A INTRODUCTION

Multilayer structures of porous silicon open a broad range of new applications. They are well suited for the formation of interference filters [1 4], waveguides [5, 6] and device structures containing these [7, 8]. The main advantage is the almost free combination of layers with different microstructure in depth and therefore different optical properties which allows the design of complex optical components [9]. In the next sections the formation process and a classification of different types of multilayer structures will be given. Also the interface roughness, interface sharpness and the depth homogeneity will be briefly discussed.

B FABRICATION

The understanding of the formation of multilayer structures offers deep insight into the understanding of the basic etch process: for most of the existing etch models (for details of different models see [10]) the already etched structure is not affected except for a slight chemical etching. In the quantum model [11, 12], the diffusion model [13] or the Beale model [14] this means for example that the silicon skeleton in the already etched structures is depleted of holes so that the remaining structure could not be affected any further during the anodisation. The formation of different, nearly independent layers in depth can be realised.

For the formation of different layers below each other the influence of the etch parameters has to be taken into account. The multilayer structures can be classified by the way the porosity is changed...