Research on the Lifetime of Extreme UV Optics

Although the EUVL mirrors will be very large, up to 35 cm in diameter, they are also incredibly precise nano-structured devices. This exotic wavelength of light can only be efficiently reflected using a constructive interference technique that requires deposition of about 50 bilayers of molybdenum-silicon each only 7 nanometers thick with sub-nanometer resolution. This challenging combination of near-atomic-scale control over relatively immense lateral expanses places a price tag exceeding $1 million for each of the 6 projection optics in a single commercial stepper. Amazingly these daunting fabrication specifications have been achieved, but one of the potential show-stoppers for EUVL is finding a way to prevent these precisely engineered structures from deteriorating in the harsh production environment throughout the 30,000 hr lifetime required for commercial viability. The intense 13.5 nanometer wavelength radiation used to expose the microchip pattern, combined with traces of water vapor or hydrocarbons at levels typically found in semiconductor production facilities, can roughen and produce oxide and carbon growth on the EUVL mirror surfaces. If this deterioration results in a reflectivity loss of only 1-2%, the mirror must be replaced in order to maintain the nanometer resolution of the optical system. To meet the industry goal of EUVL production by 2010, new protective capping layers for these mirrors must be rapidly developed and tested. NIST is in a unique position to meet these needs of the EUVL community.

In 2002 NIST established a dedicated beamline at its Synchrotron Ultraviolet Radiation Facility III for lifetime testing of multilayer mirrors. The facility delivers radiation intensities comparable to those in the EUVL production environment as well as controlled levels of water vapor and other trace contaminants. Initial tests established that standard mirrors topped with silicon would have lifetimes of just minutes to hours, while ruthenium-capped mirrors had lifetimes of a few tens of hours, still a thousand times less than industry's requirement.

To determine how damage scales with various parameters, NIST researchers recently exposed EUVL mirrors supplied by SEMATECH to varying levels of light intensity, water, and hydrocarbon concentrations. Contrary to expectations, they found that increasing amounts of water vapor caused less mirror damage, a discovery with important implications. NIST researchers believe that for reasons not yet completely understood, increasing the water vapor partial pressure also increased the amount of trace hydrocarbon contamination which reduced the amount of damaging, EUV-induced oxidation associated with the water vapor. Indeed, subsequent experiments have shown that deliberately introducing trace amounts of methanol significantly mitigates the damage from the water vapor.

These findings indicate that accelerated-lifetime tests of EUV optics must be done under tight control of both the level of water vapor and the hydrocarbon contamination level in the ambient background. The interaction between these two chemical species may offer a starting point to finding a solution to the problem of EUVL mirror lifetime. To further pursue this avenue of research, the NIST scientists are commissioning a new beamline devoted to accelerated testing and will add a second branch to the existing beamline that will provide broadband illumination (wavelengths of approximately 11 nm to 50 nm) into a single spot at approximately 100 times the intensity of the current system.