JPH0458137A - Optical system for heterodyne particulate detector - Google Patents

Abstract

PURPOSE:To increase the SN ratio by specifying the distribution ratio of the measurement beam and reference beam of a 1st beam splitter and the composition ratio of the scattered light and interference beam of a 2nd beam splitter. CONSTITUTION:Shot noises due to respective beams which are inputted to an avalanche photodiode 7 are calculated. Namely, the laser beam from a light source 1 is split by the 1st beam splitter 2a into the measurement beam and reference beam and the scattered light due to particulates in sample air and the interference beam obtained by shifting the reference beam by a proper shift frequency are put together by the 2nd beam splitter 2b to generate a beat signal of shift frequency. Then the distribution ratio of the measurement beam and reference beam of the 1st beam splitter 2a and the composition ratio of the scattered light and interference beam of the 2nd beam splitter 2b are set almost to 0.0:0.1. Consequently, the decrease of the beat signal is minimized and the SN ratio which is as large as possible is obtained.

Description

[Detailed Description of the Invention] [Industry-1- Use of bone! l! This invention relates to an optical system for a heterodyne type particle detector, and more specifically to an optical system in which the distribution ratio of a beam splitter to a laser beam is set so that the S/N of the output current of a photoreceiver becomes an optimum value. be.

[Prior Art] Fine and precise electric components such as semiconductor ICs deteriorate in quality if they become contaminated with particulates such as dust, so they are manufactured in a lean room with good cleanliness.

The cleanliness level of the clean room was determined by a particle detector to be 711.
11. Conventional particle detectors have exclusively used a method in which sample air collected from a clean room is irradiated with a laser beam and the scattered light is received to detect particles. Recently, as the degree of integration of ICs has increased, there has been a demand to improve the detection sensitivity of particle detectors and to further reduce the minimum detectable particle size. As a promising method for this, “1988-63
No. 944, “Method and Apparatus for Measuring Dust in Semiconductor Processing” has been published as a patent application.

FIG. 3 shows the configuration of one embodiment of the measuring device according to the above-mentioned patent application publication. A laser beam with a frequency f from the laser light source 1 is split by a first beam splitter (BSI) 2a into a measurement beam LS and a reference beam LR, and the measurement beam LS is incident on the detection cell 3. The sample air SA is orthogonal to this, and the scattered light SS of the particles is focused by the condensing lens system 4 and sent to the second beam splitter (BS
2) Enter in 2b. On the other hand, the reference beam LR is transmitted to the acousto-optic modulator (AOM) 5 by the oscillation X (O8C) 5a
It is softened by a sine wave signal with a frequency Δf of
The light is input to the BS2 via the BS2 and is combined with the scattered light SS.

The synthesis generates a beat signal (light) with a shift frequency Δf, which is converted into a beat signal current IB by the avalanche photodiode 7, and fine particles are detected.

Now, assuming that the amplitudes of the electric fields of the interference beam LR' and the scattered light SS are ER and ΔE, respectively, the power P of the composite signal from BS2 is expressed by the following equation, ignoring the phase difference between the two.

P=ER2+ΔE2 +28EeERcos (2πΔft)−・−・・
(+) Here, the first . The second term is the interference beam and '7
11j is the DC component of the constant beam. The third term indicates the power of the beat signal, and the amplitude ΔE of the scattered light SS is multiplied by 2ER by the interference beam, and since ER can be much larger than ΔE, it has better sensitivity than directly detecting the scattered light SS. It is assumed that the beat signal current IB can be changed.

[Problem to be solved] In order to increase the beat signal in the third term of equation (1) in 1-, the distribution ratio of BSl and BS2 should be appropriately selected to increase 8E-ER. . Now, the power output from the laser light source 1 is PO, the distribution ratio of BS1 is rH:n2, and the distribution ratio of BS2 (same as combining ratio) is rrB:
Let it be m2. n2:1-rB, m2=l-m!
where the subscript 1 indicates the measurement beam side, and the subscript 2 indicates the reference beam or interference beam side. Here, ΔE2 is (nl
rrB P□ ), and ER2 is (n2m2Po
) is obvious. Therefore, ΔE@ER
is proportional to (n1m102m2)//2Po. For convenience, the distribution ratios of both beam splitters are the same, nl =
fnl:Nl, n2:m2 =N2, Nl/N
(nr mt n2m2) J/2= (N
By calculating the change in t N2 ), the curve shown in FIG. 4 is obtained. Nl = N2 = 0.5, that is, Nt /N2 =
1, (NI N2 ) becomes 0.25, and Nl/
When N2 is smaller than 1 or larger than (NI
N2) become smaller. That is, when the distribution ratios of both beam splitters are each 0.5, the beat signal has a maximum value.

