Imprint Step Research Articles

Study of the behaviour of monomers in thermal nanoimprint lithography

Thermoplastics are commonly used in thermal nanoimprint lithography (NIL) but their high viscosity leads to inhomogeneities of residual thickness in patterns with various densities. Monomers exhibit low viscosity and are imprinted easily and polymerized with UV–NIL processes. These monomers can be also used for thermal NIL. We have imprinted A-POSS material which is spontaneously polymerized at 170 °C. The inorganic part of this monomer is interesting for pattern transfer and for permanent applications. Thermal properties of this molecule are presented in this paper. It is shown that polymerization occurs at 170 °C, and that the viscosity is 1330 mPa s at ambient temperature. Imprint experiments have demonstrated that A-POSS flows over larger surfaces during imprint step, compared to thermoplastics. Patterns with different densities have been studied and different filling regimes have been observed depending on material viscosity. They are induced by a competition between material flow and mold deformation. Finally, we imprinted some nanoelectrodes simultaneously with millimetric large connection pads, and it was demonstrated that complete filling was obtained with monomers whereas this was not possible with thermoplastics.

Pinning at template feature edges for step and flash imprint lithography

We present a theory of the pinning of an air-liquid interface at the feature edge of a template during the imprint step of step and flash imprint lithography (SFIL). Pinning occurs when the pressure at the air-liquid interface reaches the pressure of the bulk liquid. At this condition, there is no pressure gradient or driving force to move the liquid and fill the feature. We examined several parameters, including residual layer thickness, wettability of the feature edge, applied force on the template, and the radius of curvature of the feature edge, to study their effect on pinning. An optimal SFIL process window and template modifications are proposed that minimize the pinning at the feature edge while still preventing any extrusion along the mesa (pattern containing area on the template) edge.

Feature filling modeling for step and flash imprint lithography

The authors analyze the feature filling phenomena for step and flash imprint lithography (SFIL) via diffusion of a gas, entrapped in the features, through liquid imprint resist. The model and simulation for the dynamics of feature filling including gas dissolution during the SFIL imprint step is presented. Several factors including the gas concentration profile across the liquid resist drop, the filling progression, and the total filling time for different pattern configurations are investigated to quantify feature filling. Simulation results show that initial filling is pressure controlled and very rapid [∼O(1ms)]. The rest of the feature filling is diffusion controlled, but fast enough [∼O(1s)] to conclude that diffusion of entrapped gas is not a cause for nonfilling of features. Also, the result that the smaller size features fill faster than the larger ones (with the same depth) makes SFIL the prime contender for high resolution and high throughput next generation lithography.

XPS study of the degradation mechanism of fluorinated anti-sticking treatments used in UV nanoimprint lithography

Thanks to their low surface energy, fluorinated anti-sticking layers are widely used in UV nanoimprint lithography (UV-NIL) to treat the mold and facilitate its separation from the imprinted resist. However, it has been reported that release properties of the stamp deteriorate with repeated imprint operations. In this paper, X-ray photoelectron spectroscopy is used to study the mechanism of the fluorinated treatment degradation. A specific experimental protocol is used in order to avoid further degradation under X-ray exposure. It has been observed that a large amount of fluorinated molecules are removed in the first imprint steps and deposited on the surface of the imprinted resist. After this first stage, we observed that fluorinated molecules are progressively degraded along their chain during the NIL process.

Template-Assisted Fabrication of Free-Standing Nanorod Arrays of a Hole-Conducting Cross-Linked Triphenylamine Derivative: Toward Ordered Bulk-Heterojunction Solar Cells

Free-standing nanorod arrays of a thermally cross-linked semiconducting triphenylamine were fabricated on conductive ITO/glass substrates via an anodic aluminum oxide (AAO) template-assisted approach. By using a solution wetting method combined with a subsequent thermal imprinting step to fill the nanoporous structure of the template with a cross-linkable triphenylamine derivative, a polymeric replication of the AAO was obtained after thermal curing and selective removal of the template. To obtain well-aligned and free-standing nanorod arrays, aggregation and collapse of the nanorods were prevented by optimizing their aspect ratio and applying a freeze-drying technique to remove the aqueous medium after the etching step. Because of their electrochemical properties and their resistance against organic solvents after curing, these high density nanorod arrays have potential application in organic photovoltaics.

