Resist rehydration during thick film processing

Abstract

The demand for ultra thick photoresist formulations is steadily increasing. Many application such as wafer bumping and MEMB (micro electro-mechanical systems) require films in the range of 50 to 100 micrometers . In our study we measure the dissolution rates of the exposed resist as a function of re- hydration time and other process conditions. Film thickness, processing conditions and developer type are varied for two commercially available resists, AZP4620 and AZ9260 photoresists. It is found that the hydration status of the film has a defining influence on the dissolution behavior. Films that have had insufficient time to re- hydrate cannot be completely dissolved even at very high doses and long development times. Investigation of the water sorption rates by quartz crystal microbalance experiments yielded a diffusion constant of 0.12 micrometers /sec, in good agreement with literature data on similar systems. Calculations of the water concentration of thick films show that re-hydration times can take several hours. Under these conditions, there is insufficient water in the lower part of the resist film to react with the ketene generated by photolysis of the DNQ sensitizer. The IR trace of the indenylidene ketene C=C=O stretch band can be observed directly under normal atmospheric humidity conditions. From the observed kinetics, conditions can be derived for re- hydration from the gas or liquid phases that lead to practical process conditions applicable to ultrathick film processing in the fab.