Piezoelectric AlN Thin Films Research Articles

ScxAl1-xN piezoelectric film grown at room temperature

In order to overcome the shortcomings of AlN piezoelectric thin films such as low piezoelectric coefficient and low electromechanical coupling coefficientrealize, ScxAl1−xN thin films with CMOS process compatibility were prepared by pulsed DC reactive magnetron sputtering. High performance ScxAl1−xN piezoelectric thin films were prepared at room temperature by optimizing process parameters including power, gas flow ratio, substrate temperature, seed layers and so on. The phase separation phenomenon was investigated through XRD, TEM, and XPS. We found that the phase separation is related to the appearance of rocksalt ScN and led to the degeneration of film properties. The existence of ScN phase is a key factor affecting the performance of ScxAl1−xN. The results show that the piezoelectric constant d33 of ScAlN film achieved 27.5 pC/N and the full width at half maximum (FWHM) of the rocking curve is 1.9° in room temperature when the content of Sc reaches 35%.

Research on piezoelectric simulation characteristics based on ALN piezoelectric thin film structure

AlN piezoelectric thin films possess exceptional performance and find extensive applications in the fields of underwater acoustic communication and hydroacoustic sensing. To enhance the output voltage values of AlN piezoelectric thin films and optimize their structure, comprehensive simulations and analyses were carried out by using COMSOL on both circular and square film configurations. These investigations specifically focused on examining the impact of dimensional variations on resonance frequency and output voltage, as well as exploring the relationship between vibration frequency and output voltage. The findings revealed that by employing an appropriate cavity radius, increasing the thickness of the thin film can effectively enhance the output voltage. However, if the thickness surpasses 1.5 μm, the output voltage will gradually decline. Notably, the square film exhibited a significantly higher output voltage measurement of 2158 pm compared to the circular film.

Isomorphic contact resonance force microscopy and piezoresponse force microscopy of an AlN thin film: demonstration of a new contact resonance technique

We present a new contact resonance force microscopy (CRFM) imaging technique, isomorphic contact resonance (iso-CR), that acquires data at a constant contact resonance (CR) frequency, and hence constant tip-sample contact stiffness across the scan area. Constant CR frequency is obtained by performing force versus distance measurements to vary the applied force at each pixel (i.e. force-volume mapping mode). The CR frequency increases with increasing applied force; thus, a carefully selected target frequency will be reached for most pixels at some point in the force versus distance curve. In the iso-CR mode, the cantilever maintains an invariant vibrational shape and a constant environmental damping, thus simplifying interpretation of amplitude and quality factor contrast compared to conventional CRFM. Iso-CR imaging of a piezoelectric AlN thin film sample is demonstrated. Iso-CRFM images were obtained by mechanically driving the base of the cantilever, and iso-CR piezoresponse force microscopy (iso-CR-PFM) images were obtained by electrically biasing the tip. The PFM phase images reveal that the sample contains nanoscale Al-polar (or ‘up’) and N-polar (or ‘down’) domains, with ≈180° phase contrast between oppositely polarized domains. The PFM amplitude and Q-factor images also show ‘up’ vs. ‘down’ domain contrast, which decreases with increasing CR frequency. The frequency-dependent amplitude and Q contrast is ascribed to a frequency-dependent electrostatic contribution to the signal. Domain contrast is not observed in the CRFM (mechanically driven) images. To summarize, the iso-CR capability to control the resonance frequency across multiple excitation schemes helps elucidate the origin of the electromechanical and nanomechanical image contrast.

Preparation of high-quality stress-free (001) aluminum nitride thin film using a dual Kaufman ion-beam source setup

We proposed and demonstrated a preparation method of (001) preferentially oriented stress-free AlN piezoelectric thin films. The AlN thin films were deposited by a reactive sputtering technique at substrate temperatures up to 330 °C using a dual Kaufman ion-beam source setup. We deposited the AlN on Si (100), Si (111), amorphous SiO2, and a (001) preferentially oriented Ti thin film and compared their crystallographic, optical, and piezoelectric properties. The AlN thin films deposited on the (001) preferentially oriented Ti thin films have the highest crystallographic quality. The stress-free AlN reached a high value of the piezoelectric coefficient d33 = (7.33 ± 0.08) pC·N−1. The properties of the AlN thin film prepared at such low temperatures are suitable for numerous microelectromechanical systems, piezoelectric sensors, and actuators monolithically integrated with complementary metal-oxide-semiconductor signal-processing circuits.

