Fully roll-to-roll gravure printed electronics: challenges and the way to integrating logic gates

Printed electronics are estimated to revolutionize the manner of electronics manufacture in the future. ¹⁾ The potential impact of printed electronics is monumental considering its low cost of fabrication, compatibility with flexible substrates, and suitability in the case of applications where a large and compliant footprint is essential. ²⁾ Silicon-based electronic circuits are of great advantage in that they display incredible computational power and are frequently employed for high-frequency communication. However, the increased cost of production for applications due to an increase in the chip area and limited flexibility eclipses the capabilities of such circuits. The primary idea of flexible electronics is the assimilation of electronics depending on whether it is organic/inorganic on a flexible plastic or thin metal substrate to minimize weight and enhance flexibility, and portability. ^3–5) The benefits of flexible electronics including its versatility to various shapes make it vital to different applications including unconventional substrates to either adapt to curvy surfaces or to degrade naturally. ^6–9)

A promising approach towards the fabrication of flexible electronics includes the roll-to-roll (R2R) printing method, which allows the mass production of flexible devices by bypassing the requisite of 2-dimensional large substrates through a continuous web supply along the 1-dimensional direction. ¹⁰⁾ As a common and shared technology, the R2R printing technique holds the capability for a continuous and superior throughput method for material deposition on large substrate rolls. ^11,12) Printing technologies are crucial in the way that they facilitate the manufacture of electronic devices and sensors with significant area by R2R methodology in an economical method. ¹³⁾ The development of organic/polymer-based electronic devices ^14–16) was made feasible owing to the rapid advancement of reliable R2R technology with more patterning tools. The latest advances in flexible electronics applications have called for further economical, higher throughput, and superior resolution micro/nanofabrication techniques. ¹⁷⁾

A long-standing approach in the field of micro-patterning is photolithography, which is a comparatively simple, economical, particularly reproducible, and well-regulated, completely automated high-resolution patterning method having well-established registration conventions. Regardless of these advantages, the use of photolithography for organic electronic systems is restricted owing to a paucity of chemical compatibility with most organic electronic inks, and thin-film transistors (TFTs) suffer damage interacting with strong chemical reagents either via the photoresist deposition or exclusion i.e., etching or lift-off stage. Another constraint of conventional photolithography is the requirement of masks and custom-made optical components, which is expensive thereby making the method viable only for large-scale manufacturing. ^18,19) Conventional photolithographic-based fabrication methods including spin coating have shortcomings related to material waste, energy, time-consumption, area limitation, or being economically non-viable. ²⁰⁾ Inkjet printing ²¹⁾ and shadow mask evaporation ²²⁾ have been able to make remarkable progress in micro-patterning for electronic systems; however, these methods carry their shortcomings. Although inkjet printing promises high throughput, there are certain constraints regarding the resolution in addition to problems with the resulting film uniformity. A common drawback of both these techniques is the dearth of registration, specifically with the orientation and placement of patterns on the substrate, that renders the conception of multilayer devices exceptionally difficult. ²³⁾

The contact printing methods including screen printing and gravure printing, usually customized for R2R processing, permit the scaling up of the process to a higher level and can bring in a significantly advanced throughput at a superior speed in comparison to the no-contact printing procedures, such as inkjet printing as they can typically be modified for R2R processing. ²⁴⁾ Figure 1 shows a schematic illustration of such contact printing methods.

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In customized contact printing methods, monitoring surface functionalization, and patterning of receiver substrate ^25,26) enable the configuration control of the printed nanotube-based device, paving way for large-area manufacturing in the R2R printing process, such as in the case of a 4-bit code generator fabricated by the R2R printing technique shown in Fig. 2. ²⁷⁾

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Gravure printing is the most robust and reliable method employed for consistent ink transfer from the target image on the printing roll to the running web. In this method, the functional inks are transferred directly via physical interaction between the substrate and the engraved structures. The advantage of this method is that it produces superior patterns in an economical method characteristic of an R2R process. For magazine and packaging printing, the R2R gravure is considered the most efficient throughput printing technique, having the highest printing speed of 600 m min⁻¹. ²⁸⁾

