line-edge roughness, topcoats among key challenges facing immersion photoresist
lithography has emerged as the industry's imaging technology of choice
for staying on the 45- and 32-nm process-node roadmaps. Multiple research
programs in universities on three continents as well as projects at TSMC,
IBM, IMEC, Sematech, and the equipment and materials companies have propelled
the wet lithography proposition forward as aggressively as any technological
solution the industry has ever seen. One key to the ultimate success of
immersion is the development of robust, high-performance photoresists.
the early prospects for immersion resists look quite good, many issues
remain unresolved. The leaching of resist components during their first
contact with water, the refinement of protective topcoats, and line-edge
roughness and other defect sources are among the concerns facing resist
developers and users. This issue's Hot Button panel of experts shed some
light on many of the challenges surrounding the development and implementation
of advanced photoresists for 193-nm ArF immersion lithography.
CONLEY (advanced lithography group, ARDL, Dan Noble Center, Freescale
Semiconductor): Argon-fluoride (ArF) liquid immersion lithography
(IML) has the potential to extend optical lithography to meet the needs
of the 32-nm semiconductor device nodes and beyond. Water is the immersion
fluid of primary interest because of its low intrinsic absorption at 193
nm. Research to date has established the feasibility of the technology,
with no critical showstopper technical barriers.
next phases of R&D will require the availability of prototype exposure
tools operating at the 193-nm wavelength of an ArF excimer laser. Such
tools will allow for the initial exploration into such issues as photoresist
and materials interaction. Early efforts are attempting to understand
the effects of resist components leaching, water absorption, and general
image quality of current resist formulations. Other important issues under
investigation include polarization, high-numerical-aperture (NA) effects,
and general imaging requirements. The use of interferometric imaging has
been a useful tool for understanding materials and optics issues.
leaching of resist components could contaminate and even damage
the lens element that comes in contact with the immersion fluid.
the early prospects look very good, many IML-related resist issues are
yet to be resolved. One issue is the leaching of the resist components
during the initial contact with water. Researchers are developing analytical
techniques from simple water extraction to complex micro-pH measurements.
Water-soluble photoproducts such as acids can easily leach out of the
surface of the film. Other resist additives, such as quenchers or contrast-enhancement
materials, can also leach out of the film.
researchers are paying particular attention to these issues because of
their impact on the imaging system. The leaching of resist components
could contaminate and even damage the lens element that comes in contact
with the immersion fluid. Recent work focused on the measurement of "leached"
products indicates that the photoacid generator and base quencher can
be engineered to minimize or possibly even eliminate leaching.
resist manufacturers have introduced base-developable topcoats, which
act as a protective layer during the immersion imaging process. The topcoat
material is spin-applied on the resist layer and soft baked. The protective
overcoat and resist do not intermix and the protective coat is insoluble
in water. Once the wafer is exposed and postexposure baked, the protective
coat is removed during the develop process and is designed not to affect
remain the first and foremost IML issue. A number of full-field exposure
tools are being built, and several have already been delivered. As the
first full-field systems are installed, a deeper understanding of the
impact on device yields will develop. Until then, many aspects can be
addressed without a full-field system. Over the past year, the first >0.8-NA
ArF dry systems have become available for sub-90-nm process development
and manufacturing. These processes will be used for comparison of yield-contributing
issues as well as for evaluation of the technology's general usefulness.
outlook for immersion lithography is extremely promising. The technology
is attractive because much of the infrastructure already exists, and the
existing materials and tools have great potential for extendability. The
progression of tool and material technology continues at a frantic pace.
Tool manufacturers are designing >1.0 NA systems for availability in
the 2006 time frame. Many in the industry are already proposing further
extensions of ArF immersion with higher NAs (>1.2) and new higher-index
immersion fluids and resists for the further extension of optical lithography.
RONSE (director, lithography department, IMEC): The removal of
157-nm (F2) lithography from most of the industry's roadmaps
and its replacement by 193-nm (ArF) immersion lithography has certainly
caused relief among the photoresist suppliers, who did not know how to
meet the requirements put forward in the International Technology
Roadmap for Semiconductors (ITRS). Furthermore, the resist
companies knew even less about whether they would ever get any returns
on their 157-nm investments.
consensus is growing among the industry that chemically amplified resists,
which have become the mainstream products since the introduction of 248-nm
(krypton fluoride, or KrF) lithography, start to face some serious challenges
in further scaling down the dimensions toward the 45-nm technology node
and beyond. Chemically amplified resists were introduced to meet the required
sensitivity that ensured optical lithography was still cost-effective
when 365-nm (i-line) lithography reached its limits. Especially for extreme
ultraviolet (EUV) lithography (13.5-nm wavelength), where the source power
may go down to protect the lifetime of the optics, it is important that
this high sensitivity (5 mJ/cm2) is kept , although ArF lithography
cannot afford a significant reduction of sensitivity either.
chemically amplified resists work on the following principle: acid, generated
by the exposure reaction, initiates a catalytic reaction during the subsequent
bake step. During that period, the high to low concentrations of acid
can diffuse through the resist structure and, as such, reduce the contrast
obtained during the exposure step. Typical diffusion lengths are on the
order of 40 nm or higher and do not cause too many worries for 90-nm technologies
and above. However, it is quite clear that such diffusion lengths can
no longer be tolerated for 45-nm and smaller technologies without complete
loss of contrast.
