Femtosecond laser ionization mass spectrometric analysis
of layers grown by pulsed laser deposition
Florenta Costache *, Markus Ratzke, Dirk Wolfframm 1, Jurgen Reif
BTU Cottbus LS Experimentalphysik II and IHP/BTU JointLab, Konrad-Wachsmann-Allee 1, 03046 Cottbus, Germany
www.elsevier.com/locate/apsusc
Applied Surface Science 247 (2005) 249–255
Available online 17 February 2005
Abstract
Femtosecond laser-induced charged particle emission and detection by time-of-flight mass spectrometry are used for
elemental analysis and in-depth profiling of multi-layered metal-oxide-semiconductor (MOS) structures grown by pulsed laser
deposition (PLD): thin films of a high-k material – praseodymium oxide, Pr6O11 – deposited on a silicon(1 0 0) surface
and covered by an aluminum contact layer. An efficient emission of singly charged positive ions to large clusters was found at
low incident fluence. The simultaneous monitoring of the main ion yields with the number of pulses exhibits pronounced anti-
correlations, revealing several interfaces. This indicates not only that the layers were resolved but also additional layers
were detected. Surface morphology investigation shows that, upon single- or multi-pulse exposure, localized holes and nano-
particles were created on the irradiated area. The multiple interfaces may account for in-homogeneities within the PLD layer
and/or a surface rearrangement due to the nano-particles formation. We demonstrate that, with this method, a depth resolution in
the nanometer range can be achieved. Fundamental mechanisms of femtosecond laser ablation from hetero-structures are
addressed.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Femtosecond laser ablation; Time-of-flight mass spectrometry; PLD layers; Metal-oxide-semiconductor structure
1. Introduction
Ultra-short laser pulses, delivering high intensities
onto the target, enable material removal with sub-
micrometer precision and carry the potential of a few
* Corresponding author. Tel.: +49 355 69 3983;
fax: +49 355 69 3985.
E-mail address: [email protected] (F. Costache).1 Present address: Siltronic AG, Berthelsdorfer Str.113, 09599
Freiberg, Germany.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved
doi:10.1016/j.apsusc.2005.01.064
monolayers depth resolution if the irradiation condi-
tions are chosen appropriately. Basically, the femto-
second laser irradiation of metals [1], semiconductors
[2], or dielectrics [3] yields a considerable amount of
charged particles from atomic ions to large clusters
and neutrals. The intrinsic mechanisms leading to
particle emission are, however, strongly dependent on
the material, photon energy, pulse duration, and
intensity range applied.
For ablation with long pulses (t > 50 ps), the rapid
heat conduction increases the area and the depth of
.
F. Costache et al. / Applied Surface Science 247 (2005) 249–255250
the ablated region. The femtosecond laser ablation
takes place within <100 fs, faster than the typical
electron-lattice thermalization (�0.5 ps). Conse-
quently, the latter yields a sharper fluence threshold,
ionization closer to the near-surface region, smaller
spot size, meaning an enhanced in-depth ion
confinement and an improved spatial resolution. In
this case, an ablation rate of a sub-micrometer scale
down to a few monolayers should be expected.
Femtosecond laser ionization in combination with a
time-of-flight mass spectrometer (TOF-LIMS) is
currently used in analytical chemistry as a diagnostic
tool for depth profile analysis of multi-layered
structures. Previous studies [4,5] dealt with well-
known samples, metal layers on metal/semiconductor
substrate, grown by conventional methods. Although
the depth resolution reported in femtosecond-LIMS
reaches a few monolayers [4], the method is less
interface-sensitive. Therefore, secondary ionization
mass spectrometry (SIMS) is the technique of choice
for in-depth analysis. Alternatively, the low frag-
mentation induced by the femtosecond laser pulse
appears to be useful when studying the interface
composition.
In the present paper, we show the application of
femtosecond laser ablation to the analysis of a hetero-
structure type metal oxide semiconductor (MOS): a
high-k dielectric material (Pr6O11) deposited by
pulsed laser deposition (PLD) on Si(1 0 0) surface
and an aluminum contact layer. There are valuable
reasons that make these targets particularly interesting
for analysis [6,7]: (1) since the incessant reduction of
the CMOS devices’ size, new materials such as PrxO y
are highly considered to potentially replace the SiO2
as the gate dielectric; (2) the high-k of Pr6O11 may be
reproduced into the thin films grown by PLD due to
the PLD’s advantage in preserving the stoichiometry
of the target; (3) further research on improving the
quality of the PLD layer requires supplementary ways
of analysis. Proceeding from the complexity of the
ablation mechanism, corroborating ion emission and
surface morphology investigation, the present paper
shows the potential of using LIMS on these hetero-
structures for elemental analysis and depth profiling.
