Two-dimensional and three-dimensional periodic templates through holographic interference lithography

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.Includes bibliographical references (leaves 121-133).In this thesis a simple technique for controlling structure via holographic interference lithography was established and implemented. Access to various space groups including such important structures as the level set approximations to the Diamond, the Schwartz P structure, the FCC, and the non centrosymmetric Gyroid structures were demonstrated. The ability to make 3D structures over a large area, with low defect densities and periodicities on the sub/i scale opens a whole range of opportunities including such diverse areas as photonic crystals, phononic crystals, drug delivery, microtrusses, tissue scaffolds, microfluidics and colloidal crystallization. A correlation between structure and photonic band gap properties was established by systematically exploring the 11 FCC space groups. This resulted in a technique to search for photonic band gap structures. It was found that a fundamental connectivity caused by simple Fourier elements tended to support gaps. 2-3, 5-6 and 8-9 gaps were opened in the f.c.c lattices. The F-RD and 216 structures were newly shown to have complete band gaps. Two of the three previously established champion photonic crystal structures, viz. the Diamond and the Gyroid presented practical fabrication challenges, approximations to these structures were proposed.(cont.) A scalable P structure and the 3-FCC structure were fabricated by single and multiple exposure techniques. Both negative and positive tone photoresist systems were demonstrated. Line defects were written into the negative tone system using two-photon lithography. The single crystalline, porous nature of the structures was exploited to examine the possibility for their use as hypersonic phononic crystals and microfluidic microlenses. Two dimensional single crystalline patterns were created using interference lithography. Their phononic band structure was probed by Brillioun light scattering to yield a phononic band diagram, which clearly demonstrates the effect of periodicity on the phononic density of states. The ability to control the density of states at these length scales holds the potential for control over thermal properties. The two dimensional structures fabricated in negative photoresist were also tested as microlenses with the integrated pores acting as microfluidic channels. This combination resulted in a structure reminiscent to that of the biological species ophiocoma wendtii.by Chaitanya K. Ullal.Ph.D

Elastomeric Network/Air Strucutes for Mechanically Tunable Hypersonic Phononic Crystal

Hypersonic phononic crystals allow control over high frequency phonons, which is crucial for a whole range of applications from acousto-optics to thermal management and high resolution nondestructive evaluation techniques. The ability to fabricate phononic crystals with a band diagram that can be modified reversibly and repeatedly opens an interesting possibility to create tunable acoustic devices. In this talk we will describe the use of submicron elastomeric PDMS (poly(dimethylsiloxane))/air network structures as tunable phononic crystals operating in hypersonic frequency regime. The structures were fabricated from interference lithography templates, which were infiltrated with PDMS precursor and then after crosslinking the photoresist template was removed in water-based basic solution. Brillouin light scattering was used to monitor the modification of the phononic band diagram of these elastomeric structures as a function of the direction and degree of reversible mechanical deformation. The influence of symmetry and anisotropic sound velocities on the features of the phononic band diagram will be discussed

Spirothiopyran-Based Reversibly Saturable Photoresist

Super-resolution lithography holds the promise of achieving three-dimensional (3D) nanopatterning at deep subwavelength resolutions with high throughput. 3D super-resolution lithography schemes demonstrated thus far have all been serial in nature, primarily due to the lack of a photoresist chemistry that not only couples a saturable reversibly switchable reaction with a writing step but also has a low saturation threshold. Here, we demonstrate that combining the reversible photoisomerization of spirothiopyran with the thiol-Michael conjugate addition reaction achieves the necessary photochemical characteristics. Green light was found to saturate inhibition of the thiol-Michael addition writing step at very low intensity thresholds. By formulating a spirothiopyran-functionalized polyethylene glycol copolymer, we demonstrate spatial control over cross-linking using inhibition by green light. Kinetics measurements combined with photokinetic simulations show that interference lithography on a spirothiopyran maleimide-based writing system using conventional light sources (e.g., a 2 W green laser) should deliver super-resolution features (∼45 nm wide lines) in thick films (tens of microns) over large areas (hundreds of microns on a side). The unique combination of reversible photochromic switching of spirothiopyran with the thiol-Michael addition reaction marks an important step toward realizing a highly parallelized 3D super-resolution writing system

Wetting Regimes for Residual-Layer-Free Transfer Molding at Micro- and Nanoscales

Transfer molding offers a low-cost approach to large-area fabrication of isolated structures in a variety of materials when recessed features of the open-faced mold are filled without leaving a residual layer on the plateaus of the mold. Considering both macroscale dewetting and microscale capillary flow, a proposed map of wetting regimes for blade meniscus coating provides a guide for achieving discontinuous dewetting at maximum throughput. Dependence of meniscus morphology on the azimuthal orientation of the stamp provides insight into the dominant mechanisms for discontinuous dewetting of one-dimensional (1-D) patterns. Critical meniscus velocity is measured and residual-layer-free filling is demonstrated for 1-D patterned soft molds (stamps) with periods ranging from 140 nm to 6 μm. Transfer of isolated lines, and multilayer woodpile structures were achieved through plasma bonding. These results are relevant to other roll-to-roll compatible processes for scalable production of high-resolution structures across large areas

Shape control of multivalent 3D colloidal particles via interference lithography

We present a new route for the fabrication of highly nonspherical complex multivalent submicron particles. This technique exploits the ability of holographic interference lithography to control geometrical elements such as symmetry and volume fraction in 3D lattices on the submicron scale. Colloidal particles with prescribed complex concave shapes are obtained by cleaving low volume fraction connected structures fabricated by interference lithography. Controlling which Wyckoff sites in the space group of the parent structure are connected assures specific "valencies" of the particles. Two types of particles, 2D "4-valent" and 3D "6-valent" particles are fabricated via this technique. In addition to being able to control multivalent particle shape, this technique has the potential to provide tight control over size, yield, and dispersity.close252

Dynamic Imaging of Colloidal-Crystal Nanostructures at 200 Frames per Second

The dynamic noninvasive imaging of colloidal nanostructures has been precluded by the diffraction-limited resolution of (confocal) light microscopy. Using Fast Stimulated Emission Depletion (STED) microscopy, we demonstrate the ability to resolve the formation of a colloidal crystal (monolayer) from particles of 200 nm size, where the voids in the crystal are as small as 30 nm. With a temporal resolution of 5 ms, we exemplify the technique by visualizing the annealing of potential point defects during the formation of the colloidal crystal