Micro-Oscillators for Ultra-Sensitive Force Detectors

Recently, we have improved the microfabrication process for ultra-sensitive double-torsional mechanical micro-oscillators. Starting with silicon-on-insulator wafers (with 300 nm Si surface layers), we grow a protective layer of oxide, then pattern 2-mm $\times$ 0.5-mm ``windows" that result in a thin Si film after two-sided wet chemical etching. Patterning these film windows with electron-beam lithography then provides the final micro- oscillator structures. Our designs include multimode structures, particularly double-torsional modes; we have achieved excellent geometric symmetry and small sizes ($\sim 30\ \mu$m laterally and 300~nm thick). These oscillators have excellent force sensitivity, yet provide stronger mechanical structures than typical ultrafloppy cantilevers designs. For example, an antisymmetric double-cantilever mode provides a minimum detectable force of $1.6\times 10^{-16}$~N/$\sqrt{\rm Hz}$ at room temperature, corresponding to $F_{\rm min} = 5 \times 10^{-18}$~N/$\sqrt{\rm Hz}$ at $^3$He temperatures. A double torsional mode typically provides enhanced sensitivity, so minimum detectable forces on the order of $10^{-19}$--$10^{-20}$~N/$\sqrt{\rm Hz}$ are now targeted.