MANHASSET, New York — The dilemma of shrinking pixel size in CMOS image sensors' without degrading performance and image quality is the big hurdle today in the steeplechase among at least a half-dozen competitors to miniaturize digital imaging technology.
OmniVision Technologies, Inc., the largest CMOS image sensor manufacturer today, believes it has found an answer in a novel design that adopts backside illumination (BSI) technology.
Together with Taiwan Semiconductor Manufacturing Corp. (TSMC), OmniVision developed "process tweaks" that allow OmniVision to offer a CMOS sensor with improved image quality while extending its pixel roadmap down to 0.9 micron pixels, according to the company.
OmniVision said that it's ready to start sampling next month an 8-Megapixel product using the new BSI-based sensor design called OmniBSI.
But to describe it as a development that "turns the imaging world upside down," as indicated in a press release (issued today), borders on hyperbole. The BSI technology concept is not proprietary to OmniVision and there is every indication that OmniVision's competitors may be catching up.
BSI itself is not a novel concept.
A quick search on the Internet reveals that several related patents have been already granted to companies, including OmniVision's competitors.
They include: "Backside illumination of CMOS image sensor"; "CMOS image sensor with backside illumination and method for manufacturing the same"; "Methods of avoiding wafer breakage during manufacture of backside illuminated image sensors." The first one (United States Patent 6429036 ) is held by Micron Technology, Inc., the second (United States Patent 20070152250) by MagnaChip; and the third (United States Patent 20080044984) by TSMC.
To be fair, OmniVision was quick to acknowledge that backside illumination concepts have been studied for over 20 years. "But we are the first to turn the 'science project' into a process technology," thus taking it to mass production, said Michael Hepp, product marketing manager at OmniVision, in an interview with E.E.Times.
The BSI concept is straightforward. Instead of illuminating a CMOS image sensor from the top (front) side of the silicon die, backside illumination collects photons from the backside of the pixel area. By eliminating obstacles — such as many metal and dielectric layers " which tend to obscure the pixel area, BSI helps stop the loss of photons and maximize the area in which photons are collected.
Advantages of BSI-based sensor design include: improved light absorption; thinner camera modules (due to lower stack height); increased sensitivity and F stops; and reduced crosstalk.