Fabrication of electronically switchable and tunable bulk acoustic wave resonators with graphene electrodes

This work is devoted to the fabrication of multilayer hetero-structure for switchable and tunable bulk acoustic wave resonators. This structure uses a ferroelectric material in the paraelectric phase as the resonator layer. The switchable and tunable abilities are achieved by varying the external DC electric field applied. The intermediate electrode chosen is single layer graphene with an ultra-thin Ti/Au capping layer. The device uses all metal Bragg reflectors to confine the acoustic wave within the resonator layer. The device structure is silicon with 90nm oxide layer/Bragg reflectors/graphene/metal/BST/graphene/top electrode. Chemical Vapour Deposition (CVD) has been used to grow the graphene, PLD has been used to deposit the BST layer, a Mantis sputtering system has been used to grow the Bragg reflector layer and the electrode metal layers, and photolithography, ion milling and reactive ion etching have been used to pattern the device. It has been found that for the graphene/metal bilayer, graphene with Ti (5nm) + Au (5nm), is the most suitable combination for this project. The measured resistance is 6.5Ω, and the graphene underneath is preserved after the sputtering of Ti. High temperature annealing of graphene is carried out to examine the mechanisms that cause the elimination of graphene during the growth of TiN. It is suspected that the difference in thermal expansion coefficient between the graphene and the silicon substrate creates cracks on the graphene layer, and the subsequent sputtering process destroys the graphene. For the growth of Bragg reflectors, Ti/Ru has been chosen for the growth of switchable and tunable TFBARs due to durability for the high temperature growth process. For the BST based full device, resonance has been observed under an applied external ACbias at various frequencies, but the crystallinity of the BST layer can be further optimised to achieve higher quality factor.Open Acces

Fabrication of ferroelectrics based MEMS structures for electronically switchable bulk acoustic wave resonators

The thesis describes the research carried out into fabrication of multilayer microwave capacitance structure with ferroelectric films in paraelectric state; and confirmation of the possibility to develop on their base an electronically switchable bulk acoustic wave (BAW) resonator. Different eigenmodes of acoustic resonances can be excited and switched electronically through the application to ferroelectric layers of the resonator unidirectional or oppositely directed dc biased electric fields.The resonator was fabricated out of a SrRuO3/SrTiO3/SrRuO3/YSZ multilayer structure deposited on top of Si substrate. Pulsed Laser Deposition, Magnetron Sputtering, Photolithography, Argon Ion Beam Milling, and Reactive Ion Etching were the fabrication methods used to make this resonator.This novel device is a demonstrator that will contribute to the telecommunications industry’s demand for flexibility in both microwave frequency switching and tuning. The Si MEMS concept of this resonator allows easy circuit board integration into many electronics products.Open Acces

Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

Recently, piezoelectric thin films including zinc oxide (ZnO) and aluminium nitride (AlN) have found a broad range of lab-on-chip applications such as biosensing, particle/cell concentrating, sorting/patterning, pumping, mixing, nebulisation and jetting. Integrated acoustic wave sensing/microfluidic devices have been fabricated by depositing these piezoelectric films onto a number of substrates such as silicon, ceramics, diamond, quartz, glass, and more recently also polymer, metallic foils and bendable glass/silicon for making flexible devices. Such thin film acoustic wave devices have great potential for implementing integrated, disposable, or bendable/flexible lab-on-a-chip devices into various sensing and actuating applications. This paper discusses the recent development in engineering high performance piezoelectric thin films, and highlights the critical issues such as film deposition, MEMS processing techniques, control of deposition/processing parametres, film texture, doping, dispersion effects, film stress, multilayer design, electrode materials/ designs and substrate selections. Finally, advances in using thin film devices for lab-on-chip applications are summarised and future development trends are identified.The authors acknowledge support from the Innovative electronic Manufacturing Research Centre (IeMRC) through the EPSRC funded flagship project SMART MICROSYSTEMS (FS/01/02/10), Knowledge Transfer Partnership No KTP010548, EPSRC project EP/L026899/1, EP/F063865/1; EP/F06294X/1, EP/P018998/1, the Royal Society-Research Grant (RG090609) and Newton Mobility Grant (IE161019) through Royal Society and NFSC, the Scottish Sensing Systems Centre (S3C), Royal Society of Edinburgh, Carnegie Trust Funding, Royal Academy of Engineering-Research Exchange with China and India, UK Fluidic Network and Special Interest Group-Acoustofluidics, the EPSRC Engineering Instrument Pool. We also acknowledge the National Natural Science Foundation of China (Nos. 61274037, 51302173), the Zhejiang Province Natural Science Fund (No. Z11101168), the Fundamental Research Funds for the Central Universities (No. 2014QNA5002), EP/D03826X/1, EP/ C536630/1, GR/T24524/01, GR/S30573/01, GR/R36718/01, GR/L82090/01, BBSRC/E11140. ZXT acknowledges the supports from the National Natural Science Foundation of China (61178018) and the NSAF Joint Foundation of China (U1630126 and U1230124) and Ph.D. Funding Support Program of Education Ministry of China (20110185110007) and the NSAF Joint Foundation of China (Grant No. U1330103) and the National Natural Science Foundation of China (No. 11304209). NTN acknowledges support from Australian Research Council project LP150100153. This work was partially supported by the European Commission through the 6th FP MOBILIS and 7th FP RaptaDiag project HEALTH-304814 and by the COST Action IC1208 and by the Ministerio de Economía y Competitividad del Gobierno de España through projects MAT2010-18933 and MAT2013-45957R

