24 research outputs found
Silicon doping techniques using chemical vapor dopant deposition
No full textUltrashallow junctions are essential for the achievement of superior transistor performance, both in MOSFET and bipolar transistors. The stringent demands require state-of-the-art fabrication techniques. At the same time, in a different context, the accurate fabrication of various n type doping profiles by low-temperature Si epitaxy is a challenge due to autodoping. In this thesis, these two, apparently unrelated, problems are both addressed as the layer of CVD surface-deposited dopant atoms is used as a doping source. It is demonstrated that a layer of dopants deposited on the Si surface can be used as a doping source by either thermal or laser drive-in for the fabrication of both deep and ultrashallow defect-free junctions. In low-temperature CVD epitaxy, autodoping is a consequence of dopant surface segregation and doping from the surface layer. This process has been characterized, and consequently excellent controllability is achieved. In addition, new results related to the CVD of dopants itself are obtained, and two theoretical achievements are made: the analytical model of arbitrarily shallow junctions is derived, and a new C-V profiling technique suitable for the characterization of ultrashallow junctions is developed. The analytical model of arbitrarily shallow junctions, presented in Chapter 2, is derived based on the basic transport mechanisms, with the junction depth as a free parameter, allowing the junction to be fully depleted or disappear. In this way, a closed-form model is obtained that unifies the standard p-n junction and Schottky junction models. With such a unified description, the characteristics of extremely shallow diodes that lie at the cross-over between the standard p-n and Schottky diodes are for the first time analytically described, and new phenomena are observed: both forward and reverse characteristics exhibit specific nonidealities. In that respect, this model can be used to predict the onset of nonidealities as a function of the doping levels, junction depth and biasing conditions. The process of dopant CVD is central to the experimental work presented in this thesis, and it is described in detail in Chapter 3, together with other experimental techniques. Particular focus is given to the deposition of As, the deposition dynamics of which have been empirically modeled to enable accurate sub-monolayer deposition at 800 °C, and the ability to achieve substantial deposition has been demonstrated down to 300 °C. The high-temperature step prior to deposition was found to be unnecessary if Marangoni drying is used. Also, techniques for the removal of As have been investigated: thermal desorption and different wet cleaning solutions. With dopant CVD and drive-in, there are two high-temperature steps to which windows in SiO2 to the Si are exposed: first, for surface cleaning before dopant deposition, and second, for dopant drive-in; and both can have detrimental effects on the definition of deposition windows and on the Si surface. The kind and extent of disfiguration that appears in these two processes has been described in Chapter 4. At temperatures above 900 °C, significant lateral widening of deposition windows is observed. At 1100 °C, also micrometer-deep cavities in the Si form. These spikes typically form in the corners or around the edges of deposition windows, and their generation is catalyzed by the presence of dopants. Contrary to the CVD of B, As and P depositions are limited to a maximum of a single monolayer. This is nevertheless a substantial dose, well suited for the ultrashallow junction formation, which has been demonstrated by thermal and laser drive-in, and the results are presented in Chapter 5. Thermal diffusion was characterized in the temperature range from 700 °C to 900 °C. Due to the high concentration gradient, low diffusivity and SiO2 encapsulation, nanometer-shallow junctions are formed, while no defects are introduced. The diffused dose, however, is only a fraction of a monolayer, and the junctions can even be fully depleted, exhibiting characteristics predicted by the model of arbitrarily shallow junctions. Nevertheless, they can be very effective in modulating the effective Schottky barrier height. With laser annealing, at laser energy densities above the Si melt limit, successful drive-in of the complete monolayer is achieved. Thereby, junctions 10 nm to 15 nm deep, with 3 nm/dec or better vertical abruptness, doped above 3x10^20 cm-3, with sheet resistance around 300 ohm/sq indicative of 100% activation are formed in an essentially room-temperature process. Such exceptionally good results are enabled by the fact that the defect-inducing ion implantation is not used, thus no defect annealing steps are needed. While the doping profiles satisfy the ITRS requirements, the low defect density and low processing temperature make this, in combination with As deposition at 300 °C, a promising solution for junction formation after metallization and for backside junction formation in the SOG substrate-transfer technology. For the low-temperature RPCVD Si epitaxy, an empirical model of As surface segregation, adsorption, and incorporation from the surface layer has been developed, and is presented in Chapter 6. Together with the pure As deposition and As wet chemical removal, this model enables very accurate fabrication of doping profiles of nearly arbitrary shape, the most important restriction being that the profiles can be both continuously or discretely increasing in the growth direction, but only discretely decreasing using ex-situ As cleaning. Based on this model, profiles inversely proportional to the square of the doping depth are fabricated for the novel varactor diodes with state-of-the-art linearity. For the