White light interferometer (WLI) is an optical surface profiling instrument that uses broadband (white) light interference to measure 3D surface topography with sub-nanometer vertical resolution — combining the speed of non-contact optical measurement with the vertical precision of interferometry for semiconductor surface characterization, MEMS metrology, and packaging inspection.
What Is a White Light Interferometer?
- Definition: An optical microscope-based instrument that splits white (broadband) light into reference and sample beams, recombines them to create an interferogram, and uses coherence scanning (vertical scanning interferometry, VSI) to build a 3D height map of the surface with <0.1nm vertical resolution.
- Principle: White light has short coherence length (~1 µm) — interference fringes only appear when the optical path difference is near zero. By scanning vertically and tracking the fringe envelope peak for each pixel, the instrument maps surface height with extreme precision.
- Also Known As: SWLI (Scanning White Light Interferometry), VSI (Vertical Scanning Interferometry), CSI (Coherence Scanning Interferometry).
Why White Light Interferometers Matter
- Non-Contact: No stylus contact means no surface damage, no probe wear, and no contamination — measuring delicate semiconductor and MEMS surfaces safely.
- 3D Measurement: Full-field 3D surface maps rather than single-line profiles — capturing topography over areas from 50×50 µm to 10×10 mm.
- Speed: Captures millions of height data points in seconds — much faster than point-by-point stylus profilometry for full-area measurements.
- Versatility: Measures rough and smooth surfaces, steps, trenches, pillars, and complex 3D structures across a wide height range.
Applications in Semiconductor Manufacturing
- MEMS Topography: 3D profiling of MEMS cantilevers, membranes, hinges, and cavities — measuring deflection, curvature, and critical dimensions.
- Bump Height: Measuring solder bump and copper pillar heights in advanced packaging — verifying uniformity across entire substrates.
- Surface Roughness: Non-contact measurement of surface roughness parameters (Sa, Sq) on polished wafers, deposited films, and CMP surfaces.
- Etch Depth: Measuring etch trench depths and profiles without contact — preserving fragile post-etch structures.
- Wafer-Level Packaging: TSV (Through-Silicon Via) reveal height, RDL (Redistribution Layer) step heights, and micro-bump coplanarity.
WLI Specifications
| Parameter | Typical Value |
|---|---|
| Vertical resolution | <0.1 nm |
| Vertical range | 0.1 nm to 10+ mm |
| Lateral resolution | 0.3-5 µm (objective-dependent) |
| Field of view | 0.05×0.05 mm to 10×10 mm |
| Measurement speed | 1-30 seconds per field |
Leading Manufacturers
- Zygo (Ametek): NewView and Nexview series — industry standard for production and research WLI.
- Bruker: ContourGT and NPFLEX series — versatile optical profilers.
- Sensofar: S neox — multi-technique profiler combining WLI, confocal, and focus variation.
- KLA: Zeta optical profilers for semiconductor and electronics applications.
White light interferometers are the fastest non-contact 3D surface measurement tools in semiconductor manufacturing — delivering sub-nanometer vertical resolution across wide fields of view for the surface topography characterization that process development and quality control demand.
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