Fabrication of Surface Micromachined AlN Piezoelectric Microstructures and its Potential Apllication to RF Resonators

We report on a novel microfabrication method to fabricate aluminum nitride (AlN) piezoelectric microstructures down to 2 microns size by a surface micromachining process. Highly c-axis oriented AlN thin films are deposited between thin Cr electrodes on polysilicon structural layers by rf reactive sputtering. The top Cr layer is used both as a mask to etch the AlN thin films and as an electrode to actuate the AlN piezoelectric layer. The AlN layer is patterned anisotropically by wet etching using a TMAH (25%) solution. This multilayer stack uses silicon-di-oxide as a sacrificial layer to make free-standing structures. One-port scattering paramenter measurement using a network analyzer show a resonant frequency of 1.781 GHz on a clamped-clamped beam suspended structure. The effective electromechanical coupling factor is calculated as 2.4 % and the measured bandwidth is 13.5 MHz for one such a doubly clamped beam (990x30) μm2

Development of surface micromachined Aluminum Nitride air-bridges for piezoelectric MEMS/NEMS applications by Metal Organic Vapor Phase Epitaxy techniques

Group III-nitrides have attracted considerable attention for piezoelectric Micro/Nano electromechanical (MEMS/NEMS) applications due to their excellent bio compatibility, well developed growth techniques for high quality thin films and structural stability at high temperatures when compared to the commonly used piezoelectric metal oxides. Among the group III-nitrides Aluminum Nitride (AlN) possess superior material properties such as highest piezoelectric coefficient and good mechanical properties. Growth techniques for fabricating group III-nitride MEMS/NEMS by metal organic vapor phase epitaxy (MOVPE) techniques have involved sacrificial layers such as epitaxial group III-nitrides/ alloys, nanocrystalline films and porous interlayers. However, the material properties of the MOVPE grown films on the amorphous sacrificial layers such as silicon oxide have not been adequately investigated to evaluate potential MEMS/NEMS devices such as piezoelectric micro/ nanofluidic channels.;This work demonstrates a process for the fabrication of Aluminum nitride (AlN) thin film air-bridges using MOVPE techniques on silicon templates. Micro-FTIR techniques were used to study the crystallographic orientation of the AlN thin film air-bridges with lateral dimensions as low as 100 mum. FTIR results also show that the wet etching process to remove the underlying sacrificial layer also improves the material properties of the AlN films on SiOx. The study indicates that AlN air-bridges are polycrystalline in nature and are preferentially c-axis oriented after wet etching. Lateral field excitation of the piezoelectric films and laser Doppler vibrometer techniques were combined to investigate the piezoelectric response of the AlN films on the sacrificial layer. Lateral field excitation of the AlN films grown on the amorphous sacrificial layer shows that the AlN films exhibit piezoelectric properties. The displacement of the AlN air-bridges obtained by lateral field actuation is around 1 nm over an air-gap of 130 nm after the removal of the sacrificial layer. However, the mismatch in the coefficient of thermal expansion between the substrate and thin films induces significant residual stress in the heterostructure. The AlN air-bridges on silicon substrate exhibit fracture due to the tensile residual stress exceeding the fracture limit

Fabrication and Characterization of AlN-based, CMOS compatible Piezo-MEMS Devices

This paper details the development of high-quality, c-axis oriented AlN thin films up to 2 {\mu}m thick, using sputtering on platinum-coated SOI substrates for use in piezoelectric MEMS. Our comprehensive studies illustrate how important growth parameters such as the base Pt electrode quality, deposition temperature, power, and pressure, can influence film quality. With careful adjustment of these parameters, we managed to manipulate residual stresses (from compressive -1.2 GPa to tensile 230 MPa), and attain a high level of orientation in the AlN thin films, evidenced by < 5deg FWHM X-Ray diffraction peak widths. We also report on film surface quality regarding roughness, as assessed by atomic force microscopy, and grain size, as determined through scanning electron microscopy. Having attained the desired film quality, we proceeded to a fabrication process to create piezoelectric micromachined ultrasound transducers (PMUTs) with the AlN on SOI material stack, using deep reactive ion etching (DRIE). Initial evaluations of the vibrational behavior of the created devices, as observed through Laser Doppler Vibrometry, hint at the potential of these optimized AlN thin films for MEMS transducer development

