ANALYSIS OF THE DEPTH HOMOGENEITY OF P-PS BY REFLECTANCE MEASUREMENTS

We have investigated changes in the etch rate of p-PS with increasing etching time as well as changes of the porosity of buried layers with depth. These effects can be attributed to the influence of chemical et ching and variations in the electrolyte composition with depth. To stu dy these changes, first the porosities of layers above and below layer s with different thicknesses were determined by a fit of the reflectan ce spectra of these layer systems using the effective medium theory. S econdly we have measured oscillations in the reflectance during the fo rmation of PS layers caused by the increasing layer thickness. Using t hese experimental results we are able to give a functional description of the changes in the optical thickness with depth. In addition, the influence of the chemical etching and changes of the HF concentration on the optical thickness can be estimated, As a result a method for ch anging the current with depth can be given, which can be used to minim ize porosity gradients. (C) 1997 Elsevier Science S.A.