A novel design for high brightness planar technology light-emitting diodes (LEDs) and LED on-wafer arrays on absorbing substrates is proposed. The design integrates features of passive dielectric cavity deposited on top of an oxide– semiconductor distributed Bragg reflector (DBR), the p–n junction with a light emitting region is introduced into the top semiconductor λ/4 DBR period. A multilayer dielectric structure containing a cavity layer and dielectric DBRs is further processed by etching into a micrometer–scale pattern. An oxide–confined aperture is further amended for current and light confinement. We study the impact of the placement of the active region into the maximum or minimum of the optical field intensity and study an impact of the active region positioning on light extraction efficiency. We also study an etching profile composed of symmetric rings in the etched passive cavity over the light emitting area. The bottom semiconductor is an AlGaAs–AlAs multilayer DBR selectively oxidized with the conversion of the AlAs layers into AlOx to increase the stopband width preventing the light from entering the semiconductor substrate. The approach allows to achieve very high light extraction efficiency in a narrow vertical angle keeping the reasonable thermal and current conductivity properties. As an example, a micro-LED structure has been modeled with AlGaAs-AlAs or AlGaAs-AlOx DBRs and an active region based on InGaAlP quantum well(s) emitting in the orange spectral range at ~610 nm. A passive dielectric SiO2 cavity is confined by dielectric Ta2O5/SiO2 and AlGaAs-AlOx DBRs. Cylindrically–symmetric structures with multiple ring patterns are modeled. It is demonstrated that the extraction coefficient of light to the air can be increased from 1.3% up to above 90% in a narrow vertical angle (full width at half maximum (FWHM) below 20°). For very small oxide–confined apertures ~100nm the narrowing of the FWHM for light extraction can be reduced down to 5°. Consequently high efficiency high brightness arrays of micro-LEDs becomes possible. For single emitters the approach is particularly interesting for oscillator strength engineering allowing high speed data transmission and for single photonics applying single quantum dot (QD) emitters and allowing >90% coupling of the emission into single mode fiber. We also note that for longer wavelength (~1300nm) QDs the thickness of the layers and surface patterns significantly increase allowing greatly reduced processing tolerances and applying further simplifications due to the possibility of using high contrast GaAs-AlOx DBRs.
KEYWORDS: Silicon, Thin film solar cells, Solar cells, Thin films, Antireflective coatings, Diffraction gratings, Numerical simulations, Finite element methods, Thin film solar cells, Refractive index, Interfaces, Polarization, Maxwell's equations, Absorption
Hexagonal sinusoidal nanotextures are well suited to couple light into silicon on glass at normal incidence, as we have shown in an earlier publication [K. Jäger et al., Opt. Express 24, A569 (2016)]. In this manuscript we discuss how these nanotextures perform under oblique incidence illumination. For this numerical study we use a rigorous solver for the Maxwell equations. We discuss nanotextures with periods between 350 nm and 730 nm and an aspect ratio of 0.5.
Maxwell solvers based on the hp-adaptive finite element method allow for accurate geometrical modeling and high numerical accuracy. These features are indispensable for the optimization of optical properties or reconstruction of parameters through inverse processes. High computational complexity prohibits the evaluation of the solution for many parameters. We present a reduced basis method (RBM) for the time-harmonic electromagnetic scattering problem allowing to compute solutions for a parameter configuration orders of magnitude faster. The RBM allows to evaluate linear and nonlinear outputs of interest like Fourier transform or the enhancement of the electromagnetic field in milliseconds. We apply the RBM to compute light-scattering off two dimensional photonic crystal structures made of silicon and reconstruct geometrical parameters.
Optical chirality has been recently suggested to complement the physically relevant conserved quantities of the well-known Maxwell's equations. This time-even pseudoscalar is expected to provide further insight in polarization phenomena of electrodynamics such as spectroscopy of chiral molecules. Previously, the corresponding continuity equation was stated for homogeneous lossless media only. We extend the underlying theory to arbitrary setups and analyse piecewise-constant material distributions in particular. Our implementation in a Finite Element Method framework is applied to illustrative examples in order to introduce this novel tool for the analysis of time-harmonic simulations of nano-optical devices.
An efficient numerical method for computing angle-resolved light scattering off periodic arrays is presented. The method combines finite-element discretization with a Schur complement solver. A significant speed-up of the computations in comparison to standard finite-element method computations is observed.
Fields such as optical metrology and computational lithography require fast and efficient methods for solving the time-harmonic Maxwell's equation. Highly accurate geometrical modelling and numerical accuracy at low computational costs are a prerequisite for any simulation study of complex nano-structured photonic devices. We present a reduced basis method (RBM) for the time-harmonic electromagnetic scattering problem based on the hp-adaptive finite element solver JCMsuite capable of handling geometric and non-geometric parameter dependencies allowing for online evaluations in milliseconds. We apply the RBM to compute light-scattering at optical wavelengths of periodic arrays of fin field-effect transistors (FinFETs) where geometrical properties such as the width and height of the fin and gate can vary in a large range.
Oxide–confined apertures in vertical cavity surface emitting laser (VCSEL) can be engineered such that they promote
leakage of the transverse optical modes from the non– oxidized core region to the selectively oxidized periphery of the
device. The reason of the leakage is that the VCSEL modes in the core can be coupled to tilted modes in the periphery if
the orthogonality between the core mode and the modes at the periphery is broken by the oxidation–induced optical field
redistribution. Three–dimensional modeling of a practical VCSEL design reveals i) significantly stronger leakage losses
for high–order transverse modes than that of the fundamental one as high–order modes have a higher field intensity close
to the oxide layers and ii) narrow peaks in the far–field profile generated by the leaky component of the optical modes.
