This thesis is about the application and characterisation of spark ablation generated nanoparticles in microelectronics. It opens in chapter 1 with a general motivation of the need for advanced materials and for nanotechnology in particular. It then describes what nanoparticles are and why they are promising materials. Some of their advantages are a high chemical reactivity, a high specific surface area, and the display of quantum effects at that scale. The chapter ends by presenting the research questions and giving the thesis structure. Chapter 2 provides a technological background for the rest of the thesis based. It describes several applications of nanoparticles within microelectronics not researched during this PhD project: as die-attach materials, chemical sensors, or catalysts. It continues with the description and discussion of several competing nanoparticle synthesis methods and goes in-depth on the theory of spark ablation generation. It describes the effects of various parameters that govern the mass generation rate, particle size, and composition. This theory is important to be able to interpret the results in the other chapters. Impaction deposition is then described in this chapter since it is the method of printing all samples in this thesis. It explains how this method prints dots or lines of nanoparticles, that they have a Gaussian cross-section profile, and how specific deposition parameters affect the deposit. Lastly, the chapter gives a detailed description of the spark ablation synthesis and deposition equipment with which all the experiments in this thesis are performed. The generator, components, gasses, pressures, and materials are all described with diagrams and specifications. Typical synthesis and deposition settings for the generation of Au nanoparticle deposits are given (1 kV, 5 mA, 1.5 L min.¡1 Ar or N2 and 1mmnozzle distance). The first chapter with results, chapter 3, presents a method to measure the mass deposition rate of the nanoparticle printer. Measuring the mass of microgram scale deposits is challenging due to the high sensitivity required for an accurate measurement. Balances are sensitive to changes in pressure, temperature or humidity that can already give too big errors. One solution already applied in thin film deposition methods is the use of quartz crystal microbalances (QCMs). Their resonance frequency is dependent on their mass, and thus, we can use the frequency shift during deposition to measure a mass change. The Sauerbrey equation that is used for that conversion must be valid, so a special method was developed to comply to all of its conditions. A concentric circular pattern of Au nanoparticles was printed on 10 MHz QCMs to measure the mass deposition rate. It was found that the deposition rate scales linearly with the generation current of the spark, as expected from theory, but also showing the losses in the system are either constant or scale linearly too. The film density was surprisingly constant for all tested synthesis and deposition settings, at 15.95 g cm¡3, or a porosity of £p Æ 0.18. The density was compared to models presented in literature, and it is proposed that the impaction energy likely compacts the porous structure during deposition until this density is reached. The QCM method can be applied for process monitoring using commercially available equipment and open-source software. The first applications of printed conducting nanoparticle films are discussed in chapter 4. It describes the conductive properties of such films and the effect of annealing on their conductivity. It was found that an untreated Au film conducts 22 times worse than bulk Au. Several applications are then discussed. Here it was demonstrated that printed Au nanoparticle lines can be applied as interconnect materials as an alternative to wire bonding. Next, a method was presented to miniaturize the deposits even further by using lithography and lift-off. This reached a line width at the minimum of the lithography equipment available, at 1.2 ¹m, without significantly changing the nanostructure. Chapter 5 deals with the application of spark ablation generated nanoparticles as thermoelectric materials. It describes in detail the synthesis and characterization of Bi2Te3 nanoparticles and their thermoelectric properties. The main finding was that the thermal conductivity was drastically lower than bulk Bi2Te3 and comparable to the state of the art for Bi2Te3 nanostructured materials, reaching a minimum of 0.2Wm¡1 K¡1 at room temperature. Unfortunately, the electrical conductivity was reduced by at least a factor 1000, easily undoing any efficiency gains from reduced thermal conductivity. Suggestions are given to possibly improve this trade-off. Additionally, this chapter shows how quickly nanostructured materials like the ones in this thesis oxidize after synthesis. From the moment the sample is printed, it gains mass and loses conductivity, so this must be counteracted if a non-noble metal is to be applied. The final chapter before the conclusions, chapter 6, showcases another application of printed nanoparticles: as UV-sensing material. It shows the results obtained using ZnO nanoparticles to create a UV sensor that is insensitive to visible light. The nanoparticles were deposited over electrodes to fabricate a resistor that has two orders of magnitude electrical resistance reduction when exposed to 265 nm UV light. The response was slow, with 79 seconds to reach 90% of the maximum response and 82 seconds to get back to 10% again. This is attributed to the adsorption and desorption of oxygen under the influence of UV light and can be prevented by packaging the sensor. The contact behaviour between the metal electrodes and ZnO nanoparticles proved to be too unpredictable to reliably create a Schottky diode, which would have had a higher response. This dissertation ends with a list of the conclusions, the answers to the research questions, and finally, some suggestions for future work. @en

