Electrochemical anodization of p-type silicon

The central step used in our silicon machining process is electrochemical anodization in a dilute HF solution. This isotropic etching results in the progressive formation of porous silicon, which is a sponge-like structure with disordered pores and nanocrystalline silicon located at inter-pore regions. See Refs. [1,2] for detailed accounts of all relevant aspects of anodization, porous silicon formation and account of the uses of porous silicon.

The schematic below shows the basic anodization set-up. In the dissolution process, the silicon wafer is the anodic electrode. A voltage supply drives a current of electrical holes through the silicon wafer to the front surface where they result in dissolution of the surface which is turned into porous silicon.

Schematic of electrochemical anodization set-up.

In n-type silicon, holes need to be created by illumination of the sample, where electron-hole pairs are broken apart by the light to release more liberated holes. However, in p-type silicon, holes are majority charge carriers which are easily driven to reach the wafer surface to participate in the reaction. Thus, p-type silicon is the preferred material for our machining process.

The backside of the p-type silicon wafer is connected to a copper wire to achieve an Ohmic contact. The cathode is typically a platinum grid which is inert in HF. Ethanol is usually added to the HF electrolyte as a surfactant to prevent bubbles accumulating on the silicon surface.

Suggested references for further reading:
[1]     V. Lehmann, 2002 Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications Wiley-VCH, New York
[2]     M. J. Sailor,  2011 Porous Silicon in Practice: Preparation, Characterization and Applications Wiley-VCH, Weinheim