JPH0528769B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a wavelength modulation type differential sensor that can accurately measure the amount of change in a bright line peak in optical spectrum intensity measurement using a wavelength modulation device, 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.

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. 7a. Then, the intensity of the bright line spectrum A is continuously measured by fixing the detection wavelength to the bright line spectrum A, for example, 3082 Å, which is prominent in the measured spectral characteristics. Then, as shown in Figure 7b, this wavelength 3082
The etching process is stopped at the time when the intensity of the bright line spectrum A of 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 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 time of completion of etching. Even with this kind of thinking, Figure 7a
Comparing the spectral characteristics at the time of etching and the spectral characteristics at the end of etching in Figure b,
An emission line spectrum B near 4000 Å is discovered. Therefore, by adjusting the detection wavelength λ of the spectrometer to the center wavelength λ _O of the bright line spectrum B 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 described above, by fixing the measurement wavelength λ to λ _O and continuously measuring the intensity change of the bright line spectrum B with a spectrometer, it is possible to solve the problem of plasma etching treatment. There are also the following problems when detecting the end. That is,
As shown in the figure, the emission line spectrum B of wavelength λ _O is often superimposed on a wide background spectrum. For example, in Fig. 7a, 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 variation that protrudes from the background spectrum is a, then in the end of etching detection, the final variation intensity is a
It is necessary to measure the amount of change in However, in an actual spectrum spectrometer, the total spectral intensity c including the background spectral intensity b
is measured.

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

As mentioned above, in the plasma etching process of semiconductor IC elements, when the value of the fluctuation intensity a becomes undetectable as shown in FIG. 7b, it is determined that etching has ended. If the determination cannot be made accurately, it may be determined that the fluctuation strength a has become zero even though the etching process has not been completed, and the etching process may be stopped. In this case, there was a concern that some portions of the semiconductor surface would remain where the etching process had not been completely performed, resulting in a decrease in the yield of IC products.

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

[Means for Solving the Problems] The wavelength modulation type differential spectrometer of the present invention includes a wavelength modulation device that vibrates a luminous flux of light to be measured at a predetermined frequency, and a modulated luminous flux output from the wavelength modulation device. A wavelength scanning mechanism that rotates the diffraction grating to continuously change the incident angle of the output light beam to the diffraction grating, and a wavelength scanning mechanism that receives the diffraction spectrum output from the diffraction grating. a photoelectric converter that converts an optical modulation signal included in a diffraction spectrum into an electrical signal; a synchronous detection circuit that synchronously detects the electrical signal output from the photoelectric converter at a frequency twice the vibration frequency of the wavelength modulation device; It consists of 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 wavelength modulation device or twice the frequency.

Furthermore, the wavelength modulation device includes a U-shaped tuning fork;
An electromagnetic coil for vibrating the U-shaped tuning fork, a first plane mirror attached to one free end of the U-shaped tuning fork to reflect a beam of light to be measured, and the other free end of the U-shaped tuning fork. a second plane mirror attached to the camera for detecting the reflected light and vibration amplitude; a light emitting device for irradiating the second plane mirror with detection light for detecting the vibration amplitude of the reflected light beam; The amplitude measuring device receives the detected light with a position sensor and measures the vibration amplitude of the reflected detected light with a change detection circuit. It consists of a drive circuit that controls the excitation of the electromagnetic coil.

[Operation] In the wavelength modulation type differential spectrometer configured as described above, the beam of light to be measured is vibrated at a predetermined frequency F by the wavelength modulation device and input to the diffraction grating. Therefore, since the angle of incidence θ on the diffraction grating oscillates at a frequency F around the central incidence angle θ _O , the electrical signal obtained by the photoelectric converter that receives the output spectrum of this diffraction grating is at the central incidence angle θ O. With respect to the measurement center wavelength λ _O at θ _O , it becomes a vibration spectrum signal that oscillates at a frequency F and a wavelength of λ _O ±Δλ. This vibration spectrum signal is passed through a high-pass filter at a frequency of 2F.
When synchronous detection is performed at , the variation of the spectrum at the measurement center wavelength λ _O can be obtained.

Furthermore, when the output electrical signal of the photoelectric converter is sampled at a frequency of 2F via a DC imager, a DC component of the spectrum at the center wavelength λ _O is obtained. Furthermore, when the wavelength scanning mechanism is driven, the central incident angle θ _O of the oscillated light beam with respect to the diffraction grating changes, so the measurement center wavelength λ _O changes.

