Stamp Structures Research Articles

Dry Transfer of van der Waals Junctions of Two-Dimensional Materials onto Patterned Substrates Using Plasticized Poly(vinyl chloride)/Kamaboko-Shaped Polydimethylsiloxane.

Two-dimensional (2D) materials can be transferred onto substrates with various surface structures, opening up multiple functions and applications for 2D materials in the form of suspended membranes. In this paper, we present a method for transferring exfoliated 2D crystal flakes from SiO2 substrates onto patterned substrates using a poly(vinyl chloride) (PVC) layer mounted on a polydimethylsiloxane (PDMS) stamp structure. 2D crystal flakes can be transferred onto various patterned structures such as grooves, round holes, and periodic hole or groove patterns. Our method can also be used to fabricate suspended van der Waals (vdW) heterostructures by assembling 2D crystal flakes on the PVC/PDMS stamp and then transferring them onto patterned substrates. The adhesiveness and curvature of the PVC/PDMS stamp were tuned, and a high successful transfer rate was realized due to the use of kamaboko-shaped (semicylindrical) PDMS and the addition of an appropriate amount of a high-viscosity plasticizer to the PVC layer. Taking advantage of this method, we demonstrate the facile fabrication, simply by transferring a vdW heterostructure onto an Au-coated groove substrate, of a suspended vdW field-effect transistor device with the carrier density tuned using ionic gating. This method enables the transfer of 2D crystal flakes and vdW heterostructures onto various patterned substrates, and hence it should help to advance suspended 2D materials research.

Gap-directed chemical lift-off lithographic nanoarchitectonics for arbitrary sub-micrometer patterning.

We introduce a unique soft lithographic operation that exploits stamp roof collapse-induced gaps to selectively remove an alkanethiol self-assembled monolayer (SAM) on Au to generate surface patterns that are orders of magnitude smaller than structures on the original elastomer stamp. The smallest achieved feature dimension is 5 nm using a micrometer-scale structured stamp in a chemical lift-off lithography (CLL) process. Molecular patterns retained in the gaps between stamp features and their circumscribed or inscribed circles follow mathematical predictions, and their sizes can be tuned by altering the stamp structure dimensions, including height, pitch, and shape. These generated surface molecular patterns can function as biorecognition arrays or be transferred to the underneath Au layer for metallic structure creation. By combining CLL process with this gap phenomenon, soft material properties that are previously thought as demerits can be used to achieve sub-10 nm features in a straightforward sketch.

Guiding Chart for Initial Layer Choice with Nanoimprint Lithography.

When nanoimprint serves as a lithography process, it is most attractive for the ability to overcome the typical residual layer remaining without the need for etching. Then, ‘partial cavity filling’ is an efficient strategy to provide a negligible residual layer. However, this strategy requires an adequate choice of the initial layer thickness to work without defects. To promote the application of this strategy we provide a ‘guiding chart’ for initial layer choice. Due to volume conservation of the imprint polymer this guiding chart has to consider the geometric parameters of the stamp, where the polymer fills the cavities only up to a certain height, building a meniscus at its top. Furthermore, defects that may develop during the imprint due to some instability of the polymer within the cavity have to be avoided; with nanoimprint, the main instabilities are caused by van der Waals forces, temperature gradients, and electrostatic fields. Moreover, practical aspects such as a minimum polymer height required for a subsequent etching of the substrate come into play. With periodic stamp structures the guiding chart provided will indicate a window for defect-free processing considering all these limitations. As some of the relevant factors are system-specific, the user has to construct his own guiding chart in praxis, tailor-made to his particular imprint situation. To facilitate this task, all theoretical results required are presented in a graphical form, so that the quantities required can simply be read from these graphs. By means of examples, the implications of the guiding chart with respect to the choice of the initial layer are discussed with typical imprint scenarios, nanoimprint at room temperature, at elevated temperature, and under electrostatic forces. With periodic structures, the guiding chart represents a powerful and straightforward tool to avoid defects in praxis, without in-depth knowledge of the underlying physics.

