Comparative Study of Inkjet-Printed Silver Conductive Traces With Thermal and Electrical Sintering

Thermal sintering has traditionally been the most popular sintering method to enhance conductivity after the printing process in the manufacturing of printed electronics. Nevertheless, in recent years, there has been a growing interest in electrical sintering as an alternative method to overcome some of the limitations of thermal curing. This paper makes a comparative study of both sintering methods in terms of surface morphology, electrical dc conductance, and radiofrequency performance for different applied voltage waveforms. To this end, microstrip transmission lines have been inkjet-printed using nanoparticle-based silver ink on flexible polyimide substrate. The traces have been tested under different sintering conditions, achieving electrical sintering resistivity values only 2.3 times higher than that of bulk silver. This implies a 62% reduction in comparison with the best resistivity value achieved using thermal sintering in our samples. The main novelty of this contribution lies in the analysis of RF behavior as a function of electrical sintering conditions. Lower resistivities have been achieved with slower voltage ramps or allowing higher density current during sintering. It has also been proved that electrically sintered lines have similar RF performance than high-temperature thermally sintered lines in terms of insertion losses, regardless of their very different surface topology. Therefore, we can take advantage of the benefits that electrical sintering offers over thermal sintering regarding significant shorter sintering times maintaining suitable RF performance.This work was supported in part by the Spanish Ministry of Economics and Competitiveness under Grant CTQ2016-78754-C2-1-R

FORMULATION AND PROCESSING OF CONDUCTIVE INKS FOR INKJET PRINTING OF ELECTRICAL COMPONENTS

A novel process utilizing specialized continuous inkjet (CIJ) printing technology and innovative conductive inks to fabricate three-dimensional electronic products is introduced in the current investigation. The major advantage of the CIJ process is that it not only provides a fast and cost-effective method for applying electronic components, but it also allows the printing of conductive traces in three-dimensional space. The greatest challenge for inkjet printing of electrical circuits is the formulation of the conductive inks, which define not only the convenience of the materials being deposited, but also the final comprehensive properties and the major expense. Several conductive inks were investigated and compared mainly based on their electric conductance and mechanical adherence to a substrate. Two different particle-free solution conductive inks were specifically researched for low cost, deposition convenience and improved properties. A novel aqueous solution of silver nitrate with a corresponding adhesion promoter is introduced for the first time. It has been found that the traces produced by the process have excellent adherence and have an electrical resistivity of only 2.9 times that of bulk silver. Low temperature curing plus further annealing of a specified metallo-organic decomposition (MOD) ink produces close-packed silver crystal substructures. The electrical conductance of the final conductive trace was close to that of bulk silver, and wearability was significantly improved from ductile deformation. These two particle-free solution inks are expected to find a number of applications in various industries.Fundamental concepts in formulating and post-processing of inkjet printing are fully discussed. The two primary problems faced by inkjet printed components are low deposited thickness and porosity. An interlayer oxide film was found to be necessary to bond the precious metal layer with the glass substrate. Porosity can be significantly reduced through high temperature annealing, which not only increases electrical conductance, but also mechanical strength. Special curing methods are proposed to consolidate the printed conductors and avoid overheating of temperature sensitive substrates

Multilevel metallization scheme using printing technologies for IC fabrication using discrete oxide TFTs

Despite the emergence of flexible electronics as a technology for manufacturing integrated circuits at lower costs than traditional silicon microelectronics, costly and time-consuming clean room processes are still needed to interconnect the various transistors that make up the circuits. In this context, this dissertation presents an alternative, based on printing processes of multi-layer metallic interconnections, that allow to connect thin film transistors (TFTs) in functional circuit blocks, thus bringing a significant cost reduction and great flexibility for the prototype of new circuits. For such, a silver ink and polyvinylpyrrolidone (PVP) solution were used to form the conductive and insulating layer respectively. Using an inkjet printer, it was possible to pattern, on glass, silver lines with 100 and 50 μm width and a resistivity of 7.12 × 10-8 Ωm using 200 ºC for 30 min. PVP was deposited with a thickness of 3.5 μm to ensure insulation between conductive layers and a laser etching process was implemented to create vias between different metallization levels. Finally, the developed process was demonstrated in several digital circuit blocks based on oxide TFTs

Electro-mechanical Reliability of CNT-based Conductive Films on Flexible Substrates

