[time-nuts] Helium and MEMS oscillators don;t mix well
jimlux at earthlink.net
Thu Nov 1 12:14:48 EDT 2018
On 11/1/18 8:47 AM, Bob kb8tq wrote:
> A lot of Radon and *really* poor ventilation….
> There are a lot of ways for He to show up. In normal use, issue is hanging on to it.
> It tends to run away from its source very quickly. Maintaining a measurable concentration
> in something like a normal room …. not very easy at all.
a couple interesting things to think about (I personally think the
original story has some other confounding factor they forgot):
1) MEMS pressure sensors have been around for decades, and they're used
with helium all the time.
2) MEMS accelerometers (which have moving parts, vs the pressure sensor)
have also been around for a long time. I've not checked, but I'll bet
some are in hermetic packages which get He leak tested. If there was a
Helium problem, you'd have heard about it.
3) There *are* stories about trace contaminants affecting the
performance of MEMS RF switches, specifically water vapor - it affects
the stiction of the moving contacts.
4) What is the proposed mechanism for Helium affecting the oscillator?
Here's an article from 2006 discussing SiTime's stuff
They discuss how hydrogen diffuses *out* of the area where the resonator is.
Now, it's possible that in the interests of saving fractions of a penny,
Apple is using resonators that aren't packaged as well as the SiTime
units (which are awfully cheap). (although the news stories say Apple is
using SiTime's parts)
"Apparently, SiTime also is aware of this problem and says its newer
devices are “impervious to all small-molecule gasses.” But they admit
older parts were not immune." I'd be interested in the context for that
off SiTime's FAQ page:
How effective is the hermetic seal of MEMS oscillators??
One of the key elements enabling extremely stable MEMS resonators is
SiTime’s EpiSeal™ process which hermetically seals the resonators during
wafer processing, eliminating any need for hermetically sealed ceramic
packaging. SiTime’s EpiSeal resonator is impervious to the highest
concentration elements in the atmosphere, nitrogen and oxygen, and
therefore acts as a perfect seal. Previous generations of EpiSeal
resonators may have been impacted by large concentrations of
small-molecule gas. Newer EpiSeal resonators are impervious to all
small-molecule gases. Please contact SiTime in case you are planning to
use a SiTime device in large concentrations of small-molecule gas, so
that we can recommend an appropriate, immune part.
ANother paper on packaging
More information about the time-nuts