- Assessing Damage for UV-Laser-Resistant Fused Silica
Dr. Johannes Moll
Studying laser-induced damage in silica may help scientists improve its resistance and provide
another level of flexibility for lens designers.
The heart of any optical lithography tool is its illumination and projection system.
The illumination system preconditions the laser beam and provides the optical path
from the laser to the photomask, and the projection lens images the pattern from
the photomask onto the wafers. The majority of required optical elements are made
of high-purity fused silica, and the remaining ones are made of CaF2 to correct
chromatic aberrations. Because lithography tools are generally expected to have
a lifetime of approximately 10 years or, in terms of laser exposure of the optical
system, 100 billion to 400 billion laser pulses, it is important to understand how
deep-UV laser radiation influences these lens materials.
Lens materials are expected to withstand about 100 billion to 400 billion laser pulses
to be cost-effective. By exposing fused silica samples at 2000 Hz, automated laser-exposure
test benches help quantify laser-induced absorption, wavefront distortion and birefringence.
Laser-induced absorption is well-understood, and
quantitative models exist to forecast the long-term performance of fused silica
materials when exposed to KrF and ArF excimer-laser radiation. Research has improved
the understanding of excimer-laser-induced density and refractive index changes
in fused silica.
Photo-induced breaking and forming
of molecular bonds is at the root of all laser damage in fused silica. As molecular
bonds change, so do both the structural arrangement and the chemical composition
of the glass. Understanding this collective reaction becomes critical as lithography
tools begin to employ shorter wavelengths, because the energy at these wavelengths
increases the effects of laser radiation on the lens material. Primary examples
High-purity fused silica lenses represent the majority of optical
elements in both the illumination and projection systems in lithography steppers.
• Increased optical absorption
caused by color-center formation.
• A change in the optical index
of refraction resulting from variations in the chemical composition.
• The change in density of the
glass caused by structural rearrangement and alteration of its chemical composition.
Any density change in the material
alters the index of refraction and introduces stress-induced birefringence and surface
deformation of the exposed glass. Laser-induced optical absorption leads not only
to reduced optical transmission, but also to lens heating. This promotes additional
index changes and deformation of the lens elements.
Research at Corning Inc. has led to
the development of predictive models for the performance of fused silica lens material
after long-term exposure to deep-UV radiation. Although these models address exposure
trends for a specific grade of fused silica glass, they are representative of the
response of fused silica materials in general.
Exposure of fused silica to ArF and KrF laser
radiation gives rise to the formation of color centers, specifically E' centers
(undercoordinated silicon atom, ≡Si•) and nonbridging oxygen hole centers
(NBOHC, ≡Si-O•).1 Both types are created by ArF and KrF laser light, but,
because the absorption band of nonbridging oxygen hole centers (peaking at 260 nm)
does not extend to 193 nm, only E' centers (peaking at 215 nm) are relevant for
laser-induced absorption at the ArF wavelength (193 nm). The laser-induced absorption
at the KrF wavelength (248 nm), on the other hand, mostly results from formation
of nonbridging oxygen hole centers.
The models account for the following
reactions and processes:2 Absorbed laser radiation leads to the formation of excitons
in the glass. Although most of these excitons simply decay — returning the
material to its ground state — others become trapped in localized states
called precursors. Each precursor decays to form an E' center, as well as a nonbridging
oxygen hole color center, both of which react with the molecular hydrogen in the
glass to form SiH and SiOH, respectively.
During exposure of the glass, equilibrium
develops between the two types of color centers on the one side and SiH and SiOH
on the other. The color centers again react with hydrogen to form SiH and SiOH,
which are convertible to color centers and hydrogen in a photo-induced reaction
(photolysis). The relative number of color centers and SiH and SiOH, and thus the
amount of absorption at 193 nm and 248 nm, is fluence-dependent because of the photolysis.
Eventually, after sufficiently long
exposure, all precursors that were initially present in the glass react to form
color centers, and the induced absorption levels off to a steady state. Because
one of the reactions in the equilibrium is photolysis, the relative number of color
centers — and thus the optical absorption in the glass — depends on
the fluence to which the glass is exposed, even after the steady-state stage has
At the very low fluence levels used
in projection lenses, typically 0.1 mJ/cm2/pulse or less, the number of laser pulses
required to reach steady-state induced absorption in the fused silica studied can
reach from tens to hundreds of billions. Some lens elements exposed to the lowest
fluence levels in a projection lens, however, may never reach steady-state induced
absorption within the expected lifetime of the equipment.
The Corning model parameters (rate
constants) lead to agreement between model calculations and experimental data for
the glass for both ArF and KrF wavelengths, and over a wide range of laser fluences
and laser pulse frequencies.3 Although the parameter set is specific to the glass
type studied, the models are general. Still, it is likely that the parameters could
differ in other fused silica material because of contrasts in the manufacturing
A number of reactions in the glass
contribute to marathon laser-induced absorption, although only two are photo-induced.