[Example] An example 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. 1 in the diagram is a semiconductor IC
This is a plasma generation source such as an etching processing section in a device, etc., and a luminous flux 2 of the light to be measured emitted from this plasma generation source 1 is passed through an entrance slit 5 by a condenser lens 4 fitted into an entrance window of a case 3. The light is focused on a first plane mirror 8 attached to one of a pair of vibrating pieces of a vibrator 7 constituting the wavelength modulator 6 .

This vibrator 7 is configured as shown in FIG. 2, for example. That is, an electromagnetic coil 10 is disposed inside the U-shaped tuning fork 9, and a core 11 is inserted into the axis of the electromagnetic coil 10. The first plane mirror 8 and the second plane mirror 12 are attached to a pair of free ends of the U-shaped tuning fork 9, that is, a pair of vibrating pieces, respectively. However,
This vibrator 7 vibrates at a constant frequency F determined by the shape and weight of the U-shaped tuning fork 9, and at an amplitude W determined by the value of the excitation current flowing through the electromagnetic coil 10.

The light beam 2 reflected by the first plane mirror 8 of the vibrator 7 is incident on the collimator 13. As described 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 oscillating light that vibrates at a constant frequency F and a constant amplitude W. The oscillating 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 disposed at a position where the vibration spectrum reflected by the collector 15 forms an image, and adjacent to the back surface of the exit slit 16, the vibration spectrum reflected by the collector 15 forms an image on the exit slit 16. A photoelectric converter 17 is provided to receive the vibration spectrum. 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 sampling circuit 19 converts the input amplified electrical signal f to a frequency output from the timing circuit 20.
Sample with 2F sampling pulse g.
Note that a synchronization signal h having a frequency 2F, which is twice the vibration frequency F of the vibrator 7 output from the vibrator drive circuit 21 that drives the vibrator 7 of the wavelength modulation device 6, is input to the timing circuit 20. The timing circuit 20 adjusts the output timing of the synchronization signal h and sends it to the sampling circuit 19 as a sampling pulse g. sampling circuit 19
Electrical signal f sampled at frequency 2F at
The sampling data is input as the DC component intensity e of the measured spectrum to a control calculation circuit 22 comprised of a microprocessor or the like.

Further, the electrical signal f output from the photoelectric converter 17 is passed through a high-pass filter consisting of a capacitor 23 to remove the DC component, and then sent to a synchronous detection circuit 2.
4. The alternating current component of the electric signal f is synchronously detected by this synchronous detection circuit 24 at the frequency 2F of the synchronous signal h emitted from the vibrator drive circuit 21, and is inputted to the control calculation circuit 22 as the fluctuation intensity a of the measurement spectrum. Ru.

In addition, 25 in the figure is a wavelength scanning mechanism that rotates the diffraction grating 14 to continuously change the central incident angle θ _O of the oscillating light incident on the diffraction grating 14 from the collimator 13. In this wavelength scanning mechanism 25, A threaded rod 25 that is screwed into a screw hole provided in the case 3
A motor drive circuit 25 that controls a drive motor 25b attached to one end of a by a control calculation circuit 22.
When the threaded rod 25a is rotated, the horizontal position of the locking member 25d attached near the other end of the threaded rod 25a is moved. Locking member 25
When the horizontal position of d moves, the diffraction grating 1, which is rotatably attached to the fixed part at the rotation axis 25e,
The rotation arm 25f of No. 4 moves, and the diffraction grating 14
rotates. As a result, the central incident angle θ _O of the oscillating light changes. When the central incidence angle θ _O changes, the value of the center wavelength λ _O 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 detected by a rotation angle detector 26 and inputted to the control calculation circuit 22.

In the wavelength modulation device 6, as shown in the figure, detection light 29 output from a light emitting device 28 constituted by 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. Ru. The detection light 29 reflected by the second plane mirror 12 becomes vibration light with a constant frequency F, and is input to an amplitude measuring device 30 composed of a position sensor and the like that measures the vibration amplitude of the detection light 29. . Detection light 2 measured by this amplitude measuring device 30
The vibration amplitude signal 9 is input to a vibrator drive circuit 21 that sends an excitation current to an electromagnetic coil 10 incorporated inside the U-shaped tuning fork 9 of the vibrator 7.

