We report the use of delta-doped charge-coupled devices (CCDs) for direct detection of electrons in the 50–1500 eV energy range. We show that modification of the CCD back surface by molecular beam epitaxy can greatly improve sensitivity to low-energy electrons by introducing an atomically abrupt dopant profile to eliminate the dead layer. Using delta-doped CCDs, we have extended the energy threshold for detection of electrons by over an order of magnitude. We have also measured high gain in response to low-energy electrons using delta-doped CCDs. The effect of multiple electron hole pair production on the observed signals is discussed. Electrons have been directly imaged with a delta-doped CCD in the 250–750 eV range

Elliott, S. Tom

Jones, Todd J.

Nikzad, Shouleh

Smith, Aimée L.

Tombrello, T. A.

Yu, Qiuming

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 23 7 DECEMBER 1998Direct detection and imaging of low-energy electrons
with delta-doped charge-coupled devices
Shouleh Nikzad, Qiuming Yu,a) Aime´e L. Smith,b) Todd J. Jones, T. A. Tombrello,c)
and S. Tom Elliott
Center for Space Microelectronic Technology, Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, California 91109
~Received 24 July 1998; accepted for publication 6 October 1998!
We report the use of delta-doped charge-coupled devices ~CCDs! for direct detection of electrons in
the 50–1500 eV energy range. We show that modification of the CCD back surface by molecular
beam epitaxy can greatly improve sensitivity to low-energy electrons by introducing an atomically
abrupt dopant profile to eliminate the dead layer. Using delta-doped CCDs, we have extended the
energy threshold for detection of electrons by over an order of magnitude. We have also measured
high gain in response to low-energy electrons using delta-doped CCDs. The effect of multiple
electron hole pair production on the observed signals is discussed. Electrons have been directly
imaged with a delta-doped CCD in the 250–750 eV range. © 1998 American Institute of Physics.
@S0003-6951~98!03549-9#There is great interest in detecting and imaging elec-
trons, especially low-energy electrons ~tens of eV to thou-
sands of eV! for scientific spectroscopy applications, such as
low-energy electron diffraction spectroscopy and reflection
electron energy-loss spectroscopy at reflection high-energy
electron diffraction energies.1,2 In addition, there are space
science applications for low-mass, low-power plasma detec-
tors and imagers. Imaging systems for low-energy particles
generally use microchannel plate electron multipliers fol-
lowed by position-sensitive solid-state detectors, or phos-
phors and position-sensitive photon detectors. These systems
work well and can process up to 106 electrons/s; however,
they have difficulties with gain stability, require high volt-
ages, and the dynamic range and spatial resolution of these
compound systems is considerably less than that of a solid-
state imaging detector.
Because of their high resolution, linearity, and large dy-
namic range, silicon charge-coupled devices ~CCDs! could
make major advances in particle detection. CCDs have been
used to meet the needs of a wide range of scientific imaging
applications which require accurate photometric imaging at
low light levels with high dynamic range. They have been
remarkably successful as imagers of x-ray, UV, visible, and
near-IR photons.3 As low-energy particle detectors and im-
agers, CCDs can make a great impact in many scientific
fields. However, their use as particle detectors has been ham-
pered by the inherent problems existing in the frontside-
illuminated CCDs. Both the rapid radiation degradation
caused by energetic electrons passing through the frontside
gates and gate insulator structure, and the large dead layer to
the low-energy electrons presented by the thick frontside-
gate structure make frontside-illuminated CCDs unsuitable
as electron detectors.
While backside-illuminated, thinned CCDs offer the
a!Currently at Kansas State University, Manhattan, KS 66506.
b!Currently at Massachusetts Institute of Technology, Cambridge, MA
02139.
c!Also with the Division of Physics, Mathematics, and Astronomy, Califor-
nia Institute of Technology, Pasadena, CA 91125.3410003-6951/98/73(23)/3417/3/$15.00
Downloaded 16 Dec 2005 to 131.215.225.9. Redistribution subject topossibility of detecting low-energy electrons, they inherently
possess a back surface dead layer associated with the back-
side potential well ~caused by positive charge at the interface
between Si and SiO2). The problem is similar to the detec-
tion of UV photons because a significant fraction of the en-
ergy of incident electrons is deposited within a few hundred
nm of the surface ~e.g., 100% of energy lost within 17 nm for
1 keV electron beam4!. A number of techniques have been
explored to eliminate the backside potential well, such as
negative-surface charging or biasing ~e.g., UV flooding and
