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The aim of the FUPUSET project was to achieve a fundamental physicochemical understanding of the kinetics an mechanism of chemical and electrochemical dissolution of silicon in aqueous KOH with a view to develop the next generation of ultra-high precision anisotropic wet chemical etching techniques with enhanced reliability in both anisotropic ratios and surface finish for large area devices.
The project proved successful in having achieved its goals, i.e. improved surface finish of micro machined components. A novel process step has been developed which reduces the surface roughness without deteriorating the anisotropic ratio during etching. Several methods to suppress the formation of pyramids without reducing the etch rate of Si(100) have been developed. This can be realised by addition of oxidants to the etching solution (OCl- seems to be ideally suited) or by controlling the electrochemical potential. It has been shown that the OCP of the KOH/Si(100) interface can be controlled when a galvanic cell is formed and hence pyramid formation can be suppressed during the etching of micro-machined components even without the use of an external electrical circuit.
Advances have also been made concerning the improvement of anisotropic ratios. It has been shown that the Si(100)/Si(111) junction in micro-machined components acts as a local electrochemical cell in which the anisotropic ratio is determined by the relative surface area of the different crystallographic orientations. These findings are a key to improve existing design rules in order to achieve the desired anisotropic ratios. For the case of p-silicon it was demonstrated, that the anisotropic ratio between Si(100) and Si(111) can be improved by the appropriate choice of the electrochemical potential during etching. Under industrial conditions, reliable and improved anisotropic ratios have been evaluated with the use of solution agitation, lower concentration solutions and the use of suitable tank materials.
A systematic study of surface finish and anisotropic ratios under industrial conditions has shown that the quality of the KOH and the water, used for anisotropic etching, influences the etching process significantly. However, it turned out that surface finish and anisotropic ratio of etched silicon are inversely related. That is to say the surface finish deteriorates when the anisotropic ratios are improving. For that reason a compromise between anisotropic ratios and surface finish, which can be tailored to the specific needs of the product, has to be tolerated during the anisotropic etching process. However, it has been shown that defects (micro-pyramids), which are formed to some degree under strongly anisotropic conditions, can be removed in the presence of oxidants at the end of the etch and hence the surface finish can be improved without the loss of the anisotropy during the etching process.
An in depth understanding of the etching mechanism has been gained from several in-situ methods, which were applied to the Si(100)/etchant interface (FTIR- and impedance-spectroscopy, transistor techniques, photo-luminescence
.). A surface state intermediate with an energy level high in the band gap is formed during etching. For both, p- and n-Si(100), the injection of electrons from this etching intermediate into the conduction band is the main source of the electrochemical current. Isotope exchange experiments have shown, that the rate-determining step in both, the chemical- and the electrochemical etching mechanism involves the breakage of an Si-H bond.
Video microscopy studies have demonstrated, that the adhesion of hydrogen bubbles, which are formed during the etching process, is responsible for the formation of defects on Si(100).The bubble adhesion and hence the defect formation is extremely reduced at anodic potentials and at high KOH concentrations. This was explained by FTIR results, which have shown, that the hydrogen coverage of the Si(100) surface is reduced with increasing electrode potential and hence the surface is getting more hydrophilic at anodic potentials.
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