We demonstrate humidity sensors based on optical resonances sustained in sub-wavelength thick dye-doped polymer coatings on reflecting surfaces. As a result of coupling between dye molecular absorption and Fabry-Perot resonances in the air-coating-surface cavity, the absorption spectra of such thin-film structures show a strong resonant peak under certain illumination conditions. These resonances are sensitive to the structural and material properties of the thin-film, metal underlayer and ambient conditions and hence can be used for gas and vapor sensing applications. Specifically, we present our proof of principle experimental results for humidity sensing using a thin-film structure comprising Rhodamine6G-doped polyvinyl alcohol (PVA) films on silver substrates. Depending on the PVA film thickness, dye-concertation and angle of incidence, the resonant absorption peak can undergo either red-shift or blue-shift as RH level increases in the range 20% to 60%. Also, the absorption magnitude at certain wavelengths near to resonance show almost linear reduction which can be used as the sensing signal. Our simulation studies show a very good agreement with the experimental data. The spectral and temporal sensitivity of this thin-film structure is attributed to the changes in the thickness of the PVA layer which swells by absorbing water molecules
Recent work has investigated the resonant dielectric reflectors for high-aspect ratio (AR) imaging and the necessary requirements for index-matching liquids (IMLs) at the prism/sample interface when imaging using a solid immersion Lloyd’s mirror interference lithography (SILMIL) system. These past results showed that SILMIL systems require a prism/IML refractive index (RI) (real component) mismatch less than approximately ±0.02 and an imaginary RI component ≤5×10−5 to achieve good reproducibility and uniformity of high-AR resist structures in the ultrahigh-NA (UHNA) regime without system gapping control. Here, we present simulated and experimental results for an index-matched prism/IML combination and for an IML with an imaginary RI component of ∼10−5. These results present the first SILMIL system that can produce both low- and high-AR resist structures over larger exposure fields than previously reported and without gapping control. We also present simulated and experimental results that show the AR process latitude, which further highlights the improved control compared with previous SILMIL research. Finally, we present simulated results from a materials survey that show new potential candidate dielectric underlayer materials that can accommodate high-AR imaging in the UHNA regime, some of which are better geared for the semiconductor industry.
Recent work with dielectric resonant reflector underlayers has shown improvements in high aspect ratio imaging via solid
immersion evanescent interference lithography in the ultra-high numerical aperture (UHNA) regime. Controlling the gap at
the prism/sample interface has proven to be a problem for good reproducibility, uniformity and quality of UHNA regime,
high aspect ratio, resist structures. Here we present simulated and experimental results for fine tuning of the prism/sample
interface to achieve greater uniformity of high aspect ratio resist structures over large exposure fields in the evanescent
regime. These results highlight our solid immersion Lloyd’s mirror interference lithography (SILMIL) system’s limitations in
the absence and presence of an index-matching layer (IML) through the use of gap control measurements and attenuated total
internal reflection (ATR) measurements with various index mismatched prism/IML pairs. Finally, we present simulated
results for a system that will produce uniform, high aspect ratio resist structures over large exposure fields.
In the last year our Solid Immersion Lloyd's Mirror Interference Lithography (SILMIL) system has proved to be a
successful tool for evanescent interferometric lithography (EIL). The initial goal was to use SILMIL in conjunction with
the surface plasmon polariton (SPP) surface states at the resist-metal interface. Through this resonance, we aimed to
counter the decay of evanescent images created using EIL. By analyzing the theory in greater detail we were able to
develop a better understanding of the resonance phenomena. In this paper, details of the design of SILMIL and how one
may utilize it to produce ultra-high numerical apertures (NAs) are given, as well as an introduction to the resonance
phenomena and the mechanism behind it. We introduce a new method that requires a gain medium (one that has a
negative loss) to achieve significant enhancements, and present an effective gain medium by using a high-index
dielectric on low-index media. We present results at λ = 405 nm using such an effective gain medium and also provide a
feasible design example at the lithography standard λ = 193 nm.
Evanescent-wave imaging is demonstrated using solid-immersion Lloyd's mirror interference lithography at λ = 325 nm to produce 44-nm half-pitch structures (numerical aperture, NA = 1.85). At such an ultrahigh NA the image depth is severely compromised due to the evanescent nature of the exposure, and the use of reflections from plasmonic underlayers is discussed as a possible solution. Simulations and modeling show that image depths in excess of 100 nm should be possible with such a system, using silver as the plasmonic material. The concept is scalable to 193 nm illumination using aluminium as the plasmonic reflector, and simulation results are shown for 26-nm half-pitch imaging into a 37-nm thick resist layer using this scheme.
