**3. Towards on-chip integration**

Integrating the different deposition techniques on one single substrate constituted the final step towards the obtainment of fully printed and hybrid circuits, with tuned capabilities and functionalities. Flexible tags, as the one presented earlier, however, are characterized by an inherently weaker link: the connection between the integrated circuits and the printed devices. Research in solving the interconnection issues, or eliminating them by realizing inherently flexible circuits, has seen a tremendous surge in the last decade.

In every application, in which it is not possible to construct an entirely flexible circuit, the necessity to solder (or, more correctly, to "attach") monolithic integrated circuits to the printed tag arises. The immediate problematics to be faced are mainly three: attaching the component with no harm to the flexible substrate, achieving a low contact resistance and retaining the flexibility of the overall circuit, at no performance loss. The most straightforward approach for the integration of ICs onto flexible circuits, is the adaptation of industrial soldering processes, with the employment of low-temperature soldering alloys. In a systematic and well-presented work by Andersson et al. [39], a series of SMD components with different packaging was soldered keeping the reflowing temperature below 150°C. The study showed mixed results: on the one hand, it proved the feasibility of soldering on paper with standard industrial devices, obtaining contact resistances in the order of few ohms; on the other hand, it was partially inconclusive, as it showed the inevitable presence of cracks after soldering and bending (shown in **Figure 12a**). Furthermore, the soldering yield was rather low (ca. 80% for the smallest packaging, much lower for bigger components), which might be a serious issue in commercial applications. An apparently more complicated—although much more appealing and promising—solution, stems out from treating flexible electronic chips for what they are: non-classical electronic systems, with the need of dedicated solutions. In this avenue, a very interesting work from Quintero et al. compares two technologies with a similar common denominator: the employment of conducting adhesive layers [40]. In particular, the present a selective etching of an isotropic conductive adhesive and the stencil-printing of Anisotropic Conductive Adhesives (ACAs), applied to the bonding of two separate flexible chips. The latter approach, extensively studied and applied in many material combinations, is the one that, so far, has been attracting the highest interest. ACA-based interconnects are obtained in a simple three-step procedure: first, the adhesive conducting film is deposited through a stencil, then the target component is placed on top of it, finally, the temperature of the substrate is raised to the bonding temperature (usually lower than 150°C). When the resin is heated up, it creates an electrical bond between carrier substrate and component, and creates a mechanically stable link. Nilsson et al. [41], present a remarkable application of printed circuits with ACA cre

connected chips: a semipassive RFID chip, used to log eventual intrusions in carton packages. The system includes a resistive sensing network, an active microcontroller used to record and log the sensor data, powered up by a flexible battery, and a passive communication system. The connecting lines were ink-jet printed and screen-printed, while the hybrid interconnections were obtained with means of a commercial ACA, as shown in **Figure 13**. An extensive study on the stability and reliability of similar solutions has been presented, already in 2014, by Happonen et al. [42], who thoroughly investigated the resiliency of conducting adhesives employed for the connection of separate flexible foils. They explored different bonding solutions and performed a live measurement of the DC 4-wires resistance during several thermal and bending cycles. Interestingly, they show that the stability of hybrid interconnections is enhanced by the presence of supportive, non-conductive adhesives. These hybrid structures can undergo more than 1000 thermal cycles (0–100°C and back to 0°C in 1 h) and bending cycles (with bending radius down to 20 mm). The number of samples and the statistical analyses behind this analysis are solid and prove how flexible to flexible interconnects can be sufficiently reliable for consumer electronics applications. In spite of such promising results, however, the interconnections remain a significant point of failure.

for simplicity of the process but in order to obtain an antenna with larger read range, screen

Therefore, the manufacturing choice will depend on the restrictions of each application, in terms of printing technology availability, performance, area and materials and processes

Integrating the different deposition techniques on one single substrate constituted the final step towards the obtainment of fully printed and hybrid circuits, with tuned capabilities and functionalities. Flexible tags, as the one presented earlier, however, are characterized by an inherently weaker link: the connection between the integrated circuits and the printed devices. Research in solving the interconnection issues, or eliminating them by realizing inherently

