Towards Direct Fabrication of Functional Patterns on Large-Area Substrates for Flexible Devices

Nowadays there is a growing interest to develop flexible devices that can be used in applications spanning from displays to energy harvesting. This is due to the potential of such devices to be integrated onto various-shaped surfaces and also to be roll-to-roll processed. However, there are still a few challenges to be tackled before their full commercialization. In particular, the direct fabrication of micro/nano-scale functional features on a large-area flexible substrate remains a critical challenge. This is because it requires both low temperature processing and precise control of the feature size.;During this study an investigation of different fabrication methods, such as hydrothermal growth and dip-pen nanolithography (DPN), is conducted in order to assess their suitability for the large-area deposition of ZnO-based simple geometrical patterns on flexible substrates. These model micro/nano structures are characterized optically, electrically, and mechanically. In addition, the preliminary fabrication and characterization of force sensors based on flexible substrates is reported.;Experimental results from this work highlight the potential of DPN as a scalable processing technique for flexible devices. In particular, the repeatable fabrication of circular micron-sized functional features on polyester substrates is reported. It was found that both the viscosity of the starting ink material and the wettability of the starting substrate play the most critical role for the successful fabrication of such features. In addition, humidity and starting ink stability must be carefully controlled. It is believed that using DPN in a scalable manner will be the key to realize the next generation of large-area nanopatterned flexible devices

High-speed atomic force microscopy for nanofabrication

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Electrostatic actuation based modulation of polar molecules and associated force interaction studies

Seamless integration of artificial components with biological systems to form an elegant biotic-abiotic interface or smart surface has promising application potential in biomedical engineering. The specific aim of this study is to implement the actuation and modulation of binding behavior between biomolecules under electrostatic stimuli, and investigate the corresponding force interaction between the complementary pairs. The nanofabrication technology was utilized to establish the patterned binding pair of thrombin and DNA aptamer on gold substrate, and different electrical fields were applied on the system to evaluate electrostatic influence. The atomic force microscopy (AFM) surface imaging was then used to explicate the surface height change after the removal of the electrical fields. The height change of the surface showed that positive electrical fields can successfully break the bonds between thrombin and aptamer, while moderate negative electrical fields kept the integral structure. The experimental studies implement the idea of electrostatic actuation and modulation of the complementary pair. The force interaction between the pair was then investigated through AFM based dynamic force spectroscopy (DFS). The open circuit DFS experiment was conducted first to clarify the magnitude of single molecule level force interaction between thrombin and aptamer, and the linear dependence of rupture force on logarithmic loading rate was observed. A single energy barrier model was used to understand the binding physics and kinetics. By fitting the model with experiment data, we could acquire important kinetic parameters toff and xβ. Then in-situ electrochemical atomic force microscopy (ECAFM) based DFS experiment was conducted to investigate the electrostatic influence upon molecular force interaction between thrombin and aptamer. The force interaction difference showed that positive electrical fields lowered the dissociation force between thrombin and aptamer, while negative electrical fields held similar force level with zero potential. The ECAFM experimental studies further support the conclusion of electrostatic actuation and modulation of the complementary pair. Besides, the root cause for the change of binding behavior and force interaction between the biomolecules under electrostatic fields is the conformational transition of the molecules, which might be illustrated by the molecular dynamics (MD) simulation. Therefore, a MD based computational study was performed on self-assembled monolayer (SAM) with polar end group under the application of electrical fields to clarify the conformational transition and associated friction change of the monomolecular thin films. The simulation results showed that positive electrical fields can generate larger conformational transition of the SAMs, which led to a greater frictional coefficient drop of the surface, while negative electrical fields kept similar conformational state and frictional response as the zero potential. The simulation result provides another explanation of the electrostatic actuation based modulation of polar molecule functionalized surface

Polycrystalline diamond micro-electromechanical systems (MEMS) for passive micro-rheology and sensor applications

