Technologies for printing sensors and electronics over large flexible substrates: a review

Printing sensors and electronics over flexible substrates is an area of significant interest due to low-cost fabrication and possibility of obtaining multifunctional electronics over large areas. Over the years, a number of printing technologies have been developed to pattern a wide range of electronic materials on diverse substrates. As further expansion of printed technologies is expected in future for sensors and electronics, it is opportune to review the common features, complementarities and the challenges associated with various printing technologies. This paper presents a comprehensive review of various printing technologies, commonly used substrates and electronic materials. Various solution/dry printing and contact/non-contact printing technologies have been assessed on the basis of technological, materials and process related developments in the field. Critical challenges in various printing techniques and potential research directions have been highlighted. Possibilities of merging various printing methodologies have been explored to extend the lab developed standalone systems to high-speed roll-to-roll (R2R) production lines for system level integration

Microheater Array Powder Sintering (MAPS) for Printing Flexible Electronics

Microheater array powder sintering (MAPS) is a novel additive manufacturing process that uses an array of microheaters to selectively sinter powder particles. MAPS shows great promise as a new method of printing flexible electronics by enabling digital curing of conductive inks on a variety of substrates. MAPS operation relies on establishing a precision air gap of a few microns between an array of microheaters, which can reach temperatures of 600°C, and a layer of conductive ink which can be deposited onto a flexible substrate. This system presents challenges, being: the fabrication of a microheater that can reach suitable temperatures in an acceptable time frame and is reliable, electronic control of a single microheater, electronic control of an array of microheaters, and precise control of the position of the array of microheaters relative to the substrate. This work describes the design and fabrication of a printer which uses this novel technology to print flexible circuit boards. Various simulations are discussed which are used to explore the parameters affecting the MAPS printing process. Then, a small microheater array is fabricated and controlled using an electronic circuit using a PID feedback loop. This microheater array is used in an experimental proof of concept machine to print conductive lines onto a flexible substrate. Finally, a prototype MAPS printer is developed which is capable of using an improved microheater array to print simple circuits onto flexible substrates

Three Dimensional Digital Alloying with Reactive Metal Inks

3D printing of multifunctional components using two or more materials is a rapidly growing area of research. Metallic alloy inks have been used with various 3D printing techniques to create functional components such as antennas, inductors, resistors, and biocompatible implants. Most of these printing techniques use premixed metallic alloy inks or nanoalloy particles with a fixed composition to fabricate the functional part. Since the properties of alloys vary with changes in the elemental composition, a printing process which could digitally dispense alloy inks having specific desired compositions would enable different functionalities and be highly desirable. Using the binary copper-nickel system as an example, the formation of alloy with metal precursor inks is presented. Since copper and nickel both have a face centered cubic (FCC) structure and show complete miscibility in each other, formation of their nanoalloy is, in theory, relatively easy. By printing metal precursor inks rather than nanoparticle suspensions, problems associated with the nanoparticle inks such as ink stability and nozzle clogging can be avoided. Copper and nickel precursor inks were formulated having rheological properties suitable for inkjet printing. Reduction of metal inks was studied under various conditions. The sintered metal and alloy structures were characterized using thermal analysis, infrared spectroscopy, energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction. Nickel, a ferromagnetic metal, showed novel microstructures such as aligned nanowires and nanowire grids when reduced in the presence of a magnetic field. These microstructures had enhanced anisotropic electrical and magnetic properties along the direction of the nanowire. The reduction of combined ink solutions (copper and nickel) showed formation of a two phase with copper as one phase and a nickel rich alloy as other. These structures demonstrated no change in electrical resistivity when exposed to an oxidation rich environment. To achieve a homogeneous alloy formation, the copper phase and the nickel rich phase were diffused together at high temperatures. Copper nickel alloy inks with ratios Cu30Ni70, Cu50Ni50, and Cu70Ni30 were formulated and reduced at 230 °C and later high temperature diffusion was achieved at 800 °C. The lattice parameter of the alloy phase for the inks with ratio Cu30Ni70 was 3.5533Å, Cu50Ni50 was 3.5658 Å, and Cu70Ni30 was 3.5921 Å. Using Vegard’s law, the composition of the alloy phases for the three samples were estimated to be Cu32Ni68, Cu46Ni54, and Cu75Ni25. This formation of the desired alloy composition can open the door to numerous applications in biomedical and electronics sectors, among other

