Substrate Conformal Imprint Lithography Research Articles

Nanopatterned Back-Reflector with Engineered Near-Field/Far-Field Light Scattering for Enhanced Light Trapping in Silicon-Based Multijunction Solar Cells.

Multijunction solar cells provide a path to overcome the efficiency limits of standard silicon solar cells by harvesting a broader range of the solar spectrum more efficiently. However, Si-based multijunction architectures are hindered by incomplete harvesting in the near-infrared (near-IR) spectral range as Si subcells have weak absorption close to the band gap. Here, we introduce an integrated near-field/far-field light trapping scheme to enhance the efficiency of silicon-based multijunction solar cells in the near-IR range. To achieve this, we design a nanopatterned diffractive silver back-reflector featuring a scattering matrix that optimizes trapping of multiply scattered light into a range of diffraction angles. We minimize reflection to the zeroth order and parasitic plasmonic absorption in silver by engineering destructive interference in the patterned back-contact. Numerical and experimental assessment of the optimal design on the performance of single-junction Si TOPCon solar cells highlights an improved external quantum efficiency over a planar back-reflector (+1.52 mA/cm2). Nanopatterned metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal triple-junction solar cells via substrate conformal imprint lithography and characterized optically and electronically, demonstrating a power conversion efficiency improvement of +0.9%abs over the planar reference. Overall, our work demonstrates the potential of nanophotonic light trapping for enhancing the efficiency of silicon-based multijunction solar cells, paving the way for more efficient and sustainable solar energy technologies.

Bottom-up Filling of Nanosized Trenches with Silver to Fabricate Transparent Conducting Electrodes

Transparent conducting electrodes (TCEs) are essential in many optoelectronic devices including solar cells, sensors, displays, and LEDs. The use of transparent metal oxide layers, in particular Indium tin oxide (ITO), is the most common used approach. However, ITO comes with numerous drawbacks such as the relatively rarity of indium and hence high cost, fragility, strong UV absorption and relative high sheet resistance. An exciting alternative TCE is the use of interconnected metallic nanowires, where their sub-wavelength cross-sections makes them transparent to visible light, where especially silver nanowires are a good candidate because of their high conductivity.Here we show a promising cost-effective method called selective-area electrochemical deposition (SAEC) for the fabrication of these metal nanostructures. SAEC is based on a through-the-mask electrochemical deposition method, where the mask is made using substrate conformal imprint lithography. Next to the inherent low fabrication cost of the electrochemical deposition, it has numerous advantages such as bottom-up growth, scalability, operate at ambient conditions, control over nucleation, and yields high purity deposits. While submicron sized features have been already demonstrated by SAEC, obtaining homogeneous void free filling of features below 100 nm using direct plating is a major challenge because of the terminal effect and the poor control of nucleation of the seed layer.In this work, we have explored the SAEC fabrication of large area silver nanowire grids on ITO substrates and on TOPCon solar cells. We show that the nucleation density of the silver nanoparticles is the key parameter for the successful homogenous void-free filling of the trenches. The nucleation density namely determines the coalescence thickness, which sets a minimum thickness for the silver nanowires in order to be connected. We control the nucleation density of the silver nanoparticles by making use of the double pulse method, where a high overpotential nucleation pulse is applied for controlling the nucleation density. After nucleation, we merge the silver nuclei by applying a low overpotential growth pulse.After the successful filling of the trenches, we have compared the optical and electrical performance of the SAEC fabricated silver nanowire grids to those using conventional thermal or e-beam evaporation followed by lift-off. The SAEC-fabricated TCEs show high transmission (>96%) and extreme low resistance (as low as 3 Ω/sq), resulting into a superior figure of merit (FoM). Due to the bottom-up nature of this technique, arbitrary high aspect ratio nanowires can be achieved and therefore the relationship of decreasing transmission with decreasing sheet resistance is broken.Furthermore, we have also incorporated the silver NW grids in TOPCon cells and compared the performance to similar cells based on ITO. This work represents a low-cost and scalable strategy to fabricate sub 100 nm predefined metal structures, enabling the commercial viable fabrication of nanostructure based transparent conductive electrodes for optoelectronic devices.

