Despite years of research and development, the fundamental processes of photoionization, secondary electron generation, recombination, diffusion, and resist switching are poorly understood at the atomic level for EUVL. Multiscale modeling of these physical and chemical processes can provide answers to questions that are difficult or impossible to answer with experiment alone. A modeling pipeline that includes Monte Carlo modeling of photon- and electron-matter interactions, along with density functional theory calculations of chemical switching will be introduced in this proceeding. The Hf4O2(OMc)12 nanocluster resist will be presented as a case study. Photon and secondary electron yields, electron energy and spatial distributions, and a quantum chemical pathway for negative tone switching will be presented. Fundamental learning from studies like this can be used to improve resist design including improving contrast of these materials.
As lithography tools continue their progress in both numerical aperture and wavelength in pursuit of Moore’s law, we have reached the point where the number of features printed in a single pass can now easily surpass one trillion. Statistically, one should not be surprised to see some members of such a population exhibit fluctuations as great as 7σ. But what do these fluctuations look like? We consider the problem in terms of variations in the effective local resist sensitivity caused by feature-to-feature differences in absorbed photons and resist component counts, modeling these as a normal distribution. As the CD versus dose curve is generally nonlinear over large ranges, the normal distribution of the local effective sensitivity then maps to a nonnormal distribution in CD. For the case of individual vias printed near the resolution limit, it results in many more undersized or completely closed vias than one would expect from a normal distribution of the CDs. We show examples of this behavior from both EUV exposures in the fab and ebeam exposures in the lab.
As lithography tools continue their progress in both NA and wavelength in pursuit of Moore’s law, we have reached the point where the number of features printed in a single pass can now easily surpass 1 trillion. Statistically, then, one should not be surprised to see some members of such a population exhibit fluctuations as great as 7σ. But what do these fluctuations look like? We consider the problem in terms of variations in the effective local resist sensitivity caused by feature-to-feature differences in absorbed photons and resist component counts. We model such variations as a normal distribution, rather than the CDs themselves. As the CD vs. Dose curve is generally nonlinear over large ranges, the normal distribution of the local effective sensitivity then maps to a non-normal distribution in CD. For the case of individual vias printed near the resolution limit, this results in many more such undersized or completely closed vias than one would expect from a normal distribution of the CDs. We show examples of this behavior from both EUV exposures in the fab, and ebeam exposures in the lab. For the latter, results from a simple resist with a somewhat low quencher loading produce approximately the CD variation one would expect by modeling the quencher as distributed normally in the resist.
Extreme ultraviolet lithography (EUVL) technology continues to progress and remains a viable candidate for next generation lithography1, which drives the need for EUV resists capable of high resolution with high sensitivity and low LWR. While chemically amplified resists (CARs) have demonstrated the ability to pattern 12nm half-pitch features2, pattern collapse continues to limit their ultimate resolution. We have taken multiple approaches to extend resist capabilities past these limits. Recent results in pattern collapse mitigation using a resist encapsulation and etch back strategy will be discussed. We continue to investigate EUV patterning of semi-inorganic resists to simultaneously increase EUV photon absorption and extend mechanical strength beyond CAR capabilities. The limitations of metal oxide-based nanoparticle photoresists have been investigated, and have provided key insights to further understanding the mechanism of this class of materials.
Recently, both PSI1 and ASML2 illustrated champion EUVL resolution using slow, non-chemically amplified inorganic resists. However, the requirements for EUVL manufacturing require simultaneous delivery of high resolution, good
sensitivity, and low line edge/width roughness (LER/LWR) on commercial grade hardware. As a result, we believe that
new classes of materials should be explored and understood. This paper focuses on our efforts to assess metal oxide based
nanoparticles as novel EUV resists3. Various spectroscopic techniques were used to probe the patterning
mechanism of these materials. EUV exposure data is presented to investigate the feasibility of employing inorganic
materials as viable EUV resists.
DUV, EUV and e-beam patterning of hybrid nanoparticle photoresists have been reported previously by Ober and coworkers. The present work explores the underlying mechanism that is responsible for the dual tone patterning capability of these photoresist materials. Spectroscopic results correlated with mass loss and dissolution studies suggest a ligand exchange mechanism responsible for altering the solubility between the exposed and unexposed regions.
Relative ligand binding energies were determined for a series of common ligand types with hafnium
oxide nanoparticles, and from these results a series of novel strong binding ligands were developed. The
relative equilibrium concentrations of two competing ligands bound to the nanoparticles were measured using
nuclear magnetic resonance spectroscopy (NMR). For each ligand type, equilibrium constants and relative
binding energies were then calculated and compared. Methane sulfonic acid was found to have the strongest
binding energy, 2.0 Kcal/mol stronger than acetic acid. A group of three sulfonate ligands capable of freeradical
crosslinking were made, along with three sulfonate ligands capable of creating aqueous developable
nanoparticles. One of these ligands resulted in insoluble nanoparticles, however, the other two ligands
resulted in nanoparticles that coated well on a silicon substrate and had dissolution rates greater than 100 nm
per second.
