Digital devices have now been introduced in many X-ray imaging applications, replacing slowly traditional photographic films. These devices are preferred as they offer real time imaging, easy handling and fast treatment of the images. However, the performance of the detectors still have to be improved in order to increase the image quality, and possibly reduce the X-ray dose, a vital parameter for medical use. In this thesis, three different new detector concepts have been investigated. All designs use pore arrays, which are ideal starting structures to form pixellated detectors. Electrochemical etching of n-type silicon in aqueous hydrofluoric acid solution (HF) has been studied to form these pore arrays. A broad range of pores have been fabricated with diameters varying from 200 nm to 40 µm and with depths reaching almost the wafer thickness, thus leading to very high aspect ratios. The technique was also found to be suitable for the formation of other types of structures such as pillars and tubes on the sub micrometer scale. The etching is based on the dissolution of silicon in HF under anodic bias and a supply of positive electrical carriers (holes). As holes are the minority carriers in n-type silicon, they are usually photo-generated. In this work an alternative technique, based on hole injection from a forward-biased pn junction, has been successfully pioneered. The first X-ray imaging detector concept presented in the thesis consists of a silicon charge coupled device (CCD) in proximity with a scintillating screen. The screen is made from a pore array having reflective pore walls and filled with CsI(Tl), emitting photons at a wavelength of 550 nm under X-ray exposure. The secondary emitted photons are light-guided by the pore walls and then detected by the CCD pixels. Detectors were fully fabricated and characterized. This concept provides good spatial resolution with negligible cross talk between adjacent pixels. The dependences of the detector efficiency on pore depth and on the coating of the pore walls are presented. Although most of the produced detectors had a detective quantum efficiency of about 25%, some detectors indicate that efficient scintillating screens can be achieved approaching the theoretical limit as set by poissonian statistics of the X-ray photons. The two other detector designs require the formation of vertical pn junctions, i.e. in the pore walls. In one concept the secondary emitted photons are detected by photodiodes located in the pore walls. This would lead to high charge collection efficiency as the photons do not have to be guided to one end of the pore. However, high noise due to the direct detection of X-rays in the diodes is expected. The other concept is based on generation of electron-hole pairs in a semiconductor and the ‘3D’ detector, where an array of vertical electrodes is used to separate the charges via an electric field. To uniformly dope the inside of deep pores, both boron diffusion and low-pressure chemical vapor diffusion of boron-doped poly-silicon were shown to be successful techniques. This was confirmed by SIMS profiles taken through the pore wall thickness. Finally, the possibility to form individual junction in each pore was shown. The diodes were electrically characterized, demonstrating good rectifying behavior and sensitivity to light. / QC 20100831

<p><p>The solar wind interaction with the magnetosphere-ionosphere system continuously drives plasma convection in the polar regions of the ionosphere. The flow velocity and the shape of the convection pattern are closely dependent on the interplanetary conditions, in particular the direction of the interplanetary magnetic field (IMF). The main driver of the system is considered to be magnetic reconnection between the IMF and the terrestrial field, a process that is most efficient during southward IMF when the magnetic fields at the dayside magnetopause are anti-parallell, and less efficient but still present when the IMF is northward. Additional driving may be caused by waves at the magnetopause flanks, where viscous effects can lead to an energy, momentum and plasma exchange across the boundary.</p><p>In this work, we make use of the characteristics of the ionospheric convection and particle precipitation to investigate the nature of the driving dynamos, and large statistical data sets for steady solar wind conditions are used to derive the general behavior of the driving processes and their dependence on interplanetary conditions. The results show that the primary dynamo responsible for the convection in the boundary layer is closely dependent on the sign of the IMF Bz component, the average potential over the boundary layer region increases from <1 kV for steady southward IMF up to the order of 10kV for strictly northward conditions with reconnection poleward of the cusps, whereas the magnitude of magnetic field only has a minor influence at most. This could for example indicate that the magnetopause is more unstable to Kelvin-Helmholtz waves for parallel rather than anti-parallel magnetic fields, or that magnetic reconnection on the dayside suppresses other processes.</p><p>It is well known that the ionospheric potential drop saturates during strong driving conditions and southward IMF. The results presented here also show that the same phenomenon occurs when the IMF is northward. This gives additional information on the physics governing the solar wind-magnetosphere-ionosphere interaction, and may impose new restrictions on the theories explaining the saturation.</p></p>

