DC sputter deposition of amorphous indium–gallium–zinc–oxide (a-IGZO) films with

DC sputter deposition of amorphous indium –gallium –zinc –oxide (a-IGZO)films with H 2O introduction

Takafumi Aoi a ,Nobuto Oka a ,Yasushi Sato a ,Ryo Hayashi b ,Hideya Kumomi b ,Yuzo Shigesato a ,⁎

a Graduate School of Science and Engineering,Aoyama Gakuin University,5-10-1,Fuchinobe,Sagamihara,Kanagawa 229-8558,Japan b

Canon Inc.,3-30-2Shimomaruko,Ohta-ku,Tokyo 146-8501,Japan

a b s t r a c t

a r t i c l e i n f o Available online 13October 2009

Keywords:

Amorphous indium gallium zinc oxide

Transparent amorphous semiconductor oxide H 2O

dc magnetron sputtering Thin film transistor

Amorphous indium –gallium –zinc –oxide (a-IGZO)films were deposited by dc magnetron sputtering with H 2O introduction and how the H 2O partial pressure (P H 2O )during the deposition affects the electrical properties of the films was investigated in detail.Resistivity of the a-IGZO films increased dramatically to over 2×105Ωcm with increasing P H 2O to 2.7×10−2Pa while the hydrogen concentration in the films increased to 2.0×1021cm −3.TFTs using a-IGZO channels deposited under P H 2O at 1.6–8.6×10−2Pa exhibited a field-effect mobility of 1.4–3.0cm 2/Vs,subthreshold swing of 1.0–1.6V/decade and on –off current ratio of 3.9×107–1.0×108.

©2009Elsevier B.V.All rights reserved.

1.Introduction

Transparent amorphous oxide semiconductors (TAOSs)have attracted much attention as high performance channel materials for thin film transistors (TFTs)[1–4].TAOSs can be fabricated on plastic substrates at low temperature by physical vapor deposition methods such as the conventional dc sputtering method.In particular,the carrier density of amorphous indium gallium zinc oxide (a-IGZO)can be precisely controlled at very low levels,which is stable under various conditions.Thus,a-IGZO has been the strongest candidate for the semiconductor layer in TFTs.Yabuta,et al.have demonstrated high-performance TFTs using a-IGZO films deposited by rf sputtering on flexible plastic substrates [5].It is well known that 10−3–10−4Pa of H 2O vapor exists in the sputter chambers normally used in the industry and the precise control of P H 2O should be important in optimizing the electrical properties and surface morphologies of transparent conducting oxide (TCO)or TAOS films.We have previously reported that the electrical properties of ZnO-based TCO films deposited by sputtering were strongly affected by the H 2O partial pressure (P H 2O )of the residual gas in the vacuum chamber [6].On the other hands,there have been attempts to use industrially applicable H 2O as a reactive gas to improve the electrical properties or etching rate of Sn-doped In 2O 3(ITO)films [7–9].Therefore,it must be important to investigate the effects of H 2O introduction into the deposition processes to yield a-IGZO films with the electrical properties appropriate for TFT applications.In this study we

investigated the effects of P H 2O on the electrical properties of a-IGZO films and on the TFT's device characteristics.2.Experimental details

2.1.Deposition of the a-IGZO films by dc sputtering with H 2O introduction 200-nm-thick a-IGZO films were deposited on fused silica glass and alkali-free glass (Corning #1737)substrates by dc magnetron sputtering using a polycrystalline InGaZnO 4ceramic sputtering target (Toshima MFG Co.,Ltd.).The sputtering gas was a mixture of Ar and H 2O vapor,whose total pressure was maintained at 0.5Pa.These depositions were carried out under various P H 2O from 8.0×10−4to 6.0×10−2Pa.As illustrated in Fig.1,the P H 2O during sputter deposition of a-IGZO films was precisely controlled using a precision needle valve and quadrupole mass spectrometer (QMS),where P H 2O can be monitored quantitatively.The distance between the substrate and target was 60mm,and the sputtering power was kept at 50W.2.2.Fabricating TFTs using a-IGZO films

Bottom gate,bottom contact-type TFT devices were fabricated on alkali-free glass (Corning #1737)substrates.As shown in Fig.2,the TFT is composed of a gate electrode of Mo film (thickness:100nm),a gate insulator of SiO 2film (thickness:200nm),a channel layer of a-IGZO film (thickness:30nm),source and drain electrodes of an ITO film (thickness:30nm)and a channel protection layer of SiO 2film (thickness:200nm).The channel width and length were 180μm and 30μm,respectively.The a-IGZO channel layer was deposited by dc magnetron sputtering with H 2O introduction,where the P H 2O was

Thin Solid Films 518(2010)3004–3007

⁎Corresponding author.Tel.:+81427596223;fax:+81427596493.E-mail address:yuzo@chem.aoyama.ac.jp (Y.

Shigesato).0040-6090/$–see front matter ©2009Elsevier B.V.All rights reserved.doi:

10.1016/j.tsf.2009.09.176

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