Substrate conformal imprint fabrication process of synthetic antiferromagnetic nanoplatelets

Methods to fabricate and characterize monodisperse magnetic nanoplatelets for fluid/bio-based applications based on spintronic thin-film principles are a challenge. This is due to the required top-down approach where the transfer of optimized blanket films to free particles in a fluid while preserving the magnetic properties is an uncharted field. Here, we explore the use of substrate conformal imprint lithography (SCIL) as a fast and cost-effective fabrication route. We analyze the size distribution of nominal 1.8 μm and 120 nm diameter platelets and show the effect of the fabrication steps on the magnetic properties which we explain through changes in the dominant magnetization reversal mechanism as the size decreases. We show that SCIL allows for efficient large-scale platelet fabrication and discuss how application-specific requirements can be solved via process and material engineering.

Nanopatterning of Perovskite Thin Films for Enhanced and Directional Light Emission.

Lead-halide perovskites offer excellent properties for lighting and display applications. Nanopatterning perovskite films could enable perovskite-based devices with designer properties, increasing their performance and adding novel functionalities. We demonstrate the potential of nanopatterning for achieving light emission of a perovskite film into a specific angular range by introducing periodic sol–gel structures between the injection and emissive layer by using substrate conformal imprint lithography (SCIL). Structural and optical characterization reveals that the emission is funnelled into a well-defined angular range by optical resonances, while the emission wavelength and the structural properties of the perovskite film are preserved. The results demonstrate a flexible and scalable approach to the patterning of perovskite layers, paving the way toward perovskite LEDs with designer angular emission patterns.

Novel optical metrology for inspection of nanostructures fabricated by substrate conformal imprint lithography

Substrate conformal imprint lithography (SCIL) technology enables the fabrication of complex and non-trivial 3D nanostructures such a slanted gratings and metasurfaces with sub-10 nm resolution over large areas for industrial-scale production, which can be fabricated in a single lithography step. This technology utilizes novel composite silicone rubber stamps that provide versatility in addition to high precision. To inspect the quality and reproducibility of the nanostructures that are fabricated using SCIL, a novel optical characterization method using Fourier microscopy is proposed. In this method, nanostructures are illuminated under a microscope objective using a collimated light beam at different incident angles and the properties of the reflected and/or diffracted beams are analysed to extract the critical dimensions of the nanostructures. This fast and non-destructive method has the potential for being used as an in-line inspection technology to extract the critical dimensions of the nanostructures over large areas and improve the overall properties of nanostructured surfaces.

X-ray verification of sol-gel resist shrinkage in substrate-conformal imprint lithography for a replicated blazed reflection grating

Surface-relief gratings fabricated through nanoimprint lithography (NIL) are prone to topographic distortion induced by resist shrinkage. Characterizing the impact of this effect on blazed diffraction efficiency is particularly important for applications in astrophysical spectroscopy at soft x-ray wavelengths (λ ≈ 0.5 − 5 nm) that call for the mass-production of large-area grating replicas with sub-micron, sawtooth surface-relief profiles. A variant of NIL that lends itself well for this task is substrate-conformal imprint lithography (SCIL), which uses a flexible, composite stamp formed from a rigid master template to imprint nanoscale features in an inorganic resist that cures thermodynamically through a silica sol-gel process. While SCIL enables the production of several hundred imprints before stamp degradation and avoids many of the detriments associated with large-area imprinting in NIL, the sol-gel resist suffers shrinkage dependent on the post-imprint cure temperature. Through atomic force microscopy and diffraction-efficiency testing at beamline 6.3.2 of the Advanced Light Source, the impact of this effect on blaze response is constrained for a ∼160-nm-period grating replica cured at 90°C. Results demonstrate a ∼2° reduction in blaze angle relative to the master grating, which was fabricated by anisotropic wet etching in 〈311〉-oriented silicon to yield a facet angle close to 30°.

