Development of mechanical characterization methods for thin films and interfaces

Jussi Aav
Abstract & Cover

Probably the most important thin film characteristic is its' adhesion to the used substrate. It is  very important to understand the fundamental mechanics of adhesion-related failures, and by  having suitable characterization methods to detect any problems as early as possible. Tailored or  correctly conducted quantitative analysis of adhesion is required for building reliable devices. In  many conventional test methods significant loading to the substrate is applied which can result in  problems especially with brittle substrates where the substrate can break before the film is  delaminated. The characterization of interfacial mechanical properties of increasingly thinner films  is challenging with many practical shortcomings and thus method development is needed.  In this thesis, three measurement methods where the loading to the substrate is minimized are  presented and demonstrated for the interfacial and mechanical testing of especially atomic layer  deposited (ALD) thin films. (i) Microelectromechanical system (MEMS) test chip assisted shaftloaded  blister testing through a hole in the substrate to the backside of the thin film, (ii)  microrobotic manipulation of embedded microspheres using lateral loading mostly to the thin film  and to the interface and (iii) a combination of nanoscratch and scanning nanowear for minimized  interaction volume of loading to the substrate. The relationship between adhesion and cohesion  as competing processes during film/ substrate failure is shown. When the film-interface-substrate  system is under loading the energy will dissipate through the path of least resistance. This will  happen either (i) through plastic deformation of the coating/ substrate, (ii) film fracture (decohesion) or (iii) delamination (de-adhesion) of the film. Usually, however the energy is  dissipated as a combination of these three different mechanisms, unless some of the mechanisms  is dominant in the energy release.  The presented characterization methods are mostly generic, and can be applied for the evaluation  of mechanical and interfacial properties, such as adhesion, between practically any materials of  choice with some limitations. Compared to some of the existing methods, the quantitative nature  of these characterization methods enables a more in-depth possibility for the analysis, understanding, tuning and improvement of the properties of the thin films and processes aiding  in maintaining and improving product and process quality. The main outcome of this thesis is that  the authors have demonstrated the potential and versatility of especially the MEMS test structures  and microrobotic testing systems, either on their own or as a combination as a solution to  developing new tailored interfacial and mechanical characterization methods for current and future  needs of research and the industry. 

Aalto University School of Chemical Engineering
(Espoo, Finland)
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