MEMS DesignDesigning MEMS devices requires multi-disciplinary knowledge including mechanical, electrical and chemical engineering, and physics representing the different aspects of the devices.
|Spider mite on mirror assembly.|
Courtesy of Sandia National Laboratories, SUMMiT™ Technologies, www.mems.sandia.gov
Differences between the micro- and the macro-worldImagine you have a box filled with several glass marbles. Now you give the box a good shake and place it on a table. When opening the box now, where would you expect to find the marbles? Well, our everyday experience tells us, they will be on the bottom of the box.
Now imagine we use the same box but replace the glass marbles by tiny ones with a diameter of several micrometer (the thousandth part of a millimetre). Now we close the box again and give it a good shake. When opening the box now, where would you expect to find the marbles?
On the bottom again?
This is where our macro-world experience mislead us. The marbles would be equally distributed on each surface of the inside of the box, sticking to bottom, side-walls and the lid. This is due to the fact that when reducing the size of a body to micro-dimensions the gravitational force becomes more and more unimportant compared to other forces acting on the body.
More specific, the gravitational force being a volumetric force scales with L3, whereas surface forces like the electrostatic force scale with L2. This means when reducing the dimensions of a body by a factor of 1000, the gravitational force is reduced by a factor of 1000-3 = 10-9 whereas the electrostatic force is reduced by 10-6.
Therefore, the weight of a body can be neglected in almost all instances when dealing with MEMS devices, except in special devices such as accelerometers.
The other major differences when dealing with the world of MEMS are
- Surface tensions, which causes surfaces to stick together and is a common critical failure for MEMS devices.
- Mixing of fluids is very difficult on a micro-scale as most fluid flows are laminar rather than turbulent. Special designs are required to mix two substances.
- The stability of manufactured structures. When looking at MEMS devices often they look as if they can impossibly survive, such as bridges which are just 0.5 um thick, 40 um wide and 1000um long. (In the macro world this would equal an unsupported bridge 0.5 m thick, 40 m wide and 1 kilometer long, which would collapse at once due to its weight).
|Courtesy of Sandia National Laboratories, SUMMiT™ Technologies, www.mems.sandia.gov|
In general MEMS manufacturing technologies can be divided into these categories:
- Bulk micromachining, the first MEMS manufacture technology, in which the Silicon wafer is etched to create structures such as groves , bridges and apertures with near 90 degree sidewall angles.
- Surface micromachining, which uses predominantly additive processes using the Silicon wafer as substrate. Devices formed using surface micromachining tend to be considerably thinner than bulk or HAR devices.
- High aspect ratio micromachining (HAR) combines some of the aspects of both surface and bulk micromachining . A process which is commonly associated with this technology is the DRIE-process (Deep Reactive Ion Etching), w hich allows for silicon structures with extremely high aspect ratios through thick layers of Silicon (hundreds of nanometers up to hundreds of micrometers). This is achieved through a cycled etch process in which the deposition of a passivation material on the sidewalls of the etched material and the actual etching process alternate.
- LIGA process, in which a thick photoresist (up to several mm thick) is used to create a shape over a seed-layer. (Examples of commonly used photoresists are SU-8 and PMMA.) This shape is then filled using an electroplating process. After removing the photoresist, the remaining metal-shape is used as a master for a moulding process to create large amounts of negatives. Several versions of the LIGA process exist: X-Ray LIGA, UV-LIGA and Laser-LIGA which use X-radation created in synchrotrons, ultraviolet radiation and laser radiaton, respectively.
- Specialised processes for niche applications.
PackagingCompared to a CMOS circuit in which only electrical signals are fed into and out of the system, the packaging of MEMS devices is far more complex. Depending on the type of MEMS various types of inputs and outputs to the system are required (electrical, mechanical, acceleration, fluids, gases, radiation etc.) which are usually custom to that particular device. Therefore standardisation of packaging solutions is considerably more difficult than for ASIC devices. For this reason a considerable amount of the costs of a MEMS device is spent on the packaging of the device rather than the manufacture of the actual device.
ApplicationsIn general MEMS can be divided into two major groups, sensors and actuators:
The major MEMS based sensors currently on the market are:
Used to measure acceleration, originally mostly used in airbags and recently integrated into gaming interfaces to allow improved motion control and into mobile phones for screen-rotation, improved GPS functionality etc. The use of acceleromters in consumer electronic devices is expected to maintain the status of accelerometers as the most important MEMS device.
- Pressure sensors
Used for example for tyre pressure monitoring.
Used for picture stabilization in digital cameras.
- Gas flow sensors
- Remote temperature sensors
- and many more
ActuatorsA large number of possible actuation principles are being used in MEMS devices some of them inverse effects used in sensors, some used solely for actuators. Amonst the commonly actuation principles are: electrostatic, thermal, piezoelectric and magnetic.
Suggested further reading
- Fundamentals of Microfabrication: The Science of Miniaturization,
By Marc J. Madou, Published by CRC Press, 2002, ISBN 0849308267
- The Mems Handbook,
By Mohamed Gad-el-Hak, Published by CRC Press, 2002, ISBN 0849300770
- Microsystem Technology,
By Wolfgang Menz, J. Mohr, Oliver Paul, Published by Wiley-VCH, 2001, ISBN 3527296344
- Microsystem designBy Stephen D. Senturia, Published by Springer, 2000, ISBN 0792372468
- Wikipedia article on MEMS