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The Structure of CCDs

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A CCD is an array of photosensitive elements, each one of which generates photoelectrons and stores them as a small bucket of charge. When requested, the elements form a bucket brigade; each row of charges is passed from element to element down the columns and horizontally along the final row to be measured in turn and recorded digitally. Each picture element (pixel) is typically 20 to 30 square. CCDs in use as astronomical detectors for the past five years have about 200,000 pixels, usually arranged in a format of about 500 rows by 400 columns, and the active area is thus about 1 cm. Successful observations have now been made with a new generation of CCDs 5 times this size, with 1500 to 2000 pixels.

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The advanced semiconductor technology which allows all this to happen is sketched in Fig 1. The device is a silicon integrated circuit consisting of an oxide-covered silicon substrate upon which an array of closely- spaced electrodes is formed. These electrodes are made from polysilicon (polycrystalline silicon) which is transparent to light. Photons imaged onto the surface penetrate the electrode structure into the substrate where they generate hole-electron pairs in numbers precisely proportional to the numbers of incident photons. The holes are conveniently lost by diffusion down into the depths of the substrate, while the electrons migrate rapidly to the nearest biased electrode where they collect as a single charge-packet. The array of ``horizontal'' electrodes above ``vertical'' charge-transfer channels (columns) defined by column isolators, or channel-stop regions, is known as buried-channel architecture, a design which permits rapid and efficient charge-transfer. A group of three electrodes between column isolators forms a pixel, the central electrode carrying the (positive) bias voltage during exposure, and defining the pixel centre by virtue of this bias.

Fig 2 is the schematic representation of a very little CCD - 5x4 pixels - to demonstrate the charge-coupled aspect of CCDs and how frame- transfer is organized. To read out the CCD after exposure, the charge packets are shifted down the channels (columns) by taking the voltage on the adjacent electrode to a high level and then reducing the voltage on the first electrode. The process is known as clocking, and a three-phase system of voltage waveforms and electrode triplets is needed to achieve this (Fig 2). Three is the minimum number of phases: with double- electrodes and two phases, the charge packet would not know which way to go.

As each row reaches the bottom of the array it enters the horizontal register. This is structured (Fig 2) with triplet electrodes as for the signal pixels; and the horizontal clocking waveforms transfer the packets along this register in the same way. On reaching the end-of-the-line, each charge packet is detected as a voltage across a capacitance, the voltage is amplified by an on-chip amplifier, and digitized with the resulting numbers placed sequentially on a storage medium such as disk or tape. The next row is driven down into the horizontal register, and the horizontal readout repeated. With the CCD cleared of charge in this way, the final array of numbers represents the image.

This digital or slow-scan mode of operation requires one to several seconds to read out a frame, the digitization process taking the time. Analogue operation is used for CCDs in television cameras, the original motivation for CCD development in the 1970's. Here the voltage developed at output by each charge packet is amplified and constitutes the video signal which is recorded directly on video tape. TV frame rate is 30 images/second, and the charge-clocking is thus required to be some 100 times faster; digitization takes far too long.