Performance requirements for EUV resists will necessitate the development of entirely new resist platforms. As outlined
in the ITRS, the new resists for EUVL must show high etch resistance (to enable pattern transfer using thinner films),
improved LER and high sensitivity. A challenge in designing these new resists is the selection of molecular structures
that will demonstrate superior characteristics in imaging and etch performance while maintaining minimal absorbance at
EUV wavelengths. We have previously described the use of inorganic photoresists in 193 nm and e-beam lithography.
These inorganic photoresists are made of HfO2 nanoparticles and have shown etch resistance that is 25 times higher than
polymer resists. The high etch resistance of these materials allow the processing of very thin films (< 40 nm) and will
push the resolution limits below 20 nm without pattern collapse. Additionally, the small size of the nanoparticles (< 5
nm) leads to low LER while the absorbance at EUV wavelengths is low. In this presentation we show that these
inorganic resists can be applied to EUV lithography. We have successfully achieved high resolution patterning (<30 nm)
with very high sensitivity and low LER.
Chemically amplified resists (CARs) are the current workhorse for photolithography, where higher
resolution and smaller feature size represent a continual driving force for the semiconductor industry. As
the feature size decreases to sub-30 nm, LWR and gate critical dimension (CD) control become serious
concerns. In order to reach the goals in the ITRS, an unprecedented level of control of photoacid diffusion
while maintaining the high resist sensitivity and resolution during image formation is required. CARs
require a post exposure bake (PEB), typically performed on a hot plate at 90-150°C for 30-120 seconds, to
complete the resist deprotection after photoacid generation. This bake step is a primary influence on resist
performance as the time/temperature profile controls both the diffusion of photogenerated acids and the
deprotection of the resist backbone. Sufficient time must be provided to achieve the level of deprotection
required for the solubility switching in a developer, but the seconds timeframe of conventional hot plate
PEB leads to an undesirable amount of acid diffusion. As long as the activation energy of diffusion is less
than that for deprotection, higher temperatures for optimized time durations will result in reduced diffusion.
However, traditional hot plate PEB cannot access times shorter than a few seconds. We utilize a laser (CO2)
based scanned heating system to achieve sub-millisecond to milliseconds in heating durations with
temperatures up to the thermal decomposition limit of the resist. This research is aimed at using synthetic
techniques to vary the structure of the photoacid generator (PAG) in order to learn about the role of PAG
size and structure on acid diffusion during sub-millisecond heating. A variety of PAGs with different anion
sizes have been synthesized and tested on the CO2 laser system, and their lithographic performance and
effect on acid diffusion has been studied.
The chemical waste generated in today's microelectronic fabrication process can be released into the environment and
cause environmental and health concerns. It is therefore necessary to develop an environmentally friendly process that
can eliminate the use of toxic chemical solvents. Silicone fluids are linear methyl siloxanes that only contain carbon,
hydrogen, oxygen and silicon. They are low in toxicity, not ozone-depleting and contribute little to global warming.
They degrade into naturally occurring compounds instead of accumulating in the atmosphere and can be recycled. Their
unique physical and chemical properties have also made them promising developers for lithography. For example, their
low surface tension can eliminate pattern collapse problems associated with high aspect-ratio features. Silicone fluids are
non-polar solvents and their solvent strength is weaker than that of saturated hydrocarbons but stronger than that of the
commercially available saturated hydrofluorocarbons and may be enhanced by adding other solvents. Two conventional
photoresists used in this study, PBOCST and ESCAP are both insoluble in silicone fluids before or after exposure.
However, the solubility of PBOCST and ESCAP in silicone fluids can be increased by using a silicon-containing
additive. In this paper, we demonstrate this novel and environmentally friendly development of conventional
photoresists in silicone fluids.
We describe the development of new triphenylsulfonium photoacid generators (TPS PAGs) with semifluorinated
sulfonate anions containing glucose or other natural product groups, and their successful application to patterning sub-100 nm features using 254 nm and e-beam lithography. The TPS PAGs with functionalized octafluoro-3-oxapentanesulfonate were synthesized efficiently in high purity and high yield by utilizing simple and unique chemistries
on 5-iodooctafluoro-3-oxapentanesulfonyl fluoride. The PAGs has been fully evaluated in terms of chemical properties,
lithographic performance, environmental friendliness or toxicological impact. The PAGs are non-toxic and it is
susceptible to chemical degradation and to microbial attack under aerobic/anaerobic conditions. These new PAGs are
very attractive materials for high resolution photoresist applications and they are particularly useful in addressing the
environmental concerns caused by PFOS and other perfluoroalkyl surfactants.
