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New Type of Fatigue Discovered in Silicon 108

Posted by ScuttleMonkey
from the beat-on-anything-long-enough dept.
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."
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New Type of Fatigue Discovered in Silicon

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  • by zappepcs (820751) on Wednesday November 28, 2007 @06:35PM (#21510929) Journal
    or did anyone else see 'silicon fatigue' and immediately think of something more mammalian in nature?
    • Re: (Score:3, Funny)

      by davidsyes (765062)
      I was thinking "New Type of Fatigue Discovered in Silicon VALLEY".

      But, now that you're thinking (alluding to breasts/mothers' milk; or rampantly-chewing boys or men on females) "siliCONE", why not think these?:

      "New Type of Fatigue in Silicon ALLEY, or"

      "New Type of Fatigue (CastroEderItis) RE-Discovered in Castro District"?

      "New Type of Parking Discovered in Rear" (More Parking in Rear)...

      Such a lovely day...for gags... ummm chokes... umm JOKES...
      • by davidsyes (765062)
        ""New Type of Fatigue (CastroEderItis) RE-Discovered in Castro District"?"

        Meant:

        "New Type of Fatigue (CastroEndTenderItis) RE-Discovered in Castro District"?
      • I was in Belgrade, Serbia (long forsaken motherland) this summer and was quite impressed (but only briefly) that there was a downtown district called the Silicone Valley! The locals even proudly pronounced it in English: 'Da Silikon Voli!'. I thought GREAT, finally there are some BIG signs of upward turns in economy that has been ravaged in the last 15 war torn years. But Serbs being Serbs...

        Yes, there were some HUGHE silicone installations in the district, but it had absolutely nothing to do with tech,
      • by loconet (415875)
        That is exactly what I thought as well. I thought they had found a new type of work fatigue in workers at Silicon Valley which eventually forced them into doing crack.
    • by snowraver1 (1052510) on Wednesday November 28, 2007 @06:43PM (#21511043)
      I think it's just you. I think that I can speak for most of us here when I say that the word "silicon" is mentally associated with "sexy" things. Things like my xbox 360, CPLDs, diodes, LEDs... I should stop there... I'm turning myself on... Oh baby... TTL chips, CMOS... Good 'ol CMOS never say no when you call her late at night.
    • by wattrlz (1162603)
      Hopefully it's just you. How exactly does a gel exhibit fatigue? My first reaction was, "Oh noes! My RAM!"
    • Now we finally know what was causing the pain in all the diodes down his left side.
    • Re: (Score:2, Interesting)

      by Auraiken (862386)
      I was actually starting to think that maybe "tiny gears" and "vibrating reeds" might be defective by design. Maybe the forms themselves cannot take the stresses very well and lead to fatigue. Is there any research for finding stresses in single forms? like break/crack points between edges and whatnot depending on how they are shaped and how much force is put on them relative to the rest of the structure? or is it 4:30am and i should've went to bed hours ago?
      • by zappepcs (820751)
        I don't have links handy to throw at you, but yes, there is much data with regard to cogs and gears and stress. If you look at gearing of any kind, you will see that it is not simply square symmetrical construction. the beveling and other tapers on the teeth are there to reduce wear and stress while providing as much strength as needed. Some of the shaping of gear teeth is to lessen the slack and provide more precision in the geared movement. It is possible that the silicon gears are not able to be shaped
  • 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.
  • by Radon360 (951529) on Wednesday November 28, 2007 @06:40PM (#21511001)

    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.

    • I don't think so, they whacked their sample with a hammer until it cracked.

      On the other hand, using half the pressure but cycling the test hundreds of thousands of times revealed a gradually increasing pattern of surface damage at the indentation site--clear indication of mechanical fatigue.

      The tiny mirrors on the DLP are just going to flex in a magnetic field.
      I can grab a piece of wood and wobble it for hours (like Rolf Harris!) without a crack appearing but I can also pound it with a hammer and do a Jack
      • by davidsyes (765062)
        Are you trying to excite snowraver1 (1052510)? Did you read his/her:

        "Things like my xbox 360, CPLDs, diodes, LEDs... I should stop there... I'm turning myself on... Oh baby... TTL chips, CMOS... Good 'ol CMOS never say no when you call her late at night."?

