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Micromechanical devices made from
Si are rapidly gaining in importance. Their production process utilizes
most everything used in microelectronics, plus a few special processes. |
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Again, we will not discuss
MEMS in this Hyperscript, but show only a few pictures of what can be
made. |
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Let's look at mechanical MEMS first. On top, a microscopic
gear wheel systems from Sandia Labs. It
could be used for mechanically "locking" your computer; which might
be more secure than just software protection. |
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On the bottom, more or less the same
thing with a dust mite on it. This is the little animal that lives in your rug,
bed and upholstery and gives a fair share of us the infamous dust
("Hausstaub") allergy |
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Pictures: Courtesy of Sandia National Laboratories,
SUMMiTTM Technologies, www.mems.sandia.gov" |
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While gear wheels look very good, the real use of
MEMS so far is in sensors, in particular for acceleration. The sensor
exploding your air bag when you wrap your car around a tree is the paradigmatic
MEMS product. |
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If we look at optical MEMS, we are mostly also looking on a
mechanical microstructure, in this case at arrays of little mirrors which can
be addresses individually and thus "process" a light beam pixel by
pixel. |
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Courtesy of "ISiT"
(Fraunhofer Institut Silizium Technologioe; Itzehoe, Germany) |
Courtesy of Texas Instruments |
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On the left we have an array of microscopic
mirrors that can be moved up and down electrically (from the ISiT in
Itzehoe). The central mirror is removed to show the underlying structure |
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On the right a schematic drawing of the
"mechanical" part of Texas Instruments ("DLP" = digital light
processing) chip, the heart of many beamers. |
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But many other things are possible with
MEMS, suffice it to mention "bio-chips", micro-fluidics,
sensors and actuators for many uses, microlenses and lens arrays, and tunable
capacitors and resonators, and, not to forget, very down-to-earth products like
the micro-nozzles for ink jet printers. |
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There are many more applications,
most in the development phase, that exploit the exceptional quality of large
Si crystals, the unsurpassed technology base for processing, or simple
emerging new features that might be useful. Here are few examples: |
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While there are no conventional
lenses for X-rays or neutron beams, some optics is still possible by
either using reflection (i.e. imaging with mirrors) or diffraction. |
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An good X-ray mirror, like
any mirror, must have a roughness far smaller than the wavelength. For useful
applications (like "EUV" = Extreme Ultraviolet) lithography
(it is really X-ray lithography, but this term has been "burned" in
the 80ties and is now a dirty word in microelectronics), this quickly
transfers into the condition that the mirrors must be more or less atomically
flat over large areas. This can be only done with large perfect single
crystals, so your choice of materials is no choice at all: You use
Si. |
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If you want to "process" a
neutron beam, e.g. to make it monochromatic, you use Bragg diffraction at a
"good" crystal. Again, mostly only large and perfect single crystals
of Si meet the requirements |
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Si is fully transparent for IR
light and is thus a great material for making IR optics. In this field,
however, there is plenty of competition from other materials. But Si is
the material of choice for mirrors and prisms needed for IR
spectroscopy. |
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Since about 1990 "porous
Si" is emerging as a totally new kind of material. It is
electrochemically made form single-crystalline Si and comes in many
variants with many, partially astonishing properties (optically activity,
highly explosive, ...) |
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A review about this stuff can be found in the
link. Here we simply
note that a number of projects explores possible uses as for example electrodes
for fuel cell, very special optical and X-ray filters, biochips, fuses
for airbags, "normal" and biosensors, or special actuators. |
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© H. Föll
