JPH0528332B2 - - Google Patents

Description

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

[Technical Background of the Invention] For example, in plasma etching, plasma ashing, 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 are methods of detecting changes in the plasma spectrum of the emitted etched material.

For example, when plasma etching is performed on an aluminum wiring pattern of an IC element, the spectral characteristics of the plasma emitted during the etching process are measured using an emission spectrum analyzer, as shown in FIG. 6a. Then, the detection wavelength λ is fixed to the bright line spectrum A of 3082 Å which is prominent in the measured spectral characteristics, and the intensity of this bright line spectrum A is continuously measured. Then, as shown in FIG. 6b, the etching process is stopped at the time when the intensity of the bright line spectrum A with a wavelength of 3082 Å changes significantly.

However, since plasma spectral characteristics generally include 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 include a broadband broad spectrum and a linear emission line spectrum. Therefore, it is necessary to search the entire spectral characteristic for a bright line spectrum in which there is a clear difference in intensity between the time of etching and the time of completion of etching. Even with this kind of thinking, the 6th
Comparing the spectral characteristics at the time of etching shown in Figure a and the spectral characteristics at the end of etching shown in Figure b, an emission continuous spectrum B with a wavelength λ ₀ near 4000 Å is found. Therefore, the detection wavelength λ of the emission spectrum analyzer is set to the center wavelength λ ₀ of this emission line spectrum B.
The end of the etching process can be detected by detecting the change in the intensity of the bright line spectrum B.

[Problems with the Background Art] However, as described above, by fixing the measurement wavelength λ to λ ₀ and continuously measuring the intensity change of the bright line spectrum B using an emission spectrum analyzer, it is possible to determine the end of the plasma etching process. Even in the case of detection, the following problems still exist. That is, the bright line spectrum B of wavelength λ ₀ is often superimposed on a wide background spectrum as shown in the figure. For example, in Fig. 6a, if the intensity of the entire emission line spectrum B is c, the intensity of the background spectrum is b, and the intensity of the fluctuating portion that sticks out from the background spectrum is a, then in the end of etching detection, the final fluctuation intensity It is necessary to measure the amount of change in a. but,
In an actual emission spectrum analyzer, the entire spectrum intensity c including the background spectrum intensity b is measured.

Generally, the measurement accuracy of spectral intensity in conventional emission spectrum analyzers is on the order of 0.1% at most. Therefore, the fluctuation intensity a of the spectrum to be detected is equal to the intensity b of the background spectrum.
If a/b<10 ^-3 for then the fluctuation strength a
It is impossible to measure changes in Moreover, even if it can be measured based on data, the measured value lacks reliability. Furthermore, the greater the intensity b 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 plasma etching process of semiconductor IC devices, when the value of the fluctuation intensity a becomes undetectable as shown in FIG. 6b, it is determined that the etching is completed. If accurate measurement is not possible, 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 operation may be stopped. In this case, some parts of the semiconductor surface remain where the etching process has not been completely performed.
There were concerns that the yield of IC products would decline.

[Object of the Invention] The present invention has been made based on the above circumstances, and its purpose is to achieve a target emission line spectrum by using a tuning fork oscillator, even if the intensity of the background spectrum is large. or minute spectral changes such as absorption peaks,
An object of the present invention is to provide a wavelength scanning emission spectrum analyzer that can perform measurements with high precision and responsiveness.

[Summary of the invention] The wavelength scanning emission spectrum analyzer of the present invention has the following features:
A tuning fork oscillator that vibrates a beam of light to be measured at a predetermined frequency, a diffraction grating that disperses the modulated reflected beam obtained by the tuning fork oscillator and outputs a diffraction spectrum, and a diffraction grating that rotates the diffraction grating. a wavelength scanning mechanism that continuously changes the incident angle of the reflected light beam with respect to the diffraction grating; and a photoelectric conversion device 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. and a synchronous detection circuit that synchronously detects the electric signal output from the photoelectric converter at a frequency twice the vibration frequency of the tuning fork oscillator, the wavelength scanning emission spectrum analyzer comprising: This is to detect the spectral intensity at .

