Fracture behavior and characterization of free-standing metal silicide thin films
Airat Shafikov is a PhD student in the research group XUV Optics. Supervisors are prof.dr.ir. J.P.H. Benschop and prof.dr. F. Bijkerk, co-supervisor is dr.ir. R.W.E. Kruijs from the Faculty of Science & Technology.
Mechanical reliability is an important property in many applications of free-standing thin films including EUV pellicles, X-ray and electron transparent membranes, and MEMS/NEMS devices. The work presented in this thesis is aimed to help improve the understanding of mechanical failure in brittle, free-standing thin films. It describes the experimental investigation of the link between microstructure and strength in thin films of polycrystalline metal silicides, and the development of techniques which enable detailed studies of crack propagation in free-standing thin films.
The first study presented in this thesis is motivated by the need for experimental and analytical procedures for the characterization of fracture strength. To do this, an experimental investigation into the failure of brittle Si3N4 membranes was conducted using the membrane bulge test method. It was found that brittle thin films exhibit two distinct modes of failure. These modes were identified by analyzing the distribution of strength values and the membrane residuals left on the frame, which indicate the origin of the fracture. The first mode is characterized by a narrow strength distribution and the fracture originates at the very edge of the membrane, where the generated stress is the highest due to bending. It is suggested that this failure mode characterizes the intrinsic fracture strength of the film, or its ability to resist crack nucleation under tensile stress in the absence of critical extrinsic defects. This notion is supported by the negligible spread and high average value of the fracture strength, which approaches the ideal strength estimate. The second type of failure was found to originate away from the edge, in the free-standing part where the generated stress is significantly lower than the stress at the edge. This type of failure is characterized by a large spread in the fracture strength values, which indicates that it is triggered by sparsely distributed extrinsic defects. The ability to distinguish between these failure modes is key to being able to use the bulge test for studies of intrinsic fracture strength of thin films. By introducing compressive residual stress at the surface of the membrane, the high bending stress generated near the membrane edge was reduced. This essentially delayed the intrinsic fracture mode, allowing a membrane without critical extrinsic defects to withstand a higher load without failing. Burst pressure of such strengthened membranes was successfully explained using the weakest link principle, i.e. by assuming that fracture of either of the two layers, the compressed surface layer and the unmodified main layer, is initiated independently and leads to the burst of the whole membrane.
Following the study of Si3N4 membranes, the bulge test method was used to investigate mechanical properties of transition metal silicides (MexSi1-x; Me = Zr, Nb, Mo; 0.6<x<0.8) prepared by magnetron sputtering and then annealed at 500 °C. It was found that the fracture strength of the three metal silicides shows similar dependence on the metal to silicon content ratio. Films with x=0.7 had the lowest strength while deviating from this composition resulted in a monotonous strength increase. The structure of the films, which was formed upon annealing, was investigated using XRD and, for selected Zr xSi1-x films, using TEM-EDX. It was found that upon annealing the films form a homogenous nanocrystalline structure corresponding to the respective disilicide: C40 for Nb xSi1-x and Mo xSi1-x C49 for Zr xSi1-x. It was concluded that variations of the fracture strength with changing composition are caused primarily by the differences in size and morphology of grain boundaries and the density of point defects, which are induced by the excess Me and Si atoms. Short and serrated grain boundaries are more resistant to crack nucleation because they can more evenly spread the plastic damage which is produced during grain boundary sliding and localized at grain boundary serrations and triple junctions. The presence of point defects is believed to reduce the resistance to plastic deformation in metal silicides and, therefore, help to delay crack formation.
The second part of the thesis describes the development of experimental techniques to study the resistance to crack propagation in brittle free-standing thin films. The existing crack-on-a-chip method was modified and adapted for ZrxSi1-x films. First, crack initiation was induced by a FIB milling step, which allowed to conduct the test in vacuum conditions and avoid the influence of the release step on the extracted toughness value. The second modification is the improved analysis, which takes into account the out-of-plane buckling of the free-standing thin film structures. It is demonstrated, that buckling has a strong influence on the results of toughness calculation due to the relaxation of compressive in-plane stresses that form in the thin film structure and, therefore, cannot be ignored.
Another development presented in this thesis is the addition of on-chip drawbridge-like actuation structures, which convert macroscopic bending deformation of the chip into a microscopic displacement. The main merit of this technique is that it enables direct observation and characterization of stable crack growth in brittle free-standing films, while keeping the specimen preparation and testing procedures simple. As a proof of concept, the technique was applied to study fracture of thin film SiN. This study revealed some key aspects of crack propagation, which are important for the correct interpretation of experimentally measured fracture energy (toughness). Specifically, it was shown that rapid cracks and cracks propagating under mode III loading are characterized by increased fracture energy compared to slow (quasistatic) mode I cracks.
