Sn-based solder alloys are extensively used in electronic devices to form interconnects between different components to provide mechanical support and electrical path. The formation of a reliable solder interconnects fundamentally relies on the metallurgic reaction between the molten solder and solid pad metallization in reflowing. The resultant IMC layer at the solder/pad metallization interface can grow continuously during service or aging at an elevated temperature, uplifting the proportion of IMCs in the entire solder joint. However, the essential mechanical properties of interfacial IMC (i.e. Cu6Sn5, Cu3Sn) layers, such as Young s modulus and hardness, are drastically different in comparison with Sn-based solder and substrate. Therefore, the increasing fraction of interfacial IMCs in the solder joint can lead to significant deformation incompatibility under exterior load, which becomes an important reliability concern in the uses of solder joints for electronic interconnects. In the past decades, extensive research works were implemented and reported regarding the growth of interfacial IMC layers and its effect on the mechanical integrity of solder joints. But, the following fundamental issues in terms of mechanical and microstructural evolution in the uses of solder joints still remain unclear, demanding further research to elaborate: (1) The protrusion of IMCs: Though the growth of interfacial IMC layers along the diffusion direction in solder joints were studied extensively, the growth of IMCs perpendicular to the diffusion direction were reported in only a few papers without any further detailed investigation. This phenomena can crucially govern the long-term reliability of solder interconnects, in particular, in the applications that require a robust microstructural integrity from a solder joint. (2) Fracture behaviour of interfacial IMC layers: The fracture behaviour of interfacial IMC layers is a vital factor in determining the failure mechanism of solder joints, but this was scarcely investigated due to numerous challenges to enable a potential in-situ micro-scale tests. It is therefore highly imperative to carry out such study in order to reveal the fracture behaviour of interfacial IMC layers which can eventually provide better understanding of the influence of interfacial IMC layers on the mechanical integrity of solder joints. (3) Volume shrinkage: The volume shrinkage (or solder joint collapse) induced by the growth of interfacial IMC layers was frequently ascribed as one of the main causes of the degradation of mechanical reliability during aging due to the potentially resulted voids and residual stress at the solder/substrate interface. However, very few experimental works on the characterisation of such type of volume shrinkage can be found in literatures, primarily due to the difficulties of observing the small dimensional changes that can be encountered in the course of IMCs growth. (4) Residual stress: The residual stress within solder joints is another key factor that contributes to the failure of solder joints under external loads. However, the stress evolution in solder joints as aging progresses and the potential correlation between the residual stress and the growth of interfacial IMC layers is yet to be fully understood, as stress/strain status can fundamentally alter the course of total failure of a solder joint. (5) Crack initiation and propagation in solder joints: Modelling on the mechanical behaviour of solder joints is often undertaken primarily on the stress distribution within solder joints, for instance, under a given external loading. But there is lack of utilising numerical analysis to simulate the crack initiation and propagation within solder joints, thus the effect of interfacial IMC layers on the fracture behaviour of the solder joints can be elaborated in further details. In this thesis, the growth of interfacial IMCs in parallel and perpendicular to the interdiffusion direction in the Sn99Cu1/Cu solder joints after aging was investigated and followed by observation with SEM, with an intention of correlating the growth of IMCs along these two directions with aging durations based on the measured thickness of IMC layer and height of perpendicular IMCs. The mechanism of the protrusion of IMCs and the mutual effect between the growth of IMCs along these two directions was also discussed. The tensile fracture behaviour of interfacial Cu6Sn5 and Cu3Sn layers at the Sn99Cu1/Cu interface was characterised by implementing cantilever bending tests on micro Cu6Sn5 and Cu3Sn pillars prepared by focused ion beam (FIB). The fracture stress and strain were evaluated by finite element modelling using Abaqus. The tensile fracture mechanism of both Cu6Sn5 and Cu3Sn can then be proposed and discussed based on the observed fracture surface of the micro IMC pillars. The volume shrinkage of solder joints induced by the growth of interfacial IMC layers in parallel to the interdiffusion direction in solder joint was also studied by specifically designed specimens, to enable the collapse of the solder joint to be estimated by surface profiling with Zygo Newview after increased durations of aging. Finite element modelling was also carried out to understand the residual stress potentially induced due to the volume shrinkage. The volume shrinkage in solder joints is likely to be subjected to the constraint from both the attached solder and substrate, which can lead to the build-up of residual stress at the solder/Cu interface. Depth-controlled nanoindentation tests were therefore carried out in the Sn99Cu1 solder, interfacial Cu6Sn5 layer, Cu3Sn layer and Cu with Vickers indenter after aging. The residual stress was then evaluated in the correlation with aging durations, different interlayers and the locations in the solder joint. Finally, finite element models incorporated with factors that may contribute to the failure of solder joints, including microstructure of solder joints, residual stress and the fracture of interfacial IMC, were built using Abaqus to reveal the effect of these factors on the fracture behaviour of solder joints under applied load. The effect of growth of IMC layer during aging on the fracture behaviour was then discussed to provide a better understanding of the degradation of mechanical integrity of solder joints due to aging. The results from this thesis can facilitate the understanding of the influence of interfacial IMC layers on the mechanical behaviour of solder joints due to long-term exposure to high temperatures.

