On Nov 21,  1:52pm, Tom Benedict wrote:

...edited

> The problem:
>
> We've run into problems with the 148um line spacing.  Etch times for
> 30% KOH at 60 C are around 4-5 hours, and by the time we reach the
> bottom of the grooves the <111> planes look pretty ragged.
>
> Ideas:
>
> One problem I can identify right now is that we aren't as closely
> aligned as we'd like.  We used an alignment technique with the 25um
> mask that worked quite well, but with the coarser mask it doesn't work
> as well (more on that in a sec.)  We need to align our mask to the
> lattice better than we are.
>
> Another problem is that we got the 30% concentration and 60 C
> temperature from another paper on KOH etching, but really don't know
> if those are ideal for our purposes.  I've spent the past few days
> combing a number of journals, hoping to find an article with a
> parametric survey for KOH with respect to the <111> surface quality,
> but haven't had any luck.
>
edited...
>
> Thanks,
>
> Tom Benedict                          McDonald Observatory at Mount Locke
> [email protected]               Department of Astronomy
> Tel: 512-471-3337                     University of Texas at Austin
> Fax: 512-471-6016                     Austin, Texas  78712


First there are a couple of papers that describe special geometries useful for
aligning a mask to the crystal planes in silicon.  One I remember was by Tom
Poteat [I don't recall the journal, but his patent was US Patent # 4,470,875].
 Personally though, I have found that about any long feature, even a scratch
made with a diamond scribe, with edges intentionally misoriented,  will work
just as well when allowed to etch until it forms a long v-groove.  Simply make
it as long as the field of view in your mask aligner allows, and align the
edges of the groove to lines on the mask.

I have to wonder if the rough surfaces you see are really due to misalignment.
 This should be easy to verify by measuring the width of your long channels
after etch.  The closer they are to your mask dimension, the better the
alignment.  But in my experience misalignment usually just leads to a wider
groove that still has very smooth {111} surfaces, as long as the etch time is
long enough.

Several years ago in a previous job where I routinely etched deep cavities
(with ~30% KOH), I did once over a brief period see very rough {111} surfaces
after etch.  Under a microscope it appeared that there were clumps of some sort
of multi-faceted crystals emerging from the otherwise smooth {111} surfaces in
irregular patterns.  Some wafers were very bad, while others in the same lot
showed very little or no evidence of this at all.  In some cases we even saw
areas of good and bad cavities within the same wafer.

Our problem did not seem to be related to any misalignment [we didn't see any
stair-stepping at the top edges],  although those cavities with rough sidewalls
did usually seem to be overetched (ie larger than expected) as well.  After
spending several days searching through journals, we were beginning to lean
towards some sort of oxygen precipitate forming in the wafers, as had been
reported in several papers.  Before we could study the problem any further
though, it went away.  I changed jobs shortly after that, so I don't know if it
ever resurfaced.

I'm sorry that I can't offer any more than this, but perhaps something may help
you out a little.  Good luck.


Marc Straub
Automotive Components Division, Ford Motor Co., Dearborn, MI
[email protected]