26~27, Oct., 2006 Jeju ICC 전산재료과학분과 심포지엄 제일원리계산에 의한 금속이 혼입된 DLC 박막의 결합특성 고찰 한국과학기술연구원 미래기술연구본부 최정혜, 이승철, 이광렬

Diamond-like carbon (DLC) films Amorphous Carbon Film Mixture of sp 1, sp 2 and sp 3 Hybridized Bonds High Content of Hydrogen (20-60%) Synonyms –Diamond-like Carbon –(Hydrogenated) amorphous carbon (a-C:H) –i-Carbon –Tetrahedral Amorphous Carbon (ta-C) ta-C sp 2 H sp 3 DAC PAC ta-C:H GA C No film

High hardness High wear resistance Low friction coefficient Optical transparency Chemical inertness Smooth surface Bio-compatibility High hardness High wear resistance Low friction coefficient Optical transparency Chemical inertness Smooth surface Bio-compatibility Hard disk Video Head Drum Coronary Artery Stent Hip Joint Protective coating Bio materials Protective coating Bio materials Diamond-like carbon (DLC) films

Disadvantages of DLC films Hard disk Before deposition After deposition M. W. Moon, Acta Mater., (2002). High residual compressive stress (6~20 GPa) High residual compressive stress (6~20 GPa) poor adhesion Substrate bending Delamination

Structure and property relationship Hardness Substrate biasing Post-annealing Metal incorporation ; Ti, W, Mo, Cr, Al…. Substrate biasing Post-annealing Metal incorporation ; Ti, W, Mo, Cr, Al….

Metal-incorporated DLC films A.-Y. Wang APL (2005). Carbon (2006). 2 nm 0.1 at. % Ag 1.9 at. % W a-C:W a-C:Ag H.-W. Choi, unpublished work Not fully understood yet !!! Mechanism ?

Purpose of this work The effect of metal incorporation on the stress reduction  atomic bond characteristics ?? DLC ; sp 3, sp 2, sp bonding distorted sp 3 ; primary cause of the residual stress Diamond ; ideal sp 3 bonding o ≠109.5 o First-principles calculations - the dependency of total energy of the system on the bond angle - the electronic structure and its effects on the stress reduction behavior of a-C films

Tetrahedron bond model tetrahedral bonding of carbon(or Me)-carbon  structure relaxation  total energy calculation ; reference state tetrahedral bonding of carbon(or Me)-carbon  structure relaxation  total energy calculation ; reference state Bond angle distortion  bond distance relaxation  total energy calculation Bond angle distortion  bond distance relaxation  total energy calculation o Me 90 o ~ 130 o Me 90 o ~ 130 o C o C  E C-C  E Me-C Me; Mo, Ag, Al

Calculation condition by VASP  DFT scheme  E cut = 550 eV  Exchange-correlation potential; GGA (PBE)  Projector Augmented-Wave (PAW) potential  Gaussian smearing factor = 0.05 eV  Spin-unrestricted calculations  Convergence = eV  Ionic relaxation; CG method (force < 0.01 eV/Å)  Gamma point calculation (15x15x15 Å 3 )

Total energy change by the bond angle distortion  Increase in total energy drastically decreases by Me-incorporation.  Noble metal shows lower increase in total energy by the bond angle distortion than transition metal.  Al shows a negative energy change by the bond angle distortion.  Increase in total energy drastically decreases by Me-incorporation.  Noble metal shows lower increase in total energy by the bond angle distortion than transition metal.  Al shows a negative energy change by the bond angle distortion.

Charge density of HOMO C  Max =1.05 d= o z Covalent bonding [e/Å 3 ] [Å]

Charge density of HOMO MoAg  Max =0.69 d=2.10  Max =0.63 d=2.27 C  Max =1.05 d=1.54 bonding nonbonding antibonding Al  Max =0.69 d=2.05 ionic

Partial density of states s, p, d MoAg C Al bonding nonbonding antibonding ionic s, p, d orbitals

Al a-C:Al films Residual stress Hardness Young’s modulus sp 3 sp 2  E 90 < 0 P. Zhang, J. Vac. Sci. & Tech. A (2002).

Summary Mo Ag  Max =0.69 d=2.10  Max =0.63 d=2.27 C  Max =1.05 d=1.54 Al  Max =0.69 d=2.05 Atomic bond structure  role of metal incorporation in a-C films  a guideline for the choice of a metal element to control the residual stress of a-C films without a substantial degradation in the mechanical properties. Atomic bond structure  role of metal incorporation in a-C films  a guideline for the choice of a metal element to control the residual stress of a-C films without a substantial degradation in the mechanical properties. Hardness Sp fraction Sp 3 fraction Residual compressive stress C Mo Al Ag