Electrically-assisted nanoimprint of block copolymers

Guiding of the phase separation of a block copolymer (BCP) by an electric field perpendicular to the substrate is investigated in order to obtain vertical structures that can provide a mask for subsequent etching. Because of practical aspects, the substrate is bare Si without any neutral brush and the process time is limited to 1 h. A polystyrene-block polymethylmethacrylate lamellar material is employed in the study. For a unique guiding of the lamellar phase, an ordering mechanism orthogonal to the electric field is introduced by the interaction with the stamp in a thermal nanoimprint process. The naturally low surface energy of the stamp shall induce the formation of lamellae along the sidewalls of linear cavities. In order to fully utilize these two ordering mechanisms, the stamp sidewalls and the electric field, the imprint process is conducted in such a way that no residual layer remains below the stamp structures and the whole BCP is accumulated inside the cavities which are just partly filled. The electrically-assisted imprint process is studied analytically, considering the capacitive effects due to the local electric field in the cavity and in particular in the BCP. In addition, a numerical simulation is performed for the actual experimental conditions to compute the electric vector field in the BCP. In this way, an extensive understanding of the situation is gained which is the basis for choosing optimal experimental conditions for electrically-assisted thermal nanoimprint. Furthermore, the ambiguity of the electric field in a thermal nanoimprint process with partly filled cavities is addressed. The field shall induce vertical phase separation but, due to instabilities, it also may induce capillary bridges that represent replication defects. An improvement of the vertical phase separation by applying an electric field as high as 25 V/μm could be identified under specific experimental conditions. However, the guiding effect within the cavities and thus the long-range order of the lamellae remained limited. This may be due to a field strength too low in the BCP; in the present configuration, higher field strengths are prohibited by an electrical breakthrough.

Adaptable Microcontact Printing via Photochromic Optical‐Saturable Lithography

AbstractA versatile and easily customizable mold platform for the microcontact printing of 1D and 2D patterns of dyes is developed. The mold is obtained by optical‐saturable lithography, which is optimized to give a fast process on millimeter size areas. Supported by a kinetic and an electro–magnetic model, 1D and 2D patterns are designed and successfully transferred on the photoresist layer and copied with polydimethylsiloxane (PDMS) to fabricate stamps that are to be used in the printing process. Fluorescence patterning, by means of these PDMS stamp, is validated and compared with the stamp structure.

Stamp Structure on the Conductive Grid Patterns Prepared by Microcontact Printing

Stamp Structure on the Conductive Grid Patterns Prepared by Microcontact Printing

Инновационная технология производства заготовок широкохордных лопаток газотурбинных двигателей

The method of design of wide-chord billets for blades of aircraft gas turbine engines, deliberately fabricated without twist of the airfoil profile, is described. The feasible options of manufacturing sequence of their processing are considered. The stamp structure for pen twist of the blade billets and stabilizing the obtained geometric dimensions are described.

Flexible composite stamp for thermal nanoimprint lithography based on OrmoStamp

Flexible stamps are common for roll-to-roll processing but less common with planar processing, although they offer a number of benefits as, e.g., an improved conformal contact at reduced pressure. A simple way to realize such a flexible stamp is to use a two layer system with a structured top layer and a flexible backplane. The structured top layer is most easily obtained by molding, the backplane provides the flexibility envisaged. For use in a thermal nanoimprint process, a high thermal stability is required for both. This investigation addresses the preparation of flexible composite stamps with OrmoStamp as the structured top layer and polyimide as the flexible backplane. The process recommended for stamp preparation with OrmoStamp has to be modified to avoid bending after the hard bake that is required after ultraviolet-curing of the material to obtain a high stability of the top layer. Reduction of bending is advised, in particular, for large area stamps, where the hard bake step is in conflict with preparation of a flat stamp and limits applicability for thermal nanoimprint. A small scale almost flat flexible composite stamp is prepared and employed for low-pressure nanoimprint; it provides full area imprint at a reduced pressure of 25 bar. The replicated structures, lines of 300 nm, are of high quality and image the stamp structures. Stamps prepared in this way allow a tuning of the flexibility simply by changing the thickness of the backplane.

Sub-100 μs nanoimprint lithography at wafer scale

We present here an ultrafast thermal NIL technology, which enables the patterning of full wafers on the 100μs time-scale. This technique makes use of stamps with a heating layer integrated beneath their nanostructured surfaces. Injecting a single, short (<100μs), intense current pulse into the heating layer causes the surface temperature of the stamp to raise suddenly by hundreds of degrees°C, resulting in the melting of the thermoplastic resist film pressed against it and the swift indentation of the nanostructures. This paper introduces the main aspects of this technology, namely the process concept, the stamp structure, and the main features of the equipment by which the process at the wafer scale was implemented.

Effect of residual stress on replication fidelity with nanoimprint

Relief of residual stress in an imprinted polymer may affect the replication fidelity by leading to recovery. The level of stress induced in the polymer depends on the method of imprint. For example, a “soft” imprint with an elastomeric stamp uses capillary forces to fill the cavities whereas a “hard” imprint with a rigid stamp relies on external pressure. To study the effect of residual stress after imprint, both methods are applied with different imprint times to vary the level of residual stress, as the stress remaining relaxes with imprint time. To visualize the residual stress a temperature treatment is performed after imprint. This temperature treatment allows recovery within a convenient experimental time. A comparison of the shape of the imprinted structures before and after temperature treatment clearly shows that with a hard imprint at short imprint times a considerable amount of stress remains in the polymer, in particular when the residual layer is thin and the imprinted stamp structures are wide. With a soft imprint residual stress is not evident. Similar results are obtained with a thermoplastic material and a crosslinking material (SU-8); however, with the latter recovery remains limited due to a decrease of mobility during crosslinking. Residual stress is of major importance for the replication fidelity with short imprint processes.

