Drying of flowing liquid film on rotating disk

Abstract

Recently, development of high technology has been required for the formation of uniform thin film in manufacturing processes of semiconductor as the precision instruments become more sophisticated. A method called spin coating is often used for spreading photoresist on a wafer surface and drying photoresist film. In spin coating process, photoresist is uniformly spread on the wafer surface by centrifugal force caused by rotating wafer. However, it is a serious concern that streaky lines, which are caused by spiral vortices, appear on the wafer surface and prevent the formation of uniform film in the case of high rotating speed. On the other hand, in the case of low rotating speed, a small hump of the film is formed near the wafer edge. The main purpose of this study is to make clear the drying characteristics of the flowing liquid film on the rotating wafer. Temperature distribution of the flowing liquid film is captured by an infrared thermal video camera and radial gradient of the film temperature is introduced in order to evaluate the drying characteristic of the flowing film under steady state condition. Effects of the flow rate of the liquid film on the film temperature are investigated. The film temperature gradually decreases in the radial direction in all cases. At low rotating speed, the radial gradient of the film temperature is almost constant widely. On the other hand, at high rotating speed, the radial gradient of the film temperature takes a certain maximum value. It is found that the location of the gradient peak corresponds with the transition region of the air boundary layer, in which spiral vortices swirl, and shifts to the inner side of the disk with an increase of the liquid flow rate.