<p>The thesis "Modeling and Characterization of an All-Digital PLL" aims to create a behavioral model of an All-Digital Phase-Locked-Loop (ADPLL). The model should be able to perform accurate and time-effective simulations. Based on the model, a sub-block requirement will be presented as decision basis for test chip manufacturing.</p><p>The wireless communications industry has grown tremendously in the recent years, leading to strong demand for smaller, faster, better and less power consuming circuits. Digital circuits have better properties in these aspects, which have resulted in great interest for more digitally intensive circuits. Since frequency synthesis is an essential part of any wireless system an all digital PLL is very attractive.</p><p>Traditional simulation tools are unable to simulate a complex system like an ADPLL. Since production costs are high and it is necessary to verify the integrity of the design and the circuit behavior before first prototype, an alternative solution is needed. One solution is to use an event-driven simulation technique that only focus on the events that occur at each clock flank. The difficulty lies in creating a realistic model of behavior.</p><p>The project has focused on meeting the phase noise requirements imposed on a WCDMA / HSDPA application. The event-driven model is implemented in Matlab because of its high flexibility during development, and large variety of analytical tools. The proposed model is based on a previously published model that has been evolved in ways that were interesting for the project. The model’s construction and accuracy have been verified against the appropriate theory. By constructing a comprehensible user interface around the model, it is convenient to examine how different parameters affect system performance.</p><p>The simulation results of the model establish how the different parameters affect the phase noise spectrum of the ADPLL. The TDC architecture has big influence on the phase noise and it is of big importance to use high precision in the entire system to prevent an increased in-band noise level.</p><p>A time-effective simulation tool has successfully been constructed and a sub-block requirement specification has been presented.</p> / <p>Examensarbetet “Modeling and Characterization of an All-Digital PLL” har som syfte att skapa en beteendemodell av en All-Digital Phase-Locked-Loop (ADPLL). Modellen ska kunna generera noggranna och tidseffektiva simuleringar. Utifrån modellen ska sedan en kravspecifikation för de olika delblocken skapas för att utgöra ett beslutsunderlag för eventuell tillverkning av testchip.</p><p>Bakgrunden till projektet är att den trådlösa kommunikationsindustrin under de senaste åren har vuxit enormt vilket lett till stor efterfrågan på mindre, snabbare, bättre och energisnålare kretsar. Digitala kretsar har bättre egenskaper i dessa avseenden vilket resulterat i ett stort intresse för kretsar av denna typ. Eftersom frekvenssyntetiseringen utgör en central del i alla trådlösa system är en helt digital PLL mycket attraktiv.</p><p>Traditionella simuleringsverktyg har inte möjlighet att simulera ett så komplext system som en ADPLL. Då tillverkningskostnaderna är höga och det är nödvändigt att kontrollera designens egenskaper och uppförande innan första prototyp, är det ett måste att finna alternativa lösningar. En lösning är då att använda en händelsestyrd simuleringsteknik som endast fokuserar på de händelser som sker vid respektive klockflank. Svårigheten ligger i att skapa en realistisk beteendemodell.</p><p>Projektet har fokuserat på att klara de krav på fasbrus som ställs på en WCDMA/HSDPA applikation. Den händelsestyrda modellen har skapats i Matlab på grund av dess stora flexibilitet under utveckling samt stora flora av analysverktyg. Den föreslagna modellen utgår från en tidigare publicerad modell som har utvecklats i de avseenden som varit intressanta för projektet. Modellens uppförande och noggrannhet har kunnat verifieras mot adekvat teori. Ett överskådligt användargränssnitt runt modellen möjliggör undersökning av olika parametrars påverkan på systemets prestanda.</p><p>Simuleringsresultaten av modellen fastställer hur olika parametrar påverkar ADPLL fasbruset. TDC-arkiteturen har stor påverkan på fasbruset och det är viktigt att använda hög upplösning på hela systemet för att förhindra att ”in-band” brusnivån ökar.</p><p>Med gott resultat har ett tidseffektivt simuleringsverktyg skapats och en kravspecifikation för de olika delblocken har presenterats.</p>