In contrast to the conventional printing method, the ink transfer process is a crucial technical aspect in printed electronics as it is imperative to attain consistent and reliable electrical characteristics including conductivity. ²⁹⁾ The analysis of the transfer process is even more difficult considering the mixed composition of the base ink with active pigments such as metallic nanoparticles, metal oxide nanoparticles, nanotubes, etc. Moreover, the efficiency of drying and curing processes also influences the quality of the transferred ink state. The fundamental requirement for printed electronics applications is the need for sufficient substrate smoothness. ^30–33) The substrate stability and microstructure (including the formation of cracks) are influenced by the solvents employed to transfer materials along with the drying conditions, which are in turn affected by the ambient humidity and temperature constraints. ³⁴⁾ An increase in humidity results in concave profiles due to the coffee-ring effect in printed electronic devices, whereas higher humidity levels lead to convex profiles. ³⁵⁾ Likewise, enhanced humidity results in a continuous shift in the ΔV_th value of printed TFTs. ³⁶⁾ It is also imperative to analyze the thermal behaviour of the materials that comprise a printed electronic device to understand the power utilization and voltage variation. ³⁷⁾

In the R2R printing process, the crucial parameters that influence the thickness and roughness of the printed pattern are the mechanical constraints, namely operating speed and tension, and chemical factors, namely curing conditions, surface tension, and viscosity of ink. ³⁸⁾ To materialize the application of R2R gravure in printed electronics, it is imperative to characterize how the parameters such as the scalability of printed features and overlay printing registration accuracy (OPRA) vary corresponding to the system's physical constraints, such as plastic substrates and inks. ³⁹⁾ Additionally, the control of web tension is crucial during material transport with different operating speeds as it ensures the quality of the finished web products. ⁴⁰⁾

One of the most challenging problems encountered in printed electronics is micro-patterning and developing fluid-based materials (inks) for electronic and optoelectronic systems, especially integrating logic gates. ^22,41) The significance of logic gates, for instance, inverter, NAND, and NOR lie in the fact that they are the basic building blocks used to achieve intricate circuits. ⁴²⁾ Metal oxide-based semiconductors have garnered much attention since they are economical, possess good mechanical flexibility, low fabrication temperatures, and elevated carrier mobilities. However, a shortcoming of such a device exists in the fact that the circuits are designed in n-type logic only due to the unavailability of p-type metal oxide semiconductors performing equally well as their n-type counterparts. ^43,44) Researchers have developed a complementary metal-oxide-semiconductor (CMOS) device, which consists of both n-type and p-type semiconductors to design the logic functions. The advantage of this design is that it forfeits the use of a pull-up resistor and ensures lower power consumption. Moreover, the near-symmetric response of the CMOS device between the transitions from low to high rails, and vice versa makes it highly noise-immune. Figure 3 depicts one such CMOS device built on n-(In-Ga-Zn-O) and p-type (SnO_x ) active oxide semiconductors, on a compliant, reusable paper dielectric/substrate with In-Zn-O as the gate electrode. ⁴⁵⁾

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During the integration of logic gates via R2R gravure printing, it was observed that V_th variation, which affects the overall integration level of the printed logic gates, is affected by the oxide layer capacitance. ^46,47) Similarly, the difference in orientation between the two electrodes- the drain-source and gate- owing to the OPRA constraint also impacts the variations of V_th. ²⁷⁾ In comparison to conventional printing, for printing logic gates, the preliminary condition regarding OPRA is to maintain the accuracy within the permissible limit (less than ±30 μm). ²⁸⁾ A minimum of four layers is required to print a TFT to retain reliable electronic current behaviour by gate modulation. ³⁹⁾ Also, the drying/curing time for all the electronic inks included (metallic ink, dielectric ink, and semiconductor-based ink) in the R2R gravure method should be uniform. ⁴⁸⁾

