Library-Based Ocd

Library-Based OCD (Optical Critical Dimension) metrology is a technique that matches measured optical spectra to pre-calculated theoretical spectra libraries — enabling fast, accurate measurement of multiple structure parameters simultaneously by comparing experimental diffraction patterns against simulated reference database, the standard approach for inline semiconductor process control.

What Is Library-Based OCD?

  • Definition: Optical metrology using pre-computed spectral libraries for parameter extraction.
  • Method: Match measured spectrum to best-fit library entry.
  • Output: Multiple parameters (CD, height, sidewall angle) from single measurement.
  • Speed: Fast measurement via library lookup vs. real-time fitting.

Why Library-Based OCD Matters

  • Inline Capability: Fast enough for production monitoring (seconds per site).
  • Multi-Parameter: Measures CD, height, sidewall angle simultaneously.
  • Non-Destructive: Optical measurement preserves wafer.
  • High Throughput: Enables 100% wafer sampling if needed.
  • Cost Effective: Lower cost per measurement than electron microscopy.

How It Works

Step 1: Build Parametric Model:
- Structure Definition: Define geometry (trapezoid, rectangle, complex shapes).
- Parameters: CD (critical dimension), height, sidewall angle, material properties.
- Parameter Ranges: Define min/max values for each parameter.
- Material Stack: Specify all layers and optical properties.

Step 2: Generate Spectral Library:
- Simulation: Use RCWA (Rigorous Coupled-Wave Analysis) to compute spectra.
- Parameter Space: Calculate spectra for combinations of parameter values.
- Grid Sampling: Typically 5-10 points per parameter dimension.
- Computation Time: Hours to days depending on complexity.
- One-Time Cost: Library generated once per structure type.

Step 3: Measure Sample Spectrum:
- Illumination: Broadband light at specific angle(s).
- Detection: Measure reflected/diffracted spectrum.
- Wavelength Range: Typically 200-1000nm.
- Polarization: Multiple polarizations for more information.
- Measurement Time: 1-5 seconds per site.

Step 4: Library Matching:
- Search: Find library entry with best spectral match.
- Metric: Minimize χ² or other goodness-of-fit measure.
- Interpolation: Interpolate between library points for precision.
- Output: Best-fit parameter values.
- Speed: Milliseconds for library lookup.

Advantages

Speed:
- Library Lookup: Much faster than real-time regression.
- Throughput: Enables high-sampling density.
- Inline Use: Fast enough for production monitoring.

Multi-Parameter Measurement:
- Simultaneous: All parameters from single measurement.
- Correlation: Captures parameter correlations.
- Efficiency: No need for multiple metrology tools.

Robustness:
- Pre-Validated: Library entries are pre-computed and validated.
- Convergence: No optimization convergence issues.
- Repeatability: Consistent results, no fitting variability.

Limitations

Model Accuracy:
- Assumption: Model must accurately represent real structure.
- Simplifications: Real structures more complex than models.
- Impact: Model errors propagate to measurements.
- Mitigation: Validate with reference metrology (SEM, TEM, AFM).

Library Coverage:
- Parameter Space: Library must cover actual parameter range.
- Out-of-Range: Extrapolation unreliable if parameters outside library.
- Grid Density: Trade-off between accuracy and library size.
- Solution: Adaptive libraries, expand as needed.

Interpolation Accuracy:
- Between Points: Must interpolate between library grid points.
- Nonlinearity: Spectral response may be nonlinear.
- Error: Interpolation introduces uncertainty.
- Mitigation: Denser grids in sensitive regions.

Computational Cost:
- Library Generation: Days of computation for complex structures.
- Storage: Large libraries require significant storage.
- Updates: New library needed for process changes.
- Solution: Efficient simulation, library compression.

Alternative: Real-Time Regression

Method:
- On-the-Fly: Optimize parameters to fit measured spectrum in real-time.
- No Library: No pre-computation required.
- Flexibility: Handles any parameter combination.

Trade-Offs:
- Slower: Minutes per measurement vs. seconds for library.
- Convergence: May fail to converge or find local minima.
- Flexibility: Better for R&D, process development.
- Use Case: When library impractical or parameters unknown.

Applications

Lithography Process Control:
- After Develop: Measure resist CD, height, profile.
- Feedback: Adjust exposure, focus based on measurements.
- Sampling: Multiple sites per wafer, every wafer.

Etch Process Control:
- After Etch: Measure final feature dimensions.
- Endpoint: Verify etch depth, profile.
- Uniformity: Map CD and height across wafer.

CMP Monitoring:
- Remaining Thickness: Measure film thickness after polish.
- Uniformity: Ensure uniform removal across wafer.
- Endpoint: Verify target thickness achieved.

Advanced Patterning:
- Multi-Patterning: Measure each patterning step.
- Overlay: Combined with overlay metrology.
- 3D Structures: FinFETs, GAA, complex 3D geometries.

Library Optimization

Adaptive Sampling:
- Dense Sampling: More points in sensitive parameter regions.
- Sparse Sampling: Fewer points where response is smooth.
- Benefit: Smaller library with maintained accuracy.

Dimensionality Reduction:
- PCA: Principal component analysis of parameter space.
- Sensitivity: Focus on parameters with high spectral sensitivity.
- Benefit: Reduce library size, faster generation.

Incremental Updates:
- Add Points: Expand library as new parameter ranges encountered.
- Refinement: Add points where interpolation error high.
- Benefit: Start with coarse library, refine over time.

Validation & Calibration

Reference Metrology:
- CD-SEM: Validate CD measurements.
- AFM: Validate height and sidewall angle.
- TEM: Cross-section for complex 3D structures.
- Correlation: Establish correlation between OCD and reference.

Model Validation:
- Goodness of Fit: Check χ² values for library matches.
- Residuals: Analyze spectral residuals for systematic errors.
- Outliers: Identify measurements with poor fits.

Periodic Recalibration:
- Drift: Optical properties may drift over time.
- Process Changes: Update library for process modifications.
- Frequency: Quarterly or after significant process changes.

Tools & Vendors

  • KLA-Tencor: SpectraShape, SpectraCD OCD systems.
  • Nova Measuring Instruments: Integrated metrology solutions.
  • Nanometrics (Onto Innovation): Atlas OCD systems.
  • ASML: Integrated metrology in lithography scanners.

Library-Based OCD is the workhorse of semiconductor metrology — by pre-computing spectral libraries, it enables fast, accurate, multi-parameter measurements that make inline process control practical, providing the measurement speed and throughput required for high-volume manufacturing at advanced nodes.

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