Ultra high frequency thin film saw devices

Durante los ��ltimos a��os el flujo de datos en la transmisi��n que tiene lugar en los sistemas de comunicaci��n ha aumentado considerablemente de forma que d��a a d��a se requieren m��s aplicaciones trabajando en un rango de frecuencias muy alto (3-30 GHz). Muchos de estos sistemas de comunicaci��n incluyen dispositivos de onda ac��stica superficial (SAW) y por tanto se hace necesario el aumento de frecuencia a la que ��stos trabajan. Pero este incremento de frecuencia de los dispositivos SAW no s��lo es utilizado en los sistemas de comunicaci��n, varios tipos de sensores, por ejemplo, aumentan su sensibilidad cuando la frecuencia a la que trabajan tambi��n lo hace. Tradicionalmente los dispositivos SAW se han fabricado sobre cuarzo, LiNbO3 y LiTaO3 principalmente. Sin embargo la principal limitaci��n de estos materiales es su velocidad SAW. Adem��s, debido a la alta temperatura a la que se depositan no pueden ser integrados en la tecnolog��a de fabricaci��n CMOS. El uso de la tecnolog��a de capa delgada, en la que un material piezoel��ctrico es depositado sobre un substrato, se est�� utilizando en las ��ltimas d��cadas para incrementar la velocidad SAW de la estructura y poder obtener dispositivos trabajando en el rango de frecuencias requerido en la actualidad. Por otra parte, esta tecnolog��a podr��a ser integrada en el proceso de fabricaci��n CMOS. Durante esta tesis nos hemos centrado en la fabricaci��n de dispositivos SAW trabajando a muy alta frecuencia. Para ello, utilizando la tecnolog��a de capa delgada, hemos utilizado la estructura nitruro de aluminio (AlN) sobre diamante que permite conseguir velocidades SAW del sustrato que no se pueden alcanzar con otros materiales. El dep��sito de AlN se realiz�� mediante sputtering reactivo. Durante esta tesis se han realizado diferentes experimentos para optimizar dicho dep��sito de forma que se han obtenido los par��metros ��ptimos para los cuales se pueden obtener capas de AlN de alta calidad sobre cualquier tipo de sustrato. Adem��s todo el proceso se realiz�� a baja temperatura para que el procesado de estos dispositivos pueda ser compatible con la tecnolog��a CMOS. Una vez optimizada la estructura AlN/diamante, mediante litograf��a por haz de electrones se fabricaron resonadores SAW de tama��o nanom��trico que sumado a la alta velocidad resultante de la combinaci��n AlN/diamante nos ha permitido obtener dispositivos trabajando en el rango de 10-28 GHz con un alto factor de calidad y rechazo fuera de la banda. Est��s frecuencias y prestaciones no han sido alcanzadas por el momento en resonadores de este tipo. Por otra parte, se han utilizado estos dispositivos para fabricar sensores de presi��n de alta sensibilidad. Estos dispositivos son afectados altamente por los cambios de temperatura. Se realiz�� tambi��n un exhaustivo estudio de c��mo se comportan en temperatura estos resonadores, entre -250��C y 250��C (rango de temperaturas no estudiado hasta el momento) diferenci��ndose dos regiones una a muy baja temperatura en la que el dispositivo muestra un coeficiente de retraso en frecuencia (TCF) relativamente bajo y otra a partir de los -100��C en la que el TCF es similar al observado en la bibliograf��a. Por tanto, durante esta tesis se ha optimizado el dep��sito de AlN sobre diamante para que sea compatible con la tecnolog��a CMOS y permita el procesado de dispositivos trabajando a muy alta frecuencia con altas prestaciones para comunicaciones y sensores. ABSTRACT The increasing volume of information in data transmission systems results in a growing demand of applications working in the super-high-frequency band (3���30 GHz). Most of these systems work with surface acoustic wave (SAW) devices and thus there is a necessity of increasing their resonance frequency. Moreover, sensor application includes this kind of devices. The sensitivity of them is proportional with its frequency. Traditionally, quartz, LiNbO3 and LiTaO3 have been used in the fabrication of SAW devices. These materials suffer from a variety of limitations and in particular they have low SAW velocity as well as being incompatible with the CMOS technology. In order to overcome these problems, thin film technology, where a piezoelectric material is deposited on top of a substrate, has been used during the last decades. The piezoelectric/substrate structure allows to reach the frequencies required nowadays and could be compatible with the mass electronic production CMOS technology. This thesis work focuses on the fabrication of SAW devices working in the super-high-frequency range. Thin film technology has been used in order to get it, especially aluminum nitride (AlN) deposited by reactive sputtering on diamond has been used to increase the SAW velocity. Different experiments were carried out to optimize the parameters for the deposit of high quality AlN on any kind of substrates. In addition, the system was optimized under low temperature and thus this process is CMOS compatible. Once the AlN/diamond was optimized, thanks to the used e-beam lithography, nanometric SAW resonators were fabricated. The combination of the structure and the size of the devices allow the fabrication of devices working in the range of 10-28 GHz with a high quality factor and out of band rejection. These high performances and frequencies have not been reached so far for this kind of devices. Moreover, these devices have been used as high sensitivity pressure sensors. They are affected by temperature changes and thus a wide temperature range (-250��C to 250��C) study was done. From this study two regions were observed. At very low temperature, the temperature coefficient of frequency (TCF) is low. From -100��C upwards the TCF is similar to the one appearing in the literature. Therefore, during this thesis work, the sputtering of AlN on diamond substrates was optimized for the CMOS compatible fabrication of high frequency and high performance SAW devices for communication and sensor application.

