**5. Conclusion**

150 Microelectromechanical Systems and Devices

18°, which conforms with the included angle of two (221) planes. Moreover, the curvature radius of less than 10 nm is achieved without another oxidation-sharpening treatment.

Fig. 19. An SEM image of the etched tip with a cap.

Fig. 20. A completely etched sharp tip of high aspect ratio.

In this chapter, firstly the etch rate anisotropy in surfactant-modified etch solution is investigated, showing intriguing properties that are different from that of pure alkaline solutions. The etch rates of exact and vicinal {100} planes are almost unaffected when the surfactant is added, while the etch rates of exact and vicinal {110} planes are reduced significantly. The improved anisotropy ({mnl}/{100}) at high temperature provides better conformity to the mask profile for the formation of a micro cavity. The activation energy of TMAH + Triton (0.1-0.2 eV) is lower than that of pure TMAH (or KOH) solution (0.5-0.7 eV), showing to some extent diffusion-controlled etching process.

Secondly, the underlying effect of the surfactant in etching is understood microscopically and proved macroscopically that enables manufacturing of advanced and exciting structures for MEMS. Thicknesses of surfactant layers are investigated depending on the variation of orientation, temperature et al. The pre-adsorbed surfactant layers are formed and their effects on etch rate, surface roughness and corner undercutting indicate that the dissolved surfactant is adsorbed on the surface during etching.

Finally three applications by using surfactant-modified etching process are exhibited, involving the fabrication of conformal structures, scalloping removing for vertical microlens, and sharp tips with high aspect ratio. Much effort will be dedicated on other potential aspects in MEMS, and more advanced devices made by this etching technique could be anticipated in future.
