31-01-2013, 04:50 PM
MEMS ACCELEROMETERS
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.
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].
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 100m 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].
MEMS accelerometers
The basics
There are many different ways to make an accelerometer. Some accelerometers use the piezoelectric
effect - they contain microscopic crystal structures that get stressed by accelerative
forces, which causes a voltage to be generated. Another way to do it is by sensing changes in
capacitance [3]. This seminar is focused on the latter.
Capacitive interfaces have several attractive features. In most micromachining technologies
no or minimal additional processing is needed. Capacitors can operate both as sensors and
actuators. They have excellent sensitivity and the transduction mechanism is intrinsically insensitive
to temperature. Capacitive sensing is independent of the base material and relies on the
variation of capacitance when the geometry of a capacitor is changing.
Applications
Accelerometers are being incorporated into more and more personal electronic devices such as
media players and gaming devices. In particular, more and more smartphones (such as Apple’s
iPhone and the Nokia N95) are incorporating accelerometers for step counters, user interface
control, and switching between portrait and landscape modes. They use accelerometers as a
tilt sensor for tagging the orientation to photos taken with the built-in camera. The Nokia 5500
sport features a 3D accelerometer that can be used for tap gestures, for example to change
to next song by tapping through clothing when the device is in a pocket. Camcorders use
accelerometers for image stabilization. Still cameras use accelerometers for anti-blur capturing.
Some digital cameras, such as Canon’s PowerShot and Ixus range contain accelerometers to
determine the orientation of the photo being taken and also for rotating the current picture when
viewing [12].
MEMS fabrication
Micro-fabrication is the set of technologies used to manufacture structures with micrometric
features. This task can unfortunately not rely on the traditional fabrication techniques such
as milling, drilling, turning, forging and casting because of the scale. The fabrication techniques
had thus to come from another source. As MEMS devices have about the same feature
size as integrated circuits (IC), MEMS fabrication technology quickly took inspiration from
microelectronics. Techniques like photolithography, thin film deposition by chemical vapor deposition
(CVD) or physical vapor deposition (PVD), thin film growth by oxidation and epitaxy,
doping by ion implantation or diffusion, wet etching, dry etching, etc have all been adopted by
the MEMS technologists. Moreover, MEMS also grounded many unique fabrication techniques
that we will describe in this seminar like bulk micromachining, surface micromachining, deep
reactive ion etching (DRIE), etc [2].
In general, MEMS fabrication tries to use batch process to benefit from the same economy
of scale that is so successful in reducing the cost of ICs. As such, a typical fabrication process
starts with a wafer (silicon, polymer, glass...) that may play an active role in the final device or
may only be a substrate on which the MEMS is built. This wafer is processed in a succession
of processes (Table 4.1) that add, modify or remove materials along precise patterns [2].
Conclusion
Although some products like pressure sensors have been produced for 30 years, MEMS industry
in many aspects is still a young industry. MEMS will undoubtedly invade more and more
consumer products. Size of MEMS is getting smaller, frequency response and sense range
are getting wider. MEMS are more and more reliable and their sensitivity better every day.
Prices of MEMS accelerometers and other MEMS devices aren’t excessive, but they still have
to drop a lot if we want to expand massive consumption. Standardization of production, testing
and packaging MEMS would certainly do a big part at it. The relatively long and expensive
development cycle for a MEMS component is a hurdle that needs to be lowered and also less
expensive micro-fabrication method than photolithography has to be pursued.
We can be sure that the future for MEMS is bright. At least because, as R. Feynman stated
boldly in his famous 1959 talk, which inspired some of the MEMS pioneers, because, indeed,
"There’s plenty of room at the bottom!" [2].