Available on-demand - F.EL01.05.05
Surface Contamination Impact on Nanodiamond Seeding of Plasma-Treated Tantalum Films
Ken Haenen1,2,Paulius Pobedinskas1,2,Wim Dexters1,2,Jan D'Haen1,2
Hasselt University1,IMEC vzw2
The deposition of closed nanocrystalline diamond (NCD) thin films on non-diamond substrates requires diamond nucleation sites. An established technique for creating nucleation sites is a surface treatment with a water-based colloidal solution of ultra-dispersed nanodiamond (ND) particles.1 In ND seeding the most important factors are the surface charge of the substrate and the zeta-potential of the used colloid. Together, they determine the nucleation density, which can lead to an extreme difference in the observed seeding density.2–6 However, surface contamination by hydrocarbons, which is an unavoidable and quick process even in a cleanroom environment,7 is often not taken into consideration. Such contamination can alter the surface wetting properties8 and surface charge9, and thus the eventual seeding density. In this work we investigate and model the impact such surface contamination has on the ND seeding efficiency.
Tantalum is a metal well known for its resistivity to corrosion, bioinertness, biocompatibility and excellent osteoconductivity.10 After sputtering a thin film of Ta on silicon substrates, these stacks were exposed to the ambient atmosphere for different times, allowing to achieve various degrees of surface contamination. Then, the Ta surface wettability was measured, samples were seeded with NDs, and the subsequent seeding density evaluated. As surface coverage by hydrocarbons increased, the wetting properties gradually turned from hydrophilic to hydrophobic. The ND seeding density first decreased and then increased again. To understand this correlation, we propose an analytical model that describes the dynamics of surface contamination and hydrocarbon contaminant interaction with the surface, and the resulting ND seeding density.
We assume that when a hydrocarbon (HC) molecule lands on the Ta surface it attracts negative surface charge towards it, while creating a positive surface charge around it. Due to said dipole formation, the top part of the HC molecule is charging negatively. Solving differential equations for the dynamics of the normalized surface coverage number density of the HC molecules, n, and surface charge density, σ, we derive σ (n,η) = n + (1 – n)(1 + η), where η is a ratio between an area of the depleted charge around the HC molecule and the area of the HC molecule itself. The assumption that the contact angle, θ, is linearly proportional to the surface coverage by HC, n = θ /θsat, makes wettability measurements a valid method for determining the degree of surface contamination. If the sum of electrostatic double-layer and Van der Waals interaction energies becomes positive, a potential energy barrier for a ND particle approaching a surface exists. ND particles must surmount a positive potential peak for a deposition onto the surface to occur. This occurs with a probability exponentially proportional to the potential barrier. By fitting the experimental data, we find that the ND seeding density is exponentially proportional to the calculated positive surface charge number density, which forms around HC molecules. This constitutes 57% of the HC molecule area, while the created barrier height is estimated to be ~ 0.18 eV.
 O.A. Williams, et al., Chem. Phys. Lett. 445, 255 (2007).
 O.A. Williams, et al., ACS Nano 4, 4824 (2010).
 P. Pobedinskas, et al., Appl. Phys. Lett. 102, 201609 (2013).
 J. Hees, et al., Nanotechnology 24, 025601 (2013).
 G. Degutis, et al., Chem. Phys. Lett. 640, 50 (2015).
 S. Mandal, et al., ACS Appl. Mater. Interfaces 11, 40826 (2019).
 K. Takeda, et al., Proc. 44th Annual Technical Meeting of the IEST and ISCC, 556 (1998).
 Ch. Mücksch, et al., J. Phys. Chem. C 119, 12496 (2015).
 A. Khachatourian, et al., J. Chem. Phys. 140, 074107 (2014).
 H. Matsuno, et al., Biomaterials 22, 1253 (2001).