Experimental 850–nm GaAlAs leaky VCSELs produced in the modeled design demonstrate i) single–mode lasing with
the aperture diameters up to 5μm with side mode suppression ratio >20dB at the current density of 10kA/cm2; and ii)
narrow peaks tilted at 37 degrees with respect to the vertical axis in excellent agreement with the modeling data and
confirming the leaky nature of the modes and the proposed mechanism of mode selection. The results indicate that in–
plane coupling of VCSELs, VCSELs and p–i–n photodiodes, VCSEL and delay lines is possible allowing novel photonic
integrated circuits. We show that the approach enables design of oxide apertures, air–gap apertures, devices created by
impurity–induced intermixing or any combinations of such designs through quantitative evaluation of the leaky
emission.
The finite-element method is a preferred numerical method when electromagnetic fields at high accuracy are to be computed in nano-optics design. Here, we demonstrate a finite-element method using hp-adaptivity on tetrahedral meshes for computation of electromagnetic fields in a device with rough textures. The method allows for efficient computations on meshes with strong variations in element sizes. This enables to use precise geometry resolution of the rough textures. Convergence to highly accurate results is observed.
Rigorous optical simulations of 3-dimensional nano-photonic structures are an important tool in the analysis and optimization of scattering properties of nano-photonic devices or parameter reconstruction. To construct geometrically accurate models of complex structured nano-photonic devices the finite element method (FEM) is ideally suited due to its flexibility in the geometrical modeling and superior convergence properties. Reduced order models such as the reduced basis method (RBM) allow to construct self-adaptive, error-controlled, very low dimensional approximations for input-output relationships which can be evaluated orders of magnitude faster than the full model. This is advantageous in applications requiring the solution of Maxwell's equations for multiple parameters or a single parameter but in real time. We present a reduced basis method for 3D Maxwell's equations based on the finite element method which allows variations of geometric as well as material and frequency parameters. We demonstrate accuracy and efficiency of the method for a light scattering problem exhibiting a resonance in the electric field.
KEYWORDS: Vertical cavity surface emitting lasers, Oxides, Near field optics, Near field, Refractive index, 3D modeling, Waveguides, Reflectivity, Resistance, Semiconductors
Oxide–confined vertical cavity surface emitting lasers (VCSEL) are inherently leaky structures, despite the fact that the oxidized periphery region surrounding the all–semiconductor core has a lower refractive index. The reason is that the VCSEL modes in the non–oxidized core region can be coupled to tilted modes in the selectively oxidized periphery as the orthogonality between the core mode and the modes at the periphery is broken by the oxidation–induced optical field redistribution. Engineered VCSEL designs show that the overlap between the VCSEL mode of the core and the tilted mode in the periphery can reach >30% resulting in significant leakage. Three–dimensional modeling confirms that the leakage losses are much stronger for high order transverse modes which have a higher field intensity close to the oxidized region. Single mode lasing in the fundamental mode can thus proceed up to large aperture diameters. A 850–nm GaAlAs leaky VCSEL based on this concept is designed, modeled and fabricated, showing single–mode lasing with aperture diameters up to 5 μm. Side mode suppression ratio >20dB is realized at the current density of 10kA/cm2 in devices with the series resistance of 90 Ω.
Methods for solving Maxwell’s equations are integral part of optical metrology and computational lithography setups. Applications require accurate geometrical resolution, high numerical accuracy and/or low computation times. We present a finite-element based electromagnetic field solver relying on unstructured 3D meshes and adaptive hp-refinement. We apply the method for simulating light scattering off arrays of high aspect-ratio nano-posts and FinFETs.
We address demands and challenges for GaAs–based Vertical–Cavity Surface–Emitting Lasers (VCSEL) in data communication. High speed modulation (~50Gb/s) at a high reliability can be realized with a proper VCSEL design providing a high differential gain. In cases where extreme temperatures are required electrooptic modulation in duo– cavity VCSELs can be applied as the modulation speed and the differential gain are decoupled. Single mode operation of VCSELs is necessary to counteract the chromatic dispersion of glass fibers and extend distances to above 1 km while using standard multimode fibers. Oxide layer engineering or using of photonic crystals can be applied. Parallel error–free 25Gb/s transmission over OM3 and OM4 multimode fiber (~0.5 and 1 km, respectively) is realized in large aperture oxide–engineered VCSEL arrays. Passive cavity VCSELs with gain medium placed in the bottom DBR and the upper part made of dielectric materials a complete temperature insensitivity of the emission wavelength can be realized. Engineering of the oxide aperture region enables near field vertical cavity lasers. Such devices can operate in a high– order transverse mode with an effective mode angle beyond the angle of the total internal reflection at the semiconductor–air interface. Near filed coupling to optical fibers and waveguides becomes possible in this case.
A new generation of polycrystalline silicon thin film solar cells is currently being developed in laboratories, employing a combination of novel laser or electron beam based liquid phase crystallization (LPC) techniques and single side contacting systems. The lateral grain size of these polycrystalline cells is in the millimeter range at an absorber thickness of up to 10 μm. In this contribution we present a comparative simulation study of several 1D, 2D and 3D nano-optical designs for the substrate / illumination side interface to the several micrometer thick back contacted LPC silicon absorber material. The compared geometries comprise multilayer coatings, gratings with step and continuous profiles as well as combinations thereof. Using the transfer matrix method and a finite element method implementation to rigorously solve Maxwell’s equations, we discuss anti-reflection and scattering properties of the different front interface designs in view of the angular distribution of incident light.
