Adhesion Techniques for Films on Flexible Substrates

Deformation and interfacial adhesion of thin films on polymer substrates govern the performance and reliability of many designed systems such as flexible microelectronic devices, electrotextiles, and flexible displays. Promising concepts use state-of-the-art technologies such as organic light emitting diodes, thin film transistors, and flexible liquid crystal displays. Of top most importance in designing reliable flexible devices is to understand interface properties between these materials. Advances must be made in the relationship between film adhesion, mechanical properties and fracture which are currently poorly understood for metal films on compliant polymer substrates in order to provide the basic understanding required for design guidelines. The centerpiece of the proposed three year project is the development of a quantitative technique to determine the "adhesion energy" (energy release rate) and interface strength of metal thin films deposited on polymer substrates. This will be realized by combining in situ tensile experiments with electron microscopy and X-ray diffraction techniques and a mechanics approach to determine the adhesion energy. Adhesion will be measured from the well- defined buckle morphology that forms during a tensile test. While the local strain release by buckling can be estimated, X-ray diffraction including synchrotron experiments will be used to obtain quantitative measures of the local strain and stress distribution at the buckle and delamination front. This will help to establish a sound quantitative model for the adhesion energy. To elucidate some of the factors that affect adhesion, studies will be performed that examine the role of interface chemistry, interface and film structure as well as the thermal, environmental, and viscoelastic properties of the substrates. Interface chemistry, interface structure and film structure will be analyzed with Auger electron spectroscopy and advanced transmission electron microscopy. Substrate properties will be examined at high temperatures, different degrees of humidity, and strain rates. Furthermore, the possibility of self-healing cracks in metal lines by electromigration will be explored. Through the movement of atoms with an electric current, cracks formed during straining could be bridged and allow for continued reliability of flexible devices. The proposed research will be completed in three years by high quality cooperating national partners within Austria in the fields of thin film deposition, mechanics, X-ray diffraction, and materials science. During the course of the proposed research, the results will be distributed at international conferences with presentations and with articles in well-recognized journals. This proposal will provide insight into the adhesion and failure mechanisms of metal-polymer interfaces as well as encourage design of superior and robust flexible devices.

Flexible electronics will change the way people communicate and work. These devices include rollable and foldable screens or tablets, wearable smart phones, and medical sensors that attach to the skin like a sticker. Their flexibility is due to the manufacturing process where the electronics are printed on soft bendable polymer (plastic) substrates, with the thin metal electrical conducting lines made from gold or copper. Unfortunately, there is a problem with sticking/adhering metals and polymers to each other. When the metals and polymers do not adhere well to one another, the flexible device will not function due to electrical and mechanical failures. Within this project, a new measurement technique was developed to measure the adhesion strength of metal-polymer interfaces that utilizes stretching of a metal thin film-polymer substrate system until the film delaminates; these areas of delamination can be used to calculate the adhesion strength of the interface. The new method was shown to work extremely well for different metal-polymer systems. A further development of the measurement method was that the stretching behavior could be simulated using sophisticated computer models. The simulations help to better understand and illustrate the delamination processes of the metals.Other external parameters such as temperature, stretching speed, and moisture levels were also investigated to determine their effect on the adhesion strength. It was found that temperature and moisture have the largest effect on the adhesion because the metal-polymer interface changes at the atomic scale with exposure to higher temperatures and in moisture rich environments. The most important finding of the project was that the use of adhesion promoting metals to encourage sticking between metal and polymer was not necessary.Adhesion promoting metals, such as titanium or chromium, bond to polymers better than copper or gold. However, these are very brittle metals that can easily break when stretched or flexed. When these layers are used, they reduce the mechanical strength of the system as well as the electrical conductivity necessary for the flexible electronic device to function. The results from this project are ground-breaking and provide a deeper understanding of the interface strength between metals and polymers, which can be used to improve the design and production methods of flexible electronic devices that are not only more reliable and less expensive, but also medically and environmentally friendly.