Now, in the above measurement device, the avalanche photodiode 7 used to photoelectrically convert the beat signal has an amplification effect similar to a photomultiplier tube, and also does not require high voltage, so it can be used for a long time. Although this photoelectric conversion element has excellent features such as a large saturation limit, its drawback is that it generates 78t noise due to optical input. On the other hand, the direct current components ER2 and ΔE2 included in the power P of the composite signal expressed by the formula (1) in -1- are input to the photodiode 7 as they are, causing shot noise. In addition, inside the detection cell, there are Rayleigh scattered light due to air molecules of the sample air and background noise light, which are also inputted and cause shot noise. Therefore, taking into account the shutoff caused by these,
It is necessary to set the distribution ratio N1:N2 of the beam splitter (7) so that the S/N of the beat signal is maximized.

This invention calculates the optimal distribution ratio of a beam splitter in consideration of the noise characteristics of an avalanche photodiode, and constructs an optical system of a heterodyne type particle detector using a beam splitter with such a distribution ratio. This is the first objective.

[Means for Solving the Problems] In this invention, a laser beam from a light source is split into a measurement beam and a reference beam by a first beam splitter, and the sample air is orthogonal to the measurement beam. A second beam splitter generates a beat signal at the shifted frequency by combining light scattered by the particles and an interference beam obtained by shifting the reference beam at an appropriate shift frequency.The beat signal is received by an avalanche photodiode. An optical system in a heterodia type particle detector for detecting particles, which has a distribution ratio between a measurement beam and a reference beam of a first beam splitter, and a combination ratio of scattered light and interference beam of a second beam splitter, The ratio is approximately 0°9:0.1, respectively.

[Operations] In this invention, shot noise due to each beam input to the avalanche photodiode is calculated,
As a result, it was found that the stray light inside the detection cell and the Rayleigh scattered light of the sample air are both minute and can be ignored, and that the shot noise caused by the scattered light of particles and the beat signal is small compared to that of the interference beam. Therefore, the ratio (j) of the distribution ratio of the beam splitter to (S/N)P of the power S of the beat signal and the power N of the shot noise due to the -1 beam is calculated, and this is approximately 0.920. 1
By using the first and second beam splitters having this distribution ratio, the low frequency of the beat signal can be kept to a minimum, and the S/ N can be obtained.

[Example] Figures 1(a) and 1(b) show the S of the beat signal and noise in the avalanche photodiode, which is the basis of the optical system of the heterodyne particle detector according to the present invention.
The calculation method of /N and its results are shown below.

In Figure (a), the power of the laser beam from the laser light source 1 is po, and the distribution ratio of BSI and PS2 is the same, Nl:N2. The power of the measurement beam LS divided by the BSI is N1 Pa, the scattered light SS of the particles is expressed as (kSN+Po) with kS as a proportionality coefficient, and the power PS of the scattered light transmitted through PS2 is given by the following equation.

PS = kS (Nl) 2 P□
--- (2) Next, the power of the reference beam LR (N2
PO) is F-P optical modulation n (AOM)5,
The interference beam LR' is shifted by the shift frequency Δf.
However, due to the diffraction efficiency kR (<1) of the AOM, the power decreases to (kRN2 PO), which is reflected by PS2 and applied to the photodiode (-The power PR of the beam is given by the following equation.

PR=kR(N2)" Po ・"-(
3) Next, the power pB of the beat signal is calculated using the above formula (1).
By setting ER2=PR and ΔE2=PS for the third term, the following equation is obtained.

pB = 2 (PRPS)/2 a cos(ωt)
”2 (kRks) I/2Nx N2 POacos (
ct+t)...(4) Here, ω=2πΔf. In addition, the effective value PB of pB
has a coefficient of 2, and PB = (kRkS)'/2Nx N2 PO--(
5).