Leitfähige Bahnen oder Schichten auf flexiblen Substraten sind wesentliche Komponenten für die flexible Elektronik. Sie müssen bei äußeren Verformungen ihre elektrische Leitfähigkeit und mechanische Integrität erhalten. Leiterbahnen aus herkömmlichen Materialien wie Indiumzinnoxid (ITO) und Metallen haben auf flexiblen Substraten unter großen Dehnungen nur eine begrenzte Zuverlässigkeit gezeigt. In diesem Zusammenhang könnten Kohlenstoffnanoröhren (CNTs) aufgrund ihrer hervorragenden mechanischen, elektrischen und thermischen Eigenschaften eine vielversprechende Alternative sein. Einige Studien haben berichtet, dass CNT-basierte Leiterbahnen auf flexiblen Substraten sehr großen Zugspannungen standhalten können. Es wurden jedoch nur sehr wenige systematische Studien zu ihrer elektromechanischen Zuverlässigkeit durchgeführt. Diese Doktorarbeit befasst sich mit dem elektromechanischen Verhalten von CNT-basierten Filmen bei statischen und zyklischen Verformungen und untersucht systematisch, die zugrundeliegenden Mechanismen für ihre hervorragende Zuverlässigkeit. Diese Erkenntnisse könnten für die Optimierung potenzieller flexibler Geräte unter Verwendung von CNTs wichtig sein. CNT-basierte Leiterbahnen können durch Tintenstrahldruck oder Schleuderbeschichtung hergestellt werden. Als eines der Standardsubstrate für die gedruckte Elektronik wird eine Polyethylenterephthalat (PET)-Folie als Substrat verwendet. CNT-basierte Leiterbahnen konnten erfolgreich gedruckt werden. Die elektrische Leitfähigkeit ist jedoch durch den geringeren Gehalt an CNTs in der Tinte begrenzt. Daher wurde zur Erzielung einer besseren elektrischen Leitfähigkeit eine Schleuderbeschichtung von CNT-Dispersionen angewendet. Durch eine mehrschichtige Abscheidung wurde ein Schichtwiderstand von lediglich 400 Ohm/sq. erreicht. Der wichtigste Teil dieser Doktorarbeit sind die statischen und zyklischen elektromechanischen Prüfungen sowie die Mikrostrukturcharakterisierung von CNT-basierten leitfähigen Filmen auf flexiblen Substraten. Es wurden Mikrozugversuche durchgeführt, um ihre Zuverlässigkeit unter statischen Zugspannungen zu testen, während Biegeermüdungsversuche durchgeführt wurden, um ihre Lebensdauer unter zyklischen Zugdehnungen zu bewerten. Um das elektromechanische Verhalten von CNT leitfähige Filmen unter Verformung zu untersuchen, wurden ihre Morphologie und änderungen der Mikrostruktur untersucht, indem verschiedene mikroskopische Charakterisierungsmethoden kombiniert wurden. CNT leitfähige Schichte zeigen eine ausgezeichnete Zuverlässigkeit bei hohen Dehnungen von bis zu 50%. Ihre Widerstands-Dehnungs-Abhängigkeit zeigt, dass ihre intrinsischen Leitfähigkeiten durch Strecken sogar verbessert werden. Ihre Dehnbarkeit von bis zu 50% wird durch die Überbrückungswirkung von CNTs über eventuell auftretende lokale Risse begünstigt. Biegeermüdungstests ergaben ferner sehr hohe Lebensdauern der CNT Schichten. Bei kleinen Dehnungsamplituden, von 1% bis 2%, sind die CNT leitfähigen Schichten bis zu 1 Million Biegezyklen frei von Ermüdungs-schädigung, und ihre intrinsischen Leitfähigkeiten werden sogar während der zyklischen Verformung aufgrund des Kohäsionseffekts verbessert. Bei einer höheren Dehnungsamplitude von 3% können CNT Schichten ihre elektrische Leitfähigkeit bis zu 200.000 Biegezyklen aufrechterhalten, und ihr Versagen ist nur auf die Ermüdung und den Gewaltbruch des Polymersubstrats zurückzuführen, was sich als der limitierende Faktor des ganzen Systems herausstellt. Diese Ergebnisse zeigten, dass die in dieser Studie verwendeten PET-Folien nicht die optimalen flexiblen Substrate für Biegebeanspruchungen sind und alternative Substrate, die bei höheren Dehnungsamplituden (z.B. > 3%) nicht ermüden und brechen, erforderlich sind

