Modelling of the electrochemical etch stop with high reverse bias across pn-junctions

Abstract

In this work, the electrochemical etch stop for reverse biases up to 25V at abrupt pn-junctions on 200mm silicon wafers with four and three electrode configuration is investigated. It was found that for such bias conditions across the pn-junction the etch results differ significantly from the prediction of an established model for the electrochemical etch stop under open circuit potential conditions with four electrode configuration. The established model for a semiconductor with pn-junction where the p-doped region is in contact with the electrolyte describes the interface of the p-doped region and the electrolyte with an induced shallow n+ electrical layer. Thus - for this model - the space charge region in the p-doped region introduced by the electrolyte/semiconductor interface only depends on temperature and doping concentration. A new model is introduced which enables the modeling of the electrochemical etch stop with other electrolytes (with different redox potentials), voltage conditions, and for three or four electrode configurations. The new model describes the structure of the electrolyte/semiconductor interface as a Schottky contact together with the pn-junction in the semiconductor. Experimental data from three and four electrode configuration are critically compared with predictions from the new and the established analytical models as well as with numerical simulations. The new model analytically calculates the electrochemical etch stop well for three and four electrode configuration if conditions like Fermi level pinning are included. The established model underestimates the electrochemical etch stop with four electrode configuration, but shows a good agreement with the results from three electrode configuration experiments.