Reliability of amorphous InGaZnO TFTs with ITO local conducting buried layer for BEOL power transistors
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.
References (15)
- et al.
Effects of enhanced-mobility current path on the mobility of AOS TFT
Microelectron. Reliab.
(2012) - et al.
Past and future technology for mixed signal LSI
IEICE Trans. Electron.
(2014) - et al.
A novel BEOL transistor(BETr) with InGaZnO embedded in Cu interconnects for on-chip high voltage I/O in standard CMOS LSIs
- et al.
High performance bilayer oxide transistor or gate driver circuitry implemented on power electronic devices
- et al.
Operation of functional circuit elements using BEOL transistor with InGaZnO channel for on-chip high/low voltage bridging I/O and high current switches
- et al.
High reliable BEOL-transistor with oxygen-controlled InGaZnO and gate/drain offset design for high/low voltage bridging I/O operations
- et al.
High On/Off ratio P-type oxide based transistors integrated onto Cu interconnects for on-chip high/low voltage bridging BEOL CMOS I/O
Cited by (8)
Lifetime prediction of InGaZnO thin film transistor for the application of display device and BEOL-transistors
2018, Solid-State ElectronicsCitation Excerpt :It is interesting that the β of Devices C and D are larger than those of Devices A and B. From the time dependence of ΔIDS, we extracted the device lifetime (τB) for the application in BEOL transistors, in which IDS drops 10%. In this work, a study on the channel length dependent bias-stability was not performed, it had already been known that the device degradations in short channel devices under PBS were less significant compared to long channel devices [22]. Since Low/High type I/O in power management controls a low input voltage from a core logic IC to a high voltage load, the device degradations were investigated under high stress VDS ranged from 10 V to 18 V and low stress VGS = 3 V.
Polycrystalline GeSn Films Grown by Hot Wire Chemical Vapor Deposition on SiO<inf>2</inf>/Si(001) Substrates
2024, Physica Status Solidi - Rapid Research LettersEffect of Electron Beam Annealing Duration on the Properties of Zno Thin Films
2019, IOP Conference Series: Materials Science and EngineeringCarrier and heat transport properties of poly-crystalline GeSn films for thin-film transistor applications
2019, Journal of Applied PhysicsEmerging applications using metal-oxide semiconductor thin-film devices
2019, Japanese Journal of Applied Physics