INFLUENCE OF ETCH STOPS ON THE MICROSTRUCTURE OF POROUS SILICON LAYERS

Typical optical interference filters made of porous silicon (PS), e.g. Bragg reflectors or Fabry Perot filters, consist of quarter wave (lam bda/4) layers with alternating high and low porosities. For technologi cal important applications of these filters in the infrared (I.R.) ran ge the thickness of each lambda/4 layer is of the order of 1 mu m. As a consequence the overall thickness is in the range where variations o f HF concentration with depth cannot be neglected. These changes of th e HF concentration cause a porosity gradient, which degrades the filte r performance and possibly prevents the formation of filters due to me chanical instabilities. The direction of the gradient and changes in t he microstructure with depth are investigated by Raman spectroscopy an d PL measurements. Based on these results a new method for the formati on of PS layers is performed by interrupting the etch process several times during the formation. By this method the HF concentration in the electrolyte can return to its original level, which allows the fabric ation of stable interference filters. (C) 1997 Elsevier Science S.A.