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This paper reports on the mechanical and piezoresistance characterization of hydrogenated amorphous and nanocrystalline silicon thin films deposited by hot-wire chemical vapor deposition (HWCVD) and radio-frequency plasma-enhanced chemical vapor deposition (PECVD) using substrate temperatures between 100 and 250 °C. The microtensile technique is used to determine film properties such as Young’s modulus, fracture strength and Weibull parameters, and linear and quadratic piezoresistance coefficients obtained at large applied stresses. The 95%-confidence interval for the elastic constant of the films characterized, 85.9 ± 0.3 GPa, does not depend significantly on the deposition method or on film structure. In contrast, mean fracture strength values range between 256 ± 8 MPa and 600 ± 32 MPa: nanocrystalline layers are slightly stronger than their amorphous counterparts and a pronounced increase in strength is observed for films deposited using HWCVD when compared to those grown by PECVD. Extracted Weibull moduli are below 10. In terms of piezoresistance, n-doped radio-frequency nanocrystalline silicon films deposited at 250 °C present longitudinal piezoresistive coefficients as large as −(2.57 ± 0.03) × 10−10 Pa−1 with marginally nonlinear response. Such values approach those of crystalline silicon and of polysilicon layers deposited at much higher temperatures.

Thin-film silicon allows the fabrication of MEMS devices at low processing temperatures, compatible with monolithic integration in advanced electronic circuits, on large-area, low-cost, and flexible substrates. The most relevant thin-film properties for applications as MEMS structural layers are the deposition rate, electrical conductivity, and mechanical stress. In this work, n+-type doped hydrogenated amorphous and nanocrystalline silicon thin-films were deposited by RF-PECVD, and the influence of the hydrogen dilution in the reactive mixture, the RF-power coupled to the plasma, the substrate temperature, and the deposition pressure on the structural, electrical, and mechanical properties of the films was studied. Three different types of silicon films were identified, corresponding to three internal structures: (i) porous amorphous silicon, deposited at high rates and presenting tensile mechanical stress and low electrical conductivity, (ii) dense amorphous silicon, deposited at intermediate rates and presenting compressive mechanical stress and higher values of electrical conductivity, and (iii) nanocrystalline silicon, deposited at very low rates and presenting the highest compressive mechanical stress and electrical conductivity. These results show the combinations of electromechanical material properties available in silicon thin-films and thus allow the optimized selection of a thin silicon film for a given MEMS application. Four representative silicon thin-films were chosen to be used as structural material of electrostatically actuated MEMS microresonators fabricated by surface micromachining. The effect of the mechanical stress of the structural layer was observed to have a great impact on the device resonance frequency, quality factor, and actuation force.

Microelectromechanical-system resonators have great potential as sensors for the detection of chemical and biological species in air and in liquids due to their high frequency, low dissipation and possibility of miniaturization. In this work, the influence of a fluidic environment on the quality factors and resonance frequency is studied. The changes in the dynamic behavior of thin-film hydrogenated amorphous silicon microelectromechanical disk resonators of ∼50-350 μm characteristic length and spanning a resonance frequency range between 0.1 and 20 MHz are described as a function of air pressure up to atmospheric conditions. The results show that fluidic damping behavior is dependent on the frequency of operation with respect to the penetration depth of the propagation of shear waves resulting from fluid-structure interaction. The use of higher harmonics is suggested for reduced damping in fluidic applications, independent of the particular mode shape. Significant increases in the effective stiffness are observed due to elastically clamped air associated with squeeze-film effects. Linear variation of the resonance frequency up to 40% was observed when varying the pressure. It is observed that inertial effects from the surrounding media are less prominent at higher orders and at higher frequency modes.

Thin-film silicon micro resonators are fabricated by surface micromachining at temperatures that are CMOS and large area substrate-compatible. Disk resonators offer large working surfaces and a large number of vibrational modes. The vibrational modes of micromechanical disk resonators made from hydrogenated amorphous silicon thin films were studied in this work. The dynamic behavior of these structures is shown to be mechanically described to be in the transition between a membrane and a plate due to the influence of residual stresses generated during the film growth and to thermal mismatch with underlying layers. Non-degenerate modes are observed as a consequence of the radial symmetry and their effective stiffness is related to the anchor geometry and the parity of the number of diametric nodal lines. The experimentally measured frequencies were compared with the simulated values from finite element modeling with good agreement. Investigation of the intrinsic quality factors shows that there is a dependence of the energy dissipation per cycle with the mode order that is related to the clamping anchors. Thermal annealing experiments show that enhanced quality factors can be obtained using low temperature annealing for a limited period of time.