a b s t r a c t Chemical vapour deposition (CVD) diamond offers great potential as a low-cost, high-yield, easily manufactured secondary electron emitter for electron multiplication in devices such as photomultiplier tubes. Its potential for high secondary electron yield offers several significant benefits for these devices including higher time resolution, faster signal rise time, reduced pulse height distribution, low noise, and chemical stability. We describe an experiment to characterize the secondary emission yield of CVD diamond manufactured using different processes and process parameters and discuss the degradation of secondary electron yield and experimental difficulties encountered due to unwanted electron beaminduced contamination. We describe techniques utilized to overcome these difficulties, and present measurements of secondary yield from CVD diamond dynodes in reflection mode. We discuss the application of CVD diamond dynode technology, both in reflection and transmission mode, to advanced high-speed imaging and photon-counting detectors and describe future plans in this area

D P Thompson

J Howorth

J Milnes

J S Lapington

N A Fox

P W May

V Taillandier

CiteSeerX

ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 610 (2009) 253–257Contents lists available at ScienceDirectNuclear Instruments and Methods in
Physics Research A0168-90
doi:10.1
 Corr
E-mjournal homepage: www.elsevier.com/locate/nimaInvestigation of the secondary emission characteristics of CVD diamond films
for electron amplificationJ.S. Lapington a,, D.P. Thompson a, P.W. May b, N.A. Fox b, J. Howorth c, J. Milnes c, V. Taillandier d
a Space Research Centre, Univeristy of Leicester, Leicester LE1 7RH, UK
b Department of Chemistry, Univeristy of Bristol, Bristol BS8 1TS, UK
c Photek Ltd., St. Leonards on Sea, East Sussex TN38 9NS, UK
d Commissariat a l’Energie Atomique, SPN Bruyeres-le-Chatel, 91297 Arpajon Cedex, Francea r t i c l e i n f o
Available online 12 June 2009
Keywords:
CVD diamond
Dynode
Secondary electron emission
Photomultiplier
Detector02/$ - see front matter & 2009 Elsevier B.V. A
016/j.nima.2009.05.195
esponding author. Tel.: +44116 252 3498; fax
ail address: jsl12@star.le.ac.uk (J.S. Lapington)a b s t r a c t
Chemical vapour deposition (CVD) diamond offers great potential as a low-cost, high-yield, easily
manufactured secondary electron emitter for electron multiplication in devices such as photomultiplier
tubes. Its potential for high secondary electron yield offers several significant benefits for these devices
including higher time resolution, faster signal rise time, reduced pulse height distribution, low noise,
and chemical stability.
We describe an experiment to characterize the secondary emission yield of CVD diamond
manufactured using different processes and process parameters and discuss the degradation of
secondary electron yield and experimental difficulties encountered due to unwanted electron beam-
induced contamination. We describe techniques utilized to overcome these difficulties, and present
measurements of secondary yield from CVD diamond dynodes in reflection mode.
We discuss the application of CVD diamond dynode technology, both in reflection and transmission
mode, to advanced high-speed imaging and photon-counting detectors and describe future plans in this
area.
& 2009 Elsevier B.V. All rights reserved.1. Introduction
Despite the widespread success of solid-state imaging devices
such as the ubiquitous CCD, there are many fields where the
sensitivity to detect single-quanta accurately in time and space is
required, a regime where signal amplification is essential. Despite
advances in solid-state avalanche gain technologies, the physics of
solid-state semiconductors limit the ultimate response time and
signal-to-noise ratio (SNR) compared with the fundamental
limitations for in vacuo electron gain devices, such as the
photomultiplier tube (PMT) and microchannel plate (MCP).
PMT and MCP detectors are utilized in many fields of science,
from space-based astronomical UV [1] and X-ray observations, to
microscopic analysis and diagnosis of in vivo biological processes.
PMTs are the workhorse detector for photon-counting applica-
tions, from medical imaging as PET scanners to detection of exotic
particles as neutrino detectors. MCP detectors have the capability
to achieve picosecond event timing resolution, and their planar
geometry lends itself to high-resolution imaging, with formats
exceeding 4000 4000 pixel2.ll rights reserved.
: +44116 252 2464.
.
The performance of in vacuo secondary electron gain devices
such as PMTs and MCPs is fundamentally limited by the number
of secondary electrons produced by a primary electron impacting
the dynode surface. Since the gain progression is geometric, a
technology providing improved dynode emission with the
stability to use at high signal currents would produce a step
