Reliability of Al2O3/In-Si-O-C Thin-Film Transistors with an Al2O3 Passivation Layer under Gate-Bias Stress
Authors: Kurishima Kazunori , Nabatame Toshihide , Onaya Takashi , Tsukagoshi Kazuhito , Ohi Akihiko , Ikeda Naoki , Nagata Takahiro , Ogura Atsushi 
Published: February 2020

Introduction

Bottom-gate-type thin-film transistors (TFTs) have an advantage of active channel protection because the In-Si-O-C channel can be formed in the final fabrication process. However, it still remains a big issue of poor stability because the back-side surface of the In-Si-O-C channel is directly exposed to the environmental conditions, and some charges can easily occur due to the adsorption and desorption of gaseous species in air. To overcome this problem, the formation of a passivation layer is one promising process to protect the In-Si-O-C channel. Various inorganic materials, such as SiO2, Si3N4, and Al2O3, have been characterized as passivation layer. Al2O3 layer deposited by atomic layer deposition (ALD) has exhibited excellent passivation capabilities for realizing the stable high-mobility oxide TFTs [1]. Al2O3 passivation layers, which deposited by sputtering methods or the ALD process, generally have thicker film of over 20 nm, and a post-deposition annealing process is sometimes performed to improve the quality of the Al2O3 layer [2, 3]. However, there is no report on the influences of growth temperature and thickness of the Al2O3 passivation layer on the reliability for In-Si-O-C TFTs have been extensively studied.
In this paper, we studied gate-bias stress instability of bottom-gate-type Al2O3/In-Si-O-C TFT with an Al2O3 passivation layer, which deposited by low-temperature ALD at 50 °C. We also compared transistor properties of Al2O3 gate insulator fabricated ALD and PE-ALD under gate-bias stress conditions.

Experiment

A 50-nm-thick Pt film as the gate electrode was deposited on Si substrate using an e-beam evaporator, followed by patterned by a photolithographic process. Next, a 30-nm-thick Al2O3 gate insulator was deposited on the Pt gate electrode by ALD and PE-ALD at 300 °C using trimethylaluminium precursor and H2O oxidant gas and plasma oxygen gas, respectively, subsequently patterned using a dry etching process. A 10-nm-thick In-Si-O-C channel was formed on the Al2O3 gate insulator through a stencil shadow mask at room temperature by co-sputtering using In2O3 and SiC targets under an Ar/O2 (11.0/1.0) atmosphere at 0.2 Pa. Post-deposition annealing was then performed at 300 °C for 60 min in air. An Au (100 nm)/Ti (10 nm) layer was deposited by the thermal evaporation method as the source and drain electrodes. Post-metal deposition annealing was performed at 250 °C for 10 min in O3. Finally, an Al2O3 passivation layer was covered on the back-side surface of the In-Si-O-C channel by ALD at 50 °C using TMA and H2O gases. The thickness of the Al2O3 passivation layer was varied to 2 nm (AlO-2), 5 nm (AlO-5), and 10 nm (AlO-10) by changing the number of ALD cycles.

Results and Discussion

Figure 1 shows a cross-sectional schematic of the fabricated bottom-gate-type Al2O3 insulator/In-Si-O-C channel TFT with an Al2O3 passivation layer. The channel length and width of the TFT used in this study were 350 μm and 1000 μm, respectively.

Figure
2 shows the threshold voltage shift (ΔV
th) behavior of all devices as a function of the Al2O3 passivation layer thickness under negative gate-bias stress (NBS) conditions. A significant negative V
th shift appeared with the w/o device. The negative V
th shift is generally recognized to cause hole charge trapping in the gate insulator and at the Al2O3 insulator/In-Si-O-C channel interface. The holes are thought to be due to absorbed H2O molecules on the back-side of In-Si-O-C channel. The ΔV
th of the AlO-2 and AlO-5 devices were slightly reduced compared to the w/o device. On the other hand, the ΔV
th of the AlO-10 device were exhibited negligible small value, indicating that the thickness of Al2O3 as a passivation layer is required to be 10 nm or more. Here, the Al2O3 passivation layer which contains residual carbon impurities has low quality because of low-temperature ALD at 50 °C [4]. However, noted that this Al2O3 film can effectively passivate the defects from absorbed H2O molecules. Low-temperature processable Al2O3 passivation layer can be expected in flexible substrate.

Conclusions

We investigated gate-bias stress instability of bottom-gate-type Al2O3/In-Si-O-C TFTs with the Al2O3 passivation layer, which deposited by low-temperature ALD at 50 °C. The AlO-10 device was exhibited negligible small ΔV
th, which has no influence of H2O molecules. We found that the low-temperature fabricated Al2O3 film is essential for suppressing influence of absorbed H2O molecules.

Acknowledgment

This work was supported by Grant-in-Aid for JSPS Fellows.

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Figure 1