Aluminum sputtering, and neutron activation analysis after cesium ion bombardmen

Hurlbut, F. C.

Olson, N. T.

Pigford, T. H.

Smith, H. P., Jr.

English

NASA Technical Reports Server

N66 3486 i 
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UCCESSION NUMBER) 
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QUARTERLY PROGRESS RE PORT: 
INVESTIGATION OF KltOVOLT ION SPUTTERING 
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https://ntrs.nasa.gov/search.jsp?R=19660025575 2020-03-16T20:13:10+00:00Z
QUARTERLY PROGRESS REPORT 
N A S A  CR-54908 
Series No. 7, Issue 31 
INVESTIGATION OF KILOVOLT ION SPUTTERING 
by 
Harold P. Smith, Jr . ,  F. C. Hurlbut, T. H. Pigford, 
aiid N. Thoxrias Bison 
prepared for 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
July31,  1966 
Distribution of this report is provided in the interest of 
information exchange. 
resides in the author o r  organization that prepared it. 
Responsibility for the contents 
CONTRACT NAS 3-5743 
Technical Management 
Nasa Lewis Research Center 
Cleveland, Ohio 
Electric Propulsion Office 
Mr. J. A. Wolters 
SPACE SCIENCES LABORATORY 
University of California, Berkeley 94720 
I. 
11. 
111. 
IV, 
V. 
VI. 
FIGURES 
1 -  
TABLE O F  CONTENTS 
Introduction and Summary 
Surface Density Measurements 
Velocity Spectrum Measurement 
Merciiry Ton Sputteri-n-g ' 
Aluminum Sputtering 
Ion Implantation 
References 
Distribution List 
Fig. 1. Schematic of mercury ion sputtering apparatus 
Fig. 2. Relative angular distribution of aluminum 
sputtered in the (010) f rom the (001) face by 
cesium ion bombardment at 7.5 KeV and at 
normal incidence. 
tional to the surface density of aluminum 
collected at the indicated polar angle on a 
flat plate parallel to the (001) face. 
The count ra te  is propor- 
Fig. 3. Penetration of 20 KeV Cs'into polycrystalline 
A l ,  bombarded to 1017 ions/cm2 (1 gm Al/cm2 
= 3 7 2 )  
1 
3 
5 
7 
9 
10 
12 
16 
13 
14 
15 
1 
I. INTRODUCTION AND SUMMARY 
Sputtering o r  ionic erosion of the accel electrode and focusing 
structure of the ion rocket engine can be the dominant mechanism 
limiting long t e r m  operation of the engine. 
sputtering has been known since the phenomenon of g a s  discharge was 
f i r s t  observed, no reliable theory to predict the yield, angular 
distribution, and velocity spectrum has been developed. Furthermore,  
it has only been within the past few years that experiments have been 
made under suitably defined conditions. In addition, there has been 
little work with either cesium o r  mercury beams so that it is 
difficult to predict the electrode erosion on the basis of previous data. 
For these reasons,  as well a s  applications to thin film technology 
and ion implantation, the Lewis Research Center has sponsored 
detailed investigation of ion bombardment of suitable monocrystalline 
electrode material  where target parameters such as  temperature, 
angle of incidence, etc., a r e  well known and varied over the range of 
intere st. 
Although the field of 
The University of California (Berkeley) Space Sciences 
Laboratory began an investigation of this field in 1964. This document 
is submitted as a report  of progress in this investigation for the period 
of time f rom May through July 1966. 
During the past quarter, proportional counter technique and 
operation have been improved so that it i s  now possible to  separate 
completely the oxygen characteristic x-ray peak from the associated low 
energy noise. Measurement of oxygen surface density is planned for the 
coming quarter. 
Preliminary data is presented in which the electron microprobe 
has been used to determine the angular distribution of sputtered 
aluminum. 
active t racer  technique previously developed for cesium-copper 
sputtering. 
activation analysis - after cesium ion bombardment of aluminum has 
This technique offers improved resolution over the radio- 
Preliminary data i s  also presented in  which neutron 
L--- uccii aiicces sfuiiy incorporated with the standard anodization- s t r ip  
technique. 
that encountered at low dose rate. 
As expected, the median range of penetration i s  less than 
3 
11. SURFACE DENSITY MEASUREMENTS* 
The surface density of oxygen atoms on an aluminum substrate 
