www.elsevier.com/locate/apsusc

Applied Surface Science 248 (2005) 323–328

Broadening and attenuation of UV laser ablation plumes

in background gases

Salvatore Amoruso a, Bo Toftmann b, Jørgen Schou b,*

a Coherentia-INFM and Dipartimento di Scienze Fisiche, Universita di Napoli Federico II,

Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Naples, Italyb Department of Optics and Plasma Research, Risø National Laboratory, DK-4000 Roskilde, Denmark

Available online 25 March 2005

Abstract

The expansion of a laser-induced silver plume in a background gas has been studied in a variety of gases ranging from helium,

oxygen and argon to xenon. We have measured the angular distribution of the total deposit of silver on an array of quartz crystal

microbalances as well as the time-of-flight distribution with a Langmuir probe. The angular distribution broadens for all gases

except for a minor pressure range for the helium background gas, in which a distinct plume narrowing occurs. The behavior of

the collected, ablated silver atoms integrated over the full hemisphere is similar for all gases. This integral decreases strongly

above a characteristic pressure, which depends on the specific gas. The ion time-of-flight signal shows a clear plume splitting

into a fast and a slow component except for the ablation plume in a helium gas.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Laser ablation; Plume expansion; Background gases

1. Introduction

The dynamics of an expanding laser ablation plume

in a background gas is a complicated phenomenon,

which only can be treated by complex computer codes.

Thin film production by pulsed laser deposition (PLD)

often takes place in a background gas, for example, in an

oxygen gas. A reactive gas such as oxygen may serve to

maintain a certain chemical equilibrium pressure in a

growing film during a PLD process, and oxygen is

* Corresponding author. Tel.: +45 4677 4755; fax: +45 4677 4565.

E-mail address: j.schou@risoe.dk (J. Schou).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.03.040

typically used for the deposition of metal oxide films

[1–4]. The background gas may also serve to control the

impact energy of the atoms on the growing films, since

the kinetic energy of the ablated particles is reduced

with increasing pressure, and the production of defects

in the films will therefore decrease as well.

Intuitively, one would expect the angular distribu-

tion of ablated metal atoms to broaden from the initial,

comparatively narrow ‘‘vacuum distribution’’ with

increasing pressure [1,4]. This effect has actually been

observed for many target–background gas combina-

tions [4,5], but occasionally also a narrowing of the

ablation plume has been found, in particular for heavy

.

S. Amoruso et al. / Applied Surface Science 248 (2005) 323–328324

target atoms in a light background gas [6–8]. A related

feature during film production is the decreasing

deposition rate with increasing pressure. This attenua-

tion of a flow from a target to a substrate in a

background gas is an important issue in PLD and can

also influence the growth rate of a film considerably.

This work is a continuation of earlier work on the

ablation dynamics of a silver plume in oxygen and

argon background gases [5,9]. In contrast to other

existing studies we have explored the ablation from a

simple one component metal, silver, in inert gases to

avoid chemical effects. Since oxygen perhaps is the

most important background gas in PLD experiments,

we have included measurements with an oxygen gas

background as well. In addition, the present work is

covering the largest mass range up to date, from a

silver plume interacting with light helium to heavy

xenon background gas atoms.

Fig. 1. Upper part, the experimental setup; lower part, angular

distribution of the ablated Ag particles in Xe background gas as

a function of angle with respect to the normal: (*)

P = 4.5 � 10�6 mbar; (*) P = 3.4 � 10�3 mbar. The solid curves

are fits to Eq. (1).

2. Experimental details

The measurements were carried out on the existing

setup at Risø National Laboratory [5,10]. A frequency-

tripled Nd:YAG laser at 355 nm was directed at normal

incidence onto a silver target in a chamber with a base

pressure of 10�7 mbar. The laser pulse length was 6 ns

and the fluence was 2.5 J/cm2 (corresponding to an

intensity of 4 � 108 W/cm2). The area of the circular

laser beam spot was 0.04 cm2. The angular distribution

of the ablated flux of both neutrals and ions was

measured in situ 80 mm from the target using a circular

array of 8 quartz crystal microbalances (QCMs) in the

horizontal plane, as shown in the upper part of Fig. 1.