Furthermore, the tuning fork oscillator is made up of a U-shaped tuning fork and this U-shaped tuning fork.
An electromagnetic coil for vibrating the tuning fork, and a U
A first plane mirror is attached to one free end of the U-shaped tuning fork to reflect the beam of light to be measured, and a second plane mirror is attached to the other free end of the U-shaped tuning fork to detect the vibration amplitude of the reflected light. a plane mirror, a light emitting device that irradiates the second plane mirror with detection light for detecting the vibration amplitude of the reflected light beam, and a position sensor that receives the detection light reflected by the second plane mirror, and detects the reflected detection light. It consists of an amplitude measuring device that measures the vibration amplitude of the vibration amplitude using a change detection circuit, and a drive circuit that controls the excitation of the electromagnetic coil so that the vibration amplitude value measured by the amplitude measuring device is always a constant value.

[Embodiment 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 scanning type emission spectrum analyzer 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 element, 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. The light is focused through the entrance slit 5 onto 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. In addition, U
The glyph-shaped tuning fork 7 is vibrated 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 rotatably attached to a fixed part. In this diffraction grating 11,
A spectrum of wavelengths determined by the grating dimensions and the like is separated and incident on the collector 12 . This collector 12
An exit slit 13 is disposed at a position where the vibration spectrum reflected by the collector 12 is located adjacent to the back surface of the exit slit 13.
A photoelectric converter 14 is provided for receiving the vibration spectrum reflected by the exit slit 13 and converting 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. 6 of the measured spectrum. Ru. Further, the electric signal f of the photoelectric converter 14
After the DC component is removed by a high-pass filter consisting of a capacitor 17, the synchronous detection circuit 18
is input to. The alternating current component of the electric signal f is synchronously detected in this synchronous detection circuit 18 at a frequency 2F which is twice the vibration frequency F of the tuning fork oscillator 6, and outputted to an output terminal 19 which outputs the fluctuation intensity a of the measured spectrum. be done.

Further, in the figure, 20 is a wavelength scanning mechanism that rotates the diffraction grating 11 to continuously change the central incident angle θ ₀ of the oscillating light incident on the diffraction grating 11 from the collimator 9. In this wavelength scanning mechanism 20, A threaded rod 20a that is screwed into a screw hole carved in the case 3
When the adjustment knob 20b attached to one end is rotated, the horizontal position of the locking member 20c attached near the other end of the threaded rod 20a moves. When the horizontal position of the locking member 20c moves, the rotation arm 20e of the diffraction grating 11, which is rotatably attached to the fixed part by the rotation shaft 20d, moves, and the diffraction grating 11 rotates. As a result, the central incident angle θ ₀ of the oscillating light changes. When the central incidence angle θ ₀ changes, the output from this diffraction grating 11 is transmitted to the collector 12.
The value of the center wavelength λ ₀ of the vibrational diffraction spectrum incident on the beam changes.

In the tuning fork oscillator 6, a second plane mirror 21 is attached to the other free end of the U-shaped tuning fork 7, as shown in the figure.
Detection light 24 output from a light emitting device 23 composed of a light emitting element such as an LED is incident through a condenser lens 22 . The detection light 24 reflected by the second plane mirror 21 becomes vibration light with a constant frequency F, and is input to an amplitude measuring device 25 composed of a position sensor or the like that measures the vibration amplitude of the detection light 24. . The vibration amplitude signal of the detection light 24 measured by the amplitude measuring device 25 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 driving circuit 26 sends out a synchronizing signal e ₃ having a frequency of 2F to the synchronous detection circuit 18 and controls the lighting of the light emitting device 23 .

FIG. 2 is a perspective view showing a schematic configuration of the U-shaped tuning fork 7. The U-shaped portion 31 is, for example, piano wire with an outer diameter of approximately 2 mm, and the leg portions 32 of this U-shaped tuning fork 7 are also made of the same piano wire. are using. And U
The length of the character 31 is approximately 30 mm, and the width is approximately 10 mm.
It is. 7mm x 5mm at each free end of this U-shaped portion 31.
The first and second plane mirrors 8, 21 are attached. The electromagnetic coil 10 is wound around a core 33 disposed inside this U-shaped portion 31. The vibration frequency F of this U-shaped tuning fork 7 is
Although it is determined by the shape, material, etc. of 1, by adopting the above-mentioned dimensions, the vibration frequency F can be set to approximately 2KHz.

