Fabrication of structures with high aspect ratios (HAR) enables creation of metasurface optics with ever-increasing functionality. Dielectric meta-atoms consist of transverse (x,y) shapes extruded in the z-dimension. The phase discontinuity imparted by the meta-atom is a function of the dielectric constant of the material and the z-height of the meta-atom. The performance of the meta-optic improves with larger phase swings. Here we demonstrate HAR etched structures in silicon and gallium arsenide. Initial etch development employs interferometric lithography while final patterns are realized using e-beam patterning. The paper will discuss this etch development process for both material sets.
KEYWORDS: Plasmonics, Gold, Finite element methods, Polarization, Data modeling, Phase shifts, Analytical research, Instrument modeling, Diffraction, Chemical elements
Plasmonic metasurfaces with in-plane phase elements have a limit in transmission because they only affect the electric field of incident EM radiation. Recently, a set of out-of-plane plasmonic phase elements was designed using a genetic algorithm to work in the infrared as a Huygens metasurface with significantly improved transmission efficiency. A beam-steering metasurface (i.e., blazed transmissive diffraction grating) was fabricated from this design using membrane projection lithography (MPL) and characterized for its bidirectional transmittance distribution function as a function of scatter angle for normally incident light, and linear incident and transmitted polarizations. Measurements were compared with the designed behavior as predicted by finite element method (FEM) simulations that generated near fields for each phase element and propagated them to the far field as a metasurface using a Stratton–Chu formulation, but measurements showed strong zero-order diffraction not present in the simulation along with the designed +1-diffraction order. We analyze this disagreement between measured and ideal results. Further FEM modeling included the introduction of defects into the phase elements consistent with defects expected from the fabrication process and identified lateral displacement of the plasmonic decoration in the MPL structure as a potential cause for the reduced performance of the fabricated device.
KEYWORDS: Data modeling, Polarization, Instrument modeling, Finite element methods, Plasmonics, Scatter measurement, Phase shifts, Optical components, Near field
To improve upon a fundamental limit on transmission of an in-plane plasmonic metasurface device, Out-of-Plane (OOP, i.e. 3D-thin-film) metasurface plasmonic phase elements were designed using genetic algorithm techniques to work in the infrared, λ = 8 μm, and these were fabricated as a beamsteerer using membrane projection lithog- raphy and characterized using scatterometry to measure the Bidirectional Transmittance Distribution Function (BTDF) of the device. BTDF was measured as a function of scatter angle for four different polarization con- figurations: co-polarization and cross-polarization for two orthogonal linear polarization states and simulated using a finite element method (FEM) solver to generate the near fields of each phase element of the device and a Stratton-Chu formulation to propagate to the far field. The measurements showed the designed beamsteering from the device, but also a strong zero-order diffraction not present in the simulations. This disagreement be- tween models and measurements motivated this study to understand what was causing the differences. To that end, FEM models which reduced the coupling between adjacent elements of the beamsteerer were designed to examine methods that would better simulate measurements. Details of the models are discussed. Future work will focus on finding the root cause for this decrease in coupling.
KEYWORDS: Silicon, Polymers, Metamaterials, Scanning electron microscopy, Projection lithography, Chemical mechanical planarization, Lithography, Electron beam lithography, Etching, Microelectromechanical systems
Membrane projection lithography (MPL) is a fabrication approach in which a novel process flow and mature silicon processing equipment combine to create three-dimensional metamaterials with size scales operational at optical frequencies. In its most common realization, MPL leverages microelectromechanical (MEMS) processing techniques to create cubic unit cells with silicon walls, planarized by chemical mechanical polishing (CMP), a technique that is not omnipresent in fabrication facilities. Here we show several variants of MPL, two of which do not require CMP to make the MPL process compatible with low-tech fabrication environments and open the MPL process to a wider audience.
Three-dimensional (3D) metafilms composed of periodic arrays containing single and multiple micrometer-scale vertical split ring resonators per unit cell were fabricated using membrane projection lithography. In contrast to planar and stacked planar structures such as cut wire pairs and fishnet structures, these 3D metafilms have a thickness t ~λd/4, allowing for classical thin film effects in the long wavelength limit. The infrared specular far-field scattering response was measured for metafilms containing one and two resonators per unit cell, and compared to numerical simulations. Excellent agreement in the frequency region below the onset of diffractive scattering was obtained. The metafilms demonstrate strong bi-anisotropic polarization dependence. Further, we show that for 3D metafilms, just as in solids, complex unit cells with multiple atoms (inclusions) per unit cell possess a richer set of excitation mechanisms. The highlight of these new coupling mechanisms is the excitation of the 3D analog to the 2D cut-wire-pair magnetic response.
