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SiOx/PET100μm

0.15Pa

AlOx/PET100μm

0.15Pa

ZnOx/PET100μm

0.15Pa

inner bend

outer bend

Investigation and Improvement of Cracking Mechanismin Transparent Insulating Film

Keita Umemoto, Yoshinori Shirai, Ichiro Shiono, Yuto Toshimori*, Shoubin ZhangMitsubishi Materials Corporation Sanda Plant, Sanda-shi, Hyogoken, 669-1339, Japan, TEL +81-79-568-2307, E-mail : keitaume@mmc.co.jp

* Mitsubishi Materials Corporation central research institute, Naka-shi, Ibarakiken, 311-0102, Japan

II. Introduction

III. Experimental

V. Conclusion1. A contrasting curve was obtained and the residual stress was calculated.2. It clearly shows that compressive stress is weak to inner bending. However, by adjusting the sputtering

conditions to reduce the residual stress, crack resistance could be improved.3. We have succeeded in designing a new material which is excellent in crack resistance even in 500nm

thickness.

In a flexible display, all of the materials of these stacked structures need to have flexibility. Generally,metals used for wiring materials tend to be soft, but ceramics such as insulating films tend to be hard andcrack easily. When the bending radius reach 11 mm, the cracks start to initiate first at SiO2 bridges, actingas interconnecting layers among individual TFT[1]. Moreover, in OLED flexible displays, ceramics films areoften used as barrier films for protecting the light emitting organic molecule from the transmission ofwater vapor by replacing the film substrate from glass. In such ceramic films, cracks lead to fataldeterioration of function, so that the flexibility of the film is really important issue to improve. The crackingissue and the stress of the deposited film are deeply related, and several researchers discussed therelationship between the stress and the condition of the deposition on the film substrate[2],[3]. However,there is no general method to measure stress on the film substrate.

In this study, we evaluated on the relationship between film deposition conditions and crack resistancefor basic materials AlOx, SiOx, and ZnOx by measuring the residual stresses of the deposited film usingmechanical probe method. As a result, cracking mechanisms of each materials were discussed and, wesuggest a new material which has excellent in crack resistance.

Sputtering and Analysis conditions

The residual stress measurement

The stress values showed a similar tendency for difference substrates. In the case of sputtering of a metalfilm, it is known that the stress variation pressure depends on the element weight[4].

I. AbstractCrack resistance of various transparent insulating films was investigated by

measuring the residual stress of the film using simple mechanical probe method. As aresult, the cracking mechanisms of each materials were discussed and now, wesuggest a new material, which is excellent in crack resistance.

Fig. 1. Schematic image of mechanical probe method

Thin film TargetTarget size

(mm)Power

(W)Gas

Pressure(Pa)

SiOx Si φ125 x 5 Pulse-DC 500 Ar + O2 0.15, 0.6, 1.2

AlOx Al φ125 x 5 Pulse-DC 500 Ar + O2 0.15, 0.6, 1.2

ZnOx ZnO φ125 x 5 Pulse-DC 500 Ar + O2 0.15, 0.6, 1.2, 2.4

Table 1. Sputtering conditions of SiOx, AlOx and ZnOx

*Substrate : PET 100μm, 38μm.*The amounts of oxygen was adjusted according to each target respectively.

Measurement of residual stress

X axis

Z axis

Deposited film/PET

needle

h

C

The radius of the film was calculated by formula (1).

R = (C2+4h2) / 8h (1)

C : chord of the film h : height of the arc

The residual stress was calculated by Stoney’s formula (2).

s = Ests2 / {(1-ns) 6tf R} (2)

Es : Young’s modulus of the substrate ns : Poisson's ratio of the substrate ts : thickness of the substrate tf : thickness of the deposited filmR : radius of curvature

IV. Results and DiscussionsMeasurement of curvature radius

By using the mechanical probe method, symmetrical curves were obtained.

Fig. 2 Curve equation of AlOx (200nm), SiOx (200 nm) and ZnOx (200nm) , a) PET 100μm, 0.15Pa, b) PET38μm, 0.15Pa.

Fig. 9 The relationship between the residualstress and film thickness.

