J. Olivares

Degradation of the piezoelectric response of sputtered c-axis AlN thin films with traces of non-(0002) x-ray diffraction peaks

We have observed the apparently anomalous behavior of sputtered AlN thin films that exhibit poor piezoelectric response in spite of having (0002) preferred orientation and narrow rocking curve with full width at half maximum below 2°; however, other AlN films with weak 0002 x-ray diffraction peak and rocking curve as wide as 8° provide a good value of the piezoelectric coefficient d31 (2.1pm∕V). The critical difference between both types of films is the presence of traces of non-(0002) reflections in the x-ray diffraction patterns of the former. Non-(0002) reflections may be related to defects (probably, inversion domains) that significantly reduce the net piezoelectric field.

Dependence of the IR reflectance LO absorption bands on the crystalline texture of AlN films

The infrared reflectance spectra of polycrystalline sputtered AlN films are analyzed in relation to their crystalline texture. AlN films with perfect c-axis orientation and good piezoelectric response exhibit a single peak at 887cm−1 corresponding to the A1–longitudinal-optical (LO) vibrational mode. However, {0001}-textured films with traces of 1011, 1012, and 1013 reflections in their x-ray diffraction patterns show an additional peak at 905cm−1 that we assign to an E1-A1 quasi-LO mode due to interaction of light with non-⟨0001⟩-oriented grains. The piezoelectric response deteriorates as the amplitude of the quasi-LO mode increases with respect to the A1(LO) mode.

Electrical detection of the mechanical resonances in AlN-actuated microbridges for mass sensing applications

We report the fabrication and frequency characterization of mechanical resonators piezoelectrically actuated with aluminum nitride films. The resonators consist of a freestanding unimorph structure made up of a metal/AlN/metal piezoelectric stack and a Si3N4 supporting layer. We show that the electrical impedance of the one-port device can be used to assess the vibrational behavior of the resonators, provided that the modes do not exhibit specific symmetries, for which the impedance variations cancel. Frequency shifts arise when loading the resonators with small masses. As gravimetric sensors, the microbridges exhibit mass sensitivities of 0.18fg∕Hz for vibrational modes around 2MHz.

High-acoustic-impedance tantalum oxide layers for insulating acoustic reflectors

This work describes the assessment of the acoustic properties of sputtered tantalum oxide films intended for use as high-impedance films of acoustic reflectors for solidly mounted resonators operating in the gigahertz frequency range. The films are grown by sputtering a metallic tantalum target under different oxygen and argon gas mixtures, total pressures, pulsed dc powers, and substrate biases. The structural properties of the films are assessed through infrared absorption spectroscopy and X-ray diffraction measurements. Their acoustic impedance is assessed by deriving the mass density from X-ray reflectometry measurements and the acoustic velocity from picosecond acoustic spectroscopy and the analysis of the frequency response of the test resonators.

DCS Tx filters using AlN resonators with iridium electrodes

In this paper we present the design, fabrication technology, and characterization of BAW filters for the Digital Cellular System (DCS) Tx-band at 1.75 GHz. The filters are fabricated with AlN-based solidly mounted resonators (SMR) using iridium electrodes, in an attempt to increase the effective electromechanical coupling factor of the BAW devices and achieve the bandwidth requirements of DCS filters. The design and optimization of the filters is performed with a simulation tool that uses a circuit model to compute the filter frequency response. Tx filters with balanced inputs and outputs and different topologies are designed and fabricated. The experimental filter response is compared with the simulations to determine the suitability of each design. DCS bandwidth requirements are fulfilled by using Ir/AlN/Ir stacks.

P1H-6 Picosecond Ultrasonics as a Helpful Technique for Introducing a New Electrode Material in BAW Technology: The Iridium Case

We present picosecond ultrasonic characterizations on e-beam evaporated Ir thin films and on AlN films deposited by pulsed DC reactive sputtering on Ir. Picosecond ultrasonics is an optical technique which uses ultrashort laser pulses to generate and detect very short acoustic pulses. We first investigate the properties of the Ir films by measuring samples of various thicknesses and deposition parameters. It permits to extract the longitudinal sound velocity, the acoustic impedance and the acoustic losses in the Ir electrode. Second, we focus on AlN on Ir films. We measure the thickness and the sound velocity of an AlN thin layer deposited on an Ir electrode. This work illustrates how useful the picosecond ultrasonic technique can be for introducing a new material in bulk acoustic wave 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.

BAW resonators based on AlN with Ir electrodes for digital wireless transmissions

We investigate the performance of aluminum nitride (AlN)-based solidly mounted resonators (SMR) made with iridium (Ir) electrodes for applications in WCDMA filters. Ir/AlN/Ir stacks are grown on top of insulating Bragg mirrors composed of alternate lambda/4 layers of silicon oxycarbide (SiOC) and silicon nitride (Si3N4). We have developed the technological processes for the fabrication of filters including the trimming of the Ir top electrode for the tuning of the bandwidth. The influence of the thickness of the top electrode after the tuning process in the performance of the SMRs and filters is analyzed. The performance of the devices is compared with that of SMRs and filters of identical design made with Mo electrodes.

A model for the accurate determination of the electromechanical coupling factor of thin film SAW devices on non-insulating substrates

We propose a circuital model that simulates the frequency response of surface acoustic wave (SAW) delay lines made on piezoelectric thin films on non-insulating substrates. The model, which takes into account the parasitic elements that distort the frequency response (due to the conductive substrate, airborne coupling and mounting), allows accurate values of the piezoelectric properties of the films (k/sup 2/ and /spl nu//sub s/) to be obtained. This model has been applied to SAW delay lines made on AlN thin films deposited on different silicon substrates. We show that the values deduced for k/sup 2/ if parasitic effects are not considered can be underestimated by up to 20 times.

7E-6 Aluminum Nitride Bulk Acoustic Wave Devices with Iridium Bottom Electrodes

Bulk acoustic wave (BAW) test resonators in the frequency range between 1.8 to 1.9 GHz were fabricated with piezoelectric aluminum nitride (AIN) films sputtered on iridium (Ir) bottom electrodes. The crystal structure and piezoelectric response of AIN films grown on Ir were as good as those of the best AIN films grown on other metallic electrodes, like platinum (Pt), molybdenum (Mo) or tungsten (W). Solidly mounted resonators (SMR) test devices with a single low-impedance layer of silicon dioxide (SiO2) for acoustic isolation were used for the preliminary assessment of both the piezoelectric activity of AIN and the influence of the iridium layer on the performance of the devices. The transversal electromechanical coupling factor of the AIN films was derived by fitting the electrical response of the resonators to Masons physical model, which allowed us to obtain a material dependent coupling factor. AIN films exhibited very high coupling factors (7.5 %) barely dependent on the width of the rocking curve (RC) around the AIN 00-2 reflection. The high acoustic impedance of the Ir bottom electrode reduced the mechanical losses of the BAW resonators, which exhibited higher quality factors than resonators built on lighter Mo bottom electrodes. The influence of the thickness of Ir and Mo bottom electrodes in the performance of the devices was also compared.