Smart Manufacturing Technologies for Printed Electronics

Fabrication of electronic devices on different flexible substrates is an area of significant interest due to low cost, ease of fabrication, and manufacturing at ambient conditions over large areas. Over the time, a number of printing technologies have been developed to fabricate a wide range of electronic devices on nonconventional substrates according to the targeted applications. As an increasing interest of electronic industry in printed electronics, further expansion of printed technologies is expected in near future to meet the challenges of the field in terms of scalability, yield, and diversity and biocompatibility. This chapter presents a comprehensive review of various printing electronic technologies commonly used in the fabrication of electronic devices, circuits, and systems. The different printing techniques based on contact/noncontact approach of the printing tools with the target substrates have been explored. These techniques are assessed on the basis of ease of operation, printing resolutions, processability of materials, and ease of optimization of printed structures. The various technical challenges in printing techniques, their solutions with possible alternatives, and the potential research directions are highlighted. The latest developments in assembling various printing tools for enabling high speed and batch manufacturing through roll-to-roll systems are also explored

Conducting metal oxide materials for printed electronics

Printed electronics as a manufacturing process has many advantages, mainly, it allows for the high throughput rapid fabrication of thin, flexible electronic components with minimal waste. There are many printing processes that can be utilised for printing electronics and although each process can differ vastly, the materials currently used in these processes are generally the same, silver and carbon. However, to develop printing as a more mainstream manufacturing method for electronics, a wider variety of materials are required which can provide better stability and longevity of components, new functionality for printed applications and allow for in-situ processing and tuning of components. Conducting metal oxides are a good candidate for integrating into printed electronics processes, these materials are typically semiconductors, they have bandgaps, and properties can be altered via altering the band gap. They are also oxides, so they cannot oxidise further and therefore atmospheric damage is reduced compared to pure metals. They can also be fabricated into a wide range of particle morphologies, all with advantages in different fields and electronic applications. Therefore, the ability to print these materials is valuable to the field. In this thesis, the integration of conducting metal oxide electro-ceramic materials into the field of printed electronics has been explored. This was performed through the completion of five research objectives including, the selection of appropriate materials for the research, the formulation of conductive inks with the materials, the investigation of post-processing techniques for printed films and further research into passive component fabrication and sensor applications. Firstly, following an extensive literature review, four materials were selected including three doped zinc oxide materials synthesised via different methods. The fourth material is commercially sourced indium tin oxide (ITO). A nitrocellulose vehicle was determined to be the most compatible with the oxides and selected for ink formulation. Inks were then formulated with all four materials, with optical and electrical properties analysed. Gallium doped Zinc Oxide (GZO) and ITO were selected for further investigation based on the excellent conductivity of the indium tin oxide (57.77Ω□-1) and the highly transparent optical properties of the gallium doped zinc oxide (>84% transmittance). Laser processing was selected as a post processing method. It was found that the laser processing dramatically increased conductivity. The GZO improving from a non-conductive film to 10.21% of bulk conductivity. The ITO improved from 3.46% to 40.47% of the bulk conductivity. It was also found that the laser processing invoked a carbothermal reduction process allowing for a rapid manufacturing process for converting spherical particles into useful nanoparticle morphologies (nanorods, nanowires etc). Following this, resistive and capacitive applications involving laser processing and conventionally heat-treated conductive oxide inks were developed. Combining the new materials and manufacturing processes, tuneable printed resistors with a tuning range of 50 to 20M could be fabricated. All metal oxide, ITO based capacitors were also fabricated and characterised. These were then developed into humidity sensors which provided excellent humidity sensing properties, showing linearity between 5 and 95% relative humidity (RH) and sensitivities of up to 7.76pF/RH%, demonstrating higher performance than commercial equivalents (0.2 – 0.5pF/RH%). In conclusion, this work provides a breakthrough for conductive metal oxide materials research and its place in Printed Electronics research by providing insight into the processes required to make these materials conduct and by developing useful manufacturing methods, post processing techniques and applications.</div

