Fe-42%Ni austenitic alloy as a novel substrate for flexible electronics
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Fe-42%Ni austenitic alloy as a novel substrate for flexible electronics Xiaoxiao Ma, 1 Shahrukh A. Khan, 1 Nackbong Choi, 1 Miltiadis Hatalis1 and Mark Robinson2 1. Department of Electrical Engineering, Lehigh University, Bethlehem, PA 18015 2. Ametek Specialty Metal Products Division, Lancaster, PA 17601
ABSTRACT We report Fe-42%Ni as a novel high-performance substrate for a-IGZO TFT fabrication after evaluating 8 different metals for chemical compatibility, mechanical flexing and dimensional stability. Excellent flexibility and rollability indicates that Fe-42%Ni would be a good choice as flexible substrate for R2R process. Pre-annealing process for stabilizing the substrate is studied and applied to the Fe-42%Ni foil before TFT fabrication. Staggered bottom gate a-IGZO TFTs which were fabricated on this substrate have field effect mobility of 12 cm2/V.s, threshold voltage of 2V, sub-threshold swing of 0.6V/decade and on/off current ratio exceeding 107.
INTRODUCTION TFTs with poly-crystalline silicon, amorphous silicon, and metal oxide as active material fabricated on flexible substrate such as stainless steel, plastic substrate and flexible glass have drawn great attention in recent years [1, 2]. However, glass substrate is limited by its flexibility and mechanical strength; plastic substrate needs extra passivation to avoid shrinkage and elongation induced by gas permeation through plastic and also has low thermal budget for processing [3]. Metal substrates such as stainless steel has been widely studied in flexible electronics fabrication especially in R2R process due to its high yield strength after cold rolling and even after annealing, advantages of lower CTE and higher melting point compared to plastic which enables high processing temperature with dimensional stability when going through thermal cycles, high resistivity to chemical attack, low permeability to water and oxygen, cold workability and compatibility to established low temperature TFT fabrication process. Several novel metal substrates were studied in this paper for their mechanical properties, dimensional stability, and chemical compatibility. Among all of the eight metals selected, Fe-42%Ni has been proven to be the best candidate as substrate for lightweight, shatterproof, rugged flexible electronics. Though Fe-42%Ni has lower CTE than widely used stainless steel, its permanent dimensional distortion induced by phase transformation, structural changes and residual stress relief during thermal cycles in the TFT fabrication process still pose challenges. Detailed study of dimensional stability of Fe-42%Ni along with metallographic analysis was adopted and high performance a-IGZO TFTs with maximum processing temperature of 300°C are fabricated on unpolished Fe-42%Ni substrate.
EXPERIMENTAL DETAILS A. Evaluation of 8 different metal substrates
Eight commercially available metal foils SS301, SS302, SS304, Ni, Fe-42%Ni (NiFe42), Ti, Ta and Mo prepared by Hamilton Precision Metals are examined in this work. All of those metal foils are cold rol
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