Non-contact wafer thickness measuring device

Abstract

The present invention is characterized by being provided with: a monolithic-type wavelength sweeping semiconductor laser light source (12) having a laser source (14), a laser control unit (16) for controlling the laser source (14), and a processor (18) configured to cause the laser control unit (16) to control the laser source (14) so as to oscillate laser light, the wavelength of which is changed according to a setting profile with respect to time; an optical system (20, 22) that guides the laser light to a wafer (24) and irradiates the wafer with the laser light; a detection unit (26) that detects an interference light signal of reflected light; an A / D converter (28) that converts, to a digital signal, the interference light signal detected by the detection unit (26); and a calculation unit (30) that calculates the thickness of the wafer (24) by analyzing the digital signal from the A / D converter (28), wherein the processor (18) causes the laser control unit (16) to operate on the basis of a clock signal and to oscillate laser light, from the laser source (14), so as to be wavelength-swept with the setting profile with respect to time, and the A / D converter (28) performs A / D conversion of the interference light signal by generating a sampling clock in synchronization with the clock signal or directly using the same as the sampling clock.

Description

[Technical field]

[0001] The present invention relates to a non-contact wafer thickness measurement device using a monolithic wavelength swept semiconductor laser light source. [Background technology]

[0002] In a polishing apparatus for semiconductor wafers such as silicon wafers, double-sided or single-sided polishing of the wafers is performed to process the wafers to a required thickness. Semiconductor wafers are used for highly integrated semiconductor functional devices, but increasing integration density and efforts to improve productivity require more accurate thickness measurements during and between fabrication processes. Patent Document 1 (JP Patent Publication No. 7-306018) discloses a non-contact semiconductor thickness measuring device that uses a tunable laser.

[0003] In the non-contact semiconductor thickness measurement device shown in Patent Document 1, when a light beam that passes through the semiconductor is irradiated onto a target region of the semiconductor to be measured, a portion of the light is reflected by the top and bottom surfaces and interferes, causing it to be reflected or transmitted. This reflected or transmitted interference light is guided to a detector by optical means, and the wavelength of the light beam is changed within a predetermined range. At that time, the phase (or period) of the interference light intensity changes depending on the semiconductor thickness, so the absolute value of the semiconductor thickness is calculated from the change in the intensity of the interference light. The wavelength tunable laser may be, for example, a wavelength swept laser light source as disclosed in Patent Document 2 (JP Patent Publication 2006-80384 A). Alternatively, a wavelength swept laser light source as shown in FIG. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-306018 [Patent Document 2] JP 2006-80384 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the wavelength-swept laser light source shown in Patent Document 2 uses a mechanical operating part such as a polygon mirror to obtain the swept wavelength. When a mechanical operating part such as a polygon mirror is used, there is a problem that a slight distortion of the mirror surface causes a slight shift in the reflected light, which causes a shift in the swept wavelength. In addition, the presence of a mechanical operating part causes a slight mechanical vibration, which becomes a cause of a shift in the swept wavelength. Therefore, even if an optical interference signal is obtained to measure the thickness of the same location, converted into a digital signal, and processed by fast Fourier transform, the center frequency is shifted, resulting in an accidental error in the thickness. [Means for solving the problem]

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a non-contact wafer thickness measurement device that can measure wafer thickness with high accuracy by using laser light that oscillates a linear sweep wavelength with high repeatability.

[0007] In order to achieve the above object, the present invention has the following configuration. That is, the non-contact wafer thickness measurement device according to the present invention is a wafer thickness measurement device comprising: a monolithic wavelength swept semiconductor laser light source having a laser source, a laser control unit for controlling the laser source, and a processor configured to control the laser source by the laser control unit so as to oscillate a laser light having a wavelength that changes with respect to time according to a set profile; an optical system for guiding the laser light to a measurement portion of a wafer whose thickness is to be determined and irradiating the laser light; a detection unit for detecting an interference light signal of reflected light or transmitted light obtained from the measurement portion; an A / D converter for converting the interference light signal detected by the detection unit into a digital signal; and a calculation unit for calculating the thickness of the wafer by analyzing the digital signal from the A / D converter, wherein the processor operates the laser control unit by a clock signal generated by the processor to oscillate a laser light from the laser source whose wavelength is swept with respect to time according to a set profile, and the A / D converter generates a sampling clock in synchronization with the clock signal generated by the processor or directly uses it as a sampling clock to A / D convert the interference light signal.