The systematic development of Direct Write (DW) technology for the fabrication of printed antennas for the aerospace and defence industry

Low profile, conformal antennas have considerable advantages for Aerospace and Military platforms where conventional antenna system add weight and drag. Direct Write (DW) technology has been earmarked as a potential method for fabricating low profile antennas directly onto structural components. This thesis determines the key design rules and requirements for DW fabrication of planar antennas. From this, three key areas were investigated: the characterisation of DW ink materials for functionality and durability in harsh environments, localised processing of DW inks and the optimisation of DW conductive ink material properties for antenna fabrication. This study mainly focused on established DW technologies such as micro-nozzle and inkjet printing due to their ability to print on conformal surfaces. From initial characterisation studies it was found that silver based micro-nozzle PTF inks had greater adhesion then silver nano-particle inkjet inks but had lower conductivity (2% bulk conductivity of silver as opposed to 8% bulk conductivity). At higher curing temperatures (>300°C) inkjet inks were able to achieve conductivities of 33% bulk conductivity of silver. However, these temperatures were not suitable for processing on temperature sensitive surfaces such as carbon fibre. Durability tests showed that silver PTF inks were able to withstand standard aerospace environments apart from Skydrol immersion. It was found that DW inks should achieve a minimum conductivity of 30% bulk silver to reduce antenna and transmission line losses. Using a localised electroplating process (known as brush plating) it was shown that a copper layer could be deposited onto silver inkjet inks and thermoplastic PTF inks with a copper layer exhibiting a bulk conductivity of 66% bulk copper and 57% bulk copper respectively. This was an improvement on previous electroless plating techniques which reported bulk copper conductivities of 50% whilst also enabling DW inks to be plated without the need for a chemical bath. One of the limitations of many DW ink materials is they require curing or sintering before they become functional. Conventional heat treatment is performed using an oven which is not suitable when processing DW materials onto large structural component. Previous literature has investigated laser curing as means of overcoming this problem. However, lasers are monochromatic and can therefore be inefficient when curing materials that have absorption bands that differ from the laser wavelength. To investigate this, a laser diode system was compared to a broadband spot curing system. In the curing trials it was found that silver inks could be cured with much lower energy density (by a factor of 10) using the broadband white light source. Spectroscopy also revealed that broadband curing could be more advantageous when curing DW dielectric ink materials as these inks absorb at multiple wavelengths but have low heat conductivity. Themodynamical modelling of the curing process with the broadband heat source was also performed. Using this model it was shown that the parameters required to cure the ink with the broadband heat source only caused heat penetration by a few hundred micro-metres into the top surface of the substrate at very short exposure times (~1s). This suggested that this curing method could be used to process the DW inks on temperature sensitive materials without causing any significant damage. Using a combination of the developments made in this thesis the RF properties of the DW inks were measured after broadband curing and copper plating. It was found that the copper plated DW ink tracks gave an equivalent transmission line loss to a copper etched line. To test this further a number of GPS patch antennas were fabricated out of the DW ink materials. Again the copper plated antenna gave similar properties to the copper etched antenna. To demonstrate the printing capabilities of the micro-nozzle system a mock wireless telecommunications antenna was fabricated on to a GRP UAV wing. In this demonstrator a dielectric and conductive antenna pattern was fabricated on to the leading edge of the wing component using a combination of convection curing and laser curing (using an 808nm diode laser)

Chitosan gated organic transistors printed on ethyl cellulose as a versatile platform for edible electronics and bioelectronics

Edible electronics is an emerging research field targeting electronic devices that can be safely ingested and directly digested or metabolized by the human body. As such, it paves the way to a whole new family of applications, ranging from ingestible medical devices and biosensors, to smart labelling for food quality monitoring and anti-counterfeiting. Being a newborn research field, many challenges need to be addressed to realize fully edible electronic components. In particular, an extended library of edible electronic materials is required, with suitable electronic properties depending on the target device and compatible with large-area printing processes, to allow scalable and cost-effective manufacturing. In this work, we propose a platform for future low-voltage edible transistors and circuits that comprises an edible chitosan gating medium and inkjet printed inert gold electrodes, compatible with low thermal budget edible substrates, such as ethylcellulose. We report the compatibility of the platform, characterized by critical channel features as low as 10 µm, with different inkjet printed carbon-based semiconductors, including biocompatible polymers present in the picograms range per device. A complementary organic inverter is also demonstrated with the same platform as a proof-of-principle logic gate. The presented results offer a promising approach to future low-voltage edible active circuitry, as well as a testbed for non-toxic printable semiconductors