Development of AlN thin films for breast cancer acoustic biosensors

Although the development of biosensors has become popular in the recent past, there are still many opportunities to develop specific designs that address public health problems of third world countries. This paper presents the initial efforts toward the development of an affordable acoustic biosensor for breast cancer detection. The core of the sensor consists of a piezoelectric AlN thin film that requires specific crystallographic and morphological features. In this study, the processing–structure relationship of our radio-frequency magnetron sputtering (r.f. MS) system was established. Al/AlN films were produced via r.f. MS varying the applied power and atmosphere composition. The films were analyzed by glancing angle X-ray diffraction, scanning electron microscopy+energy dispersive analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. The results indicate that applied power had a much stronger influence on the phase selection, orientation and morphology of the films and was attributed to the effect of power on ad-atom mobility on the substrate. Higher power values resulted in films better suited for biosensor applications. The presence of the Al adhesion layer favored the formation of undesirable metastable c-AlN. Preliminary results on the biofunctionalization of the films were encouraging, but require further work both in the protocol and on the effect of the film surface on this process.

Quantitative thickness measurement of polarity-inverted piezoelectric thin-film layer by scanning nonlinear dielectric microscopy

A quantitative measurement method for a polarity-inverted layer in ferroelectric or piezoelectric thin film is proposed. It is performed nondestructively by scanning nonlinear dielectric microscopy (SNDM). In SNDM, linear and nonlinear dielectric constants are measured using a probe that converts the variation of capacitance related to these constants into the variation of electrical oscillation frequency. In this paper, we describe a principle for determining the layer thickness and some calculation results of the output signal, which are related to the radius of the probe tip and the thickness of the inverted layer. Moreover, we derive an equation that represents the relationship between the output signal and the oscillation frequency of the probe and explain how to determine the thickness from the measured frequency. Experimental results in Sc-doped AlN piezoelectric thin films that have a polarity-inverted layer with a thickness of 1.5 µm fabricated by radio frequency magnetron sputtering showed a fairly good value of 1.38 µm for the thickness of the polarity-inverted layer.

Frequency response of AlN-based solidly mounted resonators under mechanical stress

We analyse the performance of solidly mounted bulk acoustic wave resonators under induced mechanical stress to explore their viability as strain sensors. The resonators are made of polycrystalline AlN piezoelectric thin films containing uniformly tilted microcrystals to excite both longitudinal and shear modes. The resonators are grown on top of silicon bars that are fixed at one or two edges using two home-made apparatus specifically designed to induce deformations of several hundreds of microstrains. The induced strain causes frequency shifts of tenths of MHz, yielding strain coefficients of the resonant frequency (SCF) up to −71% per unit strain (−0.71ppm/μe). The influence of the nature of the resonant mode (shear of longitudinal), the electromechanical coupling factor and the operation frequency on the SCF is analysed.

Effect of the Electrode Layer on the Deposition of AlN for Biosensors

In the development of thin film bulk acoustic resonators (FBAR) based on AlN piezoelectric thin films for biosensing applications, a metallic electrode below the piezoelectric film is not only an essential element for the performance of these devices, but it also influences the structure and properties of the AlN film. The present study addresses the effect of an Al electrode, which also serves as an adhesion interlayer, on the final characteristics of Al/AlN films intended for FBAR biosensors. For this purpose, various single Al and Al/AlN thin films were deposited on 304L substrates by radio frequency (r.f.) magnetron sputtering. The selected processing parameters were deposition time and applied power for the Al layers. For the case of the single Al films, the deposition time had a stronger influence than the applied power in the ranges studied. Longer deposition times, regardless of the applied power, led to the formation of rougher, thicker films. The structure of the Al films in turn strongly affected that of the final Al/AlN coatings, particularly their morphology and phase selection. The present results highlight the importance of the metallic electrode structure in the development of acoustic sensors based on piezoelectric thin films.

