Philips advocate CMOS image sensors
Feb 28, 2000 at 04:00 GMT
Philips advocate CMOS for tiny lower resolution imaging devices, although this news may not have immediate imact on digital still cameras any progress in this area will impact later.
CCD advocate Philips turns to CMOS image sensors
EETimes: LONDON Philips Semiconductors, a longtime proponent of charge-coupled devices for image sensors, has adapted a mainstream CMOS process technology for use in making lower-resolution image sensors. Philips calls its process adaptation SeeMOS, and touts it size and power advantages.
"With our new SeeMOS technology, you will one day be able to make a video camera no thicker than a pencil and about a centimeter long," said Theo Claasen, chief technology officer at Philips Semiconductors (Eindhoven, Netherlands). Claasen said the picture quality of the CMOS sensor "is excellent" at 640 by 480 pixels.
Many other companies, such as VLSI Vision Ltd. (Edinburgh, Scotland) and Tower Semiconductor (Migdal Haemek, Israel), have pursued CMOS image sensors with an interest in possibly integrating additional image-processing circuitry on the same die as the sensor to save cost, size and weight in systems.
Though previously critical of the quality of such image sensors compared with CCDs, Philips now agrees that by eliminating the need for additional chips it should be possible to build a smaller, less expensive video camera with CMOS that uses 20 percent less power than current designs.
"[SeeMOS] opens up a tremendous range of low-cost, high-volume applications," Claasen said, "especially since it operates at 3.3 volts, which can easily be supplied by a couple of batteries, as opposed to the 15 volts required by today's designs.
"The small size and tiny power requirements are perfect for handheld, battery-operated products-in particular, the next generation of mobile phones that are now being designed for introduction in 2000 and will have videophone capabilities," he said
Among the expected applications are low-cost videoconferencing for laptops and PCs, medical cameras, security cameras, image-recognition systems, solid-state camcorders and computer vision for collision-avoidance systems in cars.
Claasen said, "One interesting possible application, which the ultrasmall size and low power consumption open up, is to build one of these tiny cameras into a pair of glasses for a Web-cam or video postcards."
Philips said that the use of conventional CMOS processes to form image sensors had not taken off because of problems tuning the photodiodes to be sensitive enough across the entire visible spectrum. Philips said it is using its experience in CCD manufacturing to develop process options for high sensitivity, compatible with generic CMOS processes, that it is currently prototyping.
"Most of the other companies working in this area do not have CCD expertise or systems knowledge, nor do they have their own fabs," said Claasen.
The second problem Philips claims to have solved is figuring out how to put image-processing circuitry on an IC without reducing the area available to capture the image. The limitation makes the CMOS-based sensor intrinsically less sensitive than a CCD.
For example, a pixel in a CCD device measures 5 x 5 microns, all of which captures light. With SeeMOS technology, over half the area is taken up with circuitry and thus lost for light capture. Using techniques from its CCD technology, Philips Semiconductors places a tiny lens over each pixel to focus the light onto the light-sensitive area. Before applying the microlenses, a Bayer color filter pattern is placed on the surface of the sensor area to obtain full-color images.
Philips Semiconductors said it views the addition CMOS imaging technology as complementary to its existing CCD business. SeeMOS is not currently suitable for high-resolution imaging, the company said, so digital still cameras with requirements for more than 1 million pixels and high-resolution video cameras will continue to be served by CCDs. SeeMOS will be used for lower-resolution, small-physical-format video or digital still cameras.