CMOS challenges CCD for automotive image-sensing applications

Charge-coupled device (CCD) and complementary metal oxide semiconductor (CMOS) sensors are used for digital imaging. CCD-based image sensors currently offer the best available image quality, and are capable of resolutions ranging up to 10.2 megapixels (3,872 × 2,592 pixels) or even higher, making them the prevalent technology for still cameras and camcorders. The downside is that, compared to CMOS sensors, they tend to consume more power, dissipate more heat and are frequently larger, in many cases, requiring additional support ICs for operation.

CMOS sensors are much smaller, consume less power and are beginning to challenge CCD image quality, with resolutions ranging from CIF (352 × 288 pixels) up to approximately five megapixels (2,592 × 1,944 pixels). Because CMOS sensors are fabricated using the CMOS manufacturing process they are much less expensive than CCD-based image sensors.

In 2005, CMOS image sensors were used in more than 95% of the world's camera phones with resolutions from VGA (640 × 480 pixel or 0.3 megapixel) to 1.3 megapixels. The high-volume cell phone market has created enormous pressure on sensor makers to continuously develop better CMOS sensor performance at reduced prices. VGA sensors, which cost $5 in 2003, now sell for less than $2, and yet their image quality and low light performance is the same or better than it was three years ago.

In terms of the automotive market, researchers forecast continued growth for CMOS sensors. Automobiles have used approximately 12 million CMOS sensors in 2005 and that number will grow to more than 40 million units in 2008. By then, automotive will account for 8% to 9% of the total 965 million-unit CMOS camera sensor market. That figure, of course, is based on a prediction that some 600 million camera phones will be sold in 2008.

The automotive industry offers a wide range of applications that require image sensors. The list includes imaging to cover blind spots behind the car, interior occupant classification systems to control airbag deployment, detecting drowsiness in the driver, adaptive cruise control, lane tracking, high-beam dimming and other headlight control. This means there could be 20 image sensors in a car: six for smart airbags, two to four for front and rear monitors, two to four for side monitors, and multiple sensors for lane tracking, adaptive cruise control, and driver authentication.

Most of these automotive sensor systems need only VGA resolution at a maximum of 30 frames per second, which can be easily provided with CMOS technology. Admittedly, the automotive market presents an interesting set of challenges with extreme conditions of stress, vibration, motion, weather, lighting and temperature (spanning -40 °C to +105 °C). Fortunately, CMOS processes can meet such challenges. Low light conditions had previously been an issue for CMOS sensors, but innovations such as Avago Technologies' enhanced performance pixel and array architecture, image pipe and adaptive tone mapping address this problem, resulting in clearer, sharper color photos at all light levels.

What does this mean for new cars in the next five years? The rapidly decreasing price and increasing performance of CMOS image sensors will enable a whole new automotive mainstream market to take off. By mainstream, I mean automobiles in the $20,000 to $35,000 price range, which represents more than 50% of automobile sales volume. At $15 per CMOS-based camera we're talking $300 per vehicle. This compares with an estimated $20 per equivalent CCD-based camera for a total $400 per vehicle. That's 33% more expensive than CMOS. And, in the consumer world, price is a deciding factor.