Wonhyo Kim

Fabrication of Suspended Silicon Nanowire Arrays

A method to fabricate suspended silicon nanowires that are applicable to electronic and electromechanical nanowire devices is reported. The method allows for the wafer-level production of suspended silicon nanowires using anisotropic etching and thermal oxidation of single-crystal silicon. The deviation in width of the silicon nanowire bridges produced using the proposed method is evaluated. The NW field-effect transistor (FET) properties of the device obtained by transferring suspended nanowires are shown to be practical for useful functions.

Well controlled assembly of silicon nanowires by nanowire transfer method

Efforts to date in silicon nanowire research have primarily focused on the nanowire synthesis and the demonstration of individual nanowire-based devices exhibiting interdisciplinary potential spanned from electrical (Duan et al 2003 Nature 425 274?8; Cui and Lieber 2001 Science 291 851?3; Morales and Lieber 1998 Science 279 208?11) through biomedical applications (Cui et al 2003 Science 293 1289?92; Zheng et al 2005 Nature Biotechnol. 23 1294?301). However, the realization of integrated nanowire devices requires well ordered assembly of a silicon nanowire (Huang et al 2001 Science 291 630?3; Whang et al 2003 Nano Lett. 3 1255?9) as well as simple and cost effective fabrication. Here we describe a simple fabrication scheme and a large-scale assembly of silicon nanowires by combining top-down fabrication with nanowire transfer onto another insulator substrate for device manufacture. Our innovative fabrication method enables us to obtain well defined silicon nanowires as a freestanding bridge structure with a diameter of 20?200?nm and a length varying from 5 to 100??m using micro-machining processes. Direct transfer of the freestanding nanowires simply provides large-scale assembly of silicon nanowire on various substrates for highly integrated devices such as high-performance thin-film transistors (TFTs) (Duan et al 2003 Nature 425 274?8; Ishihara et al 2003 Thin Solid Films 427 77?85) and nanowire-based electronics (Cui and Lieber 2001 Science 291 851?3). Electrical transport properties of the transferred silicon nanowire were also investigated.

Quantitative measurements of C-reactive protein using silicon nanowire arrays

A silicon nanowire-based sensor for biological application showed highly desirable electrical responses to either pH changes or receptor-ligand interactions such as protein disease markers, viruses, and DNA hybridization. Furthermore, because the silicon nanowire can display results in real-time, it may possess superior characteristics for biosensing than those demonstrated in previously studied methods. However, despite its promising potential and advantages, certain process-related limitations of the device, due to its size and material characteristics, need to be addressed. In this article, we suggest possible solutions. We fabricated silicon nanowire using a top-down and low cost micromachining method, and evaluate the sensing of molecules after transfer and surface modifications. Our newly designed method can be used to attach highly ordered nanowires to various substrates, to form a nanowire array device, which needs to follow a series of repetitive steps in conventional fabrication technology based on a vapor-liquid-solid (VLS) method. For evaluation, we demonstrated that our newly fabricated silicon nanowire arrays could detect pH changes as well as streptavidin-biotin binding events. As well as the initial proof-of-principle studies, C-reactive protein binding was measured: electrical signals were changed in a linear fashion with the concentration (1 fM to 1 nM) in PBS containing 1.37 mM of salts. Finally, to address the effects of Debye length, silicon nanowires coupled with antigen proteins underwent electrical signal changes as the salt concentration changed.

Development of a carbon nanotube-based touchscreen capable of multi-touch and multi-force sensing

A force sensing touchscreen, which detects touch point and touch force simultaneously by sensing a change in electric capacitance, was designed and fabricated. It was made with single-walled carbon nanotubes (SWCNTs) which have better mechanical and chemical characteristics than the indium-tin-oxide transparent electrodes used in most contemporary touchscreen devices. The SWCNTs, with a transmittance of about 85% and electric conductivity of 400 Ω per square; were coated and patterned on glass and polyethyleneterephthalate (PET) film substrates. The constructed force sensing touchscreen has a total size and thickness of 62 mm × 100 mm × 1.4 mm, and is composed of 11 driving line and 19 receiving line channels. The gap between the channels was designed to be 20 µm, taking visibility into consideration, and patterned by a photolithography and plasma etching processes. The mutual capacitance formed by the upper and lower transparent electrodes was initially about 2.8 pF and, on applying a 500 gf force with a 3 mm diameter tip, it showed a 25% capacitance variation. Furthermore, the touchscreen can detect multiple touches and forces simultaneously and is unaffected by touch material characteristics, such as conductance or non-conductance.

