Nonlinear mechanisms in passive microwave devices

Premi extraordinari doctorat curs 2010-2011, àmbit d’Enginyeria de les TICThe telecommunications industry follows a tendency towards smaller devices, higher power and higher frequency, which imply an increase on the complexity of the electronics involved. Moreover, there is a need for extended capabilities like frequency tunable devices, ultra-low losses or high power handling, which make use of advanced materials for these purposes. In addition, increasingly demanding communication standards and regulations push the limits of the acceptable performance degrading indicators. This is the case of nonlinearities, whose effects, like increased Adjacent Channel Power Ratio (ACPR), harmonics, or intermodulation distortion among others, are being included in the performance requirements, as maximum tolerable levels. In this context, proper modeling of the devices at the design stage is of crucial importance in predicting not only the device performance but also the global system indicators and to make sure that the requirements are fulfilled. In accordance with that, this work proposes the necessary steps for circuit models implementation of different passive microwave devices, from the linear and nonlinear measurements to the simulations to validate them. Bulk acoustic wave resonators and transmission lines made of high temperature superconductors, ferroelectrics or regular metals and dielectrics are the subject of this work. Both phenomenological and physical approaches are considered and circuit models are proposed and compared with measurements. The nonlinear observables, being harmonics, intermodulation distortion, and saturation or detuning, are properly related to the material properties that originate them. The obtained models can be used in circuit simulators to predict the performance of these microwave devices under complex modulated signals, or even be used to predict their performance when integrated into more complex systems. A key step to achieve this goal is an accurate characterization of materials and devices, which is faced by making use of advanced measurement techniques. Therefore, considerations on special measurement setups are being made along this thesis.Award-winningPostprint (published version

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and Technology, SST-105156.R1 2018

Aluminum nitride deposition/characterization & pMEMs/SAW device simulation/fabrication

Aluminum Nitride (AlN) is a promising material for piezoelectric MicroElectroMechanical Systems (pMEMS) and Surface Acoustic Wave (SAW) devices. AlN is a direct bandgap semiconductor possessing moderate piezoelectric coefficients, a high Curie temperature, and a high acoustic velocity. Potential applications of AlN thin film devices include high temperature pMEMS microvalves for use in Solid Oxide Fuel Cell (SOFC) flow control systems and high frequency/sensitivity SAW platforms for use in biosensors.;Since AlN is a robust material capable of operating at high temperatures and harsh environments, it can be used in settings where other widely used piezoelectrics such as Lead Zirconate Titanate (PZT) and Zinc Oxide (ZnO) fail. Piezoelectric beams are commonly used in MEMS and have many possible applications in smart sensor and actuator systems. In this work, the results of 3-dimensional Finite Element Analysis (FEA) of AlN homogeneous bimorphs (d31 mode) are shown. The coupled-field FEA simulations were performed using the commercially available software tool ANSYSRTM Academic Research, v.11.0. The effect of altering the contact geometry and position on the displacement, electric field, stress, and strain distributions for the static case is reported.;Surface acoustic wave devices have drawn increasing interest for use as highly sensitive sensors. Specifically, SAW platforms are being explored for chemical and biological sensor applications. Because AlN has one of the highest acoustic velocities of all the piezoelectric materials, high frequency (and thus highly sensitive) sensors are feasible. In this work, AlN SAW Rayleigh wave platforms were designed, fabricated, and tested. The insertion loss of the SAW platforms for two InterDigitated Transducers (IDTs) separation distances is also presented

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’