The crystals were positioned at angles of 108, 158,23.58, 308, 37.58, 458, 608 and 758 with respect to the

surface normal, and each crystal had a 6-mm diameter

active area. The accuracy of the measurements of the

resonance frequency with the QCMs was about 0.1 Hz,

corresponding to 1.2 � 1013 Ag-atoms/cm2. Each run

was made on a fresh target spot, typically with 200–

1000 pulses at a repetition rate of 0.1 Hz and with a

subsequent relaxation time for the crystals of 4–6 h. A

planar Langmuir probe consisting of a 2 mm � 2 mm

square copper plate insulated on the rear side similar to

the probes in Refs. [5,11] was utilized for measure-

ments of time-of-flight (TOF) signals of the plume ions.

The probe was oriented to face the target spot and

placed 75 mm from the target at angle of 208 with

respect to the normal. During the ion collection the

probe was biased at �10 V.

The laser incidence along the normal and the

circular beam spot mean that the plume is rotationally

symmetric with respect to the normal [10]. Thus, we

can determine the total number of atoms arriving at the

spherical shell from inside by integrating over the full

hemisphere.

The background gas pressure ranged from 10�6 to

1 mbar.

3. Results and discussion

3.1. Analysis

We have analysed the data for the angular

distribution F(u) in terms of Anisimov’s model

S. Amoruso et al. / Applied Surface Science 248 (2005) 323–328 325

[5,10,12,13]:

FðuÞFð0Þ ¼ ð1 þ tan2 uÞ3=2½1 þ k2

z tan2 u��3=2 (1)

where the fitting parameter kz = Zinf/Xinf is the ratio of

Fig. 2. Angular distribution of the collected ablation yield as a

function of the background gas pressure. Each distribution has been

normalized to its own maximum. The curves are fits to Eq. (1). Panel

(a): xenon, (*) P = 4.5 � 10�6 mbar; (4) P = 3.4 � 10�3 mbar;

(^) P = 1.1 � 10�2 mbar. Panel (b): argon, (*) P = 5.0 �

the limiting value of the position of the cloud front

along the Z-axis in forward direction into vacuum and

the value of the front in horizontal direction along the

X-axis. This distribution accounts for the deposition

per unit area on a hemisphere in contrast to the

expressions given in Ref. [13]. In Anisimov’s model

the gas plume expands in vacuum as a self-similar

ellipsoidal cloud based on a system of hydrodynamic

equations. The theory was developed for a neutral gas

plume, but we have been encouraged to use it also for

an expanding plasma in view of the good agreement

with our data for the ion flow from a silver plume in

vacuum [12].

3.2. Plume broadening

The angular distribution of silver ions in vacuum

and in a low-pressure xenon gas at a pressure of

3.4 � 10�3 mbar is shown in Fig. 1. Each point

indicates the deposited number of silver atoms per

shot on a specific crystal. One notes that Eq. (1) is a

very good fit to the points for the vacuum distribution

(kz = 2.8 0.2) as well as that for the xenon back-

ground gas (kz = 2.5 0.2). It is a general trend that

10�6 mbar; (4) P = 1.0 � 10�1 mbar; (^) P = 2.3 � 10�1 mbar.

Panel (c): oxygen, (*) P = 1.2 � 10�6 mbar; (4) P = 1.0 �

(i) th 10�1 mbar; (^) P = 3.5 � 10�1 mbar. Panel (d): helium, (*)

�6 �1

e collected yield on the crystals decreases with

increasing pressure;

P = 1.0 � 10 mbar; (5) P = 2.5 � 10 mbar; (!) P = 5.0 �

�1

(ii) th 10 mbar; (^) P = 1.6 mbar.

e distribution broadens with pressure except for

a minor pressure range for a helium gas.

