Measurement system and measurement method

Abstract

The present invention measures the thickness of an object by using a spectral interference method. The object is placed on a table. As measurement light, circularly polarized light beams or a combination of two linearly polarized light beams orthogonal to each other is output toward the object. Measurement light components reflected by the object are received. The thickness of the object at a point corresponding to the reflection point of the measurement light is measured on the basis of received signals of the reflected light components.

Description

Measurement system and measurement method CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This international application claims priority based on Japanese Patent Application No. 2024-017832, filed with the Japan Patent Office on February 8, 2024, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a measurement system and a measurement method.

[0003] A conventional technique for measuring wafer thickness is the spectral interferometry, which irradiates a wafer with measurement light and measures the wafer thickness based on an interference signal of the reflected light, which is measurement light reflected from the front and back surfaces of the wafer.

[0004] However, the wafer being measured may have birefringence. In this case, the wafer being measured exhibits different refractive indices for ordinary and extraordinary light. Due to birefringence, the power spectrum of the reflected light splits the interference signal peak, which should be a single peak, into multiple peaks.

[0005] To address the problem of not being able to accurately measure the thickness of a wafer made of birefringent material due to peak splitting, a method has been devised in the past to measure the thickness using the centroid or midpoint of multiple peaks (see, for example, Patent Documents 1 and 2).

[0006] JP 2009-198361 A JP 2015-059750 A

[0007] The spiral scan method is also known as a technique for measuring wafer thickness. In the spiral scan method, a wafer is placed on a rotating table and irradiated with measurement light while rotating the wafer. At this time, the measurement light irradiating the wafer is moved in the radial direction, so that the measurement light is irradiated spirally relative to the wafer, and the thickness of the entire surface of the wafer is measured.

[0008] However, in this method, even if the wafer is irradiated with linearly polarized measurement light, the orientation of the linearly polarized light relative to the wafer changes as the wafer rotates, taking birefringence into consideration.If the wafer has birefringence, the two peaks corresponding to the interference signal in the power spectrum will fluctuate in a complementary manner as the wafer rotates.

[0009] In an environment where such peak fluctuations occur, it is difficult to measure the wafer thickness accurately even if the wafer thickness is measured using the center of gravity or midpoint of multiple peaks as in the past.

[0010] Therefore, according to one aspect of the present disclosure, it is desirable to provide a technique that can suppress the influence of birefringence and measure thickness with high accuracy in an apparatus that measures the thickness of an object using a spectral interference method.

[0011] According to one aspect of the present disclosure, there is provided a measurement system for measuring the thickness of an object using spectral interference, the measurement system including a rotation stage, an optical system, a light receiving unit, and a control unit.

[0012] The rotation stage is configured to support the object, the optical system is configured to output measurement light toward the object, and the light receiving unit is configured to receive a reflected component of the measurement light from the object.

[0013] The control unit is configured to control the optical system to output measurement light toward the object rotating together with the turntable. The control unit is further configured to measure the thickness of a point on the object corresponding to the reflection point of the measurement light based on the light receiving signal of the reflected component input from the light receiving unit. The optical system is configured to output, as the measurement light, a circularly polarized wave or a combination of two linearly polarized waves that are orthogonal to each other.

[0014] With this measurement system, it is possible to suppress the fluctuation of the split peaks caused by the rotation of the object, which is a phenomenon in which the peaks of the interference signal split in the power spectrum of the received light signal due to birefringence. Therefore, with this measurement system, it is possible to measure the thickness of the object with high accuracy while suppressing the influence of birefringence.

[0015] According to an aspect of the present disclosure, the measurement system may further include a polarizer. The polarizer may be configured to rotate around an axis parallel to the propagation direction of the measurement light at the same angular velocity as the rotation table in a propagation path of the measurement light between the optical system and the rotation table. By rotating at the same angular velocity as the rotation table, the polarizer may be configured to convert the measurement light from the optical system into linearly polarized light whose polarization plane is oriented in a constant direction relative to the rotation table.

[0016] With this measurement system, the direction of the measurement light relative to the object is constant, so that fluctuations in the peak of the interference signal in the power spectrum of the received light signal due to birefringence can be suppressed. Furthermore, by adjusting the rotation phase, it is possible to emphasize either the peak due to ordinary light or the peak due to extraordinary light. Therefore, with this measurement system, the influence of birefringence can be suppressed and the thickness of the object can be measured with high accuracy.

[0017] According to one aspect of the present disclosure, the control unit may be configured to measure the thickness of the object at multiple points on the object by focusing on one specific peak among multiple peaks that occur in the power spectrum of the received light signal due to interference of reflected components from the front and back surfaces of the object.

[0018] This measurement system can suppress fluctuations in peaks split by birefringence that occur as the object rotates, making it possible to measure the thickness of the object with high accuracy by focusing on a single peak.

[0019] According to one aspect of the present disclosure, the control unit may be configured to measure the thickness of the object at multiple points by focusing on a peak derived from ordinary light as one specific peak among the multiple peaks.

[0020] According to one aspect of the present disclosure, the control unit may calculate, as the measurement value of the thickness of the object, a thickness corresponding to a local maximum point of the received light intensity, which is identified by functional fitting using a polynomial function to the received light intensities of multiple frequency components around a peak of interest in the power spectrum. With a measurement system configured in this manner, it is possible to calculate a measurement value of the thickness of the object with higher accuracy.

