Scientific Advisory Board - Erich Schmid Institute

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$dissertation global and local fracture properties of metal matrix ...$

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE was found. With all of these material and testing conditions in mind, cr and pet have been shown to be ideal materials for flexible electronics. As thin layers, less than 20nm, are typically used as adhesion layers, the 15nm cr film has exhibited ideal mechanical behavior at room temperature. All of these results indicate that the cr coated pet is a robust material system through a wide variety of mechanical testing conditions and environments illustrating its large potential for flexible electronic applications. Fig . 15 SEM image of the 70 nm Cr film strained to 6% at room temperature (a) and at 180°C (b). The images were recorded at the same magnification and clearly illustrate different buckling modes and buckle spacings at higher temperatures. At room temperature the buckles are pointed and cracked while at 180°C the buckles are rounded and un-cracked. Electro-Mechanically Self-Healing Films it has been demonstrated that the electrical resistance of a 200nm cu film on pet recovers after straining. coupled 4-point-probe resistance measurements and uni-axial tensile straining were utilized to record the electro-mechanical properties during loading, unloading, and after 24hrs of the film-substrate system. during tensile straining the resistance increases more than predicted by the constant volume approximation mostly due to plastic deformation and channel crack formation. the growth of resistance of the strained films follows the analytical Fig . 16 Relative resistance vs. relative elongation for three values of maximum strain: 5% (blue), 10% (red), and 20% (black). The error bars correspond to the standard deviation of 10 measurements. The solid circle, square and triangle show the final resistances and elongations measured after 24 hours for peak strain of 5%, 10%, and 20%, respectively. The area L/L 0=(1.00 to 1.05) is enlarged in the inset. (b) FIB cross-section of a crack in Cu film. Upon yielding, the Cu film first thinned locally and at higher strains, the channel crack formed at an angle to the surface. page 30 <strong>Scientific</strong> RepoRt 2012
ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE constant volume approximation up to approximately 2% strain, then dramatically increasing after (fig. 16a). the faster growth can be attributed to the lack of ductility because of the small grain size. Upon unloading and after a 24hrs period, the amount of resistance recovery varies depending on the maximum applied strain. for a maximum strain of 5% the resistance of the film after straining is lower than originally measured. the resistance recovery is attributed to a reduction of the defect density and grain boundary defects as well as crack closure (fig. 16b). it was found that a compressive stress was measured in the film at a load of 0n (approximately -400 mpa) and an assumed constant compressive stress after removal from the tensile stage. plastic deformation in the cu film causes the compressive stress and the viscoelastic behavior of the pet is the reason behind the continued recovery after straining. in order to properly characterize and compare the electro-mechanical behavior of metal films on polymers for flexible electronic applications, not only the behavior at a maximum stress or strain should be examined, but also during unloading and at least several hours afterward. Decrease in Adhesion with Altered Interface Microstructure titanium layers are used to promote adhesion between polymer substrates for flexible electronics and the cu or Au conducting lines. good adhesion of conducting lines in flexible circuits is critical in improving circuit performance and increasing circuit lifetime. nominally 50 nm thick ti films on polyimide (pi) are investigated by fragmentation testing under uniaxial tensile load in the as-deposited state, at 350°c, and after annealing. the cracking and buckling of the films show clear differences between the as-deposited and the thermally treated samples, cracks are much straighter and buckles are smaller following heat treatment. these changes are correlated to a drop in adhesion of the samples following heat treatment. Adhesion values are determined from the buckle dimensions using a total energy approach. cross-sectional transmission electron microscopy (tem) of the ti/pi interface found evidence of a 5 nm thick interlayer between the largely columnar ti and the amorphous pi. the tem study infers that the difference in adhesion caused by heat treatment of these ti films is linked to the crystallisation of an interlayer between the ti and pi (fig. 17). the implications of this for the processing, use and lifetimes of flexible electronic circuits are extensive. Additionally, this greater understanding of how ti acts as an effective adhesion layer between polymers and metallisation materials such as Au and cu could lead to more failure-resistant, flexible circuitry. Fig. 17 Cross-sectional TEM micrographs of the Ti/PI interface of an as-deposited sample(left) and (right) and a 350°C tested sample. <strong>Scientific</strong> RepoRt 2012 page 31
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