10.1098/rsta.2003.1301
Sputtering: survey of observations
and derived principles
By R a ú l A. Baragiola
Laboratory for Atomic and Surface Physics, University of Virginia,
Charlottesville, VA 22904, USA (raul@virginia.edu)
Published online 25 November 2003
We review the most salient observations and physical principles of knock-on and electronic sputtering and the role of sputtering in several astrophysical settings and applications in research and technology. In addition, we emphasize some unsolved problems, propose experiments and provide guides to representative literature reviews
and significant recent publications.
Keywords: sputtering; atomic collisions; surfaces; SIMS; depth profiling
1. Introduction
Sputtering is the removal of material from objects by energy transfer in collisions of
energetic atomic projectiles. It occurs widely in nature, where it causes the erosion
of the surface of airless astronomical bodies (interstellar dust particles, the Moon,
etc.) subject to ambient energetic radiation. The first record of sputtering in the
laboratory (Grove 1852) indicated the formation of a deposit in the anode of a
gaseous discharge and its removal when the polarity of the electrodes was reversed.
It is interesting to note that the discoverer of sputtering, William Robert Grove, is
best known for his role as a distinguished lawyer and for his invention of the fuel cell.
For almost a century after Grove’s discovery, most of the observations on sputtering
were made using gas discharges; isolation and characterization of the process using
controlled ion beams in a vacuum and characterized materials started only a few
decades ago.
Since sputtering has been reviewed so extensively, it would not be valuable to
have another review here in such a limited space. Rather, the aim of this paper is to
provide a useful guide to the review literature, an overview of the main phenomena,
new views on some unsolved problems and suggestions for experiments. The task is
made easier because I am excused from dwelling on theory by the several theoretical
papers in this issue, which allows me to concentrate on observations and their physics
content. The reader should not expect exhaustive references or coverage of all topics.
Many sub-topics of sputtering, such as chemical sputtering, reactive-ion etching, the
effect of sample temperature, phase and porosity, to name a few, are not treated
here.
One contribution of 11 to a Theme ‘Sputtering: past, present and future. W. R. Grove 150th Anniversary
Issue’.
c 2003 The Royal Society
Phil. Trans. R. Soc. Lond. A (2004) 362, 29–53
29
30
R. A. Baragiola
In spite of 150 years of sputtering, and the impression that we may get from the
review literature, most of the field is poorly understood. Notable examples are the
sputtering of ceramics, polymers and composite materials, the ejection of molecules,
ions and excited atoms and the whole area of electronic sputtering. The interesting
history of the topic can be seen in the reviews by Massey & Burhop (1956) and
Carter & Colligon (1968). Other historically important reviews are those by Kaminsky (1965) and the volume series Sputtering by particle bombardment (Behrisch 1981,
1983; Behrisch & Wittmaack 1991).
Sputtering is a particular case of radiation damage. Displacement of atoms from
their equilibrium lattice positions is produced either by a single collision with the
projectile or, more generally, as a result of a collision cascade in the material. The
repulsive forces required to dislodge atoms or molecules from the lattice can occur
during close ‘knock-on’ collisions initiated by a projectile with sufficient momentum
or, indirectly, by electronic excitations that lead to antibonding states. These cases
result in knock-on sputtering and electronic sputtering, respectively. The related
case of electronic sputtering by energetic electron or photon impact, which generally only involves a single surface collision, will not be treated here. The interested
reader should look into the following reviews, which have individual flavours: Knotek
(1984), Baragiola & Madey (1991), Ramsier & Yates (1991), Baragiola (1992) and
Ageev (1994). This type of electronic sputtering is often named in the literature
as electronic desorption and photodesorption or, more generally, DIET (desorption
induced by electronic transitions). For readers who wish an expanded horizon, it is
worth mentioning that photodesorption is directly related to surface photochemistry
(Zhou et al . 1991), a topic of considerable fundamental and practical importance.
The physics of sputtering can be grasped most easily by studying analytical theories even though they have limited accuracy compared with the most accurate computer simulations. Analytical theories divide the sputtering process into three steps:
(i) initial collisions with the projectile that generate a recoiling target atom that
may be ejected directly; (ii) a cascade of collisions in the solid involving fast recoils;
and (iii) the escape of recoils through a surface barrier. The calculation of step (i) is
straightforward for knock-on sputtering (but extremely hard for electronic sputtering), since interatomic potentials for close collisions are relatively well known after
decades of heated debates. The collision cascade of step (ii) can be simulated simply
with a computer, except for poorly known interatomic potentials and electronicenergy losses relevant in slow collisions. Escape through the surface barrier has been
treated in an ad hoc fashion in analytical theories and most computer simulations;
however, for molecular-dynamics simulations, escape is just a specific instance of
step (ii).
A schematic of the manifestations of sputtering is shown in figure 1. Typically,
an ion beam hits a target surface, eroding it; most of the ion beam is implanted in
the target and a small fraction is reflected after suffering a range of energy losses.
In the area of the sample hit by the ion beam, a crater forms that contains microscopic cratelets produced by each ion impact. With ion-bombardment time, the crater
becomes deeper and its bottom rougher. The ejecta, which have a wide angular and
energy distribution, originate mostly from the top surface layer, with a small percentage of atoms coming from the second layer and a negligible contribution from
deeper layers. The ejecta can be intercepted by another surface (a collector), on which
a deposit forms. This process, called sputter deposition, is the primary method of
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
target
31
sputtered
particles
projectiles
deposition
substrate
Figure 1. Schematic of the sputtering process.
producing thin films. Secondary effects include reflection of a small fraction of the
ejecta at the collector, and sputtering of the deposit by the most energetic sputtered
atoms and by reflected projectiles. The ejecta can be analysed with mass spectrometry or optical techniques to obtain information on the composition of the bombarded
surface. The sputtered particles will also coat other surfaces besides the purposely
situated collector, and become part of the background gas contaminating it.
2. Experimental methods
The purpose of this section is to give the reader some criteria that can be used to
evaluate experimental reports. The important quantities in sputtering are the sputtering yield Y (number of atoms ejected per projectile), the composition of the ejecta
with respect to species (atoms, molecules, ions), the angle and the ejection velocity.
