JP2696562B2 - Elimination method of fluctuation noise of laser in particle detector - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for eliminating laser fluctuation noise in a particle detector, and more particularly, to a laser beam generated by sample air in a particle detector using an external mirror type laser oscillator. The present invention relates to a method for detecting only fine particle scattered light while eliminating intensity fluctuation noise.

[Prior art] With the miniaturization of IC pattern sizes in semiconductor IC manufacturing factories, the control of dust particles in clean rooms has become increasingly strict, and the detection performance of particle detectors has been further improved. Is needed.

The particle detector detects the scattered light of the particles by making the sample beam containing the particles such as dust orthogonal to the laser beam, but the intensity of the scattered light is proportional to that of the laser beam. In order to achieve this, it is desirable to use a laser beam having the greatest possible intensity. As the laser light source, a gas laser tube is usually used, and there are an internal mirror type and an external mirror type as its configuration method. The former uses only a few percent of the intensity of the oscillated laser beam, while the latter is advantageous because the intensity can be dramatically increased using the oscillation intensity itself.

2 (a) and 2 (b) show a main part of a particle detector using an external mirror type laser oscillator. In FIG. 2 (a), a laser oscillator 1 is a gas laser tube 1a filled with a laser gas such as helium / neon. , An internal mirror b is fixed to one end, and a Brewster window 1c is attached to the other end, and this is inserted into the detection cell 2 and fixed. The detection cell 2 is provided with an external mirror 1d at a position corresponding to the Brewster window 1c, and a laser beam L oscillates between the external mirror 1d and the internal mirror 1b. On the other hand, the sample air A from the center nozzle 3a
And make them orthogonal. In order to make the sample air A a laminar flow, the outer sheath nozzle 3b is used to clean the sheath air B.
Is also ejected, and the sheath air B is discharged to the outside from the discharge nozzle 4 together with the sample air A. FIG. 2B shows a scattered light receiving system 5 for the scattered light of the fine particles. The light receiving lens 5a, the slit plate 5b and the photoelectric converter 5 are shown.
c, and by the slit S 'of the slit plate 5b,
The field of view at a location orthogonal to the sample air A and the laser beam L is defined as a particle detection area S, and stray light from locations other than S is eliminated to improve the S / N ratio.

[Problem to be solved] Generally, in order to improve the detection performance of a particle detector,
In addition to increasing the intensity of the laser beam using the external mirror oscillator described above, a method of increasing the focusing range or sensitivity of the light receiving system is performed, but the air molecules of the sample air have a random density distribution, In the above-mentioned external mirror type laser oscillator, the oscillation intensity fluctuates because air molecules randomly intercept the laser beam and irregularly reflect it, and this fluctuation becomes noise, and the S / N ratio of the received signal of the scattered light receiving system becomes noise. There is a problem that decreases.

The present invention has been made in order to solve the above-described problems. In a fine particle detector configured by an external mirror type laser oscillator, noise of fluctuation of a laser beam due to sample air is eliminated, and only scattered light of fine particles is detected. The purpose of the present invention is to provide a method for performing the following.

Means for Solving the Problems According to the present invention, an external mirror is provided outside a gas laser tube having an internal mirror at one end to constitute an external mirror type laser oscillator, and sample air is supplied to a laser beam of this oscillator. At right angles, the field of view is limited by a slit with respect to the orthogonal portion to form a fine particle detection area, and in this area, scattered light of fine particles contained in the sample air is received by a scattered light receiving system to eliminate laser fluctuation noise. For the laser beam in which the oscillation intensity fluctuates due to the sample air and fluctuating noise is generated, the fluctuating noise slit is used to set the fluctuating noise detection area by limiting the field of view to the vicinity of the fine particle detection area. The fluctuation noise in this area is received by the fluctuation light receiving system, and the fluctuation light is received by the output signal of the scattered light reception system. This is to detect only scattering of fine particles by subtracting a fluctuation noise signal output from the system.

[Operation] As described above, in the external mirror type laser oscillator, a fluctuation noise is generated in the laser beam due to the randomness of the air molecule density of the sample air, and this is output together with the signal of the fine particle scattered light from the scattered light receiving system. On the other hand, the fluctuation of the oscillation intensity of the laser beam in the fluctuation noise detection area set near the scattered light detection area is received by the fluctuation light receiving system and a fluctuation noise signal is output. Since this signal and the fluctuation noise contained in the output signal of the scattered light receiving system are similar and in phase, the fluctuation noise is eliminated by taking the difference between the output signals of each light receiving type, and only the scattered light of the fine particles is detected. Is what is done.