Owing to its unique mechanical and electrical properties, diamond is an attractive candidate for use in micro-electro-mechanical systems (MEMS) devices. This thesis pertains to the development, fabrication and characterisation of polycrystalline diamond (PCD) micro-electromechanical systems (MEMS) devices for passive micro-rheology and sensor applications. Intrinsic PCD and boron doped PCD (BDD) materials are investigated. Micro-rheology is the study of soft matter rheological properties, often performed by observing interactions with mechanical devices, such as micro-cantilevers, at the micro scale. In order to overcome significant fluid dampening, these devices are actuated at or around their resonant frequency, and several measurements are taken at different frequencies to build a data set. We present an intrinsic diamond-based micro-cantilever micro-rheometer device, the passively actuated thermal fluctuations of which can be characterised in a fluid at least up to the viscosity of water (8.90 × 10−4 Pa.s ). A possible data analysis method to extract a fluid’s viscoelastic properties from the power spectrum of the thermal fluctuations of a device submerged in the fluid is also presented. This method negates the requirement for measurements at multiple actuation frequencies and provides useable data up to the sample rate of the data acquisition system. Intrinsic PCD cantilevers for passive micro-rheology were fabricated from polished (~3 nm Ra) 500 nm thick PCD on Silicon substrate films. Cantilever dimensions range from 5 μm to 150 μm in length and 1 μm to 4 μm in width, the highest height/width/length ratio cantilevers yet reported. PCD samples were patterned using electron beam lithography and highly anisotropic diamond etching was achieved using an RIE Ar/O2 plasma etching method. A new fabrication process to minimize cantilever undercut is presented. The thermal fluctuations of the free-standing cantilever structures in air and water at room temperature were successfully captured by a laser Doppler vibrometer system. Resonant frequencies of devices are presented, ranging from 38 – 554 kHz in air and 42 – 148 kHz in water, comparable to that of similar single crystal diamond devices. Polycrystalline Diamond MEMS for Passive Micro-rheology and Sensor Applications ii PCD micro-cantilevers have been investigated extensively in different sensor applications. Recently, boron-doped diamond micro cantilevers exhibiting piezoresistive behavior have been fabricated from multi-layer PCD material. We present a boron-doped PCD micro-cantilever piezoresistive sensor fabricated from a single layer of BDD thin film on silicon. BDD material was electrically characterised and found to be electrically stable for up to at least 60 seconds within the I/V ranges investigated. BDD micro-cantilevers were fabricated from polished (~3 nm Ra) 480 nm thick BDD on Silicon substrate films. The Ushaped cantilever’s dimensions ranged from 60 μm to 110 μm in length with legs 4 μm in width. The deflection sensitivity of the fabricated cantilever devices is reported, ranging from 0.029 mΩ/Ω-μm to 0.063 mΩ/Ω-μm. An analysis of the nature of the piezoresistive mechanism in the BDD devices is presented

Tribochemical investigation of microelectronic materials

To achieve efficient planarization with reduced device dimensions in integrated circuits, a better understanding of the physics, chemistry, and the complex interplay involved in chemical mechanical planarization (CMP) is needed. The CMP process takes place at the interface of the pad and wafer in the presence of the fluid slurry medium. The hardness of Cu is significantly less than the slurry abrasive particles which are usually alumina or silica. It has been accepted that a surface layer can protect the Cu surface from scratching during CMP. Four competing mechanisms in materials removal have been reported: the chemical dissolution of Cu, the mechanical removal through slurry abrasives, the formation of thin layer of Cu oxide and the sweeping surface material by slurry flow. Despite the previous investigation of Cu removal, the electrochemical properties of Cu surface layer is yet to be understood. The motivation of this research was to understand the fundamental aspects of removal mechanisms in terms of electrochemical interactions, chemical dissolution, mechanical wear, and factors affecting planarization. Since one of the major requirements in CMP is to have a high surface finish, i.e., low surface roughness, optimization of the surface finish in reference to various parameters was emphasized. Three approaches were used in this research: in situ measurement of material removal, exploration of the electropotential activation and passivation at the copper surface and modeling of the synergistic electrochemical-mechanical interactions on the copper surface. In this research, copper polishing experiments were conducted using a table top tribometer. A potentiostat was coupled with this tribometer. This combination enabled the evaluation of important variables such as applied pressure, polishing speed, slurry chemistry, pH, materials, and applied DC potential. Experiments were designed to understand the combined and individual effect of electrochemical interactions as well as mechanical impact during polishing. Extensive surface characterization was performed with AFM, SEM, TEM and XPS. An innovative method for direct material removal measurement on the nanometer scale was developed and used. Experimental observations were compared with the theoretically calculated material removal rate values. The synergistic effect of all of the components of the process, which result in a better quality surface finish was quantitatively evaluated for the first time. Impressed potential during CMP proved to be a controlling parameter in the material removal mechanism. Using the experimental results, a model was developed, which provided a practical insight into the CMP process. The research is expected to help with electrochemical material removal in copper planarization with low-k dielectrics

