CD Overlay Metrology

Critical Dimension (CD) Overlay Metrology provides a critical measurement capability that is integral to maintaining control over semiconductor fabrication processes.  These measurements provide critical alignment verification and process control capability on bonded multi-layer wafers.

AST’s advanced metrology platforms can be configured to image wafers using Infrared (IR) imaging technology in the NIR (visible to 1050nm) through SWIR (900nm – 1700nm) wavelengths (see figure 1).

SWIR Spectrum

The ability to select the best optimized IR wavelength providing the best transmission through a multi-layer bonded wafer or die enables overlay measurements to be taken with extremely high precision based on achieving the highest degree of transmission to provide the best image quality for executing CD overlay measurements. 

In addition to performing alignment verification, AST’s advanced CD Overlay capability provides the ability to perform critical inspection processes including detection of cracks and delamination, sub-surface feature-based measurements, aperture measurements, as well as other metrology and inspection applications.

Figure 2: Example of a Multi-Layer Wafer Image

The image shown in Figure 2 demonstrates the capability to optimize the IR wavelength to enable the alignment targets that are present on successive layer below the top layer of the wafer (or die) to be imaged based on the transmission properties of the wafer or die material. Without the utilization of IR imaging, these alignment targets would be blocked by the top layer of the wafer preventing them from being imaged and alignment verified. See figure 3 for wavelength transmission properties based on wafer material.

CD Overlay SWIR Image Alignment Target

Figure 3: Wavelength Transmission Chart

AST’s ability to highly tune and optimize the wavelength based the wafer and die material properties & characteristics and to be able to penetrate thicker and more highly doped materials, provides AST with a significant performance capability. Wavelength based optimization is achieved through automated illumination & filter techniques, configurable camera options including Cooled InGaAs (900-1700nm), Silicon-based options for NIR applications (740-1100nm), as well as other state-of the art camera and optics technology.

CD overlay Wavelength Transmission

CD Overlay Application & Metrology Process Examples

CD Overlay Metrology is critical for inspection & measurement on Flip Chip / Hybridization Wafers where the capability to image through the wafer (or die) layers is needed to verify alignment.  VCSEL (Vertical Cavity Surface Emitting Laser) wafers and dies utilize AST’s advanced IR imaging capability to verify alignment, perform aperture measurements, inspection for defects / corrosion, and other critical process control validation.  Additional metrology & inspection applications include those where 3D stacking processes are deployed across a broad range of wafer & die designs and use cases.  Inspection and Metrology of MEMS, NEMS, and Silicon Photonics wafers and dies.

Fully Integrated CD Overlay Metrology Systems for Production & Development Environments

AST’s state-of-the-art turnkey metrology systems provide the required accuracy and stability to perform these critical measurements using advanced positioning / motion control technology, IR and visible imaging options, and high-speed image capture and processing capability.  

These systems are designed for 24/7 production environments and are also capable of being deployed into a development environment where optimum flexibility is required. 

Automated wafer and part-handling options are available for 150mm, 200mm, and 300mm wafers.  Product solutions range from fully integrated high-performance turnkey systems to integrated workstations with automated wafer loader.

Semiconductor Metrology Experts

Advanced Spectral Technology (AST) is a leading provider of metrology solutions for semiconductor manufacturing. We have decades of semiconductor and defect inspection experience and strategic partnerships with other industry leaders that enable us to provide cutting-edge semiconductor metrology solutions.

Learn More

cd overlay semiconductor

CD Overlay Metrology Products

Advanced Metrology System


NIR/SWIR Inspection & Metrology Integrated Workstation


AST-200-IR Inspection & Metrology Systems for CD Overlay applications provide flexible NIR & SWIR imaging capability as a fully-integrated workstation that can be optimized to achieve optimum performance and capability based on the material properties of the wafer(s) and/or die(s) being processed. This flexibility extends by supporting multiple objective magnifications, IR camera options, and manual and motorized filter-wheel options that can be controlled through AST’s ScopeViewer3 software.

NIR/SWIR Workstation Inspection & Metrology Systems can be configured with manual and/or motorized positioning stages that can be utilized as a semi-automated or fully automated solution for use in production, development, or failure analysis (FA) environments.

Benchtop NIR/SWIR Inspection & Metrology Systems

AST offers NIR & SWIR benchtop solutions that can be configured and optimized to meet specific CD overlay inspection and metrology applications, performance specifications, imaging capability, and budget requirements.
Benchtop IR inspection systems provide a flexible solution that can include AST’s ScopeViewer3 software for executing inspection & metrology scripts and sequences that can be easily programed to include image capture, data analysis, reporting, and other critical application & product specific results.

Benchtop IR systems can be configured and optimized with a number of NIR/SWIR camera options, objective magnifications, and filter selections.

These systems can be supported by existing customer benches, passive vibration, or active vibration isolation tables. AST can also offer these options for supporting our benchtop IR inspection & metrology systems based on magnification, performance specifications, and environment where the system will be installed.

Knowledge & Terms

Critical Dimension (CD) is the minimum feature size that a process can reliably produce in the semiconductor manufacturing process. As semiconductor technology progresses, this dimension continues to decrease, enabling higher transistor density per chip.

The CD is a crucial parameter as it influences the number of transistors that can be fabricated on a chip, thereby affecting the chip’s processing power, energy efficiency, and cost. Furthermore, the CD has an impact on the intricacy of the fabrication process; smaller CDs generally demand more complex and precise manufacturing techniques.

CD is a fundamental metric that requires precise measurement and control during the manufacturing process. Variations in the CD can lead to differences in the electrical properties of the devices being fabricated, which can impact the performance of the final product. For this reason, sophisticated metrology tools are needed to ensure that the CD is maintained within stringent tolerances.

Overlay Metrology is the method of quantifying the alignment between successive layers in a multi-layer semiconductor fabrication process. Since semiconductors are built by stacking multiple layers of various materials, it’s imperative to align each layer accurately with the previous ones to ensure optimal device functionality.

Overlay refers to the accurate alignment of these layers. Any deviations relative to the underlying layer result in an overlay error, which could be a consequence of equipment inaccuracies, process fluctuations, or thermal and mechanical stresses on the wafer.

Our overlay metrology tools measure these minuscule displacements using advanced optical techniques that generate high-resolution images of wafer layers, followed by complex image analysis algorithms to identify and measure any overlay errors.

This measurement data then feeds into alignment adjustments for subsequent fabrication stages, correcting the overlay errors. This feedback mechanism is critical to maintaining the extreme precision required in contemporary semiconductor processes, where feature sizes are often at nanoscale.

Differential Interference Contrast (DIC), also known as Nomarski Interference Contrast, is a beam-shearing interferometry technique used in light microscopy to enhance the contrast in unstained, transparent samples.

In semiconductor metrology, DIC is valuable due to its ability to produce high-contrast images of wafer features without the need for staining or other alterations of the sample. DIC creates an image with apparent 3D-like quality, allowing for precise examination of wafer topography.

The technique works by splitting a beam of polarized light into two orthogonal, slightly displaced beams that traverse the same path but interact differently with the sample due to the small displacement. When the beams recombine, they interfere, producing an image where contrast is a function of the gradient of the optical path difference across the sample, making DIC extremely sensitive to slopes and edges.

DIC can be used to visually inspect or measure features on a wafer, including defects, layer thickness, and overlay, and is often integrated into metrology tools used in semiconductor manufacturing to enable high-contrast imaging of wafer surfaces.


All of our CD overlay and Metrology systems can be fully customized to meet your requirements. If you have questions about our product offerings or capabilities, please contact us for more information.