AlN ja Sc0.2Al0.8N ohutkalvojen märkäkemiallinen etsaus

Aluminium nitride is a piezoelectric material commonly used in piezoelectric microelectromechanical systems (MEMS) in the form of thin films deposited by sputtering. AlN-based devices are found in wireless electronics in the form of acoustic filters, but they also have prospective applications in a wide variety of sensor systems. To enhance the piezoelectric properties of AlN, some of the Al can be replaced with scandium, which is required for next-generation devices. However, addition of Sc makes both the deposition and patterning of the film more difficult. This work focuses on patterning of AlN and Sc0.2Al0.8N thin films with wet etching. Both materials are etched anisotropically, which in theory enables etching the materials with little deviation from the mask dimensions. However, in practise, undercutting at the mask edges occurs easily making the structures narrower compared to the etch mask. This work investigates and compares the mechanisms and etch rates of AlN and Sc0.2Al0.8N. Tetramethyl ammonium hydroxide was mostly used for etching, but also H3PO4 and H2SO4 were tested. Addition of 20 atom-% Sc lowered the etch rate of the material and resulted in more undercutting. The causes behind mask undercutting were examined by using 11 differently deposited etch masks, and the undercutting was minimized by optimizing the mask deposition, using thermal annealing, and optimizing the etching temperature. Finally, the work identifies and discusses the relevant factors in depositing and patterning the AlN, ScxAl1-xN and mask films

Xenon difluoride etching of amorphous silicon for release of piezoelectric micromachined ultrasonic transducer structures

Piezoelectric micromachined ultrasonic transducers (PMUT) are devices, which are based on the piezoelectric effect and are used for sensing applications. A typical PMUT structure has diaphragm with a piezoelectric material between thin high conductivity electrode layers. There are several methods which can be used for PMUT structure fabrication, including back- and front-side etching, wafer bonding, and sacrificial layer release. The state-of-the-art methods used currently for PMUT structure fabrication still face several problems. Xenon difluoride (XeF2) etching is a fluorine-based dry vapour etch method that provides highly selective isotropic etch. It is an ideal solution for the release of self-supporting layers within MEMS devices. In this work, XeF2 etching of amorphous silicon (a-Si) for the release of PMUT structures was investigated. Different designs with varying dimensions were tested and characterized. The XeF2 etching process demonstrated to be efficient and very fast compared to other methods used for PMUT/MEMS release etching. Results from the optimization tests on the XeF2 process demonstrated total etching of 2 µm thick a-Si. Structures with sizes from 50 to 500 µm diameter were completely released after only 20 minutes of etching. Additionally, this work demonstrates that the etching rate of XeF2 is also influenced by the size, shape and location of the via openings. Furthermore, sputtered aluminium nitride AlN piezo layer process optimization and residual stress control contributed to the fabrication of suspended structures. All observed structures from 50 to 500 µm diameter which used AlN in the structural layer were suspended after release

Thin-film ultraviolet light-emitting diodes realized by electrochemical etching of AlGaN

Ultraviolet (UV) light sources have a direct impact on everyone’s life. They are used\ua0to sterilize surfaces as well as for water purification. In addition, they are used in\ua0green houses to enhance health-promoting substances in plants, for phototherapy to\ua0treat skin diseases, for sensing and material curing. Today, most of these applications\ua0use mercury lamps that are fragile, bulky and toxic. AlGaN-based UV light-emitting\ua0diodes (LEDs) have the potential to solve all these issues, but their implementation\ua0has been limited due to their low electrical to optical power conversion efficiency\ua0(PCE) being below 10%. Blue-emitting GaN-based LEDs have already found their\ua0way into everyone’s home through general lighting. This was made possible by the tremendous performance improvements, reaching PCEs close to 90%. Unfortunately, the device concepts for achieving highly efficient GaN-based LEDs, such as the thin-film flip-chip (TFFC) design that can greatly improve light-extraction efficiency, are not easily transferred to AlGaN-based UV LEDs.In this work, we demonstrate a new device platform to realize UV LEDs with a\ua0TFFC design based on electrochemical etching to remove the substrate. In the first\ua0part of this work, electrochemical (EC) etching of AlGaN layers with a high Al content up to 50% was demonstrated, which enabled the separation of epitaxial LED\ua0layers from their substrate while maintaining the high quality of the active region.The second key technological step was the integration of EC etching in a standard\ua0UV LED fabrication process, which required protection schemes to prevent parasitic\ua0electrochemical etching of the LED structure and the development of a device design compatible with flip-chip bonding. Finally, this work was completed by the first\ua0demonstration of a TFFC UVB LED using electrochemical etching

Characterisation and modification of optoelectronic substrate surfaces for enhanced adhesive flow control

Optoelectronics manufacturers are under continuous pressure for miniaturisation of optoelectronic modules. One route to further miniaturisation is to reduce the spacing between the optical and optoelectronic components in the optical path adhesively mounted to ceramic carriers. Flow control of the adhesives over the ceramic surface is then imperative. Uncontrolled wetting can lead to an excessive adhesive footprint which interferes in the application of other adhesives for subsequent components. However, insufficient wetting can lead to low strength bonds vulnerable to thermal fatigue and shear failure. The goal of the work was to minimise the potential for uncontrolled wetting while maintaining unmodified bond properties. In addition positional stability of adhered parts on cure and in-service must not be detrimentally affected. [Continues.

