Distinct Image Recognition Under Low-light Conditions

ROHM develops a high-sensitivity broadband image sensor in collaboration with the National Institute of Advanced Industrial Science and Technology

Kyoto, Japan – Semiconductor manufacturer ROHM CO., LTD., together with the Tsukuba, Ibaragi Prefecture-based National Institute of Advanced Industrial Science and Technology (AIST), have developed a CIGS (CuInGaSe2) image sensor that delivers wider-band performance and higher sensitivity than conventional CCD or CMOS image sensors. The new sensor delivers image recognition performance in low-light conditions equivalent to 0.001 lux (comparable to starlight).

Image recognition in conditions of less than 0.1 lux is often problematic for silicon-based CCD and CMOS sensors generally used in automotive and security applications (i.e. night vision cameras), giving users little choice but to either ensure a minimum level of illumination or rely on expensive infrared cameras. The market for products in both sectors is poised for rapid growth.

Focusing on the fact that CIGS (CuInGaSe2) – noted for its potential benefits in next-generation photovoltaic cell technology – has a light absorption coefficient some 100 times greater than that of silicon, ROHM and AIST set out to develop a high-sensitivity, broadband image sensor using CIGS as a photoelectric conversion element.

Previous progress by the partners in creating thin-film laminates of copper, indium, gallium, and selenium and confirming the viability of the technology for use in image recognition applications set the stage for this latest advancement, in which researchers succeeded in amplifying photon-converted electric charges inside the device and creating a high-sensitivity image sensor prototype. Key innovations enabling the breakthrough include optimization of the ratios in which the constituent materials are combined and improvements in associated process technologies. These achievements have resulted in a dramatic increase in sensitivity, surpassing the performance of silicon-based image sensors such as CCD and CMOS devices by a factor of approximately 100. Thanks to a combination of image recognition capability that extends to the near-infrared region and high sensitivity in low (0.001 lux) light conditions, the new image sensors are expected to see use in automotive and security cameras designed to operate in a wide range of lighting conditions, from bright to dim, as well as in biometric technologies for iris and vein authentication.

ROHM's continued development of CIGS technology for commercialization includes efforts to (1) increase imaging resolution through miniaturization and (2) reduce picture element variation by stabilizing process technologies.

·Terminology

• Photoelectric conversion element
  An element that converts light into electric signals; also called a light-sensitive element.
• Visible spectrum
  The range of light visible to the human eye, consisting wavelengths from 400 to 800 nanometers.
• Near-infrared spectrum
  The range of light with wavelengths longer than visible light but shorter than infrared light, specifically from 800 to 2,500 nanometers.

·Images Capture Comparison Under Low-light Conditions

	Images Captured with a Si-CMOS Sensor	Images Captured with a CIGS Image Sensor
0.1 lux equivalent
0.05 lux equivalent
0.001 lux equivalent

·Electric Signal Amplification Inside the Photoelectric Conversion Element	·Comparison of Spectral Sensitivity with Silicon-based Devices
·CIGS Image Sensor Cross-section	·CIGS Image Sensor Photo