Microstructures etched in doped TMAH solutions

Tetra-methyl ammonium hydroxide, or TMAH, is an anisotropic silicon etchant that is gaining more and more attention in the fabrication process of mechanical microstructures and device isolation, as an alternative to the more usual KOH and EDP etchants: because of its high compatibility with conventional IC processes, due to the absence of metal ions in it. The possibility to passivate the aluminium metalisation in properly saturated TMAH solution has also been demonsrated by doping the solution with appropriate amounts of silicon or silicic acid. This increases the range of application of these etchants, simplifying both the post processing and the etch set-up configuration. In this paper we present some different technological solutions adopted for the fabrication of 3D structures using as anisotropic etchant doped TMAH solutions. The effects of the additives on etch uniformity and surface roughness are studied. Using the etching results under different doping conditions of the TMAH solutions, we obtained silicon bulk-micomachined structures with controlled surface roughness. Furthermore the aluminium passivation permits to etch devices with no protection of the metalisation layers as some applications require (i.e., bolometers).