The most simple alignment key I can think of for a KOH etch into (100)
silicon would be to use four small squares that are placed to form the
corners of a larger square, placed in such a way as to leave small gaps
between the squares.  This way, the gap between the squares forms a
"crosshair" shape. I have given a basic diagram of the structure below
(please forgive me, but I am limited to ASCII text):

[] []

[] []

Make sure that the area inside each smaller square is the area being etched.
Since squares maintain their shape under a KOH etch (just growing smaller as
you get deeper into the silicon), the "crosshair" will be maintained.  You
can theoretically create a "crosshair" of any size, but bear in mind that
KOH will etch silicon in ALL directions; the (111) plane is just etched
extremely slowly (0.0035 um / min, vs. 1.4 um / min).  If you are performing
an etch that goes most of the way through the wafer, then the time required
to etch 500 um of silicon (~6 hours, by the rate mentioned above), then the
KOH will etch through 1.25 um in the (111) plane.  So, if you've made your
crosshairs 2 um wide, then they will have been etched away by the time your
etch is complete (since the crosshair will be attacked from both sides).
However, the feature will still be somewhat visible, as you will still have
the sharp "edge" of the crosshair feature, where the (111) planes on each
side of the crosshair structure intersect.

If you are using positive resist as your patterning method, then all you
will need to do is place similar structures on successive masks, and align
one crosshair directly over the other.  If you are using negative resist,
then you can create chrome boxes (assuming you are using a chrome on glass
mask, of course) that are complements to the crosshair feature.

Best Regards,

Chad Brubaker



EV Group-Technology, Tel: (602) 437 9492 x 119, Fax: (602) 437 9435
E-mail: C.Brubaker@evgroup.com  ,  Web:
www.EVGroup.com  , 01/21/02

                -----Original Message-----
                From:   Brian Semak [mailto:bse@hymite.com]
                Sent:   Monday, January 21, 2002 8:06 AM
                To:     mems-talk@memsnet.org
                Subject:        [mems-talk] Allignment marks for proper
orientaion of KOH mask

                Hello everyone

                Can anyone help me with some allignment marks. I am looking
for allignment
                marks, which ensures proper orientation of a KOH mask in the
100 plane of a
                wafer. The masks I am looking for, should be etched into the
wafer, thus be
                used for alignment of the mask for subsequent KOH etching.
If anyone here as
                some ideas and suggestions, I would be very gratefull.

                Med venlig hilsen/Kind regards

                Brian S. Semak, M.Sc.
                R&D Silicon Processing

                Hymite Aps
                DTU, Bygn. 325, 1st floor
                2800 Lyngby

                Telephone(Direct): + 45 4525 9147
                Fax:                     + 45 4525 9156
                E-mail: BSE@hymite.com
                www.hymite.com
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