Small-scale fracture behavior of four model alloy systems were investigated in the order of increasing microstructural complexity, namely: (i) a Ni-based Bulk Metallic Glass (Ni-BMG) with an isotropic amorphous microstructure; (ii) a single-phase high entropy alloy, HfTaTiVZr, with body centered cubic (BCC) microstructure; (iii) a dual-phase high entropy alloy, AlCoCrFeNi2.1, with eutectic FCC (L12) -BCC (B2) microstructure; and (iv) a Medium-Mn steel with hierarchical microstructure. The micro-mechanical response of these model alloys was investigated using nano-indentation, micro-pillar compression, and micro-cantilever bending. The relaxed Ni-BMG showed 6% higher hardness, 22% higher yield strength, and 26% higher bending strength compared to its as-cast counterpart. Both the as-cast and corresponding relaxed BMGs showed stable notch opening and blunting during micro-cantilever bending tests rather than unstable crack propagation. However, pronounced notch weakening was observed for both the structural states, with the bending strength lower by ~ 25% for the notched samples compared to the un-notched samples. Deformation behavior of HfTaTiVZr was evaluated by micropillar compression and micro-cantilever bending as a function of two different grain orientations, namely [101] and [111]. The [111] oriented micropillars demonstrated higher strength and strain hardening rate compared to [101] oriented micropillars. The [111] oriented micropillars showed transformation induced plasticity (TRIP) in contrast to dislocation-based planar-slip for the [101] oriented micropillars, explaining the difference in strain hardenability for the two orientations. These differences in deformation behavior for the two orientations were explained using Schmid factor calculations, transmission electron microscopy, and in-situ deformation videos. For the dual-phase AlCoCrFeNi2.1 high entropy alloy, the L12 phase exhibited superior bending strength, strain hardening, and plastic deformation, while the B2 phase showed limited damage tolerance during bending. The microstructure and deformation mechanisms were characterized for a few different medium-Mn steels with varying carbon (0.05-0.15 at%) and manganese (5-10 at%) content. The alloy with 10 at% Mn and 0.15 at% C (1015 alloy) showed hierarchical microstructure of retained austenite and ferrite with lamellae 200 nm to 300 nm wide. Micro-pillar compression at different strain levels for this alloy revealed that deformation in austenite is primarily accommodated through transformation to martensite, thereby increasing the strain hardening rate.

The objective of this study is to develop a robust portable nano-calorimeter sensor for detection of energetic materials, primarily explosives, combustible materials and propellants. A micro-cantilever sensor array is actuated thermally using bi-morph structure consisting of gold (Au: 400 nm) and silicon nitride (Si3N4: 600 nm) thin film layers of sub-micron thickness. An array of micro-heaters is integrated with the microcantilevers at their base. On electrically activating the micro-heaters at different actuation currents the microcantilevers undergo thermo-mechanical deformation, due to differential coefficient of thermal expansion. This deformation is tracked by monitoring the reflected ray from a laser illuminating the individual microcantilevers (i.e., using the optical lever principle). In the presence of explosive vapors, the change in bending response of microcantilever is affected by the induced thermal stresses arising from temperature changes due to adsorption and combustion reactions (catalyzed by the gold surface). A parametric study was performed for investigating the optimum values by varying the thickness and length in parallel with the heater power since the sensor sensitivity is enhanced by the optimum geometry as well as operating conditions for the sensor (e.g., temperature distribution within the microcantilever, power supply, concentration of the analyte, etc.). Also, for the geometry present in this study the nano-coatings of high thermal conductivity materials (e.g., Carbon Nanotubes: CNTs) over the microcantilever surface enables maximizing the thermally induced stress, which results in the enhancement of sensor sensitivity. For this purpose, CNTs are synthesized by post-growth method over the metal (e.g., Palladium Chloride: PdCl2) catalyst arrays pre-deposited by Dip-Pen Nanolithography (DPN) technique. The threshold current for differential actuation of the microcantilevers is correlated with the catalytic activity of a particular explosive (combustible vapor) over the metal (Au) catalysts and the corresponding vapor pressure. Numerical modeling is also explored to study the variation of temperature, species concentration and deflection of individual microcantilevers as a function of actuation current. Joule-heating in the resistive heating elements was coupled with the gaseous combustion at the heated surface to obtain the temperature profile and therefore the deflection of a microcantilever by calculating the thermally induced stress and strain relationship. The sensitivity of the threshold current of the sensor that is used for the specific detection and identification of individual explosives samples - is predicted to depend on the chemical kinetics and the vapor pressure. The simulation results showed similar trends with the experimental results for monitoring the bending response of the microcantilever sensors to explosive vapors (e.g., Acetone and 2-Propanol) as a function of the actuation current.

