seen that the shift to the long wavelength side of the interference waveform by the Si film after Fourier transform also moves at a constant speed. . This state is constantly monitored during polishing.

FIG. 5 shows a method for refining the waveform of the periodic component F ₁ that moves corresponding to the polishing rate. For example, the periodicity may be sharpened by setting a half-value width d for the obtained periodic component F ₁ and deleting the periodic component f distributed outside. Further, with respect to the periodic component F ₁ , the periodic component far away is deleted while being compared with the estimated polishing rate that will actually change, and only through the estimated periodic component based on the estimated polishing rate (filtering), the periodic component May be subjected to inverse Fourier transform to obtain a corrected reflectance waveform (FIG. 4D).

  Since the waveform that interferes through the Si optical path is basically a waveform with periodicity, the waveform is crushed in the corrected waveform after inverse Fourier transform by refining the waveform with emphasis on periodicity. However, the waveform can be restored faithfully (FIG. 4B). Based on the restored waveform, the polishing end point is obtained by comparing with a reference waveform (predetermined reflectance waveform) corresponding to the vicinity of the polishing end point in advance.

  In the collation of the corrected waveform and the reference waveform restored here, the cross correlation between waveforms must not be evaluated. This is because the actually measured reflectance waveform may not be able to obtain sufficient reflectance intensity due to scattering or absorption on the intermediate optical path. However, even when sufficient reflectivity intensity cannot be obtained, it is possible to accurately perform collation at a predetermined film thickness position in the present invention by clarifying the peak position and the valley position.

  (A), (b), and (c) of FIG. 6 show steps for collating the corrected reflectance waveform with a reference waveform (predetermined reflectance waveform). Regarding the step of collation, a certain reference wavelength region is set, and the peak position of the waveform is matched in the wavelength region, and the wavelength at the peak position is compared and compared between the reference waveform and the actually measured corrected reflectance waveform (FIG. 6). (B)). A wavelength shift amount l (L small letter) at each peak position from the reference position is calculated, and a sum S thereof is obtained as shown in the table of FIG. 6C and the following equation (1).

... (1)
If the sum S thereof is close to zero, it is determined that most of the peak positions of the waveforms are collated, and the polishing end point is detected based on the collation. If the wavelength shift amount at each peak position from the reference position is a value sufficiently larger than zero, it is determined that the collation has not been performed yet and the polishing is continued.

  In this waveform matching process, it is possible to roughly predict how much polishing is required before matching. For example, when both waveforms are arranged on the basis of a certain mountain, if both waveforms are not completely matched, the following form is obtained. That is, the peak positions of both the references are collated, but the wavelength is slightly shifted at the position of the next peak portion, and the wavelength is further shifted at the next peak portion than before, The reference position shifts almost proportionally with respect to zero shift amount.

  Temporarily, the amount of wavelength shift at both peak positions is not a simple proportional relationship. For example, in the case of a shift from a specific peak position and then a fixed shift amount, the vicinity where the fixed shift amount is obtained. Thus, it is estimated that the number of peaks was dropped when correcting. In such a case, if the waveform is refined again, the waveform is corrected (FIG. 6A), and it is possible to completely match the reference waveform.

  Even when waveforms are collated in this way, depending on whether the change in the deviation of the waveform to be collated shifts based on a uniform increment, or whether it changes in a constant step and then maintains that constant change, etc. By using the matching step shown in the present invention, it is possible to accurately determine whether the waveform matching is insufficient or whether the previous waveform correction process is insufficient. Become.

  Finally, in the reflectance waveform actually measured for the wafer W having the SOI structure, the waveform is refined based on the reflectance waveform as it is (FIG. 7A) and its periodicity after Fourier transformation, An example of a reflectance waveform (FIG. 7B) obtained by performing Fourier transform to accurately extract and correct a periodic waveform component is shown. In this way, a clean periodic waveform is not necessarily obtained in the actual reflectance spectrum obtained from the Si surface (FIG. 7A), but the periodic component becomes remarkable by the subsequent correction processing, and the peak portion that can be verified. It can be seen that has been restored (position ↓ in FIG. 7B). In the case where correction processing is not performed in this manner, the end point cannot often be accurately determined even if collation processing with a reference waveform is performed, and it has not been practically usable.

  Note that the standard reflectance waveform data as the reference waveform is not theoretically measured data, but theoretical reflectance waveform data that can be calculated from optical characteristics such as wafer thickness, translucency, and film surface reflectance. May be. Further, a plurality of light projecting / receiving light paths or optical systems may be provided to collate a plurality of reflected light reflectance waveforms with a reference reflectance waveform.

  Various methods are conceivable for determining the timing of completion of polishing. For example, it is possible to detect a change in the degree of coincidence at an inflection point of a predetermined reflectance waveform, and obtain the degree of coincidence from the deviation of each inflection point.

  Moreover, the peak of coincidence can be reliably detected by including a step where the degree of coincidence is improved and a step where the degree of coincidence is deteriorated in the changing part of the coincidence degree of the reflectance waveform.

