R2R Processes Research Articles

Roll-to-roll reactive ion etching of large-area nanostructure arrays in Si: Process development, characterization, and optimization

Roll-to-roll (R2R) nanofabrication processes are recognized as key enabling-technologies for many next-generation applications in flexible electronics, displays, energy generation, storage, as well as healthcare. However, R2R processing techniques reported in the literature currently lack a scalable method of performing high-throughput nanoscale pattern transfer of geometry requiring a high degree of fidelity in terms of critical dimension resolution, etch uniformity, and aspect ratio. Reactive ion etching (RIE) addresses the need for sub-10 nm pattern transfer with large-area uniformity in wafer-scale semiconductor manufacturing, but adapting plasma etch systems for use in R2R nanopatterning has proven to be nontrivial. Moreover, robust models for simulating R2R RIE do not exist, which is an obstacle to the creation of computational approaches to design, control, and scale-up of nanoscale R2R equipment and processes. To address these challenges, we demonstrate a process flow for fabricating Si nanopillar arrays utilizing a combination of nanoimprint lithography and RIE with all pattern transfer steps performed using a R2R plasma reactor system. Specifically discussed are process development details for etching imprint resist and Si including etch rates, cross-web etch uniformity, etch directionality, and etch selectivity at varying gas chemistries, powers, and pressures. 2k full-factorial Design of Experiments (DoEs) and ordinary least-squares regression analysis are also employed to study influence of process parameters on multiple outgoing etch quality characteristics and generate stochastic models of the R2R RIE pattern transfer process into Si. Utilizing these DOE-based models and desired targets for etch quality characteristics, we describe a bounded multivariate inverse-optimization scheme for automated etch process parameter tuning. The culmination of these efforts, to the best of the authors' knowledge, is the first reported RIE-based pattern transfer of 100 nm-scale features performed in continuous R2R fashion with control of feature geometry over large area. The methodology employed herein may be applied similarly to additional materials and geometries for future applications.

Impact of winding on nanoimprinted surfaces in roll‐to‐roll nanoimprint lithography

AbstractRoll‐to‐Roll manufacturing aims at scaling ultraviolet‐ and thermally cured nanoimprint lithography (UV‐NIL, T‐NIL) to commercial production speeds and volumes. Winding is the only convenient way of storing large quantities of nanoimprinted webs as they await unwinding in sequential R2R processes with distinct transport speeds. At production speeds, the imprinted resin is still chemically evolving when the imprinted web enters the winder, through a phenomenon called dark curing. The viscoelastic resin at various curing stages deforms under the contact pressure due to winding. This study is concerned with the impact of the contact pressure on the imprinted peak heights and potentially the functionality of the nanoimprinted surface. We develop a multiscale numerical model of the winding of the imprinted web. First the evolving properties of the resin through time are characterized, combining the effect of dark curing and viscoelasticity on the time‐dependent properties. Second, a finite element model of the imprinted web uses the resin mechanical properties to determine the effective properties of the imprinted web. Finally, the winding model determines the pressure and resulting strain of the imprints in the wound roll. The surface creep is quantified. This prediction will establish how and how long the imprinted materials should be wound.

Application of Monitoring Module Three-in-One Microsensor to Real-Time Microscopic Monitoring of Polarizer Sheet in Roll-to-Roll Process

The Roll-to-Roll (R2R) process refers to a high-efficiency, low-cost, continuous production method. The roll material used for processing is a flexible plastic or metal film. In many R2R processes, polarizing films are high-precision products with a high output value. In the production of conventional polarizers, product inspection will only be carried out after the production of the polarizing film is completed. The principal raw material of a polarizer sheet is a hydrophilic polymer, the properties of which may be influenced by water vapor, which degrades its quality. Whether or not the product is impacted can be ascertained by means of a quality inspection, but it must be performed after the process is finished. However, it is already too late when a defective product is detected: the production cost is increased, the schedule is influenced and the delivery date is delayed. The focus of this research was on environmental monitoring of the production drying process oven, but the commercial all-in-one sensors currently on the market cannot tolerate the temperature of the factory’s high-temperature oven. In particular, a commercial flow sensor is rarely suited to high-temperature applications. Some are expensive and cannot be widely distributed. Therefore, this study aimed to develop an integrated sensor to measure the internal environment of the drying process oven for the real-time monitoring of a polarizer sheet in the Roll-to-Roll (R2R) process. This study used micro-electro-mechanical systems (MEMS) technology to make a flexible three-in-one (temperature, humidity and flow) micro-sensor. We monitored the temperature, humidity and flow uniformity in a laboratory oven to simulate the environment of the actual factory oven, with the aim to provide data to confirm whether or not a polarizer sheet has dried. Our system can be monitored instantly by Arduino, or even Raspberry Pi 3, to realize the flexible micro-sensor layout and field verification, in order to optimize the R2R process and to enhance the yield and performance of the polarizer sheet process.

