The exposure modeling depends on the treatment of the changes in the optical properties of resist with depth and time based on Isw (у; t) and photochemical changes. The positive photoresist normally contain a base resin, a solvent, a photoactive compound (PAC), and a reaction product. PAC reduces the development rate of the resist and is hence called as inhibitor. Three parameters: (A) exposure dependent absorption, (B) exposure independent absorption (by resist) and (C) representing rate of exposure or PAC destruction define exposure.
The final relative inhibitor (PAC) concentration m(x, y) after an exposure gives the exposure state. The development process is modeled as a surface controlled etching reaction, that is determined by the local value of m. If R(x, y) is the development rate at a point P(x, y), the position of P at a time t+At is given by
where и is the surface normal unit vector. R(x, y) is the development rate in ^m/minute calculated from one of the known models.