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Characterization, modeling and reliability of RF MEMS Switches and Photovoltaic Silicon Solar Cells
Full text linkThe main goal of this thesis is the failure and reliability investigation of RF-MEMS switches and photovoltaic solar cells. For technical developer people the reliability issue is often consider a secondary problem in electronic devices since it is not considered an important factor in the production chain. This concept is changing is the last years because reliability studies are considered an important technological step to improve the production process. This fact is confirmed by the investments that companies adopt to test their products. In the particular case of this thesis, we can easily mention the solar cell production line where the cells are subjected to reliability tests that extrapolate the efficiency and the fill factor in order to study the performances and to consequently improve the production process.
Concerning RF MEMS
Wireless communication systems for space applications require electronic components with a high level of reliability, a low power consumption and they should be as small as possible in order to be better integrated in satellites. Radio Frequency Micro Electro Mechanical System (RF-MEMS) can be considered one of the best candidates to comply with previous requirements and, under certain conditions, they can completely replace an entire solid-state circuit. RF-MEM devices in general are characterized by a good miniaturization, an easily integration in a standard solid-state circuit, an almost zero power consumption, a good RF linearity and a high quality factor Q.
Concerning RF-MEMS switches RF performances, they exhibit a very low insertion loss, lower than 0.1 dBm up to 60 GHz and, at the same time, a good isolation, more than 20 dBm. From an electrical and mechanical point of view the power consumption of these switches is close to zero because of an “on-state” current around pA and they are almost unaffected by high level of acceleration or deceleration because of their mass that is extremely small. The possibility to integrate the production of these devices in the standard foundry silicon processes and their integration with mature semiconductor technology are a great advantage for their spread making possible to produce them in an easy and cheap way.
Over the last 10 years important developments on MEMS switches have been done all over the world. As a matter of fact, these switches are quite attractive since they combine excellent RF performances and low power consumption of mechanical switches with the small size and low weight of semiconductor devices. However, the appearance of MEMS switches on the market has been hindered by the need for specific packaging as well as by reliability issues. Reliability is a major issue for any satellite since it is almost impossible to envisage any repair work once the spacecraft has been launched. Hence, reliability is a key driver when designing any RF equipment. If we consider a RF-MEMS switch, we have to guarantee that his electromechanical performances will be the same after an intensive usage in harsh environment, for instance after millions or billions of cycles and after the exposure to different kind of radiations. In case of their application in a redundancy scheme, they have to be completely operative even after a long period of activity or inactivity.
The aim of this thesis is to perform an electrical characterization and several reliability tests on different kind of RF-MEMS switches in order to analyze which are the weaknesses and the strengths of this new technology. Electrical characterizations have been done using two different measurement systems. The first, based on a vector network analyzer and a power supply, has been used to test the RF performances of the devices and to extract the actuation and deactuation voltages. The second set up, based on the internal RF signal generator of the VNA, an 8-GHz digital signal oscilloscope and a profilometer (polytec MSA 500), has been used to characterize the electrical performances like actuation time, release delay and dynamic performances.
Cycling stress, one of the most common test used to understand the robustness of this kind of devices, has been performed on different topologies of switch in order to better understand how some parameters of the RF MEMS switch, such as the shape of the beams or the actuation voltage, impact on the reliability of the device. Furthermore, the influence of continuous actuation stress on the reliability of dielectric-less switches has been investigated, comparing different designs and studying the variation of the main electrical parameters induced by the stress and the successive recovery phase.
Concerning PV solar cells
A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. The operation of a photovoltaic (PV) cell requires 3 basic attributes: (i) the absorption of light, generating either electron-hole pairs or excitons, (ii) the separation of charge carriers of opposite types and (iii) the extraction of those carriers to an external circuit.
Over the last decades, many research groups have tried to improve the conversion processes in order to increase the efficiency of solar cells and to reduce the parasitic effects that limit the energy conversion. This has generated a real challenge to the best conversion efficiency. The average efficiency of multicrystalline silicon solar cells at the beginning of 2014 was about 16% but in research labs different solar cells have exceeded the 20% with records over 24%. The continuous growth of the solar cells efficiency has been achieved thanks to the reliability study of the single cells and to the degradation analysis of the real photovoltaic systems. These studies have revealed the critical points of PV solar cells and have led to a constant improvement of the production processes.
The aim of this thesis is the study of the reliability problems related to a single solar cell and to a string of solar cells subjected to different illumination conditions. Different characterization procedures have been developed in order to study the failure mechanisms and to study the weaknesses and the strengths of the technology. Four types of measurement set-ups have been utilized: (i) the first system is able to extract the IV curves in dark and light conditions. This simple measurement procedure has to be opportunely calibrated in order to obtain right results in term of efficiency and fill factor. (ii) The second system extracts the thermographic image of a single solar cell. It can be used to analyze hot spot and other failure mechanisms in the silicon structure. (iii) The third system extracts the electroluminescence and the photoluminescence of a single solar cell. It is able to extract and analyze the defects in the crystalline structure of the materials. (iv) The fourth is the LOANA system: a commercial tool able to extract the External Quantum Efficiency and the Internal Quantum Efficiency with the measurement of the reflectance.
