Attenuated Phase-Shift Mask (AttPSM) is a photomask technology where the normally opaque regions of the mask are replaced with a partially transmitting material that also shifts the phase of transmitted light by 180°. This improves image contrast at the wafer compared to standard binary (chrome-on-glass) masks.

How AttPSM Works

• In a binary mask: Chrome blocks ~100% of light. Glass transmits ~100%. The contrast at feature edges is determined by this simple light/dark transition.
• In an AttPSM: The "dark" regions transmit a small amount of light (typically 6–8%), but this light is 180° out of phase with the light from the clear regions.
• At the boundary between clear and phase-shifted regions, the transmitted light waves destructively interfere, creating a very sharp intensity null (dark line) — improving edge contrast and resolution.

Why 6% Transmission?

• Zero transmission (binary mask) provides decent contrast but no phase benefit.
• Higher transmission (>10%) improves the destructive interference effect but causes unwanted background intensity ("sidelobe printing").
• 6% is the sweet spot — enough transmitted light to provide meaningful phase cancellation without causing printable sidelobes.

AttPSM Materials

• MoSi (Molybdenum Silicide): The standard AttPSM material for decades. Provides ~6% transmission with 180° phase shift at 193 nm wavelength.
• Thin Chrome + Phase Layer: Alternative constructions using separate absorber and phase-shifting layers.

Advantages Over Binary Masks

• Better Contrast: The phase-induced destructive interference sharpens feature edges.
• Better Depth of Focus: Improved aerial image contrast enables printing over a wider focus range.
• Simple Implementation: Only a single exposure is needed — no additional process complexity compared to binary masks.
• Universal Adoption: AttPSM is the default mask type for DUV (193 nm) critical layers.

Limitations

• Sidelobe Printing: At very tight pitches or isolated features, the 6% background transmission can cause unwanted printing. Requires careful SRAF and OPC management.
• Phase-Transmission Coupling: Changing the material thickness to adjust phase also changes transmission, limiting optimization freedom.

Attenuated PSM has been the workhorse mask technology for 193nm lithography since the 130nm node — virtually every critical DUV layer at advanced fabs uses AttPSM rather than binary masks.