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| Photoresist modeling and simulation requires knowledge of the exposure and dissolution properties of a resist and the characteristics of the exposure tool. The sequence steps in the simulation are shown in Figure 6. Exposure modeling involves the calculation of the laser image intensity as a function of position (I(x)). The propagation of that intensity function into the photoresist is described by Beer's law and requires knowledge of the real and complex index of refraction (n and k) of the resist and substrate. An increment of dose is applied at the film surface. The energy deposited (# of photons) in any volume element E(x,z) is then calculated from the knowledge of the resist and substrate n, k. The energy is allowed to react with the PAC to convert a proportion of it to a photoproduct and to create a local relative PAC concentration in the film, m(x,z). This process is continued until a prescribed exposure dose is accumulated. The interference of light due to reflections at the resist-substrate interface creates an oscillation in the exposure energy and thus creates an oscillation in the PAC concentration. The PAC is then "diffused" by a baking process to create a final PAC concentration distribution m'(x,y,z). Knowledge of the dissolution rate as a function of PAC concentration (R(m)) then allows the simulation to remove volume elements at the appropriate rate to simulate the dissolution and thereby generate a resist profile. |
