High-precision air gap thickness measurement based on spectral reflectometry

Spectral reflectometry is widely used to measure the crucial parameters such as thickness and refractive index of film, surface morphology and grating constant in a convenient way. In this work, we focus on the measurement of air gap between two parallel plates. The transfer matrix method (TMM) and rigorous coupled-wave analysis (RCWA) are independently used to build the theoretical model of the air gap with micro and nano scale between two parallel plates, and then we compare the results. Through the simulation of reflectance spectrum based on optimized TMM. The influence of interfacial roughness of multilayer structure is analyzed. Then we introduce a merit function which includes air gap and interfacial roughness and then optimize them using genetic algorithm to solve the measured air gap value. The absolute measurement of air gap thickness and subsequent displacement measurement result indicate that our method has the advantage of high precision, resolution and stability.