Predicting the lifetime of ultraviolet optics

Extreme ultraviolet lithography (EUVL) may be the next-generation patterning technique used to produce smaller and faster microchips with feature sizes of 32nm and below. However, durable projection optics must be developed before this laboratory technique can become commercially viable. As part of its long-standing effort to develop EUVL metrology and calibration services, the

(NIST) is creating a measurement system for accelerated lifetime testing of the mirrors used in EUVL.

The light to be used in EUVL has a wavelength of 13nm. It can only be efficiently reflected with mirrors consisting of 50 alternating bi-layers of molybdenum and silicon, each 7nm thick and deposited with near-atomic-scale precision. So although the EUVL mirrors will be very large, up to 35cm in diameter, they are actually incredibly precise nanostructured devices. A single commercial lithography instrument may require six of these mirrors at a cost of more than $1 million each.

The mirrors are delicate, but the EUV radiation they must reflect is intense and damaging. The combination of this harsh radiation with the trace levels of water vapour and hydrocarbons typically found in the vacuum environment of EUV first-generation exposure tools can lead to rapid corruption of the EUVL mirror surfaces. And a loss of just one percent to two percent of a mirror's reflectivity renders the optical system useless for efficient production of nanometre resolution circuit features.