[0008] The monolithic wavelength swept semiconductor laser light source is characterized in that it does not have any mechanically moving parts. The wavelength sweep frequency can be set to 1 to 40 kHz. The wavelength range to be swept can be from 1200 to 1400 nm. The frequency of the clock signal may be in the range of 10 MHz to 1 GHz. The thickness of the wafer can be measured by continuously detecting the interference light signal while moving the wafer or the probe at a constant speed.

[0009] The present invention can be used as a wafer thickness measuring device when wafers are polished by a double-sided polishing machine or a single-sided polishing machine. The semiconductor monolithic wavelength swept laser light source may be a light source equivalent to SLE-101 manufactured by Insight Photonic Solutions, Inc., USA.

[0010] A circuit for monitoring the source using a reference wafer can be provided. Alternatively, a light source monitoring circuit using an MZI interferometer can be provided. The light source monitoring circuit can monitor the amount of laser light emitted from the laser source from the average value of the voltage output from the detection section. Alternatively, the light source monitoring circuit can be used to monitor the sweep wavelength accuracy, which can be grasped by the average value, PP value, or deviation value of a set number of measurements, for the frequency of the FFT peak value of the interference waveform acquired by the detection unit. Effect of the Invention

[0011] According to the present invention, the following advantageous effects are obtained. That is, according to the present invention, by using a monolithic wavelength-swept semiconductor laser light source as the light source, the swept wavelength is repeated with high accuracy every time, and the processor generates a clock signal, and an interference signal is obtained by the laser light which continuously and linearly sweeps the wavelength based on this clock signal. In addition, a sampling clock is generated in synchronization with this clock signal, or a sampling clock is directly used as the sampling clock to perform A / D conversion of the interference light signal, so that the interference action of the laser light due to the swept wavelength and the analysis of the interference light signal by fast Fourier transform are performed in synchronization, thereby enabling good thickness measurement of the wafer with little accidental error. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a block diagram showing the entire system of a non-contact wafer thickness measuring device according to the present embodiment. [Diagram 2] Figures 2A to D show examples of overlapping interference waveforms obtained by measuring the same spot on a wafer ten times. [Diagram 3] Figures 3A and 3B are graphs showing the results of three repeated measurements of the wafer thickness in the radial direction (Figure 3A). [Figure 4]FIG. 4 is a block diagram showing the entire system of a non-contact wafer thickness measuring device provided with a light source monitoring circuit using a reference wafer. [Diagram 5] FIG. 5 is a block diagram showing the entire system of a non-contact wafer thickness measuring device provided with a light source monitoring circuit using an MZI interferometer. [Figure 6] FIG. 6 is a block diagram of a system having a conventional mechanical operation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing the entire system of a non-contact wafer thickness measuring device 10 according to the present embodiment. The system 10 includes a monolithic wavelength-swept semiconductor laser source (hereinafter sometimes simply referred to as a laser source) 12 . The monolithic wavelength swept semiconductor laser light source 12 includes a semiconductor laser (laser source) 14 with a wavelength sweep function, a laser control unit 16 that controls the laser source 14 , and a processor 18 that controls the entire system 10 .

[0014] The laser light source 12 is configured as a so-called monolithic wavelength swept semiconductor laser light source, which does not have a mechanical operating part such as a polygon mirror, and is configured so that a laser control unit 16 controls the laser source 14 so as to oscillate a laser beam having a wavelength that changes with time according to a set profile by a processor 18. As such a monolithic wavelength swept semiconductor laser light source 12, an SLE-101 light source manufactured by Insight Photonic Solutions, Inc. of the United States can be used.

[0015] Specifically, the current value supplied to the wavelength-tunable semiconductor laser (laser source) 14 is changed (tuned) so that the wavelength of the laser light emitted from the laser source 14 changes to a predetermined set profile (preferably linearly). This series of controls is shown in, for example, JP2014-522105A.

