JPS61219839A - Wavelength modulating sepectroscope - Google Patents

Abstract

PURPOSE:To measure minute spectral change with good accuracy, by vibrating luminous flux to be measured at predetermined frequency in a predetermined modulation width by using the flat mirror provided to a tuning fork oscillator. CONSTITUTION:The luminous flux 2 to be measured emitted from a plasma generation source 1 is incident on the flat mirror 8 provided to a tuning fork 7 through a lens 4 and a slit 5 and receives the modulation of a wavelength at predetermined frequency by said flat mirror 8. The luminous flux 2 is further spectrally diffracted to a spectrum B with an objective wavelength lambda0 by a blaze type diffraction grating 11 through a collimator 9 to be incident to a photoelectric converter 14 through a collector 12 and a slit 13. The output (f) of the converter 14 is amplified by a DC amplifier 15 to output whole intensity (c) to a terminal 16. The output (f) passed through a high pass filter 17 and synchronously detected at frequency two times tuning fork frequency F by a synchronous detection circuit 18 and the variation intensity (a) of the spectrum B is outputted to a terminal 19. Because the tuning fork 7 is vibrated at definite amplitude by using a light emitting apparatus 13, an amplitude detection apparatus 25 and a reflective mirror 21, detection can be performed with good accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a wavelength modulation spectrometer that uses a tuning fork oscillator to increase the measurement sensitivity of emission lines or absorption peaks in an optical spectrum.

[Technical Background of the Invention] For example, in plasma etching, plasma aeration, etching, reactive ion etching, etc. in the manufacturing process of semiconductor IC devices, etc., a trace amount of etching is used as a means of detecting the completion of the etching process. There is a method of detecting changes in the plasma spectrum of the etched material emitted during etching.

For example, when performing plasma etching on an aluminum wiring pattern of an IC element, as shown in FIG. is continuously measured using a spectrometer fixed at the wavelength of this emission line spectrum A of 3082. And Fig. 8 (b
), this wavelength 3082 people's emission line spectrum A
The etching process is stopped at the time when the intensity of the etching changes significantly.

However, since the plasma spectrum generally includes spectra having various wavelengths from substances other than the target substance, interference spectra due to these substances are superimposed on the target spectrum. As a result, the intensity of the target spectrum may not be measured correctly. These interference spectra are a mixture of broadband broad spectra and linear bright line spectra. Therefore, it is necessary to search for a bright line spectrum that shows a clear difference in intensity between the time of etching and the end of etching from the overall spectral characteristics. Based on this idea, when we compare the spectral characteristics during etching in Figure 8(a) and the spectral characteristics at the end of etching in Figure 8(b), we find that the wavelength λ0 near 4000
An emission line spectrum B is discovered. Therefore, by detecting a change in the bright line spectrum B, it is possible to detect the end of the etching process.

[Problems with the Background Art] However, even when detecting the end of the plasma etching process by continuously measuring the intensity change of the bright line spectrum B using a spectrometer with the measurement wavelength fixed at λ0 as described above, it is still difficult to detect the end of the plasma etching process. There are the following problems. That is, the bright line spectrum B of wavelength λD is often superimposed on a wide background spectrum as shown in the figure.

For example, in FIG. 8(a), the intensity of the entire bright line spectrum B is C, the intensity of the background spectrum is b,
Assuming that the intensity of the fluctuating portion that sticks out from the background spectrum is a, it is necessary to finally measure the amount of change in the fluctuating intensity a in detecting the end of etching. However, in an actual spectrometer, the entire spectral intensity C including the background spectral intensity S is measured.

Generally, the measurement accuracy of optical spectrum intensity in a conventional spectrometer is on the order of 0.1% at most. Therefore, if the fluctuation intensity a of the spectrum to be detected is a/b<10-3 with respect to the intensity of the background spectrum, it is impossible to measure the change in the fluctuation strength a. Moreover, even if it can be measured based on data, the measured value lacks reliability. Furthermore, the greater the intensity of the background spectrum, the more difficult it is to measure the variation intensity a even if the value of the variation intensity a to be measured is large.