Progress made in the first 120 years of sputtering led to refinement in experimental
techniques, as methods producing errors or unreliable results were discarded. For
instance, sputtering was discovered in electrical discharges in gases and, for nearly
a century, it was mostly studied under those circumstances. Although the study of
sputtering in a discharge is important per se, a more clear control of the process is
achieved with measurements using ion beams in a vacuum. The more ideal conditions are more suitable for testing theoretical models. Even under those conditions,
several problems became apparent over the years. For instance, prolonged ion irradiation leads to accumulation of implanted projectiles in the target and with it a
time-dependence of sputtering, particularly in cases of low sputtering yield. At moderate pressures (0.1–10 µbar), gases adsorbed from the ambient atmosphere (usually
water vapour but even vacuum pump oil) form a contaminant layer that alters sputtering. The use of dynamic conditions, where the sputtering process itself prevents
accumulation of adsorbed layers, leads often to the problem of contamination of
the surface by implantation of recoil impurity atoms. In addition, dynamic cleaning
usually causes the development of topographical features (roughness) on the surface, which in turn may affect sputtering. A large roughness affects sputtering yields
by locally varying the angle of incidence. It also alters the angular distribution of
the ejecta as sputtered particles become trapped in surfaces adjacent to the emission
point. The same consideration applies to sputtering of naturally rough surfaces, from
powdered solids to the Moon’s regolith (Hapke 2001).
Another potential problem, usually not discussed, affects measurements using polycrystalline targets. A reduced-energy deposition by channelling effects near the surface will occur if there is a preferred orientation of the grains (texturing) on the
Phil. Trans. R. Soc. Lond. A (2004)
32
R. A. Baragiola
surface: a common occurrence in foil or ribbon targets, where it is caused by the
lamination process. This problem and that of surface contamination in insufficient
vacuum are probably at the root of the large spread in experimental results that
can be discerned in compilations of experimental sputtering yields (Andersen & Bay
1981; Yamamura & Tawara 1996).
The excellent review by Andersen & Bay (1981) gives many details of important
experimental issues that need to be addressed for accurate measurements of total
sputtering yields but does not stress enough the need for very low pressures. The
crucial criteria for good measurements are the purity of the ion beam (single mass,
absence of multiply charged ions), the accurate measurement of the incident ion
flux, the characterization of the target, and ultrahigh vacuum (less than 0.01 µbar).
Unfortunately, most of the studies reported in the literature do not fulfil these criteria. To study the fundamental aspects of sputtering it is better not to modify the
target material significantly during irradiation; this implies very low bombardment
doses (projectiles per unit area). On the other hand, for specific applications such
as depth profiling or ion-beam machining, the quantity of interest is the amount of
material removed after irradiation with a large dose that produces significant erosion,
roughening and contamination of the target by implanted ions (Lehrer et al . 2001).
To understand those cases, experiments must be done under those conditions.
For low-dose sputtering, two experimental methods are notable and are mentioned
briefly here: the quartz-crystal microbalance (QCM) for absolute measurements of
total sputtering yields and angular distributions, and sensitive laser techniques that
are capable of measuring yields and the angular, mass and velocity distributions of
the ejecta. The QCM is a simple and sensitive technique for measuring mass loss per
unit area due to sputtering, allowing the use of a very small fluence of projectiles. The
sensitivity can be better than 1% of a monolayer/second under irradiation (Balaji
et al . 1990; Westley et al . 1995). The technique has some limitations, the most
important being that the material must be deposited as a thin film onto the QCM so
that bulk samples cannot be studied. Such films show an initial decrease in sputtering
yield with dose as a few monolayers are sputtered away, most likely due to the removal
of weakly bound adatoms (Oliva-Florio et al . 1983). The QCM can also be used to
collect the ejecta on a plate to measure the amount of material sputtered. As with
other collection techniques, it is important to quantify the sticking coefficient versus
deposit thickness and to evaluate re-sputtering of the deposit by energetic reflected
projectiles. Sensitive alternatives to the QCM technique for quantifying sputtered
deposits are Auger or X-ray electron spectroscopies, Rutherford backscattering and
radioisotope techniques.
Using pulsed ion beams, it is possible to measure the velocity distribution of sputtered particles from their time of flight over a defined path. The elegant mechanical
method developed by Thompson (1987) has now given way to laser techniques (Betz
& Wien 1994). Depending on the specific set-up, the velocity distribution can be measured by the Doppler shift of laser-induced fluorescence, or by time-of-flight analysis
of ions produced by laser ionization of the sputtered species. Examples of modern laser-based instrumentation can be found in Husinsky (1985), Wahl & Wucher
(1994), Ma et al . (1995) and Pacholski & Winograd (1999). The laser techniques are
much more complex (thus prone to error) and costly than the QCM method, but are
preferable because not only are they at least as sensitive as the QCM for total yields
but also they provide data on velocity distributions.
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
33
sputtering yield (1 + γ )−1
2.0
1.6
Ag
Cu
1.2
Cr
0.8
Al
0.4
Be
V
Si Ti
Ni
Co
Fe
Ge
Mo
Zr
Pd
Rh
Ru
Nb
C
0
20
Au
Pt
Re
Ir
Os
Hf
Ta W
40
60
atomic number
80
U
Th
100
Figure 2. Sputtering versus the atomic number of the target for 400 eV Ne+ projectiles (from
Laegried & Wehner (1961)). γ is the secondary electron yield, which should be 0.5 (Baragiola
1994b).
The next two sections survey observables for the cases of knock-on and electronic
sputtering.
3. Knock-on sputtering
(a) Total sputtering yields
Figure 2 gives an example of sputtering yields induced by 400 eV Ne+ ions for a
wide range of elements across the periodic table (Laegried & Wehner 1961). The
oscillations with the atomic number are mainly due to variations in the surface
binding energy U . Figure 3 shows the energy dependence of sputtering of Ni by a
variety of ions at normal incidence (Biersack & Eckstein 1984). It can be seen that
the sputtering yields rise from a threshold energy below which sputtering does not
occur, passing through a maximum and then falling at high energies. For energies
much
larger than the threshold, sputtering follows the energy dependence of Sn =
σ(T ) dT , the nuclear stopping cross-section (σ(T ) is the cross-section for energy
transfer T ).