issue is line-edge or line- width roughness (LER or LWR), to which variable
sources contribute, such as resist polymer structure, image contrast,
and diffusion length. The trends in LER reduction do not keep up with
the required LER numbers listed in the ITRS guidelines. On the other hand,
for 90-nm technologies, the electrical performance of the devices does
not seem to suffer too much from LER. It is expected that this will change
for the 45-nm and smaller technologies. Moreover, reducing the diffusion
length and LER are typically conflicting requirements, just like increasing
the sensitivity and reducing LER are generally competing trends.
conclusion, even though lithographers can stick with ArF resist chemistry
for a while, now that ArF immersion is steadily taking off, and EUV lithography
is supposed to be able to use traditional KrF resists as a starting point,
resist development in both cases faces some major challenges. Creative
solutions are required to overcome those challenges, which are more wavelength
related than technology node related and thus quite generic in nature.
SLEZAK (technology manager, lithography group, JSR Microelectronics):
As 65-nm-node technologies transition from R&D groups to early production,
we are forced to make tough decisions regarding our choices for the next
technology nodes. The 45-nm node is quickly approaching and our viable
options are narrowing. It was thought that technologies such as 157 nm
and EUV lithography would be ready; however, as we get closer to making
capital investment decisions, it is clear that neither of these technologies
will be ready in time. Immersion lithography is here, and it is time to
get our feet wet.
with high-NA dry lithography, IML systems are becoming more realistic
as we set our sights on solutions for the 45-nm node. However, there are
still many questions. One prevailing question we hear as a resist supplier
involves topcoats. Many customers ask if topcoats are necessary to move
this technology from an R&D state to a production state. The fears
associated with not using a topcoat are not only resist extractables damaging
the lens, but also the degradation of the aerial image caused by changing
the optical properties of the water. By omitting a topcoat, the lens protection
resides solely with the water handling, which worries many tool vendors
results indicate that the use of a topcoat greatly reduces the amount
of low-molecular-weight components, such as photoacid generators or quenchers,
which can leave the resist and enter the water. Along with protecting
the imaging system's lens, other positives associated with using a topcoat
for immersion are the ability to tune surface tension characteristics
between the water and the wafer (contact angle), postexposure delay protection
from airborne contamination, and facilitation of defect-mitigation strategies.
questions associated with immersion lithography deal with the type of
fluid used between the optical lens and wafer. Early work on first-generation
immersion systems using water as the fluid, with an index of refraction
of 1.44, have shown depth of focus (DOF) improvements from 1.5 to 2 times
better than equivalent-NA dry systems. With this same premise in mind,
higher-index fluids will not only improve the DOF relative to dry systems,
but also allow for significant resolution enhancements. Early work at
our company has shown imaging down to 32-nm half-pitch when using a high-refractive-index
fluid—the first imaging below 35-nm half-pitch.
we get our feet wet with immersion, it seems that the positives of topcoat
technology outweigh the process complexity, and the introduction of new
high-index fluid materials should allow the industry to use ArF immersion
below the 45-nm node.
CROWELL (product marketing manager, Clean Track business unit, Tokyo Electron
[TEL]): It is now certain that 193-nm immersion lithography is
the path to the 65- and 45-nm process nodes. Inevitably, resist processes
will reach unprecedented complexity in order to fully realize the potential
of this emerging technology. Some industry experts predict that this technology
is extendable to the 32-nm node.
initial resist processes for IML include some form of topcoating that
serves as a protective barrier between the immersion fluid and the imaging
resist film. The topcoat material has two primary functions: one, to prevent
resist components from leaching out into the immersion fluid that could
degrade pattern resolution; and two, to help protect the bottom scanner
lens element from potentially detrimental resist effluents. Particular
attention must be focused on ensuring that the topcoat process does not
adversely affect the underlying imaging layer. Additionally, removal of
the topcoat prior to development must be complete to ensure minimal impact
to the final resist image. The inclusion of topcoat application and removal
adds process modules to the lithography flow, which affects overall cluster
throughput and cost of ownership.
challenge facing lithographers is line-edge roughness. Although it has
been a component of past technology nodes, LER accounts for an increasingly
larger portion of the overall CD budget at the 65- and 45-nm technology
nodes. To compensate for measurement errors caused by LER, lithographers
must explore increased sample rates or alternate metrology, including
optical digital profilometry and scatterometry. Since there isn't a single
factor that appears to cause LER, incremental relief is likely to come
from several sources within the process and materials or perhaps from
a postimaging treatment.
only will lithographers continue to whittle away at defect densities at
increasingly smaller sizes, but they will also need to attack novel categories
of anomalies. Defects are likely to come from new sources, such as processes
associated with the application and removal of topcoats, interactions
between immersion fluids and wafer surfaces, and the formation of bubbles.
How water bubbles affect resolution locally and form defects remain to
be explored in the early evaluation phases. Complex process and material
interactions could result in unprecedented defect types and levels.
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