Study on micro hot embossing of low temperature co-firable ceramic green substrates

Multi-layered ceramic substrates with embedded micro patterns are becoming increasingly important, for example, in harsh environment electronics, enabling microsystems and microfluidic devices. Fabrication of these embedded micro patterns, such as micro channels, cavities and vias, is a challenge. This study focuses on the process aspects of patterning micro features on low temperature co-firable ceramic (LTCC) green substrates using micro hot embossing. Green ceramic tapes that possessed near-zero shrinkage in the x–y plane were used, six layers of which were stacked and laminated as a substrate. The process parameters that impact on the embossing fidelity were investigated and optimized in this study. Micro features with channel-width as small as several micrometers were formed on green ceramic substrates. The dynamic thermo-mechanical analysis indicated that extending the holding time at a certain temperature range would harden the substrates with little effect on improving the embossing fidelity. Ceramic substrates with embossed micro patterns were obtained after co-firing; the shrinkage ratios of the embossed depth and channel-width were 8–15 and 12–17%, respectively. The changes of pitches between two embossed channels were within ±1.0% due to the interlocking effect of the ceramic tapes.     

A polymer-metal hybrid flexible mould and application for large area hot roller embossing

In this paper, we report a novel approach to fabricate a low cost, large area and flexible mould and its applications in large area roller embossing. A liquid crystal polymer (LCP) film, which had a high glass transition temperature of 280°C, was clad with copper foils on both sides, was used as a starting material for mould fabrication. The LCP film and the copper foils were 50 and 36 μm thick, respectively. The LCP-Cu flexible mould was obtained through photolithographic patterning and wet etching of the copper foil on top surface of the LCP film. Using this proposed method, a polymer-metal hybrid flexible mould with an area of 150 mm × 150 mm was fabricated. The fabricated mould has a minimum feature size of 25 μm, and has been successfully used to demonstrate large area micro roller embossing. Micro channels, micro dots and micro mixers were embossed on polymeric as well as ceramic green substrates.     

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.     

Solvent-assisted low pressure room temperature lamination of low temperature cofirable ceramic green tapes for formation of embedded micro channels

Low temperature cofired ceramic substrates are becoming increasingly attractive for high density electric circuits and microsystems. Embedded micro patterns such as channels and cavities in ceramic substrates are indispensable for circuit cooling and media transportation. One of process challenges is how to make these embedded micro channels and cavities, which would be collapsed or deformed under conventional lamination. This paper reports on a novel solvent-assisted lamination that could provide low pressure and room temperature lamination of ceramic green tapes. The solvent used in this study was turpentine oil, which demonstrated a proper capability of dissolving polymeric additives on surface of green tapes without obviously changing the distribution of ceramic particles. Procedures for forming embedded micro channels in ceramic green substrates include micro embossing to create open channels, coating of turpentine solvent, followed by low pressure and room temperature lamination. Embedded micro channels with channel width ranging from 25 to 1,000 μm were obtained in ceramic green substrates; Depths of embedded channels shrank by 3–12% versus embossed depths due to turpentine-assisted lamination. The ceramic green substrates with embedded channels were then sintered under a standard cofiring process.     

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.     

Study of mechanical response in micro-indentation of laminated ceramic green substrates

Micro-indentation at elevated temperature is an effective method for studying the mechanical response of a material used in micro embossing. In this study, micro-indentation was performed on co-firable green ceramic substrates for the purpose of simulating micro embossing process and investigating the mechanical response at certain temperature. The laminated low temperature co-firable ceramic (LTCC) green tapes were used as the testing materials, the correlations of indentation depth versus applied force at various temperatures ranging from 25 to 75°C were studied. The results showed that permanent indentation cavities could be formed and retained at the temperatures ranging from 25 to 75°C; the depth of cavities created was applied force, temperature and holding time dependent. Creep during holding period occurred and made a significant contribution to the plastic deformation at elevated temperatures. There were instantaneous recoveries during the unloading and cooling processes, and retarded recovery in the first day after indentation as well. No significant pile-up around the indented cavities at the temperatures up to 75°C was observed, suggesting that there was little material flow during indentation at this temperature range. The indented cavities on green ceramic substrates were formed mainly due to the plastic deformation under compression. The findings could be used as a guideline for micro embossing of ceramic green substrates.     

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.     

Embossing of 3D ceramic microstructures

 An embossing method based on the viscous polymer processed (VPP) ceramic tape has been used to fabricate 3D ceramic microstructures with high aspect ratios. Examples of lead zirconate titanate (PZT) microrod arrays with feature sizes of 10–150 μm and aspect ratios of 3–10 have been demonstrated. Advantages of the embossing technique over conventional casting and moulding methods are discussed. Received: 10 August 2001/Accepted: 24 September 2001     

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.     