The threshold is not at the surface binding energy U but at a substantially higher
energy. Assuming that all collisions are binary (often a good approximation), it is
required that the maximum value of the energy transfer T (the centre-of-mass energy)
is larger than U . Moreover, sputtering also requires the motion of other atoms besides
the one sputtered and the reversal of momentum—the projectile moves initially
towards the surface, while the ejected atom must move outward. Typical values of
threshold energies are 15–40 eV (Malherbe 1994). Unfortunately, most measurements
at low energies with heavy ions have not used ion beams analysed in mass/charge
and therefore they may have been contaminated with multiply charged ions which,
because of their higher energy, dominate threshold behaviour, as shown by Baragiola
et al . (1991) for the sputtering of inner-shell excited atoms. The threshold behaviour
Phil. Trans. R. Soc. Lond. A (2004)
34
R. A. Baragiola
10
Xe
Ar
1
sputtering yield Y
Ne
10−1
4He
10−2
ion calc. meas.
H
D
4He
Ne
Ar
Xe
10−3
10
102
103
104
incident energy E0 (eV)
D
H
105
Figure 3. Compilation of sputtering yields of Ni by different projectiles versus projectile energy.
Also shown are values calculated using the transport of recoils and ions in matter (TRIM) Monte
Carlo simulation program. (Reproduced with permission from Biersack & Eckstein (1984).)
of sputtering, which is important in astrophysics and in many applications, is generally unexplained for heavy ions, especially on multi-component targets.
(b) Effect of impact angle and channelling in single crystals
When the projectile is incident at an oblique angle θ to the surface normal, the
sputtering yield first increases with θ, as more of the projectile energy is deposited in
the thin layer responsible for sputtering. However, increasing θ also means that more
projectiles are reflected from the surface, thereby depositing substantially less energy.
The competition of these two factors explains why the dependence of sputtering yield
with angle of incidence Y (θ) peaks at some incidence angle θm . For incidents very
close to 90◦ on a flat surface, Y should become zero, as the projectile is reflected
due to a succession of very soft collisions (surface channelling) with energy and
momentum transfer insufficient to eject a surface atom. Both θm and Y (θm ) increase
with the energy of the projectile, as shown in figure 4. The study of Y (θ) requires
flat surfaces and low doses, since surface roughness averages the angular dependence
due to a distribution of microscopic incidence angles. Most published measurements
of Y (θ) are affected by this problem at large incidence angles.
In single crystals, Y (θ) is strongly modulated close to major crystallographic directions due to channelling, where atomic potentials steer the projectiles away from
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
Ar + → Au
35
Xe + → Cu
3
S(θ )/S(0)
S(θ )/S(0)
2.5
30 keV
10 keV
6 keV
3 keV
arccos θ
2
50 keV
30 keV
10 keV
5 keV
arccos θ
2.0
1.5
1
0
20
40
60
θ (deg)
80
1.0
0
20
40
60
θ (deg)
80
Figure 4. Sputtering yield as a function of angle of incidence for Ar+ and Xe+ projectiles on
polycrystalline films evaporated onto optically flat substrates and measured at low doses to
prevent development of surface roughness. (Reproduced from Oliva-Florio et al . (1987).)
small-impact-parameter collisions, thus decreasing the energy deposition and therefore the sputtering yield (Roosendal 1981). As mentioned above, channelling can
also cause low sputtering at normal incidence for polycrystalline targets if they are
textured with grains that have a preferential surface orientation.
(c) Molecular effect and nonlinear effects
The linear dependence of the sputtering yield with nuclear stopping cross-section
Sn (Sigmund 1969) is not obeyed for high densities of energy deposition, as may be
found in the superposition of collision cascades produced by the atomic components
of molecular projectiles. The molecular effect is the difference in sputtering yield
produced by a molecular projectile and the sum of the yields of each one of the atoms
in the molecule, impinging independently in the solid. This effect usually appears as
an enhancement of the yields for fast molecular projectiles, which is largest at the
energy for maximum Sn . At low projectile velocities there is a depression of the yields,
as shown in figure 5 (Oliva-Florio et al . 1979; Andersen 1993). However, a recent
study (Yao et al . 1998) gave an unexpected enhancement in sputtering yield of Au
+
for N+
2 compared with N below 500 eV, calling for further experimentation. Since
the review of Andersen (1993) on nonlinear effects in sputtering, huge deviations from
linearity were reported for MeV heavy molecular ions (Bouneau et al . 2002). Early
comparisons with the linear theory of Sigmund (1969) suggested that nonlinearity is
due to the appearance of collisions between moving atoms in the cascade. However,
molecular-dynamics simulations show that collisions between moving atoms occur
even when yields are linear with energy deposition and that the reason for the huge
yields is shock waves, hydrodynamic flow and near thermal evaporation in the dense
cascades (Jakas et al . 2002). Figure 6 shows that deviations from linear behaviour
Phil. Trans. R. Soc. Lond. A (2004)
36
R. A. Baragiola
2.5
0.45
2.0
0.30
1.5
0.15
1.0
0
0.001
nuclear stopping cross-section Sn
(reduced units)
molecular to atomic yield ratio
0.60
0.01
0.1
energy per atom (ε units)
Figure 5. Molecular effects on the sputtering yield of Au by Xe+ and Xe+
2 ions versus reduced
energy (Sigmund 1969). The line is the result from Sigmund’s analytical theory. (Reproduced
from Oliva-Florio et al . (1979).)
Stheor / Sexpt
1
Au
0.1
0
5
(dE/dx)n (100 eV Å−1)
10
Figure 6. Ratio of sputtering yields calculated by linear theory (Sigmund 1969) to experiments
measured at low doses versus N Sn . The arrow indicates measurements at energies above the
maximum of the stopping cross-section. (Reproduced from Oliva-Florio et al . (1987).)
already occur at relatively low values of N Se ca. 100 eV Å−1 , where N is the target
atomic density (Oliva-Florio et al . 1987).
