Chromatic Dispersion Profilometry (CWL)

Chromatic Dispersion Profilometry
3D surface map of a US 1 cent coin showing overall concave shape as well as topography of individual surface features

Chromatic dispersion profilometry or Chromatic white light (CWL) profilometry is a non-contact, nondestructive analytical technique used to measure surface topography. It is particularly well suited for large-area characterization (e.g. whole wafers) requiring high vertical accuracy.

  • High vertical accuracy (within a few hundred nm for mapping, or as low as single nm for point scan)
  • Able to measure large sample areas (up to full 300mm wafers)
  • Captures 3D height map of full surface curvature, warp, and flatness
  • Effective at characterizing interfacial topographies through transmissive overlayers
  • Slower data collection for high lateral resolution mapping as compared to other surface topography methods
  • Thickness measurement of transparent / translucent films only possible under certain conditions
  • Stage translation artifacts (typically a few hundred nm) limit ability to measure extremely flat surfaces
  • Difficulty measuring steeply sloped surfaces (>30°)
  • Changes in sample reflectivity (particularly if wavelength dependent) can affect the measurement.
Example Outputs

3D surface map of a US 1 cent coin showing overall concave shape as well as topography of individual surface features

Surface profiles taken from the above 3D surface scan of a US 1 cent coin

3D CWL Model showing bowing in topography of an 8-in standard wafer

3D CWL Model generated of an assembled SMD device, showing both surface texture and larger-scale components.

From: Danfoss Silicon Power GmbH
Instruments Used for CWL
cyberTECHNOLOGIES CT300 Surface Measurement System

cyberTECHNOLOGIES CT300 Surface Measurement System

  • Maximum Sample Dimensions: 315 mm x 315 mm
  • Height Measuring Range (single-scan): 500 µm
    • Multiple images can be stitched together for larger height range
  • Maximum Lateral Resolution: 1.5 µm
Sample Requirements
  • Maximum Sample Size:
    • In-house Instrument: 315 mm x 315 mm
  • Maximum Measurable Height Range:
    • Single-Scan: 500 µm
    • Z-Stitching of Multiple Scans: approximately 22 mm
  • Maximum Sample Weight: ~25 kg
How CWL Works

Chromatic dispersion profilometers employ a confocal optical configuration in which a broadband white-light beam is focused downward onto a point area of the sample using a lens with intentionally large optical dispersion. The lens causes focal length to vary with wavelength, such that different wavelengths will be in focus at different distances from the detector.

Only the narrow band of wavelengths which are in focus at the sample surface are reflected back to the detector, allowing the system to measure surface height at the illuminated point with nm-scale vertical accuracy.

Additionally, the large-area sample stage can also translate (move) laterally to allow the measurement lens to analyze surface height at a series of points. This enables 2D or 3D modeling and mapping of surface topography across a full scanned region on the sample.

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