The Technology

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I have not yet explained what a CCD is. The device invented by Boyle and Smith was named a charged coupled device, hence CCD. It is, of course, a silicon chip -but one with a difference. Whereas most chips are covered in a black plastic casing to keep light out, a CCD chip has a window opening on its top especially to

Figure 1.1. CCD chip - note the window on the top surface to admit the light.

let the light in (see Figure 1.1). Silicon is sensitive to the visible and near infrared parts of the spectrum - outside this range light is either reflected off or passes straight through. What is meant by sensitive is that it will convert incident light (photons) into an electric charge (electrons).

The active light-exposed part of the CCD is divided into photosites or pixels in a matrix of rows and columns - a bit like a chess board with a multitude of tiny squares. I will use the term photosite in this introduction, but the term pixel is equivalent. Each photosite converts light (photons) into electrons and crucially stores them until the end of the exposure. The number of electrons produced is proportional to the light intensity. All we have to do is read out the electrons

Finger Lake Camera

Figure 1.2. State-of-the-art astronomical CCD cameras. Clockwise from top left: Starlight Xpress SXV-M25 - 6-megapixel APS sized single-shot color imager; Finger Lakes Instrumentation's high quantum efficiency (85%) entry-level ME2 camera; Santa Barbara Instrument Group's STL-11000M, 11-megapixel full frame imager; Apogee Instruments Inc.'s Alta E Series Internet remote-controllable camera.

Figure 1.2. State-of-the-art astronomical CCD cameras. Clockwise from top left: Starlight Xpress SXV-M25 - 6-megapixel APS sized single-shot color imager; Finger Lakes Instrumentation's high quantum efficiency (85%) entry-level ME2 camera; Santa Barbara Instrument Group's STL-11000M, 11-megapixel full frame imager; Apogee Instruments Inc.'s Alta E Series Internet remote-controllable camera.

from each photosite (square on our chess board), and we have the makings of a digital image. However, it is a bit more complicated than that! In a CCD, the value of a particular x-y photosite cannot be read out directly but can only be read out from the edge row. Each row is "coupled" (hence the name) to adjacent rows, and when one row has been read out, all the remaining rows are shunted down one, the next is read out, and so on.

A crucial factor in the superiority of CCDs over film is their quantum efficiency (QE). This is a measure of their efficiency in turning incoming photons into an electronic signal and is invariably represented as a percentage. A QE of 100% would be one where every incoming photon is detected and its effect is present in the output. The QE for film is of the order of only 2 to 3%, and even exotic treatments, such as hypersensitization, barely get it up to two figures. For daylight scenes that is not a problem but for astronomy and its low light levels that is profligate waste. The QE for CCDs, on the other hand, varies with wavelength but typically peaks at between 40 and 85%, and even at the blue end of the spectrum, where its efficiency drops off, it still comfortably exceeds that of film (see Figure 1.3). QE is often quoted by CCD manufacturers, but be aware that some quote relative QE and some absolute. Relative means the values given are a

100 90 80 70

40 30 20 10

0 200

Ultraviolet

Quantum Efficiency (QE)

Infrared

Frontside (Typical)

-Kodak E

-Backside Midband Coated

Backside UV Coated

Frontside (Typical)

-Kodak E

-Backside Midband Coated

Backside UV Coated

0 200

Relative Efficiency Wave Length
500 600 700 800 Wavelength (nm)

Figure 1.3. Chart showing typical quantum efficiencies for a range of CCD types. Note how the Kodak ME series almost matches the efficiency of the (expensive) back-illuminated CCDs.

proportion of the maximum efficiency of the device - so figures like 70% in the blue might mean 70% of the peak efficiency in say the red, which is actually a considerably lower QE. Bear in mind also that color CCDs have built-in filters that further reduce the QE. The underlying silicon might have a QE of 50% but the actual photons reaching the photosite will be substantially reduced before they can be detected. Inherent QE varies with the type of CCD, so it is appropriate to consider what those types are.

