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
3.2. Microbeam bending
The grains in the Ag films on the SiO 2 microbeams
were found to be columnar and had a median grain size
of about 0.9 m independent of the film thickness. The
majority of grains was (111)-oriented with a smaller
fraction of (100)-oriented grains. The latter increased
with increasing film thickness. As an example of a
typical microbeam bending experiment, Fig. 4a shows
the beam stiffness as function of the number of cycles
for a beam with a 0.8 m thick film and a maximum
strain max of 2.2%. Initially, the beam stiffness is nearly
constant. The dramatic decrease in stiffness after approximately
6×10 −5 cycles indicates the onset of
severe damage formation. Similar experiments conducted
on samples with different film thicknesses are
summarized in Fig. 4b. The filled symbols denote the
experiments in which a distinct decrease in beam stiffness
was observed within 3.9×10 6 cycles accompanied
by crack formation. Only films with a thickness of 0.6
m or more suffered from fatigue damage.
3.3. Damage morphology
Typical micrographs of fatigue damage are shown in
Fig. 5. For both Cu and Ag films, the damage morphology
can be described as follows, extrusions (marked as
E) are formed within large grains, whereas intergranular
cracks (C) were found in fine grained regions of the
film. The extrusion height is comparable to the film
thickness. Underneath the extrusions, large voids are
found at the interface between the film and the substrate.
Some more detailed observations of the fatigue
damage in the Ag films are described in [12].
Fig. 4. (a) Stiffness of a microbeam with 0.8 m thick film cycled to
max of 2.2% at a frequency of 45 Hz. The sudden decrease of stiffness
indicates the onset of damage formation. (b) Map for damage formation
as a function of Ag film thickness and maximum strain. Thick
films showed fatigue damage within 3.9×10 6 cycles (filled symbols)
whereas thin films did not show structural features of fatigue (open
symbols). The transition occurs for 0.6 and 0.8 m thick films as
function of maximum strain.
increases, i.e. the sample stiffness decreases. After
10000 cycles, the strain range has reached the level of
the bare substrate indicating that the film is no longer
contributing to the sample stiffness. The number N f of
cycles to failure in these experiments was defined as the
interception of two linear curve fits as demonstrated in
Fig. 3a. In Fig. 3b, this number is plotted versus the
total strain range. As expected, the lifetime decreases
with increasing strain range following a relationship as
given by Eq. (1).
4. Discussion
Two techniques have been developed to study the
fatigue behavior of thin films on substrates. Central to
both methods is the use of an elastic substrate, which
acts as an antagonist to an external mechanical loading.
As a result, films are deformed in tension on loading
and in compression on unloading. Another important
aspect of these experiments is that film fatigue damage
is not catastrophic because the undamaged substrate
maintains some mechanical integrity of the sample.
However, the degradation of the film, i.e. crack formation
and propagation, is manifested as a decrease in
stiffness of the film–substrate composite. Based on this
distinct decrease in stiffness, the lifetime of Cu films on
polyimide substrates was determined as a function of
applied total strain range. It was not possible to determine
the plastic strain range, since the film stress was
not measurable during the experiments. As a result,
Fig. 3b is plotted as the initial total strain range as a