452 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 5, NO. 12, DECEMBER 2009 Electrical Instability of RF Sputter Amorphous In-Ga-Zn-O Thin-Film Transistors Tze-Ching Fung, Student Member, IEEE, Katsumi Abe, Hideya Kumomi, and Jerzy Kanicki, Senior Member, IEEE (Invited Paper) Abstract—Bias-temperature-stress (BTS) induced electrical instability of the RF sputter amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) was investigated. Both positive and negative BTS were applied and found to primarily cause a positive and negative voltage shift in transfer characteris- tics, respectively. The time evolution of bulk-state density and characteristic temperature of the conduction-band-tail-states are extracted. Since both values showed only minor changes after BTS, the results imply that observed shift in TFT curves were primarily due to channel charge injection/trapping rather than defect states creation. We also demonstrated the va- lidity of using stretch-exponential equation to model both positive and negative BTS induced threshold voltage shift of the a-IGZO TFTs. Stress voltage and temperature dependence of evolution are described. Index Terms—a-IGZO, amorphous semiconductors, bias temperature stress (BTS), semiconductor device reliability, trans- parent thin-film transistors (TFTs). I. INTRODUCTION A LTHOUGH the hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) has long been the workhorse for active-matrix liquid crystal display (AM-LCD), its low mobility restricts the TFT high frequency response and current driving capability. This disadvantage becomes a limiting factor for display performance, especially when the current trends of active-matrix flat panel display (AM-FPD) development are toward high frame rate (120–240 Hz) [1] and “all solid-state” device, such as active-matrix organic light-emitting display (AM-OLED) [2], [3]. On the other hand, poly-crystalline sil- icon (poly-Si) TFT is considered as a high mobility alternative device. However, such technology usually required additional crystallization steps (e.g., excimer laser annealing [4], metal seeding [5] or solid phase crystallization [6]) which raise more concerns when manufacturing over large substrate area is considered [3]. During the past few years, there have been great interests in using amorphous In-Ga-Zn-O (a-IGZO) thin-film transistor (TFT) in AM-FPDs [7]–[11]. Because the electrons Manuscript received March 05, 2009. Current version published November 13, 2009. T.-C. Fung and J. Kanicki are with the Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109 USA (e-mail: tcfung@umich.edu; kanicki@eecs.umich.edu). K. Abe and H. Kumomi are with Canon Research Center, Canon Inc., Ohta-ku, Tokyo 146-8501, Japan (e-mail: abe.katsumi@canon.co.jp, ku- momi.hideya@canon.co.jp). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JDT.2009.2020611 are drifting through ionic metal s-orbital, which permit a band-like conduction even in amorphous phase, a-IGZO TFTs have a higher field-effect mobility than traditional cova- lent bond semiconductors (e.g., a-Si:H) [7]. These properties make a-IGZO TFTs one of the ideal choices for future large area (e.g., 82-in) ultra-definition (up to pixels, [12], [13]) display backplane technology. To ensure a robust product based on a-IGZO TFTs, it is es- sential to evaluate their electrical stability. The long-term con- stant current- temperature stress (CTS) study has shown that the a-IGZO TFT has a stable electrical properties with a threshold voltage shift much smaller (0.2 V) than the for a-Si:H TFT V under the same AM-OLED testing con- dition (3 A, 60 C, 20 hours) [14]. The proper passivation layer for the back channel was also found to play an important role in improving the TFT CTS reliability [8]. Recent studies further extended investigation of a-IGZO stability into the bias-temper- ature stress (BTS) measurements. A positive shift in TFT was observed under a positive (gate bias) BTS while the shifted to negative values for negative BTS [15]. Furthermore, a simple power law relation is able to fit the stress time trend of measured [16]. A great reduction in BTS in- duced degradation was found when additional post thermal an- nealing step during fabrication is performed. Although the na- ture of this reduction is not clear, the defect states located in bulk active layer or near the interface are suspected to be of its origin [15]. Despite all the progress in a-IGZO TFTs so far, our knowl- edge of the physical origin of its electrical stability is very lim- ited, and should be addressed more in-depth. In this paper, we report the detailed study of the initial electrical properties and BTS induced electrical instability of RF sputter a-IGZO TFTs. Proper simulation model is also proposed to describe experi- mental data. II. EXPERIMENTAL A. TFT Structure The cross-sectional view of the a-IGZO TFT used in this study is shown in Fig. 1(a). The detail processing steps were discussed elsewhere [11]. The TFT has an inverted-staggered bottom-gate structure. The Ti/Au/Ti stacking layers are used for gate and source/drain (S/D) electrodes. The SiO gate insulator and a-IGZO active layer is about 200 and 30 nm thick, respec- tively. Both layers are deposited by RF magnetron sputtering. After the island formation and S/D electrodes definition, an ad- ditional sputter SiO capping layer is added which serves as a 1551-319X/$26.00 © 2009 IEEE Authorized licensed use limited to: University of Michigan Library. Downloaded on February 17,2010 at 10:07:44 EST from IEEE Xplore. Restrictions apply.