advance in the performance. Boron-doped diamond film depos-
ited using chemical vapour deposition (CVD) techniques has been
shown to exhibit significantly enhanced secondary electron yield
(SEY), especially at higher voltages, compared to traditional
dynode materials [2] (see Fig. 1), and its application offers
significant advantages:1. Enhanced SEY: Diamond, along with several other high band-
gap semiconductors, can exhibit negative electron affinity
(NEA) with suitable surface preparation, a major factor
enhancing SEY. However, unlike other higher band-gap
materials (e.g. AlN, Ga1yAlyN), synthetic CVD diamond is easy
to produce [3].2. Simplified design: Higher SEY results in a lower number of
dynodes being required for a given gain, a distinct advantage in
terms of device size, cost, and complexity.3. Enhanced timing: Boron-doped (p-type) diamond surfaces
produce a narrow energy distribution of low-energy secondary
ARTICLE IN PRESS
Fig. 1. SEY coefficients of three common dynode materials (inset—Photonis PMT
Handbook) cf. CVD diamond for different surface terminations [2] and data (grey
squares) from our research programme.
Fig. 2. A schematic diagram of the SEY characterization apparatus.
J.S. Lapington et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 253–257254electrons [4] allowing improved confinement of trajectories
with improved time resolution and reduced background [5].4. Narrow pulse height distribution (PHD): The Poisson statistics of
high-gain diamond dynodes intrinsically produce a narrower
PHD with improved SNR, allowing detector operation at lower
gain and better energy resolution for scintillation counters, PET
scanners, etc.5. Low noise: Its wide band gap (5.47 eV) precludes the thermal
excitation of electrons at typical operating temperatures cf.
materials with comparable SEY (e.g. GaP—2.26 eV). Conversely,
it has lower thermal noise than traditional dynode materials at
high temperatures.6. Large-area: CVD deposition offers a low-cost, large-area coat-
ing technique and is applicable to shaped substrates c.f.
expensive materials such as GaN and GaP, which can only be
grown on lattice matched substrates e.g. sapphire. Inkjet
printing of diamond nanoparticles and subsequent growth
into patterned films has also been demonstrated.7. Stability: CVD diamond has a stable SEY, little affected by long-
term storage. Its SEY remains high after exposure and it can be
reactivated [6].
One potential drawback of diamond’s higher gain is the
increased voltage required per stage. However, diamond’s smaller
pulse height variance and low noise promote operation at lower
gain with a smaller number of stages, thus alleviating the overall
voltage requirement.Fig. 3. (a). A photograph of the sample arm with three samples. The SFC is the
aperture between sample 1 and 2. The montage below is the false colour SEY
image produced by scanning the electron beam. Fig. 3b is an SEY image showing
the SFC aperture (the black hole in the middle).2. Experimental setup
The SEY characterization apparatus is a bakeable stainless-
steel vacuum system with a centrally mounted, electrically
insulated conductive arm supporting up to four 1 cm2 samples
and a small Faraday cup (SFC) for beam current calibration. Fig. 2
shows a schematic diagram of the apparatus and Fig. 3a shows a
photograph of the arm with three samples and the SFC aperture.
The sample support arm can be moved laterally by 725 mm
and rotated 3601 around its axis. Lateral motion is used to position
one of the samples or the Faraday cup in the path of a beam of
electrons generated by an electron gun and rotation allows the
beam incidence angle to be varied or the samples placed out of
sight of the beam. The ELS5000 electron gun, manufactured by
PSP Vacuum Technology [7], produces a focussed electron beam
with a spot size of o50mm at 1mA, working distance of
ARTICLE IN PRESS
J.S. Lapington et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 253–257 25515–75 mm, and beam energy of up to 5 keV. The beam can be
electrostatically deflected in the xy plane by 75 mm.
Secondary electrons produced by the electron beam impinging
the sample are collected by a stainless-steel cylinder, the large
Faraday cup (LFC), which surrounds the arm. The LFC is coaxial
with the support arm and is mounted on an insulating support at
its closed end. The electron beam enters through a small aperture
in the side of the LFC. The LFC can (a) be biased and, (b) has
extended length to minimize the solid angle of the external
chamber seen by emitted secondary electrons from the sample.
Both measures serve to ensure secondary electrons from the
sample are captured by the LFC. Both the sample support arm and
the LFC are electrically insulated allowing them to be biased and
their respective collected currents to be measured using a Keithley
6514 Electrometer and 6485 Picoammeter, both operating as
precision ammeters. These can be interchanged, however, only the