In a semiconductor manufacture unit or foundry, the design rule is negotiated amongst circuit design and process engineers to deliver the geometry of an integrated circuit design economically. To establish the R2R printing foundry concept, the design rule is developed in such a way that it incorporates the physical scope and electrical constraints of the completely printed logic gates. The topological features at the boundaries between gate-dielectric, dielectric-semiconductor, and drain/source-semiconductor were sustained within a few nanometers. ²⁸⁾ However, in contrast to the Si-chip foundry, the printed devices' physical scope and electrical constraints can vary according to the rheology of the electronic inks, the ink transfer mechanism, speed of printing, web tension, and OPRA of the employed R2R printer. ^27,49) Hence, it was imperative that a complimentary design rule that incorporates both the physical scope of the printed device along with the electrical parameters be established initially, which depends on the electronic ink rheology, OPRA, and speed of printing. The preliminary design rule is to create a corresponding design rule by incorporating OPRA in specifying the physical scope of the logic gate such as the reliability of web tension, efficiency of ink transfer, speed of printing, drying time, and the rheological properties of the ink used. ²⁷⁾

A gap that exists in the literature concerning R2R printed electronics is that R2R foundry has not been able to incorporate CMOS logic gates into disposable and flexible electronic devices. Recently, researchers from the LAMP lab have succeeded in developing a printing foundry for the R2R system built on a corresponding design rule of physical scope wherein the electronic ink rheology, printing speed, OPRA, and device design are integral to each other. ²⁷⁾ However, a major issue that was encountered in printing several TFTs-based logic gates was with maintaining a narrow variation of V_th. This review paper deals with the associated challenges and the innovative ideas to deal with the same.

The traditional printing processes such as gravure, flexography, screen printing, or digital inkjet printing were utilized to deposit conductor, dielectric materials, or other thin-film components essential for the application. ⁵⁰⁾ However, the major obstacles faced during the integration of printed electronics for various applications were device stability and power dissipation. ⁵¹⁾ The complementary logic gate could resolve the problem of high-power dissipation in the case of logic circuits, ^52,53) or adding electrolyte gel as a dielectric layer could rectify the issue during the printing of TFTs. Nevertheless, it was imperative to study the design rule involving the electrical characteristics and physical scope of the printed device system, to launch the foundry notion of the R2R printing method.

Even though the issue of power dissipation was resolved by the inclusion of complementary logic gates, the ambient device stability remained a cause of concern during the printing of complementary logic gates having TFTs above ten in number. ⁵⁴⁾ This is due to the inherently different mechanisms regarding trapped charges in the case of both n- and p-type printed TFTs, which could not be absolved via the simple inclusion of p- and n-type TFTs. A solution to this issue could be the integration of identical semiconducting material in printing both n-type and p-type TFTs through a suitable encapsulation material. ^55,56) In this regard, amphoteric single-walled carbon nanotube (SWCNT) has been utilized to provide p- and n-TFTs, and another breakthrough have been the fabrication of a printable material suitable for encapsulation to analyze the outcome of stabilizing completely R2R printed SWCNT-based complementary logic gates that create 1-bit code under ambient conditions, as shown in Fig. 4. ⁵¹⁾ The authors were able to solve the issues of power dissipation and stability by employing CMOS logic gates and the multilayer encapsulation technique together. ⁵¹⁾ The fabrication of CMOS logic gates using a fully R2R printing technique with ink based on single-walled carbon nanotubes (SWCNT) reduced the power dissipation. Later, the p-type load-TFTs based on SWCNT were transformed into n-type load-TFTs based on SWCNT to operate as a CMOS 1-bit code generator, which was followed by encapsulation by polymer-based multilayer films.

