Optical Scatterometry

OCD - Optical Critical Dimensions

As semiconductor devices approach atomic-scale dimensions, controlling and verifying every nanometer of pattern geometry becomes a decisive factor for yield and performance. While imaging techniques like electron microscopy (CD-SEM) allow for direct visualization without extensive modeling, they are often lacking throughput, full-wafer coverage and tool-to-tool matching in order to be used for Statistical Process Control (SPC).

Optical Scatterometry, also known as Optical Critical Dimension (OCD) metrology, provides the essential bridge between high-resolution analysis and high-throughput manufacturing. By irradiating the wafer surface with precisely controlled light at a defined angle and detecting the reflected diffracted light spectroscopically and/ or as a function of incidence or detection angle, spectral waveforms corresponding to the pattern shape are obtained. These are compared to mathematical wavefront models in order to precisely derive critical dimensions, such as feature sizes and shapes with sub-nanometer accuracy. The technique is well-suited for periodic structures and, due to its high throughput, allows for full-wafer mapping, thus enabling inline profiling of sidewall angles, footing, corner rounding or resist loss, amongst others.

The importance of scatterometry in semiconductor manufacturing

Scatterometry has become a key metrology method for process control in lithography, etching, deposition, and CMP (Chemical Mechanical Planarization/Polishing) steps. As device structures shrink and become more complex, direct imaging is no longer practical for every wafer or every layer. OCD provides:

Fast, non-destructive measurements across large wafer areas
Sub-nanometer precision for critical dimension (CD) and overlay monitoring
High throughput, suitable for in-line integration
Model-based reconstruction that delivers full 3D profile information
Tool-to-tool matching, ideal for statistical process control (SPC)

Applications include:

Lithography focus and dose control
Etch depth and sidewall angle monitoring
Film stack and multi-layer thickness measurement
Overlay metrology and critical dimension uniformity control

The role of lasers in optical scatterometry

At the heart of every scatterometry tool lies a precisely controlled light source. Lasers are the preferred illumination for OCD because they combine coherence, stability, spectral purity, and polarization control — all essential for accurate diffraction measurements.

Key laser characteristics:

High spatial and temporal coherence → ensures sharp diffraction patterns and stable intensity signals.
Narrow linewidth → provides spectral precision for wavelength-dependent modeling.
Excellent wavelength stability → critical for reproducibility in in-line metrology.
Polarization control → linear or circular polarization enables sensitivity to structure orientation and material contrast.
Beam quality → ensures uniform illumination and repeatable diffraction conditions.
Multi-wavelength capability → extends the technique from single-layer critical dimension control to complex multi-stack analysis.

Because scatterometry relies on comparing measured optical spectra with simulation models, the accuracy and stability of the laser source directly define the metrology precision.

Laser types and wavelengths used in OCD systems

Laser Type	Wavelength Range	Application	Key Features	TOPTICA's recommendation
VIS & IR Single-mode diode lasers	420–1064 nm	Fast CD control and in-line monitoring	Excellent beam quality, compact, OEM-integration, fiber-coupled	iBeam smart, iChrome
UV Single-frequency lasers	266-405nm	Shallow trench, mask and resist layer metrology	High spatial resolution, maximum coherence length, fiber-coupled	TopWave, TopMode
HeNe lasers and HeNe replacements	632.8 nm	Calibration, reference systems	Low-cost, wavelength accuracy and long-term stability	iBeam smart WS, DFB Pro 633
Fiber lasers (Yb, Er)	1030–1550 nm	Film thickness and buried interface analysis	High-power, long coherence length, low phase noise	ALS
Broadly tunable OPO sources	190–2000 nm	Spectroscopic scatterometry for multilayer stacks	Broadband tunability, polarization control	TOPO, TOPO Smart

Visible and DUV lasers

Visible and UV lasers are used when the smallest structures — typically below 20 nm — must be resolved or when process wavelengths correspond to lithography exposure (e.g., 193 nm ArF processes).
Their short wavelength increases lateral resolution and enhances sensitivity to surface and sidewall features. DUV scatterometry is especially relevant for mask and resist characterization.

NIR lasers

Near-infrared lasers (1030–1550 nm) penetrate deeper into film stacks, enabling accurate measurement of buried interfaces, film thickness, and multi-layer optical properties. Fiber-based single-frequency lasers offer excellent long-term stability, making them ideal for production-scale OCD tools.

Emerging applications and requirements

1. EUV lithography and mask metrology

Next-generation OCD tools are being developed for EUV (13.5 nm) lithography.
To characterize EUV masks and multilayer mirrors, metrology systems increasingly rely on laser-driven plasma sources and short-wavelength optical models capable of resolving nm-scale defects and pattern distortions.

2. 3D NAND and advanced packaging

As devices evolve toward vertical architectures and high-aspect-ratio structures, scatterometry must probe deeper and interpret more complex geometries.
This drives demand for multi-angle, multi-wavelength, and polarization-resolved laser systems capable of analyzing deep trenches and non-planar topographies.

3. Machine learning and AI integration

AI-assisted reconstruction is revolutionizing scatterometry data interpretation. By combining large datasets of simulated and measured spectra, neural network algorithms can extract structural parameters in real time, reducing computational overhead and improving robustness to noise.

4. Swept-source and frequency-comb lasers

New swept-source and frequency-comb lasers offer unparalleled wavelength accuracy and repeatability. Their precisely calibrated spectral spacing enables absolute thickness and distance measurements at picometer precision — setting new standards for metrology performance.

5. Inline and In-situ metrology

Compact, industrialized fiber-coupled laser scatterometers are increasingly being integrated directly into process tools (etch, deposition, lithography), allowing real-time feedback and closed-loop process control.
This evolution turns metrology from a downstream inspection step into an active component of process optimization.

Conclusion

Optical Scatterometry (OCD) has become one of the most important metrology techniques in semiconductor manufacturing - ensuring that every feature, film, and interface meets the precision required for today’s advanced nodes.
By analyzing how laser light interacts with nanostructures, OCD delivers non-destructive, high-throughput, and sub-nanometer-accurate process control.

Lasers are the foundation of this capability: their coherence, spectral purity, tunability, and polarization control make them the ideal illumination sources for scatterometry across all technology generations.

As chip designs continue to evolve - with smaller dimensions, 3D architectures, and more complex material stacks - laser-based OCD metrology will remain indispensable in guiding semiconductor manufacturing toward the atomic scale, ensuring accuracy, efficiency, and innovation at every step.

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