FIG. 3 is a block diagram showing a tuning fork driving circuit 26 for driving the U-shaped tuning fork 7 in vibration. 34 in the diagram
is a light-emitting element drive circuit that supplies drive current to light-emitting elements such as LEDs of the light-emitting device 23; In the figure, reference numeral 35 denotes a position sensor constituting 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 is a type of photoelectric conversion element that outputs a signal proportional to the displacement of the irradiation spot on the element. The amplitude, that is, the amplitude of the second plane mirror 21, is detected by moving the directly installed irradiation spot. The output signal of the position sensor 35 is input to the change detection circuit 36 and converted into an AC signal e ₁ as a vibration amplitude signal that changes as the second plane mirror 21 vibrates. The AC signal e ₁ is converted into a DC output voltage E ₁ by the detection circuit 37 . The output voltage E ₁ output from the detection circuit 37 is input to the deviation detection circuit 39 together with the set voltage E ₂ output from the amplitude setter 38 .
The deviation detection circuit 39 has a set voltage E ₂ with respect to the output voltage E ₁ .
and outputs the deviation voltage _E3 . The deviation voltage E ₃ output from the deviation detection circuit 39 is integrated by the integrating circuit 40 and becomes the control signal voltage E ₄ of the gain control amplifier 41.
become.

On the other hand, the alternating current signal _e1 output from the change detection circuit 36 is input to the phase shift circuit 42, and this phase shift circuit 4
2, the vibration phase is matched to the U-shaped tuning fork 7. The matching signal e ₂ 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 where the waveform is shaped. and,
This waveform shaping circuit 43 sends out the synchronous signal e ₃ to the synchronous detection circuit 18 . Further, the gain controller 41 converts the input matching signal e ₂ into the control signal voltage
It is amplified with an amplification factor determined by _E4 and output as a drive current for the electromagnetic coil 10 of the U-shaped tuning fork 7.

In the tuning fork drive circuit 26 having a type of servo system configuration, the amplitude of the second plane mirror 21, that is, the output voltage _E1 of the detection circuit 37 is set by the amplitude setter 38.
When the set voltage E becomes equal to ₂ , the deviation detection circuit 3
The deviation voltage E ₃ output from 9 becomes 0. Therefore, the value of the control signal voltage _E4 output from the integrating circuit 40 does not change. As a result, the vibration amplitude W of the U-shaped tuning fork 7 does not change. Further, when the output voltage E ₂ of the detection circuit 37 is not equal to the set voltage E ₃ of the amplitude setter 38, a deviation voltage E ₃ corresponding to the difference is inputted to the integrating circuit 40. Then, the control signal voltage output from the integrating circuit 40
_E4 changes corresponding to the deviation voltage _E3 . the result,
The vibration amplitude W of the U-shaped tuning fork 7 changes so that the output voltage E ₁ of the detection circuit 37 becomes equal to the set voltage E ₂ of the amplitude setter 38 . Therefore, by changing the setting voltage E ₂ of the amplitude setting device 38, the vibration amplitude W of the U-shaped tuning fork 7 can be changed as desired.

The operating principle of the wavelength scanning type emission spectrum analyzer constructed in this way will be explained using FIGS. 4a and 4b. That is, the spectral intensity is measured while modulating the wavelength λ around the central wavelength λ ₀ of the bright line spectrum B to be measured (λ ₀ ±Δλ). The spectral intensity waveform measured at this time is a waveform D in which an AC component (ripple component) with an amplitude d is superimposed on an average DC component e, as shown in the figure. The frequency of the AC component d of this waveform D is the wavelength modulation frequency F
The frequency is 2F, which is twice that of . Therefore, if this waveform D is detected at a frequency of 2F in synchronization with wavelength modulation, the original spectral intensity can be obtained. In reality, the measured emission line spectrum B is superimposed with a broadband background spectrum of intensity b as shown in Figure 4a, so the DC component e shown in Figure 4b is
After removing it with a high-pass filter, by performing synchronous detection, an intensity d corresponding to the variation intensity a of the change in the bright line spectrum B can be obtained. Furthermore, the total intensity C of the bright line spectrum B can be obtained by directly amplifying it with a DC amplifier without passing it through a high-pass filter.

