This document summarizes a metal oxide TFT manufacturing process that can be used on existing amorphous silicon (a-Si) TFT lines with minimal equipment upgrades. The process uses the same silicon nitride gate insulator deposition and wet back channel etch steps as a-Si TFT production. It deposits a proprietary metal oxide semiconductor channel layer by sputtering without oxygen, enabling high mobility and uniformity. The metal oxide TFTs exhibit excellent stability, meeting requirements for next-generation LCD and OLED displays. They can be manufactured on a-Si TFT lines at low cost, providing a competitive advantage over incumbent a-Si and poly-Si TFT technologies.

1. Metal Oxide TFT Turnkey Manufacturing Solutions for a-Si TFT Lines
Tian Xiao, Gang Yu, Chan-Long Shieh, Jung-Woo Park, Fatt Foong, Karman Lee,
Juergen Musolf, Guangming Wang, Kristoffer Ottosson, Kaixia Yang, Jeff Wang,
Bruce Berkoff and Boo Nilsson
CBRITE Inc., Goleta, California, USA
Contact Author E-mail: tianxiao@cbriteinc.com
Abstract
There is strong incentive to upgrade a-Si TFT lines with
minimal investments to enable high-end display manufacturing.
We developed an oxide TFT manufacturing process which keeps
the high-throughput SiNx GI and wet BCE process used in a-Si
TFT lines, enabling high resolution LCD and OLED display
manufacturing with minimal equipment upgrade and
production costs.
Author Keywords
Oxide TFT; Back channel etch (BCE); Silicon nitride GI; High
mobility and stability; LCD/OLED/LED display
1. Introduction
With smartphones, tablets and on-board displays driving growth
in small and medium displays and 4K TVs driving growth in
large displays, competition in the display industry is becoming
increasingly fierce with market dynamics clearly favoring
displays that offer higher resolution and lower power
consumption at an affordable price. Despite of high expectation
for metal oxide TFT to become a winner in the competition, due
to its inherent advantage in large-area uniformity and high
mobility, which should translate to high performance/price ratio,
actual adoption of oxide TFT in the display industry has been
slower than expected. Part of the reason is that companies with
high-generation a-Si TFT lines are reluctant to make significant
investments to upgrade or re-balance their production tools to
allow for the use of different materials or process flow which are
required to manufacture IGZO TFT. For example, SiNx gate
insulator (GI) used in a-Si TFT lines has to be replaced by SiO2
gate insulator to enable IGZO TFT manufacturing, which not
only adds re-tooling costs and lowers productivity due to much
slower deposition process for PECVD-based SiO2, but also
introduces inconsistency and yield issues associated with
hydrophilic and porous SiO2 GI. When hydrogen-rich SiNx GI
is used, diffusion of hydrogen into IGZO channel causes serious
Vth shift or even channel shorting[1]
. Even if IGZO TFT on
SiNx GI can be made to work in the enhancement mode, it
usually exhibits unacceptably large negative Vth shift under
negative bias illumination stress[2]
. Switching to SiO2 GI, or at
least using SiO2/SiNx double GI with SiO2 in contact with
IGZO appears to be the industry solution at this moment.
However, this approach takes a serious toll on the yield and
consistency of IGZO TFT devices: SiO2 layer is generally more
hydrophilic than SiNx layer and easily attracts moisture at
humidity levels greater than 50%RH, severely degrading IGZO
TFT device characteristics resulting in very negative Vth,
accompanied by huge hysteresis and kinks in transfer curves[3]
.
This could easily become a killer yield issue in TFT fabs where
humidity levels are maintained at greater than 50%RH. In
addition, PECVD SiO2 GI is more porous than SiNx GI, and if
used alone could cause TFT channel to short due to diffusion of
copper from bottom gate into the IGZO channel[1]
.
