Attempt to Detect Nano Oil Film on the Surface of Polished KDP Crystal

Abstract

Abstract—The KDP (Potassium Dihydrogen Phosphate) crystal is important in high-energy optical system as a high quality non-linear optical material. The residual oil film which even is of nanometer thickness has great adverse effect on the KDP optical performance, such as laser absorption and damage threshold. In this paper, spectroscopic imaging ellipsometry (SIE) is applied to investigate the residual oil film on KDP crystal surface during the processing, and atomic force microscopy (AFM) is applied as a reference of the SIE results. Two modes of SIE are applied to detect oil film thickness. A rotating substrate measurement scheme is proposed because KDP crystal is a uniaxial anisotropic material. An optical model with thin oil film and KDP substrate is built including a top surface roughness layer, a flat oil film layer and KDP substrate, which is used to fit the detection data. Then, the single-point measurement results and thickness maps of residual oil film are established to describe the oil film distribution quantitatively. The AM-FM mode of AFM is applied to detect the oil film distribution on KDP crystal after surface cleaning, and the oil film thickness established in AFM measurement corresponds well with the SIE results. The nano oil film on KDP crystal can be measured by SIE technology accurately and the measurement scheme can also provide a reference for the similar detection.