3C-SiC Films on Si for MEMS Applications: Mechanical Properties

Single crystal 3C-SiC films were grown on (100) and (111) Si substrate orientations in order to study the resulting mechanical properties of this material. In addition, poly-crystalline 3CSiC was also grown on (100)Si so that a comparison with monocrystaline 3C-SiC, also grown on (100)Si, could be made. The mechanical properties of single crystal and polycrystalline 3C-SiC films grown on Si substrates were measured by means of nanoindentation using a Berkovich diamond tip. These results indicate that polycrystalline SiC thin films are attractive for MEMS applications when compared with the single crystal 3C-SiC, which is promising since growing single crystal 3C-SiC films is more challenging. MEMS cantilevers and membranes fabricated from a 2 μm thick single crystal 3C-SiC grown on (100)Si under similar conditions resulted in a small degree of bow with only 9 μm of deflection for a cantilever of 700 μm length with an estimated tensile film stress of 300 MPa. Single crystal 3C-SiC films on (111)Si substrates have the highest elastic and plastic properties, although due to high residual stress they tend to crack and delaminate. Introduction There is intense interest in developing robust micro-electrical-mechanical systems (MEMS) for harsh applications such as chemical and biological sensors, advanced space actuators, etc. While silicon has been the primary material for MEMS applications, limitations in the temperature of operation of this material and the lack of a chemically inert surface has resulted in the development of alternate materials for these challenging applications [1]. The high hardness and chemical inertness of SiC, coupled with its very high operating temperatures, has resulted in a high level of interest in developing SiC MEMS devices, with numerous groups around the world reporting progress with both the hexagonal and cubic polytypes of SiC [2,3]. However, the ability to grow the cubic form of SiC, namely 3C-SiC, on Si offers both a low-cost alternative for SiC MEMS coupled with the prospect to use well developed Si micromachining technology and processing to realize advanced, highly robust SiC MEMS devices [4]. While SiC is potentially an ideal material for power MEMS applications, deposition and processing techniques must be mastered before a functioning device can actually be built. It is advantageous to deposit SiC on Si wafers due to their high quality and low cost, in comparison to SiC substrates [5]. However, one has to overcome the ~22% lattice mismatch between the 3C-SiC film and the (100) Si substrate, and after growth the 8% mismatch in thermal expansion [6]. Any temperature variation will cause additional stress. Also, 3C-SiC films grown on (111) Si wafers have a larger residual stress than films grown on (100). 3C-SiC can be hetero-epitaxially grown on a highly defective SiC buffer layers formed during the carbonization step.