ccds evolve to suit vast array of scientific uses by sdfsb346f


More Info
									                                                                                                                                                       product guide

ccds evolve to suit vast
array of scientific uses
Charge-coupled devices have been developing at a rapid pace over the past few decades.
Antoinette o’grady gives an overview of what’s on offer today and provides useful guidelines
on choosing the right detector format and the right architecture for your application.
Charge-coupled device (CCD) cameras
have been around since the 1960s and
have been constantly improving in terms                                            sensor area                                          sensor area
of performance, design and ease of use.
Today, they are the detectors of choice for
a wide range of scientific applications (see                                                                                            frame-transfer area
table p33). Scientific CCDs can be divided                                         storage area
into three main formats – full-frame,
                                                                                                                                                      traditional amplifier
frame-transfer and interline devices.                                                                                  normal clock voltages          wide-dynamic-range operation
   A full-frame CCD (figure 1) is basically a                  (a) frame-transfer CCD
2D array of photo sensors, which comes in                                                                                                             EMCCD amplifier
a silicon-based semiconductor-integrated                                                                               high clock voltages            high-sensitivity operation
circuit package. Each individual photo-                                                                          (b) electron-multiplying CCD
sensor is called a pixel. Photons falling on
the CCD’s pixels can either be reflected,
pass straight through the sensitive silicon                       sensor          charge is shifted vertically
layer, or be absorbed in the sensitive silicon                    area
layer, and then converted into electrons.
   The potential for a photon of a given
wavelength to be converted to an electron
is known as the quantum efficiency (QE).                                      horizontal bi-directional shift register
The electrons can be stored in each pixel.                       low capacity amplifier                    high capacity amplifier
In order for the corresponding charge to be                    (c) full-frame CCDs can have one or two amplifiers

                                                                                                                                                (d) interline CCD
read out, it is shifted vertically from pixel to
pixel sequentially into a read-out register.                  Fig. 1: common CCD formats for scientific applications include full frame and frame transfer.
The electrons in each pixel in the read-out
register are then shifted horizontally into                   full-frame devices for imaging applications                  the need for a mechanical shutter by hav-
an output charge amplifier.                                   in order to prevent light falling on the pixels              ing a sensor that is divided into two parts
   One of the fundamental characteristics of                  while they are being read out, which would                   – an image area and a storage area. The
CCDs is that they have low read-out noise.                    result in a smearing of the image, unless                    image and storage areas are usually iden-
It is the read-out noise that determines the                  the exposure time is significantly longer                    tical in size, and the storage area is cov-
detection limit or smallest detectable sig-                   than the read-out time. However, mechan-                     ered with an opaque mask, usually made
nal, as a signal has to be larger than the                    ical shutters can have lifetime issues and                   of aluminium. The image can be trans-
noise floor in order to be seen. As the band-                 can be relatively slow.                                      ferred very quickly into the storage area.
width of the charge amplifier increases, so                      Rectangular arrays with larger pixels                     However, to achieve continuous frame
does the noise level of the device. However,                  are typically used for spectroscopy appli-                   rates, the minimum exposure time has to
keeping the bandwidth small to minimize                       cations as here the dynamic range (the                       be equal to the read-out time of the storage
the noise level of the device will limit the                  ability to see small signals adjacent to large               area. It is possible to reduce the minimum
read-out speed of the CCD and result in a                     signals) is more important than the resolu-                  exposure time to a lot less than the read-
slow frame rate.                                              tion. The bigger the pixel, the larger the well              out time of the storage area if a continuous
                                                              depth and the better the dynamic range.                      frame rate is not required – for example, if
Array formats                                                 Mechanical shutters are often not required                   only two extremely fast successive frames
The 2D array is usually in a square or rec-                   for these applications as the sensor is usu-                 were required. Typically, frame-transfer
tangular format and pixel sizes generally                     ally read out in a series of columns, rather                 devices can provide frame rates of up to
range from 6.8 to 26 µm. Typically, ima-                      than individual pixels. In this case, it does                3–4 frames/s. Full-frame and frame-trans-
ging applications use square arrays with                      not matter that light is still falling on the                fer CCDs are ideal for static low-light appli-
smaller pixels, as resolution is important.                   detector as it is being read out.                            cations with medium to long exposures
A mechanical shutter is usually used with                        Frame-transfer devices (figure 1) negate                  and for moderate time resolution on a mil-

