Imprint Time Research Articles

Nanosecond-range imprint and retention characterized from polarization-voltage hysteresis loops in insulating or leaky ferroelectric thin films

We transferred ferroelectric domain switching currents under pulses into polarization-voltage (P-V) hysteresis loops. With this transformation, it is possible to derive the remanent polarization and coercive voltage from domain switching currents after the shortest imprint and retention time of 35 ns. After the separation of film leakage current from domain switching current, we measured the P-V hysteresis loop in a semiconducting BiFeO3 leaky thin film, where the apparent coercive field highly reaches 320 kV/cm2, suggestive of a different domain switching mechanism from other insulators. This technique facilitates nanosecond-range measurements of both ferroelectric capacitive and resistive memories.

In-Situ Monitoring of Pattern Filling in Nano-Imprint Lithography Using Surface Plasmon Resonance

Nano-imprint lithography possesses the advantages of high throughput, sub-10-nm feature and low cost. In NIL, the mold filling is subjected to the applied imprinting pressure, temperature and time. Incomplete mold filling causes a detrimental effect on the final imprinted pattern dimensions. The monitoring system of imprinting is essential to control the imprinting parameters properly. Up to now, no high-sensitivity monitoring of filling rate and end point has ever been proposed. In this study, the authors apply the surface plasmon resonance to monitor the filling rate and end point during imprint process. The mold contains a layer of glass of high refractive index, a metal thin film and the pattern of low refractive index. In addition, the imprinted polymer is selected considering its refractive index, which should be lower than the glass layer of mold. When the filling rate varies, it will affect the SPR behavior, including the measurable reflectivity change and resonance angle shift. The analysis results reveal that the resonance angle is truly proportional to the filling rate. When the filling rate varies from 50% to 100%, the SPR angle shifts more than 5 degree. The analysis demonstrates this innovative method for monitoring of filling rate is effective with high sensitivity.

The biological basis and clinical significance of hormonal imprinting, an epigenetic process.

The biological phenomenon, hormonal imprinting, was named and defined by us (Biol Rev, 1980, 55, 47-63) 30 years ago, after many experimental works and observations. Later, similar phenomena were also named to epigenetic imprinting or metabolic imprinting. In the case of hormonal imprinting, the first encounter between a hormone and its developing target cell receptor—usually at the perinatal period—determines the normal receptor-hormone connection for life. However, in this period, molecules similar to the target hormone (members of the same hormone family, synthetic drugs, environmental pollutants, etc), which are also able to bind to the receptor, provoke faulty imprinting also with lifelong—receptorial, behavioral, etc.,—consequences. Faulty hormonal imprinting could also be provoked later in life in continuously dividing cells and in the brain. Faulty hormonal imprinting is a disturbance of gene methylation pattern, which is epigenenetically inherited to the further generations (transgenerational imprinting). The absence of the normal or the presence of false hormonal imprinting predispose to or manifested in different diseases (e.g., malignant tumors, metabolic syndrome) long after the time of imprinting or in the progenies.

45 nm hp line/space patterning into a thin spin coat film by UV nanoimprint based on condensation

Applicability of UV nanoimprint based on gas condensation to 45 nm line and space (L/S) patterning using a spin-coat UV curable resin was evaluated. Although high capillary pressure is generated in such fine L/S patterns when UV curable resin liquid comes in contact with the groove surface, the trapped air in those grooves is not completely compressed and results in the formation of bubble defects. Bubbles much larger than the pattern size were generated at the middle of the L/S patterns for a 108 nm thick, UV curable resin film, and incomplete filling may occur on a large area of the L/S patterns for a 19 nm thick film. Since nearly bubble-free L/S patterns have been fabricated for 48 nm thick, UV curable resin film, it might be possible to make bubble-free UV nanoimprint by prolonging the imprinting time. On the contrary, when UV nanoimprint was carried out in pentafluoropropane (PFP), no bubbles were created for all thicknesses of the UV curable resin films. Uniformity of residual layer looked quite good judging from the color uniformity of the area around the L/S patterns. It was found, from the residual layer measurement by a reflective thickness monitor, that bubble-free UV nanoimprint was carried out with a residual layer thickness of 15.2 nm. UV nanoimprinted samples were inspected with a scanning electron microscope and it was verified that UV nanoimprint using PFP is viable for the fabrication 45 nm L/S patterns.

