Multi-wavelength VIS/UV optical diffractometer for high-accuracy calibration of nano-scale pitch standards

A new optical diffractometer has been developed and set up at the Physikalisch-Technische Bundesanstalt (PTB). It offers the possibility of high-accuracy calibrations of the lateral period of gratings (pitch) in the micro- and nanometre scale. The measurement principle is based on a modified Littrow configuration, where the incident and the diffracted laser beams are almost collinear. The grating is mounted on a rotary table, and a high-precision rotary encoder is used to measure its angular positions. The profiles of the diffracted laser beams are recorded by means of a line array image detector. To determine the centre positions of the imaged laser beam profiles, different analysis methods can be applied, among others a new correlation method. A variety of laser wavelengths, ranging from 266 nm to 633 nm, can be used. Due to the optional UV wavelength, the smallest measurable pitch is about 150 nm. Depending on the quality of the sample, the measurement uncertainty can be smaller than 10 pm. For two-dimensional gratings the pitch of the two main and the diagonal directions can be measured and thus, also the angle between the two main grating orientations can be determined.