O L E • M a y 2 0 07 • o p t i c s . o r g /o l e                                                                                                                                  31
     product guide
                                                                          ●   BI CCDs give the ultimate in QE (95%)
                              fibre-optic tapered bundle
                                                                              and can be coated with an antireflection
              photocathode      MCP              CCD                          coating to further improve their sensitiv-
                                                                              ity in specific wavelength regions. These
                                                                              devices can also be coated with phos-
                                                                              phors to extend their sensitivity into the
                                                                              UV but suffer from fringing effects in the
                                                                              near-infrared (NIR) above 750 nm.
                                                                          ●   OE structures provide a good QE response
                                                                              (55–60%) over a broad wavelength
                                                                              range. They are not as expensive as BI
                                                                              CCDs and do not suffer from fringing
                                                                              effects in the NIR.
                 window                                                   ●   Deep-depletion CCDs with fringe sup-
                                                screen                        pression offer the best QE in the NIR
                      high-voltage                                            region, but have approximately ten times

                      power supply                                            higher dark current then normal BI
     Fig. 2: intensifier fibre optically coupled to a FI CCD.                 CCDs. They have to be operated at much
                                                                              cooler temperatures to get the benefit of
     lisecond timescale.                                                      their sensitivity.
        Interline sensors also consist of pho-                            ●   ITO devices are not very common and
     tosensitive and masked storage areas.                                    are only currently available in imaging
     Although in this case, the photosensitive                                sensors with small pixels. These devices
     and masked areas extend along the CCD’s                                  have a better QE compared with OE
     vertical axis so that each masked area                                   devices, but tend not to perform as well
     is adjacent to a photosensitive area. The                                as detectors with BI structures.
     charge collected in the sensitive region
     can be shifted extremely quickly into the                            intensified ccds
     storage area and is subsequently read                                Intensified CCDs (ICCDs) consist of an inten-
     out. Interline devices provide much faster                           sifier that is usually fibre-optically coupled
     frame rates but have much lower sensitiv-                            to a FI CCD (figure 2). The intensifier con-
     ity as they only have approximately 25%                              sists of a photocathode that converts the
     fill factor, which means that only 25% of                            incoming light into electrons. The high elec-
     the image area is sensitive to incoming                              tron gain of a multichannel plate amplifies
     photons. In order to try to increase the                             these electrons (gains of 10 000 are easily
     fill factor, microlenses are often used to                           achieved) and the resulting signal is con-
     focus the light onto the sensitive regions.                          verted back into photons by a phosphor on
     Interline sensors have limited dynamic                               the exit window of the intensifier. The pho-
     range and are typically 12 bit as opposed                            tons then travel through the fibre optic and
     to the 16 bit performance of full-frame and                          are detected by the CCD in the usual way.
     frame-transfer CCDs. This limits their use                              The high gain of the multichannel plate
     for the more demanding scientific applica-                           enables it to amplify the signal far above
     tions, but they are very popular for applica-                        the noise floor, essentially bestowing these
     tions such as microscopy that do not have                            devices with single-photon sensitivity and
     very low light levels.                                               making them highly sensitive detectors for
                                                                          low-light applications. However, the key
     detector sensitivity                                                 benefit of ICCDs for scientific applications is
     While the format and the type of CCD                                 their gating capability. It allows the inten-
     selected is dependant on the area, reso-                             sifier to act as a very fast shutter and pro-
     lution, dynamic range and frame rate/                                vide time resolution on a sub-nanosecond
     spectral rate required, it is the QE or the                          timescale. This is useful in applications that
     probability of detecting a photon that will                          feature a short time event of interest within
     determine the ultimate sensitivity of the                            a broadband continuum with a longer life-
     camera. The QE depends on the structure                              time. These events are usually repetitive,
     of the CCD itself with the most common                               and the intensifier can capture them and
     structures being front illuminated (FI),                             accumulate their signal while ignoring the
     back illuminated (BI), open electrode (OE),                          broadband continuum.
     indium tin oxide (ITO) and deep depletion.                              It is the intensifier rather than the CCD
     ● FI structures give a moderate QE (about                            that determines the sensitivity of an ICCD
       45%) over the visible region. Phosphors                            system and there are many different types
       can be coated on these devices to extend                           of intensifiers available depending on the
       their sensitivity into the ultraviolet (UV)                        time resolution, wavelength range and sen-
       region.                                                            sitivity required. However, the QE of inten-

