Optical Microscopy

Brightfield, Darkfield and Interference Contrast (Nomarski)

Brightfield illumination

Brightfield illumination is the normal, most even illumination mode. A full cone of light is focused by the objective on the sample. The sample is uniformly illuminated. The picture observed is the result of differences in reflectivity created by material properties of the sample, transmission and reflection through surface films, and by the surface contour of the sample (see comparison above).

Darkfield illumination

Darkfield illumination has the inner circle portion of the light blocked. The sample is illuminated entirely by light which impinges on the sample surface at a glancing angle. The scattered reflected light seen through the lens comes from irregular features on the surface, such as step edges, scratch edges, particulates like dust or contamination, metal precipitates, etc. The principle use of darkfield has been for detecting particles and scratches. Note that in the sample the spectral reflections from the metal disturbed by a probe are brighter than any other feature in the photo.

Interference contrast (Nomarski)

Interference contrast (Nomarski) is a variation of brightfield illumination. Light from the lamp is polarized. The polarized light is divided into two orthogonal polarized packets of light by a Wollaston prism located immediately before the light enters the objective lens. The two packets of light are displaced laterally very slightly as they impinge on the sample. The two packets are combined again as they pass back through the prism.

The unique characteristic of interference contrast comes from the fact that each packet intersects the sample at slightly different points. If the two points on the surface are at different elevations, each light packet travels a different round trip distance from the prism to the sample and return. That difference adds or interferes as the packets are recombined in the prism. Etch pits and cracks which are not detectable in brightfield will stand out clearly enough to be photographed in interference contrast. By rotating the initial polarization, the analyst can change the interference patterns to maximize contract in specific areas of interest. Nomarski is especially good for observing the die topography on semiconductors, VLSI, memory, logic, and microprocessors. Is capable of locating point defects, line defects, twinning, stack faults, and other silicon defects.