KEYWORDS: Plasmonics, Chemical elements, Dielectrics, 3D acquisition, 3D modeling, Light-matter interactions, Wavefronts, Interfaces, Antennas, Magnetism
Metasurface optics have offered a fresh perspective into light-matter interactions, providing an unsurpassed means to engineer the wavefront of light transiting a subwavelength interface. However, strictly planar surface architectures using conventional antenna elements have performed quite inefficiently, as they contain purely electric modes and thus do not possess the magnetic modes necessary to generate the optimal Huygens-like scattering profile. And while multi-layer stacks of these 2D sheet admittances have been consistently demonstrated as the only feasible solution to-date for plasmonic-based Huygens-like metastructures, their experimental performance is often degraded by non-analytical behavior or fabrication limitations, leaving dielectric architectures as the best hope for real-world metasurface optical applications. In this work, we propose a new alternative for highly-efficient plasmonic metasurfaces: a 3D architecture which produces a Huygens-like total field and exhibits transmittances of approximately 80% at any targeted phase retardation across the full 2π phase space. The 3D unit cell consists of a cubic silicon cavity, with the interior walls of the cavity modeled as grids of voxels. The grids are initially represented in a binary fashion as a random assortment of either a metal (gold) or a dielectric (air), then iterated through a genetic algorithm routine, flipping the value of individual voxels until a maximum transmittance was reached at the desired total field phase retardation. Optimized designs for eight phase values were chosen to construct a metasurface lens. Simulation, fabrication and experimental results of both the individual element and the lens are presented.
This paper demonstrates that another class of three-dimensional integrated circuits (3-D-ICs) exists, distinct from through-silicon-via-centric and monolithic 3-D-ICs. Furthermore, it is possible to create devices that are 3-D “at the device level” (i.e., with active channels oriented in each of the three coordinate axes), by performing standard CMOS fabrication operations at an angle with respect to the wafer surface into high aspect ratio silicon substrates using membrane projection lithography (MPL). MPL requires only minimal fixturing changes to standard CMOS equipment, and no change to current state-of-the-art lithography. Eliminating the constraint of two-dimensional planar device architecture enables a wide range of interconnect topologies which could help reduce interconnect resistance/capacitance, and potentially improve performance.
This paper demonstrates that another class of three-dimensional integrated circuits (3D-ICs) exists, distinct from through silicon via centric and monolithic 3D-ICs. Furthermore, it is possible to create devices that are 3D at the device level (i.e. with active channels oriented in each of the three coordinate axes), by performing standard CMOS fabrication operations at an angle with respect to the wafer surface into high aspect ratio silicon substrates using membrane projection lithography (MPL). MPL requires only minimal fixturing changes to standard CMOS equipment, and no change to current state-of-the-art lithography. Eliminating the constraint of 2D planar device architecture enables a wide range of new interconnect topologies which could help reduce interconnect resistance/capacitance, and potentially improve performance.
Interferometrically defined 3D photoresist scaffolds are formed through a series of three successive two-beam
interference exposures, a post exposure bake and development. Heating the resist scaffold in a reducing atmosphere to >
1000 °C, results in the conversion of the resist structure into a carbon scaffold through pyrolysis, resulting in a 3D sp3-
bonded glassy carbon scaffold which maintains the same in-plane morphology as the resist despite significant shrinkage.
The carbon scaffolds are readily modified using a variety of deposition methods such as electrochemical, sputtering and
CVD/ALD. Remarkably, sputtering metal into scaffolds with ~ 5 unit cells tall results in conformal coating of the
scaffold with the metal. When the metal is a transition metal such as nickel, the scaffold can be re-annealed, during
which time the carbon diffuses through the nickel, emerging on the exterior of the nickel as sp2-bonded carbon, termed
3D graphene. This paper details the fabrication, characterization and some potential applications for these structures.
The spectral behaviors of an externally-illuminated thermal infrared metamaterial were characterized through simulation and experimental measurement of the power transmittance and reflectance within the 6 - 20μm range. Finite-difference time domain (FDTD) simulations in both 2-D and 3-D environments were swept over a multitude of bent dipole inclusion configurations at normal incidence angles to produce a model which exhibited a dominant electrical resonance in the long-wave infrared (IR) and increased in magnitude, bandwidth and wavelength as a function of the dipole length. Despite the appearance of fabrication defects in the measured samples, it was found the experimental data was in good agreement with the 3-D FDTD simulations, though not at all with the 2-D simulations. These introductory results indicate the dipole inclusion may behave in many ways similar to an antenna in the IR, enabling spectrally- and spatially-selective control of the emission pattern.
Recently we have demonstrated membrane projection lithography (MPL) as a fabrication approach capable of
creating 3D structures with sub-micron metallic inclusions for use in metamaterial and plasmonic applications using
polymer material systems. While polymers provide several advantages in processing, they are soft and subject to
stress-induced buckling. Furthermore, in next generation active photonic structures, integration of photonic
components with CMOS electronics is desirable. While the MPL process flow is conceptually simple, it requires
matrix, membrane and backfill materials with orthogonal processing deposition/removal chemistries. By
transitioning the MPL process flow into an entirely inorganic material set based around silicon and standard CMOS-compatible materials, several elements of silicon microelectronics can be integrated into photonic devices at the
unit-cell scale. This paper will present detailed fabrication and characterization data of these materials, emphasizing
the processing trade space as well as optical characterization of the resulting structures.
