An Introduction to Micro-Electro-Mechanical Systems (MEMS) Ty Harness July 1997 - July 1999

Micro-Electro-Mechanical Systems describes the field of engineering concerned with the design and fabrication of small scale electro-mechanical devices. Device dimensions range from a few millimetres to as small as fabrication processes can achieve. Without any processing constraints, atomic dimensioned devices may be possible as postulated by Richard P. Feynman in his famous 1959 speech, "There's Plenty of Room at the Bottom [1]".

For economical reasons the design of MEMS devices generally uses the processing and materials technology of the semiconductor industry. Indeed the fabrication processes are, and remain, the concepts and developments born of Kilby's monolithic idea in 1958 [2], where an entire circuit was built from bulk germanium.

Prominent applications of MEMS are micro-sensors and micro-actuators. This web site will concentrate on one particular type of actuator, known as the comb-drive xy microstage, which is shown in figure 1.0.


Figure 1 - The xy microstage - the larger fixed areas (bond pads) have not been fully undercut by the wet etching process and the thin black features are suspended high (100um) above the silicon substrate

What is an xy microstage? It is simply a device capable of precise two dimensional positioning of a table in an x, y region of interest (ROI). Figure 1.0 shows a planar view of the xy microstage, with four comb-drive actuators and a central table. The four comb actuators are suspended from the substrate by eight spring-like suspension beams, while the central table is suspended by four orthogonal cross beams connected to each spar. A DC voltage applied to any two orthogonal electrode spars (via bond pad connections) will position the central table anywhere in a ROI by the virtue of electrostatic actuation.

Technological applications for the xy microstage include the machining and processing of semiconductors, optoelectronic elements, high density magnetic memory devices, the scanning tunnelling microscope (STM), and the atomic force microscope (AFM) [3] invented by Binnig et al. [4] in 1981.

The AFM application sums up the philosophy of MEMS, that is, engineering in a small world. The simplified principle of the AFM is a highly flexible cantilever with a fine tipped probe, which can discriminate surface detail, similar to conventional surface profilometers, but with the ability to discern individual atoms on the crystal surface. The AFM scans the surface by driving the xy stage back-and-forth, and then by using optical measurements to resolve the cantilevers deflection. Other types use electron tunnelling to image the surface, though this can only be done on conducting samples [5]. Commercial STM and AFM such as The Oxford Miniature CryoSTM [6] use piezo actuators to drive the xy microstage; the system is capable of scanning an area of five micrometers square with an x, y resolution of 0.02 nanometers.

One further application for the xy microstage is that the table could hold data bits, which would be read by a scanning process similar to the AFM principle [7]; this is Feynmans prophesy of information on a small scale.


Further development of this work can be found at Imperial College.

Download the Journal paper from Imperial College, "Characteristic modes of electrostatic comb-drive X-Y microactuators."

References:

[1] Feynman, R.P.; Theres plenty of room at the bottom, Journal of Microelectromechanical Systems, Vol.1. No 1, (1992) pp 60-66 {(Conference Transcript, 26th Dec. (1959))}

[2] Millman, J.; Microelectronics: digital and analog circuits and systems, Japan: McGraw Hill, International Student Edition, (1983) pp xxii

[3] Jaecklin, V.P.; Linder, C.; de Rooij, N.F.; Moret, J.M.; Comb actuators for xy-microstages, Sensors and Actuators, Vol. A39 (1993), pp 83-89

[4] Binning, G.; Rohrer, H.; Gerber, C.H. and Weibel; W.; Surface studies by scanning force microscopy, Physical Review Letters, vol. 49, No. 1, (1982) pp 57-61

[5] Thornton, T.J.; Micro systems technology, EPSRC Short course, unpublished,Course Module 16: Scanning Probes and Field Emitters, (1995) p 17

[6] Oxford miniature CryoSTM for use with cryostats and magnets, Internet source, www.topac.com , (1998)

[7] Christenson, G. L.; Miller, S. A.; Tran, A.T.T.D.; Haronian, D.; Lo, Y. H. and MacDonald, N. C.; Optical memory using an atomic force microscope and a surface micromachined interferometer, Conference Proceedings, IEEE Lasers and Electro-optics Society, 30 - 31 Oct., (1995) pp 262 -263

I would just like to thank some of the people that I have worked with over the past few years.
Martin Calderbank
Gan Sivamagathan
Steve White
Annie Lin
Ron Sims
during the mems tuning fork gyro project we worked on at Nottingham Trent University.


Kin Wei Lee
Regina Luttge
Marvin Zai
Sabri Saidam
Munir Ahmad
Eric Yeatman
Richard Syms
All of section 14 at IC

Re-vamped (10th March 2002)

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