Self-Healing of Open Interconnect Faults for Reliable Large Area Electronic Systems

Open circuit faults in interconnects can occur due to various reasons such as unexpected current spikes (e.g. during electrostatic discharge events), thermal stress, applied mechanical forces and fabrication faults. In the case of large area electronic systems (such as wearable electronics, active matrix systems on rigid and flexible substrates such as displays and image sensors) the occurrence of an open fault could make the system unusable. It is therefore of interest to improve the operational reliability of these systems.

To address this problem several passive and active approaches have been studied. Passive techniques such as adding redundancies, improving interconnect geometries and materials (e.g. meandering interconnects, stretchable interconnects etc.) make the electronic systems more fault tolerant. Active techniques imply the use of self healing mechanisms to completely restore electrical conductivity after the occurrence of the fault. One approach considers the encapsulation of conductive inks in dielectric shells (10.1002/adfm.201000159) embedded in the interconnect during fabrication. Upon the occurrence of an open circuit fault, the fracture in the interconnect also fractures the shells present in the local vicinity of the fault thereby spilling the conductive ink and immediately restoring conductivity. While this method provides a true healing of the fault, a downside is that the interconnect fabrication process flow must be modified to embed the shells.

We propose an alternate means to actively self heal open circuit faults using a dispersion of conductive particles in an insulating fluid. This dispersion is contained and isolated over each interconnect. When a current carrying interconnect experiences an open circuit fault, an electric field appears across the open gap. The field polarizes the conductive particles in the dispersion allowing some of them to chain up due to dipole-dipole attractive forces and create a bridge across the gap thereby healing the fault. The advantages of this approach is that the implementation does not disturb the existing techniques of fabrication of interconnects. The mechanism can be incorporated as an add-on feature.

A proof of concept of self healing using dispersion was demonstrated for printed circuits board (http://aip.scitation.org/doi/10.1063/1.4916513) but not for the much smaller length scales of integrated circuits using in large area electronic systems. The main goals of this proposal are (i) study and model the effectiveness of the self healing as we scale down particle size i.e. how quickly can we heal? how much current can the heal carry? (ii) to implement and demonstrate self healing of open faults at the much smaller length scales of interest on rigid and flexible substrates. (iii) Study the impact of the bending of a flexible substrate on the healing (iv) Implement self healing on integrated circuits used in large area electronic systems (thin film transistors on a rigid substrate) and study the performance of the circuit pre and post heal.

Apart from being of academic interest, this research would great help in developing modern manufacturing techniques that result in electronic circuits that last much longer. A wide variety of faults occurring in a wide variety of systems would benefit from this study eg. opens at solder joints in printed circuit boards, cracks in capacitive touch screen displays, open faults in photo-voltaic cell arrays, faults in foldable displays and wearable electronics etc. Certain critical systems such as health care devices, electronics used in satellites (where thermal stress is significant), electronics used to monitor dangerous or hard to reach environments etc. would operate with improved reliability. The research also has a direct impact on addressing social and environmental problems such e-wastes.

This proposal permits advances in several strategic areas of significance to the UK economy and to maintaining UK as a leader in technology and innovation.
(i) Technology Impact: The answers to the questions raised in this proposal are the key to implementing the self-healing mechanism in integrated electronic circuits and systems, in particular - wearable electronics and large area electronic systems. However, the ideas could find an eventual implementation in any class of electronic systems (such as integrated circuit chips, printed circuit boards, etc.). For example,
(a) The use of electronic systems with the ability to repair themselves has a direct impact on space technology (http://www.ibtimes.co.uk/nasa-teams-stephen-hawking-help-prepare-tiny-spacecraft-epic-20-year-journey-1596130) and electronic system located in remote/hazardous environments (low maintenance required). The UK currently has a rapidly growing industry in satellite technology for space applications (https://www.gov.uk/government/publications/uk-space-industry-size-and-health-report-2016). The work from this proposal adds to the basket of technologies for the reliability of these systems
(b) The UK has put significant effort towards the development of large area electronics, flexible electronics and wearable electronics with the EPSRC Centre for Innovation and Manufacturing in Large Area Electronics (EPSRC CIMLAE) dedicated to this cause along with successful spinoffs such as Cambridge Display Technologies, Plastic Logic, FlexEnable etc. The answers to the questions raised by this proposal permit the development of reliable large area electronics, wearable electronics and flexible electronic systems with self repair capabilities. The EPSRC CIMLAE has agreed to provide a pathway to impact that allows me to directly engage with the industry (letter of support attached).

(ii) Strategic and Economic Impact: Large area electronic systems (including flexible and wearable electronics) are still being invented. All areas from manufacturing to applications have a lot of room for new ideas. Since the proposal targets a relatively new area of research (self-healing of open circuit faults) and applies it to a rapidly growing class of electronic systems (flexible, wearables and large area electronics) it forms a rich ground for generating intellectual property. It also promises new manufacturing methods. This would have a ripple effect on jobs and economic growth.

(iii) Direct Social Impact: To gauge the potential impact of this proposal, I ran a user survey through a program called the i-team (https://iteamsonline.org/exploring-the-potential-of-self-healing-circuits-to-address-e-waste-in-the-developing-world/). The team sought to identify (by contacting potential users) the key pain points in technologies and society, and to seek out the pathways to generate maximum impact with this technology. This proposal permits advances in several strategic areas of significance to the UK economy and to maintaining UK as a leader in technology and innovation. Apart from the technology impact, a direct social impact was identified in the area of e-wastes. The add-on feature of this technology helps refurbish many mildly damaged electronic devices (eg. display touch screens) and permits recycling. Moreover, if the technology matures to eventual deployment in electronic systems, it would significantly increase their lifetime thereby reducing e-waste.