Laser Annealing of Transparent semi-Conductive Oxides: A platform towards flexible
and large area processing for transparent TFT devices
Spilios Dellis
1
, Salem Elhamali
1
, Peter Downs
2
, Nikolaos Kalfagiannis
1
, Catherine
Ramsdale
2
, Richard Price
2
, Demosthenes Koutsogeorgis
1
*
1
School of Science and Technology, Nottingham Trent University, Nottingham, NG11 8NS,
United Kingdom
2
Pragmatic Printing ltd. CPI, Netpark, Thomas Wright Way, Sedgefield, TS21 3FG UK
E-mail:
.
Plastic and flexible electronics is the next big thing in electronics, making the
fabrication of high quality transparent semiconducting thin films on plastic substrates an
urgent need. Several studies on new materials that can replace Si, as well as new cost-
effective and scalable methods, have been presented. Recently, several studies have shown
that transparent amorphous IGZO thin films fabricated by physical vapour deposition
techniques is a promising alternative, as it presents higher mobility than Si and can be
deposited at low temperature. However, it is widely recognized that for the fabrication of
stable and high quality Thin Film Transistor (TFT) devices with a-IGZO a post-deposition
thermal annealing in temperatures higher than 300
o
C is required. This critical step remains a
drawback in the application of IGZO thin films as channel material in electronic devices on
plastic substrates that are very sensitive to high temperatures.
It is becoming evident a that new method of annealing, that can replace the heating
step in the fabrication process, needs to be developed in order to meet the criteria relevant to
the large area electronics field, on plastic substrates. Excimer laser annealing can be a of high
value in that context, due to its ultra-fast and macroscopically cold character in conjunction
with the ease of the laser beam manipulation and the high spatial resolution for selective
patterning/annealing that it offer.
In this work we investigate the application of KrF excimer (248 nm) laser annealing
(LA) for the quality improvement of TFT devices on PEN substrates. For this reason, top-
gate-staggered TFT devices with 20, 30 and 40 nm thick RF magnetron sputtered a-IGZO
active layer were fabricated. During the fabrication process of the TFT devices the substrate
temperature never exceeded 60
o
C, a temperature well below the limit that can be easily
tolerated by the PEN plastic substrate. The results of this study show that single-pulse laser
annealing post treatment of the sputtered a-IGZO at energy densities ranged between 15 and
60 mJ/cm
2
provided TFT of improved performance. Specifically, the I
on
to I
off
ratio of TFTs
treated with optimized LA conditions presents an increase of up to one order of magnitude
and their sub-threshold swing presents an up to 40% decrease. This improvement in the TFTs
transfer characteristics is attributed to the reduction of the resistivity of the channel layer (a-
IGZO) as well as to the reduction of the contact resistance between IGZO and the
source/drain electrodes resulting from the LA.
Overall, LA at the reported energy density in this work is very promising for TFTs
fabrication on heat sensitive plastic substrates. Moreover, since the required laser energy to
enhance IGZO TFTs characteristics is rather low, a high throughput of IGZO-TFTs could be
realised, making laser annealing a promising technique for large volume production of large
area electronics on flexible substrates
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