Particle counter
Inventors
Matsuda, Tomonobu • SHIMMURA, Masaki • Yamakawa, Yuki
Assignees
Publication Number
US-10054529-B2
Publication Date
2018-08-21
Expiration Date
2036-03-04
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Abstract
An irradiation optical system 12 irradiates a fluid flowing in a flow passage 2a with one light among a plurality of lights obtained by branching light from a light source 1 and forms the detection area. A detection optical system 13 makes scattered light with a different direction from an optical axis of the irradiation optical system enter a beam splitter 17 among the scattered lights from particles contained in the fluid in this detection area. Meanwhile, a beam expander 16 makes another light among the plurality of lights enter the beam splitter 17 as reference light. A detector 4 receives an interference light, by the scattered light and the reference light, obtained by the beam splitter 17 by light receiving elements and generates a detection signal corresponding to the interference light. A counting unit 6 counts the particles based on this detection signal.
Core Innovation
This invention provides a particle counter that counts particles in a fluid by irradiating the fluid flowing in a flow passage with one light branched from a light source and detecting scattered light from particles. The scattered light and a reference light are superimposed by a light superimposition unit, such as a beam splitter, to produce interference light which is detected by a detector to generate a signal for particle counting. This system is designed to improve the signal-to-noise ratio when counting small-size particles, including those in chemical solutions.
The problem addressed by the invention is the difficulty of counting very small particles, especially those 30 nm or less in size, in chemical solutions that generate large background scattered light, making measurement challenging. Existing particle counters that use multi-divided light receiving elements improve noise reduction to some extent but remain insufficient for smaller particles. Moreover, dynamic light-scattering devices based on Brownian motion are unsuitable for direct particle counting.
The invention achieves a particle counter capable of counting small particles at a satisfactory signal-to-noise ratio by irradiating the fluid from a direction different from the fluid flow, detecting side scattered light in the flow direction, and using interference between scattered light and a reference light. By generating two interference lights at a beam splitter and using two light receiving elements to detect these lights and calculating the difference of their electrical signals, the system amplifies the detection signal. The approach enhances detection sensitivity to small particles by converting changes in phase due to particle movement into large changes in interference intensity, thereby enabling accurate counting.
Claims Coverage
The patent includes one independent claim with several dependent claims, presenting seven main inventive features related to the particle counter's optical and detection system configuration.
Particle counting using interference of scattered light and reference light
A particle counter comprising a light source, a light superimposition unit configured to superimpose two branched lights, an irradiation optical system irradiating the fluid from a direction different from fluid flow to form a detection area, a detection optical system making scattered light enter the superimposition unit, a reference optical system making another branched light enter as reference light, and a detector generating a detection signal based on interference light obtained by superimposing scattered and reference lights. The counting unit counts particles based on this detection signal.
Detection of side scattered light along fluid flow direction
The detection optical system is configured to select the scattered lights emitted from particles specifically along the travelling direction of the fluid in the detection area to enter the light superimposition unit.
Filtering detection signal by fluid velocity frequency
The particle counter includes a filter that performs filtering on the detection signal so that frequency components corresponding to the fluid's progress velocity are passed while other frequency components are attenuated, and the counting unit counts particles based on the filtered signal.
Matching wave front shapes of scattered and reference lights
The detection optical system and the reference optical system are configured to emit scattered light and reference light such that their wave front shapes approximately match at the light superimposition unit.
Use of separate light branching and superimposition units
The particle counter employs a light branching unit disposed separately from the light superimposition unit, where the light branching unit branches light from the light source into multiple lights and the superimposition unit generates interference light.
Generation of two interference lights and differential detection
The light superimposition unit is a beam splitter configured to generate first and second interference lights by transmission and reflection combinations of scattered and reference lights. The detector receives both interference lights with two light receiving elements and generates the detection signal as the difference of their corresponding electrical signals.
Intensity control of reference light to optimize interference
The intensity of the reference light is attenuated and expanded in beam diameter to enhance interference with scattered light and produce a detection signal with sufficient amplitude within the dynamic range of the detector.
The claims collectively describe a particle counter system that uses branched light paths with separate branching and interference units, detects side scattered light along fluid flow direction, matches wave front shapes of the interfering beams, generates two opposing-phase interference lights, and processes a differential detection signal filtered by fluid velocity frequency to count small particles with improved signal-to-noise ratio.
Stated Advantages
Enables counting of particles with small particle size in a fluid at a satisfactory signal-to-noise ratio.
Improves detection sensitivity by detecting side scattered light along the fluid flow direction, increasing phase change and frequency of interference variations.
Reduces background noise through differential detection of two interference lights with opposite phase, enhancing signal amplitude.
Allows amplification of weak scattered light signals from very small particles by interference with a stronger reference light.
Filtering the detection signal by the fluid's velocity frequency component attenuates noise and further improves signal quality.
Documented Applications
Measurement and counting of small particles, including those smaller than 30 nm, in chemical solutions such as those used in semiconductor wafer manufacturing processes.
Use as a particle counter for chemical solutions, including liquids such as isopropyl alcohol, hydrofluoric acid solution, or acetone.
The particle counter can be applied to both liquid-borne and airborne particle counters.
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