Can we build a CCD image sensor using standard CMOS processe

Can we build a CCD image sensor using standard CMOS processes?(explain why,why not, or how)

Solution

CMOS 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.

Charge-coupled devices (CCDs) capture light on the small photosites on their surface and get their name from the way that charge is read after an exposure. To begin, the charges on the first row are transferred to a read out register. From there, the signals are then fed to an amplifier and then on to an analog-to-digital converter. Once a row has been read, its charges on the read-out register row are deleted. The next row then enters the read-out register, and all of the rows above march down one row. The charges on each row are \"coupled\" to those on the row above so when one moves down, the next moves down to fill its old space. In this way, each row can be read—one row at a time.

CCD 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.