The non equilibrium band model of silicon in TMAH and in anisotropic electrochemical alkaline etching solutions

This study presents the energy band diagram of silicon in contact with an a queous solution of tetra-methyl ammonium hydroxide (TMAH) during the anisot ropic electrochemical etch process. The band diagram is constructed from th e measured open circuit potential (OCP) and the measured total band bending of the silicon at the OCP which is determined by impedance measurements. C urrent-voltage as well as capacitance-voltage characteristics of silicon in contact with TMAH solutions are presented. The present study emphasizes th e non-equilibrium operation of the electrochemical cell even at the open ci rcuit potential (OCP). In previous studies of the band diagram of silicon i n alkaline anisotropic etch solutions, the Fermi levels of the silicon and the electrolyte are aligned at OCP. In contrast, in the present study the F ermi levels are not aligned since steady state rather than equilibrium prev ails at this operation point. The value of the measured OCP is used to dete rmine the separation between the two quasi Fermi levels. The assumptions le ading to the construction of the band diagram are elaborated. The relations hips between OCP, flat band voltage, passivation potentials and Fermi poten tials are discussed. The predictions of the model and the effect of conduct ivity type, doping level, temperature and illumination are compared with ex periments. The new model and methodology presented in this study is applica ble to additional anisotropic alkaline etch solutions such as KOH and EDP. It is very useful for the appropriate design of the process required for el ectrochemical etch stop for the fabrication of membranes and controlled mic romachining of silicon. (C) 1999 Elsevier Science S.A. All rights reserved.