A laser reflection method has been developed and tested for analyzing the etching of porous silicon (PS) films. It allows in situ measurement and analysis of the time dependency of the etch rate, the thickness, the average porosity, the porosity profile, and the interface roughness. The interaction of an infrared laser beam with a layered system consisting of a PS layer and a substrate during etching results in interferences in the reflected beam which is analyzed by the short-time Fourier transform. This method is used for analysis of samples prepared with etching solutions containing different concentrations of HF and glycerol and at different current densities and temperatures. Variations in the etch rate and porosity during etching are observed, which are important effects to account for when optical elements in PS are made. The method enables feedback control of the etching so that PS films with a well-controlled porosity are obtainable. By using different beam diameters it is possible to probe interface roughness at different length scales. Obtained porosity, thickness, and roughness values are in agreement with values measured with standard methods.
An etch-through process on porous silicon (PS) has been investigated. A macro-(>50 nm) and meso-(10-50 nm) PS were fabricated by electrochemical anodisation process. A normal electrochemical etching from the front-side will only yield some parts of the area being etched all the way through the wafer, due to the statistical process which leads to an uneven etch front towards the back-side of the wafer. A new method has been tried in which a thin PS layer was fabricated at the back-side before etching. The etchthrough PS apparently shows that all the pores were past through the wafer for the whole etching area (1 cm 2 ). The etch data were recorded and were plotted as voltage and temperature against time. An etchthrough thin meso PS membrane (10-50 nm diameter, 50-100 µm thick) has also been investigated. The PS were characterized by scanning electron microscopy. A proposed model for the etch-through PS is discussed.
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