ASTR 511 (O'Connell) Lecture Notes



CCD'S IN ASTRONOMY


Subaru Mosaic

Subaru CCD Mosaic 8 x (2k x 4k)


Charged coupled devices (CCD's) have been used in astronomy since the late 1970's. They represent the third of the three most important technical innovations for observational UVOIR astronomy in the second half of the 20th century (the others being space telescopes and desktop/personal computers). They are now nearly ubiquitous in observations made at wavelengths between the near-IR (~1 µ) and the X-ray. They have transformed the way astronomy is done.


I. REFERENCES FOR CCD'S

General

CCD Data Reduction and Applications


II. GENERAL DETECTOR CHARACTERIZATION

    QUANTUM EFFICIENCY

    • QE = percentage of photons incident on detector which produce measurable signals

    • Strong wavelength dependence (e.g. threshold cutoffs set by work function/band gap)

    • Typical peak values:

      • Eye: 1-2%
      • Photographic plate: 1-2%
      • Photomultiplier tube: 20-30%
      • CCD: 70-90%
      • IR array (HgCdTe): 30-50%

 
 
QE Curves

Schematic QE curves for various classes of detector


III. BRIEF HISTORY OF ASTRONOMICAL DETECTORS

PHOTOGRAPHIC ASTRONOMY:

PHOTOELECTRIC ASTRONOMY TO 1980:

SOLID STATE ARRAY DETECTORS

Wafer


IV. SEMICONDUCTORS

Semiconductors are crystalline materials which are not normally good conductors of electricity but which can be made to conduct under certain circumstances. The central useful property of semiconductors employed in astronomy is that their electrical properties change significantly after absorption of photons.

BAND GAPS: The properties of semiconductors are manipulated by changing the structure of their internal energy levels, which are spread out into "bands" by the proximity of the component atoms of the solid. The "valence" band, corresponding to the outermost electrons in a normal, unexcited atom, is the lowest energy band where electrons are able to move between ions. However, no net conduction occurs as long as the band is full. Above this lie the "conduction" bands, which are not filled, and where electrons will move freely in response to external EM fields. The separation between the valence and first conduction band is called the "band gap energy", Eg.

Different materials have a wide range of band gaps. "Conductors" have a zero gap, meaning that electrons are always available to transfer charge. "Insulators" have very large gaps, implying zero conduction except under extreme circumstances. "Semiconductors" have intermediate gaps.

Absorption of a photon can push a valence electron into the conduction band and produce a potential electrical signal. The photon energy must exceed Eg, which implies that there is a maximum wavelength for excitation given by:

Lammax = 12,400 Å/Eg(eV)

Obviously, materials with band gaps in the few eV range are of interest as potential UVOIR detectors. Band gaps and max wavelengths for some important materials are given in the following table:

Material Eg(eV) Lammax (Å)
Pure Si 1.1 11,300
GaAs 1.43 8,670
InSb 0.36 34,400
Hg1-xCdxTe 1.55x 8,000/x


DOPING: The "elemental" semiconductors are those elements in group IVa of the Periodic Table (including Si and Ge). These have four electrons in their valence shells, half the maximum allowed. These are shared among the ions in "co-valent bonds." There are many other types of "compound" semiconductors, however, composed for instance of atoms from group IIIa and Va of the Table; two of these are listed in the table above.

The electrical properties of pure semiconductors can be dramatically altered by adding ("doping with") small amounts (~1 part in 106) of an impurity. The result is called an "extrinsic" semiconductor.

  • n-type: a material with more than 4 valence electrons is added (As, from group Va, in the illustration). The extra electrons cannot be accommodated in the valence band and so occupy the conduction band. They represent a persistent set of negative carriers

  • p-type: a material with fewer than 4 valence electrons is added (e.g. B, from group IIIa). This has one fewer electron than normal and creates a small "vacuum" in the electron sea of the valence band. This is called a "hole." As valence electrons shift to fill it, the hole propagates like a positive charge in the opposite direction. The holes represent a persistent set of positive carriers.

  • In pure semiconductors, conduction electrons and holes can also exist, if electrons are excited by thermal effects, for instance. But they always occur in pairs. Electrons and holes in n- and p-type materials, respectively, have no counterparts. Extrinsic material is electrically neutral but is more responsive than pure materials to a difference in electrical potential.
By adjusting the amount of doping, the band gap of the semiconductor (donor/acceptor levels in diagram at right) can be customized. By layering n/p regions, the response to applied potentials can be adjusted to create a large variety of electronic devices.

n-type doping

p-type doping

Doping affects energy levels

Photons are primarily absorbed by electrons in the valence band. For photon energies above Eg, the electron is boosted to the conduction band, leaving a hole behind. If a positive voltage is applied at one side of the semiconductor, the electron will be attracted toward it while the hole will be repelled.


V. BASIC CCD DESIGN

Apart from sensitivity, the key design issues for solid state arrays are to localize photon-produced charges on their surfaces and then arrange to amplify and read them out without distorting the image or introducing unacceptable amounts of noise.

A CCD is a charge-transfer device. Its storage, transfer, and amplification electronics are all fabricated on a single piece of silicon. During an exposure, it traps released photoelectrons in small voltage wells. After the exposure, the electrons are shifted in a series of "charge-coupled" steps across the CCD surface, amplified, read out of the CCD, and stored in a computer memory. This is "destructive readout" --i.e. one cannot read the same signal again (unlike other array architectures, where each pixel is coupled to a separate amplifier).