If the experiments with MeV clusters were to be extended to higher projectile
masses, the results should merge with those from measurements of cratering induced
by dust grains with diameter greater than 0.1 µm accelerated to several km s−1 , used
to simulate micrometeorite impact in space (McDonnell 1992). These fast grains
‘vaporize’ the surface into a high-temperature plasma that leaves a crater at the
impact point. The collection of the charge in the sputtered plasma is used to detect
and identify dust particles by space probes (see Baragiola 1994a). A quantitative
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
37
Ti
1.0
0.8
N(E) /N(Ew )
Ni
Cu
0.6
Al
0.4
0.2
0
4
8
E (eV)
12
16
Figure 7. Energy distributions of sputtered particles from several targets bombarded with 900 eV
Ar+ ions, measured for emission close to the surface normal. (Reproduced with permission from
Oechsner (1970).)
understanding of the factors governing sputtering in the transition from atomic to
nanoparticle impacts is one of the unsolved problems in sputtering.
(d ) Ejecta
Extensive reviews of the angular and energy distribution of sputtered particles
are given by Hofer (1991), Winograd (1993) and Betz & Wien (1994). In general,
the angular distribution of sputtered particles varies as the cosine of the angle of
emission with respect to the surface normal. The cosine distribution, which also
pertains to sublimation, is a consequence of an isotropic distribution of moving recoils
just below the surface. At glancing incidence or at low projectile energies, where few
recoil collisions are important, the collision cascade is not isotropic. As a result,
emission near normal incidence is depressed and the direction of incidence of the
projectile becomes important. For multi-component targets, the angular distribution
may depend on the specific atom being sputtered, affecting the characterization of
sputtering by mass spectrometry along a specific emission angle (Wucher et al . 1988).
In crystals there is a preferential ejection of atoms along close-packed directions
that results in spotty deposits (Hofer 1991). These spot patterns are due to channelling and focusing in collision sequences (Thompson 1981, 2002; Hofer 1991). It is
significant that directional effects due to the crystalline structure are not washed out
by the apparently chaotic motion in the collision cascade. Spot patterns should lose
prominence under conditions that give nonlinear sputtering yields.
Studies of angular distributions have so far been done in conditions that produce
surface roughness, which means that emission angles were not well defined. Observations of angular and energy distributions from atomically smooth surfaces at grazing
emission angles should provide a sensitive test of models of surface barriers acting
on sputtered atoms.
Phil. Trans. R. Soc. Lond. A (2004)
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R. A. Baragiola
In addition to the observation of spot patterns in deposits, the measurements of
energy distribution of sputtered atoms Y (E) have historical importance, since they
confirmed that sputtering is due to non-equilibrated collision cascades and not to
thermal effects. For low Y the distribution follows the expression
E
dY
,
∝
dE
(E + U )3
derived with the assumption of a planar surface barrier for sputtered particles
(Thompson 1981, 2002), and generally reproduces experiments. Thompson’s expression gives a peak in dY /dE at E = U/2, which is of the order of what is observed
(see figure 7). For large Y many atoms are emitted simultaneously from a microscopically small region, and the model for a planar surface barrier becomes unrealistic.
A spherical surface barrier appears more adequate at not-too-large emission angles
and gives a shift in the peak to lower energies. This effect has often been interpreted
as an indication of thermal components to sputtering (thermal spikes). To clarify
this problem, it should be useful to measure the ejecta at large emission angles, to
maximize the difference between a cosine distribution proper of thermal evaporation
and a broad distribution resulting from a quasi-spherical surface barrier.
The expression for dY /dE given above holds only away from the maximum recoil
energy given by energy and momentum conservation in binary collisions at the specific ejection angle being measured. Near the sputtering threshold, therefore, the
energy distribution should have a different shape and be very narrow.
(e) Sputtering of ions and excited atoms
Except for the alkali metals and some ionic targets, ground-state neutrals constitute the vast majority of the ejecta with very weak emission of ions and excited atoms.
The processes responsible for efficient neutralization at surfaces and the survival of
ions and excited atoms have been reviewed by Yu (1991). Although the mechanisms
of electron transfer between sputtered particles and surfaces are in principle understood, no theory is currently able to predict yields of excited and ionized species
accurately. This is probably the most fundamental unsolved problem in sputtering.
Although the positive-ion fraction can be as low as 10−5 and is extremely dependent on the matrix, it is of great value, since ions can readily be mass analysed and
sensitively detected to give information about the composition of the sample. This
is the basis of the technique of secondary-ion mass spectrometry (SIMS; see below).
Yields of negative ions can be zero, since many elements do not have stable negative ion states. In addition, a fraction of neutrals similar to that of positive ions is
ejected in an electronically excited state. A good reference for modern techniques for
studying sputtering of excited atoms by photon emission is Cortona et al . (1999).
The sputtering of inner-shell excited atoms that decay by Auger electron emission
has been reviewed by Valeri (1993) and Baragiola (1994b).
(f ) Sputtering of clusters
The ejecta contain simple molecules and large clusters in addition to the generally
predominant atomic species. Cluster emission has been reviewed by Hofer (1991) and
Betz & Wien (1994). Recent relevant papers include those of Birtcher et al . (2000),
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
39
Rehn et al . (2001) and Staudt & Wucher (2002). The formation of small molecules
has been explained in some cases by association of atoms ejected very closely in phase
space. This mechanism is insufficient for the emission of very large molecules that
can only be explained by the simultaneous ejection of already associated atoms by
shock waves or similar mechanisms. Whether an observed molecule existed already
on the surface or was instead synthesized by sputtering has important implications
for methods of surface analysis based on sputtering. In the case of biomaterials there
is sufficient evidence of ejection of intact molecules, which has enabled a powerful method for biological analysis (see Sundqvist 1991; Ens 1993; Reimann 1993;
Håkansson 1993). In extremely dense cascades, such as those resulting from excitation tracks produced by fission fragments, molecules can be synthesized at an earlier
time and then subsequently ejected in the cascade—an example is the synthesis of
C60 molecules from PVDF polymers by swift heavy ions, a phenomenon which is far
from understood (Håkansson 1993).
