Hisatoshi Hata

Mushroom plasmonic metamaterial infrared absorbers

There has been a considerable amount of interest in the development of various types of electromagnetic wave absorbers for use in different wavelength ranges. In particular, infrared (IR) absorbers with wavelength selectivity can be applied to advanced uncooled IR sensors, which would be capable of identifying objects through their radiation spectrum. In the present study, mushroom plasmonic metamaterial absorbers (MPMAs) for the IR wavelength region were designed and fabricated. The MPMAs consist of a periodic array of thin metal micropatches connected to a thin metal plate with narrow silicon (Si) posts. A Si post height of 200 nm was achieved by isotropic XeF2 etching of a thin Si layer sandwiched between metal plates. This fabrication procedure is relatively simple and is consistent with complementary metal oxide semiconductor technology. The absorption spectra of the fabricated MPMAs were experimentally measured. In addition, theoretical calculations of their absorption properties were conducted usin...

Low-cost 320x240 uncooled IRFPA using a conventional silicon IC process

A 320 X 240 uncooled IR focal plane array (IRFPA) with series PN junction diodes fabricated on a silicon-on- insulator (SOI) wafer has been developed. Resistive bolometers, pyroelectric detectors and thermopile detectors have been reported for large scale uncooled IRFPAs, while the detector developed uses the temperature dependence of forward-biased voltage of the diode. The diode has low 1/f noise because it is fabricated on the monocrystalline SOI film which has few defects. The diode is supported by buried silicon dioxide film of the SOI wafer, which becomes a part of a thermal isolated structure by using bulk silicon micromachining technique. The detector contains an absorbing membrane with a high fill factor of 90 percent to achieve high IR absorption, and the readout circuit of the FPA contains a gate modulation integrator to suppress the noise. Low cost IRFPA can be supplied because the whole structure of the FPA is fabricated on commercial SOI wafers using a conventional silicon IC process.

Two-million-pixel SOI diode uncooled IRFPA with 15μm pixel pitch

We report the development of a 2-million-pixel, that is, a 2000 x 1000 array format, SOI diode uncooled IRFPA with 15 μm pixel pitch. The combination of the shrinkable 2-in-1 SOI diode pixel technology, which we proposed last year [1], and the uncooled IRFPA stitching technology has successfully achieved a 2-million-pixel array format. The chip size is 40.30 mm x 24.75 mm. Ten-series diodes are arranged in a 15 μm pixel. In spite of the increase to 2-million-pixels, a frame rate of 30 Hz, which is the same frame rate as our former generation (25 μm pixel pitch) VGA IRFPA, can be supported by the adoption of readout circuits with four outputs. NETDs are designed to be 60 mK (f/1.0, 15 Hz) and 84 mK (f/1.0, 30 Hz), respectively and a τth is designed to be 12 msec. We performed the fabrication of the 2-million-pixel SOI diode uncooled IRFPAs with 15 μm pixel pitch, and confirmed favorable diode pixel characteristics and IRFPA operation where the evaluated NETD and τth were 65 mK (f/1.0, 15 Hz) and 12 msec, respectively.

Uncooled IRFPA with chip scale vacuum package

We have developed an uncooled IRFPA with a chip scale vacuum package and succeeded in obtaining excellent IR images of less than 60 mK in NETD. This package consists of a device chip and a silicon lid. The chip in this study is a 160 x 120 SOI diode IRFPA with a 25 μm pixel pitch. The size of the package is 14.5(L) x 13.5(W) x 1.2(H) mm. The gap between the device chip and the lid is controlled by the thickness of the vacuum sealing material. The lid is prepared by a wafer process and diced just before vacuum sealing. We use DLC (diamond like carbon) as the AR coat because of its high IR transmittance and high endurance in the wafer process. DLC films are deposited on both sides of the silicon lid wafer, and then a ring-shaped metal pattern for solder bonding is formed on one side of the lid wafer. Solder is mounted on the metal pattern by a molten solder ejection method. The patterned thin-film getter is formed on the lid wafer. Because of the use of patterned thin-film getter, there is no need to form a cavity on the lid to allow installation of getter or to insert a spacer between the device chip and the lid. Then the lid wafer is diced into individual lids. The device wafer and the lids are set in a vacuum chamber, which has a heater to melt the solder, so as to pair each die and lid. After pumping the chamber, the patterned thin-film getters are activated and then the lids are bonded simultaneously to the device wafer. Finally the device wafer is diced into individual chips. The measured pressure of the package is less than 0.5 Pa which is sufficient for obtaining high thermal isolation. In this technique, only the good dies in a wafer are packaged in chip scale simultaneously. Thus, a reduction in the size and cost of the package has been achieved.

