Abstract

Vibration can prevent a lithographic system from achieving its full resolution/ overlay potential performance. In this talk, two types of optical metrology–laser interferometry and moire interferometry– are discussed. In comparison with accelerometer-based metrology, the optical techniques are much more accurate for low frequency motions. In addition, the optical techniques directly monitor the relative displacement of the wafer with respect to the projected image. Examples of vibrational motion in optical steppers are presented, both when the stepper is nominally stationary (static vibration) and when the stepper is executing its normal motions (dynamic vibration). The effectiveness of vibration isolation tables in reducing floor vibration is quantitatively shown. Important vibrational modes can be probed in real time by watching the stepper resonate after impulsive excitation.

© 1989 Optical Society of America