The average position can be used to more accurately determine the average position of {HEAD ~ an alignment mark} on {TAIL ~ a wafer} carried by the stage 
The average position can be used to more accurately determine the average position of { HEAD ~ an alignment mark } on { TAIL ~ a wafer } carried by the stage
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH