In-situ surface wear processes of gold coated carbon nanotube MEMS electrical contacts

Abstract

This thesis describes the development, and use, of an apparatus to investigate lifetime switching wear of micro electromechanical system (MEMS) switch contacts. The investigation is focused on a composite contact material that is composed of vertically aligned multiwalled carbon nanotubes (MWCNT) with a conductive gold film coating to form an Au/MWCNT composite. A testing system, termed the In-Situ Contact Evolution (ICE) apparatus, is developed to provide the first system capable of in-situ, nanometre scale measurement of the MEMS contact surface, during sustained high-speed, low force MEMS contact cycling required for lifetime MEMS switch contact testing. The apparatus includes instrumentation that detects the formation of the transient molten metal bridge (MMB) phenomenon known to cause contact wear when hot switching. The apparatus is tested during an investigation of thin-film Au contacts under both cold and hot switching conditions. Microscopic wear from individual switching cycles is detected and a new wear mechanism is observed where a microscale delamination of the Au film interrupts the formation of a MMB. The Au/MWCNT MEMS contact material is investigated switching below the arcing voltage (~12 V). The upper limit for switching is found and the failure mechanism characterised. Above circuit currents of 302 mA the Au surface film is thermally ruptured and as the contacts open the exposed carbon nanotubes become involved in circuit conduction. The circuit current causes thermal decomposition of the nanotubes observed as a series of discrete steps in contact resistance linked to the consecutive thermal failure of the outer walls of the conducting nanotube. The influence of circuit parameters (current and voltage) and contact texture on the energy of the MMB is characterised. In an important observation the energy of the MMB is found to have a linear relationship to circuit current, rather than the i2 relationship previously reported. The delamination event (DE) is also detected switching the Au/MWCNT contact. The DE and MMB are found to be mechanically distinct, with the DE characterised by ~20% increased separation force. Circuit conditions are shown to influence the MMB energy and number of DE. The lowest voltage and current show the highest occurrence of the DE and the lowest surface wear. Lifetime testing is carried out at low force (150 µN) under hot and cold switching conditions. Under cold switching conditions the contact is switched to 4 x109 (4 billion) cycles while the microscale wear caused by DEs is detected and quantified. Under hot switching conditions of 200 mW DC power the contact fails after 140 x106 cycles switching, by an excessive increase in contact resistance. The composites Au upper surface is found to have transferred to the opposing contact by a mixture of MMB and DEs. The results are used to develop an enhanced model for the prediction of Au/MWCNT contact lifetime which considers the influence of contact force, DE wear and the switching current. The model shows good correlation with both the results of this work and previous investigation.

More information

Identifiers

ORCID for John Mcbride: orcid.org/0000-0002-3024-0326

Catalogue record

Export record