Reducing the thermal conductivity of thermoelectric materials has been a field of intense research to improve the efficiency of thermoelectric devices. One approach is to create a nanostructured thermoelectric material that has a low thermal conductivity due to its high number of grain boundaries or voids, which scatter phonons. Here, we present a new method based on spark ablation nanoparticle generation to create nanostructured thermoelectric materials, demonstrated using Bi2Te3. The lowest achieved thermal conductivity was <0.1 W m−1 K−1 at room temperature with a mean nanoparticle size of 8±2 nm and a porosity of 44%. This is comparable to the best published nanostructured Bi2Te3 films. Oxidation is also shown to be a major issue for nanoporous materials such as the one here, illustrating the importance of immediate, air-tight packaging of such materials after synthesis and deposition.@en

Advances in semiconductor device manufacturing technologies are enabled by the development and application of novel materials. Especially one class of materials, nanoporous films, became building blocks for a broad range of applications, such as gas sensors and interconnects. Therefore, a versatile fabrication technology is needed to integrate these films and meet the trend towards device miniaturization and high integration density. In this study, we developed a novel method to pattern nanoporous thin films with high flexibility in material selection. Herein, Au and ZnO nanoparticles were synthesized by spark ablation and printed on a Ti/TiO2 adhesion layer, which was exposed by a lithographic stencil mask. Subsequently, the photoresist was stripped by a cost-efficient lift-off process. Nanoporous patterned features were thus obtained and the finest feature has a gap width of 0.6 μ fm and a line width of 2 μ fm. Using SEM and profilometers to investigate the structure of the films, it was demonstrated that the lift-off process had a minor impact on the microstructure and thickness. The samples presented a rough surface and high porosity, indicating a large surface-to-volume ratio. This is supported by the measured conductivity of Au nanoporous film, which is 12% of the value for bulk Au. As lithographic stencil printing is compatible with conventional lithographic pattering, this method enables further application on mass production of various nanoporous film-based devices in the future.

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The precise control of patterned arrays of carbon nanofibers (CNFs) is an issue of interest in a wide range of applications. In the present work, we report the synthesis of CNFs grown by plasma-enhanced CVD (PECVD) from a Ni catalyst patterned via aerosol printing of nanoparticles (NPs) by a spark ablation technique. The printing speeds were adjusted to vary the catalyst density and characterize the CNFs distribution in the pre-patterned lines. Depending on the printing speed, different CNFs distributions are obtained, ranging from dense vertically-aligned CNFs (VACNFs) to sparse individual CNFs.@en

A method is presented to directly measure the mass output of an impaction printer coupled with a spark ablation generator. It is based on a quartz crystal microbalance and shown to be reliable in quantifying mass deposition rate. Here, the method is demonstrated with an Au nanoparticle aerosol synthesized under several spark ablation and deposition settings. Changes in the deposition rate in response to changed synthesis conditions follow the spark ablation models on generation rate made in previous studies, validating this novel measurement method. In combination with the volume of a deposit, a good estimate of the film porosity can be made. The Au nanoparticle films synthesized here have a low porosity of 0.18 due to extensive restructuring and compaction on impact with the substrate. The porosity is found to be insensitive to deposition settings and is constant throughout the film. The simplicity and low cost of a quartz crystal microbalance setup make this an accessible method to determine porosity in porous thin films.

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In this work, a novel microfabrication-compatible production process is demonstrated and used to fabricate UV photoresistors made from ZnO nanoparticles. It comprises a simple room-temperature production method for synthesizing and direct-writing nanoparticles. The method can be used on a wide range of surfaces and print a wide range of materials. Here, it is used to synthesize a ZnO photoresistor for the first time. The sensor shows a two orders of magnitude lower resistance under UV-C exposure compared to darkness. The low cost and simplicity of this synthesis method enables cheap integration of UV-C sensors for human exposure monitoring or UV-output monitoring of light sources.@en

The continuous trend to integrate more multi-functions in a package often involves, Heterogeneous Integration of multi-functional blocks in some kind of 3D stacking. The conventional flip chip for die-on-substrate technology applies solder for integration. However, solder joint integration has the disadvantages of restricting height, reflow issues and re-melting at high operating temperatures. Nanometallic particle sintering offers a potential solution for these solder related issues. Nanometallic particle sintering occurs at low temperature and does not reflow and melt at higher temperatures. Hence, it can be applied for quite precise alignment and integration technologies, such as photonic components on silicon for harsh environment applications. In order to test this concept, we use sapphire and Si wafers with different mechanical properties, which can lead to the coefficient of thermal expansion mismatch. The sapphire chip can operate at a higher temperature applied for ultraviolet photonics application. This report describes a novel approach using copper nanoparticles paste patterned through photolithographic stencil printing. The photoresist acts as the stencil mask, and a photoresist lift-off process is applied to strip the photoresist stencil. This process has the advantages of lithographic form factor and precision and provides a chip to chip interconnect with a standard height of 20 µm.@en

