5. Photoresist Exposure/Bleaching Simulation

The photoresist exposure/bleaching module is the second stage within the overall lithography simulation flow illustrated in Figure 3.1. The physical problems of light propagation within the photosensitive resist thereby encountered as well as the chemical modification of the resist material induced by light absorption are extremely difficult to model. The reasons for this are, however, of different nature:

• The modeling of the photoresist exposure behavior is associated with great uncertainty because of the extreme complexity of the physical and chemical processes involved. Especially the distinct sensitivity of the resist performance to the actual chemistry and composition makes a predictive simulation very difficult. Often the only possibility to obtain reasonable results within a short time period and with an acceptable effort is to fit simple models to experimental data. The IC manufacturers keep the fitting parameters thus obtained strictly confidential due to their great importance. In the following section we present the two standard models for conventional and chemically amplified resists, which are available in the literature.
• The second demanding problem is the calculation of the light distribution within the optically nonlinear photoresist. Hereby, the physical foundations are well-known and given in the most rigorous form by the Maxwell equations. The difficulties arise from the exploding numerical costs to run full electromagnetic (EM) simulations. Depending on the required accuracy and the specific simulation problem various simplifications can be made. The most important ones are described in Section 5.2. However, as the technology is pushed closer to the physical limits, a rigorous treatment becomes necessary. Important and already existing approaches are sketched in Section 5.3, whereas a new method--first developed in this thesis--is then presented in Chapter 6.
  

Before we start the chapter as outlined, we want to point out the role of the exposure/bleaching module in the overall concept (cf. Figure 3.1). On the one hand it has to be linked to the two-dimensional/one-dimensional aerial image that is the result of the imaging module, on the other hand it yields the output for the development tool in form of a three-dimensionally/two-dimensionally distributed attribute describing the chemical state of the photoresist after exposure. Hence special care has to be given to well-designed and flexible input and output interfaces.