The CCD: Charge-coupled Device

We take a look at the evolution of the CCD and gain insight into the construction and briefly its inner workings.

 

Since Nic Wood originally wrote this article there have been great strides in the development and further advance of the Complementary Metal Oxide Semiconductor CMOS sensor, however, this article will give you insight into this sensor that is now being used in most of our digital cameras and devices. CCD or Charge-coupled Device, these are names from a component that is present in almost every digital still and video camera available on the world market today. But, what exactly is a CCD, and how does it function? We all know that it is this component that converts light into an electronic signal. By what magical process is this possible?

The story of the CCD starts in the late 1960s. Bell laboratories were working on a new form of memory chip, that chip was a CCD. However before the research was completed on the CCD as a memory devise, EEPROM memory was invented and CCD research would have become obsolete, but for one property of the CCD that made it unique. It was found to be very sensitive to photons (the particle that makes up light).

In late 1969, Dr William S. Boyle (Willard S. Boyle) and Dr George E. Smith, (George E. Smith) displayed the first working prototype of the CCD and in 2009 they were awarded along with Charles K. Kao groundbreaking achievements concerning the transmission of light in fibres for optical communication (The fibre optic cable) I have deliberately left this segment in the article as fibre optic communication is a revolution in itself and together were award the Nobel Prize in Physics.

CCD’s are constructed upon a wafer of silicon; today many CCD’s are manufactured on a single wafer of silicon and separated later. Silicon is a man-made crystal that is difficult to grow. Absolute clean-room conditions are required in the fabrication of a silicon wafer and growing wafers is a precision process. A single CCD is covered with a gridwork of pixels (or Picture Elements) that can number into the millions. To better understand the functioning of a CCD as a whole, let us look at the functioning of a single pixel.

Dr William S. Boyle

Nobel Prize in Physics – for the invention of an imaging semiconductor circuit – the CCD sensor.

 

Dr George E. Smith

Nobel Prize in Physics – for the invention of an imaging semiconductor circuit – the CCD sensor.

A pixel is a single silicon IC (integrated circuit), that is, an IC that is etched onto the surface of the silicon wafer using a lithography process. The etched silicon IC consists of two essential cells. The one cell contains electrons that are loaded to this cell prior to the exposure to light (this is the charge process). The other cell is a charge ‘well’ and is designed to catch (or attract) stray electrons. When the pixel is exposed to light, the photons travel through the silicon wafer, as they travel through the photons excite the electrons stored in the one cell of the IC, The bonds holding some of the electrons as broken and the electrons are released. The adjacent cell, which is of an opposite charge to the electrons, attracts these stray electrons into its well. 

The amount of electrons released by the photon is directly proportional to the frequency of the photon. From this, the frequency (wavelength) of the photon can be calculated. The charge collected within the well is read into an output stream, along with all the other charges collected by all the other pixel wells. Each value is then calculated and converted into a colour point, and the whole image is then reassembled. This process is repeated up to 50 times a second depending on the type of CCD being used. The first commercially viable CCD was released on the market in 1974 and had a pixel density of just 100 x 100 lines. Research continued by both NASA and the US Military. NASA wanted better quality cameras on their space probes and the Military were in the middle of the cold war with Russia and wanted to better equip their spy satellites, CCD’s offered the perfect solution.

As the resolutions have increased, meaning more pixels (silicon IC’s) cramped into the same surface area, other problems have had to be solved. One problem it reading the electrons through the output streams, without them being contaminated by light. In order for the CCD to function effectively, the previous stream is still being read out while the next image is being exposed. This was made possible with the inclusion of a light safe field that runs parallel to each of the wells. The collected charge is transferred into this field and transported down into the output stream.

CCD’s are not the only semiconductor type IC on the market. CMOS image sensors have been around as long as CCD’s, but until the mid-1990s technology was unable to manufacture CMOS chips to a comparable level of performance. Over the last few years CMOS chips, which are cheaper to produce but still of poorer quality to the CCD, have gained in popularity are today available in a large yet lower-end product range. While research into both CCD and CMOS technology continues at a rapid pace, the future advances in these two formats promise to be exciting. CMOS has great potential, improved design can make enhanced on-chip processing more of a reality in the future. It is currently possible to integrate a Digital to Analogue converter right into to surface of the CMOS chip.

Because of the almost zero loss of energy generated by the electron exchange on a CCD. CCD still has a better tonal density, generate less noise (e.g. unwanted interference) and are more light sensitive than CMOS chips Only time will tell the advances to come for both of these technologies. CMOS sensors are just now improving to the point where they reach near parity with CCD devices in some applications. CMOS cameras are usually less expensive and have great battery life.