[0021] According to an aspect of the present disclosure, the optical system may be configured to be able to change the irradiation position of the measurement light in a radial direction of the rotary table.

[0022] According to one aspect of the present disclosure, the control unit may be configured to irradiate the object rotating together with the turntable with the measurement light so as to move the irradiation position of the measurement light in a radial direction of the turntable by a spiral scan method in order to measure the thickness of the object at multiple points on the object. The control unit may be configured to measure the thickness of the object at multiple points on the movement path of the measurement light based on the light receiving signal.

[0023] This measurement system makes it possible to measure the thickness of an object with high accuracy while suppressing the influence of birefringence, while using the spiral scan method.

[0024] According to one aspect of the present disclosure, there may be provided a measurement method including at least a part of a method executed by the above-described measurement system. According to one aspect of the present disclosure, there may be provided a measurement method for measuring the thickness of an object using spectral interferometry.

[0025] The measurement method may include placing an object on a stage and controlling an optical system to output, as measurement light, a circularly polarized wave or a combination of two linearly polarized waves that are orthogonal to each other, toward the object.

[0026] The measurement method may include receiving a reflected component of the measurement light from the object, and measuring a thickness of a point on the object corresponding to the reflection point of the measurement light based on a received light signal of the reflected component.

[0027] This measurement method makes it possible to measure the thickness of an object with high accuracy while suppressing the influence of birefringence, even when the orientation of the object relative to the measurement light changes. The change in the orientation of the object relative to the measurement light here is not limited to rotation of the table.

[0028] According to an aspect of the present disclosure, the stage may be a rotating stage. In this case, outputting may include causing the optical system to output the measurement light toward the object rotating together with the rotating stage. Such a measurement method achieves the same effects as the measurement system described above.

[0029] According to one aspect of the present disclosure, a measurement method may include converting the measurement light from the optical system into linearly polarized light whose polarization plane has a constant orientation relative to the turntable, using a polarizer that rotates around an axis parallel to the propagation direction of the measurement light at the same angular velocity as the turntable, in the propagation path of the measurement light between the optical system and the turntable, and irradiating the measurement light converted into linearly polarized light onto an object.

[0030] According to this measurement method, since the direction of the measurement light relative to the object is constant, it is possible to suppress fluctuations in the peak of the interference signal in the power spectrum of the received light signal due to birefringence, and therefore, according to this measurement method, it is possible to measure the thickness of the object with high accuracy while suppressing the influence of birefringence.

[0031] FIG. 1 is a diagram showing the configuration of a measurement system of a first embodiment. FIG. 2 is a diagram showing the mechanical configuration of a measurement device. FIG. 3 is a flowchart showing measurement-related processing executed by a processor. FIG. 4 is a flowchart showing measurement processing executed by a processor. FIG. 5 is a diagram explaining function fitting using a quadratic function. FIG. 6 is a diagram showing the configuration of a measurement system of a second embodiment. FIG. 7 is an explanatory diagram regarding peaks of an interference signal in the second embodiment. FIG. 8 is a diagram explaining a modified example in which a polarizer is connected to a rotation stage. FIG. 9 is a diagram showing the configuration of a measurement system of a third embodiment.

[0032] 1,201,301...measurement system, 10...measurement device, 11...base, 13...support frame, 15...support column, 17...connection part, 20...rotating table, 21...table body, 40...driving mechanism, 41...electric rotary, 43...first adjustment stage, 45...second adjustment stage, 50...linear actuator, 60...probe, 70...optical system, 71...light source, 73...polarization controller, 75...half mirror, 77,377...quarter-wave plate, 78...condensing lens, 80...photodetector, 85...signal processor, 90...controller, 100...analysis device, 101...processor, 103...memory, 105...storage, 107...input / output interface, 109...user interface, 279,379...polarizer, 279A,379A...rotation mechanism.

[0033] Exemplary embodiments of the present disclosure will now be described with reference to the drawings.

[0034] First Embodiment A measurement system 1 of this embodiment shown in Fig. 1 is configured to measure the thickness of a sample wafer 5. The sample wafer 5 is a semiconductor wafer. Hereinafter, the semiconductor wafer as the object for thickness measurement will be referred to as the sample wafer 5. The sample wafer 5 has a thin, disk-like outer shape.

[0035] Specifically, the measurement system 1 is configured to measure the thickness of the sample wafer 5 using a spectral interference method. The measurement system 1 includes a measurement device 10 and an analysis device 100. As shown in Figures 1 and 2, the measurement device 10 includes a turntable 20 and a thickness measurement probe 60. As shown in Figure 2, the probe 60 is disposed opposite the turntable 20 that supports the sample wafer 5.

[0036] The turntable 20 is provided on the upper surface of the base 11 of the measurement device 10. The turntable 20 includes a disk-shaped table body 21 configured to support the sample wafer 5, and a drive mechanism 40 for rotating and translating the table body 21.

[0037] The table main body 21 is configured to have a rotation plane on a plane perpendicular to the height direction. The plane perpendicular to the height direction corresponds to the top surface of the base 11, the bottom surface of the measuring device 10, and the installation surface of the measuring device 10. The table main body 21 is configured so that the rotation plane can support the sample wafer 5. The sample wafer 5 is supported on the table main body 21 parallel to the rotation plane.