In every application, in which it is not possible to construct an entirely flexible circuit, the necessity to solder (or, more correctly, to "attach") monolithic integrated circuits to the printed tag arises. The immediate problematics to be faced are mainly three: attaching the component with no harm to the flexible substrate, achieving a low contact resistance and retaining the flexibility of the overall circuit, at no performance loss. The most straightforward approach for the integration of ICs onto flexible circuits, is the adaptation of industrial soldering processes, with the employment of low-temperature soldering alloys. In a systematic and well-presented work by Andersson et al. [39], a series of SMD components with different packaging was soldered keeping the reflowing temperature below 150°C. The study showed mixed results: on the one hand, it proved the feasibility of soldering on paper with standard industrial devices, obtaining contact resistances in the order of few ohms; on the other hand, it was partially inconclusive, as it showed the inevitable presence of cracks after soldering and bending (shown in **Figure 12a**). Furthermore, the soldering yield was rather low (ca. 80% for the smallest packaging, much lower for bigger components), which might be a serious issue in commercial applications. An apparently more complicated—although much more appealing and promising—solution, stems out from treating flexible electronic chips for what they are: non-classical electronic systems, with the need of dedicated solutions. In this avenue, a very interesting work from Quintero et al. compares two technologies with a similar common denominator: the employment of conducting adhesive layers [40]. In particular, the present a selective etching of an isotropic conductive adhesive and the stencil-printing of Anisotropic Conductive Adhesives (ACAs), applied to the bonding of two separate flexible chips. The latter approach, extensively studied and applied in many material combinations, is the one that, so far, has been attracting the highest interest. ACA-based interconnects are obtained in a simple three-step procedure: first, the adhesive conducting film is deposited through a stencil, then the target component is placed on top of it, finally, the temperature of the substrate is raised to the bonding temperature (usually lower than 150°C). When the resin is heated up, it creates an electrical bond between carrier substrate and component, and creates a mechanically stable link. Nilsson et al. [41], present a remarkable application of printed circuits with ACA

with a

printing would be the optimal process **Figure 11**.

flexible circuits, has seen a tremendous surge in the last decade.

**3. Towards on-chip integration**

compatibility.

104 Flexible Electronics

An altogether different approach, however, could reduce the problematics of interconnection technology to their minimum terms. The newest research in flexible electronics, in fact, shows how it is possible to develop complex and fully functional circuits, with the employment of metal-oxide n-type and carbon based p-type semiconductors. Complete circuital systems, which are inherently flexible, would restrict the need for interconnects to very few and controllable points. These points can be designed to be in positions subject to minimal mechanical stress, hence reducing the probability of failure. Most of the effort in this direction has been put in the realization of only n-type semiconducting circuits, given the superior stability of metal oxides with respect to carbon based materials [8]. Although limited by the high power consumption of unipolar circuits, these studies show the avenue to follow to reach flexible and integrated electronics with the minimal interconnection technology. A remarkable work in this context is the one presented by Hung et al. [43], where an ultra-low power RFID tag is developed on plastic foil. All the components of the tag, including logic gates, decoders,

**Figure 12.** (a) Cracks left on the paper coating, on the printed lines and at the soldering, indicated by arrows, for a 0805 packaging component (b) similar issues for QFP and SOP packages, showing that the problematic is insensitive of packaging type and, up to a certain extent, size insensitive. Image adapted from [39] with authorization.

logics. Significant breakthroughs are, nevertheless, achieved with a steady pace. Petti and coworkers recently reported a flexible full-CMOS amplifier, with a gain bandwidth product of 60 kHz, realized with sputtered IGZO and spray-deposited CNTs as n-type and p-type semiconductors, respectively [47]. The structure and characteristics of these devices are presented in **Figure 14**. The resulting amplifier is stable in ambient conditions, completely flexible and easy to integrate in other circuits, and it also shows the remarkable adaptability of solution processing (in this particular case, spray-deposition) to different substrates and pre-existent circuits. Albeit in this work the n-type material is sputtered, the specialized literature is abundant with reports of solution-processed IGZO transistors [48, 49], which could be readily

Technological Integration in Printed Electronics http://dx.doi.org/10.5772/intechopen.76520 107

The achievement of a complete set of electronic blocks—such as flexible oscillators, controllers, diving circuits, and amplifiers—is certainly a significant milestone towards a future of flexible electronics. However, considering their performances, it is also evident how printed and oxide-based circuits do not aim at substituting traditional silicon-based electronics for computationally intense tasks. What appears certain, though, is that with a careful and considerate interconnection of classical ICs and energy harvesters, with the exploitation of the advantages of each solution-processing technique, and with a systematic understanding of

the underlying processes, flexible circuits will soon be able to permeate our lives.

employed for the realization of fully printed circuits.

