Typical optical interference filters made of porous silicon (PS), e.g. Bragg reflectors or Fabry Perot filters, consist of quarter wave (lambda/4) layers with alternating high and low porosities. For technological important applications of these filters in the infrared (I.R.) range 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 of HF concentration with depth cannot be neglected. These changes of the HF concentration cause a porosity gradient, which degrades the filter performance and possibly prevents the formation of filters due to mechanical instabilities. The direction of the gradient and changes in the microstructure with depth are investigated by Raman spectroscopy and PL measurements. Based on these results a new method for the formation 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 fabrication of stable interference filters.