Hot embossing of PTFE: Towards superhydrophobic surfaces

Three types of reusable stamps with features in the form of 2D arrays of pits having lateral dimensions in the range of 2–80 μm and heights of 1.5–15 μm were successfully employed for the hot embossing of PTFE at temperatures up to 50 °C above the glass transition temperature of PTFE amorphous phase. Due to the softening of PTFE at the temperatures used in this study, we were able to decrease imprint pressure significantly when comparing with the imprint conditions reported by other authors. Impact of the imprint temperature, pressure and time on the fidelity of pattern transfer as well as on water repellency was tested. The best results of embossing were achieved by applying pressure of 10 kg/cm 2 for 2 min at 170 °C. In this case, flattening of a natural PTFE roughness and pretty accurate deep replicas of the stamp patterns were observable on the whole imprinted area. Improvement in water repellency was largest for the samples imprinted by Ni stamp patterned with a 2D array of 2 μm square pits spaced by the same dimension and having a depth of 1.5 μm. Cassie–Baxter wetting regime was observed for the deepest imprints with water contact angles up to the superhydrophobic limit.

Quantifying electronic charge trap states and the effect of imprint on ferroelectric poly(vinylidene fluoride-trifluoroethylene) thin films

Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] thin films have been extensively studied for their ferroelectric properties and are a promising material for sensing and memory applications. Imprint, the time-dependent resistance to polarization reversal, is a key material property that limits applications and is poorly understood. We have used a fast ramp rate thermally stimulated current (FR-TSC) technique to investigate the proposed link between imprint time and filling of charge trap states, and to characterize these traps in an appropriate time-frame. Thin films of P(VDF-TrFE) on oxidized Si substrates were characterized following controlled initialization, fatigue, polarization, and imprint. Traps were allowed to fill during room temperature imprint for times from 1 to 104 s. Remaining unfilled traps were subsequently filled and quantified by FR-TSC temperature cycling from 20 to 100 °C at 1 K/s. Results confirm that charge trap filling occurs both with time, under isothermal conditions, as well as with increasing temperature. The rate of charge accumulation induced by FR-TSC exhibits the same logarithmic dependence with time as the experimentally measured imprint voltage; this strongly suggests a direct link between these trapped charges and the imprint field. Furthermore, measurements of the switching speed in post FR-TSC films were found to be independent of the imprint time, confirming that thermal cycling brought all samples to a common final state. These results provide further understanding of the polarization dynamics and materials interactions affecting the ferroelectric properties of P(VDF-TrFE).

Tetrapod tracks in a marginal lacustrine setting (Middle Triassic, Argentina): Taphonomy and significance

Fossil tetrapod footprints not only provide valuable information about trackmaker paleobiology but also to give insight into details of the depositional conditions of the substrate at the time of imprinting. Therefore, in the present study the mode of formation and taphonomy of footprints in different substrates was used to investigate the gait and walking dynamics of the trackmakers as well as a source of additional information on the environmental conditions of the track-bearing beds during imprinting. The analyzed section corresponds to thick Middle Triassic lacustrine/deltaic deposits of the Ischichuca/Los Rastros Formation (Ischigualasto–Villa Unión Basin) that crops out at the Quebrada de Ischichuca in northwestern Argentina. Part of the track-bearing surfaces correspond to the top of sandy distributary channel mouth bars in a distal delta front setting that were exposed along the lake margin during a lake level fall. Cross-cutting relationships observed among ripple-marks, the footprints, and invertebrate traces of a softground suite of the Scoyenia ichnofacies suggest an omission surface. Measured trackway orientations in the sandstones are perpendicular to the paleo-shoreline, with the animals coming and going along the exposed top of the bars, probably for drinking. Laterally, the distal delta front deposits interfinger with track-bearing wackestone beds of palustrine origin deposited in a restricted local embayment lateral to the delta influenced environment. Trackway orientations in the wackestones are, in contrast, consistent with the animals moving nearly parallel to the lake border, probably along a preferred route. Evidences of a relative high groundwater table at the time of imprinting in the track-bearing surfaces are revealed by the well developed rims of extruded sediment and collapsed digits in the studied tracks and the nearly absence of associated desiccation cracks on the same surfaces. Nevertheless, temporary emergence cannot be ruled out when paleosoil formation was probably promoted as can be observed in the microstructure of both sandstones and wackestones. Moreover, footprint preservation in the wackestones might have been enhanced by partial hardening of the trampled surface during subaerial exposure. Combining ichnofossil content and taphonomy with facies analysis we identified in the lower part of the Ischichuca/Los Rastros succession a relatively rapid withdrawal of the water basinward that was probably due to a forced regression during early rifting of basin evolution. Footprints can also provide valuable information about locomotion dynamics and trackmaker behavior. Thus, the sideways deformation observed in the studied footprints, attributed to basal archosaurs and putative basal dinosaurs, can be related to an outward rotation of the foot during the step cycle, a condition that might allied to the development of the parasagittal posture in Archosauria. Besides, the densely trampled surface described herein constitutes the first documented evidence of putative social behavior among therapsid dicynodonts, the most important group of herviborous animals in the early Mesozoic terrestrial ecosystems throughout Gondwana.

