C. W. Lu

Singapore Institute of Manufacturing Technology (SIMTech), Tumasik, Singapore

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Publications (22)9.6 Total impact

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    ABSTRACT: 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 1,000μm 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.
    Microsystem Technologies 08/2010; 16(8):1501-1506. · 0.83 Impact Factor
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    ABSTRACT: 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 36μm 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 150mm×150mm was fabricated. The fabricated mould has a minimum feature size of 25μm, 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.
    Microsystem Technologies 01/2010; 16(8):1393-1398. · 0.83 Impact Factor
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    ABSTRACT: We present a wireless sensor platform that is fabricated on transparent flexible substrate by means of screen printing. The platform is battery free with data and power transmission functions. The smart sensor, fabricated on polyethylene terephtalate (PET) film, is designed based on RFID technology. Using an additive patterning process known as screen printing, metallization on the polymer substrates is created. Both sides of the polymer substrate are printed with metallic patterns and connected with micro vias filled with a conductive paste. One side of the substrate consists of printed electrical traces for components like resistors, capacitors, 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. The advantages of using polymer substrate are weight reduction and its ability to flex. This paper presents the details of manufacturing processes, component assembly and functionality test.
    01/2010;
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    ABSTRACT: A method to fabricate a low cost, large area and flexible mould and its applications in large area roller embossing are presented in this paper. A liquid crystal polymer (LCP) film, which had a glass transition temperature as high as 280°C and was clad with a copper foil on each side, was used as a starting material for mould fabrication. The LCP film and the copper foil were 50 μm and 36 μm thick, respectively. The copper foil was patterned via photolithography and copper etching process. A large polymer-metal hybrid flexible mould of 200 × 200 mm2 (effective pattern area: 150 × 150 mm2) was fabricated. The fabricated mould has a minimum feature size of 25 μm, and has been used to demonstrate large area micro roller embossing as well as ultraviolet casting. Microfluidic devices were embossed on polymeric as well as ceramic green substrates.
    01/2010;
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    ABSTRACT: In present work curing kinetics of UV-initiated cationic photo-polymerization of EPICLON series epoxy resin HP-820 and key cure process parameters, such as the extent of cross-linking and conversion, polymerization rate and the order of reactions have been studied by Photo-Differential Scanning Calorimetry (DPC). Different kinetics analysis results, including enthalpy of the reaction, induction time, peak maximum, percentage conversion were obtained for studied epoxy system at different isothermal temperatures (30-70°C), allowing calculating activation energy. Two kinetic parameters - coefficient rate (k) and the order of the initiating reaction (m) were determined, using an autocatalytic kinetics model.
    01/2010;
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    ABSTRACT: We aim to develop roll-to-roll (R2R) large area ultraviolet (UV) embossing process for structuring micro features on flexible plastic films. Critical challenges in R2R UV embossing include but not limited to (i) precise deposition of liquid UV curable resins, and (ii) realizing low temperature and low pressure micro-structuring using UV curing. We have developed a R2R UV embossing system, which consists of an unwinding unit and a rewinding one for handing flexible plastic films, a slot die coating unit, an embossing roller, and a UV lamp for exposure and curing of UV sensitive resins. The system can handle 500 mm web-width flexible films; the coating width of the slot die is 250 mm. By using different flexible moulds, various structures were fabricated via R2R UV embossing.
