Anisotropy in the surface chemistry of silicon in alkaline solution

Abstract

This thesis describes a study of the anisotropy in the surface chemistry of silicon in aqueous KOH solutions. Two main reactions are considered: chemical etching, and electrochemical oxidation and passivation. Anisotropic etching of masked (100) surfaces was used to form V-grooves exposing well-defined (111) facets. With this geometry it was possible to study both the chemical and electrochemical reactions at the (100) and (111) surfaces of the same material, simultaneously (chapter 2). The most striking result is perhaps the very strong anisotropy observed in electrochemical oxidation. The general features of these results could be accounted for by a previously proposed two-step mechanism, in which Si - H surface bonds are converted to Si - OH by an OH- catalyzed reaction and the polarized Si - Si back-bonds are subsequently attacked by water. Anodic current is due to injection of electrons into the conduction band from activated intermediates of the two chemical reactions. This concerted chemical-electrochemical mechanism accounts for the remarkably slow kinetics of anodic oxidation of n-type (111) surfaces. In chapter 3 it is shown that FTIR spectroscopy can be used very effectively to follow in situ the surface chemistry of silicon during anisotropic etching, anodic oxidation and passivation, and etch-back of the passivating oxide. While the first of the two chemical steps described above is generally considered to be rate determining for the chemical and electrochemical processes, the IR measurements show that oxide formation on ideally flat n-type (111) surfaces at room temperature is slow even though there is a considerable Si - OH coverage of the surface. The combination of potential-step and FTIR measurements is particularly effective for such studies. Anodic current transients measured after a potential step on well-defined n-type (111) electrodes can be used to obtain information about the chemical reactivity of the surface. In chapter 4 the effect of additives (hydrogen peroxide and isopropyl alcohol) on the kinetics of surface reactions is described. An electrochemical flow cell in combination with an optical microscope was designed to study in situ changes in morphology of silicon during chemical etching and electrochemical oxidation (chapter 5). The power of this approach was demonstrated by a study in real time of the evolution of the typical roof-tile morphology on Si(110) surfaces. A simple image-processing procedure was developed to quantify the changes in morphological structures. This approach, in combination with ex situ microscopic observation, gives a wealth of information about the relation between etch morphology and parameters such as KOH concentration, applied electrochemical potential and the presence of additives (H2O2). Chapter 6 shows how the V-groove geometry can be employed to measure in situ chemical etch rates, using either electrochemical measurements or in situ optical microscopy. These approaches applied to a test wafer could be used for process control in the batch fabrication of devices. Another very interesting extension of the electrochemical approach is the possibility of following in situ the change in geometry of anisotropically etched deep trenches and buried structures in wafers.