Expansion of SU-8 application scope by PAG concentration modification

In recent year SU-8 has became the most attractive photoresist in both optical and x-ray lithography. In our early work we have optimized its exposure parameters to improve the patterning quality in UV lithography and concluded that the UV absorption in SU-8 is proportional to the concentration of photoacidgenerator (PAG) and limiting the applicable SU-8 thickness in UV lithography. Actually, the PAG concentration plays an important role in all aspects of SU-8 processing in both optical and x-ray lithography. The motivation of this work is to expand the applicable thickness and application scope and improve processing control of SU-8 by optimizing its PAG concentration. In this paper we present the most recent experimental results on lithographic performance of SU-8 with different PAG concentration (varying up to 2 orders of magnitude). It includes determining the minimum bottom dose and minimum effective energy density in x-ray and UV lithography of SU-8, respectively, observing the dimensional change of SU-8 microstructure at different post exposure bake (PEB) temperature and time and measuring UV absorption spectrum of SU-8 as the function of PAG concentration. The modified SU-8 resists have moderate sensitivities and lower absorption coefficients. The application of the modified SU-8 will be addressed and demonstrated.