What Is A CCD?
The function of a CCD can be visualized as an array of buckets (pixels) collecting rainwater (photons). Each bucket in the array is exposed for the same amount of time to the rain. The buckets fill up with a varying amount of water, and the CCD is then read one bucket at a time. This process is initiated by pouring water into the adjacent empty column. The buckets in this column transfer their ‘water’ down to a final pixel where the electronics of the camera read-out this pixel (the computer measuring the bucket) and turn it into a number that can be understood and stored by a computer.
Of course, this is an oversimplification – in fact, this ‘model’ (shown above) is actually wrong in some ways, all the pixels in a CCD are actually shifted simultaneously, not one column at a time. We’ll start the explanation process by explaining how a simple pixel works.
The substrate of a CCD is made of silicon, but this is not where most of the action occurs. Photons coming from above the gate strike the epitaxial layer – essentially silicon with different elements doped into it – and generate photoelectrons. The gate is held at a positive charge in relation to the rest of the device, which attracts the electrons to it. Because of the insulating layer – essentially a layer of glass – the electrons can’t make it through to the gate, and are held in place by the positive charge above them.
The top black trace shows the ‘potential well’ for the electrons that are represented by the blue color and is low, or downhill, where the potential is high since opposites attract. As the voltage adjacent to the electron’s pixel is brought high, they begin to migrate in this direction until the voltage in the preceding gate is then brought to zero, or low, thus effectively transferring all the electrons into its neighboring pixel.Now that the electrons are held in place, they need to be moved to where the light signal they represent can be quantified. Shown at left is how this is accomplished.
Electrons are shifted in two directions on a CCD, called the parallel or serial direction. One parallel shift occurs from the right to the left ( shown at left). The serial shift is performed from top to bottom and directs the electron packets to the measurement electronics.
Many CCDs are built with multiple amplifiers at each corner of the CCD and can thus be read out faster. The image is split up into 2 or 4 different sections and read-out as shown below.
A/D electronics have limits on the largest number they can describe. For instance, an 8-bit A/D system, cannot represent a number larger than 28 = 256. 16-bit electronics can’t describe a number larger than 216 = 65536. Thus, a 16-bit camera can never show more than 65,535 ADU in any given pixel. Scientific grade CCDs can generally hold anywhere from 70,000 to 500,000 electrons in any given pixel. Since this is more than the number of ADUs that the A/D electronics can express, different gains must be used for the electronics to access the entire dynamic range of the CCD. At slow read speeds, (i.e. low noise) gains of 0.25e-/ADU are common, thus reading only a maximum of 0.25*65535 = ~16.4ke- which is much lower than the dynamic range of modern CCDs. At higher read speeds, gains of 5e-/ADU can be reached allowing full access to the CCDs dynamic range, but this sacrifices noise for extra dynamic range. All SI cameras can be read at multiple speeds to ensure access to the most important features of the measurement.
All CCDs benefit from working at lower temperatures. Thermal energy alone is enough to excite extraneous electrons into the image pixels and these cannot be distinguished from the actual image photoelectrons. This process generates noise and is called ‘dark current.’ For every 6-7°C of cooling, there is about a 2X reduction in the total dark current generation rate. This of course has its limits, most CCDs don’t function well below –120°C. Below is an example of how the CCD temperature affects dark current. Note that cooling to around –100°C nearly removes the noise generated from dark current.
This can be seen visually below as well. At –100°C, the image looks as a CCD should, with only random read noise present. Note that a bit of over scan has been included – one can tell the electronics to ‘read’ more from the CCD than there actually is to help get a sense of the electronic read-noise unrelated to the photoelectrons in the CCD pixels. As the temperature increases, more thermal electrons are generated. Remember that the electrons in any given pixel are moved across the CCD from left to right, so those electron packets accumulate more charge as they are swept in the parallel shifting of the CCD. This creates a gradient of signal increasing from left to right – a characteristic of a warm CCD. Also of note is the defect in this particular device. Towards the bottom is a column defect. The CCD manufacturing process can produce pixels that generate thermal electrons at a rate greater than their neighbors and inject charge into each electron packet swept past it.Cooling a CCD to –100°C requires that the device is thermally isolated from its environment and thus must be in an evacuated environment. CCDs are commonly cooled with liquid nitrogen, Peltier junctions (thermoelectric coolers), or mechanical pumps (cryo-coolers). Spectral Instruments offers TEC or cryo-cooling since liquid nitrogen cooling doesn’t easily allow the camera to be oriented in any direction
A back-illuminated device, which needs optical wavelength sensitivity, must have an additional coating for proper function. Every company has their own proprietary manner of performing the act of back-thinning and further coating, so variation between manufacturers can be significant. However, shown below is an example of some typical values from a CCD manufacturer (E2V) Spectral Instruments commonly uses.
The table below shows film beating out CCDs in broadband sensitivity as well, but this isn’t really fair since there is really no one piece of film that has usable sensitivity over such a broad range of the electromagnetic spectrum – CCDs can be modified to extend detection out to 125nm into the blue, but they’re not very good into the infrared past 1100nm. Film can be quite noisy even with no incident photons, but a cooled CCD can image for over 30 minutes before a single electron of dark current is generated in a pixel. Some CMOS devices generate less dark current than CCDs at a given temperature, but this is generally irrelevant because of the enhanced read noise from each CMOS pixel. As a consequence, CMOS is not usually cooled to the extent that CCDs will be.
The comparison table above is particularly geared for scientific imaging where exposure times are long and light levels are low. In a general sense, CCDs are much better where low noise is essential, but CMOS can be better in applications where speed is important and cooling is not needed (professional digital photography). CMOS is undergoing rapid changes in technology, and some of the parameters listed above are likely to change in a relatively short period of time.