The emission of clusters Xn from a solid composed of X atoms is very weak but
sufficient to make sputtering a useful method for producing clusters for subsequent
studies of the transition between the atomic and the condensed states. The sputtering yield follows a power law distribution Y (n) ∝ n−δ ; the exponent δ is large
for small clusters but the distribution falls more slowly (δ ≈ 2) for clusters with
n > 500 (Rehn et al . 2001; Staudt & Wucher 2002). The description of sputtering of molecules containing hundreds of atoms is beyond the current capabilities
of molecular-dynamics simulations, because of the difficulty and computer power
needed for sufficiently accurate quantum-mechanical descriptions of the interactions
and internal excitations.
(g) Preferential sputtering of multi-component solids
By multi-component solids we mean solids such as compounds, alloys, nanocomposites, etc., that are heterogeneous in composition on a scale smaller than the size
of the collision cascade or the sputtered depth. Sputtering of such solids occurs commonly in astrophysics (see § 5) and in depth-profiling applications. In these cases it
is found that different component atoms are removed at a different rate. This area
of sputtering, which has been reviewed by Betz & Wehner (1983), Sigmund & Lam
(1993), Malherbe (1994) and Gnaser (1996), is very undeveloped, since most extant
experiments and theories have concentrated on elemental metals and semiconductors.
Causes for preferential sputtering are differences in energy transfer with atomic
mass, differences in the binding energies of each component, and chemical alteration
with the formation and out-diffusion of volatile species. Due to preferential sputtering the target surface is enriched in the component that sputters less; this change in
composition causes a dose dependence of the sputtering yield. The simplest materials
to understand are metallic and semiconductor alloys. In general, it is not possible
to calculate the surface composition from partial sputtering yields due to segregation, chemical changes, radiation-induced diffusion, recoil implantation and cascade
mixing. Instances where complex phenomena arise are in oxides, polymers, minerals, etc.—the main reason being the changes in atomic composition, in molecular
structure and in binding energies induced by ion impact. Due to the ubiquity of
multi-component solids, the full understanding of preferential sputtering is one of
the most important needs in the field of sputtering.
Phil. Trans. R. Soc. Lond. A (2004)
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R. A. Baragiola
(h) Bombardment-induced topography
Stochastic processes in sputtering, the relaxation of the material to the generation of point defects, inhomogeneities in the sample (e.g. pores, impurities) and the
accumulation of implanted projectiles led to the development of topography in the
surface being bombarded. This is one of the most complex problems in the field of
sputtering. The effect of implanted gas on topography development, with implication
for fusion reactors, was discussed by Scherzer (1983). Cone formation on surfaces,
often attributed to impurities, has been reviewed in the book by Auciello & Kelly
(1984). Recent reviews of topography induced by low-energy ion–atom collisions
include those of Smentkowski (2000), Carter (2001), Murty (2002) and Valbusa et
al . (2002). Scanning force microscopy of initial cratering has been discussed recently
by Chey & Cahill (1997), Malherbe & Odendaal (1999) and Kim et al . (2003). Only
recently has it generally been realized that microscopic craters with rims may be
created more readily in events that do not produce sputtering, by the migration to
the surface of vacancies and interstitials generated in the collision cascade in the
bulk.
The competition of material removal with bombardment-induced surface diffusion
leads to the formation of ripples on the surface, with characteristics that depend
principally on ion energy and incidence angle. Induced diffusion can also lead to
smoothing of the surface (Chason et al . 1994; Mayr & Averback 2001). The area of
ion-beam-induced topography is one of the most active sputtering topics at present,
partly due to potential applications in nanotechnology (Chason et al . 1994; Jiang &
Alkemade 1998; Rusponi et al . 1998; Facsko et al . 1999; Batzill et al . 2000; Frost et
al . 2000; Costantini et al . 2001; Rost et al . 2001).
4. Sputtering by electronic excitations
The initiation of sputtering by the conversion of electronic energy to atomic motion
has been studied mostly in rare gas and molecular gas solids (Johnson & Schou
1993) and in alkali halides (Townsend 1983; Szymonski 1993). The particular case
of electronic-energy deposition by slow multiply charged ions has been discussed,
for example, by Varga & Diebold (1994), Schenkel et al . (2000) and Hayderer et al .
(2001). For sputtering by MeV heavy ions and cosmic rays see Betz & Wien (1994)
and Toulemonde et al . (2002).
Electronic sputtering is of great importance because it provides a rare window
into non-radiative relaxation of electronic excitations in insulators. Unlike the case
of knock-on sputtering, the yields and energy distributions of the ejecta in electronic
sputtering are extremely dependent on target properties. For this reason, we will
divide the discussion according to the type of material. Unlike knock-on sputtering,
which can occur on all materials, electronic sputtering only occurs on good insulators,
where electronic excitations are not degraded quickly by excitation of electrons to
the conduction band. In extreme conditions of electronic-energy deposition by fast
heavy ions, the decay of the electronic relaxation can be sufficiently slow in metals
(especially alloys) to produce radiation damage and sputtering.
Experimentally, electronic sputtering is separated from knock-on sputtering by its
different energy dependence: electronic sputtering is related to the electronic stopping
cross-section Se rather than Sn . The dependence on Se is different for different solids
and can vary from linear to cubic in the same material. Sputtering yields depend on
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
41
16
Ar2+
Ar+ + Ar + (e− + K.E.)
14
1
Ar* + Ar
12
Ar2*
potential energy (eV)
10
8
initial
ionization
event
M-band
luminescence
6
initial
exitation
event
4
2
2
0
2
Ar + Ar
3
4
internuclear separation (Å)
5
Figure 8. Potential-energy diagram appropriate to the sputtering of solid argon (see text).
