DEVELOPMENT OF A LASER DIFFRACTOMETER WITH IMPROVED ACCURACY FOR MEASUREMENT OF THE GRATING PERIOD

A precision laser diffractometer of a conventional-type has been devel oped for grating pitch measurements. The accuracy of the measurement d epends on the accuracies of the laser wavelength, the goniometer, and the determination of the diffraction angles. The main source of error in the pitch size measurement in conventional diffractometry has been the low resolution in the measurement of the diffraction angles. In th is paper, an improvement of the conventional measurement is reported; the rotary arm is extended, and a more reliable high-resolution goniom etor is chosen. Furthermore, a focusing lens is inserted between the g rating sample and the detection plane to prevent the error due to the parallel shift of the diffracted beam, which may take place when the r otational axis of the arm does not coincide with the grating plane. Th e values obtained for lamella-structure grating samples show quite a g ood reproducibility, whose order of magnitude is angstrom. The pitch o f the grating, whose pitch was certified as 288 nm by the manufacturer , is measured to be 287.595 with a la standard deviation of 0.017 nm a t worst. The value measured for a 700-nm pitch grating is 700.782 with the 1 sigma standard deviation of 0.054 nm at worst. The expanded unc ertainty with a coverage factor of 2 is estimated to be 0.038 nm and 0 .10 nm, respectively.