Evanescent-wave imaging is demonstrated using solid-immersion Lloyd's-mirror interference lithography (SILMIL) at
λ = 325 nm to produce 44-nm half-pitch structures (numerical aperture, NA = 1.85). At such an ultra-high NA the image
depth is severely compromised due to the evanescent nature of the exposure, and the use of reflections from plasmonic
under-layers is discussed as a possible solution. Simulations and modelling show that image depths in excess of 100 nm
should be possible with such a system, using silver as the plasmonic material. The concept is scalable to 193 nm
illumination using aluminium as the plasmonic reflector, and simulation results are shown for 26-nm half-pitch imaging
into a 37-nm thick resist layer using this scheme.
Near-field imaging through plasmonic 'superlensing' layers can offer advantages of improved working distance (i.e.
introducing the equivalent of a focal length) and control over image intensity compared to simple near-field imaging. In
a photolithographic environment at ultra-violet (UV) wavelengths the imaging performance of single- and multi-layer
silver plasmonic superlenses has been studied both experimentally and via computer simulations. Super-resolution
imaging has been demonstrated experimentally, with the sub-100 nm resolution currently being limited by issues of
roughness in the silver layers and the ability to deposit high-quality silver-dielectric multilayers. The simulation studies
have shown that super-resolved imaging should be possible using surprisingly thick silver layers (>100 nm), with the
cost of much reduced image intensity, which is something that is yet to be shown experimentally. The use of multilayer
plasmonic superlenses also introduces richness to the imaging behaviour, with very high transmission possible for certain
spatial frequency components in the image. This has been widely touted as a means for improving image resolution, but
the complexity of the spatial-frequency transfer functions for these systems does not make this a universal fact for all
classes of objects. Examples of imaging situations are given where multi-layer superlenses are actually detrimental to
the image quality, such as the case of closely-separated dark-line objects on an otherwise bright background.
Progress towards semiconductor laser frequency stabilization using optical feedback from microtoroidal resonators
is presented. A simple model of the feedback mechanism is provided, and equations of motion describing the
system fields are given. Reactive ion etcher based fabrication of microtoroidal resonators with intrinsic quality
factors as high as 1.6 x 105 is demonstrated. This fabrication technique enables improved silicon surface quality
and greater control of the physical structure of the microresonators.
Soft-lithography and plasma etching with reactive ions were used to fabricate a polymer microfluidic cell-culture bioreactor with integrated optical oxygen sensor. Platinum (II) octaethylporphyrin ketone (PtOEPK) suspended in a microporous polystyrene (PS) matrix was spin-coated to form sensor films of variable thickness from 1.1 μm to 400 nm on glass substrates. Sensor films were found to be smooth and well adhered. Arbitrary patterns with a minimum feature size of 25 μm could be routinely replicated in the PtOEPK/PS layer using polydimethylsiloxane (PDMS) elastomer stamps as etch masks in a reactive ion etcher. No effect of plasma patterning and sensor integration by plasma bonding on the sensor signal could be observed. Detection of different gaseous and dissolved oxygen concentrations with the patterned sensor followed linear Stern-Volmer behavior. Dynamic measurement of sensor intensity as a function of different oxygen concentration showed good reproducibility and a nearly instantaneous response to gas changes. For gaseous and dissolved oxygen measurement with a patterned 400 nm thick film I0/I100 ratios of 3.2 and 2.7 were found, respectively.
A performance enhancement to planar lens lithography (PLL) through the use of i-line narrowband exposures has been investigated. Experimental results show that for a 50nm silver layer the image fidelity of narrowband exposures out performs broadband exposures. This is due to the removal of off-plasmonic-resonance wavelengths, which cause unwanted background exposure and a loss of image fidelity. Dense gratings have been resolved down to 145nm periods, as well as line-pairs down to separation distances of 117nm. These results out perform the diffraction-limits that restrict traditional optical-system resolution limits.
This paper discusses the growth of silicon nanostructures on silicon (100), (110) and (111) substrates by electron beam annealing. The nanofabrication procedure involves annealing of the untreated Si substrates in the temperature range 750°C - 1200°C using a raster scanned 20 keV electron beam. Nanostructuring occurs as a result of kinetic amplification of the surface disorder induced by thermal decomposition of the native oxide. Pyramidal and truncated pyramidal nanocrystals were observed on Si(100) surfaces. The nanostructures are randomly distributed over the entire surface and square-based, reflecting the two-fold symmetry of the substrate surface. Similar square-based pyramidal structures with four equivalent facets are observed following nanostructuring of Si(110). With Si(111), nanostructure growth occurs preferentially along step-edges formed on the vicinal surfaces. Significant differences in nanostructure shapes formed on step-edges and terraces are related to the different growth mechanisms on the unreconstructed and 7x7 reconstructed domains respectively.