SU8 inverted-rib waveguide Bragg grating filter

A polymeric SU8 inverted-rib waveguide Bragg grating filter fabricated using reactive ion etching (RIE) and solvent assisted microcontact molding (SAMIM) is presented. SAMIM is one kind of soft lithography. The technique is unique in that a composite hard-polydimethysiloxane/polydimethysiloxane stamp is used to transfer the grating pattern onto an inverted SU8 rib waveguide system. The composite grating stamp can be used repeatedly several times without degradation. Using this stamp and inverter-rib waveguide structure, the Bragg grating filter fabrication can be significantly simplified. The experiment result shows an attenuation dip in the transmission spectra, with a value of -7 dBm at 1550 nm for a grating with a period of 0.492 μm on an inverted-rib waveguide with 6.6 μm width and 4 μm height.

Deformation behavior in 3D molding: experimental and simulation studies

Three-dimensional (3D) molding is a simple and effective technique using a modified hot embossing process to produce large area, hierarchical 3D micro/nanostructures in polymer substrates. However, the use of a thin intermediate polydimethylsiloxane (PDMS) stamp inevitably causes dimensional changes in the 3D molded channel, with respect to those in the brass mold protrusion and the intermediate PDMS stamp structures. Here we investigate the deformation behavior of the 3D molded poly(methyl methacrylate) (PMMA) substrate and the intermediate PDMS stamp in 3D molding through both experimentation and numerical simulation. Depending on the height, period and aspect ratio of the brass mold protrusions and the thickness of the intermediate PDMS stamp, strain contours of the intermediate PDMS stamp layer along the periphery of the 3D molded channels are varying, which leads to a nonuniform elongation of the imprinted structures in the 3D molded channel. Increasing the height and decreasing the period of brass mold protrusions leads to higher total strain of the intermediate PDMS stamp. It was found that for high aspect ratio brass mold protrusions the maximum strain of the intermediate layer occurs in the bottom center of the 3D channels. However, with decreasing aspect ratio of the brass mold protrusion the highest elongation occurs at the bottom corners of the channel causing less elongation of the intermediate PDMS stamp and imprinted structures on the bottom surface of the 3D channel. These experimental results are in good agreement with the results obtained from the numerical simulation performed with a simple 2D model.

Polymerization shrinkage stress measurement for a UV-curable resist in nanoimprint lithography

A simple method was developed to obtain the polymerization shrinkage stress exerted on the sidewalls of resist/stamp interface in ultraviolet nanoimprint lithography. This method is based on the measurements of demolding force which is the sum of adhesion and friction forces. The mean polymerization shrinkage stress on the sidewalls can readily be decoupled from overall demolding force by independently measuring the friction coefficient, adhesion force and geometries of stamp structures. The polymerization shrinkage stress on the sidewalls is overall larger than adhesion and increases by adding more cross-linking agent to the resist composition. This indicates that in addition to lowering the adhesion at the resist/stamp interface, development of resists with low degrees of shrinkage during UV curing is critical to reducing demolding force. It was also found that the shrinkage stress depends not only on the resist composition but also the stamp structure. A pillar structured stamp leads to a larger stress than a stamp with gratings with identical depth and width.

Efficient methods of nanoimprint stamp cleaning based on imprint self-cleaningeffect

Nanoimprint lithography (NIL) is a nonconventional lithographic technique that promiseslow-cost, high-throughput patterning of structures with sub-10 nm resolution.Contamination of nanoimprint stamps is one of the key obstacles to industrialize theNIL technology. Here, we report two efficient approaches for removal of typicalcontamination of particles and residual resist from stamps: thermal and ultraviolet (UV)imprinting cleaning—both based on the self-cleaning effect of imprinting process. Thecontaminated stamps were imprinted onto polymer substrates and after demolding,they were treated with an organic solvent. The images of the stamp before andafter the cleaning processes show that the two cleaning approaches can effectivelyremove contamination from stamps without destroying the stamp structures. Thecontact angles of the stamp before and after the cleaning processes indicate thatthe cleaning methods do not significantly degrade the anti-sticking layer. Thecleaning processes reported in this work could also be used for substrate cleaning.

