Lithographic semiconductor microfabrication has been employed with great success for the manufacture of mechanical structures, achieving microscopic dimensions and great complexity at low cost. However, due to the inherently planar nature of the lithographic process, this technology is limited in its capability to produce three-dimensional structures. In this work, three strategies are presented for extending thin-film microfabrication into the third dimension.
First, the pop-up mechanisms of children’s storybooks are utilized on the micro scale to assemble three-dimensional microstructures out of planar parts. Particular attention is given to hinge design in the four-level SUMMiT surface micromachining foundry process. The use of pop-up mechanisms enables more efficient assembly of fold-up MEMS structures and also creates the potential for the parallel assembly of complex three-dimensional microstructures.
Next, a conformal carbon film is produced for use as a release layer in the molding of polysilicon structures. Parylene C polymer is deposited from the vapor phase as a conformal film and then carbonized at 825 ºC in N2. Pretreatment in a CHF3 and He plasma in addition to a pre-curing step at 490 ºC are employed to minimize distortion in the carbonization process, during which a one-eighth reduction in thickness occurs. The extremely thin (0.3 μm) and conformal carbon layer yields sub-micron precision in molding. Further, the release process is dry, rapid and extremely selective, allowing large structures to be released without damage. Release is accomplished by oxidizing at 825 ºC in dry O2 gas, achieving a burnout rate of 50 μm/min for the first 2 mm of undercut.
Finally, parylene structural films are molded and subsequently released without the need for a sacrificial layer or release etch. Application of Micro-90 detergent solution to the mold prior to parylene deposition prevents adhesion. A number of strategies are explored for the aligned bonding of these parylene layers, with thermal-compression showing the most promise. One possible application is a bistable fluid valve for use in an ocular implant for glaucoma therapy. Molded fluid channels and a bistable valve element are demonstrated.
December 19, 2002
Hui, E. (2002). Assembly and Molding Processes for Three-Dimensional Microfabrication.