lol...true but they always make me suspicious.
Your work history is key to this story. After a few years with a die yield problem that was clearly litho related(and a very pissed off customer) I ran a few million alignment data points in a binning program. They were from six different steppers. At first there was clearly a 4 sigma alignment problem. My Manager was ready to shut everything down until the problem could be isolated. I separated the data based upon specific unit(s) and found that one machine had a stage position problem. Fixed that. But in the course of reviewing the data I discovered that every machine had a misalignment in a certain(common) area of operation of the stage, after that they were specific to the machine but very repeatable. Used that and created a stage optimization subroutine that accounted(and compensated) for the marginal positioning in the specific unit. Never would of found it just looking at raw data.
I had no idea you were a fellow litho engineer!
Remind me to tell you about the haunted Kasper 2001 aligner...