Munehisa Takeda

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.

Novel micro torque measurement method for microdevices

A general purpose measurement system has been developed which can measure very low torques of the order of 10-7?N?m. The new method proposed here uses wind pressure to apply a load to a turbine attached to the output shaft of a device. It can therefore be used for all rotating microdevices. The use of wind pressure reduces the loss during measurements, and makes it possible to measure low levels of torque easily by simply attaching the turbine to the device. In the present study, the measurement principle of the new system has been verified. The static friction torque was measured by the traditional method of using a thread wound around the motor shaft in order to examine the validity of the proposed method, and a comparison of the two methods with the measured data is presented. In addition, a prototype micromotor, 1.6?mm in diameter, has been fabricated and used to demonstrate that the new system is able to measure torques of the order of 10-7?N?m while the motor is in operation.

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.

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.

Micro-optical switch with uni-directional I/O fibers

The authors have developed a new 2/spl times/2 micro-optical switch. The switch features a uni-directional input/output and a drive mechanism with latch functions. The newly developed structure combines two moving mirrors and a fixed V mirror to implement a 2/spl times/2 micro-optical switch with uni-directional input/output. Furthermore, the moving mirrors have been reduced in size by integrating the drive and latch functions by the use of magnetic shaft mirrors and driving coils. The micro-optical switch has been implemented as a module mounted in a ceramic package. The modular switch is 23.5/spl times/9.86/spl times/6.76 mm in size and provides a switching time of 1.3 ms for 15 V (469 mA) and an input pulse width of 1 ms. The reflectivity of the moving mirror is 98.6% (reflection loss: 0.06 dB) and the connection loss between the fixed mirror surfaces is 5.4 dB.

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.