Dara Feili

Flexible organic field effect transistors for biomedical microimplants using polyimide and parylene C as substrate and insulator layers

Biomedical micro implants are used as neural prostheses to restore body functions after paraplegia by means of functional electrical stimulation (FES). Polymer electronic technology offers the potential to integrate flexible electronic circuits on microelectrodes in order to overcome the limit of traditional FES systems. This paper describes an approach of flexible organic transistors in order to develop a flexible biomedical micro implant for FES use. Polyimide shows excellent biocompatibility and biostability properties for flexible multi-channel microelectrodes in neural prosthetics application (Stieglitz et al 1997 Sensors Actuators A 60 240–3). Therefore, it was used as a flexible substrate on which polymer transistors have to be integrated. Gold or platinum was sputtered as the gate, drain and source. In this paper polyimide has been investigated as a gate isolator because of its high flexibility and biocompatibility. Polyimide was spin coated and imidized at different temperatures and times. Pentacene (C14H22) was evaporated at UHV and 75 °C substrate temperature as an active layer in an organic field effect transistor (OFET). Plasma activation and self-assembled monolayer surface modification were used to advance the electrical properties of organic transistors. The whole transistor was encapsulated in parylene C that was evaporated at room temperature using a standard Gorham system (Gorham 1966 J. Polym. Sci. A-1 4 3027–39). Investigation of the electrical properties of the OFET using polyimide as the isolator led to promising results.

Surface plasmon polariton model of high-spatial frequency laser-induced periodic surface structure generation in silicon

In recent years, high-spatial frequency laser-induced surfaces structures have been generated in a large variety of dielectrics. In silicon subwavelength ripples, some of which featured periodicities below 100 nm, were formed using ultrafast lasers. We demonstrate for Si(100) surfaces that generation of a dense electron-hole plasma in the focal spot of ultrashort-pulsed laser light followed by massive excitation of plasma waves provides an explanation for the formation of such high-spatial frequency surface structures. The applied Drude-like model includes carrier-carrier collisions and is in excellent agreement with the experimentally observed ripple period.

Sub-100 nm structuring of indium-tin-oxide thin films by sub-15 femtosecond pulsed near-infrared laser light

In magnetron sputtered indium-tin-oxide thin films of varying oxygen content, nanostructures were formed using tightly focused high-repetition rate near-infrared sub-15 femtosecond pulsed laser light. At radiant exposure well beyond the ablation threshold, cuts of 280-350 nm in width were generated. Illumination close to the ablation threshold resulted in periodic cuts of typically 20 nm in width at periodicities between 50 nm and 180 nm, as well as single sub-20 nm cuts. Subthreshold exposure, in combination with hydrochloric acid etching, yielded nanowires of 50 nm minimum lateral dimensions.

Sub-100 nm material processing and imaging with a sub-15 femtosecond laser scanning microscope

Low mean powers of 1–10 mW are sufficient for material nanoprocessing when using femtosecond laser microscopes. In particular, near infrared 12 fs laser pulses at peak TW/cm2 intensities, picojoule pulse energies, and 85 MHz repetition rate have been employed. Three-dimensional two-photon lithography as well as direct multiphoton ablation have been performed. Subwavelength sub-100 nm cuts have been realized in photoresists, silicon wafers, glass, polymers, metals, and biological targets. When reducing the mean power to the microwatt range, nondestructive two-photon imaging was performed with the same setup taking advantage of the broad laser emission spectrum. Multiphoton microscopes based on low-cost ultracompact sub-20 fs laser sources may become novel nonlinear optical tools for highly precise nanoprocessing and two-photon imaging.

