Kenji Yokose

Detection of Fine Particles in Liquids by Laser Breakdown Method

Basic characteristics of a particle detection method using laser breakdown were studied for a system of polystyrene standard particles dispersed in ultrapure water. The method was able to detect 0.02 µm particles. The detection sensitivity decreased with the particle size due to size dependence of the laser breakdown threshold. The plasma emission delay time from the laser pulse decreased with particle size (9.8±0.8 ns for 0.04 µm, 5.7±0.8 ns for 3.0 µm). The above results suggested the possibility of concentration and size measurement of fine particles in liquids by the proposed method.

Laser breakdown acoustic effect of ultrafine particle in liquids and its application to particle counting

The mechanism of acoustic signal generation from an ultrafine particle in liquids by laser irradiation was found to shift from the photoacoustic effect to optical breakdown of the particle as the power density of the excitation beam increased. The acoustic signal generated from a 0.085 µm polystyrene ultrafine particle in water increased discontinuously at the beam power density of 7.0 ×1010 W/cm2, corresponding to its dielectric breakdown threshold. Then, a novel method in which ultrafine particles were counted individually by counting the breakdown acoustic pulses was proposed, and its counting principle was verified using the polystyrene ultrafine particles.

Detection of Ultrafine Particles in Liquids Using a Breakdown Acoustic Effect

As water and liquid reagents used in semiconductor technology and biochemical engineering are highly purified, analytical and detecting methods for ultratrace and ultrafine particulate impurities are desired. For example, ultrafine particulate matter as small as 10-1 pm can break the insulation between wiring on a very large scale integrated circuit (VLSI), so detection, counting or determination methods are required for the water and liquid reagents used in VLSI production. However, the weight concentration of these particulate impurities is estimated to be below 10−2 ppt (number density: 101 mL−1), and this is too small for turbidimetric or even photoacoustic determination [1]. On the other hand, the detection limit size of particle counting in liquids using laser scattering is about 0.1–0.3 μm, because of the background due to medium Rayleigh scattering. We propose a new method for counting ultrafine particles in liquids using a breakdown acoustic effect.