Il Woong Kwon

A Cantilever-Type Uncooled Infrared Detector With High Fill-Factor and Low-Noise Characteristic

A capacitive microcantilever-type infrared (IR) detector having a unique structure that has high immunity to thermomechanical noise (TM-noise) is proposed. The device has a capacitive readout scheme and is compared with a conventional design using the same readout method by finite element model simulation. The total cantilever length was halved, compared with the conventional device structure, in order to increase the devices spring constant, and the IR absorber area was consequently increased as the portion of the leg in the given pixel area is decreased. Large spring constant and increased absorber area are the main causes of the TM-noise reduction. The feasibility of the device was shown by fabrication, and measured parameters demonstrated the structures superiority. It was shown that the proposed structure potentially has low TM-noise and an overall noise-equivalent temperature difference (NETD) value that is lower than that of the conventional designed device. The NETD of the proposed device was found to be 5.7 mK.

Passivation Effect for the Reduction of 1/ f Noise in Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Thin Films Based on Uncooled Type Microbolometer Applications

In this study, resistance fluctuation or 1/ f noise was measured in thin films of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The 1/ f noise of thin films is much larger than what is typical uncooled type microbolometer sensing material such as vanadium oxide films and amorphous silicon films. To use PEDOT:PSS thin films for a bolometric application, the 1/ f noise should be reduced. This study discusses the effect of passivation for reducing the 1/ f noise.

Properties of reactively sputtered nickel oxide films as a microbolometer sensing material

This study investigates the feasibility of a reactively sputtered thin nickel oxide film for application to a microbolometer. The properties of the developed thin nickel oxide film depend on the sputter process parameters. The measured resistivity of the nickel oxide films ranges from 0.3 Ωcm to approximately 50 Ωcm. Negative Temperature Coefficient of Resistance (TCR) values as high as -3.3%/ °C were acquired. The feasible 1/f noise characteristic was also measured. The magnification of the TCR value and 1/f noise of the nickel oxide films was proportional to the resistivity of the nickel oxide films. Specifically, nickel oxide film with a high resistivity showed a higher TCR value and more 1/f noise. From the measured TCR and 1/f noise values, the theoretically calculated NETD showed a value suitable for use with a microbolometer. Additionally, an analysis of sputtered thin nickel oxide films was conducted through X-ray diffraction.

Internal Bias Field in Ferroelectric Polymer Thin Film for Nonvolatile Memory Applications

Coercive voltages (<i>V</i> _C, the voltage which makes remanent polarization zero in ferroelectrics) of metal-ferroelectric polymer-metal capacitors were measured with different pulse periods. From the measured <i>V</i> _C, coercive fields (<i>E</i> _C, normalized <i>V</i> _C for thickness) and internal bias fields (<i>E</i> _BIAS) were calculated. Although <i>E</i> _C was found to be nearly constant with thickness, <i>E</i> _BIAS increased as thickness decreased. Based on these findings, it appears that <i>E</i> _BIAS can be induced from interface phenomenon and greatly affects retention performance in thin ferroelectric films used for nonvolatile memory devices.

A high fill-factor uncooled infrared detector with low noise characteristic

An uncooled capacitive type bimaterial infrared detector with high fill-factor and improved noise characteristic is investigated. Top electrode is insulated from the substrate thermally as well as electrically. Only small dimension (10μmx2μmx0.2μm) of SiO2 only layer (thermal insulation leg) assures thermal conductance of 1.06x10-7W/K, while keeping the infrared absorber (top electrode) separated from the bias signal. Due to the decreased thermal isolation leg length, high fill-factor of 0.77 is achieved. The bimaterial leg that connects the infrared absorber to the thermal insulation leg is a 38μm long cantilever structure composed of Al and SiO2 bi-layer, which has large difference in the thermal expansion coefficient (Al:25ppm/K and SiO2:0.35ppm/K). Bimaterial leg length (38μm) is quite shorter than the previously designed device, resulting in the decreased bending of the bimaterial leg. However, the increased fill-factor reduces temperature fluctuation noise term that is inversely proportional to the absorber area, and it is found by FEM simulation that the enhanced mechanical properties such as spring constant reduce the thermo-mechanical noise term of the proposed device.

A high fill-factor uncooled infrared detector with thermo-mechanical bimaterial structure

By adopting new capacitance reading scheme, a capacitive type uncooled infrared detector structure with high fill-factor and effectively controllable thermal conductance is proposed. Instead of conventional MEMS capacitor structure (i.e. an insulating gap between top and bottom electrodes), a capacitor with a floating electrode and two bottom electrodes has been applied to the infrared detector. Infrared absorber which also acts as the floating electrode of the capacitor is connected to the substrate via two bimaterial legs. These legs consist of two materials having large difference in thermal expansion coefficient (Al: 25ppm/K and SiO2: 0.35ppm/K), so that the legs are deflected according to the certain temperature change due to the infrared absorption. This legs movement results in the displacement of the top electrode of the capacitor, and infrared is sensed by measuring the capacitance change. However, the one end tip of the bimaterial leg does not contain Al and consist of SiO2, solely. This leg design enables the absorber to be separated from the substrate thermally as well as electrically, because insulators usually have low thermal conductivity than metals more than an order. The capacitance change by the result of infrared absorption is read only through two bottom electrodes which are placed right under the absorber, and also perform as infrared reflectors. The design has advantages of enlarging fill-factor of the infrared detector, effective thermal conductance controlling and high sensitivity to IR. With only small dimensions of SiO2 (10μm x 2μm x 0.2μm), the device can have low thermal conductance of 1.3x10-7W/K, so that the portion of the legs can be reduced in a pixel area. The device has fill-factor of 0.77 and 14%/K of sensitivity to infrared rays concerning 1~2K of temperature difference between the structure and the substrate.

Thin nickel oxide films for micro-bolometers

This study represents an investigation of the feasibility of thin nickel oxide film (~100nm in thickness) as a microbolometer material. Thin nickel oxide film was obtained by a heat treatment (below 400 °C) of DC-sputtered Ni film on a SiO2/Si substrate in an O2 environment. Using a parameter analyzer (4156A) with a TEC temperature controller, a spectrum analyzer and a low noise amplifier, a systemic analysis of the electrical and noise characteristics of nickel oxide film is performed. A negative temperature coefficient of resistance (TCR) value of 3.28%/oC and a feasible 1/f noise result ranging from 1Hz to 100Hz were acquired. The characteristics of the thin nickel oxide film obtained in this study are comparable to those of a-Si. Moreover, the nickel oxide thin film retained a stable state at room temperature. Thus, the thin nickel oxide, which is CMOS-compatible and yields high TCR values and proper 1/f noise characteristics through a simple fabrication process, is shown to be a promising micro-bolometric material.