Fig. 8 Optical microscope photographs of the surfaceof thin films after inner bending mechanical stress test.

Fig. 3 The relationship between the residual stress and sputtering gas pressure. (Film thickness = 200nm)

-800

-700

-600

-500

-400

-300

-200

-100

0

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Res

idu

al s

tres

ses

(MPa

)

Sputter gas pressure (Pa)

a) PET 100μm

SiOx

AlOx

ZnO

Tensile

Compressive

-800

-700

-600

-500

-400

-300

-200

-100

0

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Res

idu

al s

tres

ses

(MPa

)

Sputter gas pressure (Pa)

b) PET 38μm

SiOx

AlOx

ZnO

Tensile

Compressive

Mechanical stress test

“AZO-low n” thin film

Mechanical stress test

Radius : R=3mmFrequency : 10,000times

Testing standard：IEC 62715-6-1Flexible display devices - Part 6-1: Mechanical stress test methods

-200

-150

-100

-50

0

0 100 200 300 400 500 600

Res

idu

al s

tres

ses

(MPa

)

Film thickness (nm)

SiOx

AZO-low n

The compressive stress has little resistance to inner bending. The cracks are likely to occur at grainboundaries due to the high crystallinity of the ZnOx thin film.

0

5

10

15

20

25

30

35

40

-800 -600 -400 -200 0 200

Cra

cks

afte

r in

ner

ben

din

g(l

ines

/mm

)

Residual stress (MPa)

SiOx /PET 100μm

AlOx /PET100μm

ZnOx /PET100μm

ZnOx /PET38μm

Fig. 4 Optical microscope photographs of the surface ofthe film after bending (100x).

Fig. 5 The relationship between cracks after innerbending and residual stress. In the AlOx and the SiOx, nocrack was observed when the substrate is PET 38μm.

PET

deposited film

deposited film

PET

500μm

0

20

40

60

80

100

120

0 500 1,000 1,500 2,000 2,500 3,000 3,500

Hei

ght

(μm

)

Length (μm)

b) PET 38μm, 0.15Pa

SiOx

AlOx

ZnOx

0

20

40

60

80

100

120

0 2,000 4,000 6,000 8,000 10,000

Hei

ght

(μm

)

Length (μm)

a) PET 100μm, 0.15Pa

SiOx

AlOx

ZnOx

Concepts of AZO-low n

AZO-low n

Amorphous

material

SiO2

Soft material

ZnO

Conductivityfor target

Al2O3

Deposition Mechanical stress

Compressive stress

Compressive stress Crack(inner bend)

Tensile stress no crack(outer bend)

Hard

Soft

Hardness Crystalline

High

Low(amorphous)

Fig. 6 Comparison of deposition rate.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

SiOx AlOx ZnOx AZO-low n

Dep

osi

tio

n r

ate

(nm

/sec

)

Bending direction

a) SiOx 500nm

(-176MPa)

b) AZO-low n 500nm

(-127MPa)

500μm

Ref.) [1] K. T. EUN et al, Mod. Phys. Lett. B, 26, 1250077, (2012)[2] Y.C. Lin et al., Thin Solid Films, Vol. 517, 1701–1705 (2009)[3] Y.C. Lee et al, International journal of emerging technology and advanced engineering, 4, Issue 8, August, 454-457, (2014)

*In this paper, the deposited film comes inside is defined as inner bend, and the other side is defined as outer bend.

Fig. 7 Optical properties of AZO-low n thin film.

0.00

0.05

0.10

0.15

0.20

1.7

1.8

1.9

2.0

2.1

250 350 450 550 650 750 850

Exti

nct

ion

Co

effi

cien

t k

Ref

ract

ive

Ind

ex n

wavelength (nm)

n k

Ref.) [4] D. W. Hoffman and J. A. Thornton, J. Vacuum Sci. & Technol., 20, 355 (1982)

Residual stress

Thermalization(Sputter gas pressure) Kinetic-energy

(Voltage)

Temperature(Heat capacity of substrate)

Reference : Proceedings of International Display Workshops Volume 24, Paper No. FLXp1-4, Page 1553-1556

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