Effect of Exposure Mode on Negative Photosensitive Polyimide Patterning

The first technical obstacle encountered in the process of negative photoresist industrialization is that the lithographic pattern of the photoresist formula and lithography process optimized in the laboratory cannot be reproduced on the industrial production line. The reason for this could be the difference in exposure modes. A laboratory-prepared negative-tone photosensitive polyimide (n-PSPI) is used to prepare two polyimide photoresists with different photo-initiators. They were exposed under the same exposure energy in stepwise mode (high power, short time) and contact mode (low power, long time), respectively, being followed by solvent development for photolithography. Based on the resultant patterns, ideal lithography patterns can be obtained by stepper exposure with fast rate initiators or contact exposure with slow rate initiators. On the contrary, stepper exposure combined with slow rate initiators is prone to unclean development caused by free radical diffusion, while contact exposure combined with fast rate initiators results in low film retention rate caused by insufficient crosslinking. These undesirable graphics effects can not be improved by shortening or extending the exposure time. Therefore, the rational use of initiators is crucial for the formulation design of photoresists with different exposure modes. Combined with the theory of free radical formation and diffusion kinetics, the lithography pattern rule of n-PSPI photoresist under different exposure modes is explained. These conclusions can provide a theoretical guidance for formula modification when converting laboratory products to industrial applications.