Rational Design of In Situ Monolithic Imprinted Polymer Membranes for the Potentiometric Sensing of Diethyl Chlorophosphate – a Chemical Warfare Agent Simulant

AbstractMolecularly imprinted polymer membrane was prepared by semicovalent imprinting strategy wherein i) the template diethyl chlorophosphate (DCP), (a simulant of organophosphorous nerve agents), is covalently linked to the reactive functional monomer vinyl aniline (VA) during imprinting step followed by noncovalent rebinding and ii) in situ polymerization via single pot synthesis in presence of additional functional monomer, 2‐hydroxyethyl methacrylate (HEMA) and crosslinking monomer, ethylene glycol dimethacrylate (EGDMA) after addition of 2‐nitrophenyl octyl ether (NPOE) and 2,2′‐azobisisobutyronitrile (AIBN) as plasticizer and initiator respectively. The resulting membrane is integrated with a potentiometric transducer while designing a DCP sensor. The fabricated sensor responds over a wider concentration range of 10−6–10−2 M with a lower detection limit of 10−6 M (0.17 ppm). In addition, in situ monolithic membrane based sensor was designed by adopting noncovalent imprinting strategy also. A detailed comparison is made between semicovalent and noncovalent in situ membrane based sensors on the prime sensor performance criteria such as sensitivity, selectivity, working range, response time, reusability and reversibility. Again, the relative merits and demerits of semicovalent vis‐à‐vis noncovalent strategy based in situ monolithic membrane sensors were also highlighted. The probable molecular recognition mechanism is also discussed.

Fabrication and validation of fused silica NIL templates incorporating different length scale features

A cost effective methodology for pattering of Nano Imprint Lithography (NIL) templates incorporating different length scale features is proposed. The approach relies on selecting the optimum processing window of different technologies for cost effective micro- and nano-patterning. Very promising results were obtained when first fused silica templates were structured by F2 laser ablation at 157 nm. It was demonstrated that nano-scale features and complex 3D micro-scale features could be machined with a Focused Ion Beam (FIB) over the existing topography produced by laser ablation. Thus, a large area (up to several square centimeters) of the NIL templates is easily patterned with micro- and even meso-scale features by laser ablation while nano- and micro-scale features could be introduced locally by FIB machining. Through S-FIL process such templates were validated and the nano- and micro-structures were accurately replicated in one imprinting step.

Mr-NIL 6000LT – Epoxy-based curing resist for combined thermal and UV nanoimprint lithography below 50 °C

