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120

Table I: Estimated residual strains in SiOx films.

100 80 60 W

40 20 0

Residual strain f%)

Thickness fnm] 43

-0.51

67 90 120 320

-0.45 -0.48 -0.32 -0.29

3 4 5 6 7 8 9 10 Strain [%] Figure 1: Stress-strain curves for SiOx(43 nm)/ PET (solid line) and bare PET (dotted line). 0

600

1 2

1200

SiOx film thickness (nm) ---43 --o 67

500

5

400

300

-

"300

800

90 1200 320

600

r 0

M• 200

,,-:-'

1

2

•0

o

400 -,

0.05

0

0

0 •

200

4

Q10 0

"0

1000 ,

3

4

5

6

7

Applied strain [%] Figure 2: Crack density of SiOx films with different thicknesses.

0.1 0.15 d-"2 [nm-"/2]

0.2.

Figure 3: Thickness dependence of the fracture stress of SiOx films for initial cracks (open) and multiple cracks (filled).

the strain in the specimen was previously established by measuring the distance between two thin lines printed on a specimen. In this manner, the strains in the specimens during the tests were obtained from the stage displacement. The applied load and the stage displacement were recorded by a personal computer. Tensile tests of bare PET film were also carried out. Multiple cracking processes in SiOx films during tensile tests were observed and recorded by a VCR using in situ-optical microscopy. As an example, stress-strain curves of SiOx/PET and PET without SiOx were shown in Fig. 1. Clear transition from elastic to plastic was observed in PET at the strain of 2.2%. In addition, residual strains in SiOx films, E , were estimated from curl radii of specimens (Table I). As can be seen, Ef increases in magnitude as the thicknesses, d decreases. A detailed measurement procedure was presented in Ref. [8]. RESULTS Figure 2 shows the measured crack density as a function of the applied strain, e. It is noticed that the crack onset composite strains, ef, are higher for thinner specimens. The crack onset stress a of films were estimated by, o', = E 1((! + E), where E stands for the Young's modulus and subscripts f and s express thin film and substrates, respectively, hereafter. In Fig. 3, oa were plotted as a function of d-12 by open circles. It is shown that the initial fracture stress of Sitx films is proportional to the minus one-half power of the thickness. 34

-Delamination-

SiOx

U

'

•

' "

4

PET .......

Close up view at the crack y

__

__L_

Figure 4: A mesh model used in the finite element calculation. Table II: Material constants used in the finite element calculation.

800 Crack [Crackn L I s700

E Material SiOx PET

Young's modulus [GPa] 70.5 5.95 (c