In this Paper we investigate a tunable metallic photonic crystal filter with a novel mechanical tuning method, suitable for use in terahertz frequency applications. Tuning has been demonstrated in a micrometer-driven prototype at 70 - 110 GHz in accordance with rigorous full-vector electromagnetic simulations (finite-difference time-domain). The measured pass band has a Q of 11 and can be tuned over a 3.5 GHz range. The insertion loss is only 1.1 to 1.7 dB, while the stop band attenuation is >10 dB. The filter has the advantages of inexpensive, robust and compact construction and tunable operation that readily scales to any desired terahertz frequency.
Random deposition of conducting nanoparticles on a flat two dimensional (2D) substrate leads to the formation of a conducting path at the percolation threshold. In sufficiently small systems significant finite size effects are expected. However, in the 2D square systems that are usually studied, the random deposition means that the main effect of small system sizes is that stochastic fluctuations become increasingly large.
We have performed experiments and simulations on rectangular 2D nanoparticle films with nanoscale overall dimensions. The sample geometry is chosen to limit stochastic fluctuations in the film’s properties. In the experiments bismuth nanoparticles with mean diameters in the range 20-60nm are deposited between contacts with separations down to 300nm. At small contact separations there is a significant shift in the percolation threshold (pc) and the conducting
path formed close to pc resembles a nanowire. Percolation theory describes the experimental onset of conduction well: there is good agreement between predicted and measured values of the power law exponent for the correlation length.
Atomic clusters can be produced in a size range (100nm to 0.5nm) that bridges the gap between the limits of current lithographic fabrication technologies for integrated circuits and the atomic/molecular regime. The work presented here aims to combine established top-down device processing with bottom-up engineered cluster assembly. Conducting cluster deposition and standard optical fabrication techniques have been used to produce wires on a textured (V-grooved) substrate. The lengths of the wires (ranging from 2μm to 1mm) are defined simply by the separation of NiCr/Au contacts. The deposited nanoparticles range in size from 20-100nm and in principle define the width of the nanowire. In-situ conductance measurement allows precise control of the deposition process and the onset of conduction in the wire is readily monitored as a function of deposition time. The effectiveness of the surface templating technique is demonstrated by SEM and AFM imaging carried out after deposition. The surface coverage is seen to vary from <20% on the unpatterned (normal-to-beam) surface (which is required to be non-conducting) to >100% at the apexes of the V-grooves used to promote growth of the wire. Self assembly of the nanoparticles leads to completion of a wire between the pre-formed contacts with no possibility of a parasitic conduction path. Wires formed through this technique currently have minimum widths of ~1μm but straightforward extensions of the technique should soon allow nanowire formation.
Micromachining of ultra-high frequency waveguide structures requires etching with vertical sidewalls and flat bottoms simultaneously. The required geometries can be difficult to achieve using a single-step orientation dependent etching (ODE) process without incurring a severe mask-undercutting penalty. This may inhibit the production of isolated convex structures, such as the central pillars that are required to couple radiation into the waveguide. In this paper we will described a new technique for ODE of deep, vertical sidewall structures in (100) Si with reduced undercut etching. The process uses a two stage KOH/IPA etch with a mask pattern that is designed to compensate for the differing etch rates on the Si planes. To date we have achieved overall etch depths of 350 microns, with a lateral undercut of as little as 275 microns, compared with a 350 micron undercut for a single-stage etch. The sidewalls are at exactly 90 degrees to the surface of the (100) Si, and the bottoms of the trenches are smooth and flat. Using the process we have also been able to routinely fabricate isolated, square pillars as small as 50 X 50 square microns, and over 300 microns high. The process enables structures to be made that might previously only have been possible with high-density-plasma dry etch techniques. The new technique has clear advantages of low cost and high throughput.
Multilevel diffractive lenses give much higher efficiencies than simple binary structures by introducing a blaze. We have developed a novel micro-machining process that allows complex multi-level optics to be fabricated in silicon for use at terahertz frequencies. In this paper we demonstrate the fabrication of a four level (quaternary) lens designed for use at 1 THz. The process required two highly anisotropic dry etch stages that both used a SF6/O2 plasma at 173 K. The etch produced smooth etch surfaces with a vertical etch rate of 1 micrometers /min. The lithography for the first etch stage used a conventional positive photoresist process followed by NiCr deposition and lift-off to form an etch mask. For the second etch step the substrate was planarized and over-coated using SU-8, which was also used as a negative resist. The planarized specimen was then exposed to produce the etch mask for the next phase level etch of the lens. The process yielded a silicon:SU-8 etch selectivity of 4:1 which was more than adequate. Between the two etches a total etch depth of 115 micrometers was obtained, with steps at 38 and 77 micrometers and a minimum feature of 84 micrometers . It is anticipated that the process can be extended to give more phase levels with greater optical efficiency.
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