Atomic Structure Analysis of Diamond-like Carbon Films S.-H. Lee, S.-C. Lee, H.-S. Ahn, K.-R. Lee Korea Institute of Science and Technology 윤덕용 교수님 정년퇴임 및 최고과학기술자상 수상 기념 심포지움 (한국세라믹스학회, 서울시립대, 2005. 4. 22)

Bond Structure of Carbon 1S2 2S22P2

What is DLC ? Amorphous Solid Carbon Film Mixture of sp1, sp2 and sp3 Hybridized Bonds High Content of Hydrogen (20-60%) Synonyms Diamond-like Carbon (Hydrogenated) amorphous carbon (a-C:H) i-Carbon Tetrahedral Amorphous Carbon Heart valve Hard disk

2-D Analogy of Structure a-C:H ta-C

High Residual Compressive Stress Film Deposition

Structure and Mechanical Properties 2-D Analogy of the Structure Hardness 3-D interlink of the atomic bond network Residual Stress Distortion of bond angle and length Both are dependent on the degree of 3-D interlinks.

Hardness and Residual Stress

Hardness and Residual Stress

Stress Reduction by Si Incorporation C.-S. Lee et al, Diam. Rel. Mater., 11 (2002) 198-203

Molecular Dynamics Simulation Deposited atoms created on this plane Brenner force field for C-C bonds Tersoff force field for C-Si and Si-Si bonds Diamond substrate : 6a0 x 4.75a0 x 6a0 1,368 atoms with 72 atoms per layer Deposition Total 2,000 atoms Incident Kinetic Energy : 75 eV for both C and Si Si concentration : 0.5 % ~ 20 % Fully Relaxed Layer Fixed Layer

Snapshots after Deposition 0.0 % 3.0 % 0.5 % 5.0 % 1.0 % 10.0 % 2.0 % 20.0 %

Residual Compressive Stress Experiment : C.-S. Lee et al, Diam. Rel. Mater., 11, 198 (2002).

Atomic Bond Structure MD Simulation Raman G-peak Position Experiment : C.-S. Lee et al, Diam. Rel. Mater., 11 (2002) 198-203

Radial Distribution Function Si-Si 1st bond역시 첨가되는 실리콘의 양은 증가함에도 불구하고 감소하는 것을 볼 수 있다. 대신 Si-C 본드가 선형적으로 증가하는 것을 알 수 있다. 이러한 거동은 앞서 보였던 stress, 밀도, sp3 ratio와 유사한 거동을 보이는데 즉 meta-stable 한 상태에 있는 탄소원자가 첨가되는 Si과 결합하여 Si-C 결합을 하며 안정된 상태로 전이되는 것을 알 수 있다. Bond C-C1st Si-C1st C-Cmeta Si-Si1st C-C2nd Distance (Å) 1.35 ~1.54a),b) 1.841 2.11 2.34a) 2.54

Effect of Si Incorporation : Atoms in calculation : Silicon atom : Carbon atom

Bond Angle Distribution 93.1 Pure ta-C ta-C:Si

W-DLC by Hybrid Ion Beam Deposition Wn+ H+, Cm+ This figure is the schematic diagram of present work. The carbon and hydrogen ions are come from the ion gun by decomposing the C6H6 precursor source. The W ions are fabricated by the DC magnetron sputter gun by inducing the W target with the sputtering gas Ar. The base pressure is kept at 2.0 10-6 Torr. The substrate bias is - 200 V. Dependent on the Ar fraction, the deposition pressure and power density of target is varied in a small range of 4.2~7.3 W/cm2 and 0.6 ~ 1 10-4 Torr , respectively. The film thickness is controlled around 350 nm by varying the deposition time. Working gas: Ar, C6H6 (total: 12sccm); Power density of target: 4.2~7.3 W/cm2 Subtrate bias: 0 ~ -900V; Thickness: 350±50nm  Sputter gun: Third elements addition to DLC (W, Ti, Si …);  Ion gun: Easy controlling the ion bombardment energy with high ion flux. A.-Y. Wang et al, Appl. Phys. Lett., 86, 111902 (2005).

