Radiation Effects Letters, 1983,VOl. 76(4), pp. 137-142
0142-2448/83/7604-0137$18.50/0
01983,Gordon and Breach, Science Publishers, Inc.
Printed in the United States of America
Downloaded By: [University of Virginia] At: 14:27 12 June 2007
LOW-DOSE EFFECTS I N THE SPUTTERIN; OF EVAPORATED FIIMS
A . R . OLIVA FIIIRIO~, E.V. ALONSO, R.A.
Centro Atbmico B a r i l o c h e
8400
Rariloche, Argentina.
*
BARAGIOLA AND J . FERRON II
-
(Received for Publication April 15,1983)
Abstract:
W e r e p o r t measurements of the dose dependence of the
s p u t t e r i n g of evaporated f i l m s by 30 keV i o n s under UHV.
An
i n i t i a l (sub-monolayer) enhanced s p u t t e r i n g i s a t t r i b u t e d t o t h e
removal of weakly bound atoms; t h i s enhancement does n o t depend
on the i n c i d e n c e a n g l e of the p r o j e c t i l e .
It i s known t h a t i n most cases, the s p u t t e r i n g y i e l d depends not only
on the t y p e and energy of the p r o j e c t i l e and t h e t y p e and o r i g i n a l
s t a t e o f the t a r g e t , b u t a l s o on t h e bombarding dose. 1
This
phenomenon has i m p l i c a t i o n s i n t h e development of bombardment-induced
topography i n s o l i d s and on s u r f a c e a n a l y s i s methods which employ
sputtering for depth-profiling.
Dose e f f e c t s a r i s e because o f changes i n t h e t a r g e t which
accompany bombardment.
The p r o j e c t i l e s can not only cause damage b u t
be a l s o t r a p p e d i n s i d e the s o l i d , modifying i t s composition.
In the
vast m a j o r i t y of s p u t t e r i n g e x p e r i m e n t s , which have been performed
under moderate vacuum, f u r t h e r e f f e c t s o f t e n a r i s e due t o the presence
o f c o n t a m i n a n t f i l m s on t h e t a r g e t s u r f a c e , b e f o r e o r d u r F n g
bombardment.
With t h e a i m t o g e t more i n s i g h t i n t h i s dependence, we have
measured t h e v a r i a t i o n of s p u t t e r i n g y i e l d s S w i t h bombardment dose,
f o r Au and Cu targets and 30 keV A r S and Xes p r o j e c t i l e s .
To avoid
t a r g e t contamination we have used u l t r a - h i g h vacuum and prepared the
Deceased
11 P r e s e n t a d d r e s s : INIEC, G i i e m e s 3450, 3000 Santa F6, Argentina
Comisibn Nacional de Energia Atbmica
*
137
Downloaded By: [University of Virginia] At: 14:27 12 June 2007
138
A. R.O L N A FLORIO, E. V. ALONSO, R.A. BARAGIOLA and I . FERRON
surfaces in situ by deposition of high-purity (>99.99%) metals
onto optically polished substrates. To be able to measure at very low
doses, we have used in situ microbalance techniques so sensitive
that for Au we could take data after removal of the equivalent of only
a few percent of a monolayer of the target.
The apparatus has been described before. 2, The main experimental
aspects are the use of a differential pumping stage kept at
Torr between the accelerator and the UHV target chamber. This chamber
is pumped to a base pressure of less than 3 ~ 1 0 - 1Torr.
~
The target,
mounted on a goniometer, formed part of an electrode arrangement which
allowed the collimation of the ion beam, the suppression of spurious
currents, and the measurement of both sputtering and electron emission
during bombardment.
The dose of projectiles per unit area was
determined from the measured beam spot size and the time integral of
the ion current on the target, to an accuracy of 3%.
The targets were films evaporated onto the flat surfaces of
quartz crystals similar to those used by E e r n i ~ s e . ~The thickness of
each film was several times the ion range and such that the sputtering
yields were insensitive to the film thickness above this value.
During the evaporations, the pressure in the chamber rose to less than
4 ~ 1 0 - Torr,
~
and the deposition rates were -1 Alsec.
The film
thicknesses (rather areal masses) were determined in situ using the
target as a quartz-crystal microbalance.
If M is the mass of the
crystal, then small variations in mass, AM, are related to changes Af
in the resonance frequency of the crystal by: AM = k Af.
We
determined the sensitivity factor k for films deposited in areas
larger than the sensitive area of the crystal, by means of a
conventional microbalance.
We found k = (1.767 -+ 0.005) 10-8
gr/Hz/cm2. We also measured the differential sensitivity of the
crystal over its surface to determine the influence of the change in
impact area when the beam was smaller than the active area, or when
the target was tilted. With this data, and the measured beam spot
area, we calculated the effective sensitivity as a function of
incidence angle.
