Chi Ho Hwang

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.

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.

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.

High-Performance Pixelwise Readout Integrated Circuits for Microbolometer

In this paper, a pixelwise readout structure for a microbolometer is proposed. A current mirror gate modulation (CM) is employed as an input circuit to archive high responsivity and a current mode background suppression is used to reduce the integration capacitance. As a long integration time of more than 1 ms can be obtained using this pixelwise readout structure, the noise equivalent temperature difference (NETD) is 26 mK, which is much better than that of the conventional circuits, 67 mK.

A high SNR readout circuit design for TDI array with adaptive charge capacity control

In this paper, a novel high SNR readout circuit for a satellite TDI array is presented. Since an input range of an IR image for environmental satellites is broad and especially the cloud top temperature (CTT) that is important in understanding phenomena of atmosphere is quite low, the readout of low temperature signal is important in satellite applications. However, the noise resulted from a readout circuit is no longer ignorable compared to a detector shot noise at low IR radiation. Hence, an adaptive charge capacity control method is proposed in this paper for an improved SNR at low temperature. It is found that SNR is improved as much as 11dB at 200K and 90% background-limited infrared photodetection (BLIP) condition is satisfied over a total input range by simulation.