Hello,

I realize plenty of you have created piezo-resistive strain based
sensors and so I would like to bounce a question off this forum.  I have
done this before as well but have never undertaken the task of "precise
and accurate" calibration of the deflection versus resistance
measurement. For obvious reasons this is highly desireable. The first
thought is to manually depress the structure with a small probe and
measure the probe position versus the resistance. The probe end needs to
be on the scale of the MEMS device (say 5micron or so) and capable of
large vertical travel (say 100microns)...my first thought is that this
might require a MEMS based device itself (anyone?). A secondary problem
is the nature of the piezo material and circuit elements.Obviously
exposure to light (uv) can adversely  affect the bias of circuit
elements, thus I must wonder whether this has an effect on the
resistance value of piezoresistors (anyone?). Has anyone compared the
calibration values measured in light to those measured in dark (I can
imagine what the test jig must include for that). What about the
inherent 'noise' of the poly-structure, what is the noise floor and
spectral analysis (in other words has anyone written a paper on the
random noise in poly resistors and how to 'cancel' or compensate for it).

The second method under consideration is to 'shake' the device at
resonance and optically measure the position (this however may not
achieve sufficient deflection for my purposes...but I'm not sure).

So my questions are (aside from those i've already asked):

1. Does anyone have a test jig for the probe method. What are the
minimum steps that can be accomplished (whether it is mechanically
stepped or manually advanced). What is the minim size structured that
can be deflected (we all know how rediculously large pad probes looks
like up against some of the structures). Is the test jig commercially
available or locally fabs (a part's list would be nice).

2. What other methods have people used...what is the precision and
accuracy of such systems.

Thanks, I welcome private or open discussions on such issues if people
are interested in further exploring this line of thought.

Derek Strembicke
MEMS/ASIC Design Engineer
[email protected]