The operation of a wide variety of applications in today's modern society are heavily dependent on the magnetic properties of ferromagnetic materials and their interaction with electromagnetic fields. The understanding of these interactions and the associated loss mechanisms is therefore crucial for the improvement and future development of such applications. This thesis is concerned with electromagnetic waves in media with ferromagnetic losses. We model the dynamics of the magnetization of a ferromagnetic material with the nonlinear Landau-Lifshitz-Gilbert (LLG) equation and study stability conditions on static solutions. Furthermore, with the aid of a small signal analysis this equation is linearized around a stable static solution. From this analysis we obtain a small signal permeability, which shows that ferromagnetic material in general are gyrotropic with a resonant frequency behavior similar to that of a Lorentz material. In difference to dielectric Lorentz material, this resonance frequency can be shifted with the aid of a bias field. For a specific bias field we obtain a frequency behavior that mimics that of a material with electric conductivity losses. In terms of losses per unit volume it is then possible to define a magnetic conductivity which is independent of frequency. We treat composite materials built from ferromagnetic inclusions in a nonmagnetic and nonconductinig background material. The composite material inherits the gyrotropic structure and resonant behavior of the single particle. The resonance frequency of the composite material is found to be independent of the volume fraction, unlike dielectric composite materials. For small enough particles, typically around 100 nm, it becomes energetically favorable to form a single domain in the particle, where disturbances in the magnetization can propagate in the form of spin waves. We study the possibility of exciting spin waves and derive a susceptibility that takes spin waves into account. It is found that spin wave resonances are excited in the gigahertz range and this could offer a way to increase the losses in a composite material. We also discuss some concerns regarding stability and causality of effective material parameters for biased ferromagnetic materials. Finally, we discuss the possibility of using magnetic materials in absorbing applications. We analyze the scattering of electromagnetic waves from a metal surface covered with a thin magnetic lossy sheet. It is found that very thin magnetic layers can provide substantial specular absorption over a wide frequency band. However, magnetic specular absorbers, where the waves propagates just a fraction of the wavelength in the material, seem to require a certain amount of ferromagnetic material which make them quite heavy and thereby limit its practical use. On the other hand, for nonspecular absorbers where the waves propagates several wavelengths in the material, the amount of magnetic material required for efficient absorption seems to be substantially less than for specular absorbers. Thus, as nonspecular absorbers, magnetic lossy materials could offer very thin and light designs. / QC 20100906

Silicon carbide rectifiers and MESFET switches are commercially available since 2001 and 2005 respectively. Moreover, three inch SiC wafers can be purchased nowadays without critical defects for the device performance, four inch wafers are available and the next step of technology is set to be the six inch substrate wafers. Despite this tremendous development in SiC technology the reliability issues, like bipolar device degradation, passivation, or low MOSFET channel mobility still remain to be solved. This thesis focuses on SiC surface passivation and junction termination, a topic which is very important for the utilisation of the full potential of this semiconductor. Five dielectrics with high dielectric constants, Al2O3, AlN, AlON, HfO2 and TiO2 have been investigated. The layers were deposited directly on SiC, or on the thermally oxidized SiC surfaces with several different techniques. The structural and electrical properties of the dielectrics were measured and the best insulating layers were then deposited on fully processed and well characterised 1.2 kV 4H SiC PiN diodes. For the best Al2O3 layers, the leakage current was reduced to half its value and the breakdown voltage was extended by 0.7 kV, reaching 1.6 kV, compared to non passivated devices. Furthermore, AlON deposited on 4H SiC at room temperature provided interface quality comparable to that obtained with the thermally grown SiO2/SiC system. As important as the proper choice of dielectric material is a proper surface preparation prior to deposition of the insulator. In the thesis two surface treatments were tested, a standard HF termination used in silicon technology and an exposure to UV light from a mercury or deuterium lamp. The second technique is highly interesting since a substantial improvement was observed when UV light was used prior to the dielectric deposition. Moreover, UV light stabilized the surface and reduced the leakage current by a factor of 100 for SiC devices after 10 Mrad γ ray exposition. The experiments show also that the measured leakage currents of the order of pA are dominated by surface leakage. / QC 20100928

