An experimental facility to investigate low energy (less than 500 eV) sputtering of metal surfaces with ions produced by an ion gun is described. Results are reported on the sputtering yield of cobalt by cesium ions in the 100 to 500 eV energy range at a pressure of 1 times 10(exp -6) Torr. The target was electroplated on a copper substrate. The sputtered atoms were collected on a cobalt foil surrounding the target. Co-57 was used as a tracer to determine the sputtering yield

Handoo, A.

Ray, Pradosh K.

English

NASA Technical Reports Server

wN91-28124
LOW ENERGY SPU'ITERING OF COBALT BY CESIUM IONS*
A. Handoo and P. Ray
School of Engineering and Architecture
Tuskegee University
Tuskegee, AL
ABSTRACT
An experimental facility to investigate low energy (_:500 eV) sputtering of metal surfaces with
ions produced by an ion gun is described. Results are reported on the sputtering yield of cobalt by cesium
ions in the 100 to 500 eV energy range at a pressure of i x 10"6Torr. The target was electroplated on a
copper substrate. The sputtered atoms were collected_on a cobalt foil surrounding the target. STCo was
used as a tracer to determine the sputtering yield._ _
i
*Work supported by NASA Grant NAG8-020
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https://ntrs.nasa.gov/search.jsp?R=19910018810 2020-03-19T16:41:53+00:00Z
LOW ENERGY SPUTTERING OF COBALT BY CESIUM IONS
1. Introduction
Electron bombardment ion engines are required to have operational lifetimes of Up to 15,000
hours at 2 A beam current for successful completion of many proposed space missions [1]. From the
mission profile life tests, it was observed that the major life-limiting mechanism is the formation of flakes
of sputtered material which might be of sufficient size to cause electrical shorts or arcing in the discharge
chamber [2]. The internal surfaces of the discharge chamber where the erosion is most severe are
subjected to bombardment by ions having energies less than 100 eV, assuming that up to triply charged
ions exist in the plasma potential of 32 V within the discharge chamber where the ions are generated.
However, the data on yields from low energy sputtering are sparse and there are uncertainties due to
difficulty in detecting the extremely low sputtering yields [3-5]. In view of the scarcity of sputtering yield
data at low energies, a systematic experimental study is being undertaken using a radioactive tracer
technique. In the present study cesium ions having well-defined ion energies impinge on a cobalt target
mixed with a small amount of cobalt-57' Cobalt is chosen as the targe_ material forinitial studies because
its tracer, cobalt-57, has a relatively long half-life of 270 days and decays predominantly by emitting
low-energy (122 KeV) gamma rays. Since the total sputtering yield at low incident ion energies is
expected to be very small even after a prolonged exposure, the radioactive tracer technique appears to
be quite useful. The sputtering yield data reported are taken at a pressure of 1 x 10 6 Torr with energy
of the cesium ions varying from 100 to 500 eV.
2. Experimental Design and Procedure .......
The vacuum chamber is 10 cm in diameter, 22.5 cm long and has six ports. The ion gun enters from
the left through a 150 mm CF port. The target assembly is introduced from the right port by a linear
motion feedthrough which is connected to the vacuum chamber through a 150 mm CF adapting n_ppie.
The ionization gauge is mounted on another 70 mm CF port. A viewport is attached through a 115 mm
port on the top and there is also a provision to introduce a SIMS probe through an auxiliary 70 mm CF
port which is at a 45 ° angle.
a) Vacuum system:
Before any successful sputtering can be undertaken, the surface Should be made as free Ofadsorbed
layers of gases as possible. This can be achieved by baking and maintaining a very high vacuum in the
system. A turbomolecular pump of 170 Is"1pumping speed is usedto maintain the vacuum, All the
flanges used in the chamber are metal gaskets sealed to have minimal outgassing and diffusion rates. A
hot cathode ionization gauge with its digital readout controller is used to monitor the pressure level
inside the vacuum chamber.
b) ion Gun:
It is desirable that the ionbeam impinges on the target in a small area with a high current density.
However, space charge in the ion beam represents the fundamental limit on the ion current density.
Hence, the lower the kinetic energy of the beam, the lower is the maximum current density that can be
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generated. The ion gun used in the present setup is procured from Kimball Physics Inc. and is capable
of producing low-energy ion beam at well defined energies for both Cesium and noble gases. This will
enable us to cross check the sputtering yield data using a variety of ions.
c) Target and collector assembly:
The target and collector assembly is shown in Fig. 1. At the end of the linear motion feedthrough
is the holding rod with a tapped hole at the end. The target is cobalt (99.9% and 0.1% S7Co) electroplated
on the tip of a 4.8 mm diameter copper specimen. The edge of the copper specimen on which the cobalt
is electroplated is at an angle of 45 °, so the surface of the target is elliptical in shape with an area of
25.8 mm 2. The surface density of cobalt is approximately 50/zg / cm 2, which resulted in an activity of the
target of 110#Ci. The other end of the specimen, which is threaded, is attached to the holding rod. The
copper specimen is completely surrounded by a 19 mm diameter hollow cylinder, the inside of which is
lined with a 0.05 mm thick cobalt foil. The distance between the target and the ion gun was optimized