bias flash gating! and ion implantation.5,6 In previous studies,
untreated and treated backside-thinned CCDs have been used
for electron detection, with promising results for electrons in
the 1–20 keV range.5–8 For example, the results of the biased
flash-gate CCD study show that the average quantum effi-
ciency increases from less than 1% for an untreated CCD to
nearly 40% for a backside-treated CCD at an electron-beam
energy of 1 keV. Although these backside surface treatments
have generated good electron or UV quantum efficiency,
they suffer variously from problems of yield, response sta-
bility, hysteresis, and long-term reliability. All of these prob-
lems critically affect the detection of low-energy electrons.
Delta-doped CCDs have the potential to detect electrons
at significantly lower energies than previously possible be-
cause the dead layer associated with the backside potential
well has been eliminated.9 Delta-doped CCDs with 100%
internal quantum efficiency in the visible and UV were de-
veloped at the Jet Propulsion Laboratory.9 Further studies
have shown that the delta-doped CCDs are highly uniform
and that these devices exhibit long-term stability.10 In this
approach, only a few atomic layers of silicon, containing an
extremely high concentration of p-type dopant ~at least 2
31014 boron atoms/cm2) are epitaxially grown on the CCD
backside surface, using molecular beam epitaxy ~MBE!. The
spiked concentration of dopant atoms permanently pins the
conduction band at the back surface, eliminating the dead
layer and creating a built-in field driving free electrons to the
collection wells under the frontside gates.
Fully processed EG&G Reticon CCDs ~512 pixels35127 © 1998 American Institute of Physics
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
3418 Appl. Phys. Lett., Vol. 73, No. 23, 7 December 1998 Nikzad et al.pixels, 27 mm pixel! were modified by low-temperature mo-
lecular beam epitaxy following the process described in pre-
vious papers.9,11 To make direct comparisons between an
untreated CCD and a delta-doped CCD, a CCD that was
partially masked during the MBE process was used in our
measurements.
We report direct imaging of low-energy electrons using
CCDs. Electrons in the 250–750 eV range were imaged with
CCDs in a custom ultrahigh vacuum ~UHV! chamber. A
camera was mounted directly onto the UHV chamber to op-
erate the CCD in the imaging mode. A modified SONY cath-
ode ray tube ~CRT! with an indirectly heated cathode gun
and beam energy range of 200–2000 eV served as the source
of incident electrons with negligible light background. The
imaging mode of operation allows for observation of elec-
tron irradiation on operating parameters such as charge-
transfer efficiency ~CTE!, individual pixel response, and sur-
face charging. Because of the highly sensitive imaging mode
of operation, the incoming flux of electrons was controlled
with a mechanical shutter, with beam exposure times in the
range 0.01–2 s.
The difference in the response between a delta-doped
CCD and an untreated CCD is most apparent in the response
of the CCD that was partially masked during the MBE pro-
cess as shown in Fig. 1. Flat-field images of 500 eV electrons
with the delta-doped CCD shows excellent qualitative simi-
larity to UV images at 250 nm, with nearly identical contrast
between the delta-doped and control regions of the CCD.
The absorption length of 250 nm light in silicon is approxi-
mately 70 Å,12 and electrons with 500 eV energy have a
maximum penetration depth of 50 Å.4,13 Some small dark
blemishes are apparent in the electron flat-field image that
are not seen on the UV flat field, most likely due to dust or
debris that has been introduced to the membrane surface in
the course of handling, transporting, and storing the device in
the months following the date when the UV flat-field image
was taken. Additional studies of electron imaging with the
delta-doped CCDs are under way.
Quantum efficiency measurements for electrons in the
50–1000 eV energy range were performed with the CCD
configured as a photodiode and using three different sources
of electron beam: an indirectly-heated cathode gun described
above; a directly-heated cathode gun, ~both mounted in the
custom UHV system!; and the electron source in a scanning
electron microscope ~SEM!. In photodiode mode, a CCD is
operated in such a way as to integrate the entire signal col-
lected over the surface of the device by grounding all pins
except for the output amplifiers. The signal is then read from
the pin of one of the output amplifiers, giving the total re-
sponse of all the pixels in the irradiated region of the device
indicating the overall collection efficiency. By integrating
the response of all irradiated pixels, much of the error that
would result in a pixel-by-pixel measurement is effectively
averaged out. The ratio of CCD output current divided by
incident beam current ~measured by a Faraday cup! was used
to measure the quantum efficiency of the device.
