first oxide layer 110 followed by plasma etching substrate 200 using a chlorine based chemistry during the silicon plasma etch there is etch back of first nitride layer 215 trench 225 has a bottom 227 a first sidewall 230 a and a second sidewall 230 b first nitride layer 215 is used as a chemical mechanical polish cmp stop in subsequent processing steps in fig4 c a liner 240 is formed on bottom 227 first sidewall 230 a and a second sidewall 230 b of trench 225 trench 225 is bisected by a reference plane 245 which is equidistant from first and second sidewalls 230 a and 230 b and perpendicular to top surface 205 of substrate 200 in one example substrate 200 is p doped to a concentration of about 1e15 atm cm 3 to 1e16 atm cm 3 trench 225 is about 0 25 to 0 4 micron deep and about 0 4 to 5 microns wide first oxide layer 210 is about 8 to 12 nm thick first nitride layer 215 is about 50 to 200 nm thick and liner 240 is about 10 to 30 nm thick thermal oxide in a second example trench 225 is about 0 05 to 1 micron deep and about 0 1 to 5 microns wide in fig4 d a conformal second nitride layer 250 is deposited on top of first nitride layer 215 exposed regions of first oxide layer 210 and liner 240 which covers bottom 227 first sidewall 230 a and second sidewall 230 b of trench 225 second nitride layer 250 may be formed by.