Papers by Author: Christopher Locke

Abstract: An ever-increasing demand for biocompatible materials provides motivation for the development of advanced materials for challenging applications ranging from disease detection to organ function restoration. Carbon-based materials are considered promising candidates because they combine good biocompatibility with high chemical resistance. In this work we present an initial assessment of the biocompatibility of epitaxial graphene on 6H-SiC(0001). We have analyzed the interaction of HaCaT (human keratinocyte) cells on epitaxial graphene and compared it with that on bare 6H-SiC(0001). We have found that for both graphene and 6H-SiC there is evidence of cell-cell and cell substrate interaction which is normally an indication of the biocompatibility of the material.

Abstract: SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on either bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication, but the high residual stress created during the film grow limits the development of the material for these applications. In order to evaluate the amount of residual stress released from the epi-film, different micro-machined structures were developed. Finite elements simulations of the micro-machined structures have also been carried out in order to evaluate, in detail, the stress field inside the structures and to test the analytical model used. With finite element modeling a exponential approximation of the stress relationship was studied, yielding a better fit with the experimental data. This study shows that this new approximation of the total residual stress function reduces the disagreement between experimental and simulated data.

Abstract: The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on either bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication. In this work a comparison between single crystal and poly crystal 3C-SiC micro-machined structures will be presented. The free-standing structures realized (cantilevers and membrane) are also a suitable method for residual field stress investigation in 3C-SiC films. Measurement of the Raman shift indicates that the mono and poly-crystal 3C-SiC structures release the stress in different ways. Finite element analysis was performed to determine the stress field inside the films and provided a good fit to the experimental data. A comprehensive experimental and theoretical study of 3C-SiC MEMS structures has been performed and is presented.

Abstract: SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). In order to understand the impact that the growth rate has on the residual stress of CVD-grown 3C-SiC hetero-epitaxial films on Si substrates, growth experiments were performed and the resulting stress was evaluated. Film growth was performed using a two-step growth process with propane and silane as the C and Si precursors in hydrogen carrier gas. The film thickness was held constant at ~2.5 µm independent of the growth rate so as to allow for direct films comparison as a function of the growth rate. Supported by profilometry, Raman and XRD analysis, this study shows that the growth rate is a fundamental parameter for low-defect and low-stress hetero-epitaxial growth process of 3C-SiC on Si substrates. XRD (rocking curve analysis) and Raman spectroscopy show that the crystal quality of the films increases with decreasing growth rate. From curvature measurements, the average residual stress within the layer using the modified Stoney’s equation was calculated. The results show that the films are under compressive stress and the calculated residual stress also increases with growth rate, from -0.78 GPa to -1.11 GPa for 3C-SiC films grown at 2.45 and 4 µm/h, respectively.

Abstract: 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 3C-SiC 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.

Abstract: The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication. This approach puts together the standard silicon bulk microfabrication methodologies with the robust mechanical properties of 3C-SiC. Using this approach we were able to fabricate SiC cantilevers for a new class of pressure sensor. In the present research, chemical vapour deposition (CVD) in the low pressure regime of 3C–SiC on silicon substrates was carried out using silane (SiH4), propane (C3 H8) and hydrogen (H2) as the silicon supply, carbon supply and gas carrier, respectively. The resulting bow in the MEMS structures was evaluated optically and the residual stress in the films calculated using the modified stoney equation and determined to be approximately 300 MPa.

Abstract: Electron channeling contrast imaging (ECCI) has been utilized to evaluate the surface morphology and crystalline quality of 3C-SiC films grown by chemical vapor deposition (CVD) on (100) and (111) Si substrates. ECCI in this study was performed using an electron backscatter diffraction (EBSD) system equipped with forescatter diode detectors and mounted inside a commercial scanning electron microscope (SEM). This nondestructive method permits direct dislocation imaging through local fluctuations in forescattered electron yield attributable to lattice strain. Coordinated ECCI, SEM, and EBSD analysis of film surfaces allowed correlations between film orientation, surface morphology, and dislocation behavior. Evidence of lateral dislocations parallel to <110> directions and atomic step pinning by dislocations was observed.

Abstract: A novel method for growing highly-crystalline 3C-SiC on an oxide release layer via a poly-Si seed layer is reported. Silicon carbide’s potential role as a ubiquitous material for MEMS fabrication lies in its dual role as an electronic and mechanical material. Unfortunately, due to residual stresses and crystal defects stemming from the large lattice constant mismatch and the thermal expansion coefficient difference between SiC and Si, the use of SiC in Si-based MEMS fabrication techniques has been very limited. The growth of 3C-SiC on a poly-Si seed layer deposited on oxide on (111)Si substrates (i.e., p-Si/ SiO2/(111)Si) provides an alternative fabrication method to expensive, traditional SOI bonding techniques for producing free-standing 3C-SiC MEMS structures. 3C-SiC grown with a poly-Si seed layer on SiO2 should experience reduced residual stress and far fewer defects due to the compliance of the SiO2 layer. Although poly-Si is utilized as a seed layer in this process, a well-ordered monocrystalline 3C-SiC layer was achieved and the process and film properties reported.

Abstract: We have developed a high-quality growth process for 3C-SiC on on-axis (111)Si substrates with the ultimate goal to demonstrate high quality and yield electronic and MEMS devices. A single-side polished 50 mm (111)Si wafer was loaded into a hot-wall SiC CVD reactor for growth. The 3C-SiC process was performed in two stages: carbonization in propane and hydrogen at 1135°C and 400 Torr followed by growth at 1380°C and 100 Torr. X-ray diffraction rocking curve analysis of the 3C-SiC(222) peak indicates a FWHM value of 219 arcsec. This is a very interesting result given that the film thickness was only 2 µm, thus indicating that the grown film is of very high quality compared with published literature values. X-ray polar figure mapping was performed and it was observed that the micro twin content was below the detection limit. Therefore TEM characterization was performed in plan view to allow assessment of the stacking fault density as well as confirmation of the very low micro twin concentration in this film. TEM analysis indicates a low concentration of stacking faults in the range of 104 cm-1.

Abstract: Silicon Carbide (SiC) is a very promising material for the fabrication of a new category of sensors and devices, to be used in very hostile environments (high temperature, corrosive ambient, presence of radiation, etc.). The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on bulk material or on the SiC surface. The hetero-epitaxial growth of 3CSiC on silicon substrates allows one to overcome the traditional limitations of SiC microfabrication. This approach puts together the standard silicon bulk microfabrication methodologies with the robust mechanical properties of 3C-SiC. Using this approach we were able to fabricate SiC cantilevers for a new class of pressure sensor. The geometries studied were selected in order to study the internal residual stress of the SiC film. X-Ray Diffraction polar figure and Bragg- Brentano scan analysis were used to check to crystal structure and the orientations of the film. SEM analysis was performed to analyze the morphology of the released MEMS structures.