Product
Review
Selecting a CCD Camera Since charge-coupled devices (CCDs) became commercially available in 1973, they have been widely used in applications rangingfromtelevision cameras to spy satellites to astronomy and analytical spectroscopy. Scientific-grade devices cooled to subzero temperatures reduce the detector noise. CCDs also provide high quantum efficiency and a large dynamic range. Each camera system comprises the CCD device and read-out electronics. In most cases, the camera manufacturer is not the device manufacturer. The user selects the most appropriate CCD for a given application; the camera manufacturer obtains that device and provides the appropriate read-out electronics. Therefore, a wide variety of devices can be combined with many types of read-out electronics.
J e f f r e y H. Giles T r e n t D. Ridder Robert H. W i l l i a m s David A. J o n e s M . Bonner D e n t o n University of Arizona
Many factors should be considered when choosing a CCD camera system. Several considerations go into selecting a CCD camera system. This Product Review discusses many of these considerations to help potential users select the correct CCD and read-out electronics. Table 1, which is not necessarily comprehensive, lists representative manufacturers of CCD camera systems. An expanded version of Table 1 indicating the range of each company's product line can be found in supporting information on the Analytical Chemistry Web site (http://pubs.acs.org). Quantum efficiency and wavelength range
A CCD detects llght through the creation of electron-hole pairs in silicon. The electrons are trapped by an imposed electric field, ,ead out ta sharge ey the eamera electronics, and converted to computerreadable form by an analog-to-digital converter (ADC). At the temperatures at which scientific CCDs usually operate
(-30 °C to -120 °C), the indirect bandgap of silicon is —1.14 eV. Therefore, only photons of wavelengths ~250 nm, absorption results in a single electron, and the maximum possible QE is 100%. Farther down in the UV and into the soft X-ray region, the creation of multiple electrons can result in QE >100%. (Alternatively, QE is die percentage of incident photons that generate measurable electrons. This definition does not allow QE > 100%.. Essentially, QE is a measure of the sensitivity of the device. QE varies with wavelength and from CCD to CCD. Therefore, one of the most important documents to examine when considering a camera purchase is a plot of
Analytical Chemistry News & Features, October 1, 1998 663 A
Product
Review
Table 1 . Representative CCD c a m e r a system manufacturers. Andor Technology 435 Buckland Rd. Rosewood Bldg. South Windsor, CT 06074 860-648-1085 www.andor-tech.com RSN 401
MedOptics 4585 S. Palo Verde Rd. Ste. 405 Tuscon, AZ 520-750-0256 www.azstarnet.com/ -medoptx RSN 406
Santa Barbara Instrument Group 1482 East Valley Rd., Ste. 33 Santa Barbara, CA 93150 805-969-1851 www.sbig.com RSN 411
Apogee Instruments 3340 N. Country Club, Ste. 103 Tucson. AZ 85716 520-326-3600 www.apogee-ccd.com RSN 402
Olympus America Precision Instrument Division Two Corporate Center Dr. Melville, NY 11797-3157 800-446-5967 www.olympus.com RSN 407
Silicon Mountain Design 5055 Corporate Plaza Dr. Colorado Springs, CO 80919 719-599-7700 www.smd.com RSN 412
Eastman Kodak Motion Analysis Systems Division 11633 Sorrento Valley Rd. San Diego. CA 92121 619-535-2908 www.masdkodak.com RSN 403
Photometries 3440 East Brittania Dr. Tucson, AZ 85706 520-889-9933 www.photomet.com RSN 408
Spectral Instruments 1802 W Grant Rd. Ste. 120 Tucson, AZ 85745 520-884-8821 www.specinst.com RSN 413
EG&G Reticon 345 Potrero Ave. Sunnyvale. CA 94086-4197 408-738-4266 www.egginc.com/reticon RSN 404
PixelVision 14964 N.W. Greenbrier Pkwy. Beaverton, OR 97006 503-629-3210 www.pv-inc.com RSN 409
SpectraSource 31324 Via Colinas Ste. 114 Westlake Village, CA 91362 818-707-2655 www.optics.org/spectrasource RSN 414
Instruments SA 3880 Park Ave. Edison, NJ 08820 732-494-8660 www.instrumentssa.com RSN 405
Princeton Instruments 3660 Quakerbridge Rd. Trenton, NJ 08619 609-587-9797 www.prinst.com RSN 410
Xillix Technologies 13775 Commerce Parkway, #300 Richmond, BC Canada V6V 2V4 604-278-5000 www.xillix.com RSN 415
RSN—Reader Service Number
QE versus wavelength. Such plots should be available from all camera manufacturers, but QE measurements are often made by the CCD manufacturer and then simply passed along. In selecting the appropriate camera system, the CCD performance should be weighed against the requirements of the user's application. Most camera manufacturers offer various CCD types with differing QE curves. Examples of QE curves appear in Figure 1. One reason QE is limited is that silicon has a reflectivity of -30% at 1090 nm, reaching 50% below 400 nm. Applying antireflective (AR) coatings to the CCD surface would seem to be the obvious solution, but the photosensitive silicon (referred to as the epitaxial layer, or epi) is covered by a gate structure that imposes the voltages for electron trapping and readout. This gate structure is made from materials, such as silicon dioxide or silicon nitride that have different refractive indices than silicon, thus making AR coat664 A
ing of the front mostly ineffective. One way around the reflectivity limitation is to illuminate the back of the CCD— the side of the epi without the gate structure. The back can easily be AR coated because its composition is homogeneous. Unfortunately, with conventional integrated circuit fabrication techniques, the epi is grown on a thick substrate that must be etched away by mechanical and chemical processes. Such CCDs are referred to as back-thinned; they are significantly more costly than die usual front-illuminated variety. With applications in which high S/N is desired at low incident light levels, however the expense of back-illuminated CCDs is often justified. Another limitation on QE is that the depth at which a photon penetrates the epi before creating an electron-hole pair is wavelength-dependent. At 275 nm, the penetration depth (the depth at which 90% of the photons have been absorbed) is only about 20 A, but it rises exponentially to
Analytical Chemistry News & Features, October 1, 1998
~ 30 um at 800 nm and reaches several hundred micrometers by 1000 nm. In the near-IR (NIR), QE is attenuated because many of the incident photons pass through the epi, which is often only ~20 vim thick, without creating detectable electron-hole pairs. In addition, detection requires that the photoelectron be trapped in the electric field (often referred to as the "well") imposed by the gates. The penetration depth of thisfieldinto the epi, referred to as the depletion depth, is often only 2-5 um. Thus, many of the electron-hole pairs created in the epi but below the depletion depth will recombine and become undetectable. Furthermore, charge generated far from the electrodes might migrate to an adjacent pixel's well reducing spatial resolution One solution to this problem is to increase the depletion depth and possibly the epi thickness These CCD modifications are called deer; depletion and thick eoi respectively and improve NIR sensitivitv
systems with very low noise can be more Penetration depth also creates a serious problem for UV photons. For ffont-illuminated expensive, so simply buying the best system available is not always a cost-effective CCDs, the penetration depth is such that absorption takes place in the gate structure solution. before UV photons can reach the epi and The two main types of noise associated be trapped in a well. In many devices, the with a CCD camera system are read noise QE becomes essentially zero for photons of and dark noise (the latter is also called wavelength
