Rubber-assisted embossing of polymer thin films using molds with through-thickness microchannels

A flexible microfluidic chip is difficult to fabricate using the standard hot embossing technology. In this study, rubber-assisted embossing of polymer thin films using molds with through-thickness microchannels was investigated. The polymer film was thermoformed into the microchannels by rubber as a soft counter-tool. Different processing conditions, as well as material selections, affecting the thickness uniformity and replicated depth were examined. Results indicated that smoother surfaces on the embossed articles were created, and the thickness uniformity and the depth of the embossed channel were significantly affected by the embossing temperature, the embossing pressure, and the rubber hardness. The embossed film was sealed on one side with a layer of transparent adhesive film to form closed microchannels, and desired 3-D flow characteristics were obtained with this flexible microfluidic chip.     

A micro roller embossing process for structuring large-area substrates of laminated ceramic green tapes

This paper presents a micro roller embossing process for patterning large-area substrates of laminated green ceramic tapes. The aim of this research is to develop a large-area microstructure formation technique for green ceramic substrates using a thermal roller laminator, which is compatible with screen printing apparatus. A thin film nickel mold was developed via photolithographic patterning and nickel electroplating on a 75-μm-thick nickel film. The mold had an effective panel size of 150 mm × 150 mm with the height of plated protrusive patterns being about 38 μm. Formation of micro patterns was successfully demonstrated over the whole panel area on laminated green ceramic tapes using roller embossing. Micro patterns for inductors, heaters as well as interconnection with 50 μm line-width were embossed on green ceramic substrates. By means of tuning process parameters including roller temperature, applied pressure and feeding speed, we have demonstrated that micro roller embossing is a promising method for patterning large-area green ceramic substrates.     

Roll manufacturing of flexible microfluidic devices in thin PMMA and COC foils by embossing and lamination

Microfluidics on foil is gaining momentum due to a number of advantages of employing thin films combined with the capability of cost-effective high-volume manufacturing of devices. In this work, ultra-thin, flexible Y-microreactors with microchannels of 100 μm width and 30 μm depth were fabricated in thermoplastic polymer foils. The fluidic pattern was hot roll embossed in 125 μm thick poly-methyl-methacrylate (PMMA) and 130 μm thick cyclic-olefin-copolymer (COC) films using a dry-etched microstructured silicon wafer as a flat embossing tool in a laminator. The sealing of the channels was performed with two different techniques, one based on lamination of SU8 dry film resist (DFR) and the other one based on spin-coated poly-dimethylsiloxane (PDMS). Testing of the interconnected microreactor was carried out using two dye colorant solutions to demonstrate mixing.     

Tensile testing of microfabricated thin films

 Mechanical properties of titanium thin films of 0.5 μm thickness and aluminum thin films of 1.0 μm thickness, microfabricated by magnetron sputtering, were measured by using a novel tensile machine. These thin films are difficult to handle because they are fragile, so the thin film specimens were fabricated by using semiconductor manufacturing technology in a silicon frame to protect them. The test section of these specimens was 300 μm in width and 1400 μm in gauge length. By gripping the thin film specimen with a new device using a micrometer, it could be mounted on the tensile machine easily. The stress-strain diagrams of both thin films were measured continuously in the atmosphere at room temperature. The experimental results indicated that the titanium thin film and the aluminum thin film had a smaller breaking elongation although they had a larger tensile strength than bulk pure titanium and bulk pure aluminum, respectively. Received: 31.10.96/Accepted: 14.11.96     

Roll-to-roll hot embossing of microstructures

In this paper we present a new roll-to-roll embossing process allowing the replication of micro patterns with feature sizes down to 0.5 μm. The embossing process can be run in ‘continuous mode’ as well as in ‘discontinuous mode’. Continuous hot embossing is suitable for the continuous output of micro patterned structures. Discontinuous hot embossing has the advantage that it is not accompanied by waste produced during the initial hot embossing phase. This is because in ‘discontinuous mode’, embossing does not start before the foil has reached the target temperature. The foil rests between two parallel heating plates and foil movement and embossing starts only after the part of the foil resting between the heating plates has reached a thermal steady state. A new type of embossing master is used which is based on flexible silicon substrates. The embossing pattern with sub-μm topographic resolution is prepared on silicon wafers by state of the art lithography and dry etching techniques. The wafers are thinned down to a thickness of 40 μm, which guarantees the mechanical flexibility of the embossing masters. Up to 20 individual chips with a size of 20 × 20 mm2 were assembled on a roller. Embossing experiments with COC foils showed a good replication of the silicon master structures in the foil. The maximum depth of the embossed holes was about 70% of the master height.     

