Planarization during spray coating: Numerical study

Spin coating has been mainly used to produce photoresist-coated layers on wafers in the semiconductor fabrication process due to its simplicity of operation, and uniformity and thinness of coated layers. To improve productivity, the wafer size has increased to 300 mm recently, and it keeps increasing. The spray-coating system, which uses a spray nozzle transversing over a wafer while dispensing the photoresist solution, was developed to tackle the coating defect problem and limit of the current spin coating system in handling the 450 mm wafers, which will come in use from the year 2010. A new simulation model to predict the mean thickness and the uniformity of the photoresist film coated by the spray-coating system was developed and validated with experimental data. The new model takes advantage of the simplicity of a one-dimensional model to estimate the uniformity of the final film thickness. The new model captures the effects of various operation parameters including the wafer-spinning speed, the amount of photoresist solution applied, the initial viscosity of the solution and the dispensing phase duration on the mean film thickness and uniformity. The evaporation control was found to be most important to obtain a uniform film profile, so that it was recommended to shorten the dispensing phase by increasing the dispensing rate.