Charge Collection Mechanism in MEMS Capacitive Switches

The present paper investigates the effect of stressing bias magnitude and stressing time on the discharging process in MEMS capacitive switches. The calculation of discharge current through the dielectric film is based on monitoring the rate of shift of bias for up-state minimum capacitance. The data analysis shows that the discharge current lies in the range of femto-Amperes and the calculated discharge time constant depends directly on the time window of observation and on the stressing conditions. Moreover the analysis reveals an increase of trapped charge that remains in the bulk of the dielectric film for very long time as the stressing bias increases. The dominant discharge process, taking place under an intrinsic field of about 10^3 V/cm, is found to be the hopping effect

A MIM CAPACITOR STUDY OF DIELECTRIC CHARGING FOR RF MEMS CAPACITIVE SWITCHES

MIM capacitors are considered equally important devices for the assessment of dielectric charging in RF MEMS capacitive switches.Beside the obvious similarities between the down state condition of RF MEMS and MIM capacitors there are also some important differences. The paper aims to introduce a novel approach on the study of dielectric charging in MEMS with the aid of MIM capacitors by combining experimental results obtained by the application of DC, Charging Transient and Kelvin Probe techniques.The strengths and weaknesses are discussed in conjuction with experimental results obtained on SiNx based MIM capacitors ans MEMS capacitive switches fabricated under the same conditions

RF signals over field emission currents: a theoretical study for MEMS capacitive switches

The existence of (sub)micrometer scale gaps in Micro-Electro-Mechanical-Systems (MEMS) gives rise to field emission currents and this is already considered a reliability issue resulting in device degradation and/or failure. This work aspires to offer another perspective with respect to the field emission related phenomena in Radio Frequency (RF)-MEMS, focusing the attention prior to the failure and emphasizing on a reliability aspect affecting the signal integrity. This stems from the non-linear nature of the field emission currents and instigated during their simultaneous presence with RF signals, particularly of high power. Theoretical calculations reveal that this combination results in the generation of new harmonics in addition to the stimulated one. This effect dependents on the distortion induced in the field emission current by the simultaneous excitation by both the DC and the RF biases. Apart from the applied biases, additional parameters contributing indirectly, such as the operation frequency and the device characteristics are having a major role. These outcomes should therefore be considered when designing (high-power) RF MEMS applications.The existence of (sub)micrometer scale gaps in Micro-Electro-Mechanical-Systems (MEMS) gives rise to field emission currents and this is already considered a reliability issue resulting in device degradation and/or failure. This work aspires to offer another perspective with respect to the field emission related phenomena in Radio Frequency (RF)-MEMS, focusing the attention prior to the failure and emphasizing on a reliability aspect affecting the signal integrity. This stems from the non-linear nature of the field emission currents and instigated during their simultaneous presence with RF signals, particularly of high power. Theoretical calculations reveal that this combination results in the generation of new harmonics in addition to the stimulated one. This effect dependents on the distortion induced in the field emission current by the simultaneous excitation by both the DC and the RF biases. Apart from the applied biases, additional parameters contributing indirectly, such as the operation frequency and the device characteristics are having a major role. These outcomes should therefore be considered when designing (high-power) RF MEMS applications