may be determined by detecting and counting characteristic oxygen 
x-rays produced by bombardment with a 100-KeV proton beam using a 
gas proportional counter, coupled with suitable pulse height amplifying 
and p d s e  couiiting eqiiipme~t. 
The low energy of the characteristic oxygen x-rays (500 eV) places 
stringent requirements on the x-ray detection system. The proportional 
counter window must be extremely thin for good transmission and yet be 
able to withstand sufficiently high counter gas pressure for adequate gas 
amplification. As previously reported' a thin (4,000 %) alumina window 
assembly, which allows 6070 transmission of characteristic oxygen x-rays 
and can withstand 100 Torr  counter gas pressure,  has been constructed. 
However, with maximum g a s  amplification, i. e. proportional counter 
gas pressure of 100 Tor r  andmaximum counter voltage, the pulse heights 
corresponding to characteristic oxygen x-rays were less than the electronic 
noise level of standard pulse amplifiers. 
pulse amplifying equipment was replaced by a very low noise commercial 
field effect transistor preamplifier and mating pulse shaping amplifier. 
Characteristic oxygen x-rays were produced by bombardment of 
Consequently, the standard 
an anodized aluminum target with the duoplasmatron ion beam. 
90% argon, 10% methane proportional counter gas, the pulse height 
spectrum was obtained with a 400-channel pulse height analyzer. 
Using 
At 
.I - 
The work reported in this section was performed by R. R. Hart 
4 
maximum gas amplification, the characteristic oxygen x-ray peak was 
resolved, but the peak width (500 eVat  half maximum) was so broad 
that the leading edge of the peak was not completely clear of the electronic 
noise level. 
470 isobutane, which produced a much narrower peak (300 eV at half 
maximum) apparently due to greater penetration of the characteristic 
oxygen x-rays into the active volume of the proportional counter. 
peak was then clear of the electronic noise level. 
The proportional counter g a s  was changed to 9670 helium, 
The 
A target holder assembly that will permit up to ten targets 
of varying thicknesses of anodized high purity aluminum to be individually 
positioned in the proton beam within the ultra high vacuum target chamber 
has been designed and partially constructed. 
target capability, the time required for characteristic oxygen x-ray 
yield measurements as a function of oxygen atom surface density will 
be greatly reduced, since only one pump down to ultra high vacuum will 
be required for each set of ten targets. 
In comparison to a single 
In order  to separate the proton beam component f rom the total 
duoplasmatron ion beam, an analyzing magnet has been installed. 
It is  expected that characteristic oxygen x-ray yield measurements 
as a function of oxygen atom surface density will be performed next 
quarter. 
5 
I 11. VELOCITY SPECTRUM MEASUREMENT* 
In performing an analysis of the veloci-g spectrum f sputter e d 
particles, the parent ion &ani is first modulated by  electrostatic 
techniques and then allowed to impinge on a target as a ser ies  of 
discrete pulses. 
magnitude in comparison with flight times of the sputtered particles 
and since the ion flight time from modulator to target is known, spectra 
of arr ival  times of sputtered particles on a detector can be translated 
into spectra of velocities for these particles. 
straightforwardly if  the parent ion beam is monoenergetic. 
elements of an arrival time analysis system a re  modulated ion source, 
mass spectrometer, multiplier detector, signal amplifier train,  and 
data acquisition system. 
of these system elements is indicated below. 
Since the time lapse in sputtering is of negligible 
Analysis proceeds 
The 
Progress  in the last quarter in the development 
Following breakdown of the previous ion source, a new source 
has recently been obtained. 
design no prolonged difficulties in the redesign of the lens and beam 
transport  system are anticipated. 
accomplished by the extractor pulse method. 
the required square pulse has  been obtained and the amplifier previously 
built for this project will be used to provide the final pulse to the 
extractor electrode. 
Although the new source is of different 