Arrows labelled ‘1’ and ‘2’ symbolize repulsive paths that may lead to sputtering.
many sample properties besides surface binding energies, such as lifetimes of electronically excited states, hole mobilities, the presence of minute amounts of impurities
and, importantly, on the precise shape of the intervening intermolecular potential
curves. In addition, the dependence on energy deposition is different for projectile
velocities below and above the maximum of Se . For these reasons, the understanding
of electronic sputtering is still rather uncertain.
(a) Rare-gas solids
The electronic sputtering of rare-gas solids (Johnson & Schou 1993) has been
well studied because of its fundamental simplicity, as these materials are elemental
solids and do not undergo chemical modification during irradiation. As an example,
consider the case of solid argon, where sputtering can be explained with the aid of the
interatomic energy curves shown in figure 8. The impact of a fast ion produces a track
Phil. Trans. R. Soc. Lond. A (2004)
42
R. A. Baragiola
of ionizations (electron–ion pairs) and excitations (excitons) in the solid, following
which the atomic ions and excitons diffuse primarily by resonant electron transfer
with the lattice atoms. A diffusing Ar+ ion can strongly attract ground-state atoms
+
+
forming an Ar+
2 dimer (or an Ar3 , Ar4 multimer) that becomes trapped in 1–10 ps
by a structural relaxation assisted by lattice vibrations. After electrons have slowed
down sufficiently (in ca. 0.1 ns), they can undergo dissociative recombination, with
Ar+
2 producing excited Ar atoms (Ar*), ground-state Ar atoms, and kinetic energy
(pathway ‘1’ in figure 8). If this recombination occurs near the surface, it can produce
immediate sputtering of the Ar or Ar* involved or even of neighbouring atoms struck
by the separating pair. Excitons can also be produced directly by the projectile, by its
associated electronic-collision cascade, or by Auger recombination of an ion and an
electron. Regardless of how an Ar* is formed, it can pair with a neighbouring groundstate atom in an attractive or repulsive state. If the interaction is repulsive and at
the surface, the excited Ar* can desorb by a process called cavity ejection. In the
attractive state, Ar* combines with a neighbour, again assisted by lattice vibrations,
to form the Ar2 * excimer. The vibrationally relaxed Ar2 * excimer will then decay
by emission of a 9.8 eV M-band photon to the ground state of Ar2 in ca. 3 ns (1.4 µs)
for singlet (triplet) excimer states. Since the interatomic spacing of the excimer is
significantly smaller than that of the ground-state atoms, the decay is to the repulsive
part of the potential-energy curve. The kinetic energy released in this decay (path ‘2’
in figure 8) can also produce sputtering if the decay occurs near the surface. The total
sputtering yield is proportional to Se , since the number of ionizations and excitations
produced by the projectile per unit path length is proportional to Se . The production
of excited states by the ion is very efficient; half of the electronic energy deposited by
the ions is transformed into the 9.8 eV excimer luminescence (Grosjean et al . 1997).
The simultaneous measurement of luminescence, charge collection and sputtering in
solid Ar by Grosjean et al . (1995) allowed separation of the contribution of electron–
ion pairs and direct excitations to sputtering. The study of the effect of the substrate
and overlayers on the sputtering of thin Ar films (Grosjean et al . 2000) has produced
further elucidation of the sputtering mechanisms and the role of diffusion and drift
of excitons and holes.
(b) Alkali halides
A step up in complexity from the rare-gas solids is the alkali halides, which have
also been studied extensively under electron and photon impact. These materials offer
the advantage of easy preparation of crystalline samples. A distinguishing aspect is
that the targets are modified by the formation of molecular halides and metallic
alkalis; the highly volatile halogen molecules can leave the sample even at moderately low temperatures, whereas the alkalis can evaporate at room temperature due
to their high vapour pressure. This leads to an interesting temperature dependence
of sputtering. The mechanisms for halogen emission have similarities with the case
of rare-gas solids (Townsend 1983; Szymonski 1993; Betz & Wien 1994) but there
is, in addition, a thermal component of sputtering. The process starts with ionization and excitations that lead to the formation of a trapped molecular exciton that
decays into an electron trapped at a halogen site (F-centre) and an energetic halogen (H-centre). From there, there are two possible pathways, sputtering either by a
replacement-collision sequence or by thermalization and diffusion out of the solid.
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
43
If the temperature of the sample is kept low, an alkali metal overlayer forms that
inhibits electronic sputtering. At temperatures where the alkalis can desorb thermally, sputtering proceeds stoichiometrically. Contrary to what might be expected
from such ionic solids, the vast majority of the sputtered particles are neutral.
(c) Other insulators
For condensed molecular gases, where binding energies are small and sputtering
yields are large, it is often found that yields grow faster than linearly with deposited
electronic energy (Johnson & Schou 1993). In the astronomically important case of
ice, which we have recently reviewed (Baragiola et al . 2003), sputtering is proportional to the square of Se and results in the emission of H2 O and synthesized O2 and
H2 molecules with a strong temperature dependence. The transition from the atomic
to condensed phases is being investigated using large clusters (Bobbert et al . 2002).
Other condensed gases, such as CO2 , SO2 , etc., show complex behaviour due to the
multiplicity of chemical reactions that occur in the solid as a result of the radicals
produced by the projectile. The sputtering of oxides by electronic excitations has
recently been discussed by Itoh & Stoneham (2001) and Matsunami et al . (2002).
This topic is important in astrophysical settings, the subject of § 5.
5. Sputtering in space
The role of sputtering in astronomy has been reviewed by Johnson (1990), Tombrello
(1993) and Hapke (2001). Energetic ions are ubiquitous in space and produce sputtering on any surface not protected by a substantial atmosphere. For instance, solarwind ions (protons, alpha particles and some heavier ions at ca. 1 keV/amu) sputter
the surface of Mercury, our Moon, asteroids, comets, interplanetary dust and the
satellites of the outer planets. The flux of the solar wind is low, ca. 2×108 ions cm−2 s
at Earth, and decreases with the square of the distance to the Sun. However, in astronomical time-scales doses can be similar to the high doses achieved in the laboratory,
i.e. ca. 6 × 1018 ions cm−2 in 1000 years. Laboratory data suggest that preferential
sputtering of volatile components occurs in silicate minerals typical of the Moon
(Johnson & Baragiola 1991) and asteroids (Dukes et al . 1999). In these objects sputtering competes with erosion by micrometeorite impact; the relative removal rates
are mostly unknown.
