We developed silicon-on-insulator (SOI) diode-based uncooled infrared focal plane arrays (IRFPAs), in which single-crystal pn junction diodes formed in an SOI layer are used as temperature sensors. These diodes, based on silicon large-scale integration technology, offer excellent uniformity, and have led to the use of high-performance uncooled IRFPAs in a wide variety of applications. In order to extend the pitch to less than 12 μm, a scalable new pixel structure has been developed to reduce the pixel size, based on a novel thermally isolated structure, which is fabricated above a CMOS processed wafer. The pn junction diodes used as a temperature sensor are separated from the underlying substrate by supporting legs made from thin metal wire, forming a cavity. To reduce the pixel size, we are developing a new diode structure by optimizing the ion implantation condition, thinning the SOI layer, and redesigning the supporting legs, achieving a smaller pixel size even with ten serially connected diodes. We also evaluated a new readout circuit architecture that enables an increase in sensitivity by generating a larger change in the diode forward voltage at a given temperature with no change in the number of diodes in the SOI layer. The effectiveness of the proposed readout circuit architecture was verified using a fabricated test element. The sensitivity of the test element was 128% of that for existing circuit structures, and further increases are expected with circuit structure optimization. These techniques have greatly enhanced the performance of our SOI diode based uncooled IRFPAs.
A three-dimensional plasmonic metamaterial absorber (3-D PMA) was theoretically investigated and designed for the performance enhancement of wavelength selective uncooled infrared (IR) sensors. All components of the 3-D PMA are based on thin layers of plasmonic metals such as Au. The post produces a narrow gap, such as a few hundred nanometers, between the micropatch and the metal plate. The absorption properties of the 3-D PMA were investigated by rigorous coupled-wave analysis. A strong wavelength selective absorption is realized by the plasmonic resonant mode of the micropatch and the narrow-gap resonant mode between the micropatch and the plate. The disturbance of the post for both resonance modes is negligible. The absorption wavelength is defined mainly by the size of the micropatch, regardless of the micropatch array period and is longer than the micropatch array period. The absorption mode can also be controlled by the shape of the micropatch. Through-holes can be formed on the plate area, where there is no gap resonance to the micropatch. The thickness of each component can be reduced considering the skin depth effect and there is no added absorption of materials such as SiO2. A small pixel size with reduced thermal mass can be realized using a 3-D PMA structure. The results obtained here will contribute to the development of high-performance uncooled IR sensors for multicolor imaging.
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 ,
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.
Scalable new SOI diode structure has been proposed and developed for beyond 17μm pixel pitch mega-pixel-class SOI
diode uncooled infrared focal plane arrays (IRFPAs). Conventionally, each p+n vertical diode is formed between a p+diffusion and an n-body in each SOI active area, and 8-10 diodes are serially connected with interconnections. In the
proposed new structure, we employ two kinds of diodes, namely, p+n and n+p vertical diodes. First, two regions of an nbody
and a p-body are prepared in an SOI active area. In the n-body, a p+ diffusion is formed apart from the n-body /pbody
boundary. In the p-body, an n+ diffusion is formed apart from the boundary. In this way, a p+n vertical diode and an
n+p vertical diode are formed together in an SOI active area. Moreover, a contact hole, which is formed in touch with
both n- and p-bodies, electrically connects these two kinds of diodes. With this new structure which is named "new 2-in-
1 SOI diode structure", we have realized remarkable reduction of the diode area. It leads to significant increase of the
diode series number in a pixel, which increases infrared responsivity of the pixel. As a result, designing a 15μm pixel
pitch IRFPA with the new structure, 12 series diodes can be arranged in a pixel, although 10 series diodes have been
used even in the case of our 25μm pitch generation pixel.
To confirm the ability of the new diodes, test elements of 12-17μm pitch pixels were fabricated and evaluated.
Furthermore, the fabrication of 17μm pixel pitch 320 x 240 IRFPAs with the new diodes was carried out and their
favorable FPA operations were successfully verified.