[0038] The drive mechanism 40 includes an electric rotary 41 for rotating the table body 21, a first adjustment stage 43 for moving the table body 21 in the lateral direction, and a second adjustment stage 45 for moving the table body 21 in the vertical direction. The lateral direction is a plane perpendicular to the vertical direction, i.e., a direction along the rotation plane of the table body 21.

[0039] The electric rotary 41 is configured to be able to drive the table body 21 to rotate the table body 21 around its central axis. This rotation of the table body 21 rotates the sample wafer 5 supported by the table body 21. In Figure 2, an example of the rotation direction of the sample wafer 5 is indicated by a dashed arrow RA.

[0040] The rotation of the sample wafer 5 changes the relative position of the sample wafer 5 with respect to the probe 60. The electric rotary 41 is used to realize thickness measurement at a plurality of points in the circumferential direction of the sample wafer 5.

[0041] The first adjustment stage 43 is disposed below the electric rotary 41 and is configured to be capable of moving the table body 21 two-dimensionally in the lateral direction. The first adjustment stage 43 is used to position the table body 21 at a specified position in the internal space of the measurement device 10.

[0042] The second adjustment stage 45 is disposed below the first adjustment stage 43 and is configured to be able to move the table body 21 in the height direction. The second adjustment stage 45 is used to adjust the focal length of the measurement light that is output from the probe 60 and is used for thickness measurement.

[0043] 2, the probe 60 is mounted on a support frame 13 fixed to the upper surface of the base 11. The support frame 13 includes a pair of support columns 15 and a connecting portion 17. The pair of support columns 15 are erected on the upper surface of the base 11 with a gap between them so as to sandwich the rotating table 20. The connecting portion 17 is fixed to the upper end portions of the pair of support columns 15 so as to be parallel to the upper surface of the base 11.

[0044] A linear actuator 50 is provided at the connecting portion 17 of the support frame 13. The linear actuator 50 supports a probe 60 so that the probe 60 can move linearly in the lateral direction. The probe 60 is driven by the linear actuator 50 to move one-dimensionally in the lateral direction (the direction indicated by the solid double-headed arrow RB in FIG. 2 ) above the sample wafer 5 supported on the table main body 21.

[0045] That is, the linear actuator 50 is configured so that the irradiation position of the measurement light output from the probe 60 on the sample wafer 5 can be changed in the radial direction of the turntable 20 by moving the probe 60 .

[0046] In this way, the measuring device 10 is configured so that the sample wafer 5 rotates around the central axis through the rotation of the electric rotary 41. The probe 60 is arranged to be movable via the linear actuator 50 in parallel to the rotation plane of the table body 21, or more specifically, in the radial direction of the sample wafer 5 along the surface of the sample wafer 5.

[0047] As the sample wafer 5 rotates, the irradiation position of the measurement light irradiated onto the sample wafer 5 from the probe 60 changes in the circumferential direction. As the probe 60 moves by the linear actuator 50, the irradiation position of the measurement light on the sample wafer 5 changes in the radial direction.

[0048] In this embodiment, the turntable 20 rotates when measuring the thickness of the sample wafer 5. The probe 60 irradiates the sample wafer 5, which rotates together with the turntable 20, with measurement light while moving linearly laterally. As a result, the irradiation position of the measurement light, which serves as the thickness measurement point, changes in a spiral pattern on the surface of the sample wafer 5. In other words, the irradiation position changes in a spiral pattern from the center of the sample wafer 5. In this embodiment, the thickness of the entire surface of the sample wafer 5 is measured by scanning the measurement light in this manner.

[0049] This measurement method is called a spiral scan method. That is, in the spiral scan method, measurement light is irradiated onto the sample wafer 5 that rotates together with the turntable 20 so that the irradiation position of the measurement light moves in the radial direction of the turntable 20, and the thickness of the sample wafer 5 at multiple points on the movement path of the measurement light is measured based on the light-receiving signals.

[0050] Next, the detailed configuration of the probe 60 will be described with reference to Fig. 1. The probe 60 includes an optical system 70 configured to output measurement light, a light receiver 80 configured to receive a reflected component of the measurement light from the sample wafer 5 (hereinafter simply referred to as reflected light), and a signal processor 85 for calculating the power spectrum of the received light signal.

[0051] The optical system 70 includes a light source 71 , a polarization controller 73 , a half mirror 75 , a quarter-wave plate 77 , and a condenser lens 78 .

[0052] The light source 71 is configured to output light in a predetermined wavelength band used to generate measurement light. The light output from the light source 71 is converted into linearly polarized light by a polarization controller 73 and then propagates to a half mirror 75.

[0053] The polarization controller 73 converts the light input from the light source 71 into linearly polarized light. The polarization controller 73 converts the light input from the light source 71 into linearly polarized light tilted at 45 degrees with respect to the crystal axis of the quarter-wave plate 77 so that the linearly polarized light is converted into circularly polarized light through the quarter-wave plate 77.

[0054] The half mirror 75 is configured to reflect light incident from the light source 71 through the polarization controller 73 toward the sample wafer 5 supported on the rotation surface of the turntable 20, and to transmit the reflected light from the sample wafer 5 toward the photodetector 80.

[0055] The quarter-wave plate 77 is disposed so as to convert the linearly polarized light propagating from the light source 71 through the polarization controller 73 and half mirror 75 into circularly polarized measurement light. The circularly polarized measurement light from the quarter-wave plate 77 is condensed through a condenser lens 78 and irradiated onto the sample wafer 5. The measurement light is incident on the sample wafer 5 parallel to the height direction.