Microstructure and mechanical properties of stress-tailored piezoelectric AlN thin films for electro-acoustic devices

Nanoindentation measurements along with atomic force microscopy, X-ray diffraction, and residual stress analyses on the basis of Raman measurements have been performed to characterize stress-tailored AlN thin films grown using reactive RF magnetron sputtering. The intrinsic stress gradient caused by the growing in-plane grain size along film thickness was minimized by increasing the N2 concentration in the Ar/N2 gas mixture during the growth process. The increase of N2 concentration did not degrade the device-relevant material properties such as crystallographic orientation, surface morphology, piezoelectric response, or indentation modulus. Due to comparable crystallographic film properties for all investigated samples it was concluded that mainly the AlN crystallites contribute to the mechanical film properties such as indentation modulus and hardness, while the film stress or grain boundaries had only a minor influence. Therefore, by tailoring the stress gradient in the AlN films, device performance, fabrication yield, and the design flexibility of electro-acoustic devices can be greatly improved.

Magnetron sputtering of piezoelectric AlN and AlScN thin films and their use in energy harvesting applications

This paper reports on the deposition of AlN and AlXSc1źXN films by pulse magnetron sputtering. The first part will focus on the AlXSc1źXN deposition process in comparison to the already established AlN process. The effect of doping AlN with Sc regarding piezoelectric and mechanical properties is presented. The films show the expected increase of piezoelectric properties as well as the softening of the material with higher Sc concentrations. Above a threshold concentration of around 40 % Sc in the AlXSc1źXN films, there exists a separation into two phases, an Al-rich and a Sc-rich wurtzite phase, which is shown by XRD. At Sc concentrations higher than 50 %, the films are not piezoelectric, as the films are composed primarily of the cubic ScN phase. The second main part of this paper evaluates the films for application in energy harvesting. Especially the Sc doping allows a significant increase in the energy generated in our test setup. Directly measuring the AC voltage at resonance depending on load resistance with base excitation of ±2.5 µm, 350 µW power have been generated under optimum conditions compared to 70 µW for pure AlN. For a more application oriented measuring setup, a standard and a SSHI-based ("Synchronised Switch Harvesting on Inductor") AC/DC converter circuit have been tested. The SSHI interface showed a significant improvement to 180 % compared to the standard interface.

Preparation of (001) preferentially oriented titanium thin films by ion-beam sputtering deposition on thermal silicon dioxide

We propose the ion-beam sputtering deposition providing Ti thin films of desired crystallographic orientation and smooth surface morphology not obtainable with conventional deposition techniques such as magnetron sputtering and vacuum evaporation. The sputtering was provided by argon broad ion beams generated by a Kaufman ion-beam source. In order to achieve the optimal properties of thin film, we investigated the Ti thin films deposited on an amorphous thermal silicon dioxide using X-ray diffraction, and atomic force microscopy. We have optimized deposition conditions for growing of thin films with the only (001) preferential orientation of film crystallites, and achieved ultra-low surface roughness of 0.55 nm. The deposited films have been found to be stable upon annealing up to 300 °C which is often essential for envisaging subsequent deposition of piezoelectric AlN thin films.

Highly piezoelectric co-doped AlN thin films for wideband FBAR applications.

We report piezoelectric materials composed of charge-compensated co-doped (Mg, β)(x)Al(1-x)N (β = Zr or Hf) thin films. The effect of the dopant element into AlN on the crystal structure, and piezoelectric properties of co-doped AlN was determined on the basis of a first-principles calculation, and the theoretical piezoelectric properties were confirmed by experimentally depositing thin films of magnesium (Mg) and zirconium (Zr) co-doped AlN (Mg-Zr-doped AlN). The Mg-Zrdoped AlN thin films were prepared on Si (100) substrates by using a triple-radio-frequency magnetron reactive co-sputtering system. The crystal structures and piezoelectric coefficients (d33) were investigated as a function of the concentrations, which were measured by X-ray diffraction and a piezometer. The results show that the d33 of Mg-Zr-doped AlN at total Mg and Zr concentrations (both expressed as β) of 0.35 was 280% larger than that of pure AlN. The experimentally measured parameter of the crystal structure and d33 of Mg-Zr-doped AlN (plotted as functions of total Mg and Zr concentrations) were in very close agreement with the corresponding values obtained by the first-principle calculations. Thin film bulk acoustic wave resonators (FBAR) employing (Mg,Zr)0.13Al0.87N and (Mg, Hf)0.13 Al0.87N as a piezoelectric thin film were fabricated, and their resonant characteristics were evaluated. The measured electromechanical coupling coefficient increased from 7.1% for pure AlN to 8.5% for Mg-Zr-doped AlN and 10.0% for Mg- Hf-doped AlN. These results indicate that co-doped (Mg, β)(x)Al(1-x)N (β = Zr or Hf) films have potential as piezoelectric thin films for wideband RF applications.

