압축잔류응력을 이용한 극미세 DLC박막의 탄성계수 평가 한국과학기술연구원 조성진, 정진원, 이 광 렬

Contents Introduction to Residual Stress of Thin Films Industrial Application and Residual Stress of DLC Films Elastic Modulus Measurement by a Simple Micro-Fabrication

Residual Stress of Thin Films Thin films typically support very high stresses due to the constraint of the substrate to which they are attached Normally at near failure stress! Affects the mechanical behaviors of the coating and devices (elastic distortion, plastic deformation, fracture, adhesion) Origin of the Residual Stress Any process that changes the in-plane dimension of the film relative to that of the substrate

Relative Dimensional Changes Substrate Interaction Stresses Intrinsic Stresses Due to Property Misfit Thermal Stress Epitaxial Stress Structure Evolution During Growth Growth Stress

Thermal Stress Condition : Difference in thermal expansion coeff. Difference in temperature

Condition : Coherency with different lattice parameters Epitaxial Strains Condition : Coherency with different lattice parameters

Intrinsic Stress (Growth Stress)

Relative Dimensional Changes Substrate Interaction Stresses Intrinsic Stresses Due to Property Misfit Thermal Stress Epitaxial Stress Structure Evolution During Growth Growth Stress

Bending due to Residual Stress However, high residual compressive stress of ta-C films results in poor adhesion and the limitation of thickness. For example, like the left figure, the flat wafer strip was considerably bended after the deposition of ta-C film. As shown in the right figure, this high residual compressive stress makes ta-C film delaminate from the Si substrate and resulted in poor adhesion. It greatly limits the usefulness of ta-films for many applications. Many attempts have been reported to reduce the stress of the ta-C films without changing the other mechanical properties. In the present work, we attempt the study on the addition of silicon into ta-C to overcome the drawback of pure ta-C films. DLC Films Deposited by Filtered Vacuum Arc.

Measurement of Residual Stress Assumption 1-D Treatment of Elastic Equilibrium Sufficient Adhesion df << ds ds << R ds df Curvature (R)

Stress Measurement

Properties of Solid Carbon Property Diamond DLC Graphite Density (g/cm3) 3.51 1.8 – 3.6 2.26 Atomic Number Density (Mole/cm3) 0.3 0.2 – 0.3 0.2 Hardness (Kgf/mm2) 7000 - 10000 2000 - 8000 <500 Friction Coeff. 0.05 0.03 – 0.2 Refractive Index 2.42 1.8 – 2.6 2.15 – 1.8 Transparency UV-VIS-IR VIS-IR Opaque Resistivity (Wcm) >1016 1010 - 1013 0.2 – 0.4

경질박막의 내마모 윤활특성 DLC WC TiN CrN TiCN 마모도 마찰계수. 2.0 1.6 1.2 0.8 0.4 (상대비교치) 0.2 0.4 0.6 0.8 1.0

Applications of DLC Film

DLC Films Deposited by Filtered Vacuum Arc. Residual Stress However, high residual compressive stress of ta-C films results in poor adhesion and the limitation of thickness. For example, like the left figure, the flat wafer strip was considerably bended after the deposition of ta-C film. As shown in the right figure, this high residual compressive stress makes ta-C film delaminate from the Si substrate and resulted in poor adhesion. It greatly limits the usefulness of ta-films for many applications. Many attempts have been reported to reduce the stress of the ta-C films without changing the other mechanical properties. In the present work, we attempt the study on the addition of silicon into ta-C to overcome the drawback of pure ta-C films. DLC Films Deposited by Filtered Vacuum Arc.

Deposition Method for DLC Films Impact Energy (eV) 1 10 100 1000 Amorphous Carbon (sp2) Dense Source Hydrocarbon Hydro- Polymer Like Plasma Polymers Ion Source Energy Cold Substrate

Intrinsic Stress (Growth Stress)

Typical Behavior of Residual Stress of DLC Films ta-C by FVA a-C:H by rf-PACVD

Self Delamination of DLC Films K.-R. Lee et al., Diam. Rel. Mater. 2 (1993) 208. M.-W. Moon et al., Acta Mater., 50 (2002) 1219.

Failure Due to Residual Stress Constant Temperature and Humidifier Motor Fn Ft At 90% R.H.

