Operating Principles of CCDs

A CCD sensor is made up of a number of photosensitive semiconductor sensors that create electrical charges proportional to the intensity of the incoming light and of analog shift registers (a type of a chain of logic electric circuits used mainly for storage of data and their transfer from one to another end of the chain) that move these charges to the output of the device. The photosensitive part of the CCD sensor, its area imaging device, is a mosaic of several thousand photosensitive cells. Each of these cells corresponds to one element of the picture—that is, to one pixel. It is obvious that the resolution of a given CCD sensor will depend directly upon the number of its photosensitive elements. For standard definition television, it is usual to have 500 to 1,000 cells in each row. The number of rows is dictated by the number of lines of a given television system.

Each element of the mosaic consists of a specially treated silicon plate (called

P-type silicon) on which a thin layer of silicon dioxide is applied. That insulating

silicon dioxide layer bears an electrode connected to the voltage source. If the voltage applied to the electrode is positive, then a region of low potential energy (a depletion) is created just under the junction of two silicon layers (see Figure 9.7). That low potential energy region acts as a sort of well or empty container attracting all free electrons in the region. The higher the voltage applied to the electrode, the deeper the depletion and therefore the more attractive the well will be.

9.2 Sensors 143

Metal electrode

Silicon dioxide

Silicon substrate Depletion region

Figure 9.7 The CCD cell.

Silicon substrate Photosensitive cells Electrons (a) (b) ++++ ++ + ++++

Figure 9.8 CCD operation: (a) storing the charge and (b) coupling and transfer of

charges.

Free electrons appear in the silicon substrate under the impact of light. The number of free electrons is proportional to the intensity of the incoming light. All these free electrons will be attracted and the depletion will store the accumulated charge, acting as an analog memory device (see Figure 9.8a).

The next step in the CCD operation is the coupling and transfer of charges. As we mentioned earlier, each element of the mosaic has its own electrode. However, the elements share the same P-type silicon substrate. If we decrease the positive voltage on the first electrode and apply a higher voltage level to a neighboring one, the free electrons will start to migrate from the first to the second element (Figure 9.8b). At one point, all electrons will be transferred to the second element of the mosaic. By applying a similar procedure to electrodes of the following elements of the mosaic, we enable the negative charge induced by the effect of light to travel through all elements organized as analog shift registers.

According to the organization of registers and the way of transferring charges, we can differentiate three main groups of CCD sensors:

1. frame transfer CCDs (FT) 2. interline transfer CCDs (IT)

3. frame interline transfer CCDs (FIT)

Frame transfer CCDs were developed for professional applications. They are con-

stituted by three zones (three groups of elements) forming three registers (see Figure 9.9a). The first zone—the image zone—is exposed to the incoming light. The other two zones are always protected from any incoming light. The image zone is made exclusively of active, photosensitive elements. The incoming light conse- quently creates charges in each of these active elements that are proportional to the intensity of light falling on that particular element. During the vertical blank- ing interval, a mechanical shutter, similar to the one installed in film cameras, interrupts the light flux from the lens to the sensor. At the same time, transfer voltages are applied to the electrodes and all charges are transferred from zone one to zone two—the memory zone. At the end of the vertical blanking interval and at the beginning of the next field, the shutter is open and the light falls again on the active surface. Meanwhile, and for the duration of the whole field (1/50 or 1/60 of a second), the charges from the memory zone are transferred, line by line, into the third, horizontal, or read-out register, which delivers the resulting video signal to the output terminal.

Interline transfer CCDs were initially developed for consumer cameras (see

Figure 9.9b). Each active photosensitive element of the mosaic has an associ- ated element belonging to a shift register and protected from the impact of light. Active and register elements create a number of parallel vertical columns. Dur- ing the vertical blanking period, all charges from active elements are transferred to the register ones. During the next field, while the light impact creates new charges in photosensitive and unprotected cells, the charges accepted by the shift register cells during the previous vertical blanking are transferred “downward” along vertical shift register columns and transmitted to the horizontal read-out shift register.

