Arash A. Fomani

Metastability mechanisms in thin film transistors quantitatively resolved using post-stress relaxation of threshold voltage

A new approach is presented to resolve bias-induced metastability mechanisms in hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). The post stress relaxation of threshold voltage (VT) was employed to quantitatively distinguish between the charge trapping process in gate dielectric and defect state creation in active layer of transistor. The kinetics of the charge de-trapping from the SiN traps is analytically modeled and a Gaussian distribution of gap states is extracted for the SiN. Indeed, the relaxation in VT is in good agreement with the theory underlying the kinetics of charge de-trapping from gate dielectric. For the TFTs used in this work, the charge trapping in the SiN gate dielectric is shown to be the dominant metastability mechanism even at bias stress levels as low as 10 V.

Giant Magneto-Impedance Thin Film Magnetic Sensor

A thin film sensor for sensing magnetic fields bellow 10 Gausses is designed and fabricated based on a giant magneto-impedance (GMI) structure. Analytical equations describing GMI effect have been employed to design the sensor over the designated magnetic field and signal frequency. The GMI multilayer is comprised of an Au layer (200 nm) sandwiched between two Co73Si12B15 magnetic layers (400 nm). Various structures with different shapes and ratios have been studied to achieve the optimum performance. The sensors are fabricated on a glass wafer employing thin-film micro-fabrication processes. The paper also presents a new post-processing step to magnetize the GMI multilayer. The impedance of the GMI sensors as a function of external magnetic and excitation frequency is reported and discussed. The sensor has been utilized to change the state of an On/Off circuit in the presence/absence of magnetic field.

Monolithically Integrated Multiport RF MEMS Switch Matrices

The design methodology and performance of a miniature-size monolithically integrated RF microelectro-mechanical systems switch matrix is reported. The switch matrix has the form a of cross-bar configuration that can be easily expanded to realize a large size switch matrix. Three single-pole single-throw RF switches coupled to coplanar waveguide transmission lines are employed to construct the unit cell with dimensions of only 320 × 320 ¿m2. The compact design of the proposed cell permits the high frequency operation of large switching networks. A six-mask fabrication process has been developed to construct the entire structure on a single side of the wafer. The impact of bias line resistance on the RF performance and the switching speed of the devices were studied. An excellent RF performance is achieved for a fabricated 4×4 switch matrix using high-resistive phosphorous-doped hydrogenated amorphous silicon semiconductor as the material of choice for the biasing lines. Over a frequency range from DC to 40 GHz, the worstcase measured results obtained for the insertion loss, return loss, and isolation are -1.8, -17, and 26 dB, respectively. A wide-band operation is predicted for an 8 × 8 switch matrix version constructed from 64 switching units.

Monolithic MEMS T-type Switch for Redundancy Switch Matrix Applications

This paper presents a novel approach to monolithically implementing RF MEMS T-type switches for redundancy switch matrix applications. The T-type switch performs three operational states: two turning states and one crossover state. A six-mask fabrication process is adapted to fabricate the proposed design. Novel RF circuits were used to implement the entire system, including series contact cantilever beams, RF crossover, 90 degree turns and four-port cross junctions. The measured results for the entire T-type switch demonstrate an insertion loss of 1.5 dB, a return loss of better than -20 dB and an isolation higher than 28 dB for all states for frequencies up to 30 GHz. To our knowledge, this is the first time an RF MEMS T-type switch has ever been reported.

Miniature RF MEMS switch matrices

A novel miniature-size switching unit is reported for application as the building block of multiport switch matrices. The cell consists of 3 cantilever-beam contact type MEMS devices coupled to CPW transmission lines. A major feature of the proposed switch cell is that in each of the operating states there is only one MEMS switch located in the path of signal inducing a similar loss for all switching states. The construction of the entire system is carried out by a six-mask fabrication process. To minimize the unwanted coupling of the RF signal through the bias lines of MEMS devices, high-resistive phosphorous-doped hydrogenated amorphous silicon (n+ a-Si:H) is selected as a material of choice for the dc bias lines. The switching unit has been employed to build a 4×4 switch matrix measuring 1.45 × 1.45 mm2 in dimensions. The system presents an excellent RF performance with the worst-case insertion loss, return loss, and isolation of −1.8dB, −17dB and 26dB up to 40GHz, respectively.