Room-temperature capillary-imprint lithography for making micro-/nanostructures in large areas

Compared to imprint techniques under the driving condition of external imprint pressure, imprint lithography under capillary force has the advantages of being suitable to produce large feature patterns, simple mold structure, very low imprint pressure, and conformal contact between the mold and the substrate. However, in existing capillary-force lithography, the necessary heating process will bring problems, such as separation between the mold and the substrate, long imprint time, shorter lifetime of the mold, etc. In this article, the authors propose a technique of room-temperature capillary-imprint lithography which is applied to making both micro- and nanostructures simultaneously in large areas under low temperature. In order to prevent the polydimethylsiloxane mold from swelling in the imprint process, a new ultraviolet curable resist is introduced in the imprint process. This resist contains no solvents and has a wide range of viscosity. Through the experiment, three-dimensional microstructures are replicated in large areas on the flat substrate as well as on curved substrate. Moreover, by utilizing a resist with low viscosity, combination features with linewidth of 1 μm and 100 nm are reproduced successfully in the resist by the imprint technique.

Analysis of pattern height development in hot embossing process

The pattern height of hot embossing was analyzed based on the polymer power–law material assumption for better controlling product quality. The time development of the polymer substrate height, pattern height and the imprint pressure were obtained. The imprint temperature with respect to the pattern height was discussed. Theory analysis shows that in a constant imprint pressure, the pattern height has a power relationship with the imprint time. At the beginning imprint stage, as for the elastic response of polymer, the pattern height increases rapidly. With the time increasing, the polymer deform is mainly caused by creep and this results in low imprint speed. On the other hand, the increasing friction force between the polymer and the mold will further reduce the imprint speed. The theoretical data were compared with experiments and the results show that this model can well predict the rule of polymer substrate thickness variation.

Release Properties and Durability of Release Layer on UV Nanoimprint Lithography

Two photo-curable resins (PAK-01 and PAK-02) have been evaluated by a primitive test and a precise test. The primitive test is simple manual transfer process and the precise test is using UV nanoimprint lithography (UV-NIL) machine. At first, nano order high aspect ratio glassy carbon (GC) molds are prepared and then manual transfers are carried out. Both PAK-01 and PAK-02 have good filling and release properties. Next, a silane coupling agent C8F17C2H4SiCl3 is used for the release treatment of the quartz mold. To investigate of affinity between photo-curable resin and release layer, repetition manual transfers are carried out. In this release layer, durability is not sufficient for PAK-02. Therefore, more precise durability tests are carried out using PAK-01. The release treatment samples with various contact angles are prepared by changing the treatment of the silane coupling agent. The contact angle of less than 10 degrees is obtained by UV ozonation cleaning. The contact angle of 40 degrees is obtained by acetone cleaning. The contact angles of more than 80 degrees are obtained by silane coupling agent at 5 minutes treatment. The contact angles of more than 100 degrees are obtained by silane coupling agent at 2 hours treatment. After the preparation of the mold treatment, repeatedly imprints are carried out using UV-NIL machine. In the case of contact angles with less than 40 degrees, PAK-01 sticks to the surface of the quartz within several imprint times. On the other hand, in the case of contact angles with more than 80 degrees, PAK-01 does not stick on quartz surface more than 1000 times. Therefore, the combination of this release treatment and PAK-01 is suitable for mass-production.

Contact angles in a thermal imprint process

In a thermal imprint process, contact angles may evolve in two situations, between the stamp and the polymer, when a cavity remains unfilled, or, in cases where the polymer locally dewets the substrate, between the substrate and the polymer. For two polymers of different polarity, such contact angles are determined experimentally and compared to values calculated from surface energy data. In doing so, the specific temperature dependence of surface energies of a thermoplastic polymer as well as the typical course of an imprint process, where cooling times often exceed the imprint times, are considered. The differences between experimental and theoretical values are critically discussed. The results indicate that the respective adhesive layers govern the surface status of the substrate and the stamp as well.

