Methodology for the formation of photoresist films with uniform thicknesses of several hundred micrometers

To enhance the capabilities of photolithography in the field of microfluidic technology, this study establishes a method to produce a photoresist film with a uniform thickness spanning several hundred micrometers. Herein, the target thickness of the trial production films is 400 μm. The SU-8 3000 series and 4-inch silicon wafers were adopted as the photoresist and substrates, respectively. It was found that when a large amount of SU-8 is formed using the normal procedure, and the film thickness is biased in a certain direction and becomes non-uniform. The reason for this film thickness bias has not been clarified, but it is confirmed that the bias is induced during the soft baking of the resist and not during the coating process. Accordingly, this study proposes a new method to remediate and equalize the biased films. Before film remediation, the photoresist should be adequately soft-baked. However, it is difficult to do this inside of the thick film by simple heating. Thus, some techniques used to facilitate baking thick resist layers are also introduced. Trial production films that are appropriately remediated are obtained. It is shown that in the center region of a remedied film with a 40-mm diameter, the deviation of the local thickness is within 1%, i.e., the thickness bias is completely eliminated. The thickness can also be controlled by remediation with a reproducibility of 1%. The results indicate that the proposed method used to form thick photoresist films is effective for practical applications.