Stress & Mechanical Properties 170±15 GPa 21±3 GPa The stress evolution as a function of W concentration is shown in this figure. It is apparent that the W incorporation significantly reduce the residual stress, especially when the W concentration is small. When the W concentration is increased from 0 to 4.2 at.%, the stress abruptly dropped from 3 GPa to 1.5 GPa. Further increasing W concentration to 5.2 at.% causes a temporary increase in the stress. thereafter, the stress monotonically decreases with the increase of W concentration up to 12. 5 at.%. To date, such a curious stress jumping phenomenon has never been reported on the other Me-DLC films. For the ease of next discussion, the position of 4.2 at.% W is defined by a stress jumping point and the total stress reduction area is divided into three regions.

TEM Microstructures 1.9 4 nm 4 nm 3.6 -W2C (101) Nano-crystalline -W2C phases evolve. W atoms are dissolved in a-C:H matrix. Amorphous to crystalline WC1-x transition occurs. 4 nm 2.8 8.6 4 nm -W2C(102) -W2C(101) High resolution TEM provide us more information on the microstructure of the films as a function of W concentration. During the first region, even the tungsten carbides is formed due to the strong tendency in carbide forming of W, they are still in the dominated amorphous structure, as identified by the diffuse diffraction rings. However, in region 2, the sharpen diffraction ring indicates the emergence of crystalline carbides, even in the manner of weak crystallinity. Entering into the region 3, it is evident that the fraction, grain size and crystallinity of carbides increase with increasing W concentration. the sharpen diffraction rings point at the -W2C. This kind of evolution of tungsten carbides can also be demonstrated by the grazing incidence XRD spectra. -W2C

Raman & EELS Spectra Ip/Is = 0.550.1 Raman and EELS spectra provide us more information on the atomic bond structure. From the Raman spectra, it is apparent that the peak intensity decreases due to the decrease in total carbon content in the film with W increasing. A similar result is also visible in EELS spectra. By fitting the Raman spectra, it is found that the G-peak position almost keeps constant regardless of the W concentration, which reflects the unvaried sp3 and sp2 fraction in the carbon network. In addition, the EELS spectra also demonstrate the sp2/sp3 ratio changes slightly in the film as a function of W concentration. In this case, it can be said that the nature of carbon matrix does not change with W incorporation, and which can’t account for the stress reduction behavior. There should be other changes of freedom degree of the structure.

TEM Microstructures 1.9 4 nm 4 nm 3.6 -W2C (101) Nano-crystalline -W2C phases evolve. W atoms are dissolved in a-C:H matrix. Amorphous to crystalline WC1-x transition occurs. 4 nm 2.8 8.6 4 nm -W2C(102) -W2C(101) High resolution TEM provide us more information on the microstructure of the films as a function of W concentration. During the first region, even the tungsten carbides is formed due to the strong tendency in carbide forming of W, they are still in the dominated amorphous structure, as identified by the diffuse diffraction rings. However, in region 2, the sharpen diffraction ring indicates the emergence of crystalline carbides, even in the manner of weak crystallinity. Entering into the region 3, it is evident that the fraction, grain size and crystallinity of carbides increase with increasing W concentration. the sharpen diffraction rings point at the -W2C. This kind of evolution of tungsten carbides can also be demonstrated by the grazing incidence XRD spectra. -W2C

Role of W atoms- ab initio calculation H W C H The left two figures are the schematic drawing of the calculation system. The four carbon atoms were arranged as a tetrahedron, with a carbon or w atoms located at the center. For simplicity, unbonded carbon bonds were passivated by hydrogen atoms. The total energy of the system was calculated using one of the bond angles being distorted from the equilibrium angle of tetrahedral bond over the range 90-130 degree.

Conclusions Various properties of a-C films generated by MD simulation agrees well with those of experimentally obtained a-C films. Brenner force field for C-C bond Tersoff force field for Si-Si and Si-C bond Stress reduction mechanism based on the atomic scale structure analysis Small amount of Si incorporation in a-C network prohibits carbon atoms from locating at a metastable site. W atoms dissolved in a-C matrix play a role of pivot site where the atomic bond distortion can occur without inducing a significant increase in elastic energy.