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