The aim of the work presented here was to develop curing polymers for nanoimprint lithography (NIL) enabling short cycle time, low imprint temperature, and an isothermal imprint process. The result is mr-NIL 6000LT: A photochemically curing polymer system for isothermal imprinting by combined thermal and UV nanoimprint lithography. It allows a lower imprint temperature than materials presented previously [C. Schuster, M. Kubenz, F. Reuther, M. Fink, G. Grützner, mr-NIL 6000 – New epoxy-based curing resist for efficient processing in combined thermal and UV nanoimprint lithography, in: Proceedings of SPIE 6517 2007, 65172B.; D.W. Johnson, H. Miller, M. Kubenz, F. Reuther, G. Grützner, Nanoimprinting with SU-8 Epoxy Resists, in: Proceedings of SPIE 6517 2007, 65172A.]. The material system chosen is based on a blend of epoxy resins and a photo acid generator. Such epoxy resists cure during the imprint step in combined thermal and UV nanoimprint lithography. Initiated by UV exposure the cationic polymerisation occurs at elevated temperature forming a polymer pattern with significantly increased thermal stability compared to the uncured system. Apart from the material development leading to mr-NIL 6000LT the correlations between the parameters imprint temperature, exposure time and post exposure hold time are investigated in this work. With the applied resin combination a T g of −15 °C is obtained. This enables the formation of solid films at room temperature after spin-coating and prebake and nevertheless imprint temperatures in the range of 45–50 °C, which is a distinct decrease compared to the 100–110 °C needed for the previously introduced mr-NIL 6000 [C. Schuster, M. Kubenz, F. Reuther, M. Fink, G. Grützner, mr-NIL 6000 – New epoxy-based curing resist for efficient processing in combined thermal and UV nanoimprint lithography, in: Proceedings of SPIE 6517 2007, 65172B.] or the 65–70 °C necessary for defect-free imprinting of the epoxy-based polymer described in [D.W. Johnson, H. Miller, M. Kubenz, F. Reuther, G. Grützner, Nanoimprinting with SU-8 Epoxy Resists, in: Proceedings of SPIE 6517 2007, 65172A.]. mr-NIL 6000LT exhibits good dimensional stability at 120 °C after curing during the imprint process. This is sufficient for an isothermal imprint process as well as subsequent processes, e.g. metallization or etching.

The replication of three dimensional structures using UV curable nanoimprint lithography

The study investigates the use of ultraviolet nanoimprint lithography (UV-NIL) for patterning three dimensional (3D) structures. Generating the 3D structures is a challenging task especially on an insulating substrate such as quartz. These transparent molds are essential for the UV-NIL process. The 3D profiles were created on the negative tone photoresist, Microresist ma-N2403 using a Raith150 electron beam lithography (EBL) tool in a single step variable dose controlled exposure. The developed 3D resist profiles subsequently were utilized as the 3D masking layer. The 3D patterns were transferred into the quartz mold substrates by a single-step reactive ion etching (RIE). The replication of the 3D mold structure by using the UV-NIL technique requires a two-step imprint process. The master mold profile was replicated onto a Microresist Ormocomp US-S4 resist on the first imprint to become the soft mold. The cured Ormocomp soft mold, subsequently was used as a mold for replicating the 3D pattern structures on the Microresist mr-UVCur06 resist in the second imprint step to create a positive replica of the original mold. A test pattern of a 3D pyramid-shaped array with multilevel features was successfully been replicated using this technique. The replicated 3D pattern on the resist could be utilised as the final 3D masking layer for the pattern transfer onto final substrate using RIE.

Patterning of polyfluorene based polymer light emitting diodes by reversal imprint lithography

The authors demonstrated the fabrication of patterned arrays of polymer light emitting diodes (PLEDs) with a polyfluorene based emissive layer using reversal imprint lithography. A stack of patterned metal and polymer films is transferred to a glass substrate in a single reversal imprint step. Two different techniques for coating the patterned Si mold are shown, one involving the spin coating of polymer layers directly onto the mold and one involving inking of the polymers onto the mold from a spin-coated PDMS inkpad. Using these techniques, PLED devices with a minimum feature sizes as small as 1μm and luminescence starting at 3.5V have been demonstrated.

Toward 1Tdot∕in.2 nanoimprint lithography for magnetic bit-patterned media: Opportunities and challenges

Nanoimprint lithography presents unique opportunities for patterned media applications due to its advantages of sub-10nm resolution capability, patterning of a whole disk in a single imprint step with reasonably high throughput, and the relatively low capital cost in comparison to other next generation lithography technologies. However, there are several critical issues that still remain very challenging. This article will briefly discuss these challenges in general and then focus on imprint lithography work including the fabrication of templates and demonstrate the imprinted results. In this work two types of polarities of high-density templates (pillar tone and hole tone) have been fabricated on fused silica substrates for the UV imprint process. The difficulties and limitations in each of the template fabrication processes will be discussed. The authors have successfully demonstrated template fabrication followed by imprinted results with a pitch of 24nm (1.1Tdots∕in.2) for both tones of templates. Initial imprinted results of dense dot patterns with a pitch as small as 18nm (2.0Tdots∕in.2) have been achieved. High-resolution scanning electron microscopy images are used as the primary metrology for both the dot size uniformity and the placement accuracy analysis. The difficulties and the limitations in template fabrication, the imprint process, and associated metrology will be discussed.

