W.I. Milne

Is stress necessary to stabilise sp3 bonding in diamond-like carbon?

Abstract The role of compressive stress in producing sp3 bonding in diamond-like carbon is of interest both technologically and scientifically. Stress limits the maximum thickness of adherent films, and it is desired to produce much thicker films for protective coatings and for making micro-electromechanical systems. Stress is important theoretically, because it is often linked to the deposition process. A strong correlation between macroscopic stress and sp3 fraction in diamond-like carbons has been noted, particularly for tetrahedral amorphous carbon (ta-C). However, a survey of data shows that a given stress produces films with sp3 contents between 20 and 85%, while for a given sp3 content, stresses between 2 and 19 GPa have been found. We propose that the main cause of stress is ion bombardment, and that a low energy of only 20 eV/ion is needed to produce films with an sp3 content over 70%. We discuss the various models linking stress and the sp3 fraction in ta-C. The role of densification vs. compressive stress in stabilising sp3 bonding is also discussed.

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Abstract Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm3) and sp3 content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Youngs modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Youngs modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm2, producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.

Development of all metal electrothermal actuator and its applications

The in-plane motion of microelectrothermal actuator (heatuator) has been analysed for Si-based and metallic devices. It was found that the lateral deflection of a heatuator made of a Ni-metal is about -60% larger than that of a Si-based actuator under the same power consumption. Metals are much better for thermal actuators as they provide a relatively large deflection and large force, for a low operating temperature, and power consumption. Electroplated Ni films were used to fabricate heatuators. The electrical and mechanical properties of electroplated Ni thin films have been investigated as a function of temperature and plating current density, and the process conditions have been optimised to obtain stress-free films suitable for MEMS applications. Lateral thermal actuators have been successfully fabricated, and electrically tested. Microswitches and microtweezers utilising the heatuator have also been fabricated and tested.

Optimisation of amorphous zinc tin oxide thin film transistors by remote-plasma reactive sputtering

The influence of the stoichiometry of amorphous zinc tin oxide (a-ZTO) thin films used as the semiconducting channel in thin film transistors (TFTs) is investigated. A-ZTO has been deposited using remote-plasma reactive sputtering from zinc:tin metal alloy targets with 10%, 33%, and 50% Sn at. %. Optimisations of thin films are performed by varying the oxygen flow, which is used as the reactive gas. The structural, optical, and electrical properties are investigated for the optimised films, which, after a post-deposition annealing at 500 °C in air, are also incorporated as the channel layer in TFTs. The optical band gap of a-ZTO films slightly increases from 3.5 to 3.8 eV with increasing tin content, with an average transmission ∼90% in the visible range. The surface roughness and crystallographic properties of the films are very similar before and after annealing. An a-ZTO TFT produced from the 10% Sn target shows a threshold voltage of 8 V, a switching ratio of 108, a sub-threshold slope of 0.55 V dec−1...

Room temperature sputtering of inclined c-axis ZnO for shear mode solidly mounted resonators

ZnO films with a c-axis significantly inclined away from the surface normal were grown by a remote plasma sputtering technique at room temperature. The films were used to make solidly mounted resonators (SMRs) operating in shear mode at a resonant frequency of 1.35 GHz. Control of the ZnO microstructure was achieved using a polycrystalline AlN seed layer which can be added on top of a sputtered acoustic mirror to give a complete SMR device. The ZnO was reactively sputtered in an atmosphere of argon and oxygen from a zinc target. The c-axis of the ZnO was estimated to be at an angle of ∼45° to the surface normal. SMRs were measured to have quality factors (Q) of up to 140 and effective electromechanical coupling coefficients of up to 2.2% in air. Although an inclined c-axis can be achieved with direct growth onto the acoustic mirror, it is shown that the AlN seed layer provides higher coupling coefficients and narrower inclination angular distribution. The responses of the devices in liquids of different v...