We have developed a transparent, high refractive index inorganic photoresist with significantly higher etch resistance
than even the most robust polymeric resist. As feature sizes continue to decrease, film thickness must be reduced in
order to prevent pattern collapse. Normally thinner films prevent sufficient pattern transfer during the etch process,
creating the need for a hardmask, thus increasing production cost. Compared to PHOST, we have shown over 10 times
better etch resistance. Organic photo-crosslinkable ligands have been attached to a hafnium oxide nanoparticle core to
create an imageable photoresist. This resist has shown superior resolution with both E-beam and 193 nm lithography,
producing sub-50 nm patterns. In addition to improved etch resistance, the inorganic photoresist exhibits a high
refractive index, increasing the depth of focus (DOF). The nanoparticle size of ~ 1-2 nm has the potential to reduce line
edge roughness (LER).
Solvent development of chemically amplified (CA), negative tone photoresists depends on several factors including
molecular weight of the photoresist, the strength of polymer-solvent interactions, and the strength of polymer-polymer
interactions in the undeveloped regions. Absent are the ionic interactions present in the aqueous base development of CA
resists that greatly aids dissolution and image contrast. In its place, strong hydrogen bonding of the exposed photoresist
leads to effective resistance to dissolution in non-polar developers. These effects are discussed in the context of Flory-
Huggins theory. As part of a study of low environmental impact developers several, non-polar solvents have been
investigated with negative tone, chemically amplified photoresists. These include supercritical CO2, hydrofluoroethers
and silicone fluids. Each of these solvents has low surface energy, unique dissolution characteristics and is capable of
developing sub-50 nm patterns. Performance aspects of these developers will be described.
As the semiconductor industry moves forward, resolution limits are being pushed to the sub-30 nm regime. In order to
meet these demands, radical new resist design and processes must be explored. We have developed a molecular glass
system for all-dry processing conditions. Physical vapor deposition (PVD) has been used for film formation onto silicon
wafers. PVD deposits a uniform film of controlled thickness free from impurities that are often introduced by casting
solvents used in traditional spin coating methods. Thermal development is used as an alternative to processing in
solvents in order to prevent resist swelling and pattern collapse by capillary forces. The deposited molecule is designed
to crosslink upon E-beam irradiation without additives, and therefore form a homogeneous, single component film.
PAG-attached molecular glasses have been synthesized in order to promote film homogeneity as well. By tethering PAG
directly to the molecular glass core, issues such as PAG aggregation can be remedied. Acid migration, which increases
blur and LER, can also be hindered.
The trend of ever decreasing feature sizes in subsequent lithography generations is paralleled by the need to reduce resist
thickness to prevent pattern collapse. Thinner films limit the ability to transfer the pattern to the substrate during etch
steps, obviating the need for a hardmask layer and thus increasing processing costs. For the 22 nm node, the critical
aspect ratio will be less than 2:1, meaning 40-45 nm thick resists will be commonplace. To address this problem, we
have developed new inorganic nanocomposite photoresists with significantly higher etch resistance than the usual
polymer-based photoresists. Hafnium oxide nanoparticles are used as a core to build the inorganic nanocomposite into an
imageable photoresist. During the sol-gel processing of nanoparticles, a variety of organic ligands can be used to control
the surface chemistry of the final product. The different ligands on the surface of the nanoparticles give them unique
properties, allowing these films to act as positive or negative tone photoresists for 193 nm or electron beam lithography.
The development of such an inorganic resist can provide several advantages to conventional chemically amplified resist
(CAR) systems. Beyond the etch resistance of the material, several other advantages exist, including improved depth of
focus (DOF) and reduced line edge roughness (LER). This work will show etch data on a material that is ~3 times more
etch-resistant than a PHOST standard. The refractive index of the resist at 193 nm is about 2.0, significantly improving
the DOF. Imaging data, including cross-sections, will be shown for 60 nm lines/spaces (l/s) for 193 nm and e-beam
lithography. Further, images and physical characteristics of the materials will be provided in both positive and negative
tones for 193 nm and e-beam lithography.
The semiconductor industry is pushing the limits of resolution to sub-30nm through the extension of 193nm lithography
as well as next generation techniques such as EUV lithography. Molecular glass photoresists may provide enhanced
resolution and performance advantages compared to traditional polymeric resists. These organic compounds have a low
molecular weight but still display high glass transition temperatures (Tgs). Enhanced design aspects are employed to
give beneficial resist properties such as transparency, high Tg and etch resistance. Asymmetrical, rigid structures are
used to create amorphous structures with high Tg molecular glasses, such as branched structures and carborane inclusion
complexes. Alicyclic cyclodextrin ring compounds have also been employed for 193nm lithography. Unconventional
atoms such as boron have been incorporated to increase etch resistance while supercritical CO2 was employed as an
environmentally friendly solvent free developer. Exploring structural aspects and their effect on resist performance is
important in the design of new molecules for next generation lithography and will be discussed.
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