The vibrator drive circuit 21 includes the control calculation circuit 2
The amplitude setting signal i sent from 2 is input. Then, the measured amplitude of the vibrator 7 converted from the amplitude vibration signal input from the amplitude measuring device 30 is
The value of the drive current supplied to the electromagnetic coil 10 of the vibrator 7 is controlled so that the set amplitude W corresponds to the set signal i. Therefore, the vibration amplitude of the vibrator 7 always becomes the amplitude value W corresponding to the setting signal i sent from the control calculation circuit 22. Further, the vibrator drive circuit 21 sends a synchronization signal h having a frequency 2F twice the vibration frequency of the vibrator 7 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 FIGS. 3a and 3b. That is, the center wavelength λ _O of the emission line spectrum B to be measured
The spectral intensity is measured while modulating the wavelength λ around this center (λ _O ±Δλ). The spectral intensity waveform measured at this time becomes a waveform D in which an AC component (ripple component) with an amplitude d is superimposed on a DC component e, as shown in FIG. The frequency of the AC component of this waveform D is twice the wavelength modulation frequency F.
It will be on the 2nd floor. Therefore, this waveform D has a frequency of 2F
If the detection is performed in synchronization with the wavelength modulation, the original spectrum 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 a.
After removing the DC component e shown in FIG. 3B with a high-pass filter, synchronous detection is performed to obtain an intensity corresponding to the variation intensity a of the change in the bright line spectrum B.

Furthermore, if the waveform D on which the AC component of amplitude d remains superimposed is sampled at a frequency of 2F, it is possible to extract the original spectral intensity c. If the waveform D is sampled at the timing of S ₁ , S ₂ , S _{3 .} . . in Figure 3b, that is, f ₁ , f ₂ , f _{3 .} . ., the original spectral intensity c at the wavelength λ _O 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 has a value of about 2 KHz. Since this value is sufficiently high compared to the wavelength scanning speed by the wavelength scanning mechanism 25, the sampling data output from the sampling circuit 19 can be regarded as a substantially continuous value.

Therefore, in the wavelength modulation type differential spectrometer of the embodiment shown in FIG.
The incident angle θ of the oscillating light incident on the diffraction grating 14 is also within a certain angle range (θ _{O ±}
Δθ). As a result, the exit slit 16
A vibration spectrum of wavelength λ _O ±Δλ is imaged above.
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 waveform D in FIG. 3b. This waveform D is converted into an electrical signal f by a photoelectric converter 17. Therefore, the DC component e is removed by the capacitor 23,
A signal synchronously detected by the synchronous detection circuit 24 using the synchronous signal h having a frequency of 2f becomes a DC signal corresponding to the fluctuation intensity a of the bright line spectrum B in FIG. 3a.

In addition, the sampling circuit 19 generates a sampling pulse g with a frequency of 2F (sampling point S ₁ ,
S ₂ , S _{3 .} . . ) sampled data values correspond to the DC components f ₁ , f ₂ , f ₃ .

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.
is input.

Next, the motor drive circuit 25 of the wavelength scanning mechanism 25
When the drive motor 25b is rotated at a constant speed at step c, the center wavelength λ _O of the diffraction spectrum shifts sequentially with time t, as shown in FIG. As a result, the wavelength (λ _O ±Δλ) included in waveform D in Figure 3b
are gradually shifted. Therefore, when this waveform D is sampled with a sampling pulse g having a frequency of 2F, the waveform λ corresponding to the sampled data becomes λ ₁ , λ ₂ , λ ₃ , ..., λn, with time t.
It gradually changes to... 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 λ _O is inputted to the control calculation circuit 22 from the rotation angle detector 26. Therefore, the DC intensity c of the spectrum corresponding to each wavelength λ is input to the control calculation circuit 22.

Also, when the center wavelength λ _O of waveform D changes sequentially,
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 is performed on a SiO ₂ film on a semiconductor wafer. In this way, for the wavelength λ, the DC intensity c
It is possible to simultaneously obtain the DC characteristic E corresponding to the fluctuation intensity a and the fluctuation characteristic F corresponding to the fluctuation intensity a. 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, it is possible to remove the DC component e in Fig. 3 and separate and detect only the AC component d, so the bright line spectrum B
By removing the intensity b of the background spectrum at , only the fluctuation intensity a can be measured with high accuracy. Therefore, it is possible to significantly improve measurement accuracy compared to the case where the wavelength value to be measured is fixed to λ _O and the spectral intensity c is directly measured as in the case of a conventional spectrum spectrometer.

