Field distortion characterization using linewidth or pitch measurement

Field distortion in steppers and mask pattern generators can be a serious source of placement errors, which subsequently translate to overlay errors if different steppers/masks (with different distortion) are used in different layers of wafer printing. Such distortion is often determined using an expensive coordinate measuring system (e.g., Leica LMS2020 optical metrology system) which may not be readily available. We describe a simple and inexpensive technique for measuring field distortion using readily available instruments. The essence of the technique is to build up a pattern using multiple exposures with different portions of the field overlapped in the complete pattern, so that the resulting linewidth or pitch variation across the field gives the difference between the field distortion and a shifted field distortion. We can then reconstruct the field distortion by measuring the linewidth/pitch. We have applied this technique to two optical steppers, and have shown that the reconstructed field disto...