To overcome this dilemma, CBRITE has developed a robust
metal oxide semiconductor material which is less sensitive to the
hydrogen-rich SiNx GI and demonstrated in SID’2015 that high-
throughput SiNx GI can be used in combination with organic
etch-stopper to manufacture high-performance metal oxide
TFT[4]
. This year, we went a step further by keeping the high-
throughput SiNx GI while eliminating the ES process, further
reducing production costs and improving display performance
(higher resolution and lower power consumption). By sticking to
PECVD SiNx as gate insulator, depositing channel layer with
sputtering tool originally set up for ITO, and keeping the high-
throughput wet BCE process, metal oxide TFT can be
manufactured on existing a-Si TFT lines with capacity
comparable to that for a-Si TFT, without the need for significant
capital investments to upgrade or re-balance the production
tools. Moreover, much higher performance than IGZO-based
TFT has been demonstrated, in terms of both mobility and
reliability, which exceeds the demanding specifications of next-
generation LCD and OLED display products. Thus, metal oxide
TFT turnkey manufacturing solutions for a-Si TFT lines can be
implemented at low costs, maximizing the competitive edge of
oxide TFT over the incumbent a-Si or poly-Si TFT technologies.
2. Experiments and Results
Inverted staggered BCE type TFT structure, as shown in Fig.1,
was used to fabricate CBRITE metal oxide TFT.
Figure 1. Structure of CBRITE Metal Oxide TFT
Table 1 summarizes the tools used to process each layer of
CBRITE metal oxide TFT, along with comparison with a-Si
TFT process. By using high mobility metal oxide
semiconductors with ionic X-O bonds reinforced by strong
covalent bonds formed between metals or metalloids and oxygen
(Y-O) as channel layer, stable localized X-O-Y structure is
formed, with the strong Y-O bonds stabilizing the X-O bonds
nearby, making the channel layer highly resistant to commercial
26-1 / T. Xiao Invited Paper
318 • SID 2016 DIGEST ISSN 0097-966X/16/4701-0318-$1.00 © 2016 SID

2. wet etchants such as PAN (mixture of phosphoric, acetic and
nitric acids) or copper etchants during S/D etching. At the same
time, the channel layer also becomes highly tolerant of hydrogen
Table 1. Tools Used to Process CBRITE Metal Oxide TFT
Fig.2. Transfer Curves and Vth during Heating in Pure N2
(BCE oxide TFT on SiNx GI with W=8µm and L=3µm)
diffusion from the hydrogen-rich SiNx GI during annealing.
Examples of element “X” include In, Ga, Zn, Cd etc., and
examples of element “Y” include B, Si, Ge and Al etc. Fig.2
shows the transfer curves of such BCE type metal oxide TFT on
SiNx GI (W=8µm and L=3µm) during heating from room
temperature to 140C in pure N2 (0%O2) environment.
Strong X-O-Y bonds in the channel layer make them highly
tolerant of high concentrations of hydrogen in PECVD SiNx GI
and help inhibit oxygen loss in an oxygen-free environment at
high temperatures, resulting in excellent temperature stability.
Note that 140C is only the heater limit for in-situ testing of
transfer curves on the probe station, and stable and positive Vth
in oxygen-free environment is expected at much higher
temperature judging from the Vth vs. temperature trend in Fig.2.
Also note that the off current is below the detection limit of the
test system up to 140C, with current on/off ratio greater than 109
at Vg=±15V.
In general, robustness of the channel layer material is
proportional to its concentration of covalent bond forming “Y”
elements, as evidenced by the rapidly slowing etching rate (e.g.,
in oxalic acid) with the increase of “Y” concentration. An
example of this is illustrated in Fig.3, where the “Y” element in
this case is aluminum. However, higher concentrations of “Y”
elements also tend to decrease the carrier density and mobility,
therefore a delicate balance needs to be made between the ionic
“X-O” bonds and the covalent “Y-O” bonds when designing a
channel composition with desired mobility and stability.
