Femtosecond laser ionization mass spectrometric analysis of layers grown by pulsed laser deposition

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).


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,


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


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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Femtosecond laser ionization mass spectrometric analysis of layers grown by pulsed laser deposition

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