35.4L / G. Yu
35.4L: Late-News Paper: Metal-Oxide Thin Film Transistor
with High Performance and Good Operation Stability
Gang Yu, Chan-Long Shieh, Fatt Foong, Guangming Wang, Aaron Kuo, Kaixia Yang,
Jian Wang, Frankie Chang, James Peng and Boo Nilsson
CBRITE Inc., 421 Pine Ave., Goleta, CA 93117, USA
1.
Introduction
Although LTPS have been used for backpanels in portable
AMOLED products, a low cost, high operation stability and high
uniformity backpanel technology is still needed for next
generation flat-panel TV products based on AMLCD or
AMOLED. LTPS TFT does not provide competitive cost
structure and amorphous silicon TFT does not provide the needed
performance, neither mobility nor operation stability.
Metal-Oxide Thin Film Transistor (MOTFT) has attracted
significant attention for its high carrier mobility and potentials for
next generation AMLCD and AMOLED [1].
Several
demonstrations based on InGaZnO TFT have been disclosed
recently with impressive performance [2]. However, the mobility
and the Vth stability are yet to be improved to fulfill the demands
for the next generation display applications. Other challenges
include process stability, process simplicity and process
compatibility with existing TFT manufacture lines.
CBRITE has developed a metal-oxide TFT technology for
advanced display applications. We have developed a series of
non-InGaZnO type of MOTFT with electron mobility >80
cm2/Vsec, Ion/Ioff >1010 and S<0.2 V/dec. Such TFT shows
excellent stability under OLED and LCD operation conditions.
CBRITE has also developed a MOTFT process flow based on
PVD and wet etching patterning. AMOLED and AMLCD
backpanels have been demonstrated from a Gen-2.5 size, color
filter production line. Bottom emission, full-color AMOLED is
demonstrated in 4.8”, QVGA (320xRGBx240) with aperture ratio
larger than 50%. Power efficiency over 6 lm/Watt and >20
lm/Watt were achieved in full pixel on condition and in video
operation condition.
2.
Experiments and Results
Non-IGZO type metal-oxides were used for the channel layer.
The device architecture and process steps are similar to that socalled 5 masks, bottom-gate, etch-stopper configurations known
in a-Si TFT field. Figure 1 shows a typical Id-Vgs plot from a
10
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d
Non-InGaZnO based metal-oxide TFT is developed with high
performance and good stability. Manufacturability was
demonstrated with a high capacity, Gen-2.5 size, color-filter
production line. Superb TFT performance was achieved with
>80 cm2/Vsec, Ion/Ioff >1010 and S<0.2 V/dec. This TFT shows
high stability under OLED and LCD operation conditions. A 4.8”
bottom emission AMOLED is demonstrated with aperture ratio
larger than 50%. Power efficiency >6 lm/Watt at >500 nits was
achieved with high display uniformity. The average power
efficiency under video operation is beyond 20 lm/Watt.
TFT with W/L=6.7 and Vds=10V. Id reached 1 mA at Vgs near
5V. The highest mobility recorded was > 80 cm2/Vsec. The
device architecture and parameters have been tuned to best fit
with AMOLED and AMLCD operation conditions with stability
better than LTPS TFT reported in literature, sufficient for
commercial applications. A TFT with W/L=6.7 was stressed at
room temperature with initial current of 230 A (which is ~100x
accelerated comparing to that needed for peak brightness in an
OLED TV) for over 41 hours, the current varies within 1% during
entire test time. Noticing the oscillating type variation with 24
hours period, this ~1% variation was mainly due to temperature
change in the test room rather than biasing field or charge motion.
The corresponding Vth is < 0.2V, as shown in Figure 2 along
with Id(t). A similar TFT was also stressed at 60 oC with initial
current of 140 A for 21 hours, the current drops approximately
20 % with Vth shifts for less than 0.6 V during stress period.
I (A)
Abstract
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5
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Vgs (V)
Fig. 1 Id-Vgs of a TFT with W/L=6.7 (Vds=10V).
Such TFT is also stable under high voltage bias stress at
Vgs=+20V and Vds=0.1V. Figure 3 shows an Id-Vgs data set
collected during 48 hours stress at room temperature. The Id-Vgs
data were collected every 30 minutes under Vds=10V. The Vth
shifts at a rate of ~ 8.7 mV/h. The Id collected at Vgs=20V and
Vds=10V drops only ~2%, from 1.93mA to 1.89mA, over 48
hours stress period. The mobility of this test device is ~ 35
cm2/Vsec and the Ion/Ioff is over 1010. PBTS at 60 oC has also
been carried out under the same biasing condition. The Vth shifts
~ +0.8V after 17 hrs; i.e., a rate of ~47 mV/h.
ISSN 0097-966X/11/4201-0483-$1.00 © 2011 SID
SID 11 DIGEST • 483
35.4L / G. Yu
The maximum processing temperature used in fabricating these
TFT was below 300 oC. We note that such MOTFT can also be
fabricated at temperature below 180 oC. TFT fabricated on PAN
and PEN foils with 180 oC process shows similar IV
characteristics as shown in Fig. 3 with S<0.2 V/dec and Ion/Ioff
>109.
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I(6V)
V(t)@1nA
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5 10
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10
20
30
40
Time (hour)
Fig.2 Vth and Id as function of stress time (Vgs=6V
and Vds=5V).
