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New Type of Fatigue Discovered in Silicon
Invisible Pink Unicorn writes "Researchers at the National Institute of Standards and Technology (NIST) have discovered a phenomenon long thought not to exist. They have demonstrated a mechanical fatigue process that eventually leads to cracks and breakdown in bulk silicon crystals. Silicon — the backbone of the semiconductor industry — has long been believed to be immune to fatigue from cyclic stresses because of the nature of its crystal structure and chemical bonds. However, NIST examination of the silicon used in microscopic systems that incorporate tiny gears, vibrating reeds and other mechanical features reveals stress-induced cracks that can lead to failure. This has important implications for the design of new silicon-based micro-electromechanical system (MEMS) devices that have been proposed for a wide variety of uses. The article abstract is available from Applied Physics Letters."
or did anyone else see 'silicon fatigue' and immediately think of something more mammalian in nature?
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However, NIST examination of the silicon used in microscopic systems that incorporate tiny gears, vibrating reeds and other mechanical features reveals stress-induced cracks that can lead to failure.
I can agree with this. In my personal experience, crack inevitably leads to failure.
The theory of relativity doesn't work right in Arkansas.
A) No, certain grades of silicon are not cheap. (Price out solar panels some time.)
B) This affects the longevity of systems that were assumed to never wear out and limits the applications that they can be used in.
C) When is disposability an excuse for waste?
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Are TI's DLP mirror arrays subject to this? Don't know for sure if DLP is presently the largest MEMS rollout (if it is considered a MEMS) to the consumer market right now, but I wonder if anyone has reported mirror failures after a number of longer operating hours?
Just curious.
Study was conducted on the micro-mechanical objects modeled after mechanical objects in the macro- world. So, in essense, small gears will wear down and break just like big gears do. This isn't really a discovery, all large mechanical devices are subjected to a rigorous set of conditions that they will encounter. Just because a group of scientists never subjected the micro-versions to the macro-equivalent test doesn't mean this is new type of stress, it means that nobody though to check it.
And before anybody posts anything about flash memory or processors, this doesn't apply. Memory and processors are "solid state electronics", not "Micro mechanical devices", and are not vulnerable to the same type of stresses (i.e. those caused by friction, shear, or centrifugal forces).
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They're talking about displacements of hundreds of micrometers... it's not clear that any silicon actually displaces that much under any sort of normal operation. Even in common MEMS parts like accelerometers (like those controlling your car airbag or Wiimote), the displacements are tiny -- typically on the order of one micrometer -- although they do happen hundreds of thousands of times per second.
Ever heard of plastic versus elastic deformation? Elastic is when it's small enough to come back to it's original state (no permanent effect). Plastic is when the material is permanently reorganized. They're at a huge displacement scale, so it's not clear how this applies to modern MEMS systems which are moving two orders of magnitude less.
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Got news for you, it's everywhere. I've got stress fatigue from converting SQL scripts.
A feeling of having made the same mistake before: Deja Foobar
LEDs are not made of silicon. They are either gallium arsenide or boron nitride, depending on the color.
Now we finally know what was causing the pain in all the diodes down his left side.
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Now that a stress issue has been found that places a limit on how the materials can be used and how much MEMS devices can be shrunk etc.
Engineering is the art of compromise.
I'll be concerned when they start putting "gears" in my Intel chips. The new Intel Geartron processor - now with gears! Um - no.
...and as for DLP, it's a valid question especially given that they oscillate rapidly thousands of times a second to simulate brightness levels (they're pulse width modulated to full reflect or full absorb mirror positions). However, the NIST abstract says that their test is done with a spherical indenter presumably imparting impulsive loads of some magnitude. I don't know how big the sphere is or what material it's made of since I don't have the full article, but I'll assume it's some microscale silicon ball; hopefully they didn't do something like ceramic shattering glass easily with little force. DLP stresses would normally be torsional stress along the micromirror hinge of a magnitude dependent on the deceleration at the limit of the DLP motion and the mass of the mirror. Now, if TI was clever and didn't modulate the mirror past the elastic limit of the material, they might be able to largely overcome this problem. Cantilever-style micromirrors might not fare as well because the material is always being deformed, though I again assume they do a stress-strain plot to ensure they don't go past the elasticity limit. On that note and to come full circle, one would assume that sensors do not exceed their ductile elasticity limit except in critical situations, such as high shock as is found in an abrupt movement of an accident. Then again, they're typically single-use.
It's old fashioned fatigue, and it isn't new. This paper quotes (2nd para) 1992 work that demonstrated fatigue in micron-sized silicon specimens.
Silicon is a typical low ductility material that does not tolerate cracks very well because there is very little plastic deformation at the crack tip (the process zone). Fracture mechanics is based on an energy balance, when the amount of energy absorbed by the creation of the fracture surfaces (the surface energy) plus the amount of energy required to do that plastic work in the pz is equal to the amount of strain energy in the structure that's released when the crack gets bigger (the strain energy release rate), the crack becomes unstable and the part goes bang.
The strain energy release rate varies with the load and crack size, for a given crack size at loads lower than the critical load, pre-existing cracks (there are always cracks even if they are microscopic) open a bit and the pz deforms. When the load is released, the pz doesn't go back to it's original configuration. Repeating the apply-load remove-load cycle progressively grows the pz which causes the crack to get bigger in some complicated ways. But think of it this way, the crack tip is theoretically infinitely sharp (the limit is the inter-atomic distance of the material). This discontinuity causes infinite theoretical stress which causes the atomic bonds to break at the tip. Process zones have been the subject of countless PhD theses.
In a low ductility material the energy absorbed by the pz is small compared to the energy absorbed by the surfaces created when the crack grows. Remember the pz is responsible for fatigue growth, the pz plus the surface energy is responsible for unstable crack propagation. So a small pz means you have to load the material close to the crack instability load to get fatigue growth. With a small enough pz it's impossible to load the material accurately enough to grow the crack without breaking the part. So THATS what they mean by silicon being immune to fatigue.
It seems like the reason this is not the case in microscopic silicon specimens is another PhD topic, the explanation is complicated. Oxidation caused by humidity in the air is a factor, as well as loading in the compression mode.
Again, all this has been known for many years.
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