Flash processing of copper electrodes printed from copper oxide nanoparticle ink

Deposizione di inchiostro di nanoparticelle di rame ossido tramite tecnologie di stampa tradizionali e conversione e sinterizzazione delle nanoparticelle tramite tecnologia "intense pulsed light

Pulsed Photonic Curing of Conformal Printed Electronics

As next-generation electronic products emerge, there is a need to create more electronic functionality in compact spaces. One of the techniques to achieve this is by integrating electronic circuitry on mechanical stress bearing parts of electro-mechanical products. Direct-write printing processes like inkjet printing and aerosol jet printing can be used to print conductive inks on conformal surfaces of mechanical components. Advanced curing/sintering processes such as pulsed photonic curing can be used to cure/sinter printed inks to produce conductive traces. However, the use of photonic curing on conformal surfaces introduces two sources of variability into the process, which are the distance and slope between the flash lamps and the conformal substrate. This research studies the effects that distance and slope between the flash lamps and substrate have on the characteristics of the photonically cured material. Screen printed samples of copper nanoparticle ink on paper substrates were photonically cured at various distances and slope settings in a Novacentrix Pusleforge 3300 machine. Analysis of the experimental data reveals that there is significant decrease in the conductivity of the cured copper ink with increase in both the distance and slope between the flash lamps and the substrate. The lowering of conductivity of the coupons with increase in distance was correlated to the reduction in the intensity of pulsed light with distance from the source. Similarly, the lowering of conductivity of the coupons with increase in slope was correlated to the reduction in the intensity of pulsed light with increase in angle between the incident light and the surface normal. A spectrophotometer was used to correlate the lowering of the conductivity of the printed coupon to the reduction in the amount of light absorbed by the coupon surface with increase in the slope from the flash lamps. This research highlights that distance and slope variations are important considerations to achieve uniform electrical properties in conformal printed electronics undergoing photonic curing

Improving the Manufacture by Flexographic Printing of RFID Aerials for Intelligent Packaging

Flexography is a well-established high-volume roll-to-roll industrial printing process that has shown promise for the manufacture of printed electronics for smart and intelligent packaging, particularly on to flexible substrates. Understanding is required of the relationship between print process parameters, including ink rheology, and performance of printed electronic circuits, sensors and in particular RFID antenna. The complexity of this printing process with its shear and extensional flows of complex inks and flexible substrates can lead to undesirable surface morphology to the detriment of electronic performance of the print. This thesis reports work that progresses the understanding of the complex relationships amongst relevant factors, particularly focusing on the printability of features that have an impact on printed RFID antenna where increases in resistance increase the antennas resonant frequency. Flexography was successfully used to print RFID antenna. However, the large variation in print outcomes when using commercial inks and the limits on resistivity reduction even at the optimal print parameters necessitated the systematic development of an alternative silver flake ink. Increases in silver loading and TPU polymer viscosity grade (molecular weight) increased the viscosity. The ink maintained its geometry from the anilox cell between rollers, on to the substrate and print surface roughness increased. This, however, did not increase resistance of the track due to the high silver loading. Better understanding of the relationship between print parameters, print outcomes, ink rheology and performance of an RFID antenna has been achieved. Increases in silver loading up to 60wt.% improved conductivity. However, further increasing the silver loading produced negligible additional benefit. An adaption of Krieger-Dougherty suspension model equation has been proposed for silver at concentrations over 60wt.% after assessing existing suspension models. Such a model has proven to better predict relative viscosities of inks than Einstein-Batchelor, Krieger-Dougherty and Maron-Pierce equations. Increasing TPU viscosity grade was found to be a promising ink adjustment in the absence of changing print parameters, to produce a more consistent print. Better prediction of ink behaviour will allow for improved control of ink deposition, which for RFID applications can improve ink conductivity, essential for good response to signal. Further developments such as addition of non-flake particles and formulation refinement are required to enable the model ink to match the resistivity of the commercial ink