Microfabrication of thick tungsten films for use as absorbers of deep X-ray lithography masks

 We fabricated thick (5 μm) tungsten (W) film patterns by sputtering and dry etching, and realized a new deep X-ray lithography mask. The X-ray mask with 5-μm-thick W absorbers could expose about 1-mm-thick resist structures. In the deposition process of W films, the column structure of about 0.2 μm grain size, from which pattern edge roughness originates, disappeared by adding nitrogen into the sputtering gas. W film etching was carried out by reducing gas pressure and cooling the substrate (−40 °C), and a side etch width of below 0.2 μm was obtained. From the results of the pattern edge roughness and the side etch width, a pattern fabrication accuracy below ±0.5 μm was achieved. Furthermore, film stress, which induces pattern distortion, was reduced to below 50 MPa by controlling the sputtering gas pressure. The obtained mask achieved a pattern distortion below ±0.3 μm. Received: 7 July 1999/Accepted: 29 May 2000     

A roller embossing process for rapid fabrication of microlens arrays on glass substrates

This paper reports an innovative technique for rapid fabrication of polymeric microlens arrays based on UV roller embossing process. In this method, a thin flat mold is fabricated by electroforming of nickel against a microlens master. The thin Ni mold with microlens cavities is then wrapped onto cylinder to form the roller. During rolling operation, the roller pressing and dragging the UV-curable photopolymer layer on the glass substrate through the rolling zone, the microlens array is formed. At the same time, the microlens array is cured by the UV light radiation while traveling through the rolling zone. The technique can be developed to an effective roll-to-roll process at room temperature and with low pressure. In this study, a roller embossing facility with UV exposure capacity has been designed, constructed and tested. Under the proper processing conditions, the 100×100 arrays of polymeric microlens, with a diameter of 100 μm, a pitch of 200 μm and a sag height of 21 μm can be successfully fabricated.     

Collapse-free thermal bonding technique for large area microchambers in plastic lab-on-a-chip applications

Bonding is an essential step to form microchannels or microchambers in lab-on-a-chip applications. In this paper, we present a novel plastic thermal bonding technique to seal and form large area microchambers (planar characteristic width and length on the order of 1 mm and characteristic thickness on the order of 10–100 μm) without collapse by introducing a holed pressure equalizing plate (HPEP) that includes holes of the same size and shape as the microchambers. To demonstrate the proposed technique, two types of large area microchambers [(1) 20 × 10 mm and 40 μm thick and (2) 12 × 2.5 mm and 120 μm thick] with microchannels were designed and replicated on plastic substrates by means of hot embossing and injection molding processes with prepared two nickel mold inserts. The replicated large area microchambers as well as the microchannels in the plastic lab-on-a-chip were successfully sealed (i.e., no leakage) and formed without any collapse by the proposed thermal bonding technique with the help of the HPEP.     

Development of precision transfer technology of atmospheric hot embossing by ultrasonic vibration

We succeeded to transfer a precise micro-pattern combining with an ultrasonic vibration in an atmospheric hot embossing on the almost same condition as a vacuum hot embossing. This paper reports the effect of the ultrasonic vibration that was verified experimentally. In the conventional method, a metallic mold and a plastic sheet are heated more than the glass transition temperature of the plastic, and the softened plastic is flowed into the pattern only by applying a load. On the other hand, a longitudinal ultrasonic vibration is added in the molding process of an ultrasonic-vibration hot embossing. The synergy effect of the load and the ultrasonic vibration enables flowing of the plastic into a more precise pattern of the metallic mold. The longitudinal wave generated by an ultrasonic vibration system of the frequency 15 kHz and output 900 W. A pattern of the Ni mold used in the experiment was a pyramid hole in which a peak was cut and sidewalls were rounded. Entrance lengths of pyramids were from 100 to 530 μm and its all of the depth were 260 μm. A polycarbonate was chosen with a replication material. Compared with the condition that the ultrasonic vibration was not used, a contact force and a contact time could be reduced to about 1/3 and 1/12, respectively.     

