I'm happy to report that STS has responded to this question and is
actively assisting our contractor in resolving the problem.

Thanks to all who offered their experience.

HFL

On Apr 1, 2005, at 2:56 PM, Harry Lockwood wrote:

> Our processing contractor has been having problems getting controllable
> results in etching high-aspect channels through Si wafers using an STS
> machine.  Channel dimensions are typically about 25 um diameter by >
> 300 um deep.
>
> We are getting a "wine-glass" shape in the cross section.  Many
> parameter changes later, the effect persists.  Si loading of the sparse
> pattern, etch/passivate cycle changes, wafer backing, power levels have
> all been modified, to no avail.