차세대 박막 트랜지스터를 위한 효율적인 용액공정기반 반도성 단일벽 탄소나노튜브의 박막 형성 방법들에 관한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2020. 8. 홍용택.As the demand for flexible and stretchable electronic devices increases, underpinning technologies of system implementation based on the solution-process have been tremendously developed. In particular, solution-processed random networks of semiconducting single-walled carbon nanotubes (SWCNTs) have been widely studied as suitable semiconducting materials for thin-film transistors (TFTs) in next-generation novel applications such as large-area active matrix for lighting emitting devices or biosensors due to their superior electrical and mechanical properties. Furthermore, low-cost and low-temperature processability of highly purified semiconducting SWCNTs with dispersed ink facilitates realizing large-area flexible and stretchable electronic systems. Among various deposition methods of SWCNT based on the solution-process, the easiest and the most effective method to fabricate the SWCNT TFT is directly dipping the substrate into the SWCNT ink, resulting in highly-feasible and highly-uniform deposition of SWCNT onto the large-area substrate, which is called direct dipping method. However, there are limitations to utilize it for the mass-production and commercialization at the related industry. First, this method exhibits low fabrication throughput due to a very long deposition time of SWCNT. And moreover, additional patterning process is needed because the substrate is wholly immersed into the SWCNT solution during dipping process. Therefore, to overcome these issues, some engineering modifications of strategy should be developed. In this Ph. D. dissertation, I developed two facile and effective fundamental technologies, which are multi-dipping technique and self-patterning technique (inkjet-printing of PLL technique), to resolve the aforementioned issues of direct dipping method. Multi-dipping technique is repeatedly both soaking a substrate into SWCNT solution with a very short time and rinsing the substrate each time for dramatically significant reduction of total deposition time of SWCNTs networks instead of soaking the substrate into the solution with a very long time. Compared to the conventional dipping method, this technique reduced the overall process time by more than half and improved the electrical characteristics of SWCNT TFTs at the same time. In addition, in order to achieve simultaneous patterning of the SWCNT layer during the direct dipping process, I inkjet-printed the surface functionalization material, especially the poly-L-lysine (PLL) material, enhancing the attachment of the semiconducting SWCNT at the region where we want to attach the SWCNTs. Only on top of PLL-patterned region, the networks of semiconducting SWCNTs were formed during dipping process although the substrate was wholly immersed into the SWCNT solution. Then, I combined the newly-desired two techniques for effectively fabricating the SWCNT TFT based