Monotonic and fatigue testing of freestanding submicron thin beams application for MEMS

A freestanding submicron thin film specimen is designed and fabricated here to carry out a series of monotonic and fatigue testing. This freestanding beam was loaded by performing monotonic loading/unloading or closed-loop load controlled tension–tension fatigue experiments on it. Loading was applied using a piezoelectric actuator with 0.1 μm resolution connected to the test specimen. Loads were measured by connected a capacitor load cell with a resolution of less than 0.1 mN. The modulus, yield stress and maximum stress of tested submicron thin films at room temperature were found from monotonic loading/unloading tests. The results of 300 nm copper thin films fatigue experiments demonstrated a trend of decreasing cycles to failure with increasing loading amplitude and increasing mean stress.     

Resin micromachining by roller hot embossing

The roller hot embossing is an efficient process of manufacture in which patterns are continuously transcribed on film, etc. Recently, the application of the embossing roll to the manufacturing processes of micro parts is paid attention. In this paper, we examined the development of the embossing roll with patterns of micron level and we tried to make the embossing roll mold by using the LIGA process. In this study, instead of producing embossing patterns directly on the roll surface, we fabricated a flexible thin mold with micro-patterns, which was then wrapped onto a cylinder to form an embossing roll, and tested the soft-mold roller hot embossing method. First, by optimizing UV exposure conditions of UV lithography, we prepared a resist pattern of numerous dots with a diameter of 10 μm, a sag height of 8 μm and a pitch of 20 μm. By Ni-electroforming this pattern, a 50 μm-thick thin mold was successfully fabricated. The 50 μm-thick mold was then wrapped onto a cylinder to form an embossing roll. In the roller hot embossing process, the 10 μm-diameter dot shape was successfully replicated on PET sheets.     

Ammonia sensing properties of polypyrrole thin films at room temperature

Polypyrrole thin films were synthesized in situ by chemical polymerization. Fourier transform infrared spectroscopy revealed formation of polypyrrole. The morphological studies by scanning electron microscopy showed formation of uniform granular structure with average grain size of 0.6 μm. The film composition was characterized by X-ray photoelectron spectroscopy for chemical composition in polypyrrole film. These films were investigated for their sensing behaviour towards NH₃ and NO at room temperature. It has been observed that these films are selective for NH₃ and the sensitivity exhibited a linear response in range of 4-80 ppm.     

Fabrication of microfluidic channel utilizing silicone rubber with vacuum casting

Microfluidic patterns of 100 μm in width and 50 μm height were replicated from a master using vacuum casting with silicone rubber. These silicone copies are subjected to thorough analysis for dimensional accuracy against the master pattern. Analysis of experimental results shows repeatability of the silicone rubber molds. To test the limits of vacuum casting with silicone rubber, an attempt was undertaken to replicate 8 μm microchannel and submicron features. The results show that casting of microfluidic channel via vacuum casting has high repeatability.     

Technology of microthermoforming of complex three-dimensional parts with multiscale features

Thermoforming parts with micro-scale design features require use of thin polymer films. Low heat capacity and fragility of thin (<100 μm) polymer films requires new technical developments for precise heating and form pressure application. This paper will present results of an improved process based on a new replication machine. Temperature stability and pressure distribution is highly improved, even at reduced cycle times. With an upgraded temperature range microthermoforming of high temperature polymers is possible. This technology is well suited for the replication of three-dimensional micro parts with multiscale features. Replication of hollow parts with multiscale features is also possible. Outer dimension in macroscopic length scale and functional design features in microscopic length scale are combined with nanoscale surface structures in an affordable technology.     

Morphology and magnetic properties of electroplated Co-rich Co-Zn thin films

This work explores the microstructure and magnetic properties of electrodeposited Co-Zn thin films. Using pulse-reverse electroplating technique, Co-rich Co-Zn films are deposited 0.4–1.9 μm thick from aqueous sulfate-based baths at low temperature (55°C). The influence of current density (25–100 mA/cm²) and electrolyte Zn concentration (0–0.28 M) on the microstructure and magnetic properties are investigated. All of the Co-Zn films exhibit higher out-of-plane coercivity, as compared to in-plane. With increasing current density, the out-of-plane coercivity decreases from 50 to 40 kA/m (628–500 Oe). The influence of the Zn concentration in the electrolyte is more pronounced, affecting the grain size, film composition, and magnetic properties. The best magnetic properties were obtained from a bath with 0.21 M Zn and an average current density of 25 mA/cm², resulting in a Co₉₇Zn₃ composition and an out-of-plane coercivity of 92 kA/m (1,160 Oe).     

