Selective chemical vapor deposition of tungsten for microelectromechanical structures

Abstract A selective chemical vapor deposition (CVD) tungsten process is used to fabricate three-dimensional micromechanical structures on a silicon substrate. Patterned structures are formed in silicon dioxide trenches by selective nucleation and growth of tungsten from the bottom of the trench. Examples are shown for selective growth of tungsten on single-crystal silicon, on thin films of silicon and on silicon-implanted, silicon dioxide layers. This high deposition-rate selective CVD tungsten process had been used to fabricate tungsten micromechanical structures greater than 4 μm thick. Tungsten patters with features of 0.3 μm × 0.3 μm in 0.6 μm of tungsten are fabricated using electron beam lithography. As an example of an electromechnical structure, cantilever beams have been fabricated to make micromechanical tweezers that move in two dimensions by the application of potential differences between the beams (lateral motion), and between the beams and the silicon substrate (vertical motion). Tungsten microtweezers 200 μm in length with a cross-section of 2.7 μm × 2.5 μm closed with an applied voltage of less than 150 V.