Engineering Science - Doctoral Theses

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Nitride materials for use in downscaled interconnect technology
(University College Cork, 2022) Nies, Cara-Lena; Nolan, Michael; Science Foundation Ireland
Copper interconnects will struggle to keep up with the advances in transistor miniaturisation. This creates a significant limiting factor in trying to keep Moore’s Law on track into the 2020’s. At smaller dimensions, Cu prefers to form non-conducting Cu islands. To prevent this, a liner material that promotes 2D conducting Cu needs to be deposited in the interconnect via in addition to the diffusion barrier that prevents the migration of Cu into the surrounding dielectric. However, the lowest, and smallest, levels of interconnects have extremely high aspect ratios and the bilayer of diffusion barrier and liner material takes up too much volume to allow for the deposition of a sufficient amount of Cu. A change to an alternative metal could solve some of these issues, but this brings its own challenges. Additionally, Cu is expected to remain in use at global interconnect level. For these reasons it is essential to optimise the performance of Cu interconnects and extend the use of Cu as the interconnect metal for as long as possible. One approach to this is to replace the separate diffusion barrier and liner bilayer for a single material that exhibits both properties and can be deposited in an ultra-thin layer, e.g. using atomic layer deposition. In this thesis I explore, using first principles simulations, two different approaches to developing such a combined barrier+liner material. One is to study how known liner and barrier materials could be combined through doping and passivation to create a new material that delivers both properties. Our model system is TaN (diffusion barrier) modified with Ru, Co and W. Secondly, an intrinsically ultra-thin 2D material that has both barrier and liner properties could be used to replace the barrier/liner bilayer. MoS2 has diffusion barrier properties but its liner properties have not been studied and is thus used as a model system for this approach. The literature on studying barrier and liner materials using theoretical methods is quite limited. Therefore, this thesis also presents an approach for predicting thin film morphology using density functional theory (DFT). Applying this method to Cu adsorption on TaN modified with Ru, Co and W as well as monolayers of MoS2 showed that the most important factor in controlling thin film morphology is the competition between metal-metal and metal-substrate interaction. Results show that 50% doping of TaN with Ru and 25% doping with Co or W in the surface layer can promote liner properties in TaN and thus promote the growth of 2D, conducting Cu while also preventing Cu diffusion through the material. The study of Cu as well as Ru and Co, which are of interest as alternative interconnect metals, on MoS2 showed that the metal-substrate interaction of Cu and Ru on MoS2 must be improved in order to promote a 2D film, while Co shows very strong metal-substrate interactions and should thus form a 2D film on MoS2.
Development of thermoelectric materials and micro-devices for cooling and power generation
(University College Cork, 2020-12) Lal, Swatchith; Mahmood Razeeb, Kafil; Horizon 2020
Thermoelectric materials have been widely used for solid-state cooling (as thermoelectric cooler) and power generation (as thermoelectric generator) applications. Integration of thermoelectric thin-film materials into micro-device fabrication offers various advantages compared to bulk thermoelectric devices such as miniaturized size, high-density integration and substantial reduction of the material usage that helps in reducing the weight of the system. Thermoelectric coolers play a vital role in optoelectronic devices. Active photonic device (e.g. lasers) generate incredibly high heat flux levels (~10 kW/cm2) that must be efficiently removed to maintain their performance and reliability; furthermore, active photonic devices must be controlled with a precision of ±0.1 °C. Today’s photonic integrated circuits (PICs) employ macro thermoelectric coolers (TECs) that are inefficient in the thermal management of the device and cannot scale with the growing trends of miniaturisation and high-density integration. On the other hand, micro-thermoelectric coolers (µ-TECs) integrated directly on the laser or other photonic component can more efficiently perform the thermal management of the device. Similarly, these micro-thermoelectric devices can be applied to convert heat into usable electricity as a thermoelectric generator, which has a wide range of applications including wearable electronics and biomedical devices. These micro-thermoelectric generators (µ-TEGs) can convert the body heat to usable electrical energy which can, in turn, be used for powering various wearable vital health monitoring systems, particularly using the low-temperature gradients. This thesis deals with the development of high-performance room temperature thermoelectric materials using electrodeposition technique that offers cost-effectiveness, ease of process control and industrial batch production compatibility. Further, this work aims to integrate the developed materials in the micro-thermoelectric devices, both for cooling and power generation, particularly using available low-temperature gradients near room temperature regimes. As part of this work, p-type BiSbTe material has been developed using a pulse amperometry technique employing a suitable nitric acid bath, and the thermoelectric properties of the developed material are enhanced using additives, particularly sodium do-decyl sulfate surfactant. From the following investigations, it was observed that the overall power factor of the developed materials with the surfactant was 149% higher than the material with no surfactant added into the system due to the densification of the films. Later, this material is optimised using an annealing time-temperature profile to achieve better thermoelectric properties. The inclusion of the Te material layer in between the BiSbTe layers prevented the loss of Te during annealing and helped