electrometer input can be biased.
Two independent methods can be used to calibrate the
electron beam current, (i) the beam can be collected completely
in the SFC, the sample current equalling the beam current,
confirmed by a zero LFC current, or, (ii) the beam current can be
calculated from the sum of sample and LFC currents. The former
method requires the electron beam to be sufficiently well
focussed to completely pass through the SFC aperture of
0.5 mm. Focussing was carried out by scanning the beam in two
dimensions across the SFC aperture using the beam deflection
capability, and imaging the aperture by means of the measured
SEY, calculated using the sample and LFC currents. The SEY was
then given by
SEY ¼
ILFC
Isample þ ILFC
(1)
The focus was adjusted to achieve zero SEY over the aperture,
the expected result when the beam is fully collected in the SFC.
Elsewhere, the phosphor-bronze shim from which the SFC was
constructed produced an SEY of 1.0. This technique also allowed
the beam profile to be calculated by deconvolution of the circular
SFC pinhole from the imaged SEY (see Fig. 3b).3. Results
Initially the system was pumped with a Leybold turbo pump
and rotary backing pump, reaching an ultimate pressure of
107 mbar. Early on in the experiment it was noted that the
measured SEY from any sample material, unless already very low,18
18.5
19
19.5
20
20.5
21
0.00
Extracted 
S
ec
on
da
ry
 E
le
ct
ro
n 
Y
ie
ld
0.50 1.00
Fig. 4. A plot showing the degradation of SEdegraded rapidly over time-scales of minutes as a result of
exposure to the electron beam. Evidence in the form of visible
contamination, known as electron beam-induced contamination
[8], from areas subject to long-term exposure, pointed strongly to
the presence of organic contamination. This occurred in two
forms, both highly structured, suggesting interaction of the beam
with adsorbed contaminants on the sample surface, and as diffuse
shadows whose geometry suggested either the presence of a
large-angle component in the beam distribution or isotropic
evolution of contaminants from a remote source back along the
beam line. At this stage the vacuum system was cleaned and the
pump set replaced with an oil-free system comprising an Edwards
Maglev turbo and scroll pump, though the ultimate vacuum
achieved was not appreciably lower.
Another mechanism for SEY degradation is hydrogen deso-
rption from the diamond surface, which causes the disappearance
of NEA [9]. Since both electron beam-induced contamination and
hydrogen desorption are produced by exposure to the electron
beam and are sensitive to total beam flux, they are difficult to
distinguish, but their effect can be alleviated if the total flux can
be reduced. This approach was used to enable measurement of the
nominal SEY of an initially ‘‘clean’’ material. The overall flux was
reduced by both pulsing the beam and lowering the beam current.
LabView was used for the software platform to automate the
measurement procedure, which was used to control the electron
gun HV, the xy beam deflection, the beam focus, and to initialize
and take synchronised measurements from the Keithley am-
meters, process the data, and save and display results. Several
modes of operation allowed: (1) automated beam focussing, (2) xy
area beam scan and SEY image capture, (3) energy scan at a single
point to produce SEY versus beam energy data, and (4) other
calibration procedures. The low-noise Keithley ammeters and
good electrical insulation allowed electron beam currents in the
1–10 nA range to be used even when several hundred volt biases
were applied to the LFC. The HV was typically pulsed for 0.3 s with
the ammeters taking measurements every 10 ms. The time profile
of measurements was used to select a window where the currents
had reached a stable regime.
These modifications drastically reduced the rate of SEY
degradation with total beam charge. Fig. 4 shows a plot of the
variation of SEY for a single 100mm spot illuminated by the
electron beam versus total deposited charge. The contribution of
the two SEY degradation mechanisms to this data is uncertain,
however, we have qualitative proof that, despite the
decontaminated vacuum system and oil-free pump set, electron
beam-induced contamination is still present. This is evidenced byCharge (mC)
1.50 2.00 2.50
Y with increasing electron beam charge.
ARTICLE IN PRESS
J.S. Lapington et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 253–257256Fig. 5, which shows a photograph of the SFC after prolonged
exposure to the electron beam, producing a diagonal dark stain.
However, concurrent SEY imaging of the area around the SFCFig. 5. A photograph of the SFC before (left) and after (right) prolonged
illumination with the electron beam.
0
2
4
6
8
10
12
14
16
18
0
Electron Bea
S
ec
on
da
ry
 E
le
ct
ro
n 
Y
ie
ld
0 deg
15 deg
30 deg
45 deg
60 deg
75 deg
0.5 1
Fig. 7. Measured SEY versus energy for hydrogen-terminated B-doped CVD diamond d