As mentioned above, in the semiconductor plasma etching process, when the value of the fluctuation intensity a becomes undetectable as shown in FIG. 8(b), it is determined that etching has ended. If accurate measurement is not possible, the fluctuation intensity aI may be measured even though the etching process is not completed.
There are cases where it is determined that fi has become zero and the etching processing operation is 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.

[Object of the Invention] The present invention has been made based on the above circumstances, and its purpose is to eliminate the target emission line spectrum or absorption spectrum even if the intensity of the background spectrum is large by using a tuning fork oscillator. The object of the present invention is to provide a wavelength modulation spectrometer that can accurately measure minute spectral changes such as peaks.

[Summary of the Invention] The wavelength modulation spectrometer of the present invention has a plane mirror for reflecting a beam of light to be measured, vibrates the reflected beam reflected from the plane mirror at a predetermined frequency, and also vibrates the reflected beam at a predetermined frequency. a tuning fork oscillator having a vibration amplitude detection device for detecting the vibration amplitude of the reflected light beam captured by the tuning fork oscillator; a diffraction grating for dispersing the modulated reflected light beam obtained by the tuning fork oscillator and outputting a diffraction spectrum; a photoelectric converter that receives a diffraction spectrum output from the diffraction grating and converts an optical modulation signal included in the diffraction spectrum into an electrical signal; A wavelength modulation spectrometer equipped with a synchronous detection circuit that performs synchronous detection at a frequency twice the frequency,
It detects spectral intensity.

[Embodiments of the invention] 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 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 luminous flux 2 of the light to be measured emitted from this plasma generation source 1 enters a condenser lens 4 fitted into an entrance window of a case 3. Inlet slit 5
The light is focused on a first plane mirror 8 attached to one free end of a U-shaped tuning fork 7 constituting a tuning fork oscillator 6. Then, it is reflected by this first plane mirror 8 and enters the collimator 9. Note that the U-shaped tuning fork 7 is imaged by an electromagnetic coil 10 disposed inside the tuning fork at a constant frequency F and a constant amplitude W determined by the shape, weight, etc. of the tuning fork. Therefore, the light beam 2 reflected by the first plane mirror 8 becomes oscillating light with a constant frequency and constant amplitude.

The oscillating light reflected by the collimator 9 is incident on a blazed diffraction grating 11. This diffraction grating 11 separates the spectrum of wavelengths determined by the grating dimensions, etc., and the collector 1
2. An exit slit 13 is disposed at a position where the vibration spectrum reflected by the collector 12 forms an image, and adjacent to the back surface of the exit slit 13, the vibration spectrum reflected by the collector 12 is focused on the exit slit 13. A photoelectric converter 14 is provided which receives the imaged vibration spectrum and converts the intensity of this spectrum into an electrical signal.

The electrical signal f containing a DC component and an AC component output from the photoelectric converter 14 is amplified by a DC amplifier 15 and then output to an output terminal 16 that outputs the total intensity C in FIG. 8 of the measurement spectrum. Ru. Further, the electric signal f from the photoelectric converter 14 is inputted to a synchronous detection circuit 18 after a DC component is removed by a bypass filter consisting of a capacitor 17 . The alternating current component of the electric signal is synchronously detected by this synchronous detection circuit 18 at a frequency 2F, which is twice the vibration frequency F of the tuning fork oscillator 6, and an output terminal 19 outputs the fluctuation strength a of the measured speed vector. Output to.

