Stepper & Scanner Basics
How exposure tools work — step-and-repeat vs step-and-scan
How Lithography Tools Work
How Lithography Tools Work
Lithography exposure tools project a circuit pattern from a reticle (mask) onto the wafer through a precision optical system:
- Stepper (step-and-repeat): Exposes one die field at a time, then steps to the next position. Older technology, simpler but lower throughput.
- Scanner (step-and-scan): Both the reticle and wafer move simultaneously during exposure — the reticle scans through a narrow slit of illumination. This allows a larger exposure field while keeping the optics compact. All modern tools are scanners.
Key performance metrics:
- Resolution: Minimum feature size (determined by wavelength and NA)
- Overlay: Alignment accuracy between successive layers (< 2 nm for advanced nodes)
- Throughput: Wafers per hour (150–300+ WPH depending on tool and process)
- Focus uniformity: Depth of focus across the exposure field (critical for yield)
Key Concept: The Wafer Stage
The wafer stage positions the wafer with sub-nanometer precision while moving at speeds up to 500 mm/s. It uses laser interferometry for position measurement and magnetic levitation for vibration isolation. This is one of the most precise mechanical systems ever built.
Throughput, Focus, and Overlay Budgets
Throughput, Focus, and Overlay Budgets
A modern scanner is a balance between three competing budgets — every nanosecond, nanometer, and milliwatt is fought over.
1. Throughput (WPH)
Time per wafer breaks down roughly as:
- Load / unload / align: 2–4 s, hidden by dual-stage TWINSCAN architecture (one wafer exposes while the next aligns)
- Exposure scan: dose × (field area) / source power — the dominant term at EUV
- Stage step-and-settle: ~150 ms per field for 96 fields = ~15 s
An NXT:2050i DUV scanner reaches ~295 wafers/hour; an EUV NXE:3800E targets ~220 WPH.
2. Focus (CD uniformity)
Depth of focus (DOF ≈ k₂ · λ / NA²) is only ~80 nm at 193 nm immersion. The scanner measures wafer height at every die with multiple optical sensors, then tilts and translates the chuck within tens of nanometers in Z to keep each field in focus.
3. Overlay
Overlay error is the misalignment between the current exposure and previous layers. Budget components:
- Scanner contribution: 1–2 nm (stage, lens distortion, reticle alignment)
- Wafer/process contribution: 1–3 nm (CMP-induced topography, thermal warping)
- Reticle contribution: ~0.5 nm
Total tool-to-tool overlay must stay under ~2 nm for 3 nm-class logic.
Key Concept: Mix-and-Match Overlay
Different layers may be patterned on different tool types (EUV for critical layers, DUV for relaxed layers). Modern scanners share alignment-mark fingerprints in a global model so that an EUV exposure can still land within 2 nm of a DUV layer below — the "mix-and-match overlay" challenge.
Knowledge Check
Knowledge Check
1 / 2What is the key difference between a stepper and a scanner?