Project Leader: Dr. Oleksandr Glushko

Project number: P27432-N20

Project duration: 06.2015- 12.2018

The aim of this project is to understand the mechanisms of microplasticity, fatigue, and fracture in polymer supported films subjected to cyclic mechanical strain and to uncover the relationships between topological damage and electrical degradation.

1. How topological damage of a thin film influences its resistance?

It is evident that formation of through-thickness cracks in a thin film should lead to its electrical degradation, i.e. to an increase of electrical resistance. It can be also shown that severe surface roughening and extrusion formation does not cause a significant resistance increase if no through-thickness cracks are formed [1]. Fig. 1 shows electrical resistance signal recorded in-situ during cyclic straining of evaporated Au film (a), evaporated Cu films (b), and ink-jet printed Ag films (c). The corresponding SEM images with FIB cross-sections (d-f) demonstrate severe roughening of Au films without through-thickness crack formation (d), propagating extrusion/crack couples in Cu films (e) and long straight brittle-like cracks of printed Ag (f). Note that the y-scales in (a-c) are different, to make comparison easier, the horizontal dashed line mark the resistance increase by 25%.

Fig. 1. In-situ resistance signal vs. cycle number for Au(a), Cu(b) and printed Ag (c). The strain amplitudes are given on the plots. Representative corresponding SEM images with FIB cuts are given in (d-f). Horizontal dashed lines in (a-c) show the same resistance growth of 25%. For more details see [1]

Qualitative relationship between mechanical and electrical damage is pretty straightforward and logical: more intensive cracking – more electrical degradation. But is it possible to find a quantitative relationship? The answer is yes. With the help of finite element simulations and comparison with experiment [2] it was shown that resistance increase can be represented as second-order polynomial function of crack length and crack density:

                                             

where R/R0 is relative resistance, Cl is the linear crack density, and l0 is the length of a single crack (or average crack length). Linear crack density and average crack length can be combined in a single factor – the cracking factor (CF) – giving rise to a simple equation connecting electrical degradation and topological degradation of a thin film:

Some examples of how to use Eqs. (1) and (2) to extract useful information about the evolution of mechanical damage in thin films are given in [3].

2. Room-temperature grain coarsening induced by cyclic strain.

Thin Au films deposited on polyimide substrates were employed in a systematic study of room-temperature grain coarsening induced by cyclic strain. Three film thicknesses (250, 500 and 1000 nm) were subjected to up to 30000 cycles with three cyclic strain amplitudes (0.5%, 1%, 2%). The evolution of the microstructure with the cycle number was recorded using EBSD either quasi-in-situ or post-mortem. Typical microstructural evolution is shown in Fig. 2 (grain orientation maps in TD direction!). Significant grain coarsening is observed with the average grain size increasing by almost an order of magnitude. At the same time, the final grain size distribution is close to normal, in contrast to frequently reported “abnormal” grain growth. Interestingly, initially GB migration and grain coarsening occur without any indications on the sample surface, as evidenced by the SEM images (e-h). First when the grains have grown over 1 µm in size, the slip steps and extrusions develop on the surface.

Using the statistically significant collected data about the evolution of the microstructure of different thin films under different strain amplitudes, a model explaining the grain coarsening effect and its stagnation is proposed. The model suggests that thermodynamic driving force arises from the difference in elastic strain energy density between the grains with different sizes or orientations. For more details please see [4–6].

Fig. 2. Grain orientation maps in TD direction (parallel to the applied strain) for the same area of 1 µm thick Au film in as-deposited state (a), after 500 cycles (b), after 1000 cycles (c), and after 5000 cycles (d) with 1% peak strain. The corresponding SEM images are shown in (e)-(h). The corresponding grain size distributions are shown in (i)-(l). The dashed circles in (a) and (e) depict the tip of a FIB-milled marker used to find the same area after each portion of straining cycles. Four white arrows in (d) point at the grains which were not clearly detectable in the initial microstructure but became relatively large during cyclic straining. The straining direction is horizontal.

3. In-operando fatigue of Au metallization.

Although fatigue behavior of polymer-supported films is more or less well understood, a question of great practical relevance remains still open: how the fatigue behavior changes if high electric current is applied simultaneously with the external mechanical load? To estimate it, a set of experiments on “in-operando” fatigue behavior of Au metallization lines on polyimide was designed and conducted. Fig. 3 summarizes the evolution of electrical resistance of 50 µm wide and 500 nm thick Au metallization lines subjected to simultaneously cyclic strain and direct current with different magnitudes. Initial resistance increase is due to Joule heating. It can be seen that the lines were able to sustain the current of 70 mA (current density 0.28 MA/cm2) and 10000 cycles with 1 % strain without significant electrical degradation. At the current of 80 mA the films failed reproducibly due to local overheating and thermal failure of the polyimide substrate (inset). For more details see [7].

Fig. 3. Dependence of averaged normalized resistance vs. time during in-operando fatigue experiments.

4. Initiation of fatigue damage in thin films with different microstructures.

Despite over 150 years of fatigue research there are still unsolved problems and unanswered questions. One example is the question of how the local microstructure such as grain size, grain orientation, grain boundary properties as well as global microstructure (texture, grain size distribution, misorientation angle distribution) of ultra-fine grained thin films influence the process of fatigue damage initiation. Au, Cu and Al films with different thicknesses and microstructures were deposited on polyimide substrates and subjected to cyclic strain. The number of cycles and the strain amplitude were adjusted in a way that only sparsely localized plasticity events are induced in the film. Then EBSD analysis of surrounding microstructure is performed to find our possible correlations. Some examples of such localized damage initiation sites in films with different microstructures are shown in Fig. 4. Detailed report entitled “Initiation of fatigue damage in ultrafine grained metal films” will be published soon.

Fig. 4. Fatigue damage initiation sites in evaporated Au film (a), sputtered Au film (b), evaporated Cu film with bi-modal grain size distribution (c), sputtered Cu film with log-normal grain sizes. Straining direction is horizontal.