Fig.3. Etch Rate in Oxalic Acid (40°C) vs. Aluminum Oxide
Concentration in Metal Oxide Semiconductor
Fig.4 shows the mobility curves together with the transfer curves
for a high-mobility BCE type metal oxide TFT on SiNx GI with
W=8µm and L=3µm. With a sharp sub-threshold slope of
0.15V/Dec, current and mobility rises very rapidly with the gate
voltage. Fig.5 shows the 5-point Id-Vg uniformity on the same
sample, where the active layer thickness non-uniformity has
been determined to be larger than 25%. It can be seen that great
Vth uniformity can still be maintained (ΔVth<0.5V) despite of
such large variation in active layer thickness, suggesting great
process latitude in manufacturing environment where film non-
uniformity can usually be controlled to under 10%.
One of the reasons for the high mobility (33 cm2
/Vs at
Vg=Vth+10V) and superb uniformity is the fact that the metal
oxide semiconductor layer is sputtered without flowing oxygen,
contrary to the conventional practice in IGZO sputtering.
Sputtering with oxygen partial pressure could cause negative
oxygen ion bombardment on the film being deposited, causing
non-uniform damage which translates to divergent transfer
Layer Tool Comparison to a-Si TFT
Gate
Sputter
Wet Etch
Same tools, materials and etchants as a-Si
TFT
GI PECVD Same tool and material (SiNx) as a-Si TFT
Channel
Sputter
Wet Etch
Same sputter tools and wet etchants as ITO
process in a-Si TFT lines, but using
proprietary channel materials with XOYO
composition and X-O-Y bonds
S/D
Sputter
Wet Etch
Same sputter tools and wet etchants as a-Si
TFT, but with no need for n+
layer dry etch
Passivation (PV)
PECVD
or Coating
Option of either PECVD PV layer or slit
coated (or spin coated) organic PV layer
Planarization (PLN)
(for High PPI LCD)
Coating
Same tools and materials for PLN process
(slit coating or spin coating)
Pixel Electrode
Sputter
Wet Etch
Same tools, materials and etchants as a-Si
TFT
Invited Paper 26-1 / T. Xiao
SID 2016 DIGEST • 319

3. curves with lower mobility. Sputtering in oxygen environment
also accelerates the change in conductivity and chemical
stoichiometry of target surface, further contributing to non-
uniform device characteristics across the substrate with
prolonged use of target. Sputtering in oxygen can also become a
major maintenance and safety issue if cryopump is used in the
sputtering system. Therefore sputtering with no oxygen not only
enhances TFT performance, but also ensures long-term stability
of active layer sputtering process, while greatly improving the
TFT fab operational efficiency and safety.
Fig.4. Mobility and Transfer Curves for BCE Type Metal
Oxide TFT on SiNx GI with W=8µm and L=3µm
Fig.5. Five-point Id-Vg Uniformity on 3” Substrate with
Active Layer Thickness Non-uniformity > 25% (BCE type
metal oxide TFT on SiNx GI with W=8µm and L=3µm)
Fig.6 shows the transfer curve uniformity of CBRITE BCE type
metal oxide TFT on 400mm x 500mm glass substrate
manufactured from an actual production line using commercial
etchant, which exhibits even tighter Vth spread (ΔVth=0.3V)
compared to Fig.5.
Fig.6. Five-point Id-Vg Uniformity of CBRITE BCE Type
Metal Oxide TFT Manufactured on 400mm x 500mm
Substrate at Production Line
Fig.7 demonstrates the superior BTS stability (especially NBTIS
stability) at 60C for CBRITE BCE type metal oxide TFT on
SiNx GI with W=8µm and L=3µm.
There is usually a trade-off between mobility and NBTIS
stability, and Table 2 summarizes the overall performance of a
“high mobility” version and a “high NBTIS stability” version
BCE process on SiNx GI intended for TFT-LCD display
products.
