Research Academics  
 
   
 

Dependence of the Elastic Properties of Thin 3C-SiC Films  on their
Crystallinity and the Deposition Conditions  

 

Contact Person : Dipl.-Ing. G.Wieczorek

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.

Young`s modulus Poisson`s ratio

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.

Thermal stress a)Thermal stress b)

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.
  

       

Load deflection characteristics

Figure 3: Schematic of the Test Structure and Load-Deflection Characteristics for Two of the 3C-SiC Diaphragms.