Modulation of the ion beam will be 
A generator to  produce 
3k 
The work reported in this section was performed by G. Cowell, 
M. Kosaki, and N. T. Olson 
The high vacuum system in which the experiment will be 
performed has been maintained at 6 x l o a 9  Torr .  
been reoriented so that the newly acquired source may be operated in 
a vertical position (necessitated by source design). 
The system has 
Although the quadrupole mass spectrometer had broken down 
several  weeks ago, it is now operating properly. 
The detection and counting system is in i ts  final stage of 
development. 
pulser is currently under construction. 
that is undergoing repair ,  the rest  of the system - electron multiplier, 
charge-sensitive preamplifier, pulse-shaping amplifier, timing 
discriminator, two scalers  and a variable time-delay gate-pulse 
generator - is assembled and in operable order.  
A scaler gating circuit to be triggered by the ion beam 
Except for a power supply bin 
6 I 
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.b 
IV. MERCURY ION SPUTTERING-” 
The apparatus for  mercury ion sputtering has been completed 
The system can now provide a magnetically and successfully tested. 
analyzed mercury ion beam of about 10 - 20 pa/cm2 at  the target 
with a target chamber pressure below 2 x 10 
at  Hg+ ion energies of 5.0, 7.5, and 10  KeV can be made without any further 
changes; while measurements for 1.0 and 2.5 KeV must await additional 
refinements in focusing. 
the higher energy measurements has been initiated. 
-7  Torr .  Measurements 
The preparation of the copper crystals for 
Insertion of a circular aperature at the exit of the magnet 
chamber has decreased the energy spread in the ion beam. 
retarding potential technique has been used to determine the energy 
spread of the Hg+ ion beam at the target assembly. 
in the 7.5 KeV Hg+ ion beam was found to be l e s s  than 50 eV. Thus, 
in the least  favorable case (ion energy of 1 KeV), AE/Eo will be l e s s  
than five percent. It should be recalled that the ion beam is transported 
at a high energy (7.5 KeV), with the target assembly biased to provide 
the desired ion energy at the target. 
A 
The energy spread 
A schematic diagram of the mercury  ion apparatus is given in 
Figure 1 .  
represents  a top view with a horizontal cut through the beam center 
line. 
six inch oil diffusion pump and a cylindrical cryogenic pump a s  vacuum. 
Since the ion beammoves in a horizontal plane, the schematic 
Although not shown, the source chamber has a cryogenically baffled 
.I. -r 
The work reported in this section was performed by R. G. Musket 
l -  
pumping elements. 
distance is about 120 cm. 
stainless steel while the isolation valve is  made of brass.  
isolation valve allows the pressures in the ends of the system to be 
completely independent of one another. 
ment has been completed, the valve is closed and the target 
chamber brought to atmospheric pressure so that the collector 
assembly may be changed while the source remains at  vacuum. 
After the collector is changed, the target end is pumped down, and 
when proper vacuum conditions a r e  obtained the valve may be opened 
for  another measurement. The source end employs O-rings for 
vacuum seals ,  while i t  is possible to use only metal seals  in the 
target end. 
of purposes but was specifically designed to accomodate a quadrupole 
mass  spectrometer. 
It should be noted that the total ion beam path 
All chambers a r e  constructed of 304 
The 
After a sputtering measure- 
The auxiliary port of the target end may serve a variety 
Alignment requirements dictated that all chambers 
~ 
I through which the ion beam passes be rigidly connected. One flexible 
section has been used to join the target chamber to the vac-ion pump. 
It is hoped that most of the angular distribution measurements 
on copper, nickel, and molybdenum can be completed during the coming 
quarter. 
9 
v. ALUMINUM SPUTTERING* 
1 The apparatus previously described was assembled and tested 
at operating conditions. 
were determined at various energies between 1 and 10 KeV. 
these parameters,  data was taken to determine the angular distribution 
of aluminum atoms sputtered by cesium ions of 2. 5, 5.0, 7. 5 and 