[0016] More specifically, a current is applied to the laser source 14 while tuning the current to be supplied, the oscillated laser light is detected by a detector (not shown) and fed back to the processor 18, and the value of the current to be supplied to the laser source 14 is readjusted, thereby performing feedback control so that the wavelength of the laser light changes continuously, for example, linearly. This series of feedback controls is shown in, for example, JP2014-522105A.

[0017] The processor 18 generates a clock signal of, for example, 400 MHz (at 2.5 nsec intervals), and operates the laser control unit 16 using the clock signal generated by the processor 18 to tune the current value, thereby causing the laser source 14 to emit laser light having a wavelength that changes with time according to a set profile. In the manner described above, the laser source 14 emits laser light whose wavelength changes continuously according to a desired profile (preferably linearly).

[0018] The laser light emitted from the laser source 14 is irradiated onto a measurement site of a wafer 24 through a circulator 20 and a probe 22. The circulator 20, the probe 22, etc. constitute an optical system. The laser light (interference light) reflected from the front and back surfaces of the wafer 24 is detected by a photodiode (detection unit) 26 via the probe 22 and the circulator 20, converted into an electrical signal (interference light signal) by the photodiode 26, and this electrical signal is further amplified by an amplifier.

[0019] The laser light reflected from the front surface and the back surface of the wafer 24 interfere with each other and are observed as interference light having a desired phase. Fig. 2A shows interference waveforms superimposed by measuring the same spot on wafer 24 ten times. Fig. 2B shows an enlarged waveform of part a in Fig. 2A, Fig. 2C shows an enlarged waveform of part b in Fig. 2A, and Fig. 2D shows an enlarged waveform of part c in Fig. 2A.

[0020] As is clear from Figure 2, the interference waveforms measured 10 times at the same point on the wafer 24 and superimposed on each other are almost identical from start to finish. The reason why the interference waveforms are almost identical is believed to be that the use of a monolithic wavelength-swept semiconductor laser light source as the light source makes it possible to stably vary the emission wavelength of the laser light continuously and linearly.

[0021] The interference light signal is converted into a digital signal by an A / D converter 28 and input to a computer (arithmetic unit) 30 . The A / D converter 28 receives an external trigger signal generated by the processor 18 and the clock signal (the clock signal that operates the laser control unit 16), and generates a sampling clock in synchronization with the clock signal or directly uses it as the sampling clock to A / D convert the interference light (electrical) signal. It is also possible to observe interference light between the laser light passing through the wafer 24 and the laser light reflected from the rear surface of the wafer 24, then reflected from the front surface and passing through the rear surface side.

[0022] The computer 30 calculates the thickness of the measurement portion of the wafer 24 from the input interference (electrical) signal by known steps. As a known step, for example, as shown in the above-mentioned Patent Document 1 (JP Patent Publication No. 7-306018), the thickness of the wafer 24 can be measured by detecting the phase change φ of the interference (electrical) signal from an interference relational equation (including the refractive index of the wafer).

[0023] Also, as shown in Patent Document 1, the thickness of the wafer can be obtained by frequency analysis without directly calculating the phase change φ. That is, when the wavelength of the tunable laser is changed by a certain amount Δλ, the interference signal with the wavelength on the horizontal axis becomes a waveform with a small frequency modulation on the center frequency. The thickness of the wafer can be measured from the relational expression between the center frequency and the refractive index. Alternatively, the interference signal can be processed by fast Fourier transform to calculate the center frequency, and the thickness of the wafer 24 can be measured by converting it using a calibration curve obtained in advance and the wafer refractive index.

[0024] Table 1 shows data on the thickness of the measurement portion of the wafer 24 measured by processing the interference signal by fast Fourier transform when the same portion of the wafer 24 is measured ten times. As shown in Table 1, for a wafer with a thickness of approximately 722 μm, good thickness measurements with little variation were obtained with a standard deviation of 2.99 nm in thickness measurements where interference waveforms obtained 10 times at the same position were processed by fast Fourier transform. For a wafer with a thickness of approximately 776 μm, good thickness measurements with little variation were obtained with a standard deviation of 3.03 nm in thickness measurements where interference waveforms obtained 10 times at the same position were processed by fast Fourier transform.

