1、Designation: E 2642 08Standard Terminology forScientific Charge-Coupled Device (CCD) Detectors1This standard is issued under the fixed designation E 2642; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A
2、 number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This terminology brings together and clarifies the basicterms and definitions used with scientific grade cooled charge-coupled device
3、(CCD) detectors, thus allowing end users andvendors to use common documented terminology when evalu-ating or discussing these instruments. CCD detectors aresensitive to light in the region from 200 to 1100 nm and theterminology outlined in the document is based on the detectiontechnology developed a
4、round CCDs for this range of thespectrum.2. Referenced Documents2.1 ASTM Standards:2E 131 Terminology Relating to Molecular Spectroscopy3. Significance and Use3.1 This terminology was drafted to exclude any commer-cial relevance to any one vendor by using only general termsthat are acknowledged by a
5、ll vendors and should be revised ascharge-coupled device (CCD) technology matures. This termi-nology uses standard explanations, symbols, and abbreviations.4. Terminology4.1 Definitions:advanced inverted mode operation (AIMO), na commer-cial tradename given to a method of reducing the rate ofgenerat
6、ion of dark current. Also known as multi-pinnedphase operation.analog-to-digital (A/D) converter, nan electronic circuitryin a CCD detector that converts an analog signal into digitalvalues, which are specified in terms of bits that can bemanipulated by the computer.anti-blooming structure, na struc
7、ture built into the pixel toprevent signal charge above full-well capacity from bloom-ing into adjacent pixels.DISCUSSIONAnti-blooming structures bleed off any excess chargebefore they can overflow the pixel and thereby stop blooming. Thesestructures can reduce the effective quantum efficiency and i
8、ntroducenonlinearity into the sensor.antireflective (AR) coating, na coating applied to either thefront surface of the CCD or the vacuum window surfaces, tominimize the amount of reflected energy (or electromagneticradiation) so as to maximize the amount of transmittedenergy.back-illuminated CCD (BI
9、 CCD), na type of CCD that hasbeen uniformly reduced in thickness on the side away fromthe gate structure (see Fig. 1b) and positioned such that thephotons are detected on that side.DISCUSSIONA BI CCD leads to an improvement in sensitivity toincoming photons from the soft X-ray to the near-infrared
10、(NIR)regions of the spectrum with the highest response in the visible region.However, compared to a front-illuminated CCD, it suffers from higherdark currents and interference fringe formation (etaloning) usually inthe NIR region. Also called back-thinned CCD.binning, nthe process of combining charg
11、e from adjacentpixels in a CCD prior to read out.DISCUSSIONThere are two main types of binning: (1) verticalbinning and (2) horizontal binning (see Fig. 2). Summing charge on theCCD and doing a single readout results in better noise performancethan reading out several pixels and then summing them in
12、 the computermemory. This is because each act of reading out contributes to noise(see noise).charge-coupled device (CCD), na silicon-based semicon-ductor chip consisting of a two-dimensional matrix of photosensors or pixels (see Fig. 3).DISCUSSIONThe matrix is usually referred to as the image area.E
13、lectronic charge is accumulated on the image area and transferred outby the application of electrical potentials to shielded electrodes. Thesize of pixels in the sensor is typically 26 m 3 26 m; however,sensors can be manufactured in a variety of different pixel sizes rangingfrom 6 m 3 6mto50m3 50 m
14、. Although mathematicallyincorrect, the dimension unit of a square pixel is typically given in1This terminology is under the jurisdiction of ASTM Committee E13 onMolecular Spectroscopy and Separation Science and is the direct responsibility ofSubcommittee E13.08 on Raman Spectroscopy.Current edition
15、 approved Nov. 1, 2008. Published December 2008.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright AS
16、TM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.square microns (for example, a pixel of dimension 26 m 3 26 m isspecified as 26 3 26 m2).charge transfer, nthe process by which a CCD moveselectrons or charge from one pixel to the next.charge trans
17、fer efficiency (CTE), nmeasure of the abilityof the CCD to transfer charge from the point of generation tothe device output.DISCUSSIONIt is defined as the fraction of the charge initially storedin a CCD element that is transferred to an adjacent element by a singleFIG. 1 Cross Sections of Front-Illu
18、minated (a) and Back-Illuminated (b) CCDsFIG. 2 Example of a 2 3 2 Vertical and Horizontal Binning MethodologyE2642082clock cycle. The value for CTE is not constant but varies with signalsize, temperature, and clock frequency.column, na line of pixels in the CCDs image area that isperpendicular to t