10.0 KeV. 
such that the poisoning ratio was 0.1 o r  less. 
7. 5 KeV and 77O K was also made. 
using the electron microprobe technique and a r e  presently being 
analyzed. 
shown in Fig. 2. 
Optimum parameters for ion beam fccusing 
Using 
These were made at room temperature under conditions 
2 A further run at 
The distribution data were obtained 
Data for  the run at 7.5 KeV and room temperature a r e  
Preliminary arrangements for  the determination of the 
sputtering yield and angular distribution of A1 0 2 3  
Methods of monitoring the beam current and neutralizing the surface 
charge buildup on the dielectric material have been devised. 
current will be determined by periodically swinging a beam flag into 
the beam with a solenoid. The relative constancy of the ion current 
in this apparatus has been verified during the above runs. 
surface charge will be neutralized by electron bombardment using a 
heated tantalum wire. 
the electron microprobe whereas the yield will be determined using the 
neutron activation technique, both of which have been fully described in 
previous progress  reports.  
have been made. 
The 
The 
Angular distributions will be determined using 
.L Tp 
The work reported in this section was performed by E. H. Hasseltine 
10 
-1. 
VI. ION IMPLANTATION-p 
The ear l ier  problems of surface irregularities resulting 
f r o m  extensive ion bombardment appear to have been solved through 
proper preparation of target surfaces and by sweeping the ion beam 
far beyond the apertures defined by the focusing electrodes. 
sweeping system has been improved by the construction of a linear 
sweeping circuit, which moves the beam at 1000 cycles/sec in one 
dimension and at 60 cycles/sec in the other dimension of the target 
surface. 
without visible alteration of the microscopically clean surfaces. 
The beam 
Aluminum targets have been irradiated to 1017 ions/cm2 
Several targets of polycrystalline and monocrystalline 
aluminum have now been bombarded with 15 and 20 KeV cesium ions 
and have been neutron irradiated for activation analysis of the implanted 
cesium. 
oxidation, followed by chemical dissolution of the oxidized layer,  to 
determine the location and relative amounts of implanted cesium. 
present the amount of aluminum removed in each such surface treatment 
is determined from the calibration reported by Davies. 
calibration using bulk impurity activation in the aluminum will soon 
be made in this laboratory for the high ion bombardments characteristic 
of the present experiments. 
These samples are now undergoing electrochemical surface 
At 
3 A new 
.I- -*
The work report in this section was performed by G. Moreau, 
T. H. Pigford, W. Siekhaus, and B. Southworth 
1 1  
Preliminary results for  polycrystalline cesium bombarded 
to lo1' ions/cm' by 20 KeV cesium, obtained by measuring 
radioactivity levels of C s  
are shown in Figure 3. 
and shape of the penetration curve appear reasonable for  20 KeV 
cesium onto aluminum. 
bombarded aluminum of the same initial preparation is yet to be made. 
134 in the anodizing and stripping solutions, 
The indicated depth of penetration 
The comparison with penetration in lightly 
Focusing electrodes in slit geometry a re  now being constructed 
for the radioactive-ion source. 
tested with krypton and argon. 
This source has been successfully 
REFERENCES 
1. H. P. Smith, J r . ,  F. C. Hurlbut, and T. H. Pigford, "Quarterly 
P r o g r e s s  Report: Investigation of Kilovolt Ion Sputtering", 
NASA CR-54907, April 30, 1966. 
2. A. E. Andrews, E. H. Hasseltine, N. Thomas Olsen, andHarold 
P. Smith, J r . ,  Cesium Ion Sputtering of Aluminum, J. Appl. Phys. 
(in press). 
3. J. A. Davies,  J. Friesen, and J. D. McIntyre, Can. J. Chem.,  
- 38, 1526 (1960). 
13 
ELECTROSTATIC ION -- TRANSPORT LENSES 
ISOLATION VALVE 
MAGNET CHAMBER 
g?EU&!'%O 
MECHANICAL PUMP 
AUXILIARY PORT 
I A W C  I 
ANGULAR DISTRH.JTK)N 
COLLECTOR 
ALL=METAL 
VALVE 
STAINLESS 
STEEL 
BELLOWS 
TO 200LB 
VAC-ION 
PUMP 
FIGURE 1 : Schematic of mercury ion sputtering apparatus. 
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Investigation of kilovolt ion sputtering Quarterly progress report, May - Jul. 1966

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