We will briefly provide a history of photolithography in Chapter 4, and outline some of the limitations of the existing lithographic methods. In this introductory chapter, we highlight some of the advantages of bottom-up approaches to materials synthesis.
Bottom-up Fabrication via Self-Assembly
Colloidal Assembly
There are a number of reported examples that combine colloidal assembly and lithography to yield novel nanostructures.[12, 13]. Despite these drawbacks, there are many cases in which colloidal assembly provides a robust approach to fabricating periodic micro- and nanostructures.
Block Copolymer Assembly
Periodic Nanostructured Materials via ROMP
Well-defined lamellae, cylinders, and spheres could be accessed by varying the relative volume fraction of the two blocks. A library of BCs constructed from these monomers was synthesized by varying the molar incorporation of the imidazolium monomer from 4 to 58% [27].
Hybrid Organic/Inorganic Composites
Extensive SAXS analysis of the BC-derived morphologies was used to determine the phase diagrams of the various morphologies. This was believed to be due to the strong segregation parameters exhibited by these BCs.
Challenges
Synthesis of isocyanate-based brush block copolymers and their rapid self-assembly into infrared-reflecting photonic crystals”. We have demonstrated the rapid self-assembly of brush block copolymers into multilayer nanostructures with tunable domain periodicities down to several hundreds of nanometers.
Synthesis of Brush Polymers
In the graft-to strategy, it is difficult to achieve complete engraftment due to the dramatic steric congestion associated with increased conversion. 4, 5] investigated the graft-through (co)polymerization of polystyrene (PS), polyacrylate (PA) and PLA macromonomers, end-functionalized with exo-norbornene chain end groups, mediated by the very fast initiating catalyst 3.
A Brief Introduction to Photonic Crystals
The graft-from approach inherently yields non-uniform architectures due to the difficulty in achieving quantitative and uniform growth with a polymeric initiator. The dielectric contrast between the layers is proportional to the reflected intensity at each interface R = (n1−n2/n1 +n2)2, as well as to the width of the photonic band gap.
Controlled Assembly of Brush Block Copoly- mer Photonic Crystalsmer Photonic Crystals
In the case of the film cast by VKM, a disordered morphology is observed in the SEM image. We observed the first-order reflection peak to be a linear function of MW for all self-assembly techniques used.
Optical Modeling
Despite the moderate to weak lamellar alignment observed in the SEM cross section, our 1D transfer matrix simulations can qualitatively predict reflection spectra, enabling the design of optical components using this platform.
Tunable Assembly from Rigid Polymer Brushes
Thin films of the polymers were prepared through controlled evaporation from DCM, THF, CHCl 3 or toluene. Plots of (b) reflectance versus wavelength and (c) maximum peak wavelength of reflectance versus blend weight fraction (% BCP-2) of the various brush block copolymer blends.
High-fidelity Multilayers from Dendritic Copoly- mersmers
The reduced MW of the dendronized polymer would presumably increase diffusion rates and increase the rate of self-assembly. The GMRs of the 1D photonic crystals produced from dendronized BCPs were much smaller (GMR = 9-18%) than those of the isocyanate-based BCPs4b,c (GMR = 17-27%) for PCs transmitting light over the visible reflected spectrum (λmax = 334-768 nm).
A Modular Blending Strategy for Improved AssemblyAssembly
BBCPs could even swell to a periodicity that was 180% of the value of the corresponding unmixed BBCP (Figure 2.17(a)). These phenomena can be explained by the inherent polydispersity of the BBCP backbone length.
Applications
The reflection wavelength can be optimized by adjusting the refractive index of the materials and the periodicity of the layers. Specifically, the concept aims to deliver a window layer that is transparent to visible light (85%) and rejects 97% of infrared radiation, while being installed at a fraction of the cost of current dielectric films.
Introduction
The sharp resonance is due to coupling to the resonant modes of the colloidal array.
Fabrication
Solar Cell Fabrication
A silica colloidal monolayer on a solar cell. a) Schematic of the sphere deposition on the solar cell using the Langmuir-Blodgett method. The influence of the separation between the sphere array and the active layer is discussed in Reference [15].
Simulation & Measurement
Illumination at Normal Incidence
To account for the measured sphere size distribution, we weight the series of simulations by the Gaussian distribution shown in Figure 3.2(c). The resonant characteristics of the experimental spectrum and the weighted average spectrum are in very good agreement (Figure 3.3(a)).
Angle-dependent Performance
Reflectance measurements of a 2D colloidal crystal on a Si wafer support the proposed mechanism of enhancement (Figure 3.5). measured absorption of a resonant dielectric structure on flat Si. a) Map of the absorption as a function of the wavelength for the different sphere diameters over the measured size distribution. The observed EQE enhancement bands account for the variation of the structure's resonances as a function of the angle of incidence.
Anti-Reflective Coating of a High Performance Cell
Despite the fact that the nominal thickness of PVA is only 2-3% of the diameter of the sphere, the colloidal crystals were effectively transferred to the surface of the device with high reliability and good yield. The performance of the solar cell with and without the sphere array is shown in Figures 3.7(c) & 3.7(d).
Conclusion
Some of the well-suited chemical mechanisms behind photolithography will be discussed, particularly in the context of UV-activated thin films. We describe some of the limitations of existing photo modeling methodologies, mainly related to the variety of materials that can be used.
