Substrate Conformal Imprint Lithography Research Articles

Stamp degradation for high volume UV enhanced substrate conformal imprint lithography (UV-SCIL)

In this work we investigate degradation effects occurring on soft PDMS stamps while performing UV enhanced substrate conformal imprint lithography (UV-SCIL) with dual-curing UV Sol–Gel resists. First, we describe and optimize the imprint process to obtain an optimum curing time for a given layer thickness and imprint pattern. With this set of optimized parameters we investigate the physical degradation of the stamp during multiple imprinting runs and could show that diffusion of resist components is in the end a limiting factor for the end of life of a stamp in an imprint cycle. The use of optimized imprint parameters guarantees high pattern accuracy and enables us to reach the shortest cycle time reported so far for SCIL of 3min 30s for 75 consecutive imprints.

Light Coupling and Trapping in Ultrathin Cu(In,Ga)Se2 Solar Cells Using Dielectric Scattering Patterns.

We experimentally demonstrate photocurrent enhancement in ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells with absorber layers of 460 nm by nanoscale dielectric light scattering patterns printed by substrate conformal imprint lithography. We show that patterning the front side of the device with TiO2 nanoparticle arrays results in a small photocurrent enhancement in almost the entire 400-1200 nm spectral range due to enhanced light coupling into the cell. Three-dimensional finite-difference time-domain simulations are in good agreement with external quantum efficiency measurements. Patterning the Mo/CIGSe back interface using SiO2 nanoparticles leads to strongly enhanced light trapping, increasing the efficiency from 11.1% for a flat to 12.3% for a patterned cell. Simulations show that optimizing the array geometry could further improve light trapping. Including nanoparticles at the Mo/CIGSe interface leads to substantially reduced parasitic absorption in the Mo back contact. Parasitic absorption in the back contact can be further reduced by fabricating CIGSe cells on top of a SiO2-patterned In2O3:Sn (ITO) back contact. Simulations show that these semitransparent cells have similar spectrally averaged reflection and absorption in the CIGSe active layer as a Mo-based patterned cell, demonstrating that the absorption losses in the Mo can be partially turned into transmission through the semitransparent geometry.

Patterning of Polymer Arrays with Enhanced Aspect-Ratio Using Hybrid Substrate Conformal Imprint Lithography

Ultraviolet Substrate Conformal Imprint Lithography (SCIL) is an economic nanolithography technique of imprinting high-fidelity patterns over large areas. It is an improvement to conventional UV-Nanoimprint lithography (NIL) with high resolution, flexibility and conformal imprinting. However the aspect-ratio of SCIL is limited due to its patterning by capillary forces and the PDMS material has very low Young’s modulus (<2Mpa) hence, resulting in deformation of the feature sizes during the imprint process. In general the residual layer between the imprinted structures is etched away to obtain high aspect ratio. But in industrial applications an array of different 3D patterns are imprinted in a single step, thus, the residual layer of each structure depends on its volume and viscosity of the imprint material, thereby making the etching process impossible to increase the feature size and aspect-ratio of each microstructure individually. Hence in this paper a new SCIL stamp technique is demonstrated to implement an enhancement of aspect ratio. In this process, a flexible polymer material, Polydimethylsiloxane (PDMS) is used as the stamp material. The heights of the structure on the stamp are supported by a metal layer (higher Young’s modulus), which provides higher stiffness and rigidity to the stamp and thereby avoiding distortion of the patterns on the stamp. Also the metal layer acts as an UV blocking mask: hence while patterning on an UV curable imprint material, the residual edges of the microstructures, which are subjected to decrease the feature size, are removed in the development process of the uncured polymer areas. Thus, irrespective of the number and shape of the microstructures, an enhancement of aspect ratio is attained in a single step. Conventional imprinting of circular polymer arrays with dimensions 90μm / 70nm on their master template resulted in increased feature size up to 150 μm, while the height varied between 90-66nm and thereby decreasing the aspect ratio. Imprinting with the proposed SCIL technique resulted in 96 μm / 74nm dimension, thus around 60 μm of feature size improvement is achieved

Highly uniform residual layers for arrays of 3D nanoimprinted cavities in Fabry–Pérot-filter-array-based nanospectrometers