A numerical investigation of a two dimensional integrated fiber grating coupler capable of exciting several LP fiber modes in both TE and TM polarization is presented. Simulation results and an assessment of the numerical complexity of the 3D, fully vectorial finite element model of the device are shown.
KEYWORDS: 3D modeling, Solar cells, Error analysis, MATLAB, Finite element methods, Chemical elements, Nanophotonics, Metrology, Scattering, Device simulation
Rigorous optical simulations are an important tool in optimizing scattering properties of nano-photonic devices and are used, for example, in solar cell optimization. The finite element method (FEM) yields rigorous, timeharmonic, high accuracy solutions of the full 3D vectorial Maxwell's equations1 and furthermore allows for great flexibility and accuracy in the geometrical modeling of these often complex shaped 3D nano-structures. A major drawback of frequency domain methods is the limitation of single frequency evaluations. For example the accurate computation of the short circuit current density of an amorphous silicon/micro-crystalline multi-junction thin film solar cell may require the solution of Maxwell's equations for over a hundred different wavelengths if an equidistant sampling strategy is employed. Also in optical metrology, wavelength scans are frequently used to reconstruct unknown geometrical and material properties of optical systems numerically from measured
scatterometric data. In our contribution we present several adaptive numerical integration and sampling routines and study their efficiency in the context of the determination of generation rate profiles of solar cells. We show that these strategies lead to a reduction in the computational effort without loss of accuracy. We discuss the employment of tangential information in a Hermite interpolation scheme to achieve similar accuracy on coarser grids. We explore the usability of these strategies for scatterometry and solar cell simulations.
KEYWORDS: Glasses, Chemical elements, Finite element methods, Solar cells, Scattering, Electroluminescent displays, Light scattering, 3D modeling, Silicon, Multijunction solar cells
In many experimentally realized applications, e.g. photonic crystals, solar cells and light-emitting diodes, nanophotonic systems are coupled to a thick substrate layer, which in certain cases has to be included as a part of the optical system. The finite element method (FEM) yields rigorous, high accuracy solutions of full 3D vectorial Maxwell's equations1 and allows for great flexibility and accuracy in the geometrical modelling. Time-harmonic FEM solvers have been combined with Fourier methods in domain decomposition algorithms to compute coherent solutions of these coupled system.2, 3 The basic idea of a domain decomposition approach lies in a decomposition of the domain into smaller subdomains, separate calculations of the solutions and coupling of these solutions on adjacent subdomains. In experiments light sources are often not perfectly monochromatic and hence a comparision to simulation results might only be justified if the simulation results, which include interference patterns in the substrate, are spectrally averaged. In this contribution we present a scattering matrix domain decomposition algorithm for Maxwell's equations based on FEM. We study its convergence and advantages in the context of optical simulations of silicon thin film multi-junction solar cells. This allows for substrate lighttrapping to be included in optical simulations and leads to a more realistic estimation of light path enhancement factors in thin-film devices near the band edge.
KEYWORDS: Finite element methods, Electromagnetism, Computer simulations, Metrology, Stanford Linear Collider, Silver, Near field, Sensors, Numerical analysis, Chemical elements
A smart light trapping scheme is essential to tap the full potential of polycrystalline silicon (poly-Si) thin-film solar cells. Periodic nanophotonic structures are of particular interest as they allow to substantially surpass the Lambertian limit from ray optics in selected spectral ranges. We use nanoimprint-lithography for the periodic patterning of sol-gel coated glass substrates, ensuring a cost-effective, large-area production of thin-film solar cell devices. Periodic crystalline silicon nanoarchitectures are prepared on these textured substrates by high-rate silicon film evaporation, solid phase crystallization and chemical etching. Poly-Si microhole arrays in square lattice geometry with an effective thickness of about 2μm and with comparatively large pitch (2 μm) exhibit a large absorption enhancement (A900nm = 52%) compared to a planar film (A900nm ~ 7%). For the optimization of light trapping in the desired spectral region, the geometry of the nanophotonic structures with varying pitch from 600 nm to 800 nm is tailored and investigated for the cases of poly-Si nanopillar arrays of hexagonal lattice geometry, exhibiting an increase in absorption in comparison to planar film attributed to nanophotonic wave optic effects. These structures inspire the design of prospective applications such as highly-efficient nanostructured poly-Si thin-film solar cells and large-area photonic crystals.
A method for automatic computation of parameter derivatives of numerically computed light scattering signals is demonstrated. The finite-element based method is validated in a numerical convergence study, and it is applied to investigate the sensitivity of a scatterometric setup with respect to geometrical parameters of the scattering target. The method can significantly improve numerical performance of design optimization, parameter reconstruction, sensitivity analysis, and other applications.
KEYWORDS: Monte Carlo methods, Absorption, Interfaces, Solar cells, Chemical elements, 3D modeling, Computer simulations, Transparent conductors, Finite element methods, Prisms
Light trapping techniques are one of the key research areas in thin film silicon photovoltaics. Since the 1980s randomly rough textured front transparent oxides (TCOs) have been the methods of choice as light trapping strategies for thin-film devices. Light-trapping efficiency can be optimized by means of optical simulations of nano-structured solar cells. We present a FEM based simulator for 3D rigorous optical modeling of amorphous silicon / microcrystalline silicon tandem thin-film solar cells with randomly textured layer interfaces. We focus strongly on an error analysis study for the presented simulator to demonstrate the numerical convergence of the method and investigate grid and finite element degree refinement strategies in order to obtain reliable simulation results.
