Delta-doped CCDs for Ultraviolet and Low-energy Particle Detection

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Delta-doped CCDs for Ultraviolet and Low-energy Particle Detection Powered By Docstoc
					                     Delta-doped CCDs for Ultravioletand Low-energy Particle Detection
       Shouleh Nikzad, Aimee Smith, Qiuming Yu, Todd J. Jones, Paula Grunthaner, and S.Tom Elliott
                                Center for Space Microelectronics Technology
        .'    Jet Propulsion Laboratory, California institute of Technology, Pasadena, CA 9 1 109
                      Abstract                            Processing of delta-doped CCDs was described
                                                          previously.12 A 2.5 nm delta-doped Si layer is
Delta-doped CCDs developed                     at the     grown onthe back surface of thinned, fully-
Microdevices Laboratory at the Jet Propulsion             processed CCDs at low-temperature. Delta-doped
Laboratory to address quantum efficiency and              CCDs have been extensively tested and have
quantumefficiencyhysteresis           in UV CCDs.         shown 100% internal quantum efficiency in the
Using molecular beam epitaxy, fully-processed             ultraviolet and visible  part of the spectrum
thinned CCDs were modified by growing 2.5 nm              indicating that the deleterious backside potential
of Boron-doped silicon on their back surface.             well responsible for the detector dead layer has
Because of the sharply-spiked dopant profile in           been effectively eliminated. Because the delta-
the thin epitaxial layer, these devices are called        dopedlayerisincorporateddirectlyinto          the
delta-doped CCDs and they exhibit stable and              silicon lattice, themodified CCDs are robust
uniform, 100% internal quantum efficiency in the          enough to withstand direct deposition of anti-
visible and ultraviolet regions of the spectrum. In       reflection coatings for enhanced UV quantum
addition,because delta-doped
                the              layer               is   efficiency.
incorporated in the lattice, it is possible to directly
deposit antireflection coatings on these CCDs to            UV characterization of Delta-doped CCDs
further enhance the total quantum efficiency. UV
and charged-particle detection with delta-doped           The quantum efficiency(QE) and stability of
CCDs, and data from field measurements will be            delta-doped CCDs in the UV and visible regions
presented.                                                of the spectrum has been extensively measured.
                                                          Figure 1 shows the typical quantum efficiency in
                    Introduction                          the 250-700 nm region of the spectrum and the
                                                          enhancement of the QE in the 300-400 nm region
Because of their large format, high resolution,low        by direct deposition of single layer HfOz.2 The
noise, and maturityof their technology, CCDs are          solid line in figure 1 is the silicon transmittance
the detector      of choice for    many scientific        which represents 100% internal quantum or the
applications.  However,      frontside-
                       standard                           maximum QE that be   can obtained          without
illuminated CCDs do not respond in the UV                 addition of antireflection coatings. We have also
because of short absorption of photons in this            measured the QE of delta-doped CCDs in the
wavelength range. Untreated back-illuminated              121.6-3 10 nm region of the spectrum. It was
silicon CCDs have limited sensitivity to radiation        shown in thosemeasurements that the delta-doped
with short penetration depth (e.g., UV photons            CCD shows 100% internal QE throughout the
and low-energy particles), due to the surface             entire 120-700 nm waveband.
depletion caused by the inherent positive charge
in the native oxide. Because of surface depletion,        Applications in astronomy require stable device
internally-generated electrons are trapped near the       performance. Figure 2 shows quantum efficiency
irradiated surface therefore     cannotbe                 data over a three-year period. No degradation of
transported to the detection circuitry. This surface      the device quantum efficiency was observed. The
potential can be eliminated by low-temperature            device        with
                                                                stability respect            to historyof
molecular beam epitaxial(MBE) growth of a                 illumination has also been examined. Increasing
delta-doped layer onthe Si surface. This effect           the exposure time by a factor of 100 and returning
hasbeen demonstrated through achievement of               tothe original exposure time yielded identical
 1 0 0 % internal quantum efficiency for UV photons       quantum efficiency for the delta-doped CCD,
detected with delta-doped CCDs.                                          thatquantum
                                                          demonstrating no          efficiency
                                                          hysteresis existsin the device.