Novel Use of Columnar Porous Silicon Carbide Structures as Nanoimprint Lithography Stamps

Columnar porous Si-face 6H-SiC substrates were prepared by a photo-electrochemical etching method and applied as nanoimprint lithography (NIL) stamps. The diameter of the pores in the porous region was about 20 nm and the center-to-center separation between pores was about 60 nm. The columnar porous SiC substrates were subjected to a vapor phase silanization treatment whereby a monolayer of perfluorooctyltrichlorosilane (FOTS) was deposited in order to keep the stamps from sticking to the substrates during the imprint step. Subsequently, the porous SiC stamps were used to imprint polymethylmethacrylate (PMMA) at elevated temperatures and pressures. The imprinted PMMA could then be used to transfer the nanopattern on the columnar porous SiC to other substrates for various purposes; e.g. templates for GaN regrowth, catalysts for nanowire growth by vapor-liquid-solid type methods (VLS), etc. SiC is not typically used for NIL stamps since etch processing of SiC is less mature than that of Si. However, as demonstrated here, there is no reason why SiC cannot be used as a material for NIL stamps. The superior mechanical properties to Si make the use of SiC alluring as a master template for NIL processing.

Inverted Microcontact Printing on Polystyrene-block-Poly(tert-butyl acrylate) Films: A Versatile Approach to Fabricate Structured Biointerfaces Across the Length Scales

The combination of the recently introduced soft lithographic technique of inverted microcontact printing (i-muCP) and spin-coated films of polystyrene- block-poly( tert-butyl acrylate) (PS 690- b-P tBA 1210) as a reactive platform is shown to yield a versatile approach for the facile fabrication of topographically structured and chemically patterned biointerfaces with characteristic spacings and distances that cross many orders of magnitude. The shortcomings of conventional muCP in printing of small features with large spacings, due to the collapse of small or high aspect ratio stamp structures, are circumvented in i-muCP by printing reactants using a featureless elastomeric stamp onto a topographically structured reactive polymer film. Prior to molecular transfer, the substrate-supported PS 690- b-P tBA 1210 films were structured by imprint lithography resulting in lateral and vertical feature sizes between >50 microm-150 nm and >1.0 microm-18 nm, respectively. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and water contact angle measurements provided evidence for the absence of surface chemical transformations during the imprinting step. Following the previously established hydrolysis and activation protocol with trifluoroacetic acid and N-hydroxysuccinimide, amino end-functionalized poly(ethylene glycol) (PEG-NH 2), as well as bovine serum albumin and fibronectin as model proteins, were successfully transferred by i-muCP and coupled covalently. As shown, i-muCP yields increased PEG coverages and thus improved performance in suppressing nonspecific adsorption of proteins by exploiting the high local concentrations in the micro- and nanocontacts during molecular transfer. The i-muCP strategy provides access to versatile biointerface platforms patterned across the length scales, as shown for guided cancer cell adhesion, which opens the pathway for systematic cell-surface interaction studies.

Soft photocurable nanoimprint lithography for compound semiconductor nanostructures

Soft photocurable nanoimprint lithography is used to transfer nanoscale features to GaAs substrates. Only ambient conditions without externally applied imprinting pressure are required during the imprinting step. The pattern which is transferred consists of a dense array of 100nm features, either one-dimensional lines or two-dimensional holes. Both wet etching and dry etching are used to etch the GaAs for pattern transfer. Examination by atomic force microscopy and scanning electron microscopy confirms faithful pattern transfer; defects are few enough and patterned areas are large enough for optoelectronic device applications.

