Method to promote adhesion of DLC thin film on silicon substrate Jarkko Etula 09.03.2016

Adhesion of protective DLC coatings on silicon Thick and hard films are ideal for long wear life  Compressive stress in DLC films from high energy deposition  Stress concentrates on the weak substrate interface A thin film of thickness h delaminates when the film stress elastic energy σ exceeds surface fracture energy γ  Adhesion is good as long as: γ> σ 2 h 4 𝐸 Stress, modulus and hardness are proportional to each other  Maximum hardness = Maximum compressive stress  Limited film thickness [Robertson 2002]

Titanium adhesion layer: TEM image Figure 1. Cross-section TEM image of Si/Ti/DLC interlayer. [Laurila et al. 2014]

Mechanism 1/2: Soft Ti adhesion layer Relative increase of surface fracture energy γ by: Soft intermediary layer of titanium Will locally yield and absorb the compressive stress Very good adhesion to both Si and DLC Also forms carbides Ti 1−𝑥 C 𝑥 Increased adhesion [Robertson 2002, Laurila et al. 2014] Figure 2. Thickness profile of Si/Ti/DLC adhesion layer

Mechanism 2/2: Gradient nature of Ti adhesion layer Relative increase of surface fracture energy γ by: Relocating compressive stress on substrate interface Gradient interface creates stress relief mechanisms over larger volumes Varying stoichiometry across  Hardness gradient Figure 3. Thickness profile of the gradient nature of Ti/DLC interface layer [Robertson 2002]

Revisited: Ti adhesion interlayer Figure 4. Cross-section TEM image of Si/Ti/DLC interlayer. [Laurila et al. 2014]

Questions ? References γ> σ 2 h 4 𝐸 Robertson, J. 2002, "Diamond-like amorphous carbon", Materials Science and Engineering: R: Reports, vol. 37, no. 4-6, pp. 129-282. Laurila, T., Rautiainen, A., Sintonen, S., Jiang, H., Kaivosoja, E. & Koskinen, J. 2014, "Diamond-like carbon (DLC) thin film bioelectrodes: Effect of thermal post-treatments and the use of Ti adhesion layer", Materials Science and Engineering: C, vol. 34, no. 0, pp. 446-454.

Information Slide (1/2) Compressive stress is created during high sp3 content film depositions due to high energy carbon ion sub-implantation model. Elastic energy due to stress σ depends a lot on the deposition technique: Hard DLC films have nevertheless high compressive stress and high elastic modulus: poor adhesion. Addition of most metal dopants decreases sp3 content in DLC films. This significantly decreases the compressive stress therefore increasing adhesion. Some metals, such as Si, Cr, Ti of W are carbide formers that make hard stoichiometric compounds in the interlayers: Strong adhesion. Gradient mixing is important especially for soft substrates to provide a good contact and larger stress absorbing volumes. Effect of film grain size on hardness: Hall-Petch equation.  On the contrary, nanogranular materials are softer due to increased intergranular slipping In ideal adhesion, under wear, the film fractures from within the substrate. [Robertson 2002]

Information Slide (2/2): Additional solutions Additional strategies to maximize adhesion between the substrate and film: Surface preparation: Argon cleaning of the surface to remove adsorbed gases, surface oxides and contaminants. Ion beam mixing between the film and the substrate to create a mixed interface (gradient). Heterogeneous films to create various stress relief mechanisms and load balance. Multilayers to provide internal stress relief and balance between multiple interlayers. [Robertson 2002]