High Aspect Pattern Formation by Integration of Micro Inkjetting and Electroless Plating

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 micrometers 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 200 degrees C 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 micrometers /min, and the thickness of traces was plated up to 84 micrometers. This study demonstrates that the integration of inkjetting with plating is an effective method to form high aspect patterns at the demand location.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/handle/2042/16838

Metallisation and structuring of low temperature Co-fired ceramic for micro and millimetre wave applications

The recent developments in Low Temperature Co-fired Ceramic (LTCC) as a substrate material enable it to be used in the micro and millimetre wave range providing low dissipation factors at high frequencies, good dielectric properties and a high degree of integration for further miniaturised devices. The most common metallisation method used in LTCC technology is screen printing with high cost noble metals such as silver and gold that are compatible with the high sintering temperatures (8500C). However, these techniques require high capital cost and maintenance cost. As the commercial world requires convenient and low cost process technologies for mass production, alternative metallisation methods should be considered. As a result, electroless copper plating of fired LTCC was mainly investigated in this research. The main goals of this project were to carry out electroless plating of fired LTCC with sufficient adhesion and to extend the process to metallise closed LTCC channel structures to manufacture Substrate Integrated Waveguide (SIW) components. The objectives were focused on electroless copper deposition on fired LTCC with improved adhesion. Electroless deposits on the Sn/Pd activated LTCC surface showed poor adhesion without any surface pre-treatments. Hence, chemical etching of fired LTCC was carried out using concentrated NaOH solution. NaOH pre-treatment of LTCC led to the formation of flake like structures on the LTCC surface. A number of surface and chemical analysis techniques and weight measurements were used to investigate the mechanism of the modification of the LTCC surface. The results showed that the flake like structures were dispersed in the LTCC material and a material model for the LTCC structure was proposed. SEM EDX elemental mapping showed that the flake like structure consisted of aluminium, calcium, boron and oxygen. Further experiments showed that both the concentration of NaOH and the immersion time affect the surface morphology and the roughness of fired LTCC. The measured Ra values were 0.6 m for untreated LTCC and 1.1 m for the LTCC sample treated with 4M NaOH for 270 minutes. Adhesion tests including peel test and scratch test were carried out to examine the adhesion strength of the deposited copper and both tests indicated that the NaOH pre-treatment led to an improvement, with the best results achieved for samples treated with 4M NaOH. A second aspect of the research focused on the selective metallisation of fired LTCC. Excimer laser machining was used to pattern a resist film laminated on the LTCC surface. This process also roughened the substrate and created channels that were characterised with respect to the laser operating parameters. After patterning the resist layer, samples were activated using Sn/Pd catalyst solution followed by the electroless copper deposition. Electroless copper was selectively deposited only on the patterned LTCC surface. Laser parameters clearly affected the copper plating rate. Even with a similar number of shots per area, the tracks machined with higher repetition rate showed relatively more machining depth as well as good plating conditions with low resistance values. The process was further implemented to realize a complete working circuit on fired LTCC. Passive components including a capacitor and an inductor were also fabricated on LTCC using the mask projection technique of the excimer laser system. This was successful for many designs, but when the separation between conductor lines dropped below 18 m, electroless copper started to deposit on the areas between them. Finally, a method to deposit copper films on the internal walls of closed channel structures was developed. The method was first demonstrated by flowing electroless copper solutions through silane treated glass capillaries. A thin layer (approx. 60 nm) of electroless copper was deposited only on the internal walls of the glass capillaries. The flow rate of the electroless copper solution had to be maintained at a low level as the copper deposits tended to wash away with higher flow rates. The structures were tested for transmission losses and showed low (<10dB) transmission losses in the terahertz region of the electromagnetic spectrum. The process was further applied to deposit electroless copper on the internal walls of the LTCC closed channel structures to manufacture a LTCC Substrate Integrated Waveguide (SIW)

Sensitivity alteration of fiber Bragg grating sensors through on-fiber metallic coatings produced by a combined laser-assisted maskless microdeposition and electroless plating process

