Toshihisa Watabe

Stacked Image Sensor With Green- and Red-Sensitive Organic Photoconductive Films Applying Zinc Oxide Thin-Film Transistors to a Signal Readout Circuit

A vertically stacked image sensor composed of green (G)- and red (R)-sensitive organic photoconductive films, each having a thin-film transistor (TFT) that uses a transparent zinc oxide (ZnO) channel to read out a signal generated in the organic film, was fabricated. The effective number of pixels of the ZnO-TFT circuits was 1410 (47 times 30), and their pitch was 600 mum. The current on/off ratio and turn-on voltage of the ZnO-TFT were over 105 and 1.5 V, respectively. The G- and R-sensitive organic photoconductive films showed excellent wavelength selectivity: the peak wavelength of the G-sensitive film was 540 nm, and that of the R-sensitive one was 700 nm. A color image with a resolution corresponding to the number of pixels was obtained by a shooting experiment with the fabricated image sensor, which clearly demonstrated color separation in the depth direction of the image sensor, using a stacked structure of wavelength-selective organic films with ZnO-TFT readout circuits.

A 33-Megapixel 120-Frames-Per-Second 2.5-Watt CMOS Image Sensor With Column-Parallel Two-Stage Cyclic Analog-to-Digital Converters

A 33-megapixel 120-frames/s (fps) CMOS image sensor has been developed. The 7808 × 4336 pixel 2.8-μm pixel pitch CMOS image sensor with 12-bit, column-parallel, two-stage, cyclic analog-to-digital converters (ADCs) and 96 parallel low-voltage differential signaling output ports operates at a data rate of 51.2 Gb/s. The pipelined operation of the two cyclic ADCs reduces the conversion time. This ADC architecture also effectively lowers the power consumption by exploiting the amplifier function of the cyclic ADC. The CMOS image sensor implemented with 0.18-μm technology exhibits a sensitivity of 0.76 V/lx·s without a microlens and a random noise of 5.1 erms- with no column amplifier gain and 3.0erms- with a gain of 7.5 at 120 fps while dissipating only 2.45 and 2.67 W, respectively.

A 128×96 Pixel Stack-Type Color Image Sensor: Stack of Individual Blue-, Green-, and Red-Sensitive Organic Photoconductive Films Integrated with a ZnO Thin Film Transistor Readout Circuit

A color image was produced by a vertically stacked image sensor with blue (B)-, green (G)-, and red (R)-sensitive organic photoconductive films, each having a thin-film transistor (TFT) array that uses a zinc oxide (ZnO) channel to read out the signal generated in each organic film. The number of the pixels of the fabricated image sensor is 128×96 for each color, and the pixel size is 100×100 µm2. The current on/off ratio of the ZnO TFT is over 106, and the B-, G-, and R-sensitive organic photoconductive films show excellent wavelength selectivity. The stacked image sensor can produce a color image at 10 frames per second with a resolution corresponding to the pixel number. This result clearly shows that color separation is achieved without using any conventional color separation optical system such as a color filter array or a prism.

New signal readout method for ultrahigh-sensitivity CMOS image sensor

We propose a new signal readout method that uses a charge-transfer circuit. Its application is to an ultrahigh-sensitivity CMOS image sensor on which an avalanche-mode photoconductive film is overlaid. The charge-transfer circuit makes it possible to obtain high signal-to-noise ratio features by transferring signal charges accumulated in each photodiode to a parasitic capacitance that is small compared with the photodiode capacitance. A 138 /spl times/ 138 passive-pixel prototype sensor that had the charge-transfer circuit in each column was fabricated and tested. The prototypes column-to-column fixed-pattern noise and random noise were, respectively, 56.7 and 58.4 dB below the saturation signal level, which demonstrated its potential as a signal readout circuit for a next-generation ultrahigh-sensitivity CMOS image sensor.