Comparison of Sintering Methods and Conductive Adhesives for Interconnections in Inkjet-Printed Flexible Electronics

Increasing demands for flexibility and stretchability for electronic devices are driving the research for novel fabrication technologies. Inkjet-printing is one of these novel electronics fabrication techniques studied and developed globally in recent years and it has some interesting benefits over traditional lithography-based techniques, mainly its additive and digital nature. Traditional manufacturing methods are mature techniques and the processes are well defined and optimized for large scale manufacturing and inkjet-printing is not going to replace the lithography as such for large scale manufacturing. Inkjet-printing does, however, enable whole new ways of electronics fabrication, such as high part-to-part customization and 3D processability, which have previously been either very challenging or even impossible.So far research has focused mainly on inkjet-printing itself and the jetting process is understood fairly well. However, at the moment printed semiconductor materials are far inferior to traditional semiconductor components and can not enable the same level of functionality or connectivity. Hybrid systems, combining the high performance of traditional semiconductor components and benefits of inkjet-printing, are studied as a solution for fabricating high performance devices with novel fabrication techniques. Hybrid systems require the ability to attach external components to the printed structures and this integration was chosen as one of the main topic for this thesis work as it had not been studied previously and the knowledge was required for developing inkjet-printing.This thesis analyzes inkjet-printed hybrid systems and focuses on system level integration. The work is done on interconnections including both the sintering of metallic nanoparticles as well as external component interconnections and circuit board to circuit board connections. Sintering research is focused on alternative sintering methods to traditional thermal sintering and evaluation of their usability in electronics fabrication. Electrically conductive adhesives are studied as the main method of forming external connection to components and to other circuit boards.In the research related to this thesis alternative sintering methods were found to be suitable replacements for traditional thermal sintering with the advantages and disadvantages varying between different technologies. Laser and intense pulsed lighting were generally found to be the most promising techniques for inkjet-printed structures. External connections to traditional surface mounted components as well as other printed circuit boards were also successfully demonstrated in the related publications using electrically conductive adhesive materials. Both the electrical performance and long term reliability of the conductive adhesives were found to be inferior to solder-based interconnections but observations show that the difference is caused by the adhesive material itself, not by the use of inkjet-printing. Thus adhesives can be considered as a viable method for forming external interconnections on inkjet-printed structures

Comparative reliability of inkjet-printed electronics packaging

This article compares the thermomechanical behavior of 3-D inkjet-printed microelectronics devices relative to those fabricated from traditional methods. It discusses the benefits and challenges in the adoption of additive manufacturing methods for microelectronics manufacture relative to conventional approaches. The critical issues related to the design and reliability of additively manufactured parts and systems stem from the change in the manufacturing process and the change in materials utilized. This study uses numerical modeling techniques to gain insight into these issues. This article is an extension of the same topic presented at the 2018 IEEE Electronics Packaging Technology Conference. An introduction providing an overview of the area, covering salient academic research activities and discussing progress toward commercialization is presented. The state-of-the-art modular microelectronics fabrication system developed within the EU NextFactory project is introduced. This system has been used to manufacture several test samples, which were assessed both experimentally and numerically. A full series of JEDEC tests showed that the samples were reliable, successfully passing all tests. The numerical model assessing the mechanical behavior of an inkjet-printed structure during layer-by-layer fabrication is presented. This analysis predicts that the stresses induced by the UV cure process are concentrated toward the extremities of the part and, in particular, in the lower layers which are constrained by the print platform. Subsequently, a model of a multilayer microelectronics structure undergoing JEDEC thermal cycling is presented. The model assesses the differences in mechanical properties between a conventional FR4/copper structure and an inkjet-printed acrylic/silver structure. The model identified that the influence of the sintering process on subsequent material properties, behavior of the inject-printed structure, and reliability of the inject-printed structure is significant. Key findings are that while stresses in the conventional and inkjet boards are relatively similar, the inkjet-printed board exhibits significantly greater deformation than the standard board. Furthermore, the mechanical stresses in the inkjet fabricated board are strongly dependent on the elastic modulus of the sintered silver material, which, in turn, is dependent on the sintering process