Rubber-assisted micro forming of polymer thin films

Polymer thin films patterned with microstructures at a characteristic size greater than the film thickness are difficult to fabricate using the standard hot embossing technology. This study investigated a rubber-assisted embossing process for structuring polymer thin films. The main advantages of a rubber support, instead of a hard counter-tool, include simplification of the embossing tool, protection of the embossing master, buildup of uniform embossing pressure, and ease of demolding. The testing pattern for rubber-assisted embossing was a microgroove pattern with a characteristic size of 100 μm on a 25-μm thick polystyrene film. Results showed that the uniformity and replicability of the embossed pattern were significantly affected by the embossing temperature, the rubber hardness, and the embossing pressure. With an embossing temperature about 20°C above the glass transition temperature and appropriate rubber hardness and embossing force, uniform microgroove patterns were successfully replicated.     

Fabrication of metallic micromolds by laser and electro-discharge micromachining

A method of creating metallic micromolds with features that have high-aspect ratios is described in this paper. The proposed manufacturing process utilizes laser micromachining to cut the negative two-dimensional profiles of the desired microfeatures and fluidic network patterns on a 100 μm thick brass sheet. The positive relief of the cut pattern is then created by using electro-discharge micromachining (micro-EDM) die-sinking the metallic mask onto a brass substrate. The final substrate with the desired relief pattern becomes the molding tool used for either elastomer casting or thermoplastic hot embossing. To validate the proposed fabrication methodology and evaluate the quality of surface finishes, a brass mold master of a T-channel micromixer (50 μm width, 25 μm height) is developed and multiple replicated devices are cast on this mold using poly-di-methyl-siloxane (PDMS). The surface finish of both the original micromold master and final molded channels on PDMS are measured using an optical profiler and found to have a roughness of approximately 400 nm Ra. The ability of the proposed fabrication technique to create high-aspect ratio features is illustrated by manufacturing a Y-channel micromixer with an aspect ratio of 4. Experimental results are discussed and suggestions for improvement are presented.     

Replication of nanostructures on microstructures by intermediate film mold inserted hot embossing process

The present study proposes a simple method to replicate nano/micro combined multiscale structures using an intermediate film mold and anodic aluminum oxide (AAO) nanomold in hot embossing process. The proposed method is simply to add an intermediate film mold with microscale thru-hole patterns to the ordinary mold system, on which nanostructures are patterned, in the hot embossing process. The intermediate film mold is inserted between polymer substrate and AAO nanomold. During the hot embossing process, the polymer first fills microscale thru-hole patterns in the intermediate film mold and subsequently fills nanopores in AAO nanomold, resulting in the nano/micro combined structures. The intermediate film molds, which have microscale thru-hole patterns were fabricated by micro-milling, laser ablation, etching methods and/or LIGA process. The nano/micro combined structures were successfully replicated by the proposed method.     

Hot embossing of microstructure with moving induction heating and gas-assisted pressuring

A new apparatus for a moving induction heating and gas-assisted hot embossing apparatus has been developed. A mechanism was designed and implemented to move the platform in and out the wrapped coil, on which the sealed box for substrate/mold was placed. A chamber of 195 mm diameter and 221 mm length was machined. The movable platform, the sealed box with substrate/mold stack, wrapped coil and cooling fan were all implemented in the high pressure chamber. The nine-point thermocouples attached on the mold, thus, a temperature history of the moving induction heating can be obtained and study the influence of the moving path and power on the heating rate and temperature distribution. The micro V-cut structure hot embossing experiment were performed to prove the potential of this moving induction heating and gas-assisted pressuring hot embossing for fast fabrication of microstructure onto polymeric substrates. As a results, replication rates were all above 95% at 200 °C and 5 kgf/cm² and the cycle time was less than 4 min and the optic measurement shows the replicated V-cut film can enhance the 36.8% illuminance. The experiment results show the manufacturing potential of this apparatus.

     

Microstructure formation on low temperature co-fired ceramic green substrates using micro embossing

Microstructures were formed on low temperature co-fired ceramic (LTCC) green substrates with high fidelity using micro embossing. The impact of embossing temperature and pattern density against the embossed profiles was investigated. The increase in pattern density resulted in a macro deformation in addition to embossed micro-depth. The macro deformation can be decreased by careful management of pattern density as well as pressure ramp and temperature ramp. The embossed ceramic green substrates were debinded and co-fired with a supplier-recommended process; the dimension shrinkage of embossed channels after co-firing ranges from 20 to 22% in depth and from 10 to 13% in width. The achievements of this investigation demonstrated that micro embossing is a promising process for fabricating ceramic-based microstructures and devices, including embedded cavities and channels.     

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.     