Ferroelectric-on-Silicon Switchable Bulk Acoustic Wave Resonators and Filters for RF Applications.

Todays’ multi-band mobile phones’ RF front ends require separate transceivers for each frequency band. Future wireless mobile devices are expected to accommodate a larger number of frequency bands; therefore using the existing transceiver configurations becomes prohibitive. One of the key RF components in wireless devices is the image reject and band-selection filter. Today’s multi-band mobile phones use bulk acoustic wave (BAW) filters in conjunction with solid-state or MEMS-based RF switches for selecting the frequency band of operation. This approach results in very complex circuits. As number of frequency bands increases, ferroelectric BST, operating at its paraelectric phase, has recently been utilized in designing intrinsically switchable BAW resonators and filters due to its voltage induced piezoelectricity. The intrinsically switchable BAW resonators and filters are suitable for designing compact multiband and frequency agile transceivers as they can be switched on and off by simply controlling the dc bias voltage across the ferroelectric layer instead of using separate MEMS or solid-state based RF switches. In this thesis, composite ferroelectric resonators are studied to improve the Q of intrinsically switchable BAW resonators. Intrinsically switchable BAW resonators with record Q values based on ferroelectric-on-silicon composite structures have been demonstrated. In addition, two types of intrinsically switchable BAW filters using ferroelectric-on-silicon composite structure: electrically connected filters and laterally coupled acoustic filters are studied. In the first part of this thesis, the design, fabrication and measurement results for high-Q composite film bulk acoustic resonators (FBARs) are discussed. Subsequently, an intrinsically switchable electrically connected filter based on ferroelectric-on-silicon composite FBARs is presented. Finally, an intrinsically switchable laterally coupled acoustic filter with a ferroelectric-on-silicon composite structure is presented. The reported laterally coupled acoustic filter represents the first demonstration of a BST based intrinsically switchable acoustically coupled filter.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107289/1/siss_1.pd

Plasma-Assisted Growth and Characterization of Piezoelectric AlN and Sc(x)Al(1-x)N Films for Microwave Acoustic Sensor Applications

The use of surface acoustic wave (SAW) sensors in high temperature harsh environments such as those found in power plants, industrial manufacturing, or aerospace applications allows for monitoring of internal conditions at locations where traditional sensors do not operate or are unreliable. Surface acoustic wave resonator (SAWR) sensors are based on piezoelectric materials and feature a small passive low-profile self-powered design that can operate and wirelessly transmit data to monitor parameters such as temperature, pressure, or strain. SAWR sensors typically consist of a series of metal electrodes fabricated onto a bulk crystal piezoelectric such as langasite (La3Ga5SiO14). However, there are major advantages in using thin film piezoelectrics such as AlN and ScxAl1-xN rather than bulk single crystal piezoelectrics, including the ability to fabricate devices on a wider range of substrates allowing for greater tuning of devices properties. This thesis investigates the film growth, materials characterization, and surface acoustic wave resonator (SAWR) device behavior of AlN and ScxAl1-xN thin film piezoelectric materials. AlN has many properties that make it an ideal candidate for harsh environment SAW sensors, including the ability to remain piezoelectric up to 1200oC, stability in air up to 700oC, and relatively high phase velocity and low acoustic loss. In this work, piezoelectric AlN and ScxAl1-xN films were synthesized at 930oC using a nitrogen plasma-assisted e-beam evaporation growth method, and the influence of substrate preparation, Al flux, Sc flux, N-plasma flux, and the use of a TiN (111) seed layer were investigated. The films contain epitaxial (0002) oriented grains that yield piezoelectric coupling when integrated into SAWR devices, and the specific film growth parameters that determine epitaxial film quality are correlated with SAWR response and the film electromechanical coupling coefficient (k2). The piezoelectric strength of AlN can be enhanced by alloying with Sc to form a ScxAl1-xN film and this increases the magnitude of electromechanical coupling by up to 400%. ScxAl1-xN films were grown with Sc compositions ranging from 8% to 57% and the electromechanical coupling constant, k2, extracted from SAWR device measurements was found to be significantly increased compared to AlN. A prototype Sc0.13Al0.87N-based SAWR temperature sensor was fabricated and packaged at the Frontier Institute for Research in Sensor Technologies (FIRST) and tested on an exhaust baffle in the UMaine Steam Plant for over 1000 hours, demonstrating the transition of the research from a Technology Readiness Level of ‘experimental proof of concept’ to ‘system prototype demonstration in an operational environment’

