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PhD thesis, Chalmers Univeristy of Technology, 2017.
The constant miniaturization and steady performance improvement of electronics devices have generated innovative ideas such as internet of thing (IoT), which also includes devices with integrated energy sources.
The high performance is conceived by the high density of the devices on a chip leading to a high density of interconnects, to connect these devices to outside world. Since the size and the pitch of the interconnects have decreased, the current density in interconnect has increased, posing challenges on the existing copper pillar interconnect technology, such as intermetallic compound formation and electro-migration resulting in open circuit. The challenges are forecasted to increase on further down scaling due to bridging of the solder between pillars. Moreover, the environmental pollution and the threat of vanishing of fossil fuel have prompted to find cheap and efficient alternating energy sources and energy storage systems.
Carbon nanomaterials such as carbon nanotubes and carbon nanofibers have unprecedented electrical, mechanical and thermal properties, high resistance to corrosion and high surface area have been proposed for the solution of above mentioned challenges.
In this thesis, vertically aligned carbon nanofibers (VACNFs) have been grown by direct current plasma enhanced chemical vapor deposition (dc-PECVD) at complementary metal oxide semiconductor (CMOS) compatible temperatures for on chip application. In addition, the catalyst to grow VACNFs is deposited using innovative low-cost polymer–Pd nanohybrid colloidal solutions by an effective coating method.
Also, due to controllable DC behavior and good mechanical reinforcement properties of solder-CNFs, the solderable micro-bumps of VACNFs have been shown to potentially yield the acceptable electrical resistances. Moreover the CNFs bumps can be made in submicron size range, which can comply with further down scaling of interconnect. In addition, advanced CNF based adhesives, produced by coating CNFs with low temperature polymers, have been investigated as alternating anisotropic conducting film for anisotropic connection, using a thermo-compression bonding. The shearing strength of the bonded chip qualifies the MIL-STD-883 standards of bonding strength in microelectronics devices.
Further, supercapacitor are the energy storage devices having high energy density, and high power density due to quick intake and release of charges and long cycles life of about 1 million. On-chip integrated solid-state parallel-plate capacitor and supercapacitor are demonstrated based on VACNFs. The preliminary capacitance of the parallel-plate capacitor and supercapacitor are 10–15 nF/mm2 and 10 μF/mm2, respectively. The profile of parallel plate capacitor is below 10 micrometers, which enables integration even in advance 2.5 and 3D heterogeneous packaging. The on-chip capacitor can work as decoupling capacitor to resolve the energy fluctuation related issues and also power up devices on the chip.
Then, along with other properties, high aspect ratio and ease of fabrication, the carbon nanotubes (CNTs) are considered as potential electrode material for future high performance supercapacitor. The CNTs are directly grown on electrospun CNFs giving the specific capacitance of 92 F/g, i.e. twice the capacitance of bare CNFs. Finally, a complete energy storage device coin-cell supercapacitor is made by directly growing VACNFs on the current collector and the capacitance is 15 times higher than the capacitance without CNFs. Thus such supercapcaitor is suitable to be combined with harvester to collect energy to the level of operating power of the devices and can provide durable solution to the frequent change of battery in the devices mounted at sensitive or airborne locations.