Rebecca Cheung

Sub-diffraction-limited patterning using evanescent near-field optical lithography

Patterning at resolution below the diffraction limit for projection optical lithography has been demonstrated using evanescent near-field optical lithography with broadband illumination (365–600 nm). Linewidths of 50 nm and gratings with 140 nm period have been achieved. Ultrathin photoresist layers in conjunction with conformable photomasks are employed and a reactive ion etching process using SF6 has been developed to transfer the patterns to a depth of more than 100 nm into silicon. Full electromagnetic field simulations of the exposure process show that a high contrast image is present within the resist layer, and that the exposure is dominated by one polarization for the grating structures studied.

Fluorination of carbon nanotubes in CF4 plasma

The effect of CF4 gaseous plasma exposure to single-wall carbon nanotubes (CNTs) has been studied. Raman spectroscopy results show that CNTs have gained more disordered sp3 bonds associated with functionalization, as both the flow rates of gas in the plasma and exposure time in the plasma are increased. Scanning electron microscopy images indicate the CNTs have been preserved after CF4 plasma exposure. X-ray photoelectron spectroscopy provides evidence of carbon to fluorine bonds (C–F) on the CNTs samples after CF4 plasma exposure. Semi-ionic and covalent C–F bonds are prevalent on the CNTs after CF4 exposure with the intensity ratio of the semi-ionic to covalent C–F bond decreasing as the flow rate of CF4 and exposure time in the CF4 plasma is increased.

Silicon carbide microelectromechanical systems for harsh environments

This unique book describes the science and technology of silicon carbide (SiC) microelectromechanical systems (MEMS), from the creation of SiC material to the formation of final system, through various expert contributions by several leading key figures in the field. The book contains high-quality up-to-date scientific information concerning SiC MEMS for harsh environments summarized concisely for students, academics, engineers and researchers in the field of SiC MEMS. This is the only book that addresses in a comprehensive manner the main advantages of SiC as a MEMS material for applications in high temperature and harsh environments, as well as approaches to the relevant technologies, with a view progressing towards the final product.

Inductively coupled plasma etching of SiC in SF6/O2 and etch-induced surface chemical bonding modifications

4H silicon carbide (SiC) substrates were dry etched in an inductively coupled plasma (ICP) system, using SF6/O2 gas mixtures. Etch rate and etch mechanisms have been investigated as a function of oxygen concentration in the gas mixture, ICP chuck power, work pressure, and flow rate. Corresponding to these etch conditions, surface information of the etched SiC has been obtained by x-ray photoelectron spectroscopy measurements. The fact that no obvious Si–Si and Si–F bonds were detected on the etched surface of SiC in all our etch experiments suggests efficient removal of Si atoms as volatile products during the processes. However, various kinds of C–F bonds have been detected on the etched SiC surface and the relative intensities of these bonds vary with the etch conditions. In addition, the nature of the incorporated F atoms on the etched surface also depends strongly on etch conditions, which was identified by the change of the relative ratio between semi-ionic and covalent carbon fluorine bonds. The electrical behavior for different bond structures on the etched SiC surface can be one of the basic reasons affecting related devices.

Passivation of donors in electron beam lithographically defined nanostructures after methane/hydrogen reactive ion etching

We show that while the methane/hydrogen gas mixture is capable of etching GaAs and AlGaAs controllably and with little residual damange, it leads to the passivation of donors in both semiconductors. The passivation can, however, be annealed out with a short thermal cycle to 300 °C. The effects of passivation are illustrated and characterized by processing uniform epilayers and quantum wires in n+‐GaAs and modulation doped AlGaAs/GaAs heterostructures.