A 1.7-in, 33-Mpixel, 120-frames/s CMOS Image Sensor With Depletion-Mode MOS Capacitor-Based 14-b Two-Stage Cyclic A/D Converters

A 1.7-in, 33-Mpixel, 120-frames/s, 14-bit CMOS image sensor has been developed. The 7936 (H) × 4412 (V) pixel CMOS image sensor, which uses 14-b depletion-mode MOS (DMOS) capacitor-based two-stage cyclic A/D converters (ADCs) and 64 parallel scalable low-voltage signaling output ports, operates at a data rate of 63.8 Gb/s. DMOS capacitors have a high capacitance density, but it is difficult to achieve high bit resolutions in ADCs with these capacitors because their capacitance depends on the applied voltage. Column-parallel two-stage cyclic ADCs overcome this difficulty using a split-sampling DMOS capacitors architecture. The two-stage cyclic ADC with the DMOS capacitors at a 6.4-μm column pitch exhibited a differential nonlinearity of 0.95/-0.80 least significant bit (LSB); the integral nonlinearity was 2.57/-28.27 LSB at a 14-b resolution. The CMOS image sensor implemented with a 90-/65-nm technology exhibited a sensitivity of 5.22 V/lx·s and a random noise of 3.6 e-rms with a gain of 3.3 at 120 frames/s while dissipating 3.2 W.

Stacked color image sensor using wavelength-selective organic photoconductive films with zinc-oxide thin film transistors as a signal readout circuit

Our group has been developing a new type of image sensor overlaid with three organic photoconductive films, which are individually sensitive to only one of the primary color components (blue (B), green (G), or red (R) light), with the aim of developing a compact, high resolution color camera without any color separation optical systems. In this paper, we firstly revealed the unique characteristics of organic photoconductive films. Only choosing organic materials can tune the photoconductive properties of the film, especially excellent wavelength selectivities which are good enough to divide the incident light into three primary colors. Color separation with vertically stacked organic films was also shown. In addition, the high-resolution of organic photoconductive films sufficient for high-definition television (HDTV) was confirmed in a shooting experiment using a camera tube. Secondly, as a step toward our goal, we fabricated a stacked organic image sensor with G- and R-sensitive organic photoconductive films, each of which had a zinc oxide (ZnO) thin film transistor (TFT) readout circuit, and demonstrated image pickup at a TV frame rate. A color image with a resolution corresponding to the pixel number of the ZnO TFT readout circuit was obtained from the stacked image sensor. These results show the potential for the development of high-resolution prism-less color cameras with stacked organic photoconductive films.

Lag Characteristics of Flat Image Sensor Consisting of Field Emitter Array and High-Gain Avalanche Rushing Amorphous Photoconductor Target

We have been studying a flat image sensor consisting of a Spindt-type field emitter array (FEA) and a high-gain avalanche rushing amorphous photoconductor target. This image sensor has the advantage of having high-sensitivity, but has the disadvantage of image lag due to its signal-charge readout by the electron beam. We investigated the lag characteristics to overcome this problem and proposed a new FEA driving method for suppressing the lag. We confirmed that the lag was drastically suppressed by eliminating the remaining charges when using an extra in-line scan during the horizontal blanking period.

A new readout circuit for an ultra high sensitivity CMOS image sensor

We have developed a new readout circuit for highly sensitive CMOS image sensors. The circuit makes it possible to obtain high signal-to-noise ratio (S/N) by effectively transferring signal charges accumulated in the photo-diode (PD) to a smaller capacitance. We fabricated and tested a CMOS image sensor with the readout circuit, and confirmed that it has higher sensitivity than conventional passive-type CMOS image sensors.

3-Layered Au/SiO 2 hybrid bonding with 6-μm-pitch au electrodes for 3D structured image sensors

A three-layered daisy-chain test device in which each layer is electrically interconnected with 6-μm-pitch Au electrodes is fabricated using a direct-bonding technique. The experimental results show a series of electrical interconnections exceeding 986,000 contacts, with the Au contact resistance for a single connection being approximately 92.8 mΩ.

In-pixel A/D converters with 120-dB dynamic range using event-driven correlated double sampling for stacked SOI image sensors

We report in-pixel analog-to-digital converters (ADCs) using event-driven correlated double sampling (CDS) for stacked silicon-on-insulator (SOI) image sensors. The pulse-frequency-modulation ADCs enable a pixel-parallel operation that leads to superior imaging performance. We designed a novel CDS for an ADC comprising comparators, capacitors, and timing control logic circuits to generate clocks for in-pixel operation to suppress reset noise. The developed ADC is successfully confirmed to exhibit an excellent linearity in a wide dynamic range of 120 dB and it shows noise reduction effects, indicating the feasibility of high-performance pixel-level imaging for next-generation image sensors.