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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?
If it's for-profit but free, you're not the customer -- you're the product (e.g., the Slashdot Beta's "audience").
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).
~Sticky
/Duh
Diamonds are forever~
I'm not defending I'm observing! You can't argue that a capitalist system (such as, well, all of global trade) is geared toward profits and maximising commodification? I'm no economist so I'm not going to express anything more specific than that, but you can't argue that the general trend in electronics has been toward production/consumption over maintainability.
-1 not first post
Our silicon-bsed overlords can bite my carbon-based ass! Oh wait - they're too fatigued, and crack up under even microscopic stress.
Seriously, this may have implications for the non-existence of silicon-based life. After all, silicon-based "dna" might be more liable to failure.
Kevin Smith on Prince
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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Our tired silicon overlords are welcome to crash on my couch for a while, 'til they feel a bit less fatigued.
Going on means going far
Going far means returning
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.
- None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
And who is to blame for that? Consumers/Corporate buyers?
You may find the headlong trend towards buying new computers slowing quite a bit. You only really need so much horsepower to edit a document or twiddle numbers in a spreadsheet. Adding memory is quite effective rather than just junking the old box. Microsoft's strategy of building bigger versions of windows, which require bigger versions of PCs is flattening out on the curve, with Vista adoption quite slow.
This all means we're trying to get more life out of our computers. As for other electronics, I only buy when I need to replace or upgrade considerably. We aren't all like the japanese buyers of G3 phones who tossed them every few months for the next eye-catcher.
A feeling of having made the same mistake before: Deja Foobar
Twitter supports and protects racists - by smearing their critics with the "Hate Speech" label.
So they are now reporting on the back strain of supermodels? That was my first thought reading the headline...
I wonder where they would get the idea that silicon wouldn't fatigue? Silicon exhibits both electrostriction and magnetostriction, and hence the lattice undergoes strains with changes in electric or magnetic field (as appropriate). Ideally these chips either have no dislocations or maybe just a single screw dislocation. Or rather, the substrate would be that way. But all of the gates and switches and what not will have interfacial dislocations. It would be unusual if dislocation multiplication couldn't take place under micro-plastic flow near these interfaces.
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.
"Microsoft's strategy of building bigger versions of windows, which require bigger versions of PCs is flattening out on the curve, with Vista adoption quite slow."
To that, I'll add:
http://www.urbandictionary.com/define.php?term=Windows&defid=1581637
Previously: "Linux... Toward the Sunrise..." Now: "Linux... Toward the-- No, now, part of Every Sunrise"
This has always been the explanation for why our digital watches run faster every year. Only a second or two, but faster.
>has long been believed to be immune to fatigue from cyclic stresses because of the nature of its crystal structure and chemical bonds Um, did someone forget about the laws of thermodynamics?
My first thought was circuit boards in avionics. Surely all those cycles -- nevermind exposure to extreme cold -- are going to put mechanical stresses on boards, which is kinda scary when you consider the level of computer-control in modern airplanes.
I want to see silicon nano cars failing and blowing up as they crash into another nano cars.
But, seriously... I want to see more pictures and video. You never see that with these "cool" really tiny things.
I'll be concerned when they start putting "gears" in my Intel chips. The new Intel Geartron processor - now with gears! Um - no.
Everyone knew these devices wear out, they just didn't know what the cause was. It may be interesting, but not earth shattering.
Sounds like some of the ladies in Hollywood might be worried about the future value of their assets!
Can't silicon be recycled ? Consumerism is driven by cheap, short-lived junk that's promptly replaced by more cheap junk. If we can't make these built-to-break gadgets recyclable, we're headed for a brick wall.
-Billco, Fnarg.com
While silicon wafers in principle could be recycled, silicon computer chips can't be. In a typical fabrication process, silicon is coated and exposed to a variety of other contaminants that can't be easily removed. Dopants like arsenic and phosphorous, dielectric coatings like silicon and hafnium oxides, plus a variety of metals for connections would all have to be removed from the chip before silicon could be thrown back into the melt. It's just not worth it for such a small amount of silicon.
...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.
Some people might realize why I'm a bit leery of carbon fiber. We don't understand their properties as well as we think we do. Back to vacuum tubes for me. Then we really can say the internet is a series of tubes.
What?
And who is to blame for that? Consumers/Corporate buyers?
You may find the headlong trend towards buying new computers slowing quite a bit. You only really need so much horsepower to edit a document or twiddle numbers in a spreadsheetYou see, the system works, and it's called "capitalism." Demand for the next widget is slowing because existing widgets work just fine. Supply adjusts, prices stabalize, yadda yadda yadda.
Point is, in a capitalist system, you can't waste something valuable for long - somebody's going to go broke.
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The only thing I know of that can't be cracked is the thinking of those who believe that some things will never wear out.
The torsion hinges that support the micromirror on Digital Micromirror Device (DMDs) are said to be good for at least a trillion (10^12) operations. Source: Wikipedia, which has no footnote link to an authoritative source
Wiki also says that, at least for a DLP projector, the "DMD chip can be easily repaired or replaced".
What does a trillion operations means to you or me? Is that MTBF, or some other metric? No idea. Back-of-the-napkin (assumption: 1 operation/frame):
- 10^12 ops divided by (30 ops/sec) = 33,333,333,333 secs = 9,259,259 hours = 1,057 years.
If it's 100 operations per mirror per frame instead of 1, then that's still 10 continuous years of operation.
There is probably a nice function that can tell you how quickly the actual failure rate per mirror translates into visible degradation of picture quality over the whole screen over time. Does anyone know if the useful life of the screen would be longer or shorter than the MTBF of an individual mirror? Things to consider: MTBF/mirror, deviation of MTBF/mirror, # of mirrors/pixel (wobulation), # or mirrors, and statistics that I don't know.
But in any event, mirror life seems reasonable. At a minimum, it doesn't seem to be enough of a worry to be publicized widely (vs. everyone knowing there were concerns about the life of early plasmas)
Did anyone else notice that the abstract quotes a scientist from a University in Spain called - Extremadura? How fitting for a fatigue testing expert. Is this coincidence? "University of Extremadura finds silicon is not extremely durable after all"
Si being one of the most abundant elements on the planet weights for a lot on the not-worth-it side... but the other rarer elements deposited on the chips could be a different story. Much of these could be recovered my melting thousands of ICs into a single ingot and passing it through an induction oven a few times to separate elements by sedimentation. Since most of the valuable elements used in Si-based ICs are heavier than Si, a fair amount of them would settle near the bottom.
In the days of pre-flip-chip ICs, scavengers were taking apart ICs to recover the gold bonding wires and gold-plated components because there was more gold per kg in electronics than raw ore. Depending on how prices for the other rare elements go, the above recycling process could become economically viable.
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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This will come as a big surprise to all those people actually working in the semiconductor industry who regularly apply cyclic stresses as part of our typical reliability testing...
Seriously, no one who works with silicon thought it was immune to fatigue from cyclic stresses.
The only thing that is new here is that with semiconductors the stresses are incidental due to temperature and pressure events, where with MEMS the stresses are direct and part of the function of the device.
Maybe this is why I've had several multiaxis MEMS angle sensors freak out or just die on me for no apparent reason. Good power, good grounding, good data acquisition box, all operational conditions within spec, but after use, they just crap out.