[Embodiments] FIGS. 1A, 1B and 1C show an entire configuration and a partial configuration diagram of an embodiment of a laser fluctuation noise elimination system according to the present invention, and waveform diagrams of respective output signals. 2A, the gas laser tube 1 is similar to FIG.
a, internal mirror 1b, Brewster window 1c and external mirror 1
An external mirror type laser oscillator 1 is constituted by d, and a laser beam L oscillates between the mirrors 1d and 1b. The sample air A and the sheath air B are jetted orthogonally to the laser beam L, and the scattered light is scattered by the fine particles. The scattered light receiving system 5 is also configured in the same manner as described with reference to FIGS. 2A and 2B, and scattered light of fine particles in the fine particle detection region S, Rayleigh scattered light by air molecules of the sample air (laser beam intensity). (Variation noise due to fluctuation) is received by the scattered light receiving system 5, and the signal es is output from the photoelectric converter 5c. On the other hand, a fluctuating light receiving system 6 is provided. The fluctuating light receiving system 6 shares the light receiving lens 5a of the scattered light receiving system 5 and a fluctuating noise slit N 'is provided in the vicinity of the conventional slit S' of the slit plate 5b so as to be in the vicinity of the scattered light detecting area S. A fluctuation noise detection area N is set, and the fluctuation of the emission intensity of the laser beam in the area N is received by the photoelectric converter 6a, and the fluctuation noise signal n
Is output. FIG. 4B shows the relationship between the scattered light receiving system 5 and the fluctuating light receiving system 6 and the tracing of each light ray. The fine particles in the area S are not mixed into the area N, and the respective light rays are clearly distinguished from each other. Both are arranged at an appropriate interval so that they are separately received. The signals es and n obtained as described above are level-adjusted by the amplifiers 7a and 7b so that the fluctuation noise component included in es becomes equal to n and input to the comparator 8, and as shown in FIG. Differential signal (es
By taking -n), a detection signal ep for the scattered light of the fine particles is obtained. In this case, the detection waveform p of the fine particles in the signal es
1, p2, p3, etc. are superimposed on the fluctuation noise n and the peaks rise and fall. Some of the peaks are masked like p2, but the difference signal ep eliminates the fluctuation noise n and the S / N ratio is reduced. Improved and any peak value is correctly detected. The signal ep is input to the signal processing unit 9 to detect the particle diameter of each fine particle.

[Effects of the Invention] As is apparent from the above description, according to the laser fluctuation noise elimination method according to the present invention, the fluctuation noise of the laser beam included in the output signal of the scattered light receiving system is reduced by the fluctuation light receiving system. Is subtracted by subtracting the output signal to detect only the scattered light of the fine particles. The effect of improving the detection performance of the fine particle detector constituted by an external mirror type laser oscillator is significant.

[Brief description of the drawings]

1 (a), 1 (b) and 1 (c) are an overall configuration diagram and a partial diagram of a laser noise elimination system in a particle detector according to an embodiment of the present invention, a waveform diagram of each output signal,
2 (a) and 2 (b) are a cross-sectional view of a main part of a particle detector constituted by an external mirror type laser oscillator and a perspective view of a scattered light receiving system. 1 external mirror type laser oscillator, 1a gas laser tube, 1b internal mirror, 1c Brewster window, 1d external mirror, 2 detection cell, 3 injection nozzle, 3a center Nozzle, 3b: outer layer nozzle, 4: discharge nozzle, 5: scattered light receiving system, 5a: light receiving lens, 5b: slit plate, 5c, 6a: photoelectric converter, 6: variable noise receiving system , 7a, 7b ... amplifier, 8 ... comparator, 9 ... signal processing unit.

Claims (1)

(57) [Claims] An external mirror is provided outside a gas laser tube having an internal mirror at one end to constitute an external mirror laser oscillator. The sample air is made orthogonal to the laser beam of the oscillator, and the laser beam is orthogonalized by a slit. The particle detection area is limited by limiting the field of view to the location, and in a particle detector that receives the scattered light of the fine particles contained in the sample air in the fine particle detection area by a scattered light receiving system, the oscillation intensity fluctuates due to the sample air. For the laser beam in which the fluctuation noise has occurred, a fluctuation noise slit is used to set a fluctuation noise detection area by limiting the field of view to the vicinity of the fine particle detection area, and the fluctuation noise in the fluctuation noise detection area is changed by the fluctuation light. The variable noise signal output from the variable light receiving system is received from the light receiving system and is output from the scattered light receiving system. The subtracted and detects only the scattered light of the particles, the laser fluctuation noise elimination method in particle detector.