Multi-color imaging with silicon-on-insulator diode uncooled infrared focal plane array using through-hole plasmonic metamaterial absorbers

This paper reports a silicon-on-insulator diode uncooled infrared focal plane array (IRFPA) with through-hole plasmonic metamaterial absorbers (TH-PLMAs) for multi-color imaging with a 320×240 array format. Through-holes formed on the PLMA can reduce the thermal mass while maintaining both the single-mode and high absorption due to plasmonic metamaterial structures, which results in fast response and high responsivity. The detection wavelength of the PLMA with through-holes can be controlled over a wide range of the IR spectrum by varying the size of the micropatches on the top layer.

Pixel scaling for SOI-diode uncooled infrared focal plane arrays

Pixel scaling for SOI diode uncooled infrared focal plane arrays (IRFPAs) was investigated in order to achieve the realization of small size and low cost IRFPAs. Since the SOI diode pixel has two different layers -- one for the temperature sensor and the thermal isolation structure, and the other for the infrared absorption structure -- each layer can be independently designed. Hence, a high fill factor can be maintained when reducing pixel size without changing the basic structure of the pixel, which is advantageous in reducing the pixel size. In order to verify this, the authors have developed an SOI diode IRFPA with the pixel size of 28 μm x 28 μm which is 49% of the previous pixel size (40 μm x 40 μm) and achieved a noise equivalent temperature difference (NETD) of 87 mK. In order to further reduce the pixel size and to improve device sensitivity, we propose a new pixel structure. In this structure, a reflector is fabricated between the infrared absorption structure and support legs. Therefore, the infrared rays which are incident on the support legs, which do not sufficiently function as a reflector, can be used effectively. A new pixel structure with a pixel size of 25 μm x 25 μm was fabricated and realized the thermal conductance of 1.0 x 10-8 W/K and the infrared absorption structure was then verified for its effectiveness.

Novel readout circuit architecture realizing TEC-less operation for SOI diode uncooled IRFPA

We have developed a novel readout circuit architecture realizing a TEC-less (Thermo-Electric Cooler) operation for an SOI diode uncooled infrared focal plane array (IRFPA). Through the fabrication of an SOI diode uncooled 320 x 240 IRFPA adopting the readout circuit architecture with our existing 25μm pixel-pitch technology, we demonstrate that the variation of the output DC level of the pixels is successfully suppressed in environmental temperatures from -10°C to 50°C. The developed TEC-less technology greatly enhances the ability of the SOI diode uncooled IRFPA, which inherently possesses excellent uniformity and low noise features.

High-performance mushroom plasmonic metamaterial absorbers for infrared polarimetric imaging

Infrared (IR) polarimetric imaging is a promising approach to enhance object recognition with conventional IR imaging for applications such as artificial object recognition from the natural environment and facial recognition. However, typical infrared polarimetric imaging requires the attachment of polarizers to an IR camera or sensor, which leads to high cost and lower performance caused by their own IR radiation. We have developed asymmetric mushroom plasmonic metamaterial absorbers (A-MPMAs) to address this challenge. The A-MPMAs have an all-Al construction that consists of micropatches and a reflector layer connected with hollow rectangular posts. The asymmetric-shaped micropatches lead to strong polarization-selective IR absorption due to localized surface plasmon resonance at the micropatches. The operating wavelength region can be controlled mainly by the micropatch and the hollow rectangular post size. AMPMAs are complicated three-dimensional structures, the fabrication of which is challenging. Hollow rectangular post structures are introduced to enable simple fabrication using conventional surface micromachining techniques, such as sacrificial layer etching, with no degradation of the optical properties. The A-MPMAs have a smaller thermal mass than metal-insulator-metal based metamaterials and no influence of the strong non-linear dispersion relation of the insulator materials constant, which produces a gap in the wavelength region and additional absorption insensitive to polarization. A-MPMAs are therefore promising candidates for uncooled IR polarimetric image sensors in terms of both their optical properties and ease of fabrication. The results presented here are expected to contribute to the development of highperformance polarimetric uncooled IR image sensors that do not require polarizers.

160×120 Uncooled IRFPA for Small JR Camera

We have developed a 160 times 120 SOI (silicon on insulator) diode uncooled IRFPA (Infrared Focal Plane Array) with 25 mum pixel pitch for a small IR camera. The IRFPA has a highly responsive pixel structure and is packaged in a chip scale vacuum package (CSVP) in order to reduce the package size. The size of the package is 14.5(L) times 13.5(W) times 1.2(H) mm. An infrared image of less than 60 mK in NETD (Noise Equivalent Temperature Difference) with f/1.0 optics has been obtained by the developed IRFPA.

Performance of new handheld IR camera using uncooled bolometer FPA

A camera using an uncooled infrared image sensor has been developed. This image sensor is a bolometer focal plane array (FPA), of which the readout circuit is designed to minimize the temperature drift or the pattern noise caused by the changes of the ambient temperature. The circuit has a bolometer for the load resistor, which has the same temperature coefficient of resistance as that of the pixel bolometer. Therefore the signal change induced by the temperature change of the FPA substrate is reduced because the resistance change of the load bolometer compensates for that of the pixel bolometer. The effectiveness of the drift- compensating circuit has been confirmed with a prototype handheld camera.