Low Temperature Annealing with Solid-State Laser or UV Lamp Irradiation on Amorphous IGZO Thin-Film Transistors

Abstract

Instead of the conventional furnace annealing process with a temperature higher than 300°C, two low temperature annealing methods are successfully demonstrated to suppress the instability problem of amorphous indium gallium zinc oxide ͑IGZO͒ thin film transistors ͑TFTs͒. With adequate Nd:yttrium aluminum garnet laser ͑266 nm͒ annealing energy density or Xe excimer UV lamp ͑172 nm͒ irradiation time, the on voltage shift is greatly suppressed from over 10 to 0.1 V. The influence of laser energy density and UV lamp irradiation time on the performance of IGZO TFTs is also investigated and explained. The proposed methods are promising for the development of amorphous IGZO TFTs on flexible substrates.Recently, transparent amorphous InGaZnO 4 ͑IGZO͒ thin-film transistors ͑TFTs͒ have become promising candidates to realize flexible display on plastic substrates because they can be deposited at a temperature lower than 150°C. 1 Compared to amorphous silicon TFT, IGZO TFT exhibits a higher mobility ͑3-30 cm 2 /V s͒ even in the amorphous phase. 2 However, in many articles, low temperature deposited IGZO TFT suffers from significant instability problems. 3,4 Its current-voltage ͑I-V͒ characteristics shift during electrical measurement. To suppress the characteristics' shift, furnace annealing higher than 300°C is usually used. It was reported that the annealing process reduces the tail state defects, rearranges the amorphous structure, and improves oxygen compensation in the nonstoichiometric film. 3,4 However, an annealing temperature higher than 300°C makes the process unsuitable for flexible substrates. A low temperature annealing process is essential for the development of stable IGZO TFTs on flexible substrates.One method utilizing excimer laser annealing ͑ELA͒ to improve IGZO TFT characteristics was proposed recently. 5 Although the authors did not mention stability issues, they successfully demonstrated the feasibility of using ELA on IGZO TFTs. In our study, two low temperature annealing methods are proposed to effectively solve the stability problem. One method is the solid-state Nd:yttrium aluminum garnet ͑YAG͒ laser annealing method; the other method is a simple UV lamp irradiation method. The Nd:YAG solid-state laser operates at a wavelength of 266 nm, annealing the IGZO TFT at a low laser energy density of 10.7 mJ/cm 2 . This low energy density enables the future design of a large laser beam size with a low production cost. To further reduce the production cost, another annealing process is proposed. This annealing process uses a low power ͑50 mW/cm 2 ͒ Xe excimer lamp with a wavelength of 172 nm ͑UV light͒. The UV lamp irradiation effectively anneals the devices. The proposed method is a promising candidate to realize large-area, low cost UV lamp annealing for IGZO TFTs. In this article, the influences of laser energy density and UV lamp irradiation time on the performance and stability of IGZO TFTs are investigated. The bias-stress effects of an unannealed device, a laserannealed device, and a UV-annealed devic...

Pulse Thermal Processing for Low Thermal Budget Integration of IGZO Thin Film Transistors

“…3 shows the time dependence of V T shift with positive bias stress. The time dependence of the V T shift is in agreement with the stretched-exponential equation [13], [24] rather than a logarithmic time dependence model [25]. The stretched exponential equation has been developed for the charge trapping mechanism in a-Si TFTs, which hypothesizes the emission of trapped charges toward deep states in the bulk dielectric for longer stress time and larger stress bias [24], [26].…”

“…In general, furnace annealing above 300 • C is effective for obtaining good transfer characteristics and stable IGZO-TFTs [9]- [11], but processing temperatures above 150 • C are not compatible with most flexible polymer substrates. To address the low temperature integration issue, excimer laser annealing (ELA) has been explored [12], [13]. However, the small laser beam size is a major limitation to meet the high throughput and low-cost demands of large area roll-to-roll manufacturing.…”

“…However, the small laser beam size is a major limitation to meet the high throughput and low-cost demands of large area roll-to-roll manufacturing. Attempts have been made to use ultraviolet (UV) lamp irradiation [13]. Although bias stress stability improves, the TFT characteristics have been found to degrade as a function of UV irradiation time.…”

High-Performance Light-Erasable Memory and Real-Time Ultraviolet Detector Based on Unannealed Indium–Gallium–Zinc–Oxide Thin-Film Transistor

“…An a-IGZO TFT fabricated at low temperature exhibits an unstable threshold voltage under operation [3], [4]. Generally, efforts such as post-annealing and passivation are necessary to stabilize the a-IGZO TFT [4], [5].…”

Light-Enhanced Bias Stress Effect on Amorphous In-Ga-Zn-O Thin-film Transistor with Lights of Varying Colors

“…1,2 Furthermore, IGZO is predicted to be insensitive to visible light, thereby meeting the requirement of certain electric-optical applications. In this study, we used visible light and infrared light for illumination to understand the actual response of an IGZO TFT to the sub-band-gap photons.…”