Jeremy L. Dunning

Mechanical properties of epitaxial 3C silicon carbide thin films

Microscale tensile specimens of epitaxial 3C silicon carbide (3C-SiC) thin films were fabricated on Si substrates and tested to provide measurements of strength and elastic modulus. Samples were fabricated using both micromolding and reactive ion etching (RIE) processes to pattern the 3C-SiC films. All specimens were on the (100) plane with a <110> tensile direction. Testing was performed on a microsample tensile testing machine previously used on other materials. The samples had a thickness of 0.5 to 2 /spl mu/m, a gauge length of 4 mm, and a minimum width of 600 /spl mu/m. Testing results show an average strength of 1.19 /spl plusmn/ 0.53 GPa and 1.65 /spl plusmn/ 0.39 GPa for micromolded and RIE patterned specimens, respectively. The elastic modulus was measured to be 424 /spl plusmn/ 44 GPa, which was consistent with but slightly lower than the elastic modulus calculated with single crystal elastic constants found by ab initio calculations.

A bio-inspired, chemo-responsive polymer nanocomposite for mechanically dynamic microsystems

This paper reports the development of a biologically-inspired, variable-modulus nanocomposite material for mechanically dynamic biomedical microsystems. This nanocomposite is comprised of a poly(vinyl acetate) matrix polymer that is reinforced with rigid cellulose nanofibers, and becomes very flexible when exposed to water. A direct-write CO2 laser was used to pattern structures in this chemical- and temperature-sensitive material. Tensile testing of laser-cut, micron-scale nanocomposite beams was performed using a custom-built tensile tester. These samples displayed a significant reduction in Youngs modulus from 4.1 GPa to 6.1 MPa when the nanocomposite was exposed to phosphate buffered saline. Additionally, the modulus change was observed to be reversible upon drying of soaked tensile samples. As a well-suited application of this nanocomposite, cortical probes utilizing this material as a substrate were fabricated. Gold-coated, dual-shank cortical probes utilizing this nanocomposite as a substrate were shown to record action potentials from a single neuron in a cockroach brain.

Low Stress Polycrystalline SiC Thin Films Suitable for MEMS Applications

This paper details the development of low residual stress and low stress gradient unintentionally doped polycrystalline SiC (poly-SiC) thin films. The films were deposited in a large-volume, low-pressure chemical vapor deposition (LPCVD) furnace on 100 mm-diameter silicon (Si) wafers using dichlorosilane (SiH 2 Cl 2 ) and acetylene (C 2 H 2 ) as precursors. We found that the flow rate of SiH 2 Cl 2 could be used to control the residual film stress in the as-deposited films. Wafer curvature measurements for ∼2 μm-thick films indicated that tensile stress ranging from 4 to 55 MPa across a 25 wafer boat had been achieved. A variety of micromachined structures including lateral resonant structures, stress pointers and cantilevers were fabricated for characterization of the deposited SiC films. The average Youngs modulus was found to be 403 GPa. Residual stress measurements were consistent with those obtained using a wafer curvature technique. Interferometric measurements of cantilever beams indicated stress gradients with an upper bound of 52 MPa/μm for ∼2 μm-thick films with tensile stress less than 55 MPa.

Development of a Microfabricated Flat Interface Nerve Electrode Based on Liquid Crystal Polymer and Polynorbornene Multilayered Structures

This paper reports on the development of a mechanically-flexible microfabricated flat interface nerve electrode using liquid crystal polymer (LCP) and polynorbornene (PNB) as the structural materials. The device consists of two electrode arrays each fabricated on a LCP base with thin film Pt electrodes and a photolithographically patterned PNB capping layer. The two arrays are inserted into a silicone housing designed to create a flat interface between the electrodes and the nerve bundle. Electrical tests showed that the resistance of the thin film Pt electrode interconnect traces are unaffected by flexing around a 1.5 mm radius. Electrical testing in PBS shows that the resistance of the traces is about 1 kOmega. A 10 day leakage current test in PBS indicates that the PNB absorbs moisture but still maintains its insulating behavior. These and other tests indicate that the LCP/PNB multilayer may be a viable material system for microfabricated electrodes.

