Micro- and Nano-electromechanical Systems (MEMS and NEMS) consist of devices which can sense and actuate on the micrometer and nanometer scales. A number of MEMS devices have been commercialized, including accelerometers, gyroscopes, pressure sensors, and micromirror displays. The most common structural layer used in this technology is polycrystalline silicon, which is adequate for MEMS/NEMS devices operating in ambient environments; however, the use of a more robust material, such as silicon carbide, would permit micromechanical devices to function in a variety of harsh environments, including high temperatures, high pressures, and highly abrasive and corrosive conditions. In order for SiC to become a standard micromachining material and thus enable harsh environment sensors and actuators, deposition of high-quality SiC thin films on standard wafers with controlled electrical and mechanical properties must be possible.
This work describes the development and characterization of a horizontal hot-wall low pressure chemical vapor deposition reactor to deposit polycrystalline 3C-SiC (poly-SiC) thin films from the precursors 1,3-disilabutane (1,3-DSB) and dichlorosilane (DCS). Deposition is performed on 100 and 150 mm silicon wafers at 800°C and 40-400 mTorr. Using the standard open boat geometry, the film uniformity is found to be poor. Upon identification of the two dominant reaction pathways, the reaction channel which leads to the non-uniform growth is quenched by the use of a closed-boat geometry. In this way, highly uniform films across individual wafers and between wafers are achieved.
The stress and strain gradient are quantified and related to the process parameters through wafer curvature measurements and a number of microfabricated devices. In the absence of DCS, highly stressed films result regardless of deposition conditions. Varying the flow rate ratio of DCS to 1,3-DSB is found to control residual stress and reduce strain gradient. Electron probe microanalysis shows that added dichlorosilane increases the silicon-to-carbon ratio of the films. Transmission electron micrographs (TEM) of film cross-sections, plane view atomic force micrographs (AFM), and plane-view scanning electron micrographs (SEM) reveal a changing crystallinity and film morphology with dichlorosilane addition. A model is developed to fit the data based on thermal stress, intrinsic stress due to changing Si:C ratio, and intrinsic stress due to grain boundary effects.
Ammonia is used for in-situ doping of the SiC films. By varying the ammonia flow rate and subsequent annealing temperature, the resistivity of the films is controlled and ranges from over 2 MΩ·cm to 18 mΩ·cm. Secondary ion mass spectroscopy shows that increased ammonia flow rate leads to increased nitrogen incorporation in the films. Over the range examined, film resistivity is found to decrease with both increased nitrogen incorporation and higher annealing temperatures. The effect of doping on strain and strain gradient is also investigated. SIMS and XPS analyses indicate the change in mechanical properties upon annealing is correlated to oxygen impurity levels and the bonding state of the incorporated nitrogen atoms.
Investigation of the deposition and annealing of poly-SiC reveals a rich phase space of electrical and mechanical properties with no universal linear correlations between resistivity, residual stress, and strain gradient. Semi-insulating SiC (resistivity greater than 2 MΩ·cm) can only be achieved with high residual stress (greater than 1.0 GPa tensile) and high strain gradient (magnitude greater than 2.7×10^−3μm-1). Resistivity levels between 200 Ω·cm and 20 mΩ·cm can be achieved with moderately tensile stress (300 ± 30 MPa tensile) and low strain gradient (magnitude less than 7.7×10^−3μm-1) films. Lower resistivity can be achieved by annealing, but this yields compressively stressed films (as large as 240 MPa compressive) with large strain gradients (magnitude as great as 1.8×10^−2μm-1).
The ability to deposit uniform and doped SiC films on 100 and 150 mm wafers with controlled electrical and mechanical properties enables MEMS/NEMS designers to create a variety of devices for harsh environment applications.
December 31, 2007
Roper, C. S. (2007). Silicon Carbide Thin Films Via Low Pressure Chemical Vapor Deposition for Micro- and Nano-electromechanical Systems. (n.p.): University of California, Berkeley.