Piezoelectric AlN Thin Films Research Articles

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.

Highly piezoelectric AlN thin films grown on amorphous, insulating substrates

AlN thin films were grown by reactive sputtering on amorphous SiO2 thin films. Film texture, x-ray rocking curve width, mechanical stress, and the clamped piezoelectric constant d33,f were studied as a function of rf bias power and substrate roughness. A high d33,f of 5.0 pm/V was achieved at low substrate roughness and low mechanical AlN film stress. Increasing substrate roughness and stress leads to a deterioration of d33,f, which is correlated with a higher density of opposite polarity grains detected by piezoresponse force microscopy. Extrapolating to 100% uniform polarity, a d33,f of 6.1 pm/V is derived as highest possible value, probably corresponding to the d33,f=e33/c33E of perfect single crystalline material. Growth mechanisms are proposed and underlined by high resolution transmission electron microscopy to explain the observed phenomena.

Fabrication and characterization of a shear mode AlN solidly mounted resonator-silicone microfluidic system for in-liquid sensor applications

A shear mode AlN solidly mounted resonator (SMR) microfluidic sensor system was fabricated and characterized. The AlN SMR fabrication process is fully IC compatible and uses reactive sputtering to deposit piezoelectric AlN thin films with a non-zero mean inclination of the c-axis, which allows in-liquid operation through the excitation of the shear mode. Silicone encapsulation bonded on top of the Si sensor chip includes a microfluidic system to transport the analyte and confine the flow to the active area of the sensor chip. The sensor operation in air, water, glycerol and acetone was characterized. The resonator had a resonance frequency of around 1.2 GHz and a Q value in water of around 100. Results concerning the stability and resolution are also presented. The results indicate a potential of highly sensitive low-cost microfluidic sensor systems for applications in, e.g. point-of-care testing.

Piezoelectric thin films and their applications for electronics

ZnO and AlN piezoelectric thin films have been studied for applications in bulk acoustic wave (BAW) resonator. This article introduces methods of forming ZnO and AlN piezoelectric thin films by radio frequency sputtering and applications of BAW resonators considering the relationship between the crystallinity of piezoelectric thin films and the characteristics of the BAW resonators. Using ZnO thin films, BAW resonators were fabricated for a contour mode at 3.58 MHz and thickness modes from 200 MHz to 5 GHz. The ZnO thin films were combined with various materials, substrates, and thin films to minimize the temperature coefficient of frequency (TCF). The minimum TCF of BAW resonators was approximately 2 ppm/°C in the range −20 to 80 °C. The electromechanical coupling coefficient (k2) in a 1.9 GHz BAW resonator was 6.9%. Using AlN thin films, 5–20 GHz BAW resonators with an ultrathin membrane were realized. The membrane thickness of a 20 GHz BAW resonator was about 200 nm, k2 was 6.1%, and the quality factor (Q) was about 280. Q decreased with increasing resonant frequency. The value of k2 is almost the same for 5–20 GHz resonators. This result could be obtained by improving the thickness uniformity, by controlling internal stress of thin films, and by controlling the crystallinity of AlN piezoelectric thin film.

Two-step reactive sputtering of piezoelectric AlN thin films

A series of two-step pulsed-DC reactive sputtering methods were employed to explore the possibility of making fully (0002)-textured AlN thin films with an independent control of its film quality and deposition rate at the same time. By varying the process parameters, e.g., working pressure, discharge power, and reactive atmosphere, of the first-step sputtering, while those of the second-step sputtering kept constant, we observed that the FWHM (full width at half maximum) of AlN (0002) rocking curve became smaller and the deposition rate closer to constants, than that of the corresponding one-step sputtering. The residual stress in two-step working pressure method also became smaller in magnitude and varied less with pressure compared to that of its corresponding one-step counterpart. These phenomena were rationalized in terms of film thickness effects and deposition mechanisms.

물성변화에 따른 압전형 마이크로스피커의 특성

This paper reports the characteristics of piezoelectric microspeakers that are audible in open air with high quality piezoelectric AlN thin film according to the materials properties. When we use a tensile-stressed silicon nitride diaphragm as a supporting layer, the Sound Pressure Level (SPL) is relatively small and constant at low frequency region and shows about 70 dB at 10 kHz. However, in case of a compressively stressed composite diaphragm, the SPL of the fabricated microspeakers shows higher output pressure than those of a tensile-stressed diaphragm. It produces more than 66 dB from 100 Hz to 15 kHz and the highest SPL is about 100 dB at 9.3 kHz with <TEX>$20V_{peak-to-peak}$</TEX>, sinusoidal input biases and at 10 mm distances from the fabricated microspeakers to the reference microphone. From the experimental results, it is superior to have a compressively composite diaphragm in order to produce a high SPL in piezoelectric microspeaker.

