white light interferometry

White light interferometry (WLI) is a non-contact optical metrology technique that measures surface topography with sub-nanometer vertical resolution by analyzing interference patterns created when white (broadband) light reflects from both the sample surface and a reference mirror. Operating principle: (1) white light from a broadband source is split into two beams by a beam splitter in a Michelson or Mirau interferometer objective, (2) one beam reflects off the sample surface, the other off a precision reference mirror, (3) the two beams recombine, creating interference fringes, (4) because white light has short coherence length (~1-2μm), constructive interference (bright fringes) only occurs when the optical path lengths match to within the coherence length, (5) by scanning the objective vertically (z-scan) while recording the interference signal at each pixel on a camera, the software determines the exact height where maximum fringe contrast occurs at each lateral position—this is the surface height at that point. Performance: (1) vertical resolution 0.1-1nm (sub-angstrom possible with advanced algorithms), (2) lateral resolution 0.5-5μm (limited by optical diffraction), (3) vertical measurement range up to several millimeters, (4) field of view depends on objective magnification (100μm × 100μm to 10mm × 10mm). Semiconductor applications: (1) CMP step height and dishing measurement (quantify post-CMP topography across test structures and product features), (2) etch depth measurement (trench depth, via depth, feature profile characterization), (3) MEMS structure characterization (membrane deflection, cantilever profiles, 3D structural metrology), (4) wafer bow and warp measurement (full-wafer surface mapping for stress analysis), (5) bump height and coplanarity (flip-chip bump metrology for packaging). Advantages over contact profilometry: no sample contact (no scratching or damage), faster area measurement (2D surface map vs. 1D line trace), applicable to soft or delicate surfaces. WLI is complementary to AFM (which provides higher lateral resolution but smaller field of view).