It has become clear that although line edge roughness and resolution are important for future lithographic technology
nodes, other issues such as pattern collapse must be addressed as well. One of the primary modes of pattern collapse is
pattern collapse caused by loss of adhesion of the resist from the substrate. The main forces which govern pattern
collapse by adhesion failure are related to substrate/resist interactions. Though several methodologies to improve pattern
collapse have been investigated, such as the use of surfactants during the final rinse, the use of such methods virtually all
suffer from some serious drawback. To this end, we have developed a reactive surface modifier capable of covalently
attaching to a positive tone resists containing hydroxystyrene groups. A vinyl-ether-modified silane was prepared and
effectively applied using a solution silanization reaction. A hydroxystyrene-based positive tone copolymer resist was
applied and subjected to a post apply bake to allow sufficient time for reaction with the surface modifier to occur prior to
patterning using e-beam lithography. Ultimately, it was determined that covalent attachment of the surface modifier to
the photoresist during the post apply bake resulted in enhanced resist/substrate adhesion of photoresist lines as evidenced
by improved pattern collapse performance in high resolution imaging experiments.
As integrated circuit fabrication continues to advance towards the 32 nm node and below, it has become increasingly
apparent that the use of ultrathin films and polymer features will be required. Though it has been widely accepted that
the properties of polymers on the nanoscale can differ significantly from their bulk counterparts, the extent of such
deviation is the subject of much debate and concern. Furthermore, most studies have focused on elucidating the
differences in the thermal properties of micro- and nano-scale polymer films as determining the mechanical properties of
ultrathin films can be somewhat cumbersome. In order to study the modulus of polymer thin films we have implemented
a thin film buckling technique wherein a polymer film is floated onto a pre-strained PDMS substrate. Release of the
strain, results in the buckling of the polymer film and provides the opportunity to accurately determine the modulus of
polymer thin films with thicknesses down to 20 nm. This thin film buckling strategy was also used to probe the effect of
thickness on the modulus of the ESCAP-1 thin films. Finally, a reactive rinse method was employed whereby the hydroxyl functional groups of the resist were cross-linked via a dicarboxylic acid using carbodiimide chemistry as a potential method to ultimately enhance lithographic patterning performance. The effect of the reactive rinse on the modulus of the ESCAP-1 thin films was analyzed and it was found that the application of the reactive rinse resulted in a clear increase in the modulus of the polymer films. Also, quartz crystal microbalance (QCM) supporting the confinement of the crosslinking agents to the surface will be discussed.
Two different types of non-traditional molecular resists were synthesized and characterized. A positive-tone cross-linked
molecular resist was made that functions by first forming an etch resistant film via thermal cross-linking of vinyl ether
functionalized small molecules followed by patterning of the film via acid catalyzed cleavage of the resulting acetal
bonds. DPA-2VE, a single multi-functional molecular resist of this type, showed DUV sensitivity of 7 mJ/cm2 and a
contrast of 5.2 for development in either organic solvent or aqueous base. Using high resolution patterning with a 100
keV e-beam, it was possible to demonstrate feature resolutions down to 40 nm. When 0.26N TMAH was used as a
developer, the dose-to-size was 84 μC/cm2 with a 3σ LER of 14.2 nm. Using MIBK as a developer, the dose-to-size was 104 μC/cm2 and the 3σ LER was 7.4 nm. A series of non-chemically amplified molecular resists based on using 2-
nitrobenzyl ethers as photosensitive protecting groups were also made. One formulation showed a DUV sensitivity of 1 mJ/cm2, while another formulation which showed the best contrast of 8.3 obtained at a sensitivity of 10 mJ/cm2.
However, under 100 keV e-beam patterning, the 2-nitrobenzyl ether protected materials showed little to no response even up to 3000 μC/cm2.
Pattern collapse is becoming a critical issue as integrated circuit fabrication continues to advance towards the 32 nm
node and below. Though line edge roughness and resolution are certainly important in moving forward, pattern collapse
by both deformation and adhesion failure must be addressed. In this work, a post-development strategy to reduce pattern
collapse by bending was developed whereby the hydroxyl functional groups on the surface of the resist were crosslinked
via a dicarboxylic acid using carbodiimide chemistry. The pattern collapse of a hydroxystyrene-based, positive tone
resist was then studied before and after the application of the reactive rinse. SEM analysis of the samples showed that
application of the reactive rinse resulted in a clear increase in the printing capabilities of the resist, as the photoresist
lines could be printed with smaller space widths corresponding to higher stresses after the rinse treatment.
