Aca - Departamento de Física - Universidad Técnica Federico Santa ...

V Encuentro Sud Americano de Colisiones Inelásticas en la Materia
Depth profiling of thin films using Coulomb explosion
S. M. Shubeita 1 , P. L. Grande 1 and J. F. Dias 1
1 Instituto de Física da Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil
e-mail address corresponding author: grande@if.ufrgs.br
Depth profiling of heavy elements in
thin films can be performed by backscattering
spectrometry. In this well established
technique the depth is given in units of
length assuming the knowledge of the density
of the target. Otherwise “depth” stands as
an abbreviation for the number of atoms per
unit area (density) over the distance traversed
(length) in the target [1]. A method to
determine the absolute depth without the
knowledge of the density is useful since the
density is a physical quantity that can have
different values for the same material.
In this work we explore the Coulomb
explosion occurring when energetic H 2
+
ionic clusters interact with thin layers of
dielectric materials (LaScO 3 , HfO 2 and LaAlO
3, thickness < 100 Å; and Si 3 15 N 4 and
Al 2 O 3 , hundreds of Å). The molecules dissociate
after passing the first monolayer and
get stripped of all their electrons. The moving
ionic fragments repel each other via mutual
quasi Coulomb forces and excite the
electronic medium coherently. The Coulomb
explosion leads to a broadening of the energy-loss
distribution of the ionic fragments,
and can be evaluated through an additional
energy-loss straggling.
The information obtained by Coulomb
explosion of H 2 + clusters in this approach
can provide the dwell time of the
ionic fragments in thin layers after the breakup.
In this way, the Coulomb explosion can
work as a clock where the start is given by
the electron striping and the stop by the
backscattering. Thus the determination of
the absolute thickness of the thin films based
in the dwell time (fig. 1) can be achieved.
Figure 1. The experimental results of Coulomb explosion
for HfO 2 as a function of the depth traversed
in comparison with calculations considering
screened and non-screened Coulomb potentials.
For this purpose, high energy resolution
backscattering experiments (MEIS: Medium
Energy Ion Scattering) were carried
out as a function of the incoming projectile
energy, covering an energy range between
100 and 200 keV/nucleon. Also, NRP (Nuclear
Reaction Profiling) experiments were
carried out on Si 3 15 N 4 ( 15 N(p,αγ) 12 C at 429
keV/nucleon) and Al 2 O 3 ( 27 Al(p,γ) 28 Si at
992 keV/nucleon) targets.
The results obtained are compared
with theoretical calculations and show the
potentiality of a Coulomb explosion profiling
technique.
References
[1] W. -K. Chu, J. W. Mayer, M. A. Nicolet,
Backscattering Spectrometry (New York: Academic
Press) (1978).
22 Valparaíso, Chile