In the tuning fork oscillator 6, as shown in the figure, a second plane mirror 21 is attached to the other free end of the U-shaped tuning fork 7, and a condenser lens 2 is attached to the second plane mirror 21.
2, a light emitting device 2 composed of a light emitting element such as an LED.
Detection light 24 output from 3 is incident. Then, the detection light beam 4 reflected by the second plane alt21 becomes vibration light of a constant frequency F, and an amplitude measuring device 2 composed of a position sensor or the like measures the vibration amplitude of this detection light 24.
5. The vibration amplitude signal of the detection light 24 measured by the amplitude measuring device 26 is input to a tuning fork drive circuit 26 that drives an electromagnetic coil 10 incorporated inside the U-shaped tuning fork 7. This tuning fork drive circuit 26 is the synchronous detection circuit 1
8 and sends a synchronization signal with a frequency of 2F to the luminescent bag! Controls the lighting of 23.

FIG. 2 is a perspective view showing a schematic configuration of the U-shaped tuning fork 7, in which the 0-shaped portion 31 is, for example, a piano wire with an outer diameter of approximately 2IIIR, and the leg portions 32 of this U-shaped tuning fork 7 are also the same as the piano wire. using lines. The length of the 0-character portion 31 is approximately 30jlIII, and the width is approximately 10am. The first and second plane mirrors 8.21 each having a shape of 7 m x 5 Jll+ are attached to each free end of this 0-shaped portion 31.

The electromagnetic coil 10 is wound around a core 33 arranged inside this 0-shaped portion 31. The vibration frequency F of this U-shaped tuning fork 7 is determined by the shape, material, etc. of the 0-shaped part 31, but by adopting the above-mentioned dimensions, the vibration frequency F can be increased to approximately 2kHz.
It can be set to z.

FIG. 3 is a block diagram showing a tuning fork drive circuit 26 that drives the U-shaped tuning fork 7 in vibration. 34 in the figure is a light emitting device f2
This is a light emitting element drive circuit that supplies a drive current to the light emitting elements such as LEDs of No. 3. In the figure, 35 is a position sensor that constitutes the amplitude measuring device 25. This position sensor 35 is a type of photoelectric conversion element that outputs a signal proportional to the displacement of the irradiation spot on the element, and the amplitude of the detected light 24, That is, the amplitude of the second plane &1t21 is detected by directly moving the irradiation spot. Position sensor 35
The output signal is input to the change detection circuit 36 and converted into an alternating current signal el as a vibration amplitude signal that changes as the second plane 121 vibrates. The AC signal el is sent to the detection circuit 3
7, it is converted into a DC output voltage El. Detection circuit 37
The output voltage E1 output from the amplitude setter 38 is input to the deviation detection circuit 39 together with the set voltage E2 output from the amplitude setter 38. The deviation detection circuit 39 receives the DC signal E1 and the set voltage E.
A deviation voltage E3 from 2 is output. The deviation voltage E3 outputted from the deviation detection circuit 39 is integrated by the integrating circuit 40, and becomes the control signal voltage E4 of the gain control amplifier 41.

On the other hand, the alternating current signal el outputted from the change detection circuit 36 is input to a phase shift circuit 42, where it is matched to the vibration phase of the U-shaped tuning fork 7.

The matching signal e2 output from the phase shift circuit 42 is input to the gain control amplifier 41, and is also input to the waveform shaping circuit 43.
The signal is input to and waveform shaped. Then, this waveform shaping circuit 43 sends out the synchronous signal e3 to the synchronous detection circuit 18. Further, the gain control amplifier 41 amplifies the input matching signal e2 with an amplification factor determined by the control signal voltage E4, and generates a driving current of the electromagnetic coil 10 of the U-shaped tuning fork 7.
Output as .

In the tuning fork drive circuit 26 having such a kind of servo system configuration, the amplitude of the second plane &I21, that is, the detection circuit 3
When the output voltage El of No. 7 becomes equal to the set voltage E2 of the amplitude setter 38, the deviation voltage E3 output from the deviation detection circuit 39 becomes zero. Therefore, the value of the control signal voltage E4 output from the integrating circuit 40 does not change. As a result, U
The vibration amplitude of the shaped tuning fork 7 does not change. Further, when the output voltage E2 of the detection circuit 37 is not equal to the set voltage E3 of the swing 11ifl regulator 38, a deviation voltage E3 corresponding to the difference is input to the integrating circuit 40. The control signal voltage E4 output from the integrating circuit 4o is the deviation voltage E3.
changes in response to. As a result, the vibration amplitude of the U-shaped tuning fork 7 changes so that the output voltage E1 of the detection circuit 37 becomes equal to the set voltage E2 of the amplitude setter 38. therefore,
Conversely, by changing the set voltage E2 of the amplitude setter 38, the amplitude W of the U-shaped tuning fork 7 can be changed arbitrarily.

