Hot embossing of diffractive optically variable images in biaxially-oriented polypropylene

The hot embossing of grating-based optically variable devices has been demonstrated in biaxially-oriented polypropylene (BOPP). Embossing of the grating structures was examined over a range of temperatures (80-155 °C) at 135 kN force. However, only at temperatures above the glass transition temperature, T_g, was high quality replication achieved over a full embossing area of 80 × 80 mm. The embossing of several different types of optically variable device has been examined including portrait, non-portrait and 3-dimensional images. The images embossed into BOPP have displayed an optically variable effect when viewed in transmitted light.     

Optically variable micro-mirror arrays fabricated by graytone lithography

The fabrication of micro-mirror based portrait and non-portrait images has been demonstrated by graytone lithography. The depiction of a portrait image was achieved by a novel modulation of the width of micro-apertures in the variable transparency mask. The non-portrait image was encoded as micro-mirrors of an opposing slope and orientation. An optical switch effect between the portrait and a non-portrait image was encoded into the mask by the arrangement of the micro-mirrors as a separate interleaved channel for each image. The lithographic exposure of the pattern into a layer of thick resist has produced an array of micro-mirrors with a depth of 10-30 μm. Various configurations of the micro-mirror arrays have been examined in relation to the type of micro-apertures and their orientation. Optically variable effects achieved in the micro-mirror devices have included a portrait to non-portrait switch, a positive to negative image switch and a non-portrait to non-portrait image switch. The micro-mirror structures have been hot embossed into polypropylene sheet using a Ni shim to create high quality optically variable images.     

Hot embossing of micrographic elements in polypropylene

Micrographic features generated by electron beam lithography have been hot embossed in biaxially-oriented polypropylene (BOPP). Each micrographic element consisted of a sub-field of 30 × 30 μm with a specific configuration of symbols, lettering and graphics. The micrographic elements were integrated with arrays of diffractive gratings in the form of an optically variable device. A high quality embossing of various micrographic elements has been demonstrated in features with a linewidth dimension ?365 nm and depth ?255 nm. The replication of arrays of micrographic elements was achieved uniformly over an area of 20 × 20 mm in the BOPP film by embossing at a temperature of 130 °C.     

Optically variable watermark (OVW) microstructures for transparent substrates

We report on the novel fabrication of optically variable watermarks (OVW) in a transparent substrate. The OVW is incorporated into the substrate via an embossing process. The master embossing die is produced by a combination of electron beam lithography, greytone photolithography, and precision electroplating techniques. The micro-embossing into polypropylene film is performed at 130 °C and 100 kN, a similar pressure to that used in intaglio printing. The optical effect produced by the device corresponds to a switch from one image to another as the device is observed in transmission when held in front of a light source and moved up and down. The image switching mechanism is a result of the refraction of the incident light through the embossed interlaced rows of micro-prisms, with the two different images corresponding to opposite prism slopes. Specific results reported here include an OVW image switch from a “$” to a “50” symbol and a second test image corresponding to an image switch from a portrait to a logo style image. Applications of the OVW technology include anti-counterfeiting protection for polymer banknotes, identity cards with transparent regions and labels attached to brand name products.     

Scanning probe microscope analysis of microstructures in optically variable devices

The fine-scale features of optically variable devices (OVDs) fabricated in resist by electron beam lithography have been examined using scanning probe microscopy (SPM). These features have included patterns of gratings, micro-text and geometrical images. Scanning probe microscopy has provided information on the groove angle, depth of profile and spatial frequency of the features as determined by the details of processing of the image. The OVD patterns formed in EBR-9 and X-AR-P 7400 resists exhibited a more rounded profile with a lower side-wall angle than in ZEP-7000 and PMMA resists.     

