Andrew Bunting

Low frequency tantalum electromechanical systems for biomimetical applications

The integration of p-channel metal-oxide-semiconductor transistors and tantalum bridge structures for the fabrication of resonant gate transistors (RGTs) that operate in the audible frequency range has been developed. Resonant gate transistors with channel length of 15 μm and clamped-clamped tantalum bridges of 0.5 mm to 1.6 mm in length have been fabricated. The measured first modal frequency of the bridges has been found to be higher than the expected theoretical value. From the experimental and theoretical analysis of the first three modes, the stress in the bridges has been extracted and found to be tensile with values of 3 MPa – 10 MPa. Finite element simulation has validated the extracted stress and the mode shapes of the tantalum bridges. The modulation of conductance in the channel region between the source and drain by the tantalum bridge of the RGT has been demonstrated. The threshold voltage and transconductance of the fabricated p-channel RGT have been measured to be −37 V and 6.84 μS, respect...

Piezoelectrically driven silicon carbide resonators

Silicon carbide cantilever beam resonators have been designed with top electrodes made of piezoelectric lead zirconium titanate (PZT). The devices have been simulated, fabricated, and tested. Piezoelectric actuation has been performed by applying an alternating actuation voltage to the PZT electrodes, thus inducing vertical displacements. The devices have been fabricated with a beam length of 150 and 200 μm, and driven into resonance at frequencies in the kilohertz range. The devices’ resonance has been detected by monitoring the impedance of the actuating electrode. Simulations and measurements have shown that the electrode length on top of the beam influences the magnitude of the deflection and the resonant frequency of the devices. Furthermore, the electrical feedthrough capacitance presented by the piezoelectric electrode has been observed to strongly influence the output impedance of the resonators. The obtained results show the importance of the electrode design for the optimization of the performan...

Microelectromechanical systems for biomimetical applications

An etch release process capable of releasing long resonant gate transistor bridges from a sacrificial layer has been studied as a step towards developing a system to mimic the cochlear mechanism inside the human ear. The developed etch release process involves the use of a gentle etch tool that is capable of a clean and damage-free etch release. The influence of temperature and oxygen/nitrogen gas flow rates on the undercut etch rates and the profiles of photoresist and polyimide sacrificial layers have been investigated. An array of aluminum bridges of length 0.278–1.618 mm, which cover the frequencies from 1 to 33.86 kHz, has been designed and released from a sacrificial layer. The resonating beams have been measured.

Modulating patterned adhesion and repulsion of HEK 293 cells on microengineered parylene-C/SiO2 substrates

This article describes high resolution patterning of HEK 293 cells on a construct of micropatterned parylene-C and silicon dioxide. Photolithographic patterning of parylene-C on silicon dioxide is an established and consistent process. Activation of patterns by immersion in serum has previously enabled patterning of murine hippocampal neurons and glia, as well as the human hNT cell line. Adapting this protocol we now illustrate high resolution patterning of the HEK 293 cell line. We explore hypotheses that patterning is mediated by transmembrane integrin interactions with differentially absorbed serum proteins, and also by etching the surface substrate with piranha solution. Using rationalized protein activation solutions in place of serum, we show that cell patterning can be modulated or even inverted. These cell-patterning findings assist our wider goal of engineering and interfacing functional neuronal networks via a silicon semiconductor platform.

Patterning human neuronal networks on photolithographically engineered silicon dioxide substrates functionalized with glial analogues

Interfacing neurons with silicon semiconductors is a challenge being tackled through various bioengineering approaches. Such constructs inform our understanding of neuronal coding and learning and ultimately guide us toward creating intelligent neuroprostheses. A fundamental prerequisite is to dictate the spatial organization of neuronal cells. We sought to pattern neurons using photolithographically defined arrays of polymer parylene-C, activated with fetal calf serum. We used a purified human neuronal cell line [Lund human mesencephalic (LUHMES)] to establish whether neurons remain viable when isolated on-chip or whether they require a supporting cell substrate. When cultured in isolation, LUHMES neurons failed to pattern and did not show any morphological signs of differentiation. We therefore sought a cell type with which to prepattern parylene regions, hypothesizing that this cellular template would enable secondary neuronal adhesion and network formation. From a range of cell lines tested, human embryonal kidney (HEK) 293 cells patterned with highest accuracy. LUHMES neurons adhered to pre-established HEK 293 cell clusters and this coculture environment promoted morphological differentiation of neurons. Neurites extended between islands of adherent cell somata, creating an orthogonally arranged neuronal network. HEK 293 cells appear to fulfill a role analogous to glia, dictating cell adhesion, and generating an environment conducive to neuronal survival. We next replaced HEK 293 cells with slower growing glioma-derived precursors. These primary human cells patterned accurately on parylene and provided a similarly effective scaffold for neuronal adhesion. These findings advance the use of this microfabrication-compatible platform for neuronal patterning.

