{TAIL ~ A first portion} of {HEAD ~ the intrinsic base layer} is masked while a second portion of an intrinsic base layer is etched 
{ TAIL ~ A first portion } of { HEAD ~ the intrinsic base layer } is masked while a second portion of an intrinsic base layer is etched
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH