Thermal assembly of polysilicon microstructures

Thermal microassembly techniques are demonstrated which extend the capabilities of surface micromachining technology. Bridges are cleanly severed by application of a single 30 mA, 100 mu s pulse. Delicately suspended microstructures are supported by tee bridges during wet etching of phosphosilicate glass, in order to reduce yield loss due to breakage and stiction to the substrate during rinsing and drying. The supports are subsequently severed to release the structure. Mechanical contacts are welded together with 30 mA of current, with a microprobe used to force together the contacts. Qualitative destructive tests indicate that the welded contact is robust. Electrostatic force applied by interdigitated electrodes is insufficient to initiate welding of polysilicon, possibly because the native oxide film must be penetrated to allow current to pass through the contact. Current vs. voltage measurements of polysilicon microbridges agree well with a first-order model, in which heat conduction and convection in air are neglected. Values of the voltage needed to melt the bridge are found to vary with bridge dimensions, because the bridge anchors are not perfect heat sinks.<
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