We report a new surface micromachining technology to fabricate monocrystalline silicon membranes covering a vacuum cavity for applications like piezoresistive pressure sensors. The main process steps are: (i) local anodic etching of layered porous silicon with different porosities, (ii) tbermal rearrTgement of the porous silicon, and (iii) epitaxial growth of the silicon membrane layer. In contrast to conventional bulk micromachining the new technology has the benefit of a considerable freedom in the design of monocrystalline silicon membranes. The membrane geometry is only determined by the porous region. Further, the new fabrication method is fully CMOS compatible. In fact, except for anodic etching, all process steps are part of a standard mixed signal IC production line. Various aspects of the used key process steps are discussed, particularly with regard to the oxygen and fluorine desorption during the porous silicon annealing. A piezoresistive pressure sensor with integrated ASIC based on the new fabrication method is demonstrated.
Mechanical properties of meso-porous silicon are studied using topographic measurements and finite element simulations. Our approach is based on an original analysis of the strain at the free surface of porous silicon tub embedded in bulk Si regions allowing the determination of the Young’s modulus of the porous layers. In particular, the internal stress in the porous Si region is evaluated from the corresponding deformation of the monocrystalline Si adjacent region which mechanical parameters are well known. Moreover, a mechanical anisotropy of the columnar nanostructured porous Si is brought to the fore from the characteristic shape of the strained porous layer profile. Moderately oxidized, 70% in porosity, porous silicon patterns were investigated. Correlation of our measurements with x-ray data reported early in literature shows the macroscopic strain being close to the silicon lattice relative increase revealing an elastic deformation regime. The porous layers exhibit an unexpected low and strongly anisotropic Young’s modulus for all samples. Young’s modulus values of 1.5 and 0.44 GPa are found in parallel and perpendicular directions of the columnar structure, respectively. Finally, a phenomenological model for such a mechanical behavior taking into account porosity and percolation strength factor of the randomly arranged as-prepared and partially oxidized porous Si nanostructures is proposed.
We have fabricated a textured monocrystalline Si solar cell with a thickness of 15Á5 lm and a con®rmed ef®ciency of 12Á2% using porous silicon (PSI) for layer transfer. The PSI process avoids photolithography and high-temperature oxidation. The cell has a surface that is textured with randomly positioned inverted pyramids for light trapping. The device does not yet fully exploit the light-trapping capability of this ®lm shape, owing to a small back-surface re¯ectance.
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