In the custom ultrahigh vacuum chamber, a Faraday cup
and phosphor screen were mounted vertically on a manipu-
lator from top to bottom, facing the electron beam. Both the
Faraday cup and CCD outputs were measured with a digital
Downloaded 16 Dec 2005 to 131.215.225.9. Redistribution subject tovoltmeter through an amplifier with 10.5 mV/nA gain. Dur-
ing the measurements with the indirectly-heated cathode gun
the electron beam was first focused into a ;1 mm spot in the
center of the phosphor screen, then the Faraday cup and the
CCD were moved down in sequence to measure the current
of the incident electron beam and the CCD output current,
respectively. After the CCD measurement, the incident
electron-beam current was measured again using the Faraday
cup to check the beam stability. To ensure that both the CCD
and the Faraday cup were exposed to the same area of elec-
tron beam, an aperture with diameter of 0.64 cm was cen-
tered in front of each. A plate with negative bias was put in
front of the Faraday cup to repel secondary electrons. Typi-
cal beam currents were in the hundreds of pA range. The
typical vacuum levels during the measurements were in the
low 731028 – 231027 Torr range. A directly-heated cath-
ode gun or a flood gun was used as a source of electrons with
energies in the tens of eV. This electron source also produces
a large light background that was distinguished for the elec-
tron signal by measuring the CCD response before and after
magnetically deflecting the electron beam. For this reason,
the response of the delta-doped CCD in this energy range is
only reported qualitatively. Another set of measurements
were performed in a SEM to take advantage of its highly
focused electron beam for mapping the response of the CCD.
The SEM measurements were performed using a JEOL JSM-
6400 SEM as the source of the electrons, which could pro-
vide beam energies from 0.2 to 40 keV. Both the beam cur-
rent and the CCD output current were measured with a
Keithley 485 picoammeter. The incident beam current was
measured with a Faraday cup that could be rotated in and out
of the beam path in front of the CCD. The beam current for
the measurements were in the range of 2–40 pA. The pres-
sure during the measurements was approximately 1025 Torr.
The responses of a delta-doped CCD and an untreated
backside-thinned CCD to electrons were repeatedly mea-
sured in the range of 200–1000 eV using the modified CRT
and the SEM as sources. In Fig. 2, the CCD quantum effi-
ciency is plotted as a function of electron incident energy.
Quantum efficiency was calculated by dividing the measured
current from the CCD configured in photodiode mode to the
measured electron-beam current ~measured by a Faraday
FIG. 1. Images of a partially masked delta-doped CCD with 250 nm uni-
form light ~a! and 500 eV electrons ~b!. Brightness in the two images is
adjusted differently; however, qualitatively the two images are the same.
The dark area on the left of both images is untreated ~masked during MBE
growth! and the delta-doped region is bright, indicating sensitivity to UV
photons and electrons. The electron beam was slightly off center and a
portion of the CCD corresponding to the bottom-left corner of the image
was not exposed to the electron beam. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
3419Appl. Phys. Lett., Vol. 73, No. 23, 7 December 1998 Nikzad et al.cup!, which is equivalent to the number of electrons detected
divided by the number of incident electrons. Because por-
tions of the delta-doped CCD were masked during process-
ing to serve as control regions, data taken in the UHV system
were corrected to account for the fraction of untreated ex-
posed CCD area. The untreated backside-thinned CCD
showed a dramatically lower quantum efficiency than the
delta-doped CCD. The response of the untreated CCD to
electrons was unstable, decaying with a time constant on the
order of 20 min at an incident electron energy of 1 keV and
it showed no response at 300 eV. The measured quantum
efficiency of the delta-doped CCD increases with increasing
energy of the incident beam. The dependence of quantum
efficiency on incident energy is due to the complicated inter-
action of electrons with silicon, which results in the genera-
tion of multiple electron–hole pairs in the cascade initiated
by each incident electron. Multiple electron–hole pair pro-
duction, also known in the literature as quantum yield, is
observed in the measured UV and x-ray response of delta-
doped CCDs and other devices. Quantum yield greater than
unity has been previously observed in backside-illuminated
CCDs modified using both the flash-gate5 and ion
implantation14 techniques at electron energies greater than 1
keV.
To the best of our knowledge, the delta-doped CCD is
the first CCD shown to respond to electrons with energies
lower than 300 eV. At 1 keV, which is the previously re-
ported lower limit for the flash-gate CCD,5 the quantum ef-
