AlN-based BAW resonators with CNT electrodes for gravimetric biosensing

Solidly mounted resonators (SMRs) with a top carbon nanotubes (CNTs) surface coating that doubles as an electrode and as a sensing layer have been fabricated. The influence of the CNTs on the frequency response of the resonators was studied by direct comparison to identical devices with a top metallic electrode. It was found that the CNTs introduced significantly less mass load on the resonators and these devices exhibited a greater quality factor, Q (>2000, compared to ∼1000 for devices with metal electrodes), which increases the gravimetric sensitivity of the devices by allowing the tracking of smaller frequency shifts. Protein solutions with different concentrations were loaded on the top of the resonators and their responses to mass-load from physically adsorbed coatings were investigated. Results show that resonators using CNTs as the top electrode exhibited a higher frequency change for a given load (∼0.25 MHz cm2 ng−1) compared to that of a metal thin film electrode (∼0.14 MHz cm2 ng−1), due to the lower mass of the CNTelectrodes and their higher active surface area compared to that of a thin film metal electrode. It is therefore concluded that the use of CNTelectrodes on resonators for their use as gravimetric biosensors is a significant improvement over metallic electrodes that are normally employed.

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.

Ta2O5/SiO2 insulating acoustic mirrors for AlN-based X-band BAW resonators

This work describes the performance of AlN-based bulk acoustic wave resonators built on top of insulating acoustic reflectors and operating at around 8 GHz. The acoustic reflectors are composed of alternate layers of amorphous Ta2O5and SiO2 deposited at room temperature by pulsed-DC reactive sputtering in Ar/O2 atmospheres. SiO2 layers have a porous structure that provides a low acoustic impedance of only 9.5 MRayl. Ta2O5 films exhibit an acoustic impedance of around 39.5 MRayl that was assessed by the picoseconds acoustic technique These values allow to design acoustic mirrors with transmission coefficients in the centre of the band lower than -40 dB (99.998 % of reflectance) with only seven layers. The resonators were fabricated by depositing a very thin AlN film onto an iridium bottom electrode 180 nm-thick and by using Ir or Mo layers as top electrode. Resonators with effective electromechanical coupling factors of 5.7% and quality factors at the antiresonant frequency around 600 are achieved.

Growth of AlN oriented films on insulating substrates.

This work describes the structural and piezoelectric assessment of aluminum nitride (AlN) thin films deposited by pulsed-DC reactive sputtering on insulating substrates. We investigate the effect of different insulating seed layers on AlN properties (crystallinity, residual stress and piezoelectric activity). The seed layers investigated, silicon nitride (Si3N4), silicon dioxide (SiO2), amorphous tantalum oxide (Ta2O5), and amorphous or nano-crystalline titanium oxide (TiO2) are deposited on glass plates to a thickness lower than 100 nm. Before AlN films deposition, their surface is pre-treated with a soft ionic cleaning, either with argon or nitrogen ions. Only AlN films grown of TiO2 seed layers exhibit a significant piezoelectric activity to be used in acoustic device applications. Pure c-axis oriented films, with FWHM of rocking curve of 6º, stress below 500 MPa, and electromechanical coupling factors measured in SAW devices of 1.25% are obtained. The best AlN films are achieved on amorphous TiO2 seed layers deposited at high target power and low sputtering pressure. On the other hand, AlN films deposited on Si3N4, SiO2 and TaOx exhibit a mixed orientation, high stress and very low piezoelectric activity, which invalidate their use in acoustic devices.

Fabrication of high frequency SAW resonators using AlN/Diamond/Si technology

The synthesis of AlN on diamond is a great challenge, not only because of the between an AlN/diamond interface, but also because of the high surface roughness of the diamond layers [8, 9]. In the case of microcrystalline diamond, the last problem was solved by polishing. However, polishing nanocrystalline diamond is not straightforward. For the diamond synthesis by CVD, silicon was used as a substrate. The diamond/Si interface presents a smoother diamond than the diamond/air interface. This paper reports on the fabrication of high frequency SAW resonators using AlN/Diamond/Si technology.

Influence of Crystal Quality on the Excitation and Propagation of Surface and Bulk Acoustic Waves in Polycrystalline AlN Films

We investigate the excitation and propagation of acoustic waves in polycrystalline aluminum nitride films along the directions parallel and normal to the c-axis. Longitudinal and transverse propagations are assessed through the frequency response of surface acoustic wave and bulk acoustic wave devices fabricated on films of different crystal qualities. The crystalline properties significantly affect the electromechanical coupling factors and acoustic properties of the piezoelectric layers. The presence of misoriented grains produces an overall decrease of the piezoelectric activity, degrading more severely the excitation and propagation of waves traveling transversally to the c-axis. It is suggested that the presence of such crystalline defects in c-axis-oriented films reduces the mechanical coherence between grains and hinders the transverse deformation of the film when the electric field is applied parallel to the surface.