Integrated ZnO Film Based Acoustic Wave Microfluidics and Biosensors

Lab-on-a-chip (LOC) is one of the most important microsystems with promising applications in microanalysis, drug development and diagnosis, etc. We have been developing a LOC biodetection system using acoustic wave as a single actuation mechanism for both microfluidics and biosensing using low cost piezoelectric ZnO film. Surface acoustic waves (SAW) coupled into the liquid will induce acoustic streaming, or move the droplet on the surface. These have been utilized to make SAW-based micropumps and micromixers which are simple in structure, easy to fabricate, low cost, reliable and efficient. SAW devices and thin film bulk acoustic resonators (FBAR) have been fabricated on nanocrystalline ZnO thin films deposited using sputtering on Si substrates. A streaming velocity up to ~5cm/s within a microdroplet and a droplet moving speed of ~1cm/s have been achieved. SAW based droplet ejection and vaporization have also been realized. SAW devices and FBARs have been used to detect antibody/antigen and rabbit/goat immunoglobulin type G molecules, showing their high sensitivity. The results have demonstrated the feasibility of using a single actuation mechanism for the LOC.

Dual ion plasma-beam sources used to maximise sp3 C–C bonds in carbon nitride

Amorphous carbon nitride films grown by plasma or ion sources never achieve the limit of β-C 3 N 4 , because the nitrogen fraction saturates below 57% and the carbon tends to become sp 2 -bonded at high N content. When a-CN x is grown from a single ion or plasma beam, high nitrogen pressures are needed to promote higher N contents, but this leads to a loss of ionisation. We use a dual ion beam method to grow a-CN x with a filtered cathodic arc (FCVA) to supply carbon and a low pressure, high plasma density electron cyclotron wave resonance (ECWR) source to supply atomic nitrogen ions. The film composition and fraction of unsaturated π * bonding at C and N sites was measured by electron energy loss spectroscopy. We find that the C sp 3 fraction decreases linearly with nitrogen content, rather than showing the sharp fall at N/C = 0.08-0.1 found by others. Thus, we achieve the highest C-C sp 3 content for a-CN x films with N/C > 0.1. We find a rather sharp increase in the fraction of empty N π * states, from 10 to 30% at N/C = 0.2. Whereas previous work suggests that N contents above the critical value of 0.1 induce a transition for all C sites to sp 2 , in our results only those C sites bonded to N revert to sp 2 . The film density was observed to change in a similar way to the density of ta-C films with respect to the C sp 2 fraction. However, strong differences in the optical gap are observed.

Acoustic Wave Based Microfluidics and Lab-on-a-Chip

Microfluidics refers to a set of technologies that control the flow of minute amounts of liquids, typically from a few picolitres (pls) to a few microlitres (μls) in a miniaturized system [1,2]. Lab-on-a-chip (LOC) systems typically consist of a set of microfluidics and sensors with dimensions from a few square millimetres (mm2) to a few square centimetres (cm2). Microfluidics handles liquids through droplet generation, transportation and mixing of liquid samples, chemical reactions etc. Sensors may include biochemical sensors, gas sensors and physical sensors such as humidity and temperature sensors, flow meter and viscometers etc. Therefore, LOCs are microsystems with a much broader meaning, and generally perform single or multiple laboratory processes and functions on a chip-scale.

Electrochemical communication with the inside of cells using micro-patterned vertical carbon nanofibre electrodes

With the rapidly increasing demands for ultrasensitive biodetection, the design and applications of new nano-scale materials for development of sensors based on optical and electrochemical transducers have attracted substantial interest. In particular, given the comparable sizes of nanomaterials and biomolecules, there exist plenty of opportunities to develop functional nanoprobes with biomolecules for highly sensitive and selective biosensing, shedding new light on cellular behaviour. Towards this aim, herein we interface cells with patterned nano-arrays of carbon nanofibers forming a nanosensor-cell construct. We show that such a construct is capable of electrochemically communicating with the intracellular environment.

Anomalous improved electron field emission from hybridised graphene on Mo tip arrays

A new, efficient electron field emitter geometry, based on monolayer graphene coated well aligned Mo tip arrays, is here reported. The rather anomalous, yet nonetheless beneficial contributions of this hybridized nanostructured film morphology is evaluated and discussed. Efficient and stable field emission with low turn on fields has been observed. Incorporation of graphene and Mo tip array results in noteworthy improvements in emission of these nanoscale heterostructure devices.