Investigation of electrical properties of thin film transistor

Aim of the present thesis was to investigate the role of polycrystalline material properties on the electrical behaviour of polysilicon TFTs. The electrical properties have been investigated by the analysis of the transfer and the transient characteristics of TFTs in the linear operation regime. The transfer characteristics analysis reveal that although the influence of grain boundaries and extended defects on the transport properties could be minimized by the use of the properly orientation, their presence was found to determine the device performance through thermal activation of carriers. The activation energy of the thermally activated parameters was found to be material related. The undershoot effect in poly-Si TFTs occurs when the device gate is switched from strong to weak or moderate inversion. The analysis of the undershoot currents suggest the trapping and emission of electrons in the band gap states as the responsible mechanism. Both trapping and emission processes were found to obey the stretched exponential law, while thermally activated time constants, with material related activation energies, were obtained. Based on the understanding of the transient currents in polycrystalline silicon TFTs, a new approach for experimental determination of band gap states density (DOS), by Deep Level Transient Spectroscopy (DLTS), has been presented. An asymmetric U-shaped distribution of states in the band gap has been obtained. The results are in agreement with theoretical predictions and others experimental observations. The temperature analysis of the transfer characteristics has been also implemented in the investigation of the effects of α-particles irradiation on the electrical properties of the devices. As indicated by the thermally activated parameters, generation of states deep in the band gap usually attributed to dangling or floating bonds, is responsible for the device degradation. Furthermore a negative threshold voltage shift is observed attributed to positively charged oxygen vacancies in SiO2 introduced by irradiation Finally the role of back gate on the front channel operation of p-channel double gate devices has been investigated. The results suggest that the presence of back gate can adjust significantly the front gate parameters and also control their temperature dependence allowing the desirable electrical behaviour of double gate TFTs in wide temperature range.Η διδακτορική διατριβή είχε ως στόχο τη μελέτη των ηλεκτρικών ιδιοτήτων λεπτών υμενίων (30nm, 50nm, 100nm) πολυκρυσταλλικού πυριτίου που είχαν δομηθεί με διάφορες μεθόδους ανόπτησης με λέιζερ και της επίδρασης των ιδιοτήτων των υμενίων στα ηλεκτρικά χαρακτηριστικά διατάξεων TFT (Thin Film Transistor).Η μελέτη πραγματοπoιήθηκε με τη καταγραφή και ανάλυση των στατικών χαρακτηριστικών και μεταβατικών ρευμάτων τύπου overshoot και τύπου undershoot, στη γραμμική περιοχή λειτουργίας και για θερμοκρασίες από 120 έως 440 Κ. Η θερμοκρασιακή εξάρτηση των μεγεθών που προέκυψαν από τις χαρακτηριστικές IDS-VGS καμπύλες όπως το ρεύμα διαρροής, η υποκατώφλια κλίση, η τάση κατωφλίου και η ευκινησία, ανέδειξαν την ύπαρξη θερμικά διεγειρόμενων μηχανισμών, υποβοηθούμενων από τις καταστάσεις του χάσματος. Οι μηχανισμοί αυτοί παρουσιάζουν χαρακτηριστική ενέργεια ενεργοποίησης και θερμοκρασία κατωφλίου, που υπολογίστηκε σε διαφορετικές καταστάσεις λειτουργίας της διάταξης (OFF, Sub-threshold, ON). Η παραπάνω τάση επιβεβαιώθηκε από την ανάλυση του μεταβατικού ρεύματος τύπου overshoot. Επιπλέον η καταγραφή και μοντελοποίηση του μεταβατικού φαινομένου undershoot στην περίπτωση των πολυκρυσταλλικών TFTs ανέδειξε επιπλέον πληροφορίες για τη φυσική των καταστάσεων του χάσματος, καθώς αποδόθηκε στη σύλληψη και σταδιακή εκπομπή φορέων στις καταστάσεις αυτές. Η σύγκριση της εξάρτησης των στατικών χαρακτηριστικών και της μεταβατικής απόκρισης από τη θερμοκρασία, οδήγησε στην ανάπτυξη μεθοδολογίας για τον προσδιορισμό της κατανομής των ατελειών στο ενεργειακό χάσμα με την εφαρμογή της μεθόδου “φασματοσκοπία βαθέων σταθμών” (DLTS). Τα αποτελέσματα έρχονται σε συμφωνία με θεωρητικά μοντέλα αλλά και με άλλες κοινώς αποδεκτές πειραματικές τεχνικές. Επιπλέον η επίδραση των ατελειών στα ηλεκτρικά χαρακτηριστικά των διατάξεων μελετήθηκε και με την ανάλυση της επίδρασης ιονίζουσας σωματιδιακής ακτινοβολίας, σωματίων α. Η ανάλυση των παραμέτρων συναρτήσει της θερμοκρασίας ανέδειξε μεταβολή στην ενέργεια ενεργοποίησης των θερμικά διεγειρόμενων μηχανισμών υποδηλώνοντας ως μηχανισμό υποβάθμισης των διατάξεων τη δημιουργία ακόρεστων δεσμών στο υμένιο. Τέλος μελετήθηκαν νέες διατάξεις με δύο πύλες οι οποίες έλεγχαν την πρόσθια και οπίσθια διεπιφάνεια του υμενίου. Τα αποτελέσματα ανέδειξαν πως με κατάλληλη εφαρμογή τάσης στη πίσω πύλη είναι δυνατό να οριστεί με ακρίβεια η τάση κατωφλίου και η θερμοκρασιακή εξάρτηση των παραμέτρων της επάνω πύλης

Interface barriers at Metal – TiO₂ contacts

Metal-oxides combine a unique ensemble of properties presenting great potential to meet the diverse requirements of modern electronics and in particular of brain-inspired applications. Among others, TiO2 is without a doubt one of the most celebrated materials. The ability of TiO2 to obtain different microstructures (i.e. amorphous, rutile etc.) and thus a plethora of electronic properties that can be determined/controlled by the fabrication and/or biasing conditions augmented its use in practical applications, such as memristors, TFTs and sensors. Notwithstanding the importance of the active layer, identifying appropriate metal contacts and deciphering their interfacial role is also of paramount importance to a device’s electrical behaviour. This paper aims to present a detailed quantitative electrical characterization study of Metal-TiO2 interface characteristics.&amp; more...<br/

