Kazunori Miyakawa

50×50μm pixel magnetic focus field emitter array image sensor with high-gain avalanche rushing amorphous photoconductor target

A 50×50μm pixel field emitter array image sensor with a highly sensitive avalanche-mode photoconductive target and a scanning electron focusing system consisting of permanent magnets were fabricated and tested as a step toward the development of ultrahigh-sensitivity compact image sensors for high-definition television cameras. The experimental results revealed that the magnetic focusing conditions of the prototype sensor were in accordance with those estimated from the simulation, and the prototype sensor could obtain both enough resolution for its pixel size and high sensitivity by focusing the electrons emitted from the field emitter array onto the highly sensitive photoconductive target.

Photoconductive properties of organic films based on porphine complex evaluated with image pickup tubes

We have fabricated two types of organic photoconductive films; a layered structure of tetra(4-methoxyphenyl) porphine cobalt complex (Co–TPP) and bathocuproine (BCP) (target A), and another of Co–TPP, tris-8-hydroxyquinoline aluminum (Alq3) and BCP (target B). The photoconductive properties of each film have been measured using image pickup tubes. The dark current was drastically reduced in target B compared with that in target A at the same applied electric field. The external quantum efficiency reached 20% in target B, which is twenty times higher than that in target A. Image pickup of a test chart from target B at standard TV operation was also demonstrated. High resolution and excellent tone sufficient for television use has been obtained.

Image Pickup from Zinc Phthalocyanine/Bathocuproine Double-Layer Film Using Pickup Tube

We have demonstrated image pickup from zinc phthalocyanine/bathocuproine double-layer film incorporated into a pickup tube for both standard television systems and high-definition television (HDTV) systems. A limiting resolution of more than 800 television lines was obtained, which is sufficient for HDTV. The peak external quantum efficiency was 14.7% under 620 nm irradiation. The results indicate that organic molecules have great potential for use in imaging devices.

Active-matrix Spindt-type field emitter array with faster response time for image sensor with high-gain avalanche rushing amorphous photoconductor target

A new active-matrix Spindt-type field emitter array for a FEA-HARP (field emitter array - high-gain avalanche rushing amorphous photoconductor) image sensor was designed. Simulation and experimental results showed that the new active-matrix drive circuit can shorten the response time and can potentially meet the HDTV standard.

High-Sensitivity Image Sensors Overlaid With Thin-Film Gallium Oxide/Crystalline Selenium Heterojunction Photodiodes

We developed a stacked CMOS image sensor overlaid with a thin-film gallium oxide (Ga2O3)/crystalline selenium (c-Se) heterojunction photodiode. Uniform c-Se films laminated on the CMOS circuits were fabricated through a low-temperature (200 °C) tellurium-diffused crystallization process. This stacking structure has several advantages, such as a high aperture ratio, low crosstalk, and high sensitivity. We successfully controlled the size of the polycrystalline particles to much smaller than the pixel size of the image sensors by adjusting the thickness of the c-Se films. Therefore, we herein present the first high-resolution images obtained with such a device. Furthermore, the dark current, which is induced by the carrier injection from an external electrode, was significantly reduced by employing a wide bandgap Ga2O3 film as a high hole-injection barrier.

Double-Gated, Spindt-Type Field Emitter With Improved Electron Beam Extraction

We have developed a highly sensitive, compact image sensor that comprises a field emitter array (FEA) and a high-gain avalanche-rushing amorphous photoconductor target, with the ultimate aim of developing an ultrahighly sensitive, compact, high-definition television camera. Double-gated FEs have an advantage of having a compact electron beam focusing system; however, image intensities reproduced by the sensor were nonuniform due to low electron beam current. Furthermore, the simulated electron beam current disagreed with the measured current. The electron beam current characteristics of two types of double-gated, Spindt-type FEs (both with improved electron beam current extraction) are discussed for possible use within the sensor: convex-structured and volcano-structured. A highly accurate simulation model of the image sensor using a double-gated, Spindt-type FE has been examined; the simulated electron beam currents extracted from the double-gated, Spindt-type FE are in agreement with the measured electron beam currents when the initial electron velocity is assumed, thus suggesting that the simulated anode current-anode voltage characteristic conforms to the measured one. For example, the electron beam currents extracted from the convex-structured and volcano-structured FEs when the focusing electrode is placed 0.2 μm below the gate electrode opening at a focusing electrode voltage of 15 V are, respectively, 1.8 and 1.9 times larger than that extracted from the previously used FEs when the focusing electrode is stacked 1.5 μm above the gate electrode. The results show potential for reducing the degradation of the uniformity of the reproduced images intensity, and show that the highly accurate simulation model of the sensor is valid to design the double-gated FEAs for the sensor.