This work addresses a versatile modeling of complex photonic integrated circuits (PICs). We introduce a co-simulation solution for combining the efficient modeling capabilities of a circuit-level simulator, based on analytical models of PIC sub-elements and frequency-dependent scattering matrix (S-matrix) description, and an accurate electromagnetic field simulator that implements the finite element method (FEM) for solving photonic structures with complicated geometries. This is exemplified with the model of a coupled-resonator induced transparency (CRIT), where resonator elements are first modeled in the field simulator. Afterwards, the whole structure is created at a circuit level and statistical analysis of tolerances is investigated.
An overview on recent applications of the finite-element method Maxwell-solver JCMsuite to simulation tasks in nanooptics is given. Numerical achievements in the fields of optical metamaterials, plasmonics, photonic crystal fibers, light emitting devices, solar cells, optical lithography, optical metrology, integrated optics, and photonic crystals are summarized.
Nanostructures, like periodic arrays of scatters or low-index gratings, are used to improve the light outcoupling from organic light-emitting diodes (OLED). In order to optimize geometrical and material properties of such structures, simulations of the outcoupling process are very helpful. The finite element method is best suited for an accurate discretization of the geometry and the singular-like field profile within the structured layer and the emitting layer. However, a finite element simulation of the overall OLED stack is often beyond available computer resources. The main focus of this paper is the simulation of a single dipole source embedded into a twofold infinitely periodic OLED structure. To overcome the numerical burden we apply the Floquet transform, so that the computational domain reduces to the unit cell. The relevant outcoupling data are than gained by inverse Flouqet transforming. This step requires a careful numerical treatment as reported in this paper.
Numerical simulations are an important tool for the design of opto-electronical components and devices. In
order to obtain realistic results, a multitude of physical effects and theories have to be included, e.g., Maxwell's
equations for lasing mode computations, heat transfer in active devices, and electronic transport. In our contribution
we perform coupled electro-thermal simulations of high power diode lasers. We analyze the temperature
dependence of the mode profile and far field characteristics. Our results will be compared to experimental
measurements of broad area lasers and will quantitatively describe the effect of thermal blooming.
We present a finite element method (FEM) solver for computation of optical resonance modes in VCSELs. We
perform a convergence study and demonstrate that high accuracies for 3D setups can be attained on standard
computers. We also demonstrate simulations of thero-optical effects in VCSELs.
For nanostructures such as semiconductor quantum dot emitters or biological systems like light harvesting complexes
(photosynthesis) the coupling between individual constituents leads to the formation of delocalized exciton
states. Coherent two dimensional spectroscopy is a versatile tool to investigate the structure of the excitonic
states, whereas nanoplasmonics allows to localize optical fields on a nanoscale: We combine these two methods
in a theoretical study and propose new experiments, such as the two dimensional spectra containing spatial
resolution via localized fields. Using post processing of different spectra with localized fields, we can enhance
certain spectroscopic features in standard coherent spectroscopy, e.g. by suppressing unwanted resonances.
EUV scatterometry is performed on 3D patterns on EUV lithography masks. Numerical simulations of the
experimental setup are performed using a rigorous Maxwell solver. Mask geometry is determined by minimizing
the difference between experimental results and numerical results for varied geometrical input parameters for
the simulations.
Simulations of light scattering off an extreme ultraviolet lithography mask with a 2D-periodic absorber pattern
are presented. In a detailed convergence study it is shown that accurate results can be attained for relatively
large 3D computational domains and in the presence of sidewall-angles and corner-roundings.
Finite element methods (FEM) for the rigorous electromagnetic solution of Maxwell's equations are known to be
very accurate. They possess a high convergence rate for the determination of near field and far field quantities
of scattering and diffraction processes of light with structures having feature sizes in the range of the light
wavelength. We are using FEM software for 3D scatterometric diffraction calculations allowing the application
of a brilliant and extremely fast solution method: the reduced basis method (RBM). The RBM constructs a
reduced model of the scattering problem from precalculated snapshot solutions, guided self-adaptively by an error
estimator. Using RBM, we achieve an efficiency accuracy of about 10-4 compared to the direct problem with
only 35 precalculated snapshots being the reduced basis dimension. This speeds up the calculation of diffraction
amplitudes by a factor of about 1000 compared to the conventional solution of Maxwell's equations by FEM.
This allows us to reconstruct the three geometrical parameters of our phase grating from "measured" scattering
data in a 3D parameter manifold online in a minute having the full FEM accuracy available. Additionally, also
a sensitivity analysis or the choice of robust measuring strategies, for example, can be done online in a few
minutes.
Optical metrology by scatterometry usually bases on the comparison of experimental and modeled light field
data. When solving inverse scatterometric problems, often not only a single simulation has to be carried out, but
multiple electromagnetic field solutions have to be computed for varying material and geometrical parameters
of the system under consideration. Then, high computational times for a single forward solution can make the
complete simulation task infeasible. Table based parameter reconstruction on the other hand has the disadvantage
of long offline computational times for creation of the library. Also an increasing number of variable parameters
can not be handled efficiently.
In this contribution we introduce the reduced basis method for creation of highly accurate reduced order
models of parametrized electromagnetic scattering problems. We apply our method to a real-world EUV metrology
application and show speed up factors of about 3000 in reconstruction time. Instead of several minutes or
hours EUV mask parameters can now be obtained in seconds, i.e., in real-time.