19、he horizontal plementary metal oxide semiconductor (CMOS),ntechnology widely used to manufacture electronic de-vices and image sensors similar to CCDs. In a CMOS sensor,each pixel has its own charge-to-voltage conversion circuit,and the sensor often also includes amplifiers, noise-correction, and di
20、gitization circuits. Due to the additionalcomponents associated with each pixel, the sensitivity tolight is lower than with a CCD, the signal is noisier, and theuniformity is lower. But the sensor can be built to requireless off-chip circuitry for basic operation (see Fig. 4).correlated double sampl
21、ing, na readout sampling tech-nique used to achieve higher precision in CCD readout.DISCUSSIONThe sampling circuit is set to a predetermined referencelevel and then the actual pixel voltage is sampled in order to find thedifference between the two. The resulting correlation minimizes readnoise, espe
22、cially in ultra-low-noise CCD detectors.cosmic event, na spurious signal caused by a cosmic ray orparticle hitting the CCD sensor. It is typically observed toFIG. 3 Typical 1024 3 256 (26 3 26 m2pixel) Element CCD Sensor Used for SpectroscopyFIG. 4 Typical Architectures of CCD and CMOS SensorsE26420
23、83result in a high intensity signal coming from a single pixel orsmall group of pixels.dark current, na current that occurs naturally through thethermally generated electrons in the semiconductor materialof the CCD. It is intrinsic to semiconductors and is indepen-dent of incident photons.DISCUSSION
24、Dark current is dependant on the CCDs temperature. Itis expressed in electrons/pixel/unit time.dark noise, nthe shot noise associated with the dark currentfor the given exposure time, and is approximately equal tothe square root of the dark current times the exposure timeused. It is usually expresse
25、d in terms of number of electrons.deep depletion CCD, na CCD that has been designed witha thicker active area to provide enhanced sensitivity in theNIR and hard X-ray regimes.DISCUSSIONBoth front-illuminated and back-illuminated CCDs canbe manufactured with a deep depletion process to enhance the NI
26、Rresponse; however, such devices cannot be operated in AIMO and arealso more susceptible to cosmic rays. A back-illuminated deep deple-tion CCD will have reduced etaloning effects that are typicallyobserved in back-illuminated devices exposed to NIR signals (see Fig.5).dynamic range, nthe ratio of t
27、he full well saturation chargeto the system noise level. It represents the ratio of thebrightest and darkest signals a detector can measure in asingle measurement.DISCUSSIONA true 16-bit detector will have a dynamic range of65 535:1.electron-multiplying CCD (EMCCD), ntype of CCD thathas a two-way re
28、adout register, that is, the shift register andthe gain register, each with its own output amplifier. Whenthe charge is read out through the shift register, the detectorworks like a standard CCD detector, and when the charge isread out through the gain register, it undergoes chargeamplification as a
29、 result of a different electrode structureembedded underneath the pixels of this register (see Fig. 6).DISCUSSIONPassing charge through the gain register allows thesignal to be amplified before readout noise is added at the readoutamplifier, thus improving the signal-to-noise ratios making the camer
30、ahighly sensitive in the low-light regime.etaloning, na phenomenon by which constructive and de-structive interference fringes are produced in a back-illuminated CCD caused by internal reflections between thetwo parallel surfaces of the CCD. Typically BI CCDsexperience etaloning effects when subject
31、ed to NIR signals(see Fig. 5).DISCUSSIONThis effect causes the device to become transparent toincoming photons in the NIR region.exposure time, nthe length of time for which a CCDaccumulated charge.frame, none full image that is read out of a CCD.frame-transfer CCD, na type of CCD whose active image
32、area is divided into two sections, that is, image area and thestorage area. The image area is the light sensitive area of theCCD and the storage area is masked to make it insensitive tolight (see Fig. 7).DISCUSSIONDuring operation the charge accumulated in the imagesection is rapidly transferred to
33、the storage section at the end of theFIG. 5 Cross-Sections of Back-Illuminated (a) and Back-Illuminated Deep Depletion (b) DevicesE2642084exposure time. The storage area is then readout as the image sectionaccumulates charge for the next exposure. This type of CCD reduces oreliminates the need for a
34、 shutter, depending on the speed of the transferfrom image to storage.front-illuminated CCD (FI CCD), na type of CCD inwhich the photons are detected through the gate structurelocated in front of the silicon material of the semiconductor(see Fig. 1a).DISCUSSIONThis type of CCD has moderate quantum e
