AVS 45th International Symposium
    The Science of Micro-Electro-Mechanical Systems Topical Conference Monday Sessions
       Session MM+PS-MoM

Paper MM+PS-MoM9
Application of the Footing Effect in the Microfabrication of Self-Aligned, Free-Standing Structures

Monday, November 2, 1998, 11:00 am, Room 324/325

Session: MEMS Processing and Deep Si Etch Technology
Presenter: A.A. Ayon, Massachusetts Institute of Technology
Authors: A.A. Ayon, Massachusetts Institute of Technology
K. Ishihara, Massachusetts Institute of Technology
R. Braff, Massachusetts Institute of Technology
H. Sawin, Massachusetts Institute of Technology
M.A. Schmidt, Massachusetts Institute of Technology
Correspondent: Click to Email
The footing or notching effect is observed when dry etching silicon or polysilicon layers on buried dielectric films.@footnote 1@ This effect is usually considered an undesirable feature for most applications, although it is frequently small in conventional reactive ion etching (RIE) tools due to the low density of the plasmas utilized. However, with the new generation of inductively coupled plasma etching tools the notching effect can extend laterally several microns depending not only on operating conditions but also on the aspect ratio@footnote 2@ and extent of overetching time. The suppression of this effect depends in a critical manner on achieving a balance between etching and deposition of passivating films.@footnote 3@ Deviations from such balance promote the appearance of grass or even excessive deposition of passivating films. We review the dependence of footing effect on etching conditions in a time multiplexed deep etcher (TMDE) and suggest specific operating conditions to preclude the appearance of notching even when overetching for as much as 85%. Additionally, we introduce the application of the footing effect in the microfabrication of free-standing structures, by demonstrating the micromachining of self-aligned, wafer-free electrostatic actuators for which etching, releasing, ashing and passivating (dielectric isolation) were done in the same piece of equipment. All processes needed to produce cantilevered structures are done in situ using VLSI compatible plasma chemistries only. The measured pull-in voltage for a 1000 µm cantilevered beam, of the order of 85 V, agrees with predicted values. The novel low-temperature, soft-mask scheme presented here, is compatible with other VLSI processes and can be easily integrated in the microfabrication of intelligent sensors and actuators. This robust new concept allows unparalleled fabrication simplicity while permitting the fabrication of sctructures and devices in an efficient and timely fashion. Electrostatic actuators with or without interdigitated fingers, valves, pumps and relays, to name but a few, are applications that immediately benefit with this technique. @FootnoteText@ @footnote 1@G. S. Hwang and K. P. Giapis, J. Vac. Sci. Technol., B 15 (70) 1997. @footnote 2@T. Nozawa, T. Kinoshita, T. Nishizuka, A. Naral, T. Inoue and A. Nakaue, Jpn. J. Appl. Phys., 34 (2107) 1995. @footnote 3@J. P. Chang, Ph. D. Thesis, Massachusetts Institute of Technology, 1997.