Dust particles generated during the process of producing semiconductors degrades the products. Therefore, semiconductor manufacturing equipments are normally equipped with a particle counter for counting in real time the number of particles generated in the process chamber (refer to Patent Document 1).
The particle counter is generally provided in an exhaust pipe from the process chamber and includes: a light source for emitting a laser light to a measurement area in the exhaust pipe; a detector for detecting the light scattered by a particle irradiated with the laser light; a discriminator for determining whether or not a particle exists by comparing the detection signal of the detector with a predetermined specific discrimination threshold; and other units.
The exhaust pipe is provided with a light incident window, through which laser light from the light source is delivered to the measurement area. The scattered light generated by a particle irradiated with the laser light enters the detector through the light detection window. While most of the particles flowing in the exhaust pipe pass through the measurement area with the emitted gas, some particles are adhered to the light incident window and light detection window (hereinafter, both of which will be simply called “windows”) by electrostatic force and other factors. When particles are thus adhered to the windows, the stray light, which is generated by the laser light scattered by other places in the exhaust pipe, is scattered by the particles adhered to the windows and, as well as the laser light scattered by the particles passing through the measurement area, enters the detector.
The stray light scattered by the particles adhered to the windows becomes a factor of high frequency component of the scattered light signal detected and generated by the detector. Hence, in conventional particle counters, the contamination level of the windows is determined based on the magnitude of the high frequency component of the scattered light detection signal.*
Patent Document 1: Japanese Unexamined Patent Application Publication No. H06-26823