Porous silicon formation by stain etching

An empirical study was carried out on the porous silicon layer (PSL) formation process by stain etching on p- and n-type silicon wafers having different doping concentrations. PSL formation differs from the electrochemical etching process, since the top surface of the porous layers is continuously etched during formation. A porosity gradient is developed in the porous layers formed on p-, p +-, n- and n +-type silicon because stain etching, as a wet chemical etching method attacks the pore walls. This complex process can result in limited thickness for PSLs depending on the doping type and concentration of the substrate. The total mass of the silicon dissolved from the top surface and from the pores, was measured by gravimetry. The porosity and thickness values extracted from spectroscopic ellipsometrical (SE) measurements and with the measured mass of dissolved silicon are used for studying the etching process. The structure of the layers was characterized by backscattering spectrometry (BS) and cross-sectional transmission electron microscopy (XTEM). The PSLs exhibit amorphous structure on p-type silicon, while p + layers have crystalline structure according to ion beam channeling experiments, and XTEM images.