Excellent opportunity here for a fully funded 3.5 year PhD: Lifetime prediction of implanted electronics operating at increased relative humidity.
Lifetime prediction of implanted electronics operating at increased relative humidity – Kings College London.
Aim of the project:
Active implants, like cochlear implants, must guarantee decades of safe operation in the body, surrounded by fluid. Without protection, electronics exposed to such conditions would rapidly corrode. Most implants, irrespective of the clinical application, achieve long-term reliability by sealing the electronics inside a hermetic enclosure, to prevent the ingress of moisture.
However, no enclosure is perfectly hermetic, water vapor does penetrate at a very slow rate. With technological advances and the miniaturisation of electronics (future implants will have free internal volumes < 1 mm3), even this slow rate is becoming a challenge. Operating at elevated relative humidity introduces new failure mechanisms, we need new understandings, to develop a new method to predict microimplant lifetime [Vanhoestenberghe 2013].
We believe that it is possible to create micro-implantable-devices that operate safely for the required lifetimes despite the internal relative humidity being elevated, and the aim of this project is to design experiments to rigorously evaluate this claim. The outcome will be a new understanding of the lifetime of such microdevices, enabling us to deliver a range of new clinical applications.
This project is a study of the reliability, and failure mechanisms, of electronics in biomedical applications, specifically electronics packaged in hermetic or semi-hermetic enclosures. This is a technology development project, at this stage no specific clinical application is targeted. Our aim is to ensure the safety and long-term reliability of the next generation of active implantable micro-devices, for applications such as brain computer interfaces, wearable sensors, spinal cord stimulators. This very timely work will support the rapidly developing field of bioelectronics medicine.
Our focus is on the failure of ICs over time as a function of environmental conditions typically found in implants and wearable devices. There will be several competing failure mechanisms to study, we expect in particular to see both wirebond failure and corrosion of the integrated circuits (IC) [Gan 2014]. One challenge will be to conceive an experimental protocol that can discriminate between these failure mechanisms to evaluate their relative contribution to the failure rate. A second challenge is the need to accelerate the failure rate, since implants should operate safely for decades, yet it would be impractical to run an experiment for such a long period. Instead, the environmental stresses (temperature and relative humidity) are increased, to accelerate the failure rate and observe failures within months [Hallberg 1991].
The candidate will design an innovative experimental protocol and build the associated equipment, to control the accelerated aging environment and automatically collect data on the failure rate of electronics. We are interested in the effect of the residual ionic contaminants that remain after the final cleaning steps during assembly. The student will validate their equipment and prepare the samples in year 1 (Y1), run the experiment (Y2) and analyse the data to acquire a new understanding of the time to failure, and acceleration, based on a comparison with measurements taken on day 0 (Y3).
As with every novel equipment, the final ageing protocol is unknown at this stage, it will be designed by the candidate. We anticipate that for IC corrosion, the samples will be CMOS ICs with InterDigitated Electrodes (IDE) on the top metal layer. Monitoring the impedance between the electrodes gives information about the corrosion of the IC, a method we and others have used successfully in previous work [Vanhoestenberghe 2013, Lamont 2021]. Aged wirebond reliability will be evaluated using a different test sample design, that will be developed by the student. After a period under test, the samples will be characterised by complementary methods, such as SEM, Focused Ion Beam imaging including Transmission Electron Microscopy and Time of Flight Secondary Ion Mass Spectrometry as appropriate. Some of these methods may be available in house, other characterisation will be performed collaboratively with research partners, such as the Fraunhofer Institute for Reliability and Microintegration IZM in Berlin.
This project is multi-disciplinary, and therefore we are open to candidates with a broad range of backgrounds, whether in electronics, electrochemistry, biomedical engineering or material sciences. What we look for is commitment to rigorous scientific enquiry, and a desire to conduct research that can make a difference in people’s life.