#### Introduction
The availability of accurate data about the elastic properties of a material is essential for the development of micromechanical devices such as membrane based pressure
sensors. For high temperature applications sensors using the high bandgap material SiC are of great potential. Therefore the influence of the most important parameters such as the deposition
temperature, the crystallinity, and the thickness of the 3C-SiC films on the elastic properties
(Young's modulus, and the thermal and the residual stress) were
investigated.
Analytic calculation of the elastic properties
The anisotropic behavior of Young's modulus E and Poisson's ratio was calculated from the elastic coefficients of single crystal 3C-SiC
(FIGURE 1). The elastic in-plane properties of single crystal 3C-SiC films were obtained by averaging the calculated values over all directions in the given plane. The elastic properties for polycrystalline cubic silicon carbide were determined by averaging over all crystal
orientations. The average value obtained for Young's modulus in the (100)-plane was 307 GPa. For polycrystalline material E equals 329 GPa.
Figure. 1: Calculated Anisotropic Behavior of Young's Modulus E
(Left) and Poisson's Ratio (Right) for Single Crystal 3C-SiC and Silicon in the (100)-Planes.
####
#### Finite element analysis of the thermal stress
Calculation of the thermal stress using FEA shows that for 3C-SiC films deposited on silicon the stress is always tensile and increases with decreasing film thickness and increasing deposition temperature
(FIGURE 2). Since polycrystalline material has a higher value of E compared with single crystal 3C-SiC the thermal stress is higher in polycrystalline
films.
Figure 2: Dependence of the Thermal Tensile Stress for Single and Polycrystalline 3C-SiC Films on Deposition Temperature
(Left)
and Film Thickness (Right).
#### Experimental
Using the load-deflection measurement technique, the deformation of thin SiC diaphragms was determined as a function of the applied pressure
(FIGURE 3). Utilizing membrane theory, which gives a defined relationship between the deflection of a thin membrane and an external
load, both Young's modulus E and the residual stress were determined. The experimental results show good agreement with the calculated
values. The measured values for Young's modulus of single crystal (100)-textured and polycrystalline films were approximately 315 GPa and 380 GPa,
respectively, independent of the membrane dimensions. The residual tensile stress for single and polycrystalline 3C-SiC films was approximately 150 MPa and 260 MPa,
respectively.
Figure 3: Schematic of the Test Structure and Load-Deflection Characteristics for Two of the 3C-SiC
Diaphragms. |