  In order to determine that the inflection point of the waveform is the maximum inflection point, it should be performed after a certain amount of time has passed, so that the maximum inflection point can be recognized without error. Since the required amount can be predicted from the polishing rate or the like, the above time can be constant and may be determined from the required polishing time of the wafer obtained by another film thickness measurement method.

  The changing part of the coincidence degree of the reflectance waveform may be the maximum inflection point of the inflection point group of the waveform or the approximate curve, thereby further reducing the influence of disturbance such as noise.

  For calculating the degree of coincidence of reflectance waveforms, an inner product of waveforms obtained by differentiating wavelengths may be used. Also in this case, there is an effect of reducing the influence of a spiked noise portion or the like.

  The polishing state monitoring method / polishing state monitoring apparatus of the present invention and another polishing state monitoring method are used in combination, and the polishing state monitoring method of the present invention is used within a certain range of the results of another polishing state monitoring method. May be.

  Note that the present invention is not limited to the above-described embodiment, and various modifications are possible within the technical scope of the present invention, and the present invention naturally extends to those modified ones.

The perspective view of a CMP apparatus. Sectional drawing of a platen and a pad. It is a figure which shows the structural example of an SOI wafer, and the typical reflectance waveform example, (a) is sectional drawing which shows the structural example of an SOI wafer, (b) is the typical reflectance waveform example. It is a figure for demonstrating the method of correct | amending the measured reflectance waveform, (a) is an example of a measured reflectance waveform, (b) is the reflectance waveform restored | regenerated by correction | amendment, (c) is divided by Fourier transformation. (D) is a reflectance waveform restored by inverse Fourier transform of the filtered waveform. The figure for demonstrating the method to refine | purify the periodic part which changes. It is a figure for demonstrating the method of collating a correction | amendment reflectance waveform with a reference waveform, (a) is a reflectance waveform which restored the position of a mountain and valley by refining a waveform, (b) is a reference with an actually measured correction reflectance waveform. The figure which shows the aspect of the comparison contrast with a waveform, (c) is a table | surface which shows the example of the deviation | shift amount of the wavelength calculated as a result of the comparison contrast. It is a figure which shows the example of an actual reflectance waveform, and the example of a reflectance waveform after correction | amendment, (a) is an example of an actual reflectance waveform, (b) is after correcting the reflectance waveform of FIG. Example of reflectance waveform.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 CMP apparatus 2 Platen 3 Motor 4 Drive mechanism 5 Pad
6 Polishing Head 7 Hole 8 Hole 9 Window 10 Objective Lens W Wafer

Claims (5)

A SiOx film is formed on Si, and a Si / SiOx / Si film structure on which a Si film is formed is irradiated with light, and the polishing state during polishing is monitored based on the reflected light. In the polishing state monitoring method to
A means for irradiating the wafer surface with white light; and a means for spectroscopically reflecting the reflected light from the wafer surface; during the polishing, a Fourier transform of the reflectance waveform with respect to the wavelength or wave number reflected from the wafer surface;
Dividing the Fourier-transformed waveform into a periodic component due to the Si film thickness and a periodic component due to the SiOx film thickness;
Of these, focusing on the periodic component due to the Si film thickness, refining the changing periodic part,
Having a step of inverse Fourier transforming the waveform of the Si film thickness after filtering to restore the reflectance waveform;
In two of the predetermined reflectance waveform obtained at the end of polishing and the reflectance waveform during polishing, the peak and valley positions are collated in a predetermined area, and the polishing end point is detected based on the collation result. A polishing state monitoring method. 2. The polishing state monitoring method according to claim 1, wherein the predetermined reflectance waveform is a theoretical waveform. With respect to the Fourier transformed reflectance waveform during polishing, the step of purifying the periodicity unit amount of change in the polishing state monitoring according to claim 1 or 2, wherein the purifying based on predicted polishing rate Method. With respect to the Fourier transformed reflectance waveform during polishing, claim thickness by inverse Fourier transform on only the periodic component that varies by polishing, characterized by the step of restoring the reflectance waveform 1 , 2 or 3 polishing state monitoring method. A SiOx film is formed on Si, and a Si / SiOx / Si film structure on which a Si film is formed is irradiated with light, and the polishing state during polishing is monitored based on the reflected light. In the polishing state monitoring device to
Means for irradiating the wafer surface with white light, means for spectrally reflecting the reflected light from the wafer surface, and means for Fourier transforming the reflectance waveform for the wavelength or wave number reflected from the wafer surface during polishing;
A means for dividing the Fourier transformed waveform into a periodic component caused by the Si film thickness and a periodic component caused by the SiOx film thickness,
Among them, paying attention to the periodic component due to the Si film thickness, and means for purifying the changing periodic part,
It has means to restore the reflectance waveform by inverse Fourier transform of the waveform of the Si film thickness after filtering,
There are means to check the position of the peaks and valleys in a predetermined area in the predetermined reflectance waveform obtained at the end of polishing and the reflectance waveform during polishing, and the polishing end point is detected based on the comparison result A polishing state monitoring device.

Images