Flexible heartbeat sensor for wearable device

We demonstrate a flexible strain-gauge sensor and its use in a wearable application for heart rate detection. This polymer-based strain-gauge sensor was fabricated using a double-sided fabrication method with polymer and metal, i.e., polyimide and nickel-chrome. The fabrication process for this strain-gauge sensor is compatible with the conventional flexible printed circuit board (FPCB) processes facilitating its commercialization. The fabricated sensor showed a linear relation for an applied normal force of more than 930 kPa, with a minimum detectable force of 6.25Pa. This sensor can also linearly detect a bending radius from 5mm to 100mm. It is a thin, flexible, compact, and inexpensive (for mass production) heart rate detection sensor that is highly sensitive compared to the established optical photoplethysmography (PPG) sensors. It can detect not only the timing of heart pulsation, but also the amplitude or shape of the pulse signal. The proposed strain-gauge sensor can be applicable to various applications for smart devices requiring heartbeat detection.

Fabrication of Silicon Nanowire for Biosensor Applications

Single crystalline silicon nanowire (SiNW) array was fabricated by lithography and oxidation based technology. Uniform and well-defined single crystalline silicon nanowires were obtained, which have sub-100 nm and 5 to 200 mum in diameter and length, respectively, just using conventional micro-machining process such as stepper photolithography, anisotropic etching and thermal oxidation. Our approach to nanowire fabrication simply provides large-scale and well-controlled silicon nanowire array required for nanowire based electronics and biosensor applications without complex and expensive facilities and trivial nanowire assembly process. The contact electrodes for silicon nanowire device were fabricated using sputtered Ti/Au with 50 nm/200 nm thickness and I-V characteristics were recorded. Several types of chemical or biological functional groups were introduced onto silicon nanowire surface. The feasibility for sensor applications of nanowire was investigated.

High Accuracy Open-Type Current Sensor with a Differential Planar Hall Resistive Sensor

In this paper, we propose a high accuracy open-type current sensor with a differential Planar Hall Resistive (PHR) sensor. Conventional open-type current sensors with magnetic sensors are usually vulnerable to interference from an external magnetic field. To reduce the effect of an unintended magnetic field, the proposed design uses a differential structure with PHR. The differential structure provides robust performance to unwanted magnetic flux and increased magnetic sensitivity. In addition, instead of conventional Hall sensors with a magnetic concentrator, a newly developed PHR with high sensitivity is employed to sense horizontal magnetic fields. The PHR sensor and read-out integrated circuit (IC) are integrated through a post-Complementary metal-oxide-semiconductor (CMOS) process using multi-chip packaging. The current sensor is designed to measure a 1 A current level. The measured performance of the designed current sensor has a 16 kHz bandwidth and a current nonlinearity of under ±0.5%.

Comparison of two types of tactile sensing layer in touch screen panel for force sensitive detection

Here we present two types of Touch Screen Panels (TSPs) consisted of silicone gel and glycerin as the transparent Tactile Sensing Layer (TSL) measuring touch force(z axis) and touch position(x-y axis). The principle of the TSP is based on capacitive methods in which the distance between top and bottom substrates is varied by touch or interaction force leading the capacitance change between two substrates. Silicone gel as the TSL showed the force detection resolution of about 50gf and the dynamic range of 0-500gf. For the same test, Glycerin showed the detection resolution of 10gf and the dynamic range of 200gf. These relatively excellent results are caused from the permittivity and hardness of the new two TSL materials.