Nam-Ki Min

Comparison of Effective Working Electrode Areas on Planar and Porous Silicon Substrates for Cholesterol Biosensor

Porous silicon-based biosensors were originally developed to further stet the miniaturization of a host of devices. In this paper, we describe the relationship between the enlargement of an electrodes area and its sensitivity for the determination of cholesterol concentrations with covalent binding to immobilized enzymes. For comparison, we conducted a series of experiments using a planar silicon electrode and a porous silicon electrode. We determined the effective surface area of the electrodes using the Randles–Sevcik equation. The active surface area of the planar electrode was approximately 0.1608 cm2, and that of the porous electrode was approximately 0.5054 cm2. Cholesterol oxidase was covalently immobilized on each electrode by silanization. The sensitivities were 0.08567 µA/mM for the planar sensor and 0.2656 µA/mM for the porous sensor. The calculated effective surface area and sensitivity of the porous electrode were about 3.1-fold larger than those of the planar electrode.

Etching Characteristics and Mechanism of InP in Inductively Coupled HBr/Ar Plasma

Investigations of InP etch characteristics and mechanisms in HBr/Ar inductively coupled plasma were carried out. The etch rates of InP and the photoresist were measured as functions of HBr/Ar mixing ratio at fixed gas pressure (5 mTorr), input power (800 W), and bias power (200 W). Langmuir probe diagnostics and zero-dimensional (global) plasma modeling provided the information on plasma parameters, plasma composition, and fluxes of active species. It was found that, with variations in gas mixing ratio, the InP etch rate follows the changes in Br atom density and flux, but shows opposite behavior of the changes in H atoms and positive ions. These findings allow one to conclude that, under a given set of input process parameters, the InP etch process is not limited by the ion-surface interaction kinetics and that Br atoms are the main chemically active species.

Percolated pore networks of oxygen plasma-activated multi-walled carbon nanotubes for fast response, high sensitivity capacitive humidity sensors

We report on the preparation of capacitive-type relative humidity sensors incorporating plasma-activated multi-wall carbon nanotube (p-MWCNT) electrodes and on their performance compared with existing commercial technology. Highly open porous conductive electrodes, which are almost impossible to obtain with conventional metal electrodes, are fabricated by spray-depositing MWCNT networks on a polyimide layer. Oxygen plasma activation of the MWCNTs is also explored to improve the water adsorption of the MWCNT films, by introducing oxygen-containing functional groups on the CNT surface. Polyimide humidity sensors with optimized p-MWCNT network electrodes exhibit exceptionally fast response times (1.5 for adsorption and 2 s for desorption) and high sensitivity (0.75 pF/% RH). These results may be partially due to their percolated pore structure being more accessible for water molecules, expending the diffusion of moisture to the polyimide sensing film, and partially due to the oxygenated surface of p-MWCNT films, allocating more locations for adsorption or attraction of water molecules to contribute to the sensitivity.

Effect of gas mixing ratio on etch behavior of ZrO2 thin films in Cl2-based inductively coupled plasmas

This article reports a study carried out on a model-based analysis of the etch mechanism for ZrO2 thin films in a BCl3∕He inductively coupled plasma. It was found that an increase in the He mixing ratio at a fixed gas pressure and input power results in an increase in the ZrO2 etch rate, which changes from 36to57nm∕min for 0–83% He. Langmuir probe diagnostics and zero-dimensional plasma modeling indicated that both plasma parameters and active species kinetics were noticeably influenced by the initial composition of the BCl3∕He mixture, resulting in the nonmonotonic or nonlinear behaviors of species densities. Using the model-based analysis of etch kinetics, it was demonstrated that the behavior of the ZrO2 etch rate corresponds to the ion-flux-limited etch regime of the ion-assisted chemical reaction.

Fabrication, characterization and application of a microelectromechanical system (MEMS) thermopile for non-dispersive infrared gas sensors

We report on the design, fabrication, and characterization of a non-dispersive infrared (NDIR) gas sensor using an integrated thermopile on a micromachined silicon nitride membrane. The NDIR sensor consists of an optical cavity with new specular reflectors around the light bulb. The multi-layer absorber showed an absorptance of over 90% at 3.3–4.9 µm. The thermopile with this absorber has an output voltage of 144.83 mV at a 5 mW incident power and a sensitivity of 30 V W−1. The sensitivity of the thermopile packaged with a Fresnel lens was 51 V W−1, approximately 1.7 times higher than that of a thermopile with only an absorber. This is due to the decrease in thermal mass and heat loss from a hot junction, and due to the increase in absorptance. Using this newly fabricated thermopile, we developed a small and sensitive NDIR gas detector module for accurate air quality monitoring systems for energy-saving buildings and automotive applications. Our novel sample cavity design is configured to uniformly emit collimated light into the entrance aperture of the cavity to enhance the sensitivity of the NDIR gas detector.