The Process Optimization of Smart Nanostructured AlN Thin Films Sputtered by Pulsed DC

A ‘Smart’ coating design, based on the incorporation of an AlN piezoelectric thin film sensor into a tribological coating system, has been explained in this paper. As part of the overall coating system, piezoelectric AlN thin films with highly preferred (002) orientations have been deposited by a pulsed closed field unbalanced magnetron sputtering system using Cr, Mo and Pt as the seed/adhesion layer. The texture, orientation and piezoelectric properties of AlN films were characterized by means of x-ray diffraction, field emission scanning electron microscopy and laser interferometry. A Michelson laser interferometer was designed and constructed to obtain the converse piezoelectric response of the deposited AlN thin films. It was found that the incorporation of different seed layers at various working pressure in the sandwiched structure, significantly affected the (002) orientation and the piezoelectric response of AlN thin films.

Effect of pre-deposition RF plasma etching on wafer surface morphology and crystal orientation of piezoelectric AlN thin films.

In this work, we describe the design and operation of a planarized capacitively coupled RF plasma module and investigate the effects of non-reactive RF plasma etching on Si (100) wafer surface morphology and crystal orientation of Al bottom electrodes and subsequently deposited AlN films. To ensure formation of highly (111) textured Al electrode, a thin 25-nm AlN seed layer was grown before the Al deposition. The seed layer's orientation efficiency improved with increasing the RF power from 70 to 300 W and resulted in narrowing the Al (111) rocking curves. AFM and XRD data have shown that crystal orientations of both the electrode and reactively sputtered AlN film are considerably improved when the substrate micro roughness is reduced from an ordinary level of a few nanometers to atomic level corresponding to root mean square roughness as low as about 0.2 to 0.3 nm. The most perfectly crystallized film stacks of 100-nm Al and 500-nm AlN were obtained in this work using etching in Ar plasma optimized to create an atomically smooth, epi-ready Si surface morphology that enables superior AlN seed layer nucleation conditions. X-ray rocking curves around the Al (111) and AlN (0002) diffraction peaks exhibited extremely low FWHM values of 0.68° and 1.05°, respectively.

Effect of Mg and Zr co-doping on piezoelectric AlN thin films for bulk acoustic wave resonators

This paper reports the crystal structures and piezoelectric properties of Mg and Zr co-doped AlN (MgZr-doped AlN) thin films. MgZr-doped AlN thin films on Si (100) substrates were fabricated by using a radio-frequency magnetron reactive cosputtering system. The relations between dopant concentration and the crystal structures measured by X-ray diffraction, transmission electron microscopy, and the piezoelectric constant d 33 measured with a piezometer were investigated. Up to total Mg and Zr concentrations of 34.8 at%, the MgZr-doped AlN films maintained the c-axis-oriented wurtzite structure and were likely formed by substituting Mg and Zr atoms into Al atom sites. The d 33 of the MgZr-doped AlN with a total Mg and Zr concentration (Mg + Zr) of 34.8 at% was about three times that of pure AlN. The experimental results on the relation between total Mg and Zr concentration and the crystal structure and the d 33 were in close agreement with the results of first-principles calculations. Finally, thin film bulk acoustic wave resonators (FBARs) that used MgZr-doped AlN as a piezoelectric thin film were fabricated and compared with the AlN-based FBAR. A total Mg and Zr concentration of 13 at% was found to improve the electromechanical coupling coefficient of AlN from 7.1% (for pure AlN) to 8.5%. The results from this study suggest that the MgZr-doped AlN films have potential as piezoelectric thin films for wideband and high-frequency RF applications.