On the lateral excitation of shear modes in AlN layered resonators

In this paper we describe the fabrication and frequency characterization of different structures intended for the lateral excitation of shear modes in AlN c-axis-oriented films, which are at the same time designed to minimize the excitation of longitudinal modes. Laterally excited resonators were built on partially metallic (SiO2, W) and insulating (SiOC, Si3N4) acoustic mirrors built on silicon substrates, and on insulating mirrors (SiO2, TaOx) built on insulating glass plates. TiOx seed layers were used to stimulate the growth of highly c-axis oriented AlN films, which was confirmed by XRD and SAW measurements. Coplanar Mo electrodes of different geometries were defined on top of the AlN films to excite the shear modes. All the structures analyzed displayed a clear longitudinal mode, corresponding to an acoustic velocity of 11000 m/s, but a null or extremely weak shear response corresponding to a sound velocity of around 6350 m/s. The simulation of the frequency response based on Mason's model confirms that the shear resonance is extremely weak. The observed longitudinal modes are attributed either to the field applied between the electrodes and a conductive plane (metallic layer or Si substrate) or to the electric field parallel to the c-axis in the edges of the electrodes or in tilted grains. The low excitation of shear modes is attributed to the very low values of electric field strength parallel to the surface.

Piezoelectric and electroacoustic properties of Ti-doped AlN thin films as a function of Ti content

In this work we present the assessment of the structural and piezoelectric properties of Al(0.5-x)TixN0.5 compounds (titanium content menor que6% atomic), which are expected to possess improved properties than conventional AlN films, such as larger piezoelectric activity, thermal stability of frequency and temperature resistance. Al:Ti:N films were deposited from a twin concentric target of Al and Ti by reactive AC sputtering, which provided films with a radial gradient of the Ti concentration. The properties of the films were investigated as a function of their composition, which was measured by electron dispersive energy dispersive X-ray spectroscopy and Rutherford backscattering spectrometry. The microstructure and morphology of the films were assessed by X-ray diffraction and infrared reflectance. Their electroacoustic properties and dielectric constant were derived from the frequency response of BAW test resonators. Al:Ti:N films properties appear to be strongly dependent on the Ti content, which modifies the AlN wurtzite crystal structure leading to greater dielectric constant, lower sound velocities, lower electromechanical factor and moderately improved temperature coefficient of the resonant frequency.

Effect of substrate-target distance and sputtering pressure in the synthesis of AlN thin films

In this work, we analyze the influence of the processing pressure and the substrate–target distance on the synthesis by reactive sputtering of c-axis oriented polycrystalline aluminum nitride thin films deposited on Si(100) wafers. The crystalline quality of AlN has been characterized by high-resolution X-ray diffraction (HR-XRD). The films exhibited a very high degree of c-axis orientation especially when a low process pressure was used. After growth, residual stress measurements obtained indirectly from radius of curvature measurements of the wafer prior and after deposition are also provided. Two different techniques are used to determine the curvature—an optically levered laser beam and a method based on X-ray diffraction. There is a transition from compressive to tensile stress at a processing pressure around 2 mTorr. The transition occurs at different pressures for thin films of different thickness. The degree of c-axis orientation was not affected by the target–substrate distance as it was varied in between 30 and 70 mm.

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 μm 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.

Super-high-frequency SAW resonators on AlN/Diamond

This letter describes the procedure to manufacture high-performance surface acoustic wave (SAW) resonators on AlN/diamond heterostructures working at frequencies beyond 10 GHz. In the design of SAW devices on AlN/diamond systems, the thickness of the piezoelectric layer is a key parameter. The influence of the film thickness on the SAW device response has been studied. Optimized thin films combined with advanced e-beam lithographic techniques have allowed the fabrication of one-port SAW resonators with finger width and pitch of 200 nm operating in the 10–14 GHz range with up to 36 dB out-of-band rejection.

Role of surface trap states on two-dimensional electron gas density in InAlN/AlN/GaN heterostructures

In order to clarify the effect of charged dislocations and surface donor states on the transport mechanisms in polar AlInN/AlN/GaN heterostructures, we have studied the current-voltage characteristics of Schottky junctions fabricated on AlInN/AlN/GaN heterostructures. The reverse-bias leakage current behaviour has been interpreted with a Poole-Frenkel emission of electrons from trap states near the metal-semiconductor junction to dislocation induced states. The variation of the Schottky barrier height as a function of the AlN layer thickness has been measured and discussed, considering the role of the surface states in the formation of the two dimensional electron gas at AlN/GaN interface.