purpose of characterization of ultrashallow junctions, beyond the limitations of SIMS resolution, a new C-V profiling technique has been developed and is presented in Chapter 7. It relies on fabricating and measuring two diodes, one Schottky and one p-n diode, both with the same background doping profile, in order to obtain first the background and then the ultrashallow doping profile. It has been demonstrated that this can be achieved if the background profile contains abrupt changes in doping, e.g. in the form of steps, even if it is considerably less abrupt and less highly doped. Using an epitaxially grown step-like background profile, the ultrashallow B profile formed only by pure B CVD has been measured to have the junction depth of 7 nm, and the slope of around 2.5 nm/dec.Microelectronics & Computer EngineeringElectrical Engineering, Mathematics and Computer Scienc
C-V profiling of ultra-shallow junctions using a buried layer with stepped doping
No full textThe paper investigates two-sided capacitance-voltage (C-V) technique for application in doping profile characterization of Si ultra shallow p+-n junctions. Stepped doping profile in the n region is designed for the accurate determination of xn0, a crucial parameter for the extraction of the doping profile in the p region. Medici simulations are carried out for the C-VR relationships of the p+-n and n-Schottky junctions with the same step-like n profile. The xn0 can be determined with an accuracy of 1.7 nm by a criteria developed in this work. And the doping profile in the p+ doped region can finally be extracted and shown to be in good agreement with the Medici simulation results.Electrical Engineering, Mathematics and Computer Scienc
Analytical model of I–V characteristics of arbitrarily shallow p-n junctions
No full textFor the first time, an analytical model of arbitrarily shallow p-n junctions is presented. Depending on the junction depth, electrical characteristics of ultrashallow p-n junctions can vary from the characteristics of standard Schottky diodes to standard deep p-n junctions. This model successfully unifies the standard Schottky and p-n diode expressions. In the crossover region, where the shallow doping region can be totally depleted, electrical characteristics phenomenologically substantially different from typical diode characteristics are predicted. These predictions and the accuracy of the presented model are evaluated by comparison with the MEDICI simulations. Furthermore, ultrashallow n+-p diodes were fabricated, and the anomalous behavior in the crossover regime was experimentally observed.Microelectronics & Computer EngineeringElectrical Engineering, Mathematics and Computer Scienc
Coronary Microvascular Dysfunction and Hypertension: A Bond More Important than We Think
No full textCoronary microvascular dysfunction (CMD) is a clinical entity linked with various risk factors that significantly affect cardiac morbidity and mortality. Hypertension, one of the most important, causes both functional and structural alterations in the microvasculature, promoting the occurrence and progression of microvascular angina. Endothelial dysfunction and capillary rarefaction play the most significant role in the development of CMD among patients with hypertension. CMD is also related to several hypertension-induced morphological and functional changes in the myocardium in the subclinical and early clinical stages, including left ventricular hypertrophy, interstitial myocardial fibrosis, and diastolic dysfunction. This indicates the fact that CMD, especially if associated with hypertension, is a subclinical marker of end-organ damage and heart failure, particularly that with preserved ejection fraction. This is why it is important to search for microvascular angina in every patient with hypertension and chest pain not associated with obstructive coronary artery disease. Several highly sensitive and specific non-invasive and invasive diagnostic modalities have been developed to evaluate the presence and severity of CMD and also to investigate and guide the treatment of additional complications that can affect further prognosis. This comprehensive review provides insight into the main pathophysiological mechanisms of CMD in hypertensive patients, offering an integrated diagnostic approach as well as an overview of currently available therapeutical modalities
POST-PCI LAD ANEURYSM IN A SARS-COV2 IgG POSITIVE PATIENT
No full textABSTRACT
Since its outbreak in December 2019, COVID-19 infection became a pandemic disease, spreading in more than 200 countries within a short period of time. Despite the usual presentation as a respiratory infection, most commonly as bilateral pneumonia, the extra pulmonary manifestations are in greater focus for the last few months. Understanding the pathophysiological mechanisms of different clinical presentations and possible complications are of great importance for further treatment and prognosis. Cardiovascular complications are various including myocardial injury, arrhythmias, myocarditis, arterial, and venous thrombotic events, mainly presented as acute coronary syndrome and pulmonary embolism. Recently, specific hyperinflammatory syndrome within COVID-19 infection has been described in children and occasionally in adults, with features similar to those of Kawasaki disease, leading to multiorgan failure and shock. Although it’s presenting similarly to Kawasaki disease in terms of symptoms, the cases of Kawasaki like hyperinflammatory syndrome with the development of coronary artery aneurysms haven’t been described yet in adults. In this case report, we review a case of SARS-CoV-2 IgG positive 63-year old patient with a large aneurysm of the left anterior descending artery (LAD) after percutaneous coronary intervention and signs of possible viral myocardial involvement.