Introduction
Photolithography
A photoresist is a material system capable of undergoing a photo-activated chemical change, so that the properties of the irradiated domains contrast those of the starting material. Today, chemically enhanced resists are less common in integrated circuit manufacturing, in part because one of the most valuable properties of a photolithographic property is resolution.
Towards Functional Lithography
We report a negative-tone photoresist using a photoactivated olefin metathesis catalyst, which can be rapidly prepared in a one-pot synthesis from commercially available starting materials. The resist is based on a ruthenium vinyl ether complex, which is generally considered to be inactive with respect to olefin metathesis.
Introduction
A typical quenching procedure uses excess vinyl ether and immediate precipitation of the polymer to remove the catalyst. This can be considered a "chemically enhanced" resist, in that the photoactive species is a catalyst for the cross-linking of the polymer matrix.
Results
The presence of some excess vinyl ether is beneficial in mitigating dark polymerization of the resist material. While we were able to work with a few groups of the resist under ambient conditions for many weeks. a).
PLOMP 1.0 - Recipe
Control Experiments
The shift of the PLOMP resist is within 1 ppm of the Ru-vinyl ether complex reported by Louie. First, the formally 14-electron ruthenium vinyl ether species can be prepared by quenching the second-generation Grubbs-Hoveyda catalyst with ethyl vinyl ether.
Strategies for Functional Group Incorporation
This approach offers alternative architectures that can provide unique chemical properties, such as chain-end coupling or tailored rheology. One use of chain transfer agents has been to incorporate multiple polymerization mechanisms into a single polymeric backbone, which can be useful in designing tailored material properties.[3].
Improving the Pre-catalyst Stability
The general conditions for these studies involved the partial polymerization of DCPD via catalyst 2, quenching at the desired approximate viscosity with ethyl vinyl ether and one of the ligands. Interestingly, the crystal structure of 1,10-phenanthroline adduct complex 6 revealed an isomerization of the chloride ligands from trans to cis.
PLOMP 2.0 Recipe
This enabled many of the studies of multiphoton PLOMP, described in detail in Chapter 7, as it both provided an extended working window and removed many of the limitations of resin composition. Overnight, the pink solution turned ruby red, which we believe was due to the formation of complex 6.
Compatibility Scope of Molecular Additives
Second, the glass transition temperature of the partially polymerized poly(DCPD) in the PLOMP 2.0 resist is likely to be much higher than the poly(COD)-based PLOMP 1.0 resist. Another ongoing challenge within this project is the characterization of the final composition of the patterned film.
Progress Towards Robust Chemical Etch Masks
This particular image shows the crystallization of the alcohol-functional monomer N-(hydroxyethanyl)-cis-5-norbornene-exo-2,3-di-carboximide with 1 wt% loading. A number of considerations must be made to move forward with this application, including resist surface adhesion, interconnect density, accessible film thicknesses, and the procedure for removing the resist after the etch process.
Peptide-Functional Surfaces toward Cell Scaf- foldsfolds
While there is evidence of adipogenic differentiation for our RGD-functionalized films, the improvement over controls is not particularly pronounced. Further studies will be required to investigate the dependence of peptide concentration on stem cell growth.
Nanoimprint Lithography via PLOMP
The side of the nanopatterned PDMS stamp was in contact with the PLOMP resist, and the two sides were pressed together using binder clips. For many of the applications discussed, nanoimprint lithography could provide an attractive route to high performance.
Electrocatalytic Films for Selective CO 2 Re- ductionduction
This demonstrates the ability of the PLOMP process to attach catalysts to electrode surfaces and withstand the acidic conditions required for the CO2RR. As shown in Figure 6.15, there are several possible architectures for incorporation of the catalyst into the PLOMP photoresist.
Future Directions
The rate of two-photon excitation is proportional to the intensity squared, and the intensity of a focused beam falls off with the square of the distance from the focal plane. Combining these, the two-photon excitation decays as the fourth power of the distance from the focal plane, enabling precision excitation and sub-diffraction limited resolution.[3].
Direct Laser Writing
The top spot is the result of emission from 2-photon absorption, while the bottom reveals the entire excitation path in the case of single-photon excitation.
Early Attempts
Much of the inspiration to find a more stable photocatalyst came from these initial efforts at Nanoscript. One factor contributing to the polymerization threshold phenomenon is the inhibition of radical polymerization by oxygen from the environment.
Control Experiments
For PLOMP 1.0 resists, ethylidene norbornene is the most volatile component with a boiling point of 146 °C. This suggests that there is a significant amount of unreacted DCPD remaining in the PLOMP 2.0 resist.
Multiphoton PLOMP Results
After concentrating a typical PLOMP 2.0 resist to 50 millitorr overnight, the remaining solids accounted for only ∼15% of the resist mass. Details of the synthetic preparation can be found in Section 8.4, and we have named this variant PLOMP 2.1.
Remaining Challenges
Conclusions
Appendices
- Detailed Procedures for Chapter 2
- Detailed Procedures for Chapter 3
- Detailed Procedures for Chapter 6
- Detailed Procedures for Chapter 7
The number of alternating layers in the simulations was estimated from the SEM cross-section of the corresponding films. When the surface of the trough is large, the surface water pressure is about 4 mN/m.