To explain this principle with respect to the wavelength scanning emission spectrum analyzer shown in FIG. It is reflected by the collimator 9 and input to the diffraction grating 11. Since the U-shaped tuning fork 7 vibrates with a constant amplitude W and a frequency F, the incident angle θ of the light beam 2 incident on the diffraction grating ₁₁ also falls within a constant angle range (θ ₀ ± Δθ).
As a result, the wavelength λ ₀ ±Δλ
The vibration spectrum of is imaged. 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 waveform D in FIG. 4b. This waveform D is converted into an electrical signal f by a photoelectric converter 14. Therefore, the DC component e is removed by the capacitor 17, and the synchronous detection circuit 1
8, the signal synchronously detected by the synchronous signal _e3 from the tuning fork drive circuit 26 having the frequency of 2F becomes a DC signal corresponding to the fluctuation intensity a of the bright line spectrum B in FIG. 4a.

In addition, a DC amplifier 1 that directly amplifies the electrical signal f
The output signal of No. 5 has a value corresponding to the total intensity c, which is the sum of the background intensity b of the bright line spectrum B and the fluctuation intensity a.

Next, when the adjustment knob 20b of the wavelength scanning mechanism 20 is rotated, the center wavelength λ ₀ of the diffraction spectrum is sequentially shifted. As a result, spectral characteristics over a wide wavelength range as shown in FIG. 6 are obtained.

Figure 5 shows an example of actual measurement. In FIG. 5, E is the spectrum obtained from the output signal of the DC amplifier 15, F is the spectrum obtained from the synchronous detection circuit 18, and the peak G in the spectrum E is a large peak H on the spectrum F. I can understand what is being detected.

In this way, it is possible to remove the DC component e in Figure 4 and separate and detect only the fluctuation component d, so it is possible to remove the background intensity P in the bright line spectrum B and detect only the fluctuation intensity a. Can be measured accurately. Furthermore, with this measurement method, even if the background intensity b varies, this variation will not affect the measurement accuracy of the variation intensity a of the bright line spectrum B. Therefore, it is possible to significantly improve measurement accuracy compared to the case where the wavelength value to be measured is fixed to λ ₀ and the spectral intensity is directly measured as in the conventional optical spectrum analyzer.

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 element 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 mirror 8 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 scanning emission spectrum analyzer of the embodiment uses a U-shaped tuning fork 7 having the shape and dimensions as shown in FIG. 2 as a vibration source. It is possible to obtain a high vibration frequency of about 2KHz. Therefore, it is possible to improve response characteristics.

Further, the amplitude of the vibration can be measured by the second plane mirror 21 and the 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. It is also possible to change the amplitude value W to an arbitrary value. Furthermore, since no mechanically movable parts are employed, reliability does not deteriorate due to durability deterioration.

Note that as a means of detecting the vibration amplitude of a tuning fork oscillator using a U-shaped tuning fork, a method using a magnetic head for pickup has been proposed (Japanese Patent Laid-Open Publication No.
However, the output of the magnetic head is easily affected by temperature changes, and the relationship between the vibration amplitude value and the actual output value of the magnetic head is non-linear. It is also an indirect measurement in which the amplitude is detected using the excitation voltage (current) of the magnetic head. Furthermore, due to pick-up installation errors, head performance (temperature characteristics, manufacturing variations), and the non-linear characteristics mentioned above, complicated calibration between the measured detection voltage (current) value and amplitude value is required. There are issues such as whether it is necessary.

When detecting the end of etching in the plasma etching process of a semiconductor IC element using the wavelength scanning emission spectrum analyzer configured as described above, first, the wavelength scanning mechanism 20 shown in FIG. The plasma vector characteristics shown in FIG. 6a during the etching process are obtained by changing the wavelength to short wavelengths and output from the output terminal 16 via the DC amplifier 15. Next, using the same operating procedure, the plasma spectrum characteristics shown in FIG. 6b at the time when the etching process is clearly considered to have been completed are determined. Then, the bright line spectrum B in which the difference between the two characteristics is most remarkable is determined. and,
The wavelength scanning mechanism 20 sets the measurement wavelength λ of the analyzer to the center wavelength λ ₀ of the bright line spectrum B.

