Hargsoon Yoon

Passive wireless sensors using electrical transition of carbon nanotube junctions in polymer matrix

This paper presents the design and development of a passive wireless sensor for the detection of bio-hazard materials and vapors using chemiresistive thin films. Composite polymer thin film with functionalized carbon nanotubes (f-CNT) and polymethylmethacrylate (PMMA) is employed as a sensing material. It is investigated that resistance change is determined with the concentration of dichloromethane vapors diffused into composite thin film, due to electrical transition from direct contact to tunneling in carbon nanotube nanojunctions. The chemiresistive film is integrated into a passive wireless system which works based on the change in phase of the reflected RF signal. Measurement results of sensors in a wireless sensing system show a large differential phase shift, which can be utilized for remote monitoring of bio-hazard vapors in real time.

Aligned nanowire growth using lithography-assisted bonding of a polycarbonate template for neural probe electrodes

This research presents a fabrication method of vertically aligned nanowires on substrates using lithography-assisted template bonding (LATB) towards developing highly efficient electrodes for biomedical applications at low cost. A polycarbonate template containing cylindrical nanopores is attached to a substrate and the nanopores are selectively opened with a modified lithography process. Vertically aligned nanowires are grown by electrochemical deposition through these open pores on polyimide film and silicon substrates. The process of opening the nanopores is optimized to yield uniform growth of nanowires. The morphological, crystalline, and electrochemical properties of the resulting vertically aligned nanowires are discussed using scanning electron microscopy (SEM), x-ray diffraction (XRD), and electrochemical analysis tools. The potential application of this simple and inexpensive fabrication technology is discussed in the development of neural probe electrodes.

Ku-band antenna array feed distribution network with ferroelectric phase shifters on silicon

This paper presents the design, fabrication, and experimental results of a 1 : 4 monolithic power distribution network for Ku-band array antenna applications. The network integrated on a high-resistivity silicon (HRS) substrate surface stabilized by polysilicon consists of three Wilkinson power dividers, four dc blocking filters, and four coplanar waveguide (CPW)-to-microstrip (MS) transitions. Each output ports are fed with a barium-strontium-titanate phase shifter. It is found that the introduction of the polysilicon layer between the oxide and HRS reduces RF losses significantly, which will enable the monolithic integration of high-power controller modules onto silicon because of the existence of the oxide layer, preventing any degradation of RF performances. The individual components show insertion losses ranging from 0.4 to 2.6 dB at 15 GHz, and the interconnecting CPW lines result in a loss of 0.064 dB/mm. This network was successfully integrated with MS patch antennas monolithically, showing good performance of 32-dB return loss at 14.85 GHz, and 10/spl deg/ beam-steering capability.

Biomechanical Strain Analysis at the Interface of Brain and Nanowire Electrodes on a Neural Probe

The viability of neural probes with microelectrodes for neural recording and stimulation in the brain is important for the development of neuroprosthetic devices. Vertically aligned nanowire microelectrode arrays can significantly enhance the capabilities of neuroprosthetic devices. However, when they are implanted into the brain, micromotion and mechanical stress around the neural probe may cause tissue damage and reactive immune response, which may degrade recording signals from neurons. In this research, a finite-element model of the nanowire microelectrode and brain tissue was developed. A rigid body method was provided, and the simulation efficiency was significantly increased. The interface between the microelectrode and brain tissue was modeled by contact elements. Brain micromotion was mimicked by applying a displacement load to the electrode and fixing the boundaries of the brain region. It was observed that the vertically aligned nanostructures on the electrode of the neural probe do increase the cellular sheath area. The strain field distributions under various physical coupling cases at the interface were analyzed along with different loading effects on the neural electrode.

