1. Characterization of Mechanical Properties of Diamond-like Carbon Films by Using Residual Compressive Stress
Sung-Jin Cho, Jin-Won Chung, Myoung-Woon Moon and Kwang-Ryeol Lee
Korea Institute of Science and Technology
Thanks Ravi,
I am very pleased to be here and appreciate inviting me as a speaker of this meeting.
In this presentation, I will present a method to characterize the mechanical properties of DLC films
Of high residual stress.
SMAC2004, ~10, Trento, Italy

2. Residual Compressive Stress of DLC Film
Film Deposition
There would be no doubt that the high residual compressive stress of DLC film is one of the major concerns in the DLC community.
Yes, DLC film typically has very high residual stress upto 10 or 20 GPa. As you can see this picture, flat Si wafer of thickness 200um is bended like this after depositing only 1um thick ta-C films. If you deposit thicker film, for examples 2um, you can break the Si wafer if the film has sufficient adhesion.
However, when the interfacial force due to the stress is beyond the interfacial toughness, in other words when the adhesion is not sufficient, the film delaminate from the substrate. I will show you a short movie of the delamination that occurs spontaneously without any external force.

3. Telephone Cord Buckling
M.W.Moon et al , Acta Mater., 50 (2002) 1219.

4. Buckling Configurations

5. Delamination of Floor Paint
Bottom of parking lot of a chinese restaurant in Vancouver (2004)

6. Off-Piste Run in Hoghfügen

7. What can we do with this phenomenon?

8. Quantitative Analysis
K.-R. Lee et al , Diam. Rel. Mater., 2 (1993) 218.

9. What can we do with this phenomenon?
Can be a useful tool to estimate the mechanical properties of thin films and the fundamental interface toughness (adhesion)

10. What can we do with this phenomenon?
For Isotropic Thin Films

11. Measurement of Residual StressDs Df
Curvature (R)

12. What can we do with this phenomenon?
For Isotropic Thin Films

13. DLC Bridges by Micro Fabrication
SiO2 Isotropic
Wet Etching
Wet Cleaning
DLC film
Deposition
( on SiO2 )
DLC Patterning
Strain Estimation

14. Microstructure of DLC Bridges
C6H6, 10mTorr, -400V, 0.5mm
150mm

15. Strain of the Buckled Thin FilmsZ X
2A0

16. Effect of Bridge Length
60mm

17. DLC Bridges
100 V
250 V
400 V
550 V

18. Biaxial Elastic Modulus
Bridge Method

19. Preparation of Free Overhang
Si Etching
(by KOH Solution)
Wet Cleaning
DLC film
Deposition
Cleavage along
[011] Direction
Strain Measurement

20. Free Overhang Method Strain of the free overhang
Biaxial elastic modulus

21. A0 / λof Free-hang at 546 nm I II
III
a-C:H, C6H6 -400V

22. Effect of Etching Depth
5.6 ㎛
11.3 ㎛
t=546 nm
2 ㎛
11 ㎛
t=55 nm

23. Elastic Modulus for Various Ion Energies
Nanoindentation t>1.0 ㎛
그래서 본 실험에서는 버클링 현상과 필름의 탄성특성을 이용하여 fundamental adhesion 에너지를 정량적으로 평가 하려고 합니다. 또한 이렇게 평가된 정량적인 값이 얼마나 타당하여 이 방법이 얼마나 타당한지에 대해 확인해 보려고 합니다.

24. Advantages of This Method
Completely Exclude the Substrate Effect
Can Be Used for Very Very Thin Films
In contrast to the other measurements method, the present technique has many advantages.
The most important advantage is that
the elastic property of thin film can be measured without the substrate effect, because we can completely exclude the substrate effect by etching process.
So we can accurately measure the elastic modulus very thin films, using this method.

25. Substrate Effect is Significant.
Nano-indentation
Substrate
However, the difficulties of nano-indentation for very thin films / arise from the high sensitivity to the substrate,
especially when applying to the system of large difference in mechanical properties between the substrate and the film.
The substrate effect is more significant in measuring the elastic modulus than in measuring the hardness
The elastic strain field is much wider than the plastic strain field.
Hence the elastic behavior during unloading is dominated by the elastic behavior of the substrate.
The elastic strain field >> the plastic strain field
Substrate Effect is Significant.

26. Substrate Effect on the Measurement

27. Advantages of This Method
Completely Exclude the Substrate Effect
Can Be Used for Very Thin Films
In contrast to the other measurements method, the present technique has many advantages.
The most important advantage is that
the elastic property of thin film can be measured without the substrate effect, because we can completely exclude the substrate effect by etching process.
So we can accurately measure the elastic modulus very thin films, using this method.

28. Elastic Modulus of Very Thin Films
The free overhang method was successfully employed to measure the biaxial elastic modulus of very thin DLC film.
The left figure is the elastic modulus of a-C:H film made by rf-PACVD, and the Right figure is that of ta-C film made by Filtered Vacuum Arc.
Here, the a-C:H film is polymeric, but the ta-C film is very hard.
Using this method, we could successfully measure the elastic modulus of the film about 33nm thickness.
The more important observation is that , in contrast to ta-C films, the elastic modulus of the film decreased when the film thickness was very small, in a-C:H film.
In our previous work, we showed that the decrease in elastic modulus of very thin film is not due to the interfacial layer but due to the structural evolution during the initial stage of the film growth.
These results show that the mechanical property measured in thick film cannot be always used for very thin film.
Therefore, the mechanical properties of the film and the structural evolution during the initial stage of the film growth should be carefully investigated for a specific deposition condition.
a-C:H, C6H6 -400V
ta-C (Ground)
J.-W. Chung et al, Diam.Rel. Mater. 10 (2001) 2069.

29. Biaxial Elastic Modulus
100
166
233 20
This Figure shows the dependence of the biaxial elastic modulus on the film thickness
A fixed elastic modulus was observed only at red point, hard and dense carbon film deposited
In both case of higher or lower value of V / root P, decreasing the elastic modulus was observed in very thin films.
The observed elastic modulus shows that the structural evolution during the initial stage of the film deposition is significant in the films of high content of polymeric or graphitic component.

30. Structural Evolution of DLC Films
Si Substrate
233
166
100 20
Si Substrate
Si Substrate
In polymeric and graphitic films, the elastic behavior of very thin film is similar. The biaxial elastic modulus decreased with decreasing film thickness.
But the reason for the decrease of elastic modulus is not the same.
In polymeric film, more polymeric film reduced the elastic modulus
In graphitic film, more graphitic film reduced the elastic modulus
J.-W. Chung et al, Diam.Rel. Mater., 11, 1441 (2002).

31. Residual Stress of ta-C film

32. Biaxial Elastic Modulus of ta-C film

33. What can we do with this phenomenon?
Can be a useful tool to estimate the mechanical properties of thin films and the fundamental interface toughness (adhesion)

34. Fundamental Adhesion l
First of all, we analyzed the delaminaton buckling with considering one-dimensional buckling pattern. Buckling geometry can be expressed as shown. And the total work of adheision can be expressed with buckling geometry and the elastic constant.

35. Fundamental Adhesion
DLC on Glass

36. Delamination of Patterned Substrate
SEM FEM
2b* = 2.573mm
2b0

37. Conclusions
Can be a useful tool to estimate the mechanical properties of thin films and the fundamental interface toughness (adhesion)