Optical Scatterometry

Optical Scatterometry is a method of characterizing unknown properties of a sample by measuring the reflection of broadband light from an object. The reflection varies by wavelength (color), polarization, and angle-of-incidence. Nova’s scatterometry tools deliver highly accurate measurements at very high rate, taken over user-defined small spot-size on the samples. Even though speed/accuracy/spot-size are often a contradictory set of requirements, Nova’s continuous developments and innovations allow all these to coexist despite the continual pressure to improve these as technology node size shrinks.

Highlights & Benefits

  • High Measurement Speed: Measurement speed is usually very high with optical scatterometry, due to the availability of strong light sources, and very sensitive detectors. This eventually yields fast measurement with a high signal-to-noise ratio.
  • Nondestructive: Optical scatterometry is a non-destructive type of measurement, and has no impact on the production line.
  • Accurate interpretation: Since the interaction of light with the sample is well-known, the interpretation of the scattered signal is much more accurate than with other metrology technologies.

Optical Scatterometry Inner Workings, Capabilities and Challenges

A broad-band light source is focused onto a sample. The light reflected off the sample is focused onto a device that can separate it into its spectral and polarization components (represented schematically by a prism). Each component is then transformed into an electrical signal using a specialized detector. The spectral data, which is the signal level for each wavelength, is then delivered to a processing unit. Using the physics describing how matter interacts with light (Maxwell’s equations) as well as machine-learning mathematical models, the processing unit evaluates the properties of the sample that best matches the measurement.

The typical dimensions being measured, are those associated with state-of-the-art transistors. These are based on the most advanced industry design node of both logic and memory devices. Even though these sizes are much smaller than the wavelength of the light that is used, optical scatterometry with sophisticated methods enable to cover this gap. In addition, measurement of most critical dimensions is done with a high degree of accuracy, very fast, and while focusing the light onto a very small area. These three requirements, that are often considered contradictory, are reached by several approaches. These include: Clever target designs, use of a broad spectral band, complex optical design, educated control of light polarization, sophisticated algorithms and more.