Comparison to direct microscopical measurements of the reconstructed geometry demonstrate the good performance
and maturity of our method.
In many implementations of transparent boundary conditions for resonance problems, spurious modes arise.
We have developed a transparent boundary condition based on the pole condition that has one complex
tuning parameter. Numerical experiments suggest that the artificial eigenvalues are due to badly converged
solutions in the exterior domain and thus are strongly dependent on variations of this parameter while
physical solutions are well converged and thus almost invariant. Hence it is possible to differentiate between
spurious and physical solutions by doing a sensitivity analysis of the eigenvalues.
KEYWORDS: Monte Carlo methods, Silicon, Absorption, Transparent conductors, Interfaces, Solar cells, Glasses, Chemical elements, Silicon films, Optical simulations
We apply a hybrid finite element / transfer matrix solver to calculate generation rate spectra of thin film
silicon solar cells with textured interfaces. Our focus lies on interfaces with statistical rough textures. Due to
limited computational domain size the treatment of such textures requires a Monte Carlo sampling of texture
representations to obtain a statistical average of integral target quantities. This contribution discusses our choice
of synthetic rough interface generation, the Monte Carlo sampling and the need for an incorporation of the cell's
substrate into optical simulation when illumination of the cell happens through the substrate. We present results
of the numerical characterization and generation rates for a single junction cell layout.
We present algorithmic details and applications of the reduced basis method as efficient Maxwell solver to
nanophotonic applications including examples from mask optimization in photolithography and parameter retrieval
in inverse problems, e.g., in optical metrology. The reduced basis method is a currently studied approach
to the multiple solution of problems depending on a number of geometrical, material and source parameters.
Such problems occur frequently in optimization tasks where parameters have to be adjusted in order to minimize
some error functionals or in production environments where deviations from ideal structures have to be
controlled.
Image modeling and simulation are critical to extending the limits of leading edge lithography technologies used
for IC making. Simultaneous source mask optimization (SMO) has become an important objective in the field of
computational lithography. SMO is considered essential to extending immersion lithography beyond the 45nm
node. However, SMO is computationally extremely challenging and time-consuming. The key challenges are due
to run time vs. accuracy tradeoffs of the imaging models used for the computational lithography.
We present a new technique to be incorporated in the SMO flow. This new approach is based on the reduced
basis method (RBM) applied to the simulation of light transmission through the lithography masks. It provides a
rigorous approximation to the exact lithographical problem, based on fully vectorial Maxwell's equations. Using
the reduced basis method, the optimization process is divided into an offline and an online steps. In the offline
step, a RBM model with variable geometrical parameters is built self-adaptively and using a Finite Element
(FEM) based solver. In the online step, the RBM model can be solved very fast for arbitrary illumination
and geometrical parameters, such as dimensions of OPC features, line widths, etc. This approach dramatically
reduces computational costs of the optimization procedure while providing accuracy superior to the approaches
involving simplified mask models. RBM furthermore provides rigorous error estimators, which assure the quality
and reliability of the reduced basis solutions.
We apply the reduced basis method to a 3D SMO example. We quantify performance, computational costs
and accuracy of our method.
Optical resonances in 1D photonic crystal microcavities are investigated numerically using finite-element light
scattering and eigenmode solvers. The results are validated by comparison to experimental and theoretical
findings from the literature. The influence of nanometer-scale geometry variations on the resonator performance
is studied. Limiting factors to ultra-high Q-factor performance are identified.
Optical properties of circular grating resonators in a silicon-on-insulator system are investigated numerically.
These structures are of interest as building-blocks of integrated photonic devices. We use a time-harmonic 3D
finite-element solver for studying transmission of waveguide modes through the system. We compare numerical
results to experimental findings from the literature.
Optical properties of hybrid plasmonic waveguides and of low-Q cavities, formed by waveguides of finite length
are investigated numerically. These structures are of interest as building-blocks of plasmon lasers. We use
a time-harmonic finite-element package including a propagation-mode solver, a resonance-mode solver and a
scattering solver for studying various properties of the system. Numerical convergence of all used methods is
demonstrated.
A bottleneck for computational lithography and optical metrology are long computational times for near field
simulations. For design, optimization, and inverse scatterometry usually the same basic layout has to be simulated
multiple times for different values of geometrical parameters.
The reduced basis method allows to split up the solution process of a parameterized model into an expensive
offline and a cheap online part. After constructing the reduced basis offline, the reduced model can be solved
online very fast in the order of seconds or below. Error estimators assure the reliability of the reduced basis
solution and are used for self adaptive construction of the reduced system.
We explain the idea of reduced basis and use the finite element solver JCMsuite constructing the reduced
basis system. We present a 3D optimization application from optical proximity correction (OPC).
Numerical design inverse reconstruction and parameter estimation of optical systems usually involves the multiple
solution of an e.g. geometrically parameterized system. Long computational times however can rule out many
possible applications like inverse scatterometry.
The reduced basis method allows to split up the solution process of an e.g. geometrically parameterized
system into an expensive offline and a cheap online part. In the offline phase the reduced basis is computed selfadaptively
by solving the underlying model several times. During the real-time application the reduced system
is solved in the order of seconds even for 3D problems. Error estimators assure the reliability of the reduced
basis solutions.
In our contribution we explain general ideas of the reduced basis method and apply it to the simulation of
light scattering from 2D and 3D parameterized photo masks. We compare computational times and accuracy of
reduced basis and rigorous finite element simulations.