Multilayer Giant Magneto-Impedance sensor for low field sensing

In this paper, several Giant Magneto Impedance (GMI) magnetic sensors have been designed, fabricated, post processed and tested to work in low field intensities (miliTesla range). The sensors are multilayer GMI sensors having CoSiB as GMI material surrounded with two thinner gold layers. The conventional thin film microfabrication process is employed to fabricate the sensors on a glass wafer. A post-processing thermal and magnetic treatment is suggested to magnetize GMI material and enhance performance of the sensors. The suggested post-processing step will decrease fabrication cost of GMI sensors and improve their performance effectively.

Development and Characterization of Multisite Three-Dimensional Microprobes for Deep Brain Stimulation and Recording

Novel 3-D multichannel microprobes are presented for deep brain stimulation and recording applications. The microprobes offer independent electrode sites around the shaft of the implant, providing the capability to control the profile of injected charge into the tissue. The devices are composed of planar flexible microprobes folded over cylindrical polyurethane shafts with diameters as small as 750 μm . A dramatic enhancement in the density/number of channels and a precise control over the dimensions of the electrode sites are achieved using this approach. The fabricated devices host 16 stimulating and 16 recording channels. The impedance characteristics and long-term behavior of electrodes were studied in acidic and saline solutions under prolonged pulse stress tests. To enhance the charge delivery capacity or reduce the impedances of the channels, iridium (Ir) was electroplated on gold electrode sites. Both Ir and gold channels demonstrate stable characteristics after pulse stress tests longer than 100 million cycles. The in vitro experiments in the whole hippocampus of a C57BL/6 mouse demonstrate the potential application of fabricated microprobes in simultaneous neural stimulation and recording.

Effect of gate dielectric scaling in nanometer scale vertical thin film transistors

A short channel vertical thin film transistor (VTFT) with 30 nm SiNx gate dielectric is reported for low voltage, high-resolution active matrix applications. The device demonstrates an ON/OFF current ratio as high as 109, leakage current in the fA range, and a sub-threshold slope steeper than 0.23 V/dec exhibiting a marked improvement with scaling of the gate dielectric thickness.

Flexible neural microelectrode arrays reinforced with embedded metallic micro-needles

Neural microprobes with 16 stimulation/recording channels are fabricated employing surface micromachining techniques. Each device integrates a flexible polyimide-based interconnection cable, the array of bonding pads, and 4 parallel shanks hosting the electrode sites, all constructed in a single 3-mask fabrication process. In order to provide the shanks with enough mechanical strength for insertion into the tissue, a 15 µm thick gold micro-needle is embedded inside each shank as the reinforcement structure. A polyimide shell encapsulates the micro-needles from electrode sites and the interconnection lines. The shanks are 4 mm long, 100 µm wide at the base, and 24 µm thick. The electrode sites measuring 20 µm × 20 µm are addressed through gold interconnection lines to reduce the electrodes impedances and consequently improve the signal to noise ratio of the recording signal. This paper presents the details of microprobe fabrication and the preliminary characterization results.

Distributed Phase Shifter with Enhanced Variability and Impedance Matching

A reconfigurable phase shifter has been developed with enhanced phase variation without compromising the impedance matching performance. The phase shifter is based on a distributed MEMS transmission line (DMTL). A slow-wave structure is coupled with the DMTL to improve the phase shift and to reduce the dimensions of the device. The optimisation process has been performed using a lumped element model extracted from electromagnetic simulations. The proposed structure has been studied in order to validate the theoretical analysis. A significant increase of 2 times in the effective dielectric constant improves the phase variability by 42% compared to a traditional design with a similar MEMS capacitive ratio. The proposed concept offers an attractive solution for applications demanding low return loss and a wide degree of phase variability.