Laser light scattering (LLS) to observe plasma impact on the adhesion of micrometer-sized particles to a surface

TL;DR

This study employs Laser Light Scattering combined with microscopy to monitor micrometer-sized particle behavior on surfaces during plasma exposure, revealing effects on adhesion, morphology, and composition with high sensitivity and size measurement accuracy.

Contribution

It introduces an in-situ LLS method for real-time monitoring of particle adhesion and morphological changes under plasma exposure, calibrated for different particle sizes and compositions.

Findings

LLS can detect individual particle release, shrinkage, or fragmentation during plasma exposure.

The method achieves a noise level of about 3% for 5.26 μm particles.

Size measurement accuracy varies from 50% for 2.14 μm to 10% for 5.26 μm particles.

Abstract

Laser Light Scattering (LLS) method, combined with a long-distance microscope was utilized to detect micrometer-sized particles on a smooth substrate. LLS was capable to detect individual particle release, shrink, or fragmentation during exposure to a plasma or a gas jet. In-situ monitoring of hundreds of particles was carried out to investigate the effect of hydrogen plasma exposure on particle adhesion, morphology, and composition. LLS was calibrated with monodisperse melamine resin spheres with known sizes of 2.14 um, 2.94 um, and 5.26 um in diameter. The lowest achievable noise level of approximately 3% was demonstrated for counting 5.26 um spherical melamine particles. The accuracy for melamine particle size measurements ranged from 50% for 2.14 um particles to 10% for 5.26 um particles. This scatter was taken as the imprecision of the method. Size distribution for polydisperse particles with known refractive index was obtained by interpolating to an effective scattering cross-section of a sphere using Mie theory. While the Abbe diffraction limit was about 2 um in our system, the detection limit for Si particles in LLS according to Mie approximation was assessed to about 3 um, given the limitations of the laser flux, microscope resolution, camera noise, and particle composition. Additionally, the gradual changes in forward scattering cross-sections for Si particles during the exposure to the hydrogen plasma were consistent with Si etching reported in the literature.