Piezoelectric aluminum nitride thin films for microelectromechanical systems

This article reports on the state-of-the-art of the development of aluminum nitride (AlN) thin-film microelectromechanical systems (MEMS) with particular emphasis on acoustic devices for radio frequency (RF) signal processing. Examples of resonant devices are reviewed to highlight the capabilities of AlN as an integrated circuit compatible material for the implementation of RF filters and oscillators. The commercial success of thin-film bulk acoustic resonators is presented to show how AlN has de facto become an industrial standard for the synthesis of high performance duplexers. The article also reports on the development of a new class of AlN acoustic resonators that are directly integrated with circuits and enable a new generation of reconfigurable narrowband filters and oscillators. Research efforts related to the deposition of doped AlN films and the scaling of sputtered AlN films into the nano realm are also provided as examples of possible future material developments that could expand the range of applicability of AlN MEM

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

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

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

Piezoelectric aluminum nitride thin films for microelectromechanical systems

This article reports on the state-of-the-art of the development of aluminum nitride (AlN) thin-film microelectromechanical systems (MEMS) with particular emphasis on acoustic devices for radio frequency (RF) signal processing. Examples of resonant devices are reviewed to highlight the capabilities of AlN as an integrated circuit compatible material for the implementation of RF filters and oscillators. The commercial success of thin-film bulk acoustic resonators is presented to show how AlN has de facto become an industrial standard for the synthesis of high performance duplexers. The article also reports on the development of a new class of AlN acoustic resonators that are directly integrated with circuits and enable a new generation of reconfigurable narrowband filters and oscillators. Research efforts related to the deposition of doped AlN films and the scaling of sputtered AlN films into the nano realm are also provided as examples of possible future material developments that could expand the range of applicability of AlN MEMS

Reactive sputtering of AlScN thin Ulms with variable Sc content on 200 mm wafers

Sc-doped AlN polycrystalline films are attractive active layers for high frequency (GHz range) acoustic resonators owing to the significant enlargement of the AlN piezoelectric activity with the increasing Sc content. To sputter homogenously doped AlScN films on 200 mm Si wafers we use a configurable cathode containing a variable number of embedded Sc pellets to fine tuning the Sc content in the films. The method was implemented in an Endeavor-AT™ cluster tool from OEM Group, adapted for sputtering on 200 mm wafers. 1 µm thick AlScN films with uniform Sc content (around 7 at.%), high crystal quality and good piezoelectric response have been sputtered over 200 mm production-level wafers

Combined assessment of Al1-xScxN thin films by RBS, XRD, FTIR and BAW frequency response measurements

Al0.7Sc0.3N films were reactively sputtered from Al-Sc segmented targets by ac powered dual-cathode S-gun magnetron. Films with homogeneous Sc concentration within 30 +/- 0.5 at. % were grown at ambient temperature directly on 200-mm (100) silicon wafers and on 100-mm silicon substrates covered with SiO2/Mo-based acoustic reflectors terminated by highly (110) textured Mo electrodes. The piezoelectric assessment derived from the frequency response of bulk acoustic resonators yields values of the electromechanical coupling factor k(2) up to 12.8%. Infrared absorption and X-ray diffraction measurements reveal that tiny structural changes may lead to deviation in the value of k(2) across the wafer, which can be reduced by performing a post-processing heat treatment at around 600 degrees C

Effects of Post-Deposition Vacuum Annealing on the Piezoelectric Properties of AlScN Thin Films Sputtered on 200 Mm Production Wafers

Sc-doped AlN polycrystalline films are attractive active layers for high frequency (GHz range) acoustic resonators owing to the significant rise in the material piezoelectric activity with the increasing Sc content. AlScN films are sputtered on 200 mm Si wafers using configurable cathode containing a variable number of embedded Sc pellets which allows controlling both the Sc content in the films and the composition homogeneity across the wafer. The method is implemented in an Endeavor-AT? cluster tool from OEM Group adapted for sputtering on 200 mm wafers. The piezoelectric activity of the as-deposited AlScN films appears to improve after a 15-minute post-deposition annealing at 600ºC, leading to a 20% increase in the electromechanical coupling factor