-
139
LOW-DOSEEFFECTS IN THE SPUTTERING OF EVAPORATED FILMS
The s p u t t e r i n g y i e l d w a s determined from t h e frequency change Af
Downloaded By: [University of Virginia] At: 14:27 12 June 2007
measured a f t e r a dose increment AD (ions/cm2) from:
s =
where Nav
,-
Nav Af
M, AD
T
M,
M,
i s Avogadro’s number, and M, and M,
t h e masses of p r o j e c t i l e
and t a r g e t atoms, r e s p e c t i v e l y . The mass i n c r e a s e due t o t r a p p i n g of
p r o j e c t i l e s i n t h e t a r g e t is taken i n t o account by T , assumed t o be
u n i t y i n t h e d a t a presented h e r e , but which is a function of dose.
Each measurement was performed a f t e r t h e c r y s t a l s t a b i l i z e d t o
frequency d r i f t s l e s s than 0.1 Hz/min.
During bombardment, we
measured t h e e l e c t r o n y i e l d y which provided an i n d i c a t i o n of t h e
s t a t e of c l e a n l i n e s s of t h e s u r f a c e . W e found no evidence f o r a dose
dependence of y, and n e i t h e r f o r a change of Y due t o t h e exposure of
t h e t a r g e t t o t h e r e s i d u a l gases i n t h e UHV t a r g e t chamber between
succesive i r r a d i a t i o n s .
From measurements of y over extended p e r i o d s
of
time,
we determined t h a t
t h e time c o n s t a n t f o r adsorption of
25
1
Xe
*Ot
13
15
04
10’~
0 0
I
10IL
-
30 keV X e on Cu.
t
Cu
0 0 0000000000000000000
I
1015
Dose (ions/cm2)
I
10l6
FIGURE 1. S p u t t e r i n g y i e l d s vs accumulated dose D.
o
a
1
1
10’~
30 keV A r on Au;
140
A. R. OLNA FLORIO,E. V. ALONSO, R. A. BARAGIOLA and I . FERRON
Downloaded By: [University of Virginia] At: 14:27 12 June 2007
monolayer of r e s i d u a l gases w a s of t h e o r d e r of
lo4
seconds.
Examples of t h e dose dependence of s p u t t e r i n g measured a t normal
incidence over a very wide dose range, a r e shown i n Fig.1.
We now a n a l y s e reasons f o r t h e observed dose dependences.
One
e f f e c t i s through t h e unknown v a r i a t i o n with dose of t h e c o e f f i c i e n t T
i n Eq.1, which was ignored i n t h e e v a l u a t i o n of S , where we took ~ = l .
This q u a n t i t y , which a t zero dose i s t h e t r a p p i n g c o e f f i c i e n t and -1,
d e c r e a s e s with D and reaches zero a t doses where f o r each p r o j e c t i l e
t h a t impinges on t h e t a r g e t another atom, p r e v i o u s l y implanted, i s
sputtered.
The i n f l u e n c e of t h i s e f f e c t i n S can be bound t o t h e
r a t i o between M,/M2 and t h e f i r s t term i n Eq.1; from our measured
v a l u e s , t h e r a t i o is -3% f o r A r on Au and 14% f o r X e on Cu.
In t h i s
l a t t e r c a s e , t h e change of S between t h e low and high dose regimes,
due t o t h e dose-dependent t r a p p i n g p r o b a b i l i t y T, should then be more
visible.
I f we d e f i n e high doses t h o s e a t which t h e s p u t t e r e d depth
i s l a r g e r than t h e mean i m p l a n t a t i o n depth p l u s i t s s t r a g g l i n g , w e
f i n d f o r 30 keV Xe' on Cu, a dose of 2x1015 ions/cm2 f o r t h e onset of
t h e h i g h dose regime, and, using T=O, we o b t a i n good agreement between
our measured values of S and values r e p o r t e d i n t h e l i t e r a t u r e . 1
I n t h e case of 30 keV A r on Au, we found t h a t f o r doses < l o i 4
ions/cm2 t h e r e i s a region with S=12.
Then, a t r a n s i t i o n region
appeared which extended u n t i l t h e e q u i v a l e n t i n mass of
3 monolayers
was s p u t t e r e d away, where S drops t o a value of S=10. For t h e f i r s t
r e g i o n , w e n o t e t h a t t h e q u a n t i t y of s p u t t e r e d Au atoms is l e s s than
the e q u i v a l e n t of a monolayer.
-
This i n i t i a l dose dependence cannot be a t t r i b u t e d t o a l a r g e
s p u t t e r i n g y i e l d of adsorbed gas s i n c e experiments w e r e made under
UHV, nor t o t h e presence of trapped A r , s i n c e t h e A r c o n c e n t r a t i o n a t
t h e s u r f a c e i s extremely low f o r t h e s e doses.
