EUV-specific Overlay Issues
Because EUV operates in a vacuum and requires reflective optics, EUV lithography tools have special overlay concerns, recently studied by IMEC, along with ASML. Electrostatic chucks must be used instead of conventional vacuum chucks. Therefore the wafer clamping variability on the electrostatic chuck needs to be dealt with. A backside coating of 200 nm silicon nitride (which must be removed later to allow backside cooling and heatsinking) was found to be helpful. Other than this additional step, which also requires first protecting the device layers already patterned, zone alignment (using all alignment marks across the wafer, not a standard subset) also provided some improvement. The vacuum environment required by EUV also leads to heating of the wafer without much dissipation. A sacrificial first wafer was found to be necessary to stabilize the chuck temperature. Moreover, the local overlay corrections due to exposure heating requires the use of a second wafer. Thus, an extra wafer per lot is required for overlay stabilization in EUV lithography. The use of reflection causes wafer exposure position to be extremely sensitive to the reticle flatness and the reticle clamp. Reticle clamp cleanliness is therefore required to be maintained.
Read more about this topic: Extreme Ultraviolet Lithography
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