In the 1970’s, electronic recording media started to replace photographic plates; today, the CCD-detector is in common use. The spatial resolution of electronic recording media was, however, not nearly as high as for holographic plates.
Scatterometry is an optical metrology technique designed for the characterization of test samples from lithography. The diffraction-based method involves ellipsometric measurements of the light diffracted from the samples in various experimental configurations. A data fit to theoretical predictions provides the most probable values for the sample parameters; sensitivity analysis allows us to achieve maximum information and sensitivity from existing equipment.
A sample consists of a reflective diffraction grating with a linewidth lw equivalent to the CD of the device under study. The linewidth is the most important parameter of the sample to be determined, but others include grating height and pitch, as well as the height of the antireflection coating beneath. The light backdiffracted by the grating falls into multiple orders. Because the zeroth order is generally the strongest, it is usually the only one that is measured. Also, in the conditions of the continuous miniaturization trend in the microelectronics industry, this order is oftentimes the only one available.
A typical scatterometer consists of a laser source, a photodiode detector, and a sample holder that turns about its vertical axis and horizontal axis (see figure bellow). During the measurement process, the sample holder rotates about the vertical axis to effectively scan the beam through a range of incident angles, and about its horizontal axis to effectively scan the beam through a range of azimuthal angles.
