Stepper
A Stepper is a lithography tool that projects a reticle (mask) pattern onto photoresist-coated wafers using a step-and-repeat process — exposing one die (or a small group of dies) at a time through a high-precision reduction lens system (typically 4× or 5× reduction), then physically stepping the wafer stage to the next die position and repeating the exposure, building up the complete wafer pattern one field at a time.
What Is a Stepper?
- Definition: A projection lithography system where the reticle image is projected through a reduction lens onto the wafer in a stationary (non-scanning) exposure — the entire field is illuminated simultaneously, and after exposure, the wafer stage "steps" to the next die position.
- The Name: "Stepper" comes from the step-and-repeat motion — expose one field, step to the next position, repeat across the entire wafer. Each exposure covers one "exposure field" (typically 22×22mm to 26×33mm).
- Reduction Optics: The reticle pattern is 4× or 5× larger than the printed pattern on the wafer, allowing easier mask fabrication and tighter wafer-level resolution from the demagnification.
How a Stepper Works
| Step | Action | Detail |
|---|---|---|
| 1. Illuminate | Light source illuminates the reticle | DUV excimer laser (248nm KrF or 193nm ArF) |
| 2. Project | Reduction lens projects reticle image onto wafer | 4× reduction (reticle features 4× larger than wafer features) |
| 3. Expose | Entire exposure field printed simultaneously | Stationary wafer during exposure |
| 4. Step | Wafer stage moves to next die position | Interferometer-controlled precision (~1nm) |
| 5. Repeat | Expose next field | Continue across all die positions on wafer |
| 6. Align | Alignment marks checked at each field | Ensures overlay to previous layers |
Key Specifications
| Specification | Typical Value | Significance |
|---|---|---|
| Numerical Aperture (NA) | 0.5 - 0.93 (dry) | Higher NA = finer resolution |
| Wavelength | 365nm (i-line), 248nm (KrF), 193nm (ArF) | Shorter wavelength = finer features |
| Resolution | ~150nm (i-line) to ~65nm (ArF) | Minimum printable feature size |
| Exposure Field | 22×22mm to 26×33mm | Maximum die size per shot |
| Overlay Accuracy | 5-20nm | Alignment precision between layers |
| Throughput | 40-100 wafers/hour | Production speed |
| Reduction Ratio | 4× or 5× | Reticle size to wafer pattern ratio |
Stepper vs Scanner
| Feature | Stepper | Scanner |
|---|---|---|
| Exposure Method | Full field illuminated at once | Slit scans across reticle and wafer |
| Exposure Field | Limited by lens field size (22×22mm typical) | Larger fields (26×33mm standard) |
| Resolution | Limited by full-field lens quality | Better — lens only optimized for narrow slit |
| Throughput | Lower (for large dies) | Higher (continuous scan motion) |
| Overlay | Excellent field-to-field | Excellent (comparable or better) |
| Dominant Era | 1980s-1990s | 2000s-present |
| Current Use | Older nodes (>90nm), specialty applications | All advanced manufacturing (<90nm) |
Steppers were the workhorse of semiconductor lithography through the 1990s — establishing the step-and-repeat projection paradigm with 4× reduction optics that enabled the semiconductor industry to shrink from micron-scale to sub-100nm features, before being superseded by scanning systems (scanners) for advanced nodes where larger exposure fields and better aberration control became critical for volume manufacturing.