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Image Grating Metrology Using a Fresnel Zone Plate

Scanning-beam interference lithography (SBIL) is a novel concept for nanometeraccurate grating fabrication. It can produce gratings with nanometer phase accuracy over large area by scanning a photoresist-coated substrate under phase-locked image grating. For the successful implementation of the SBIL, it is crucial to measure the spatial period, phase, and distortion of the image grating accurately, since it enables the precise stitching of subsequent scans. This thesis describes the efforts to develop an image grating metrology that is capable of measuring a wide range of image periods, regardless of changes in grating orientation. In order to satisfy the conditions, the use of a Fresnel zone plate is proposed. The Fresnel zone plate is a circularly symmetric diffraction grating that contains a wide range of spatial frequencies. The work begins with a basic understanding of the zone plate, followed by the physics of the image metrology. Closed mathematical solution is obtained to describe the diffraction pattern of incident plane waves by a Fresnel zone plate. For a given zone plate, a range of image periods that can be measured is also investigated. The question regarding to contrast variation of optical power by varying image period is also discussed. The detailed theoretical analysis is followed by the experiments designed and performed to verify the findings. Period measurements are conducted in two cases: homodyne and heterodyne measurements. In the homodyne case, two different image periods are measured using the same amplitude zone plate. In the heterodyne measurement, the zone plate is installed in the current SBIL prototype system, the Nanoruler, and is used to measure the period of the image grating with fringes that moves at frequency difference between two beams. The contrast variation is also verified by the experiment. With the two conditions for image metrology to satisfy: immune to grating orientations and capability of measuring a wide range of periods, the use of a zone plate would be an excellent choice. However, one should be careful to use a properly designed zone plate such that the image period to be measured is within the range of high contrast for a given zone plate. 3 Thesis Supervisor: Mark L. Schattenburg Title: Principal Research Scientist