Hi all,

I am looking for some advice on the avoidance of surface defects when etching
silicon with KOH.

We are using a pretty standard KOH etch (80°C, 40%) to etch large surface areas
down by about 100µm on standard SSP (100) silicon material.
We are looking for smooth surface after etching the polished wafer side. Mostly
this works quite well but sometimes we do see the formation of small point
defects all over the wafer, tpyically in a radial distribution.

These defects are small recesses (not hillocks) in the range of 10-50µm
diameter, perfectly circular shaped (unlike you would expect in anisotropic
etching). The depth of the structure is very low (we assess to <100nm) but
clearly visible macroscopically and in the microscope.
We see different sizes of these defects leading us to believe that this is not
an effect started on the surface but is a volume effect (the 'early' defects
becoming larger as we etch down).


We do batch processing and sometimes complete batches are affected, sometimes
only a few wafers out of the batch. So we would link this mainly to the silicon
material and not to the process. For this we control the material spec closely
and the wafers should be very much identical. As we fear an influence by doping
the resistivity is defined to be ~10 Ohmcm.


Our material supplier is puzzled as well. As we do see a mainly radial
distribution of defect occurrence (typical max in the center area, sometimes
donut shaped).
Could this maybe be an effect from polishing the wafers in manufacturing which
is sometimes more severe?
Any ideas are welcome!

Thanks & best regards
Ulli
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