For example, in the same figure, the emission line spectra G and H that exist at wavelengths λ = 4510 Å and λ = 4820 Å almost disappear at the end of etching.
By monitoring the fluctuation characteristic F value of the bright line spectrum at the same wavelength, the etching end time can be reliably detected.

In addition, in addition to the fluctuation characteristics F, the DC characteristics E can also be determined at the same time, so the target value of the bright line spectrum B can be observed while observing the overall spectral characteristics, so the fluctuation information of the bright line spectrum B can be more accurately grasped. can.

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

Furthermore, in the embodiment, the timing circuit 2
The output timing of the sampling pulse g from 0 to the sampling circuit 19 was set at the moment when the fluctuating portion of the waveform D crosses the center position shown by the dashed line as shown in FIG. 4, but as shown in FIG. It may be set at the moment when the fluctuating portion of waveform D reaches its maximum value and minimum value. If you set it like this,
The wavelengths corresponding to the sampled data are the wavelengths B ₁ , B ₂ , B ₃ . . . at the turning points of the waveform D. The amount of change in data at the turning point of waveform D (dλ/
dt) is small, it is possible to set a wide sampling gate time, and therefore it is possible to improve the measurement accuracy of each sampled data.

In this case, the wavelength during sampling will deviate from the center wavelength λ _O by ±Δλ, but it can be calculated from the amplitude W of the oscillator 7 which is Δλ at that time, so
The control calculation circuit 22 may correct the wavelength.

[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 emission line spectra can be measured with high precision, and the absolute value of the spectrum can also be measured simultaneously, and the target can be achieved. It is possible to easily distinguish the emission line spectrum and accurately grasp the amount of change.

Further, 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 improved, so that the measurement accuracy of the final spectral change can be further improved.

[Brief explanation of the drawing]

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.
The figure is a diagram for explaining the operation of the same embodiment, FIG. 5 is a diagram showing actual measured values, and FIG. 6 is a diagram for explaining the operation of a wavelength modulation type differential spectrometer according to another embodiment of the present invention. , FIG. 7 is a diagram showing 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...electromagnetic coil, 12...second
plane mirror, 13...collimator, 14...diffraction grating, 15...collector, 16...exit slit,
17...Photoelectric converter, 18...DC amplifier, 19
... Sampling circuit, 20 ... Timing circuit, 21 ... Vibrator drive circuit, 22 ... Control calculation circuit, 23 ... Capacitor, 24 ... Synchronous detection circuit, 25 ... Wavelength scanning mechanism, 26 ... Rotation angle Detector, 28... Light emitting device, 29... Detection light, 30
...Amplitude measuring device.

Claims (1)

[Claims] 1. A wavelength modulation device 6 that vibrates a beam of light to be measured at a predetermined frequency, and a diffraction grating 14 that disperses the modulated beam output from the wavelength modulation device and outputs a diffraction spectrum. and a wavelength scanning mechanism 25 that rotates the diffraction grating to continuously change the incident angle of the output light beam with respect to the diffraction grating.
a photoelectric converter 17 that receives the diffraction spectrum output from the diffraction grating and converts the optical modulation signal included in the diffraction spectrum into an electrical signal; and converts the electrical signal output from the photoelectric converter into an electrical signal at the wavelength. A synchronous detection circuit 24 that performs synchronous detection at a frequency twice the vibration frequency of the modulator, and a DC amplifier 1 that amplifies the electrical signal output from the photoelectric converter.
8, and a sampling circuit 19 that samples the output signal of the DC amplifier in synchronization with the vibration frequency of the wavelength modulation device or twice the frequency, the wavelength modulation device comprising: , a U-shaped tuning fork 9, an electromagnetic coil 10 for driving the U-shaped tuning fork in vibration,
a first plane mirror 8 attached to one free end of the U-shaped tuning fork to reflect and output the luminous flux of the light to be measured; and a first plane mirror 8 attached to the other free end of the U-shaped tuning fork to reflect and output the beam of light to be measured; the second to detect
a plane mirror 12, and a light emitting device 2 that irradiates detection light for detecting the vibration amplitude of the reflected light beam onto the second plane mirror.
8, an amplitude measuring device 30 for receiving the detection light reflected by the second plane mirror with a position sensor and measuring the vibration amplitude of the reflected detection light with a change detection circuit; A wavelength modulation type differential spectrometer comprising: a drive circuit 21 that controls excitation of the electromagnetic coil so that the vibration amplitude value measured by the electromagnetic coil always has a constant value.