We show evidence of nano-particle formation upon
femtosecond laser interaction with the thin film that
may, to some extent, affect the accuracy of a depth
profile analysis.
2. Experimental
The p-polarized output from a Ti:sapphire laser
system (800 nm, �100 fs at FWHM, 1 kHz, pulse to
pulse stability <3%) focused by a quartz lens
( f = 300 mm) was used to ablate the target positioned
at 458 incidence in a high vacuum chamber (base
pressure �10�10 mbar). The emitted charged particles
were led into a linear time-of-flight mass spectro-
meter, directed normally to the target surface in order
to warrant an axial ion flow to the microsphere plate
detector. An extraction field of about 1000 V/cm was
applied between the target holder and the entrance
aperture of the mass spectrometer. Two ion lenses
maximized the yield of the ablated ions. Details of the
apparatus were published previously [3].
The targets consisted of Pr6O11 films deposited by
nanosecond PLD (using the third harmonic of 355 nm
of a Nd:YAG laser; pulse width �5 ns) on a p-type
Si(1 0 0) substrate (for details on the sample prepara-
tion see [6]). Subsequently, the sample was con-
ventionally covered with an aluminum contact layer.
The sample had approximately �20 nm and �30–
35 nm thickness for the Al layer and PrxO y layer,
respectively. The intensity range used in these
measurements was 1011–1012 W/cm2 (fluence range
of 20–200 mJ/cm2).
Positive ion mass spectra were recorded always
from a fresh surface. In order to achieve their real time
development, the ion signals of interest were
simultaneously monitored using a combination of
two gated boxcar integrators. Further, the crater
morphology was investigated ex situ by means of
atomic force microscopy (AFM).
3. Results
The success in the use of LIMS on multi-layered
structures requires a good lateral and depth resolution,
hence the necessity of appropriately selecting the
fluence range applied. The complex combination of
materials included in our MOS target makes it difficult
to establish an optimum fluence, as the individual
materials exhibit different fluence thresholds and
ablation mechanisms. At 800 nm and 100 fs, the
fluence threshold for aluminum was found to be
approximately F th = 0.1 J/cm2 [1] and for silicon
F. Costache et al. / Applied Surface Science 247 (2005) 249–255 251
F th = 0.2 J/cm2 [2]. It is more difficult to obtain an
accurate value for the fluence threshold of Pr6O11.
Previous Ultraviolet Photoelectron Spectroscopy,
Raman spectroscopy and AFM measurements on
the Pr6O11 – PLD layer [6,7] – revealed an
inhomogeneous and relatively porous layer of
amorphous PrxO y possibly divided into its stable
fragments (PrO2 and Pr2O3). The difficulty in applying
a LIMS analysis comes not only from the probability
of reaching the silicon substrate due to the layer
inhomogeneity but also from the direct heating of the
silicon substrate.
The surface morphology of the PLD grown layer
shows large splashes of about 20 nm average height
(�5% surface coverage). By Raman spectroscopy
measurements, these splashes were found of crystal-
line nature [6]. As previously demonstrated for
dielectric materials and silicon [8–10], the positive
ions are emitted at high kinetic energies upon
irradiation at laser intensities below the ablation
threshold. In this intensity regime, the ablation rate is
very low, <1 nm/pulse. Certainly, we have to find a
modified threshold due to the sample inhomogeneity
and thus multiple precursors that decrease the single-
and multi-shot thresholds for the individual materials.
3.1. Ion emission
Already, at fluences below the ablation threshold
for aluminum ( F th = 0.1 J/cm2), the desorbed ion
yield almost saturates our detector. Fig. 1 shows a
Fig. 1. Positive ion mass spectra obtained upon femtosecond abla-
tion from the Al/PrxOy/Si target (average over 1000 pulses).
typical positive mass spectrum taken upon accumula-
tion of pulses when the silicon substrate was reached.