MOS capacitor
BASIC STRUCTURAL ELEMENT: The basic element in a CCD design is a "Metal-Oxide-Semiconductor" capacitor. See the illustration above. This serves both to store photoelectrons and to shift them wholesale. The bulk material is p-silicon on which an insulating layer of silicon-oxide has been grown. P-silicon can be made to have very few free electrons ("high resistivity") before exposure to light; this is important for best performance. A set of thin conducting electrodes of semitransparent "polysilicon" are applied.

Before exposure, the central electrode is set to a positive bias while the two flanking electrodes are set negative. This creates a "depletion" region under the central electrode containing no holes but a deep potential well to trap electrons. The region shown is about 10 µ thick.

OPERATION SEQUENCE: During exposure (controlled by a separate shutter), light enters through the "front-side" electrodes. Photoelectrons generated under the central electrode will be attracted toward the electrode and held below it. The corresponding holes will be swept away into the bulk silicon. Good performance requires little diffusion of electrons away from the potential well.

    The surface of the CCD is covered with MOS capactitors in a pattern like that at the right. In this particular design, there are three electrodes per pixel. A single pixel is shown shaded in the diagram. Typical pixel sizes are 10-40 µ. The "parallel shift" registers are shown as rows running across the whole face of the CCD. These are separated by insulating "channel stops."

    At one end is a column of "serial shift" electronics and an output amplifier. There is only one amplifier in this design. Contemporary large chip designs involve several amplifiers (but always many fewer than the number of pixels!).

    At the end of the exposure, readout of the collected electrons is accomplished by cycling ("clocking") the voltages on the electrodes such that the charge is shifted bodily to the right along the rows. The figure at the right shows how this is done. Good performance depends on near-100% transfer of the electrons to/from each electrode with no smearing or generation of spurious electrons.

    Each parallel transfer places the contents of one pixel from each row into the serial register column. This column is then shifted out vertically through the output amplifier and into computer memory before the next parallel transfer occurs.

 

 

"BUCKET BRIGADE": The resulting transfer and readout process is illustrated in the animation below:

ADU CONVERSION: For storage in memory, the electrical signal generated by the amplifier must be digitized. This is done by an "analog-to-digital converter". This is normally adjusted such that one digital unit corresponds to more than one photo-electron. Typical values of this conversion are 2 to 8 electrons per stored digital unit.

The stored values are called "ADU's", for analog-to-digital-unit. The corresponding constant of transformation, normally quoted in units of "electrons per ADU", is often called the "Gain" (although this is confusing nomenclature because a larger Gain results in reduced ADU values).

Note that the use of such a conversion importantly affects the statistical properties of the recorded signal. If x is the recorded signal in ADU's, y is the original signal in photo-electrons, and G is the gain, then from Lecture 8 we see that:

Var(x) = Var(y)/G2


VI. CCD DESIGN ISSUES

CCDs have undergone a long optimization process since 1980. Contemporary designs have excellent performance but still require careful calibration in order to overcome inherent limitations. There is also only a handful of reliable manufacturers of professional-grade chips.

Here are some of the issues affecting electronic design and manufacture of CCDs:


VII. ADVANTAGES OF CCD DETECTORS


VIII. LIMITATIONS/DISADVANTAGES OF CCD'S

  • RESPONSE NONUNIFORMITIES ("FLAT FIELD" EFFECTS): Caused by small variations in masks used to manufacture chips, deposition irregularities, thinning variations, etc. Typically 5% pixel-to-pixel. Color-dependent. Requires extensive calibration, with color-matching to targets. Use special exposures ("dome flats" or "twilight flats"). Special observing procedures to suppress flat field effects include:

    • Drift Scanning: see above.

    • Multiple Offset: Break exposure into 4-5 parts, offset ~50 pixels between exposures. Combine exposures during data reduction. "Dithering" is a related technique involving smaller offsets to achieve sub-pixel spatial resolution (see below). These methods result in reduced field of view because not all parts of the original field will have uniform exposures.

  • FRINGING: see above. Caused by interference effects within chip.

  • SENSITIVITY TO COSMIC RAYS: High energy particles produce strong, localized signals in CCDs, especially in thick chips. Their effects must be removed during processing. CR's are a major problem for spacecraft detectors (e.g. on HST). Mitigation requires that exposures be broken into multiple parts (called "CRSPLITs" on HST) so that CR events can be identified. CR hits can be removed from processed data frames, but this always leaves "holes" which have less exposure than other parts of the image.

  • SENSITIVITY TO X-RAYS: An advantage for X-ray astronomy, but some materials in vicinity of CCD's, e.g. special glasses used for windows, can produce a diffuse background of X-rays which add noise to observations.

Flatfield

CCD Flatfield Frame (AAO)

 

Flatfield

CR's on HST/WFPC2 2400 sec exposure


IX. EXAMPLE CCD SYSTEMS


X. CCD HIGH PRECISION CALIBRATION PROCEDURE

A. DATA REQUIRED

B. REDUCTION AND DATA-TAKING PROCEDURES

C. EXAMPLE SCIENTIFIC APPLICATIONS

Drizzle

XI. NON-UVOIR USE OF CCD & RELATED DEVICES

A. X-RAY: CHANDRA/ACIS DETECTOR

B. INFRARED ARRAYS




ADDITIONAL WEB LINKS

CCD-World (forum for discussions about CCDs and their use in astronomy)

Basics:

Example Systems & Development Efforts:


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Last modified June 2017 by rwo


Bandgap images from Britney's Guide to Semiconductors. MOS capacitor illustration from Molecular Expressions. Bucket brigade animation and front/back illumination drawing by C. Tremonti (UWisc). CCD design drawings from C. MacKay, Annual Reviews (1986). Most other images from public observatory sites. Text copyright © 1989-2017 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 511 at the University of Virginia.