Nanoscale spatial limitations of large-area substrate conformal imprint lithography

We demonstrate a soft-imprint nanofabrication technique offering nanometer resolution over an area as large as a 150 mm diameter wafer. It makes use of a composite imprint stamp composed of a quaternary siloxane-modified poly-di-methyl-siloxane patterned rubber layer with a relatively high Young’s modulus that is laminated on a thin glass support. The in-plane stiffness of the stamp avoids pattern deformation over large areas, while out-of-plane flexibility allows conformal contact to be made over the entire substrate area. The stamp is used in conjunction with a novel tetra-methyl-ortho-siloxane/methyl-tri-methoxy-siloxane sol-gel imprint resist material developed to replicate nanoscale features in rigid silica at room temperature. We demonstrate better than 10 nm resolution in imprinted line gratings and individual pillars with aspect ratio as high as 5:1. Gaps as small as 6 nm can be reproduced. The patterns can be used as an etch mask to pattern 150 mm diameter silicon and quartz substrates while maintaining sub-10 nm resolution.

Submicrometer Top-Gate Self-Aligned a-IGZO TFTs by Substrate Conformal Imprint Lithography

Thin-film transistors (TFTs) are the fundamental building blocks of today's display industry. To achieve higher drive currents and device density, it is essential to scale down the channel lengths of TFTs. To be able to fabricate short-channel TFTs in large volumes is also equally important in order to realize lower fabrication costs and higher throughput. In this paper, we demonstrate the application of substrate conformal imprint lithography (SCIL) to pattern top-gate (TG) self-aligned (SA) amorphous indium gallium zinc oxide TFTs down to channel length LG = 450 nm with good device scaling properties resulting in average field-effect mobility (μFE) = ~10 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> = ~ 0.5 V, and subthreshold swing (SS) = ~0.3 V/decade. The device performance as a function of channel length outlines the importance of dopant diffusion control for realizing submicrometer SA TFTs. The results demonstrate the compatibility of SCIL-based large-area patterning for the realization of submicrometer TG SA TFTs with a potential for high throughput.

One-step nanoimprinted Bragg grating sensor based on hybrid polymers

Bragg grating sensors are used for real-time analysis of gases or liquids. Complex processing methods are typically required for the sensor fabrication. A reduction of process steps and costs during their fabrication is essential in order to broaden the field of application. Within this work, we demonstrate an innovative process for a one-step fabrication of integrated Bragg grating sensors and their successful application for temperature and refractive index sensing. UV-enhanced substrate conformal imprint lithography (UV-SCIL) is used to replicate surface relief Bragg grating (SBG) sensors on a full wafer scale. Multiple chips of a planar waveguide system including couplers and junctions were etched into a silicon wafer. Nanostructured SBGs were locally added on the waveguides by focused ion beam processing. The sensor structures were replicated by UV-SCIL into a commercially available hybrid polymer (OrmoComp®). Reflection measurements with different Bragg gratings were performed and compared to simulations. The results reveal narrow-band Bragg reflections, coinciding with the simulations. Further, the temperature dependence of the SBGs was investigated. The imprinted grating structures featured a very high temperature sensitivity of −202 pm/°C. The sensor response to a varying refractive index of the surrounding medium was determined for index matching liquids and aqueous solutions.

The realization of isotropic multiplexing diffraction patterns and polarization property by two-dimensional imprinting silica periodic photonic crystals