32                                                                                       O L E • M a y 2 0 07 • o p t i c s . o r g /o l e
                                                                           product guide

 comparing detectors
 CCD: full frame and frame transfer                 ICCD: full frame
 Architecture: FI; BI; OE                           Architecture: Gen II, Gen3 intensifiers fibre-
 Key characteristics: high sensitivity; low         optically coupled to FI device
 frame rates; low read-out speeds                   Key characteristics: High sensitivity; gating
 Typical applications: astronomy;                   capability
 fluorescence absorbance and transmission           Typical applications: laser-induced breakdown
 spectroscopy; nanotechnology; Raman                spectroscopy (LIBS); laser-induced
 spectroscopy                                       fluorescence (LIF); pulsed-laser deposition;
                                                    pressure-sensitive paint; particle imaging
 CCD: interline                                     velocity (PIV)
 Architecture: FI
 Key characteristics: low fill factor; low          CMOS: full frame
 sensitivity; high frame rates                      Architecture: FI
 Typical applications: Bose–Einstein                Key characteristics: lowest sensitivity; low fill
 condensation; microscopy                           factor; high speed                                    WHY ARE EO’S
                                                    Typical applications: industrial imaging
 EMCCD: frame transfer                                                                                   NEW MECHANICS
 Architecture: FI; BI; ITO                          HSD: full frame
 Key characteristics: high-sensitivity with         Architecture: FI; BI                                    BETTER?
 high frame rates                                   Key characteristics: high sensitivity
 Typical applications: single molecule              Typical applications: single molecule
 detection; Bose–Einstein condensation;             fluorescence; single molecule detection;             • Finger Cuts for Easier
 adaptive optics; intracellular ion signalling      time-resolved spectroscopy; confocal Raman             Placement of Optics
 microscopy                                         spectroscopy
                                                                                                         • Larger Tip/Tilt Range
                                                                                                         • Greater Accuracy with
                                               noise can be amplified by up to 1000 times
                                                                                                           Fine-Pitch Screws
“ccds are the                                  and can be easily identified provided that
                                               it is greater than the electron-multiplying               • Compatible Mounting
detectors of                                   CCD’s generated noise factor. The key ben-
                                               efit of EMCCDs is that they overcome the

choice for a wide                              limitations of conventional CCDs by pro-
                                               viding excellent detection limits with fast

range of scientific                            frame rates. They are ideal for extremely
                                               low-light-imaging applications that
                                               require high frame rates. Their key limi-
applications.”                                 tation is that their detection limit is set by
                                               their noise factor, which is a noise source
Antoinette o’grady                             due to the amplification process. For appli-
                                               cations where the light levels are above the
                                               CCD read-out noise, EMCCDs offer no ben-
sifier tubes is significantly less than the QE efit and a conventional CCD is often a better
of conventional CCDs. ICCDs also suffer choice in this instance.
from the disadvantages of additional noise
due to the amplification process (known as Best of both worlds
the noise factor), crosstalk, “chicken wire” Hybrid-sensor devices utilizing CCD and                        LOWER PRICES!
and halo effects.                              CMOS devices represent the next generation
                                               of detectors. They combine the sensitivity
electron-multiplying ccds                      benefits of scientific CCDs with the read-out                   Request
Electron-multiplying CCDs (EMCCDs) are capabilities of CMOS devices without having
                                                                                                                a FREE
manufactured using standard fabrication the limitation of increased read-out noise or
techniques. They differ from conventional noise factors. Therefore, they will be able to                       Catalog!
CCDs in that they have an additional, or offer high dynamic range, high sensitivity
gain, register inserted between the end of and extreme detection limits independent of                   Germany: +49 (0)721 6273730
the usual shift register and the amplifier. read-out speed.                                        
Higher-voltage amplitudes than normal                                                                       UK: +44 (0) 1904 691469
are used in the gain register to generate Antoinette O’Grady is the business manager for          
electrons (the gain) by impact ionization. spectroscopy at Princeton Instruments. See
Because this is done before the amplifier, or e-mail aogrady@piacton.
a signal that is lower than the read-out com for more information.

O L E • M a y 2 0 07 • o p t i c s . o r g /o l e                                                   33

To top