Reverse-tone step and flash imprint lithography (S-FIL/R) requires materials that can be spin-coated onto patterned substrates with significant topography and that are highly planarizing. Ideally, these planarizing materials must contain silicon for etch selectivity, be UV or thermally curable, and have low viscosity and low volatility. One such unique material, in particular, a branched and functionalized siloxane (Si-12), is able to adequately satisfy the above requirements. This paper describes a study of the properties of epoxy functionalized Si-12 (epoxy-Si-12) as a planarizing layer. An efficient synthetic route to epoxy-Si-12 was successfully developed, which is suitable and scalable for an industrial process. Epoxy-Si-12 has a high silicon content (30.0%), low viscosity (29 cP at 25°C), and low vapor pressure (0.65 Torr at 25°C). A planarizing study was carried out using epoxy-Si-12 on trench patterned test substrates. The material showed excellent planarizing properties and met the calculated critical degree of planarization (critical DOP), which is a requirement for a successful etch process. An S-FIL/R process using epoxy-Si-12 was demonstrated using an Imprio® 100 (Molecular Imprints Inc., Austin, Texas) imprint tool. The results indicate that epoxy-Si-12 works very well as a planarizing layer for S-FIL/R.
One of the major concerns with nanoimprint lithography is defectivity. One source of process-specific defects is associated with template separation failure. The addition of fluorinated surfactants to the imprint resist is an effective way to improve separation and template lifetime. This study focuses on the development of new reactive fluorinated additives, which function as surfactants and also have the ability to chemically modify the template surface during the imprint process and thereby sustain a low surface energy release layer on the template. Material screening indicated that the silazane functional group is well suited for this role. The new reactive surfactant, di-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silazane (F-silazane) was synthesized and tested for this purpose. The material has sufficient reactivity to functionalize the template surface and acceptable stability (and thus shelf-life) in the imprint formulation. Addition of F-silazane to a standard imprint resist formulation significantly improved template release performance and allowed for significantly longer continuous imprinting than the control formulation. A multiple-imprint study using an Imprio® 100 tool confirmed the effectiveness of this new additive.
One of the major concerns with nanoimprint lithography is defecivity. One source of process specific defects is
associated with template separation failure. The addition of fluorinated surfactants to the imprint resist is an effective
way to improve separation and template lifetime. This study focuses on the development of new reactive fluorinated
additives, which function as surfactants and also have the ability to chemically modify the template surface during the
imprint process and thereby sustain a low surface energy release layer on the template. Material screening indicated that
the silazane functional group is well suited for this role. The new reactive surfactant, di-(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)silazane (F-silazane) was synthesized and tested for this purpose. The material has sufficient
reactivity to functionalize the template surface and acceptable stability (and thus shelf-life) in the imprint formulation.
Addition of F-Silazane to a standard imprint resist formulation significantly improved template release performance and
allowed for significantly longer continuous imprinting than the control formulation. A multiple-imprint study using an
Imprio® 100 tool confirmed the effectiveness of this new additive.
Reverse-tone Step and Flash Imprint Lithography (S-FIL/R) requires materials that can be spin coated onto patterned
substrates with significant topography and that are highly-planarizing. Ideally, these planarizing materials must contain
silicon for etch selectivity, be UV or thermally curable, have low viscosity, and low volatility. One such novel material
in particular, a branched and functionalized siloxane (Si-12), is able to adequately satisfy the above requirements.
This paper describes a study of the properties of epoxy functionalized Si-12 (epoxy-Si-12) as a planarizing layer. An
efficient synthetic route to epoxy-Si-12 was successfully developed, which is suitable and scalable for an industrial
process. Epoxy-Si-12 has a high silicon content (30.0 %), low viscosity (29 cP @ 25 °C), and low vapor pressure (0.65
Torr @ 25 °C). A planarizing study was carried out using epoxy-Si-12 on trench patterned test substrates. The material
showed excellent planarizing properties and met the calculated critical degree of planarization (critical DOP), which is a
requirement for a successful etch process. An S-FIL/R process using epoxy-Si-12 was demonstrated using, an ImprioR
100 (Molecular Imprints Inc., USA) imprint tool. The results indicate that epoxy-Si-12 works very well as a planarizing
layer for S-FIL/R.
The use of conventional thermally cross-linked materials in advanced lithography and nano-imprinting techniques, such
as negative photo resist, anti reflective coatings and planarizing layers, does not guarantee that a high degree of
planarization will be obtained. Additionally, iso-dense thickness biases can create problems by narrowing process
latitudes.
This presentation focuses on the correlation between simulated and experimental analyses and how planarization is
affected. The factors we have identified that influence a material's planarizing capability are; coating spin speed, spin
time and the relationship between the solvent concentration of the material and it's via filling properties. Through
optimization of these factors, an appreciable reduction in via topography was achieved. Based on our results, novel, UV
cross-linkable materials have been developed and optimized for improving planarity in via applications.
The step-and-flash imprint lithography process requires the clean separation of a quartz template from a polymer, and the force required to create this separation must be minimized to prevent the generation of defects. Fluorinated surfactant additives to the imprint fluid address this problem by migrating to the template-polymer interface and forming a local layer with ideal properties for adhesive fracture. Tensile and four-point bend fracture experiments show that surfactants lower the modulus of the imprint polymer and decrease the fracture energy. The fracture energy is further decreased by using a nonreactive, liquid surfactant versus a surfactant that reacts with the polymer matrix. Angle-resolved X-ray photoelectron spectroscopy results indicate that surfactant migration is more effective with a fluorinated surface treatment compared to an untreated quartz surface. This result shows that the use of fluorinated surfactants must be accompanied by a surface treatment that produces a similar energy or polarity to induce migration and lower the adhesive strength.