1.E-02
0 hr
1.E-03
4 hrs
8 hrs
1.E-04
12 hrs
A process flow has been established on a Gen-2.5 size
(370mmx470mm), color filter manufacture line; based on existing
PVD tool, proximity photo exposure tool and wet etching
patterning process used for color filter manufacture. Such
approach enables AMOLED displays be made with a cost
structure competitive to that for AMLCD. Figure 4 shows a photo
of a completed backplane in Gen-2.5 size with several
development vehicles on it. A major development vehicle used in
the 12 months was a 4.8”, full-color QVGA display for
AMOLED. The pixel circuit comprises two transistors and one
capacitor. The pitch size was 300umx100um with 52 % aperture
ratio. It is worth to note that such high aperture ratio has only
been achieved previously in AMOLED with top-emission
configuration [3].
Figure 5 and Table 1 shows the photo and the specification chart
from a 4.8” AMOLED with a white OLED emitter. Power
efficiency over 6 lm/W and >20 lm/Watt was achieved under
100% pixel on at full brightness and video operation conditions.
This power budget is of great significance for battery powered
portable electronic products [4]. This engineering prototype also
confirms peak luminance over 600 nits with Vdd-Vcom less than
8 V, with 9 point uniformity over 89%.
16 hrs
1.E-05
20 hrs
Id(A),Vd=10V
1.E-06
24 hrs
28 hrs
1.E-07
32 hrs
1.E-08
36 hrs
40 hrs
1.E-09
44 hrs
1.E-10
48 hrs
1.E-11
1.E-12
1.E-13
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0
5
10
15
20
Vg(V),step=0.2V
5.0
2.5E‐03
4.0
2.0E‐03
3.0
Vg (Id=1E‐9)
1.0
1.5E‐03
Vth
0.0
Id
‐1.0
1.0E‐03
‐2.0
‐3.0
Id (A) at Vds=10V, Vgs=20V
2.0
5.0E‐04
‐4.0
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0.0E+00
0
5
10
15
20
25
30
35
40
45
50
55
Stress Time (Hours)
Fig.3 PBTS at Vgs=20V, Vds=0.1V, at RT: IdVgs (up-panel), and Vth and Id vs. stress
time (low-panel).
484 • SID 11 DIGEST
Fig. 4 370mmx470mm MOTFT backpanel processed from a Gen-2.5 color filter production line
(left); and a 4.8” FC AMOLED backpanel on
5”x5” glass (top-right). >50% aperture ratio is
well seen from the high transmission of the 4.8”
panel.
35.4L / G. Yu
The aperture ratio has been improved further to 65% in a recent
AMOLED design (see the top-center portion of the Gen-II panel
in Fig. 4 with higher transmission). The power efficiency and the
operation lifetime showed corresponding improvement. Such
MOTFT backpanel enables the full-color OLED technology used
in current PM OLED products being upgraded for AMOLED
products without further development.
CBRITE AMOLED Module
11” EDTV (480p)
18.5” HDTV (720p)
26” FHDTV (1080p)
Fig. 5 A photo of the 4.8” AMOLED with a white
OLED emitter. This display is equivalent to a tile
in 11” EDTV (480p), or 18.5” HDTV (720p), or
26” FHDTV (1080p).
Table 1. Specification Table of 4.8” AMOLED panel
3.
Discussion and Remarks
The data showing in this paper demonstrate that CBRITE’s
MOTFT is an ideal technology for next generation flat-panel
displays. It offers mobility > 30 cm2/Vsec with superb operation
stability necessary not only for pixel drivers but also for
peripheral driver circuits. Data from Figure 2 is equivalent to a
total charge of 34 Coulombs passed through the TFT. These data
suggest that an AMOLED TV with pixel peak current of 2 A,
can be operated continuously with 100% pixels on at full
brightness for over 4,700 hours without noticeable circuit
parameter change. Using a mean usage factor of 20% for video
contents, such OLED display can be used for OLED TV for
continuously operating over 23,000 hours. Assuming 8 hours per
day operation, such TV can be operated over ~ 7 years.
The data shown in Figure 3 also confirms that CBRITE MOTFT
can also be used for next generation AMLCD products with
advanced liquid crystal technologies.
It is also worth to note that such high mobility, stable MOTFT
was made with a non-IGZO based metal-oxide channel layer.
The process demonstrated in a Gen-II size, high capacity color
filter line confirmed that MOTFT can be made with a production
line with substantially lower capital investment than that used for
a-Si TFT. It also confirms that existing a-Si TFT lines can be
upgraded for MOTFT when the time arrives.
The performance table also implies that a 48” OLED TV panel
can be operated with mean power substantially less than 50 Watt.
Display Size
4.8” diagonal
Display Format
FC QVGA: 320x3x240
Total Pixel Count
230.4K
Pitch Size (sub-pixel)
300 µm x 100 µm
Active Area
96 mm x 72 mm
Architecture
Bottom emission
Aperture Ratio
>50 %
5.
Frame Rate
60, 120, 240 Hz
Sub-pixel current
>3µA @ Vds<3V
[1] For early IGZO works, see, for examples, K. Nomura et al.,
Nature 432, 488 (2004); H.Q. Chiang et al., Appl. Phys. Lett.
86, 13503 (2005)
Vds(T2)@Ipixel=3 µA
<3V
Peak Brightness
>600 nits
Power efficiency
6 lm/W (~ 20 lm/W Movie)
Homogeneity
>89% (5 and 9 point tests)
4.
Summary
In summary, we presented a MOTFT technology with
performance and stability better than IGZO and LTPS TFTs; and
with manufacture cost as good as, or even better than that for a-Si
TFT. The mobility, the operation voltage and the operation
stability meet the specifications for next generation OLED/LCD
TV applications.
References
[2] For engineer prototypes based on IGZO TFT, see the
corresponding works presented in Digest of the Technical
Papers in SID2009 and SID2010.
[3] Specification of SONY XEL-1 OLED TV, User’s manual.
[4] G. Yu, invited talk, Asia FPD 2000, August 26-28 (2000)
Guangzhou, China
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