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
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.
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).
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.
In immersion lithography, high index fluids are used to increase the numerical aperture (NA) of the imaging system and
decrease the minimum printable feature size. Water has been used in first generation immersion lithography at 193 nm to
reach the 45 nm node, but to reach the 38 and 32 nm nodes, fluids and resists with a higher index than water are needed.
A critical issue hindering the implementation of 193i at the 32 nm node is the availability of high refractive index (n >
1.8) and low optical absorption fluids and resists. It is critical to note that high index resists are necessary only when a
high refractive index fluid is in use. High index resist improves the depth of focus (DOF) even without high index fluids.
In this study, high refractive index nanoparticles have been synthesized and introduced into a resist matrix to increase the
overall refractive index. The strategy followed is to synthesize PGMEA-soluble nanoparticles and then disperse them
into a 193 nm resist. High index nanoparticles 1-2 nm in diameter were synthesized by a combination of hydrolysis and
sol-gel methods. A ligand exchange method was used, allowing the surface of the nanoparticles to be modified with
photoresist-friendly moieties to help them disperse uniformly in the resist matrix. The refractive index and ultraviolet
absorbance were measured to evaluate the quality of next generation immersion lithography resist materials.
A critical issue preventing the implementation of 193nm immersion lithography (193i) to the 32nm node is the
availability of high refractive index (n > 1.8) and low optical absorption fluids. To overcome these issues, we have
synthesized high refractive index nanoparticles and introduced them into the immersion fluid to increase the refractive
index. Hydrolysis and sol-gel methods have been implemented to grow high refractive index nanoparticles with diameters of 3-4nm. Depending on the synthetic route, it is possible to produce stable suspensions of nanoparticles in either aqueous or organic solvents, making it possible to synthesize a stable high-index immersion fluid.