Film Creation

The lattice model for a photoresist film is based upon fundamental and measurable resist properties. To specify the polymer component, both the average degree of polymerization and the standard deviation of the distribution of degree of polymerization are input. It is also necessary to know the average fraction of the polymer repeat units that are chemically protected. Concentrations of other photoresist components, most notably the PAG loading and presence of residual casting solvent, must also be quantified. Knowledge of the chemical composition of the photoresist enables calculation of the appropriate population of each component in the lattice. Each component can then be added sequentially until the desired concentration is present. In the first step of lattice creation, chains are added to the lattice by stringing cells together to the desired chain length and chain length distribution. These chains are then randomized via many simulated reptation-type moves that have the effect of introducing bends into the chains and eventually randomizing their conformation. Periodic boundary conditions are employed so that the simulation is effectively modeling an infinite plane of resist material. After randomization, pendant groups are added to random positions on the chains to represent blocked sites on the polymer chains. This approach produces a normal distribution of blocking fractions for the chains. Photoresists typically consist of random copolymers, so this distribution of blocking fractions is an accurate representation of what would be found in an actual resist. Finally, PAG molecules are included by adding PAG units to random lattice vacancies until the required concentration of PAG has been achieved. Other components, such as added base and residual casting solvent, may be added in a similar manner.

Click here to download a movie that illustrates how chains are added to an empty simulation lattice (15.7MB).
After creating the resist model in this manner, the lattice approximates the configuration of the film after the spincoating step. It is then necessary to simulate the post apply bake (PAB) step. During the PAB, intermolecular interactions are important in the processes of solvent evaporation and film shrinkage. A Monte Carlo simulation is performed that allows the effects of specific intermolecular interactions to be considered. (Details of this simulation have been published, and the references are available on this website.)


Click here to download a movie showing solvent evaporation and film shrinkage during the PAB simulation (3.4MB).
After the PAB simulation, the resist lattice model contains a realistic configuration of polymer chains, photoactive molecules, solvent, free volume, etc. This lattice model is the starting point for the next simulation module, the exposure step.
Next: Exposure Module