Miniaturized optical spectrometers can be implemented by an array of Fabry–Perot (FP) filters. FP filters are composed of two highly reflecting parallel mirrors and a resonance cavity. Each filter transmits a small spectral band (filter line) depending on its individual cavity height. The optical nanospectrometer, a miniaturized FP-based spectrometer, implements 3D NanoImprint technology for the fabrication of multiple FP filter cavities in a single process step. However, it is challenging to avoid the dependency of residual layer (RL) thickness on the shape of the printed patterns in NanoImprint. Since in a nanospectrometer the filter cavities vary in height between neighboring FP filters and, thus, the volume of each cavity varies causing that the RL varies slightly or noticeably between different filters. This is one of the few disadvantages of NanoImprint using soft templates such as substrate conformal imprint lithography which is used in this paper. The advantages of large area soft templates can be revealed substantially if the problem of laterally inhomogeneous RLs can be avoided or reduced considerably. In the case of the nanospectrometer, non-uniform RLs lead to random variations in the designed cavity heights resulting in the shift of desired filter lines. To achieve highly uniform RLs, we report a volume-equalized template design with the lateral distribution of 64 different cavity heights into several units with each unit comprising four cavity heights. The average volume of each unit is kept constant to obtain uniform filling of imprint material per unit area. The imprint results, based on the volume-equalized template, demonstrate highly uniform RLs of 110 nm thickness.

Optimization of substrate conformal imprint lithography (SCIL) and etching for nanostructure

The UV-SCIL fabrication process was developed and optimized to improve the quality of the nanostructures on the hard substrate transferred with substrate conformal imprint lithography (SCIL) technology. In particular, the key steps such as coating imprint resist, exposure time and etching time were investigated thoroughly. The experiment’s results illustrate that imprint resist could well serve as an etching mask for the dry etching process without oxygen plasma. The optimized etching condition and SCIL technology could also be used to transfer nanostructures on different substrates for metal nanostructured biosensors or nanophotonics.

Anti-Reflective Porous Silicon Features by Substrate Conformal Imprint Lithography for Silicon Photovoltaic Applications

Silicon photovoltaic cells require anti-reflection treatments in order to minimise optical losses and improve cell efficiencies. Commercially, the silicon surface is textured using a chemical etchant followed by the addition of an anti-reflective coating to further suppress reflectivity. We present a process using metal assisted etching to create porous silicon features capable of reducing reflectivity to less than 5%. A method for producing porous silicon using Substrate Conformal Imprint Lithography (SCIL) has been developed in order to pattern the nanoscale anti-reflective structures onto silicon wafers.

Bioactivation of Plane and Patterned PDMS Thin Films by Wettability Engineering

This study shows how surface properties of polydimethylsiloxane (PDMS) thin films, such as wettability and cellular adhesive behavior, can be influenced by surface modification and patterning, to improve the suitability of such modified PDMS for biomedical applications. For that purpose, different types of PDMS (i.e., soft (S-) and hard (H-) PDMS) and differently patterned surfaces were prepared by a commercially available tool which is originally used to produce patterned PDMS stamps for substrate conformal imprint lithography. To increase surface wettability and cellular adhesive behavior, PDMS surfaces were modified by O2, Ar/O2, Ar, and forming gas (N2/H2) plasma treatment. It is shown that, especially, plasma treatment using N2/H2 is a promising method to modify PDMS surfaces towards enhanced wettability. Such modified PDMS surfaces exhibit water contact angle values (WCA) of nearly 0° and demonstrate enhanced attachment of melanoma cells. Differences between as-prepared and plasma-treated S- and H-PDMS can be observed, where H-PDMS shows smaller WCA than S-PDMS. Even plasma-treated PDMS surfaces, however, exhibit only temporary hydrophilicity, due to hydrophobic recovery. To overcome this problem, plasma-treated PDMS films were stored in ethanol and water, where low and constant WCAs for several months could be achieved. Finally, the influence of surface topography of different patterned PDMS thin films on the adhesion behavior of melanoma cells was investigated. It can be concluded that chemical as well as structural modifications of PDMS enable to gradually control the adhesive properties of cells on their surfaces. This has a high technological impact since regulated and topologically stable adhesion and repulsion of cells by their scaffolds has high potency for in vitro as well as in vivo applications.

A New Mold Structure to Replicate Patterns Over 1 <I>μ</I>m in Depth for Substrate Conformal Imprint Lithography

This paper shows an improved mold replication process that uses polyurethane acrylate (PUA) and polyethylene terephthalate (PET) for the fabrication of an ultraviolet (UV) imprinting mold used in substrate conformal imprint lithography (SCIL). With the conventional replication process, which uses hard polydimethylsiloxane (h-PDMS) as a pattern layer, it is difficult to detach the mold from a silicon master for metal oxide semiconductor field effect transistor (MOSFET) that has patterns with over 1-micron depth. However, the method proposed in this paper allows us to easily replicate patterns that have more than 1-micron depth. The key idea of this method is to use PET film as a bonding layer to attach the PUA layer to the polydimethylsiloxane (PDMS) cushion layer to overcome the weak the adhesion force between the PUA and PDMS layer. We demonstrate how to make the modified replica mold and present imprinting results obtained using this replica mold in the SCIL process.