The operating principle of the wavelength modulation spectrometer configured in this way is described in the fourth section.
This will be explained using Figures (a) and (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. As shown in the figure, the spectral intensity waveform measured at this time has a waveform in which an AC component (ripple component) with an amplitude d is added to the average DC component e by 1 sigma. The frequency of the AC component d having this waveform is a frequency 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 corresponding spectral intensity can be obtained. In reality, the measured emission line spectrum B includes the fourth
As shown in Figure (a), the broadband background spectrum of intensity S is superimposed, so if the direct current component e shown in Figure (b) is removed with a bypass filter and synchronous detection is performed, the bright line spectrum B can be obtained. An intensity d corresponding to the variation intensity a of the change is obtained.

To explain this principle with respect to the wavelength modulation spectrometer shown in FIG. is reflected and input to the diffraction grating 11. Since the U-shaped tuning fork 7 vibrates at a constant amplitude W9 and a frequency F, the incident angle θ of the light beam 2 incident on the diffraction grating 11 also vibrates within a constant angular range (θ±Δθ).

As a result, an imaging spectrum of wavelength λ0±Δλ is imaged on the exit slit 13. Therefore, the spectrum leaking from the exit slit 13 formed at the center of the vibration spectrum has a waveform in which the AC component d is superimposed on the DC component e, as shown in the waveform of FIG. 4(b). This waveform is converted into an electrical signal f by a photoelectric converter 14. Therefore, the signal whose direct current component e is removed by the capacitor 17 and which is synchronously detected by the synchronous detection circuit 18 using the synchronous signal e3 from the tuning fork drive circuit 26 having the frequency of 2F has the bright line spectrum shown in FIG. 4(a). This becomes a DC signal corresponding to the fluctuation intensity a of B.

The output signal of the DC amplifier 15, which directly converts the electrical signal t, has a value corresponding to the total intensity C, which is the sum of the background intensity of the bright line spectrum B and the fluctuation intensity a.

In this way, since it is possible to remove the DC component e and separate and detect only the fluctuation component d, it is possible to remove the background intensity in the bright line spectrum B and measure only the fluctuation intensity a with high accuracy. . Also, with this measurement method, even if the background intensity fluctuates, this fluctuation will be I1g! This does not affect the measurement accuracy of the fluctuation intensity a of the spectrum B. Therefore, it is possible to significantly improve the measurement accuracy compared to the case where the wavelength values to be measured are fixed to two and the spectral intensity is directly measured as in the case of a conventional spectrometer.

Furthermore, in order to obtain the fluctuation intensity a of the bright line spectrum B as a DC voltage, it is necessary to apply the signal after synchronous detection to a low-pass filter in the synchronous detection circuit 18. According to experiments conducted by the inventors, in order to maintain the S/N ratio of the measured fluctuation intensity a above a certain level, the value of the time constant τ of this low-pass filter should be set to the minimum value, τ, with respect to the synchronous detection frequency 2F.
It is necessary to set it large so that the relationship is >200/2F or more. In order to quickly detect the end point of plasma etching of the semiconductor IC device mentioned above, it is necessary to make the above-mentioned time constant τ as small as possible, and therefore it is necessary to make the wavelength modulation frequency F high. That is, the vibration frequency F of the first plane 1118 attached to the U-shaped tuning fork 7 may be increased.

Conventionally, there is a galvanometer-type device that vibrates a plane mirror at a high frequency, but since this device has a rotating shaft and bearings, there are problems with durability when operating continuously for a long time, and the vibration amplitude It is difficult to control freely.

