Search

Thin film materials used in flexible electronics are deposited on polymer substrates and must withstand a variety of static and dynamic mechanical loading conditions to ensure adequate reliability of the device. Tribological loads are also among these loading conditions, and suitable characterization methods and strategies are required for analyzing friction and wear for a variety of tribological contact situations. In the present work, Mo films were deposited on polyimide substrates by high-power impulse magnetron sputtering and then pre-conditioned by straining to several strain levels, including crack onset strain and strains within the crack saturation regime. Subsequently, ball-on-disk tests against different counterpart materials, namely glass, steel, and polymer, were performed to evaluate different tribological contact situations. The comparison of the results of morphologies and characteristics of the films using surface images for strained and unstrained samples provide insight into how increasing straining of the films and crack formation affect the enhanced fracture of the deposited Mo films, which served as a model system in these investigations. [ABSTRACT FROM AUTHOR]

In order to assess the longevity of foldable electronics, folding or bending tests on model systems need to be performed. However, not all bending tests are created equal in that different configurations lead to different amounts of mechanical damage and thus different electrical responses. Two 90° bending instruments were compared using two model metallic thin film systems on polyimide to establish if the two seemingly similar bending tests yield the same results. The two film systems, namely 300 nm Mo and 130 nm Al on 50 nm Mo, were magnetron-sputtered on polyimide substrates and tested in the custom-built FLEX-E-TEST and the commercially available YUASA test that is capable of in situ resistance measurements. For statistics, 10–12 samples were tested of each film system on each folding device using the same applied bending strain and number of cycles. Samples were intermittently characterized with confocal laser scanning microscopy and electrical resistance to correlate the amount of mechanical damage (crack density) with the electrical normalized resistance ratio of the damaged area. The results show that even with the same bending radius, a similar but not identical amount of mechanical damage forms for both bending devices. Additionally, the resistance as a function of cycles also differs after 10,000 cycles. A closer examination of the damage, especially in the Al/Mo film system, indicates that the speed of the bending, and if the samples experience spring back, can alter the received mechanical damage. The in situ resistance data of the YUASA test were further examined and a suggestion of standardizing how folding or bending test results are reported is provided. [ABSTRACT FROM AUTHOR]

Featured Application: Void quantification in the metallization layer of microelectronic chips. In many applications, voids in metals are observed as early degradation features caused by fatigue. In this publication, electropolishing is presented in the context of a novel sample preparation method that is capable of accessing voids in the interior of metal thin films along their lateral direction by material removal. When performed at optimized process parameters, material removal can be well controlled and the surface becomes smooth at the micro scale, resulting in the voids being well distinguishable from the background in scanning electron microscopy images. Compared to conventional cross-sectional sample preparation (embedded mechanical cross-section or focused ion beam), the accessed surface is not constrained by the thickness of the investigated film and laterally resolved void analyses are possible. For demonstrational purposes of this method, the distribution of degradation voids along the metallization of thermo-mechanically stressed microelectronic chips has been quantified. [ABSTRACT FROM AUTHOR]

Rapid progress in the reduction of substrate thickness for silicon-based microelectronics leads to a significant reduction of the device bending stiffness and the need to address its implication for the thermo-mechanical fatigue behavior of metallization layers. Results on 5 μm thick Cu films reveal a strong substrate thickness-dependent microstructural evolution. Substrates with hs = 323 and 220 µm showed that the Cu microstructure exhibits accelerated grain growth and surface roughening. Moreover, curvature-strain data indicates that Stoney's simplified curvature-stress relation is not valid for thin substrates with regard to the expected strains, but can be addressed using more sophisticated plate bending theories. [ABSTRACT FROM AUTHOR]

More and more flexible, bendable, and stretchable sensors and displays are becoming a reality. While complex engineering and fabrication methods exist to manufacture flexible thin film systems, materials engineering through advanced metallic thin film deposition methods can also be utilized to create robust and long-lasting flexible devices. In this review, materials engineering concepts as well as electro-mechanical testing aspects will be discussed for metallic films. Through the use of residual stress, film thickness, or microstructure tailoring, all controlled by the film deposition parameters, long-lasting flexible film systems in terms of increased fracture or deformation strains, electrical or mechanical reliability, can be generated. These topics, as well as concrete examples, will be discussed. One objective of this work is to provide a toolbox with sustainable and scalable methods to create robust metal thin films for flexible, bendable, and stretchable applications. [ABSTRACT FROM AUTHOR]