Matrix-addressable, active electrode arrays for neural stimulation using organic semiconductors—cytotoxicity and pilot experiments in vivo

Organic field effect transistors can be integrated into micromachined polyimide-based neural stimulation electrode arrays in order to build active switching matrices. With this approach, a matrix of N x M electrode contacts requires only N + M interconnects to a stimulator when active switching elements are used instead of N x M interconnects. In this paper, we demonstrated that pentacene-based organic field effect transistors (OFETs) can be used to drive stimulation currents through neural electrodes in a physiological-like environment. In order to prove the general applicability as an implant material, the cytotoxicity of pentacene was evaluated with respect to potential effects on cell viability. The results of these tests indicate that extracts from pentacene inhibit neither proliferation nor metabolism of the tested mouse fibroblasts. However, some effect on cell spreading was observed when cells were in direct contact to pentacene for 48 h. In pilot experiments it was demonstrated for the very first time that pentacene transistors can be used as switching elements, acting as voltage-controlled current sources, capable of driving currents suitable for electrical stimulation of a peripheral nerve via a tripolar cuff electrode.

Periodic nanostructures on Si(100) surfaces generated by high-repetition rate sub-15 fs pulsed near-infrared laser light

Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.

Electrical crosstalk in two-port piezoelectric resonators and compensation solutions

Crosstalk is an impediment to electrically interfaced two-port resonators. The overall output function of two-port piezoelectric resonator is a superposition of the mechanical resonance behavior and electrical crosstalk, the latter coming mainly from the coupling feedthrough capacitance. In this paper, two crosstalk compensation solutions have been developed for an aluminum nitride-based doubly clamped beam resonator. The first solution demonstrates an on-chip self-cancellation technique of the feedthrough capacitance by using a compensation electrode and applying a complementary voltage to it, while the second solution applies an adjustable compensation voltage to the common bottom electrode. A specifically designed compensation-readout circuit is presented. Experimental investigations of the output signal have proved the efficiency of both crosstalk compensation solutions.

A MEMS-based hot-film thermal anemometer with wide dynamic measurement range

This paper describes the fabrication and characterisation of a micromachined thermal flow sensor based on platinum thin film resistors. The spatial design and geometric dimensions of the sensing elements result in a very short response time of less than 1 ms and in a very wide measurement range of flow velocities from below 1 m/s up to 180 m/s with a high sensitivity of 72 mV/slm at low flow rates and an acceptable sensitivity of 7.34 mV/slm even at high flow velocities. The heater power was varied from 4.4 to 34.9 mW. The same platinum-resistor is used for heating and sensing and an additional resistor is used for temperature compensation. Geometrically identical resistors were fabricated on silicon wafers as well as on flexible glass substrates and were tested in an air flow. The measured temperature coefficient of resistance (TCR) of the platinum resistor is 0.0027 K-1. A constant temperature anemometer (CTA) setup is used to measure the air flow velocity.

Resonance-mode effect on piezoelectric microcantilever performance in air, with a focus on the torsional modes

A high quality factor is preferred for a microresonator sensor to improve the sensitivity and resolution. In this paper we systematically investigate the performance of the microcantilever in different resonance modes, which are the first three flexural modes, the first lateral mode, and the first and the second torsional modes. An aluminum nitride-based piezoelectric cantilever is fabricated and tested under controlled pressure from an ultra-high vacuum to a normal atmosphere, using a custom-built vacuum chamber. From the experiment results, it can be seen that the torsional modes exhibit better quality factors than those of the flexural and lateral ones. Finally, an analytical model for the air damping characteristics of the torsional mode cantilever is derived and verified by comparing with experimental results.

Investigations on the high temperature compatibility of various adhesion layers for platinum

In this paper we report on the high temperature compatibility of various adhesion layers for plat inum (Pt ) thin films. We investigated different adhesion layers, such as titanium (Ti), tantalum (Ta), aluminium nitride (AlN), aluminium oxide (Al2O3) and titanium oxide (TiO2). All films were deposited on SiO2/Si substrate by using the sputter technique. After deposition the films were annealed in air at 800°C for different time lengths up to 16 h ours. After annealing, Al2O3 and TiO2 showed a dense oxide layer between Pt and SiO2/Si and they seem to be suitable as adhesion layers for Pt at high temperatures. AlN is not suitable as adhesion layer for Pt at high temperatures. Ti and Ta are also not suitable for high temperatures, diffusing strongly into Pt layers and leading to the format ion of oxide precipitates (TiOx or TaOx) in the Pt grain boundaries. In addition, the format ion of Pt-crystallites (hillocks) on the surface was common in all the films.