EUV Mirrors: The Smoothest Objects Ever Manufactured
Because 13.5nm extreme ultraviolet light is absorbed by virtually all materials including glass, ASML's EUV lithography machines use reflective optics built from 40-80 alternating molybdenum-silicon layers that achieve roughly 70% reflectivity via Bragg interference. Surface roughness is specified at 2.3 silicon atoms average bump height, making them the smoothest macroscopic objects ever made.
Extreme ultraviolet lithography at 13.5nm cannot use conventional lenses — the wavelength is absorbed by glass, air, and most metals. ASML's machines therefore rely entirely on mirrors, manufactured by the German optics company Zeiss under a parallel development program lasting decades. The mirrors work via Bragg reflection: alternating layers of molybdenum and silicon, each roughly 3-4 nanometers thick, stacked 40-80 layers deep. Each layer thickness is tuned so partial reflections from each interface add up in phase, yielding maximum reflectivity around 70% per mirror. A current-generation EUV machine has six mirrors plus the reticle (functioning as a seventh mirror), so total light reaching the wafer is approximately 8% of what the source produces — a brutal optical budget that drives the requirement for ever-more-powerful tin-plasma light sources. The surface-roughness specification is the part with no precedent. Average bump height across the polished surface must be no greater than 2.3 silicon atoms. Scaled up, if a single mirror were the size of Germany, the largest bump on its surface would measure about one millimeter. For the new high-NA EUV mirrors, the standard is even tighter: if the mirror were the size of Earth, the largest bump would be the thickness of a playing card. The mirrors are polished using ion-beam sputtering, removing material atom by atom with corrections informed by precision metrology. Zeiss also engineered a pico-radian sensor and actuator network — described as a 'nervous system in the optics' — that allows each mirror to self-correct for thermal drift in real time, with a pointing accuracy comparable to aiming a beam at one side of a dime sitting on the surface of the Moon.