Light transmission through a 2D-periodic array of small rectangular apertures in a film of highly conductive
material is simulated using a finite-element method. It is demonstrated that well converged results are obtained
using higher-order finite-elements. The influence of the array periodicity and of corner roundings on transmission
properties is investigated.
Light transmission through circular subwavelength apertures in metallic films with surrounding nanostructures
is investigated numerically. Numerical results are obtained with a frequency-domain finite-element method.
Convergence of the obtained observables to very low levels of numerical error is demonstrated. Very good
agreement to experimental results from the literature is reached, and the utility of the method is demonstrated
in the investigation of the influence of geometrical parameters on enhanced transmission through the apertures.
We have developed an interface which allows to perform rigorous electromagnetic field (EMF) simulations with
the simulator JCMsuite and subsequent aerial imaging and resist simulations with the simulator Dr.LiTHO.With
the combined tools we investigate the convergence of near-field and far-field results for different DUV masks.
We also benchmark results obtained with the waveguide-method EMF solver included in Dr.LiTHO and with
the finite-element-method EMF solver JCMsuite. We demonstrate results on convergence for dense and isolated
hole arrays, for masks including diagonal structures, and for a large 3D mask pattern of lateral size 10 microns
by 10 microns.
KEYWORDS: Scattering, Maxwell's equations, Photomasks, Radio propagation, Waveguides, Chemical elements, Scatterometry, Magnetism, Electromagnetism, Systems modeling
Rigorous electromagnetic field simulations are an essential part for scatterometry and mask pattern design.
Today mainly periodic structures are considered in simulations. Non-periodic structures are typically modeled
by large, artificially periodified computational domains. For systems with a large radius of influence this leads to
very large computational domains to keep the error sufficiently small. In this paper we review recent advances
in the rigorous simulation of isolated structures embedded into a surrounding media. We especially address the
situation of a layered surrounding media (mask or wafer) with additional infinite inhomogeneities such as resist
lines. Further we detail how to extract the far field information needed for the aerial image computation in the
non-periodic setting.
Extreme ultraviolet (EUV) lithography is seen as a main candidate for production of future generation computer
technology. Due to the short wavelength of EUV light (≈ 13 nm) novel reflective masks have to be used in the
production process. A prerequisite to meet the high quality requirements for these EUV masks is a simple and
accurate method for absorber pattern profile characterization.
In our previous work we demonstrated that the Finite Element Method (FEM) is very well suited for the simulation
of EUV scatterometry and can be used to reconstruct EUV mask profiles from experimental scatterometric
data.
In this contribution we apply an indirect metrology method to periodic EUV line masks with different critical
dimensions (140 nm and 540 nm) over a large range of duty cycles (1:2, ... , 1:20). We quantitatively compare
the reconstructed absorber pattern parameters to values obtained from direct AFM and CD-SEM measurements.
We analyze the reliability of the reconstruction for the given experimental data. For the CD of the absorber
lines, the comparison shows agreement of the order of 1nm.
Furthermore we discuss special numerical techniques like domain decomposition algorithms and high order
finite elements and their importance for fast and accurate solution of the inverse problem.
Scatterometry, the analysis of light diffracted from a periodic structure, is a versatile metrology for characterizing
periodic structures, regarding critical dimension (CD) and other profile properties. For extreme ultraviolet (EUV) masks,
only EUV radiation provides direct information on the mask performance comparable to the operating regime in an EUV
lithography tool. With respect to the small feature dimensions on EUV masks, the short wavelength of EUV is also
advantageous since it increases the sensitivity for small structural details. Measurements using PTB's EUV reflectometer
at the storage ring BESSY II showed that it is feasible to derive information on the absorber line profile in periodic areas
of lines and spaces by means of rigorous numerical modeling with the finite element method (FEM). A prototype EUV
mask with fields of nominally identical lines was used for the measurements. In this contribution we correlate the
scatterometry data to CD-SEM and surface nano probe measurements of the line profiles as provided by the mask
supplier. We discuss status of the determination of CD and side-wall geometry by scatterometry using rigorous FEM
calculations of EUV diffraction and directions for further investigations.
We formulate the mask modeling as a parametric model order reduction problem. We then apply a robust reduction
technique to generate the compact mask model. Since this model is based on first principle, it naturally includes diffraction,
polarization and couplings, important effects that are poorly handled by the modified thin-mask model (MTMM). The
model generation involves only a few sampling solves of the governing equation, much fewer than that needed to generate
MTMM. Though the model evaluation takes marginally more CPU time than MTMM, the accuracy and the robustness of
the new model are based on much more rigorous theoretical foundation.
Rigorous computer simulations of propagating electromagnetic fields have become an important tool for optical
metrology and design of nanostructured optical components. A vectorial finite element method (FEM) is a good
choice for an accurate modeling of complicated geometrical features. However, from a numerical point of view
solving the arising system of linear equations is very demanding even for medium sized 3D domains. In numerics,
a domain decomposition method is a commonly used strategy to overcome this problem. Within this approach
the overall computational domain is split up into smaller domains and interface conditions are used to assure
continuity of the electromagnetic field. Unfortunately, standard implementations of the domain decomposition
method as developed for electrostatic problems are not appropriate for wave propagation problems. In an earlier
paper we therefore proposed a domain decomposition method adapted to electromagnetic field wave propagation
problems. In this paper we apply this method to 3D mask simulation.
