BSI Technology Takes A Major Leap Forward

Since their efforts were first reported in June 2008, CMOS image sensor specialists at OmniVision have been very busy perfecting their backside illumination technology (see “BSI Technology Flips Digital Imaging Upside Down” at www.electronicdesign.com, ED Online 19160).

Teaming with semiconductor specialist TSMC, the company demonstrated an 8-Mpixel sensor built on a 0.11-µm process technology. At that time, the technology promised shorter lenses, lower z-height modules, easier zoom capabilities, and significantly smaller camera sizes.

Now, OmniVision has unveiled what it calls the world’s first 0.5-in., 5-Mpixel system-on-a-chip (SoC) image sensor based on its OmniBSI technology. Tailored for the mobile-phone market, the sensor carries features and specifies performance levels one would expect to find in higher-end digital cameras, one of which is support for high-definition video.

BSI Review Primarily the staple of military and other high-end applications, backside illumination (BSI) basically flips the image sensor upside down so it absorbs light from the backside. As an alternative to the more common front-side illumination (FSI) technique, it offers the most direct path for light to strike the pixel.

This produces a significantly better fill factor, higher quantum efficiency, and dramatically lower color cross talk. OmniVision’s BSI technology also reduces stack height, increasing the chief ray angle to enable greater zoom tolerances and significantly thinner camera modules.

The initial advantage of BSI is that light entering the sensor takes the shortest path to the detector, through the color filter only (Fig. 1). There are no metal layers or transistors to block or reflect light. Yet the architecture’s most significant feature has to do with pixel size.

Since light strikes the silicon directly, the sensor’s fill factor improves significantly. As a side effect, low-light sensitivity also increases dramatically. The combination of an improved fill factor and a higher low-light sensitivity yields overall better light absorption, which in turn creates a 1.4-µm BSI pixel. A typical FSI pixel measures 1.75 µm.

Enter the OV5642 BSI isn’t a new technology. Due to the company’s proprietary architecture, though, it is now viable for lower-cost, high-volume applications, particularly cell-phone cameras.

Introduced as the first 0.25-in., 5-Mpixel SoC CMOS image sensor, the OV5642 combines OmniVision’s 1.4-µm BSI technology with its top of the line TrueFocus image signal processor (Fig. 2). The BSI architecture enables a low-light performance in excess of 500 mV/Lux-s.

The embedded TrueFocus processor provides all the functionality of a digital camera, including extended depth of field (EDoF). To flatten color noise without affecting details, it relies on unique de-noise, gamma, and color correction algorithms plus imagesharpening techniques that minimize color aliasing.

Further performance enhancements come by way of an anti-shake engine for image stabilization. The sensor supports thumbnailing and Scalado tagging for fast image previews and zooming. It also integrates an auto focus controller, programmable via an internal microcontroller and controllable through the device’s general-purpose I/O.

In addition to still photography, the OV5642 delivers a full-resolution video output at 15 fps and supports 720p high-definition (HD) video at 60 fps and 1080p HD at 30 fps. Also on board is support for bridging and daisy chaining, features that enable external cameras to share the sensor’s TrueFocus processor via the digital video port while delivering simultaneous output through a dual-lane MIPI interface.

In certain processor-sharing situations that involve bandwidth-limited interfaces, the sensor relies on an integrated JPEG-compression engine to ease data transfers. Additionally, a one-time programmable memory is available for any identification and storage chores.

Bottom Line Judging by performance levels and its feature set, it’s fairly impressive that the OV5642 targets the mobile camera phone market. With the ability to deliver near-pro quality still images and support for HD video, it seems the only limitation would be the storage (memory) capabilities of the end product.

That aside, it won’t be long before we’re walking around with a high-end film studio in our pockets, one we can also use to phone home. Sampling now, expect volume production of the OV5642 this quarter.