This thesis is concerned with sensitivity alterations of Fiber Bragg Grating (FBG), sensors through additive coatings produced by a combined Laser-Assisted Maskless Micro-deposition (LAMM) and electroless plating process. The coatings can also protect the brittle FBG used in harsh environments. The thesis encompasses design, fabrication procedures, modeling and comparison of experimental and modeling results to gain insight into the advantages and short-comings of the approach. Starting with the opto-mechanical modeling, a program is written in MAPLE to analyze the effect of different on-fiber metallic materials and coating thicknesses on the sensitivity of FBGs to temperature and axial force. On the basis of the proper material and thickness, the sensitivity of FBG at different thermal and loading strains are predicted. The optimal theoretical data suggests that if the thickness of the Ni layer is 30–50 μm, maximum temperature sensitivity is achieved. Some experiments are proposed to test the feasibility of the coated FBG sensors. LAMM is used to coat bare FBGs with a 1-2 μm thick conductive silver layer followed by the electroless nickel plating process to increase layer thickness to a desired level ranging from 1 to 80 μm. Our analytical and experimental results suggest that the temperature sensitivity of the coated FBG with 1 μm Ag and 33 μm Ni is increased almost twice compared to a bare FBG with sensitivity of 0.011±0.001 nm/°C. On the contrary, the force sensitivity is decreased; however, this sensitivity reduction is less than values reported in the literature

Fabrication and Applications of Printed and Handwriting Electronics

The accelerating arrival of the Internet of Things (IoT) era creates a rapidly growing demand for printed electronic. As a low-cost and green substrate, cellulose paper has become the most attractive choice for the printing of sustainable and disposable electronics. However, manufacture of high quality circuits with high conductivity on cellulose paper remains a challenge due to the substrate’s high porosity and roughness. In this thesis, a method for facile fabrication of hybrid copper-fiber highly conductive features on low-cost cellulose paper with strong adhesion and enhanced bending durability is introduced. With three-dimensional electroless deposition (ELD) of copper, the as-fabricated circuits show ultra-low sheet resistance down to 0.00544 Ω/sq. Taking advantages of the porous structure of paper, together with the precise control of the inkjet droplets, highly conductive vertical interconnected accesses (VIAs) are fabricated for multilayered devices without physically drilling holes or depositing additional dielectric material. To further utilize the unique porous structure of cellulose paper, a scalable fabrication method for flexible, binder-free and all-solid-state supercapacitors is proposed based on the low-cost chemical engraving technique, to construct CuxO nanostructure in-situ on the three-dimensional metallized cellulose fiber structures. Benefitting from both the “2D Materials on 3D Structures” design and the binder-free nature of the fabricated electrodes, substantial improvements to electrical conductivity, aerial capacitance, and electrochemical performance of the resulting supercapacitors (SCs) are achieved, fulfilling the increasing demand of highly customized power systems in the IoT and wearable electronics industries. The above-mentioned work all use inkjet printing for materials deposition. However, as a solvent-based printing technique, inkjet printer has strict requirement of ink properties and suffer from inevitable nozzle clogging. To address these challenges, a fabrication method based on solvent-free laser printing technique is proposed, pushing the manufacture of printed electronics towards an environmentally benign and more cost-efficient manor. Lastly, a one-step react-on-demand (RoD) method for fabricating flexible circuits with ultra-low sheet resistance, enhanced safety and durability is proposed. With the special functionalized substrate, a real-time synthesize of the 3D metal-polymer (3DMP) conductive structure is triggered on demand. The as-fabricated silver traces show an ultralow sheet resistance down to 4 mΩ/sq

Materials and processes to enable polymeric waveguide integration on flexible substrates

