JPH0442039A - Detected-signal processing method for particulate detector - Google Patents

Abstract

PURPOSE:To remove the noise in a beat signal and to improve an A/N ratio by performing synchronous detection of the synthesized beat signal of the scattered light from particulates and the reference beam of the output of a modulating part with the synchronizing signal obtained by multiplying the limited wave signals of two frequencies from a transmitter. CONSTITUTION:Oscillators 6c and 6d transmit oscillating frequencies f1 and f2. The laser beam having a frequency f0 from a laser light source 1 is split into two beams through a beam splitter 2. One beam orthogonally crosses sample air in a detecting region D of a detecting cell 3. The scattered light from particulates is made to pass through a condenser lens system 4 and inputted into a beam splitter 5 as the parallel beam. the other beam is shifted into a frequency (f0 + f1 - f2) and becomes a reference beam. Both beams are synthesized in the splitter 5, and the particulate is detected by the beat signal of the synthesized frequency (f1 - f2). At this time, the frequencies f1 and f2 are multiplied in a multiplier 9. The synchronizing signal having the frequency (f1 - f2) is obtained in a bandpass filter 10. The beat signal undergoes synchronous detection. Thus, the noises are removed, and the S/N ratio is improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a detection signal processing method for a particle detector,
Specifically, the present invention relates to a method for processing a particle detection signal in a heterodyne type particle detector.

[Prior Art] In the manufacture of precision parts such as semiconductors, the work is carried out in a lean room with good cleanliness because fine particles such as dust deteriorate the quality of the product. The cleanliness of the clean room is measured by a particle detector. The basic principle of a particle detector is to sample air from indoor air, introduce it into the detector's detection cell, and project a laser beam onto it.

Since the particles contained in the sample air scatter the laser beam, the particles are detected by receiving this scattered light. In response to the recent high degree of integration of semiconductors, particle detectors are required to have a high level of detection performance, and various improvements have been made to the configuration based on the above basic principle to improve detection performance to a certain extent. However, a heterodyne method has been proposed as a method that can further improve the performance, and a patent application publication No. 1983-83944 titled "Method and Apparatus for Measuring Dust in Semiconductor Manufacturing Process" is disclosed.

FIG. 3 shows the configuration of one embodiment of the dust measuring device according to the above patent application. A laser beam of frequency fo from a laser light source 1 is split into two by a beam splitter 2, one of which is perpendicular to the sample air in the detection area of the detection cell 3, and the scattered light of the particles is scattered. The scattered light is condensed by the condenser lens 4a of the condenser lens system 4, stray light is removed by the slit of the slit plate 4b, and is converted into a parallel beam by the collimator lens 4c, which is sent to the beam splitter 5.
Enter. The other half divided into two is input to the modulation section 6, and the first
In the second acousto-optic modulator 6a, it is shifted to the frequency (fo + fx) by the frequency fl from the oscillator 6c, and further shifted to the frequency (fo + fl - f2) by the frequency f2 from the oscillator 6d in the second acousto-optic modulator 6b. The reference beam is reflected by the mirror 7 and input to the beam splitter 5, where it is combined with the parallel beam of the scattered light. The frequency (f
A beat signal (l-fz) is generated, which is received by the light receiver 8, and the detection signal S detects the particles. Note that in the above, the modulation section 6 can naturally generate the above shift frequency (fo + fx - fz) using one set of oscillator and modulator, but the reason why two sets of oscillator and modulator are used in this case is The optical modulator requires a high frequency wave with a large power, and this causes noise to be mixed into the electronic elements of the device, reducing the S/N ratio, so this is said to be done to prevent this.

The heterodyne method described above has already been used in radio receivers and other devices for a long time, and when the strength of the signal wave to be detected is very weak, it is amplified by the modulated wave and can significantly increase the sensitivity. It is extremely effective to apply this method to weak particle detection signals. However, in the heterodyne system, in addition to the generally useful signals, there are unnecessary and harmful signals generated due to modulation, and it is of course necessary to completely remove these signals, but the beat signal obtained by the above synthesis Since there are various noise signals mixed in from the modulation section and the like, it is absolutely necessary to remove them using an appropriate method.

