Optical proximity effects (OPE) are the phenomenon where the printed feature size and shape on the wafer depend not just on the designed dimensions but also on the pattern's local environment — the size, shape, and distance of neighboring features. Identical designs print differently depending on surrounding context.

Why OPE Occurs

• Lithographic imaging is a diffraction-limited process. The optical system can only capture a finite number of diffraction orders from the mask, which limits the spatial frequency content in the aerial image.
• Dense features (closely packed lines) have different diffraction patterns than isolated features (single lines far from neighbors). The same designed width will print at different sizes.
• Pattern-dependent diffraction means the aerial image of any given feature is influenced by features within a range of roughly λ/NA (~500 nm for ArF immersion) from its edges.

Types of Optical Proximity Effects

• Iso-Dense Bias: The most common effect. A 100 nm line in a dense array (surrounded by other lines) prints at a different width than an identical 100 nm isolated line. The difference can be 10–30 nm without correction.
• Line-End Shortening: Lines are shorter on the wafer than designed due to diffraction-induced rounding at the endpoints.
• Corner Rounding: Square corners in the design print as rounded curves on the wafer.
• Pitch-Dependent CD: Feature width varies continuously as a function of pitch (spacing to neighbors).
• Proximity-Induced Placement Error: Feature positions shift due to interactions with nearby patterns.

Correction: Optical Proximity Correction (OPC)

• Rule-Based OPC: Apply fixed bias corrections based on the local pattern environment (e.g., add 5 nm to isolated lines, subtract 3 nm from dense lines).
• Model-Based OPC: Use a calibrated lithography simulation model to predict OPE and compute per-edge corrections. More accurate but computationally intensive.
• Serifs and Hammer-Heads: Add small square features at corners and line-ends to counteract rounding and shortening.
• SRAFs: Add sub-resolution assist features near isolated features to make their optical environment resemble dense features.

OPE in EUV

• EUV has different OPE characteristics than DUV due to its shorter wavelength and lower-NA optics.
• Mask 3D effects in EUV add additional pattern-dependent variations on top of standard OPE.

Optical proximity effects are the fundamental reason computational lithography exists — without OPC, sub-wavelength patterning would be impossible.