A sub-micron metallic electrothermal gripper

We report the design, fabrication, and characterization of a multiple bent beam, sub-micron metallic electrothermal gripper. A bottom electroplating mold for electrodes was patterned using electron beam lithography in an SU-8, followed by nickel electroplating. A top electroplating mold for a sub-micron metallic gripper with high aspect ratio bent beams (thickness of 1 μm, width of 350 nm) was prepared using electron beam lithography in a polymethyl methacrylate (PMMA), followed by nickel electroplating and dry release of the top and bottom molds. The sub-micron gripper was characterized using a nanomanipulator system installed in a dual column scanning electron microscopy/focused ion beam system. The ability of the jaw to close up to 1.39 μm displacement with high precision and reliability has been reproducibly observed at an applied current of 28 mA, corresponding to the maximum power consumption of 11.2 mW. Finite element modeling displacement results performed using ANSYS for effective bent beam widths of 370 nm showed a good agreement with the measured displacement results. The sub-micron gripper demonstrated herein will enable the reproducible manipulations with nano-scale resolution displacement and could provide an effective means of interface between nano-scale objects and the micro/macro scale robotic systems.     

Fabrication of wireless sensors on flexible film using screen printing and via filling

Wireless sensors are fabricated on flexible plastic films by means of screen printing and via-hole filling. The wireless sensors are battery free with data and power transmission functions. The sensors, fabricated on polyethylene terephtalate films, are designed based on RFID technology. Using an additive patterning process known as screen printing, metallization on polymer films is created. Both sides of a polymer film are printed with metallic patterns and connected with micro vias filled with conductive paste. One side of the film consists of printed electrical traces for discrete components like resistors and transistors that would be mounted onto it; the other side consists of a printed inductive coil used for wireless data and power transmission. The micro vias, which have a diameter of 120 μm, are formed by mechanical punching and filled with conductive silver paste. The size of one sensor unit is approximately 2 cm × 1.5 cm; an array of 4 × 7 sensor units are printed over an area of 15 cm × 15 cm on a PET film. Details of manufacturing processes, component assembly and functionality test are presented in this paper.     

Design and fabrication of an electrostatically suspended microgyroscope using UV-LIGA technology

A micromachined electrostatically suspended gyroscope, with a wheel-like rotor housed by top stator and bottom stator, using UV-LIGA microfabrication technology, was presented. The designed structure and basic operating principle of the gyroscope are described. The key steps in the fabrication process, such as wet etching of Pyrex glass pits for soldering, and integration of thick nickel structures by removal of SU-8 mold, were considered in detail and well solved. Cr/Pt/photoresist was used as etching mask and the etched pits, in depth of near 30 μm, with aspect ratio (depth to undercutting) of 0.75, were obtained. With metal foundations constructed for consolidation, successful integration of the nickel structures, in thickness of 200 μm, was achieved by successful removal of the SU-8 mold using oleum. After the two stators and the rotor were fabricated separately, they were assembled and soldering bonded to form axial and radial small gaps, hence, the initial prototype of the microgyroscope was realized. The key techniques described in this paper can be applied to fabrication of other micro devices. The metal foundation method, associated with removal of SU-8 mold by oleum, is expected to make SU-8 wider applications in making integrated microstructures with fabricated circuitry on the same chip.     

Replication of precise polymeric microlens arrays combining ultra-precision diamond ball-end milling and micro injection molding

A simple and novel combination of ultra-precision diamond ball-end milling and micro injection molding technique is described to produce precise microlens arrays out of polycarbonate (PC), polymethylmethacrylate (PMMA) as well as polystyrene (PS). The microlens arrays consist of 100 lenses in a 10 × 10 array with a lens radius of 273 μm, a lens diameter of 300 μm and a lens depth of 45 μm. Pitch between the lenses is fixed at 800 μm. The injection molding parameters were optimized to get precise microlens geometries with low surface roughness. The results show a precise diamond milled mold insert and injection molded microlens arrays with minor deviations in radius and surface roughness of the microlenses, particularly for microlens arrays out of PMMA.     

Fabrication of RF MEMS variable capacitors by deep X-ray lithography and electroplating

Radio frequency micro electro-mechanical systems (RF MEMS) vertical cantilever variable capacitors fabricated using deep X-ray lithography and electroplating are presented. Polymethylmethacrylate (PMMA) layers of 100 μm and 150 μm have been patterned and electroplated with 70 μm and 100 μm thick nickel. A 3 μm thick titanium layer was used as plating base as well as etch time-controlled sacrificial layer for the release of the cantilever beam. The parallel plate layout includes narrow gaps and cantilever beams with an aspect ratio in nickel of up to 60 for 1 mm long features. Auxiliary structures support the beams and gaps during the processing. Room temperature electroplating significantly reduces the risk of deformations compared to the standard process temperature of 52°C. The capacitors operate in the 1–5 GHz range, and demonstrate good RF performance, with quality factors on the order of 170 at 1 GHz for a 1 pF capacitance.