SOLID STATE LIGHT SOURCES FOR 3D PROFILERS

Solid state light sources such as light emitting diodes (LEDs) are replacing Halogen lamps in 3D profilers such as Scanning White Light Interferometers (SWLI) [1]. LEDs have long lifetimes, stable spectra and can be pulsed for stroboscopic measurements [2]. LEDs suffer from long coherence lengths (single color LEDs) and a non-continuous spectrum (white LEDs) [1], which affect measurement precision and make resolving closely packed layers difficult. We apply two new light sources for SWLI: a) multiple LED assembly emitting in different spectral regions, and b) a source that combines light from laser pumped phosphor with LEDs. The sources produce wide, continuous spectra which create tight interferograms without side peaks. This allows imaging thin-layer structures. The source spectra can rapidly be tuned for maximum penetration into different materials.

The sources were tested by measuring known step heights with our SWLI setup. The measured step heights and measurement precision were similar to measurements using a halogen lamp source. The localized interference was demonstrated by imaging the top and bottom surfaces of a 4 μm silicon membrane simultaneously.