Christopher S. Roper

Covalent Attachment of Organic Monolayers to Silicon Carbide Surfaces

This work presents the first alkyl monolayers covalently bound on HF-treated silicon carbide surfaces (SiC) through thermal reaction with 1-alkenes. Treatment of SiC with diluted aqueous HF solutions removes the native oxide layer (SiO2) and provides a reactive hydroxyl-covered surface. Very hydrophobic methyl-terminated surfaces (water contact angle theta = 107 degrees ) are obtained on flat SiC, whereas attachment of omega-functionalized 1-alkenes also yields well-defined functionalized surfaces. Infrared reflection absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy measurements are used to characterize the monolayers and show their covalent attachment. The resulting surfaces are shown to be extremely stable under harsh acidic conditions (e.g., no change in theta after 4 h in 2 M HCl at 90 degrees C), while their stability in alkaline conditions (pH = 11, 60 degrees C) also supersedes that of analogous monolayers such as those on Au, Si, and SiO2. These results are very promising for applications involving functionalized silicon carbide.

Advances in silicon carbide science and technology at the micro- and nanoscales

Advances in silicon carbide microfabrication and growth process optimization for silicon carbide nanostructures are ushering in new opportunities for microdevices capable of operation in a variety of demanding applications, involving high temperature, radiation, or corrosive environment. This review focuses on the materials science and processing technologies for silicon carbide thin films and low dimensional structures, and details recent progress in manufacturing technology, including deposition, metallization, and fabrication of semiconductor microdevices, with emphasis on sensor technology. The challenges remaining in developing silicon carbide as a mainstay materials platform are discussed throughout.

Stress control of polycrystalline 3C-SiC films in a large-scale LPCVD reactor using 1,3-disilabutane and dichlorosilane as precursors

Control of residual stress and strain gradient of polycrystalline SiC films deposited via low-pressure chemical vapor deposition on 100 mm Si wafers is achieved by varying dichlorosilane (DCS) and 1,3-disilabutane (DSB) fractions in the inlet gas mixture. For films deposited at 800 °C and 45 sccm DSB, stress decreases from 1.2 GPa tensile with no added DCS to 240 MPa tensile with 40 sccm DCS added to the inlet gas stream. The lowest magnitude strain gradient achieved is 3.1 × 10−5 µm−1 with 20 sccm DCS added. Electron probe microanalysis indicates that the films change from being slightly carbon-rich in the absence of DCS to successively more silicon-rich with the addition of DCS.

Characterization of polycrystalline 3C-SiC films deposited from the precursors 1,3-disilabutane and dichlorosilane

Polycrystalline 3C-SiC thin films are deposited via low pressure chemical vapor deposition from the precursors 1,3-disilabutane (DSB) and dichlorosilane (DCS). Elemental composition, microstructure, surface morphology, and residual stress are characterized as functions of DCS flow rate fraction. Elemental composition varies linearly with DCS fraction, while microstructure changes drastically with slight changes in DCS fraction. Residual stress varies from 1.2GPa tensile to 240MPa tensile, and the causes of which are traced to variations in elemental composition and grain size. A model of residual stress accounting for the effects of elemental composition and grain size on intrinsic stress is developed and found to be in reasonable agreement with experimental data.

Single-Source Chemical Vapor Deposition of SiC Films in a Large-Scale Low-Pressure CVD Growth, Chemical, and Mechanical Characterization Reactor

The development and characterization of a silicon carbide (SiC) deposition process from a single source precursor, 1,3-disilabutane, in a large-scale reactor is described. Deposition was performed simultaneously on fifteen, 4 in. Si wafers in a 4 or 6 in. wafer-capable horizontal low-pressure chemical vapor deposition reactor. Amorphous SiC is obtained at temperatures of 750°C and below, while some polycrystallinity is obtained at temperatures of 800°C and above. Highly uniform and relatively smooth films are realized using a closed wafer boat. A maximum growth rate of 0.45 μm/h is attained at 750°C. Residual stress in the film is characterized and found to be greater than 1.3 GPa (tensile) across a wide range of deposition temperatures. Stress profiling is performed to investigate the stress distribution throughout the films. Microfabrication on the wafer level using SiC as a structural layer is also demonstrated.