Reverse-tone step and flash imprint lithography SFIL-R
shows promise as a cost-efficient, high-resolution patterning technique;
however, the generation of satisfactory patterns requires the successful
application of a planarizing topcoat over topography through spincoating.
Photopolymerizable nonvolatile fluids are ideal topcoat materials because
they planarize better than volatile fluids during spincoating and
can continue to level after spincoating. Fluid mechanics analyses indicate
that complete planarization using capillary force is slow. Therefore,
defining the acceptable or critical degree of planarization DOPcrit becomes
necessary. Finite difference simulation of the spincoat and postspin
leveling processes was used to determine the planarization time for
various topographic and material property combinations. A new material,
Si-14, was designed to have ideal planarization characteristics low
viscosity-15.1 cP; low shrinkage-5.1% and satisfy SFIL-R processing
requirements oxygen etch resistance-33 wt% silicon, photocurable
and was used to validate our models through profilometry and interferometry
experiments. During spincoating, minimizing the spin speed generates
more planar films; however, this increases the spin time. To rectify
this problem, a two-stage spincoating process-a first step with high spin
speeds to achieve the target thickness quickly and a second step with
low spin speeds to improve planarization-was proposed and experimentally
demonstrated.
The step and flash imprint lithography (SFIL) process requires the clean separation of a quartz template from a polymer
imprint, and the force required to create this separation must be minimized to prevent the generation of defects.
According to fracture mechanics principles, decreasing both the imprint polymer modulus and the interfacial fracture
energy are beneficial for reducing the separation force. Adjusting the crosslinker concentration in the imprint
formulation decreases the modulus but does not significantly impact the facture energy. On the other hand, fluorinated
surfactant additives to the imprint fluid lower the modulus of the imprint polymer and decrease the fracture energy. The
fracture energy is further decreased by using a nonreactive, liquid surfactant versus a surfactant that reacts with the
polymer matrix. Angle-resolved X-ray photoelectron spectroscopy (XPS) results indicate that surfactant migration is
more effective with a fluorinated surface treatment compared to an untreated quartz surface. This result shows that the
use of fluorinated surfactants must be accompanied by a surface treatment that produces a similar energy or polarity to
induce migration and lower the adhesive strength.
Step and flash imprint lithography (SFIL) is low cost, high resolution patterning process and has found its way into a multitude of front end of the line (FEOL) and back end of the line (BEOL) applications. SFIL-R, a reverse tone variant of SFIL, and imprintable dielectrics are examples of such applications, and both require the design of specialized, silicon-based materials. Polyhedral oligomeric silsesquioxane (POSS) liquids were modified through a dual functionalization strategy to introduce photosensitive acrylate and thermally curable benzocyclobutane (BCB) groups to the molecule. The optimal functional group ratio was observed to be 3:5 acrylate to BCB, and the result was an imprintable dielectric with good mechanical properties and minimal post-exposure shrinkage. Thermal gravimetric analysis (TGA) revealed good thermal stability with minimal mass loss under annealing conditions of 400°C for 2 hours. Si-14 was designed to be a non-volatile, etch-resistant planarization layer for SFIL-R application. A polydimethylsiloxane (PDMS) derivative was modified to introduce acrylate functional groups and side branching for photosensitivity and low viscosity, respectively. Characterization of the material showed ideal planarization characteristics - low volatility (0.77 Torr at 25°C), low viscosity (15.1 cP), and minimal post-exposure shrinkage (5.1%).
Modern integrated circuit fabrication uses the dual damascene process to create the copper
interconnects in the Back End of the Line (BEOL) processing. The number of wiring levels is
increasing to eight or more in advanced microprocessors, and the complexity and cost of the
BEOL processes is growing rapidly. An approach to dual damascene processing using Step and
Flash Imprint Lithography (S-FIL®) in conjunction with Sacrificial Imprint Materials (SIM) offers the
ability to pattern two levels of interconnect structures simultaneously. By using a multi-level
imprint template built with both the via and trench structures, one imprint lithography step can
produce the same structures as two photolithography steps, greatly reducing the number of
patterning process steps in the BEOL layers. This paper presents progress in formulation of new
sacrificial imprint materials and the development of S-FIL and etch processes to incorporate the
SIM strategy. The SIM is formulated as a two-component system, with a tunable etch rate
adjusted by the ratio of the monomer and cross-linker components. High quality imprints were
produced with a multi-level template on wafers with blank films of black diamond® dielectric
material. The quality of the multi-level pattern transfer from the SIM into black diamond was
evaluated.
Understanding the dynamics of thin film planarization over topography is a key issue in the reverse-tone step and flash imprint lithography (SFIL-R) process. Complete planarization of a film over large, isolated topography poses an enormous challenge, since the driving force for planarization, the capillary pressure, continuously weakens as the film becomes more planar. For SFIL-R, only a specific degree of planarization (DOP) needs to be achieved before pattern transfer is possible. This paper presents the derivation of an inequality statement describing the required extent of planarization for successful pattern transfer. To observe how this critical DOP value (DOPcrit), and its corresponding leveling time (Tcrit) vary with materials and topographic properties, finite difference simulation was utilized to model planarization of a thin film over isolated topography after the spincoating process. This model was verified experimentally for various film thickness to substrate height ratios using interferometry to monitor silicon oil planarization over isolated trenches and lines. Material and topographic parameters were shown to not have a dramatic impact on DOPcrit; however, the critical leveling time increased considerably at DOPcrit values above 60 percent.
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