In conclusion, the proposed and developed new SOI diode technology is very promising for beyond 17μm pixel pitch
mega-pixel-class uncooled IRFPAs.
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.
An uncooled infrared focal plane array (IR FPA) is a MEMS device that integrates an array of tiny thermal infrared
detector pixels. An SOI diode uncooled IR FPA is a type that uses freestanding single-crystal diodes as temperature
sensors and has various advantages over the other MEMS-based uncooled IR FPAs. Since the first demonstration of an
SOI diode uncooled IR FPA in 1999, the pixel structure has been improved by developing sophisticated MEMS
processes. The most advanced pixel has a three-level structure that has an independent metal reflector for interference
infrared absorption between the temperature sensor (bottom level) and the infrared-absorbing thin metal film (top level).
This structure makes it possible to design pixels with lower thermal conductance by allocating more area for thermal
isolation without reducing infrared absorption. The new MEMS process for the three-level structure includes a XeF2 dry
bulk silicon etching process and a double organic sacrificial layer surface micromachining process. Employing
advanced MEMS technology, we have developed a 640 x 480-element SOI diode uncooled IR FPA with 25-μm square
pixels. The noise equivalent temperature difference of the FPA is 40 mK with f/1.0 optics. This result clearly
demonstrates the great potential of the SOI diode uncooled IR FPA for high-end applications. In this paper, we explain
the advances and state-of-the-art technology of the SOI diode uncooled IR FPA.
This paper describes the structure and performance of a 25-micron pitch 640 x 480 pixel uncooled infrared focal plane array (IR FPA) with silicon-on-insulator (SOI) diode detectors. The uncooled IR FPA is a thermal type FPA that has a temperature sensor of single crystal PN junction diodes formed in an SOI layer. In the conventional pixel structure, the temperature sensor and two support legs for thermal isolation are made in the lower level of the pixel, and an IR absorbing structure is made in the upper pixel level to cover almost the entire pixel area. The IR absorption utilizes IR reflections from the lower level. Since the reflection from the support leg portions is not perfect due to the slits in the metal reflector, the reflection becomes smaller as the support leg section increases in reduced pixel pitches. In order to achieve high thermal isolation and high IR absorption simultaneously, we have developed a new pixel structure that has an independent IR reflector between the lower and upper levels. The structure assures perfect IR reflection and thus improves IR absorption. The FPA shows a noise equivalent temperature difference (NETD) of 40 mK (f/1.0) and a responsivity non-uniformity of less than 0.9%. The good uniformity is due to the high uniformity of the electrical characteristics of SOI diodes made of single crystal silicon (Si). We have confirmed that the SOI diodes architecture is suitable for large format uncooled IR FPAs.
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.
Spectral responsivity of an uncooled IRFPA with SOI diode detectors has been measured using a free electron laser (FEL), which is a new optical source with tunable wavelegnth. Light from the FEL has a complicated pulse structure, and the beam has spatial non-uniformity. These features make it difficult to evaluate the responsivity of the thermal detector with the FEL. To measure the responsivity, images of the FEL beam are recorded and analyzed by an image processor. The spectral resonsivity obtained is flat in the wavelength from 5μm to 16.5μm, and the effect of the optical resonant is smaller than that of a two-level microbolometer.
