□ Silicon nitride ■ Silicon lp' i ■ Titanium siliaje □ (¡old
Fig. 13.2. (a) SEM photgraph of a piezoresistive cantilevers. Ten cantilevers were designed on the two sides of a channel, and the cantilever size is 120^mx 338^m, and (b) schematic cross section of cantilever and substrate (Yu et al. 2002)
Piezoelectric material is considered to be smart because it enables the driving and sensing of mechanical resonance to be done electrically. Because of this quality, we can use a Pb(Zr0.52Ti0.48)O3 (PZT) monolithic microcantilever to electrically measure the direct mechanical resonance. Fig. 13.3 shows some fabricated nanomechanical PZT cantilevers. The monolithic PZT thin film microcantilevers are fabricated in eight main steps. In the first step, the substrates that form the PZT capacitors were 4 inch Si (100) wafers covered with 1.5 |im thick low-stress silicon nitride (SiNx) deposited by LPCVD. The PECVD silicone oxide was then deposited with a thickness of 2000 ^m. The bottom electrode was prepared by sputtering a thin Ta adhesion layer followed by a Pt layer with a thickness of 1500 ^m. The PZT films were deposited by a diol-based sol-gel route. For a metal-
ferroelectric-metal capacitor structure, a Pt layer was deposited for the top electrode by DC sputtering. The Pt for the top electrode and the PZT for the piezoelectric material were etched by RIE and ICP, respectively. The patterning of the Pt bottom electrode was carried out with RIE etching. SiO2 thin film was then deposited and patterned for an electrical passivation layer via hole patterning. Cr/Au with a 30/100 nm for electrical pads was formed by evaporation and a liftoff process. The cantilever window and the back SiNx window were patterned with RIE. The bulk silicon was then wet etched using a TMAH silicon etchant. Finally, SiNx etching was carried out with RIE for forming cantilever.
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