Etching characteristics and mechanism of Ge2Sb2Te5 thin films in inductively coupled Cl2∕Ar plasma

This work reports the investigations of both etch characteristics and mechanisms for the Ge{sub 2}Sb{sub 2}Te{sub 5} (GST) thin films in the Cl{sub 2}/Ar inductively coupled plasma. The GST etch rates and etch selectivities over SiO{sub 2} were measured as functions of the Cl{sub 2}/Ar mixing ratio (43%-86% Ar), gas pressure (4-10 mTorr), and source power (400-700 W). Langmuir probe diagnostics and zero-dimensional (global) plasma modeling provided the information on plasma parameters and behaviors of plasma active species. From the model-based analysis of surface kinetics, it was found that with variations of the Cl{sub 2}/Ar mixing ratio and gas pressure, the GST etch rate follows the changes of Cl atom density and flux but contradicts with those for positive ions. The GST etch mechanism in the Cl{sub 2}-containing plasmas represents a combination of spontaneous and ion-assisted chemical reactions with no limitation by ion-surface interaction kinetics such as physical sputtering of the main material or the ion-stimulated desorption of low volatile reaction products.

A front-side dry-etched thermopile detector with 3–5 μm infrared absorber and its application to novel NDIR CO 2 gas sensors

We present a front-side micromachined thermopile with high sensitivity in the 3-5 mum window, and discuss its application to a novel non-dispersive infrared (NDIR) CO2 gas sensor with a light source emitting collimated light. The micromachined thermopile shows a measured sensitivity of 30 mV/W and a D* of 0.3 times 108 cmradicHz/W. Using this newly fabricated thermopile, we also have successfully developed a small, sensitive NDIR CO2 detector module for accurate air quality monitoring systems in energy-saving building and automotive applications. The novel sample cavity comprising specular reflectors around the light bulb is configured to uniformly emit collimated light into the entrance aperture of the cavity in order to enhance the sensitivity of NDIR CO2 detector.

A locally cured polyimide-based humidity sensor with high sensitivity and high speed

A novel capacitive-type humidity sensor based on polyimide thin films locally cured using MEMS microhotplate was developed. The locally cured polyimide films have been characterized using infrared spectroscopy and compared to those of films cured using a conventional thermal process. The polyimide locally cured at temperature over 350degC for 1 hour was fully cured. There were no significant differences in the polyimide thin films between locally cured on microhotplate and cured in convection ovens. The measured sensitivity for a sensor with a 1400 aring-thick polyimide film is 0.78 pF/%RH. Measurements show an offset drift of less than 1% RH at 50% RH and 37degC, and a hysteresis of <0.6%RH over a range of 25-85% RH, and a time response of 2.5 s. These results can be attributed to both locally-cured polyimide and sensor design and show the possibility of locally-cured polyimide films as high-speed, high-sensitivity humidity sensors.

Single wall carbon nanotube electrode system capable of quantitative detection of CD4+ T cells

Development of CNT-based CD4+ T cell imunosensors remains in its infancy due to the poor immobilization efficiency, lack of reproducibility, and difficulty in providing linear quantification. Here, we developed a fully-integrated single wall carbon nanotube (SWCNT)-based immunosensor capable of selective capture and linear quantification of CD4+ T cells with greater dynamic range. By employing repeated two-step oxygen (O2) plasma treatment processes with 35 days of recovery periods, we achieved the enhanced functionalization of the CNT surface and the removal of the byproduct of spray-coated SWCNTs that hinders charge transfer and stable CD4+ T cell sensing. As a result, a linear electrochemical signal was generated in direct proportion to the bound cells. The slope of a SWCNT electrode in a target concentration range (102~106cells/mL) was 4.55×10-2μA per concentration decade, with the lowest detection limit of 1×102cells/mL. Since the reduced number of CD4+ T cell counts in patients peripheral blood corresponds to the progression of HIV disease, our CD4+ T cell-immunosensor provides a simple and low-cost platform which can fulfill the requirement for the development of point-of-care (POC) diagnostic technologies for human immunodeficiency virus (HIV) patients in resource-limited countries.

Carbon nanotube based electrochemical immunosensors for high-sensitive detection of E. coli

This paper outlines the fabrication and characterization of an immunosensor based on electrodchemical, biological, and nano-material techniques to achieve high sensitive detection of Escherichia coli (E.coli). The working electrode consists of E.coli antibody/antigen/horse-radish peroxidase(HRP) labeled antibody immobilized on plasma-functionalized multiwalled carbon nanotube (pf-MWCNT) film. Immunosensors were characterized by cyclic voltammetry using 10 mM K3Fe(CN)6R/ 3 M KCl and 3,3,5,5-tetramethylbenzidine (TMB). This immunosensor shows very high sensitivity to E. coli O157:H7 and detection limit as low as 0.031 ng/ml. This is 100 times lower than that achievable with standard spectrophotometric enzyme-linked immunosorbent assay (ELISA) using the same immunochemicals.