Except for those, the same rate constants are used to calculate induced absorption
at both wavelengths. These results support the validity of the model because both
types of color centers contributing to induced absorption at these wavelengths originate
from the same precursor.
Two types of density changes occur in fused silica
when it is exposed to deep-UV laser radiation. The first effect, generally known
as densification, or compaction, is an increase in material density.4 The second,
recently discovered during a laser damage study conducted by Sematech International
at Cymer Inc. in San Diego, is expansion, or rarefaction, which signifies a decrease
Compared with compaction, expansion
is significant only at very low fluences, which is why it was overlooked in earlier
laser damage studies that applied higher fluences. The effect is thought to be the
result of a radiation-induced process that forms β-hydroxyl (SiOH) in the
glass.6 This process requires hydrogen; so, aside from laser fluence, hydrogen content
is one of the key factors determining the amount of expansion in fused silica.
Compaction and expansion occur simultaneously
in an exposed piece of glass, but the dominance of one or the other depends on glass
parameters and exposure conditions. Nevertheless, both affect lens performance.
For example, a change in density alters the index of refraction, induces surface
deformation of the lens and incurs stress-induced birefringence. The magnitude of
density effects is a function of the geometry of the glass element and the exposure
pattern on the glass because unexposed glass surrounding the exposure region reduces
the ability of the exposed glass to densify or expand.
The material property used to study
density changes and to compare experiments is called the unconstrained density change.
It is the change that one would observe in the absence of any constraining material
surrounding the exposure region. The parameter can be calculated from the measured
density change by using a conversion factor determined through finite-element analysis
and by considering the elastic properties of the glass.
As described above, β-hydroxyl
formation decreases the density and changes the chemical composition of the material,
altering its index of refraction. However, these effects counter one another. Lower
density decreases the index of refraction, whereas the β-hydroxyl formation
increases it via a photorefractive effect.
In recent studies, this photorefractive
effect became apparent, with contradictory results found in samples exposed under
certain conditions. Specifically, birefringence measurements indicate a net decrease
in density. Interferometric measurements of wavefront distortion in the same samples,
however, show a net increase of the exposed material’s optical path length,
which, in the absence of any other effects, indicate a density increase. This apparent
contradiction can be understood only if there is a positive index change not associated
with a density change, such as with a photorefractive effect. Only compaction and
expansion contribute to the measured laser-induced birefringence. But three effects
— compaction, expansion and photorefraction — contribute to the measured
laser-induced wavefront distortion.
When examining net wavefront distortion in typical fused silica as measured interferometrically,
expansion is dominant at very low fluence, and the wavefront inside the damage region
is advanced. At high fluence, compaction and photorefractive effect are dominant,
and the wavefront inside the damage region is retarded.
The distinction between expansion and
the photorefractive effect is important because photorefraction is not caused by
a density change and thus, unlike compaction and expansion, is not subject to the
constraint of surrounding material. Therefore, without knowledge of the magnitude
of the photorefractive effect, calculating any unconstrained density change from
wavefront distortion measurements is not possible.
One technique for to separating density
changes from the photorefractive effect is to measure the laser-induced birefringence
in and around the exposure region. This birefringence is stress-induced; that is,
the result of compaction and/or expansion, but not of the photorefractive effect.
So the magnitude of induced birefringence, considering the same constraint from
surrounding material as previously mentioned, can help determine the amount of unconstrained
density change. Moreover, the pattern of the direction of the slow or fast axis
of birefringence can help determine whether the net density change of the exposed
material is positive or negative.
Understanding the type and magnitude
of such laser damage effects in fused silica can facilitate development of material
grades tailored for optimal performance in microlithography applications. A lens
designer may be able to reduce the impact of these effects on lens performance.
It is even conceivable that some of these effects could be made to counteract each
1. T.E. Tsai, D.L. Griscom, E.J. Friebele (1988).
PHYS. REV. LETT., 61:444.
2. R.J. Araujo, N.F. Borrelli, C.M.
Smith (1998). Proc. SPIE, 3424:2.
3. J. Moll (2001), Proc. SPIE,
4. N.F. Borrelli et al (1997). JOURNAL
OF THE OPTICAL SOCIETY OF AMERICA, B 14:1606.
5. C.K. van Peski, R. Morton, Z. Bor
(2000). JOURNAL OF NON-CRYST. SOLIDS, 265:285.
6. C.M. Smith et al (2001). APPL.
PHYS. LETT., 78:2452.
Meet the author
Johannes Moll is a senior development scientist
at the Sullivan Park Research and Development facility of Corning Inc. in Corning,
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