[time-nuts] Etching of quartz crystals (was: Rakon HSO-14)

Attila Kinali attila at kinali.ch
Mon Feb 5 08:54:51 EST 2018

On Sun, 4 Feb 2018 09:21:54 -0500
Bob kb8tq <kb8tq at n1k.org> wrote:

> > The images on this page gives a good impression about the current
> > skill-level in that area:
> > 
> > 	https://www.azonano.com/article.aspx?ArticleID=2740 <https://www.azonano.com/article.aspx?ArticleID=2740>
> The gotcha is shown in the pictures. First point is that they are etching *very*
> small features. A 5 MHz 3rd overtone blank is way thicker than what they are 
> playing with. The second issue is that even at small scale the walls are going 
> non-parallel.

That's exactly the issue here. While SAW resonators benefit quite a lot
from the processing skills learned from semiconductor fabrication, these
skills do not translate into BAW manufacturing. SAW resonators are built
etching or depositing small features ontop of a SiO2 wafer that is supposed
to be as flat as possible. On the other hand BAW oscillators are 3D structures
by themselves. They are lens shaped (thus not flat) to keep the oscillation
energy trapped in the center of the slap, thus allowing the edges to be used
for mounting/contacting, with minimal damping of the oscillation.

Yes, the shapes are simple. But not only because that's the only shapes
we know how to build, but also because these shapes allow us to calculate
how the crystal will oscialate and because the simpler the structure the
easier it is to build it with high precision and accuracy.

It would be possible to use edging of surface structures into the
crystal to form a Bragg reflector (instead of the lense shape).
But I have no idea how well it works. Considering that it is easier
to build a slap that is flat and then etching structures on it, than
to form a 3D structure, I wonder why I have not read about anyone
doing exactly that (beside for SAW structures). 

				Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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