Micromechanisms Using Sidewall Beams

A process technology is developed for surface micromachining that achieves sub-micrometer linewidths without the use of advanced lithography equipment. This technique is based upon the sidewall spacer technique that has been used extensively in integrated circuits. The sidewall spacer technique, stated simply, is to deposit a conformal film over a vertical step on the substrate. The conformal film is then anisotropically etched leaving a siclewall spacer or beam. Traditionally these sidewall "stringers" were a nuisance in surface micromachining great care had to be expended to remove these structures. These sidewall beams, however, have many favorable characteristics that motivated this work. First, the lateral conlpliance per unit area is dramatically reduced. This has direct implications for structures that potentially could be fabricated in arrays, such as accelerometers, gyros, and micropositioners. Second, the sidewall beams have a high aspect-ratio and thus high cross-axis selectivity. Finally, the sidewall beams can be cornbined with other thin structural layers to form beams with coniples cross sections such as 0,L_ ], L, and I beams. Polysilicon is the primary film used in creating sidewall beams in the process technology that is developed. Polysilicon is by no means the only film that can be used. In fact, any film with reasonable conformality should work well.
Improved designs of micromechanical structures require better models of the behavior of structures. In this dissertation, the folded-flexure lateral comb drive resonator is used as a test vehicle of the sidewall beam process technology. An extensive analysis of this device, which points out many second-order effects, is presented. Finite element analyses are performed to check the analytical results and to compare the results with experimental data.
Three applications of sidewall beams are investigated as proofs of concept. The first application is called self-adjusting microstructures (SAMS). This structure is a sidewall beam bimorph of polysilicon and silicon nitride. Once the structure is released, it deflects laterally due to an intensional residual stress gradient. The goal is to create a self-actuated in situ assembly actuator that will reduce intercomponent lateral clearances and apply lateral bearing pre-loads. The second application is called hollow beam lateral resonators. Sidewall beams are cornbined with thin planar beams to create a hollow beam. These hollow lateral resonators have a higher stiffness to mass ratio than conventional polysilicon lateral resonators and thus a higher resonant frequency. The third appLication is called highly compliant suspensions using sidewall beams. These devices use only a polysilicon sidewall beam as the mechanical suspension. The goal is to maximize compliance while minimizing area.
Publication date: 
April 30, 1994
Publication type: 
Ph.D. Dissertation
Judy, M. W. (1994). Micromechanisms Using Sidewall Beams. United States: University of California, Berkeley.

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