Pattern Generation by Using Multistep Room-Temperature Nanoimprint Lithography

We have demonstrated multistep room-temperature nanoimprint lithography (RTNIL) using polystyrene (PS, average molecular weight 97 kg/mol) as the imprint polymer layer on a silicon substrate for imprinting complex patterns. Single, double, and multiple (up to ten) sequential imprint steps were performed at imprint pressures between 1 to 30 MPa in separate experiments. We also transferred the imprinted patterns from the PS layer into the silicon substrate by means of an reactive-ion etching (RIE) process. To accomplish this demonstration, we designed and built a tool that controllably and repeatedly translated and pressed a sample into a stationary mold. The demonstrated interstep alignment accuracy of this tool ranged between 80 nm and 380 nm. These experiments revealed that polymer deformation results when nanoimprint is used to further deform a previously structured surface. The molds used in these experiments consisted of 400-nm-period diffraction gratings, as well as of rectangular structures of varying aspect ratios, ranging from 150 to 300 nm wide.

Improved mold fabrication for the definition of high quality nanopatterns by Soft UV-Nanoimprint lithography using diluted PDMS material

An improved mold fabrication process that utilizes toluene diluted polydimethylsiloxane (PDMS) as flexible mold material was developed. Various toluene concentrations and their implication on the pattern definition using the Soft UV-Nanoimprint process were analyzed and discussed. Dots with a resolution of 50 nm are well replicated and an excellent imprint homogeneity across a 4 in. wafer with one imprint step only is demonstrated.

Ordered three-dimensional hierarchical nanostructures by nanoimprint lithography

This paper describes a technique for the fabrication of ordered three-dimensionalhierarchical nanostructures in polymer films via nanoimprint lithography. The hierarchicalstructure is obtained by a series of sequential imprinting steps, where smaller structures areimprinted on top of larger imprinted structures. Higher order hierarchy is achieved bysequentially adding imprinting steps. An important feature of this fabrication technique isthat the subsequent imprinting is carried out at a temperature below the glass transitiontemperature of the polymer film and below the imprinting temperature of the precedingimprint without the assistance of any solvents or plasticizers. Various formats ofhierarchical structures can be obtained by varying the mould geometries and mouldorientations in the sequential imprinting. It is found that the secondary or higher-orderimprints are enabled by a reduction in the modulus of the polymer after the primaryimprinting.

Comparison of multilayer stamp concepts in UV–NIL

Template fabrication is one of the crucial issues in nanoimprint lithography. In this paper three different multilayer stamp concepts are compared that allow the imprinting of high resolution pattern on wafer scale with one imprint step only. Fabrication recipes as well as an analysis of local and global deformation, resolution and area homogeneity in an UV–NIL process are given. Advantages and disadvantages of the concepts are discussed.

Fabrication of multi-dimensional colloid crystals on raised surfaces via reversal nanoimprint lithography

Fabrication of multi-dimensional colloidal crystals on raised polymer substrate has been achieved by reversal nanoimprint technique. The combine effects of the feature size of the mold, particle diameter and imprinting steps control ordering of the colloidal particles. It is shown that using ‘Reversal nanoimprint lithography’, 3D colloidal particles can be selectively patterned on soft (polymer) substrates. Reversal nanoimprint lithography offers a relatively easy, fast and versatile method for patterning of colloidal particles.

Nanoelectrochemical transducers for (bio-) chemical sensor applications fabricated by nanoimprint lithography

Nanometer-structured transducers for commercial use in pharmaceutical, medical or (bio-) chemical analysis have so far been hardly accessible since they could not be produced by parallel lithography techniques at reasonable costs. We introduce here a method on how to fabricate nanometer-structured interdigitated array electrodes including interconnections and bond pads in the micrometer range in a single imprint step on 2-in. wafer scale. The method enables the mass production of those devices at lowest cost opening a new field for the commercial use of nanometer-structured sensor systems.