Pattern replication in polypropylene films by hot embossing

The hot embossing of electron beam generated structures with linewidths of 0.35-3.25 μm has been examined in biaxially-oriented polypropylene (BOPP). The individual test elements consisted of pixels of 30 × 30 μm containing a specific configuration of symbols and lettering. The embossing of these features has been performed as a function of temperature (80-140 °C) and applied pressure (2-20 kPa). Significant increases in both embossed depth and sidewall angle were evident over the temperature range 100-130 °C with a leveling off at higher temperatures attributed to the onset of a regime of viscous liquid flow. At temperatures within this regime, a critical level of pressure was required to fill the mold features. Accurate embossing of medium and coarse lettering (0.50-0.65 μm linewidth) and geometric symbols (1.25-3.25 μm linewidth) has been demonstrated at a temperature of 130 °C and an applied pressure ?13 kPa. The depth of the finest lettering (0.35 μm linewidth) was incompletely embossed under these conditions.     

Fabrication of gold sub-wavelength pore array using gas-assisted hot embossing with anodic aluminum oxide (AAO) template

This paper reports a novel and effective method for the fabrication of gold sub-wavelength pore array using gas-assisted hot embossing. The novel fabrication comprises the fabrication of AAO template with high order pore array, sputter-coating of gold and gas-assisted hot embossing. AAO template is fabricated by a two-step anodization process. It is then coated with gold (100 nm thick). The gold sub-wavelength pore array is finally formed by using gas-assisted hot embossing process. The transmission spectra of gold sub-wavelength pore array are measured in several conditions such as air, water and glycerol/water mixture. The detection shows significant shift in the wavelength of resonance peak. The gold sub-wavelength pore array can be employed as optical biosensors.     

Low-temperature embossing technique for fabrication of large-area polymeric microlens array with supercritical carbon dioxide

This study reports an effective process for low temperature replication of polymeric microlens array using carbon dioxide (CO₂) as softening solvent and then as embossing pressure. Supercritical CO₂ is employed to soften the surface of polymer substrate and then apply pressure on it. Accordingly, microlens can be formed on the polymer substrate at low temperature. Using gas as embossing pressure can provide uniform pressure for large-area replication. During the embossing process, an array of convex microlenses is generated by partial protrusion of the substrate into the micro-holes of the mold under the action of embossing pressure and surface tension. The uniformity of large-area fabrication and optical property of the fabricated refractive microlens array on a 260 mm × 110 mm poly (methyl methacrylate) (PMMA) substrate have been verified. Little residual stress can be observed by polariscope. With the proposed method, the large-area embossing can be performed without heating and cooling. Moreover, the problems of non-uniform pressure and residual stress can be overcome.     

Impact of molecular weight of polymers and shear rate effects for nanoimprint lithography

Linear polystyrene of different weight averaged molecular weight (M_w = 58 kg/mol and 353 kg/mol) and a narrow molecular weight distribution was imprinted to point out differences in the flow behaviour caused by the polymer’s molecular weight in a typical hot embossing/thermal nanoimprint process. Residual layer thickness and uniformity as well as defects were taken as a measure to qualify the imprint process. Since polymer viscosity is addressed as the main material parameter, a full thermo-mechanical characterisation of the two polymers via parallel plate dynamic viscosimetry was performed prior to imprint. The main process parameter responsible for viscosity, the temperature, was varied from about 30 to 150 K above the glass transition. In addition the amount of polymer supplied for the process was varied, i.e. the initial spin-coated polymer layer thickness. Situations representing ‘underfilling’, ‘filling’ and ‘overfilling’ of the stamp cavities in specific regions were investigated. Squeezed flow simulations were performed taking into account the increase of pattern size and the filling of cavities during the ongoing imprint, which have substantial impact on the residual layers obtained. The results show that shear thinning is an issue in a high pressure hot embossing system. It governs the initial phase of the imprint featuring a rapid decrease of the polymer height. From theoretical considerations, a model could be derived for the imprint procedure under shear thinning in a constant pressure system which was able to interpret the observed phenomena. Despite the macroscopic nature of the thermo-mechanical analysis, the standard viscosimetric data for the polymers appeared to be sufficient to understand the experimental results.     

