To achieve high resolution depends on the ability to squeeze the zones close together, with a placement accuracy no less than one-third the width of the zones themselves. Accurate placement of 15-nanometer-wide zones allows no more than 5‑nanometer leeway. In fact the Nanowriter is capable of placement accuracy to within 2 nanometers, allowing for even greater improvements in resolution in the future.
Unfortunately, no matter how accurately it is aimed, even a tight beam of electrons spreads out when it hits the resist. Electron scattering, combined with inherent limits in the resolution of the resist itself, makes it impossible at this time to maintain high contrast and optical separation between features. Previously the best separation between zones the Nanowriter could achieve to make the XM-1's current objective lens was 25 nanometers.
To overcome this limit, the CXRO researchers decided to overlay and combine two different zone-plate patterns. Opaque zones are typically given even numbers, so in this scheme the first pattern contains zones 2, 6, 10, 14, and so on, and the second contains zones 4, 8, 12, 16, and so on. The first pattern is carved into the resist-coated wafer; then the zones formed by the electron patterning are filled with gold. The wafer is coated with resist again to make the second pattern.
When combined, the critical outer zones of the combined patterns were less than 15 nanometers apart, accurately placed to within less than 2 nanometers.