Young's Modulus and Residual Stress of Polycrystalline 3C-SiC Films Grown by LPCVD and Measured by the Load-Deflection Technique

Polycrystalline silicon carbide (poly-SiC) thin films were grown in a large-volume, lowpressure chemical vapor deposition (LPCVD) furnace using dichlorosilane (SiH2Cl2) and acetylene (C2H2) precursors. The deposition temperature was fixed at 900°C and the pressure was varied between 0.46 and 5 Torr. The load-deflection technique was used to determine the Young’s modulus and residual stress of the as deposited poly-SiC films using bulk micromachined poly-SiC suspended diaphragms. The results indicated the residual stress of poly-SiC films changed from high tensile to low tensile stress as the deposition pressure increased from 0.46 to 2.5 Torr, which was consistent with results obtained from wafer curvature measurements. The Young’s modulus of the films was independent of deposition pressure and averaged 396 GPa. Bent-beam strain gauges were also fabricated and used to measure the stresses in films exhibiting low compressive and tensile residual stresses. Measurements from these structures were consistent with the wafer curvature and, in the case of tensile films, the load-deflection measurements.

A Polynorbornene-Based Microelectrode Array for Neural Interfacing

This paper reports the development of a mechanically-flexible microfabricated flat interface nerve electrode using polynorbornene (PNB) as the structural material. The device consists of two electrode arrays each fabricated on a photolithographically-defined PNB base with thin film Pt electrodes and a photolithographically patterned PNB capping layer. The two arrays are inserted into a silicone housing designed to create a flat interface between the electrodes and the nerve bundle. Electrical tests showed that the resistance of the Pt electrode interconnect traces are unaffected by flexing around a 1.8 mm radius. Electrical testing in phosphate-buffered saline (PBS) shows that resistance of the traces is about 3 kOmega. A 10 day leakage current test in PBS did not produce a detectable change in leakage current. These and other tests indicate that a PNB multilayer system may be viable for microfabricated electrodes.

Characterization of Polycrystalline SiC Thin Films for MEMS Applications using Surface Micromachined Devices

This paper details the characterization of polycrystalline SiC (poly-SiC) thin films using surface micromachined devices. The films were deposited in a large-volume, low-pressure chemical vapor deposition (LPCVD) furnace on 100mm-diameter silicon (Si) wafers using dichlorosilane (SiH2Cl2) and acetylene (C2H2) as precursors. The lowest average residual stresses across the wafers were obtained for films deposited at 900oC, a pressure of 2 torr, and flow rates of SiH2Cl2 and C2H2 at 35 sccm and 180 sccm (5% in H2), respectively. Wafer curvature measurements for ~2 μm-thick films indicated tensile stresses ranging from 4 to 55 MPa across the boat. A variety of micromachined structures including lateral resonant structures, stress pointers and cantilevers were fabricated. The average Young’s modulus was found to be 403 GPa. Residual stress measurements were consistent with those obtained using a wafer curvature technique. Interferometric measurements of cantilever beams indicated stress gradients with an upper bound of 52 MPa/μm.

Development of a High-Throughput LPCVD Process for Depositing Low Stress Poly-SiC

