MECHANICS OF THIN FILMS FOR FREESTANDING MICROFABRICATED INTERLOCKING STRUCTURES

Abstract

In this work, several major accomplishments are outlined which center around the large deflection modeling of cantilevers under point loading. The first accomplishment was using large deflection cantilever analysis to create several novel designs of microfabricated interlocking structures for the joining of microdevices as an improvement to a process called heterogeneous integration. The second accomplishment was developing a large deflection model for curved cantilevers under contact point loading to model force versus displacement of compliant hooks like those in VelcroTM and related hooks found in nature like in plant fruits and in bird feathers, something that has never before been done in the field of applied mechanics. This was done by extending the same mathematical methods used in the design of interlocking structures towards the design of structures with initial nonzero curvature. Current techniques of joining microdevices require the use of chemical adhesives like epoxy which have a tendency to fail when operated at high temperatures. Mechanical attachment using interlocking structures will remedy the many problems of traditional adhesives while simplifying the assembly requiring only mechanical force to join components. This innovation promises to simplify the assembly process of microelectronics using currently available fabrication techniques. Two viable designs are presented, one which uses flat cantilevers allowing for rework of components on assemblies, and design with non-flat cantilevers which enables a low force to join components while having a high pull-out force enabling a strong joint used in permanent connections. Analytical modeling predicts that interlocking structures with flat cantilevers would produce a bond strength of 6.3 kPa. From modeling in COMSOL Multiphysics, structures with non-flat cantilevers would have a maximum bond strength of 28 kPa which is comparable to commercially available VelcroTM. From analysis, it was determined that the cantilevers would deform plastically. To make the design viable it is suggested to use a compliant structure and a rigid structure, where the rigid structure is fabricated onto an assembly or device which is desired to be reused, and the compliant structures are fabricated onto devices which can be disposed of. This is the key to making interlocking structures for heterogeneous integration a viable approach to joining. Along with design, the process for manufacturing is presented along with the selected materials which were copper, titanium and gold. Perhaps the most important result of the large deflection modeling of hooks and curved cantilevers was the discovery of an apparent linear relationship between nondimensional force and nondimensional displacement when under a sliding point contact condition. This allows description of the curved cantilever with a simple equation based on linear fits to the force and displacement, another novel advancement for compliant system modeling. Testing of hooks fabricated from stainless steel showed that analytical model was able to predict the force versus displacement behavior for hooks with an error no more than ±5%.