FMC8 - 1

Ultra thin glass roll for flexible AMOLED display
Takashi Murata, Tomoki Yanase , Shinkichi Miwa and Hiroki Yamazaki
Nippon Electric Glass Co., Ltd. 7-1 Seiran 2-chome Otsu city 520-0833, JapanABSTRACT
Ultra thin glass roll, 50 microns thick, 800mm
wide and over 100m of long, has beensuccessfully produced. The roll has high flexibility,smooth surface, high thermal stability andexcellent impermeability to gases. The roll isuseful for flexible AMOLED display driven byamorphous oxide TFTs.
1. INTRODUCTION
1.1 BackgroundThe flexible display has been much anticipated
as a new display device which opens up a newmarket. Many electronics companies are actively
researching technologies in this field, especially ,such as liquid crystal display (LCD) and organic
light emitting diodes (OLEDs).
Since LCD requires a precise cell gap between
two substrates to guarantee a quality of image, itwould be hard to control the cell gap when theLCD panel is bent.
AMOLED has the advantages of fast motion
picture response time, vivid color, high contrast,
compact structure and lightweight.
The challenges of AMOLED are large size
image uniformity and reliability, which are strongly
affected by TFT technology. Although a-Si TFT
shows good uniformity, electron mobility is notsufficient. On the other hand, p-Si TFT showshigher electron mobility, but uniformity throughout
a large area is not sufficient [1].
An
amorphousoxide
semiconductor,a-InGaZnO4 (a-IGZO), facilitates flexible andtransparent
TFTs
withmuch
improvedperformance. Compared to conventional a-SiTFTs, the TFTs show higher mobility and uniform
performance over large areas [1, 2]. Thus, it isthought that AMOLED driven by amorphous oxide
TFTs is suitable for the flexible display.

performance TFTs, display fabrication requires
thermal treatment over 400 ﾟC [3]. The flexible
substrate must have thermally stabledimensions.
Gas barrier – impermeability is important toguarantee the OLED performance. OLED quality
is deteriorated by H2O or O2 gas.
Smooth surface- a continuous even surface isimportant for fine patterning.
Flexibility- this property is essential not only
for flexible displays but also for roll-to-rollprocess.
2. Properties of Ultra thin glass roll
Ultra thin glass roll, developed by Nippon
Electric glass, is a promising material for
substrates for flexible display. In this section theproperties of the ultra thin glass roll will be
discussed.
2-1 Size of Ultra thin glass roll
Figure 1 is an ultra thin glass roll. Thickness
and width of the thin glass are 50 micron and800mm, respectively. Ultra thin glass roll over
100m has been successfully rolled. We believe
this glass roll can be applied to the roll-to-rollprocess. This ultra thin glass is produced by anoverflow down draw method.

1.2 Requirements for substrate

The following substrate properties are required
for flexible display manufacturing.
Thermal stability - In order to obtain highFigure 1 A photo of ultra thin glass roll.

ISSN-L 1883-2490/18/1735 © 2011 ITE and SID

IDW ’111735

2-2 Surface state of the glass
Figure 2 is a schematic of the overflow down
draw process. The great advantage of theprocess is that glass with ultra smooth surfaces
can be obtained without a polishing process. The
surfaces of molten glass do not make contact withany other material during the forming process.

Figure 4 AFM image of a glass surface
polished with cerium oxide.
2-3 Properties of the thin glass

Figure 2 Schematic of overflow down draw
forming process.
The AFM images in Figure 3 and Figure 4 area glass surface formed by the overflow down draw
method and polished surface respectively. The
surface roughness (Ra) of Figure 3 of unpolishedthin glass is around 0.1-0.2nm, which is lower
than that of polished glass surface.

The glass code of the thin glass roll is OA-10G.
This glass has been widely used as substrate forLCD.
The properties of OA-10G are listed in Table
1.The strain point of OA-10G is 650 ﾟC. Thistemperature is high enough to withstand heattreatment temperatures used in the production
process of amorphous oxide TFT.
Table 1 Properties of OA-10G
OA-10G
3Density [g/cm ]2.46

Thermal expansion coefficient

38-7[×10 /ﾟC]（30-380ﾟC）

Strain point[ﾟC]

Young’s modulus
Poisson’s ratio

650
[GPa]

[-]

0.2

Transmittance(λ=550nm)
Refractive Index(nd)1736

IDW ’11[%]

[-]

92
1.52

Volume resistivity at 350ﾟC [Ω・cm]

12Permittivity (1MHz・25ﾟC)

5.3

tan δ (1MHz・25ﾟC)

Figure 3 AFM image of a glass surface formedby the overflow down draw method.

73

[-]

[-]

0.001

Stress on the top of curvature σ(MPa)

2-4 Minimum core diameter of the thin glassroll
The relationship between the bending
curvature and the resultant stress induced isshown in Figure 5. In general, the average
breaking stress of glass is around 100-200MPa.
However, the breaking stress can be degraded byedge condition. Taking the safety range of thebreaking stress of the glass into consideration, if
the safety limit of breaking stress is assumed tobe 50MPa, thin glass of 50micron thickness canbe rolled at a radius of 40mm.
13012011010090807060504030201000.70mm

σ ＝（E＝（ E・ T / 2 ） / R
E : Young's Modulus
T : Glass thickness

0.50mm
0.30mm0.10mm (100μm)
0.05mm (50μm)

050

100150200

250300

350

400

450

Curvature radius of bending R (mm)Figure 5. Stress at the top of curvature versus
curvature radius of thin glass.
2-5 Gas permeability of the thin glass
The permeability of 30 micron ultra thin glass to
H2O or O2 gas can not be detected by the API-MS
method (Sumika Chemical Analysis Service). Thisultra thin 30-micron glass was a specially
produced glass for this gas permeability test. Thelower limits for H2O and O2 gas are <7 x 10-7 g / m2

day (85ﾟC, 85%RH)and < 8 x 10-3 cc / m2 day,(40 ﾟ C, 90%RH) respectively. The results
indicate that the ultra thin glass shows excellent
impermeability to H2O and O2 gas, even though
the thickness is only 30 microns. This is believedto be the first data on the gas barrier propertiesof ultra thin glass.
3. Summary
Ultra thin glass roll has been successfully
produced by Nippon Electric Glass. The glassroll exhibits attractive features: flexibility, smoothsurfaces, impermeability to gas and thermal
stability. These features are quite useful for
obtaining high quality and reliable TFT
performance.
We believe the introduction of ultra thin glassroll will establish a new standard of performancefor flexible AMOLED display.
4. References[1] T.Tsujimura, S. Mizukoshi, N. Mori, K. Miwa,Y. Maekawa, M. Kohno, K. Onomura, K.
Mameno, T. Anjiki, A. Kawakami and S.
Vanslyke “Scalable AMOLED technologiesfor TV application” Proc. IDW ’08, pp.
145-148 (2008).
[2] Tetsufumi Kawamura, Hiroyuki Uchiyama,Shin-ichi Saito, Hironori Wakana, Toshiyuki
Mine and Matsuko Hatano “Low-voltageoperating amorphous oxide TFTs” Proc.
IDW ’09, pp. 1689-1692 (2009).
[3] Toshio Kamiya, Kenji Nomura and Hideo
Hosono “Impact of subgap states onpeculiar characteristics of Amorphous oxidethin-film transistor” Proc. IDW ’09, pp.
1673-1676 (2009).

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