Cover Picture: phys. stat. sol. (a) 205/5

AbstractThe progress of CVD diamond growth technologies has allowed for acousto‐electronic applications based on diamond. Nowadays, thin (∼1 μm) and smooth nanocrystalline diamond films are used in new acousto electronic sensor applications such as flexural plate wave (FPW) sensors, microcantilever sensors and composite thin film bulk acoustic wave resonator (FBAR) sensors. The cover pictures illustrate this development. The upper picture shows an SAW filter made on a thick (∼100 μm) freestanding diamond layer coated with a piezoelectric AlN thin film. The lower picture shows an array of diamond/AlN micro‐cantilevers that can, for example, be used as pressure and temperature sensors. For more details on these devices see the Feature Article by Vincent Mortet et al. (pp. 1009–1020). The first author started to work on CVD diamond for electronic applications in 2001 and has been part of the wide band gap materials group in the Institute for Materials Research (IMO) of the Hasselt University and the Division IMOMEC, IMEC vzw in Belgium since 2004. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effects of outgassing on the reactive sputtering of piezoelectric AlN thin films

A series of study of the effects of outgassing on pulsed-DC reactive sputtering of highly (0002)-textured AlN thin film was conducted with systematically adjusted sputtering parameters like working pressure, atmosphere, and temperature. The film quality was evaluated by its rocking curve width and residual stress, both utilizing X-ray diffraction methods, as well as by SEM and XPS analyses. It is found that with lower outgassing all the above-mentioned sputtering parameters become less effective on both rocking curve width and residual stress. The rocking curve FWHM (Full Width at Half Maximum) measurements even exhibit apparently insensitive regions to the sputtering parameters, and accompanied threshold behaviors as well. XPS analysis reveals higher oxygen content while SEM observation shows thinner and slanter columnar structure in the AlN film when outgassing is higher upon sputtering.

Property characterization of AlN thin films in composite resonator structure

AlN thin films with c-axis orientation have been investigated for fabricating thin film bulk acoustic wave resonators in the past few years. Characterization of thin film material properties including density, elastic modulus, and piezoelectric coefficient is essential in processing study and for predicting the performance of the acoustic devices. In this paper, we present our results on the fabrication of highly c-axis oriented AlN thin films on Pt∕Ti∕Si (100) substrates by dc reactive magnetron sputtering method. The crystalline structure and the surface morphology of AlN films are characterized by x-ray diffraction and scanning electron microscopy. The effective piezoelectric coefficient d33eff of the AlN films was measured by a laser interferometer method and the piezoelectric coefficient d33 was estimated. A recently developed resonance spectrum method is applied to characterize the electromechanical properties of AlN thin films based on the input electrical impedance equation derived by one-dimensional transmission line theory for composite resonators. Using the experimental impedance spectrum data, the density and elastic constant of the piezoelectric AlN thin film in the four-layer composite resonator structure are evaluated. The calculated results reveal that the piezoelectric coefficient d33, density, and velocity of the c-axis oriented AlN thin film are 4.19pm∕V, 3187.3kg∕m3, and 10631m∕s, respectively.

Deposition of AlN films by reactive sputtering: Effect of radiofrequency substrate bias

Piezoelectric AlN thin films were deposited on Silicon substrates by triode reactive sputtering. The variation of residual stress versus bias voltage on the substrate was investigated. A compressive stress was always observed with a maximum value for a negative substrate bias of 50 V. For higher negative bias voltage values, the compressive stress decreases. X-ray diffraction measurements showed two kinds of growth orientation. First, without bias voltage, films are well crystallized and (002) oriented. Second, with bias voltage, the (002) orientation disappears and a small peak appears (situated in the 2 θ = 32°–33° range) which can be attributed to (100) orientation. Finally, the influence of compressive stress and ion bombardment on the change of orientation is discussed.

Growth and characterization of piezoelectric AlN thin films for diamond-based surface acoustic wave devices

We report on the preparation and structural characterization of piezoelectric films of aluminium nitride onto diamond substrates. The samples were fabricated by sequential radio frequency reactive diode sputtering processes, carried out at various temperatures, in a head vacuum system starting from stechiometric targets. The structural characterization of the films was performed by energy dispersive X-ray diffraction analysis. The deposition temperature was found to play a relevant role to obtain highly textured films with the c-axis perpendicular to the substrate surface, as required by surface-acoustic-wave applications. In particular, a minimum substrate temperature of 300 °C was needed in order to obtain any internal order along the c-axis while, increasing the temperature, the AlN <002> orientation becomes preferential. The rocking curve analysis revealed a good crystalline quality of the AlN films whose degree of epitaxy can be well described by a linearly increasing function of the temperature at which the films are grown.