When the above preparatory work is completed, the fluctuation intensity a of the bright line spectrum B from the start of the actual plasma etching process is continuously measured using the DC voltage from the output terminal via the synchronous detection circuit 18. Then, the time when the output characteristic value changes to zero is determined to be the etching end time.

As described above, the present invention makes it possible to perform high-speed wavelength modulation, making it possible to measure minute spectral fluctuations with high precision and responsiveness even in plasma etching, plasma CVD, etc., where the emission spectrum changes rapidly. .

Note that the present invention is not limited to the embodiments described above. An absorption cell is placed in front of the condenser lens 4, and the gas concentration can be determined from changes in the absorption spectrum of the gas or liquid, as in the ammonia analyzer described above.
It is also possible to carry out measurements of liquid concentration.

[Effects of the Invention] As explained above, according to the present invention, by using a tuning fork oscillator, even if the intensity of the background spectrum is large, minute spectral changes such as the target emission line spectrum or absorption peak can be detected with high accuracy. The response characteristics can be measured easily and with good sensitivity.

Furthermore, the absolute value of the amplitude of the U-shaped tuning fork that vibrates the measurement light is directly measured using an optical method such as a position sensor, and the amplitude is controlled to be constant. Therefore, compared to other measurement methods using, for example, a magnetic head, the stability and measurement accuracy of the amplitude measurement of the U-shaped tuning fork are increased, so that the measurement accuracy of the final spectral change can be further improved. .

[Brief explanation of drawings]

The figures show a wavelength scanning emission spectrum analyzer according to an embodiment of the present invention, in which Fig. 1 is a schematic diagram showing the overall configuration, Fig. 2 is a perspective view showing a U-shaped tuning fork, and Fig. 3 is a perspective view showing a U-shaped tuning fork. The figure is a block diagram showing the tuning fork drive circuit, and Figure 4 is a diagram for explaining the operating principle.
FIG. 5 is a diagram showing an example, and FIG. 6 is a plasma spectrum characteristic 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, 1
0... Electromagnetic coil, 11... Diffraction grating, 12...
Collector, 13... Exit slit, 14... Photoelectric converter, 15... DC amplifier, 17... Capacitor, 18... Synchronous detection circuit, 20... Wavelength scanning mechanism, 21... Second plane mirror, 23... ...light emitting device,
24...detection light, 25...amplitude measuring device, 26...
Tuning fork drive circuit, 31...U-shaped part, 32... leg part,
34...Light emitting element drive circuit, 35...Position sensor, 36...Change detection circuit, 37...Detection circuit, 38...Amplitude setter, 39...Difference detection circuit, 40...Integrator circuit, 41... . . . gain control amplifier, 42 . . . phase shift circuit, 43 . . . waveform shaping circuit.

Claims (1)

[Scope of Claims] 1. A tuning fork oscillator 6 that vibrates a luminous flux of light to be measured at a predetermined frequency, and a diffraction grating 11 that disperses the modulated luminous flux output from the tuning fork oscillator and outputs a diffraction spectrum. a wavelength scanning mechanism 20 that rotates the diffraction grating to continuously change the angle of incidence of the output beam with respect to the diffraction grating;
a photoelectric converter 14 that receives the diffraction spectrum output from the diffraction grating and converts an optical modulation signal included in the diffraction spectrum into an electrical signal; In a wavelength scanning emission spectrum analyzer equipped with a synchronous detection circuit 18 that performs synchronous detection at a frequency twice the vibration frequency, the tuning fork oscillator 6 includes a U-shaped tuning fork 31 and a U-shaped tuning fork 31;
Electromagnetic coil 10 for vibrating the glyph tuning fork
and a first one attached to one free end of the U-shaped tuning fork to reflect and output the luminous flux of the light to be measured.
a second plane mirror 21 attached to the other free end of the U-shaped tuning fork for detecting the vibration amplitude of the reflected light; and a second plane mirror 21 for detecting the vibration amplitude of the reflected light beam to the second plane mirror. a light emitting device 23 that emits detection light; and an amplitude measuring device that receives the detection light reflected by the second plane mirror with a position sensor and measures the vibration amplitude of the reflected detection light with a change detection circuit. 25, and a drive circuit 26 for controlling the excitation of the electromagnetic coil so that the vibration amplitude value measured by the amplitude measuring device is always a constant value. .