Polymeric waveguide-on-flex has the potential to replace complex and costly light-turning devices in optoelectronic applications. As light signals are propagated and confined through the definition of core-cladding interface, the light guiding structure is required to adhere well and ensure long term interfacial stability. This thesis addresses the gap that has emerged in the fundamental material issues such as the polymeric optical waveguide materials deposited on the flexible substrates. In addition, this thesis investigates the feasibility of a new approach using electrostatic-induced lithography in micro-patterning of polymer, in optical waveguide fabrication. Plasma treatment is applied to enhance interfacial adhesion between flex substrates and optical cladding layers. The modified flex surfaces of polyimide KaptonHNTM and liquid crystal polymer VecstarTM materials are characterised. In addition, sonochemical surface treatment is evaluated on these flexible substrates. ToF-SIMS depth profiling has confirmed the interface reaction mechanisms where it has shown that plasma treatment increases the interfacial interpenetration. The larger interfacial width increases the possible entanglement mechanism between the polymer chains. These results, together with the double cantilever beam testing, indicate the strengthening of the polymeric interface upon plasma treatment, which is essential for long term optical and mechanical stability of waveguide-on-flex applications. A new method of micro-pattering of polymer material has been adopted for fabricating multimode waveguide-on-flex. The method, using an electrostatic-induced lithography, is developed to produce 50 μm x 50 μm arrays of polysiloxane LightlinkTM waveguide on flex. This thesis looks at various process recipes of the technique and reports the pattern formation of polymeric optical core. By adjusting the spin-coated liquid core thickness, pre-bake condition, UV exposure and applied voltage, the aspect ratio and profile of the optical core microstructure can be varied. As the electrostatic pressure overcoming the surface tension of spin-coated waveguide material induces the optical core formation, the core structure is smooth, making it ideal for low scattering loss waveguide. The propagation loss of fabricated waveguide is measured at 1.97 dB/cm at 850 nm wavelength. The result shows that the use of electrostatic-induced lithography in optical polymer is a promising approach for low cost and low temperature (<150 °C) processing at back end optical-electrical integrated circuitry assembly

Flexible and Stretchable Electronics

Flexible and stretchable electronics are receiving tremendous attention as future electronics due to their flexibility and light weight, especially as applications in wearable electronics. Flexible electronics are usually fabricated on heat sensitive flexible substrates such as plastic, fabric or even paper, while stretchable electronics are usually fabricated from an elastomeric substrate to survive large deformation in their practical application. Therefore, successful fabrication of flexible electronics needs low temperature processable novel materials and a particular processing development because traditional materials and processes are not compatible with flexible/stretchable electronics. Huge technical challenges and opportunities surround these dramatic changes from the perspective of new material design and processing, new fabrication techniques, large deformation mechanics, new application development and so on. Here, we invited talented researchers to join us in this new vital field that holds the potential to reshape our future life, by contributing their words of wisdom from their particular perspective

Designing a Hybrid Inkjet and Laser Manufacturing System for the Digital and Non-Contact Fabrication of Emerging Nanotechnology Based Devices

Though many VA-CNT based applications have recently been developed, there is still a research gap in the scalable manufacture of VA-CNT applications in industry. In this research, a hybrid additive and subtractive direct-write digital fabrication platform has been developed to deposit and pattern catalyst for VA-CNT Growth. Inkjet printing has been explored as the additive technology for depositing magnetite nanoparticles as catalyst, and laser ablation has been studied as a subtractive technique to pattern the catalyst for growing VA-CNT structures, using chemical vapour deposition as an assistive growth procedure. The research was conducted in three phases. In the feasibility phase the CVD growth conditions were established and a comparison was made using SEM and AFM analysis. A variation in size between the annealed nano-islands formed from the nanoparticle film and the PVD deposited iron film was identified. The magnetite ink properties were then characterised for inkjet printing. In the specification phase inkjet printing and laser patterning experiments were conducted. It was determined that VA-CNTs can be grown by printing 2.19 \% w/w ink concentration on UV treated 10 nm alumina coated substrates at a droplet spacing of 45 µm. SEM analysis identified that even though the nano-islands formed from the inkjet printed catalyst are larger in size than those formed from the PVD deposited catalyst, VA-CNTs were successfully grown. In the laser patterning experiments a femtosecond laser fluence range between 0.48 J/cm_2 and 0.64 J/cm_2 was found to successfully pattern the nanoparticle catalyst. Varying laser fluence resulted in a change in the annealed catalyst nano-island size, giving varying growth trends. In the conceptualisation phase, an industrially scalable laser system by M-Solv Ltd was successfully demonstrated for patterning the catalyst. Gas sensing experiments were also conducted proving a response of the VA-CNTs to changes in nitrogen dioxide.EPSRC Cambridge and Cranfield Doctoral Training Centre in Ultra Precision Centre (EPSRC Reference : EP/K503241/1) and M-SOLV Ltd