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To: Aaron Cooperband who wrote (40558)	11/17/1997 9:12:00 PM
From: Paul Engel	Read Replies (2) \| Respond to of 186894

Aaron - Re: "could this be what theta refers to?, " Yes, you are correct - in both cases - Theta refers to rotation in the x-y plane - the plane of the wafer. As you noted (and I didn't discuss it) the "z" height of the stage is adjustable in order to insure proper depth of focus. The focal plane is very shallow - a micron and usually less - with the high numerical aperture lenses used in these steppers. Accordingly, the plane of the wafer's top surface is munutely adjusted (z axis) to insure proper focus. This is also done on a die-by-die basis. Paul