This paper describes recent results from the Extremely High Temperature Photonic Crystal
System Technology (XTEMPS) technology program. The XTEMPS program has developed a
Photonic Crystal (PhC) based high efficiency IR emitter array for use in the emerging generation
of wide field of view high performance scene projectors. Cyan's approach provides high
dynamic range, multispectral emission from SWIR to LWIR and is uniquely capable of
accurately simulating very realistic system spectral signatures. The PhC array is fabricated from
refractory materials to provide high radiance and long service lifetime. Cyan is teamed with
Sandia National Laboratories for design and fabrication of the emitter and with Nova sensors to
utilize their advanced Read In Integrated Circuit (RIIC). PhC based emitters show improved inband
output power efficiency when compared to broad band "graybody" emitters due to the
absence of out-of-band emission. Less electrical power is required to achieve high operating
temperature, and non-Lambertian emission pattern puts a large fraction of the emitted energy
into a straight ahead beam. Both effects significantly boost effective radiance output. Cyan has
demonstrated pixel designs compatible with Nova's medium format RIIC, which ensures high
apparent output temperatures with modest drive currents and low operating voltages of less than
five volts. Unit cell pixel structures for high radiative efficiency have been demonstrated and
arrays using PhC optimized for up to four spectral bands have been successfully patterned and
fabricated into high yield wafers.
The magnetostatic response of a variety of 3D metallic loop traces are studied numerically by evaluating the Biot-
Savart law as a first step in understanding the radiative behavior of such structures. These numerical studies confirm
that the magnetostatic behavior of localized planar and non-planar current distributions are equivalent to magnetic
dipoles in the far field, however the near-field behavior of these traces can be quite different.
Membrane projection lithography (MPL) has been demonstrated as a fabrication method for creation of layers of 3D unit cell metamaterials. Here we report an extensive modeling study of the electromagnetic behavior of split ring resonator (SRR) based metamaterial layers using rigorous coupled wave analysis, with particular attention to the MPL fabrication related aspects.
This paper describes results from the Extremely High Temperature Photonic Crystal System
Technology (XTEMPS) program. The XTEMPS program is developing projector technology
based on photonic crystals capable of high dynamic range, multispectral emission from SWIR to
LWIR, and realistic band widths. These Photonics Crystals (PhC) are fabricated from refractory
materials to provide high radiance and long device lifetime. Cyan is teamed with Sandia
National Laboratories, to develop photonics crystals designed for realistic scene projection
systems and Nova sensors to utilize their advanced Read In Integrated Circuit (RIIC). PhC based
emitters show improved in-band output power efficiency when compared to broad band
"graybody" emitters due to the absence of out-of-band emission. Less electrical power is
required to achieve high operating temperature, and the potential for nonequilibrium pumping
exists. Both effects boost effective radiance output. Cyan has demonstrated pixel designs
compatible with Nova's medium format RIIC, ensuring high apparent output temperatures,
modest drive currents, and low operating voltages of less than five volts. Unit cell pixel
structures with high radiative efficiency have been demonstrated, and arrays using PhC
optimized for up to four spectral bands have been successfully patterned.
Straightforward extension of canonical microwave metamaterial structures to optical and IR frequency dimensions is
complicated by both the size scale of the resulting structures, requiring cutting edge lithography to achieve the requisite
line-widths, as well as limitations on assembly/construction into final geometry. We present a scalable fabrication
approach capable of generating metamaterial structures such as split ring resonators and split wire pairs on a micron/sub-micron
size scale on concave surfaces with a radius of curvature ~ SRR diameter. This talk outlines the fabrication
method and modeling/theory based interpretation of the implications of curved metamaterial resonators.
Cyan Systems is developing a new Extremely High Temperature Projector System Technology (XTEMPS). The XTEMPS is a multispectral emitter array based upon photonic crystals, providing high radiance and tailored spectral emission in infrared (IR) bands of interest. Cyan has teamed with a state of the art MEMS fabrication facility, Sandia National Laboratories, to develop metallic photonics crystals designed for scene projection systems. Photonic crystals have improved output power efficiency when compared to broad band "graybody" emitters due to limiting the emission to narrow bands. Photonic crystal based emitter pixels have potential for higher effective radiance output, while filtering out energy in the forbidden bandgap. Cyan has developed pixel designs using a medium format RIIC from Nova Sensors that ensures high apparent output temperatures with modest drive currents, and low voltage requirement goals of < 5 V. Cyan has developed a pixel structure for high radiative efficiency of the photonic lattice, while suppressing undesired IR sidelobes. Cyan will provide XTEMPS system performance metrics and illustrate with test structures.
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