attenuated psm (attpsm)

**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.