Photoresist spectral sensitivity
The wavelength-dependent absorption coefficient of resists (spectral sensitivity) is an important factor in thick film lithography. To microstructure resists with steep sidewalls, one needs to get acquainted with optical absorption datasheets. So-called i-line resists are only sensitive in the range of the i-line (365 nm), a strong emission peak characteristic of mercury-based sources. Broadband resists also absorb the g- (435 nm, violet) and h- (405 nm) lines. Thus, they can also be exposed monochromatically within their spectral sensitivity. On the other hand, it is well known that mixed wavelengths are ideal for thicker resists. Besides theoretical aspects and technical know-how, process optimisation necessarily involves trial-and-error tuning steps. Finding an appropriate exposure recipe can therefore be tedious.