Accuracy of wafer level alignment with substrate conformal imprint lithography

Substrate conformal imprint lithography (SCIL) is an innovative soft lithography method for the transfer of large area nanostructures. This technology was originally invented by Philips Research. With the implementation of the SCIL process on mask aligners, the standard alignment options of a mask aligner were also available for SCIL. This work presents studies on overlay alignment accuracy of the SCIL process on mask aligners. The utilized alignment markers were common box-in-box fiducials. With these structures, the alignment accuracy was determined. The experiments showed that the SCIL process induces a shift of the stamp with respect to the stage in imprint direction during the initial movement of the stamp from the SCIL actuator to the substrate. This shift is reproducible and depends on the distance between the actuator and the substrate. With an offset correction of the stage, this shift could be compensated. The achieved alignment accuracy was then below one micrometer, limited just by the mechanical capabilities of the mask aligner. Besides, global and local distortions of the imprinted structures with respect to the original master structures were identified and analyzed within this work. It was found that there is no systematic distortion of structures in the stamp due to the imprint process itself despite using the flexible composite SCIL stamps with polydimethylsiloxane pattern layer. Only nonsystematic local distortions could be found. Finally, the accuracy of the stamp manufacturing process for the SCIL stamps was investigated, and it was found that large distances in the centimeter range between single features are reproduced with a variation of 0.06‰. All together, the results of this work show that an accurate and reproducible overlay alignment of imprinted lithography layers is possible using SCIL as a wafer scale imprint technology.

Functional epoxy polymer for direct nano-imprinting of micro-optical elements

This work presents a functional resist for the direct fabrication of micro-optical elements with UV-enhanced substrate conformal imprint lithography (UV-SCIL). This functional resist is based on a UV-curable epoxy polymer where TiO2 nanoparticles are enclosed. Adding these particles to the polymer raises the refractive index of the functional resist with raising particle content from 1.52 up to 1.64 at a wavelength of 633nm. Therefore, the refractive index can be matched for many micro-optical applications. Here, multilevel Fresnel lenses from a 200mm silica substrate are transferred into the functional resist by UV-SCIL with nanometer resolution and high structure fidelity.

Ordered arrays of nanoporous silicon nanopillars and silicon nanopillars with nanoporous shells

The fabrication of ordered arrays of nanoporous Si nanopillars with and without nanoporous base and ordered arrays of Si nanopillars with nanoporous shells are presented. The fabrication route is using a combination of substrate conformal imprint lithography and metal-assisted chemical etching. The metal-assisted chemical etching is performed in solutions with different [HF]/[H2O2 + HF] ratios. Both pore formation and polishing (marked by the vertical etching of the nanopillars) are observed in highly doped and lightly doped Si during metal-assisted chemical etching. Pore formation is more active in the highly doped Si, while the transition from polishing to pore formation is more obvious in the lightly doped Si. The etching rate is clearly higher in the highly doped Si. Oxidation occurs on the sidewalls of the pillars by etching in solutions with small [HF]/[H2O2 + HF] ratios, leading to thinning, bending, and bonding of pillars.

Ordered arrays of nanoporous gold nanoparticles

A combination of a “top-down” approach (substrate-conformal imprint lithography) and two “bottom-up” approaches (dewetting and dealloying) enables fabrication of perfectly ordered 2-dimensional arrays of nanoporous gold nanoparticles. The dewetting of Au/Ag bilayers on the periodically prepatterned substrates leads to the interdiffusion of Au and Ag and the formation of an array of Au–Ag alloy nanoparticles. The array of alloy nanoparticles is transformed into an array of nanoporous gold nanoparticles by a following dealloying step. Large areas of this new type of material arrangement can be realized with this technique. In addition, this technique allows for the control of particle size, particle spacing, and ligament size (or pore size) by varying the period of the structure, total metal layer thickness, and the thickness ratio of the as-deposited bilayers.

Novel organic polymer for UV-enhanced substrate conformal imprint lithography

In this work, a novel kind of UV curing polymer is introduced as promising resist for UV-enhanced substrate conformal imprint lithography (UV-SCIL). This fully organic polymer is an epoxy based material and can crosslink via UV exposure to form a solid layer. The curing time of 17s for this epoxy based resist is ten times shorter compared to commonly used resists for UV-SCIL. Imprints with this material are shown in this work as well as results of an HBr dry etch process of silicon, where the material served as etching mask. Using this polymer as resist for UV-SCIL enables shorter process times whereby the fidelity of the structures remains high.

