Process Optimization of Single-Step Photolithography: An Updated and Novel Approach

Thickness control and uniformity optimization of the photoresist during the photolithography process are thought to be important topics for process engineers to take into consideration due to the precision and sizing requirements of popular semiconductor devices today. In the Integrated Nanosystems Research Facility at the California Institute for Telecommunications and Information Technology (Calit2), 8 silicon wafers were coated with a positive photoresist named Shipley 1827 in a systematic way to improve uniformity and thickness during single step photolithography. In this paper, the parameters affecting photoresist thickness are thoroughly analyzed and an economic and novel approach improving uniformity and thickness is presented. First, peeling off is commonly observed during developing processes and it can be minimized by adding HMDS to promote adhesion. Second, all exhaust lids are sealed to control the air flow. Finally, laser beam check and edge distance measurement are performed to properly position the wafer. The author also discusses several other essential but easily ignored factors that can affect the uniformity and thickness of the photoresist, such as recoating, bubbles, and so on. By following this approach, high quality single-layer silicon wafer can be coated at ease.