The second feature is shown in great detail in Fig. 2

for a silver plume in the four gases. In all cases the

angular distribution has been normalized to the ma-

ximum to show clearly the broadening. However, as

pointed out in Ref. [8], there is a distinct sharpening of

the angular distribution of the ablated silver atoms in

helium in the range from 0.2 to 0.4 mbar.

The broadening of the angular distribution of the

collected plume atoms is seen in Fig. 3, where the full

width at half maximum, Du, of the angular distribution

is plotted for the four background gases. Du can be

found directly from the fitting parameter kz:

Du ¼ 2 arctan ½ð22=3 � 1Þðk2z � 22=3Þ�1=2 (2)

Obviously, the broadening starts at a lower pressure

(2.5 � 10�3 mbar) in the heaviest gas, xenon, than in

oxygen and argon (at (4–8) � 10�3 mbar). The

surprising feature is that the angular distribution

shows a distinct saturation in xenon at about 558,whereas argon and oxygen may have a saturation level

around 808. However, the existence of a saturation

level is not demonstrated completely for these two

gases in view of the relatively few data points for the

distributions in the high-pressure region. For helium it

is even more uncertain what happens for the highest

pressure, but the width of the distribution reaches at

least a value of about 508.

S. Amoruso et al. / Applied Surface Science 248 (2005) 323–328326

Fig. 3. The full width at half maximum Du of the angular distribu-

tion of the collected Ag atoms as a function of background gas

pressure: (a) xenon; (b) argon; (c) oxygen; (d) helium. The dotted

lines have been inserted to guide the eye.

Fig. 4. Total collected yield (integral) of Ag atoms as a function of

the background gas pressure. The collected yield has been obtained

from an integration of Eq. (1) over the full hemisphere. The vertical

bar shows the statistical uncertainty of the data. The dotted lines

have been inserted to guide the eye.

3.3. Plume attenuation

The effect of the increasing pressure of the

background gas is also visible in the total yield of

collected silver atoms obtained by integrating Eq. (1)

over the full hemisphere. This integral is shown in

Fig. 4 as a function of the background pressure. One

notes that the integral is constant up to a pressure of

0.003 mbar for xenon and up to a somewhat higher

pressure for the lighter gases. For helium the integral

decreases at about 0.13 mbar. This attenuation is a

result of the enhanced scattering of the plume atoms

from atoms or molecules in the background gas, and

the enhanced scattering means that the plume atoms

never reach the collecting surface at a radius of

80 mm. The particles diffuse out of the detection

volume and are pumped away or trapped on the

surfaces of the vacuum chamber.

The similarity of the behavior of the integral is

striking as well. The integral shows the same behavior

for all gases except that the value of the ‘‘bending

point’’, i.e. the pressure at which the integral starts to

fall below the vacuum value, depends critically on the

molecular mass. In fact, the ‘‘bending point’’ is

inversely proportional to the atomic mass of the

background gas ( p�m�1g ).

3.4. Ion probe measurements

The occurrence of plume splitting of ions during

plume expansion in a background gas has been

observed by different authors [14–17]. This effect

consists in the formation of two separated ion

components, in a specific pressure range, as a result

of scattering processes between plume particles and

background gas atoms. More recently, it has been

shown to occur also for neutrals by using time-

resolved optical emission spectroscopy [18,19].

Except for work made by the present authors the

plume splitting effect has only been studied with

Langmuir probes for silicon in two background gases,

argon and helium [14,15] and for YBCO

(YBa2Cu3O4�d) in an oxygen background gas [20].