[0056] The light incident on the sample wafer 5 is reflected by the front and back surfaces of the sample wafer 5. The reflected light passes through the condenser lens 78 and the quarter-wave plate 77, propagates to the half mirror 75, transmits through the half mirror 75, and propagates to the light receiver 80.

[0057] The light receiver 80 is configured to receive the light reflected from the sample wafer 5 and input the received light signal to the signal processor 85. The signal processor 85 is configured to calculate the power spectrum of the received light signal and output the power spectrum. The thickness of the sample wafer 5 is measured by analyzing this power spectrum.

[0058] As described above, the reflected light from the sample wafer 5 includes surface reflected light, which is a component of the measurement light reflected from the surface of the sample wafer 5, and back surface reflected light, which is a component of the measurement light reflected from the back surface of the sample wafer 5.

[0059] When the physical distance (i.e., thickness) between the front and back surfaces of the sample wafer 5 is D, there exists an optical distance n×2D between the front and back surfaces according to the refractive index n of the sample wafer 5. This optical distance n×2D appears as a phase difference between the back surface reflected light and the front surface reflected light.

[0060] That is, the received light signal includes an interference signal corresponding to the phase difference caused by interference between the front-surface reflected light and the back-surface reflected light. The frequency of this interference signal corresponds to the optical distance n×2D of the sample wafer 5.

[0061] Therefore, in an environment where the refractive index n of the sample wafer 5 is known, the thickness of the sample wafer 5, i.e., the physical distance D between the front and back surfaces of the sample wafer 5, can be determined by identifying the frequency of the interference signal based on the power spectrum.

[0062] However, if the sample wafer 5 exhibits birefringence, the sample wafer 5 exhibits different refractive indices for ordinary and extraordinary light. This causes multiple peaks to appear in the power spectrum as peaks corresponding to the interference signal. These multiple peaks may have an undesirable effect on thickness measurement.

[0063] Therefore, in this embodiment, the adverse effects on thickness measurement due to the birefringence of the sample wafer 5 are suppressed by irradiating the sample wafer 5 with circularly polarized measurement light. As a reference example, the characteristics of the power spectrum when the measurement light output from the probe 60 is not circularly polarized but is a single linearly polarized wave will be described.

[0064] According to this embodiment, the sample wafer 5 is rotated during thickness measurement. Therefore, the two orthogonal birefringence axes present in the sample wafer 5 rotate with respect to the linearly polarized wave output from the probe 60. In other words, when the measurement light is linearly polarized, the relative angle between the two birefringence axes present in the sample wafer 5 and the linearly polarized wave changes as the sample wafer 5 rotates.

[0065] When the direction of the linearly polarized wave irradiated onto the sample wafer 5 coincides with either of the two birefringence axes, a single peak corresponding to the interference signal appears in the power spectrum. When the sample wafer 5 is rotated so that the direction of the linearly polarized wave irradiated onto the sample wafer 5 does not coincide with either of the two birefringence axes, the linearly polarized wave is decomposed into components corresponding to the two birefringence axes, i.e., ordinary light and extraordinary light.

[0066] In this case, the optical receiver 80 receives reflected light containing ordinary light and extraordinary light. Therefore, two peaks corresponding to the interference signal appear in the power spectrum. The two peaks include one due to ordinary light and one due to extraordinary light. In other words, the single peak is split into a peak due to ordinary light and a peak due to extraordinary light.

[0067] Even if the power of the linearly polarized wave irradiated onto the sample wafer 5 is constant, the power of the interference signal corresponding to the ordinary light and the power of the interference signal corresponding to the extraordinary light change depending on the angle of the linearly polarized wave with respect to the birefringence axis.

[0068] Due to the change in the angle of the linearly polarized wave relative to the birefringence axis, the power of the two peaks in the power spectrum fluctuates with rotation. When the linearly polarized wave coincides with one of the two birefringence axes, one of the peaks disappears in the power spectrum. When such peak fluctuations occur, the thickness of the sample wafer 5 cannot be measured accurately using conventional thickness measurements that use the center of gravity or midpoint of the two peaks.

[0069] Therefore, in this embodiment, circularly polarized waves are used as the measurement light, so that the probe 60 can irradiate the measurement light onto the sample wafer 5 so that the measurement light is an isotropic input polarized wave even if the birefringence axis of the sample wafer 5 rotates.

[0070] If the measurement light is isotropic with respect to the birefringence axis of the rotating sample wafer 5, the two peaks of the power spectrum do not fluctuate due to the rotation of the sample wafer 5. Therefore, by focusing on one of the two peaks, either the high or low frequency one, and identifying the frequency of that peak, it is possible to measure the thickness of each point on the sample wafer 5.

[0071] The frequency relationship between the peak due to ordinary light and the peak due to extraordinary light depends on the material of the sample wafer 5. When the material of the sample wafer 5 is known, the peak due to ordinary light can be identified from the magnitude of the frequencies of the two peaks. In this case, if the thickness measurement is performed while focusing on the peak due to ordinary light, more stable thickness measurement can be achieved.

[0072] According to this embodiment, the measuring device 10 is provided with a controller 90. The controller 90 is configured to perform control necessary to measure the thickness of each measurement point on the sample wafer 5 by the spiral scan method, specifically, control the drive mechanism 40, the linear actuator 50, and the optical system 70, based on instructions from the analyzing device 100. The control of the optical system 70 includes control related to the output of the measurement light from the light source 71 and control of the polarization angle of the polarization controller 73.