On the other hand, the tuning fork oscillator 6 used in the wavelength modulation spectrometer of the embodiment uses a U-shaped tuning fork 7 having the shape and dimensions as shown in FIG. It is possible to obtain high vibration frequencies. Also, the amplitude of that vibration is measured on the second plane! ! 21 and an amplitude measuring device 25, and the tuning fork drive circuit 26 can control the vibration amplitude W of the U-shaped tuning fork 7 to always be constant, and change the amplitude value W to an arbitrary value as necessary. It is also possible. Furthermore, since no mechanically movable parts are employed, reliability does not deteriorate due to durability deterioration.

Note that a method using a magnetic head for pickup has been proposed as a means for detecting the vibration amplitude of a tuning fork oscillator using a U-shaped tuning fork (Japanese Patent Laid-Open No. 14124/1983).
This proposal has drawbacks such as the output of the magnetic head being greatly affected by temperature changes and the vibration amplitude value and the output value of the magnetic head being non-linear.

Furthermore, the inventors have developed a spectrometer for trace gas analysis using a vibrating slit using a tuning fork (Patent No. 1247126).
) or a vibrating mirror using a tuning fork (Japanese Patent Laid-Open No. 56-1412
No. 4) was used. However, with these spectrometers,
There have been problems such as the inability to arbitrarily change the slit width and the difficulty in precisely controlling the wavelength modulation width.

As described above, the wavelength modulation spectrometer of the embodiment uses the second plane 1I21° amplitude measuring device 2 as a means for controlling the vibration amplitude.
5 and the tuning fork drive circuit 26, the above drawbacks could be overcome.

FIG. 5 shows a mechanism for controlling the vibration amplitude of a tuning fork oscillator of a wavelength modulation spectrometer according to another embodiment of the present invention. The detection light 24 outputted from the light emitting device 23 is reflected by the second plane 121 of the U-shaped tuning fork 7 and input to the light amount detector 54 which is composed of a photoelectric conversion element and an amplifier. By placing a knife 51 as shown in the figure between the second plane mirror 21 and the light amount detector 54 and blocking a part of the optical path of the detection light 24, the amount of light detected as the U-shaped tuning fork 7 vibrates. The alternating current component changes depending on the amplitude of the second plane, the mirror 21. Since this alternating current component changes in accordance with the amplitude of the second plane mirror 21, it is possible to control the vibration amplitude W of the U-shaped tuning fork 7 in the same manner as in the previous embodiment.

FIG. 6 is a diagram showing a vibration amplitude control I11 structure of the tuning fork oscillator 6 in a wavelength modulation spectrometer of still another embodiment. In this embodiment, a knife 52 is attached to the other free end of the U-shaped tuning fork 7 instead of the second plane mirror 21, and the knife 52 allows detection light to be output from the light emitting device 23 and incident on the light amount detector 54. A part of 24 is shielded from light. Even with such a configuration, a signal corresponding to the imaging amplitude W of the U-shaped tuning fork 7 can be extracted from the light amount detector 54.

FIGS. 7(a) and 7(b) are diagrams showing a vibration amplitude control ltl structure of a tuning fork oscillator of a wavelength modulation spectrometer in still another embodiment of the present invention. In this example, the second plane 1
Instead of j121, one 0-shaped part 31 of the U-shaped tuning fork 7
is output from the light emitting device 123 and sent to the light amount detector 54.
A part of the detection light 24 incident on the sensor is blocked. Note that a dummy mirror 53 is attached in place of the second plane mirror to balance the weight. Even with such a configuration, it is possible to extract a signal corresponding to the imaging amplitude W of the U-shaped tuning fork 7 from the light amount detector 54.

In particular, when the wavelength modulation spectrometer of the present invention is applied to an ammonia analyzer that requires high stability of the wavelength modulation width (for example, the Bitter Tokko Publication No. 56-4349), its effects can be maximized. It is possible.

