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High-aspect ratio (HAR) silicon molds formed with deep-reactive ion etching (DRIE) technique are difficult to use as masters for replication, especially for replication of high-aspect ratio microstructures. Microstructures in silicon molds made by DRIE have high surface energy and wavy vertical side walls, both of which make demolding difficult. In the research reported here, microstructured silicon master molds were made with the DRIE technique; the mold microstructures were "trenches" with maximum depth and width of 146 μm and 9.8 μm, respectively. Silicone rubber replicas could not be successfully demolded from such a high aspect ratio (15) silicon master without further surface modification. A second similarly etched mold was plasma treated with C4F8 for 120 seconds after etching (postpassivated). This treatment was found to be successful; five faithful replicas in silicone rubber were successfully demolded from the postpassivated mold whereas the replica mold broke during demolding from the unpostpassivated master, leaving the master mold microstructure filled with silicone rubber. At higher aspect ratio of 18, the silicon mold had to be plasma treated with C4F8 for at least 400 s before five replicas could be demolded successfully. Post-passivation with C4F8 was found to reduce the root mean square sidewall roughness of the silicon master mold trenches by about halves. The nanotribology of the silicon surface was studied with atomic force microscopy. The friction coefficient of the silicon surface was found to be reduced six-fold by the postpassivation technique. XPS analyzes showed that when post-passivation timing was increased, concentration of C-F increased while that of C-CF decreased and the thickness of the fluorinated coating increased. These results show that postpassivation of silicon microstructures made by DRIE can be used to fabricate much higher aspect ratio molds that has heretofore been considered possible using this substrate material and fabrication technique.
Date of Publication: Feb. 2006