10.1117/2.1201609.006687 Extreme-UV interference lithography at the Paul Scherrer Institute Elizabeth Buitrago, Roberto Fallica, Daniel Fan, Waiz Karim, Michaela Vockenhuber, Jeroen van Bokhoven, and Yasin Ekinci Advantages such as high resolution and throughput, along with insen- sitivity to misalignment, make extreme-UV interference lithography a powerful enabling technology for academic and industrial research. Extreme-UV interference lithography (EUV-IL), at a wavelength of 13.5nm, has proved to be a powerful technique due both to its relative simplicity and record-high-resolution patterning capabilities. In an EUV-IL setup, a mask with transmission- diffraction gratings is illuminated by a spatially coherent beam of EUV light from an undulator synchrotron source. Periodic images are then created by the interference of two or more dif- fracted coherent beams (see Figure 1). The sinusoidal aerial im- age produced by two-beam IL has a period that is half of the original mask grating period when first-order diffracted beams are used. Consequently, this method provides a demagnification of grating patterns that are written by electron beam lithography (EBL). Moreover, versatile periodic patterns and quasi-periodic patterns can be obtained by using multiple beams and by con- trolling their phases. 1, 2 The ultimate resolution (i.e., half-pitch, HP) that can be achieved with EUV-IL, however, is limited by light diffraction. Although it is therefore theoretically possible to resolve features to below 4nm with EUV-IL, 3 the achievable resolution is limited by the grating resolution and by other factors. In addition to high resolution and throughput, EUV-IL offers many other advantages, such as achromaticity, insensitivity to misalignment, and infinite depth of focus, making this lithogra- phy technique extremely useful. 4 For example, with EUV-IL, it is possible to pattern high-resolution periodic images to create highly ordered and dense nanostructures. Such structures can be difficult or time-consuming to pattern by EBL, but are inter- esting for a wide range of applications, such as nanocatalysis, 5 Figure 1. Schematic diagram of an extreme-UV interference lithogra- phy (EUV-IL) setup, in which first-order diffraction is used to create an aerial image on a resist, by interference.  ,  1 ,  2 : Diffraction angles. m 0;1;2 : Diffraction orders. (Adapted from Mojarad et al., 2015. 7 ) electronic 6 and photonic devices, 7 and fundamental materials analysis. 8 In this work, 9 we discuss the long-term performance and ca- pabilities of EUV-IL activities at the Paul Scherrer Institute (PSI), Switzerland. We also describe some of the state-of-the-art re- search that has been conducted at PSI. In particular, we focus on the X-ray Interference Lithography (XIL) beamline at PSI, which is illuminated by the co-located Swiss Light Source. We have been able to achieve resolution down to 10nm HP by using masks that consist of silicon oxide diffraction gratings. 10, 11 These gratings are directly written on silicon nitride membranes Continued on next page