Remote plasma enhanced atomic layer deposition of ZnO for thin film electronic applications S.M. Sultan a,⇑ , O.D. Clark a , T.B. Masaud a , Q. Fang b , R. Gunn b , M.M.A. Hakim a , K. Sun a , P. Ashburn a , H.M.H. Chong a a Nano Research Group, University of Southampton, Hampshire SO17 1BJ, UK b Oxford Instruments Plasma Technology, North End, Yatton, Bristol BS49 4AP, UK article info Article history: Available online 28 May 2012 Keywords: Remote plasma Atomic layer deposition (ALD) ZnO Thin film transistor abstract This paper describes a systematic approach to analyze the simultaneous impact of various reactant plasma parameters of remote plasma enhanced ALD (PEALD) on the ZnO thin film properties. Particular emphasis is placed on the film stoichiometry which affects the electrical properties of the thin film. Design of Experiment (DOE) is used to study the impact of the oxygen plasma parameters such as the RF power, pressure and plasma time to realize semiconductor quality of ZnO thin film. Based on the opti- mized plasma condition, staggered bottom-gate TFTs were fabricated and its electrical characteristics were measured. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide is a metal-oxide semiconductor with considerable potential for applications in electronics, optoelectronics and sen- sors due to its wide and direct bandgap of 3.37 eV, large excitonic binding energy of 60 meV and its low cost [1,2]. There have been many reports on the deposition of ZnO using pulsed laser deposi- tion (PLD) [3], molecular beam epitaxy (MBE) [4] and chemical vapor deposition (CVD) [5]. However, these depositions require high temperatures and it is difficult to perform large area deposi- tion on low cost substrates. Recent attention has been focused on conventional ALD as the preferred growth method due to the low growth temperature, good crystallinity and good control of thick- ness, composition and uniformity [6–10]. ZnO thin film transistors fabricated with ALD ZnO have shown excellent electrical character- istics [6–8]. In the ALD process, diethyl zinc, DEZ and water are normally used as metal precursor and reactant, respectively. This is called thermal ALD and has been extensively studied and can be consid- ered as a model system for ALD [6–10]. Recently, plasma assisted ALD method has been explored [11–20] and it is found that the use of plasma species as reactants allows more freedom in process- ing conditions and wider range of material properties compared with the conventional thermal ALD. The use of plasma also signif- icantly reduced the OH impurity which affect the conductivity of the semiconductor film and induce defects in dielectric materials [16]. In the advent of this, remote PEALD has been widely reported to form high quality dielectric films, particularly for high-k dielec- tric materials [18–20]. Very few have reported using remote PEALD for semiconductor layer especially ZnO film [11,12]. In this work, we study the remote PEALD process to deposit semiconducting and high quality ZnO layer for electronic device applications. Various plasma parameters were varied and their effects on ZnO film properties were investigated. 2. Experimental methods 2.1. Remote PEALD Generally, ALD deposition technique on metal oxide is a cyclic and based on two self-limiting reactions: (1) metallization and (2) oxidation. These reactions are separately executed on a sub- strate surface at fixed temperature of 150 °C. DEZ (Zn (C 2 H 5 ) 2 ) is used as the Zn metal precursor. During metallization cycle, DEZ is injected into the reactor at 50 ms and Zn (C 2 H 5 ) 2 is chemisorbed to the substrate surface. After reaching saturation, residual Zn (C 2 H 5 ) 2 and reaction products are removed by Ar purge. Subse- quently, the –C 2 H 5 ligands of the chemisorbed Zn (C 2 H 5 ) 2 species are removed by a reaction with oxidant in the oxidation step. In this work, oxygen radicals were created by inductively coupled plasma source which is generated outside the deposition chamber. The O 2 flow was kept constant at 60 sccm during this cycle. To en- sure only one reactant is present in the ALD chamber at a given time, oxygen is purged using Ar for 4 s before the next DEZ injec- tion into the chamber. This constitutes one cycle and the substrate surface is ideally left with one monolayer of ZnO. Remote PEALD combines a high reactivity with low ion energy, typically below 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.04.019 ⇑ Corresponding author. E-mail address: sms08r@ecs.soton.ac.uk (S.M. Sultan). Microelectronic Engineering 97 (2012) 162–165 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee