The laser diffractometer is an effective instrument for calibrating pitch standard of a grating structure. A conventional diffractometer based on the Littrow configuration cannot measure a grating whose pitch is less than half of the laser wavelength when the diffractometer is operated in the atmosphere. This study proposes an immersion diffractometer to raise the refractive index of the environment. Thus the new approach can overcome the limit of one-half wavelength. A 288 nm grating was measured using an immersion diffractometer with a 633 nm laser and using a conventional diffractometer with a 543 nm laser to demonstrate the feasibility and effectiveness of the proposed technology. The difference between the pitches obtained by these two methods is around 0.05 nm.
A Traceable Atomic Force Microscope (TAFM) to calibrate the pitch standards is presented. The TAFM consists of an atomic force microscope, a three-axis active compensation flexure stage, two laser interferometers, an L-shape mirror, a vibration isolator, and a super-Invar metrology frame. A test specimen is laid on the same plane of laser interferometers to eliminate the Abbe-offset. The displacements of X and Y axes are taken by the laser interferometers, the Z movement is controlled by AFM cantilever and the displacement is taken by a capacitance sensor while the flexure stage moves the specimen in X and Y axes motions. A water circulator is used to control the TAFM at 20 • C. Measuring results of a standard pitch sample show that this TAFM can be used for measuring of pitch standards. A pitch standard with nominal value of 292 nm was served as a test sample. The combined standard uncertainty was 1.2 nm.
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