Molybdenum deposited on Si (111), SiO2, and amorphous Si through RF magnetron sputtering

Molybdenum (Mo) commonly serves as an electrode layer for the highly columnar growth of AlN piezoelectric thin films used to fabricate film bulk/surface acoustic resonators. The structure of the bottom Mo electrode directly influences the texture of the top AlN thin film, whereas the surface morphology and texture of the Mo electrode are inversely influenced by the substrate materials. In this study, the effects of different underlays on the characteristics of Mo films deposited through RF magnetron sputtering are compared. The results show that Mo thin film deposited on SiO2 has well-textured crystallization and a small residual stress of 95MPa. Mo deposited on amorphous Si (α-Si)/SiO2 has the smallest surface roughness of 1.63nm, but its columnar texture is slightly tilted to the substrate. Mo thin film deposited on Si (111) shows relatively moderate characteristics compared with the other two thin films.

Crystallographic Texture of Submicron Thin Aluminum Nitride Films on Molybdenum Electrode for Suspended Micro and Nanosystems

The structural properties of piezoelectric AlN/Mo/AlN stacks have been investigated for obtaining an improved c-axis oriented, piezoelectric AlN thin film on Mo electrodes to be employed in suspended nano- and micromechanical devices. With increasing thickness of the AlN interlayer up to 80 nm, the texture of the overgrown Mo layer improves significantly, marked by a significant reduction in full-width-at-half-maximum of the rocking curve obtained around 220 Bragg reflections. The improved orientation of Mo crystallites promotes the growth of preferentially c-axis oriented columns in the overgrown AlN film as evidenced by analyzes of crystal orientation determined by scanning nano-beam transmission electron diffraction. Atomic-Force Microscopy revealed only a slight deterioration of the surface roughness of AlN films upon introduction of the interlayer. The whole film stack has a total thickness of less than 1 μm and is, thus, promising for suspended piezoelectric active RF devices.

The Liquid Sensor Using Thin Film Bulk Acoustic Resonator with C‐Axis Tilted AlN Films

Dual‐mode thin film bulk acoustic resonator (TFBAR) devices are fabricated with c‐axis tilted AlN films. To fabricate dual‐mode TFBAR devices, the off‐axis RF magnetron sputtering method for the growth of tilted piezoelectric AlN thin films is adopted. In this report, the AlN thin films are deposited with tilting angles of 15° and 23°. The frequency response of the TFBAR device with 23° tilted AlN thin film is measured to reveal its ability to provide dual‐mode resonance. The sensitivities of the longitudinal and shear modes to mass loading are calculated to be 2295 Hz cm2/ng and 1363 Hz cm2/ng with the mechanical quality factors of 480 and 287, respectively. The sensitivities of the longitudinal and shear modes are calculated to be 0 and 15 Hz cm2/μg for liquid loading.

Nonlinear excitation of a rotational mode in a piezoelectrically excited square-extensional mode resonator

We present an experimental study of the nonlinear behavior of a square-extensional (SE) mode microelectromechanical resonator, actuated with a piezoelectric AlN thin film. The acoustic vibration fields of the device are characterized using laser probing. A nonlinear vibration behavior of the SE mode is observed above a drive power level of −10 dBm such that the vibration amplitude of the SE mode saturates and a rotational in-plane vibration mode is excited at a significantly lower frequency (0.725 MHz) than the SE mode (16.670 MHz). Interestingly, the measured ∼10 nm saturation amplitude of the SE mode is more than a decade below the amplitude value at which mechanical or electromechanical nonlinearities are estimated to become significant.

Designing a Piezoelectric Vibration-Energy Harvester Used for a Rapid IgE Allergy Sensing System Platform

This investigation examines a novel means of integrating high-performance AlN piezoelectric thin films with a flexible stainless steel substrate (SUS304) to fabricate a double-sided piezoelectric transducer for wind-power generators. The double-sided piezoelectric transducer is constructed by depositing AlN piezoelectric thin films on both the front and the back sides of SUS304 substrate. The titanium (Ti) and platinum (Pt) layers were deposited using a dual-gun DC sputtering system between the AlN piezoelectric thin film and the back side of the SUS304 substrate. Scanning electron microscopy and X-ray diffraction of AlN piezoelectric films reveal a rigid surface structure and highly c-axis-preferred orientation. To fabricate a transducer with a resonant frequency of about 90 Hz, a cantilever length of 1 cm and a vibration area of 1 cm2 are designed, based on the cantilever vibration theory. The maximum open circuit voltage of the power transducer is approximately 14.6 V.