                                                                  Low-energy particle detection withdelta-doped
                                                                 Imaging systems for low energy        particles
                                                                 generally involve the use of microchannel plate
                                                                 electron multipliers followed by position sensitive
                                                                 solid state detectors,or phosphors and CCDs.
        40                                                       These systems work well and can process up to
                                          Ban &doped CCD          106 electrons/sec., however the spatial resolution
                       Si transmittance
                                                             -   of these compound systems is considerably less
                                                                 than that of a directly imaged CCD. Also, these
            0-                                                   systems have difficulties with gain stability and
                 300            400          500           600   they require high voltages.
                             Wavelength (nm)
                                                                 Similar to UV photons,low-energyparticles
Figure 1. Quantum efficiency of a bare delta-doped CCD           depositasignificantfraction      of theirenergy
(circles) compared with solid line (Si transmittance) shows      within a few nanometers of thesurface, therefore,
1 0 0 % internal QE. QE is enhanced by the addition of anti-
                                    300.400 nm regions.
reflection coatings optimized for the
                                                                 frontside-illuminated untreated
                                                                                       or      back-
                                                                 illuminated CCDs cannotdetectlow-energy
                                                                 particles. Quantum efficiency measurements in
X-raymeasurements performed
                  were        on                                 the UV indicate that electrons generated near the
 1024x1024 pixel, 9 pm thick CCDs using Fe, Ti,                  surface of delta-doped        are
                                                                                          CCDs detected
Ca, K, Si, and A1 targets. these
                               From                              efficiently and delta-doped CCDs are promising
measurements, it was seen that very low-energy x                 as imaging detectors of low-energy particles. We
                                                                 have extended the characterization of delta-doped
rays,such as thealuminum K a line,canbe                          CCDs to detection of electrons in the 50-1500 eV
detected.      measurements were                                 energy range using both a custom UHV chamber
performedusingcarbonandfluorinexrays.                            and a scanning electron microscope.3J
Because of their short absorption length, these x
rays                      that very
           photo-electrons are                                   To gain an understanding of different aspects of
vulnerable to backsiderecombinationeffects.                      low-energyelectronresponse        of delta-doped
These tests showed that no significant surface                   CCDs, we performed measurements using various
recombination was occurring.                                            sources
                                                                 electron and        device
                                                                 configurations. One set of measurements was
                                                                 performed inan SEM to take advantage of its
                                                                 highly-focused      beam.SEM
                                                                               electron The
                                                                 apparatus was a JEOL, model JSM 6 4 0 0 , and the
                                                                 measurements were made with beam energies
                                                                 ranging between 200 eV and 1 keV. While it was
                                                                 not possible for modifications to be made to the
     60-                                                         SEM in order to accommodate the electronics
                                                                 necessary for collecting CCD images, performing
                                                                 photo-diode measurements               was quite
                                                           -     straightforward and informative. Another set of
                                m After8-doping                  measurements wasmade in a UHV system in
                                  16 Months after &doping'                 mode.
                                                                 photo-diode                 For thisof
                                  3 y n after8-doping            measurement, each CCD in turn was mounted in
                                                                 plane with a Faraday cup and a phosphor screen
      !#O        3
                 h     3iO     4h    4h        5
                                               h   5iO   6b          a
                                                                 onto manipulator. Using the custom UHV
                             Wavelength (nm)                     system afforded the use of two different electron
                                                                 sources, one of very low energy and one of
Figure 2. QE measuredover a three-yearperiodonthe                      energies
                                                                 similar               as used i n the SEM
samedelta-doped 5 12-by-512-pixel Reticon CCD. The               measurements. The low-energy electron gun is a
CCD was stored unprotected in a laboratory environment.          hot-filamentcathode      that produceselectron
The bars         the
  error represent accuracy               (f5%) of the            energies of several 10 eV while generating a
measurement systems used.                                        stronglight background. Comparisonwasmade
between the observed response ofthe CCD and            implantation6 at electron energies greater than 1
the response of the CCD with the electron beam         keV.
