Bistable Micro Caps Fabricated by Sheet Metal Forming

Traditional fabrication techniques are not suitable for the manufacturing of three-dimensional micro-scale structures. In this work, we report on a modified self molding punch-less sheet forming technique allowing fabrication of initially curved, non-planar, shallow micro shells. These structures, when appropriately configured, may exhibit the coexistence of two equilibria under the same loading. A ≈ 400 nm thick layer of Al, or ≈ 2.5 µm of Cu was deposited on top of a Si wafer. Next, circular openings of the radii varying between ≈ 100 µm and up to ≈ 700 µm were created in the Si substrate by means of the deep reactive ion etching (DRIE), with the metallic layer serving as the etch stop. Finally, the circular free-standing metallic sheet was pressed by a soft uniform foam layer serving as a stamp and pushing the metal foil into the opening in the wafer. Induced plastic deformation resulted in a residual strain shaping an initially planar metal sheet into a shallow cap with the midpoint elevations up to ≈ 50 µm. Metallic as well as metal-polymer bimorph caps, of the thickness varying between ≈ 400 nm and up to ≈ 2.5 µm and of the desired curvature, were successfully fabricated by the suggested process. Snap-through and snap-back of the fabricated devices under mechanical and electrostatic actuation was demonstrated experimentally.