Oleksandr Glushko

Electrical Resistance of Metal Films on Polymer Substrates Under Tension

The coupling of electrical and mechanical property measurements for thin films is a relatively new area of research. It is particularly useful to know how electrical resistance could be altered with mechanical straining for flexible electronic applications. While coupled resistance measurements have been made in conjunction with tensile straining, a clear description of the measurement technique is rarely provided. Explained here is a four-point probe resistance measurement that has been incorporated into tensile straining grips for accurate and repeatable in situ resistance measurements. Also described is an analytical explanation of how to correct for constant resistances that do not change during the experiment. The new technique has been employed on Cu films on polyethylene terephthalate (PET) to illustrate its use and to discuss how resistance will recover after 24 h.

Electro-Mechanical testing of conductive materials used in flexible electronics

The use of flexible electronics has increased in recent years. In order to have robust and long lasting flexible displays and sensors, the combined electro-mechanical behavior needs to be assessed. The most common method to determine electrical and mechanical behavior of conductive thin films used in flexible electronics is the fragmentation test, or uniaxial tensile straining of the film and substrate. When performed in situ fracture and deformation behavior can be determined. The use of in situ electrical resistance measurements can be informative about the crack onset strain of brittle layers, such as transparent conductors, or the stretchability of metal interconnects. The combination of in situ electrical measurements with in situ X-ray or confocal laser scanning microscopy can provide even more information about the failure mechanisms of the material systems. Lattice strains and stresses can be measured with X-rays, while cracking and buckle delaminations can be studied with confocal laser scanning microscopy. These new combinations of in situ methods will be discussed as well as methods to quantify interfacial properties of conductive thin films on polymer substrates. The combined techniques provide valuable correlated electrical and mechanical data needed to understand failure mechanisms in flexible devices.

Monotonic and cyclic mechanical reliability of metallization lines on polymer substrates

Mechanical stability of Ag and Cu printed and evaporated metallization lines on polymer substrates is investigated by means of monotonic tensile and cyclic bending tests. It is shown that lines which demonstrate good performance during monotonic tests fail at lower strains during a cyclic bending tests. Evaporated lines with the grain size of several hundreds of nanometers have good ductility and consequently good stability during monotonic loading but at the same time they fail at low strains during cyclic bending. Printed lines with nanocrystalline microstructure, in contrast, demonstrate more intensive cracking during monotonic loading but higher failure strains during cyclic bending. Apart from the grain size effect, the effect of film thickness on the saturation crack density after cyclic bending is also demonstrated. Thinner films have higher crack density in accordance with the shear lag model.

The driving force governing room temperature grain coarsening in thin gold films

Abstract Strong room-temperature grain coarsening in gold films on polyimide induced by cyclic uniaxial mechanical strain is demonstrated. Detailed electron backscatter diffraction analysis revealed that, in contrast to the predictions of shear-coupled grain boundary migration model, the grain coarsening is isotropic and coarsened grains do not exhibit any specific crystallographic orientations or misorientations to the neighboring grains. It is shown that a thermodynamic model where the driving force appears due to the difference in yield stresses between the grains with different sizes provides an adequate explanation of the experimental data.

Explicit relationship between electrical and topological degradation of polymer-supported metal films subjected to mechanical loading

For a comprehensive characterization of mechanical reliability of metallization layers on polymer substrates, both electrical and mechanical degradation should be taken into account. Although it is evident that cracking of a conductive film should lead to electrical degradation, the quantitative relationship between the growth of electric resistance and parameters of the induced crack pattern has remained thus far unexplored. With the help of finite element modelling, we were able to find an explicit and concise expression which shows that electrical resistance grows with the fourth order of the crack length and second order of the areal crack density. The discovered relationship was verified by comparison with the experimental results of tensile testing of polymer-supported thin metal films. The presented model is independent of the length scale and can be applied to films with different thicknesses as long as Ohms law is valid. It is demonstrated that the linear crack density is an ambiguous parameter,...

Electromigration in Gold Films on Flexible Polyimide Substrates as a Self-healing Mechanism

The study of electromigration (EM) in metallisations for flexible thin film systems has not been a major concern due to low applied current densities in todays flexible electronic devices. However, the trend towards smaller and more powerful devices demands increasing current densities for future applications, making EM a reliability matter. This work investigates EM in 50 nm Au thin films with a 10 nm Cr adhesion layer on a flexible polyimide substrate at high current densities. Results indicate that EM does occur and could be used as a self-healing mechanism for flexible electronics. GRAPHICAL ABSTRACT

Crack Initiation of Printed Lines Predicted with Digital Image Correlation

Printing of metallic films has been preferred over vacuum technologies for roll-to-roll processes because of faster processing times and lower processing costs. Films can be produced by depositing inks containing suspended metallic particles within a solvent and then heating the films to both remove the solvent and sinter the particles. The resulting printed structure and electrical and mechanical behavior of the printed films has been studied to better understand their electro-mechanical response to loading and eventual brittle fracture. This study evaluated the electro-mechanical behavior of 1.25-μm printed Ag films using in situ resistance and in situ imaging methods. Digital image correlation was utilized with confocal laser scanning microscope images to better visualize crack initiation during tensile straining. This technique showed that cracks initiated earlier in the thicker areas of the film (crests) than in lower areas (troughs) because of a higher density of printing defects and the increased thickness.