Super‐high‐resolution transfer printing for full‐color OLED display patterning

Abstract— A transfer‐printing method for the patterning of thin polymer layers is described. A hard stamp with a raised feature is brought into contact with a spin‐coated organic film under elevated pressure and temperature to break the films. The patterned film is then transfer printed onto the devices. This method is used to print red/green/blue subpixel arrays with a pattern size as small as 12 μm at a resolution of 530 ppi to demonstrate its ability for full‐color organic light‐emitting‐display fabrication. Devices with printed organic layers have similar performance to spin‐coated controls under optimized printing temperature and pressure settings. The critical physical parameters include a soft intermediate plate for the sharp breaking of edge patterns, control of surface energies, and printing at moderate temperature and pressure to achieve intimate contact between the printed layer and the underlying substrate.     

The electrical and optical properties of AZO thin film under different post-annealing temperatures

In this study, the Aluminum element doped zinc oxide (Al:ZnO) thin film was deposited on the Corning glass substrate by RF magnetron sputtering technology and annealing treatment. After sputtering, all thin films are then annealed on nitrogen atmosphere and temperature of 300, 500 and 550 °C, respectively. The structural, electric and optical characteristics were then investigated. All films illustrate strong (002) for ZnO and (335) for Al preferential orientation by using XRD analysis. The lower resistivity can be observed at nitrogen annealing and temperature of 400 °C. The transmittance property of AZO thin film exhibited an excellent transparency in the visible light range. The transmittance reached to nearly 81.4 % for all Al:ZnO film. It can be clearly observed that the grain size of AZO thin film is very uniform by utilizing SEM technology.     

Nano- and microsized metal oxide thin film gas sensors

Functional micro- and nanosized metal oxide thin film structures are very promising candidate for future gas-sensors. Their reduced size offers an increased surface to volume ratio thus improving sensitivity and sensor performance. Whilst most experimental nanostructures are produced using a bottom-up approach, a top-down sputtering technique for structuring nano-sized gas sensitive metal oxide areas is presented in this letter. Oxidised silicon wafers were used as substrates. The silicon dioxide film of 1 μm thickness was prepared by thermal oxidation in order to insulate the gas sensing elements from the substrate. The sensor chips had an overall size of (1.5 × 1.5) mm² onto which a Ta/Pt film (20/200 nm thickness) was deposited and patterned to act as electrodes, heater and temperature sensor. In a second step micro-scaled tin dioxide layers (60 nm thick, 5 μm width) were deposited by sputtering techniques and photolithographical patterning between the platinum micro-electrodes (4 μm gap). Finally, the width of the stripes was reduced using focused ion beam technology to obtain the desired size and structure. This enables the control of the dimensions of the structures down to the resolution limit of the FIB-system which is about 10 nm. The structural and electrical characterisation of the sensors and their responses during exposure to several test gases including O₂, CO, NO₂ and H₂O are presented as well.     

Micro-metalforming with silicon dies

 The introduction of forming technology into MEMS manufacturing demands forming dies being characterized by a high strength and hardness, a good microstructurability, a low surface roughness, and a high precision of the microgeometry to be molded. Silicon structured by lithography and etching processes meeting these requirements especially concerning precision and surface roughness. For micro-metalforming silicon dies with different structural dimensions (>1 μm) have been used. The microstructures could be molded in different materials using cold and superplastic embossing. The precision and surface quality of the formed parts correspond to the high quality of the microstructured die. Both the low surface roughness and the accurate edges of the silicon structures are nearly represented in the molded structures. However, in particular during cold embossing die wear or even die failure could be observed limiting the implementation of silicon for micro-metalforming.     

1-3 Piezocomposites realised from small feature size, high aspect ratio, hot embossed moulds. Part I. Mould development

1-3 Piezocomposites offer many advantages over monolithic transducers, however replicating their fine scale structure is demanding. Viscous polymer (VP) embossing is a technique that has already been demonstrated as being capable of achieving the small feature size and high aspect ratio features that are required for piezocomposites. However, the process utilises a lost mould technique and is therefore limited by the ability to make cost effective sacrificial moulds. Hot embossing has been identified as a technique that is suitable for replicating these moulds, and piezocomposites made from them are presented here. This paper dimensionally characterises the piezocomposites through each stage of the VP embossing process. The piezocomposites described here have feature sizes of 30 μm, volume fractions 0.5 and operating frequencies of 12.3 MHz. For the first time piezocomposites have been made by VP embossing into cost effective moulds.