inline metrology
**Inline Metrology** is the **real-time measurement of critical process parameters (critical dimension, overlay, film thickness, composition) on product wafers during manufacturing without removing them from the production flow** — providing the process control data that enables engineers to detect drift, tighten process windows, and maximize yield before defective lots reach final test. Inline metrology is the sensory nervous system of the semiconductor fab, converting manufacturing process uncertainty into actionable feedback.
**Why Inline Metrology Is Critical**
- Advanced nodes (5nm, 3nm) have process tolerances of ±1–2 nm for gate length and overlay.
- A 3nm CD shift can change transistor threshold voltage by 30–50 mV → circuit timing failure.
- Without inline measurement, a drifting process would produce many bad wafers before final test reveals the problem.
- Inline data enables: lot disposition, process correction (APC), equipment qualification, and yield learning.
**Key Inline Metrology Types**
**1. CD-SEM (Critical Dimension Scanning Electron Microscopy)**
- Measures line width, trench width, contact diameter at nm precision.
- Resolution: 1–2 nm (line/space); 3–5 nm (contact/via).
- Throughput: 30–100 sites/wafer, 2–5 wafers/hour.
- Limitation: 2D only (no depth), slow for full wafer coverage.
**2. OCD/Scatterometry (Optical CD)**
- Measures CD, sidewall angle, film thickness of periodic structures using diffracted light.
- Non-destructive, fast (1–3 sec/site).
- Requires reference model (regression against library of simulated spectra).
- Sensitivity: 0.1–0.3 nm CD; also measures resist profile, underlayer thickness.
**3. Overlay Metrology**
- Measures misalignment between current and previous layer patterning.
- Tools: Imaging-based (KLA Archer) or diffraction-based (ASML YieldStar, μDBO).
- Precision: 0.1–0.3 nm (3σ) for advanced DUV/EUV.
- Target types: Box-in-box (imaging), µDBO (diffraction) — µDBO preferred at 5nm and below.
**4. Film Thickness (Ellipsometry/Reflectometry)**
- Measures thin film thickness (0.1–10,000 nm range) using polarized light.
- Ellipsometry: Measures ψ and Δ → solve for n, k, thickness.
- Reflectometry: Measures spectral reflectance → fit to model for thickness.
- Applications: Oxide, nitride, photoresist, low-k ILD, metal film monitoring.
**5. XRF (X-Ray Fluorescence)**
- Measures elemental composition and metal film thickness.
- Used for: Cu, W, TaN, TiN film thickness monitoring.
- Non-destructive, no sample prep; typical precision ±0.5% thickness.
**Inline Metrology Flow in a Fab**
```
Wafer enters process step (e.g., litho)
↓
Process step completes
↓
Sampled wafers → inline metrology tool
↓
Measure CD / overlay / thickness
↓
Data → APC (Advanced Process Control) system
↓
APC adjusts next lot: exposure dose, focus, etch time, etc.
↓
Out-of-spec lots → hold for engineering review
```
**Sampling Strategy**
- **Full sampling**: Every wafer, every lot — highest control, highest cost.
- **Statistical sampling**: 1-in-N lots; efficient for stable processes.
- **Skip-lot**: Only measure lots flagged by SPC (statistical process control) rules.
- At advanced nodes: More critical layers require full sampling (EUV layers, gate etch, active area).
**Metrology Tooling at Scale**
| Tool | Vendor | Layer Application | Throughput |
|------|--------|-----------------|----------|
| CD-SEM | HITACHI, Applied | Gate CD, fin, contact | Low-medium |
| OCD/Scatterometry | KLA, Nova | Grating CD, film | High |
| Overlay | KLA, ASML | Every litho layer | High |
| Ellipsometry | KLA, Onto | Every film deposition | High |
Inline metrology is **the precision feedback loop that closes the gap between intended and manufactured dimensions** — without it, the ±1 nm tolerances required at 3nm and below would be unachievable, and every wafer would be a gamble rather than a controlled, data-driven manufacturing outcome.