Modelling, Simulation and Control of Roll-to-Roll Physical Vapor Deposition Processes

Roll-to-roll (R2R) nano-manufacturing processes are critical in the manufacturing of electronic components for consumer products such as displays and flexible electronics. However, there are significant quality and process control challenges inherent to the control of R2R processes at nanometer scales. These challenges arise from the strong nonlinearities in the underlying phenomena, high levels of noise, and large measurement delays. This paper analyzes and evaluates the effectiveness of several control strategies in mitigating the challenges with control of R2R process steps. The proposed control strategies were tested on an experimentally derived model of an R2R physical vapor deposition (PVD) process, which simulates forward and double-pass R2R sputtering deposition procedures.

Measuring nonuniform web tension for roll-to-roll manufacturing of flexible and printed electronics

High-quality roll-to-roll (R2R) manufacturing for flexible and printed electronics often requires uniform web tension. Nonuniformity in web tension can lead to nonuniform performance of printed electronic devices across the width of the web, and excessive nonuniformity in web tension can lead to web wrinkling. Here we develop and test a noncontact resonance (NCR) method and a gentle contact stiffness mapping (GCSM) method for measuring the average web tension and its linear variation across the width of the web. The NCR method uses the lowest symmetric and anti-symmetric frequencies of a web with a closed-form expression to obtain its linearly varied tension. The closed-form expression includes the significant effects of air loading on web vibrations through accurate hydrodynamic functions. While the GCSM method is based on nonlinear regression of the contact stiffness on multiple locations of the web. Both methods are accurate, reliable, and inexpensive, and are compatible for a wide range of web properties, web path, web tension, measurement configurations, and environmental conditions. We cross-validate the two methods on a stationary test stand and in-line test the NCR method in two spans of a moving commercial R2R system. We measure up to 35.58% cross-span tension variation in that system, and both average tension and its linear variation can vary in different spans of the same R2R system. We expect the results presented in this article can improve quality control of R2R processes for flexible and printed electronics and maximize device yields.

Roll‐to‐Roll Processes for the Fabrication of Perovskite Solar Cells under Ambient Conditions

Recent progress in the engineering and fabrication optimization of hybrid perovskite solar cells (PSCs) has led to power conversion efficiencies (PCEs) of up to 25.2%. The properties of perovskite solutions enable low‐temperature, scalable coating methods that are compatible with the roll‐to‐roll (R2R) fabrication process. Despite this, there is still a significant lag in efficiency in large‐area devices fabricated using the R2R process, which achieves PCEs of 50% of the record value. Direct translation of research results to large‐scale commercially acceptable levels is crucial for commercialization, along with long‐term durability and processing reproducibility of the different device layers. Herein, the recent progress of several coating and R2R processes for the formation of PSC layers, detailing the key variables involved in the process, with an emphasis on the nucleation and crystallization of the perovskite film, is considered. The barriers and challenges in producing high‐quality perovskite films and improving the stability of devices are discussed. In addition, stability protocols for PSCs are discussed as they are key to unifying PSC stability and performance assessment, which is required to gain a better understanding of the different compounds and layers used in cell fabrication and the approaches necessary for their optimization.

