A cross sectional view of the device: The LSI is located under the ZnO and CIGS layers, which act as a photodiode.
A cross sectional view of the device: The LSI is located under the ZnO and CIGS layers, which act as a photodiode.
[Click to enlarge image]
The avalanche effect is enhanced as the reverse bias voltage increases.
The avalanche effect is enhanced as the reverse bias voltage increases.
[Click to enlarge image]

Rohm Co Ltd and Japan's National Institute of Advanced Industrial Science and Technology (AIST) presented the technical details of the high sensitivity CIGS image sensor, which they announced in April 2008, at IEDM 2008.

They unveiled the device structure, the requirements to enable the high sensitivity, which is approximately 100 times higher than that of a Si-based image sensor, and the technique to reduce the dark current.

Rohm and AIST are currently developing a high sensitivity image sensor that uses "CIGS," a compound semiconductor composed of copper (Cu), indium (In), gallium (Ga) and selenium (Se) (See related article). When the sensor was exhibited at CEATEC 2007 in October 2007, its resolution was 352 x 288 (CIF). But it was improved to 640 x 480 (VGA) in April 2008.

This time, they revealed the device structure of the image sensor and its production method as well as the factor that contributed to the high sensitivity. According to their presentation, the sensor has a layer structure composed of n-type ZnO, i-type ZnO, CdS, CIGS and Mo, which are arranged in this order from the top surface. The sensor is placed on a Si LSI via the molybdenum (Mo) layer serving as an electrode. The ZnO layer was formed using RF sputtering.

Its high sensitivity, which is about 100 times higher than that of a Si-based image sensor, was achieved by applying a reverse bias voltage of -10V. This is based on the "avalanche phenomenon," in which a large number of electron-hole pairs are created like an avalanche when one photon is absorbed by a reverse bias voltage.

"If a reverse bias voltage is applied, the avalanche phenomenon occurs in Si-based sensors, too," Rohm said. "The major difference is that the CIGS layer requires less voltage because it is thinner."

In general, the application of reverse bias voltage increases the dark current, or noise. Rohm and AIST discovered that the dark current reduces when the thickness of the i-ZnO layer in the sensor is increased and improved the net sensitivity, they said.