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
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
- 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.
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
illuminated CCDs cannotdetectlow-energy
particles. Quantum efficiency measurements in
were on the UV indicate that electrons generated near the
1024x1024 pixel, 9 pm thick CCDs using Fe, Ti, surface of delta-doped are
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
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
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
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
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
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
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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