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In recent years, the R2R printing process has attained a resolution of 50–100 nm at a laboratory level, allowing accessibility to superior-resolution flexible devices, although 5 to 10 nm levels of OPRA are required to print logic gates with the proposed resolution. In turn, the process requires proper moderation of web speed and tension, since the printed patterns would be damaged by the variation of these parameters, or else the web itself may be broken or bent. ⁵⁷⁾ Consequently, the speed and tension of the web are the two fundamental variables that alter the quality of the fabricated products. ⁵⁷⁾ Additionally, it has been observed that the design rule of the foundry in R2R printing ought to encompass features of the employed ink and R2R printer equally, such that the printed CMOS-based active devices via R2R can constantly show designated functions. In this context, researchers have developed a low-bit code generator by the R2R technique that emerges as the initial phase in launching the printing foundry for fabricating logic gate-based compliant devices. Thus, the primary step in the R2R printing foundry is to develop the preliminary design rule before finalizing the practical design rule that defines electrical constraints and physical scope of the R2R printed devices within the permitted deviations of V_th based on the integration level. It has been observed that the V_th variations of the R2R system depend on the rheological properties of the ink, web tension, printing speed, and OPRA; hence the predominant component responsible for instigating ΔV_th in the printing foundry needs to be designated and encompassed in the preliminary design rule. ^27,39)

Specifically, the parameters under consideration in the printed TFTs are OPRA and isotropic printability since they are vital in retaining the narrow variations in V_th. Even though a maximum printing speed of 600 m min⁻¹ is possible via the R2R gravure method, taking into consideration the OPRA, electrical, and drying constraints, the speed of printing would be just above 6 m min⁻¹ in practice. Additionally, for maintaining the surface topology of printed layers, it is imperative to maintain the interface topology between dielectric-semiconductor, drain/source-semiconductor, and gate-dielectric within a few nanometer scales. Logic gates and TFT active matrices are printed via a fully R2R gravure keeping in mind the above-mentioned concepts, designing a compliant electrophoretic display model which could be enhanced for large-area signage, as depicted in Fig. 5.

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Considering the effect of the above-mentioned factors on the overall efficiency of R2R printing, each parameter is analyzed in detail in the forthcoming sections.

2.1. Ink rheology

Conventional gravure printing which is the representative method to produce publications such as newspaper do not require high-quality printing technology like the printing defects in the range of 10 μm that cannot be distinguished by the human eye ⁵⁸⁾ can be ignored, and it thereby facilitates economical mass production. However, in the case of electronic printing, minor defects in any part of the circuit can cause malfunction of the device. Hence, the precision printing technology that prints thin-width lines or patterns with low OPRA(or without defect) is required to fabricate high-level integrated logic gates. ⁵⁹⁾ The key point for successful printing is the accurate transfer of ink on the substrate in the desired pattern at the correct location. However, since ink transfer is a complex nonlinear phenomenon affected by various factors such as free boundaries, moving contact lines, rheology, cell geometry, and printing conditions, it is difficult to accurately control the process. ⁶⁰⁾ Therefore, to reduce defects and perform accurate printing, it is essential to understand how these factors influence ink transfer.

As the first step for successful printing, various theoretical and experimental studies are being carried out to analyze the fundamental physical mechanism of ink transfer. Several researchers have simplified the curved surface of the roll into a parallel plate to check the effect of contact angle and parameters such as plate velocity, ink density, and viscosity on ink transfer. ^61–64) Studies have also been carried out to analyze the effect of cell inclination angle, depth, and parameters on ink transfer in a plate with cells, as shown in Figs. 6(a) and 6(b). ^61,64–66) Furthermore, both extensional (vertical) and shear (horizontal) motion were implemented to replicate the phenomenon similar to the roll to analyze the ink transfer mechanism, as shown in Fig. 6(c). ⁶⁷⁾ A thorough understanding of the underlying phenomena allows us to further examine the impact of different factors on ink transfer.

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Since the ink used for printing is a non-Newtonian fluid containing different additives such as functional additives, binders, and active pigments, ^67,68) it is also imperative to check the effect of rheological properties on ink transfer. Several numerical studies are being conducted to check ink transfer between the plate and cell considering the non-Newtonian rheological properties of ink, ^69,70) however, only the shear rate-dependent non-Newtonian model is considered. Hence, further research is required to evaluate the effect of the complex rheological properties of real inks on ink transfer. In addition, since the rheological properties of fluids are extremely sensitive to temperature and humidity, it is necessary to maintain appropriate ambient conditions during printing to minimize instability. ⁷¹⁾ In addition, the surface tension or surface energy of the substrate is related to the adhesive or cohesive force acting between the plate and the ink during the ink transfer process, and these forces affect the ink transfer ratio. ^72,73) Consequently, it is imperative to select the ink with appropriate surface tension by adding an additive or surfactant and substrate with suitable properties through surface treatment.