An ethanol gas sensor based on carbon nanofibers (CNFs) with various densities and nanoparticle functionalization was investigated. The CNFs were grown by means of a Plasma-Enhanced Chemical Vapor Deposition (PECVD), and the synthesis conditions were varied to obtain different number of fibers per unit area. The devices with a larger density of CNFs lead to higher responses, with a maximal responsivity of 10%. Furthermore, to simultaneously improve the sensitivity and selectivity, CNFs were decorated with gold nanoparticles by an impaction printing method. After metal decoration, the devices showed a response 300% higher than pristine devices toward 5 ppm of ethanol gas. The morphology and structure of the different samples deposited on a silicon substrate were characterized by TEM, EDX, SEM, and Raman spectroscopy, and the results confirmed the presence of CNF decorated with gold. The influence of operating temperature (OT) and humidity were studied on the sensing devices. In the case of decorated samples with a high density of nanofibers, a less-strong cross-sensitivity was observed toward a variation in humidity and temperature.@en

Continuous monitoring of tissue microphysiology is a key enabling feature of the organ-on-chip (OoC) approach for drug screening and disease modeling. Sensing charged species in OoC tissue microenvironments is thereby essential. However, the inherently small (i.e., cm) size of OoC devices poses the challenging requirement to integrate miniaturized and highly sensitive in situ charge sensing components to maximize signal extraction from small volumes (nL to L, range) of media used in these devices. Here we meet this need by presenting a novel dual-gate field-effect transistor-based charge sensor integrated within an optically transparent microelectromechanical (MEM) OoC device. Post-process mask-less decoration of Ti sensing electrodes by spark-ablated Au nanoparticle films significantly increases the effective electrode surface area and thus sensor sensitivity while retaining the CMOS-compatibility of the wafer-level fabrication process. We validate the biocompatibility of the sensor and its selective response to poly-D-lsine and KC1, and provide a perspective on monitoring cultures and differentiation of hiPSC-derived cortical neurons on our OoC device.

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The growing diversity in the used materials in semiconductor packaging provides challenges for achieving good interconnection. Particularly the very soft substrates, such as paper and polymers, and very hard, such as silicon carbide, offer unique challenges to wire-bonding or formation of vertical interconnects. Complementary technologies are therefore needed. Here, a method to direct-write metal tracks on the top and sides of dies is demonstrated. It is based on a spark ablation aerosol printing process entirely performed at room temperature and without any applied force. Therefore, it is suitable for use on soft or temperature-sensitive substrates. The printed metal lines consist of pure Au nanoparticles, without surfactants or contaminants, and do not require any further curing, cleaning, or other processing. The process is demonstrated on Si dies and paper, but is theoretically applicable on a wide variety of substrate materials. It can provide an alternative method to create interconnects or vias on soft materials, temperature sensitive materials, irregularly shaped materials, or curved surfaces.@en

Porous layers composed of nanoparticles (NPs) have a wide range of (potential) applications including catalysts, (chemical) sensors, thermoelectric materials and electronics. These application domains will profit strongly from our NP printing process which is flexible with respect to i) the composition of the NPs and ii) the possibility of composing arbitrary mixtures of different NPs (external mixtures). Our printer is a combination of a spark ablation NP generator supplying unagglomerated 5 nm particles with a hypersonic impactor equipped with an xyz stage. The profiles of printed lines are measured, and the impact velocity is described by theory. Printed gold lines on a polymer contact lens are sufficiently sintered by the impact energy to show the plasmon mode of gold (“golden“ color).@en

The CaAlSiN₃:Eu²⁺ red phosphor and its silicone/phosphor composite are very promising materials used in the high color rendering white light-emitting diode (LED) packaging. However, the reliabilities of CaAlSiN₃:Eu²⁺ and its composite are still being challenged by phosphor hydrolysis at high humidity application condition. A fundamental understanding of the interface adhesion between silicone and CaAlSiN₃:Eu²⁺ is significant for the developments and applications of this material. In this work, the mechanical properties of silicone/pristine CaAlSiN₃:Eu²⁺ and silicone/hydrolyzed CaAlSiN₃:Eu²⁺ composites are experimentally measured and compared firstly, in which both the tensile strength and Young's modulus of composite are increased after the hydrolysis reaction. Then, the first principles Density Functional Theory (DFT) calculations are used to investigate the adhesion behaviors of the silicone molecular on both the pristine and the hydrolyzed CaAlSiN₃[0 1 0] at atomic level. The results show that: (1) The silicone molecular is weakly adsorbed on the pristine CaAlSiN₃[0 1 0] via Van der Waals (vdW) interactions, while silicone molecular is much stronger absorbed on the hydrolyzed CaAlSiN₃[0 1 0] due to the formation of hydrogen bonding at the interface; (2) The transient state calculations indicate that the sliding energy barrier of silicone on the hydrolyzed CaAlSiN₃[0 1 0] is higher than that on the pristine one, as the increased adsorption energy and surface roughness. Generally, the findings in this paper can guide the phosphor selection, storage and process in LED packaging, and also assist in improving the reliability design of LED package used in high moisture condition.

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