Multi Optical Passes Method for Measuring 2-D Particle Size Distribution in Plasmas

The laser light scattering (LLS) is a powerful method for size measurement of dust particles in plasmas. Several two dimensional measurement tools have recently been developed, but measurement of particle size < 100 nm was limited because of the weak incident beam intensity attributed from implementing a sheet beam as the incident laser beam. In this work, multi optical passes were adopted using two spherical mirrors, which enabled detection of small particles as small as about 50 nm. Vertical multi passes could be obtained by tilting the two mirrors, and the 2-D detection was realized using a CCD camera as a detector. The 2-D particle size distribution was obtained in various operating conditions in low pressure capacitively-coupled silane plasmas. Experiments showed that particles near the sheath boundary had almost constant size, while particles above the sheath grew in size and diffused repetitively. The geometrical center of the particle trap was found through a 2-D contour plot.