[0073] The controller 90 is communicatively connected to the analytical device 100. The analytical device 100 is configured to be a computer system such as a personal computer. The analytical device 100 is configured to control the measurement device 10 to irradiate the sample wafer 5 with measurement light in a spiral manner using a spiral scan method, analyze the power spectrum of each of the multiple measurement points obtained thereby, and measure the thickness of the sample wafer 5 at each measurement point.

[0074] 1, analysis device 100 includes processor 101, memory 103, storage 105, input / output interface 107, and user interface 109. Memory 103 includes RAM, and is used as a work area when processor 101 executes processing.

[0075] The storage 105 is configured by, for example, a hard disk drive or a solid state drive, and stores computer programs executed by the processor 101 and data used when the computer programs are executed.

[0076] The input / output interface 107 is configured to be able to input a control signal to the measurement device 10 and further to be able to acquire the power spectrum of each measurement point on the sample wafer 5 from the measurement device 10. The input / output interface 107 can be an interface for serial communication with the measurement device 10.

[0077] The user interface 109 is configured to be able to receive operations from the user and to display various information to the user. The user interface 109 includes a display for displaying various information to the user and operation devices such as a keyboard and a mouse for receiving operations from the user.

[0078] When the sample wafer 5 is set on the turntable 20 and preparations for measuring the thickness of the sample wafer 5 are complete, and an instruction to execute measurement-related processing is input through the user interface 109, the processor 101 starts the measurement-related processing shown in FIG. 3 in accordance with the computer program stored in the storage 105.

[0079] When the measurement-related processing is started, the processor 101 starts the rotation control of the turntable 20 in the measurement device 10 (S110). The processor 101 further starts the drive control of the linear actuator 50 for irradiating the measurement light onto the sample wafer 5 in a spiral pattern using a spiral scan method (S120).

[0080] 4 for each of the plurality of measurement points on the movement path of the measurement light on the sample wafer 5 (S130). When the measurement process for all measurement points is completed (Yes in S140), the processor 101 outputs measurement data recording the thickness of the sample wafer 5 at each measurement point (S150), and ends the measurement-related process shown in FIG.

[0081] In S150, the processor 101 can record the measurement data in the storage 105. The processor 101 can display a screen visually representing the thickness distribution of the sample wafer 5 to the user via the user interface 109.

[0082] The processor 101 can perform rotation control of the turntable 20 and drive control of the linear actuator 50 so as to stop the drive of the turntable 20 and the linear actuator 50 when the acquisition of power spectra for all measurement points is completed.

[0083] 4, in the measurement process for each measurement point, the processor 101 controls the optical system 70 via the controller 90 to cause the optical system 70 to output measurement light. The measurement light is irradiated onto the sample wafer 5 rotating together with the turntable 20 (S210). The processor 101 further causes the signal processor 85 to generate a power spectrum of the received light signal input from the photodetector 80 (S210).

[0084] As a result, the processor 101 acquires the power spectrum of the received light signal from the signal processor 85 via the controller 90 (S210). The power spectrum is data that represents the power of the received light signal (i.e., the intensity of received light) for each frequency.

[0085] The power spectrum represents the distribution of power versus frequency for the received light signal. This power spectrum corresponds to the power spectrum of the reflected light corresponding to the measurement light at the measurement point. As described above, the power spectrum includes the interference signal between the front-surface reflected light and the back-surface reflected light.

[0086] Next, the processor 101 detects the peak of the interference signal in the acquired power spectrum (S220). Here, the processor 101 detects the peak derived from ordinary light as the specific peak based on the information about the material of the sample wafer 5 input in advance.

[0087] As described above, when the sample wafer 5 is made of a birefringent material, the power spectrum contains, as peaks related to the interference signal, a peak derived from ordinary light and a peak derived from extraordinary light. Based on information about the material of the sample wafer 5 obtained in advance, it is possible to identify which peak is the ordinary light peak from the magnitude of the frequencies of the two peaks.

[0088] The reason for detecting the peak resulting from ordinary light is that it is expected that the intensity fluctuation of the ordinary light peak will be smaller than that of the extraordinary light peak, and therefore it is expected that the thickness can be measured with high accuracy.

[0089] The processor 101 then extracts multiple data points around the peak, specifically the data point with the maximum power and a predetermined number of data points before and after it on the frequency axis, as reference data points (S230).

[0090] For example, as shown in Figure 5, the processor 101 can extract three data points P1, P2, and P3 as reference data points P1, P2, and P3: the data point P1 with the maximum power and the data points P2 and P3 one before and one after it on the frequency axis.

[0091] The processor 101 calculates a quadratic function that approximates the frequency vs. power distribution of the extracted reference data points P1, P2, and P3 by fitting the data points P1, P2, and P3 to an upward convex quadratic function (S240).

[0092] Thereafter, the processor 101 identifies the frequency at the maximum point of the approximated quadratic function, and calculates the thickness of the sample wafer 5 at the measurement point based on the identified frequency (S250). That is, the processor 101 calculates the thickness corresponding to the maximum point of the quadratic function as the thickness of the sample wafer 5.

