Traceable and long-range grating pitch measurement with picometer resolution

Abstract

Traceable and long-range measurement method for grating pitch is proposed using a scanning grating method, which means an in situ equal-period grating is mounted on a moving stage that is continuously measured with a laser interferometer. The interference signal generated from the moving grating can be measured with a long distance to form thousand-period interference pattern which has a nano-resolved positioning using a laser interferometer, thus for a single period, the repeatability can be calculated with picometer resolution. Both the mean pitch and the uniformity of the grating should be measured quantitatively. The method was applied to measure a transmission grating and a reflection grating with the nominal groove density value of 1480 grooves/mm and 600 grooves/mm, where the mean periods are measured to be 675.610 nm ±7 pm in 30 mm range with the relative repeatability of 29.44 ppm (parts per million) and 1667.039 nm ±6 pm in 20 mm range with the relative repeatability of 8.76 ppm. The pitch with the picometer resolution and ppm repeatability not only guarantees the strict requirement of pitch value for most advanced lithographic equipment, but also provides reliable ruler for picometer optical measurement in various pico-optical applications.