Fig.7. BTS stability at 60C for BCE Type Metal Oxide TFT
on SiNx GI with W=8µm and L=3µm
26-1 / T. Xiao Invited Paper
320 • SID 2016 DIGEST

4. Table 2. Performance Summary of Two Versions of BCE
Type Metal Oxide TFT on SiNx GI (W=8µm and L=3µm)
The metal oxide TFT summarized in Table 2 is ideal for high
pixel count TFT-LCD retina displays for mobile phones/pad
phones/tablets/2-in-1s/laptops applications. In addition, such
TFT is also suitable for next generation TV products with
4Kx2K and 8Kx4K formats and with high frame rate. It is also
worth noting that the use of much denser SiNx GI (as opposed to
more porous SiO2 GI required by IGZO TFT) also helps to
suppress the copper diffusion from copper gate lines used in
large size high resolution TV products.
With a passivation layer and optional PLN layer following S/D
patterning, and a pixel electrode over, one could achieve high
aperture ratio backplane for LCD or for top emission OLED. For
large size LCD and bottom emission OLED TV, one could
complete entire backplane with 4-5 masks. One of the designs
with metal oxide layer for both channel layer and transparent
pixel electrode has been disclosed[5]
. Backplane for IPS-LCD
can be achieved with a six mask process.
Fig.8 shows the uniformity and transfer/output characteristics of
a BCE type oxide TFT on SiNx GI (W=5µm, L=6µm) tailored
for OLED/LED display applications, exhibiting high mobility of
over 45 cm2
/Vs and excellent transfer and output characteristics.
Fig.8. Uniformity and Transfer/Output Characteristics of
BCE Type Metal Oxide TFT on SiNx GI with W=5µm and
L=6µm Tailored for OLED/LED Display Applications
Fig.9. Constant Current Stress Stability at 60C of BCE
Type Metal Oxide TFT on SiNx GI with W=5µm and
L=6µm Tailored for OLED/LED Display Applications
Fig.9 shows the constant current stress stability at 60C of such
TFT, which is sufficient to meet the lifetime requirements for
portable and TV OLED displays, or LED display products.
3. Conclusion
We developed an oxide TFT manufacturing process which keeps
the high-throughput SiNx GI and wet BCE process used in a-Si
TFT lines, enabling high resolution LCD and OLED/LED
display manufacturing with minimal equipment upgrade capital
expenditures and production costs.
With exceptionally wide processing latitude and superb
consistency enabled by the robust metal oxide semiconductor
material coupled with manufacturing friendly sputtering
deposition process, such metal oxide TFTs not only possess high
mobility and large current switch ratio, but also exhibit
exceptional stability sufficient for next generation LCD and
OLED/LED displays either for portable products or for large
size TVs. They are also promising for many non-display
applications including radiation imagers and
chemical/biochemical sensors.
4. References
[1] Chun Wei Wu et al., “Improvement of Stability on a-IGZO
LCD”, SID 2013 Digest, p.97-99
[2] Jae Kyeong Jeong, “Photo-bias instability of metal oxide
thin film transistors for advanced active matrix displays”,
J. Mater. Res., 28/16, 2013, p.2071-2084
[3] Ken Hoshino, Bao Yeh and John F. Wager, “Impact of
humidity on the electrical performance of amorphous oxide
semiconductor thin-film transistors”, Journal of the SID,
21/7, 2013, p.310-316
[4] Gang Yu, Chan-Long Shieh, Tian Xiao, Karman Lee,
Fatt Foong, Guangming Wang, Juergen Musolf,
Zhao Chen, Frankie Chang, Kristoffer Ottosson,
Jung-Woo Park, Jason Chen and Chen-Yue Li, “High
Throughput MOTFT with Organic Etch-Stopper and SiNx
Gate Insulator”, SID 2015 Digest, p.296-299
[5] Chan-Long Shieh, Fatt Foong and Gang Yu, US Patent
8,187,929
Invited Paper 26-1 / T. Xiao
SID 2016 DIGEST • 321