The thesis "Modeling and Characterization of an All-Digital PLL" aims to create a behavioral model of an All-Digital Phase-Locked-Loop (ADPLL). The model should be able to perform accurate and time-effective simulations. Based on the model, a sub-block requirement will be presented as decision basis for test chip manufacturing. The wireless communications industry has grown tremendously in the recent years, leading to strong demand for smaller, faster, better and less power consuming circuits. Digital circuits have better properties in these aspects, which have resulted in great interest for more digitally intensive circuits. Since frequency synthesis is an essential part of any wireless system an all digital PLL is very attractive. Traditional simulation tools are unable to simulate a complex system like an ADPLL. Since production costs are high and it is necessary to verify the integrity of the design and the circuit behavior before first prototype, an alternative solution is needed. One solution is to use an event-driven simulation technique that only focus on the events that occur at each clock flank. The difficulty lies in creating a realistic model of behavior. The project has focused on meeting the phase noise requirements imposed on a WCDMA / HSDPA application. The event-driven model is implemented in Matlab because of its high flexibility during development, and large variety of analytical tools. The proposed model is based on a previously published model that has been evolved in ways that were interesting for the project. The model’s construction and accuracy have been verified against the appropriate theory. By constructing a comprehensible user interface around the model, it is convenient to examine how different parameters affect system performance. The simulation results of the model establish how the different parameters affect the phase noise spectrum of the ADPLL. The TDC architecture has big influence on the phase noise and it is of big importance to use high precision in the entire system to prevent an increased in-band noise level. A time-effective simulation tool has successfully been constructed and a sub-block requirement specification has been presented. / Examensarbetet “Modeling and Characterization of an All-Digital PLL” har som syfte att skapa en beteendemodell av en All-Digital Phase-Locked-Loop (ADPLL). Modellen ska kunna generera noggranna och tidseffektiva simuleringar. Utifrån modellen ska sedan en kravspecifikation för de olika delblocken skapas för att utgöra ett beslutsunderlag för eventuell tillverkning av testchip. Bakgrunden till projektet är att den trådlösa kommunikationsindustrin under de senaste åren har vuxit enormt vilket lett till stor efterfrågan på mindre, snabbare, bättre och energisnålare kretsar. Digitala kretsar har bättre egenskaper i dessa avseenden vilket resulterat i ett stort intresse för kretsar av denna typ. Eftersom frekvenssyntetiseringen utgör en central del i alla trådlösa system är en helt digital PLL mycket attraktiv. Traditionella simuleringsverktyg har inte möjlighet att simulera ett så komplext system som en ADPLL. Då tillverkningskostnaderna är höga och det är nödvändigt att kontrollera designens egenskaper och uppförande innan första prototyp, är det ett måste att finna alternativa lösningar. En lösning är då att använda en händelsestyrd simuleringsteknik som endast fokuserar på de händelser som sker vid respektive klockflank. Svårigheten ligger i att skapa en realistisk beteendemodell. Projektet har fokuserat på att klara de krav på fasbrus som ställs på en WCDMA/HSDPA applikation. Den händelsestyrda modellen har skapats i Matlab på grund av dess stora flexibilitet under utveckling samt stora flora av analysverktyg. Den föreslagna modellen utgår från en tidigare publicerad modell som har utvecklats i de avseenden som varit intressanta för projektet. Modellens uppförande och noggrannhet har kunnat verifieras mot adekvat teori. Ett överskådligt användargränssnitt runt modellen möjliggör undersökning av olika parametrars påverkan på systemets prestanda. Simuleringsresultaten av modellen fastställer hur olika parametrar påverkar ADPLL fasbruset. TDC-arkiteturen har stor påverkan på fasbruset och det är viktigt att använda hög upplösning på hela systemet för att förhindra att ”in-band” brusnivån ökar. Med gott resultat har ett tidseffektivt simuleringsverktyg skapats och en kravspecifikation för de olika delblocken har presenterats.

<p>This thesis presents work performed on explosively driven magnetic flux compression generators. This kind of devices converts the chemically stored energy in a high explosive into electromagnetic energy in the form of a powerful current pulse. The high energy density of the high explosives makes flux compression generators attractive as compact power sources. In order to study these devices a generator was designed at FOI in the mid-90ies. Two generators remained unused and became available for this licentiate work.</p><p>The thesis reports experiments with, and simulations of, the operation of the two remaining generators. The aim was to fully understand the performance of the generator design and be able to accurately simulate its behaviour. The generators were improved and fitted with various types of diagnostics to monitor the generator operation.</p><p>Two experiments were performed of which the first generator was operated well below its current capability limits while the second was stressed far above its limits. Since the generator generates a rapidly increasing current, a current measurement is the most important diagnostic revealing the current amplification of the generator and its overall performance. Further it is important to measure the timing of various events in the generator. With a common time reference it is possible to combine data from different probes and extract interesting information which cannot be directly obtained with a single measurement.</p><p> </p><p> </p>

