Tomohiro Ishikawa

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.

Performance of 320 x 240 uncooled IRFPA with SOI diode detectors

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.

Monolithic uncooled infrared image sensor with 160 by 120 pixels

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.

Silicon infrared focal plane arrays

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.

Spectral responsivity measurement of uncooled IRFPA using free electron laser

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.

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.

Microscopic electric field measurement for microwave high-power FETs by using a noncontact electro-optic probing technique

Direct observation of nonuniform operation in GaAs microwave high-power FETs has been realized by introducing a new electro- optic (E-O) probing system. In the system, ZnTe is used as a longitudinal external E-O crystal in order to make high sensitive measurement. The spatial resolution of the E-O probing system with an electrically synchronized laser diode is as small as 5 micrometers . We apply this system to the measurement of the electric field at the microscopic region of drain electrodes of an X-band 1W FET consisting of 10 FET unit cells (73 micrometers separation). The electric field concentration to the center (1.6 times) and the most outer cells (1.1 times) has directly been measured. The electric fields on the unbalanced FET cells which were damaged artificially by the focused ion beam are also reported.