resist sensitivity

**Resist sensitivity** (also called photospeed) measures the **amount of exposure energy required** to produce the desired chemical change in a photoresist — specifically, the dose (energy per unit area, typically measured in mJ/cm²) needed to properly expose the resist and produce the target feature dimensions after development. **What Resist Sensitivity Means** - **High Sensitivity (Low Dose)**: The resist requires less energy to achieve the desired pattern. Example: a resist requiring only 20 mJ/cm² is highly sensitive. - **Low Sensitivity (High Dose)**: The resist requires more energy. Example: a resist requiring 80 mJ/cm² is less sensitive. - Sensitivity is inversely related to the dose required: more sensitive = less dose needed. **Why Sensitivity Matters** - **Throughput**: More sensitive resists require lower exposure doses, allowing the scanner to expose wafers faster. For EUV lithography (where photon generation is expensive), sensitivity directly impacts **wafers per hour** and cost per wafer. - **Shot Noise Tradeoff**: Higher sensitivity means fewer photons are used, increasing **photon shot noise** and stochastic variability. This creates the fundamental **sensitivity-resolution-roughness tradeoff**. **The RLS Tradeoff** The dominant challenge in resist development is the **RLS (Resolution, Line Edge Roughness, Sensitivity) tradeoff**: - **Resolution** (R): Smallest feature the resist can resolve. - **Line Edge Roughness** (L): Random roughness on feature edges. - **Sensitivity** (S): Dose required for exposure. Improving any two parameters typically degrades the third. A more sensitive resist (lower dose) tends to have **worse roughness** (fewer photons → more noise) and/or **worse resolution** (more chemical blur). **Factors Affecting Sensitivity** - **PAG Loading**: More PhotoAcid Generator molecules per volume → higher sensitivity. But excessive PAG can degrade optical properties. - **Chemical Amplification**: CARs amplify the effect of each absorbed photon through catalytic acid reactions — multiple deprotection events per photon. - **Quantum Yield**: How many chemical events (acid molecules generated) per absorbed photon. - **EUV Absorption**: Resists with higher EUV absorption (e.g., metal-oxide resists containing Sn, Hf) capture more photons per unit thickness. **Typical Sensitivity Values** - **DUV (193 nm) CARs**: 15–40 mJ/cm². - **EUV CARs**: 20–50 mJ/cm². - **EUV Metal-Oxide Resists**: 15–40 mJ/cm² (comparable to CARs but with potentially better etch resistance). Resist sensitivity is at the **center of the main tradeoff** in lithography — it connects economic throughput requirements to fundamental physics limits on patterning quality.