Reduction of photoresist usage during spin coating

To reduce the photoresist usage and understand the film spreading process, this study performs flow visualization experiments and numerical simulations. This paper is the first work to show that in the early stage of the spin coating process, the spreading of photoresist is mainly governed by the photoresist injection. Then, instability fingers are formed due to the centrifugal force. Accompanied by the growing of fingers in length, the Coriolis force broadens the width of the fingers. The numerical results agree with the measured liquid front history at very short times. The difference between the numerical results and experiment data gradually arises due to the formation of instability fingers. The critical injection rate for fully coating a wafer increases with decreasing injection volume. Under a fixed wafer rotating speed, increasing the injection rate can significantly reduce the photoresist usage. To assist in the design and operation of the spin coating process, a regime map for injection rate and injection volume is provided.