Direct stamp fabrication for NIL and hot embossing using HSQ

Commonly stamps or masters for nanoimprinting are made by electron beam lithography (EBL) and subsequent reactive ion etching into silicon. Here we present a single step procedure to prepare stamps suitable for nanoimprinting and hot embossing. The stamps are directly fabricated in HSQ (hydrogen silsequioxane), a negative EBL resist, which has a high lateral resolution and good mechanical properties. We demonstrate successful pattern transfer in both bulk PMMA and PCL by hot embossing with features down to 20 nm. Such pattern transfer is useful for biological applications. Also, we demonstrate that this approach can make stamps suitable for nanoimprint lithography and have achieved features as small as 35 nm. It was found that the stability and strength of the HSQ could be improved by annealing and that the application of a non-stick coating was not necessarily required although it aided the demoulding.     

BSRF上的微细塑铸研究

利用北京同步辐射装置 (BSRF)上的LIGA技术制作了镍模具 ,试验了溶液浇铸、反应浇铸、大气模压、真空模压四种不同的塑铸方法。结果表明采用自制模压装置 ,采用排气法可以获得与真空模压相近的效果 ;真空模压在这四种方法中具有最好的精度 ,但若不进行工艺参数优化 ,也会导致变形等问题     

A parameter study on the micro hot-embossing process of glassy polymer for pattern replication

As a promising processing method for glassy polymer, hot-embossing process has attracted more attention in the micro/nano manufacturing field. However, the relationship between process condition and final quality has not been thoroughly and systematically investigated. Since the replication quality of the process is sensitive to the control parameter, it is necessary to study on the optimal process condition. For the four main factors influencing on the final replication fidelity of the hot-embossing process, we carried out a series of experiments with different sets of parameters, in which micro pattern arrays were directly replicated from a glassy carbon (GC) mold to a commercially available glassy polymer substrate (polycarbonate, PC) with self-made micro hot-embossing system. sWe adopt Taguchi method to minimize the sets of experiments and made an analysis of the result using ANOM and ANOVA. As the result shows, the most significant factor is the embossing temperature and the optimal process parameters are the embossing temperature at 170 °C, heating time at 120 s, embossing force at 70 kgf and force holding time at 90 s. The experiment with these parameters shows a good result.     

Deformation behavior of solid polymer during hot embossing process

Though hot embossing is a well known technique for the fabrication of polymer based micro-device, the deformation behavior of solid polymer during hot embossing process is not investigated clearly. In this paper, the deformation behavior of solid polymer was observed by two methods, synchronous observation and asynchronous analysis. A finite element simulation and a phenomenological model were used to evaluate the deformation behavior of solid polymer during hot embossing. Results showed that the deformation of solid polymer during embossing process included two stages. One was a stress concentration and strain hardening stage, which occurred during the heating and applying pressure process. The “swallowtails” induced by incomplete filling generate at this stage. The other was a stress relaxation and deformation recovery stage, which occurred during the remaining temperature and pressure process. The “swallowtails” were eliminated at this stage. The second stage was significant for improving replication precision, but it had not been reported before.     

Metal direct nanoimprinting for photonics

In this paper metal direct nanoimprinting (embossing) for the production of metallic microparts is discussed, with a main focus on its suitability for the fabrication of metal-containing optical devices such as photonic crystals, plasmon waveguides or chiral structures. Silver and gold were chosen, since they have the lowest light absorption in the near infrared and visible range. They are also easily formable due to their good ductility, which can be further enhanced by processing at elevated temperature. The mold material, which may form part of the optical device, usually consisted of silicon, but other dielectric materials such as silicon oxide and silicon nitride were also successfully tested.Cylindrical and line-shaped holes with lateral dimensions down to 250 nm and aspect ratios of up to 5 were etched in silicon wafers. All the structures were successfully filled with silver and gold, and the filling of smaller dimensions is also deemed possible. This technique is therefore suitable for producing metal-containing optical devices working in the infrared, at least down to the standard telecom wavelength of 1.5 μm, which requires metallic dimensions of 200-300 nm.     