Formulization of long-time domain switching around the coercive field from imprint measurements on ferroelectric thin films

The time interval for the completion of slow polarization reversal in ferroelectric thin films measured in the order of magnitude of 10−9–100s is broadened by more than two decades as the applied field approaches the coercive field of domain switching. The domain-switching kinetics is conjectured to change from the classical Kolmogorov–Avrami–Ishibashi (KAI) model at high fields to nucleation-limited-switching (NLS) model at low fields. However, the true voltage drop across the film from the real-time measurement of the oscilloscope remains constant at a coercive voltage Vc during polarization reversal irrespective of the applied voltage. Therefore, the above high-field assumption for KAI does not exist. From our measurements, a long-time effect of imprint can occur for any slow processes of physical phenomena, even if the imprint field is very weak. The imprinted Vc can be estimated either from the voltage shift in the curve of switched polarization versus the applied voltage or directly from the height variation in domain-switching current plateau after an imprint voltage Vimp for time timp (1μs<timp<10ms). Vc reduces continually with timp under Vimp>0 to exhibit a linear time dependence in a logarithmic scale above an initial time on the order of 1.0μs, even if under Vimp<Vc, which foretells the possibility of the initial unswitched domains with negative polarizations to switch into positive states with the prolonged imprint time under a positive voltage of Vimp. After formulation of the Vc-timp dependence under Vimp according to imprint principles, we derive the time evolution of slow polarization reversal without evoking KAI and NLS approaches once the Vc distribution is obtained for a genuine thin film. The spectrum derivation is completely comparable to experimental measurements, which suggests the common physics between imprint and long-time domain switching of ferroelectric thin films.

EXPERIMENTAL STUDY ON THE HOT EMBOSSING POLYMER MICROFLUIDIC CHIP

Experiments are used to study the fabrication of polymer microfluidic chip with hot embossing method.The pattern fidelity with respect to the process parameters is analyzed. Experiment results show that the relationship between the imprint temperature and the microchannel width is approximately exponential.However,the depth of micro channel isn't sensitive to the imprint temperature.When the imprint pressure is larger than 1 MPa and the imprint time is longer than 2 rain,the increasing of imprint pressure and holding time has little impact on the microchannel width. So over long holding time is not needed in hot embossing.Based on the experiment analysis,a series of optimization process parameters is obtained and a fine microfluidic chip is fabricated.The electrophoresis separation experiment are used to verify the microfluidic chip performance after bonding.The results show that 100bp-ladder DNA sample can be separated in less than 5 min successfully.

Nanosecond-range measurements of imprint effect for Pt∕IrO2∕Pb(Zr0.4Ti0.6)O3∕IrO2∕Pt thin-film capacitors

The determination of the coercive voltage from the height of domain-switching current plateau in ferroelectric thin films provides the liability to estimate the coercive-voltage shift with imprint time shortly on the order of polarization-reversal time. The voltage shift exhibits a linear time dependence in a logarithmic scale above an initial time (∼1μs), below which the voltage is nearly constant. The modeling of imprint on the basis of the interfacial charge injection at different stressing voltages strongly supports Schottky emission as a dominant mechanism, instead of Frenkel-Poole emission and Fowler-Nordheim tunneling.

Thermal Imprint Process of Parylene for MEMS Applications

This study investigated a thermal imprint technique to pattern parylene microstructures over an area of 2525 mm2. A nickel mold having arrays of 25 m-high, 10 m-wide and 1 mm-long lines with 10 m spacing was fabricated using the deep RIE silicon etching followed by the electroplating process. Imprint tests were then carried out under different conditions of temperature, imprint-hold time and applied pressure to investigate a thermal imprint condition for the complete filling of parylene. Good release results without damage or deformation in parylene microstructures were achieved by the help of a release agent in the imprint temperature range of 160 oC to 250oC. With increasing temperature, the depths of imprinted structures increased and their distribution came to be homogeneous. Complete filling was obtained under the imprint temperature of 250oC, applied load of 195 kgf (3 MPa) and imprint hold time of 1800 s.

