> A question about KOH etching. If two exactly co-centered
> squares are opened on both sides of a DSP 100 nitride coated
> wafer, the sizes are different, but the etching deepth will
> overlap with each other, and a through hole is expected. The
> question is what happens at the intersection point of the two
> piramid hole? Will the sharp edges be etched away or well
> kept? If there is misalignment of the two square holes, then
> what will happen at the meet point?
>
> Any idear is appreciated, esp. those direct experiences on this.


I have done this and taken cross-sectional SEMS to view the progress of the
etch.

Etching starts vertically downward from the top of the wafer
and vertically upward from the bottom of the wafer in <100> directions.
The areas of the etch fronts get smaller as the etch proceeds due to
the {111} planes barely etching, forming an inverted pyramid from the top
of the wafer and an upright pyramid from the bottom of the wafer.

Where these pyramids meet, there a sharp edge is formed.
I think of this edge as having all different planes/directions exposed.
The fastest-etching plane/directions varies and is often quoted as <411>-type
directions.
It is easier, however, to envision the effect on <110> directions
(which are parallel and perpendicular to the wafer flat).
In the <110> directions, the etch goes about twice as fast to the sides
as it went vertically in the <100> directions.

If the sizes of the squares where the etch fronts meet,
there will be a "ledge" until it is etched away.

If the wafer is etched for a long time, the sideways etch will proceed
until {111}-type planes are reached, making the inside of the hole
wider than it is at the wafer surfaces, with the original holes at the surfaces
and the terminal planes inside the wafer forming a hexagon.

If there misalignment between the original square mask holes,
etching will proceed until the above-described {111} planes intersect the wafer
surfaces
(unless the wafers are pulled out of the KOH before the etch is complete).
Some interesting sloped walls can be made in this fashion.

        --Kirt Williams Agilent Technologies