We discuss realization, properties and performance of the adaptive finite element approach to the design of optical
waveguides. Central issues like the construction of higher-order vectorial finite elements, local error estimation,
automatic and adaptive grid refinement, transparent boundary conditions and fast linear system solution by
domain decomposition techniques will be discussed.
We present rigorous 3D EMF simulations of isolated features on photomasks using a newly developed finite-element
method. We report on the current status of the finite-element solver JCMsuite, incorporating higher-order
edge elements, adaptive refinement methods, and fast solution algorithms. We demonstrate that rigorous
and accurate results on light scattering off isolated features can be achived at relatively low computational cost,
compared to the standard approach of simulations on large-pitch, periodic computational domains.
Scatterometry, the analysis of light diffracted from a periodic structure, is a versatile metrology for characterizing
periodic structures, regarding critical dimension (CD) and other profile properties. For extreme ultraviolet (EUV) masks,
only EUV radiation provides direct information on the mask performance comparable to the operating regime in an EUV
lithography tool. With respect to the small feature dimensions on EUV masks, the short wavelength of EUV is also
advantageous since it provides more diffraction orders as compared to UV. First measurements using PTB's EUV
reflectometer at the storage ring BESSY II showed that it is feasible to derive information on the line profile in periodic
areas of lines and spaces by means of rigorous numerical modeling. A prototype EUV mask with a matrix of test fields
each divided into subfields containing among others test fields with lines & spaces was used for the measurements. In this
contribution we summarize our present results in determining line profile parameters using scatterometry and
reflectometry to provide the input data for the determination of CD and side-wall geometry using rigorous calculations of
EUV diffraction. Particularly, we present a first investigation on the influence of line edge roughness and CD uniformity
by correlating in-plane scatterometry data for the discrete diffraction orders corresponding to the pitch of the structure to
out-of-plane measurements of diffusely scattered light induced by line edge roughness and CD uniformity. We
demonstrate the influence of diffuse scattering on the determination of CD and side-wall geometry using only the
discrete in-plane diffraction orders. To this aim we perform finite element (FEM) simulations on 2D computational domains.
Extreme ultraviolet (EUV) lithography is seen as the main candidate for production of next generation computer
technology. Due to the short wavelength of EUV light (≈ 13 nm) novel reflective masks have to be used in the
production process. The high quality requirements for these EUV masks make it necessary to measure and
characterize their pattern profile.
Here we present numerical simulations of EUV masks with the finite element method (FEM) which allow the
reconstruction of geometrical mask parameters like critical dimension (CD), sidewall angles, layer thicknesses
from experimental scatterometry data.1 Special numerical techniques like domain decomposition algorithms and
high order finite elements become very important to obtain accurate numerical results in small computational
time.
We analyze the sensitivity of scatterometry with respect to the geometrical line profile. We demonstrate the
determination of line profiles from experimental scatterometry data and compare our values to direct microscopic
measurements using CD-SEM and AFM.
Rigorous computer simulations of propagating electromagnetic fields have become an important tool for optical
metrology and optics design of nanostructured components. As has been shown in previous benchmarks some of
the presently used methods suffer from low convergence rates and/or low accuracy of the results and exhibit very
long computation times1, 2 which makes application to extended 2D layout patterns impractical. We address 3D
simulation tasks by using a finite-element solver which has been shown to be superior to competing methods by
several orders of magnitude in accuracy and computational time for typical microlithography simulations.2 We
report on the current status of the solver, incorporating higher order edge elements, adaptive refinement methods,
and fast solution algorithms. Further, we investigate the performance of the solver in the 3D simulation project
of light diffraction off an alternating phase-shift contact-hole mask.
Adaptive finite elements are the method of choice for accurate simulations of optical components. However as
shown recently by Bienstman et al. many finite element mode solvers fail to compute the propagation constant's
imaginary part of a leaky waveguide with sufficient accuracy. In this paper we show that with a special goal
oriented error estimator for capturing radiation losses this problem is overcome.
Hollow-core holey fibers are promising candidates for low-loss guidance of light in various applications, e.g., for the use in laser guide star adaptive optics systems in optical astronomy. We present an accurate and fast method for the computation of light modes in arbitrarily shaped waveguides. Maxwell's equations are discretized using vectorial finite elements (FEM). We discuss how we utilize concepts like adaptive grid refinement, higher-order finite elements, and transparent boundary conditions for the computation of leaky modes in photonic crystal fibers. Further, we investigate the convergence behavior of our methods. We employ our FEM solver to design hollow-core photonic crystal fibers (HCPCF) whose cores are formed from 19 omitted cladding unit cells. We optimize the fiber geometry for minimal attenuation using multidimensional optimization taking into account radiation loss (leaky modes).
We present rigorous simulations of EUV masks with technological imperfections like side-wall angles and corner roundings. We perform an optimization of two different geometrical parameters in order to fit the numerical results to results obtained from experimental scatterometry measurements. For the numerical simulations we use an adaptive finite element approach on irregular meshes. This gives us the opportunity to model geometrical structures accurately. Moreover we comment on the use of domain decomposition techniques for EUV mask simulations. Geometric mask parameters have a great influence on the diffraction pattern. We show that using accurate simulation tools it is possible to deduce the relevant geometrical parameters of EUV masks from scatterometry measurements. This work results from a collaboration between AMTC (mask fabrication), Physikalisch-Technische Bundesanstalt (scatterometry) and ZIB/JCMwave (numerical simulation).
We perform 3D lithography simulations by using a finite-element
solver.