    01/2010;
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    ABSTRACT: The authors demonstrated the feasibility of patterning conductive inks on sintered low temperature cofired ceramic (LTCC) substrates using ink-jetting technology. The objective of this study is to realize drop-on-demand metallization on plain or structured ceramic substrates. The sintered LTCC substrates can be viewed as a porous ceramic medium with rough surface finishes, and its porosity is one of the major impediments for achieving high pattern quality via ink-jetting in terms of pattern uniformity and resolution. To improve the uniformity and minimize the dimension of ink-jetted patterns, the authors proposed and demonstrated three technical solutions, i.e., ink-jetting on patterned ceramic substrates, heating substrates during ink-jetting, and coating substrates with a dielectric layer. Improvement in resolution and uniformity of ink-jetting was achieved using the proposed approaches.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2009; 27(3). · 1.36 Impact Factor
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    ABSTRACT: Micropunching and laser drilling are conventional techniques for patterning microcavities as well as microvias on low temperature cofirable ceramic (LTCC) green tapes. Micro-hot-embossing is, however, an emerging and promising method for fabricating microstructures on prefired ceramic green tapes. This article presents experimental studies of influential process parameters in hot embossing for micropattern formation on LTCC green substrates. By means of dynamic mechanical thermal analysis, the impacts of process temperature and holding time on embossed profiles were studied and their correlation was discussed. When the temperature exceeds a certain range, the organic additives in the LTCC green substrates start to decompose at the isothermal condition, and this loss of organic additives increases the difficulty in embossing. Micropatterns with various dimensional scales, including microchannels, were formed on green ceramic substrates under optimal process parameters, and the patterned ceramic substrates were obtained via sintering.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2009; 27. · 1.36 Impact Factor
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    ABSTRACT: Large-area (150 mm × 150 mm) patterning of green ceramic substrates was demonstrated by using micro roller embossing; micro patterns such as inductors and channels with a smallest line width of 50 µm were formed on the green ceramic substrates. This paper focuses on process challenges and optimizations of micro roller embossing for improving the quality of large-area patterning. By increasing the embossing temperature, both the embossed depth and uniformity within a pattern unit as well as all over the substrate panel were enhanced. The defects induced during embossing were minimized by increasing the substrate thickness from four tapes to six tapes. The dimensions and locations of the embossed patterns, for instance, the channel length and unit size were characterized and compared with the designed values. The dimensions of embossed green substrates were scaled up by 0.4–0.5% in the x- and 0.6–0.8% in the y-directions against the mold. The dimensions of sintered substrate shrank by 0.6–0.7% in the x- and 0.1–0.3% in the y-direction against the green substrate. These scaling factors could be used as a guideline for actual device design and fabrication.
    Journal of Micromechanics and Microengineering 12/2008; 19(1):017001. · 1.79 Impact Factor
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    ABSTRACT: 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.
    Microsystem Technologies 09/2008; 14(9):1405-1409. · 0.83 Impact Factor
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    ABSTRACT: In this paper, we will report our achievements in developing large area patterning of multilayered ceramic green composites using roller embossing. The aim of our research is to pattern large area ceramic green composites using a modified roller laminating apparatus, which is compatible with screen printing machines, for integration of embossing and screen printing. The instrumentation of our roller embossing apparatus, as shown in Figure1, consists of roller 1 and rollers 2. Roller 1 is heated up to the desired embossing temperature ; roller 2 is, however, kept at room temperature. The mould is a nickel template manufactured by plating nickel-based micro patterns (height : 50 $\mu$m) on a nickel film (thickness : 70 $\mu$m) ; the substrate for the roller embossing is a multilayered Heraeus Heralock HL 2000 ceramic green composite. Comparing with the conventional simultaneous embossing, the advantages of roller embossing include : (1) low embossing force ; (2) easiness of demoulding ; (3) localized area in contact with heater ; and etc. We have demonstrated the capability of large area roller embossing with a panel size of 150mmx 150mm on the mentioned substrate. We have explored and confirmed the impact of parameters (feed speed, temperature of roller and applied pressure) to the pattern quality of roller embossing. Furthermore, under the optimized process parameters, we characterized the variations of pattern dimension over the panel area, and calculated a scaling factor in order to make the panel compatible with other processes. Figure 2 shows the embossed patterns on a 150mmx 150mm green ceramic panel.
    06/2008;
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    ABSTRACT: This paper reports on formation of high aspect micro patterns on low temperature co-fired ceramic (LTCC) substrates by integrating micro inkjetting with electroless plating. Micro inkjetting was realized by using an inkjetting printer that ejects ink droplets from a printhead. This printhead consists of a glass nozzle with a diameter of 50 mum and a piezoelectric transducer that is coated on the nozzle. The silver colloidal solution was inkjetted on a sintered CT800 ceramic substrate, followed by curing at 200degC for 60 minutes. As a result, the silver trace with a thickness of 200 nm was obtained. The substrate, with the ejected silver thin film as the seed layer, was then immersed into a preinitiator solution to coat a layer of palladium for enhancing the deposition of nickel. Electroless nickel plating was successfully conducted at a rate of 0.39 mum /min, and the thickness of traces was plated up to 84 mum. This study demonstrates that the integration of inkjetting with plating is an effective method to form high aspect patterns at the demand location.
