Abstract
The drive for miniaturisation and personalisation of electronic devices demand challenging manufacturing methods with greater performing new materials. Carbon nanotubes (CNTs) have proven to possess electronic and mechanical properties that are critically beneficial to be utilised in a wide range of electronic applications. The ability to design directionally-aligned, high aspect ratio, type and geometrically selective CNT-based hybrid structures greatly match the complex and demanding needs of electronic and electromechanical systems. This work explores a hierarchical materials design approach, based on hybrid, multi-functional, aligned-CNT structures to improve the performance of microelectromechanical architectures and to minimise the limitations and complexity of conventional metal and/or semiconductor-based systems. First, this thesis demonstrates on-chip fabrication of arrays of vertically-aligned multiwall carbon nanotubes (MWCNTs) deposited via photo-thermal chemical vapour deposition (PT-CVD) at temperatures that are fully compatible with the integrated circuit processes. CNTs can have the ability to act as compliant small-scale springs or as shock resistance micro-contactors. This work investigates the performance of vertically-aligned CNTs (VA-CNTs) as micro-contactors in electromechanical testing applications for testing at wafer-level chip-scale-packaging (WLCSP) and wafer-level-packaging (WLP). Fabricated on ohmic substrates, 500-µm-tall CNT-metal composite contact structures are electromechanically characterised. The probe design and architecture are scalable, allowing for the assembly of thousands of probes in short manufacturing times, with easy pitch control. The effect of the metallisation morphology and thickness on the compliance and electromechanical response of the metal-CNT composite contacts is discussed. Pd-metallised CNT contactors show up to 25 μm of compliance, with contact resistance as low as 460 mΩ (3.6 kΩ/µm) and network resistivity of 1.8 x 10-5 Ω cm, tested up to 25000 touchdowns, with 50 μm of over-travel, displaying reproducible and repeatable contacts with less than 5% contact resistance degradation. Failure mechanisms are studied in-situ and after cyclic testing show that, for the top cap and sides metalised contacts, the CNT-metal shell provides stiffness to the probe structure in the elastic region, whilst reducing the contact resistance. It is demonstrated here that the stable, low resistance achieved combined with the high repeatability and endurance of the manufactured probes make hybrid CNT micro-contacts a viable candidate for small pitch (< 50 μm) electromechanical probing applications. Whilst this research project initially focused on the fabrication and the characterisation of CNT micro-contact, it also explored CNT-based flexible and wearable strain sensors for human motion detection. Recent interest in the fields of human motion monitoring, electronic skin and human-machine interface technology demand strain sensors with high stretchability/compressibility (ε > 50%), high sensitivity (or gauge factor (GF > 100) and long-lasting electromechanical compliance. However, current metal and semiconductor-based strain sensors have very low (ε < 5%) stretchability or low sensitivity (GF < 2), typically sacrificing the stretchability for high-sensitivity. Composite elastomer sensors are a solution where the challenge is to improve the sensitivity to GF > 100. In this thesis, a simple, low-cost fabrication of mechanically compliant, physically robust hybrid CNT/polydimethylsiloxane (PDMS) strain sensors is proposed. The process allows the alignment of CNTs within the PDMS elastomer, permitting directional sensing. Aligning CNTs horizontally (HA-CNTs) on the substrate before embedding in the PDMS reduces the number of CNT junctions and introduces scale-like features on the CNT film perpendicular to the tensile strain direction, resulting in improved sensitivity compared to VA-CNT-PDMS strain sensors under tension. The CNT alignment and the scale-like features modulate the electron conduction pathway, affecting the electrical sensitivity. Resulting GFs are 594 at 15 % and 65 at 50 % strains for HA-CNT-PDMS and 326 at 25 % and 52 at 50 % strains for VA-CNT-PDMS sensors. Under compression, VA-CNT-PDMS show more sensitivity to small-scale deformation than HA-CNT-PDMS due to the CNT orientation and the continuous morphology of the film, demonstrating that the sensing ability can be improved by aligning the CNTs in certain directions. Furthermore, mechanical robustness and electromechanical durability are tested for over 6000 cycles to up to 50 % tensile and compressive strains, with good frequency response with negligible hysteresis. Finally, both types of sensors are shown to detect small-scale human motions, successfully distinguishing various human motions with reaction and recovery times of as low as 130 ms and 0.5 s respectively.