The coefficient kR in the following is the diffraction efficiency of the acousto-optic modulator described above, and in this case, assuming that two stages of modulators are used, it is 0.
It is about 72. Also, the coefficient kS is kS = ap ωc
tS/sB −−(G), σ
p is the integral scattering cross section of the fine particles, and is a known number calculated from the particle size of the fine particles, the wavelength of the laser, and the like. ωC is the solid angle of convergence of the condenser lens system 4, tS is the transmission efficiency of the condenser lens system, and sB is the cross-sectional area of the measurement beam, which depends on the configuration of the detector.

Now, assuming that the particle size of the fine particles is 0.05 μm, kS is normally very small, on the order of 10 to the 112th power. Therefore, (kRkS)//2 in equation (5) is -(3 to 4)
It is a power order.

Next, avalanche photodiode 7 for optical input
The output current and shot noise will be explained. Generally, the output current i for the optical input power P is I=RMP (
7). R is the sensitivity coefficient (A/W), and M is the multiplication factor. Therefore, the effective value IB of the output current (beat signal current) with respect to the optical power PH of the beat signal is given by formula (7)
Inserting formula (5) into, IB = RM (kRkS) I/2NIN2 PO...
...(8).

Next, the effective value 12 of the power of shot noise caused by the current IL due to the input light is generally 12=2q IL
*MxOB −・” (9) Here, q is the electron charge (coulombs) and is a known number, X is the excess noise figure, B is the noise frequency bandwidth (Hz), and the above It is a constant uniquely determined for the avalanche photodiode together with R and M.

Now, as mentioned above, kS is very small, and (kRkS
) I/2 is also quite small, so PS in equation (2) and equation (5
) can be ignored. In addition, although the calculation process is omitted, shot noise due to Rayleigh scattered light of the sample air in the detection cell can also be ignored, and background noise cannot be evaluated here because it depends on the structure of the detection cell. Ignore it as small.

As described above, it is assumed that the problematic shot noise is caused only by the current IR due to the power PR of the interfering beam shown in equation (3). IR is the formula (3) and (7)% formula% (), and the shot noise power iR2 is iR2:2
QIR@Mχ・B =2qRM・MχB kR(N2) 2Po =(1
2). Now, let us consider the power of the beat signal current IB.
The square of C is taken, and the ratio between this and the shot noise iR2 is set as the S/N of the power.(S/N)P is converted into the table r, and the following equation is obtained.

(S/N)P = (IB)2/iR2=PokS(
Nx )2 (RM/2qMχB)...(1
3) According to the above equation, (S/N)P is proportional to the laser power PQ of the light source, the proportionality coefficient kS, and the square of the distribution ratio N1, respectively. Further, (RM/2qMχB) is a constant of the avalanche fist photodiode. Therefore, according to this equation, the optimal value of the distribution ratio of the beam splitter can be determined by considering only Nl. Since Nl can take a value from 0 to 1, if it is set to 1, the maximum S/N will be obtained. However, when N1=1, N2=0, and the interference beam cannot be changed, so a heterodyne system cannot be configured. Therefore, referring to the change curve of the beat signal explained in Fig. 4, it is necessary to leave only the necessary minimum amount of the crossing beam and to increase the S/N as much as possible. (!3)
(Nl)2 is transformed as follows.

(Ni)2= (NI N2)・(Nl/N2)
(14) In this equation, (NI N2 ) is the curve shown in FIG. 4 described above, and this is rewritten in FIG. 1(b). The curve is (Nl/N
2) gradually decreases from 1 to 5, becomes about one-third at Nl/N2:10, and then rapidly decreases. On the other hand (N1)2
increases somewhat rapidly when Nl/N2 reaches 1 to 10, but after that it becomes saturated and does not increase much. This invention focuses on this point and sets the distribution ratio of BSl and BS2 to approximately 0.9:0゜1 (Nl /N2 = 9) to obtain an optimal S/N that does not significantly reduce the beat signal. .

FIG. 2(a) shows the hetero gain of $☆' according to the present invention.
3 shows the configuration of an embodiment of the optical system of the end detector.