All these characterization procedures have been utilized to study the evolution of the failure mechanisms when a single solar cell is subjected to reverse biasing stresses. The study of the catastrophic degradation of solar cells submitted to reverse current stress is of crucial importance since the failure can lead to the rapid increase of the temperature with a consequent risk of fire and to the breaking of the entire PV system. This particular situation can occur when the PV system is not uniformly illuminated and the solar cells of the system present not uniform shunt resistance.
Additional studies have been performed in the modelization of a solar cell with the two-diode model. The study and modeling of solar cells allow to obtain right results in term of efficiency and fill factor extrapolation. Moreover, the modelization allows the study of string of solar cells working in particular conditions in which the illumination level is not uniform in a whole panel. The simulations allow to predict the dangerous situations and to design appropriate prevention systems
Real-time Identification and Tracking Using Kinect. Multimodal Interaction and Performance Analysis
No full textMicrosoft Kinect motion sensor can capture information of different nature
and it originated several applications in the field of human-computer interaction.
To achieve natural interaction, an application to detect and follow the
user’s movements and simultaneously track his voice is proposed when more
than one person is in front of the sensor. Using gesture recognition to identify
the active speaker, the microphone array is virtually pointed to him through a
beamforming technique to follow his movements. Consequently, the user can
speak without the necessity of being close to the sensor. Excellent results are
obtained regarding accuracy and execution speed
TECNICHE DI INTELLIGENZA COMPUTAZIONALE PER RILEVAZIONE E ANALISI DI DEPOSITI DI POLVERE IN AMBIENTI MUSEALI
No full text
Electro-Mechanical Characterization of the Dynamic Behavior of Ohmic RF MEMS Switches
No full textThe influence of the bias signal waveform on
the electro-mechanical dynamic response of ohmic
RF-MEMS switches is here investigated. Laser Doppler
Vibrometer and electrical measurements were performed to
characterize the actuation time, as well as bounces
occurrences and reliability to cycling stress. A practical
solution to limit bounces without compromise the actuation
time is also presented
Preconditioning Procedure for the Better Estimation of the Long-Term Lifetime in Microelectromechanical Switches
No full textThe study of long-term reliability of RF-MEMS switches subjected to continuous biasing involves different failure mechanisms: 1) short-to-medium-term stress inducing permanent phenomena, such as permanent mechanical degradation, and 2) long-term stress inducing recoverable phenomena related
to mechanical viscoelastic mechanism. The lifetime prediction should consider all the long-term effects after the saturation of
all the short-to-medium-term degradation effects. In fact, the lifetime estimation is affected by both the different short-term
and long-term degradation mechanisms and not separating the different effects may lead to wrong lifetime prediction. In this
paper, we define a preconditioning technique to be applied to each RF-MEMS switch before any stress test. The main purpose
is to eliminate all the short-term permanent effects to obtain a lifetime estimation only related to the long-term phenomena
A Comprehensive Study of MEMS Behavior under EOS/ESD Events: Breakdown, Dielectric Charging, and Realistic Cures
No full textThe breakdown characterization of both out- and in-plane electrostatically actuated RF-MEMS switches with air-gaps from 1.0 to 6.7 μm was studied. The emitted electromagnetic field during the testing was also analyzed, in order to have a certain indication of the air-breakdown occurrence. Furthermore, we studied the effect of TLP and HBM events on the dielectric charging of tested MEMS, furnishing the experimental evidence that ESD events should not be responsible of this important reliability problem for MEMS, and that ESD tester parasitic elements can seriously influence the MEMS electromechanical behavior characterization. Finally, a simple, but effective, varistor based protection structure was explored
A Combined Mechanical and Electrical Characterization Procedure for Investigating the Dynamic Behavior of RF-MEMS Switches
No full textThis paper shows the potentialities of two characterization procedures on the electrical and mechanical characterizations of ohmic RF microelectromechanical systems (MEMS) switches. The first is a "fast electrical" procedure that uses an electrical stimulus and monitors the RF signals at the input and output ports of the switch; the second is a "fast hybrid" procedure, i.e., electrical and mechanical, with an electrical input adopted to actuate the device and a mechanical measurement conducted with an optical profilometer, which monitors the displacement and the velocity of the moving membrane when the input electrical signal is applied. Both systems are validated on cantilever and clamped-clamped resistive RF-MEMS switches. We developed these measurement procedures to speed up the measurement process and, consequently, to limit charge trapping during the characterization process. In future analyses, the procedure will be systematically applied to investigate reliability issues when the switch is subjected to multiple impacts and long-term actuation. The use of such procedures will permit separating electrical and mechanical failure mechanisms
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