first time etching small pieces, mounting advice needed

[Mark Wistey]

At 2:36 PM -0700 10/25/06, Patrick Lu wrote:
>I usually etch entire wafers, but now I have some smaller 
>quartz-substrate chips. Anyone have advice on how I should mount 
>these to my handle wafers? Tape? resist?

It depends on the uniformity you need.  For fairly large pieces, or 
if you don't need good uniformity in your etch, you can use 
double-sided *conductive* copper tape (see the instructions in the 
PQuest manual for the 3M part number).  Cover the entire backside of 
your sample with the tape, but don't expose any of the copper. 
(Copper is bad for many people's projects.)  The sample will get hot 
around the edges and corners, so it will etch faster there.

If you need better uniformity, we've had very good results tightly 
packing the sample with fairly large dummy pieces, and using no tape 
under the sample itself.  You want good thermal conduction between 
your sample and the dummy pieces.  The dummies can be held on with 
copper tape, but we've had better results soldering them down with 
indium on a hotplate.  For best results, the mounted height of the 
dummy should equal the height of your sample.  Set the chiller and 
chuck to a low temperature (-10 and 0, respectively).  Again, make 
sure there's no exposed indium, because exposed metal will change the 
plasma characteristics near your wafer.  We've gotten <10% etch 
variation over ~95% of a 1" sample this way, compared with ~30% 
variation using copper tape.

Whichever route you take, of course, make sure you're starting with 
one of the prime p-doped Si wafers specifically stocked for the 
PQuest, since other types are likely to fall off the pins and/or 
break.
					 - Mark