A second asymmetric etching process is performed to remove {HEAD ~ a portion} of the first oxide layer by using the remaining nitride layer as {TAIL ~ a mask} until about 50% to 90% portion of the first oxide layer is removed 
A second asymmetric etching process is performed to remove { HEAD ~ a portion } of the first oxide layer by using the remaining nitride layer as { TAIL ~ a mask } until about 50 % to 90 % portion of the first oxide layer is removed
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH