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