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Effect of excimer laser annealing on a-InGaZnO thin-film transistors passivated by solutionprocessed hybrid passivation layers
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 J. Phys. D: Appl. Phys. 49 035102 (http://iopscience.iop.org/0022-3727/49/3/035102) View the table of contents for this issue, or go to the journal homepage for more
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Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 49 (2016) 035102 (7pp)
doi:10.1088/0022-3727/49/3/035102
Effect of excimer laser annealing on a-InGaZnO thin-film transistors passivated by solution-processed hybrid passivation layers Juan Paolo Bermundo1, Yasuaki Ishikawa1, Mami N Fujii1, Toshiaki Nonaka2, Ryoichi Ishihara3, Hiroshi Ikenoue4 and Yukiharu Uraoka1 1
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan 2 Merck Performance Materials G.K. Arco Tower 11F, 1-8-1 Shimomeguro, Meguro-ku, Tokyo 153–8605, Japan 3 Faculty of Electrical Engineering, Mathematics and Computer Science (EEMCS), Delft University of Technology, Feldmannweg 17, 2628CT Delft, The Netherlands 4 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan E-mail:
[email protected] Received 7 September 2015, revised 24 November 2015 Accepted for publication 25 November 2015 Published 22 December 2015 Abstract
We demonstrate the use of excimer laser annealing (ELA) as a low temperature annealing alternative to anneal amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) passivated by a solution-processed hybrid passivation layer. Usually, a-IGZO is annealed using thermal annealing at high temperatures of up to 400 °C. As an alternative to high temperature thermal annealing, two types of ELA, XeCl (308 nm) and KrF (248 nm) ELA, are introduced. Both ELA types enhanced the electrical characteristics of a-IGZO TFTs leading to a mobility improvement of ~13 cm2 V−1 s−1 and small threshold voltage which varied from ~0–3 V. Furthermore, two-dimensional heat simulation using COMSOL Multiphysics was used to identify possible degradation sites, analyse laser heat localization, and confirm that the substrate temperature is below 50 °C. The two-dimensional heat simulation showed that the substrate temperature remained at very low temperatures, less than 30 °C, during ELA. This implies that any flexible material can be used as the substrate. These results demonstrate the large potential of ELA as a low temperature annealing alternative for already-passivated a-IGZO TFTs. Keywords: excimer laser annealing, amorphous InGaZnO thin-film transistors, hybrid passivation, solution process (Some figures may appear in colour only in the online journal)
1. Introduction
transparent flexible displays and electronics [1–7]. Because un-annealed a-IGZO thin-film transistors (TFTs) typically have inferior electrical characteristics and stability, annealing up to 400 °C is often necessary [8–11]. This high temperature annealing method, however, is not compatible with most
A desired feature of amorphous InGaZnO (a-IGZO) is its low temperature processability. Combined with its wide band gap, both features enable more advanced applications in 0022-3727/16/035102+7$33.00
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© 2016 IOP Publishing Ltd Printed in the UK
J P Bermundo et al
J. Phys. D: Appl. Phys. 49 (2016) 035102
flexible substrates and thus developing an annealing method with lower temperatures less than 50 °C, especially at the substrate, is required. Conventional annealing methods based on furnace annealing are much preferred over other methods because of their simplicity. However, their non-selective heating limits the type of substrates that can be used. In general, more flexible substrates become available as the process temperature is decreased. For instance, polyimide which is stable at maximum process temperatures of up to 350 °C, Polyetheretherketone (260 °C), Polyethylene naphtalate (200 °C), and Polyethylene terephthalate (150 °C) are flexible substrates compatible with a-IGZO at lower process temperatures [12]. Paper has also been demonstrated as a suitable substrate [3, 13] by further reducing the process temperature. Several methods to lower annealing temperature have been previously proposed by either using special precursors activated at lower temperatures of 230 °C [14] or by utilizing combustion synthesis [15, 16]. Although microwave annealing has been shown to further decrease the process temperature to 87 °C, reported mobilities (μ) were lower than 10 cm2 V−1 s−1 [17, 18]. Excimer laser annealing (ELA) has been demonstrated as another low temperature annealing alternative [19]. While ELA induces high temperatures at the impact site, this high temperature can be localized far from the substrate. This is achieved by tuning the fluence energy (E), using appropriate materials, and depositing buffer layers with sufficient thicknesses over the substrate. Thus, ELA is a popular annealing alternative because it induces a low local temperature below 50 °C at the substrate. Application of ELA on poly-Si TFTs [20–22] and a-Si:H [23] is prevalent. It has also been used to transform Si ink to poly-Si on paper [24]. Likewise, ELA has been successfully used in oxide semiconductors such as in annealing a-InZnO TFT [25] and a-IGZO TFTs [26–30] and in direct high resolution patterning of sol– gel a-IGZO thin films [31]. ELA at typical E of 150 mJ cm−2 induces temperatures of up to 1500 °C in a-IGZO while maintaining low substrate temperatures below 50 °C [27]. Although it is well known that passivation layers are mandatory to protect the sensitive a-IGZO backchannel, previous reports employed ELA on unpassivated TFTs. In this study, two types of ELA with different wavelengths: XeCl or KrF ELA, were used to anneal a-IGZO TFTs passivated with polysilsesquioxane (PSQ). The effects of both types of ELA on the properties and electrical characteristics of a-IGZO TFTs passivated with PSQ were examined. Additionally, as understanding the laser-induced heating phenomenon during ELA on a-IGZO TFTs is important, simulation was also performed to analyze the heating localization, identify possible degradation regions, and to determine if substrate temperature during ELA is below 50 °C.