This paper reports the development of a high-throughput, LPCVD reactor and processes for depositing low residual stress, low stress gradient poly-SiC on large area substrates. The system consists of a resistively-heated, horizontal LPCVD furnace capable of holding up to 100, 150 mmdiameter substrates. Poly-SiC depositions were performed using a SiH2Cl2 flow rate of 54 sccm and a C2H2 flow rate of 180 sccm (5% in H2) at temperatures from 800 to 900°C and pressures from 0.46 to 5 Torr. Stoichiometric poly-SiC films were deposited over this entire range. The poly-SiC films exhibited a strong (111) 3C-SiC, polycrystalline texture regardless of temperature and pressure. The surface roughness ranged from 5.6 nm for a film grown at 800°C and 0.46 Torr to 12.6 nm for a film deposited at 900°C and 2.5 Torr. Films having a thickness of up to 1.5 μm were uniform, with a 5% variation across both the wafers and the boat. Single-layer mechanical properties test structures were fabricated from 500 to 600 nm-thick films and successfully released using a Si etchant. The residual stresses in these films exhibit a strong dependence on deposition pressure, ranging from roughly 700 MPa at 0.46 Torr to –100 MPa at pressures above 3.5 Torr. Introduction SiC is well known for its excellent material properties, making it an outstanding addition to the microelectromechanical systems (MEMS) technology material toolbox. Increasing interest in SiC for coating and structural device applications, combined with recent demonstrations of SiC surface micromachining processes [1-2], have provided substantial impetus for developing deposition and process technologies similar to those for polysilicon. This paper reports the development of a highthroughput, LPCVD reactor and process for depositing low stress poly-SiC films on large-area substrates with low stress gradient through the SiC thickness. While our effort is not the first to develop an LPCVD process for poly-SiC [3-5], our system is likely the first high-throughput system designed specifically to produce poly-SiC films for MEMS. Experimental The system consists of a resistively-heated, horizontal LPCVD furnace capable of holding up to 100, 150 mm-diameter substrates, which are vertically mounted in close-pack configuration inside SiC boats. The reaction chamber was large by SiC deposition standards, measuring 2007 mm in length and 225 mm in inner diameter. Wafers were held in a SiC boat that rested on a paddle attached to the front flange. Two small-diameter tubes, one for SiH2Cl2 and the other for C2H2, were used to inject these gases underneath the boat, which was placed near the center of the furnace tube during deposition. The deposition system utilizes SiH2Cl2 and C2H2 (5% in H2) as Si and C precursors, H2 as a carrier gas, and N2 and NH3 as doping gases. The reactor base pressure can reach levels less than 1 mTorr when fully loaded. The first set of depositions was performed for 2 hr at temperatures from 800°C to 900°C and roughly 0.46 Torr. A second set was performed at 900°C using pressure settings from 0.46 Torr to 5 Torr. Flow rates of SiH2Cl2 and C2H2 (5% in H2) were held constant at 54 sccm and 180 sccm, respectively, for all deposition runs. Following each deposition, the thickness of the films was measured optically using a Nanospec 4000 AFT Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 305-308 doi:10.4028/www.scientific.net/MSF.457-460.305

Mechanical properties and morphology of polycrystalline 3C-SiC films deposited on Si and SiO2 by LPCVD

Poly-SiC films were deposited on Si and SiO 2 substrates in a high-throughput, low pressure chemical vapor deposition (LPCVD) furnace using dichlorosilane (DCS) and acetylene precursors. The deposition temperature and pressure were fixed at 900°C and 2 Torr, respectively, while the flow rate of DCS was varied between 18 and 54 sccm. Poly-SiC deposition rates on both Si and SiO 2 were nearly identical to each other and increased as a function of DCS flow rate. Consistent with both substrate materials, the following observations were made. A slope change of the deposition rate versus DCS flow rate was observed around a DCS flow rate of 35 sccm. Residual stress varied with respect to the deposition rate, with tensile stresses occurring at lower deposition rates and compressive stresses at higher deposition rates. The tensile-to-compressive stress transition corresponded to the slope change of the deposition rate versus DCS flow rate. The surface morphology consisted of pyramidal grains, as observed under an SEM. TEM analysis for poly-SiC films grown on Si substrates showed that microstructural differences exist for poly-SiC films having tensile and compressive stresses.

Stress and strain gradient control of polycrystalline SiC films

This work reports the development of very low residual stress and low strain gradient polycrystalline SiC (poly-SiC) thin films deposited by low pressure chemical vapor deposition (LPCVD). Using dichlorosilane (DCS, SiH<inf>2</inf>Cl<inf>2</inf>) and acetylene (C<inf>2</inf>H<inf>2</inf>) as precursors, it was found that the flow rate of DCS can be used to adjust the residual stress from tensile to compressive in as-deposited films. The resulting poly-SiC films, with tensile stresses lower than 50 MPa and strain gradients as low as 1.3×10⁻⁴ °m⁻¹, are well-suited for MEMS and NEMS structural materials.