Numerical Simulation of an Ozone-Based Wet-Chemical Etching

In the PV-industry, wet-chemical baths are used for surface structuring, conditioning and cleaning. Besides cleaning the wafers, ozone-based wet-chemical cleaning processes show the ancillary effect of intendedly etching back the emitter and/or rounding of textured surfaces. To simulate and predict this rounding of micro-pyramids during the cleaning process, a two dimensional process simulation model was built. With the finite element method of COMSOL Multiphysics the diffusion and convection based mass transport of ozone along the pyramids is described. Important parameters for the evaluation, obtained from experimental results, are the etch rate and resulting roundness (R) of the micro-pyramids tips. A parametric study was carried out with the purpose of adjusting the model. The simulation represents the experimental data accurately, e.g. after 2 min exposure time R = 67 nm for the experimental and R = 66 nm for the simulation data.