on the direct dipping method and investigated the feasibility and applicability of the combined technology for the implementation of high-throughput and high-resolution SWCNT TFTs. I defined it as fast and self-patterning technique. In this dissertation, verification was conducted based on two criteria which are large-area scalability and micro-patternability, and for the verification of the latter one, I utilized electrohydrodynamic (EHD)-based printing technology that enables effective fabrication of fine pattern. The two primary purposes of this dissertation are to develop the technologies for effective deposition of in-situ patterned high-quality SWCNT film at the desired area onto the large-area substrate and to investigate the feasibility of the integrated technique for the implementation of future electronic applications. Furthermore, low-temperature processability and excellent mechanical flexibility of SWCNT networks could provide a guideline for implementing roll-to-roll (R2R) fabrication of high-throughput, high-resolution, and high-performance SWCNT TFT array, which is ultimately for the commercialization of future advanced flexible/stretchable electronic applications.최근 유연 및 신축성 전자 소자 응용에 대한 수요가 증가함에 따라, 용액공정기반의 시스템을 구현하는 기반 기술들이 계속해서 발전해오고 있다. 특히, 용액공정기반으로 제작된 반도성 단일벽 탄소나노튜브의 랜덤한 네트워크는 전기적, 기계적으로 우수한 성질을 가지고 있어, 대면적 액티브 매트릭스 기반의 발광 소자 및 센서와 같은 다양하고 새로운 차세대 응용 소자의 기본이 되는 박막 트랜지스터 (TFT)의 반도체층 물질로서 널리 연구되어 왔다. 또한, 저비용, 저온 공정이 가능한 분산된 형태의 고순도 반도성 단일벽 탄소나노튜브용액은 대면적의 유연/신축성 전자 소자로의 응용을 가능하게 해주었다. 이 때, 용액 공정 기반으로 탄소나노튜브 네트워크를 형성할 수 있는 여러 방법들 중에서, 박막 트랜지스터 소자의 채널 영역을 쉽고 효율적으로 제작하기 위한 방법 중 하나는, 직접 기판을 단일벽 탄소나노튜브 용액에 담그는 직접 담그는 방법(Direct dipping method)의 형태이며, 이는 대면적에 단일벽 탄소나노튜브를 매우 쉽고 균일하게 형성할 수 있다. 하지만 이 방법에는 크게 두 가지의 기술적인 한계점들이 존재한다. 우선, 수용액 형태의 용액을 사용하게 되면 매우 긴 공정 시간으로 인해 그 수율이 매우 낮아지게 되며, 다른 하나는 기판 전체가 용액으로 들어가기 때문에 이로부터 추가적인 패터닝 과정이 필요로 한다는 점이다. 따라서, 이 문제점들을 해결하기 위하여, 본인은 다양한 공학적인 접근 방법들을 시도해 보았다. 본 학위 논문에서는, 위에서 언급된 직접 담그는 방법의 문제점들을 해결하기 위한 새로운 쉽고 효과적인 기반 기술들을 개발하였다. 첫 번째 기술 (multi-dipping technique, 반복적 담금기법)은 기판을 용액에 오랜 시간 담가 두는 이전의 방법과는 달리, 기판을 탄소나노튜브 용액 안에 짧은 시간 동안만 담그고 이를 탈 이온수를 이용하여 세정한 뒤, 앞에서 언급한 공정을 계속해서 반복하여 준다. 이는 기존의 직접 담그는 방법과는 달리 단일벽 탄소나노튜브 기반의 박막 트랜지스터의 공정 시간을 절반 이상 감소시켜줄 뿐 아니라, 그 전기적인 특성 또한 향상시켜준다. 또한, 두 번째 기술 (self-patterning technology, 표면처리물질 인쇄공정기법)은, 반도성 단일벽 탄소나노튜브와 기판 사이의 접착력을 좋게 해주는 표면처리물질을 채널을 형성하고자 하는 부분에만 잉크젯프린팅을 해주어 원하는 패턴을 형성한다. 특히 본 논문에서는 표면처리물질로서 poly-L-lysine (PLL)을 이용하였다. 이 기술을 이용할 경우, 기판 전체를 탄소나노튜브 용액에 담금에도 불구하고 반도성 단일벽 탄소나노튜브 네트워크는 오로지 추가적인 패터닝과정 없이도 공정 과정 내에서 스스로 표면처리물질 (PLL)이 인쇄된 부분에만 형성됨을 확인할 수 있었다. 또한, 단일벽 탄소나노튜브 기반의 박막 트랜지스터를 보다 효율적으로 형성해주기 위하여 두 기술을 결합하여 새로운 하나의 기술로 정의 (fast and self-patterning technique)한 후, 해당 기술이 높은 수율 및 해상도를 가지는 단일벽 탄소나노튜브 기반 박막 트랜지스터를 형성할 수 있을지에 대한 실현 및 적용 가능성을 검증하였다. 검증과정에서는, 대면적 소자 제작에 적용 가능할 수 있는지에 대한 부분과 미세화된 소자 제작에 적용 가능한지에 대한 부분, 총 두 가지 기준점을 가지고 연구를 진행하였으며, 이 중에서 미세 패턴 가능성을 보기 위해서는 미세 패턴을 효율적으로 형성할 수 있는 전기수력학방식의 프린팅 기법 (EHD printing)을 활용하였다. 