Overlay Process Window defines the range of overlay errors within which the device still functions correctly — specified by overlay tolerance or budget, the process window is the maximum allowable registration error between layers before shorts, opens, or electrical failures occur.

Overlay Budget Components

• Scanner Contribution: Stage positioning accuracy, lens distortion, inter-field stitching — the lithography tool's overlay error.
• Process Contribution: Wafer distortion from thermal processing, film stress, CMP — process-induced overlay errors.
• Metrology Contribution: Measurement uncertainty — the error in measuring the overlay itself.
• Total Budget: $OV_{total}^2 = OV_{scanner}^2 + OV_{process}^2 + OV_{metrology}^2$ — RSS (root sum square) combination.

Why It Matters

• Yield Cliff: Overlay errors beyond the process window cause catastrophic yield loss — edge placement errors create shorts or opens.
• Shrinking Budget: <5nm nodes require <2nm total overlay — every component must improve.
• Design Rules: Overlay budget determines minimum design rules for contacts-to-gates and via-to-metal connections.

Overlay Process Window is the alignment tolerance budget — the total allowable registration error partitioned across tool, process, and metrology contributions.