Q. On the Nikon Steppers, which direction will the stage move if +Focus command is given?
A. Positive (+) offset will cause the stage to move up, and Negative (-) will make it go down.
This question came up back in 1997 when I was working as a Field Service engineer for Nikon (NPI). One of our customers asked about the direction that the wafer stage moves when a + Focus offset is given. Most of us including me replied UP but one person said down. We could not positively confirm or deny either answer and the question was ultimately given to the application folks. I did not follow up but the question stayed with me for a long time.
Fast forward to 2001…. I was working for another company and we wanted to move a model Body-9 Nikon from one location to another. I decided to run an experiment on this tool which was no longer in production.
The focus motor on the Body-9 stepper is located in front of the stage, behind a black panel. If this panel is removed, the motor and block assembly can be seen. The motor and the lead screw are horizontal and block moves side ways on a ramp which will cause the stage to go up or down. I activated the manual stage movement and pressed the (+) Focus button on the control panel. This caused the block on the lead screw to move to the left. Pressing (-) caused the block to move right.
While in manual mode, I also selected +Limit on the software to confirm that the block moved to the left; it did.
Now it was time to determine if the movement of the block to the left meant that the stage is moving up. I connected a piece of scotch tape from the stage to the lens block. Moving of the block to the left, caused the scotch tape to bunch up (wrinkle) in the middle (stage was getting closer to the lens). Movement of the block to the right caused the tape to become disconnected from the lens block; meaning that the distance between wafer stage and the lens was increasing.
Note: If the focus is determined through Focus test to be say (- 0.3 micron), this would mean that the stage is too high and the optimum focus is at 0.3um below current 0 position. Therefore, the stage will need to be lowered. That is why (-.3) is added to the existing offset to make the stage go down in order to make the focus zero.
I have heard that other steppers manufacturers (Cannon, ASML, Ultratech) have it the opposite of Nikons. Having no experience with them, I hope someone can enlighten me.
Let me know if this was of your interest.
F.O.
(Lithoguy)
