Effect of SiC X-ray masks on alignment accuracy of heterodyne alignment

This paper describes the results of a study to investigate the applicability of silicon carbide (SiC) X-ray masks to an optical-heterodyne alignment technique. SiC was deposited at a thickness of 2 mu m on a 4 inch silicon (Si) wafer. It's surface roughness was improved from 15 nm Ra (geometrical average) to 0.2 nm Ra using a polishing method. Using this SiC material, X-ray masks with Ta absorber patterns for alignment marks and overlay measurement were fabricated. To obtain a high overlay repeatability, we have deposited an anti-reflection coating (ARC) on both sides of the X-ray mask, which increased an optical transmission at a wavelength of 785 nm from 37% to 73%. We have also deposited an opaque coating (OPC) on the mask alignment mark. Using the X-ray mask, the overlay repeat ability by mix-and-match method was evaluated. The overlay repeat ability near the X, Y and theta alignment marks was 21 nm, 21 nm, and 49 nm (3 sigma) for the corresponding axes. The overlay repeatability of the X and Y directions was 61 nm and 54 nm (3 sigma) in a wafer. These results are equivalent to the results obtained using SIN X-ray masks. From these results, we consider that SiC X-ray masks are applicable to optical heterodyne alignment, and can also be used in practical X-ray lithography.