Hello Ramji,
I don't know whether we can control the side wall angle in etching the si
anisotropically.
But regarding getting the vertical walls in a (100) silicon wafer using KOH,
the trick here is to align the rectangular/square mask opening at at angle
of 45 degrees to the crystal wafer flat which is <110>. By doing so the
{100} faces are introduced as sidewalls. the {110} planes etch faster than
the {100} planes and are not introduced as sidewalls at any point. as the
bottom and sidewalls are both from {100} gp, lateral undercut equals the
vertical etch rate and vertical walls will be produced.
Regards
Rahul agarwal
University of South Florida
Electrical Engineering Dept.
Micro Electro Mechanical Systems Lab