Fabrication of thin-film transistors on polyimide films - Polyimides and Other High Temperature Polymers - page 457

Fabrication of thin-film transistors on polyimide films

HELENA GLESKOVA ∗ and SIGURD WAGNER
Department of Electrical Engineering, Princeton University, Princeton, NJ 08544

Abstract—We describe polyimide films as substrates for active thin-film electronics. Amorphous
silicon thin-film transistors were fabricated on 51 µ m thick polyimide foil (Kapton ® E) at a maxi-
mum process temperature of 150 C. Kapton E was selected for its chemical stability, high softening

or glass transition temperature, relatively low coefficient of humidity expansion, negligible heat
shrinkage, low water and oxygen permeability, coefficient of thermal expansion comparable to that
of thin-film electronics, and low surface roughness. The fabricated transistors have off-current of ~
2 x 10 -12 A, on-off current ratio ~ 10 7 , threshold voltage ~ 2 V, electron mobility ~ 0.5 cm 2 V -1 s -1 ,
and subthreshold slope ~ 0.5 V/decade. These performance parameters are similar to those of tran-
sistors fabricated at 150 ° C on glass substrates.

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Keywords : Polyimide; flexible electronics; amorphous silicon thin-film transistors.

1. INTRODUCTION

Novel display applications, such as electronic paper, smart labels, and displays for
vehicular applications, require flexible thin-film transistor (TFT) backplanes [1-
10]. Future sensor skins and electrotextiles will be based on flexible circuits. For
these applications, the traditional glass substrate for the thin-film transistors of ac-
tive-matrix liquid crystal displays must be replaced with foils of organic polymers
or metals. Stainless steel foils are suitable as substrates for amorphous [11],
nanocrystalline, and polysilicon thin film transistors [12] without much change in
the processes used for making them on glass. Since steel is an electrical conductor
and its surface roughness is much larger than that of glass, it must be electrically
insulated and planarized by using, for example, spin-on-glass.
Another large class of flexible substrates are organic polymers. Their low cost,
transparency in the visible part of the light spectrum, and wide variety are attrac-
tive attributes. However, in thin-film electronics other characteristics, such as
chemical stability, high softening or glass transition temperature, a low coefficient
of thermal expansion (comparable to that of the materials used for thin-film elec-