Low voltage tunable capacitors for RF MEM filters and antenna applications

The design and experimental evaluation of a very low voltage tunable capacitor is presented in this paper. Interdigital capacitors are designed and fabricated on high resistance silicon substrate. Tunable ferroelectric thin film is deposited on to the device and its tunability is measured using an RF network analyzer. It is observed that the capacitor has 80% tunability at 6 volts. In conventional micro fabrication techniques, metal is deposited on the top of a ferroelectric thin film and then develops the device layout by photolithographic and etching technique. However, in this paper, the thin film is deposited on the top of IDT. This helps to reduce the bias voltage and to eliminate the device micro fabrication problems on ferroelectric films. High dielectric constant films can dramatically reduce the device size and it could be possible to achieve desired size of microsensors using these devices. These low voltage tunable ferroelectric films are also useful for the design of phase shifters for phased array antennas and tunable RF MEM filters.

Heterostructured IrO2/Au Nanowire Electrodes and Unit Recordings From Hippocampal Rat Brain

This research presents the development of neural recording electrodes with high sensitivity using nanotechnology and the evaluation of their electrochemical sensing properties. Neural electrodes with heterostructured IrO2/Au nanowires have been fabricated on a flexible needle probe using a polyimide substrate. The outer, functional layer of the nanoelectrodes was made with a thin iridium oxide layer on vertically aligned Au nanowire cores providing low impedance and high charge storage capacity, which can enhance neural sensing and stimulating efficiency. Acute recording from the hippocampus of a rat brain has confirmed the neural sensing functionality of the heterostructured nanowire electrodes.

CPW phase shifter using barium strontium titanate thin film on silicon substrate

In this paper the design and development of a coplanar waveguide phase shifter on high resistivity silicon substrate is presented. This design makes use of BaSrTiO/sub 3/ thin film (50% dielectric tunability) grown on SiO/sub 2//poly-Si by RF sputtering method. A new process flow has been developed to enable full compatibility for direct integration with current CMOS technology. Other important features of this design are the bilateral interdigital structure, and the inclusion of a polysilicon layer for improved performance.

Design and performance of bilateral interdigital coplanar waveguide phase shifter for rf communication applications

A new design of a bilateral interdigital coplanar waveguide (BI-CPW) phase shifter using tunable barium strontium titanate thin film is presented and analyzed. This new design has shown a phase shift of 203°/cm at 25 GHz. Compared to a normal coplanar waveguide (CPW) type, a two-thirds size BI-CPW phase shifter is realized by interdigital capacitive loading. The phase shifter is fabricated with processes fully compatible with current SiGe technology. In addition, it is shown that operating voltages can be reduced significantly within a practical range by the inclusion of a polysilicon layer as a control electrode.

Development of a nanoscale heterostructured glucose sensor using modified microfabrication processes

We discuss the development of a novel amperometric sensor to detect glucose concentrations in solution. Inorganic, vertically aligned nanowire arrays were employed as the sensing electrode in place of planar electrodes to utilize the unique properties of nanostructures, resulting in enhanced sensing signal from a smaller area. Heterostructured gold/platinum nanowires were used so that the dual functions of the nanoelectrodes for covalent immobilization of glucose oxidase and enhanced oxidation of hydrogen peroxide can be achieved using modified microfabrication methods. Two different enzyme immobilization methods—using self-assembled monolayers of alkanethiols and a porous conducting polypyrrole matrix—were investigated as methods for functionalizing the electrodes. Glucose sensing results were compared for planar and nanowire electrodes and the heterostructured nanowire electrodes. The results indicate that the unique structure of the sensing electrode delivers superior sensing performance from a smaller geometric area of electrodes, thus enabling further miniaturization of the sensor.

Nanowire sensor applications based on radio frequency phase shift in coplanar waveguide

This paper presents the design and development of a gas sensor based on phase monitoring of reflected waves at radio frequencies for various dichloromethane vapors. Composite thin film with functionalized carbon nanotubes (f-CNT) and polymethylmethacrylate (PMMA) was employed as a sensing material on a coplanar waveguide and its impedance was monitored for various concentrations. Conductivity change of the composite due to absorption of dichloromethane vapors was clearly observed by resistance measurements. When the f-CNTs/PMMA composite is exposed to dichloromethane with low vapor concentration, phase monitoring of reflected waves from resistive load exhibited higher sensitivity than resistance measurements. With high sensitivity at radio frequency, a wireless gas sensing network integrated with power divider and antenna is introduced.