Multiplexing optical devices realized by 2D photonic crystals is a feasible way and hot research topic. Especially, dielectric photonic crystals with isotropic multi-point far-filed diffraction patterns and high diffraction efficiency are essential for the development of multiplexing optical system in optical communication field. However, the fabrication of high quality photonic crystals based on SiO2 dielectric material with large surface area and strict periodicity remains a great challenge. This paper presents a single-step nanoimprint route to fabricate two-dimensional silica photonic nanostructures using silica sol-gel solution combined with substrate conformal imprint lithography (SCIL) technique. The imprinted silica photonic nanostructures, with strictly periodicity of cylindrical imprinted pattern and uniform aspect ratio in a glass substrate with 4 inches, have a good multi-channel diffraction performance in transmission mode and the transmission efficiency can be reached up to 80% at a broad wavelength band. Furthermore, the change of polarization direction in output multi diffraction beams is consistent with the variation of the polarization direction of the injected laser light, which shows potential application as multiplexing devices. The finding provides a low cost and fast imprint method to fabricate high quality 2D silica photonic nanostructures and their outstanding optical performance make them become important components or fundamental building blocks in integrated on chip optical or optoelectronic devices, such as light division multiplexing system.

Overlay accuracy limitations of soft stamp UV nanoimprint lithography and circumvention strategies for device applications

In this work multilevel pattering capabilities of Substrate Conformal Imprint Lithography (SCIL) have been explored. A mix & match approach combining the high throughput of nanoimprint lithography with the excellent overlay accuracy of electron beam lithography (EBL) has been exploited to fabricate nanoscale devices.An EBL system has also been utilized as a benchmarking tool to measure both stamp distortions and alignment precision of this mix & match approach. By aligning the EBL system to 20 mm × 20 mm and 8 mm × 8 mm cells to compensate pattern distortions of order of 3 μm over 6 in. wafer area, overlay accuracy better than 1.2 μm has been demonstrated. This result can partially be attributed to the flexible SCIL stamp which compensates deformations caused by the presence of particles which would otherwise significantly reduce the alignment precision.

Fabrication and characterization of multi-stopband Fabry–Pérot filter array for nanospectrometers in the VIS range using SCIL nanoimprint technology

Miniaturization of optical spectrometers can be achieved by Fabry–Perot (FP) filter arrays. Each FP filter consists of two parallel highly reflecting mirrors and a resonance cavity in between. Originating from different individual cavity heights, each filter transmits a narrow spectral band (transmission line) with different wavelengths. Considering the fabrication efficiency, plasma enhanced chemical vapor deposition (PECVD) technology is applied to implement the high-optical-quality distributed Bragg reflectors (DBRs), while substrate conformal imprint lithography (one type of nanoimprint technology) is utilized to achieve the multiple cavities in just a single step. The FP filter array fabricated by nanoimprint combined with corresponding detector array builds a so-called “nanospectrometer”. However, the silicon nitride and silicon dioxide stacks deposited by PECVD result in a limited stopband width of DBR (i.e., < 100 nm), which then limits the sensing range of filter arrays. However, an extension of the spectral range of filter arrays is desired and the topic of this investigation. In this work, multiple DBRs with different central wavelengths (λc) are structured, deposited, and combined on a single substrate to enlarge the entire stopband. Cavity arrays are successfully aligned and imprinted over such terrace like surface in a single step. With this method, small chip size of filter arrays can be preserved, and the fabrication procedure of multiple resonance cavities is kept efficient as well. The detecting range of filter arrays is increased from roughly 50 nm with single DBR to ~ 163 nm with three different DBRs.

Polymerization related deformations in multilayer soft stamps for nanoimprint

This paper presents a study concerning the macroscale deformation of soft stamps for nanoimprint lithography. Composite stamps consisting of two or three layers were prepared with varying process parameters, and the bow of the stamps was investigated with an optical profilometer. Deflection measurements were compared with results from a mechanical model in order to prove its validity. The composite stamps were prepared from polydimethylsiloxane (S-PDMS) and OrmoStamp® layers on different substrate materials (silicon or AF-32-glass). The influence of selected preparation parameters on the deformation was investigated, such as layer thickness, curing, and hardbake temperatures, as well as UV-exposure intensities. It is discussed how an appropriate choice of preparation parameters can reduce the deformation, too. By decreasing the curing intensity of a thin OrmoStamp layer, coated on top of a silicon wafer, from 200 mW/cm2 to 2 mW/cm2, the deformation was reduced from 6 μm to 0. It is shown that the mechanical model is capable of predicting the deformation of composite stamps consisting of OrmoStamp layers on top of a silicon substrate. Further on, deformation results for S-PDMS top layers are presented and discussed. As a promising process option for the reduction of stamp deformations, a new, room temperature curable PDMS recipe is introduced. For composite stamps prepared with this material, the deformation value was reduced below 10 μm, compared to values ranging from 10 μm up to 127 μm for thermally cured S-PDMS (50 °C or 70 °C). Finally, different two-layer stamps were coated with an additional third layer and measured again, in order to investigate the deflection for tri-layered cases. The research is of high relevance for the fabrication and application of substrate conformal imprint lithography stamps with minimized deflection.

