Double Coating Process Using the Single Photoresist and the Thickness Prediction

The thicker photoresists are usually used in the manufacture processes of micro electro mechanical systems (MEMS), bipolar CMOS DMOS (BCD), and insulated gate bipolar transistor (IGBT), due to the thicker film need to be etched, high-dose implant process or other extreme processes have been widely used. This paper deal with the double coating process using the single commercial photoresist, this is an effective and simple approach to achieve the thicker photoresist thickness for the lithography process. We focus on the prediction of the thickness of the double-layer photoresist, which has seldom been reported in past studies. Although the final photoresist cross-section looks like one layer, we divide it theoretically into two layers, refer to as the first layer photoresist and the second layer one. Combined this model, first, an original equation for predicting the final thickness is deduced from the model developed by Emslie et al. Second, we conduct the experiment with different photoresists, soft bake temperatures, and soft bake times, and compare the theoretical predicted thickness to the actual one obtained from the experiment, moreover, a modified equation with satisfied accuracy is proposed, we can prepare the required photoresist thickness quickly and accurately by using this equation, not only can development cost be saved, but also can development cycle be shortened.