Materials, processes and ink formulation towards viable, manufacturable passive- and active-printed electronics components

Printable electronics is a disruptive technology when compared to conventional electronics manufacturing. It allows high throughput manufacturing, rapid prototyping, and can utilise thin, flexible substrates. As an additive manufacturing method, it also presents cost reduction possibilities to mass manufacturing and, in some contexts, potentially lessen environmental impact when compared to existing conventional methods. Printing technologies vary significantly in deposition method, rate, and resolution thus offering multiple options to manufacturers in application design and manufacture. Currently, the available market and usage of conductive inks is dominated by carbon and silver for conductive tracks and electrodes which limits both the choices available to product designers, and the electronic components possible. Opportunities exist, therefore through interdisciplinary fields spanning materials chemistry, nanoparticle physics, ink formulation and complex mixtures, printing techniques, thin film electronics, traditional electronics characterisation and design, to progress and facilitate a wider adoption of printing technologies through materials understanding, processing and fabrication towards viable printed device and systems manufacture.

In this thesis, active and passive electronic components are explored through the various stages of materials processing, ink formulation, and component design. Printed active components have been less-widely studied with fewer available ink materials and so were approached from the materials processing and ink formulation perspective. Printed passive components, more studied and established, were therefore approached in terms of component design, targeted improvements and identified gaps in performance. Five research objectives were constructed: identification of target active and passive components and materials, the development of materials processing methods, formulation and optimisation of inks, exploration of component design in passive components, and the fabrication of printed devices to be tested and characterised.

Following an extensive literature review, silicon was selected as the target material for active component development due to its prominence in conventional electronics and current absence from printed electronics literature. The relatively poor performance of currently used printable semiconductors which favour organic materials was found to present considerable opportunity for fundamental work with inorganic conventional semiconductors. A process was developed to prepare silicon powders of known particle size with minimal contamination from doped silicon wafers. Further, a treatment process involving hydrofluoric acid was developed in order to remove the insulating native oxide layer that grows on every silicon particle. Nitrocellulose was identified as a favourable binder material for ink formulation, and a suitable vehicle was formulated. Design of experiments software was employed to carry out a series of ink formulation studies, starting from stress testing to screening, then finally an optimisation. A finalised silicon ink formulation presented reasonable conductivity (19.5kΩcm) relative to the wafer starting material (5.8Ωcm) following low temperature curing. The finalised silicon ink formulation was used to demonstrate semiconductor properties through the fabrication of Schottky diodes which presented appreciable rectification (up to 141x).

Carbon resistors were identified as the target passive component for study with an aim of improving manufactured tolerance, reducing thermally-induced resistance variation on thin plastic substrates and thus increasing power dissipation stability and capability. A series of carbon resistor designs were studied, additional printed layers of zinc oxide (ZnO) and silver (Ag) were introduced. The incorporation of additional layers was shown to notably improve the tolerance compared to carbon resistors without additional layers. The incorporation of the additional layers also improved peak power dissipation before deviation from a linear current-voltage relationship. Two ZnO layers and one Ag improved peak power by 52.1% relative to the basic carbon resistor design. Finally, in sustained power dissipation tests, the incorporation of additional layers enabled the resistors to dissipate 500mW of power for 100 hours, whilst the basic carbon resistor catastrophically failed within 24 hours.

In conclusion, this thesis provides the first presentation of formulated silicon inks and their use in semiconductor devices as well as the first documentation of printed thermal variance mitigation materials to improve tolerance, stability and power capabilities of manufacturer printed components (resistors) on thin plastic substrates.