JPS62135737A - Wavelength modulation type differential spectrometer - Google Patents

Abstract

PURPOSE:To discriminate easily a bright line spectrum of a target, and to grasp exactly its variation quantity by providing a wavelength modulating device, a diffraction grating, a wavelength scanning mechanism, a photoelectric converter, a synchronous detecting circuit, a sampling circuit. etc. CONSTITUTION:A wavelength modulating device 6 vibrates a reflected luminous flux by a plane mirror 8 attached to a vibrator 7, by a prescribed frequency by a vibrator driving circuit 21, and also measures its vibration amplitude. Subsequently, a diffraction grating 14 outputs a diffraction spectrum by dispersing the reflected luminous flux which is brought to a vibration modulation by the device 6. Also, a wavelength operating mechanism 25 varies continuously an incident angle to the grating 14 of the reflected luminous flux by turning the grating 14. On the other hand, a photoelectric converter 17 photodetects the diffraction spectrum which is outputted from the grating 14 and converts an optical modulation component contained therein, to an electric signal, and a synchronous detecting circuit 24 brings the electric signal which has been outputted from the converter 17, to a synchronous detection by a frequency of two times of an oscillation frequency of the vibrator 7, and also brings it to a DC amplification 18. Next, an output signal of the amplifier 18 is brought to sampling 19 by synchronizing it with the oscillation frequency of the vibrator 7 or the frequency of two times.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a wavelength modulation type differential spectrometer that uses a wavelength modulation device to accurately measure the amount of change in a bright line peak in optical spectrum intensity measurement, and can also measure the absolute amount at the same time. Regarding the meter.

[Prior Art] For example, in plasma etching, plasma ashing, reactive ion etching, etc. in the manufacturing process of semiconductor IC devices, changes in the plasma spectrum of the material being etched are used as a means of detecting the completion of the etching process. There is a way to detect it.

For example, when an aluminum wiring pattern of an IC element is subjected to plasma etching, a spectrometer is used to measure the plasma spectrum emitted during the etching process, as shown in FIG. 7(a).

Then, the intensity of the bright line spectrum A is continuously measured by fixing the detection wavelength to, for example, the bright line spectrum 8 of 3082 people, which is prominent among the measured spectral characteristics. Then, as shown in FIG. 7(b), the etching process is stopped at the time when the intensity of the fluorescence spectrum A of 3082 wavelengths changes significantly.

However, since plasma spectral characteristics generally include spectra having various wavelengths from substances other than the target substance, interference spectra caused by these substances interfere with the target spectrum. As a result, the intensity of the target spectrum may not be measured correctly. These interference spectra include a wide band broad spectrum and a linear emission line spectrum. Therefore, it is necessary to search the entire spectral characteristic for a bright line spectrum that shows a clear difference in intensity between the time of etching and the end of etching. Based on this idea, when the spectral characteristics during etching shown in FIG. 7(a) are compared with the spectral characteristics at the end of etching shown in FIG. 7(b), an emission line spectrum B near 4000 is discovered. Therefore, by adjusting the detection wavelength λ of the spectrometer to the center wavelength λ0 of the bright line spectrum 8 and detecting the change in the intensity of the bright line spectrum B, it is possible to detect the end of the etching process.

[Problems to be Solved by the Invention] However, as mentioned above, by fixing the measurement wavelength λ to λ0 and continuously measuring the intensity change of the ray B with a spectrometer, plasma etching processing can be performed. Even when detecting the end of the process, there are still problems as follows. In other words, I! with two wavelengths! As shown in the figure, the i-line spectrum B is often centered on a wide background spectrum. For example, in FIG. 7(a), if the intensity of the entire bright line spectrum B is C, the intensity of the background spectrum is b, and the intensity of the variation that sticks out from the background spectrum is a, then in the end of etching detection, the final It is necessary to measure the amount of change in the fluctuation strength a. However, in an actual spectrum spectrometer, the entire spectral intensity C including the background spectral intensity is measured.

Generally, the measurement accuracy of spectral intensity in conventional spectrometers is on the order of 0.1% at most. Therefore, when the t-dynamic intensity a of the spectrum to be detected is smaller than the intensity gb of the background spectrum, it has been impossible to accurately measure changes in the variation intensity a.

As mentioned above, in the plasma etching process of semiconductor IC devices, when the value of the fluctuation intensity a becomes undetectable as shown in FIG. 7(b), it is determined that etching has ended. If the value cannot be measured accurately, it may be determined that the fluctuation intensity a has become zero even though the etching process has not been completed, and the etching process may be stopped. In this case, there remains a portion on the semiconductor surface where the etching process has not been completely performed, and there is a concern that the yield of IC products will decrease.

