Ultrathin resonant cantilevers are promising for ultrasensitive detection. A technique is developed for high-yield fabrication of single-crystalline-silicon cantilevers as thin as 12 nm. The formed cantilever resonators are characterized by resonance testing in high vacuum. Significant specimen size effect on Young’s modulus of ultrathin (12–170 nm) silicon is detected. The Young’s modulus decreases monotonously as the cantilevers become thinner. The size effect is consistent with the published simulation results of direct-atomistic model, in which surface effects are taken into consideration.

Microflow devices including microvalves, micropumps and microflow sensors fabricated by micromachining are reviewed from the point of view of the actuating principle and structures. Integration of microflow control devices and microflow sensors allowed very precise control of small flow. High performance liquid dosing microsystems and sophisticated chemical analysing microsystems were demonstrated by the combination of microflow devices and microsensors. Applications of microflow devices and systems are also introduced.

Wafer level packaging plays many important roles for MEMS (micro electro mechanical systems), including cost, yield and reliability. MEMS structures on silicon chips are encapsulated between bonded wafers or by surface micromachining, and electrical interconnections are made from the cavity. Bonding at the interface, such as glass-Si anodic bonding and metal-to-metal bonding, requires electrical interconnection through the lid vias in many cases. On the other hand, lateral electrical interconnections on the surface of the chip are used for bonding with intermediate melting materials, such as low melting point glass and solder. The cavity formed by surface micromachining is made using sacrificial etching, and the openings needed for the sacrificial etching are plugged using deposition sealing methods. Vacuum packaging methods and the structures for electrical feedthrough for the interconnection are discussed in this review.

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Surface effects in ultrathin single-crystal silicon cantilevers of 170 nm thickness, which are optically actuated mainly by the light pressure effect, are investigated under ultrahigh vacuum (UHV) condition. Annealing the cantilevers at 1000 °C for 30 s in UHV results in an over 1 order of magnitude increase of the quality factor (Q factor), up to about 2.5×105 for cantilevers of 30–90 μm in length. The improvement of Q factor was found to be associated with the deoxidization of the surface, as determined by x-ray photoelectron spectroscopy. These results suggest that the surface effects in the ultrathin cantilevers dominate their mechanical behavior. With the promising mechanical behavior, the cantilever can be easily actuated by a laser beam (beam size: about 300×100 μm2) with power down to less than 40 μW at a wavelength of 680 nm, corresponding to 480 nW, i.e., 1.64×1012 photons/s, irradiated on the cantilever surface (60×6 μm2). This provides a rather simple way to operate the ultrathin cantilevers dynamically in UHV. Atomic scale force resolution (4.8×10−17 N) at 300 K is also expected with these cantilevers.

Ultrathin single-crystalline silicon cantilevers with a thickness of 170 nm as a resonating sensor are applied to mass sensing. The hydrogen storage capacity of a small amount of carbon nanotubes (CNTs), which were mounted on an ultrathin resonator by a manipulator, is measured from the resonant frequency change. The resonator is annealed in ultrahigh vacuum to clean the surface and increase the quality factor, and exposed to oxygen gas to oxidize the surface for long-term stability. The resonator can be electrostatically actuated, and the vibration is measured by a laser Doppler vibrometer in ultrahigh vacuum. The mass of the CNTs is determined by the difference of resonant frequencies before and after mounting the CNTs, and the hydrogen storage capacity is determined from the frequency change after exposure to high-pressure hydrogen as well. The obtained hydrogen storage capacitance is 1.6%–6.0%. The available mass resolution and the achieved stability of the resonance of the 170-nm-thick resonator are below 10−18 g and 5 Hz/days, respectively.

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