Comparison to White Light Interferometry

WLI vs Zeta 3D Optical Profilometry

 Read MoreOptical Profiler with ZDot vs WLI.pdf

White Light Interferometry schematic

White Light Interferometry (WLI) has been adopted across multiple industries for 3D measurements.  WLI is capable of measuring surface heights from nanometers to millimeters.  A major advantage of WLI over other optical techniques is that the vertical resolution is independent of magnification or field of view. This allows very high z-resolution while maintaining large fields of view.  Conventional confocal techniques have a z-resolution that is dependent on the field of view, this inherently limits the z-resolution of confocal techniques to small fields of view. Therefore, WIL is the go-to technology for applications with low topographic features on smooth, flat surfaces.  

However, WLI has some limitations for 3D optical profiling of complex surfaces with a range of topographic features. WLI imaging capability is partially limited due to the lower light throughput design of the optical system. Samples with transparent and multiple layers, low reflectivity, high roughness, large height or large reflectivity variations are generally better fit for a confocal-like technique.

Zeta optical profiler schematic

The technology for the Zeta optical profiler overcomes the limitations of WLI with an optical design that has high light throughput.

High light throughput and optical efficiency are essential for surfaces that have low reflectivity or high aspect ratio features, such as vias or deep trenches. Optical profilers based on WLI have limited light throughput because of their optical design. Zeta optical profilers (right) use a brighter light source and are designed to use the same optical path for focusing and imaging.

The Zeta optical profiler employs an unique optical design that uses a common path for focusing and imaging as well as a patented and proprietary method for sensing the focal plane. The Zeta imaging technology uses a confocal grid structured array illumination (CGSI) to generate contrast that is independent of the sample, enabling robust Z-height measurements on almost any surface.