Electrochemical Pore Formation

In some ways electropolishing and electrochemical pore formation can be understood as the two sides of the same coin. In the first case the rate-limiting species in the chemical reaction is HF, while in the second it is the supply of holes from the electrode. If we assume a rough silicon wafer surface and a reaction that is... 

Pore formation is a common feature of many metal and semiconductor electrodes under anodic conditions in various electrolytes. Common products, for example aluminum capacitors, have been manufactured for decades using electrochemical pore formation techniques. Nevertheless in many cases the physics of pore initiation and propagation is poorly understood. 

A common feature of all electrochemical pore formation processes in solid-state electrodes of a homogeneous chemical composition is the remarkable difference in dissolution rate between pore tip and pore wall. This is usually discussed in terms of an active-passive transition between the pore tip interface and the pore wall interface. But this still leaves the question open as to what quality of the pores makes their tips active and the remaining surface passive. On a basic level the active-passive transition has been ascribed to three distinct causes [Le31] ... 

A basic requirement for electrochemical pore formation is passivation of the pore walls and passivity breakdown at the pore tips. Any model of the pore formation process in silicon electrodes has to explain this difference between pore tip and pore wall conditions. Three different mechanisms have been proposed to explain the remarkable stability of the silicon pore walls against dissolution in HF, as shown in Fig. 6.1. 

The pore formation models discussed in Section 6.2 are based on the semiconducting properties of the electrode but not on its chemical identity. Consequently electrochemical pore formation has been observed for many semiconductors. 

---