To proof applicability to real 3D problems we investigate
DUV light propagation through a structure of size 9μm x 4μm x 65nm.
On this relatively large computational domain we
perform rigorous computations (No Hopkins) taking into account
a grid of 11 x 21 source points with two polarization directions
each.
We obtain well converged results with an accuracy of the
diffraction orders of about 1%.
The results compare well to experimental aerial imaging results.
We further investigate the convergence of 3D solutions towards
quasi-exact results obtained with different methods.
Rigorous numerical simulations of Maxwell's equations are extremely helpful in the understanding of physical effects in nano-optics and are essential for the design of nano-optical devices. We have developed a finite-element (FEM) package for the solution of eigenvalue and scattering problems resulting from Maxwell's equations. We have implemented higher order vectorial elements, adaptive mesh refinement, transparent boundary conditions based on the Pole condition, and fast algorithms. In this contribution we discuss the application of the FEM solvers to recent experiments in nano-photonics.
Miniaturized optical resonators with spatial dimensions of the order of the wavelength of the trapped light offer prospects for a variety of new applications like quantum processing or construction of meta-materials. Light propagation in these structures is modelled by Maxwell's equations. For a deeper numerical analysis one may compute the scattered field when the structure is illuminated or one may compute the resonances of the structure. We therefore address in this paper the electromagnetic scattering problem as well as the computation of resonances in an open system. For the simulation effcient and reliable numerical methods are required which cope with the infinite domain. We use transparent boundary conditions based on the Perfectly Matched Layer Method (PML) combined with a novel adaptive strategy to determine optimal discretization parameters like the thickness of the sponge layer or the mesh width. Further a novel iterative solver for time-harmonic Maxwell's equations is presented.
An extremely fast time-harmonic finite element solver developed for the transmission analysis of photonic crystals was applied to mask simulation problems. The applicability was proven by examining a set of typical problems and by a benchmarking against two established methods (FDTD and a differential method) and an analytical example. The new finite element approach was up to 100 times faster than the competing approaches for moderate target accuracies, and it was the only method which allowed to reach high target accuracies.
KEYWORDS: Magnetism, Split ring resonators, Geometrical optics, Near infrared, Light scattering, Chemical elements, Metamaterials, Finite element methods, Polarization, Gold
It seems to be feasible in the near future to exploit the properties of left-handed metamaterials in the telecom or even in the optical regime. Recently, split ring-resonators (SRR's) have been realized experimentally in the near infrared (NIR) and optical regime.1, 2 In this contribution we numerically investigate light propagation through an array of metallic SRR's in the NIR and optical regime and compare our results to experimental results. We find numerical solutions to the time-harmonic Maxwell's equations by using advanced finite-element-methods (FEM). The geometry of the problem is discretized with unstructured tetrahedral meshes. Higher order, vectorial elements (edge elements) are used as ansatz functions. Transparent boundary conditions (a modified PML method3) and periodic boundary conditions4 are implemented, which allow to treat light scattering problems off periodic structures.
This simulation tool enables us to obtain transmission and reflection spectra of plane waves which are incident onto the SRR array under arbitrary angles of incidence, with arbitrary polarization, and with arbitrary wavelength-dependencies of the permittivity tensor. We compare the computed spectra to experimental results and investigate resonances of the system.
We present a finite-element simulation tool for calculating
light fields in 3D nano-optical devices. This allows to solve challenging problems on a standard personal computer. We present solutions to eigenvalue problems, like Bloch-type eigenvalues in photonic crystals and photonic crystal waveguides, and to scattering problems, like the transmission through finite photonic crystals.
The discretization is based on unstructured tetrahedral grids with
an adaptive grid refinement controlled and steered by an error-estimator. As ansatz functions we use higher order, vectorial elements (Nedelec, edge elements). For a fast convergence of the solution we make use of advanced multi-grid algorithms adapted for the vectorial Maxwell's equations.
We present our simulation tool JCMmode for calculating propagating modes of an optical waveguide. As ansatz functions we use higher order, vectorial elements (Nedelec elements, edge elements). Further we construct transparent boundary conditions to deal with leaky modes even for problems with inhomogeneous exterior domains as for integrated hollow core Arrow waveguides. We have implemented an error estimator which steers the adaptive mesh refinement. This allows the precise computation of singularities near the metal's corner of a Plasmon-Polariton waveguide even for irregular shaped metal films on a standard personal computer.
We review key algorithms for the numerical solution of waveguide eigenvalue problems and discuss their application to a typical simulation problem in integrated optics - the computation of eigenmodes of a MQW laser structure. Here we focus mainly on a self adaptive realization of the codes supplying solutions with prescribed
accuracy in a CPU-time as short as possible. In a brief outlook we show how to extend these principles to solve general time-harmonic and time-dependent scattering problems in an adaptive finite element context.
The finite element method is a successful tool to investigate integrated optics devices, both for stationary as well as for wave propagation problems. Despite the fact that different functionals and discretizations are considered in the literature, in practice most of these approaches use piecewise linear basis functions to approximate the true solution. However, in the case of wave propagation these functions may become numerically inefficient. Therefore our proposal is to construct basis functions fitting the local situation better than the linear standard functions. We introduce new basis functions as the product of linear polynomials and local phase functions. These local phases functions are exponential functions characterized by a wave number, which in general changes in space but is assumed to be constant over a single finite element. The closer the a-priori fixed wave number resembles the true local wave number, the more efficient the simulation will be. The multilevel finite element scheme supplies a well-suited frame to determine the local wave number in an adaptive manner.
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