Micromechanical characterization of ALD thin films

Abstract

Atomic layer deposited (ALD) films have become essential for various microelectromechanical systems (MEMS) due to their excellent properties: ALD films are conformal, uniform, dense, and pin-hole free. The main requirement for any film to be applied in MEMS is to exhibit good mechanical properties. Good mechanical properties mean that film has low residual stress, high fracture and interfacial strengths, and known elastic properties under applied mechanical load. MEMS devices are often subjected to the environmental stress. Therefore, it is important to evaluate mechanical properties also after environmental stress conditions. In this doctoral dissertation, the mechanical properties of ALD thin films are evaluated by means of bulge and MEMS shaft-loaded techniques (SLT). Both techniques are very valuable because mechanical properties of thin films are extracted without influence of underlying substrate. The bulge method is a non-contact method, in which overpressure is applied to load free-standing membrane until it fractures.In the MEMS SLT, the integrated shaft loads free-standing membrane facilitating the extraction of mechanical properties.The developed technique is attractive for characterization mechanical properties of variable thin films due to offered repeatability, precision, and non-piercing nature (the premature fracture by sharp indenter tip is avoided). In this doctoral dissertation, MEMS SLT was employed, in addition, for quantitative and qualitative evaluation of interfacial strength between two thin films. A new method to study adhesion between extra thin films and various substrates was developed (when conventional scratch testing is not appropriate: when substrates or coatings break before the coating is delaminated). The solution was to embed micro-spheres into the coating. These spheres were laterally detached using microrobotic set-up. This approach facilitated the extraction of interfacial mechanical properties, such as critical load and critical stress needed for removal of a coating. This doctoral dissertation describes the mechanical properties of ALD Al2O3, Al2O3/TiO2 nanolaminates, AlxTiyOz mixed oxide and graphene/ALD Al2O3 composites. These materials are promising for MEMS as suspended membranes in thermal devices like bolometers, in chemical sensors like microhotplates and as windows in X-ray optics. The adhesion properties between sputtered films and ALD Al2O3 were measured with MEMS SLT. A new method with the lateral displacement of microspheres led to extraction of interfacial properties between ALD TiO2 and glass substrate. This information is important to prevent debonding events when fabricating or using MEMS structures.