Toshiki Seto

640 x 480 pixel uncooled infrared FPA with SOI diode detectors

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.

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.

High-performance 1040- x 1040-element PtSi Schottky-barrier image sensor

We have developed a monolithic 1040 X 1040 element PtSi Schottky-barrier infrared image sensor. This device uses the Charge Sweep Device readout architecture with four parallel outputs. The pixel size is 17 X 17 micrometers 2, which is 56% of that of our 512 X 512 element PtSi image sensor. In order to keep sufficient sensitivity with such a small pixel, we have developed a 1.5 micrometers Schottky-barrier process technology and improved the fill factor. The fill factor of this device is 53%. As a result of this improvement, a high differential temperature response of 9.6 X 103 electrons/K and a low noise equivalent temperature difference of 0.1 K have been achieved with f/1.2 optics. We have also improved the saturation characteristics of the device by optimizing the impurity concentrations of the isolation region and guard ring. The saturation level is 1.6 X 106 electrons at a detector reset voltage of 4 V.

SOI diode uncooled infrared focal plane arrays

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.

1040 X 1040 infrared charge sweep device imager with PtSi Schottky-barrier detectors

The authors have developed a 1-Mpixel infrared charge sweep device (IRCSD) imager for thermal imaging in the 3- to 5-[mu]m band. The device of this imager is a 1040 x 1040 monolithic PtSi Schottky-barrier (SB) array using the charge sweep device (CSD) readout architecture. The pixel size is 17 x 17 [mu]m[sup 2] and the fill factor of this device is 44%. In this imager system, four video signals are read out from four independent channels on the device. The processing of these four outputs, such as sample and hold (S/H), and offset control and image correction, is performed in parallel, after which these outputs are combined to produce high-definition TV (HDTV; 1,125 lines, 30 Hz) format thermal image in real time. The noise-equivalent temperature difference (NETD) with f/1.2 optics at 27 C background is 0.13 C at the HDTV output stage.

Monolithic Schottky-barrier infrared image sensor with 71% fill factor

An improved 512 x 512-element PtSi Schottky-barrier IR image sensor (512 x 512 IRCSD) has been developed using the charge sweep device (CSD) readout architecture and 1.2-[mu]m minimum design rules. Finer pattern process technology enhances the advantage of the CSD readout architecture, enlarging the fill factor without sacrificing the saturation signal level. A large fill factor of 71% is achieved in spite of a small pixel size of 26 x 20 [mu]m[sup 2]. At the Schottky-barrier detector reset voltage of 4 V, the differential temperature response with f/1.2 optics at 300 K and saturation signal level were 3.2 [times] 10[sup 4] electrons/K and 2.9 [times] 10[sup 6] electrons, respectively. The noise equivalent temperature difference was estimated as 0.033 K with f/1.2 optics at 300 K. The improved 512 x 512 IRCSD was designed to be operated either in the field or frame integration interlace modes for versatility.

PtSi FPA with improved CSD operation

Over the past ten years we have been developing PtSi focal plane arrays (FPAs) using the charge sweep device (CSD). FPAs are going to high resolution and the power of the FPAs are on an upward trend. Now we have developed a low-power CMOS CSD scanner (LOCCS) for a high resolution FPA. The conventional CSD scanner operates at the same frequency as that of the horizontal CCD to prevent fixed pattern noise (FPN), and generates a frequency pulse higher than the minimum requirement. The LOCCS is a kind of CMOS dynamic shift resistor, which generates clock pulses for vertical signal transfer without the low frequency input pulses that cause FPN. Because the LOCCS generates multi-phase clock pulses, the power consumption can be reduced. We have fabricated test devices to evaluate the improved CSD operation by the LOCCS, and confirmed that the devices operate normally and the reduction of power consumption is in good agreement with the theory. We also applied the LOCCS to a 256 by 256 PtSi FPA and obtained thermal images.

Portable high-performance camera with 801 x 512 PtSi-SB IRCSD

We have developed an 801 by 512 element PtSi Schottky-barrier FPA with low-power multiphase CSD readout for a compact high performance infrared camera. The power consumption of this detector is about a half of the conventional IRCSD and the number of pixels is 1.5 times of that. The cryocooler is a high efficiency and compact size Stirling-cycle cooler that has also been developed for this imager at the same time. The cooling capacity is just match for the new IRCSD. This paper describes the basic imager design and features.

High-fill-factor monolithic infrared image sensor

A 256 X 256 element platinum silicide monolithic image sensor with a large fill factor has been developed as a high sensitivity infrared image sensor. It is essential to increase the maximum signal charge of the staring infrared image sensor to obtain higher sensitivity. We used the Charge Sweep Device readout architecture and improved operations of the floating diffusion amplifier to increase the maximum signal charge. A 52 X 40 micrometers 2 pixel using a minimum features size of 2 micrometers has a fill factor of 66%. Evaluating the performance of the device, we confirmed the effectiveness of the improved technologies. The measured saturation level is 2.8 X 106 electrons which is determined by the storage capacity of the detector. We estimated from a measurement point at low background temperature that the noise equivalent temperature difference with a f/1.2 optics is 0.036 K at 300 K background.

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.