Micro-poutres résonantes à base de films minces de nitrure d’aluminium piézoélectriques, application aux capteurs de gaz gravimétriques

Resonant MEMS and NEMS are excellent candidate for the realization of low cost and high resolution gas sensing systems that have several applications in security, defense, and environment and health care domains. However, the question of the transduction technique used to couple micro or nano scale signals to the macro scale is still a key issue. Piezoelectric transduction can be advantageously exploited but has been rarely studied at the nano-scale. The objective of this PhD is thus to progress toward the realization of high-resolution gas sensor using piezoelectric micro/nano cantilevers resonators and cover the whole prototyping chain from device fabrication to proof of principle experiment. Our first contribution in this research relates the analytical modeling of the sensing performance and the system and design optimization. In particular we demonstrate that decreasing the piezoelectric active film thickness below 100 nm is particularly beneficial. The second contribution relates the fabrication, characterization and demonstration of the high sensing performances of 80 μm long cantilevers embedding a 50 nm thick piezoelectric AlN film for transduction. These devices exhibit state of the art performances in terms of resonance frequency deviation down to the 〖10〗^(-8) range. They allow thus the detection of Di-Methyl-Methyl-Phosphonate vapors, a sarin gas simulant, with concentration as low as 10 ppb. Although the level of integration of our sensing system is not sufficient for real life application, these results prove the high potential of these piezoelectric cantilever resonators for future industrial development.Les MEMS et NEMS résonants sont d'excellents candidats pour la réalisation de systèmes de détection de gaz haute résolution et faible couts ayant des applications dans les domaines de la sécurité, la défense, l'environnement et la santé. Cependant, la question du choix des techniques de transduction est toujours largement débattue. La transduction piézoélectrique pourrait être avantageusement exploitée mais elle est encore peu connue à l'échelle nanométrique. L'objectif de cette thèse est donc de progresser vers la réalisation de capteur de gaz à haute résolution à l'aide résonateurs à base de micro / nano poutres piézoélectriques en couvrant la chaîne de prototypage complète depuis les techniques de dépôt des matériaux jusqu'à l'expérience de preuve de principe de mesure de gaz. Pour cela, notre première contribution concerne la modélisation analytique des performances et l'optimisation, design et système, d'un capteur de gaz à base de poutres résonantes piézoélectriques. En particulier, nous démontrons que la diminution de l'épaisseur du film piézoélectrique actif sous la barre des 100 nm permet d'atteindre les meilleures performances. La deuxième contribution concerne la fabrication, la caractérisation et la démonstration des performances capteur de poutres résonantes de 80 μm de long exploitant un film piézoélectrique en AlN de 50 nm d'épais. Ainsi nous avons démontré expérimentalement la stabilité fréquentielle exceptionnelle de ces dispositifs atteignant des déviations standard de l'ordre de 〖10〗^(-8), au niveau de l’état de l'art. Ainsi, ils permettent la détection de vapeurs Di -Methyl -méthyl- phosphonates, un simulateur de gaz sarin, avec des concentrations aussi faibles que 10 ppb. Bien que le niveau d'intégration de notre système de détection ne soit pas suffisant, ces résultats prouvent le fort potentiel de ces résonateurs cantilever piézoélectriques pour un développement industriel futur

Acoustic Waves

The concept of acoustic wave is a pervasive one, which emerges in any type of medium, from solids to plasmas, at length and time scales ranging from sub-micrometric layers in microdevices to seismic waves in the Sun's interior. This book presents several aspects of the active research ongoing in this field. Theoretical efforts are leading to a deeper understanding of phenomena, also in complicated environments like the solar surface boundary. Acoustic waves are a flexible probe to investigate the properties of very different systems, from thin inorganic layers to ripening cheese to biological systems. Acoustic waves are also a tool to manipulate matter, from the delicate evaporation of biomolecules to be analysed, to the phase transitions induced by intense shock waves. And a whole class of widespread microdevices, including filters and sensors, is based on the behaviour of acoustic waves propagating in thin layers. The search for better performances is driving to new materials for these devices, and to more refined tools for their analysis