Inkjet and screen printing for electronic applications

Abstract

Printed electronics (PE) is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography, and inkjet. Electrically functional, electronic or optical inks are deposited on the substrate, creating active or passive devices.

PE offers a great advantage when compared to traditional processes or microelectronics due to its versatility, low manufacturing cost and the possibility of generating flexible circuit componentsi. Furthermore, these techniques are suitable for roll-to-roll processes and open the possibility for printing large areas and in a large-scale production. .

The selection of the printing technique is crucial to achieve a good result and it will largely depend on both, the material needed and dimensional and functional requirements pursued. Each technology offers different possibilities in terms of resolution, complexity, versatility, speed, layers thickness, materials, reliability and scalability. This work aims to dig deeper into two of the main techniques in the world of printed electronics: screen printing and inkjet printing for different applications and for the manufacturing of different devices. In addition, the capabilities of a technology that is currently in growing development (inkjet) are analyzed in comparison with the mature screen printing technique to give a wider insight of the advantages and limitations that this technology offers. The totally knowledge in this technique is still in progress and it arises to be a trend in technological and scientific aspects due to the barely availability of functional materialsii and the difficulties in achieving a precise control on the drop formation and its interaction in the final system. A better understanding of these technological issues, as well as the approaching to current difficulties in electronic applications is accomplished in this thesis. Up-to-date issues as the reliability of flexible resistive gas sensors, solution-based synthesis of absorber layers in thin film solar cells and the tuning and area reducing for inductors in RF applications are tackled.

The main objective in this thesis arises from the need of expanding the knowledge on inkjet printing by exploring affordable new possibilities, taking as starting point the previous knowledge of screen printing. To pursue this goal, the comparison between SP and IP is presented along the thesis. The framework of this thesis is not solely an overview of the

development of functional materials for both techniques, but also the investigation of its final implementation reliability in several devices for different electronic applications.

The structure of this thesis dissertation can be divided in two well defined blocks. In chapter 2 and 3, both printing techniques are explained in detail, while in Chapter 4, 5 and 6, the potential of both technologies are studied for different electronic applications by means of the fabrication and characterization of different devices.

In chapter 2, the main topic is the analysis of the inkjet printing technique. This chapter follows the attainment of two different objectives: the establishment of a quality evaluation guideline for any inkjet ink and as example of it, the formulation of our own silver ink developed in our laboratory. In this sense, the most important properties for the functional materials which should be under control during its formulation are reviewed; as well as the fundamentals and main parameters during the printing process, which affect the outcome quality. Instead, in chapter 3, the fundamentals of screen printing technique are quickly overviewed due to the consolidation of the technique knowledge and the previous studies on it done in the field, and specifically at FAE Company.

In the second block of this thesis, the description of the printing techniques leads to the implementation of both, inkjet and screen printing, in different electronic fields. The chapters involving this block are focused in the printing step during its fabrication, the printing and functional material quality characterization, and the influence of this printing step in the functional performance of the devices.

Chapter 4 is a comparison between low-cost flexible resistive sensor platforms with heater fabricated by both, SP and IP techniques. The performance of these sensor platforms was checked by long-term characterization and aging tests to identify the causes of the device failure. Chemical degradation of silver is observed in SP-devices due to the flake-like morphology of the deposits but not in the smooth sintered silver tracks deposited by IP. However, the IP very thin film promotes failure by hot spot phenomena. Design improvements are, hence, implemented to overcome the drawbacks of silver corrosion and power consumption. The final devices turned to be sturdy, wearable and reliable gas sensor platforms.

In Chapter 5, IP is implemented in the step of the absorber layer synthesis for the fabrication of kesterite thin film solar cells. Copper-Zinc-Tin-Sulfur (CZTS) precursor ink is formulated and optimized for the enhancement of the solar cell performance. The influence of the formulation and the printing process is analyzed. Finally, the thickness of the deposited precursor was

modified until obtained a cell with 6.55% efficiency, the higher efficiency reported with this absorber type using IP as deposition method.

In the last chapter, Chapter 6, spiral inductors are fabricating using the two printing technologies in LTCC (low-temperature-cofired-ceramics). IP, although turning out to be a suitable technology for enhancing the accuracy of narrower tracks than SP and thus, for increasing the number of turns within a concrete area, presents difficulties to achieve a certain value in electrical conductivity due to the deposition of very thin conductor layers. For this reason, in this part of the thesis, a combination of IP with electroless copper deposition is used to overcome this limitation and to develop equivalent performances using SP and IP devices.

La electrónica impresa permite la impresión de dispositivos electrónicos ofreciendo una gran ventaja en comparación con procesos tradicionales o microelectrónica debido a su versatilidad, bajo coste de producción y posibilidad de generar circuitos flexibles.

La selección del método de impresión es crucial a la hora de alcanzar un buen resultado y depende de los materiales necesarios y de los requerimientos dimensionales y funcionales. En esta tesis, la serigrafía, una técnica de impresión fiable y consolidada en la industria desde hace años, es comparada con la inyección de tinta (inkjet), que aún muestra un gran desafío en cuanto a rendimiento y reproducibilidad. Cada tecnología ofrece posibilidades diferentes en complejidad, resolución, grosor de capas y materiales.

En la primera parte de la tesis se describen la inyección de tinta y la serigrafía en términos de fundamentos, parámetros y formulación de materiales. En la segunda parte, el potencial de ambas tecnologías se ha estudiado en diferentes escenarios mediante la fabricación de diversos dispositivos electrónicos. En el estudio de la fiabilidad y robustez de plataformas sensoras flexibles se ha encontrado una relación directa entre la morfología de la plata depositada y su causa de fallo en funcionamiento prolongado. La sinterización de las nanoparticulas depositadas por inkjet forma una capa lisa y con poca porosidad que evita parcialmente la corrosión, a diferencia de la pasta de plata impresa por serigrafía. Sin embargo, su bajo provoca defectos puntuales que puede causar puntos calientes. También, inkjet se ha empleado para la síntesis de precursores de la capa absorbente para celdas solares de capa fina. Se ha formulado una tinta de precursores de cobre, zinc, estaño y azufre (CZTS) para la formación de kesterita obteniéndose celdas de 6.55% de eficiencia, siendo la más alta reportada hasta la fecha utilizando este tipo de absorbente y tecnología. Sin embargo, en aplicaciones donde la conductividad es crucial para altas prestaciones, como en radiofrecuencia, queda patente la desventaja del inkjet sobre la serigrafía, donde su escaso grosor de capa es un claro hándicap para la obtención de conductividades elevadas. Dicho factor limitante es abordado con la combinación de la inyección de tinta con la deposición química (electroless) de níquel y cobre, consiguiéndose inductores equivalentes a los serigrafiados.