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."

Comments

Is it just me?

[zappepcs]

or did anyone else see 'silicon fatigue' and immediately think of something more mammalian in nature?

Re:Is it just me?

[snowraver1]

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.

Re:Is it just me?

[haystor]

I put on my robe and wizard hat.

Re:Commodity

[Valdrax]

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?

DLP TV/Projectors, the first consumer victim?

[Radon360]

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.

Re:DLP TV/Projectors, the first consumer victim?

[StickyWidget]

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" 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:Small gears vs. Large gears?

[caerwyn]

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.

NOT LEDs!!!

[mangu]

LEDs are not made of silicon. They are either gallium arsenide or boron nitride, depending on the color.

Not "New Type"...

[florescent_beige]

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.