본 학위논문은 다양한 기판에서 원하는 위치에 효과적으로 고품질의 단일벽 탄소나노튜브를 붙여줄 수 있는 기술들을 개발하고, 그 기술들을 결합하여 미래의 전자 소자 제작에 적합한지에 대한 적용가능성을 평가해 보는 것을 주 목적으로 하였다. 또한, 단일벽 탄소나노튜브 물질의 저온 공정 가능성과 물질 자체가 가지고 있는 우수한 기계적 특성은 본인이 개발한 기술과 함께, 가까운 미래에 차세대 유연/신축성 소자를 위한 높은 수율, 높은 해상도, 높은 성능을 가지는 단일벽 탄소나노튜브 어레이를 롤투롤 방식의 공정 기법으로 형성하는 것에 대한 지침을 제공해줄 수 있으며, 결과적으로 차세대 유연 응용 전자 소자 구현에 큰 기여를 할 수 있을 것으로 기대된다.Chapter 1. Introduction 1 1.1. Solution-processed Thin-film Transistors 1 1.2. Solution-processed Single-walled Carbon Nanotube Transistor 5 1.2.1. Semiconducting Single-walled Carbon Nanotube 5 1.2.2. Various Deposition Methods of Semiconducting SWCNTs with Solution-process 9 1.2.3. Main Issues of Direct Dipping Methods 12 1.3. Organization of this Dissertation 14 Reference 18 Chapter 2. Multi-dipping Technique of Semiconducting Single-walled Carbon Nanotubes for Rapid Fabrication and Performance Improvement of Solution-processed SWCNT TFTs 25 2.1. Introduction 25 2.2. Experimental Methods 29 2.2.1. Fabrication Process 29 2.2.2. Measurement Details 30 2.3. Results and Discussion 32 2.3.1. Comparision of Fabrication Time and Electrical Performances of SWCNT TFT 32 2.3.2. Mechanisms of SWCNT Network Formation 36 2.3.3. Channel Morphology 41 2.4. Chapter Summary 45 Reference 46 Chapter 3. Self-patterning Technique of Semiconducting SWCNT based on Inkjet-printing of Surface Treatment Material 49 3.1. Introduction 49 3.2. Experimental Methods 51 3.2.1. Fabrication Process 51 3.2.2. Measurement Details 52 3.3. Results and Discussions 54 3.3.1. Optimization of Printing Conditions of PLL 54 3.3.2. SEM analysis of Successful Patterning of SWCNT 58 3.3.3. Electrical Characteristics 59 3.3.4. Two Main Parameters for Inkjet-printing of PLL 61 3.3.4.1. Effect of Water-stain of PLL 61 3.3.4.2. Effect of Pattern Size of PLL 62 3.3.5. Array Implementation 65 3.4. Chapter Summary 67 Reference 68 Chapter 4. Fast and Self-patterning Technique of Semiconducting SWCNT for High-throughput and High-resolution Solution-processed TFT 70 4.1. Introduction 70 4.2. Verifying Large-area Scalability of Two Techniques for implementing SWCNT TFTs array 73 4.2.1. Fabrication Process 73 4.2.2. Electrical Characteristics 74 4.2.3. Sub-chapter Summary 78 4.3. Verifying Micro-patternability of Two Techniques for implementing SWCNT TFTs array 79 4.3.1. Concept of High-resolution and EHD printing technique 79 4.3.2. Optimization of Each Layer of TFTs with EHD printing 81 4.3.2.1. Electrode 85 4.3.2.2. Dielectric 87 4.3.2.3. Surface Treatment Materials 89 4.3.3. All-EHD-printed SWCNT TFT with Two Techniques 94 4.3.3.1. Fabrication Process 94 4.3.3.2. Electrical characteristics 96 4.3.4. Sub-chapter Summary 98 4.4. Chapter Summary 101 Reference 102 Chapter 5. Conclusion 105 5.1. Summary 105 5.2. Limitations and Related Works 108 5.2.1. Detailed Network Analysis of Multi-dipping Technique 108 5.2.2. Reliablilty and Encapsulation for All-EHD-printed TFT 112 5.3. Recommendation for Future Researches 114 Reference 117 Appendix 119 Publication List 120 Abstract in Korean 125Docto