The present invention has been made based on these circumstances, and its purpose is to reduce the minute amount in the emission line spectrum even if the intensity of the background spectrum is large, by using a wavelength variable position and a sampling circuit. To provide a wavelength modulation type differential spectrometer that can accurately measure spectral changes and simultaneously measure the absolute value of the spectrum, easily distinguish the target emission line spectrum, and accurately grasp the amount of change. be.

[Means for Solving the Problems 1] The wavelength modulation type differential spectrometer of the present invention includes a plane mirror for reflecting the beam of light to be measured on the vibrating element of the vibrator,
A wavelength modulation device having an amplitude measuring means for vibrating a reflected light beam reflected by a plane mirror at a predetermined frequency using a vibrator drive circuit and measuring the vibration amplitude of the vibrated reflected light beam, and the wavelength modulation device. a diffraction grating for dispersing the vibration-modulated reflected light beam obtained by the device and outputting a diffraction spectrum; a wavelength scanning mechanism for rotating the diffraction grating to continuously change the incident angle of the reflected light beam with respect to the diffraction grating; , a charging converter that receives the diffraction spectrum output from this diffraction grating and converts the optical modulation component included in the diffraction spectrum into an electrical signal; A synchronous detection circuit that performs synchronous detection at twice the frequency, a DC amplifier that amplifies the electrical signal output from the photoelectric converter, and a synchronous detection circuit that synchronizes the output signal of this DC amplifier with the vibration frequency of the vibrator or the twice the frequency. The sampling circuit includes a sampling circuit for sampling.

[Function] If the wavelength modulation type differential spectrometer is configured in this way,
The luminous flux of the light to be measured is adjusted to a predetermined frequency F by a wavelength modulation device.
is vibrated and input to the diffraction grating. Therefore, since the incident angle θ to the diffraction grating oscillates at the frequency F around the central incident angle θ0, the electrical signal obtained by the photoelectric converter that receives the output dregs vector of this diffraction grating is
0 and the measurement center wavelength λ0, and the wavelength at frequency F is λ0±Δ
It becomes a vibration spectrum signal that vibrates at λ.

When this vibration spectrum signal is synchronously detected at a frequency of 2F via a bypass filter, the variation of the spectrum at two measurement center wavelengths is (q).

Furthermore, when the output electrical signal of the photoelectric converter is sampled at a frequency of 2F via a DC amplifier, a DC component of the spectrum at the center frequency λa is obtained. Further, when the wavelength scanning mechanism is driven, the central incident angle θD of the vibrated light beam with respect to the diffraction grating changes, so the measurement central wavelength λG changes.

[Example] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram showing a schematic configuration of a wavelength modulation type differential spectrometer according to an embodiment. In the figure, reference numeral 1 indicates a plasma generation source such as an etching processing section in a semiconductor IC device, etc., and a constant light beam 2 of 111 emitted from this plasma generation source 1 is transmitted to a condenser lens 4 fitted in an entrance window of a case 3. The light is focused through the entrance slit 5 on a first plane mirror 8 attached to one of the pair of vibrating pieces of the vibrator 7 that constitutes the wavelength modulation vL arrangement 6 .

This vibrator 7 is configured as shown in FIG. 2, for example. That is, an electromagnetic coil 1 is disposed inside the U-shaped tuning fork 9, and a core 11 is inserted into the axis of the electromagnetic coil 10. Then, the first plane mirror 8, the second constant frequency F and the electromagnetic coil 10 are attached to a pair of free ends of the U-shaped tuning fork 9, that is, a pair of vibrating pieces, respectively.
It vibrates with an amplitude W determined by the excitation current 1lIII flowing through the oscillator.