Parallel and serial generated patterned DLC surfaces for creating three-dimensional polymeric stamp structures

This study demonstrates pattern generation on a highly durable and flat diamond-like-carbon (DLC) film with micro/nano-scale resolution using the Atomic Force Microscope (AFM). Parallel processing (masked lithography) and serial local probe processing (maskless lithography) have both been utilized to produce a range of structure shapes at different length scales. The AFM is operated in the electrical conductivity mode which induces oxidation on the DLC surface. The technique offers features with structure depths as small as 20 nm (serial processing) and pattern replication of many centimeters (parallel processing). Moreover parallel processed structures may be further modified via serial patterning using the same instrumentation. As a result, complex shapes can be produced with a depth being controlled by the DLC film thickness and/or by the bias voltage parameters. The patterned DLC structures can be used as a template for fabrication of 3 dimensional polymeric structures.

Recovery prevention via pressure control in thermal nanoimprint lithography

In order to investigate the nonuniformity occurring below wider patterns during thermal nanoimprint lithography, the pressure situation is analyzed in detail. A balance of vertical forces shows that the gas pressure within the cavities is negligible, whereas the effective pressure acting on the stamp structures in contact to the polymer is increased compared to the externally applied pressure. This effective pressure has a high hydrostatic component, which may result in local stamp deformation. Imprint with a low molecular weight polymer provides evidence that the stamp structures become compressed under high pressure, resulting in a nonuniform residual layer beneath the imprinted patterns. In contrast, bending deformation of single stamp structures is found to be negligible. Pressure reduction is effective to reduce stamp compression, improving the uniformity of the residual layer. With typical imprint polymers of medium molecular weight, however, pressure reduction reduces the overall imprint depth.

Nanoimprint lithography using vertically aligned carbon nanostructures as stamps

Nanoimprint lithography using vertically aligned carbon nanostructures asstamps is reported. The functionality of the stamp is demonstrated throughlift-off and etch-back processes after pattern replication. The imprint process isrobust and the stamp structures survived more than 50 consecutive imprints.In this paper we demonstrate this for feature sizes ranging from 80 nm to200 µm where the aspect ratio of the individual nanostructures surpasses 1:5 with a pitch down to100 nm. This demonstration opens up the possibility of utilizing vertically grown carbonnanostructures for manufacturing extremely high aspect ratio and small pitch stamps fornanoimprint lithography.

Reflection of the Voronezh crystalline massif in the seismic noises of neotectonic structures

In this paper, we describe the results of investigation of the correlation between the seismic noise parameters and the geological structure combined with geodynamic conditions of neotectonic structure formation. This investigation is based on data on the characteristics of seismic noise at 102 points located in the central and eastern parts of the VCM over an area exceeding 75 000 km 2 . The massif, which is part of the East European Platform, is a differentiated block-dome elevation of the Precambrian basement covered with a sedimentary layer from 50 to 600 m thick. The study region is located in the conjunction zone between the two largest neotectonic elements: the Middle Russia Rise and Oka‐Don Depression [3]. Elements of the second and higher orders were distinguished within these structures (Fig. 1). In the genetic relation they can be related to the structures of the stamp type, which are formed in the sedimentary cover owing to free-bulk motions of the basement [4]. The boundaries of stamp structures related to the fracture zones in the crystalline basement are presented in the sedimentary cover by the regions of dynamic influence of these fractures. In the NeogeneQuaternary structural material complex within the investigated territory, these regions are presented by flat flexures, linear troughs that are frequently complicated by local depressions and elevations. Superfracture structural parageneses of the study region formed at the boundaries between modern stamp structures are related to large and wide fracture zones in the Precambrian basement, which appeared in different stages of the basement formation and were subjected to multiple activations. The zones important in neotectonics are related to the Saltykovskii and Krivoborskii linear flexures, which are oriented in the submeridional direction, and separate the Middle Russia Rise from the Oka‐Don Depression. While characterizing the modern structure of the territory, it is worth noting the following peculiarities:

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.

Quantitative characterizations of a nanopatterned bonded wafer: force determination fornanoimprint lithography stamp removal

The nanoimprint lithography process consists of two mechanical steps: molding and stampremoval. While many publications dealing with anti-sticking layer properties or theunderstanding of polymer flow during imprinting have recently been published, only a fewstudies have been carried out to deeply characterize the demolding step. Regarding thesmall amount of theoretical work dedicated to this issue, in this paper both experimentaland first theoretical approaches are proposed to characterize the demolding process in apeeling scheme. Full 200 mm stamp and imprinted wafers were used to identify theexperimental limitation of such a full wafer peeling demolding scheme. A rectangularstamp and substrate samples with or without nanoscale features combined with anaugmented beam theory are proposed to extract quantitative data for the requireddemolding force as well as the friction stress along the feature sidewall. Thereforeboth adhesion and friction forces were characterized on single stamp structures.