Andrew S. Bunting

An investigation of the influence of deposition and post-fabrication processes on the final mechanical structure of tantalum beams is reported in the present study. The final deflection profiles of doubly supported beams made from compressive and tensile-stressed films have been studied experimentally. An optimum finite element model has been developed to predict the buckling behaviour of the doubly supported beams by considering the boundary conditions in the form of a compressive stress and an applied load. No matter which etch release method has been used, the initial stress state of the as-deposited films has been found to have a significant impact on the final deflection profile of the fabricated device. The compressive-stressed films have presented larger deflection in the final released beams than the tensile-stressed films. Taking into account the type of etch release methods, the beams that have been released in the dry etch release processes have been found to deform more vertically than those released in the wet-etch release method. Moreover, it has been found that the amplitude of vertical deflection increases with the increase of the beam length and thickness. The results indicate that optimum profiles of tantalum suspended structures can be obtained from the tensile-stressed films that have been released by the wet etching method with an aspect ratio of 1:48.

Conductive carbon films are highly attractive for use as electrodes in electrochemistry and biosensing applications. Patterned photoresist films can be transformed into carbon electrodes using standard photolithographic techniques followed by pyrolysation of the photoresist in a furnace under a reducing atmosphere. Previous studies have been made of the electrical properties of blanket carbon films created using this method of fabrication. However, there is a need to investigate pattern dependent effects, particularly the extent to which the dimensions of the patterned films shrink during the high temperature processing. This study applies microfabricated test structures to the process characterisation of conductive carbon produced from standard positive photoresists.

The enhancement of the second flexural mode in a monolayer graphene resonator by an inhomogeneous electrostatic actuation force has been observed. The devices have been fabricated by transferring the graphene onto a poly‐Si/SiO ₂ /Si substrate whereby the poly‐Si has been released to produce graphene resonators. Enhancement of the second harmonic has been demonstrated by varying the actuation voltage, achieving an amplitude enhancement of up to 95% of the fundamental mode. The reported findings open new perspectives for graphene resonant sensors with enhanced sensitivity.

Electrodeposited alloys of nickel and iron are prone to suffer from the development of undesirable-stress gradients resulting from process steps and heat treatments following deposition. As a result, the yield associated with devices using permalloy (Ni-Fe, 80%-20%) films can be severely compromised. Although electrodeposition recipes have been formulated that greatly reduce stress in Ni-Fe films, any stress gradients through the thickness of permalloy can pose considerable problems during the release of cantilevers. The development of such gradients is typically the direct result of the selected design and process architecture and/or problematic material choices. This paper presents an improved architecture for electroplated Ni-Fe freestanding microcantilever structures. This architecture minimizes the development of stress gradients in cantilever beams and facilitates the use of suspended permalloy cantilever structures in magnetically actuated microelectromechanical systems (MEMS) switches.

Cell patterning platforms support broad research goals, such as construction of predefined in vitro neuronal networks and the exploration of certain central aspects of cellular physiology. To easily combine cell patterning with Multi-Electrode Arrays (MEAs) and silicon-based ‘lab on a chip’ technologies, a microfabrication-compatible protocol is required. We describe a method that utilizes deposition of the polymer parylene-C on SiO2 wafers. Photolithography enables accurate and reliable patterning of parylene-C at micron-level resolution. Subsequent activation by immersion in fetal bovine serum (or another specific activation solution) results in a substrate in which cultured cells adhere to, or are repulsed by, parylene or SiO2 regions respectively. This technique has allowed patterning of a broad range of cell types (including primary murine hippocampal cells, HEK 293 cell line, human neuron-like teratocarcinoma cell line, primary murine cerebellar granule cells, and primary human glioma-derived stem-like cells). Interestingly, however, the platform is not universal; reflecting the importance of cell-specific adhesion molecules. This cell patterning process is cost effective, reliable, and importantly can be incorporated into standard microfabrication (chip manufacturing) protocols, paving the way for integration of microelectronic technology.

This paper reports the application of test structures to the evaluation of tantalum nitride (Ta-N) as a material for integration with high temperature electronics. The test structure fabrication involves the reactive sputtering of Ta-N and its consequent annealing in a vacuum to reach the target specifications of low temperature coefficient of resistance (TCR). A test wafer has been designed to both evaluate the temperature performance of thin films and to study the possibility of integrating different metal films into a single sensing device. The Ta-N resistors resulting from this work have a TCR of -150 ppm/°C which remains stable after 6 hours of annealing at 600°C.