Testing the adhesion of thin film to substrate

Testing the adhesion of thin film to substrate Student Talk 30. 3. 2016 Tia Lohtander

Buckling delamination of DLC film on Si substrate. Image from: http: //diamond. kist. re. kr/~mwmoon/gallery. htm

Adhesion • Durability and performance of thin film device • Sum of several factors • There is no one specific quantitative parameter – Contradictory results from different tests Possible types of interfaces Image from: Franssila S. Introduction to Microfabrication. John Wiley & Sons. 2010. Chapter 7. ISBN: 978 -0 -470 -74983 -8

Selection of test method • Simple tests – Industrial use – Estimation of thin film quality – Simple set-up • Complex tests – Research use – More precise quantitative results – Sample preparations and expensive equipment • For example: Pulsed laser or cross-section indentation

Scratch test set-up Image from: https: //www. pvd-coatings. co. uk/wp-content/uploads/scratch-adhesiontest. jpg

Results • Critical load = “adhesion value” • Frictional force, depth and visual changes Image from: Lee J-E. et al. Assessment of adhesion between thin film and silicon based on a scratch test. Journal of Material Science and Technology. 2010. doi: 10. 1007/s 12206 -009 -1124 -7

Limitations and problems • Most useful for hard coatings deposited on softer substrates • Standards only for some cases Lack of data • Safest compare data to results from same test method SEM image from scratch test of 100 nm Al film on stainless steel. Image from: Bull S. J. & Berasetegui E. G. An overview of the potential of quantitative coating adhesion measurement by scratch testing. Tribology International. 2006. doi: 10. 1016/j. triboint. 2005. 04. 013

References [1] Bull S. J. & Berasetegui E. G. An overview of the potential of quantitative coating adhesion measurement by scratch testing. Tribology International. 2006. doi: 10. 1016/j. triboint. 2005. 04. 013 [2] Franssila S. Introduction to Microfabrication. John Wiley & Sons. 2010. Chapter 7. ISBN: 978 -0 -470 -74983 -8 [3] Lee J-E. et al. Assessment of adhesion between thin film and silicon based on a scratch test. Journal of Material Science and Technology. 2010. doi: 10. 1007/s 12206 -009 -1124 -7 [4] https: //www. pvd-coatings. co. uk/pvd-coatingtechnology/testing-equipment/scratch-tester/

Detailed information: Why to test? • 2 main failure modes for thin films: buckling and wedge spallation • Adhesion is sum of many things. For example it depends on pre treatments like cleaning the substrate, deposition technique, substrate roughness, will the atoms are chemisorped or physisorped to substrate and if adhesive film is used or not etc. Interface structure (see slide 3) also impacts on adhesion • We want to prevent debonding and delamination because it impacts directly to the lifetime and performance of thin film device

Detailed information: How to test? • Basic adhesion= chem. bonds across the interface • Practical adhesion= Film adhesion is often characterized by the strain energy released per unit in delamination area, practical adhesion basically gives method-specific measurement. • Various ways to test: simple ones (peel tape test, pulloff, scratch) and more difficult ones (cavitation, laser acoustics) selection depending on how accurate results needed

Detailed information: Scratch test (1) • Rockwell diamond tip moves across the film at a constant velocity. At the same time applied normal force is steadily increased. The diamond tip causes stress between the thin film/substrate interface causing delamination of film. • The load that breaks the film is called critical load and can be used to describe adhesion in scratch tests. – Depends on set-up parameters and materials of film and substrate • The beginning of delamination and the critical load can be detected by observing changes in friction, viewing the sample with optical microscope and measuring the depths of scratches by AFM – The hardness of substrate and film affects how big critical load is needed. – The scratch test most suitable for hard coatings

Detailed information: Scratch test (2) • The test set-up is quite simple but comparing the results is not reliable always, especially between different tests • Standards for some cases: e. g. ASTM C 1624 -05 for hard thin ceramic coatings on metal or ceramic substrates • If any questions, send to tia. lohtander@aalto. fi