3.1 Modeling Phases and Basic Simulator Structure

Basically, three distinct modeling phases are required for photolithography simulation, namely imaging of the photomask, photoresist exposure/bleaching, and photoresist development also including optional or required post-exposure bake steps. In a typical lithography simulator, each of the three fundamental processes is accounted for by a separate module as shown in Figure 3.1. In the overall simulation flow the three modules are called sequentially as well-defined interfaces exist. Fortunately, the modules can be treated independently, which is of crucial importance because very specific simulation methods are required for each step. The individual tasks are briefly characterized as follows:

• The imaging module describes the illumination of the photomask. The light propagation through the optical system and the light transmission through the photomask has to be simulated. The output of the imaging module is the aerial image, which is the light intensity incident on top of the wafer.
• The exposure/bleaching module simulates the chemical reaction of the photosensitive resist. Thereby the light propagation within the optically nonlinear resist as well as electromagnetic scattering effects due to a nonplanar topography have to be modeled. The result of the exposure/bleaching module is the latent bulk image.
• The development module describes an isotropic etching process, whereby the etch rate is determined by the previously calculated bulk image. Depending on the situation a post-exposure bake step has to be considered by a diffusion simulation to model the smoothing of the variations in the bulk image. The final developed resist profile is the result of the overall photolithography simulator.

  

Figure 3.1: Basic modules of a typical photolithography simulator (after [47]). The three modules of imaging, exposure and development are called sequentially by the simulator.