5.1.3 Modeling Interaction between Droplet and Wafer Surface

There are two main types of depositions which have been examined in the literature and in Section 4.1. One type relies on the droplets being transported very near the surface, where they undergo evaporation and the resulting vapor causes a CVD-like deposition process on the silicon surface. The other type relies on the droplet reaching the surface before it is fully evaporated and sticking on the silicon wafer while simultaneously spreading. The former is commonly the result of a PSD deposition process, while the latter is common for ESD processes, where the droplets are accelerated at much higher speeds and therefore have enough force to overcome the retardant Stokes and thermophoretic forces to reach the substrate as a liquid.

5.1.3.1 Deposition Model

The model for the deposition of a thin film using a PSD process, which acts as a CVD process is performed by following the flux from the nozzle and its expected trajectory as it reaches the wafer area. At this moment, at the substrate surface a CVD process is modeled, where the flux direction and sticking probability are the model parameters. The thickness of the deposited film depends on the amount of droplets reaching the surface, the deposition time, and the temperature used.

Figure 5.4: Droplet impact onto a heated surface, resulting in the spreading of a thin film in a disc-like shape.

In order to model the deposition for a porous film, which is deposited as a result of an ESD spray pyrolysis system, each droplet's trajectory is tracked until it reaches the LS surface. At this point, at the location of the droplet's contact, a disc whose size is relative to the droplet's radius is deposited on the surface, shown in Figure 5.4 [171]. The disc is generated using evenly distributed particles which accelerate towards the LS surface and stick to generate the desired pattern.