This work was partially funded by TUM Graduate School.

\*, Florin C. Loghin1

1 Institute for Nanoelectronics, Technical University of Munich, Munich Germany

2 Faculty of Science and Technology, Free University of Bolzen-Bolzano, Bolzen Italy

dating of Paleolithic art in 11 caves in Spain. Science. 2012;**336**:1409-1413

[1] Pike AW, Hoffmann DL, García-Diez M, Pettitt PB, Alcolea J, De Balbin R, et al. U-series

\*Address all correspondence to: almudena.rivadeneyra@tum.de

and Aniello Falco<sup>2</sup>

**Acknowledgements**

**Conflict of interest**

**Author details**

**References**

Almudena Rivadeneyra<sup>1</sup>

Authors declare no conflict of interest.

**Figure 13.** (a) A complete tag with RFID chip, antenna and flexible battery for a complete sensing kit (b) realization of an anti-intrusion system with sensing elements integrated in the sealing tape (c) photo of chip connected to printed line with an ACA. Figure adapted from [41] with authorization.

**Figure 14.** (a) Cross-section of the CMOS amplifier, with the different layers emphasized in different colors (b) photograph of a flexible substrate with many devices (c) voltage gain as a function of bias voltages, measured in ambient air (d) bode plot for different bias voltages, showing how the gain can be gate tuned. Figure adapted from [47] with authorization.

memory and clock generator were fabricated employing amorphous-IGZO FETs and were functioning at 1 V. Such systems, does not need any external driving element, and, thus, no further interconnection is necessary. In a similar direction, Myny et al. presented a flexible NFC barcode tag, with direct clock division circuit, which is compliant with ISO14443-A [44].

To further extend the spectrum of possibilities of printed, interconnectionless electronics, the group led by Prof. Jan Genoe, for instance, has demonstrated the feasibility of flexible control, driver and conversion electronics for photovoltaics-powered micro LCD screens, suggestively integrated onto a contact lens [45]. In this article they demonstrate the great potential of oxidebased electronics, fabricating a system which would have needed a high number of interconnects, and it would have not been realizable without this enabling technology. Finally, although flexible sensors have been often presented in many works, their biggest limitation was the necessity to connect them to external amplifiers and read-out electronics. Recent efforts in literature have shown how stable and reliable amplifier can be obtained employing a-IGZO FETs [46], although they still present the classic limitation of unimodal pseudo-CMOS logics. Significant breakthroughs are, nevertheless, achieved with a steady pace. Petti and coworkers recently reported a flexible full-CMOS amplifier, with a gain bandwidth product of 60 kHz, realized with sputtered IGZO and spray-deposited CNTs as n-type and p-type semiconductors, respectively [47]. The structure and characteristics of these devices are presented in **Figure 14**. The resulting amplifier is stable in ambient conditions, completely flexible and easy to integrate in other circuits, and it also shows the remarkable adaptability of solution processing (in this particular case, spray-deposition) to different substrates and pre-existent circuits. Albeit in this work the n-type material is sputtered, the specialized literature is abundant with reports of solution-processed IGZO transistors [48, 49], which could be readily employed for the realization of fully printed circuits.

The achievement of a complete set of electronic blocks—such as flexible oscillators, controllers, diving circuits, and amplifiers—is certainly a significant milestone towards a future of flexible electronics. However, considering their performances, it is also evident how printed and oxide-based circuits do not aim at substituting traditional silicon-based electronics for computationally intense tasks. What appears certain, though, is that with a careful and considerate interconnection of classical ICs and energy harvesters, with the exploitation of the advantages of each solution-processing technique, and with a systematic understanding of the underlying processes, flexible circuits will soon be able to permeate our lives.
