Stress and strain gradient control of polycrystalline SiC films

Abstract

This work reports the development of very low residual stress and low strain gradient polycrystalline SiC (poly-SiC) thin films deposited by low pressure chemical vapor deposition (LPCVD). Using dichlorosilane (DCS, SiH 2 Cl 2 ) and acetylene (C 2 H 2 ) as precursors, it was found that the flow rate of DCS can be used to adjust the residual stress from tensile to compressive in as-deposited films. The resulting poly-SiC films, with tensile stresses lower than 50 MPa and strain gradients as low as 1.3×10 -4 µm -1 , are well-suited for MEMS and NEMS structural materials.

Use of Vacuum as a Gate Dielectric: The SiC VacFET

“…If the gate were to significantly deflect, the gap thickness would vary as a function of both gate dimensions, resulting in a non-uniform field strength and anomalies in the creation of the inversion layer. With rigidity in mind, the choice to use poly-SiC as the gate bridge was based principally on the fact that the Young's modulus of our poly-SiC (>300 GPa, [10]) would provide 2× more stiffness than polysilicon (150-170 GPa) to minimize electrostatic deflection. Also, the mechanical properties of poly-SiC change negligibly at elevated temperatures [11], enabling predictable mechanical behavior of the VacFET architecture into temperature regimes of 600 °C and beyond.…”

“…Timoshenko's text [12] was used to calculate the maximum deflection, y max , of the gate, with the constraint that y max should be a negligible fraction of the vacuum thickness in order to lend credibility to characterization. The effect of residual tensile stress in our poly-SiC films [10] where α is the coefficient of deflection (values are empirically derived for various ratios of plate dimensions, see [12]); q is the pressure due to electrostatic force, F e ; W and L are the gate width and length, respectively; E is Young's modulus of the gate material; H is the gate thickness; and ν is Poisson's ratio of the gate material. The electrostatic force is expressed as:…”