Development of FOP-HARP imaging device

The high-gain avalanche rushing amorphous photoconductor (HARP) camera tube achieves ultrahigh-sensitivity by using the avalanche multiplication. The applications of this tube extend beyond broadcasting into other fields. It is attracting a great deal of attention especially for radiation diagnosis, such as synchrotron radiation microangiography, because it can obtain high-resolution and high-contrast images with a low dose of radiation. However, in the present system, a fluorescent screen and the photoconductive film of the HARP tube are connected optically by a lens-coupling method, and low light throughput remains a big problem. To improve the light throughput by using a fiber-coupling method, we applied a fiber-optic plate (FOP) to the substrate of a HARP tube. The FOP consists of three types of glass that have differing hardnesses and elastic coefficients that make it difficult to flatten the FOP surface enough to form the HARP film. We thus introduced a new mechanical polishing method and succeeded in realizing avalanche multiplication in the FOP-HARP tube. The results of shooting experiments by applying the FOP-HARP to the microangiography showed that a spatial resolution of over 20 line pairs/mm was obtained. Moreover, rat femoral arteries of 150-200 μm in diameter could be visualized as motion pictures with a one-fourth lower concentration of contrast material than that needed for ordinary microangiography. Another potential application of the FOP-HARP is an ultrahigh-sensitivity nearinfrared (NIR) image sensor made by fiber-coupling with an image intensifier (I.I.). The image sensor provides highquality images and should be a powerful tool for NIR imaging.

Electrostatic-focusing image sensor with volcano-structured Spindt-type field emitter array

The authors have developed a highly sensitive, compact image sensor comprising a field emitter array (FEA) and a high-gain avalanche rushing amorphous photoconductor (HARP) target with the ultimate aim of developing an ultrahigh sensitivity, compact, high-definition television camera. Double-gated field emitters have the advantage of a compact electron beam focusing system; however, image intensities reproduced by a sensor with the double-gated, Spindt-type field emitter array with the focusing electrode stacked 1.5 μm above the gate electrode were nonuniform owing to low electron beam current. The minimum required electron beam current extracted from the double-gated field emitter array is considered for possible use with the sensor. Furthermore, a suitable field emitter array pitch to balance the electron beam current and the electrostatic-focusing lens strength was simulated. For the sensors design guidelines, a field emitter array pitch of approximately 3 μm would be reasonable in the case of employi...

Improvement photoelectric conversion efficiency of red light in HARP film

We enhanced the photoelectric conversion efficiency of red light in a 15-&mgr;m-thick HARP film without deteriorating image pick-up characteristics or reliability. To achieve a higher photoelectric conversion efficiency for red light, we designed a new film structure with an increased amount of doped Te, which has a narrower band gap than that of a-Se. The thickness of the LiF-doped layer for trapping holes was increased from that of the conventional red-extended HARP film in order to weaken the internal field that would otherwise be enhanced by trapped electrons in extra doped Te. The new red-extended HARP film achieved a photoelectric conversion efficiency for red light of about 22.5% at a wavelength of 620 nm, which is twice that of the conventional red-extended film. We confirmed an improvement in signal to shot noise ratio of 3 dB and a dramatic improvement in color reproduction when we experimented with an HDTV camera with a camera tube incorporating the new film.

Heat treatment to suppress image defect occurrence in amorphous selenium avalanche multiplication photoconductive film with improved red-light sensitivity

Amorphous selenium (a-Se) avalanche multiplication photoconductive film, HARP film, has been developed for the purpose of making a high sensitivity video camera. HARP film used for the red channel in a color camera is doped with tellurium (Te) to improve its red-light sensitivity. However, doping a-Se with Te causes impurity levels that trap electrons, and the built-in-field from the trapped electrons causes image defects when a camera using the film is shooting intense spotlights. A heat treatment to suppress the defect occurrence was found. We describe this treatment and discuss its mechanism.