A simple technique for optical thin film characterisation in the case of small refractive index differences between the coating and the substrate

摘要

A two-beam technique based on reflectometry is introduced to study the coating properties of SiO_x films on glass. A plane polarised light beam illuminates the coated glass sheet and simultaneously the reflectance of the front reflection (coated surface) and the reflectance of the rear reflection (uncoated surface) are measured in an angular range of 6° around Brewster's angle of the glass substrate. Based on the detection of three angles of the reflectance curves of the coated and the uncoated glass substrate (two minimum angles and the angle of the intersection point), the refractive indices of coating and substrate can be calculated directly. With these parameters, we use the multiple beam interference model to compute the coating thickness. Experimental results show that the coating thickness of thin SiO_x films on glass can be measured in a coating thickness range from 20 to 100 nm with a reproducibility of 1 nm.