[0093] The processor 101 regards the identified frequency as the frequency of the interference signal corresponding to the phase difference between the front-surface reflected light and the back-surface reflected light, and can calculate from the frequency the optical distance Z between the front and back surfaces of the sample wafer 5. The processor 101 can calculate the thickness D of the sample wafer 5 in accordance with the distance Z, a predetermined refractive index n of the sample wafer 5 for ordinary light, and the relational expression Z=n×2D.

[0094] The processor 101 can record the thickness D of the sample wafer 5 calculated in this way in the memory 103 as a measured thickness value while associating it with the position coordinates of the measurement point (S250).

[0095] As is known, the power spectrum obtained by digitally frequency-converting (e.g., fast Fourier transforming) the received light signal has discrete data points of frequency versus power at a resolution corresponding to the sampling frequency and block size of the received light signal.

[0096] By measuring the thickness of the sample wafer 5 based on the maximum point of the quadratic function obtained by fitting a function to such a power spectrum, errors caused by the resolution of the power spectrum can be suppressed and the thickness of the sample wafer 5 can be measured with high accuracy.

[0097] The processor 101 executes the measurement process for each measurement point as described above. In step S150, the processor 101 records the measurement data, which is a compilation of the thicknesses D at each measurement point, in the storage 105 and visually displays the data to the user via the user interface 109.

[0098] According to the measurement system 1 of this embodiment described above, circularly polarized waves are used as measurement light, which allows the sample wafer 5 to be irradiated with measurement light that is isotropic with respect to the birefringence axis of the rotating sample wafer 5.

[0099] Therefore, with this measurement system 1, fluctuations in the peaks of the interference signals resulting from ordinary light and extraordinary light can be suppressed in the power spectrum, and by focusing on one peak, the thickness of each measurement point on the sample wafer 5 can be measured with high accuracy. In particular, in this embodiment, the thickness of each measurement point is measured by focusing on the peak resulting from ordinary light, so that the thickness of each measurement point can be measured simply and with high accuracy.

[0100] In this embodiment, the received light intensities of multiple frequency components corresponding to multiple data points around a peak of interest in the power spectrum are function-fitted using a quadratic function. The thickness corresponding to the maximum point of the received light intensity in the quadratic function, which is identified in this way, is calculated as the thickness of the sample wafer 5. Therefore, in this embodiment, it is possible to measure the thickness of the sample wafer 5 with high accuracy while suppressing the influence of the frequency resolution of the power spectrum.

[0101] Second Embodiment Next, a measurement system 201 of a second embodiment will be described with reference to Fig. 6 and Fig. 7. The measurement system 201 of this embodiment is basically the same as the measurement system 1 of the first embodiment, except that it includes a polarizer 279 and that a controller 90 controls the rotation of the polarizer 279.

[0102] Therefore, among the components included in the measurement system 201 of this embodiment, the components corresponding to the components included in the measurement system 1 of the first embodiment are assigned the same reference numerals as in the first embodiment. In the measurement system 201, the components assigned the same reference numerals may be understood to be the same as the components with the same reference numerals included in the measurement system 1 of the first embodiment, unless additional explanation is provided.

[0103] 6 , the measurement system 201 of this embodiment includes a polarizer 279 in the probe 60 of the measurement device 10, on the propagation path of the measurement light between the optical system 70 and the rotation table 20. A rotation mechanism 279A is attached to the polarizer 279. The rotation mechanism 279A is controlled by the controller 90 to rotate the polarizer 279 around an axis parallel to the propagation direction of the measurement light at the same angular velocity as the rotation table 20.

[0104] When the turntable 20 is stationary and not rotating, the polarizer 279 converts the measurement light, which has been converted into a circularly polarized wave by the quarter-wave plate 77, into a linearly polarized wave that oscillates in a fixed direction. When the turntable 20 is rotating, the polarizer 279 rotates at the same angular velocity as the turntable 20, thereby converting the measurement light from the quarter-wave plate 77 into a linearly polarized wave that oscillates in a fixed direction in the rotating coordinate system of the turntable 20. In other words, the polarizer 279 converts the measurement light from the quarter-wave plate 77 into a linearly polarized wave that rotates synchronously with the turntable 20 at the same angular velocity in the fixed coordinate system.

[0105] As a result, even when the turntable 20 rotates, the sample wafer 5, which rotates together with the turntable 20, is irradiated with linearly polarized light as measurement light, the polarization plane of which is constant relative to the birefringence axis of the sample wafer 5.

[0106] In the processing of S110, the processor 101 can start the rotation control of the polarizer 279 together with the rotation control of the turntable 20 via the controller 90. The rotation control of the polarizer 279 is a control for rotating the polarizer 279 in synchronization with the turntable 20.

[0107] Synchronous rotation can be achieved, for example, by providing the drive mechanism 40 and the rotation mechanism 279A with sensors (e.g., rotary encoders) for measuring the rotation angle or angular velocity, and controlling the drive mechanism 40 and the rotation mechanism 279A based on the output from the sensors.

[0108] According to the measurement system 201 of the second embodiment configured as described above, if the precision error of the rotation control is ignored, the direction of the measurement light is constant with respect to the birefringence axis of the sample wafer 5. Therefore, the peaks resulting from the ordinary light and extraordinary light contained in the interference signal appearing in the power spectrum do not fluctuate and are always constant. When controlling the rotation of the polarizer 279, if the rotation phase of the polarizer 279 is adjusted so that only the peak of the ordinary light appears, only the peak resulting from the ordinary light appears in the power spectrum.