Novel processes for large area gallium nitride single crystal and nanowire growth.

III-nitrides (InN, GaN, AlN) are some of the most promising materials for making blue light emitting diodes (LED), blue laser diodes (LD) and high power, high temperature field effect transistors (FET). Current techniques produce GaN films with defect densities on the order of 107/cm2 or higher. The performance and life-time of the devices critically depend upon the defect densities and high power, high frequency devices require the defect densities to be lower than 104/cm2. So, the need for new processes to produce large size GaN crystals with defect densities less than 107/cm2 is immediate. In addition to large area single crystals (or wafers), the nanowires also present as an alternating platform for making devices. So, the processes for controlled synthesis, modifying and integrating sub 100 nm nanowires into electronic devices are of great interest. This thesis presents a new concept of ‘self-oriented growth\u27 of GaN platelets shaped crystals on molten gallium to produce near single crystal quality GaN films over large areas (\u3e 1 cm²). The process involves direct nitridation of Ga films using nitrogen plasma at low pressures (few mTorr). GaN flakes with areas over 25 mm² have been successfully obtained. Raman spectra of the resulting GaN crystals show no stress and low native donor concentration on the order of 1017/cm³. XRD texture analysis showed an overall c-axis tilt of 2.2o between GaN domains within the flake. The cross-sectional TEM micrographs showed that the resulting GaN films are free from dislocation crops inside the grains but showed diffraction contrast due to small mis-orientation between the grains. The twist and tilt angles between adjacent columnar grains were determined using convergent beam electron diffraction technique to be less than 8o and 1o, respectively. HRTEM micrographs of the grain boundaries showed sharp interfaces resulting with both twisted and perfect attachments. This thesis also presents direct synthesis approach for GaN nanowires with control on growth directions using modified nitridation conditions. The nitridation in the presence of hydrogen or ammonia resulted in oxide sheath free GaN nanowires as thin as 20 nm and long as 100 Ìm in \u3c0001\u3e direction. The nitridation using low Ga flux in a vapor transport set-up resulted in sub 100 nm GaN nanowires with \u3c10-10\u3e growth direction. The difference in the nucleation and growth mechanism allowed control on the nanowire directions. Homo-epitaxial experiments onto pre-synthesized GaN nanowires with the above two growth directions using the vapor transport of Ga and dissociated ammonia exhibited different morphologies, e.g. micro hexagonal columns for \u3c0001\u3e nanowires and micro belts for nanowires with \u3c10-10\u3e growth direction. The results further illustrate a new phenomenon of enhanced surface diffusion on nanowires in general but more pronounced for wires with \u3c0001\u3e growth direction. The results from homo-epitaxy experiments suggest that the \u3c10-10\u3e direction wires could be used as seeds for growing large area GaN crystals in vapor phase homo-epitaxy schemes

Piezoelectric Transducers Based on Aluminum Nitride and Polyimide for Tactile Applications