Table 1. Summary of PSQ passivation materials used. m and n
denote the mol% of the phenyl and methyl groups, respectively. Sample
Siloxane
Me (n)
Ph (m)
Me 100 Me 60/Ph 40
Methylsilsesquioxane Copolymer of Methylsilsesquioxane/ phenylsilsesquioxane
100 60
0 40
gate insulator, respectively. A 70 nm thick a-IGZO layer was deposited by radio frequency (RF) magnetron sputtering at room temperature. The 70 nm thickness is important since this is the estimated penetration depth of the excimer laser in a-IGZO [29]. The a-IGZO channel was then patterned by photolithography and etched to form a-IGZO islands. A stack of Mo and Pt (80 nm/20 nm) deposited by RF magnetron sputtering and patterned using the lift-off technique was used as source/drain electrodes. Contrary to the fabrication process for unpassivated TFTs we previously described in [32], 300 °C annealing for 2 h in N2/O2 ambient atmosphere was not performed on these unpassivated TFTs after source/drain electrode deposition. The a-IGZO TFTs were then passivated by PSQ passivation layer. PSQ passivation layers were used over conventional vacuum-processed passivation materials because PSQ passivation layers are fabricated by solution process and enhance a-IGZO TFT stability [32]. Their solution processability enables applications in printed roll-to-roll processes if these future options are explored. Table 1 summarizes the two PSQ passivation layers used in this study: Me 100 and Me 60/Ph 40. Both Me 100 and Me 60/Ph 40 follow the same straightforward fabrication process. PSQ was initially spincoated on a-IGZO TFT at 3000 rpm for 15 s. A 2-step heat treatment was then performed on a hot plate: first, the samples were pre-baked at 130 °C for 90 s to evaporate the solvent and finally, post-baked at 300 °C for 1 h to cure the PSQ. Contact holes were then formed by reactive ion etching using a CF4/ O2/Ar gas mixture. Unlike the standard process we described in [32], these TFTs were not subjected to the usual O2 postannealing at 300 °C for 2 h after the dry etching process. 2.2. ELA
Me 100 passivated a-IGZO TFTs were subjected to a single shot of XeCl ELA at a set E under N2 atmosphere at room temperature. The wavelength and pulse width at full width half maximum were 308 nm and 25 ns, respectively. The beam width/length (Wb/Lb) is 0.1/0.1 cm ensuring that the beam size is much larger than a single TFT (maximum TFT dimension is ~100 μm). The inset of figure 1 illustrates the ELA of PSQpassivated a-IGZO TFT. Note that Me 100 is completely transparent at 308 nm, confirming that the excimer laser passes through it to completely irradiate and anneal a-IGZO. A single shot of a lower wavelength (248 nm), higher energy excimer laser, KrF ELA, was used for Me 60/Ph 40 passivated a-IGZO TFTs. The KrF ELA beam Wb/Lb (690/262 μm) is also larger than the size of a single TFT. Me 60/Ph 40 is almost completely transparent at 248 nm with 84%
2. Experimental procedure 2.1. Sample fabrication
Highly conductive n-type Si (resistivity