[0109] Therefore, in this embodiment, as in the first embodiment, fluctuations in the peaks of the interference signals resulting from ordinary light and extraordinary light can be suppressed in the power spectrum, and the thickness D of each measurement point on the sample wafer 5 can be measured with high accuracy by focusing on one peak, for example, the peak of ordinary light.

[0110] In this embodiment, particularly when the difference in refractive index between ordinary light and extraordinary light is small, it is possible to accurately measure the thickness of the sample wafer 5. When the difference in refractive index between ordinary light and extraordinary light is small, a peak derived from ordinary light and a peak derived from extraordinary light may be combined in the power spectrum, resulting in the appearance of a single peak.

[0111] Fig. 7 illustrates that a single peak C0 appears as a result of the combination of a peak C1 derived from ordinary light and a peak C2 derived from extraordinary light. In the frequency vs. power graph shown in Fig. 7, the peak C1 derived from ordinary light is represented by a thick solid line, the peak C2 derived from extraordinary light is represented by a dashed line, and the combined peak C0 is represented by a dashed line.

[0112] According to this embodiment, not only can the peaks resulting from ordinary light and extraordinary light be kept constant, but also the peak C2 resulting from extraordinary light can be reduced and the peak C1 resulting from ordinary light can be enhanced by phase adjustment, thereby suppressing thickness measurement errors due to peak synthesis.

[0113] Therefore, according to this embodiment, it is possible to measure the thickness D of the sample wafer 5 with high accuracy even for a material with a small difference between the ordinary and extraordinary refractive indices.

[0114] In addition, the polarizer 279 does not have to be rotated synchronously with the rotation stage 20 by the rotation control. For example, the polarizer 279 may be arranged outside the probe 60 so as to be coupled to the rotation stage 20.

[0115] 8, the polarizer 279 is fixed to the turntable 20 by a connecting member 279B, and is thereby arranged to physically rotate synchronously with the turntable 20. In this modification, the rotation mechanism 279A does not need to be attached to the polarizer 279. In this modification, an example can be considered in which the irradiation position of the measurement light on the turntable 20 and the sample wafer 5 is not changed.

[0116] Alternatively, a polarizer capable of polarizing the incident measurement light in a wide range corresponding to the movement range of the irradiation position of the measurement light may be installed as the polarizer 279. Instead of the polarizer 279, a polarizer having a movement mechanism that can move laterally on the turntable 20 in accordance with the movement of the propagation path of the measurement light accompanying the movement of the probe 60 may be provided.

[0117] Third Embodiment Next, a measurement system 301 of a third embodiment will be described with reference to Fig. 9. The measurement system 301 of this embodiment is basically the same as the measurement system 201 of the second embodiment, except that the installation position of the polarizer 379 is different from that of the second embodiment.

[0118] Therefore, among the components included in the measurement system 201 of this embodiment, the components corresponding to the components included in the measurement system 201 of the second embodiment are given the same reference numerals as in the second embodiment. In the measurement system 301, the components given the same reference numerals may be understood to be the same as the components with the same reference numerals included in the measurement system 201 of the second embodiment and the measurement system 1 of the first embodiment, unless additional explanation is provided.

[0119] As shown in FIG. 9, a measurement system 301 of this embodiment includes an optical system 370 in a probe 60 of a measurement device 10, the optical system 370 including a first optical system 370A, a polarizer 379, and a second optical system 370B.

[0120] The first optical system 370A includes a light source 71, a polarization controller 73, and a quarter-wave plate 377. In the first optical system 370A, similar to the optical system 70 of the first embodiment, the output light from the light source 71 is converted into linearly polarized light by the polarization controller 73.

[0121] In the polarization controller 73, the input light from the light source 71 is converted into a linearly polarized wave tilted by 45 degrees with respect to the crystal axis of the quarter-wave plate 377 so that the linearly polarized wave is converted into a circularly polarized wave through the quarter-wave plate 377.

[0122] The quarter-wave plate 377 is arranged to convert the linearly polarized light propagating from the light source 71 through the polarization controller 73 into circularly polarized measurement light. This circularly polarized measurement light is converted back into linearly polarized light by the polarizer 379. However, the linearly polarized light generated by the polarizer 379 rotates in synchronization with the rotation table 20.

[0123] The polarizer 379 is provided on the propagation path of the measurement light between the first optical system 370A and the rotary table 20. The polarizer 379 is provided between the first optical system 370A (particularly, the quarter-wave plate 377) and the second optical system 370B (particularly, the half mirror 75).

[0124] The polarizer 379 is provided with a rotation mechanism 379A, similar to the polarizer 279 of the second embodiment. The rotation mechanism 379A is controlled by the controller 90 to rotate the polarizer 379 around an axis parallel to the propagation direction of the measurement light at the same angular velocity as the rotation table 20.

[0125] The second optical system 370B includes a half mirror 75 and a condenser lens 78. The measurement light converted by the polarizer 379 into linearly polarized light that rotates synchronously with the rotation table 20 propagates to the half mirror 75.

[0126] The half mirror 75 is configured to reflect the measurement light from the polarizer 379 toward the sample wafer 5 supported on the rotation surface of the turntable 20 and transmit the reflected light from the sample wafer 5 toward the photodetector 80.