Large area nanoimprint by substrate conformal imprint lithography (SCIL)

Abstract Releasing the potential of advanced material properties by controlled structuring materials on sub-100-nm length scales for applications such as integrated circuits, nano-photonics, (bio-)sensors, lasers, optical security, etc. requires new technology to fabricate nano-patterns on large areas (from cm2 to 200 mm up to display sizes) in a cost-effective manner. Conventional high-end optical lithography such as stepper/scanners is highly capital intensive and not flexible towards substrate types. Nanoimprint has had the potential for over 20 years to bring a cost-effective, flexible method for large area nano-patterning. Over the last 3–4 years, nanoimprint has made great progress towards volume production. The main accelerator has been the switch from rigid- to wafer-scale soft stamps and tool improvements for step and repeat patterning. In this paper, we discuss substrate conformal imprint lithography (SCIL), which combines nanometer resolution, low patterns distortion, and overlay alignment, traditionally reserved for rigid stamps, with the flexibility and robustness of soft stamps. This was made possible by a combination of a new soft stamp material, an inorganic resist, combined with an innovative imprint method. Finally, a volume production solution will be presented, which can pattern up to 60 wafers per hour.

Bending properties of two- and three-dimensional-shaped nanoparticles fabricated via substrate conformal imprint lithography

Nanoimprinting enables the implementation of nanoparticle shapes with complex 2D shapes involving different materials. In addition to these objects, this article presents 3D-shaped nanoparticles fabricated by substrate conformal imprint technique. The imprint polymer AMONIL is used either in pure form or in combination with fluorescent dyes for the preparation of particles. The substrate conformal imprint lithography process, including etching and particle release, is conducted for both materials in a similar fashion. In this work, cuboidal particles with a high aspect ratio (1:120) are compared to particles with a T-shaped cross section with respect to their abilities to enhance or reduce their stiffness. Additionally, particles with a high aspect ratio are compared to particles with a lower aspect ratio (1:20). The local stiffness is found to depend strongly on the particle thickness and the geometry of their cross section. Thicker and 3D T-shaped particles present higher local stiffness than thinner and 2D cuboidal-shaped particles. The local bending angle was determined to be 77° for 2D-shaped particles and 83° for 3D-shaped particles, of the same total height of 176 nm. Very thin particles (<50 nm) of high aspect ratio prefer to curl finally forming loops.

Large area manufacturing of plasmonic colour filters using substrate conformal imprint lithography

This work presents the large area fabrication of plasmonic colour filters consisting of subwavelength apertures in aluminium films of different thicknesses. Wafer-scale pattern transfer was realized by a soft lithography technique (substrate conformal imprint lithography). The fabricated colour filters have an active area of up to 145 cm2 which presents a considerable increase compared to previously published results. In addition to experimental investigations, simulations of the transmission behaviour were performed using a rigorous electromagnetic field solver based on an extended RCWA approach. Furthermore, the use of a spin-coated cover layer consisting of the UV-curable hybrid polymer OrmoComp® instead of often applied PECVD-SiO2 was investigated.

