Selective Encapsulations of MEMS: Micro Channels, Needles, Resonators and Electromechanical Filters

Surface and combined surface/bulk micromachining techniques are developed for selectively encapsulated micro devices including micro flow channels, micro hypodermic needles, hermetically sealed lateral micro resonators and electromechanical filters. As part of this research, unencapsulated devices are also fabricated: as proof-of-principle demonstrations of bubble-powered micro actuators and to assist process control of passive micro residual strain gauges.
Micro flow channels consist of silicon nitride are made on silicon substrate using a surface encapsulation process. These micro channels have trapezoidal cross sections, nominally 30 um in width and 7.5 um in depth. Built-in phosphorus doped polysilicon line resistors, 50 x 2 x 0.3 um^3, are embedded at the bottom of the flow channels. These resistors function as micro heaters to generate micro bubbles for the actuation sources. An electrothermal model of the micro heaters is established to characterized bubble formation phenomena both in open environment and inside the micro channels by using water, methanol and Fluorinert as working liquids. The nucleation of bubbles, controllability of bubble sizes, bubble movements due to variable channel dimensions and Marangoni effects have all been observed and studied.
Micro hypodermic needles have been fabricated by the integration of surface encapsulation and bulk micromachining process. These microneedles have micro flow channels supported by heavily boron doped silicon and single crystal silicon underneath. They are separated from the substrate in the final form with lengths'from 1 to 6 mm. Access to the flow channels is provided at their shank and distal ends through 40 um-square apertures in the overlying silicon nitride layer. The microneedles are found to be intact and undamaged following repetitive insertion of animal-muscle tissue.
As the third example of encapsulation process, lateral microresonators including micro electromechanical filters are selectively vacuum-sealed by lowstress silicon nitride shells. This planar hermetic sealing process provides an inexpensive way to protect moving micro structures. It also increases quality factors for microresonators such that the encapsulated comb shape microresonators have a measured quality factor of 2 200. Micro electromechanical bandpass filters based on coupled, lateral comb micro resonators are demonstrated with a measured center frequency of 18.7 kHz and a bandwidth of 1.2 kHz operated in air.
Liwei Lin
Van P. Carey
Publication date: 
November 30, 1993
Publication type: 
Ph.D. Dissertation
Lin, L. (1993). Selective Encapsulations of MEMS: Micro Channels, Needles, Resonators, and Electromechanical Filters. United States: University of California, Berkeley.

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