Short CommunicationX-ray photoelectron spectroscopy analysis of the effect of photoresist passivation on InGaZnO thin-film transistors
Introduction
Metal-oxide thin-film transistors (MO TFTs) show a great potential for use in flat-panel displays (FPDs) such as active-matrix organic light-emitting diode (AMOLED) and active-matrix liquid crystal display (AMLCD) due to their high mobility and excellent uniformity [1], [2]. Especially, amorphous indium gallium zinc oxide (a-IGZO) has been recognized as an ideal channel material for TFTs requiring high mobility, high on/off ratio, and process availability at room temperature [3], [4], [5]. Though great process has been made in improving the performance of the IGZO TFTs during the past few years, the stability is still the biggest obstacle for the practical products.
It is known to all that oxide semiconductors are very sensitive to water [6], [7] or oxygen molecules [8], the adsorption/desorption effect on the back channel of active layer plays a key role in the device stability. Therefore, it is critical to reduce the interactions between the back-channel surface and ambient to attain high stable devices. Usually, there are two methods to hinder the interactions between the back-channel surface and ambient. The first strategy is to adopt the top-gate TFT [9] structure which the gate electrode and dielectric over the semiconductor play the role of passivation layer, protecting the semiconductor from the ambient. However, the electrical properties of the active channel of the top-gate TFTs will be seriously changed due to the ionic bombardment during the gate insulator formation. Verma et al. [10] made a comparison between bottom-gate and top-gate TFTs. The results showed that the top-gate TFTs exhibited more obvious Vth shift than the bottom-gate TFTs, because the former was subjected to higher energy bombardment than that in the latter. Chang et al. [11] reported top-gate IGZO TFTs with a pronounced threshold voltage shift (ΔVth) under positive bias stress. Compared to tope-gate TFTs, the BCE structure is more common in the practical application due to the following two advantages: (i) the process of BCE structure TFTs is relatively simple and compatibility with amorphous silicon TFTs production line, so the fabrication cost is low; (ii) it is easy to get the minimum device size for BCE TFTs, which has a great potential in the high-resolution displays in future. However, for the BCE TFTs without passivation, the active channel will be exposed in the air and the absorption/desorption of water or oxygen molecules on the back channel will easily happen, resulting in the poor stability. To solve this problem, it is necessary to prepare passivation on the active channel for BCE TFTs. Therefore, the second strategy is to fabricate a passivation layer on the back channel for BCE TFTs. The most common used passivation materials include inorganic materials such as SiO2 [12], [13], [14], Al2O3 [9], [15], Y2O3 [16], [17], TiO2 [18] Ga2O3 [19] and so on. Usually, the inorganic passivation exhibits excellent ability of protecting the active layer from the atmosphere (H2O or O2). But the oxide semiconductor active layer underneath is easily affected by process chemicals or by plasma exposure during the deposition of passivation layer, degrading severely the device properties. Compared to inorganic passivation, organic polymers (e.g., PDMS [20], CYTOP [21], parylene [22]) can act as an effective active channel protection without damaging device characteristics. However, organic polymers have relatively poor ability to prevent water or oxygen molecules. In addition, an improper passivation such as those polymers with hydroxyl groups (e.g., PVP [23] or PVA [24]) could also create interface states at the back surface, degrading severely the device performance. Despite the relatively poor ability to cut-off the atmosphere, the choice of suitable organic passivation will make a huge improvement on the device characteristics. Furthermore, organic passivation have a great potential on the development of flexible devices due to the low temperature process and good flexibility.
In this contribution, a-IGZO TFTs based on BCE structure were passivated by EOC photoresist (DL-1000 from TORAY, hereafter referred to PR) with a solution-based approach. The effect of PR-passivation on a-IGZO TFTs was investigated by x-ray photoelectron spectroscopy (XPS) and PR-passivation was proved to eliminate the ambient influence as well as change the element distribution of IGZO thin film for improving device performance. Furthermore, the high quality PR-passivation with excellent barrier ability against water and oxygen molecules can be obtained easily at a low temperature (100 °C) and has no influence on the intrinsic device characteristics, showing a great potential on the development of flexible electronics.
Section snippets
Experimental section
The schematic structure of the bottom-gate IGZO-TFTs with and without PR passivation layer are shown in Fig. 1. A layer of 300-nm-thick Al–Nd alloy was deposited on a glass substrate by DC magnetron sputtering, followed by an anodization process to produce a layer of Nd: Al2O3 as the gate dielectric layer [25], [26]. The IGZO active layer (In:Ga:Zn = 1:1:1) with a thickness of 30 nm was deposited on Nd: Al2O3 by RF magnetron sputtering in pure argon atmosphere with a flow rate of 25 sccm under
Results and discussion
Fig. 2 shows the transfer curves for IGZO-TFTs with and without PR passivation layer. And the corresponding properties were summarized in Table 1. For the unpassivated IGZO-TFT, a significant hysteresis about 2.42 V can be observed in ID-VGS characteristics of forward and reverse bias sweepings. For the PR-passivated TFT, typical FET behaviors were observed with suppressed hysteresis of 0.06 V. In addition, the PR-passivated TFT exhibited a higher saturation mobility (μsat) of 8.85 cm2 V−1 s−1,
Conclusions
In conclusion, the effect of PR passivation on the performance of IGZO-TFT was investigated by XPS, systemically. The PR-passivated IGZO-TFT showed a higher μsat of 8.85 cm2 V−1 s−1, a lower SS of 0.17 V, a lower hysteresis of 0.06 V and a higher Ion/Ioff of > 106, compared to the unpassivated TFT with a μsat of 6.71 V, a SS of 0.24 V, a hysteresis of 2.42 V and a Ion/off of 105. Furthermore, the PR-passivated IGZO TFT showed good electrical stability under positive gate bias stress with a
Acknowledgments
This research was funded by the financial support from the Scientific Research Starting Foundation of Foshan University (Grant Nos. Gg040926, 040973, 040928), the National Natural Science Foundation of China (Grant No. 61804029, 61471123, 61704027, 61575041), the Guangdong Natural Science Foundation (No. 2018A030310353, 2015A030313639), and Foshan Science and Technology Innovation Special Funds (Grant No. 2017EZ100111).
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