Technique for measuring the groove density of diffraction gratings using the long trace profiler

Diffraction gratings are the most efficient systems for dispersing radiation in the soft x-ray and extreme ultraviolet region. They can have a constant periodic structure or a variable line spacing. The performance of an instrument adopting optics depends on the grating manufacturing errors. These could be both slope errors (routinely measured by various instruments) and undesired groove density variation. The measurement of the groove density is mainly performed with home-made setups, which use the diffraction pattern generated by a laser or, sometimes, the images collected with an atomic force microscope or a scanning tunneling microscope. With the use of a long trace profiler (LTP), an instrument developed to measure the longitudinal profile of a mirror, a method to measure the groove density (and the groove density variation) of a surface diffraction grating was developed. It consists of the measurement of the angle of a particular diffraction order, taking advantage of the extreme angular precision of the LTP. An accuracy of one part per million, or better, in the determination of the groove density constancy/variation, is reached. This method demonstrates the great versatility of the LTP and, in particular, its infinite depth of field.