The etching of silicon carbide in inductively coupled SF6/O2 plasma

The etching mechanisms of silicon carbide in an inductively coupled plasma (ICP) reactor using a SF6/O2 gas mixture, have been investigated using optical emission spectroscopy (OES) and Langmuir probe measurements. The etching is shown to be ion induced with a high degree of anisotropy. An optimum etch rate is achieved with 20% oxygen content within the gas mixture. By studying the independent influence of the ICP power and the substrate bias voltage on the ion current density, as well as the fluorine and oxygen radical densities in the plasma, the etch mechanism is found to be dominated by the number of ions bombarding the SiC surface. The steady state sputter yield observed at P>0.7?Pa, despite the increase in F radical concentration indicates the dominant role of ion bombardment in this etch regime, while at P<0.7?Pa, the etch mechanism is limited by the number of F radicals in the plasma. The OES results have shown that the etch rate is dependent upon the concentration of reactive radicals present with the [F]/[0] ratio = 8 at the optimum. Whilst using the optimum gas composition, the parameters which dominate the physical side of the reaction, ICP power and bias voltage, produce an increase of the etch rate as the potential difference between the substrate and the plasma is increased.

Dry etching of SiC in inductively coupled Cl2/Ar plasma

Inductively coupled Cl2/Ar plasma etching of 4H–SiC has been studied. The SiC etch rate has been investigated as a function of average ion energy, Ar concentration in the gas mixtures, inductively coupled plasma power, work pressure and substrate temperature. The etch mechanism has been investigated by correlating the ion current density and relative atomic chlorine content to the etch rate under various etch conditions. For the first time, it has been found that the etch rate of SiC increases by about 50% at lower substrate temperatures (−80°C) than at high substrate temperatures (150°C) with the highest SiC etch rate of 230 nm min−1 being achieved at a substrate temperature of −80°C.

Thiolation of single-wall carbon nanotubes and their self-assembly

A method for the thiolation of single-wall carbon nanotubes has been developed by exposing a sulfur/carbon nanotubes mixture to an argon/hydrogen gaseous plasma. X-ray photoelectron spectroscopy provides evidence of the existence of sulfur attached to carbon on the carbon nanotubes samples and Raman spectroscopy results show that the carbon nanotubes’ structure has been preserved after the treatment. One added advantage of the reported method is that excess oxygen is not present on the nanotubes. The thiolated carbon nanotubes are shown to self-assemble onto gold electrodes. Our method for thiolating carbon nanotubes provides a reliable and simple way for preparing functionalized tubes for nanoelectronic circuits based on carbon nanotubes.

Fabrication of SiC microelectromechanical systems using one-step dry etching

A simple one-step inductively coupled plasma etching technique has been developed for the fabrication of SiC resonant beam structures. Straight cantilever and bridge devices have been made successfully. The structures have been actuated and resonant frequencies ranging from ∼120 kHz to ∼5 MHz have been measured. Comparison of the theoretically simulated and experimentally measured resonant frequencies shows the presence of significant tensile stress in bridge structures while the cantilever beams are free of stress. The degree of the tension in the bridge structures has been found to be independent of the bridge length.

Characterization of frequency tuning using focused ion beam platinum deposition

This paper presents and characterizes focused ion beam (FIB) platinum (Pt) deposition as a novel frequency tuning method for micromechanical resonators. FIB Pt deposited tuning was performed at room temperature and in contrast to other reported methods, frequency changes were achieved without any device failure. To perform the tuning, Pt was deposited on a 13 µm× 5 µm surface area at the free end of 3C silicon carbide (SiC) and polysilicon cantilever resonators in thicknesses ranging from 0.5 µm to 2.6 µm. To determine the amount of tuning, the resonant frequency of SiC and polysilicon devices was measured before and after Pt deposition. Frequency measurements performed before Pt deposition found that SiC resonators operated at higher resonant frequencies and quality (Q)-factors than their polysilicon counterparts. Measurements after Pt deposition on SiC and polysilicon resonators confirmed the predicted maximum frequency change of −15.5% made by FEM simulations and analytical modelling. Due to their lower mass, polysilicon resonators showed a larger frequency change than their SiC counterparts. A Q-factor decrease was observed for SiC and polysilicon resonators due to thermoelastic damping associated with the deposited Pt and surface contamination.