Comparison of V2IC control with voltage mode and current mode controls for high frequency (MHz) and very fast response applications

High switching frequencies (several MHz) allow the integration of low power DC/DC converters. Although, in theory, a high switching frequency would make possible to implement a conventional Voltage Mode control (VMC) or Peak Current Mode control (PCMC) with very high bandwidth, in practice, parasitic effects and robustness limits the applicability of these control techniques. This paper compares VMC and CMC techniques with the V2IC control. This control is based on two loops. The fast internal loop has information of the output capacitor current and the error voltage, providing fast dynamic response under load and voltage reference steps, while the slow external voltage loop provides accurate steady state regulation. This paper shows the fast dynamic response of the V2IC control under load and output voltage reference steps and its robustness operating with additional output capacitors added by the customer.

High frequency high-order Rayleigh modes in ZnO/GaAs

Strong high-order Rayleigh or Sezawa modes, in addition to the fundamental Rayleigh mode, have been observed in ZnO/GaAs(001) systems along the [110] propagation direction of GaAs. The dispersion of the different acoustic waves has been calculated and compared to the experimental data. The bandwidth and impedance matching characteristics of the multimode SAW delay lines operating at high frequencies (2.5-3.5 GHz regime) have been investigated.

Series combination of a switched dc-dc converter and a linear regulator for high frequency RF envelope amplifier

Classical linear amplifiers such as A, AB and B offer very good linearity suitable for RF power amplifiers. However, its inherent low efficiency limits its use especially in base-stations that manage tens or hundreds of Watts. The use of linearization techniques such as Envelope Elimination and Restoration (EER) allow an increase of efficiency keeping good linearity. This technique requires a very fast dc-dc power converter to provide variable voltage supply to the power amplifier. In this paper, several alternatives are analyzed to implement the envelope amplifier based on a cascade association of a switched dc-dc converter and a linear regulator. A simplified version of this approach is also suitable to operate with Envelope Tracking technique.

Nanotechnology for high frequency communications: nitrides and graphene

The achievement of higher frequencies (HF) and the reduction of energy consumption, to improve sensing, communication and computation, involve the continued scaling down to the nanometer level. This scaling is enabled by of innovative device designs, improved processing technologies and assessment tools, and new material structures. In this work, we have used all these factors to demonstrate state-of-the-art HF devices in two materials with quite different electronic properties: wide semiconductor bandgap III-nitrides for resonators and power amplifiers; and graphene, a zero bandgap material expected to revolutionize low noise and HF flexible electronics. Some issues faced during their development will be discussed during the talk.

Ion correlations due to high-frequency electric field and their effect on the nonlinear plasma conductivity

The influence of a strong, high���frequency electric field on the ion���ion correlations in a fully ionized plasma is investigated in the limit of infinite ion mass, starting with the Bogoliubov���Born���Green���Kirkwood���Yvon hierarchy of equations; a significant departure from the thermal correlations is found. It is shown that the above effect may substantially modify earlier results on the nonlinear high���frequency plasma conductivity.

Loss model for a high frequency and low load DC/DC synchronous buck converter

This work presents a behavioral-analytical hybrid loss model for a buck converter. The model has been designed for a wide operating frequency range up to 4MHz and a low power range (below 20W). It is focused on the switching losses obtained in the power MOSFETs. Main advantages of the model are the fast calculation time and a good accuracy. It has been validated by simulation and experimentally with one Ga, power transistor and two Si MOSFETs. Results show good agreement between measurements and the model.

Hybrid behavioral-analytical loss model for a high frequency and low load DC/DC buck converter

This work presents a behavioral-analytical hybrid loss model for a buck converter. The model has been designed for a wide operating frequency range up to 4MHz and a low power range (below 20W). It is focused on the switching losses obtained in the power MOSFETs. Main advantages of the model are the fast calculation time (below 8.5 seconds) and a good accuracy, which makes this model suitable for the optimization process of the losses in the design of a converter. It has been validated by simulation and experimentally with one GaN power transistor and three Si MOSFETs. Results show good agreement between measurements and the model

Sputter optimization of AlN on diamond substrates for high frequency SAW resonators

The AlN/diamond structure is an attractive combination for SAW devices and its application at high frequencies. In this work, the synthesis of AlN thin films by reactive sputtering has been optimized on diamond substrates in order to process high frequency devices. Polished microcrystalline and as-grown nanocrystalline diamond substrates have been used to deposit AlN of different thickness under equal sputtering conditions. For the smoother substrates, the FWHM of the rocking curve of the (002) AlN peak varies from 3.8�� to 2.7�� with increasing power. SAW one port resonators have been fabricated on these films, whose electrical characterization (in terms of S11 parameters) is reported.

