Microfabricated High Aspect Ratio Silicon Flexures

Christopher G. Keller, Ph.D

MEMS Precision Instruments (publisher)

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High Aspect Ratio Microfabricated Silicon Flexures for in-plane motion were made by three methods:

(1) 2-sided timed bulk etching in aqueous potassium hydroxide(KOH)
(2) deep reactive ion etching (RIE) with Cl2/He, and with SF6 (Bosch process)
(3) the HEXSIL molding process

KOH and RIE are applied to single crystal silicon, while HEXSIL yields polysilicon flexures.

Aspect ratios up to 20:1 are demonstrated. Dimensions ranged from 2 to 100 microns in width, 10 to 5000 microns in length, and 20 to 500 microns in thickness. Guidelines for robust flexure design, and processing details are provided.

Example devices presented include MICROTWEEZERS for micro pick and place assembly, fracture stress and fracture toughness specimens, and transmission electron microscope stages for thin film specimens.

The major contribution of this work to the field of MEMS is the invention and development of high aspect ratio molded low pressure chemical vapor deposited (LPCVD) polysilicon, a new process known as hexsil. Hexsil is a basic technology for the fabrication of micrometer to centimeter scale polysilicon structures. Molds are plasma etched in silicon wafers and are reusable. The high aspect ratio honeycomb geometry can be deposited economically as a conformal thin film. This allows large z dimensions while requiring only a few micrometers of conformal sidewall deposition. Up to three different beam com positions have been integrated into the same device by using mold trenches of three differ- ent widths: electroless nickel filled beams for metallic conductivity, in-situ phosphorous doped polysilicon beams for resistive thermal expansion elements, and undoped polysili con for the insulating body of the micromachine. Beams that deflect vertically after HF release were made by depositing two or more layers that have different values of residual stress. Polysilicon tubing was made using a 2-wafer mold. Molded milliscale structures have been used as the mechanical foundation for surface micromachined layers with micrometer scale features.

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