A l i k e l y cause i s
rather
t h e presence of Au atoms which have been l o c a t e d by vacuum
d e p o s i t i o n i n s p e c i a l , low binding energy s u r f a c e s i t e s , and which
a r e , consequently, more e a s i l y s p u t t e r e d . The average energy of an Au
atom "adsorbed" on a p o l y c r y s t a l l i n e gold s u r f a c e , can be estimated5
t o be -30% lower than t h e binding energy of a normal s u r f a c e atom.
Downloaded By: [University of Virginia] At: 14:27 12 June 2007
LOW-DOSE EFFECTS IN THE SPUTTERING OF EVAPORATED FILMS
141
Then, f o r low d o s e s , while t h i s o u t e r l a y e r of r e l a t i v e l y weakly bound
atoms i s not y e t removed, there should be a l a r g e r s p u t t e r i n g y i e l d
due t o the c o n t r i b u t i o n of t h i s k i n d of atoms.
T h i s e f f e c t was only
observed i n Au, f o r which a submonolayer s e n s i t i v i t y could be a t t a i n e d .
F u r t h e r evidence f o r t h e i n f l u e n c e of s p u t t e r i n g o f atoms from
p r e f e r e n t i a l l o c a t i o n s , a t low d o s e s , was found when s t u d y i n g the
v a r i a t i o n of S with i n c i d e n c e a n g l e as a f u n c t i o n of removed mass. I n
when the mass removed w a s that of l o L 5 and
F i g . 2 , we d e p i c t S vs
10 I atoms/cm2 ( e q u i v a l e n t t o -2/3 and -6.5 monolayers, r e s p e c t i v e l y )
It i s seen t h a t S is l a r g e r f o r t h e less p e r t u r b e d s u r f a c e s by
approximately a c o n s t a n t f a c t o r over a l l measured i n c i d e n c e a n g l e s .
This i s c o n s i s t e n t w i t h the i n i t i a l e x i s t e n c e of an o u t e r l a y e r o f
r e l a t i v e l y weakly bound s u r f a c e atoms , s i n c e o t h e r f a c t o r s important
.
i n determining S,
l i k e the i n i t i a l energy d e p o s i t i o n and the s p a t i a l
cos
0
FIGURE 2. S vs t h e c o s i n e of the i n c i d e n c e a n g l e f o r 30 keV A r s on Au
a t two v a l u e s of the average removed t h i c k n e s s .
Downloaded By: [University of Virginia] At: 14:27 12 June 2007
142
A. R. OLWA FLORIO, E. V. ALONSO, R. A. BARAGIOLA and I. FERRON
distribution of the cascade should not be affected by the presence of
this low-dose surface structure.
Returning to Fig.1, one can notice thet after a dose of lo“+
at/cm2 was surpassed, another region occurs where S does not vary with
D and agrees with published data,’ until a depth equivalent to -20
monolayers has been removed. Then, a gentle decrease in the measured
values begins, continuing up to the highest measured doses. At these
high doses, the sputtered depth was -2.5 times the projected range
plus range straggling. Then, any variation originated in the presence
of implanted projectiles should have levelled off, and a saturation
regime should have been reached.6 The decrease we observe here can
instead be attributed to the effect of lateral stresses created on the
quartz crystal resonator, as measured by Eernisse’ for Au bombarded
with 45 keV KrS using the same measurement method.
Finally, we should point out that the observed smooth variation
of S with dose is in contradiction with the strong oscillations
reported8 for ArS on Au in our range of energies and doses. Since our
measurements showed excellent repetitivity over different measurement
runs, we conclude that the reported oscillations were probably caused
by an undetermined experimental artifact.
References
1. H.H. Andersen and H.L. Bay, in Sputtering by Particle Bombardment
I, ed. R. Behrisch (Springer, Berlin 1981) Ch.4.
2. A. Oliva-Florio, E.V. Alonso, R.A. Baragiola, J. Ferrh and M.M.
Jakas, Rad. Effects Letters 50, 3 (1979).
3 . J. Ferrbn, E.V. Alonso, R.A. Baragiola and A. Oliva-Florio, Phys.
Rev. B 24, 4412 (1981).
4. E.P. Eernisse, J.Nucl.Mater. 53, 226 (1974).
5. D.P. Jackson, Rad. Effects 18, 1 (1983).
6 . F. Schultz and K. Wittmaack and J. Maul, Rad.Effects 18,221 (1973).
7. E.P. Eernisse,
11, 488 (1974).
8. J.S. Colligon and M.H. Patel,
32, 193 (1977).