[Effects of the Invention] As explained above, according to the present invention, by using a tuning fork oscillator, minute spectral changes such as the target emission line spectrum or absorption peak can be accurately measured even if the intensity of the background spectrum is large. .

[Brief explanation of the drawing]

1 to 4 show a wavelength modulation spectrometer according to an embodiment of the present invention, FIG. 1 is a schematic diagram showing the overall configuration, and FIG. 2 is a perspective view showing a U-shaped tuning fork. , FIG. 3 is a block diagram showing the tuning fork driving circuit, FIG. 4 is a diagram for explaining the operating principle, and FIGS. 5 to 7 are wavelength modulation spectrometers according to other embodiments of the present invention. FIG. 8 is a diagram showing an imaging amplitude control mechanism in a tuning fork oscillator, and FIG. 8 is a plasma spectrum diagram. DESCRIPTION OF SYMBOLS 1... Plasma generation source, 2... Luminous flux, 3... Case, 4.22... Condenser lens, 5... Entrance slit, 6... Tuning fork oscillator, 7... U-shaped tuning fork , 8
...first plane mirror, 9...collimator, 10...
Electromagnetic coil, 11... Diffraction grating, 12... Collector, 13... Exit slit, 14... Photoelectric converter, 1
5... DC amplifier, 17... Capacitor, 18...
- Synchronous detection circuit, 21... second plane mirror, 23...
Light emitting device, 24...detection light, 25...amplitude measuring device,
26...Tuning fork drive circuit, 31...0 character part, 32...
- Legs, 34... Light emitting element drive circuit, 35... Position sensor, 36... Change detection circuit, 37... Detection circuit, 38... Amplitude setting device, 39... Deviation detection circuit , 40... Integrating circuit, 41... Gain control amplifier, 42... Phase shifting circuit, 43... Waveform shaping circuit, 51
゜52... Naifetsuji, 53... Dummy mirror, 54
...Light level detector. Applicant's representative Patent attorney Takehiko Suzue Figure 4 (a) (1)) Liquid length λ, wavelength λ Figure 5
Figure 6 7v! J (a) (b) F3

Claims (3)

[Claims] (1) It has a plane mirror for reflecting the beam of light to be measured, vibrates the reflected beam reflected by the plane mirror at a predetermined frequency, and calculates the vibration amplitude of the reflected beam that is vibrated at the predetermined frequency. a tuning fork oscillator having a vibration amplitude detection device for detection; a diffraction grating for dispersing the modulated reflected light beam obtained by the tuning fork oscillator and outputting a diffraction spectrum; and receiving the diffraction spectrum output from the diffraction grating. a photoelectric converter that converts the optical modulation signal included in the diffraction spectrum into an electrical signal; and synchronous detection that synchronously detects the electrical signal output from the photoelectric converter at a frequency twice the vibration frequency of the tuning fork oscillator. A wavelength modulation spectrometer characterized by comprising a circuit. (2) The tuning fork oscillator includes a U-shaped tuning fork; an electromagnetic coil for driving the U-shaped tuning fork; and a coil attached to one free end of the U-shaped tuning fork for reflecting the luminous flux of the light to be measured. a second plane mirror attached to the other free end of the U-shaped tuning fork for detecting the vibration amplitude of the reflected light beam; a detection device for detecting the vibration amplitude of the reflected light beam to the second plane mirror; a light emitting device that irradiates light; and a light emitting device that receives the detection light reflected by the second plane mirror;
An amplitude measuring device that measures the vibration amplitude of the reflected detection light; and a drive circuit that controls excitation of the electromagnetic coil so that the vibration amplitude value measured by the amplitude measuring device is always a constant value. A wavelength modulation spectrometer according to claim (1), characterized in that: (3) The amplitude measuring device includes a light amount detector that receives the detection light reflected by the second plane mirror and detects a light amount corresponding to the vibration amplitude of the reflected detection light, and the light amount 2. The wavelength modulation spectrometer according to claim 2, wherein the excitation of the electromagnetic coil is controlled so that the amount of light detected by the detector is always a constant value.