electron-beam lithography

To build a quantum chip you have to draw shapes onto a wafer: the metal wires, the capacitor pads, and especially the tiny Josephson junctions that make a qubit work. Some of those features are only tens of nanometers wide, far finer than a human hair. Electron-beam lithography is how you draw them. Instead of shining light through a mask, you steer a focused beam of electrons, like the pen of an extremely precise plotter, across a thin coating of resist on the wafer, exposing exactly the lines you want, point by point.

It works because electrons can be focused to a much smaller spot than visible light, so the beam can write features a few nanometers across, with no mask needed. You first spin a layer of electron-sensitive resist onto the wafer. The beam scans the pattern, chemically changing the resist wherever it lands. A developer then washes away the exposed (or unexposed) parts, leaving a stencil. Metal is evaporated through that stencil, and dissolving the leftover resist lifts off the excess, leaving only your shapes behind. For Josephson junctions this is paired with clever resist tricks, like the Dolan-bridge method, to make the two thin films overlap with a sliver of oxide between them.

The honest catch is speed. Because the beam writes one point at a time in a serial scan, it is slow, fine for one research chip but far too slow and costly to mass-produce wafers. So in practice e-beam is used only where its resolution is truly needed, mainly the nanometer-scale junctions, while the large, coarse features, the wiring and pads, are patterned far faster with optical (photo) lithography. Combining the two is standard, and the slowness of e-beam writing is one real reason quantum chips are still made in small batches rather than churned out like ordinary processors.