KEYWORDS: Molecules, Electron beam lithography, Deep ultraviolet, Line edge roughness, Polymers, Polymerization, Glasses, Dewetting, Optical lithography, Scanning electron microscopy
A series of negative tone molecular resists was investigated for use in both organic solvent and aqueous base
development. Molecular resists designed purely for solvent development showed half-pitch resolution down to 25 nm
with sensitivities of 50 μC/cm2 and LER (3σ) down to 2.3 nm. Aqueous developable designs that used epoxide
functionalized molecules that are intrinsically water soluble showed improved contrast and comparable sensitivity, but
suffered from significant dewetting during baking due to their low molecular weight and high polarity. This inability to
form high quality films prevented their use as high resolution resists. Aqueous developable designs that used molecules
with both cross-linking and base solubilizing groups were also investigated; the initial example of this design is DPA-
2Ep, a molecular resist containing two epoxides and one carboxylic acid per molecule. It formed high quality films and
showed improved contrast compared to the purely solvent developed designs. Even after complete cross-linking of the
epoxide groups, several free carboxylic acids still remained in the network. These free acids tend to imbibe developer and appear to retain the tetramethylammonium carboxylates even after rinsing and drying the film. This imbibing of developer leads to significant failure during high resolution patterning due to swelling.
A series of nonionic photoacid generators (PAGs) are synthesized and their acid generation efficiency measured under deep ultraviolet (DUV) and electron beam exposures. The acid generation efficiency is determined with an on-wafer method that uses spectroscopic ellipsometry to measure the absorbance of an acid sensitive dye (Coumarin 6). Under DUV exposures, common ionic onium salt PAGs show excellent photoacid generation efficiency, superior to most nonionic PAGs tested in this work. In contrast, when under 100-keV high energy e-beam exposures, almost all of the nonionic PAGs show significantly better acid generation performance than the ionic onium salt PAGs tested. In particular, one nonionic PAG shows almost an order of magnitude improvement in the Dill C acid generation rate constant compared to a triarylsulfonium PAG. The high energy acid generation efficiency is found to correlate well with the electron affinity of the PAGs, suggesting that improvements in PAG design can be predicted. Nonionic PAGs merit further investigation as a means for producing higher sensitivity resists under high energy exposure sources.
A series of non-ionic PAGs were synthesized and their acid generation efficiency measured under deep ultraviolet and
electron beam exposures. The acid generation efficiency was determined with an on-wafer method that uses
spectroscopic ellipsometry to measure the absorbance of an acid sensitive dye (Coumarin 6). Under DUV exposures,
common ionic onium salt PAGs showed excellent photoacid generation efficiency, superior to most non-ionic PAGS
tested in this work. In contrast, under 100 keV high energy e-beam exposures, almost all of the non-ionic PAGs showed
significantly better acid generation performance than the ionic onium salt PAGs tested. In particular, one non-ionic PAG
showed almost an order of magnitude improvement in the Dill C acid generation rate constant as compared to a
triarylsulfonium PAG. The high energy acid generation efficiency was found to correlate well with the electron affinity
of the PAGs, suggesting that improvements in PAG design can be predicted. Non-ionic PAGs merit further
investigation as a means for producing higher sensitivity resists under high energy exposure sources.
Chemically amplified resists have served as high resolution and high photospeed patterning materials in the fabrication of modern microelectronic devices for more than two decades. A
significant amount of research during that time, and in particular more recently, has focused on minimizing line width roughness and on improving the achievable resolution and sensitivity of resist
materials. While these lithographic parameters are certainly important, the distortion of the resist pattern during wet processing and subsequent drying can have significant negative impacts on performance and is often relatively overlooked as a major resist resolution and performance limiter. Resist pattern distortion after development and during drying is mainly due to the unbalanced
capillary forces created due to pattern asymmetries which give rise to variations in liquid meniscus radii of curvature as the final rinse liquid is dried from the pattern. These capillary forces are
dependent upon the surface tension of the final rinsing solvent, the contact angle of the rinse liquid with the side wall of the resist line, and the pattern space widths and sidewall angles. The demand
for resist films with smaller feature sizes has led to a reduction in resist pattern dimensions resulting in overall poor mechanical strength and a decrease in the adhesion forces at the resist line/substrate interface. In this work, the pattern collapse behavior of a hydroxystyrene-based resist copolymer is studied. Ultra-thin film effects and the role of the feature width of the resist line on pattern collapse are also investigated.
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