The process of photoanodic etching of deep pores in lightly doped n--Si wafers was investigated. On a 6 in. wafer, more than 1 billion pores, regularly distributed over the wafer in a hexagonal array with a pitch of 3.5 μm, were obtained using an electrolytic back contact. The diameter of the pores was about 2 μm and depths up to 400 μm could be achieved. Kinetic experiments revealed that the etching process was diffusion controlled in a solution containing HF, H2O, and ethanol. Electrochemical experiments showed that H2 evolution took place at the back side of the Si wafer and that H2 and O2 evolved at the Pt cathode and anode, respectively.