Rohm Co. Ltd. exhibited a thin film multilayer image sensor featuring a wide range and a high sensitivity at CEATEC Japan 2007 as a reference presentation.
The image sensor is a joint development with Research Center for Photovoltaics of Japan's National Institute of Advanced Industrial Science and Technology (AIST). According to Rohm, the latest sensor features (1) photodiodes formed directly on an LSI chip and (2) a high sensitivity not just in the visible range but also in the infrared range, where the sensitivity of existing Si-based image sensors is lowered to an insufficient level.
CIGS solar cell on an LSI chip
The image sensor on display was fabricated by forming sensor layers composed of photodiodes directly on the LSI wiring layer of a control circuit based on the CMOS technology. Although Rohm said that details of the chip size won't be announced at the moment, it measures several millimeters on a side. The sensor was created by arranging 100,000 picture elements, each of which is a 10 μm-square.
The major difference between the latest image sensor and the existing CMOS sensors is that the former has a structure technically identical to that of a Cu-In-Ga-Se (CIGS) solar cell, instead of employing the existing Si photodiode. The CIGS solar cell is drawing attention worldwide not only because it can eliminate the use of crystalline Si, and thus is manufactured at a relatively low cost, but also because of its high conversion efficiency of about 13%.
In Japan, Honda Soltec Co. Ltd. and Showa Shell Sekiyu K.K. will shortly begin full-scale mass production. AIST, which participated in the joint development, is one of the organizations involved in the CIGS solar cell development.
Based on these techniques, the new image sensor allows a high sensitivity in a wider range. The quantum efficiency of the sensor in the visible light range is approximately double that of the case using the crystalline Si photodiode.
In addition, the sensor also has high sensitivity in the infrared range, with a wavelength of 1.0-1.3 μm or shorter, where the sensitivity of Si photodiodes decreases to nearly zero. At the exhibition site, Rohm conducted a demonstration by shooting an image pattern irradiated only with the infrared light with the new image sensor and a silicon sensor.
The major remaining issue is how to miniaturize the sensor. Products using standard CMOS sensors with picture elements measuring 2-3 μm on a side are already commercialized, whereas the element in the latest prototype is a 10 μm-square as described above. In the technology used for the latest development, grooves between the adjacent elements are made after the formation of CIGS layer.
"We need to develop a more sophisticated technique to miniaturize the picture element blocks," Rohm said.