    Design, Test, Integration and Packaging of MEMS/MOEMS, 2008. MEMS/MOEMS 2008. Symposium on; 05/2008
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    ABSTRACT: This paper presents our latest achievements in developing large area patterning of multilayered ceramic green composites using micro roller embossing. The aim of this research is to develop large area pattern technique for ceramic green substrates using a modified roller laminator, which is compatible with screen printing apparatus, for integration of micro embossing and printing in the future stage. A thin film nickel mold was developed via photolithography, nickel electroplating and photoresist strip-off. The mold had an effective panel size of 150 mmtimes 150 mm with the height of protrusive micro patterns being about 40 mum. Formation of micro patterns was successfully demonstrated over the whole panel area using roller embossing on laminated ceramic green tapes (HL2000 from Heraeus). Micro patterns for inductors, capacitors as well as interconnection with 50 mum line width were embossed on ceramic green substrates. With the optimized process parameters (including feeding speed, roller temperature and applied pressure), we have demonstrated that micro roller embossing is a promising method for large area patterning of ceramic green substrates.
    Design, Test, Integration and Packaging of MEMS/MOEMS, 2008. MEMS/MOEMS 2008. Symposium on; 05/2008
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    ABSTRACT: Multilayered ceramic substrates with embedded micro patterns are becoming increasingly important, for example, in harsh environment electronics 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 of patterning micro features on ceramic green substrates using micro embossing. A ceramic green tape that possessed near-zero shrinkage in the x-y plane was used, six layers of which were laminated as the embossing substrate. The process parameters that impact on the pattern fidelity were investigated and optimized in this study. Micro features with line-width as small as several micrometers were formed on the ceramic green substrates. The dynamic thermomechanical analysis indicated that extending the holding time at certain temperature range would harden the green substrates with little effect on improving the embossing fidelity. Ceramic substrates with embossed micro patterns were obtained after co-firing. The embedded micro channels were also obtained by laminating the green tapes on the embossed substrates.
    Design, Test, Integration and Packaging of MEMS/MOEMS, 2008. MEMS/MOEMS 2008. Symposium on; 05/2008
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    ABSTRACT: Low-temperature co-fired ceramic (LTCC) offers many advantages as a material of multi-functional substrates for high-density, high-frequency microelectronic applications, as well as for enabling microsystems. One of the key challenges in manufacturing multilayered ceramic substrates is fabricating micro patterns on unfired green ceramic sheets, and micro embossing is an alternative and promising method for this purpose. This paper presents our achievements in fabricating micro patterns over a large area on multilayered green ceramic substrates using micro roller embossing. A commercialized roller laminating machine was modified and used as the roller embossing apparatus. The feasibility of micro roller embossing on green ceramic substrates with an effective panel size of 150 mm × 150 mm has been successfully demonstrated. Micro patterns, including channels and electrical passives with 50 µm line-width, were formed over the whole panel area. The influence of main process parameters on the pattern quality of roller embossing has been investigated and confirmed.