Keywords: COVID-19, acute coronary syndrome, percutaneous coronary intervention, coronary aneurysm, Kawasaki disease, cardiac magnetic resonance
Incidence and Risk Factors for Clostridioides difficile Infections in Non-COVID and COVID-19 Patients: Experience from a Tertiary Care Hospital
No full text(1) Background: The aim of this study was to assess the incidence and the risk factors for healthcare-associated Clostridioides difficile infection (HA-CDI) in patients with COVID-19 and without this infection. (2) Methods: A single-center, prospective observational study was conducted at the University Clinical Hospital Center in Belgrade, Serbia, from January 2019 to December 2021. The entire hospital was a COVID-dedicated hospital for 12 months during the study period. The incidence density rates and risk factors for HA-CDI in patients with and without COVID-19 are presented. (3) Results: The incidence rates of HA-CDIs were three times higher in patients with COVID-19. The HA-CDI–COVID-patients were younger (69.9 ± 12.6 vs. 72.5 ± 11.6; p = 0.017), admitted from another hospital (20.5% vs. 2.9; p < 0.001), had antimicrobial therapy before CDI (99.1% vs. 91.3%, p < 0.001), received two or more antibiotics (p = 0.030) during a longer period (p = 0.035), received proton pump inhibitors (95.9% vs. 50.0%, p < 0.001) during a longer period (p = 0.012) and steroids (32.8% vs. 20.4%, p < 0.001). During the last month before their current hospitalization, a higher percentage of patients without COVID-19 disease were hospitalized in our hospital (p < 0.001). Independent predictors for HA-CDIs in patients with COVID-19 were admission from another hospital (p = 0.003), the length of antibiotic administration (0.020), and the use of steroids in therapy (p < 0.001). The HA-CDI predictors in the non-COVID patients were older age (p = 0.017), advanced-stage renal failure (p = 0.005), chemotherapy (p = 0.003), and a low albumin level (0.005). (4) Conclusion: Higher incidence rates of HAI-CDIs in COVID-19 patients did not occur due to reduced infection control precautions and hygiene measures but due to antibiotic therapy and therapy with other drugs used during the pandemic
COVID-19 Vaccination Willingness and Vaccine Uptake among Healthcare Workers: A Single-Center Experience
No full textHealthcare workers (HCWs) are at higher risk of developing COVID-19 due to their professional exposition to the SARS-CoV-2 virus. This study assesses the intention of vaccination against COVID-19 before the vaccines were approved, and the rate of vaccine uptake during the first nine months of immunization among HCWs. A cross-sectional seroprevalence study was carried out during July 2020 in University Clinical Hospital Center Bezanijska Kosa in Belgrade, Serbia that included 62.8% of all HCWs. Besides serological testing for IgG antibodies, data about HCWs’ intention to accept COVID-19 vaccination if a vaccine became available were collected. This cohort of HCWs was followed up until the end of October 2021 to assess the number of vaccinated and PCR-positive staff. In the cross-sectional study, 18.3% HCWs had positive SARS-CoV-2 IgG antibodies without difference with IgG-negative HCWs regarding age, gender, profession type, and years of service. Before vaccines became available, a significantly higher percentage of IgG-positive HCWs compared to IgG-negative HCWs was unsure whether to be vaccinated (62.5% vs. 49.0%), and significantly fewer stated that they would not be vaccinated (16.7% vs. 25.1%). When the vaccines became available in Serbia, among IgG-negative HCWs, those who stated clear positive (yes) and clear negative (no) attitude toward vaccination before the immunization period had begun were vaccinated at 28% and 20%, respectively, while 51% of unsure HCWs received a vaccine (p = 0.006). Among IgG-positive HCWs, there was no statistical difference in vaccine uptake regarding those with previous negative, positive, and unsure opinions about vaccination (p = 0.498). In multivariate analysis, independent factors associated with uptake were being female (OR = 1.92; 95%CI: 1.04–3.55), age of 30–59 years, previously vaccine-unsure (OR = 1.84; 95%CI: 1.04–3.25), and those with previous positive vaccine attitudes (OR = 2.48; 95%CI:1.23–5.01), while nurses were less likely to become vaccinated (OR = 0.39 95% CI: 0.20–0.75) These findings indicate a positive change in attitudes of HCWs towards COVID-19 vaccination
