High Performance Polysilicon Thin Film Transistor Circuits On Flexible Stainless Steel Foils
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High Performance Polysilicon Thin Film Transistor Circuits On Flexible Stainless Steel Foils Themis Afentakis1, Miltiadis K. Hatalis1, Apostolos T. Voutsas2, John W. Hartzell2 1 Display Research Laboratory, Department of Electrical & Computer Engineering, Lehigh University, Bethlehem, PA 2 Sharp Labs of America, Camas, WA ABSTRACT In recent years, there has been an increased interest in the use of flexible substrates in microelectronic fabrication. Flexible substrates, such as polymers and metals have the potential to be utilized in roll-to-roll processing, resulting in low cost, rugged systems. Thin flexible stainless steel foils offer a number of advantages over polymers for device and circuit fabrication, most significantly in the increased thermal budget tolerance that they provide. This enables the utilization of high temperature processes in the fabrication and the production of high performance devices and circuits. Thin film transistors have been fabricated on thin stainless steel foils using a variety of crystallization and gate dielectric approaches. N-channel devices with average effective mobility values of 250cm2/Vs and p-channel devices with effective mobility values in the region of 90cm2/Vs were successfully fabricated. Both digital and analog circuits, such as operational amplifiers, ring oscillators and a variety of shift register designs were also fabricated, and their basic performance characteristics will be presented in this paper. The characteristics of ring oscillators having 19 inverter stages operating with speeds of 15MHz or higher, along with the characteristics of various shift register architectures with maximum operating frequencies above 1MHz, will be reported. The impact of process and design parameters on their operation was evaluated and will be addressed in this paper. The results that are presented in this paper constitute the first successful implementation of high performance circuitry having a high degree of scalability and complexity on thin metal foils, thus making the fabrication of efficient, inexpensive and versatile systems on flexible foils for a large variety of applications a realistic prospect. INTRODUCTION During the past decade, there has been an increased interest in flexible substrates and their applications in microelectronic designs [1-3]. This interest has its source in the reduced weight and increased versatility those results from the physical properties of this class of substrates. Ultimately, it is envisioned that a unique, dedicated fabrication sequence will apply to these substrate materials, called roll-to-roll processing. This revolutionary approach of circuit fabrication will be able to produce integrated systems for a variety of applications with substantial cost savings over the traditional wafer-by-wafer approach, due to its inherent higher throughput.
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