A scattering bar is the most common type of sub-resolution assist feature (SRAF) — a thin line placed on the photomask parallel to and near a main feature to improve its imaging quality. Scattering bars are designed to be too narrow to print on the wafer, but they modify the diffraction pattern to enhance the main feature's contrast and depth of focus.

How Scattering Bars Work

• A main feature in isolation has a different diffraction pattern than the same feature in a dense array. Dense features typically image better because multiple diffraction orders interact constructively.
• A scattering bar placed near an isolated feature creates an artificial periodic environment, making the diffraction pattern resemble that of a dense array.
• The main feature benefits from improved aerial image contrast and greater depth of focus — meaning it prints more consistently across process variations.

Scattering Bar Design

• Width: Typically 40–60% of the main feature width — narrow enough to stay below the printing threshold. For example, if the main feature is 100 nm, the scattering bar might be 40–50 nm.
• Placement Distance: Positioned at a specific distance from the main feature — usually corresponding to the pitch that produces optimal diffraction conditions. This distance is determined by optical simulation.
• Number per Side: One or two scattering bars per side of the main feature is common. More may be added for very isolated features.
• Length: Usually extends the full length of the adjacent main feature.

Single vs. Double Scattering Bars

• Isolated Feature: Two scattering bars (one on each side) create the most uniform improvement.
• Semi-Isolated Feature: A scattering bar on the isolated side only, where the feature lacks a natural neighbor.
• Dense Features: No scattering bars needed — the neighboring main features already provide the periodic environment.

Practical Considerations

• Printability Verification: Must verify scattering bars don't print under worst-case conditions (maximum dose, best focus). Printing of SRAFs creates defects.
• Mask Inspection: Scattering bars must be flagged as intentional features during mask inspection to avoid being classified as defects.
• Rule-Based vs. Model-Based: Simple scattering bars use fixed design rules. Advanced approaches use model-based or ILT-based placement for optimized performance.

Scattering bars are one of the earliest and most widely used resolution enhancement techniques — they've been standard practice in lithography since the 130nm node and remain essential today.