Dissertation Defense, Po-Chin Kuo

Dissertation Defense, Po-Chin Kuo

Title:                      Flexible Electronics based on Polysilicon TFTs
                             on steel Foil: Fabrication and Characterization
                             under strain

Date:                     November 17, 2009

Time:                     10:00am, Tuesday
Location:              Packard Lab 503

Flexible electronics is an emerging technology that offers many advantages such as low profile, light weight and mechanical flexibility that enables many new applications such as flexible displays. The poly-Si TFT technology on steel foil is suitable for high performance flexible electronics because it offers high carrier mobility, excellent device reliability and compatibility with conventional CMOS process.

In the first part of this dissertation defense the development of flexible electronics by using poly-Si TFT technology on the steel foil is presented. It includes the fabrication and characterization of poly-Si TFT devices, circuits and active matrix displays on steel foils. The average mobility of n- and p-channel poly-Si TFTs are about 300 cm2/V·s and 150 cm2/V·s respectively; the propagation delay per stage of ring oscillators can be as low as 1 ns which demonstrates the potential of the poly-Si technology on steel foil, for high performance flexible digital circuits. In this work a functional AMOLED flexible display is also demonstrated.

 In the second part of this dissertation defense the effects of mechanical strain on the integrity of thin film materials and the electrical characteristics of flexible electronics are discussed. Flexible electronics will be exposed to different levels of strain due to bending and thus understanding the effects of mechanical strain is of paramount importance. The failure mechanisms and the failure strain of thin film materials including dielectric layers and metal interconnects on steel foil are analyzed. The finite element method is used to simulate the strain distribution of a poly-Si TFT under bending for failure prediction. The electrical characteristics of poly-Si TFTs are a function of strain and the response of circuit under strain is attributed to the change of TFT characteristics. Using electron microscopy the failure mode of poly-Si TFTs under tension is found to be caused by the cracking of the polysilicon layer. The TFTs are found to withstand more the compressive strain than the tensile strain.

This work validates the compatibility of poly-Si technology on steel foil substrate for large-area microelectronic applications and provides an insight on the effect of strain on the performance of flexible electronics.

 Ph.D. Committee:

Prof. Miltiadis K. Hatalis (Chair), ECE, Lehigh University
Prof. Marvin H. White, ECE, Lehigh University
Prof. Richard P. Vinci, ECE, Lehigh University
Prof. Mark Stewart, East Stroudsburg University