The aluminum clusters showed up in the spectrum at a
lower fluence range than those of silicon. Accom-
panied by the Al+ ion and the Si+ singly charged
atomic ion and clusters, the PrxO y layer yields O+, Pr+
and PrO+ as main desorption products. The spectra
show singly charged atomic ions and clusters of Prx+
and PrxO y+ (x = 2, 3, . . .; y = 1, 2, . . .) up to very large
masses, signature of layer composition and/or
fragmentation which is expected to occur at fluences
around single-pulse damage threshold. However, the
Pr6O11 compound could not be found among the main
products. A silicate Pr–Si–O layer is expected to be
Fig. 2. Anti-correlated emission of Al+/Si+ and PrO+ yields (a) and
Si+ and PrO+ yields (b) as function of number of pulses (fluence
20 mJ/cm2). The interfaces are marked by dashed lines (the first
eight pulses showed no activity; thus, they were excluded from the
graph).
F. Costache et al. / Applied Surface Science 247 (2005) 249–255252
present at the PrxO y/Si interface [7]. However, under
these experimental conditions, the silicate did not
appear in the mass spectra. Alternatively, SiO+ ions
are found and they may originate either from the
silicate fragmentation or from a silicon dioxide
formation at the Si surface, as the latter could be
caused by both hygroscopicity and oxygen diffusion at
the interface.
Fig. 3. (a) AFM images of a laser ablated spot (20,000 pulses at 40 mJ/cm2)
revealing holes; the line on which the depth profile (d) was recorded i
respectively, PrxOy layers deposited on the silicon substrate.
3.2. Layer-by-layer ablation
As previously stated, to obtain a high-depth
resolution, a low ablation rate is needed. In order to
monitor the ion yield, it is necessary to know the ion
kinetic energy and the time-of-flight distribution. For
fluences near the threshold, femtosecond laser
ablation of aluminum yields thermal and non-thermal
showing erosion; (b) detail from (a); (c) 2D image of a detail from (a)
s marked. The dashed lines indicate imaginary limits for the Al,
F. Costache et al. / Applied Surface Science 247 (2005) 249–255 253
Fig. 4. (a) AFM image of an ablated spot (single-pulse at 180 mJ/
cm2); (b) 3D image of a detail from (a) revealing nano-particles (see
text for details).
kinetic energies [1]. The ionic and neutral cluster
production in silicon and kinetic energy was recently
addressed [9,10] and measurements of the kinetic
energy showed peaks at high kinetic energy over-
lapping contributions at near thermal energies.
At only one atomic mass unit difference, as it is the
case for Al+ and Si+, the changing in the mass
resolution during ablation becomes critical. For the
fluence regime used in this particular measurement
(�20–40 mJ/cm2), i.e. a fluence of compromise for
obtaining stable signals, which is near to our detector’s
saturation limit, both ions exhibit double contributions
in their kinetic energy distribution. Therefore, the
peaks become broader and consequently less mass
resolved. This situation shows up for the first tens of
pulses applied on a fresh surface, still very much
depending on the surface preparation. The Al+ time-
of-flight distributions overlaps the Si+ peak in the first
few tens of pulses, making the mass separation
difficult. Therefore, it appeared practical to integrate
in the first few tens of pulses the Al+ and Si+ signals as
a single mass when the mass separation becomes
inaccurate, since the goal is to compare it with the
PrO+ ion, the most prominent desorption product of
the PLD layer.
Fig. 2(a) shows a recording of the integrated Al+
and Si+ ion signal on one hand, and PrO+ ion signal on
the other hand, versus the number of pulses applied
(fluence �40 mJ/cm2). For the first 100 pulses, the ion
signals show an anti-correlation, signature of a layer-
by-layer ablation as previously reported for laser
ablation of dielectric materials [11]. The aluminum
layer appears to be ejected in the first few tens of
pulses; then, the PrO+ layer is reached and further on
the silicon substrate.
The signal fluctuation and/or shifting in the time-
of-flight distribution strongly depend on the surface
preparation; therefore, the separation in time-of-flight
distributions for the Al+ and Si+ peaks remains
uncertain for the actual set-up of our time-of-flight
mass spectrometer. However, the simultaneous mon-
itoring of the Si+ and PrO+ further shows that
surprisingly, the anti-correlation reveals several addi-
tional interfaces (Fig. 2(b)).