For the precise ink transfer, preliminary steps in the gravure printing process, namely cell filling and wiping, must be accurately controlled. ⁷⁴⁾ Filling the cells in the pattern roll with ink is the first step in gravure printing. During this process, necessary precautions must be taken to fill the cell without air entrapment for accurate ink transfer, and to focus on the various factors such as contact angle, printing speed, viscosity, and cell size that affect cell filling as shown in Fig. 7(a). ^75,76) Therefore, the parameters must be properly adjusted according to the type of ink or the printing condition to fill the cell. After the cell filling, a wiping process is essential to remove excess ink remaining in the pattern or around it with a doctor blade. However, during wiping, ink is imbibed between the doctor blade and the roll surface, creating a new free surface, and as a result, residual ink remains around the pattern, forming an unwanted drag-out tail as shown in Fig. 7(b). ^76,77) Therefore, a clear pattern can be obtained through the optimization of the wiping process.

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Numerous factors affect the ink transfer process, and researchers are carrying out individual studies to focus on each effect. Any inconsistency in web tension or printing speed was found to affect the ink transfer process. ⁷⁸⁾ The ink rheology and printing process parameters influence the thickness, width, and surface roughness of the printed patterns, thereby affecting the characteristics of consequent dielectric and semiconducting layers that define the electrical characteristics of the printed TFTs. ⁵⁹⁾ However, since all printing processes are quite complex in practice, these factors comprehensively affect the printing process. Therefore, it is necessary to check the impact on the process when various factors are combined, which presents a challenging issue.

2.2. Overlay printing registration accuracy (OPRA)

Several research works have been carried out in the past to analyze the effect of enhancing the OPRA to obtain high-quality flexible electronic devices via the R2R printing process. ^10,79,80) Improving OPRA to match the printing resolution is crucial to maximizing the capability of the R2R system. ⁸¹⁾ To attain an improved OPRA for a flexible substrate, developing the web handling modules, namely tension control, guiding of the web, winding, and unwinding, along with a precise printing tool is as important as the management of each unit in the entire system. As researchers have pointed out, overlay printings require a minimum set of layers for printing TFT for retaining uniform electronic current performance via variation in the gate. OPRA depends on the micropatterning technique, and the material deposition method employed. To attain a yield of more than 90%, the width of the gate and length of the channel in TFT must have a channel aligned with the top of the gate, subject to OPRA. An increase in the registration precision can result in more integrated CMOS logic gates within a reduced area. ⁸²⁾ Specifically, the expansion/contraction of the web when passing through the drying/curing chambers should be simulated and controlled accurately to attain the OPRA of ±5 μm. The principal cause for the large value OPRA attained is the inconsistency in web tension, the thermal expansion of the polyethylene terephthalate (PET) film, and vibration/thermal expansion of the gravure system. Hence, the vibration/thermal expansion of the gravure printer and plastic substrate need to be analyzed and monitored to improve OPRA. Researchers have found a correlation between the fundamental parameters of R2R gravure printing including OPRA and the printing and ink parameters and summarized the findings as given in Table I. The dielectric ink employed in the study was BaTiO₃ based, having a viscosity of 200 cP and surface tension of 300 N m⁻². It was observed that the groove structure aspect ratio has an influence on OPRA, surface roughness, and line widening of gate electrodes in the printed system. ³⁹⁾

Table I. Correlation between the key parameters for the optimized R2R system. [Reproduced with permission from Ref. 39. Copyright 2010 by IEEE].

