DETERMINATION OF MECHANICAL PROPERTIES
FROM MICROCOMPRESSION TEST
M. Truhlář1
, T. Kruml2
, I. Kuběna3
, K. Petráčková3
, L. Náhlík3
Abstract: This paper describes a microcompression test of thin Al - 1.5 wt. % Cu thin film deposited on
Si substrate. Microcompression combines the sample preparation with the use of ion focused beam (FIB)
with a compression test carried out using nanoindenter. Cylindrical specimens (pillars) were prepared
using FIB. The diameter of pillars was about 1.3 μm and their height was about 2 μm (equal to the film
thickness). Stress-strain curves of the thin film were obtained. The results depend on crystallographic
orientation of pillar. The paper is focused to an attempt to determine as precisely as possible Young
modulus of the film using experimental data and finite element modelling.
Keywords: microcompression, thin film properties, focused ion beam, Young modulus, FEM
modelling
1. Introduction
One possibility how to perform mechanical tests on a very small scale is to prepare a small cylindrical
specimen of micrometric size by focused ion beam and to execute a compression test using
nanoindenter device equipped with a flat diamond punch. Such experiments are rather popular
nowadays for the study of effect of specimen size on its mechanical behaviour (Nix et al. 2007).
We performed successfully such tests with the aim to determine elastic and plastic properties of thin
films deposited on a substrate by PVD method (Kruml et al. 2009). In this paper, we tried to improve
calculation method for determining Young modulus with higher precision.
2. Experiment
The aim of experiment was prepare perfect cylindrical specimens with a diameter of about 1.3 μm
with height determined by film thickness. Pillars were prepared in the FEI Quanta 3D FEG
DualBeam™ system and Tescan Lyra 3 FEG microscopes. To ensure that the whole pillar is single
crystalline were pillars produced in centers of large grains. The FIB milling procedure was optimized
so that the final shape of Al part of the pillar is as close to the perfect cylinder as possible.
From the microcompression experiments an equivalent of macroscopic compression curve is obtained.
Such stress-strain curve provides information about yield stress, stress at chosen strain level or work
hardening rate. For calculation of Young modulus, finite elements modelling must be used.
3. Results
Three different pillars were measured and the resulting values of measured elastic slopes Eexp are
shown in Table 1 for each tested pillar.
1
Mgr. Michal Truhlář; Institute of Physics of Materials AS CR, Žižkova 22, 61662 Brno, Czech Republic and Masaryk
University, Faculty of Science, Kotlářská 2, 61137 Brno, Czech Republic, truhlar@ipm.cz
2
Prof. Mgr. Tomáš Kruml, CSc.; Institute of Physics of Materials AS CR, Žižkova 22, 61662 Brno, Czech Republic,
kruml@imp.cz
3
Bc. Klára Petráčková; Ing. Ivo Kuběna; doc. Ing. Luboš Náhlík, Ph.D.; Institute of Physics of Materials AS CR, Žižkova
22, 61662 Brno, Czech Republic and Brno University of Technology, Faculty of Material Engineering, Technická 2, 61669
Brno, Czech Republic, klara.petrackova@seznam.cz, kubena@ipm.cz, nahlik@ipm.cz
Finite element method (FEM) was used for numerical simulation of microcompressive test and
theoretical values of elastic slope were found (see Ep values in Table 1).
The most influencing geometric factor is the presence of Si substrate. It was estimated that it reduces
the measured elastic slope during the microcompressive testing by almost 30%. Quantitatively,
influence of the substrate is described by coefficient C1.
Second geometric factor which has influence on the elastic slope is conical shape of pillars. Numerical
expression of the influence on elastic slope is shown in Table 1 as coefficient C2. In this case the
conical geometry of pillar influences data obtained by experiment of about 5%. This factor does not
depend on material parameters of the pillar but only on the geometry. It means that multiplying
experimental data by C2 = 1.05 eliminates the influence of conical geometry with taper angle of 4
degree for any type of material tested.
Finally, it was estimated that existence of tungsten interlayer affects the elastic slope by about 1%.
Numerical expression of this effect is shown in Table 1 as C3 coefficient.
The last column in Table 1 is the Young modulus of the Al film, for the given crystallographic
orientation.
Tab. 1: Measured and calculated data.
Sample
crystal.
orient.
Eexp,1
[GPa]
Eexp,2
[GPa]
Ep
[GPa]
C1 C2 C3
Ep,cor
[GPa]
M1 [3 5 9] 48.0 56.1 52.35 1.36 1.03 0.99 66.6
M2 [2 4 5] 45.6 52.3 56.30 1.22 1.07 0.99 58.9
S2 [1 2 3] 48.7 55.5 52.22 1.33 1.04 0.99 66.7
Eexp,1 – elastic slope obtained by experimental measurements (first unloading),
Eexp,2 – elastic slope obtained by experimental measurements (second unloading),
Ep – theoretical elastic slope obtained by numerical simulation of microcompressive test. As the
input, elastic constants found in the literature for pure Al were used,
C1,C2,C3 – correcting coefficients describing influence of substrate, conical geometry, and
interlayer,
Ep,cor = Eexp,1 · C1· C2· C3, i.e. the values of Young modulus for given orientation of Al crystal
lattice
4. Conclusions
In this paper, the methodology for evaluation of Young modulus of thin film from data obtained by
microcompressive test was suggested. The influence of specific pillar geometry and presence of other
phases (W – interlayer, Si – substrate) within the tested specimen can be calculated by finite element
method and elastic slope obtained directly from experimental stress–strain curve can be corrected for
these factors. The measured value of elastic slope was taken from the first unloading part of the curve.
It was found that the elastic constants of the tested film are close to the ones reported for bulk Al
which means that the film is of a good quality and did not contain significant amount of porosity.
References
Kruml, T., Stranyanek, M., Ctvrtlik, R., Bohac, P., Vystavel, T. and Panek, P.: A new method for mechanical
testing of thin films: application on Aluminum. Journal of Material Research 24 (2009), 1353
Nix, W. D., Greer, J. R., Feng, G., Lilleodden, E. T.: Deformation at the nanometer an microme-ter length
scales: Effect of strain gradients an dislocation starvation. Thin Solid Films 515 (2007), 3152