      • IAAME (I AM a MEMS Engineer) and there are 2 quibbles I have with your statement: 1) The DLP mirrors' flexing is exactly the types of fatigue loading that most other materials eventually fail under. Just the slight imbalances in a rotating shaft cause it to experience slight up and down flexing during operation. If you've not taken into account the fatigue limit of your material, a 1 RPM shaft will eventually fail due to tiny "wobbles". 2) The "magnetic" field that causes DLP mirrors to actuate is magne
        • Thanks for the clarifications - I knew it was something like I said but couldn't think of the correct description.

          As for the breakdown of a rotating shaft *eventually* I agree with you, but since you have literal hands on experience - does this occur within products' lifespans or is the article using a sledgehammer to pound some FUD into the process?
          • My experience is academic, not hands on, so I'd happily defer to a truly experienced opinion should one surface. However, for certain load levels, some materials have an essentially infinite fatigue limit when it comes to cyclic loading. Many metals, for example, have a characteristic curve that will tell you the projected life of a part in cycles for a given load. This allows designers, without mandates for planned obsolescence, to make cyclically loaded parts that will not fail due to fatigue. So, pre
    • I have a 720p DLP set, so I'd like to know the answer here as well. However, I do recall during my due dilligence research reading that the mirrors only flex through about 10 degrees of travel, and were tested through hundreds of thousands of flexes with no perceptable damage.
      • by wattrlz (1162603)
        At 30fps, "hundreds of thousands of flexes" might get you through the LOTR trilogy, if you're really lucky.
    • by StickyWidget (741415) on Wednesday November 28, 2007 @07:04PM (#21511305)
      The answer is no, but it could be subject to other types of mechanical stress. The difference is that the experiment was done to gauge damage from differing direct pressure, DLP use something called a micro mechanical torsion spring. The experiment doesn't quite scale to the spring. However, the way the torsion spring works is that it allows twisting, kind of of like the old "bird in the cage" [bizarrelabs.com] persistence of vision trick. It's designed to accept a degree of stress from the pressure of twisting. Conceivably, if the crystal layers were aligned in a way that put differing stresses on different layers, it could be an issue. Kind of like if you do the bird trick too long you start seeing small bits of thread pop off from the main string.

      However, the kind of tolerance is *probably* already present in the DLP chips. The forces that the spring is subjected to were carefully calculated, and the technology has been in use since the 60s. You could probably take a look at the older types of DLPs and compile evidence that a large amount of cycles won't harm it.

      Caveat: I am not a micro-mechanical device engineer, but I follow developments. I figure micro-mechanical devices will need control systems of some sort someday.

      ~Sticky
      /It's all about temperature, pressure, and friction.

    • Re: (Score:2, Informative)

      by nullspace (11532)
      Actually, Analog Devices probably has a larger MEMS rollout and probably for a longer time. MEMS is incorporated into airbag systems [analog.com] (about 200 million units and the largest market share at around 60%), IBM's Active Protection System for Thinkpads [nanotechwire.com] and of course Nintendo's Wii controller [analog.com]. I would assume that this fatigue would be something worthy of further examination. Disclaimer: I work for Analog Devices, but not as a product designer.
    • Re: (Score:1, Redundant)

      by mollymoo (202721)
      The fairly recent appearance of motion sensors in everything from mobile phones to games consoles is due to MEMS technology. If you're a geek, it's quite likely you've got a MEMS device already and it's likely made by Analog Devices [analog.com].
      • by mollymoo (202721)
        That'll teach me to hit refresh before posting, nullspace got there first with basically the same information.
    • ...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
    • The torsion hinges that support the micromirror on Digital Micromirror Device [wikipedia.org] (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 [wikipedia.org] 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

    • Technically, inkjet print heads or accelerometers are a much larger MEMS rollout. They're just not as sexy as DLPs :)
  • by StickyWidget (741415) on Wednesday November 28, 2007 @06:43PM (#21511055)
    Duh.

    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

    • by wattrlz (1162603)
      Actually, as I read it, TFA seemed to say that big gears, made of silicon, don't wear down while small ones do contrary to accepted belief, thus making it news.
    • by caerwyn (38056) on Wednesday November 28, 2007 @06:53PM (#21511167)
      You didn't RTFA, did you?

      The findings are relevant to silicon precisely because the macro-level tests have *not* shown fatigue cracks. Now, the article suggests that this may be a weakness in the macro-level testing methodology, but it doesn't change the fact that silicon was considered "special" because of it's structure, and now it appears not to be.