The light beam 2 reflected by the first plane mirror 8 of the vibrator 7 is incident on the collimator 13. As mentioned above, since the vibrator 7 vibrates at a constant frequency F and a constant amplitude W, the light beam 2 reflected by the first plane mirror 8 becomes a moving light that vibrates at a constant frequency F and a constant amplitude W. . The perturbed light reflected by the collimator 13 is incident on a blazed diffraction grating 14 rotatably attached to a fixed part. The diffraction grating 14 separates a spectrum of wavelength λ determined by the grating dimensions, the incident angle θ, etc., and enters the collector 15 . An exit slit 16 is arranged at a position where the tG IJ spectrum reflected by the collector 15 forms an image.
A photoelectric converter 17 is provided for receiving the vibration spectrum reflected by the light source 5 and imaged onto the exit slit 16. The electrical signal f containing a DC component and an AC component output from the photoelectric converter 17 is amplified by a DC amplifier 18 and then input to a sampling circuit 19 . This one sampling circuit samples the input amplified electrical signal f using a sampling pulse Q of a frequency of 2F output from the timing circuit 20. Note that the timing circuit 20 receives the vibration frequency F of the vibrator 7 output from the vibrator drive circuit 21 that drives the vibrator 7 of the wave modulation device 46.
The timing circuit 20 adjusts the output timing of the synchronization signal and sends it as a sampling pulse Q to the sampling circuit 19. Frequency 2 in sampling circuit 19
The sampling data of the electrical signal f sampled at F is input as the DC component intensity e of the measurement spectrum to the control [1 calculation circuit 22, which is comprised of a microprocessor or the like.

Further, the electric signal f output from the photoelectric converter 17 is inputted to the synchronous detection circuit 24 after a DC component is removed by a bypass filter consisting of a capacitor 23 . The alternating current component of the electrical signal f is determined by the frequency 2 of the synchronous signal sent from the vibrator drive circuit 21 using the synchronous detection circuit 24.
It is synchronously detected at F and inputted to the control calculation input 22 as the fluctuation intensity a of the measured spectrum.

In addition, 25 in the figure is a wavelength scanning mechanism that rotates the diffraction grating 14 to continuously change the central incident angle θ0 of the oscillating light incident on the diffraction grating 14 from the collimator 13. When the motor drive circuit 25c controlled by the control calculation circuit 22 rotates the drive motor 25b attached to one end of the screw stud 25a which is screwed into the screw hole formed in the screw hole 3, the screw rod 25a rotates. , a locking member 2 attached near the other end of the screw foot 25a
The horizontal position of 5d moves. When the horizontal position of the locking member 25d moves, the rotation arm 25f of the diffraction grating 14, which is rotatably attached to the fixed part by the rotation shaft 25e, moves, and the diffraction grating 14 rotates. As a result, the central incident angle θ0 of the oscillating light changes. When the central incidence angle θ0 changes, the value of the central wavelength λq of the vibrational diffraction spectrum output from the diffraction grating 14 and incident on the collector 15 changes.

Note that the rotation angle of the diffraction grating 14 is determined by the rotation angle detector 2.
6 and input to the control calculation circuit 22.

In the surface wavelength modulation device fffi6, as shown in the figure, detection light 29 outputted from a light emitting device 28 composed of a light emitting element such as an LED is incident on the second plane mirror 12 of the vibrator 7 via a condenser lens 27. be done. The detection light 29 reflected by this second plane mirror 12 has a constant frequency F
The vibration amplitude of this detection light 29 is inputted to an amplitude measuring device 30 including a position sensor and the like, which measures the vibration amplitude of the detection light 29.

The vibration width signal of the detection light 29 measured by the amplitude measuring device 30 is used to drive a single element that sends an excitation current I to an electric Eff coil 10 incorporated inside the U-shaped tuning fork 9 of the vibrator 7. The signal is input to the circuit 21.

The amplitude setting signal i sent from the control calculation circuit 22 is input to the vibrator drive circuit 21 . Then, the transducer 7 is adjusted so that the measured imaging width of the transducer 7 obtained from the one-limb width vibration signal input from the vibration width 'JA11 regulator 30 becomes the setting amplitude ~V corresponding to the setting signal i. Electromagnetic coil 1
Kakeru to supply to 0! Controls the value of Il current 1. Therefore, the vibration amplitude of the vibrator 7 always becomes the amplitude value W corresponding to the setting signal 1 sent from the control calculation circuit 22. Further, the vibrator drive circuit 21 sends a synchronization signal having a frequency 2F, which is twice the vibration factor number of the vibrator 70, to the synchronous detection circuit 24 and the timing circuit 20.

The operating principle of the wavelength modulation differential spectrometer constructed in this way will be explained using FIG. 3 (a>(b)).

That is, the spectral intensity is measured while modulating the wavelength λ around two central wavelengths (λ0±Δλ) of the bright line spectrum B to be measured.

The spectral intensity waveform measured at this time is a waveform in which the AC component (ripple component) with the DC component e and the amplitude d is centered, as shown in FIG. The frequency of the alternating current component of this waveform is 2F, which is twice the wavelength modulation frequency F. Therefore, if this waveform is detected at a frequency of 2F in synchronization with the wavelength modulation, the original spectral intensity can be obtained.

