What are CMOS Image Sensors?
CMOS (pr. SEE-moss) stands for complementary metal-oxide semiconductor, the architecture of most computer CPUs and memory modules. Image sensors are silicon chips that capture and read light. High-performance CMOS image sensors use “active-pixel” architectures invented at NASA’s Jet Propulsion Laboratory in the mid 1990s. They can perform camera functions on-chip.
CCD vs. CMOS IMAGE
|The CCD shifts one whole row at a time into the readout register. The readout register then shifts one pixel at a time to the output amplifier.|
technology is now about 25 years old. Using a specialised VLSI process, a
very closely packed mesh of polysilicon electrodes is formed on the
surface of the chip. These are so small and close that the individual
packets of electrons can be kept intact whilst they are physically moved
from the position where light was detected, across the surface of the
chip, to an output amplifier. To achieve this, the mesh of electrodes is
clocked by an off-chip source.
It is technically feasible but not economic to use the CCD process to integrate other camera functions, like the clockdrivers, timing logic, signal processing, etc. These are therefore normally implemented in secondary chips. Thus most CCD cameras comprise several chips, often as many as 8, and not fewer than 3. Apart from the need to integrate the other camera electronics in a separate chip, the achilles heel of all CCD's is the clock requirement. The clock amplitude and shape are critical to successful operation. Generating correctly sized and shaped clocks is normally the function of a specialised clock driver chip, and leads to two major disadvantages; multiple non-standard supply voltages and high power consumption. It is not uncommon for CCD's to require 5 or 6 different supplies at critical and obscure values. If the user is offered a simple single voltage supply input, then several regulators will be employed internally to generate these supply requirements. On the plus side, CCD's have matured to provide excellent image quality with low noise.CCD processes are generally captive to the major manufacturers.
was actually born for the wrong reason. In the 1960s there were
computers but the inexpensive mass-produced memory they needed to
operate (and which we take for granted) did not yet exist.
Instead, there were lots of strange and unusual ways being
explored to store data while it was being manipulated. One form
actually used the phosphor coating on the screen of a display
monitor and wrote data to the screen with one beam of light and
read it back with another. However, at the time the most commonly
used technology was bubble memory. At Bell Labs (where bubble
memory had been invented), they then came up with the CCD as a way
to store data in 1969. Two Bell Labs scientists, Willard Boyle and
George Smith, "started batting ideas around," in Smith's
words, "and invented charge-coupled devices in an hour. Yes,
it was unusual—like a light bulb
going on." Since then, that "light bulb" has
reached far and wide. Here are some highlights:
Image sensors are manufactured in wafer foundries or fabs. Here the tiny circuits and devices are etched onto silicon chips. The biggest problem with CCDs is that there isn't enough economy of scale. They are created in foundries using specialized and expensive processes that can only be used to make CCDs. Meanwhile, more and larger foundries across the street are using a different process called Complementary Metal Oxide Semiconductor (CMOS) to make millions of chips for computer processors and memory. This is by far the most common and highest yielding process in the world. The latest CMOS processors, such as the Pentium III, contain almost 10 million active elements. Using this same process and the same equipment to manufacturer CMOS image sensors cuts costs dramatically because the fixed costs of the plant are spread over a much larger number of devices. (CMOS refers to how a sensor is manufactured, and not to a specific sensor technology.) As a result of this economy of scale, the cost of fabricating a CMOS wafer is lower than the cost of fabricating a similar wafer using the more specialized CCD process.
|VISION's 800 x 1000 color sensor provides high resolution at lower cost than comparable CCDs. Image courtesy of VISION.|
There are two basic kinds of CMOS image sensors—passive and active.
Inexpensive CMOS chips are being used in low-end digital cameras. There is a consensus that while these devices may dominate the low-end of the camera market, more expensive active-pixel sensors will become dominant in niches.
|Toshiba Corporation fabricates a 1,300,000 pixel complementary metal oxide semiconductor (CMOS) image sensor. Courtesy of Toshiba.|
Here are some things you might like to know about CMOS image sensors:
|Fill factor refers to the percentage of a photosite that is sensitive to light. If circuits cover 25% of each photosite, the sensor is said to have a fill factor of 75%. The higher the fill factor, the more sensitive the sensor. Courtesy of Photobit.|
The quality of any digital image, whether printed or displayed on a screen, depends in part on the number of pixels it contains. More and smaller pixels add detail and sharpen edges.