Because the semiconducting YBaCuO films which are fabricated by sputtering have a temperature coefficient of resistance (TCR) over 3%/K at room temperature, they are considered to be candidates for bolometer materials of uncooled infrared (IR) detectors. There is a problem, however, in that the resistivity of the films is over 10 (Omega) cm, which is two orders of magnitude higher than that of conventional VOX bolometer films. To decrease the resistance of the bolometers, we researched sputtering conditions of the YBaCuO films and combined them with comb-shaped electrodes. When the YBaCuO film was deposited on these electrodes by RF magnetron sputtering at room temperature in an atmosphere of 2% O2 and 98% Ar, it showed a resistivity of 90 Ωcm and a TCR of -3.2%/K; ultimately the YBaCuO bolometer resistance became 82 k(Omega) using the comb-shaped electrodes. The YBaCuO bolometer detector that contains an infrared absorbing membrane achieved a high fill factor of 90% and high infrared absorptance of 79%. Moreover, the detector showed a thermal conductance of 1.3x10-7 W/K and a responsivity of 6.8x105 V/W in a vacuum. The YBaCuO microbolometer FPA which we have developed has an array format of 320x240 pixels and a pixel pitch of 40 μm. The FPA showed a noise equivalent temperature difference (NETD) of 0.08 K with a prototype camera and f/1.0 optics.
Using Si VLSI technology, we can fabricate various kinds of infrared focal plane arrays (FPAs) which cover spectral bands from short wavelength infrared to long wavelength infrared. The Si-based technology offers many attractive features, such as monolithic integration, high uniformity, low noise, low cost, and high productivity. We have been developing Si-based infrared FPAs for more than 20 years and have verified their usefulness.
We reported a 320 x 240 uncooled IRFPA with 40 micrometers pitch having diode detectors fabricated on an SOI wafer. Since the fabrication process of the SOI diode detector is compatible with the silicon IC process, only a silicon IC fab is necessary for manufacture of the FPAs. This enables mass production of low cost uncooled FPAs. This paper focuses on the performance of the FPA. In the previous paper, we proposed a novel infrared absorbing structure which offers a very high fill factor. Although this structure exhibited a high infrared absorption because of interference absorbing components incorporated in the structure, large thermal capacitance was an issue. Thus we have improved the infrared absorbing structure in the newly developed FPA. The improved absorbing structure has been devised making use of reflection of metal interconnections including diode metal straps. A thermal time constant of 17 msec has been achieved without degrading the responsivity compared with the conventional absorbing structure.
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.
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.
A PtSi Schottky-barrier infrared focal plane array (FPA) has been developed for the short wavelength infrared radiometer (SWIR) of the advanced spaceborne thermal emission and reflection radiometer (ASTER). Six linear image sensors, which correspond to six observation bands, are integrated on the FPA to simplify the cooling system and the optics. Each linear sensor has a stagger layout of 2100 PtSi Schottky-barrier detectors and has an effective fill factor of 100%. The detector size is 20 micrometers by 17 micrometers with a cross-track pitch of 16.5 micrometers and a spacing between adjacent sensors of 1.33 mm. The charge transfer in each linear sensor is carried out by two 4-phase buried-channel CCD shift registers. The driving clock and structure of the CCD are optimized to achieve a large charge handling capacity and low transfer inefficiency. To assure a high reliability and a focal plane flatness, we have developed an original multilayer ceramic package with a filter holding structure. A focal plane flatness less than 14 micrometers and a heat shock endurance over 4000 cycles are achieved by using this packaging technology. The wavelength region between 1.5 micrometers and 2.5 micrometers is separated into six bands by optical band-pass filters attached to the package.
This paper presents the key design features of an uncooled infrared image sensor with 160 by 120 pixels. This sensor has a monolithic structure using micromachining technology. These features concern the configuration of the readout circuit, the structure of the infrared detector, and the thermal isolation structure in a pixel. The first feature is a simple readout circuit that includes neither an amplifier nor a switching transistor in the pixel. The second feature is the use of a thin film resistive bolometer made of polysilicon as the infrared detector. The detector has a P+-P--N+ diode structure which operates as a bolometer and cuts off current passes through non-selected pixels. The forward resistance of the diode can be tailored by adjusting the shape and impurity concentration of the P- region. Finally, a microbridge structure for the thermal isolation is made in each pixel by using the micromachining technology. The bolometer is monolithically integrated on this structure. Since polysilicon is generally used in the conventional Si-LSI process, this choice of detector material makes it possible to manufacture the image sensor using only current Si-LSI facilities, and realize a low cost uncooled infrared camera.