Numerical analysis of axisymmetric shells by one-dimensional continuum elements suitable for high frequency excitations

Axisymmetric shells are analyzed by means of one-dimensional continuum elements by using the analogy between the bending of shells and the bending of beams on elastic foundation. The mathematical model is formulated in the frequency domain. Because the solution of the governing equations of vibration of beams are exact, the spatial discretization only depends on geometrical or material considerations. For some kind of situations, for example, for high frequency excitations, this approach may be more convenient than other conventional ones such as the finite element method.

Low-thickness high-quality aluminum nitride films for super high frequency solidly mounted resonators

We investigate the sputter growth of very thin aluminum nitride (AlN) films on iridium electrodes for electroacoustic devices operating in the super high frequency range. Superior crystal quality and low stress films with thicknesses as low as 160 nm are achieved after a radio frequency plasma treatment of the iridium electrode followed by a two-step alternating current reactive magnetron sputtering of an aluminum target, which promotes better conditions for the nucleation of well textured AlN films in the very first stages of growth. Solidly mounted resonators tuned around 8 GHz with effective electromechanical coupling factors of 5.8% and quality factors Q up to 900 are achieved.

Estimation of functional connectivity from electromagnetic signals and the amount of empirical data required

An increasing number of neuroimaging studies are concerned with the identification of interactions or statistical dependencies between brain areas. Dependencies between the activities of different brain regions can be quantified with functional connectivity measures such as the cross-correlation coefficient. An important factor limiting the accuracy of such measures is the amount of empirical data available. For event-related protocols, the amount of data also affects the temporal resolution of the analysis. We use analytical expressions to calculate the amount of empirical data needed to establish whether a certain level of dependency is significant when the time series are autocorrelated, as is the case for biological signals. These analytical results are then contrasted with estimates from simulations based on real data recorded with magnetoencephalography during a resting-state paradigm and during the presentation of visual stimuli. Results indicate that, for broadband signals, 50���100 s of data is required to detect a true underlying cross-correlations coefficient of 0.05. This corresponds to a resolution of a few hundred milliseconds for typical event-related recordings. The required time window increases for narrow band signals as frequency decreases. For instance, approximately 3 times as much data is necessary for signals in the alpha band. Important implications can be derived for the design and interpretation of experiments to characterize weak interactions, which are potentially important for brain processing.

Electrostatic plasma instabilities excited by a high���frequency electric field

The electrostatic plasma waves excited by a uniform, alternating electric field of arbitrary intensity are studied on the basis of the Vlasov equation; their dispersion relation, which involves the determinant of either of two infinite matrices, is derived. For ��0�����������pi (��0 being the applied frequency and ��pi the ion plasma frequency) the waves may be classified in two groups, each satisfying a simple condition; this allows writing the dispersion relation in closed form. Both groups coalesce (resonance) if (a) ��0��� ��� �����pe/r (r any integer) and (b) the wavenumber k is small. A nonoscillatory instability is found; its distinction from the DuBois���Goldman instability and its physical origin are discussed. Conditions for its excitation (in particular, upper limits to ��0,k, and k���vE,vE being the field���induced electron velocity), and simple equations for the growth rate are given off���resonance and at ��0��� ��� �����pi. The dependence of both threshold and maximum growth rate on various parameters is discussed, and the results are compared with those of Silin and Nishikawa. The threshold at ��0��� ��� �����pi/r,r��� ��� ���1, is studied.

Integration of temporal and semantic components into the Geographic Information. Part II: Methodology

The overall objective of this research project is to enrich geographic data with temporal and semantic components in order to significantly improve spatio-temporal analysis of geographic phenomena. To achieve this goal, we intend to establish and incorporate three new layers (structures) into the core of the Geographic Information by using mark-up languages as well as defining a set of methods and tools for enriching the system to make it able to retrieve and exploit such layers (semantic-temporal, geosemantic, and incremental spatio-temporal). Besides these layers, we also propose a set of models (temporal and spatial) and two semantic engines that make the most of the enriched geographic data. The roots of the project and its definition have been previously presented in Siabato & Manso-Callejo 2011. In this new position paper, we extend such work by delineating clearly the methodology and the foundations on which we will base to define the main components of this research: the spatial model, the temporal model, the semantic layers, and the semantic engines. By putting together the former paper and this new work we try to present a comprehensive description of the whole process, from pinpointing the basic problem to describing and assessing the solution. In this new article we just mention the methods and the background to describe how we intend to define the components and integrate them into the GI.

Optimization of AlN thin layers on diamond substrates for high frequency SAW resonators

AlN/diamond heterostructures are very promising for high frequency surface acoustic wave (SAW) resonators. In their design, the thickness of the piezoelectric film is one of the key parameters. On the other hand, the film material quality and, hence, the device performance, also depend on that thickness. In this work, polished microcrystalline diamond substrates have been used to deposit AlN films by reactive sputtering, from 150 nm up to 3 ��m thick. A high degree of the c-axis orientation has been obtained in all cases. SAW one port resonators at high frequency have been fabricated on these films with a proper combination of the film thickness and transducer size.