Acoustic Wave Based MEMS Devices, Development and Applications

Acoustic waves based MEMS devices offer a promising technology platform for a wide range of applications due to their high sensitivity and the capability to operate wirelessly. These devices utilize acoustic waves propagating through or on the surface of a piezoelectric material. An acoustic wave device typically consists of two layers, metal transducers on top of piezoelectric substrate or thin films. The piezoelectric material has inherent capabilities of generating acoustic waves related to the input electrical sinusoidal signals placed on the transducers. Using this characteristic, different transducer designs can be placed on top of the piezoelectric material to create acoustic wave based filters, resonators or sensors. Historically, acoustic wave devices have been and are still widely used in telecommunications industry, primarily in mobile cell phones and base stations. Surface Acoustic Wave (SAW) devices are capable of performing powerful signal processing and have been successfully functioning as filters, resonators and duplexers for the past 60 years. Although SAW devices are technological mature and have served the telecommunication industry for several decades, these devices are typically fabricated on piezoelectric substrates and are packaged as discrete components. Considering the wide flexibility and capabilities of the SAW device to form filters, resonators there has been motivation to integrate such devices on silicon substrates as demonstrated in (Nordin et al., 2007; M. J. Vellekoop et al., 1987; Visser et al., 1989). One such example is illustrated in (Nordin et al., 2007) where a CMOS SAW resonator was fabricated using 0.6 m AMIs CMOS technology process with additional MEMS post-processing. The traditional SAW structure of having the piezoelectric at the bottom was inverted. Instead, the IDTs were cleverly manufactured using standard complementary-metal-oxide-semiconductor (CMOS) process and the piezoelectric layer was placed on the top. Active circuitry can be placed adjacent to the CMOS resonator and can be connected using the integrated metal layers. A SAW device can also be designed to have a long propagation path between the input and output transducer. The propagating acoustic waves will then be very sensitive to ambient changes, allowing the device to act as a sensor. Any variations to the characteristics of the propagation path affect the velocity or amplitude of the wave. Important application for acoustic wave devices as sensors include torque and tire pressure sensors (Cullen et al., 1980; Cullen et al., 1975; Pohl et al., 1997), gas sensors (Levit et al., 2002; Nakamoto et al., 1996; Staples, 1999; Wohltjen et al., 1979), biosensors for medical applications (Andle et al., 1995; Ballantine et al., 1996; Cavic et al., 1999; Janshoff et al., 2000), and industrial and commercial applications (vapor, humidity, temperature, and mass sensors) (Bowers et al., 1991; Cheeke et al., 1996; Smith, 2001; N. J. Vellekoop et al., 1999; Vetelino et al., 1996; Weld et al., 1999). In recent years, the interest in the development of highly sensitive acoustic wave devices as biosensor platforms has grown. For biological applications the acoustic wave device is integrated in a microfluidic system and the sensing area is coated with a biospecific layer. When a bioanalyte interacts with this sensing layer, physical, chemical, and/or biochemical changes are produced. Typically, mass and viscosity changes of the biospecific layer can be detected by analyzing changes in the acoustic wave properties such as velocity, attenuation and resonant frequency of the sensor. An important advantage of the acoustic wave biosensors is simple electronic readout that characterizes these sensors. The measurement of the resonant frequency or time delay can be performed with high degree of precision using conventional electronics. This chapter is focused on two important applications of the acoustic-wave based MEMS devices; (1) biosensors and (2) telecommunications. For biological applications these devices are integrated in a microfluidic system and the sensing area is coated with a biospecific layer. When a bioanalyte interacts with this sensing layer, physical, chemical, and/or biochemical changes are produced. Typically, mass and viscosity changes of the biospecific layer can be detected by analyzing changes in the acoustic wave properties such as velocity, attenuation and resonant frequency of the sensor. An important advantage of the acoustic wave biosensors is simple electronic readout that characterizes these sensors. The measurement of the resonant frequency and time delay can be performed with high degree of precision using conventional electronics. Only few types of acoustic wave devices could be integrated in microfluidic systems without significant degradation of the quality factor. The acoustic wave based MEMS devices reported in the literature as biosensors are film bulk acoustic wave resonators (FBAR) and surface acoustic waves (SAW) resonators and SAW delay lines. Different approaches to the realization of FBARs and SAW resonators and SAW delay lines used for various biochemical applications are presented. Next, acoustic wave MEMS devices used in telecommunications applications are presented. Telecommunication devices have different requirements compared to sensors, where acoustic wave devices operating as a filter or resonator are expected to operate at high frequencies (GHz), have high quality factors and low insertion losses. Traditionally, SAW devices have been widely used in the telecommunications industry, however with advancement in lithographic techniques, FBARs are rapidly gaining popularity. FBARs have the advantage of meeting the stringent requirement of telecommunication industry of having Qs in the 10,000 range and silicon compatibility

Gallium Nitride Integrated Microsystems for Radio Frequency Applications.

The focus of this work is design, fabrication, and characterization of novel and advanced electro-acoustic devices and integrated micro/nano systems based on Gallium Nitride (GaN). Looking beyond silicon (Si), compound semiconductors, such as GaN have significantly improved the performance of the existing electronic devices, as well as enabled completely novel micro/nano systems. GaN is of particular interest in the “More than Moore” era because it combines the advantages of a wide-band gap semiconductor with strong piezoelectric properties. Popular in optoelectronics, high-power and high-frequency applications, the added piezoelectric feature, extends the research horizons of GaN to diverse scientific and multi-disciplinary fields. In this work, we have incorporated GaN micro-electro-mechanical systems (MEMS) and acoustic resonators to the GaN baseline process and used high electron mobility transistors (HEMTs) to actuate, sense and amplify the acoustic waves based on depletion, piezoelectric, thermal and piezo-resistive mechanisms and achieved resonance frequencies ranging from 100s of MHz up to 10 GHz with frequency×quality factor (f×Q) values as high as 1013. Such high-performance integrated systems can be utilized in radio frequency (RF) and microwave communication and extreme-environment applications.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135799/1/azadans_1.pd