Photodefined etching of nq layers diffused on p-type silicon substrates

Non uniform nq diffused layers on p-silicon are usually obtained by a uniform doping process followed by etch back in areas defined by a photolitographic mask applied onto the sample. We present a study on an alternative technique to obtain nonuniform nq doped layers using a photoselective etching process, thus avoiding the photolitographic mask. The electrochemical behaviour of nq silicon as obtained by phosphorus diffusion into p-type samples.in HF solutions was studied, in order to evaluate the sensitivity of anodic current density to illumination, and thus predict the possibility of photocontrolled etching. Current density vs. potential j–V.curves were measured as etching proceeded, and deeper, less doped layers, were exposed to the electrolyte. Two different regimes are suggested by these measurements: at high current densities, the etch rate in the dark regions should be higher than in the illuminated regions, while at low currents the opposite should occur. The high current regime was explored in etching experiments in which several samples were subjected to a darkrilluminated pattern. The prediction of higher etch rates in the dark areas was confirmed; however, total etched depth contrast was experimentally found to be much higher than predicted from the j–V curves. Contrasts as measured by sheet resistance. of 59 VrI in the dark against 34 VrI in the illuminated regions were obtained. q1999 Elsevier Science B.V. All rights reserved