for maximum sputtering yield after conducting several initial runs under similar experimental conditions.
The operating distance was found to be about 19 mm (Fig. 2).
The vacuum chamber is pumped down to a pressure of 1 x 10 "6Torr. The ions are extracted and
accelerated to the desired energy by setting corresponding grid voltages. The beam current is then
measured with a Faraday cup which is pneumatically operated to intercept the beam. The ion beam
impinges on the target at a 45 ° angle. The target is exposed to the ion beam for a period ranging from
5 to 20 minutes depending on the beam energy level. The experiments were run for beam energies of
100, 200, 300, 400 and 500 eV for both focused and unfocused ion beams. The focused ion beam
diameter is about 1 mm whereas the unfocused beam diameter is about 3 mm. The ion beam current
ranged from 0.45 microampere at 100 eV to 1.15 microampere at 500 eV. After bombarding the target
with cesium ions, the cobalt foil is removed from the vacuum chamber and taken to a multichannel
analyzer for counting the gamma rays emitted by the disintegrating 57Co atoms deposited on the foil.
The analyzer is set so that only photoelectric peak is counted. A standard S7Co source of 0.115/zCi is
used to determine the efficiency of the counter. The amount of radioactive atoms in the sputtered
material is then a measure of the total sputtering yield.
3. Sputtering Yield Measurement
Let N be the number of total atoms counted under the photoelectric peak in time to. Then the
total number of radioactive atoms NR on the cobalt foil is given by
NR = N / (tc At/)
where 2 is the disintegration constant of 57Co and r/the efficiency of the counter. The percentage of
radioactive atoms on the target, _,, decreases with time and is determined by
-2T)'= )'o e
where _'o is the percentage of radioactive atoms on the target at the time of electroplating, and T is the
elapsed time between electroplating the sample and the day the data were taken. Assuming that all the
sputtered atoms are deposited on cobalt foil, the sputtering yield S is given by
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S = (NRq) / (7 1 t,)
where q is the charge of an ion, I the ion beam current and t, the beam exposure time of the target.
. _z ::
4. Results and Discussion
The sputtering yields of cobalt by cesium ions at 100, 200, 300, 400, and 500 eV are pre.sented in
Fig. 3 for both focused and unfocused ion beams. For compa_n,_the sputtering yield of cobalt by
mercury ions reported by Wehner is also presented, although the experiments were performed under
different conditions [3]. The sputtering yield values obtained by Wehner are 2 to 5 times higher than
those obtained from oui experiments with focused beams.
The discrepancy in the sputtering yield values can be traced to the vacuum chamber pressure and
the ion current densities at which these data were taken. Wehner's experiments were performed with
the target immersed in a low-pressure mercury plasma discharge at about 1 x 10 .6 Torr and5 mA/cm 2
ion current density. The highest current density obtained in our experiments was 0.15 mA/cm 2. At 0.15
mA/cm 2 a surface atom of the target receiv_ about 30 impacts per minute form the incident ions
whereas the residual gas molecules at 1 x 10 .6 Tort will impinge about i0 times on the surface atoms
during the same period [6]. Thus, the surface of the target will have an appreciable amount of adsorbed
gases which inhibit the sputtering process significantly.
The effect of ion current density on the sputtering yield can be clearly seen in Table 1, where the
ratios of the sputtering yields for the focused and the unfocused beams at various ion energies are listed.
At all ion energies the sputtering yields of unfocused beams are lower by factors of 10 to 14. The focused
and the unfocused ion beam densities differ by about the same values. It is obvious that a very high
vacuum is required to minimize the effect of the residual gas on the sputtering yields at low ion current
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densities. In view of this, our vacuum system is being upgraded to a pressure of 10 Torr. Currently, we
are in the process of obtaining data for sputtering yield of cobalt bombarded by low energy argon ions.
? : : ::7 =
The amounts of material sputtered at ion energies down to 50 eV are within the detection limits
using the 110/_Ci target. A higher activity target would enable us to measure sputtering yields below
100 eV.
m
Table 1.
Ratio of the Sputtering Yields for the Focused and the Unfocused Ion Beams.
ION ENERGY
(cv)
RATIO
100 10.4
200 10.0
300 13.7
400 11.9
5O0 11.i
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References
1. Austin, R.E., "Solar Electric Propulsion Systems (SEPS), Tomorrow's Propulsion System
Today", Paper no. AIAA-79-2045, Oct. 1979.
2. Bechtel, R.T.; Trump, G.E.; James, E.J., "Results of the Mission Profile Life Test", Paper
No. AIAA-82-1905, Nov. 1982, 22.
3. Wehner, G.K., Phys. Rev., 1956, 102, 690.
4. Laegreid, N.; Wehner, G.IC, J. Appl. Phys., 1961, 32, 365.
5. Rosenberg, D.; Wehner, G.IC, J. Appl. Phys., 1962, 33, 1842.
6. Chopra, ICL., Thin Film Phenomena (McGraw-Hill, New York, 1969).
475
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Distonce of the terget
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Figure 2. Variation of sputtering yield with distance.
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o _:: i _,
-1 o o
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o iOO 20o.....3o0 40o 5oo
Ion Energy, eV
Figure 3. Sputtering yield of cobalt by cesium ions at various ion energies.
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Low energy sputtering of cobalt by cesium ions

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