|M.Sc Student||Benski Gilad|
|Subject||Study of the Fabrication of 3D MEMS Structure Using Self-|
|Department||Department of Mechanical Engineering||Supervisor||Professor Eyal Zussman|
A variety of MEMS devices require stacking of several different types of chips (e.g., optical, mechanical, electrical) and forming them into a spatial structure. Alignment of the chips and controllability of the joints (material composition, geometry) between the chips can be critical to the device performance. In order to minimize damage during handling, stacking is preferably done during fabrication, i.e. at the wafer level. This work deals with the development of a self-assembly technique relying on multiple-chip solder based joining which satisfies the above. The technique offers a low-cost, high throughput batch fabrication and is flexible with respect to the choice of ambient environment. Key process steps are the creation of solder joints, the pretreatment of bonding surfaces, the pre-sticking of the chips, and solder reflow in a clean controlled ambient using a designated oven. Assemblies based on eutectic SnPb(67/37) solder and Au as the top surface metallization have been reflowed in a vacuum and formic acid ambient and have then been characterized using geometrical parameters (i.e., misalignment and the gap between neighboring chips). The experimentally-measured solder joint size-displacement relationships were compared with predictions obtained from both a geometric based model and from a minimum energy based model generated with the Surface Evolver computer code. The comparison indicates that such models are an extremely accurate means of predicting the geometry of these solder joints. Also presented is a simulation of the heat transfer in the stack which helps in studying several essential parameters of the heating process required for the assembly. The Self-assembly method, has been found to be an efficient, predictable way to assemble multi-chip 3D stacks. This method preserves the important benefits of MEMS, and therefore may be considered a promising approach for the assembly of 3D structures.