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Jun 16th, 2008
Sony and Omnivision to develop BSI architectures for CMOS image sensor markets
After recent annoucement from Omnivision & TSMC, Sony recently revealed the development of a backside illuminated CMOS image sensor (1.75µm square pixel size, 5MPixel resolution, 60 frames/s) with significantly enhanced imaging characteristics, including nearly two fold sensitivity and low noise. This improved performance has been achieved by illuminating the backside of the silicon substrate, in contrast to conventional CMOS image sensors based on front-side illumination technology.
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Front-side versus Back-side CMOS image sensor architectures
Front-side versus Back-side CMOS image sensor architectures
Sony will apply its backside-illuminated CMOS image sensor (BSI) technology in consumer digital video camcorders and digital still cameras to deliver an even higher quality image experience. With a conventional front-side illumination structure, the metal wiring and transistors at the surface of the silicon substrate reduce the amount of light collected by the photodiode located at the deep bottom of the wafer surface. Although microlenses array can help reducing this issue, this type of arhcitecture is today reaching its limits in terms of light-sensitive area (fill-factor) in the current race for decreasing pixel sizes.

A backside-illuminated structure minimizes the degradation of sensitivity to optical angle response, while also increasing the amount of light (dramatic increase of the fill-factor) that enters each pixel due to the lack of obstacles such as metal wiring and transistors that have been moved to the reverse of the silicon substrate. However, compared to conventional front-illuminated structures, backside-illuminated structures commonly causes problems such as noise, dark current, defective pixels and color mixture that lead to image degradation and also cause a decrease in the signal-to-noise ratio. To overcome this issue, Sony has newly developed a unique photo-diode structure and on-chip lens optimized for back-illuminated structures, that achieves a higher sensitivity of +6dB and a lower random noise of -2dB without light by reducing noise, dark current and defect pixels compared to the conventional front-illuminated structure.

Technology and Markets:
Backside-illumination is not a new concept as it has been developed for years in the image sensor industry. Companies e2v and Tracit Technologies (FR) have already introduced several products on niche industrial CCD sensor markets (using BSI for machine vision and scientific imaging applications) and could expand the technology concept to CMOS sensor in the future. Actually, most of all CMOS image sensor manufacturers have reported advanced developments in this direction (including Micron, Samsung, Toshiba, MagnaChip and STMicro). The successive announcement of OmniVision and Sony just confirmed the fact that the market window for BSI architectures is opening and will expand from niche (low volume) industrial applications to high volume consumer products such as high resolution camera cell-phones, high-end DSCs and DVRs.

However, several questions are remaining about how to move BSI from concept to reality such as the use of a SOI substrate or not? Grinding, CMP and plasma technologies to reach the silicon transparency through the bacskside of the wafer? How to handle the thinned wafers? What is the best permanent wafer bonding material technologies? How to connect the photodiodes to the circuit read-out? Does specific low temperature processes are required or not? Despite the few remaining cost and technical issues to be solved, it is today becoming very clear that BSI architectures are poised to impact considerably the CMOS image sensor business in the 2009-2012 time frame...



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