Higher fluxes of ions in the 1–1000 keV range exist in the planetary magnetospheres
of Jupiter and Saturn and are responsible for surface erosion and modification of
the icy satellites (Shi et al . 1995; Cooper et al . 2001). The substantial gravity of
those satellites binds a large fraction of heavy sputtered species. Water molecules
sputtered from the surface into the atmosphere can be dissociated by solar light or
by magnetospheric particles. H and H2 , being light and therefore fast, can escape the
gravitational pull. Molecular oxygen forms an atmosphere that does not condense
at the satellite temperatures, which adds to the transient water vapour atmosphere
(Shi et al . 1995).
Cosmic rays in the interplanetary and interstellar media sputter efficiently but on
very long time-scales, since fluxes are low. Sputtering of dust grains is an important
process for the balance of grain destruction and creation in interstellar space (Dwek et
al . 1996), where the dust grains are typically a few hundred nanometres or smaller in
Phil. Trans. R. Soc. Lond. A (2004)
44
R. A. Baragiola
size. For these small grains sputtering yields are enhanced, since the collision cascade
intersects not only the entrance surface but, depending on the ion energy, the side
and exit surfaces as well (Jurac et al . 1998).
A symmetrical situation occurs when a high-velocity grain goes through a gas at
rest. Such is the case for micrometre- or sub-micrometre-sized dust particles entering the Earth’s atmosphere at geocentric velocities of 0.2–1×107 cm s−1 , which are
destroyed by sputtering and evaporation in collisions with atmospheric molecules
(Meisel et al . 2002).
6. Applications
(a) Surface preparation
A common application of sputtering is cleaning surfaces for basic science studies.
For instance, one can remove the native oxide layer of a silicon sample by sputtering
with low-energy noble-gas ions (to avoid chemical effects of implantation) and then
remove the radiation damage by high-temperature annealing. Several complications
accompany sputter cleaning.
(i) Ion bombardment also produces recoils in the contaminant layer, which move
towards the bulk (recoil implantation) thereby slowing the cleaning process.
(ii) Ion implantation, damage and the production of topographical features occur,
which may not be acceptable. These can be removed by thermal annealing but
only for those materials that can take high temperatures without decomposing,
vaporizing or melting.
(iii) Sputter cleaning is generally not possible for polymers, compounds, etc., which
are chemically altered by preferential sputtering. These effects can be minimized by using the lowest projectile energy possible (e.g. 200 eV) at the expense
of cleaning time.
(iv) Impurity ions present in the sputtering beam, which originate in the ion source,
are incorporated. These can be removed by mass analysis, a precaution not
usually taken, since most ion guns in commercial surface-science equipment
lack mass analysis.
Sputtering can also be used to polish rough surfaces by using glancing incidence
accompanied by azimuthal sample rotation (Wissing et al . 1996), while the angular
distribution of reflected ions gives an indication of surface smoothness. A new method
for surface polishing is the use of low-energy cluster beams, which are also useful for
shallow ion implantation (Brown & Sosnowski 1995; Yamada 1999). As mentioned
above, cluster beams produce effects that are nonlinear in the energy deposition. For
low-energy cluster impact the effects include greatly enhanced sputtering yields and
a distorted angular distribution of sputtered particles, since the cluster intercepts
part of the ejecta.
In other cases it is preferable to have rough surfaces that can be produced by
sputtering. For instance, texturing of surfaces by ion impact is used to improve
adhesion in thin-film deposition and may be applied in biomedicine (Kowalski 2001).
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
45
(b) Depth profiling
This is one of the most important applications of sputtering. Sequential removal
of surface layers exposes the bulk of the material, allowing the generation of depth
profiles (composition versus depth) by continuous elemental analysis using any of
the standard surface-analysis techniques such as low-energy ion-scattering spectroscopy, X-ray photoelectron spectroscopy or Auger electron spectroscopy. An excellent review of the topic is given by Wittmaack (1991).
The depth resolution of the sputter removal technique is limited by ion-beam
mixing and surface roughness. In ion-beam mixing, layers at different depths are
mixed prior to their removal by the collision cascade generated by penetrating ions.
This process is complex because it depends on momentum transfer not only by
the ions but by the (enhanced) diffusion that occurs during cooling of the collision
cascade. The limitation of depth resolution by the induced roughness increases with
sputtered depth; this effect can be reduced to a large extent by rotating the sample
during sputtering, as mentioned above. Both ion-beam mixing and surface roughness
are alleviated by using very low incident energies (e.g. 200 eV) at the expense of
removal rate.
(c) Surface analysis
There are several surface-analysis techniques based on the mass spectrometry of
sputtered species. The most popular is SIMS, which relies on measuring the mass
spectra of the sputtered positive or negative ions. Description of the techniques is
outside the scope of this paper but it should be said that SIMS is the most sensitive
surface-analysis technique, that it can detect all elements (including hydrogen) and
that, unfortunately, it is very hard to quantify even with standards, due to the
extraordinary sensitivity of the ion yield to the local electronic structure of the
sputtered species at the time of sputtering (matrix effect). For extensive reviews see
Benninghoven et al . (1987), Wilson et al . (1989) and Stephan (2001). Secondary
neutral mass spectrometry, or SNMS (Oechsner 1995; Nicolussi et al . 1996; He et
al . 1997; Gnaser et al . 1998; Higashi 1999), is related to SIMS. Here the sputtered
neutrals are ionized by electrons or a laser beam and then mass analysed. This
technique has the potential to replace SIMS, since it largely circumvents the problem
of the matrix effect in ion yields. Pacholski & Winograd (1999) review the use of mass
spectrometry of sputtered particles for chemical imaging of surfaces, a fast-developing
technical area.