In the figure, the distribution ratio of BSI 2a and B522b is N:N
2, that is, the distribution ratio of the measurement beam and reference beam of BS1 and the combination ratio of the scattered light and interference beam of BS2 are set to be the same, 0.9:0.1. Due to the above argument, the beat signal will drop to approximately one-third of its maximum value, but the S/
N becomes a value close to the maximum value. Beat signal current IB from avalanche photodiode 7 and yacht noise B72
is output, generates a voltage across a suitable load resistor 8b, and is amplified by an amplifier 8a.

FIG. 2(b) shows the optical system of FIG. 2(a) in which the grain size of the fine particles η is 0.05 μm, and kS and R are used in equation (13).

A group of curves of (S/N) P calculated by giving the data of M and X and using the power po of the laser light source and the frequency bandwidth B of the shot noise iR2 as parameters are shown. In this case, since the amplifier 8a is accompanied by thermal noise, this is also taken into consideration. As shown by equation (13), (S/N)P is approximately proportional to Po. However, although (S/N)P should be inversely proportional to B, it is not necessarily inversely proportional due to the influence of thermal noise of the amplifier system.

According to this curve, Po is 15mW and B is 200kHz.
Assuming that z, fine particles with a particle size of 0.05 μm are (S
/N) It is understood that it can be detected at P~2.1.

[Effects of the Invention] As is clear from the explanation below, in this invention, the output current for the beat optical signal input to the avalanche photodiode and the shot noise for each predecessor force are evaluated, and as a result, the M output cell It was found that the stray light inside the beam and the Rayleigh scattered light of the sample air are both small and ignored, and that the scattered light of particles and the 7E4 noise caused by the beat signal are small compared to the shot noise of the F beam. , by calculating the contribution of the distribution ratio of the beam splitter to (S/N)P of the power S of the beat signal and the power N of shot noise due to the interference beam, it is optimal to set this to approximately 0.9:0.1. It was concluded that
By using the first and second beam splitters having this distribution ratio, the drop in the beat signal is kept to the necessary minimum and the S/N is as high as possible, which improves the performance of the heterodyne particle detector. There are significant effects that contribute to the direction of the world.

[Brief explanation of drawings]

FIGS. 1(a) and 1(b) are optical system configuration diagrams and curves illustrating S/N analysis for determining the optimum distribution ratio of the beam splinter in the optical system of the heterodyne particle detector according to the present invention. Figures 2(a) and (b) are
A configuration diagram of an embodiment of the optical system of the heterodyne particle detector according to the present invention, a curve diagram showing the calculated value of (S/N)P, and FIG. FIG. 4, a block diagram of the embodiment, is a curve diagram showing the dependence characteristic of the beat signal intensity on the distribution ratio of the beam splitter. ■... Laser light source, 2a... First beam splitter (BSl), 2b...
... Second beam splitter (BS2), 3... Detection cell, 4... Condensing lens system, 5... Acousto-optic modulator (AOM), 5a... Sine wave oscillator, 6...・Mirror 7...Avalanche photodiode, 8a...Amplifier,
8b...Load resistance, N1:N2...Beam splitter distribution ratio, po...Power of laser light source, f,...Laser frequency, Δf...Shift frequency,
kR...Diffraction efficiency of the acousto-optic modulator, kS...Proportionality coefficient for scattered light, PR...Power of interference beam PS...Power of scattered light IB...Beat signal current, PB... Beat signal power iR2...PR shot noise power B...
- Frequency bandwidth of shot noise. Fig. 1 (G) 1tJ NVN2+ Fig. 2 (CI) Pa (mW) Fig. 4 Fig. Goma/Tan

Claims (1)

[Claims] (1) A laser beam from a light source is split into a measurement beam and a reference beam by a first beam splitter, and a sample air is perpendicular to the measurement beam, and the scattered light due to particles contained in the sample air and the above A heterodyne that generates a beat signal of the above-mentioned shift frequency by combining the reference beam with an interference beam shifted by an appropriate shift frequency using a second beam splitter, and detects the above-mentioned fine particles by receiving the beat signal with an avalanche photodiode. In the particle detector of this type, the distribution ratio of the measurement beam and reference beam of the first beam splitter and the combination ratio of the scattered light and interference beam of the second beam splitter are approximately 0.9:0, respectively. An optical system for a heterodyne particle detector, characterized in that: .1.