Electrochemical behaviour of silicon for micromachining applications

This thesis reports on the electrochemical behaviour of Si in aqueous KOH solutions, and its implications on 3-D micromachining for transducer applications. First, wet chemical etches of Si are summarised, with etching reactions and suitable etch masks. Several ways for 3-D Si micromachining are reviewed, with special emphasis on electrochemical passivation techniques. Analysis of the electrochemical behaviour of Si requires the combination of the concepts of solid state physics and electrochemistry. The equilibration processes and the kinetics of the charge transfer at the semiconductor/solution interface are described on the energy band diagrams of the solid state physics. This is accomplished by defining an effective Fermi level for the solution, and by describing the movement of the ions and the surrounding water molecules on an electronic energy scale utilising a fluctuating energy level model. Arguments for chemical and electrochemical mechanisms for the dissolution of Si in aqueous KOH solutions are reviewed and compared. It is shown that an electrochemical approach is more consistent and accounts for the aniostropic nature and concentration dependence of the etching process. The analysis of the electrochemical behaviour of Si is based on a series of voltammetry (I-V) measurements, where the current of a Si electrode is plotted while its potential (with respect to the solution) is linearly increased with time. Different regions of the I-V characteristics and bias dependence of the etching mechanisms are analysed utilising the energy band diagram representation of the Si/KOH interface. I-V characteristics, obtained by ramping the electrode potential of a Si electrode in the same voltage interval with increasing rates, are used to examine the dynamics and the bias dependence of the passivation processes and the anodic oxide growth. Relying on the results of the voltammetry measurements, 3-D microstructures are successfully fabricated that use implantation/diffusion steps employed in a standard CMOS process. A novel approach to biasing the electrodes in an electrochemical cell is reported. With this technique, the solution is biased via a feedback loop which virtually grounds the reference electrode. This reduces the noise susceptibility of the apparatus and allows the use of samples incorporating multiple electrodes.