A novel fabrication of polymer film with tapered sub-wavelength structures for anti-reflection

This paper presents a cost-effective method to fabricate tapered sub-wavelength structures (SWSs) on the polycarbonate (PC) film using gas-assisted hot embossing with anodic aluminum oxide (AAO) template. The AAO template with array of high-aspect-ratio submicron holes was fabricated using a two-step anodization process. The soften polymer is then pressurized by gas to fill deep into the nano-holes, tapered SWSs could be generated by directly separating the PC film from AAO template during the de-molding process. The heights were up to about 260 nm. The fabricated PC film reduces the reflectivity from 8.9% in a bare film to 2% with tapered SWSs at the wavelength of 550 nm. The AAO template can be used repeatedly.     

Tailoring of the plasmonic and waveguide effect in bulk-heterojunction photovoltaic devices with ordered,nanopatterned structures

Various nano-patterned bulk-heterojunction (BHJ) films with different diameters and pitches were fabricated by a stamping method to tailor the plasmonic effect. The nanopatterned BHJ active layers exhibit regular-ordered embossing structures, which were confirmed by the surface morphological analysis with SEM and AFM. The simulation results confirm that devices with nanopatterned BHJ film with a diameter/pitch of 265/400 nm exhibit a strong improvement in E-field distribution intensity due to the combination of the plasmonic and waveguide modes compared to devices without a nanopattern, with 150/400 nm, or with 265/800 nm, which led to increased J_SC and cell efficiency in J–V curves under solar light illumination. The optimized plasmonic effect plays an important role in the light harvesting of BHJ devices.     

Polymer Planar‐Lightwave‐Circuit‐Type Variable Optical Attenuator Fabricated by Hot Embossing Process

A polymer‐based planar‐lightwave‐circuit‐type variable optical attenuator (VOA) was fabricated using a hot embossing process. With an optimized one‐step embossing process, forty micro‐channels for the guidance of light were defined on a polymer thin film with an accuracy of ° 0.5 µm. The fabricated polymeric thermo‐optic VOA shows 30 dB attenuation with 110 mW electrical input power at 1.55 µm. The rise and fall times are less than 5 ms.     

UV nanoimprint using flexible polymer template and substrate

Nano to micro-sized patterns were formed on a flexible polymer substrate using a flexible UV imprint stamp. A 6 in. diameter flexible UV nanoimprint template was fabricated using PVC hot embossing and DLC coating. Using the UV nanoimprint process with the DLC coated PVC template, nano to micro-sized patterns were clearly formed on the flexible PET substrate without a residual layer, due to the antistiction properties and high mechanical hardness of the DLC coating. By depositing a Cr layer on the imprinted resist pattern and lifting it off, Cr metal patterns were fabricated on the PET substrate.     

A non-fluorine mold release agent for Ni stamp in nanoimprint process

This study presents a novel material as an anti-adhesive layer between Ni mold stamps and polymethyl methacrylate (PMMA) substrate in nanoimprint process. A polybenzoxazine ((6,6’-bis(2,3-dihydro-3-methyl-4H-1,3-benzoxazinyl))) molecule self-assembled monolayer (PBO-SAM) considering as anti-adhesive coating agent demonstrates that non-fluorine-containing compounds can be improve the nanoimprint process in Ni/PMMA substrates. In this work, the nanostructure-based Ni stamps and the imprinted PMMA mold are performed by electron-beam lithograph (EBL) and our homemade nanoimprint equipment, respectively. To control the forming of fabricated nanopatterns, the simulation can be analyzed their effect of temperature distributions on the deformation of PBO-SAM/PMMA substrate during hot embossing lithography (HEL) process. Herein the diameter of pillar patterns is 200 nm with and 400 nm pitch on Ni stamp surface. Based on the hydrophobic PBO-SAM surface in this conforming condition, the results of Ni mold stamps infer over 90% improvement in controlling quality and quantity.