New small radius joints based on thermal shrinkage of polyimide in V-grooves for robust self-assembly 3D microstructures

A novel and simple technology for making robust three-dimensional (3D) silicon structures with small radii of bending has been developed and investigated. The proposed self-assembly method of bending 3D structures out of plane such that they stay bent without any interlocking braces is based on thermal shrinkage of polyimide in V-grooves. A wide range of bending angles for the permanent out-of-plane rotated structure can be chosen by varying the curing temperature of the polyimide. The relatively large thermal expansion of polyimide makes it possible to use the structure in a dynamic mode useful for compensation of undesired process variations. The proposed technique is compatible with both IC based surface micromachining and batch fabrication. Therefore, structures based on the new polyimide joint have general applications in micromachining and can be used in many new 3D sensors or actuators having detailed features in all three dimensions. For the tested polyimide V-groove joints, static bending angles between 0 degrees and 200 degrees have been achieved with a maximum bending angle of 35 degrees per V-groove. Bending radii smaller than 60 mu m for a 30 mu m thick out-of-plane rotated silicon plate have been measured. Electrical connections to the assembled structures have been investigated. It has been shown that the aluminium conductors crossing the V-grooves in the polyimide joint are not affected by the out-of-plane rotation.