in maintaining the proper stoichiometry of the material. Using this annealing study, the charge carrier concentration and mobility of the materials are optimised for the higher performance, which led to an increase in the power factor of the material from 11 µW/mK2 to 225 µW/mK2, when annealed for the duration of 1 hr at 350 °C in the N2 atmosphere. As a part of the n-type material development, Cu doped BiTe and Cu doped Te have been developed, both exhibiting significantly improved thermoelectric properties for the electrodeposited materials. Both the materials showcased a crystal symmetry breakdown beyond a certain percentage of copper inclusion in the system, which has led to a significant enhancement of the thermoelectric properties of the material. Cu doped BiTe and Cu doped Te showed a power factor of 3.02 mW/mK2 and 5.60 mW/mK2 respectively, which are one of the best values reported so far. The developed p- and n-type materials are integrated in a silicon-based micro- thermoelectric device for both cooling and power generation applications. Two different approaches to device fabrication have been used. The first approach deals with the reduction of overall cost of the device fabrication, for which flip-chip bonding approach has been undertaken where p- and n-type materials are fabricated on different wafers, which reduced the overall lithographic processing. In the second approach, both p- and n-type materials are fabricated on a single wafer using multiple lithographic steps. This single wafer approach has various advantages such as minimized electrical contact resistances and improved thermal contact over the first method. The micro-devices developed using this method has been employed for both cooling and power generation applications and has been thoroughly investigated. The devices fabricated using flip-chip bonding generated an output voltage of 90 mV for a 10 K temperature gradient with an average electrical resistance of 0.87 Ω for individual thermoelectric leg pair. The average electrical resistance was dropped to 0.28 Ω for a device fabricated using a single wafer approach. An average cooling of 2 K was observed for the devices in cooling mode. The improved thermoelectric materials and optimized fabrication of micro-thermoelectric devices makes them a promising system for both cooling and power generation applications.
Area selective atomic layer deposition of Si-based materials
(University College Cork, 2019) Filatova, Ekaterina A.; Elliott, Simon D.; Nolan, Michael; Hausmann, Dennis; Lam Research Corporation
Modern electronics are very small and light yet extremely powerful. This is possible due to the constant integration of new techniques and novel materials into the electronics fabrication process. In the current project we are focusing on one of the most precise deposition fabrication techniques for microchip fabrication, called atomic layer deposition (ALD). In particular, we are interested in the applications of ALD for area-selective deposition, where the material is deposited only where it is needed allowing the lateral control of the grown film. We are using the quantum mechanical modelling, density functional theory (DFT) to investigate the chemical mechanism of the area-selective ALD processes for Si-based materials (SiC, SiO2 and SiNx ), which are widely used in the semiconductor microchip fabrication process. We are also focusing on the possible SiC ALD mechanism in more detail, as this mechanism is very challenging and still not implemented in high-volume manufacturing in the semiconductor industry. In order to investigate the possibility of area-selective deposition of Si-based materials on Si, SiC, SiO2 and SiNx substrates, the difference in adsorption energies of various aminosilane precursors was first analyzed by DFT. From DFT calculations we found that it is thermodynamically favorable for aminosilane precursors to react with SiNx and SiO2 substrates but not with SiC and Si. We further experimentally corroborate these results by depositing SiNx on Si, SiO2 and SiC substrates using di(sec-butylamino)silane precursor and N2 plasma ALD and measuring the apparent SiNx thickness by spectroscopic ellipsometry. Both DFT calculations and experiment show that the aminosilane precursor adsorbs selectively on SiO2 not on Si and SiC substrates, however, after N2 plasma pulse this selectivity is lost. Further, we investigated the possibility of the ALD of SiC. First, we used DFT to screen various precursors in order to select the most favourable for SiC ALD. Then we expanded this study by analyzing how these precursors react with H-terminated and bare SiC surfaces. We predicted that precursors disilane (Si2H6 ), silane (SiH4 ) or monochlorosilane (SiH3Cl) with ethyne (C2H2 ), carbontetrachloride (CCl4 ) or trichloromethane (CHCl3 ) are the most promising for ALD of SiC. All of these precursors are predicted to react thermodynamically with bare SiC but not with the passivated surface. An additional activation step would be needed to sustain an ALD process. In order to analyze how silane plasma fragments will react with the passivated surface, the reaction pathways of neutral silane plasma fragments SiH3 and SiH2 with the H-terminated surface were analyzed. Counterintuitively, it was found that silane plasma fragments SiH3 and SiH2 react selectivity with the Si-H bond rather than with the C-H bond of the H-terminated SiC surface. Lastly, as a part of collaboration with Prof. Adrie Mackus from Plasma Materials Process group in Eindhoven Technological University, Netherlands, we used a DFT screening approach to determine the non-growth surface during ALD of transition metal oxides via a ligand-exchange mechanism in the precursor pulse. Two different precursors reacting with various OH-terminated metal surfaces were compared: a heteroleptic amidometallocene and a homoleptic alkoxide. A sample result is that HfO2 , SiO2 and GeO2 are predicted to show nucleation delay in ALD on OH-terminated W, Co and Cu surfaces. Depending on the ligands used in the ALD precursor, Ru-based or W-based substrates were predicted to resist the nucleation of all the metal oxides that were studied.

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