Comparison of Elastic Modulus of Very Thin DLC Films Deposited by r. f Comparison of Elastic Modulus of Very Thin DLC Films Deposited by r.f.-PACVD and FVA Jin-Won Chung, Churl-Seung Lee, Dae Hong Ko*, Jun-Hee Hahn** and Kwang-Ryeol Lee Future Technology Research Division, Korea Institute of Science and Technology * Department of Ceramics, Yonsei University ** Korea Research Institute of Standard Science

Applications of DLC Film

High Residual Compressive Stress of DLC Films DLC Coating 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. Causes the Instability of the Coating Affects the Physical Properties in Some Cases

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.

Key Idea of the Method For Isotropic Thin Films 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

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

Measurement of Curvature

Key Idea of the Method For Isotropic Thin Films 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

Preparation of DLC Bridge 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)

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

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.

Advantages of This Method Simple 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.

Elastic Modulus of Very Thin Films a-C:H, C6H6 -400V ta-C (-50Vb) 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. J.-W. Chung et al, Diam.Rel. Mater. 10 (2001) 2069.

Synthesis of DLC Film by r.f.-PACVD RF PACVD (13.56MHz) Precursor : CH4 Vb/ P1/2 : 20 ~ 233 Vb/mTorr1/2 Substrate : P type (100) Si Film Thickness : ~ 50nm This Figure shows a schematic diagram of the deposition equipment. DLC films were deposited on p-type Si wafer by using 13.56 MHz r.f. glow discharge of CH4. In order to measure the residual stressed of the films, thin silicon wafer was used as the substrate. The structural evolutions are investigated by Raman spectroscopy. It is well known that high energy particles play an important role in determining the structure and properties of DLC films In rf-PACVD process, the mean ion energy is proportional to V (negative bias voltage) over square root P(deposition pressure). Hence, the films of wide range of physical properties were deposited for various V over root P, from twenty to two thirty three.

Residual Stress & G-peak Position of Raman Spectra 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 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

Biaxial Elastic Modulus 100 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

Biaxial Elastic Modulus 100 166 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

Biaxial Elastic Modulus 20 233 166 100 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.

G-peak Position of Raman Spectra 20 233 166 100 20 233 166 100 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.

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

Synthesis of ta-C Films 11. We synthesized ta-C films / using filtered cathodic arc deposition method. This is the schematic of the deposition system / used in the present work. Forty-five degree bending magnet was used /to remove the macro particles. In order to obtain the ta-C films / of various residual stresses, DC negative bias voltages ranging from zero to five hundred volt were applied / to the substrate during the deposition. One hundred nanometer thick films were deposited on Silicon wafer / and thin Silicon strip for stress measurement. ta-C films on Si (100) Wafer Vb : from 0 to –500V

Elastic Modulus of ta-C film

Elastic Modulus of ta-C film

Raman Spectra

Summary Presently suggested method for the elastic modulus measurement enabled us to compare the mechanical properites and thus the atomic bond structures of very thin amorphous carbon films. ta-C films showed insignificant structural evolution during the initial period of deposition. a-C:H showed the significant structural evolution in both polymeric and graphitic film deposition condition. a-C:H film deposited in optimum ion energy condition didn’t show the structural evolution.

Applications of DLC Film