Sony developed “Exmor R”, a CMOS sensor featuring Sony’s independently developed back-illuminated structure, with the aim of creating a camera capable of taking exceptional photographs even by candlelight. “Exmor R” is approximately twice as sensitive (*1) as a conventional front-illuminated CMOS sensor and also features low noise. In a back-illuminated CMOS sensor, light is directed onto the silicon substrate from behind, allowing light to be used with a level of efficiency not possible with conventional front-illuminated pixel structures. Photographers can now create smooth, high-quality images in low light settings, including night scenes.
Issues Surrounding Conventional CMOS Sensors
“Exmor R”—Back-illuminated CMOS Sensor
Characteristics of CMOS sensors include low power consumption and high speed. While maximizing these advantages, Sony has eliminated the aforementioned problems associated with conventional front-illuminated CMOS sensors by developing breakthrough technology. In contrast with the conventional CMOS pixel structure, Sony’s back-illuminated CMOS sensor is exposed to light from the back of the silicon substrate. The result is a dramatic improvement in photographic performance, including approximately double the sensitivity (*1)and a reduction in noise. Sony has succeeded in the development of a CMOS sensor with 1.75µm square pixels, a resolution of 5 megapixels (effective), and a speed of 60 frames per second. Sony has now commenced mass-production of this new chip, the “Exmor R” back-illuminated CMOS sensor.In a back-illuminated sensor, the silicon substrate is reversed to allow exposure to light on the reverse side. Because the light is not blocked by the wiring and transistor layer, comparatively more light enters each pixel and there is no loss of sensitivity caused by changes in the angle of incidence. However, the structure of a back-illuminated sensor and the manufacturing processes can result in a number of problems that can reduce the resolution and signal-to-noise ratio of the image sensor. These include noise, dark current, defective pixels and color mixing.
Sony overcame these problems by developing a new photodiode structure and on-chip lens optimized for this back-illuminated CMOS sensor. These efforts increased the signal-to-noise ratio by 8dB compared with earlier Sony CMOS sensors with pixel structures based on pixels of the same size. Sensitivity has been improved by 6dB (*1), while noise, dark current, and defective pixels have all been reduced. This is reflected in a 2dB reduction in random noise under dark conditions. The problem of color mixing was overcome using high-precision superposition technology.