35、fficiency (seeFig. 8) over the spectral range it covers and it is also free from anyetaloning effects that occur in the back-illuminated CCD when sub-jected to NIR signals. These devices are relatively less expensive tomanufacture than the back-illuminated type.full-frame CCD, na type of CCD that us
36、es the entire siliconactive area for photon detection. A shutter is required toeliminate image smear (see Fig. 3).full well capacity, nthe maximum number of photoelectronsthat can be collected on a single pixel in the image area or inthe horizontal register of a CCD. It is typically specified interm
37、s of number of electrons.gate structure, na polysilicon arrangement of electrodesthat create pixels and move charge.horizontal binning, nthe process that allows charge from arow of pixels to be combined on the CCD chip prior toreadout (See Fig. 2). Horizontal binning is commonly usedin spectroscopy
38、to increase the signal level of a data point,when less horizontal (or wavelength) resolution is not ofconcern.horizontal register, na row of light insensitive pixels that islocated below the CCDs image acquisition area into whichFIG. 6 Typical Sketch of Full-Frame EMCCD SensorFIG. 7 Typical Sketch o
39、f a Frame-Transfer CCDE2642085charge from the pixel columns is clocked and subsequentlypassed on to the output node to be read out. Also called theserial register or readout register.indium tin oxide (ITO), na transparent conductive materialused in some CCD designs to provide an increase inquantum e
40、fficiency (QE) in the blue-green region of thespectrum.intensified CCD (ICCD), na type of CCD camera that hasan intensifier block attached in front of it. An ICCD is usedto amplify the incoming signal without varying the imagesize so as to provide single-photon sensitivity and it can beelectronicall
41、y gated down to nanosecond ranges (see Fig. 9).DISCUSSIONIntensifiers were initially designed for the military fornight-vision ability and are now being widely used in applications thatneed nanosecond gate widths or single-photon sensitivity or both. Theintensifier consists of a photocathode, multic
42、hannel plate and phosphor.A large potential difference is applied across the ends of the multichan-nel plate to amplify the signal. There are two main types of intensifiers:Gen II and Gen III. The main difference between them is in the materialused in the photocathode. The Gen III models are a more
43、advanceddesign and they provide higher quantum efficiencies than the Gen IImodels.interline transfer CCD, na type of CCD designed withcolumns of pixels alternated with masked storage registersso as to increase the rate of acquisition. The storage registersoccupy a portion of the pixel area reducing
44、the fill factor ofthe diodes under the pixels, and hence, such a CCDarchitecture has typically lower quantum efficiencies thatother types of CCDs (see Fig. 10).linear array CCD, na type of CCD that is comprised of asingle row of pixels that are used as the active area forcapturing incident photons.N
45、OTE 1Image used courtesy of E2V Technologies, 106 Waterhouse Lane, Chelmsford, Essex CM1 2QU, England, http:/.FIG. 8 Typical QE Curves for FI and BI CCD SensorsFIG. 9 Schematic of a Typical Intensifier Fiber Optically Coupled to a CCD SensorE2642086multi-pinned phase (MPP), nmode of operation in CCD
46、sthat reduces dark charge.DISCUSSIONAlso known as advanced inverted mode operation(AIMO).noise, nunwanted random variations of output signal that areadded to the real signal and are not subtractable. Noise arisesfrom the statistical variations of both thermal and photon-generated signal as well as f
47、rom electron conduction throughresistive material, and variations in the readout electronics.DISCUSSIONThe total noise in a signal measured by a CCD detectoris referred to as “system noise” and is the equal to the square root of thesum of the squares of each of the individual noise components. Thema
48、jor noise components present in CCD devices are: read noise causedby the systems output amplifier and electronics, shot noise from thelight signal itself, and dark noise (shot noise from the dark signal). Seeread noise, shot noise, and dark noise for further details.open electrode CCD (OE CCD), ntyp
49、e of front-illuminatedCCD in which the electrodes are patterned such that aportion of every pixel on the sensor remains open to directillumination from incident photons (see Fig. 11). Thisminimizes absorption of charge between layers and leads tohigher QEs.outgassing, vgradual release of gaseous molecules in avacuum chamber that degrades long-term vacuum perfor-mance.DISCUSSIONThe use of vacuum-grade materials and advancedvacuum-processing techniques will reduce the rate of outgassing andres
