Characterization, modeling and reliability of RF MEMS Switches and Photovoltaic Silicon Solar Cells

The 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

Characterisation and modelling of degradation mechanisms in RF MEMS capacitive switches during hold-down operation

RF MEMS switches represent an attractive alternative technology to current mechanical (e.g. coaxial and waveguide) and solid-state (e.g. PIN diode and FET transistor) RF switch technologies. The materials and fabrication techniques used in MEMS manufacture enable mechanically moveable devices with high RF performance to be fabricated on a miniature scale. However, the operation of these devices is affected by several mechanical and electrical reliability concerns which limit device lifetimes and have so far prevented the widespread adoption and commercialisation of RF MEMS. While a significant amount of research and development on RF MEMS reliability has been performed in recent years, the degradation mechanisms responsible for these reliability concerns are still poorly understood. This is due to the multi-physical nature of MEMS switches where multiple mechanical and electrical degradation mechanisms can simultaneously affect device behaviour with no clear way of distinguishing between their individual effects. As such, little progress has been made in proposing solutions to these reliability concerns. While some RF MEMS switches have recently been commercialised, their success has come at the expense of decreased performance due to design changes necessarily imposed to prevent device failure. However, more high performance switches could be developed if the mechanisms responsible for reliability problems could be understood and solved. The work of this thesis is focussed on the isolation and study of individual reliability mechanisms in RF MEMS capacitive switches. A bipolar hold-down technique is used to minimise the effects of dielectric charging and allow mechanical degradation to be studied in isolation in aluminium-based capacitive switches. An investigation of mechanical degradation leads to the identification of grain boundary sliding as the physical process responsible for the decreased mechanical performance of a switch. An alternative material for the switch movable electrode is investigated and shown to be mechanically robust. The effects of dielectric charging are isolated from mechanical degradation using mechanically robust switches. The isolated investigation of dielectric charging leads to the identification of two major charging mechanisms which take place at the bulk and surface of the dielectric, respectively. The exchange of charge from interface traps is identified as the physical mechanism responsible for bulk dielectric charging. An investigation of surface dielectric charging reveals how this reliability concern depends on the structure and design of a switch. Finally, electrical and material means of minimising dielectric charging are investigated. The findings and results presented in this thesis represent a significant contribution to the state-of the- art understanding of RF MEMS capacitive switch reliability. By implementing the design changes and material solutions proposed in this work, the performance and lifetime of RF MEMS capacitive switches can be greatly improved

Microelectromechanical Systems and Devices

The advances of microelectromechanical systems (MEMS) and devices have been instrumental in the demonstration of new devices and applications, and even in the creation of new fields of research and development: bioMEMS, actuators, microfluidic devices, RF and optical MEMS. Experience indicates a need for MEMS book covering these materials as well as the most important process steps in bulk micro-machining and modeling. We are very pleased to present this book that contains 18 chapters, written by the experts in the field of MEMS. These chapters are groups into four broad sections of BioMEMS Devices, MEMS characterization and micromachining, RF and Optical MEMS, and MEMS based Actuators. The book starts with the emerging field of bioMEMS, including MEMS coil for retinal prostheses, DNA extraction by micro/bio-fluidics devices and acoustic biosensors. MEMS characterization, micromachining, macromodels, RF and Optical MEMS switches are discussed in next sections. The book concludes with the emphasis on MEMS based actuators

Improvements to Micro-Contact Performance and Reliability

Microelectromechanical Systems (MEMS) based devices, and specifically microswitches, continue to offer many advantages over competing technologies. To realize the benefits of micro-switches, improvements must be made to address performance and reliability shortfalls which have long been an issue with this application. To improve the performance of these devices, the micro-contacts used in this technology must be understood to allow for design improvements, and offer a means for testing to validate this technology and determine when such improvements are ready for operational environments. To build devices which are more robust and capable of continued operation after billions of cycles requires that improved fabrication techniques be identified and perfected to allow for more sophisticated designs to be tested