Solution processing of polymer solar cells: towards continuous vacuum-free production

Roll-to-roll (R2R) production is essential for commercial mass production of organic photovoltaics, avoiding energy costs related to the inert atmosphere or vacuum steps. This work provides a complete review of various techniques and materials that have been used for the R2R production of bulk heterojunction polymer solar cells. Various fabrication parameters have been identified and discussed to improve the mass production process, aiming to obtain better-performing devices. The evolution of PCE and FF values of all-solution and partially R2R-processed modules over the last decade has been established, verifying the continuous evolution in this field. The life cycle analyses have confirmed that industrially processed modules through R2R processes have great potential to realize industrial production of high-performance R2R solar cell devices with an immediate goal of 10% PCE, 10 years durability, and less CO2 emissions with consequent lower energy payback time than other solar technologies. Currently, several laboratories have reported PCE values over 10% for small-area active layers fabricated by R2R techniques. However, the fabrication of large-area modules remains challenging. At the end of this document, based on the collected results, we propose a sequence of R2R compatible processes for an all-solution ITO-free solar device with high performance.

Continuous Manufacturing of Graphene-Contact Ion-Selective Electrodes Using Roll-to-Roll Processes: The Case for Nitrate Sensors

Soil contains various species of nitrogen, which is crucial for plant growth. Nitrate ion (NO3 -) is essential for promoting high crop yields and is widely used in fertilizers, however groundwater can be polluted by the overuse of nitrate. “Smart agriculture” can be supported by monitoring NO3 - in ground-soil using a distributed network of disposable potentiometric sensors. We have found the performance of such sensors to be enhanced by adding graphene nanoplatelets (GrNPs) as a conductive yet hydrophobic solid contact between the electrode and ion-selective membrane. Exfoliated GrNPs can be stably dispersed in ethyl acetate by liquid-assisted ball-milling, then deposited using methods compatible with the scalable roll-to-roll (R2R) production of ion-selective electrodes (ISEs). Ultra thin layers of GrNPs were formed by spray-coating onto a moving web of screen-printed Ag electrodes on polyethylene terephthalate (PET), followed by in-line deposition of a NO3 - selective membrane and silicone sealant then oven drying. Multiple electrodes from each production run were conditioned in aqueous KNO3 solutions and potentiometric characterizations of sensitivity, selectivity and stability were completed using an 80-channel analyzer. The GrNP layer serves as a hydrophobic transducer of ion adsorption and provides a high capacitance for stable potentiometric signals while also protecting SC-ISEs from damage caused by water permeation. We find that R2R processes can successfully produce graphene-contact NO3 - ISEs for agricultural applications, with extended operational lifetimes and high levels of quality control.

Parametric investigation of flexographic printing processes for R2R printed electronics

ABSTRACTRecent technological advancement in flexographic printing has allowed flexography to catch up with other R2R processes such as gravure printing in the printed electronics field. However, there are many process parameters involved in flexography that can affect the print quality. This work attempted to achieve finer line printing compared to the current 45–100 µm printed line width range of flexography by manipulating and studying these process parameters. A design of experiments was conducted to investigate the influence and interaction of various process parameters such as anilox volume, anilox force and printing force on the printed line width. After identifying the key process parameter from the design of experiments, another in-depth study of the key process parameter was conducted to further investigate how printed line width was affected. The results showed that the printing plate was elastically deformed with a range of 50–400 N printing forces. Beyond the 400-N printing force, the printing plate experienced plastic deformation.

Slot-die processing and encapsulation of non-fullerene based ITO-free organic solar cells and modules

Organic photovoltaic (OPV) devices have shown remarkable performance progress in recent years, reaching current record power conversion efficiency (PCE) values of 16.4% for single junction and 17.3% for multi junction devices, owing mostly to the impressive developments made within synthesis of new non-fullerene acceptors. This progress places organic solar cells at the forefront of thin-film photovoltaic technology. However, in order to meet industrial demands and reach high performance values in industrial settings, further research and development efforts within roll-to-roll (R2R) and sheet-to-sheet (S2S) processing of OPV devices under ambient conditions are required. Furthermore, OPV modules being manufactured through such up-scaled processing techniques should ideally be developed from low cost materials, and show good stability towards various different operational stress conditions. In this work, we demonstrate combined R2R and S2S development of ITO-free OPV devices, which are based on the non-fullerene material system PBDB-T:ITIC. The devices are processed from R2R vacuum sputtering and S2S slot-die coating at ambient conditions, and reach cell PCE values of 5.5%. In addition, we introduce a correlation between different barrier films, both commercial and sputtered inorganic coatings on ultra-clean PET, and the lifetime of the developed devices. The results therefore demonstrate an important step in the development of OPV devices from R2R and S2S processes in industrial settings.

