MEMS

Topics: Microelectromechanical systems, Silicon, Semiconductor device fabrication Pages: 25 (5800 words) Published: November 2, 2013
University of Ljubljana
Faculty for mathematics and physics
Department of physics

Seminar

MEMS ACCELEROMETERS

Author: Matej Andrejašiˇ
c
Mentor: doc. dr. Igor Poberaj
Marec 2008

Abstract:
MEMS accelerometers are one of the simplest but also most applicable micro-electromechanical systems. They became indispensable in automobile industry, computer and audio-video technology. This seminar presents MEMS technology as a highly developing industry. Special attention is given to the capacitor accelerometers, how do they work and their applications. The seminar closes with quite extensively described MEMS fabrication.

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Introduction

An accelerometer is an electromechanical device that measures acceleration forces. These forces may be static, like the constant force of gravity pulling at our feet, or they could be dynamic - caused by moving or vibrating the accelerometer. There are many types of accelerometers developed and reported in the literature. The vast majority is based on piezoelectric crystals, but they are too big and to clumsy. People tried to develop something smaller, that could increase applicability and started searching in the field of microelectronics. They developed MEMS (micro electromechanical systems) accelerometers. The first micro machined accelerometer was designed in 1979 at Stanford University, but it took over 15 years before such devices became accepted mainstream products for large volume applications [1]. In the 1990s MEMS accelerometers revolutionised the automotive-airbagsystem industry. Since then they have enabled unique features and applications ranging from hard-disk protection on laptops to game controllers. More recently, the same sensor-core technology has become available in fully integrated, full-featured devices suitable for industrial applications [2].

Micro machined accelerometers are a highly enabling technology with a huge commercial potential. They provide lower power, compact and robust sensing. Multiple sensors are often combined to provide multi-axis sensing and more accurate data [3].

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MEMS technology

What could link an inkjet printer head, a video projector DLP system, a disposable bio-analysis chip and an airbag crash sensor - yes, they are all MEMS, but what is MEMS? Micro Electro Mechanical Systems or MEMS is a term coined around 1989 by Prof. R. Howe [2] and others to describe an emerging research field, where mechanical elements, like cantilevers or membranes, had been manufactured at a scale more akin to microelectronics circuit than to lathe machining. It appears that these devices share the presence of features below 100µm that are not machined using standard machining but using other techniques globally called micro-fabrication technology. Of course, this simple definition would also include microelectronics, but there is a characteristic that electronic circuits do not share with MEMS. While electronic circuits are inherently solid and compact structures, MEMS have holes, cavity, channels, cantilevers, membranes, etc, and, in some way, imitate ‘mechanical’ parts. The emphasis on MEMS based on silicon is clearly a result of the vast knowledge on silicon material and on silicon based microfabrication gained by decades of research in microelectronics. And again, even when MEMS are based on silicon, microelectronics process needs to be adapted to cater for thicker layer deposition, deeper etching and to introduce special steps to free the mechanical structures. MEMS needs a completely different set of mind, where next to electronics, mechanical and material knowledge plays a fundamental role. Then, many more MEMS are not based on silicon and can be manufactured in polymer, in glass, in quartz or even in metals...[2]. The development of a MEMS component has a cost that should not be misevaluated and the technology has the possibility to bring unique benefits. The reasons that prompt the use of MEMS technology are for example...

References: [1] I. Lee, G. H. Yoon, J. Park, S. Seok, K. Chun, K. Lee, Development and analysis of the
vertical capacitive accelerometer, Sensors and Actuators A 119 (2005) 8-18
A (not so) short introduction
(http://memscyclopedia.org/introMEMS.html (18.2.2008))
[3] S. Beeby, G. Ensell, M. Kraft, N.White, MEMS mechanical sensors (Artech house inc.,
USA, 2004)
[4] S. E. Lyshevski, Mems and Nems: systems, devices and structures (CRC Press LLC, USA,
2002)
[5] http://www.analog.com/UploadedFiles/Obsolete_Data_Sheets/66309706ADXL05.pdf
(10.3.2008)
Papers, pp. 1169-1172, June 1997.
[7] http://www.analog.com/en/prod/0„764_800_ADXL202%2C00.html (10.3.2008)
[8] http://www.analog.com/en/content/0,2886,764%255F800%255F122115%255F0,00.html
(14.2.2008)
[9] C
and actuators (Springer, USA, 2006)
[10] http://rfdesign.com/military_defense_electronics/news/accelerometer_proves_accurate_0509/
(14.2.2008)
[11] F
accelerometers Solid-State Electronics 47 (2003) 357-360
[12] http://en.wikipedia.org/wiki/Accelerometer (14.2.2008)
[13] S. Beeby, G. Ensell, M. Kraft, N.White, MEMS mechanical sensors (Artech house inc.,
USA, 2004)
[14] http://www.sensorsmag.com/articles/0399/0399_44/main.shtml (14.2.2008)
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