Reza,

One thing that you can try is to mix precision glass spheres of the
gap size that you require into your glass frit paste.  Once you apply
pressure under heat, the spheres will become a spacer to limit and
maintain the required gap.  You may want to experiment with different
size spheres depending on the precision of the gap required.  You will
find a vendor by googling "precision glass sphere."

I used this technique some time ago making a vacuum display.

Brad


On Jan 24, 2009, at 12:18 PM, Reza Rashidi wrote:

> Hi everybody,
>
> I was trying to fabricate a high temperature capacitive pressure
> sensor. I have made two gold electrodes on pyrex and Si membrane
> connected to pads by a lines (100 um width and 150 nm thickness). I
> used a glass frit to bond two chips together. The bonding interface
> forms gap between two electrodes. According to my design, the gap
> should be 10 um. The glass frit (originally 50 um thickness) should
> flow due to force and temperature in the furnace and decreases its
> thickness. The problem was that I couldn't control the force on
> chips during heat treatment at 500C because any spring will become
> loose in high temperature. So, controling the frit thichness was
> dificult and also there was a leakage from some where that there is
> no good contact between glass frit and si or pyrex.
>
> I also tried to use high temperature epoxy. One with high viscosity
> generates a big gap and another one with low viscosity flow into the
> membrane. Also, anodic bonding (of different sample with a 10 um
> cavity) didn't work as I had connection lines of gold to pads on Si.
>
> Any advise regarding solving the problem by the above solutions or
> any other solutions will be highly appreciated.
> In case of glass frit, where do you recommend me to buy a thiner
> glass frit and how can I work with it to solve the problem?
>
> Thanks,
> Reza Rashidi
>

Brad Cantos
brad.cantos@holage.com
http://holage.com

LinkedIn: http://www.linkedin.com/in/bradcantos