Nanostructured high refractive index titanium oxide films

Titanium oxide due to its optical properties, such as high refractive index, transparency to visible light and photocatalytic properties is being used in a large number of devices such as micro-optical elements and solar cells. The fabrication of TiO2 micro and nano-patterns however requires costly clean-room technology involving a number of fabrication steps. Alternatively titania can be prepared using low cost chemical solution deposition technique using sol-gel process. Sol-gel methods, on the other hand, suffer from high densification temperatures and therefore poorer performance. We present an advanced sol-gel process based on high titanium oxide content synthesis and its one-step patterning using nanoimprint lithography. TiO2 sol-gel films were prepared using titania precursors, titanium isopropoxide (90%) and silica precursor (10%) modified by organic moieties. TiO2 nano-particles were either added (Ø20nm) to the solution or formed in-situ (Ø5nm). The films up to 1 um were spin coated and imprinted at temperatures ranging from 80 to 130C. After this process, the films were cured with UV irradiation and shrank up to 50% leaving patterned areas free of cracks. The patterns from 50 nm to 2 um size and up to 300 nm deep were obtained. Refractive index of the films after UV irradiation increased from 1.6 to 2.1. Directly patternable and UV curable, crack-free and high purity TiO2 films up to 0.5 um thick are suitable for the fabrication of high efficiency micro-components.

Low-Stress UV-Curable Hyperbranched Polymer Nanocomposites for High-Precision Devices

Valérie Geiser

The aim of this thesis was to investigate the process-structure-property relationships of UV-curable hyperbranched polymer (HBP)/silica nanocomposites. Special attention was paid to the interplay between photo-conversion, rheological behavior, shrinkage, and stress dynamics. This knowledge was used to maximize the shape fidelity and dimensional stability of imprinted nano-patterns made with these nanocomposites. Two different processing routes, resulting in different nanocomposite morphologies, were compared. The first and more conventional approach was ultrasonic, solvent-assisted mixing with solid silica nanoparticles followed by UV curing, which led to a discrete dispersion of silica particles in the matrix. The second and more novel approach was a dual-cure process combining UV and sol-gel processing with liquid tetraethyl orthosilicate. The sol-gel process, using a low-viscosity organometallic precursor, overcame the potential processing issue of the highly viscous particulate nanocomposites and led to a hybrid material with a homogeneous silica network at nanometer scale. Rheological analysis of silica nanoparticle suspensions up to the concentrated regime in the HBP showed an exponential increase of the viscosity with the particle fraction for well-dispersed discrete particle systems. A liquid-to-solid transition occurred in the 5 to 10 vol% range, which was correlated with an immobilized layer of polymer hydrogen-bonded to the silanol groups on the surface of the particles, as was confirmed by calorimetric analysis. Polymerization kinetics of HBP nanocomposites and hybrids were analyzed by means of photo differential scanning calorimetry using an autocatalytic model. A time-intensity-superposition principle with power-law dependence was established, which invalidated the classic radiation dose equivalence principle. Gelation and modulus build-up were monitored using photo-rheology. The calorimetric and rheological data were combined in the form of time-intensity-transformation diagrams. It was found that gelation was delayed with respect to conversion up to 36% when lower UV intensities were used, which favored reduce polymerization stresses. The dynamics of polymerization shrinkage and internal stress build-up were investigated using photo-hyphenated interferometry and beam-bending methods. The linear shrinkage of the HBP was as low as 4.5%, which was further reduced to 3.3% with the addition of 20 vol% nanoparticles. The residual stress of HBP/SiO2 nanocomposites was below 5.5 MPa. That is well below the level of standard non-reinforced resins, in spite of an increased stiffness. For the sol-gel hybrids with 20 vol% SiO2, stress reduction by 50% with simultaneous stiffness increase by 20% with respect to corresponding particulate composites was demonstrated. The coefficient of thermal expansion was also lowered by 30%. Finally, nanogratings with a period of 360 nm and a step height of 12 nm were fabricated using UV-nanoimprint lithography with a glass or nickel master in a low-pressure (max. 6 bar) and rapid (≈1 min) process. Stable gratings were imprinted in the composite material containing up to 25 vol% of silica nanoparticles, despite the high viscosity of such compositions. Thanks to the exudation of a HBP-rich surface layer, the shape fidelity in terms of period of the replicated patterns was within 98% of the master, with shape distortion increasing with internal stress. The obtained nanogratings were used as a substrate for grated high refractive index TiO2 waveguides and then applied as wavelength-interrogated optical sensors (WIOS). An immunoassay with the polymer WIOS showed the same ultrahigh detection sensitivity as the standard glass-based devices. Therefore, the novel polymer-based sensor should be useful to probe contaminants in liquids with concentrations as low as a few ppb.

