The bulk micromachining on (010), (110) and (111)A GaAs substrates in the 1 H2SO4:1 H2O2:8 H2O system is investigated. Focus is placed on anisotropy of 3D etching shapes with a special emphasis on convex and concave undercuts which are of prime importance in the wet micromachining of mechanical structures. Etched structures exhibit curved contours and more and less rounded sidewalls showing that the anisotropy is of type 2. This anisotropy can be conveniently described by a kinematic and tensorial model. Hence, a database composed of dissolution constants is further determined from experiments. A self-elaborated simulator which works with the proposed database is used to derive theoretical 3D shapes. Simulated shapes agree well with observed shapes of microstructures. The successful simulations open up two important applications for MEMS: CAD of mask patterns and meshing of simulated shapes for FEM simulation tools.

Analytical models for the piezoelectric excitation and for the wet micromachining of resonant cantilevers are proposed. Firstly, computations of metrological performances of micro-resonators allow us to select special cuts and special alignment of the cantilevers. Secondly the self-elaborated simulator TENSOSIM based on the kinematic and tensorial model furnishes etching shapes of cantilevers. As the result the number of selected cuts is reduced. Finally the simulator COMSOL® is used to evaluate the influence of final etching shape on metrological performances and especially on the resonance frequency. Changes in frequency are evaluated and deviating behaviours of structures with less favourable built-ins are tested showing that the X cut is the best cut for LGS resonant cantilevers vibrating in flexural modes (type 1 and type 2) or in torsion mode.

The present work is concerned with the development of etch pits on (hk0) and (hh $\ell$ ) silicon surfaces immersed in various etchants. Final shapes of small etch pits are found to be bounded by {111} facets solely. The orientation dependence of etching shapes is explained in terms of a kinematic and tensorial model involved in the simulator TENSOSIM. An agreement between experimental and theoretical etching shapes is observed showing that the simulator can generate quite accurate 3D etching shapes for pits blocked by limiting facets. After a fair adjustment of the database the simulator derives also etching shapes close to experimental shapes of large pits.

This paper is devoted to the theoretical study of the influence of thetemperature and of the doping on the piezoresistance of N-type silicon. In the firststep the fractional change in the resistivity caused by stresses is calculated in theframework of a multivalley model using a kinetic transport formulation based on theBoltzmann transport equation. In the second step shifts in the minima of theconduction band and the resulting shift of the Fermi level are expressed in terms ofdeformation potentials and of stresses. General expressions for the fundamentallinear, π11 and π12, and non-linear, π111, π112,π122 and π123, piezoresistance coefficients are then derived. Plots ofthe non-linear piezoresistance coefficients against the reduced shift of the Fermilevel or against temperature allow us to characterize the influence of doping andtemperature. Finally some attempts are made to estimate the non-linearity for heavilydoped semiconductor gauges.