When inspecting the irradiated surface by AFM,
one can distinguish optical damage only upon a larger
accumulation of pulses at this intensity range (see
Fig. 3: 20,000 pulses at 40 mJ/cm2). In the crater
center, where the intensity is higher due to the
Gaussian profile, the laser input leads to erosion with
valleys and crests, ripples and particles assembly (see
Fig. 3(a) with detail in (b)). The irradiated area
appears eroded with holes, possibly debris, a ripple
pattern perpendicular to the laser polarization of about
�1 mm period and a short-range fine ripple pattern of
�200 nm period parallel to the laser polarization.
The absence of a real crater makes it difficult to
determine an accurate ablated volume, i.e. ablation
rate per pulse, as our depth profiling analysis would
require. Nevertheless, important information about
the processes involved can be gained. The density of
deep holes in the irradiated spot matches the number
per unit area of the aforementioned crystalline
splashes found on the PLD layer, signaling that these
F. Costache et al. / Applied Surface Science 247 (2005) 249–255254
splashes are removed and the erosion continued down
to the silicon substrate. In Fig. 3(c and d), we show in
detail an individual hole and the corresponding depth
profile as revealed by the AFM analysis. The profile
indicates that the Al and PrxO y layers can be resolved
and an imaginary layer thickness can be derived from
it, which is nearly as the expected thickness of our
layers.
The astonishing appearance of the irradiated spot
led us to increase the fluence applied close to the
single-pulse ablation threshold for silicon. The AFM
image of a single-pulse ablation on a PrxO y/Si sample
obtained at 180 mJ/cm2 irradiation is displayed in
Fig. 4(a). The spot appears covered with nano-
particles of a prolonged or spherical shape although no
additional holes could be distinguished. A 3D profile
of a detail from the spot indicates the shape of nano-
particles (Fig. 4(b). Their height was found ranging
from tens to hundreds of nanometers. The nano-
particle surface density increases with the number of
pulses applied until the whole surface is covered.
4. Discussion and conclusions
Below the single-pulse threshold, the ultra-short
laser action on the hetero-structures leads to an
efficient emission of singly charged atomic ions and
clusters. The positive ion spectra give valuable
information about the composition of the PLD layer
of PrxO y and possibly the interface compounds.
From the ion yield dependence on the number of
pulses, an average depth resolution of <1 nm/pulse
can be estimated for the PrxO y layer (Fig. 2(a)),
although the recording does not show sharp interfaces.
The anti-correlated ion signals indicate the forma-
tion of additional interfaces, suggesting the presence
of significant in-homogeneities within the original
PLD layer and/or rearranged surfaces. The laser
erosion appears localized in defect points on the
surface, from which the ions and more likely neutrals
are emitted. Upon large accumulation of pulses, debris
emerges from the localized large holes. Single-pulse
ablation indicates the existence of an agglomeration of
nano-particles on the surface, which may be asso-
ciated with the interfaces found. The generation of
nano-particles of amorphous or crystalline nature by
femtosecond laser irradiation of a PLD layer has been
studied theoretically and experimentally under dif-
ferent conditions [12,13]. Different non-equilibrium
mechanisms could be responsible such as Coulomb
explosion of positive ions, which takes place at laser
intensities well below the damage threshold [8,13].
The surface is reconstructed and the nano-particles
produced on the thin film may be responsible for the
anti-correlation in the ion yields. However, in the case
of our heterogeneous system, further characterization
of the nano-particles nature is necessary in order to
prove a direct connection between the two effects.
In conclusion, we have shown the potential of
applying time-of-flight laser ionization mass spectro-
metry as a fast analytical tool. The key parameters of
femtosecond laser pulse interaction with the material
must be defined as the fluence necessary to be applied
and the kinetic energies of the ejected particles. For
making it an easy to handle method, one must
disregard further effects that appear due to the
femtosecond-laser matter interaction. Our findings,
still preliminary, show that this method can be applied
on hetero-structures type MOS and underline possible
additional effects such as the surface reconstruction
and the formation of nano-particles that limit the
method accuracy.
In order to advance LIMS as an analytical tool for
in-depth profiling of such demanding sample as the
MOS structures with layers grown by PLD, further
experiments on different samples as composition and
layer thickness are required.
Acknowledgements
We would like to thank L. Zhu and S. Eckert for
their help with these experiments and T. Arguirov for
stimulating discussions. Financial support from the
German Federal Government and the Land of
Brandenburg via an HWP program is gratefully
acknowledged.
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