NHK Image Sensor Research

Sept. 2015 issue of NHK R&D Journal is almost entirely devoted to the image sensor projects:

Fabrication of Three-Dimensional Integrated Circuits for CMOS Image Sensors with Pixel-Parallel Signal Processing
Masahide GOTO, Kei HAGIWARA, Yoshinori IGUCHI and Hiroshi OHTAKE
ABSTRACT
We studied a three-dimensional (3D)-integrated image sensor that is capable of pixel-parallel signal processing, thereby meeting the demand for high-resolution and high-frame-rate imaging. Unlike the conventional 3D-stacked devices using through silicon vias (TSVs), which are not applicable to image sensor pixels of a few micrometers or less due to its hole diameter larger than the pixel size, we have recently developed a novel 3D-structuring method by using the direct bonding that could transfer signals vertically without using TSVs. Demonstrations are performed on 3D-CMOS inverters and 3D-ring oscillators with 101 stages through the lowtemperature hybrid bonding of Au contacts embedded in a SiO2 surface. The experimental results show that the developed technology is promising for high-density 3D-integrated circuits.

Color Image Sensor using Three Stacked Organic Photoconductive Films with Silicon Nitrides as Interlayer Insulators
Hokuto SEO, Toshikatsu SAKAI and Hiroshi OHTAKE
ABSTRACT
We are developing a color image sensor with three stacked organic photoconductive films (OPFs) sensitive to only one primary color component (red (R), green (G), or blue (B)); each OPF has a signal readout circuit. In this study, we fabricated a structure with three stacked OPFs sensitive to the R, G, or B component with a thickness of 5.8μm by using 2-μmthick silicon nitrides as interlayer insulators.

Development on Low-voltage Carrier Multiplication Film using Chalcopyrite Based Materials
Kenji KIKUCHI, Shigeyuki IMURA, Kazunori MIYAKAWA, Hiroshi OHTAKE and Misao KUBOTA
ABSTRACT
There is a need for highly sensitive imaging devices for high-resolution and high-frame-rate image sensors. We have been developing low-voltage carrier multiplication film using chalcopyrite based materials, CuIn1-xGax(Se1-ySy)2 (CIGS), to increase the sensitivity of image sensors. A gallium oxide (Ga2O3) thin layer, which functioned as a hole-blocking layer, was used for the CIGS layer to achieve low dark current. The electric and optical properties of Ga2O3/CIGS photodiodes were investigated. The quantum efficiency in the visible light range (400-700nm) was 95% at an applied voltage of 4V. Moreover, carrier multiplication phenomena were observed at applied voltages under 5V.

Development of CMOS Image Sensors Overlaid with Crystalline Selenium-based Heterojunction Photodiode
Shigeyuki IMURA, Kenji KIKUCHI, Kazunori MIYAKAWA, Hiroshi OHTAKE and Misao KUBOTA
ABSTRACT
We have been investigating highly sensitive imaging devices with crystalline selenium (c-Se) as a photosensitive layer for applications to nextgeneration ultra high-definition imaging systems. By using c-Se with a high absorption coefficient over the entire visible region instead of using silicon, which has been conventionally applied to a photoelectric conversion material, highly sensitive image sensors using avalanche multiplication in a photoelectric conversion film can be realized. In this paper, we describe the first observation of avalanche multiplication in a c-Se film fabricated on the glass substrate which exhibits external quantum efficiency of greater than 100%. Furthermore, we successfully created high resolution images by applying a uniform c-Se film to the CMOS circuits.

Selenium sensor external QE