Handbook of Silicon Based MEMS Materials and Technologies

22.4. Thick Photoresist Lithography

Resists may be considered thick when they are thicker than 5 μm. The distinction to thin resists comes from exposure, or more specifically, absorbance. DNQ photoactive compound in positive resists absorbs strongly, and the surface layers prevent exposure of the layers underneath. Continued exposure causes bleaching of DNQ (absorbance reduction by a factor of 10) which enables bottom part of resist to be exposed. However, exposed DNQ exhibits residual absorption, which means that exposure times increase as a function of thickness. This sets a practical limit to positive resist thicknesses to ∼ 50 μm. In negative resists based on photoacid generator, the reactive acid molecule is produced throughout the volume ...