Twofold Variation Propagation Modeling and Analysis for Roll-to-Roll Manufacturing Systems

Roll-to-roll (R2R) manufacturing has emerged as one of the most cost-effective manufacturing techniques for high-volume production of flexible printing electronics, graphene films, thin-film batteries, and solar cells, as opposed to low-volume batch production. However, in practice, real-time monitoring of the R2R process and in situ measurement of fabricated materials during production are challenging because of the high speed and dynamic variation of the web. In addition, due to the lack of cost-effective sensors and in-line metrology systems with a large range and high resolution, effective quality control and defect diagnosis are difficult to achieve. To address these challenges, this paper aims to develop analytical models that can serve as virtual sensing and metrology tools to quantify process state variation and estimate product quality in R2R processes with limited accessibility of in situ sensor. First, the quality variation propagation mechanism in R2R processes is investigated. Second, a hybrid multistage modeling method is proposed to characterize the twofold variation propagation—product-centric and process-centric variations, and its relationship with product quality in R2R processes. Note to Practitioner s—The novel modeling method developed in this paper employs both physics-based analysis (e.g., web handling system dynamics) and regression methods [e.g., censored regression and linear/logistic regression (LG)] using multisensor signals. The estimation results from the model can serve as virtual sensing and virtual metrology tools to increase the system visibility and be applied for process monitoring and error detection in real time. A print registration unit of elastic film in a roll-to-roll system is employed to demonstrate and validate the proposed modeling method in terms of the accuracy of state estimates and quality prediction. Based on the comparison results, the hybrid modeling method shows higher accuracy in state estimation and quality prediction than do the models with data-driven methods only.

Thermomechanics of axially moving webs in roll-to-roll manufacturing processes

Understanding the thermomechanics of flexible, axially moving webs subject to heat flux is important for ensuring process stability and product quality in roll-to-roll (R2R) flexible electronics manufacturing and nano-manufacturing. These applications include processes such as drying, curing, heat annealing, laser processing, plasma processing, functional printing, etc. A mathematical model for predicting the temperature and stress distributions in an axially moving web under arbitrary shape of heat flux is presented, including axial transport, heat conduction, convective heat transfer, and thermal radiation. A MATLAB based nonlinear finite element code is developed to predict the temperature and stress fields during such processes. The experimentally measured temperature distribution in moving paper and PET webs subject to heating from an IR laser source is compared with theoretical predictions to validate the computational approach. Such computational models are expected to aid in the development of a range of R2R processes where temperature gradients and stress fields can influence the product quality or lead to mechanical instabilities such as wrinkling or excessive web vibration.

Roll-to-roll processing of film substrates for hybrid integrated flexible electronics

Roll-to-roll (R2R) processing on film substrates has been demonstrated to have the potential for achieving high throughput manufacturing of organic electronic systems at low cost. However, the ever-growing mobile devices market accompanied by the developments in information and communication technologies require high performance systems at very low power operation, sometimes on larger substrates having sizes in the range of a few metres. Organic electronics often fall short of fulfilling the required computing performance and power requirements of most of the common use cases. Hybrid integration of inorganic monocrystalline silicon chips on polymer films is a means to fulfil the aforementioned requirements. In this context, it is opportune to report our recent activities on R2R processing of plastic films for hybrid integration of flexible electronics. Hybrid integration can be performed with conventional, rigid surface mount devices as well as flexible, ultra-thin bare silicon chips. The first section of the paper is dedicated to a brief overview of R2R manufacturing of electronic devices with an example of production of radio frequency identification tags as well as to a discussion emphasising the targets for hybrid integration. Then, detailed descriptions about our processes for R2R manufacturing of metal wiring lines on films and hybrid integration are included. Three-dimensional integration of films and a temperature sensor label manufactured using hybrid integration process are also elaborated on. Furthermore, key results from fatigue reliability assessment of R2R metallised wiring lines are reported. Finally, some of the challenges in transferring the R2R processes for hybrid integration on film substrates from research labs to industrial manufacturing are highlighted.