The main drawback of the FT CCD sensors is the necessity of implementing a rotating shutter in an all-electronic camera and of coping with the inconve- nience created by that mechanical device (necessity to implement a shutter and its motor in the body of the camera, to solve the problems of vibrations, iner- tia, noise created by the rotating blade, and so on). On the other hand, the IT CCDs also has drawbacks. The surface of such a sensor is made up of not only active, light-sensitive elements but also of nonphotosensitive register ele- ments. That solution diminishes the number of photosensitive cells that can be

9.2 Sensors 145 Read-out register Output terminal Photosensor (a) Image zone Memory zone (b) Vertical shift register Photosensor Output terminal Optical mask

Horizontal read-out register

Figure 9.9 Types of CCD sensors: (a) frame transfer, (b) interline transfer, and (c) frame

Photosensor Vertical shift register Imaging area Optical mask Storage area Output terminal Horizontal read-out register

(c)

Figure 9.9 Continued.

installed on the CCD surface, thus limiting the resolution power of such a sensor. When the scene in front of the lens contains important highlight areas, the active elements of the sensor that receives these highlights are saturated and some of the generated free electrons are “spilled over” into the neighboring elements of the mosaic. Since these electrons are not the result of the light falling on the concerned element or of a controlled transfer, they generate a clearly noticeable spurious red or pink vertical line in the reproduced picture. This effect is called smear. To diminish this unpleasant effect, later IT CCD sensors have been equipped with an additional horizontal shift register whose role is to drain the overflow of free electrons resulting from highlights and to prevent the saturation of active elements.

9.2 Sensors 147

Frame interline transfer CCDs are, in a way, a combination of both FT and

IT transfer methods and operating principles (see Figure 9.9c). At first glance, FIT sensors look like FT ones since they each have three zones, or areas—the image area, open to the impact of light, and two other, permanently protected areas. The image area, composed of active and shift register cells, looks very much like an IT sensor. The incoming light creates electrical charges in active photosensitive cells. During the vertical blanking interval, these charges are transferred from active cells to shift register cells. While in the case of IT sensors, the transfer from the shift registers to the horizontal register is done continuously (at a line rate within one field, that is, for the duration of one television field), in the case of FIT sensors, a memory zone, protected by an optical mask from the incoming light, is inserted between the active field and the horizontal register. Thanks to the addition of that memory area the charges in vertical shift registers can be rapidly transferred to the memory area (during the vertical blanking interval, that is, 60 times faster than in the case of IT sensors). Once in the memory area, the charges are transferred at a line rate to the horizontal read-out register, in the same way as in FT sensors.

The net result of such a charge-transfer process is the elimination of the mechanical shutter and the considerable reduction of the unwanted smear effect. As indicated above, the smear is the result of the over-saturation of a given pho- tosensitive element and the subsequent overflow of the charge to the neighboring elements. In the case of IT sensors, the charges are transferred line by line from vertical registers to the horizontal one, and that relatively slow transfer rate is convenient for the propagation of the smear. Considering that the transfer from vertical registers for FIT sensors is 60 times faster, the visibility of the smear effect should be diminished in the same proportions. The line-by-line transfer is per- formed in the memory zone that acts as a buffer between the very fast transfer (which helps eliminate the smear) and the fixed transfer speed defined by the line-repetition rate of a given scanning system.

FIT sensors also offer the opportunity to implement an “electronic shutter.” If an additional vertical register is installed on the reverse side from the shift register, it can act as a sort of electron “drain,” discarding charges instead of conducting them toward the memory zone. The modification of the speed of change of the control voltage is in fact the modification of the integration time in active cells, which is equivalent to the exposure time of photo cameras. The integration time can vary from 1/100 to 1/1000 of a second, allowing an excellent rendition of fast-moving objects. However, the price of introducing this feature is a certain loss in sensitivity.

From the time of their introduction, CCD sensors have progressed tremen- dously. Some of their initial limitations (such as the lack of resolution, rela- tively low sensitivity, smear, hypersensitivity in the area of red wavelengths,

and the response nonuniformity) were successfully overcome by appropriate solutions. Hole-accumulated diode (HAD) sensors dramatically increased sen- sitivity and reduced the visibility of smear. Hyper-HAD sensors introduced micro lenses applied over each cell of the photosensitive mosaic. These micro lenses increased the efficiency and sensitivity of the CCD by collecting the light that otherwise would be scattered over nonsensitive parts of the sen- sors. High-definition CCD sensors became a standard product. For the moment it seems that this type of solid-state sensor still has considerable development potential.