Fabrication of photonic integrated circuits in silicon nitride using substrate conformal imprint lithography

The feasibility of nanoimprint lithography as an alternative strategy for the pattern definition of Si3N4 based PIC (photonic integrated circuit) is demonstrated in this study. The PICs were patterned by the substrate conformal imprint lithography (SCIL), a variation of UV nanoimprint lithography. The optical quality of the patterned PICs was determined by measuring the optical transmission losses of the fabricated PICs. The quality was then compared with the PICs patterned by an optical stepper. The analysis shows that the optical quality of the patterns fabricated by the SCIL process is similar to the patterns defined by an optical stepper. Our study demonstrates clearly that the SCIL is an alternative to established lithographic approaches in regards to material or pattern resolution.

Soft imprinted Ag nanowire hybrid electrodes on silicon heterojunction solar cells

We demonstrate enhanced efficiencies in front-contacted silicon heterojunction (SHJ) solar cells using silver nanowire-based hybrid electrodes. SHJ cells typically suffer from shading losses due to reflection from macroscopic sun-facing metal fingers, which must be closely spaced to avoid resistive losses in the transparent conductive electrode (TCE). Using substrate conformal imprint lithography (SCIL) we fabricate silver nanowire electrodes on practical scale (4.0 cm2) planar SHJ cells. These electrodes exhibit anomalous transmission and a 7-fold improvement in sheet conductance relative to a standard ITO layer, enabling larger finger spacings and reducing reflection losses without compromising the cell fill factor. Over 70% of the ITO is replaced with transparent SiNx, reducing the use of indium while improving the anti-reflective performance. Combined, the reduced shading and reflection raises the short circuit current density increases by 2.1 mA cm−2, yielding an absolute increase in cell efficiency of 1.0%. These engineered hybrid electrodes provide a practical pathway towards front-contacted SHJ cells with a reduced dependence on rare metals and high efficiencies.

Hybrid polymers processed by substrate conformal imprint lithography for the fabrication of planar Bragg gratings

In this work, we present an approach to use UV-enhanced substrate conformal imprint lithography (UV-SCIL) as a soft imprint technique combined with excimer laser irradiation to manufacture Bragg gratings within planar waveguides on a full-wafer scale. For the first time, different hybrid polymers (OrmoComp®, OrmoStamp, OrmoCore, OrmoClad and OrmoClear) could be successfully patterned using UV-SCIL. For OrmoComp® (showing results very similar to OrmoStamp and OrmoClad), a complete imprint process could be realized. OrmoCore formed an inhibition layer in the presence of oxygen during the imprint, as could be observed for the use of OrmoClear as well. Processing options were elaborated to reduce the inhibition effect significantly, whereby the latter is mainly due to the atmospheric oxygen-containing PDMS layer of the UV-SCIL working stamp. Further on, the successful realization of a planar Bragg grating operating at the telecom wavelength is demonstrated by tuning the refractive index (RI) of OrmoComp® using a phase mask and an UV excimer laser. FTIR measurements show that the change in RI can be clearly correlated with a change in the chemical composition of the hybrid polymer during laser exposure.

3D nanoimprint for NIR Fabry-Pérot filter arrays: fabrication, characterization and comparison of different cavity designs

We report on the fabrication of miniaturized NIR spectrometers based on arrays of multiple Fabry-Perot (FP) filters. The various cavities of different height are fabricated via a single patterning step using high resolution 3D nanoimprint technology. Today, low-cost patterning of extended cavity heights for NIR filters using the conventional spin-coated nanoimprint methodology is not available because of insufficient coating layers and low mobility of the resist materials to fill extended cavity structures. Our investigation focuses on reducing the technological effort for fabrication of homogeneous extended cavities. We study alternative cavity designs, including a new resist and apply large-area 3D nanoimprint based on hybrid mold and UV Substrate Conformal Imprint Lithography (UV-SCIL) to overcome these limitations. We compare three different solutions, i.e. (1) applying multiple spin coating of the resist to obtain thicker initial resist layers, (2) introducing a hybrid cavity (combination of a thin oxide layer and the organic cavity) to compensate the height differences, and (3) optimizing the imprint process with a novel resist material. The imprint results based on these methods demonstrate the implementation of NIR FP filters with high transmission intensity (best single filter transmission >90 %) and small line widths (<5 nm in full width at half maximum).