EPFL2010

Nanostencil Lithography - Quick & Clean: Towards a reliable scalable nanopatterning method

Jürgen Brugger, Nao Takano, Marc Antonius Friedrich van den Boogaart, Oscar Vazquez Mena

Stencil lithography is a surface patterning technique that relies on the local deposition of material through miniaturized shadow mask membranes. This method has been used since many years in various implementations for the formation of patterns, mainly structured thin metal films, in situations when lithography equipment is not available or when the surfaces don’t allow the harsh process steps typically involved in photolithography such as spin coating of photoresist, high temperature baking, development, wet or dry etching and resist stripping. Stencil lithography is down-sizable into the sub-100-nm scale which makes it very interesting as method for (in-vacuum) rapid-prototyping of nanostructures without the risk of contamination, and for laboratories without access to high-end nanolithography equipment. The major challenges in stencil lithography are following: i) the fabrication of large area nanostencils requires one high-resolution lithography step and sophisticated MEMS processes, ii) the mechanical properties of thin membranes (typically ~ 100 nm thick) requires careful handling, iii) the effect of deposited material on stencil leads to aperture clogging and membrane bending which limits the resolution capabilities, iv) the precise positioning of nanostencils to prefabricated surfaces structures for aligned multiple layer patterning, v) and eventually the recycling of used stencils. Despite these difficulties, stencils are extremely useful tools for flexible and reliable patterning of surface structures across multiple length-scales from mm to sub-100 nm. In collaboration with our project partners we have recently progressed in several of the above mentioned challenges: the fabrication of stencils is now based on a set of advanced silicon micromachining steps including DUV exposure for >200 nm apertures and a combination of DRIE and wet etching. Focused beams are used to fabricate stencils with sub-100 nm apertures. In particular the mechanical stability of the ultra-thin low-stress silicon nitride membranes could be considerably improved by topographic reinforcement rims. As a result, the membranes deform less under the stress of deposited film and consequently the surface patterns are better defined. We have studied patterning by stencil lithography of metals (e.g. Al, Au, Bi, Cr, Ti, Cu) on various surfaces (e.g. Si, SiO2, SU-8, PDMS, PMMA, freestanding SiN cantilevers, and curved surfaces. Pulsed laser deposition of metal on self-assembled monolayers (SAMs) has resulted in novel micro/nanopattern for molecular electronic devices. Parallel fabrication of thousands of sub-micrometer resonating beams that are integrated with CMOS circuitry have been realized by stencil lithography and sacrificial etching. The talk will present the current state-of-the-art of the nanostencil lithography , will highlight the strength of the method but will also discuss the current limits and challenges ahead to make it a truly reliable nanofabrication method.

2006

Showcasing the optical properties of monocrystalline zinc phosphide thin films as an earth-abundant photovoltaic absorber

Léa Buswell, Mirjana Dimitrievska, Simon Robert Escobar Steinvall, Anna Fontcuberta i Morral, Jean-Baptiste Leran, Rajrupa Paul, Elias Zsolt Stutz, Mahdi Zamani

Zinc phosphide, Zn3P2, is a semiconductor with a high absorption coefficient in the spectral range relevant for single junction photovoltaic applications. It is made of elements abundant in the Earth's crust, opening up a pathway for large deployment of solar cell alternatives to the silicon market. Here we provide a thorough study of the optical properties of single crystalline Zn3P2 thin films grown on (100) InP by molecular beam epitaxy. The films are slightly phosphorus-rich as determined by Rutherford backscattering. We elucidate two main radiative recombination pathways: one transition at approximately 1.52 eV attributed to zone-center band-to-band electronic transitions; and a lower-energy transition observed at 1.3 eV to 1.4 eV attributed to a defect band or band tail related recombination mechanisms. We believe phosphorus interstitials are likely at the origin of this band.