In reality, the measured emission line spectrum B is heavily influenced by the broadband background spectrum of the intensity as shown in (a) of the same figure, so after removing the DC component e shown in (b) of the same figure with a bypass filter, If synchronous detection is performed, the intensity corresponding to the variation intensity [a] of the change in the bright line spectrum B can be obtained.

Furthermore, if the waveform in which the AC component of amplitude d remains superimposed is sampled at frequency a2F, it is possible to extract the original spectral intensity C. 31.82 in Figure 3(b).
33. The waveform is sampled at the timing of fl, f2. By sampling f3..., the original spectral intensity C at two wavelengths can be obtained. Note that the sampling frequency 2F is twice the vibration frequency F of the vibrator 7, and the vibration frequency F of the vibrator 7 is a value of 2 K'th culmness. Since this value is sufficiently high compared to the wavelength scanning speed by the wavelength scanning mechanism 25, the sampling data output from one sampling circuit can be regarded as a substantially continuous value.

Therefore, in the wavelength modulation differential spectrometer of the embodiment shown in FIG. 1, the oscillator 7 has a constant frequency F,
Since it vibrates with a constant amplitude W, the incident angle θ of the oscillating light incident on the diffraction grating 14 also vibrates at a constant angle (θ8±Δθ) around the central incident angle 0. As a result, a vibration spectrum of two wavelengths ±Δλ is imaged on the exit slit 16. Therefore, the input light to the light receiving surface of the photoelectric converter 7 has a waveform in which an alternating current component d is superimposed on a direct current component e, as shown in the waveform of FIG. 3(b).

This waveform is converted into an electrical signal f by a photoelectric converter 17. Therefore, the DC component e is removed by the capacitor 23, and the signal synchronously detected by the synchronous signal of frequency 2f by the synchronous detection circuit 24 is a DC component corresponding to the fluctuation intensity a of the bright line spectrum B in FIG. 3(a). It becomes a signal.

In addition, in one sampling circuit, a sampling pulse g of frequency v12F (sampling point Sl, 82,
The data values sampled in S3...) are DC component elements 1 and f2 in FIG.

The value corresponds to f3...

Therefore, the control calculation circuit 22 receives the fluctuation intensity a of the bright line spectrum B and the intensity C corresponding to the DC component.

Next, when the drive motor 25b is rotated at a constant speed by the motor drive circuit 25C of the wavelength scanning mechanism 25, the center wavelength λG of the diffraction spectrum gradually shifts with time t, as shown in FIG. As a result, the wavelengths (λG±Δλ) included in the waveform of FIG. 3(b) also shift sequentially. Therefore, when this waveform is sampled with a sampling pulse q having a frequency of 2F, the wavelength λ corresponding to the sampled data changes as time t changes from λ1° to λ2. λ3.・
..., λ, n8, and so on.

This data is then sequentially input to the control calculation circuit 22. Further, information on the rotation angle of the diffraction grating 14, that is, the center wavelength λ0, is inputted to the control calculation circuit 22 from the rotation angle detector 26. Therefore, this control calculation circuit 2
2, the DC intensity C of the spectrum corresponding to each wavelength λ is input.

Further, when the two center wavelengths of the waveform sequentially change, the wavelength λ of the fluctuation intensity a output from the synchronous detection circuit 24 also changes sequentially.

As a result, two data, the DC intensity C and the fluctuation intensity a for each wavelength λ, are input to the control calculation circuit 22 at the same time.

FIG. 5 shows an example of actual measurement when reactive ion etching of 5i021J was performed on a semiconductor wafer. In this way, it is possible to simultaneously obtain the DC characteristic E corresponding to the DC intensity C and the fluctuation characteristic F corresponding to the fluctuation intensity a with respect to the wavelength λ. As is clear from this figure, the fluctuation characteristic F is a characteristic obtained by second-order differentiation of the DC characteristic E.

In this way, in the fluctuation characteristic F, the DC component e in FIG.
Since it is possible to separate and detect only the AC component d by removing the AC component d, it is possible to remove the intensity gb of the background spectrum from the bright line spectrum and measure only the fluctuation intensity a with high accuracy. Therefore, it is possible to significantly improve the measurement accuracy compared to the case where the spectral intensity C is directly measured by fixing the wavelength value 3111 to λ0 as in the case of a conventional spectrum spectrometer.

For example, in the same figure, the wavelength λ=4510 people and λ=
Emission line spectrum G that exists in 4820 people.