Characterization of Encapsulated Micromechanical Resonators Sealed and Coated With Polycrystalline SiC

This paper presents the characterization of sealed microelectromechanical devices and their packaging fabricated in a polycrystalline 3C silicon carbide (poly-SiC) thin-film encapsulation process. In this fabrication technique, devices are sealed with a nominally 2-¿m low-pressure chemical vapor deposition poly-SiC, and the device layer is simultaneously coated with a nominally 0.2- ¿m poly-SiC thin film. Device characterization includes measurement of the resonant frequency and the quality factor of double-ended tuning-fork micromechanical resonators, which have a Si-SiC composite beam structure. Experimental results show that the pressure inside the packaging can be controlled from 447 Pa to 15.5 kPa with a 400°C annealing process. The frequency drifts of the encapsulated resonators are less than the frequency noise level (±10.6 ppm) measured over 29 days at 84.6°C ±0.1°C, which suggests that the poly-SiC thin-film packaging technique can offer hermetic packaging for various applications in microelectromechanical systems including inertial sensors. In addition to the packaging performance, the temperature coefficient of Youngs modulus for poly-SiC is derived from the resonant-frequency change of resonators with temperature. The reduction of the quality factor due to the poly-SiC coating, predicted in the theoretical model, is confirmed by measurements.

Effects of Annealing on Residual Stress and Strain Gradient of Doped Polycrystalline SiC Thin Films

We report the effects of annealing at 925 and 1050°C in argon ambient on the resistivity, average residual stress, and strain gradient of highly doped polycrystalline 3C-SiC films deposited at 800°C and 170 mTorr from 1,3-disilabutane, dichlorosilane, and ammonia. Residual stress shifts from -569 to 274 MPa, compressive strain gradient shifts from 4 X 10 -4 to -0.018 μm -1 , and resistivity shifts from -0.037 to 0.030 Ω cm. Characterization of the SiC films reveals that oxygen impurity diffusion from the ambient during annealing and a change in bonding of nitrogen dopant atoms are sources of the shifts in these properties.

In situ studies of interfacial contact evolution via a two-axis deflecting cantilever microinstrument

The time-dependent assessment of two contacting polycrystalline silicon surfaces is realized using a microinstrument that allows for in situ surface analysis. The evolution in contact resistance, morphology, and chemistry is probed as a function of contact cycle. Initially, the contact resistance is found to decrease and then increase with impact cycle. Upon prolonged cycling, the fracture of Si grains is observed which grow to form a wear crater. The electrical, morphological, and chemical analyses suggest that the wear of rough polysilicon surfaces due to impact proceeds through three distinct phases, namely plastic deformation of asperities, adhesive wear, and grain fracture.

Silicon Carbide Thin Films using 1,3-Disilabutane Single Precursor for MEMS Applications - A Review

Significant progress has been made over the past several years in the development of silicon carbide (SiC) films grown via low pressure chemical vapor deposition (LPCVD) from the single precursor, 1,3-disilabutane (DSB) for microelectromechanical systems (MEMS) applications. Polycrystalline 3C-SiC (poly-SiC) deposition capabilities have been scaled up from depositions on a single 1cm × 1cm piece of Si to forty-five 150 mm Si wafers. Insitu doping with gaseous ammonia and post-deposition annealing have been developed, resulting in a lowest resistivity of ~0.02 Ω·cm to date. Plasma etch chemistries for SiC with good selectivity to silicon dioxide and silicon nitride masking layers have been discovered and characterized. Extensive mechanical, electrical, and tribological characterization of the SiC films have been performed. These SiC films have been applied as both MEMS structural layers for high-frequency resonators and thin film coating layers to improve anti-stiction properties and harsh environment performance.

Room-Temperature Wet Etching of Polycrystalline and Nanocrystalline Silicon Carbide Thin Films with HF and HNO3

Polycrystalline 3C-SiC films deposited using low-pressure chemical vapor deposition from the precursors 1,3-disilabutane and dichlorosilane (DCS) are etched at room temperature in a mixed acid solution consisting of a 1:1 mixture of HF:HNO 3 . DCS flow rate fractions from 0 to 0.47, which yield a range of films with varying grain sizes, are examined. Etch rates vary from 50 A/min for smaller grained films to 0.1 A/min for larger grained films. A kinetic model of the etch rate with first-order dependence on free silicon concentration and inverse power-law dependence on grain size is developed that fits the data well. The change in roughness of most polycrystalline SiC films is nominal upon etching; however, films deposited with 0.16 DCS fraction exhibit an increase in roughness accompanied by the exposure of larger grains at the etched surface. Furthermore, films deposited without DCS exhibit an increase in roughness accompanied by the formation of 2-4 nm deep nanocraters on the surface of individual grains. The room-temperature wet-etch capability thus presented enables new process flows for the realization of micro- and nanoelectromechanical system devices for harsh environments.