Effect of plasma treatment on the characteristics of thin film transistors with
zirconium-aluminum-zinc oxide active channels
JuHee Park, Sang-Hyuk Lee, Hyun-Sik Jun, Won Kim, and Jin-Seok Park
Department of Electronic Systems Engineering, Hanyang University at ERICA Campus
1271, Sa 3-dong, Sangnok-gu, Ansan, Gyeonggi-do 426-791, Republic of Korea
E-mail:
Thin film transistors (TFTs) using amorphous oxide semiconductors (AOSs) have recently
been studied as alternatives for conventional silicon-based TFTs because of good mobility,
low temperature process, and high transparency. Among the AOS-based TFTs, the In-Ga-Zn-
O (IGZO)-based TFT has been announced as the leading selection because of its high
performances including excellent sub-threshold swing (< 0.4 V decade
-1
), high mobility (> 10
cm
2
/Vs), and high on/off current ratio (> 10
7
). The indium (In) element used in the IGZO-
TFT plays a role of supplying electron carriers. However, In is a rare element, resulting in a
high cost. Therefore, intensive studies have been conducted to determine suitable AOS
materials as substitutes for In, such as aluminum-zinc-oxide (AZO), zinc-tin-oxide (ZTO),
and silicon-zinc-oxide (SZO). Aluminum (Al) in particular is a promising candidate for
achieving a low cost because Al is far more abundant than In. Also, gallium (Ga) is known as
a carrier suppressor in the IGZO TFT, but the weak Ga-O binding energy causes bias
instability; therefore, hafnium (Hf) and zirconium (Zr), which have strong binding energies
with oxygen, may be suitable materials to replace Ga.
In addition, various post-treatment processes such as furnace annealing, laser annealing,
rapid thermal annealing (RTA), and plasma treatment have been performed to improve the
characteristics of AOS-TFTs effectively. In particular, plasma treatment has an advantage in
that a low-temperature process with good uniformity is possible. Plasma treatment processes
using several gases such as argon (Ar), oxygen (O
2
), hydrogen (H
2
), nitrogen (N
2
) and mixed
gas have been performed. For instance, it is known that O
2
plasma treatment can improve
electrical performance and remove surface contaminants like carbon effectively. H
2
plasma
treatment is also an effective method in decreasing resistivity and subthreshold swing.
The purpose of this study is to improve the characteristics of interfaces between the gate
insulator and channel layers and between the channel and source/drain electrode layers in
TFTs using the channel layer of zirconium-aluminum-zinc-oxide (ZAZO), by employing
hydrogen (H
2
) and oxygen (O
2
) plasma treatment. O
2
plasma treatment was performed on the
thermally grown SiO
2
insulator. A 50 nm-thick ZAZO film as the channel layer was deposited
by RF magnetron co-sputtering of AZO (99.999% of purity) and ZrO
2
(99.999% of purity)
targets. Then, the source/drain pattern was formed via photolithography. A 50 nm-thick AZO
layer as the source/drain electrodes was deposited via RF sputtering, after which lift-off was
conducted. The post treatment using H
2
plasma treatment was performed on the interface
between the ZAZO channel and electrode layers.
The effects of O
2
and H
2
plasma treatment on the characteristics of the interfaces between
the channel and gate insulator layers as well as between the channel and source/drain contacts
were investigated. The visible-wavelength transmittance of ZAZO films was measured using
a UV-VIS spectroscope. Also, the changes in the microstructures of the ZAZO films were
analyzed using x-ray diffraction (XRD) and the changes in the chemical bonds of those films
were analyzed using X-ray photoelectron spectroscopy (XPS). Additionally, the surface
analysis with depth profiling was performed using Auger electron spectroscopy (AES).
Finally, the transfer characteristics of the ZAZO-TFTs were measured using a semiconductor
parameter analyzer (Keithley 4200) and their changes due to plasma treatment were also
examined. The experimental results obtained from this study indicated that the device
characteristics of ZAZO-TFTs could be effectively improved by plasma treatment.
PS1 32
-188-
