A New Dimension In Optical CD

Combining spectral measurements and interferometry for improved metrology.


One of the biggest challenges for nanoscale fabrication is how to measure devices on such a minute scale. As the semiconductor industry demands ever smaller devices, the need for reliable, robust measurements for quality control and process optimization increases.

One robust and commonly used technique in semiconductor manufacturing is optical critical dimension (OCD) metrology. Standard, already widely adopted technologies include incidence spectral reflectometry and ellipsometry (SR/SE) technologies.However, a novel technology combining spectral measurements and interferometry offers additional advantages to these well-established approaches.

Optical critical dimension metrology

Optical characterization of semiconductors is performed by measuring the sample’s reflectivity at different incidence angles, azimuths, polarizations, and wavelengths. Sometimes ellipsometry measurements are used to measure the relative phase between the reflected polarization components.1 This comprehensive suite of measurements makes it possible to determine dimensions and shapes for various structures. Such measurements are usually performed using a broadband white light source, shining it on the sample or wafer of interest, and measuring the reflected light and scattered light.

Detectors used are usually wavelength resolved and placed at normal and oblique angles relative to the incident light source. The reflectivity spectrum can be recovered using the information from the two angles and analyzed by comparing previous data of known samples or theoretical spectra. 

Fig. 1: (Left Image) Schematic of current multichannel OCD tools, including normal incidence and oblique illumination and collection channels. (Right Image) Schematic of Spectral Interferometer (SI): light is split between the sample and a reference arm and then recombined and measured with a spectrometer.

The missing phase

Optical CD metrology is flexible, efficient, and fast, which leads to its wide adoption in semiconductor manufacturing.  In addition, while “high TPT” is indeed one of the main merits of OCD, accuracy isn’t. However, one aspect of information it lacks is data on the relative phases of the incident and reflected light that can provide another dimension of information on the sample.One of the challenges in the optical domain is that phase information cannot be recovered directly. This means that measurements must be made by using interference effects.3 Nova has developed the Nova PRISM platform and the unique Spectral Interferometry technology precisely for this purpose.4

Interferometers work by taking a single beam and splitting it to form the interferometer’s two arms. One of these paths is kept as a reference, and the other interacts with the sample. The beams are then recombined or interfered, to produce a signal from which the phase information can be extracted. One of the arms’ path lengths can also be varied to scan the beams’ relative delay.3 Simply put – interferometry can transform a two-dimensional image into a three-dimensional one.


The Nova PRISM is an all-new platform incorporating Spectral Interferometry on top of traditional Optical CD capabilities. Nova PRISM makes use of an interferometry scheme based on an advanced traditional scatterometry (NI SR and oblique SE) design4 to provide access to essential phase information, which is inaccessible by other current solutions.

One of the main challenges for spectral interferometry is performing such measurements over the wide wavelength ranges typically employed for reflectance measurements. The other is precisely recording the relative optical path for the two arms. The Nova PRISM uses advanced hardware, including ultra-broadband optics and precision stages, to overcome both challenges.

As well as the instrumentation, the Nova PRISM exploits a synergy of hardware and algorithms to provide uniquely sensitive measurements of a range of sample types.

Using industry-leading model-based and machine learning algorithms, Spectral Interferometry can solve applications such as tracking cavity geometries during etch formation with much greater sensitivity than traditional scatterometry techniques.

Unique sensitivity

In this direct comparison of the new spectral interferometry set-up versus traditional scatterometry for characterization of cavity geometries, the accuracy of interferometry outperformed traditional methods for measuring the cavity dimensions and spacings by five times.The sample dimensions were measured using TEM, considered the gold standard for nanoscale sizing, and then compared to the optical results.

Cavity formation is a crucial step in the fabrication of advanced logic devices, and the cavity dimension directly impacts the functionality of the final transistor. The complexity of the object and the limited sensitivity of traditional OCD methods for such small-scale systems have left a critical metrology gap that spectral interferometry has been able to fill.3

High-end metrology

Another application in which spectral interferometry shows it correlates to TEM for characterization is in the fabrication of gate-all-around stacked nanosheets.6

These are some of the most promising directions for future logic technology nodes and require a precisely sized inner spacer to properly isolate the channel from the source and drain regions. Even for the nanosheets’ challenging size, spectral interferometry could accurately measure the accurate dimensions and complete process flow.

Alongside their instrumentation, Nova offers high-level training programs and specialist support to ensure you can maximize process improvements enabled by new technologies.


  1. Ukraintsev, V. (2012). Review of reference metrology for nanotechnology: significance, challenges, and solutions. Journal of Micro/Nanolithography, MEMS, and MOEMS, 11(1), 011010. https://doi.org/10.1117/1.jmm.11.1.011010
  2. Rasheed M. A. Azzam (1997). Mueller-matrix ellipsometry: a review”. Proc. of SPIE Vol. 3121
  3. De Groot, P. J. (2019). A review of selected topics in interferometric optical metrology. Reports on Progress in Physics, 82(5). https://doi.org/10.1088/1361-6633/ab092d
  4. Nova (2021) Nova PRISM, https://www.novami.com/nova-product/prizm/, accessed 10th March 2021
  5. Vaid et al., (2008) Scatterometry as Technology Enabler for Embedded SiGe Process, Proc. of SPIE Vol. 6922
  6. Kong, D et al. (2020). Development of SiGe Indentation Process Control to Enable Stacked Nanosheet FET Technology. ASMC (Advanced Semiconductor Manufacturing Conference) Proceedings, https://doi.org/10.1109/ASMC49169.2020.9185226

Leave a Reply

(Note: This name will be displayed publicly)