[time-nuts] Etching of quartz crystals

jimlux jimlux at earthlink.net
Mon Feb 5 10:31:38 EST 2018

On 2/5/18 5:54 AM, Attila Kinali wrote:
> 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).

Follow the money - or lack thereof - Folks are happy with the existing 
technology - If I'm flying a science mission that needs a space 
qualified Ultra Stable Oscillator - I've already budgeted my several 
million dollars, claiming that I'll just use what we already know how to 
build, and I spend no more proposal pages talking about it.  I certainly 
am not going to say "instead of spending $1M/oscillator for my 2 
oscillators, I'm going to spend $5M on an experimental process to change 
how the resonator is made, and by the way, it might not work"

Would using ion milling and other modern fabrication techniques lead to 
an oscillator with *significantly* better performance or *significantly* 
lower cost?

For those users for whom this is important, research focuses on looking 
for another qualitatively different way to get there - That's sort of 
what the CSAC and the Deep Space Atomic Clock (DSAC) are about - the ion 
trap clock for DSAC gives you long term performance BETTER than a USO. 
Although probably not at a lower cost, yet, there is potential for it to 
be so.
The CSAC gives you "good accuracy at low power", compared to an OCXO - 
less than 1/10th the power.

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