A first asymmetric etching process is performed to remove a portion of the nitride layer until {HEAD ~ a portion} of {TAIL ~ a top surface} of the first oxide layer is exposed 
A first asymmetric etching process is performed to remove a portion of the nitride layer until { HEAD ~ a portion } of { TAIL ~ a top surface } of the first oxide layer is exposed
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH