by Debra Vogler, Senior Technical Editor, Solid State Technology

August 1, 2008 - Founded in 1942 as RCA Laboratories, Sarnoff Corporation, now a subsidiary of SRI International, recently launched its Ultra-Sense backside thinning technology for back-illuminated image sensor applications. The company told SST that its new technology, which is based on SOI substrates, enables scalable fabrication techniques that achieve higher resolution, smaller pixel size, greater quantum efficiency (QE), and maximized system performance, without incurring additional costs.

"SOI costs more than a standard bulk silicon substrate so people tend to think it's more expensive," noted David Cheskis, director of business development at Sarnoff. However, using SOI avoids a number of problems encountered using standard bulk silicon, Cheskis explained.

For example, for either standard CMOS or CCD sensor processes using silicon, after processes are completed on the frontside of the wafer, the wafer backside is thinned. This backside thinning process must be very precise and tightly controlled, which is difficult to do on silicon, he explained. Additionally, more processing has to be accomplished on the wafer backside to complete the image sensor, and both the thinning and the additional processing contribute to nonuniformity that can compromise pixel quality. "These problems are not incurred with SOI substrates," said Cheskis. "SOI improves uniformity, lowers costs, and improves the yield."

Back-illumination image sensors achieve better performance over front-illuminated sensors due in part to higher quantum efficiency. Cheskis explained that the backside thinning process enables the exact amount of silicon needed for photon absorption; thinning the substrate to the appropriate thickness tailors the pixel for the particular application. Front-illuminated sensors have a much thicker amount of silicon for the photons to absorb, he noted.

Cross-section views of a back-illuminated CMOS imager fabricated on SOI. (Source: Sarnoff)

Another reason for better performance of back-illumination is higher fill factor, the amount of light that can be collected in a single pixel. The various metal layers on top of a front-illuminated sensor limit the amount of light that can be collected in a single pixel. Furthermore, as pixel sizes get smaller, the fill factor gets worse.

Targeted applications for back-illuminated high-resolution CCD or CMOS imaging systems using Sarnoff's Ultra-Sense technology include: cell phone camera modules, digital still cameras, ultraviolet (UV) and deep-UV, visible, near infrared (NIR) inspection systems, spectroscopy systems, and surveillance systems. The new technology is available through licensing and manufacturing partnerships, as well as services, including sensor design, wafer sourcing and customization, and back-end processing. -- D.V.