Room-Temperature Imprinting Poly(acrylic acid)/Poly(allylamine hydrochloride) Multilayer Films by Using Polymer Molds

Layer-by-layer assembled polyelectrolyte multilayer films of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) have been successfully patterned by room-temperature imprinting using a Norland Optical Adhesives (NOA 63) polymer mold. The proper amount of water in the PAA/PAH multilayer film can decrease the viscosity of the film and facilitate the imprinting. Many factors, such as imprinting pressure, length of imprinting time, and the structure and size of the patterns in the polymer mold, can produce an influence on the final imprinted pattern structures on multilayer films. A high imprinting pressure of 100 bar and elongated imprinting time of several hours is needed to achieve a patterned PAA/PAH multilayer film with a feature size of several tens of micrometers. With a twice imprinting, grid structures can be successfully produced when a NOA 63 mold having line structures is used. Room-temperature imprinting by using polymer NOA 63 mold provides a facile way to fabricate layered polymeric films with various kinds of pattern structures.

Three-dimensional SU-8 structures by reversal UV imprint

In this work, three-dimensional (3D) SU-8 micro- and nanostructures were fabricated using a reversal UV imprint process at low temperature and low pressure. The SU-8 polymer was coated on a patterned glass mold and then transferred onto various substrates by reversal UV imprint at a typical temperature of 50°C, pressure of 1MPa, and UV exposure of 1s. The lower temperature and pressure used compared to conventional thermal imprint shorten the imprint time and alleviate pattern distortion. A combination of silanes was used to generate a medium surface energy on the imprint molds to enable polymer spin coating and mold release after imprint. In addition, an O2 plasma was used for glass mold treatment to improve uniformity of silane coating and to increase substrate surface energy for better polymer adhesion. Using this technology, 100nm–1μm wide SU-8 gratings were fabricated on flat or patterned substrates with good fidelity. By repeating this process, multiple-level nanochannels, cavities, or air-bridging polymer structures with 400nm–10μm widths have been demonstrated. The surface energy of SU-8 was modified using an O2 plasma to promote layer adhesion for 3D stacking. This reversal UV imprint technology offers versatility and flexibility to stack polymer layers and multiple-level sealed fluidic channels.

Pattern replication of 100 nm to millimeter-scale features by thermal nanoimprint lithography

The aim of this work is to demonstrate the ability of nanoimprint lithography (NIL) to replicate patterns having feature sizes ranging from nanoscale to millimeter scale. The pattern replication process includes NIL on PMMA, PMMA RIE, metal liftoff and then silicon RIE for final pattern transfer. A tri-layer resist scheme was employed to facilitate the liftoff. We studied systematically the dependence of the maximum duplicable feature size on imprint temperature, pressure and time, which shows good agreement with a simple squeeze flow model. The maximum duplicable feature size also depends on PMMA molecular weight and the amount of PMMA RIE. For example, with NIL at 200 °C and 20 bar for 20 min and PMMA etching of 180 nm, we duplicated 1.3 mm square pattern without defects using 12 kg/mol PMMA. Such amount of PMMA RIE leads to the nanoscale grating line-width increase of 18 nm.

Characterization of imprinting polymeric temperature variation with fluorescent Rhodamine B molecule

The method to measure the imprint temperature variation of resist film with fluorescence probe technique was established. The maximal Rhodamine B concentration was 8 × 10 −4 M to achieve higher sensitivity and avoid precipitation problem. The temperature effect was a critical parameter on the temperature sensing, while imprinting pressure was not. The fluorescence intensity of Rhodamine B linearly decayed with time at various temperatures due to the formation of lactone-containing molecule during thermal stressing. The usual imprint time of less than 20 min was not affected by the signal decay effect due to limited percentage of signal variation during 100–200 °C. The technique based on fluorescence probe method was successfully applied to measure the resist temperature variation of the imprint on the resist film of 6-in. wafer.

Polymer time constants during low temperature nanoimprint lithography

We investigate low temperature nanoimprint into polymers, where adequate pressure choice helps to increase the imprint velocity of larger patterns und thus decreases pattern size effects by reduction of the effective viscosity. In order to make use of shear rate effects, the imprint has to be performed within the viscoelastic plateau region and not in the viscous flow regime of the polymer. Therefore elastic effects play an important role and may lead to shape recovery of the polymer after imprint. We address the counter play of elastic effects and viscous flow by conducting experiments very near to the glass transition temperature. The elastic behavior at the beginning of the imprint is simulated for different pattern sizes and thus different aspect ratios of the stamp. The investigations show, that a reduction of temperature has to be compensated by an increased imprint time and this time increase has to consider the reduction of viscosity on the one hand and the extension of the polymer flow time constants on the other hand. The experiments are suitable to define a lower limit for utilization of shear thinning effects.