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.