In this paper, the effects of coating conditions on the thickness and quality, especially the hardness and density, of a photoresist film are reported. The photoresist film was deposited on a Si wafer by a spin coater under various conditions, including baking temperature, rotation speed, and the number of coats. The thickness of the film was measured by a surface profilometer. The sound velocity, which is closely related to hardness, and the density of the film were obtained by acoustic resonant imaging technique. The thickness and sound velocity of the film increased with increasing the number of coats and decreased with increasing the baking temperature and the rotation speed. Furthermore, the density of the film reached its maximum value for each condition. From multiple regression analysis, it was found that, among the three parameters of coating conditions, the rotation speed affects the quality of the film the most. It was shown that a dense photoresist film is obtained by deciding the baking temperature and number of coats in advance, then choosing a suitable rotation speed.

This paper proposes an acoustic resonant imaging technique for visualizing the acoustic properties and thickness of a polymer film on a substrate. When ultrasound passes through a thin layer, transmission and reflection coefficients of sound pressure attain their extreme values at the resonant frequency. By obtaining the area information of the extreme value and resonant frequency and matching them with a theoretical model, the acoustic properties and thicknesses of a polymer film on a substrate can be visualized. Herein, this technique was applied to a photoresist film coated on a Si wafer, and in addition to visualizing fluctuations in film thickness, the differences in the film hardness that may have occurred during the curing process were successfully detected as the differences in the acoustic impedance of the film. The acoustic resonant imaging technique was successfully used to determine the frequency dependence of both the transmission and reflection coefficients.