Fatigue in thin films Lifetime and damage formation.pdf

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O. Kraft et al. / Materials Science and Engineering A319–321 (2001) 919–923
It is the aim of this paper to elucidate the fatigue
mechanisms in constrained volumes and to determine
fatigue lifetimes under these conditions. We present
two methods, which we developed to study the fatigue
behavior of thin metal films on substrates: The
first technique involves tensile testing of Cu films deposited
onto elastic polyimide substrates. Tensile testing
of these specimen results in tension–compression
cycles in the plastically deformed thin film while the
substrate is elastically deformed only. The second
technique utilizes the cyclic deflection of thin Ag films
deposited onto micromachined SiO 2 cantilever beam
using a nanoindentation system.
2. Experimental
2.1. Tensile testing
For processing of the tensile specimen, shown in
Fig. 1a, the 125 m thick polyimide substrates (Kapton,
Du Pont) were initially cleaned by rinsing with
ethanol and pressurized CO 2 . Then, the sample was
mounted in the deposition system (base pressure of
5×10 −7 mbar) and activated by an oxygen plasma
(O 2 pressure 1×10 −2 mbar, power 100 W, bias
−300 V). Without breaking vacuum, 3 m thick Cu
films were then sputter deposited at a substrate temperature
of 300°C, a rate of 60 nm per min with an
Ar pressure of 2×10 −3 mbar, a power of 150 W
and a bias of −80 V. After deposition, the samples
were annealed in a vacuum furnace (6×10 −6 mbar)
at a temperature of 400°C.
The tensile tests were performed in an electromechanical
tensile tester (Zwicki 1120, Zwick) under
load control. Samples were cyclically loaded between
a minimum load of 2 N to maximum loads in the
range of 15–60 N. Under these conditions, the film is
strained with constant total strain ranges between 0.7
and 2.1% as determined from the crosshead displacement
and monitored as a function of the number of
cycles. It is not possible to determine the film stress
using this configuration because the externally applied
force is predominantly governed by the mechanical
properties of the substrate. However, microtensile
tests on comparable samples using an X-ray diffraction
technique enabled the measurements of stress–
strain behavior during such experiments as
exemplified in Fig. 1b [11]. On loading and unloading,
the film is plastically deformed in tension and in
compression for a total and plastic strain range of 0.5
and 0.15%, respectively.
2.2. Microbeam bending
Fig. 1. Microtensile testing; (a) Sample geometry of the 125 m thick
polyimide foil substrate and the Cu film sputtered in the middle
section of the dog-bone shaped sample. (b) Typical stress-strain
behavior for a Cu film on a polyimide substrate during cyclic loading
as measured by X-ray diffraction (second cycle of 0.7 m thick Cu
film, from [11]). The total applied strain range of the cycle is 0.5%,
whereas the plastic strain range amounts only to about 0.15% as
indicated by the arrow within the hysteresis loop.
The microbeam samples are shown schematically in
Fig. 2a. They consist of a 2.83 m thick SiO 2 layer
and a thinner Ag film. The Ag films, ranging from
0.2 to 1.5 m in thickness, were sputter deposited at
a temperature of about −190°C and annealed at +
100°C. A detailed description of film deposition and
the testing procedure is given in [12]. The testing procedure
is briefly described as follows, the beams are
deflected by a commercial nanoindentation system
(Nano II, MTS Corp.), which applies a cyclic load
P=P mean +P o cos(2t), where P mean is the mean
load, P o the load amplitude and the frequency,
which was chosen to be 45 Hz. In this configuration,
the largest strain occurs at the fixed end of the beam
(as indicated in Fig. 2a). Based on elastic beam bending
theory, this strain can be calculated from the
lever length L and the applied load as:
= P(L−x) (z−q) (3)
IE
where I is the moment of inertia, E the Young’s
modulus of the substrate material, q the position of
the neutral axis, and x and z the coordinates along
the length and the thickness of the beam. The mo-