In Fig. 5 we show the ion probe signals recorded for

the different gases at pressures where plume splitting

occurs. The dashed line in the upper panel of Fig. 5

S. Amoruso et al. / Applied Surface Science 248 (2005) 323–328 327

Fig. 5. Plume splitting of the ion signals in the different background

gases. All signals have been collected with the planar probe at 208from the normal in a distance of 75 mm from the target. The dashed

curve in the upper panel shows the ion signal recorded in high

vacuum (not to scale) for comparison.

also gives the ion time-of-flight profile (not to scale) in

high vacuum ( 10�6 mbar), for comparison. The

vacuum signal peaks at about 5 ms, which corresponds

to a velocity for the silver ions of ffi1.6 � 104 m/s

(ffi120 eV). Plume splitting is clearly observed in the

heavy background gases, i.e. xenon, argon and

oxygen, while for the lighter helium it is not possible

to discern two well separated ion components even at a

pressure of few tenths of mbar. Nevertheless, even the

light helium atoms have a significant effect on the ion

signal (Fig. 5, lower panel).

For heavy gases, the leading edge of the first ion

peak is similar to an attenuated vacuum signal and is

mainly due to ions that have been transmitted through

the background gas without collisions. The second

peak is due to ions that have suffered many collisions

with the gas atoms, then transferring part of their

kinetic energy to the ambient gas eventually leading to

a shock wave formation. On the other hand, time-of-

flight spectra of the plume ions in the helium gas show

a peak of fast ions, which gradually slows down, until

a more complex structure appears at pressures of the

order of few tenths of mbar [8]. At these pressures (see

Fig. 5, lower panel), the leading edge of the ion signal

reaches its maximum at a time of 10–13 ms, while the

trailing edge is only slightly delayed.

In this respect, it is worth observing that in a single

head-on collision between a silver ion and a background

gas atom/molecule, the maximum fraction of kinetic

energy a silver ion can transfer to an atom or molecule is

0.99, 0.79 and 0.70 for xenon, argon and oxygen

background gases, respectively, while only a fraction of

0.13 is transferred to helium atoms. Thus, for heavy

gases collisions with background atoms can efficiently

slow down the plume ions after very few collisions,

while for helium the plume ions retain much of their

velocity after collisions. This indicates that the average

number of collisions needed to effectively slow down

the ions in heavy background gas is quite low (i.e. the

mean free path is only slightly smaller than the target-

probe distance). Thus, the fraction of ions suffering no

collision is still significant, leading to the formation of

the observed two-peak distribution (Fig. 5). In contrast,

for helium the collisional effects start to play a role only

after a large number of collisions. Therefore, the

fraction of ions that have suffered no collisions from the

target to the ion probe in this last case is extremely

small, leading to braking of most of the ions and the

absence of the delayed, second peak, as observed in

Fig. 5, lower panel.

4. Conclusion

The expansion of the silver plume in a background

gas has been studied systematically in a variety of

gases ranging from helium, oxygen and argon to

xenon. The angular distribution of the total deposit on

an array of quartz crystal microbalances as well as the

time-of-flight distribution with a Langmuir probe have

been measured for laser irradiation at 355 nm at a

fluence of 2.5 J/cm2. The angular distribution broad-

ens for all gases except for a minor pressure range for

the helium background gas, in which a distinct plume

narrowing occurs. The behavior of the collected,

ablated silver atoms integrated over the full hemi-

S. Amoruso et al. / Applied Surface Science 248 (2005) 323–328328

sphere is similar for all gases. This integral decreases

strongly above a pressure, which depends on the

specific gas. The ion time-of-flight signal shows a

clear plume splitting into a fast and a slow component

except for the ablation plume in a helium gas.

The case of a silver plume in an argon background

gas was previously studied in great detail [5], and we

identified three pressure regimes: (a) a low-pressure

vacuum-like with a forward directed flow and only

little scattering of the ablated atoms, (b) a transition

regime with broadening of the angular distribution and

of plume splitting of the ion signal and (c) a high

pressure regime with an expected saturation of the

broadening of the angular distribution as well as

diffusion and thermalization of the plume. Only the

expansion in the helium background gas deviates from

the general behavior because of the collision dynamics

in a helium plume. The silver atoms are so heavy

compared with the helium atoms that a substantial

number of collisions is needed to slow down or change

the direction of the flow of silver atoms in the plume.

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