[Problem to be solved] Although it is somewhat effective to simply select the frequency of the beat signal using a filter in the method of noise removal from the beat signal described above, a more reliable method is to apply the synchronous detection method. It is possible that

An object of the present invention is to provide a particulate detection signal output method that applies a synchronous detection method to remove noise in a beat signal and improve S/N.

[Means for Solving the Problems] The present invention includes an oscillator with oscillation frequencies fl and fz and two acousto-optic modulators, and the frequency f of the laser beam output from the laser light source is changed to the frequency (fo +fx - fz
) is equipped with a modulation unit that outputs a reference beam shifted to
With respect to the sample air introduced into the detection cell, the frequency fo
The beam splitter combines the light scattered by the particles contained in the sample air with the reference beam output from the modulator, and the beat signal of the frequency (fl-fz) obtained by the synthesis is used to detect the particles. This is a detection signal processing method in a heterodyne particle detector, in which two sine wave signals with frequencies f1 and fz output from an oscillator are multiplied to create a synchronized signal with a frequency (fl-fz), and a beat signal is generated. The signal is synchronously detected using this synchronous signal.

[Operation] In the detection signal processing method with the above configuration, the frequency ( The beat signal of fl-fz) is synchronously detected by a synchronizing signal of frequency (fl-fz) obtained by multiplying the oscillator frequency f1 by the sine wave signal of fz. The principle of synchronous detection is that only the components of the target signal that are synchronized with the synchronous signal are detected, and the components that are not synchronized are not detected.In this case, the beat signal is detected because it is synchronized with the synchronous signal. Therefore, unsynchronized noise contained in the beat signal is not detected, and the S/N is improved accordingly.

[Embodiment] FIGS. 1(a) and 1(b) are a block diagram of a first embodiment of a detection signal processing method for a particulate detector according to the present invention, and an explanatory diagram of the action of synchronous detection. In each of the following embodiments, the same parts as those of the conventional heterodyne particle detector shown in FIG. 3 are given the same numbers. In FIG. 1(a), a laser beam of frequency fO from a laser light source 1 is split into two by a beam splitter 2, one of which is perpendicular to the sample air in the detection area of a detection cell 3, and the scattered light of the particles is passed through a condenser lens. The beam passes through the system 4, becomes a parallel beam, and enters the beam splitter 5. The other of the two halves is input to the modulator 6, shifted to the frequency (fQ+f1-fz), becomes a reference beat, is input to the beam splitter 5 via the mirror 7, and is combined with the parallel beam of the above-mentioned scattered light. This synthesis generates a beat signal with a frequency (fl-fz), which is received by the photoreceiver 8 and becomes the detection signal S.
is output. The process up to this point is the same as above. On the other hand, in the present invention, each signal of the frequency fl of the oscillator 8C of the modulation section 6 and the frequency f2 of the oscillator 6d is applied to the multiplier 9.
The bandpass filter 1G gives the frequency (fl-
fz) is selectively extracted and used as the synchronization signal T.

Synchronous detection is performed by inputting the synchronizing signal T and the detection signal S adjusted to an appropriate level by the amplifier 8a to the synchronous detector 11, and a detection signal S' with noise removed and an improved S/N ratio is obtained. I can be hatched. To explain the noise removal effect of synchronous detection with reference to Figure (b), the detection signal S contains minutely fluctuating noise in addition to the beat signal shown by the dotted line.
It is understood that only the beat signal synchronized with the synchronization signal T becomes the detection signal S' and noise is removed.

Now, a He--Ne gas laser tube which can obtain a large power is normally used as a laser light source of a particle detector, and its types include an internal mirror type and an external mirror type.

In both cases, the laser beam also leaks to some extent on the opposite side (back side) of the laser tube to the laser output side (front side). These allow various modifications to the configuration of the optical system.

