ultraviolet lithography (EUVL) may be the next-generation
patterning technique used to produce smaller and
faster microchips with feature sizes of 32 nanometers
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 (summarized in a recent paper*), the National
Institute of Standards and Technology (NIST) is
creating a measurement system for accelerated lifetime
testing of the mirrors used in EUVL.
light to be used in EUVL has a wavelength of
only 13 nm. It can only be efficiently reflected
with mirrors consisting of 50 alternating bi-layers
of molybdenum and silicon, each only 7 nm thick
and deposited with near-atomic-scale precision.
So although the EUVL mirrors will be very large,
up to 35 centimeter (cm) 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.
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 vapor 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
1 percent to 2 percent of a mirror's reflectivity
renders the optical system useless for efficient
production of nanometer-resolution circuit features.
help the semiconductor industry meet its goal
of EUVL production by 2010, NIST has established
a dedicated beamline at its Synchrotron Ultraviolet
Radiation Facility for durability testing of
multilayer mirrors. 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
determine how damage scales with various parameters,
NIST researchers recently exposed EUVL mirrors
(provided by SEMATECH from work it co-funded)
to varying levels of light intensity, water and
to expectations, they found that increasing amounts
of water vapor caused less mirror damage, which
may be due to a simultaneous increase in the
ambient hydrocarbon levels. Subsequent experiments
have shown that deliberately introducing trace
amounts of a simple hydrocarbon like methanol
can mitigate significantly the water-induced
damage. 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.
further information, see http://physics.nist.gov/euvl.
Grantham, S.B. Hill, C. Tarrio, R.E. Vest and
T.B. Lucatorto.2005. EUV component and system
characterization at NIST for the support of extreme-ultraviolet
lithography. Proceedings of SPIE 5751, 1185-91.
Laura Ost, firstname.lastname@example.org ,