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

[Bob kb8tq]

Hi


> On Feb 5, 2018, at 8:54 AM, Attila Kinali <attila at kinali.ch> 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.

The typical quartz resonator is operating in a mode that involves 
more than a surface wave. Much of the effort involves not just that
mode but getting rid of the vast number of similar modes that can 
pop up. As you add structure complexity, you don’t just want to “improve”
the main mode. You also want to be sure you don’t encourage any 
others …..

That all said, there are indeed people out there who *do* understand how
this all works. There aren’t a lot of them, but they are out there. From what
I’ve seen, the supply of “those who know” actually exceeds the industrial 
demand for what they know. Like it or not, precision quartz resonators is
*not* a growth field. 

Bob


> 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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