magnetically deflected. Because of the strong
CCD response             background
                    to the          light,             The delta-doped CCD responds efficiently and
measurements with the hot filament electron gun        reliably to low-energy electrons. Moreover, a
beam are reported only qualitatively (50-200 eV).      delta-doped CCD responds with higher gain to
The UHV system further allowed for the later           low-energy electrons than other backside treated
attachment of the electronics necessary     for        devices (e.g., twice that of a flashgate CCD). The
operating the CCD in imaging mode. This mode           untreated                   CCD
                                                                backside-thinned showed       a
of operation allows for the observation electron       dramatically lower quantum efficiency than the
irradiation on operating parameters only apparent      delta-doped CCD (5% vs. 160% at 900 eV). The
            mode as
in imaging suchcharge      transfer                    response of the untreated CCD to electrons was
efficiency (CTE), individual pixel response, and       unstable, decaying with a time constant on the
surface charging.                                      order of 20 minutes at an incident electron energy
                                                       of 1 keV. This decay was not reversible by a
TheCCDs  used              experiments
                    in these           were            thermal anneal at -200°C.
thinned, back-illuminated EG&G Reticon CCDs.
All measurements were repeated with both delta-        In measurements conducted in our laboratory, we
doped and untreated CCDs. In some of the               reportthe use of CCDs to imageelectrons.
measurements, direct comparisons of delta-doped        Images of 500 eV electrons with the delta-doped
CCDs with untreated CCDs were made on the              CCD show excellent qualitative similarity to UV
same device, using a delta-doped CCD which             images at 250 nm, with similar contrast between
included acontrolled(untreated)    region. The
controlledregion was provided on theback               delta-doped andcontrol regions of the CCD.
surface of the array by masking off a portion of
the surface during the MBE growth. All devices
were fully-characterizedprior to the electron
measurements usingUV illumination.
Figure 3 shows the electron quantum efficiency of
adelta-dopedCCDplottedasafunction                of
incident     Quantum           was

calculated by dividing the measured current from
the CCD configured in photodiode mode to the
measured electron beam current (measured by a
Faraday cup), which is equivalent to the number
of electron-hole pairs detected divided by the
                                                             50   1                    0

number of incident             The
                      electrons.         measured
quantumefficiency of thedelta-dopedCCD
increases with increasing energy of the incident
beam. The dependence of quantum efficiency on                                 Beam Energy (eV)
incident      is to
        energydue                 the complicated      Figure 3 Ratio of detected electrons to incident electrons as
interaction of electrons with silicon which results    a function of energy. The response of the CCD increases
in the generation of multiple electron-hole pairs in   with increasing energy as result of multiple electron-hole
the cascade initiated by each incident electron. A     pair generation.
significantfraction of the incident energy is
undetected,due to backscattering of incident
electrons energy
         other dissipation                             Field observationsand feedback fromscientific
mechanisms (e.g., secondary and Auger electron                          community
emission). Multiple electron-hole pair production,
also known in the literature as quantum yield, is      Delta-doped CCDs have beenusedrecentlyin
also observed in the measured UV andx-ray              collaborations with a        scientists
                                                                              several                in a
response of delta-doped CCDs and other devices.        number of field observations. In collaboration
Quantum yield greater than unity has been              with Caltech, a delta-doped CCD wasused to
previouslyobserved       in backside-illuminated       image galaxies in the near UV at Caltech's
CCDs modified using the flashgate5 and       ion       Palomar observatory. In a sounding rocket
                                                       experiment in collaboration with the University of
Colorado,adelta-dopedCCD wasusedasthe                                  References
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              Acknowledgments                       Orlando, Fl. (1994).
                                                    3. A. Smith, Q. Yu, S.T. Elliott, T.A. Tombrello,
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