CCD vs. CMOS

CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) picture sensors are different technologies for capturing images digitally. Each has unique strengths and weaknesses giving advantages in different applications. Neither is categorically superior to the other, although vendors selling expertise have usually claimed otherwise. The current situation and outlook for both technologies is vibrant, but a new framework exists for thinking about the relative strengths and opportunities of CCD and CMOS imagers.

Both types of imagers convert light in to electric charge and process it in to electronic signals. In a CCD sensor, every pixel's charge is transferred through a limited number of output nodes (often ) to be converted to voltage, buffered, and sent off-chip as an analog signal. All of the pixel can be dedicated to light capture, and the output's uniformity (a key factor in picture quality) is high. In aCMOS sensor, each pixel has its own charge-to-voltage conversion, and the sensor often also includes amplifiers, noise-correction, and digitization circuits, so that the chip outputs digital bits. These other functions increase the design complexity and reduce the area obtainable for light capture. With each pixel doing its own conversion, uniformity is lower. But the chip can be built to require less off-chip circuitry for basic operation.

CCDs and CMOS imagers were both invented in the late 1960s and 1970s (DALSA founder and CEO Dr. Savvas Chamberlain was a pioneer in developing both technologies). CCD became dominant, primarily because they gave far superior images with the fabrication expertise obtainable. CMOS picture sensors necessary more uniformity and smaller features than silicon wafer foundries could deliver at the time. Not until the 1990s did lithography create to the point that designers could start making a case for CMOS imagers again. Renewed interest in CMOS was based on expectations of lowered power consumption, camera-on-a-chip integration, and lowered fabrication costs from the reuse of mainstream logic and memory device fabrication. While all of these benefits are feasible in theory, achieving them in practice while simultaneously delivering high picture quality has taken far more time, money, and process adaptation than original projections suggested.

Costs are similar at the chip level. Early CMOS proponents claimed CMOS imagers would be less costly because they could be produced on the same high-volume wafer processing lines as mainstream logic or memory chips. This has not been the case. The accommodations necessary for lovely imaging perfomance have necessary CMOS designers to iteratively create specialized, optimized, lower-volume mixed-signal fabrication processes--very much like those used for CCDs. Proving out these processes at successively smaller lithography nodes (0.35um, 0.25um, 0.18um...) has been slow and expensive; those with a captive foundry have an advantage

Both CCDs and CMOS imagers can offer excellent imaging performance when designed properly. CCDs have historically provided the performance benchmarks in the photographic, scientific, and industrial applications that demand the highest picture quality (as measured in quantum efficiency and noise) at the expense of method size. CMOS imagers offer more integration (more functions on the chip), lower power dissipation (at the chip level), and the likelihood of smaller method size, but they have often necessary tradeoffs between picture quality and device cost. Today there is no clear line dividing the categories of applications each can serve. CMOS designers have devoted intense work to achieving high picture quality, while CCD designers have lowered their power requirements and pixel sizes. As a result, you can find CCDs in low-cost low-power mobile phone cameras and CMOS sensors in high-performance professional and industrial cameras, directly contradicting the early stereotypes. It is worth noting that the producers succeeding with "crossovers" have always been established players with years of deep experience in both technologies.