An electrical characterisation methodology for identifying the switching mechanism in TiO₂ memristive stacks

Resistive random access memories (RRAMs) can be programmed to discrete resistive levels on demand via voltage pulses with appropriate amplitude and widths. This tuneability enables the design of various emerging concepts, to name a few: neuromorphic applications and reconfigurable circuits. Despite the wide interest in RRAM technologies there is still room for improvement and the key lies with understanding better the underpinning mechanism responsible for resistive switching. This work presents a methodology that aids such efforts, by revealing the nature of the resistive switching through assessing the transport properties in the non-switching operation regimes, before and after switching occurs. Variation in the transport properties obtained by analysing the current-voltage characteristics at distinct temperatures provides experimental evidence for understanding the nature of the responsible mechanism. This study is performed on prototyped device stacks that possess common Au bottom electrodes, identical TiO2 active layers while employing three different top electrodes, Au, Ni and Pt. Our results support in all cases an interface controlled transport due to Schottky emission and suggest that the acquired gradual switching originates by the bias induced modification of the interfacial barrier. Throughout this study, the top electrode material was found to play a role in determining the electroforming requirements and thus indirectly the devices’ memristive characteristics whilst both the top and bottom metal/oxide interfaces are found to be modified as result of this process

Electrical characteristics of interfacial barriers at Metal – TiO₂ contacts

The electrical properties of thin TiO2 films have recently been extensively exploited towards enabling a variety of metal-oxide electron devices: unipolar/bipolar semiconductor devices and/or memristors. In such efforts, investigations on the role of TiO2 as active material have been the main focus, however, electrode materials are equally important. In this work, we address this need by presenting a systematic quantitative electrical characterization study on the interface characteristics of Metal-TiO2-Metal structures. Our study employs typical contact materials that are used both as top and bottom electrodes in a Metal-TiO2-Metal setting. This allows investigating the characteristics of the interfaces as well as holistically studying an electrode’s influence to the opposite interface, referred to in this work as top/bottom electrodes interrelationship. Our methodology comprises the recording of current-voltage (I-V) characteristics from a variety of solid-state prototypes in the temperature range of 300-350 K and by analysing them through appropriate modelling. Clear field and temperature dependent signature plots were also obtained towards shinning more light on the role of each material as top/bottom electrodes in Metal-TiO2-Metal configurations. Our results highlight that these are not conventional metal-semiconductor contacts and several parameters such as the electrodes position (atop or below the film), the electronegativity, the interface states and even the opposite interface electrode material are involved on the formation of the interfacial barriers. Overall our study provides a useful database for selecting appropriate electrode materials in TiO2 based devices, offering new insights on the role of electrodes on metal-oxide electronics applications

Conduction mechanisms at distinct resistive levels of Pt/TiO_2-x/Pt memristors

Resistive random access memories (RRAMs) are considered as key enabling components for a variety of emerging applications due to their capacity to support multiple resistive states. Deciphering the underlying mechanisms that support resistive switching remains to date a topic of debate, particularly for metal-oxide technologies and is very much needed for optimizing their performance. This work aims to identify the dominant conduction mechanisms during switching operation of Pt/TiO2-x/Pt stacks, which is without a doubt one of the most celebrated ones. A number of identical devices were accordingly electroformed for acquiring distinct resistive levels through a pulsing-based and compliance-free protocol. For each obtained level the switching current-voltage (I-V) characteristics were recorded and analyzed in the temperature range of 300 K to 350 K. This allowed the extraction of the corresponding signature plots revealing the dominant transport mechanism for each of the I-V branches. Gradual (analogue) switching was obtained for all cases and two major regimes were identified. For the higher resistance regime the transport at both the high and low resistive states was found to be interface controlled due to Schottky emission. As the resistance of devices reduces to lower levels, the dominant conduction changes from an interface to core-material controlled mechanism. This study overall supports that engineering the metal-oxide/metal electrode interface can lead to tailored barrier modifications for controlling the switching characteristics of TiO2 RRAM

Conduction mechanisms in Pt/TiO₂/Pt memristors

Resistive random access memories (RRAMs) receive increasing attention as very promising candidates for the next generation of nonvolatile memories, as well as for artificial neuronetworks and reconfigurable systems developments. A key feature for the aforementioned applications is their ability to obtain multiple resistive levels by proper tuning of the biasing schemes. Therefore, clarification of the dominant conduction mechanism, which remains to date a topic of debate, is of a paramount importance. This strongly depends on the device characteristics (oxide and electrodes’ materials) and the programming bias schemes. &amp; more..