TY - BOOK

T1 - Electro-Mechanical Behavior of Flexible Thin Film Systems

AU - Kreiml, Patrice

N1 - no embargo

PY - 2020

Y1 - 2020

N2 - In recent years, flexible electrical devices are an extensively studied subject in industry and academia. Flexible electronics may potentially lead to advances in different areas, such as medicine and logistics, but also in sports, entertainment and everyday life. Among the many issues that need to be resolved for a reliable flexible electrical device, one of them are the electrodes for data and energy transfer. For flexible applications, they have to connect the individual components and maintain their function under changing load conditions (bending, stretching and twisting). This work focuses on Mo/Al/Mo multilayer stack electrodes, commonly used in thin film transistor displays. The Mo film acts as an adhesion layer and diffusion barrier, while the Al film’s function is charge carrying. For electro-mechanical testing simplified Al/Mo bilayers were deposited on polymer substrates. The Al/Mo bilayers were tested under uniaxial tensile loading (stretching) and under cyclic bending. The bilayers had a 30 nm thick Mo adhesion layer with varying thicknesses of Al (30, 75, 150, 300 nm) on top. The tensile tests showed that the inherently brittle Mo layer causes the bilayer to fracture, including the loss of conductivity, but that increasing thicknesses of the Al layers are able to alleviate and delay the fracture process. The bending tests yielded a similar behavior. However, for cyclic bending the applied bending strain and the number of bending cycles have a strong impact on the result. At 20,000 bending cycles and 0.5% applied bending strain the Al/Mo bilayers showed no fracture and almost no change in conductivity. At the same cycle number, but at 1.3% and 3.1% bending strains, the bilayers start to fracture within the first few cycles and reach crack saturation around 10,000 cycles. The saturation crack density and the loss of conductivity are proportional to the applied bending strain. At 1.3% and 3.1% bending strains the Al/Mo bilayers fail before reaching 1,000 bending cycles. To improve the brittle nature of the Mo film without severe impact on industrial process chains and the function of the Mo film in the multilayer stack, alloying with another element was investigated. Mo-Ta alloys emerged as the best combination of mechanical failure strain (∽1% increase) and sheet resistivity (higher, but similar to Mo). The Mo films in the bilayers were substituted with Mo50Ta50 alloys under high compressive stresses (-1.9 GPa). The new Al/Mo-Ta bilayers showed a roughly 1% higher failure strain under tensile load (stretching). At 1.3% bending strain the Al/Mo-Ta bilayers had no cracks when the experiment ended at 50,000 bending cycles. Only, the electrical resistance increased by 25%, probably due to fatigue, when the experiment ended. At 3.1% bending strain the Al/Mo-Ta bilayers fail, similar to the Al/Mo bilayers. The study has proven that alloying of the brittle Mo film improves the robustness and reliability of the whole electrode stack and makes the electrode more suitable for flexible applications, such as rollable TVs and foldable smartphones.

AB - In recent years, flexible electrical devices are an extensively studied subject in industry and academia. Flexible electronics may potentially lead to advances in different areas, such as medicine and logistics, but also in sports, entertainment and everyday life. Among the many issues that need to be resolved for a reliable flexible electrical device, one of them are the electrodes for data and energy transfer. For flexible applications, they have to connect the individual components and maintain their function under changing load conditions (bending, stretching and twisting). This work focuses on Mo/Al/Mo multilayer stack electrodes, commonly used in thin film transistor displays. The Mo film acts as an adhesion layer and diffusion barrier, while the Al film’s function is charge carrying. For electro-mechanical testing simplified Al/Mo bilayers were deposited on polymer substrates. The Al/Mo bilayers were tested under uniaxial tensile loading (stretching) and under cyclic bending. The bilayers had a 30 nm thick Mo adhesion layer with varying thicknesses of Al (30, 75, 150, 300 nm) on top. The tensile tests showed that the inherently brittle Mo layer causes the bilayer to fracture, including the loss of conductivity, but that increasing thicknesses of the Al layers are able to alleviate and delay the fracture process. The bending tests yielded a similar behavior. However, for cyclic bending the applied bending strain and the number of bending cycles have a strong impact on the result. At 20,000 bending cycles and 0.5% applied bending strain the Al/Mo bilayers showed no fracture and almost no change in conductivity. At the same cycle number, but at 1.3% and 3.1% bending strains, the bilayers start to fracture within the first few cycles and reach crack saturation around 10,000 cycles. The saturation crack density and the loss of conductivity are proportional to the applied bending strain. At 1.3% and 3.1% bending strains the Al/Mo bilayers fail before reaching 1,000 bending cycles. To improve the brittle nature of the Mo film without severe impact on industrial process chains and the function of the Mo film in the multilayer stack, alloying with another element was investigated. Mo-Ta alloys emerged as the best combination of mechanical failure strain (∽1% increase) and sheet resistivity (higher, but similar to Mo). The Mo films in the bilayers were substituted with Mo50Ta50 alloys under high compressive stresses (-1.9 GPa). The new Al/Mo-Ta bilayers showed a roughly 1% higher failure strain under tensile load (stretching). At 1.3% bending strain the Al/Mo-Ta bilayers had no cracks when the experiment ended at 50,000 bending cycles. Only, the electrical resistance increased by 25%, probably due to fatigue, when the experiment ended. At 3.1% bending strain the Al/Mo-Ta bilayers fail, similar to the Al/Mo bilayers. The study has proven that alloying of the brittle Mo film improves the robustness and reliability of the whole electrode stack and makes the electrode more suitable for flexible applications, such as rollable TVs and foldable smartphones.

KW - Dünnfilm

KW - Zugtest

KW - elektrischer Widerstand

KW - Fragmentierung

KW - In situ

KW - Biegen

KW - Kathodenzerstäubung

KW - Risse

KW - Adhäsion

KW - Legierung

KW - Multischichten

KW - Zwei-Schicht Systeme

KW - Thin films

KW - Tensile test

KW - Electrical resistance

KW - Fragmentation

KW - In situ

KW - Bending

KW - Sputtering

KW - Cracks

KW - Adhesion

KW - Alloy

KW - Multilayer

KW - Bilayer

M3 - Doctoral Thesis

ER -