TL;DR
Overlay errors usually come from alignment model drift, chuck temperature change, or lens heating. Run a standard overlay measurement lot and re-optimize the alignment model.
What you might see
- overlay metrology lot failing specification
- process layer misregistration visible in SEM
- gradual overlay trend over time
- lot-to-lot overlay variation increased
Likely causes
Alignment model parameter drift from wafer stage thermal change or stage calibration expiry
Lens heating effect from a recent process recipe change at different exposure dose
Reticle clamping or reticle stage repeatability degradation
Wafer chuck particle contamination under a wafer, causing local tilt
Required tools
- Overlay metrology tool (scatterometry or CD-SEM)
- Alignment model calibration reticle
- Chuck particle scanner or bright-field inspection tool
- ASML process control software for model updates
Safety first
- EUV systems operate with hydrogen purge gas in the optical column. Follow the site EUV gas handling procedure before opening any optical enclosure.
- The reticle handling robot operates in the SMIF environment. Do not reach into the robot envelope without confirming all motion is inhibited.
Procedure
- 1
Run a dedicated overlay measurement lot using the standard overlay target mask.[1]
- 2
Review the overlay vector map. Identify whether the error is systematic (one direction, one field, or radial) or random.
- 3
Check when the last alignment model calibration was run and compare to the scheduled interval. Run a full alignment model re-optimization if overdue.
- 4
Review the lens heating model parameters for the recipe. If the dose was recently changed, update the lens heating model correction values.
- 5
Inspect the wafer chuck surface with a particle scanner or bright-field illumination. Clean the chuck if particles are present.
- 6
Check the reticle clamp and reticle stage settle time logs for increased variation. Replace the reticle clamp pad if repeatability is out of spec.
- 7
Run a qualification overlay lot after corrections and verify all fields are within specification.
Sources
ASML ASML PAS 5500 / NXT 1980 / Twinscan NXE EUV Wafer Lithography / Stepper general technical documentation, ASML
ASML lithography system general overlay model calibration and alignment procedures (general)
More guides for ASML ASML PAS 5500 / NXT 1980 / Twinscan NXE EUV
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CD shift is most often a focus offset drift or a dose calibration error. Run a focus-exposure matrix and compare to the process baseline before adjusting any parameter.
How to clear a reticle handling fault on an ASML PAS 5500 / NXT 1980 / Twinscan NXE EUV
Reticle handling faults are most often caused by a reticle misidentification, a SMIF pod loading error, or a gripper vacuum fault. Check the pod barcode and gripper vacuum before attempting recovery.
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