      So, uh, you've actually got this completely backward. No one thinks it's a new type of stress, it merely wasn't expected that silicon would be susceptible to it.
      • This example was brought up in a previous comment [slashdot.org], I'm expanding on it.

        Assume the resistance of steel to pressure is X. So, I decide to put a weight that exerts a pressure of X/2 and leave it there. NOTHING HAPPENS, ever. So I conclude that due to the structure of the steel, it is immune to pressure effects. OOPS, wrong.

        Then, I decided to beat it with a hammer with an impact pressure measured at X/2. Nothing happens, for a long time, likely a very long time. However, eventually there will appear s

        • Crap.

          Me:Temperature, pressure, and friction, until you get to the atomic level, they exist and will forever be factors in design.

          I deserve a dunce cap for that one.

          Edit:Temperature, pressure, and friction, even when you get to the atomic level, they exist and will forever be factors in design.

          ~Sticky
          /Dunce

  • Diamonds are forever~
    • Re: (Score:3, Interesting)

      by SEE (7681)
      Diamond is metastable. Graphite is forever.
      • by geekoid (135745)
        Then how do you explain my pencil needing sharpening?
        • Re: (Score:3, Informative)

          by pimpimpim (811140)
          When you write with a pencil you move graphite layers around, from the pencil to the paper. They stay graphite layers, however. The point is in the layers, the layer-layer bonding is weak pi-bonding, which makes it easy to detach the layers from each other by shearing them (e.g. writing).

          The point why diamond stays like it is, is that even though it's thermodynamically unstable, it is kinetically stable. In contrast to graphite, it is very hard in diamond to break all bonds between all atoms in the lattic

    • Re: (Score:3, Funny)

      by mcpkaaos (449561)
      Well, you know what they say:

      A woman without diamonds is a parsnip.
  • 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.

  • by compumike (454538) on Wednesday November 28, 2007 @06:50PM (#21511133) Homepage
    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.
    --
    if(coder && wantToLearn(electronics)) click(here); [nerdkits.com]
    • Yes, current MEMS operate at a scale where the effect likely has no impact (probably by a few orders of magnitude). However, previously this effect was thought to not exist and thus was not a factor in future MEMS design. That really only limited MEMS devices to size constraints where there is enough mass etc to provide a measurable effect.

      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.

    • by secPM_MS (1081961) on Wednesday November 28, 2007 @08:39PM (#21512433)
      There are scale issue here. Even in metals with significant fatigue issues, such as Aluminum, if the structure is thin enough, the image forces on a dislocation suck it to the nearest free surface and you avoid the growth of dislocation tangles that result in fatigue failure. If I remember properly, the relevant thickness for Al was on the order of 100 nm. Note that I am working from memory from grad school ~ 25 years ago, when I did my Ph.D in fracture mechanics.

      TI has been working with the mirror systems for a long time now, I suspect on the order of 20 years. They should have real reliability info to work from.

  • Our tired silicon overlords are welcome to crash on my couch for a while, 'til they feel a bit less fatigued.
  • MEMS memes (Score:1, Troll)

    by Dr. Eggman (932300)
    In Soviet Russia, MEMS wear you out!
  • Microchips made of silicon will never work... the notion is ridiculous.
  • So they are now reporting on the back strain of supermodels? That was my first thought reading the headline...
  • I'm tired (Score:1, Interesting)

    by Anonymous Coward
    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
  • This has always been the explanation for why our digital watches run faster every year. Only a second or two, but faster.
    • by mollymoo (202721)
      Surely that's down to the quartz, not the silicon. This research is specific to silicon on small scales.
      • by TooTechy (191509)
        Yep. The quartz is but another form of silicon, in this case SiO2. Whether it from the same cause, I don't know.
        • by imsabbel (611519)
          Well, saying that quarz is just another form of silicon is like saying water is just another form of breathable air...

  • >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?
  • 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.
  • Oh noes! (Score:2, Funny)

    by mgabrys_sf (951552)
    I'll be concerned when they start putting "gears" in my Intel chips. The new Intel Geartron processor - now with gears! Um - no.
  • 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.
  • 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"
  • Not "New Type"... (Score:5, Informative)

    by florescent_beige (608235) on Thursday November 29, 2007 @03:08AM (#21515207) Journal

    It's old fashioned fatigue, and it isn't new. This [cwru.edu] 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.

  • Silicon -- the backbone of the semiconductor industry -- has long been believed to be immune to fatigue from cyclic stresses ...

    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 te

  • 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.

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