I built the 4 contact electrochemical etchstop described in:

Title: STUDY OF ELECTROCHEMICAL ETCH-STOP FOR HIGH-PRECISION THICKNESS
CONTROL OF SILICON MEMBRANES

Author(s): KLOECK B, COLLINS SD, DEROOIJ NF, SMITH RL

Source: IEEE TRANSACTIONS ON ELECTRON DEVICES 36 (4): 663-669 Part 1,
APR 1989

I did not see any pores in the n-epi layer -what size were yours?. The
only checking that I did was to see that the sheet resistivity of the
epi was as expected from the resistivity and thickness.

Perhaps the high temperature steps I did caused diffusion in the epi
layer and ensured the doping was continuous?

My basic process was:

1. Wet ox 1100C 30min + strip oxide to remove any contaminents around
edge of wafer perhaps from grinding (had diode problems if I didn't do
this).

2. Wet ox 1100C approx 40min for TMAH mask

3. Did phosphourous diffusion on epi side to ensure low resistance to
epi contact (which was a 5mm ring around perimeter of wafer).

4. Metalization and open oxide pattern on substrate side.





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