Cardiac Magnetic Resonance in Hypertensive Heart Disease: Time for a New Chapter
No full textHypertension is one of the most important cardiovascular risk factors, associated with significant morbidity and mortality. Chronic high blood pressure leads to various structural and functional changes in the myocardium. Different sophisticated imaging methods are developed to properly estimate the severity of the disease and to prevent possible complications. Cardiac magnetic resonance can provide a comprehensive assessment of patients with hypertensive heart disease, including accurate and reproducible measurement of left and right ventricle volumes and function, tissue characterization, and scar quantification. It is important in the proper evaluation of different left ventricle hypertrophy patterns to estimate the presence and severity of myocardial fibrosis, as well as to give more information about the benefits of different therapeutic modalities. Hypertensive heart disease often manifests as a subclinical condition, giving exceptional value to cardiac magnetic resonance as an imaging modality capable to detect subtle changes. In this article, we are giving a comprehensive review of all the possibilities of cardiac magnetic resonance in patients with hypertensive heart disease
Versatile N-Type Profile Engineering by Controlling Arsenic Surface Segregation in Silicon RPCVD
No full textWith the purpose of controlling the incorporation of arsenic during RPCVD Si epitaxy, the surface segregation of As during lowtemperature epi growth was investigated. Parameters such as the Si growth rate, As deposition rate and As incorporation rate, which in previous models were either not evaluated or assumed to be constant, were found here to depend on both the As surface coverage and the arsine partial pressure. Knowledge of these dependencies was employed in simultaneous As deposition and Si epi growth to obtain highlycontrollable doping profiles by appropriate variation of the arsine partial pressure.MicroelectronicsElectrical Engineering, Mathematics and Computer Scienc
Improved RF Devices for Future Adaptive Wireless Systems Using Two-Sided Contacting and A1N Cooling
No full textThis paper reviews special RF/microwave silicon device implementations in a process that allows two-sided contacting of the devices: the back-wafer contacted silicon-on-glass (SOG) substrate-transfer technology (STT) developed at DIMES. In this technology, metal transmission lines can be placed on the low-loss glass substrate, while the resistive/capacitive parasitics of the silicon devices can be minimized by a direct two-sided contacting. Focus is placed here on the improved device performance that can be achieved. In particular, high-quality SOG varactors have been developed and an overview is given of a number of innovative highly-linear circuit configurations that have successfully made use of the special device properties. A high flexibility in device design is achieved by two-sided contacting because it eliminates the need for buried layers. This aspect has enabled the implementation of varactors with special Ndx -2 doping profiles and a straightforward integration of complementary bipolar devices. For the latter, the integration of AlN heatspreaders has been essential for achieving effective circuit cooling. Moreover, the use of Schottky collector contacts is highlighted also with respect to the potential benefits for the speed of SiGe heterojunction bipolar transistors (HBTs).Microelectronics & Computer EngineeringElectrical Engineering, Mathematics and Computer Scienc