[0025] On the other hand, when a conventional mechanically driven light source was used, the standard deviation of the thickness measurement results for a wafer approximately 722 μm thick, in which the interference waveforms obtained 10 times at the same position were processed by fast Fourier transform, was 65.92 nm, which was a large variance, as shown in Table 1. Also, the standard deviation of the thickness measurement results for a wafer approximately 776 μm thick, in which the interference waveforms obtained 10 times at the same position were processed by fast Fourier transform, was 96.35 nm, which was a large variance.

[0026] Figure 3A is a graph showing the results of three overlapping measurements of the thickness of a φ300 mm wafer in the radial direction, using a monolithic wavelength-swept semiconductor laser source (SLE-101 source manufactured by Insight Photonic Solutions, Inc., USA) shown in Figure 1. The wafer was moved at a speed of 5 mm / sec, and the thickness was measured over the entire radial direction. As shown in Figure 3, the graphs of the three measurement results are completely overlapped with no misalignment. This indicates that the repeat wavelength sweep precision of the light source is extremely high.

[0027] FIG. 3B is a graph showing the results of measuring the thickness of a φ300 mm wafer using a thickness measuring device currently used by the applicant, Kuroda Precision Industries Co., Ltd., KURODA NANOMETRO (registered trademark) 300TT-M. The results of measurement using either device shown in FIG. 3A or FIG. 3B show good agreement in the shape of the upward convexity of the wafer.

[0028] Such good thickness measurements were made possible because the monolithic wavelength-swept semiconductor laser light source was used as the light source, and the swept wavelength was stable every time. Also, the processor 18 generates a clock signal, and obtains an interference signal from the laser light whose wavelength is continuously and linearly changed based on this clock signal, and generates a sampling clock in synchronization with this clock signal or uses it directly as the sampling clock to A / D convert the interference light signal, so that the interference action of the laser light due to the swept wavelength and the analysis by signal processing using the fast Fourier transform of the interference light signal are performed in synchronization with each other, and there is no deviation.

[0029] When measuring the thickness of the wafer 24, the wafer 24 or the probe 22 is moved at a constant speed while continuously detecting interference light signals, thereby making it possible to measure the thickness of the wafer 24. When the wafer 24 is polished by a double-sided polishing machine or a single-sided polishing machine (not shown), the thickness of the wafer 24 can be measured continuously or at regular intervals using the wafer thickness measuring device 10 during the polishing process.

[0030] The wavelength scanning frequency of the laser light can be 1 to 40 kHz. The wavelength range of the laser light can be 1200 to 1400 nm, which is transparent to silicon wafers. The sampling clock speed can be tuned from 10 MHz to 1 GHz to measure the interference waveform. Moreover, the thickness of the wafer can be measured by continuously measuring the interference waveform while moving the wafer 24 or the probe 22 at a constant speed. The present invention is also suitable for measuring the polishing rate of a wafer using a double-sided or single-sided polisher.

[0031] Furthermore, in order to accurately obtain the amount of wavelength change or the absolute value of the wavelength, it is necessary to use a high-precision laser light source 10, which increases costs. Therefore, as shown in Patent Document 1 (JP Patent Publication No. 7-306018), a reference optical system can be provided, and a reference sample having the same composition as the measurement sample and an accurately known thickness can be used to calculate the thickness of the measurement sample from the relationship between the frequency of the interference signal output from the optical system of the measurement sample, the frequency of the interference signal output from the optical system of the reference sample, the thickness of the reference sample, and the desired thickness of the measurement sample.

[0032] FIG. 4 is a block diagram of an entire system showing another embodiment of the monolithic wavelength swept semiconductor laser light source 10. In FIG. The same members as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. In this embodiment, a light source monitoring circuit 32 using a reference wafer is added. In this light source monitoring circuit 32, the laser light emitted from the laser source 14 and dispersed is irradiated onto a reference wafer 38 through a circulator 34 and a probe 36. The laser light (interference light) reflected by the front and back surfaces of the reference wafer 38 passes through the probe 36 and the circulator 34 and is detected by a photodiode 40, and is further converted into a digital signal by the A / D converter 28 and input to the computer 30.