The development of micro systems with smart sensing capabilities is paving the way to progresses in the technology for humanoid robotics. The importance of sensory feedback has been recognized the enabler of a high degree of autonomy for robotic systems. In tactile applications, it can be exploited not only to avoid objects slipping during their manipulation but also to allow safe interaction with humans and unknown objects and environments. In order to ensure the minimal deformation of an object during subtle manipulation tasks, information not only on contact forces between the object and fingers but also on contact geometry and contact friction characteristics has to be provided. Touch, unlike other senses, is a critical component that plays a fundamental role in dexterous manipulation capabilities and in the evaluation of objects properties such as type of material, shape, texture, stiffness, which is not easily possible by vision alone. Understanding of unstructured environments is made possible by touch through the determination of stress distribution in the surrounding area of physical contact. To this aim, tactile sensing and pressure detection systems should be integrated as an artificial tactile system. As illustrated in the Chapter I, the role of external stimuli detection in humans is provided by a great number of sensorial receptors: they are specialized endings whose structure and location in the skin determine their specific signal transmission characteristics. Especially, mechanoreceptors are specialized in the conversion of the mechanical deformations caused by force, vibration or slip on skin into electrical nerve impulses which are processed and encoded by the central nervous system. Highly miniaturized systems based on MEMS technology seem to imitate properly the large number of fast responsive mechanoreceptors present in human skin. Moreover, an artificial electronic skin should be lightweight, flexible, soft and wearable and it should be fabricated with compliant materials. In this respect a big challenge of bio-inspired technologies is the efficient application of flexible active materials to convert the mechanical pressure or stress into a usable electric signal (voltage or current). In the emerging field of soft active materials, able of large deformation, piezoelectrics have been recognized as a really promising and attractive material in both sensing and actuation applications. As outlined in Chapter II, there is a wide choice of materials and material forms (ceramics: PZT; polycrystalline films: ZnO, AlN; polymers and copolymers: PVDF, PVDF-TrFe) which are actively piezoelectric and exhibit features more or less attractive. Among them, aluminum nitride is a promising piezoelectric material for flexible technology. It has moderate piezoelectric coefficient, when available in c-axis oriented polycrystalline columnar structure, but, at same time, it exhibits low dielectric constant, high temperature stability, large band gap, large electrical resistivity, high breakdown voltage and low dielectric loss which make it suitable for transducers and high thermal conductivity which implies low thermal drifts. The high chemical stability allows AlN to be used in humid environments. Moreover, all the above properties and its deposition method make AlN compatible with CMOS technology. Exploiting the features of the AlN, three-dimensional AlN dome-shaped cells, embedded between two metal electrodes, are proposed in this thesis. They are fabricated on general purpose Kapton™ substrate, exploiting the flexibility of the polymer and the electrical stability of the semiconductor at the same time. As matter of fact, the crystalline layers release a compressive stress over the polymer, generating three-dimensional structures with reduced stiffness, compared to the semiconductor materials. In Chapter III, a mathematical model to calculate the residual stresses which arise because of mismatch in coefficient of thermal expansion between layers and because of mismatch in lattice constants between the substrate and the epitaxially grown films is adopted. The theoretical equation is then used to evaluate the dependence of geometrical features of the fabricated three-dimensional structures on compressive residual stress. Moreover, FEM simulations and theoretical models analysis are developed in order to qualitative explore the operation principle of curved membranes, which are labelled dome-shaped diaphragm transducers (DSDT), both as sensors and as piezo-actuators and for the related design optimization. For the reliability of the proposed device as a force/pressure sensor and piezo-actuator, an exhaustive electromechanical characterization of the devices is carried out. A complete description of the microfabrication processes is also provided. As shown in Chapter IV, standard microfabrication techniques are employed to fabricate the array of DSDTs. The overall microfabrication process involves deposition of metal and piezoelectric films, photolithography and plasma-based dry and wet etching to pattern thin films with the desired features. The DSDT devices are designed and developed according to FEM and theoretical analysis and following the typical requirements of force/pressure systems for tactile applications. Experimental analyses are also accomplished to extract the relationship between the compressive residual stress due to the aluminum nitride and the geometries of the devices. They reveal different deformations, proving the dependence of the geometrical features of the three-dimensional structures on residual stress. Moreover, electrical characterization is performed to determine capacitance and impedance of the DSDTs and to experimentally calculate the relative dielectric constant of sputtered AlN piezoelectric film. In order to investigate the mechanical behaviour of the curved circular transducers, a characterization of the flexural deflection modes of the DSDT membranes is carried out. The natural frequency of vibrations and the corresponding displacements are measured by a Laser Doppler Vibrometer when a suitable oscillating voltage, with known amplitude, is applied to drive the piezo-DSDTs. Finally, being developed for tactile sensing purpose, the proposed technology is tested in order to explore the electromechanical response of the device when impulsive dynamic and/or long static forces are applied. The study on the impulsive dynamic and long static stimuli detection is then performed by using an ad hoc setup measuring both the applied loading forces and the corresponding generated voltage and capacitance variation. These measurements allow a thorough test of the sensing abilities of the AlN-based DSDT cells. Finally, as stated in Chapter V, the proposed technology exhibits an improved electromechanical coupling with higher mechanical deformation per unit energy compared with the conventional plate structures, when the devices are used as piezo-actuator. On the other hand, it is well suited to realize large area tactile sensors for robotics applications, opening up new perspectives to the development of latest generation biomimetic sensors and allowing the design and the fabrication of miniaturized devices

Effect of AlN Seed Layer on Crystallographic Characterization of Piezoelectric AlN

Ultrathin aluminum nitride (AlN) films are of great interest for integration into nanoelectromechanical systems for actuation and sensing. Given the direct relationship between crystallographic texture and piezoelectric response, x-ray diffraction has become an important metrology step. However, signals from layers deposited below the piezoelectric (PZE) AlN thin film may skew the crystallographic analysis and give misleading results. In this work, we compare the use of a Ti or AlN seed layer on the crystallographic quality of PZE AlN. We also analyze the influence of several AlN seed layer thicknesses on the rocking curve FWHM of PZE AlN and demonstrate an larger effect of the AlN seed layer on the {\theta}-2{\theta} AlN crystallographic peak for increasing AlN seed layer thickness.Comment: 16 pages, 4 figures, 1 tabl