Up to now the wet etching of crystal is used to fabricate microstructures for sensors or resonators. Shapes of structures micromachined in crystals are found to be determined by the crystal, by substrate orientation and by direction of alignment of the mask. The wet etching applied to unmasked crystal surfaces gives also rise to formation of dissolution figures that are orientation dependent. Owing to the diversity of shapes that can be obtained by wet etching we concentrate our investigations to the etching of nano-shapes in various crystals used in electronics industry such as silicon, gallium arsenide, quartz and langasite (LGS).
Our aim is to identify crystals and etchants that give rise to micro- and nano-structures with potentially interesting shapes. For this purpose we performed a systematic experimental study of micro- and nano-structures etched (KOH, TMAH and NaOH etchants) on ten differently oriented silicon plates. Arrays of masks with square, circular and elliptic shapes were used to obtain convex and concave structures. The influence of the inter-space between consecutive masks final shapes of peaky convex structures was studied. Investigations for GaAs were concerned with (100), (110) and (111) orientations and with two acidic solutions. The langasite crystal was etched in an HCl solution. Shapes of structures etched on GaAs, LGS and quartz plates are more dependent on starting shapes of masks than for silicon because these crystals suffer dissolution processes that are characterized by anisotropies of different types. Tentative to use a self-elaborated simulator to derive etched shapes for these structures was also made.
Satisfactory theoretical results were obtained especially for silicon, GaAs and LGS crystals. Moreover observation by atomic force microscopy of dissolution figures that develop on etched surfaces revealed that it is possible for some crystals and for some orientations to obtain at the nanometer scale regular arrays of square mesa or of rods. The most interesting shapes of dissolution figures were found to be formed on specific quartz and silicon surfaces. In conclusion the wet etching of crystal can constitute an alternative way to fabricate at close micrometer and nanometer scales arrays of cavities, peaky mesas and rods.
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