For amateur CCD cameras there have been three main types of CCD available. The first is the interline device. These are prevalent in domestic video cameras and are optimized for fast readout - essential for video operations. This is achieved by each column of active photosites (pixels) being paralleled by another shielded column of inactive photosites right next to it. Fast readout is possible because at the end of each exposure the charge (electrons) from each photosite is shifted at high speed into the adjacent shielded ones. These can be read out while the next exposure is taking place. Because only half of the columns of the device are being used for photon detection, their QE is immediately halved. Manufacturers can mitigate this by having micro-lenses focusing the light onto the active part but inevitably QE is going to be reduced. Nevertheless, for amateur astronomical CCD cameras interline devices are extremely useful and their mass manufacture for video and consumer digital cameras means they are an excellent value for money. They are the enabling technology behind the one-shot color camera, which makes color planetary imaging so simple and straightforward.

The most common type of CCD on the amateur market is still probably the front-side illuminated CCD. As might be guessed from their name, the light arrives on the front side of the chip. However, the front side is where the surface channels and gate structures are laid down. Beneath these lies the bulk silicon that will absorb the photons and generate the electrons. Therein lies the problem. The light has to pass the front structures before it can be recorded. That reduces the QE of the device as some photons don't make it through, but recent advances, such as the Kodak E and ME series, have reduced the losses. The original front-sided CCDs achieved QE of around 40%, the E series pushed this to more than 70% and the ME series (the M stands for micro-lenses) have lifted it up to 85%. QEs this high were previously the preserve of the back-side illuminated CCDs. Unlike interline CCDs, all the silicon in a front-side chip is available for receiving photons. However, in the ABG (anti-blooming gate) variety this is not the case. Blooming or bleeding is the ugly vertical streaks that occur from oversaturated bright stars and result from electrons spilling into adjacent pixels. ABG overcomes this by having vertical drains which, rather like the interline CCD, are inactive areas that allow the electrons to escape. This comes with a penalty, as not all the silicon is available for detecting photons. QE is reduced by around 25%. This is a heavy price to pay and there are alternatives, such as taking short exposures and summing them, that can reduce bleeding for non-ABG CCDs when bright stars are present.

Traditionally only affordable to professionals, back-sided illuminated CCDs by SITe and Marconi have entered the amateur market. Sometimes referred to as "thinned" devices, they are inverted after manufacture, exposing the bulk silicon directly. This process involves thinning them down to around 15 microns and remounting them upside down. Light no longer has to fight its way through the gate structures. As a result QEs have been the highest available and they are therefore highly desirable to both professional and amateur astronomers. The down side is that their manufacture is expensive. A feature of these CCDs is options on coatings. These can tweak the spectral response of the device. A midband coating produces an enhanced visible/infrared sensitivity with a QE peak greater than 90%. A broadband coating enhances the blue sensitivity, while one with a UV coating takes the spectral response into the ultraviolet.

CCDs are not the only type of solid-state imaging device. As already mentioned, the CMOS (complementary metal oxide semiconductor) imager has joined the party. It is still made of silicon so its intrinsic properties are similar to that of the CCD. They have the big advantage that their manufacture is not dissimilar to that of standard computer chips so they benefit from economies of scale as they are made in standard wafer foundries ("fabs"). Where they differ from CCDs is that on each photosite there are processing electronics such as transistors, amplifiers and circuitry - the actual amount varies according to type, but they are virtually a camera on a chip with the minimum of ancillary electronics needed. The readout procedure is simpler too, so much so that subsections and even single pixels can be read out - something not possible with CCDs. So is the CCD era coming to an end already?

Well, the answer is probably yes... and no. Certainly several areas where CCDs are in use today could well be replaced by CMOS technology. CMOS imagers will probably take over the low-cost high-volume market such as consumer digital still and video cameras. High-performance low-volume markets such as astronomical and medical imaging are likely, for the foreseeable future, to be the preserve of the CCD. Early CMOS imagers were bedeviled by noise and gained themselves a very poor reputation. Like all new technologies they have matured and improved. However, intrinsically they have a lower quantum efficiency (particularly in the red) because of that circuitry over the photosite. This can be mitigated to some extent by micro-lenses focusing the light onto the active part of each pixel. Whichever technology wins, the final product is the same, i.e., a digital image, and as far as we are concerned we could well have more choice and the prospect of falling costs.

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