A novel optical diffraction based technique is proposed and demonstrated to measure deflections of the order of ~1nm in microcantilevers (MC) designed for sensing ultra-small forces of stress. The proposed method employs a double MC structure where one of the cantilevers acts as the active sensor beam, while the other as a reference. The active beam can respond to any minute change of stress, for example, molecular recognition induced surface stress, through bending (~1nm) relative to the other fixed beam. Optical diffraction patterns obtained from this double slit aperture mask with varying slit width, which is for the bending of MC due to loading, carries the deflection profile of the active beam. A significant part of the present work explores the possibility of connecting diffraction minima (or maxima) to the bending profile of the MC structure and thus the possibility to measure induced surface stress. To start with, it is also the aim to develop double MC sensors using PHDDA (Poly – Hexane diol diacrylate) because this material has the potential to achieve high mechanical deformation sensitivity in even moderately scaled down structures by virtue of its very low Young’s modulus. Moreover, the high thermal stability of PHDDA also ensures low thermally induced noise floors in microcantilever sensors. To demonstrate the proposed optical diffraction-based profiling technique, a bent microcantilever structure is designed and fabricated by an in-house developed Microstereolithography (MSL) system where, essentially one of the microcantilevers is fabricated with a bent profile by varying the gap between the two structures at each cured 2D patterned layer. The diffraction pattern obtained on transilluminating the fabricated structure by a spherical wavefront is analyzed and the possibility of obtaining the deflections at each cross section is ascertained. Since the proposed profiling technique relies on the accurate detection and measurement of shifts of intensity minima on the image plane, analysis of the minimum detectable shift in intensity minima for the employed optical interrogation setup with respect to the minimum detectable contrast and SNR of the optical measurement system is carried out, in order to justify the applicability of the proposed minima intensity shift measurement technique. The proposed novel diffraction based profiling technique can provide vital clue on the origins of surface stress at the atomic and molecular level by virtue of the entire bent profile due to adsorption induced bending thereby establishing microcantilever sensor technology as a more reliable and competitive approach for sensing ultra-low concentrations of biological and chemical agents.

Photonic bandgap structures have provided promising platform for miniaturization of modern integrated optical devices. In this thesis, a photonic crystal based ring resonator (PCRR) is proposed and optimized to exhibit high quality factor. Also, force sensing application of the optimized PC ring resonator and Dense Wavelength Division Multiplexing (DWDM) application of the PCRR are discussed. Finally fabrication and characterization of the PCRR is presented. A photonic crystal ring resonator is designed in a hexagonal lattice of air holes on a silicon slab. A novel approach is used to optimize PCRR to achieve high quality factor. The numerical analysis of the optimized photonic crystal ring resonator is presented in detail. For all electromagnetic computations Finite Difference Time Domain (FDTD) method is used. The improvement in Q factor is explained by using the physical phenomenon, multipole cancellation of the radiation held of the PCRR cavity. The corresponding mathematical frame work has been included. The forced cancellation of lower order radiation components are verified by plotting far held radiation pattern of the PCRR cavity. Then, the force sensing application of the optimized PCRR is presented. A high sensitive force sensor based on photonic crystal ring resonator integrated with silicon micro cantilever is presented. The design and modelling of the device, including the mechanics of the cantilever, FEM (Finite Element Method) analysis of the cantilever beam with PC and without PC integrated on it. The force sensing characteristics are presented for forces in the range of 0 to 1 N. For forces which are in the range of few tens of N, a force sensor with bilayer cantilever is considered. PC ring resonator on the bilayer of 220nm thick silicon and 600nm thick SiO2 plays the role of sensing element. Force sensing characteristics of the bilayer cantilever for forces in the range of 0 to 10 N are presented. Fabrication and characterization of PCRR is also carried out. This experimental work is done mainly to understand practical issues in study of photonic crystal ring resonators. It is proved that Q factor of PCRR can be signi cantly improved by varying the PCRR parameters by the proposed method. Dense Wavelength Division Multiplexing (DWDM) application of PC ring resonator is included. A novel 4-channel PC based demultiplexer is proposed and optimized in order to tolerate the fabrication errors and exhibit optimal cross talk, coupling efficiency between resonator and various channels of the device. Since the intention of this design is, to achieve the device performance that is independent of the unavoidable fabrication errors, the tolerance studies are made on the performance of the device towards the fabrication errors in the dimension of various related parameters. In conclusion we summarize major results, applications including computations and practical measurements of this work and suggest future work that may be carried out later.