Key point for the evaluation of printed silver patterns	To attain optimized printed silver patterns
Overlay Printing Registration Accuracy	Printing direction 15.7 μm
(OPRA)	Vertical to prints direction 41 μm
Specific resistance (connectivity)	-higher viscosity ink
	-higher print speed
	-lower aspect ratio of the cell
Waviness	-aspect ratio of 0.16 ± 0.022
Thickness	-ink viscosity
	-lower aspect ratio of cell
	-higher print speed
Line widening	-higher ink viscosity
	-higher print speed
	-higher aspect ratio of cell
Surface roughness	-lower viscosity
	-higher aspect ratio of cell
	-higher print speed

Studies have been conducted to control the OPRA in the machine and transverse directions at 6 m min⁻¹ by operating the servomechanism established on three charge-coupled devices [Fig. 8(a)]. The film and roll cameras will aid in the calculation of the precise timing at which the registration markers from the printing unit and gravure cylinder respectively meet the second unit of printing. Any misalignment between the two markers is monitored by the review camera, and subsequently, the actuator will monitor the second gravure cylinder speed to attain the OPRA in the machine direction. The servomechanism in the R2R system helps retain an OPRA of ±20 μm at the prescribed speed of printing at the set temperature for the PET film considered in the study, as depicted in Fig. 8(b). Figure 8(c) depicts the web tension which is maintained with 6 kgf at the prescribed printing speed by using three dancer rolls between the unwinder and rewinder (1.019 kgf cm⁻²).

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Hence, it was observed from the literature that to integrate a passivation unit of printing, a complimentary design rule is obligatory to nullify the OPRA constraint during the integration of the CMOS logic gates in the gravure printing via the R2R technique. Researchers have reported that the rheology of ink and printing conditions affect the OPRA. ⁷⁸⁾ It was also observed that the deviations in V_th are due to variations in capacitance, which are in turn caused by the fluctuation in OPRA, surface roughness, and ink rheology of the printed dielectric layers. ^59,83)

2.3. Web tension

A factor that can greatly alter the multilayer printing accuracy includes the control of web tension. ⁸⁴⁾ Researchers have evaluated the impact of the web tension on the surface traits and assessed the quality of the printed pattern using the surface roughness and thickness attained. ⁸⁵⁾ High-quality R2R fabrication for flexible and printed electronics necessitates the need for homogeneous web tension. ⁸⁶⁾ Authors have developed methods for optimizing the web tension in the R2R system by taking into consideration the elastic modulus of the substrate film and printed pattern conductance inside the dryer. ⁸⁷⁾ Regulating the transport velocity and tension of the web to retain superior product quality is the primary task of R2R systems, which is not negligible owing to the strong nonlinearities encountered. ^88–91) Previous studies found that the basis of the error in registration is the disturbance in web tension, and these are in turn triggered by the thermal disturbance from the dryer. ⁹²⁾ The inconsistency in web tension can cause the unpredictable performance of printed electronic devices throughout the width of the web, and extreme inconsistency in web tension can cause web wrinkling. The variation in web tension can also cause a change in the V_th value. The process variables of gravure printing including the web tension affect the volume of ink transferred, i.e., alter the ink transfer ratio, which modifies the thickness and width of the printed pattern on the substrate. Figure 9 shows the relation between the ink transfer method and process conditions in gravure printing.

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During the ink transfer process, the web tension affects the web surface energy and alters the shape of the printed pattern. ⁹³⁾ The ink transfer ratio is affected by the gravure printing process variables, which in turn alters the thickness and width of the printed substrate pattern. Moreover, the resistance of the printed pattern also changes with a variation in the pattern geometry, even though the ink used remains the same. ⁷³⁾ Figure 10 depicts the effect of various parameters including web tension on the resistance of printed patterns, as obtained from the literature. It can be observed that the resistance of printed patterns shows a decrease with an increase in speed and tension of the web and nip pressure.