Nanoscale electrode arrays produced with microscale lithographic techniques for use in biomedical sensing applications

A novel technique for the production of nanoscale electrode arrays that uses standard microfabrication processes and micron-scale photolithography is reported here in detail. These microsquare nanoband edge electrode (MNEE) arrays have been fabricated with highly reproducible control of the key array dimensions, including the size and pitch of the individual elements and, most importantly, the width of the nanoband electrodes. The definition of lateral features to nanoscale dimensions typically requires expensive patterning techniques that are complex and low-throughput. However, the fabrication methodology used here relies on the fact that vertical dimensions (i.e. layer thicknesses) have long been manufacturable at the nanoscale using thin film deposition techniques that are well established in mainstream microelectronics. The authors report for the first time two aspects that highlight the particular suitability of these MNEE array systems for probe monolayer biosensing. The first is simulation, which shows the enhanced sensitivity to the redox reaction of the solution redox couple. The second is the enhancement of probe film functionalisation observed for the probe film model molecule, 6-mercapto-1-hexanol compared with microsquare electrodes. Such surface modification for specific probe layer biosensing and detection is of significance for a wide range of biomedical and other sensing and analytical applications.

Test structures for characterising the integration of EWOD and SAW technologies for microfluidics

This paper presents details of the design and fabrication of test structures specifically designed for the characterisation of two distinct digital microfluidic technologies: Electro-Wetting On Dielectric (EWOD) and Surface Acoustic Wave (SAW). A test chip has been fabricated that includes structures with a wide range of dimensions and provides the capability to characterise enhanced droplet manipulation as well as other integrated functions. In particular, we detail the use of EWOD to anchor droplets while SAW excitation is applied to perform mixing.

Test Structures for the Characterization of MEMS and CMOS Integration Technology

Test structures have been used to study the feasibility of bonding MEMS to CMOS wafers to create an integrated system. This involves bonding of prefabricated wafers and creating interconnects between the bonded wafers. Bonding of prefabricated wafers has been demonstrated using a chemical-mechanical polishing enabled surface planarization process and an oxygen plasma assisted low temperature wafer bonding process. Two interwafer connection approaches have been evaluated. For an oxide bonding approach, interconnects between wafers are established through contact vias, using a standard multilevel metallization process after the wafer bonding process. Resistances of 3.8-5.2 Omega have been obtained from via chain test structures and an average specific contact resistivity of 1.7 X 10-8 Omega cm2, measured from the single via Kelvin structures. For a direct metal contact approach, electrical connections have been achieved during the bonding anneal stage due to stress relief of the aluminium film.

Test structures for the characterisation of MEMS and CMOS integration technology

Test structures have been used to demonstrate the feasibility of bonding MEMS and CMOS wafers to create an integrated system. This involves using low temperature bonding along with CMP planarisation and wafer thinning. The last step in the integration process is bringing the electrical connections to the top surface and the creation of interconnect between the wafers. Test structures to evaluate this process have been designed and fabricated resulting in 7-9 /spl Omega/ resistances for via chain structures. Via contact resistances of 6 /spl times/ 10/sup -8/ /spl Omega//spl middot/cm/sup 2/ were measured using Kelvin test structures.

The integration of EWOD and SAW technologies for improved droplet manipulation and mixing

This paper details the first reported integration of two advanced digital microfluidic technologies where 100 µm silicon cubes are transported with ElectroWetting On Dielectric (EWOD) and the droplet then held with EWOD while the silicon cubes are mixed with another liquid using a Surface Acoustic Wave (SAW). Together these two technologies provide a comprehensive lab-on-a-chip combination with well developed functionalities. These include droplet generation, splitting and transportation offered by EWOD with transportation, mixing and biosensing being potentially available with SAW. The fabrication of both EWOD and SAW structures on LiNbO3 substrates used low temperature Ta/Ta2O5/CYTOP layer deposition and patterning technologies, which enabled efficient transportation and mixing functions to be demonstrated.