Ai-Ying Wang, Hyo-Shin Ahn, Kwang-Ryeol Lee* Carbon2005 Stress reduction and structural evolution of W incorporated amorphous hydrogenated carbon films Ai-Ying Wang, Hyo-Shin Ahn, Kwang-Ryeol Lee* Future Technology Research Division, Korea Institute of Science and Technology

Diamond-like Carbon Film Amorphous Solid Carbon Film Mixture of (sp1), sp2 and sp3 Hybridized Bonds High Content of Hydrogen (20-60%) Properties High Hardness and Excellent Tribological Properties Smooth Surface with Optical Transparency Chemical Inertness and Hemo-compatibility Heart valve Hard disk

Obstacles  High residual stress and poor adhesion. 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

Approaches  Substrate bias applying Post-deposition thermal annealing B, N doped DLC films. Interfacial layer or graded multilayer Metal doped DLC films.( Ti, Si, W, Al, Co, V et al). Considerable improved performance not only on stress reduction but also on modification of thermal stability, electronical and tribological etc.

Me-DLC nanocomposite films  ~2nm Refractory phases: nc-TiN, TiC, WC … 2~10 nm Primary phases: a-C, C:H, Si3N4… Hardness, thermal stability, friction behavior, no grain boundary sliding, no dislocations activity, strong interface, strong segregation (immiscibility). Origin of stress reduction with Me incorporation is still disputable due to the difference in deposition processes and used metal sorts.

Motivation of this work Residual stress and mechanical properties of W incorporated diamond-like carbon (W-DLC) films? What is the dominant structural factor that changes the residual stress of the W-DLC film?

Hybrid ion beam system Wn+ H+, Cm+ Sputter gun: Third elements addition to DLC (W, Ti, Si …); Ion gun: Easy controlling the ion bombardment energy with high ion flux.

Deposition parameters Working gas: Ar + C6H6 (total: 12sccm) Base pressure : 2.0 10-6 Torr Substrate bias : - 200 V Power density of target: 4.2~7.3 W/cm2 Deposition Pressure : 0.6 ~ 1 10-4 Torr Thickness: 350±50nm Substrate: P-type Si(100), 500m, 100m Deposition temperature: < 200C Si sub. W-C:H

Composition of the Deposited Film Si in substrate C W Ar Deriving from the RBS spectra, the W concentration in the film as a function of Ar fraction in the gas mixture is shown in the right figure. With increasing Ar fraction, the W concentration increases monotonically in a manner of linear correlation. However, it is note that W-DLC films can’t be obtained as Ar fraction is lower than 55% due to the unstable operation of sputter gun and serious target poisoning. Also beyond the 90%, the films displays the typical characteristics of pure W films with metal gloss.

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

GIXRD 1. amorphous structure. 2. weak crystallinity of WC1-x. 3. high crysallinity of -W2C. The increase in size and crystallinity of tungsten carbides particles with increasing W concentration. From the GIXRD spectra, the amorphous structure of carbides in region 1 is typically presented by the flat curves. In region 2, the emergence of broaden peak reflects the weak crystallinity of crystalline carbides. In region 3, both the increase in peak intensity and decrease in peak width illustrate the high crystallinity and fraction increase of crystalline carbides. It is obvious that this results are agree well with the TEM results.

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.

Stress evolution in W-C:H films When W atoms were dissolved in amorphous carbon matrix without forming nanosized WC1-x crystalline phase, the reduced directionality of W-C bonds relaxed the stress of the distorted bonds. When crystalline WC1-x nanoparticles formed, the mechanical properties of the film was governed by the rule of sum between WC1-x and a-C:H phase. (incoherent stress relaxation or percolation structural transition from a-C:H rich structure to carbide-rich one. )

Conclusions  W incorporated a-C:H nanocomposite films were prepared by hybrid ion beam deposition system.  W concentration was controlled by the Ar fraction in the gas mixture.  A significant stress reduction was obtained with W incorporation without the degradation of mechanical properties. The stress was determined in terms of the dissolved W atoms in amorphous carbon matrix and the evolution of the crystalline WC1-x phase. The mechanical properties were dominated by the unchanged amorphous carbon matrix since no percolation structure transition from a-C:H to WC1-x occurred in such low range of W concentration.

Other Candidate Elements