0
5
10
15
20
25
30
35
40
45
50
0
Electron Bea
S
ec
on
da
ry
 E
le
ct
ro
n 
Y
ie
ld
0.5 1
Fig. 6. Measured SEY versus beam energy data for hydrogen-terminated B-doped CVDshowed no trace of actual elongation of the beam distribution that
this deposit implies. This would be apparent as elongation of the
SFC aperture in the SEY image owing to convolution of the beam
and aperture distributions.
With the apparatus modified to reduce the SEY degradation
effects, we measured the SEY of a variety of diamond samples
from various sources, manufactured using different methods
(CVD, inkjet-printed nanoparticles), dopants and dopant concen-
trations, and differing crystallinities. Fig. 6 shows a plot of the best
results obtained for boron-doped, CVD diamond coated on silicon
and molybdenum substrates. The best SEY value so far obtained of
45 for B-doped diamond on silicon at 2.4 keV is the highest
reported for hydrogen-terminated material and a strong driver to
apply this technology to electron multiplication. However, in the
presence of degradation due to electron beam-induced
contamination, these must be regarded as preliminary.
However, measurements of variation of SEY with angle of
incidence show unexpected behaviour (see Fig. 7). Typically, SEYm Energy (keV)
1.5 2 2.5 3
eposited on molybdenum, with the electron beam at various angles of incidence.
m Energy (keV)
Sample A
Sample B
1.5 2 2.5 3
diamond deposited on molybdenum (sample A) and silicon (sample B) substrates.
ARTICLE IN PRESS
J.S. Lapington et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 253–257 257is expected to increase with angle of incidence (defined as the
angle with respect to the surface normal). This is due to the
reduced depth of penetration normal to the surface at higher
angles, resulting in an increase in secondary electron escape
probability. Either, the high granularity of the surface plays a role
in reversing this trend or, as has been suggested by other authors,
the secondary electron escape mechanism from polycrystalline
diamond is different, utilizing transport along grain boundaries
[10].4. Future developments
We have plans to investigate the application of diamond as
secondary electron multiplication dynodes in actual detectors
operating in two modes:1. Transmission: primary electrons enter through one surface of a
thin film of diamond, and secondary electrons exit through the
other. Primary track lengths at typical energies coupled with
the minority carrier diffusion length requires (a) film thick-
nesses o200 nm, or (b) thicker films with an internal drift field
to increase the secondary diffusion length.2. Reflection: diamond is deposited on an open conductive wire
mesh or electroformed grid. Primary and secondary electrons
enter and exit through the same diamond surface. Coated mesh
dynodes (not CVD diamond) are already used in some imaging
PMTs.
The transmission technique is conceptually superior, providing
a linear detector design with minimum charge spread andguaranteed electron collision at every dynode, ideally suited for
imaging applications. It is technologically more demanding;
challenges being mechanical support of very thin films or
incorporation of internal drift fields.5. Conclusions
We have obtained SEY results from B-doped hydrogen-
terminated CVD diamond comparable or better than reported
elsewhere. However, we are still encountering electron beam-
induced contamination effects within the apparatus and are
continuing to develop counter measures to this limitation, while
in parallel, attempting to establish the relevance of SEY reduction due
to hydrogen desorption. The scale of the latter is the more serious
issue, being unavoidable in a detector context in reflection mode.
Successful demonstration of high SEY from a transmission dynode
will circumvent this problem since hydrogen desorption from the
input surface should not affect the NEA of the output surface.
References
[1] J.S. Lapington, S. Chakrabarti, T. Cook, J.C. Gsell, V.T. Gsell, Nucl. Instr. and
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[2] J.E. Yater, A. Shih, R. Abrams, J. Vac. Sci. Technol. A 16 (1998) 913.
[3] P.W. May, Phil. Trans. R. Soc. London A 358 (2000) 473.
[4] J.B. Miller, G.R. Brandes, J. Appl. Phys. 82 (1997) 4538.
[5] Jon Howorth, Photek Ltd., private communication.
[6] G.T. Mearini, I.L. Krainsky, J.A. Dayton Jr., Surf. Interface Anal. 21 (1994) 138.
[7] PSP Vacuum Technology, Macclesfield, UK, /www.pspvacuum.comS.
[8] A.E. Vladar, M.T. Postek, Microsc. Microanal. 11 (Suppl. 2) (2005) 764.
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Investigation of the secondary emission characteristics of CVD diamond films for electron amplification

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Investigation of the secondary emission characteristics of CVD diamond films for electron amplification

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