    Journal of Micromechanics and Microengineering 04/2008; 18(6):065007. · 1.79 Impact Factor
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    ABSTRACT: With recent development, multi-functional platform with substrate based on ceramics composite technology, in particular low temperature co-fired ceramic (LTCC) multilayer technology, is gaining advantages over conventional silicon technology. LTCC offers ease of integration for both homogenous and heterogeneous structures and materials. Its intrinsic properties and unique fabrication process makes 3-D meso and micro-system possible. There is, however, among the functionalities, optical access is desirable for most of the micro-electronic devices and micro-systems. Therefore, a process of integrating glass layer is always required. Glass layer could be fabricated directly on a fired ceramics composite substrate or ideally be co-sintered. In this paper, preliminary investigation on direct thermal glass bonding process on a fired LTCC substrate is presented. Bonding interfacial integrity was characterized using nondestructive scanning acoustic microscopy. At a temperature of 720degC for 30 minutes, bonding began to establish over 7 - 8 % of the interfacial area for a glass chip size of 10 mm times 10 mm. At 800degC, the bonded area reached approximately 98%. Glass sagging measurement over a number of open cavities was also conducted. Average sagging of about 42 mum was measured on a Oslash5 mm open cavity. Visible light transmittance for the open cavity after glass bonding was in the range of 89 - 92% which is closed to the reference "as received" glass of 92%. Cross-sectional micrograph at bonding interface showed a continuous metallurgical glass fused bond to the LTCC surface. Destructive shear strength test on glass/LTCC bonding was conducted. Failure was observed in the glass chip with an average shear apparent strength of about 2.5 MPa. Further investigation is required to eliminate edge deformation and to minimize sagging for large cavity.
    Electronics Packaging Technology Conference, 2007. EPTC 2007. 9th; 01/2008
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    ABSTRACT: Multi-layer lamination with carbon fugitive material to realize embedded channels in low shrinkage LTCC laminates was investigated. The sagging behaviors of suspended LTCC laminates over the cavity after lamination and co-firing were studied. Suspended LTCC laminates over the channels of width 1 to 10 mm were dimensionally stable after sintering with maximum deflection of less than 20 mum. Swelling (i.e. deflection in the opposite direction of sagging) was observed for suspended LTCC laminates over the channels of width 15 to 30 mm after co-firing. The LTCC material and carbon burnout behavior were characterized using thermal analysis techniques. Based on the thermal analyses, a multi-step burnout process was proposed to control the carbon burnout in realizing dimensionally stable embedded channels.
    Electronics Packaging Technology Conference, 2007. EPTC 2007. 9th; 01/2008
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    ABSTRACT: This paper presents our latest achievements in addressing the process challenges in developing large area patterning of green ceramic substrates using hot roller embossing. Micro roller embossing on large area ceramic green substrates was successfully demonstrated by using an electroplated film mold and feeding the sandwiched structure (mold-substrate-supporting plate) through a micro embosser, which consists of top and bottom rollers. Formation of micro patterns on ceramic green substrates over an effective panel size of 150 mm �? 150 mm was performed; and micro patterns such as inductors, channels with smallest line width of 50 ¿m were embossed on the ceramic green substrates. This paper also addresses the important process issues for improving embossing fidelity within a pattern unit, as well as the uniformity of unit-to-unit patterns. At lower embossing temperatures, the embossed depth at the center of a pattern unit was found to be lower than that at the edge of the pattern unit. By increasing the embossing temperature, both the embossed depth and the uniformity within a pattern unit were enhanced. The effect of the thickness of green substrates was investigated by increasing the green tapers from 4 layers to 6 layers. The dimensions and locations of the embossed patterns were characterized before and after co-firing, the variations of pattern dimensions and locations were compared with the designed values on the nickel mold. These results will be used as a guideline for actual device design.
    01/2008;
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    ABSTRACT: Low temperature co-fired ceramic (LTCC) technology is successfully employed for the first time to develop a vaporizing liquid propellant microthruster (VLM). The VLM has potential applications as micropropulsion system for attitude control of microsatellites. The design, fabrication and testing of the LTCC based VLM are presented. Preliminary testing is done by using water as liquid propellant. The VLM of nozzle throat size 220 μm × 200 μm produces an average thrust in the range of 34–68 μN for a flow rate of 1 mg s−1 with an input heater power varying from 7.1–9.2 W. An average specific impulse of 3.4–6.9 s is measured. An average impulse bit of 0.67–1.4 mNs, 3.40–6.70 mNs is measured for thruster firing time of 20 s and 100 s during VLM stable mode operation at sea level conditions.
    Applied Energy. 97:577–583.
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    ABSTRACT: 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 150mm×150mm with the height of plated protrusive patterns being about 38μm. 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 50μm 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.
    Microsystem Technologies 15(8):1319-1325. · 0.83 Impact Factor