[0033] The computer 30 calculates the average value of the voltage output from the photodiode 40 (average value within a set time) from the input digital signal value. The voltage value output from the photodiode 40 fluctuates in proportion to the increase or decrease in the amount of laser light emitted from the laser source 14. The fluctuation in the voltage value is appropriately monitored, and if the average value of the voltage falls below a predetermined threshold value, it is determined to be abnormal and an alarm is issued.

[0034] Alternatively, the light source monitoring circuit 32 can also be used to monitor the swept wavelength accuracy of the monolithic swept wavelength semiconductor laser light source 10. Since the thickness (and its distribution) of the reference wafer 38 is constant, if there is a deviation in the measured thickness, it can be determined that the swept wavelength accuracy of the monolithic swept wavelength semiconductor laser light source 10 has fluctuated, and this can serve as a guide for understanding the cause of the fluctuation. The swept wavelength accuracy can be understood by the average value, PP value, deviation value, etc., of the frequency of the FFT peak value of the acquired interference waveform measured a set number of times (for example, 1000 times). The timing of monitoring can be set by the program, for example, just before measuring the wafer thickness.

[0035] 5 shows yet another embodiment of the monolithic wavelength swept semiconductor laser light source 10. This embodiment is the same as the other embodiments except that an MZI (Mach-Zehnder) interferometer 42 is used as the light source monitoring circuit. In this embodiment, the swept wavelength accuracy of the monolithic wavelength swept semiconductor laser light source 10 can be grasped in the same manner as above, and can be used to monitor the light source. Furthermore, the use of a fiber-type MZI interferometer has the advantage that the refractive index of the quartz glass fiber material is approximately 1.4, which is lower than the refractive index of a silicon wafer, approximately 3.5, and therefore the change in optical path length due to temperature effects is small.

[0036] [Table 1]

Claims

1. a monolithic wavelength-swept semiconductor laser light source having a laser source, a laser control unit for controlling the laser source, and a processor configured to control the laser source by the laser control unit so as to oscillate laser light having a wavelength that changes with time according to a set profile; an optical system for guiding the laser light to a measurement portion of a wafer whose thickness is to be determined and irradiating the portion; a detection unit for detecting an interference light signal of reflected light or transmitted light obtained from the measurement portion; an A / D converter that converts the interference light signal detected by the detection unit into a digital signal; A wafer thickness measuring device comprising a calculation unit that calculates a thickness of the wafer by analyzing a digital signal from the A / D converter, the processor operates the laser control unit by a clock signal generated by the processor to oscillate a laser beam from the laser source, the wavelength of which is swept with respect to time in a set profile; the A / D converter generates a sampling clock in synchronization with a clock signal generated by the processor or directly uses the sampling clock as the sampling clock to A / D convert the interference light signal; Further comprising a light source monitoring circuit using a reference wafer; A non-contact wafer thickness measurement device characterized in that the light source monitoring circuit monitors the sweep wavelength accuracy, which can be grasped by the average value, P-P value, or deviation value measured a set number of times, for the frequency of the FFT peak value of the interference waveform acquired by the detection unit.

2. 2. A non-contact wafer thickness measuring device according to claim 1, wherein said monolithic wavelength swept semiconductor laser light source has no mechanically moving parts.

3. 3. The non-contact wafer thickness measuring device according to claim 1, wherein the wavelength range to be swept is 1200 to 1400 nm.

4. 4. The non-contact wafer thickness measuring device according to claim 1, wherein the frequency of the clock signal is in the range of 10 MHz to 1 GHz.

5. 5. The non-contact wafer thickness measuring device according to claim 1, wherein the interference light signal is continuously detected while the wafer or the probe is moved at a constant speed, thereby measuring the thickness of the wafer.

6. 6. The non-contact wafer thickness measuring device according to claim 1, which is a wafer thickness measuring device used when polishing a wafer with a double-sided polishing machine or a single-sided polishing machine.

7. 7. The non-contact wafer thickness measuring device according to claim 1, further comprising a light source monitoring circuit using an MZI interferometer.

8. A non-contact wafer thickness measuring device as claimed in any one of claims 1 to 7, characterized in that the light source monitoring circuit monitors the amount of laser light emitted from the laser source from the average value of the voltage output from the detection unit.

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