Stress Effect on MEMS Structure 0.45mm thick DLC coating Courtesy of SAIT

Key Idea of the Present Method Recently, we suggested a simple method to measure the elastic modulus of a DLC film which has a compressive residual stress This is a simple stress-strain relation for elastically isotropic thin films. In this equation, If one can measure the strain and the residual stress of the film, The biaxial elastic modulus would be obtained For Isotropic Thin Films

Relative Dimensional Changes Substrate Interaction Stresses Intrinsic Stresses Due to Property Misfit Thermal Stress Epitaxial Stress Structure Evolution During Growth Growth Stress

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

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

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

Strain of the Buckled Thin Films (I) Z X 2A0

Stain of the Buckled Thin Films (II)

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.

Elastic Modulus of Thin Films Mechanical properties of thin films are not the same as those of materials having the sample composition in bulk form High quench rate in deposition process High defect densities and textures Non-equilibrium compositions Confinement of dislocations, craction, etc. in small dimensions

Nano-Indentation of Thin Film Substrate

Nano-Indentation Initial unloading is pure elastic. Sneddon’s elastic contact theory

Bulge Test For Isotropic Film

Laser-Acoustic Technique Sonic Vibration and Laser-Acoustic Technique Sonic Vibration Laser-Acoustic

Freehang and Micro Bridge

Advantages of This Method Simple Method Completely Exclude the Substrate Effect Can Be Used for Very Thin Films The possibility of elastic modulus measurement in very thin film 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.

Nano-Indentation of Thin Film Substrate

Nano-indentation Results

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.

HDD용 Hard Disk

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, 2069 (2001).

Residual Compressive Stress & G-peak Position of Raman The left figure is measured residual stress and the right figure is G-peak positions of the Raman spectra of the thick DLC film. The residual stress of the film shows a maximum of 2.3 Gpa at the value of v over square root p of 100. This behavior agrees with the previous work in the precursor gas effect in rf-PACVD. In the previous work, EELS and electrical conductivity shows that the character of the film changed from polymeric to dense carbon and then to graphitic one as the value of V over square root P increased from 20 to 220. The structure change can be also observed by Raman spectrum analysis. The Raman spectrum analysis of DLC film includes deconvolution of the spectrum with two Gaussian peaks, G and D-peak. It is empirically known that the G-peak position illustrates the changes in atomic bond structure of the film. For example, graphitization of the film during high temperature annealing is correlated with the g-peak position shift to higher wave number. In the present work, G-peak position of the Raman spectra shifted to higher wave number as the value of V over square root P increased. We know that black point’s film has more polymeric component than the red point, the red point has dense and hard carbon bonding and the green and blue points have more graphitic component than red. From the data of this figure, it can be said that the G-peak position shifts to high wavenumber when the graphitic component increased and shifts to lower wave number when the polymeric component increased

Biaxial Elastic Modulus 100 166 233 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. 20

G-peak Position of Raman 233 166 100 This figure shows the G-peak position of Raman spectra as a function of film thickness. The red point was deposited at optimum ion energy, and had maximum residual stress. It exhibits almost fixed G-peak position regardless to the film thickness It means that there are no structural changes during the growth in red point. But in the black point of the polymeric film, the G-peak position shifts to lower wave number with decreasing film thickness. Previously, we mentioned that, as the film became more polymeric, the G-peak position shifted to lower wave number. Hence, this result shows that the structure of the film is more polymeric, when the film is very thin. On the other hand, in green and blue points of the graphitic films, The G-peak positions shift to higher wavenumber. Because the G-peak position shifted to higher wavenumber, as the film became more graphitic This result shows that the structure of the film is more graphitic, when the film is very thin. 20

Schematic Film Structure Si Substrate 233 166 100 20 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 Si Substrate J.-W. Chung et al, Diam.Rel. Mater., 11, 1441 (2002).

Conclusions 나노두께 박막의 탄성계수와 간단한 micro machining 기술을 이용하여 나노두께 박막 고유의 탄성계수 측정법을 제시하였다. 나노두께의 다이아몬드상 탄소박막의 기계적 물성은 합성조건에 따라 크게 달라졌으며, 이는 초기합성시 박막의 구조변화에 기인하였다. 나노박막의 응용과 나노 multilayer의 기계적 물성변화를 해석할 때, 이러한 문제가 주의깊게 고려되어야 한다.

Acknowledgement 원천기술개발사업, KIST (1999-2000) 선도기술개발과제, 과기처 (1994-2001) 플라즈마 응용 표면기술연구센터, 한국과학재단 (2000-2002) 권동일교수, 정증현박사 (서울대학교)