Large Area Exposure with Automatic Stitching by Alignment using conventional Mechanical Stage and Grid of Markers

Given example demonstrates that NanoMaker allows one to perform exposure of large area structures using cooperative control of stage and beam deflection system for subfields stitching in automatic mode. As a test structure the Bragg-Fresnel lens of 1+3+5 orders with overall sizes of 1250 um x 180 um and minimal zone size of 200 nm has been chosen. The job has been executed with microscope Zeiss SEM EVO-50 equipped with a conventional mechanical stage, which accuracy of moving is worse then 1 um. Exposure was carried out with stage moving over 200 micron subfields along the horizontal axis. In each subfield, before the exposure, an automatic alignment by using one marker was carried out to compensate stage moving inaccuracy. To perform such procedure, on an initial Si substrate the contrast metal markers in the form of crosses have been created by optical lithography. Marker's centre to centre distance was 200 micron, while width of crosshair was nearby to 1.5 micron. The lens has been written in PMMA-950 resist of 400 nm thickness at accelerating voltage of 15 kV. After development, 100 nm Au film has been deposited and lift-off has been performed. Evidently, from published below photos, an accuracy of fields stitching was better than 100 nm. And the limiting factor for the given procedure was quality of the markers used for alignment.

Fig. 1. Whole lens image and grating of markers. The Bragg-Fresnel lens of 1+3+5 orders with overall size of 1250 um x 180 um and minimal zone size of 200 nm. The lens was written in PMMA-950 resist of 400 nm thickness at 15 kV of accelerating voltage and transferred to 100 nm Au film by lift-off procedure. A few markers, visible at the left side of image were used for pre-alignment of sample coordinate system, deflection system and stage by semiautomatic procedure. Then exposure with automatic stitching by alignment using grid of markers was performed.

				Fig. 2. Fragment of Bragg-Fresnel lens of 1+3+5 orders.
Fig. 3. A few zones view at stitching point. Minimal zone size is 200 nm. Accuracy of stitching is better then 100 nm. The job has been done at IMT RAS in Chernogolovka by Dr. M. Knyazev and Dr. A. Svintsov

Fig. 4. Fragment of Bragg-Fresnel lens of 1+3+5 orders at stitching point.