ficiency of the delta-doped CCD is approximately twice as
great. The delta-doped CCD exhibited a response above the
noise at electron energies as low as 50 eV. The minimum
energy tested at this point represents source limitations rather
than detector limitations.
Analogous to photon quantum efficiency of CCDs, elec-
tron quantum efficiency ~QE! is the product of three quanti-
ties: the transmission coefficient, the quantum yield, and the
internal quantum efficiency.15 The transmission coefficient is
a factor representing the fraction of incident beam absorbed
in the device, which for electrons includes the backscattering
coefficient; the quantum yield accounts for the statistically
averaged number of electron–hole pairs produced by the in-
cident electron ~or photon!; and the internal quantum effi-
FIG. 2. Ratio of detected electrons to incident electrons as a function of
energy. The response of the CCD increases with increasing energy as a
result of multiple electron–hole pair generation.Downloaded 16 Dec 2005 to 131.215.225.9. Redistribution subject tociency accounts for internal losses in the CCD, such as re-
combination of electron–hole pairs at the back surface of the
CCD.14 Ultraviolet measurements of the delta-doped CCD
indicate that the internal quantum efficiency is nearly 100%,
even at 270 nm where the absorption length in silicon is only
4 nm.9 Assuming that all the generated electrons are detected
by the delta-doped CCD ~internal QE ;100%!, our measure-
ments will represent the product of the effective quantum
yield16,17 of silicon and the transmission factor for low-
energy electrons. If the transmission factor is dominated by
the backscattering coefficient, i.e., 40%–50%18 for 200–
1500 eV electrons, the effective quantum yield can be deter-
mined from our measurements.
While separating the effects of transmission and quan-
tum yield is interesting from a theoretical standpoint, the
convolution of the two, as measured in these experiments, is
the quantity of interest for solid-state electron detectors. It is
significant that, to the best of our knowledge, no other solid-
state devices detect low-energy electrons as efficiently as the
delta-doped CCD, due to the presence of a dead layer near
their surfaces. In addition to its high efficiency, the delta-
doped CCD also has the capability of imaging low-energy
particles, which may prove valuable in energy-selective par-
ticle detector applications.
The authors gratefully acknowledge the invaluable assis-
tance of Dr. M. E. Hoenck, Dr. L. D. Bell, Dr. S. Manion,
Dr. T. Van Zandt, W. Proniawicz, and Professor L. C. Ki-
merling. The work presented in this letter was performed by
the Center for Space Microelectronics Technology, Jet Pro-
pulsion Laboratory, California Institute of Technology, and
was jointly funded by the Caltech President’s Fund and the
NASA Office of Space Science.
1 S. Nikzad, S. S. Wong, C. C. Ahn, A. L. Smith, and H. A. Atwater, Appl.
Phys. Lett. 63, 1414 ~1993!.
2 S. Nikzad, C. C. Ahn, and H. A. Atwater, J. Vac. Sci. Technol. B 10, 762
~1992!.
3 J. R. Janesick, S. T. Elliott, S. A. Collins, H. Marsh, M. M. Blouke, and J.
Freeman, Opt. Eng. ~Bellingham! 26, 692 ~1987!.
4 T. E. Everhart and P. H. Hoff, J. Appl. Phys. 42, 5837 ~1971!.
5 T. Daud, J. R. Janesick, K. Evans, and T. Elliott, Opt. Eng. ~Bellingham!
26, 686 ~1987!.
6 M. K. Ravel and A. L. Reinheimer, Proc. SPIE 1447, 109 ~1991!.
7 J. C. Richard and M. Vittot, Nucl. Instrum. Methods Phys. Res. A 315,
368 ~1992!.
8 K. L. Luke and L.-J. Cheng, J. Appl. Phys. 60, 589 ~1986!.
9 M. E. Hoenk, P. J. Grunthaner, F. J. Grunthaner, M. Fattahi, H.-F. Tseng,
and R. W. Terhune, Appl. Phys. Lett. 61, 1084 ~1992!.
10 S. Nikzad, M. E. Hoenk, P. J. Grunthaner, R. W. Terhune, F. J.
Grunthaner, R. Wizenread, M. Fattahi, and H.-F. Tseng, Proc. SPIE 2198,
907 ~1994!.
11 M. E. Hoenk, P. J. Grunthaner, F. J. Grunthaner, R. W. Terhune, and M.
Fatthi, Proc. SPIE 1656, 488 ~1992!.
12 S. M. Sze, Physics of Semiconductor Devices, 2nd ed. ~Wiley, New York,
1981!, p. 42.
13 S. Nikzad, A. L. Smith, S. T. Elliott, T. J. Jones, T. A. Tombrello, and Q.
Yu, Proc. SPIE 3019, 241 ~1997!.
14 D. G. Stearns and J. D. Wiedwald, Rev. Sci. Instrum. 60, 1095 ~1989!.
15 S. Nikzad, M. E. Hoenk, P. J. Grunthanber, R. W. Terhune, R. Wizenread,
M. Fattahi, H.-F. Tseng, and F. J. Grunthaner, Proc. SPIE 2217, 355
~1994!.
16 C. A. Klein, J. Appl. Phys. 39, 2029 ~1968!.
17 L. R. Canfield, J. Kerner, and R. Korde, Appl. Opt. 28, 3940 ~1989!.
18 A. L. Smith, Q. Yu, S. T. Elliott, T. A. Tombrello, and S. Nikzad, Mater.
Res. Soc. Symp. Proc. 448, 177 ~1996!.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Direct detection and imaging of low-energy electrons with delta-doped charge-coupled devices

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Direct detection and imaging of low-energy electrons with delta-doped charge-coupled devices

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