Silicon and Dopant Ink-Based CMOS TFTs on Flexible Steel Foils
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Silicon and Dopant Ink-Based CMOS TFTs on Flexible Steel Foils Aditi Chandra1; Mao Takashima1; Arvind Kamath1 1
Thinfilm Electronics, San Jose, CA, 95134 United States.
ABSTRACT Polysilicon complementary metal oxide semiconductor (CMOS) thin film transistors (TFTs) are fabricated on large area, flexible stainless steel foils using novel ink depositions within a hybrid printed/conventional process flow. A self-aligned top gate TFT structure is realized with an additive materials approach to substitute the use of high capital cost ion implantation and lithography processes. Polyhydrosilane-based silicon ink is coated and laser crystallized to form the polysilicon channel. Semiconductor grade P-type and N-type unique dopant ink formulations are screen printed and combined with thermal drive in and activation to form self-aligned doped source and drain regions. A high refractory top gate material is chosen for its process compatibility with printed dopants, chemical resistance, and work function. Steel foil substrates are fully encapsulated to allow for high temperature processing. The resultant materials set and process flow enables TFT electrical characteristics with NMOS and PMOS mobilities exceeding 120 cm2/Vs and 60 cm2/Vs, respectively. On/Off ratios are >107. Reproducibility, uniformity, and reliability data in a production environmental is shown to demonstrate high volume, high throughput manufacturability. The device characteristics and scheme enable NFC (13.56MHz) capable circuits for use in flexible NFC and display-based smart labels and packaging. INTRODUCTION Wearable and low cost disposable devices which enable the Internet of Everything (IoE) demand thin, low-cost, durable and flexible logic circuits that communicate wirelessly. Printed electronics has received considerable attention in recent years with a view to fabricate displays, PV and sensor elements with low capital cost, high-volume manufacturing on large-area or flexible substrates 1-4. It has been previously shown that high performance polysilicon TFT devices based on printable silicon ink and screen printed dopants on high temperature flexible substrates can be used to make low cost wireless devices (with limited transistor count) in a production environment 5-7. The key enabling technologies: (1) the use of large-area coating of polyhydrosilane-based ink for silicon film formation; and (2) the use of screen printed n- and p-type silicon dopants. Silicon ink-based TFTs allow for tunable transistor threshold voltage (Vt) through the addition of dopant precursors in the formation. A linear relationship between Vt and dopant concentration is shown, demonstrating the ability to eliminate the need for ion implantation equipment and additional lithography and patterning steps. EXPERIMENT
Downloaded from https:/www.cambridge.org/core. Cornell University Library, on 10 May 2017 at 14:58:41, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/adv.2017.227
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