Life time evaluation of PDMS stamps for UV-enhanced substrate conformal imprint lithography

UV-enhanced substrate conformal imprint lithography (UV-SCIL) is a large area nano patterning technique which has the potential to find its way into industrial application. A promising application for UV-SCIL exists in the field of high brightness LEDs. Here, a surface patterning can increase the external quantum efficiency and therefore the light output of LEDs significantly. But, before UV-SCIL can be introduced as patterning technique into industrial application, process relevant parameters like the interaction between the PDMS stamp and the UV-curing material as well as the overall life time of PDMS stamps have to be evaluated. Within this work, stamps from different PDMS material compositions with a diameter of 150 mm were fabricated. These stamps were used to perform UV-SCIL with the epoxy based resist Katiobond OM VE 110707 from DELO Industrial Adhesives and it was found that the final resist layer thickness strongly depends on the PDMS material composition, the number of imprints, and the time between two subsequent imprints. This can be explained by diffusion of one or several components of the UV-curing material into the stamps, which influences the life time of the PDMS stamps. With the actual optimum PDMS material composition, 500 UV-SCIL imprints could be successfully performed without any visible degradation.

Full Field Nanoimprint on Mask Aligners Using Substrate Conformal Imprint Lithography Technique

Full Field Nanoimprint on Mask Aligners Using Substrate Conformal Imprint Lithography Technique

UV enhanced substrate conformal imprint lithography (UV-SCIL) technique for photonic crystals patterning in LED manufacturing

The global LED (light emitting diode) market reached 5 billion dollors in 2008 and will be driven towards 9 billion dollors by 2011 [1]. The current applications are dominated by portable device backlighting, e.g. cell phones, PDAs, GPS, laptop etc. In order to open the general lighting market doors the luminous efficiency needs to be improved significantly. Photonic crystal (PhC) structures in LEDs have been demonstrated to enhance light extraction efficiency on the wafer level by researchers [2]. However, there is still a great challenge to fabricate PhC structures on LED wafers cost-effectively. Nanoimprint lithography (NIL) [3] has attracted considerable attentions in this field due to its high resolution, high throughput and low cost of ownership (CoO). However, the current NIL techniques with rigid stamps rely strongly on the substrate flatness and the production atmosphere. Those factors hinder the integration of NIL into high volume production lines. UV-NIL with flexible stamps [4], e.g. PDMS stamps, allows the large-area imprint in a single step and is less-sensitive to the production atmosphere. However, the resolution is normally limited due to stamp distortion caused by imprint pressure. A novel NIL technique developed by Philips Research and Süss MicroTec, substrate conformal imprint lithography (SCIL), bridges the gap between UV-NIL with rigid stamp for best resolution and soft stamp for large-area patterning. Based on a cost-effective upgrade on Süss mask aligner, the capability can be enhanced to nanoimprint with resolution of down to sub-10 nm on an up to 6 inch area without affecting the established conventional optical lithographic processes on the machine. Benefit from the exposure unit on the mask aligners, the SCIL process is now extended with UV-curing option, which can help to improve the throughput dramatically. In this paper, the fabrication of photonic crystal structures with SCIL technique on Süss MA6 mask aligner is demonstrated. In addition, the industrialization considerations of UV-SCIL process in high volume manufacturing are briefly discussed.

Charged Particle Nanopatterning (CHARPAN) of 2D and 3D masters for flexible replication in Substrate Conformal Imprint Lithography (SCIL)

Recently, the first generation programmable Aperture Plate System with integrated CMOS electronics (CMOS-APS) featuring 43 thousand switchable beams has been inserted into a Charged Particle Nanopatterning (CHARPAN) tool. Using this configuration, the first (2D) exposure results in Hydrogen Silsesquioxane (HSQ) resist, employing 10 keV Hydrogen ion ( H 3 + ) parallel beams of 12.5 nm spot size, show that (at least) a 20 nm resolution is feasible in this system. These CHARPAN patterns have been used as masters for PDMS stamp casting. The flexible PDMS stamps have been implemented in Substrate Conformal Imprint Lithography (SCIL™). The original 20 nm resolution features have also been observed in the imprinted sol–gel material. The CHARPAN tool can be operated with heavier ions (Ar +, Xe +) as well, enabling maskless and resistless 3D nanopatterning. Here again, a flexible PDMS stamp has been cast and these deep 3D structures have been successfully applied in SCIL. The combination of CHARPAN and SCIL opens up new possibilities for low cost, fast and flexible 2D and 3D manufacturing of nano-devices.