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It was reported from the literature that to sustain a variation of less than ±3 nm in the physical dimensions of the printed layers, the rheological characteristics along with the printing speed, and web tension should be controlled well. ²⁸⁾ For a specific value of OPRA, altering the physical dimensions while maintaining a uniform printing speed and web tension was observed to provide a device yield above 98%. ⁹⁴⁾

Literature states an accurate, inexpensive, and reliable method for the measurement of web tension in the R2R process, namely the noncontact resonance (NCR) technique and a gentle contact stiffness mapping (GCSM) technique, to enhance the quality of R2R system for flexible and printed electronics. ⁸⁶⁾

2.4. R2R printing foundry

For the fabrication of economical, disposable, and large-area electronic devices where several logic gates are integrated, R2R gravure is regarded as the fundamental foundry. Though the R2R system can effortlessly print consistent patterns up to a width of 30 μm, ⁹⁵⁾ it cannot be finalized as the definitive physical dimension possible for each device printed by the R2R foundry. To completely employ R2R gravure foundry, issues related to parameters such as OPRA, ink rheology, web tension, drying condition, and the design rules should be resolved first. Hence, the design rule of the R2R gravure foundry must focus on the OPRA achieved with employed inks and web, and reliable physical measurements, which could, in turn, affect the electrical properties of printed logic gates. Also, the printing of active-matrix and logic gates on the web demands additional complex criteria, as mentioned in the previous section. Though printing consistent patterns in the range of 30 μm width are plausible using R2R gravure, ⁹⁵⁾ it cannot be set as a univocal physical dimension for all printed devices using R2R foundry. Considering this aspect, researchers have developed the initial concept of a corresponding design rule for the printing foundry via the R2R method using five different nanomaterial-based inks and PET web, as shown in Fig. 11. Herein, the complementary design rule was established via a printed CMOS-based 4-bit code generator along with an antenna and sustaining a consistent OPRA limit, the optimum printing speed, the rheology of the five diverse inks (dielectric, drain-source, gate, n-doping, and semiconductor) and web tension simultaneously. To counteract the variation in V_th in the R2R printed logic gates, the complementary design regulation was applied in TFT printing to avert electrical fluctuations initiated by the non-alignment of drain-source electrodes on the gate electrodes.

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There is ample scope for improvement for R2R gravure to reinforce the concept of high throughput foundry in the fabrication of economical, compliant, and disposable microcontrollers and smart packaging. One possibility could be the improvement of OPRA up to ±5 μm, thereby decreasing the width of the gate and length of the channel to 20 μm. The additional precision in registration would also lead to the fabrication of a larger number of integrated CMOS logic gates in a reduced space. ⁸²⁾ To establish the concept of high throughput foundry, it is imperative to have a combinatory design model between the rheology of ink used, speed of printing, and the tension in the web along with other physical dimensions. ³⁹⁾

Through this review paper, the authors have compared the status of R2R gravure printing in the fabrication of flexible and printed electronics and the future associated with it. Remarkable innovations in the printing foundry have helped R2R inline fabrication for large-scale and complaint logic devices to be available economically soon enough. Considerable progress has been made in integrating logic gate circuits via the R2R printing method. The major issue concerning the V_th fluctuation in R2R printed logic gates could be resolved by applying the complementary design rule in printing TFTs, which can avert the electrical fluctuations produced by the misalignment of drain-source electrodes on the gate electrodes. Various parameters involved in maintaining narrow ΔV_th such as ink rheology, OPRA, web tension, and printing foundry were analyzed in detail. It was noted that a breakthrough was made by the printing of CMOS-based logic gates via R2R foundry. Discretion is required to retain the capillary thinning mechanism of the inks used to maintain consistent electrical characteristics in the printed CMOS logic gates. Various findings on the R2R gravure printing method analyze the various parameters affecting the efficiency of a fabrication method for flexible and printed electronics. Printed electronics have made considerable headway to enhance the printing resolution for electronics applications. However, there are issues in developing high-resolution printed electronics, specifically further scaling, and practical operation. Moreover, progress in fabrication would necessitate further knowledge of the inherent fluid mechanics, which can be transformed into advances in materials and printing tools. The authors envision that the continuing growth of flexible and printed electronics via the R2R printing technique can overcome the challenges faced and rapidly broaden the extent of the prospective applications.

The authors appreciate the support from the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2020R1A5A1019649).