The first embodiment described above is of an internal mirror type, and FIG. 2 shows other embodiments with various modifications.

FIG. 2(a) shows a second embodiment. The laser light source 1 is of an internal mirror type, and a laser beam with a frequency fO from the front thereof is projected onto the detection cell 3, and scattered light from the particles is scattered. On the other hand, the direction of the laser beam leaked from the back side is changed by the mirror 12 and inputted to the modulation section 6. below,
As in the first embodiment, a beat signal based on the heterodyne system and a detection signal S corresponding to the beat signal are obtained, a synchronization signal T is generated by a multiplier 9, synchronous detection is performed by a synchronous detector 11, and a detection signal S' is output. Ru.

FIGS. 2(b) and 2(e) show third and fourth embodiments in which the laser light source 1 is an external mirror type. In FIG. 2(b), the mirror 1a on the back of the laser tube is An external mirror 1b is provided between the two and laser oscillation is performed between the two.
The detection cell 3 is placed between the laser tube and the external mirror 1b, the laser beam passes through the detection area of the detection cell, and the scattered light of the particles is scattered. Further, some laser beam is leaked to the external mirror 1b, and this is supplied to the modulation section 6 through the mirror seal 2 to create a reference beam and a synchronization signal. Figure (C) uses the laser beam leaked by the rear mirror 1a, and the same operation as described above is performed in both Figures (b) and (c) below.

In this invention, the frequency ro of the laser beam is doubly modulated using two sets of oscillators and an acousto-optic modulator, but it can also be applied to a heterodyne system using one set of oscillators and an acousto-optic modulator. In that case, the multiplier 9 is not necessary and the oscillation frequency of the oscillator is directly used for the synchronization signal.

[Effects of the Invention] As is clear from the above explanation, in the detection signal processing method of the particle detector according to the present invention, the particle scattered light of the frequency fo and the reference beam of the frequency (fo+f1-fz) are combined to The frequency f1 output from the two oscillators for the beat signal of the given frequency (ft-fz)
The frequency (fx
- fz) synchronization signal, only beam signals that are synchronized with the synchronization signal are detected, and unsynchronized noise contained in the beat signal is not detected, improving the S/N. Heterodyne method The selection and extraction of beam signals essential for this method can be performed well, and this contributes to the practical application of particle detectors using this method.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are a block diagram and an explanatory diagram of the synchronous detection function for a first embodiment of the detection signal processing method of a particle detector according to the present invention, and FIGS. 2(a) and (b) and (C) are block diagrams of second, third, and fourth embodiments of the detection signal processing method of the particle detector according to the present invention, and FIG. 3 is the basic structure of the heterodyne particle detector according to the patent application publication. FIG. DESCRIPTION OF SYMBOLS 1... Laser light source, 2, 5... Beam splitter, 3... Detection cell, 4... Condensing lens system, 4a... Condensing lens, 4b... Slit plate, 4c...・Collimator lens, 6...Modulation section, 6
a, 6b... Acousto-optic modulator, 8c, 6d... Oscillator, 7.12... Mirror 8... Light receiver, 8
a... Amplifier, 9... Multiplier, 10...
・Band filter, 11...Synchronized detector.

Claims (1)

[Claims] (1) It consists of an oscillator with oscillation frequencies f_1 and f_2 and two acousto-optic modulators, and the frequency f_0 of the laser beam output from the laser light source is changed to the frequency (f_0+f_1-f_
2) is equipped with a modulation unit that outputs a reference beam shifted to 2), and projects a laser beam of the frequency f_0 on the sample air introduced into the detection cell, and scatters light scattered by fine particles contained in the sample air. In a heterodyne type particle detector that combines the reference beam with the reference beam using a beam splitter and detects the particles using a beat signal of frequency (f_1-f_2) obtained by the synthesis, two of the frequencies f_1 and f_2 output from the oscillator are used. A detection signal processing method for a particle detector, characterized in that a synchronization signal of frequency (f_1-f_2) is generated by multiplying sine wave signals, and the beat signal is synchronously detected using the synchronization signal.