Nanocrack-induced leakage current in AlInN/AlN/GaN

Here we report on the study of nano-crack formation in Al1−xInxN/AlN/GaN heterostructures, on its association with composition fluctuation and on its local electrical properties. It is shown here that indium segregation at nano-cracks and threading dislocations originating from the non-pseudomorphic AlN interlayer could be the cause of the high reverse-bias gate leakage current of Ni/Au Schottky contacts on Al1−xInxN/AlN/GaN heterostructures and significantly affects the contact rectifying behavior. Segregation of indium around crack tips in Al1−xInxN acting as conductive paths was assessed with conductive atomic force microscopy.

TEM study of the AlN grain orientation grown on NCD diamond substrate

Piezoelectric AlN layer grain orientation, grown by room temperature reactive sputtering, is analyzed by transmission electron microscopy (TEM).Two types of samples are studied: (i) AlN grown on well-polished NCD (nano-crystalline diamond) diamond, (ii) AlN grown on an up-side down NCD layer previously grown on a Si substrate, i.e. diamond surface as smooth as that of Si substrates. The second set of sample show a faster lignment of their AlN grain caxis attributed to it smoother diamond free surface. No grain orientation relationship between diamond substrate grain and the AlN ones is evidenced, which seems to indicate the preponderance role of the surface substrate state.

CMOS integration of AlN based piezoelectric microcantilevers

El gran crecimiento de los sistemas MEMS (Micro Electro Mechanical Systems) así como su presencia en la mayoría de los dispositivos que usamos diariamente despertó nuestro interés. Paralelamente, la tecnología CMOS (Complementary Metal Oxide Semiconductor) es la tecnología más utilizada para la fabricación de circuitos integrados. Además de ventajas relacionadas con el funcionamiento electrónico del dispositivo final, la integración de sistemas MEMS en la tecnología CMOS reduce significantemente los costes de fabricación. Algunos de los dispositivos MEMS con mayor variedad de aplicaciones son los microflejes. Estos dispositivos pueden ser utilizados para la extracción de energía, en microscopios de fuerza atómica o en sensores, como por ejemplo, para biodetección. Los materiales piezoeléctricos más comúnmente utilizados en aplicaciones MEMS se sintetizan a altas temperaturas y por lo tanto no son compatibles con la tecnología CMOS. En nuestro caso hemos usado nitruro de alumino (AlN), que se deposita a temperatura ambiente y es compatible con la tecnología CMOS. Además, es biocompatible, y por tanto podría formar parte de un dispositivo que actúe como biosensor. A lo largo de esta tesis hemos prestado especial atención en desarrollar un proceso de fabricación rápido, reproducible y de bajo coste. Para ello, todos los pasos de fabricación han sido minuciosamente optimizados. Los parámetros de sputtering para depositar el AlN, las distintas técnicas y recetas de ataque, los materiales que actúan como electrodos o las capas sacrificiales para liberar los flejes son algunos de los factores clave estudiados en este trabajo. Una vez que la fabricación de los microflejes de AlN ha sido optimizada, fueron medidos para caracterizar sus propiedades piezoeléctricas y finalmente verificar positivamente su viabilidad como dispositivos piezoeléctricos. ABSTRACT The huge growth of MEMS (Micro Electro Mechanical Systems) as well as their presence in most of our daily used devices aroused our interest on them. At the same time, CMOS (Complementary Metal Oxide Semiconductor) technology is the most popular technology for integrated circuits. In addition to advantages related with the electronics operation of the final device, the integration of MEMS with CMOS technology reduces the manufacturing costs significantly. Some of the MEMS devices with a wider variety of applications are the microcantilevers. These devices can be used for energy harvesting, in an atomic force microscopes or as sensors, as for example, for biodetection. Most of the piezoelectric materials used for these MEMS applications are synthesized at high temperature and consequently are not compatible with CMOS technology. In our case we have used aluminum nitride (AlN), which is deposited at room temperature and hence fully compatible with CMOS technology. Otherwise, it is biocompatible and and can be used to compose a biosensing device. During this thesis work we have specially focused our attention in developing a high throughput, reproducible and low cost fabrication process. All the manufacturing process steps of have been thoroughly optimized in order to achieve this goal. Sputtering parameters to synthesize AlN, different techniques and etching recipes, electrode material and sacrificial layers are some of the key factors studied in this work to develop the manufacturing process. Once the AlN microcantilevers fabrication was optimized, they were measured to characterize their piezoelectric properties and to successfully check their viability as piezoelectric devices.