S15-O-13 10~14, Sep., 2006 Jeju, Korea IUMRS-ICA-2006 First-Principles Study on Stress Reduction Behavior & Bond Characteristics of Metal-Incorporated Amorphous Carbon Films Jung-Hae Choi, Seung-Cheol Lee, and Kwang-Ryeol Lee Future Technology Research Division Korea Institute of Science and Technology choijh@kist.re.kr http://diamond.kist.re.kr/DLC

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

Disadvantages of a-C films High residual compressive stress (6~20 GPa) poor adhesion Hard disk Before deposition After deposition Substrate bending Delamination M. W. Moon, Acta Mater., 50 219 (2002).

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

Metal-incorporated a-C films a-C:W a-C:Ag Not fully understood yet !!! Mechanism ? 1.9 at. % W 2 nm 0.1 at. % Ag A.-Y. Wang APL 86 111902 (2005). Carbon 44 1826 (2006). H.-W. Choi, unpublished work

Purpose of this work a-C; ; sp3, sp2, sp bonding Diamond ; ideal sp3 bonding 109.5o ≠109.5o a-C; ; sp3, sp2, sp bonding distorted sp3 ; primary cause of the residual stress The effect of metal incorporation on the stress reduction  atomic bond characteristics ?? 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 C 109.5o Me tetrahedral bonding of carbon(or Me)-carbon structure relaxation total energy calculation ; reference state DEC-C DEMe-C 90o~ 130o C 90o~ 130o Me Bond angle distortion bond distance relaxation total energy calculation Me; Mo, Ag, Al

Calculation condition by VASP DFT scheme Ecut = 550 eV Exchange-correlation potential; GGA (PBE) Projector Augmented-Wave (PAW) potential Gaussian smearing factor = 0.05 eV Spin-unrestricted calculations Convergence = 10-5 eV Ionic relaxation; CG method (force < 0.01 eV/Å) Gamma point calculation (15x15x15 Å3)

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.

Charge density of HOMO C 4 5 3 2 1 z Covalent bonding [e/Å3] [Å] rMax=1.05 d=1.54 Covalent bonding [e/Å3] [Å]

Charge density of HOMO C Mo Ag Al bonding nonbonding antibonding ionic rMax=1.05 d=1.54 rMax=0.69 d=2.10 rMax=0.63 d=2.27 rMax=0.69 d=2.05 bonding nonbonding antibonding ionic

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

a-C:Al films DE90 < 0 sp3 sp2 Residual stress Hardness Young’s modulus P. Zhang, J. Vac. Sci. & Tech. A. 20 390 (2002).

Summary C Mo C Mo Ag Al Ag Al d=1.54 d=2.10 Atomic bond structure Sp3 fraction Hardness Residual compressive stress C Mo Al Ag C Mo rMax=1.05 d=1.54 rMax=0.69 d=2.10 Al Ag 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. rMax=0.63 d=2.27 rMax=0.69 d=2.05