Shay is correct.  For recent experimental evidence (where Si nanopillars
start to deform plastically, when the length scale of the pillar is less
than the mean distance between defects), see:

http://www.sciencedaily.com/releases/2009/10/091008133455.htm

In my opinion, the decrease in standard deviation of the distribution of
fracture strength, measured in silicon membranes, by exposing the
membranes to high-temperature anneals in inert atmosphere, is due to the
reduction of point defects and oxygen precipitate clusters, both in the
bulk silicon, and at the surface.  But, for the moment, this is more
opinion than proven fact.

---
Albert K. Henning, PhD
Director of MEMS Technology
NanoInk, Inc.
215 E. Hacienda Avenue
Campbell, CA  95008
408-379-9069  ext 101
ahenning@nanoink.net
Thank you.

-----Original Message-----
From: Shay Kaplan [mailto:shay@mizur.com]
Sent: Wednesday, November 11, 2009 11:20 PM
To: 'General MEMS discussion'
Subject: Re: [mems-talk] Si stress-strain relationship and allowable
stress

The main cause for fracture in single crystal devices is stress
concentration - this may be design related but also, scratches, defects
surface pinhole etc are reducing the actual fracture strength of the
crystal.

Actually, the smaller the device area, the higher if fracture strength
since
statistically it will have less defects.

Shay