Photodefined etching of n+ layers diffused on p-type silicon substrates

Non uniform n + diffused layers on p-silicon are usually obtained by a unif orm doping process followed by etch back in areas defined by a photolitogra phic mask applied onto the sample. We present a study on an alternative tec hnique to obtain nonuniform n + doped layers using a photoselective etching process, thus avoiding the photolitographic mask. The electrochemical beha viour of n + silicon las obtained by phosphorus diffusion into p-type sampl es) in HF solutions was studied, in order to evaluate the sensitivity of an odic current density to illumination, and thus predict the possibility of p hotocontrolled etching. Current density vs. potential (j-V) curves were mea sured 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 hig her than in the illuminated regions, while at low currents the opposite sho uld occur. The high current regime was explored in etching experiments in w hich several samples were subjected to a dark/illuminated pattern. The pred iction of higher etch rates in the dark areas was confirmed; however, total etched depth contrast was experimentally found to be much higher than pred icted from the j-V curves. Contrasts las measured by sheet resistance) of 5 9 Ohm/square in the dark against 34 Ohm/square in the illuminated regions w ere obtained. (C) 1999 Elsevier Science B.V. All rights reserved.