Design and implementation of adaptive PI control schemes for web tension control in roll-to-roll (R2R) manufacturing

In this paper, two adaptive Proportional-Integral (PI) control schemes are designed and discussed for control of web tension in Roll-to-Roll (R2R) manufacturing systems. R2R systems are used to transport continuous materials (called webs) on rollers from the unwind roll to the rewind roll. Maintaining web tension at the desired value is critical to many R2R processes such as printing, coating, lamination, etc. Existing fixed gain PI tension control schemes currently used in industrial practice require extensive tuning and do not provide the desired performance for changing operating conditions and material properties. The first adaptive PI scheme utilizes the model reference approach where the controller gains are estimated based on matching of the actual closed-loop tension control systems with an appropriately chosen reference model. The second adaptive PI scheme utilizes the indirect adaptive control approach together with relay feedback technique to automatically initialize the adaptive PI gains. These adaptive tension control schemes can be implemented on any R2R manufacturing system. The key features of the two adaptive schemes is that their designs are simple for practicing engineers, easy to implement in real-time, and automate the tuning process. Extensive experiments are conducted on a large experimental R2R machine which mimics many features of an industrial R2R machine. These experiments include trials with two different polymer webs and a variety of operating conditions. Implementation guidelines are provided for both adaptive schemes. Experimental results comparing the two adaptive schemes and a fixed gain PI tension control scheme used in industrial practice are provided and discussed.

Ultra-slim flexible glass for roll-to-roll electronic device fabrication

As displays and electronics evolve to become lighter, thinner, and more flexible, the choice of substrate continues to be critical to their overall optimization. The substrate directly affects improvements in the designs, materials, fabrication processes, and performance of advanced electronics. With their inherent benefits such as surface quality, optical transmission, hermeticity, and thermal and dimensional stability, glass substrates enable high-quality and long-life devices. As substrate thicknesses are reduced below 200 μm, ultra-slim flexible glass continues to provide these inherent benefits to high-performance flexible electronics such as displays, touch sensors, photovoltaics, and lighting. In addition, the reduction in glass thickness also allows for new device designs and high-throughput, continuous manufacturing enabled by R2R processes. This paper provides an overview of ultra-slim flexible glass substrates and how they enable flexible electronic device optimization. Specific focus is put on flexible glass’ mechanical reliability. For this, a combination of substrate design and process optimizations has been demonstrated that enables R2R device fabrication on flexible glass. Demonstrations of R2R flexible glass processes such as vacuum deposition, photolithography, laser patterning, screen printing, slot die coating, and lamination have been made. Compatibility with these key process steps has resulted in the first demonstration of a fully functional flexible glass device fabricated completely using R2R processes.

A high-yield vacuum-evaporation-based R2R-compatible fabrication route for organic electronic circuits

Advances are described in a vacuum-evaporation-based approach for the roll-to-roll (R2R) production of organic thin film transistors (TFTs) and circuits. Results from 90-transistor arrays formed directly onto a plasma-polymerised diacrylate gate dielectric are compared with those formed on polystyrene-buffered diacrylate. The latter approach resulted in stable, reproducible transistors with yields in excess of 90%. The resulting TFTs had low turn-on voltage, on–off ratios ∼106 and mobility ∼1cm2/Vs in the linear regime, as expected for dinaphtho[2,3-b:2′,3′-f] thieno[3,2-b]thiophene the air stable small molecule used as the active semiconductor. We show that when device design is constrained by the generally poor registration ability of R2R processes, parasitic source–drain currents can lead to a >50% increase in the mobility extracted from the resulting TFTs, the increases being especially marked in low channel width devices. Batches of 27 saturated-load inverters were fabricated with 100% yield and their behaviour successfully reproduced using TFT parameters extracted with Silvaco’s UOTFT Model. 5- and 7-stage ring oscillator (RO) outputs ranged from ∼120Hz to >2kHz with rail voltages, VDD, increasing from −15V to −90V. From simulations an order of magnitude increase in frequency could be expected by reducing parasitic gate capacitances. During 8h of continuous operation at VDD=−60V, the frequency of a 7-stage RO remained almost constant at ∼1.4kHz albeit that the output signal amplitude decreased from ∼22V to ∼10V. Over the next 30days of intermittent operation further degradation in performance occurred although an unused RO showed no deterioration over the same period.

