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.

Anyone seen this effect?  I'd be most appreciative for any insights.

Harry Lockwood
[email protected]