[0127] The measurement light traveling from the half mirror 75 toward the turntable 20 is condensed by a condenser lens 78 while propagating to the turntable 20, and is then irradiated onto the sample wafer 5. The measurement light is incident on the sample wafer 5 parallel to the height direction.

[0128] As described above, the measurement light incident on the sample wafer 5 is linearly polarized light that rotates synchronously with the turntable 20, and is linearly polarized light with a constant orientation of the polarization plane relative to the turntable 20. In other words, the measurement light incident on the sample wafer 5 is linearly polarized light with a constant orientation of the polarization plane relative to the birefringence axis of the sample wafer 5.

[0129] Therefore, the same effect as in the second embodiment can be obtained in this embodiment. That is, it is possible to suppress the influence of birefringence and measure the thickness D of the sample wafer 5 with high accuracy. In particular, it is possible to measure the thickness D of the sample wafer 5 with high accuracy even for materials with a small difference between the ordinary refractive index and the extraordinary refractive index.

[0130] [Other Embodiments] The present disclosure is not limited to the above-described embodiments and can adopt various aspects. For example, the function of the analysis device 100 may be incorporated into the measurement device 10. That is, the controller 90 may calculate the thickness D of the sample wafer 5 at the corresponding measurement point based on the power spectrum.

[0131] The measurement light may be a combination of two linearly polarized waves that are orthogonal to each other, instead of a circularly polarized wave. Using such a measurement light also makes it possible to suppress the fluctuation of the two peaks resulting from ordinary and extraordinary light in the power spectrum, just as with circularly polarized waves.

[0132] In this case, the quarter-wave plate 77 can be removed from the optical system 70, and a polarization controller capable of generating two linearly polarized waves that are orthogonal to each other can be provided as the polarization controller 73. In the above embodiment, the optical system 70 includes the condenser lens 78, but the condenser lens 78 does not have to be provided.

[0133] In addition, when frequency conversion is performed on the received light signal, zero data may be added before and after the sampling data of the received light signal by zero padding processing to increase the block size of the data to be converted, thereby increasing the frequency resolution in the power spectrum.

[0134] In the function fitting in S240, a polynomial function other than a quadratic function may be used. The present disclosure is not limited to application to the spiral scan method using the rotary table 20. The technology of the present disclosure is applicable to various measurement devices that measure the thickness D of the sample wafer 5 in an environment in which the sample wafer 5 is displaced relative to the measurement light.

[0135] The function of one component in the above embodiments may be distributed among multiple components. The functions of multiple components may be integrated into one component. Part of the configuration of the above embodiments may be omitted. At least part of the configuration of the above embodiments may be added to or substituted for the configuration of another of the above embodiments. All aspects included in the technical idea identified from the wording of the claims are embodiments of the present disclosure.

Claims

1. A measurement system for measuring the thickness of an object using a spectral interference method, comprising: a turntable configured to support the object; an optical system configured to output measurement light toward the object; a light receiving unit configured to receive a reflected component of the measurement light from the object; and a control unit configured to control the optical system to output the measurement light toward the object rotating together with the turntable, and to measure the thickness of a point on the object corresponding to the reflection point of the measurement light based on a light receiving signal of the reflected component input from the light receiving unit, wherein the optical system outputs, as the measurement light, a circularly polarized wave or a combination of two linearly polarized waves that are orthogonal to each other.

2. A measurement system according to claim 1, further comprising a polarizer, wherein the polarizer is configured to rotate around an axis parallel to the propagation direction of the measurement light at the same angular velocity as the rotation table in the propagation path of the measurement light between the optical system and the rotation table, thereby converting the measurement light from the optical system into linearly polarized waves with a constant orientation of the polarization plane relative to the rotation table.

3. A measurement system according to claim 1 or 2, wherein the control unit is configured to measure the thickness of the object at multiple points on the object by focusing on one specific peak among multiple peaks that appear in the power spectrum of the received light signal due to interference of reflected components from the front and back surfaces of the object.

4. A measurement system according to claim 3, wherein the control unit is configured to measure the thickness of the object at the multiple points by focusing on a peak derived from ordinary light as the one specific peak among the multiple peaks.

5. A measurement system according to any one of claims 1 to 4, wherein the optical system is capable of changing the irradiation position of the measurement light in the radial direction of the turntable, and the control unit moves the irradiation position of the measurement light in the radial direction of the turntable using a spiral scan method in order to measure the thickness of the object at multiple points on the object, and measures the thickness of the object at multiple points on the movement path of the measurement light based on the light reception signal.

6. A measurement method for measuring the thickness of an object using a spectral interference method, comprising: placing the object on a table; controlling an optical system to cause the optical system to output a circularly polarized wave or a combination of two linearly polarized waves that are orthogonal to each other as measurement light toward the object; receiving a reflected component of the measurement light from the object; and measuring the thickness of a point on the object corresponding to the reflection point of the measurement light based on a received light signal of the reflected component.

7. A measurement method according to claim 6, wherein the table is a rotating table, and the outputting includes causing the optical system to output the measurement light toward the object rotating together with the rotating table.

8. A measurement method according to claim 7, further comprising: converting the measurement light from the optical system into linearly polarized light whose polarization plane is oriented in a constant direction relative to the rotary table, using a polarizer that rotates around an axis parallel to the propagation direction of the measurement light at the same angular velocity as the rotary table, in the propagation path of the measurement light between the optical system and the rotary table; and irradiating the measurement light converted into linearly polarized light onto the object.

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