ULTRATHIN CARBON-BASED OVERCOATS FOR EXTREMELY HIGH DENSITY MAGNETIC RECORDING

Ph.DDOCTOR OF PHILOSOPH

Laser-induced forward transfer (LIFT) of water soluble polyvinyl alcohol (PVA) polymers for use as support material for 3D-printed structures

The additive microfabrication method of laser-induced forward transfer (LIFT) permits the creation of functional microstructures with feature sizes down to below a micrometre [1]. Compared to other additive manufacturing techniques, LIFT can be used to deposit a broad range of materials in a contactless fashion. LIFT features the possibility of building out of plane features, but is currently limited to 2D or 2½D structures [2–4]. That is because printing of 3D structures requires sophisticated printing strategies, such as mechanical support structures and post-processing, as the material to be printed is in the liquid phase. Therefore, we propose the use of water-soluble materials as a support (and sacrificial) material, which can be easily removed after printing, by submerging the printed structure in water, without exposing the sample to more aggressive solvents or sintering treatments. Here, we present studies on LIFT printing of polyvinyl alcohol (PVA) polymer thin films via a picosecond pulsed laser source. Glass carriers are coated with a solution of PVA (donor) and brought into proximity to a receiver substrate (glass, silicon) once dried. Focussing of a laser pulse with a beam radius of 2 µm at the interface of carrier and donor leads to the ejection of a small volume of PVA that is being deposited on a receiver substrate. The effect of laser pulse fluence , donor film thickness and receiver material on the morphology (shape and size) of the deposits are studied. Adhesion of the deposits on the receiver is verified via deposition on various receiver materials and via a tape test. The solubility of PVA after laser irradiation is confirmed via dissolution in de-ionised water. In our study, the feasibility of the concept of printing PVA with the help of LIFT is demonstrated. The transfer process maintains the ability of water solubility of the deposits allowing the use as support material in LIFT printing of complex 3D structures. Future studies will investigate the compatibility (i.e. adhesion) of PVA with relevant donor materials, such as metals and functional polymers. References: [1] A. Piqué and P. Serra (2018) Laser Printing of Functional Materials. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA. [2] R. C. Y. Auyeung, H. Kim, A. J. Birnbaum, M. Zalalutdinov, S. A. Mathews, and A. Piqué (2009) Laser decal transfer of freestanding microcantilevers and microbridges, Appl. Phys. A, vol. 97, no. 3, pp. 513–519. [3] C. W. Visser, R. Pohl, C. Sun, G.-W. Römer, B. Huis in ‘t Veld, and D. Lohse (2015) Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer, Adv. Mater., vol. 27, no. 27, pp. 4087–4092. [4] J. Luo et al. (2017) Printing Functional 3D Microdevices by Laser-Induced Forward Transfer, Small, vol. 13, no. 9, p. 1602553

Structural, mechanical and transport characterization of organosulphur nanoscaled molecular films

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 14-09-200