MEMS Microshells for Microneedles, Microscale Fluid Visualization, and Vacuum Packaging of Microdevices

Three MEMS microshell devices are developed in this work. The first device is an isotropically etched microhypodermic injection needle (microneedle); the second device is a transparent, silicon nitride flow tube for fluid flow visualization; and the third device is a wafer-level vacuum package using permeable polysilicon. The process steps involved in creating the microshell devices, along with the design issues and experimental results, are presented.
A robust, implementable MEMS fabrication process using isotropic etching of silicon-on-insulator (SOI) wafers was developed to produce hypodermic injection needles sharper and smaller than those available in stainless steel. Initial problems with microneedle bottom roughness and tip hooking were addressed. Tip radius was found to be as small as 10 nm. Typical microneedles ranged in size from 1 to 5 mm in length, 80 to 300 pm in width, and 50 to 100 pm in thickness. The buckling strength of a 3 mm long and 300 pm wide needle was found to be as high as 0.68 N. Expected fluid flow rates through the needle are approximately 35 nL/sec for a 300 kPa pressure drop along a 3.5 mm long channel. Also, microneedles without channels were fabricated for use as miniature lancets (microlancets).
The visualization of fluid flow in MEMS devices is chalienging since the majority of fluidic MEMS devices are made on an opaque substrate. A process was developed that used the conformal nature of low-stress LPCVD silicon nitride to form tubes inside of bonded silicon wafers. Using KOH, all of the silicon substrate was removed and silicon nitride tubes remained. To nucleate bubbles, gold heating elements were provided. Wall thickness was shown to be related to geometry. For a deposition resulting in 4 um at the channel entrance, a 25 um wide channel had only 0.9 um of silicon nitride 4 mm into the channel, whereas a 100 um wide channel had 1.9 um.
Many resonant MEMS devices must be vacuum packaged. A new, robust fabrication process using permeable polysilicon etch-access windows allows fast removal of sacrificial material and eliminates the internal deposition of sealing film. Shells as wide as 1 mix have been cleared of PSG in under two minutes. Quality factors of encapsulated lateral resonators were shown to be in the order of 3000.
Publication date: 
November 30, 1998
Publication type: 
Ph.D. Dissertation
Lebouitz, K. S. (1998). MEMS Microshells for Microneedles, Microscale Fluid Visualization, and Vacuum Packaging of Microdevices. United States: University of California, Berkeley.

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