Roughness of the porous silicon dissolution interface

We present a study of the fluctuations in the dissolution front observed during the formation of porous silicon, leading finally to layer thickness inhomogeneities. Two types of fluctuations were revealed, one at the millimeter scale (waviness) and the other one at the micrometer scale (roughness). Root mean square amplitudes are comparable. In both cases fluctuations of the dissolution velocity can be invoked and we discuss their dependence on the current density and viscosity of the solution. The large scale fluctuations are attributed to planar resistivity fluctuations in the wafer. The second type of fluctuation displays a typical spatial periodicity comparable to the wavelength of the light so that a statistical characterization can be performed by optical measurements. The Davies-Bennett model quantitatively describes the induced light scattering. Remarkably, these fluctuations increase linearly with the layer thickness up to a critical value where a saturation regime is observed. In order to explain this behavior, we show the importance of the initial surface state of the wafer and of the porous medium. (C) 1997 American Institute of Physics.