QC 20110502

The streaming plasma in the solar wind is a never ending source of energy, plasma, and momentum for planetary magnetospheres, and it continuously drives large-scale plasma convection systems in our magnetosphere and over our polar ionosphere. This coupling between the solar wind and the magnetosphere is primarily explained by two different processes: magnetic reconnection at high latitudes, which interconnects the interplanetary magnetic field (IMF) with the planetary dipole field, and low-latitude dynamos such as viscous interaction, where the streaming plasma in the solar wind may trigger waves and instabilities at the flanks of the magnetosphere, and thereby allow solar wind plasma to enter into the system.This work aims to further determine the nature and properties of these driving dynamos, both by statistical studies of their relative importance for ionospheric convection at Earth, and by assessment and analysis of the Kelvin-Helmholtz instability at Mercury, utilizing data from the MESSENGER spacecraft's first and third flyby of the planet.It is shown that the presence of the low-latitude dynamos is primarily dependent on the IMF direction: the driving is close to non-existent when the IMF is southward, but increases to the order of a third of the total ionospheric driving when the IMF turns northward (here, the magnitude of the driving is also shown to be dependent on the viscous parameters in the solar wind). The work also discusses the saturation of the reconnection generated potential, and shows that the terrestrial response follows a non-linear behavior for strong solar wind driving both when the IMF is southward and northward.Comparative studies of different magnetospheres provide an excellent path for increasing our understanding of space-related phenomena. Here, study of the Kelvin-Helmholtz instability at Mercury allows us to investigate how the different parameters of the system affect the mass, energy, and momentum transfer at the flanks of the magnetosphere. The large ion gyro radius expected is shown to develop a dawn-dusk asymmetry in the growth rates, with the dawn side as the more unstable of the two. This effect should be particularly visible when the planet is close to perihelion. Mercury's smaller scale size combined with the relatively high spacecraft velocity is also shown to provide excellent opportunities for studying the spatial structure of the waves, and a vortex reconstruction that can explain all the large-scale variations in the Kelvin-Helmholtz waves observed during MESSENGER's third Mercury flyby is presented. / QC 20110405

High quality perovskite manganites, La1-xAxMnO3 (A = Ca, Sr, Ba) are very attractive materials due to their great application potential for magnetic memory, uncooled infrared (IR) microbolometer and spintronics devices. This thesis presents studies of the growth and material characterization (including structural, electrical, magnetic and noise) of epitaxial manganite films on Si and GaAs. Furthermore, investigations about strain effect on structural and electrical properties of manganites, and finally fabrication of self-supported free standing microstructures for uncooled IR bolometer are also demonstrated. To obtain high quality epitaxial manganite films on semiconductor substrates at room temperature, using a combination of La0.67Sr0.33MnO3 (LSMO) and La0.67Ca0.33MnO3 (LCMO) compounds, La0.67(Sr,Ca)0.33MnO3 (LSCMO) films were successfully grown on Si substrates with Bi4Ti3O12(BTO)/CeO2/YSZ buffers by pulsed laser deposition (PLD) technique. Crystallographic relations between layers shows cube-on-cube for BTO/CeO2/YSZ/Si and diagonal-on-side for LSCMO films on BTO layer. 4.4 %K-1 maximum temperature coefficient of resistivity (TCR = 1/ρ·dρ/dT) and 2.9 %kOe-1 colossal magnetoresistance (CMR) were obtained at room temperature. Assuming of a prototype of temperature sensor, 1.2 μK/√Hz of noise equivalent temperature difference (NETD) and 2.9×108 cm√Hz/W of detectivity are expected to achieve at 294 K, 30 Hz. For GaAs substrates, using MgO buffer layer, LCMO films shows 9.0 %K-1 of TCR at 223 K while LSMO exhibits 2 %K-1 at 327 K. Systematic strain effects on structural and electrical properties of La0.75Sr0.25MnO3 LSMO) films on BTO/CeO2/YSZ-buffered Si, Si1-xGex/Si (compressive strain, x = 0.05-0.20) and Si1-yCy/Si (tensile, y = 0.01) were investigated. The strain induced from Si1-xGex/Si and Si0.99C0.01/Si has a tendency to decrease the roughness of CMR films compared to Si sample. High resistivity and low TCR values are observed for Si0.8Ge0.2/Si and Si0.99C0.01/Si samples due to excessive strains whereas Si0.9Ge0.1/Si and Si0.95Ge0.05/Si show slight improvements of films quality and TCR value. To fabricate LSCMO manganite bolometer on Si, wet etching with KOH and BHF and dry etching methods with Ar ion beam etching (IBE) were studied. For KOH wet etching, LSCMO films show high chemical resistance with lower than 0.2 nm/min of etch rate. BHF wet etching shows high etching selectivity over photoresist mask and silicon substrates. The etch rates for LSCMO and BTO layers are 22 and 17 nm/min. For Ar IBE, LSCMO films and oxide buffer layers show similar etch rates, 16-17 nm/min that are lower compared to 24 nm/min for Si. Free standing, self-supported heteroepitaxial LSCMO/BTO/CeO2/YSZ membranes for bolometer pixels on Si was successfully fabricated by Ar IBE and ICP etching techniques using a preannealed photoresist. The structural investigation by TEM revealed the sharp interfaces between layers. The electrical property of the free standing membrane was slightly degraded due to strain release and multi-step etching effect. These results demonstrate feasibility to use heteroepitaxial oxide structures as a thermally isolated membrane with conventional photoresist patterning. / QC 20101101