A batch-based run-to-run process control scheme for semiconductor manufacturing

Run-to-Run (R2R) control has been widely used in semiconductor manufacturing to compensate for process disturbance and to improve quality. The traditional R2R controller only takes the process output in a previous run as its input and generates an optimal recipe for the next run. In a multistage semiconductor manufacturing process, variations in upstream stations are propagated to downstream stations. However, the information from upstream stations is not considered by existing controllers. In addition, most R2R processes have a limited capacity; the products must be processed in batches. Therefore, if the incoming materials could be grouped with small within-batch variations and large batch-to-batch variations and the recipes are customized for each batch to drive all batch averages toward the same target value, the output variation could be reduced and quality improved. A batch Exponentially Weighted Moving Average (EWMA) controller is proposed in this article. It employs a modified K-means algorithm to group the incoming materials into batches with a fixed and equal size while minimizing the within-batch variation. The controller then generates the control settings by taking both the batch information and the feedback quality information into account. Simulation studies show that the proposed controller could significantly reduce output variation and improve quality.

In-line spectroscopic ellipsometry for the monitoring of the optical properties and quality of roll-to-roll printed nanolayers for organic photovoltaics

The fabrication of organic electronics devices onto flexible polymer substrates by roll-to-roll (r2r) processes requires the use of in-line quality control tools for the optimization of the optical and electronic properties of semiconducting, electrodes and barrier layers and their thickness that will achieve the required performance and lifetime of the devices. This paper focuses of the investigation of the optical properties of r2r printed nanolayers for organic photovoltaics, by in-line spectroscopic ellipsometry (SE) in the visible-far ultraviolet spectral region. This is performed by a prototype very fast stand-alone SE unit adapted onto a lab-scale r2r system. First, we test the stability and precision of measurements to validate the optical measurements and the analysis procedure. Afterward, we report on the robust determination of the thickness and optical properties of barrier thin layers (a combination of SiOx and hybrid polymer bilayers), PEDOT:PSS transparent electrodes and polymer:fullerene (P3HT:PCBM) blends that were gravure printed onto flexible PET substrates in the form of rolls. Finally, we discuss on the effect of the experimental parameters on the optical properties and the quality of the printed nanolayers as determined by the modeling approaches of the in-line measurements.

Recursive Least Squares Run-to-Run Control with Time-Varying Metrology Delays

This article investigates how to adaptively predict the time-varying metrology delay that could realistically occur in the semiconductor manufacturing practice. Metrology delays pose a great challenge for the existing run-to-run (R2R) controllers, driving the process output significantly away from target if not adequately predicted. First, the expected asymptotic double exponentially weighted moving average (DEWMA) control output, by using the EWMA and recursive least squares (RLS) prediction methods, is derived. It has been found that the relationships between the expected control output and target in both estimation methods are parallel, and six cases are addressed. Within the context of time-varying metrology delay, this paper presents a modified recursive least squares-linear trend (RLS-LT) controller, in combination with runs test. Simulated single input-single output (SISO) R2R processes subject to various time-varying metrology delay scenarios are used as a testbed to evaluate the proposed algorithms. The simulation results indicate that the modified RLS-LT controller can yield the process output more accurately on target with smaller mean squared error (MSE) than the original RLSLT controller that only deals with constant metrology delays.

77.3: Invited Paper: Roll‐to‐Roll Manufacturing of Backplanes for Paper‐Like Displays

AbstractHP and Phicot are planning the world's first R2R (roll‐to‐roll) manufacturing line for display backplanes based on the SAIL (Self‐Aligned Imprint Lithography) process. Economic benefits for R2R compared to batch, cost comparisons of different R2R processes, comparison of substrate options, and necessary supply chain infrastructure developments are presented.