Soft x-ray zone plate microscopy has proven to be a valuable imaging technique for nanoscale studies. It complements nano-analytic techniques such as electron and scanning probe microscopies, and offers a unique set of capabilities including high spatial resolution, natural elemental/chemical and magnetic sensitivities, large permissible sample thickness, and a myriad of in-situ sample environments. In this paper, a new zone plate fabrication technique based on overlaying complementary, semi-dense patterns is described. The new technique permits zone plates with sub-20 nm zones, which are extremely challenging for conventional fabrication processes, to be fabricated. With the new technique, zone plates of 15 nm outermost zone width were successfully fabricated for the first time, yielding a spatial resolution better than 15 nm. Zone plates of 10 nm outer zones, as well as 3-D zone plate structures, which cannot be fabricated using conventional zone plate fabrication processes, are anticipated with the overlay technique.