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

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Albert Henning
Sent: Wednesday, November 11, 2009 7:05 PM
To: General MEMS discussion
Subject: Re: [mems-talk] Si stress-strain relationship and allowable stress

Unfortunately, there is silicon, and there is silicon.  The growth process,
and subsequent thermal steps in its fabrication, affect fracture strength.
For instance, magnetic Czochralski starting material is better than CZ,
because (I think) it has a lower concentration of oxygen, below 20 ppm.
Also, thermal processing above 1150 degC, ideally in an argon ambient,
improves fracture strength (see below).

Also, silicon is a crystal.  Fracture strength is a function of
crystallographic direction.

Also, surface texture and surface film (e.g., SiO2) affect the initiation of
a crack at a surface.

What this all means:  you must remember that there is a *distribution* of
fracture strength for any ensemble of crystalline structures, ostensibly
fabricated 'the same'.  This distribution has a mean, and a standard
deviation.  The mean can be increased somewhat using starting material with
lower point defect concentrations.  The standard deviation can, to a certain
extent, be made smaller by using thermal processing in inert ambient at
temperatures above 1150 degC.  Control of surface roughness and use/control
of surface films is also important.

See:  JMEMS reference by CA Wilson and P Beck (HP).  Various works by Alissa
Fitzgerald (AM Fitzgerald & Assoc) and Chris Muhlstein (Penn State).  I've
written some on the subject, in SPIE MEMS proceedings.

Bottom line:  there is no 'safety factor', except insofar as you build
devices, measure at what fracture strength they break (measure the
distribution function, in other words), and either specify pressures/forces
that avoid the mean of the fracture strength minus, say, 3 sigma, or
re-design the structure to survive higher pressures/forces.

PS:  It has never been shown experimentally, but I have a strong suspicion
that isotopically pure silicon will have a much higher fracture strength
than 'regular' silicon (with composition of isotopes as found in nature).
Isotopically pure silicon has been shown to have much higher electrical
mobility, and much higher thermal conductivity, so it should have higher
fracture strength. Unfortunately, isotopically pure silicon is very
expensive...

---
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