A method of producing very high resistivity surface conduction on ceramic accelerator components using metal ion implantation Page: 2 of 3
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delivered to the downstream target is several hundred mA
peak with the mean on-target beam current less than this
by the chosen duty cycle, typically of order 1%.
Note that the implantation is done in a broad-beam
mode. The diameter of the ion beam formation electrodes
("extractor grids") is 10 cm and the initial beam diameter
is almost the same. In this mode of ion implantation, the
broad area ion beam propagates line-of-sight from ion
source to target; there is no magnetic analysis or beam
bending, and the target is implanted over its complete
forward-exposed surface at one time, in contrast to the
conventional swept, focused beam approach used widely
in the semiconductor industry. Important also is that this
facility and mode of implantation are accompanied by an
'automatic' charge neutralization feature - the broad area
ion beam propagates in a self-produced sea of cold
electrons that can provide more than enough neutralization
for the potential charge build-up on the ceramic surface by
the positive ion beam .
Figure 2, Rotisserie Detail with Ceramic Insulating
Cylinder in Implanting Position
For the work described here we fabricated an
appendage to the implanter in which the ceramic
insulating cylinder was housed and which allowed the
cylinder to be continuously rotated while implanted. A
rotating cradle (the "rotisserie") held the cylinder at an
appropriate angle (35*) to the incident energetic ion flux
while continuously, slowly rotating it about its axis so
that the entire inside surface was implanted. The
mechanism was insulated and high voltage resistance was
monitored at selected times during the processing. For
this geometry the entire surface area of the cylinder was
exposed to the beam and implanted, but in order to
maximize the implantation dose symmetry and uniformity
we turned the cylinder 180' lengthwise in the cradle
halfway through the implantation process. '
The type of ceramic used throughout this work was
97% alumina with a surface finish of a few micro inches
rms. Ion source extraction voltage was 65 kV, whic for
the Pt mean ion charge state [6,91 of 2.1 gives a mean ion
beam energy of 135 keV. Note that for the 55' angle of
incidence, the ion energy normal to te surfae is
135cos35' or 77 keV. The source-to-target distance was
~25m for this implantation set-up.
The dc surface resistivity of the samples was
measured using a 0-100 kV high voltage power supply
and a current meter accurate to a few pA; most of the
measurements were done with the sample (either small
ceramic coupon or the actual cylinder) in vacuum, and at a
voltage gradient of from 1 to 10 kV/cm.
We first investigated the effect of implantation parameters
on the surface resistivity of small ceramic coupons of
dimension 1 cm x 2 cm. The metal ion species
investigated were Ti, Au and Pt; ion energy was varied
from 50 keV to 135 keV; dose was varied from 1 x 1014
cm2 to 5 x 1016 cm 2; and the angle of incidence of the
ion beam upon the ceramic substrate was either 0 (i.e.,
normal incidence) or 45 ; not all cells of this parameter
array were tested. The most important and quite basic
result from this preliminary exploration was that indeed
the surface resistivity could be controlled in this way.
Small surface cracks in the ceramic coupons, possibly
introduced in the grinding stage of sample preparation and
not at all easily visible in the unimplanted ceramic, were
found to be a frequent cause of apparent irreproducibility;
Interestingly, the cracks were considerably more visible
after implantation. The dependencies of the resistivity on
ion species, ion energy, and angle of incidence were small
(over the range investigated), and the resistivity decreased
with increasing implantation dose.
After the work on samples, we chose to do
implantation on the cylinders using Pt at 135 keV, and to
mount the cylinder at the angle required so as to expose
the entire inner surface to the beam. Platinum does not
oxidize and is a good cathode material for the vacuum arc
ion source. Our small samples indicated (with much
scatter) that Pt was slightly more effective than the
nearest competitor, gold. Future work would certainly
address further the issue of the effect of implanted species.
The surface resistivity as a function of implantation
dose, under the conditions of the cylinder implant, is
shown in Figure 3. In this processing we monitored the
total number of implantation pulses, and the dose as
shown in the figure is estimated from the pulse count
based on RBS (Rutherford Backscattering Spectrometry)
measurements  of the Pt dose implanted into small
silicon coupons mounted in place of the cylinder during a
calibration run. The required resistivity of approximately
50 Gf?/square was achieved for a dose of about 2 x 1016
ions/cm2. The data point in Figure 3 that shows an
apparent rapid drop in resistivity as a function of applied
dose seems to be real; we speculate that resistivity
tailoring by ion implantation is a complicated physical
phenomenon that may lead to a highly non-linear
resistivity vs. dose relationship, and that considerably
lower resistivities than achieved (and wanted) here may be
obtainable. No knowledge of the conductivity mechanism
exists, but a negative resistance coefficient with respect to
temperature leads to speculation that the mechanism is a
We have not measured the Pt ion implantation dose
or profile in the alumina. Measurement in the usual way,
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Liu, F.; Brown, I.; Phillips, L.; Biallas, G. & Siggins, T. A method of producing very high resistivity surface conduction on ceramic accelerator components using metal ion implantation, article, May 1, 1997; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc704135/m1/2/: accessed July 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.