Since H almost disappears at the end of etching, the end time of etching can be reliably detected by monitoring the fluctuation characteristic F value of the Lii line spectrum at the same wavelength.

In addition, since the DC characteristic E can be obtained at the same time as the fluctuation characteristic F, the value of the lfi line spectrum B can be observed while observing the overall spectral characteristic.
1iI! Fluctuation information of spectrum B can be grasped more reliably.

Note that the present invention is not limited to the embodiments described above. In the embodiment, a U-shaped tuning fork 9 is used as the vibrator 7, but it is also possible to obtain vibrating light with a constant frequency and a constant i width by using a piezoelectric actuator.

Furthermore, in the embodiment, the output timing of sampling pulse 0 from the timing circuit 20 to the sampling circuit 19 is set at the moment when the fluctuating portion of the waveform crosses the center position shown by the dashed line, as shown in FIG. but,
As shown in FIG. 6, it may be set at the moment when the fluctuating portion of the waveform reaches its maximum value and minimum value. With this setting, the wavelengths corresponding to the sampled data become the wavelengths B1, 82, B3, . . . at the turning points of the waveform.

Change in data at the turning point of the waveform fi(dλ/d
Since t) is small, it is possible to set the sampling gate time widely, and therefore it is possible to improve the measured viscosity of each sampled data.

In this case, the wavelength at the time of sampling will deviate from the center wavelength λ0 by ±Δλ, but since Δλ at that time can be calculated from the amplitude W of the vibrator 7, the control calculation circuit 22 should correct the wavelength. Bye.

[Effects of the Invention] As explained above, according to the present invention, even if the intensity of the background spectrum is large, minute spectral changes such as bright line spectra can be measured with good accuracy, and the absolute value of the spectrum can also be measured at the same time. The target emission line spectrum can be easily identified and the amount of change can be accurately grasped.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the general configuration of a wavelength modulation differential spectrometer according to an embodiment of the present invention, FIG. 2 is a perspective view showing a vibrator of the same embodiment, and FIGS. 3 and 4 are the same. A diagram for explaining the operation of the embodiment, FIG. 5 is a diagram showing actual measured values, a sixth factor is a diagram for explaining the operation of the wavelength modulation differential spectrometer according to another embodiment of the present invention, and FIG. The figure shows a plasma spectrum. DESCRIPTION OF SYMBOLS 1... Plasma generation source, 2... Luminous flux, 3... Case, 5... Entrance slit, 6... Wavelength modulation device, 7
... Vibrator, 8... First plane mirror, 9... U-shaped tuning fork, 10... rX SR coil, 12... Second plane mirror, 13... Collimator, 14... Diffraction grating,
15...Collector, 16...Output loss slip 1-117
...Photoelectric converter, 18...DC amplifier, 19...
Sampling circuit, 20... Timing circuit, 21.
...Flea Senko drive circuit, 22...Control calculation circuit, 23.
...Capacitor, 24...Synchronous detection circuit, 25...
Wavelength scanning mechanism, 26... Rotation angle detector, 28...
Light emitting device, 2...detection light, 30...amplitude meter. Applicant's representative Patent attorney Takehiko Suzue Figure 2 (a) (b) Figure 3 Figure 4 Figure 6 (a) (b) Figure 7

Claims (3)

[Claims] (1) A vibrator, a plane mirror attached to the vibrating piece of the vibrator for reflecting the beam of light to be measured, and a vibrator for vibrating the reflected beam reflected by the plane mirror at a predetermined frequency. a wavelength modulation device comprising a drive circuit and an amplitude measuring means for measuring the vibration amplitude of the reflected light beam vibrated at the predetermined frequency; a diffraction grating for outputting a diffraction spectrum; a wavelength scanning mechanism for rotating the diffraction grating to continuously change the incident angle of the reflected light beam with respect to the diffraction grating; a photoelectric converter that converts an optical modulation component included in the diffraction spectrum into an electrical signal; and a synchronization device that periodically detects the electrical signal output from the photoelectric converter at a frequency twice the vibration frequency of the vibrator. a detection circuit; a DC amplifier that amplifies the electrical signal output from the photoelectric converter; and a sampling circuit that samples the output signal of the DC amplifier in synchronization with the vibration frequency of the vibrator or twice the frequency. A wavelength modulation differential spectrometer characterized by: (2) The wavelength modulation type differential spectrometer according to claim (1), wherein the vibrator is constructed using a U-shaped tuning fork. (3) The wavelength modulation type differential spectrometer according to claim (1), wherein the vibrator is constructed using a piezoelectric actuator.