Another use of sputtered particles for surface analysis is given by the energy analysis of recoil atoms directly emitted by the projectiles. This recoil spectroscopy is a
very powerful technique for the study of adsorbates on surfaces (Bertrand & Rabalais
1994).
Finally, another view of the surface is obtained by wavelength analysis of the
light emitted from radiative decay of excited sputtered species. Optical spectroscopy
is a very powerful technique for identifying atoms and molecules, but it is not as
straightforward as mass analysis in SIMS or SNMS. If, instead of using ion beams,
we place the sample in a gas discharge and analyse the light emitted by the sputtered
atoms, the technique is called glow-discharge optical-emission spectroscopy (Payling
et al . 1997).
Phil. Trans. R. Soc. Lond. A (2004)
46
R. A. Baragiola
FOV 19 µm
× 2 µm
Figure 9. Membrane made by FIBs of an integrated circuit. Notice the topography
induced by sputtering outside the membrane. (Reproduced courtesy of Micrion Inc.)
(a)
(b)
3 nm
(c)
3 nm
0 nm
105 nm
Figure 10. Scanning force microscopy (SFM) images of localization of human serum albumin
(HSA) on a GaAs surface. (a) An array of pits milled by an In+ FIB. Small raised crater rims
with apparent heights ca. 0.4 nm surround each pit, 60 nm in diameter and greater than or equal
to 0.8 nm deep. (b) HSA adsorbed from a solution is adsorbed preferentially to the inner portion
of the rims of the pits. Sizes of (a) and (b) are 1 µm × 1 µm. (c) Close-up of HSA molecules
adsorbed on the rim of one pit. (Adapted from Bergman et al . (1998).)
(d ) Sputtering with nanoscale focused ion beams (FIBs)
The production of microstructures by controlled ion beams (Hauffe 1991; Li et al .
2001) has seen a dramatic development in the last decade or so due to the spreading
availability of nanometre-sized ion beams from field emission liquid-metal ion sources.
In the FIB accelerator, typically 20–60 keV Ga+ ion beams are focused on a spot
that can be a few nanometres in diameter. Computer control of the FIBs can be used
to machine complex structures; this application needs new modelling tools when the
lateral dimensions of the ion-milled structures become of the order of, or smaller
than, the lateral extent of the collision cascade. This implies sputtering not only
Phil. Trans. R. Soc. Lond. A (2004)
Sputtering: observations and derived principles
47
from the surface on which the FIB is incident but also from side surfaces, a situation
falling outside the range of standard Monte Carlo simulations such as TRIM. This
situation is common in the popular technique of specimen thinning for transmission
electron microscopy (Ishitani & Yaguchi 1996). See figure 9 for an application to
semiconductor devices.
Patterns formed by FIBs have widespread applications. Shallow pits on GaAs
surfaces sputtered by a computer-controlled FIB (see SFM images in figure 10) were
used by Bergman et al . (1998) to localize proteins on a surface. It must be noted
that writing surfaces with nanometre-sized ion beams is very slow and therefore
the method is not suitable for applications requiring mass production of features on
the scale of centimetres or larger. Finely focused beams can nevertheless be very
useful in making prototypes or building blocks of structures that can be replicated
by other means. Such tiny structures can have unusual properties for applications in
microelectronics, photonics, micromechanics and biomedicine.
(e) Sputter deposition
The collection of the ejecta on a substrate forms the basis for the method of
sputter deposition, one of the most widely used thin-film-deposition techniques in
the laboratory and in industry (McClanahan & Laegreid 1991; Wasa & Hayakawa
1992; Wadley et al . 2001). Sputtered films have higher adhesion to the substrate
than films produced by thermal evaporation, due to the hyperthermal energy of the
sputtered particles (see figure 7) but sputter deposition is very difficult to use to
produce stoichiometric films of multiple components, such as superconductor oxides.
Although sputter deposition with ion beams is used, it is much more common to
employ different types of gaseous-discharge apparatus. This brings us back to the
beginning of the story, the observation of sputter deposits in a discharge tube by
Grove nearly 150 years ago.
7. Outlook
It is interesting and refreshing to note how an intriguing observation by Grove has
evolved for so many unanticipated and beneficial applications. While we should
acknowledge the historical importance of Grove’s curiosity, it should be said that
most of the advances in the field have happened in the last 40 years, energized by
discoveries in the laboratories of Gottfried Wehner and M. W. Thompson. Activity
in basic experimental research has decayed lately but it nevertheless has, as reviewed
here, produced many interesting new observations, particularly when using molecular
and cluster ion beams or high-energy deposition densities. In these cases there is still
the need to know the effect of incidence angle, changes in the angular and energy
distribution of the ejecta and the threshold behaviour of the yields. Fundamental
advances might result from experiments designed to map the transition from impact
with large clusters to impact with microscopic dust particles. Further experimentation is needed in preferential sputtering of compounds, including chemical changes
to ceramics, polymers and biomaterials and, in general, in sputtering by electronic
transitions, where the high material specificity makes it difficult to reproduce experimental results.
The current understanding of sputtering has led to its applications in other sciences
and to engineering and industry. For instance, a great deal of progress is being made
Phil. Trans. R. Soc. Lond. A (2004)
48
R. A. Baragiola
in understanding the role of sputtering in space, which spans from the destruction of
interstellar dust by fast ions in supernova shocks to the production of atmospheres
around icy satellites in our Solar System. Sputtering has greatly aided surface analysis by providing an easily understandable way to sequentially etch materials with
extremely high resolution. Sputtered ions are being used routinely in SIMS, the
most sensitive technique to identify trace elements on surfaces. The full application of this technique must wait for our understanding of the, as yet unpredictable,
matrix effects. SIMS mapping with sub-micrometre resolution is proving invaluable
for research and diagnostics in microelectronics, and is likely to revolutionize the
life sciences if sufficient progress is made in understanding the sputtering of biological materials. The fact that most of the applications we have mentioned were
made possible by basic research in sputtering since the time of Grove, points to
the need to reactivate fundamental studies, not only along the lines outlined here,
but also furthering new serendipitous ideas that will result when pushing for deep
understanding.
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