Anodic oxidation during MEMS processing of silicon and polysilicon: Native oxides can be thicker than you think

The thickness and surface roughness of the native oxide on undoped and P-doped single crystal silicon and polycrystalline silicon (polysilicon) were measured after exposure to aqueous hydrofluoric acid (HF) in the presence of localized metallization of sputtered Au or Pd. Both P-doping and the presence of metallization led to an increase in the thickness of the native surface oxide and an increased surface roughness after HF exposure. An external positive (negative) potential during HF immersion increased (decreased) the rate of what is clearly electrochemical i.e., anodic corrosion. The presence of the sputtered metallization promoted anodic corrosion, particularly in HF and particularly for P-doped silicon. Porous silicon can be formed under these conditions, due to dissolution of the anodically produced surface oxide. Subsequent oxidation of the porous silicon can lead to thick surface oxide layers.