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MEMS are "Micro Electro Mechanical Systems" including also micro
optics, micro fluidics and generally meaning micro systems. | |
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MEMS uses Si substrates and technologies because "it is there
and cheap" for the non-electronic part and because electronic components can be
integrated on the same chip. | |
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Examples of high-volume MEMS products are - (Pressure) sensors.
- Accelerometers.
- Gyros
- "Beamer" chips (DLP)
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More products are to come; MEMS is an emerging
and often an enabling technology |
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Gyros are particular complex MEMS sensor products with a huge range
of applications. | |
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There must be a physical principle behind the sensor design; different approaches can be used. |
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One approach uses the Coriolis force causing detectable additional vibrations in an
oscillator with two degrees of freedom if some rotation is experienced. |
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Many MEMS devices are either sensors or actuators. |
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Looking only at mechanical MEMS, there is a need to couple mechanical movements to
electrical signals and vice verse. | |
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Ways to do this include. - Capacitive coupling
- Piezoelectric and piezoresisitive coupling.
- Thermal coupling (expansion, resistivity changes).
- Magnetic coupling.
- Optical coupling.
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There is no "ideal" coupling; all methods suffer from certain problems. |
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MEMS uses all of "known" Si technology and has some specifics
of its own. | |
Works |
Sticks |
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Making cantilevers and membranes necessitates making "large" cavities. |
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Staying absolutely planar and stress-free is essential |
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Packaging can be far more demanding than for chips (e.g. transparent tops for OMEMS,
keeping defined pressures for > 10 a in gyros). | |
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The bane of MEMS is stiction. |
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If you can't lubricate, it will stick sooner or later.
Never bring moving parts in contact! | |
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MEMS design therefore cannot just miniaturize exiting mechanical designs; it must look
for new approaches. | |
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MEMS employs some special processes and materials; they are the drivers
of progress | | |
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Anisotropic chemical etching | |
Making "large" cavities and extremely deep "holes"
Planarization
Free-standing structures |
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High-rate plasma etching ("Bosch process") |
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Chemical-mechanical polishing | |
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Sacrificial layers and removal (including chemical etching with "vapors") |
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Wafer bonding; in particular for packaging. |
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Process integration looks simple if compared to an advanced CMOS process,
but is actually rather involved due to the special processes needed and quality requirements |
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© H. Föll (Semiconductor Technology - Script)
