Elsevier

Microelectronics Reliability

Volumes 76–77, September 2017, Pages 333-337
Microelectronics Reliability

Reliability of amorphous InGaZnO TFTs with ITO local conducting buried layer for BEOL power transistors

https://doi.org/10.1016/j.microrel.2017.06.023Get rights and content

Highlights

  • Investigation on reliability of a-InGaZnO transistors with ITO conducting buried layer for BEOL transistor

  • Threshold voltage shifts, the drain current degradation, and breakdown voltage have been measured with ITO LCBL thickness and distance.

  • The devices with tick ITO and short ITO distance are desirable for a power device for High/Low type I/O bridges.

  • The devices with thin ITO and long ITO distance are desirable for Low/High type I/O bridges.

Abstract

The device reliability of a-IGZOTFTs with ITO local conducting buried layer (LCBL) has been investigated under positive gate bias stress and hot carrier stress for the application as BEOL power transistors. The drive current of a-IGZO TFTs could be controlled by the modulation of ITO LCBL thickness and distance under source/drain electrode. The threshold voltage shifts, the drain current degradation, and breakdown voltage have been measured and discussed according to the different ITO LCBL thickness and distance. The devices with thick ITO and short ITO distance are desirable for a power device for High/Low type I/O bridges. The devices with thin ITO and long ITO distance are desirable for Low/High type I/O bridges. The breakdown voltages are decreased with the increase of ITO thickness.

Introduction

Mixed signal LSI (Large Scale Integrated circuits) which is widely used for power management has progressed rapidly from discrete devices to integrated LSI and evolved into true system LSI. Power system is generally consisted of power management integrated circuits, gate diver circuitry, power transistors, and etc. However, the low breakdown voltage and low drive current delayed the on-chip integration. A BEOL (Back End Of Line) transistor is expected to be used as an embedded power transistor instead of BiC-DMOS devices in Mixed signal LSI [1]. Due to a few additional masks and a cost competitive, BEOL transistor can be used for the voltage management devices on advanced system LSI. Currently, amorphous InGaZnO thin film transistors (a-IGZOTFTs) have been attracting attention for a BEOL transistor due to high breakdown voltage and low temperature process [2], [3], [4], [5], [6]. One of the advantages is the fact that Cu metal interconnection line can be used as the gate electrode for BEOL transistor with embedded bottom-gate a-IGZO TFTs.

The voltage management devices can control a high input voltage from a load to a low voltage for core logic IC (High/Low type) and vice versa (Low/High type). In voltage management devices, high VGS and low VDS are applied to High/Low type, and low VGS and high VDS are applied for Low/High type. Therefore, a-IGZO TFTs have been degraded under positive gate bias stress (PBS) for High/Low type operation and under hot carrier (HC) stress for Low/High type operation. Due to the electron trapped charges under PBS and HC stress, the threshold voltage shifts positively and the drain current decreases with the increase of the stress time. The device instability and relatively low drive current hinder the implement of a-IGZO TFTs as BEOL transistors. In order to improve the drive current in oxide semiconductor TFTs, the comb-type device structure and bilayer oxide devices have been suggested including oxygen controlled process [3], [4]. The studies on gate-drain offset device structure and high-k dielectric materials have been reported to increase the breakdown voltage [4], [7].

Currently, the experimental works on the device characterization of a-IGZO TFTs with ITO (Indium Thin Oxide) local conducting buried layer (LCBL) under both source/drain regions and in the middle of channel region have been reported to enhance the electron mobility and resulted in high drive current [8], [9], [10]. The introducing optimized thin ITO layer between the gate insulator and active channel in a-IGZO TFT has been suggested to reduce the device instability [11]. Since one can control the drive current by modulating the length of ITO LCBL without changing the ratio of channel width and length or modifying the device structures, a-IGZO TFT with ITO LCBL is expected to be one of the most promising candidate for BEOL transistor in power management system.

In this work, the device reliabilities on a-IGZO TFTs with ITO LCBL according to different ITO thickness and distance have been investigated for the application of BEOL transistors as power management integrated circuit components. The device degradations have been characterized under PBS and HC stress, and the device breakdown characteristics were discussed.

Section snippets

Device fabrication

The schematic diagram of fabricated bottom-gate a-IGZO TFTs with LCBL has shown in Fig. 1. The devices have been fabricated using a p++-type (100) silicon wafer substrate grown on thermal oxide (SiO2) having a resistivity of 10 Ω cm. After RCA cleaning, the thickness of ITO (TITO) ranged from 10 nm to 40 nm for a-IGZO TFTs with LCBL was deposited to form LCBL using the RF magnetron sputtering method (Ar flow of 20 sccm, working pressure of 3 mTorr). According to the different space distance (DITO)

Device degradation under PBS

From the transfer curves as shown in Fig. 2, the drive currents of a-IGZO TFTs with ITO LCBL are increased with the increase of TITO and the decrease of DITO. This can be attributed to the decrease of overall series resistance by enhancing the conductivity of the inner channel layer. The device performance parameters were summarized in Table 1. The measured VTH was defined as the VGS required to reach a drain current of 2.5 μA  L / W[μA] at VDS = 2.0 V, and the field effect mobility (μFE) was

Conclusion

With increasing ITO thickness and decreasing of ITO distance, the effective mobility and drive current have been enhanced. Therefore, one can control the drive current of a-IGZO TFTs with modulation of ITO thickness and ITO distance under source/drain electrode without changing the ratio of channel width to channel length or modification of device structures and processing. The thick ITO and short ITO LCBL distance can be effective in reducing device degradation under PBS and such device is

Acknowledgement

This work was supported by the Incheon National University Research Grant in 2017.

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