DEVELOPMENT OF NEMS RELAYS IN LOGIC COMPUTATION AND RUGGED ELECTRONICS

Ph.DDOCTOR OF PHILOSOPH

CMOS Compatible Thin-Film ALD Tungsten Nanoelectromechanical Devices

This research focuses on the development of a novel, low-temperature, CMOS compatible, atomic-layer-deposition (ALD) enabled NEMS fabrication process for the development of ALD Tungsten (WALD) NEMS devices. The devices are intended for use in CMOS/NEMS hybrid systems, and NEMS based micro-processors/controllers capable of reliable operation in harsh environments not accessible to standard CMOS technologies. The majority of NEMS switches/devices to date have been based on carbon-nano-tube (CNT) designs. The devices consume little power during actuation, and as expected, have demonstrated actuation voltages much smaller than MEMS switches. Unfortunately, NEMS CNT switches are not typically CMOS integrable due to the high temperatures required for their growth, and their fabrication typically results in extremely low and unpredictable yields. Thin-film NEMS devices offer great advantages over reported CNT devices for several reasons, including: higher fabrication yields, low-temperature (CMOS compatible) deposition techniques like ALD, and increased control over design parameters/device performance metrics, i.e., device geometry. Furthermore, top-down, thin-film, nano-fabrication techniques are better capable of producing complicated device geometries than CNT based processes, enabling the design and development of multi-terminal switches well-suited for low-power hybrid NEMS/CMOS systems as well as electromechanical transistors and logic devices for use in temperature/radiation hard computing architectures. In this work several novel, low-temperature, CMOS compatible fabrication technologies, employing WALD as a structural layer for MEMS or NEMS devices, were developed. The technologies developed are top-down nano-scale fabrication processes based on traditional micro-machining techniques commonly used in the fabrication of MEMS devices. Using these processes a variety of novel WALD NEMS devices have been successfully fabricated and characterized. Using two different WALD fabrication technologies two generations of 2-terminal WALD NEMS switches have been developed. These devices have functional gap heights of 30-50 nm, and actuation voltages typically ranging from 3-5 Volts. Via the extension of a two terminal WALD technology novel 3-terminal WALD NEMS devices were developed. These devices have actuation voltages ranging from 1.5-3 Volts, reliabilities in excess of 2 million cycles, and have been designed to be the fundamental building blocks for WALD NEMS complementary inverters. Through the development of these devices several advancements in the modeling and design of thin-_lm NEMS devices were achieved. A new model was developed to better characterize pre-actuation currents commonly measured for NEMS switches with nano-scale gate-to-source gap heights. The developed model is an extension of the standard field-emission model and considers the electromechanical response, and electric field effects specific to thin-film NEMS switches. Finally, a multi-physics FEM/FD based model was developed to simulate the dynamic behavior of 2 or 3-terminal electrostatically actuated devices whose electrostatic domains have an aspect ratio on the order of 103. The model uses a faux-Lagrangian finite difference method to solve Laplaces equation in a quasi-statatically deforming domain. This model allows for the numerical characterization and design of thin- _lm NEMS devices not feasible using typical non-specialized BEM/FEM based software. Using this model several novel and feasible designs for fixed-fixed 3-terminal WALD NEMS switches capable for the construction of complementary inverters were discovered

Energy autonomous systems : future trends in devices, technology, and systems

The rapid evolution of electronic devices since the beginning of the nanoelectronics era has brought about exceptional computational power in an ever shrinking system footprint. This has enabled among others the wealth of nomadic battery powered wireless systems (smart phones, mp3 players, GPS, …) that society currently enjoys. Emerging integration technologies enabling even smaller volumes and the associated increased functional density may bring about a new revolution in systems targeting wearable healthcare, wellness, lifestyle and industrial monitoring applications

Carbon nanotube surfaces for low force contact application

This thesis focuses on developing a testing method to estimate the mechanical and electrical characteristics of CNT-based contact surfaces. In particular the use of gold thin films deposited on a CNT forest has potential to offer a very effective contact surface. Two different pieces of experimental apparatus were used in this research to determine the mechanical and electrical properties of gold/multi-walled carbon nanotube (Au/MWCNT) composites: 1) a modified nano-indentation; and 2) a PZT actuator test rig. These apparatuses were used to mimic force-displacement and contact behaviour of the MEMS relay?s contact at a maximum load of 1 mN with dry-circuit and hot-switched conditions. The surfaces were compared with reference Au-Au and Au-MWCNT contact pairs studied under the same experimental conditions. In the modified nano-indentation experiment, tests of up to 10 cycles were performed. The results showed that the Au-MWCNT pair electrical contact resistance improved when the Au-Au/MWCNT pair was used. Additionally the Au-Au/MWCNT pair electrical contact resistance (Rc) was comparable with the Au-Au contact pair. When the Au-Au/MWCNT composite surface is in contact with the Au hemispherical probe it provides a compliant surface. It conforms to the shape of the Au hemispherical probe. For a higher number of cycles, a PZT actuator was used to support Au/MWCNT planar coated surfaces. This surface makes electrical contact with a gold coated hemispherical probe to mimic the actuation of a MEMS relay?s contact at higher actuation frequencies. This apparatus allows the performance of the contact materials to be investigated over large numbers of switching cycles. Different current loads were used in this experiment, 1mA, 10mA, 20mA and 50mA at 4V supply. The Rc of these surfaces was investigated as a function of the applied force under repeated cycles. Under current loads of 1mA and 10mA the Au/MWCNT composite surface provides a stable contact resistance of up to more than a million cycles and no degradation was observed on the surface. Compared with Au-Au contact pair, degradation occurred after 220 cycles. The Au-Au contact pair shows delamination of the Au surface on the probe. The possible reason is the softening or melting of the Au surface. Furthermore, under higher current loads of 20mA to 50mA, degradation had occurred after 50 million cycles (at 20mA) and degradation had occurred at around 45 to 150 cycles for 50mA to 30mA respectively. This is because of the softening or melting of the Au and Au fatigue after a large number of cycles. This study is the first step to show the potential of CNT surfaces as an interface in low force electrical contact applications. With this research, current trends in materials used on contacts and fabrication methods can be explored and even modified or adopted. The use of CNT?s and their composites for contacts can be tested using the available apparatus to look at their performance and reliability in terms of mechanical and electrical properties. This is useful for MEMS contacts that form part of MEMS relay devices

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact