After the diffusion contact endpoint has been reached, the second etch step is initiated using a CHF3 /O2 mixture which etches {TAIL ~ silicon nitride} with a very high etch rate ratio to both BPSG and {HEAD ~ titanium disilicide} 
After the diffusion contact endpoint has been reached , the second etch step is initiated using a CHF3 /O2 mixture which etches { TAIL ~ silicon nitride } with a very high etch rate ratio to both BPSG and { HEAD ~ titanium disilicide }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH 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 HEAD HEAD HEAD HEAD HEAD HEAD