TOKYO CMOS image sensors may be nearing the mass-production stage, as Conexant Systems Inc. and Toshiba Corp. begin to roll out new Common Intermediate Format (CIF)-sized sensors. Conexant has started sampling a PC-camera chip set that includes a CIF-resolution (352 x 288 pixels) CMOS sensor, while Toshiba has announced a 1/7-inch CIF-resolution CMOS sensor.
The most immediate advantage that CMOS imagers have over entrenched charge-coupled devices (CCDs) is low power consumption, which is about one-fourth as much on a CMOS sensor as on a charge-coupled device, said Kevin Strong, general manager of Conexant's personal imaging division.
Conexant's chip set also includes a separate signal-processing engine with USB interface, and related driver and partitioning software that divides the processing load between a host CPU and camera engine. With production set to begin next month, Conexant is making the rounds of leading Japanese digital-camera OEMs.
The devices promise to make mounting a camera on a PC affordable, and perhaps open up a new category of low-cost digital cameras that draw their processing power from the PC. Also, cell-phone manufacturers may use the image sensors to integrate video-capture capabilities on next-generation terminals.
One of the companies Conexant will square off with is Toshiba, which is now sampling both a color and black-and-white version of its CMOS sensor. Sample price for the color version is approximately $25; it is expected to be available in commercial quantities by September.
Operating at a single-supply 2.8-V drive voltage, the Toshiba sensor consumes 15 mW of power. Even with its companion digital signal processor included, the sensor's power consumption is 20 percent as much as a CCD camera system, Toshiba said. The processor includes circuitry for double sampling noise reduction, gain control for stabilizing output levels and a 10-bit A/D converter.
While not disclosing per-unit costs for the Conexant sensor, Strong said he is telling Taiwanese OEMs they can build a USB PC camera for a bill-of-materials cost of less than $25 or a low-end digital camera that connects to a PC and retails for $150.
Next, video phones
Portable video phones are another emerging application. This summer, Japan's Kyocera Corp. will roll out what the company calls the first wireless video phone that features a reflective LCD and camera lens with a 110,000-pixel CMOS image sensor. The company did not disclose the supplier of the image sensor.
Conexant has supplied infrared CMOS image sensors for the U.S. military. Only recently, however, has the company come up with the process recipe for making visible sensors.
Conexant is using 0.5-micron technology for its current CIF devices, a process that requires it to use a relatively large 5.6-micron pixel size. The company plans to soon roll out a 0.25-micron CMOS process that can use smaller pixel sizes, and will start sampling an XGA resolution (1,024 x 768-pixel) sensor by the fourth quarter. Using a single 3.3-V power supply, that device will consume 100 mW of power and include a 10-bit A/D converter. The chip also boasts 30-frame/second readout capability, low noise and high sensitivity.
The biggest problems are controlling photodiode leakage current, reducing noise, controlling pixel crosstalk and getting a handle on the higher-wavelength blue response.