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2838 |
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MEMS Kinematic Clamp for Repeatable Precision Alignment |
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Micro-Electromechanical Devices |
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| Content |
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This work presents the design and fabrication of micro-scale kinematic clamps for microelectromechanical system (MEMS) applications. Kinematic clamps have been used for myriad macro-scale engineering applications where precise and repeatable positioning is required; however, micro-scale kinematic clamps and passive precision alignment are relatively new and underutilized in MEMS. Conventional methods of wafer alignment involve optical inspection, which is not highly precise and is often time consuming. In contrast, with MEMS-scale Kinematic Clamps sub-micron alignment repeatability and reduced setup times for alignment are possible. Previous research has investigated the precision performance of using a MEMS-scale elastic averaging technique; however, the use of a purely kinematic clamp is novel. A MEMS kinematic clamp will enable technologies where precise alignment is required between dies or wafers such as optical MEMS, wafer and die-level bonding, and microfluidic lab-on-a-chip applications. This paper discusses the design and fabrication of a “three-vee” MEMS-scale kinematic clamp.
Properly designed kinematic clamps are deterministic; they only make contact at a number of points equal to the number of degrees of freedom (DOF) that are to be constrained. To fully constrain a fixture, three sets of vee-groove and post pairs can be utilized (2DOF per pair). This paper begins with a review of potential microfabrication methods for various vee-grooves and post designs. The three most promising methods based on fabrication feasibility were chosen for further analysis to determine their theoretical precision capabilities, specific fabrication challenges, and possible failure mechanisms.
Of the several methods analyzed, the most effective method for the fabrication of vee-grooves and posts utilizes Deep Reactive Ion Etching (DRIE) to etch deep trenches into the substrate for vee-grooves and to etch back parts of the wafer to create posts. Both the vee-grooves and the posts are then coated using a conformal Chemical Vapor Deposition (CVD) step to blunt the sharp corners and avoid the clamp from being over constrained. The layout of the trenches is defined through a single lithographic step and the trench positions are defined on the mask to provide a stable kinematic coupling.
Using the DRIE processes with conformal CVD for the fabrication of posts and grooves, the theoretical best achievable precision in the clamps was determined to be 2.6x10^-3 degrees, using the Karl Suss mask aligner, and 9.9x10^-6 degrees when using the Crestec electron-beam lithography tool (both tools are widely used in the UC Berkeley Microlab). Clamp loading considerations—such as failure in buckling and bending—were investigated and the effect of fabrication and design on the overall MEMS-kinematic clamp performance is also discussed. |
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