Fabrication of suspended thin film resonator for application of RF bandpass filter

Characteristics of AlN thin film and thin film resonator for RF bandpass filter have been studied. AlN thin films were deposited by RF magnetron sputter system. Deposition parameters such as N 2 contents, Ar and N 2 partial pressures, and the distance between metal target and substrate were found to affect the piezoelectric response. To fabricate the suspended thin film resonator (STFR) using the piezoelectric AlN thin film, the etching of AlN and the surface micromachining process were conducted. The thickness of AlN film and membrane for the STFR are 2 and 15 μm, respectively. This membrane was fabricated by SOI technology. The device with the dimension of 160 × 160 μm 2 has a resonant frequency of 1.653 GHz, a K eff 2 of 2.4%, a bandwidth of 17 MHz, and a quality factor of 91.7. The device with the dimension of 200 × 200 μm 2 has a resonant frequency of 1.641 GHz, a K eff 2 of 1.2%, and a bandwidth of 9 MHz, and a quality factor of 50.2.

Effects of bottom electrodes on the orientation of AlN films and the frequency responses of resonators in AlN-based FBARs

Effects of bottom electrode materials, included in film bulk acoustic resonators (FBARs), on the orientation of piezoelectric AlN thin films and the frequency response characteristics of AlN-resonators are investigated. AlN films are deposited using RF reactive sputtering on various bottom metals, such as Al, Cu, Ti and Mo. Measurements of X-ray diffraction, field-emission scanning electron microscopy and atomic force microscopy show that the AlN film deposited on the Mo electrode exhibits highly desirable properties, namely, a texture coefficient value of ∼93%, crystallite size of ∼40 nm and surface roughness of ∼8.5 nm. The AlN film deposited on the Mo electrode reveals a relatively dense and well-textured columnar structure with fairly uniform grains, while the films deposited on the other electrode metals exhibit a granular type of structure with mixed small and large grains. FBAR devices employing an Al (top)/AlN/metal (bottom)/Si configuration were also fabricated and their frequency response characteristics ( S 11) were measured. The resonator using the Mo electrode was found to have a superior performance (average return loss ⩾10 dB at 3 GHz). It is concluded that the positive role of the Mo electrode in achieving the high-quality AlN films and the high-performance FBAR devices may be attributed to the smaller lattice mismatch as well as the similarity of the thermal expansion coefficient between the deposited AlN film and the Mo electrode substrate.

Effects of bottom electrodes on the orientation of AlN films and the frequency responses of resonators in AlN-based FBARs

Effects of bottom electrode materials, included in film bulk acoustic resonators (FBARs), on the orientation of piezoelectric AlN thin films and the frequency response characteristics of AlN-resonators are investigated. AlN films are deposited using RF reactive sputtering on various bottom metals, such as Al, Cu, Ti and Mo. Measurements of X-ray diffraction, field-emission scanning electron microscopy and atomic force microscopy show that the AlN film deposited on the Mo electrode exhibits highly desirable properties, namely, a texture coefficient value of ∼93%, crystallite size of ∼40 nm and surface roughness of ∼8.5 nm. The AlN film deposited on the Mo electrode reveals a relatively dense and well-textured columnar structure with fairly uniform grains, while the films deposited on the other electrode metals exhibit a granular type of structure with mixed small and large grains. FBAR devices employing an Al (top)/AlN/metal (bottom)/Si configuration were also fabricated and their frequency response characteristics (S11) were measured. The resonator using the Mo electrode was found to have a superior performance (average return loss ⩾10 dB at 3 GHz). It is concluded that the positive role of the Mo electrode in achieving the high-quality AlN films and the high-performance FBAR devices may be attributed to the smaller lattice mismatch as well as the similarity of the thermal expansion coefficient between the deposited AlN film and the Mo electrode substrate.

Morphological properties of AlN piezoelectric thin films deposited by DC reactive magnetron sputtering

This paper deals with the formation of AlN thin films successfully deposited on Si(111) substrates by using the direct current magnetron sputtering method. The films feature planes with (100) and (002) preferential orientations, low surface roughness and a homogeneous composition. The effects of the sputtering pressure, sputtering power and distance ( D) from the target to the substrate on the preferential orientation of the AlN films are also studied. The results show that a lower sputtering pressure and shorter distance D are conducive to the formation of the (002) plane. On the contrary, a higher sputtering pressure and longer distance D are beneficial for the growth of the (100) plane. Moreover, the preferential orientation of the AlN thin film is also dealt with from the viewpoint of the formation of the AlN chemical bond and the mean free path of sputtered particles.

A noble suspended type thin film resonator (STFR) using the SOI technology

The fabrication and characteristics of suspended type thin film resonators (STFRs) using surface micromachining of the SOI technology, have been studied. The size of the active part of STFRs is 160 μm×160 μm. For the piezoelectric AlN thin film, the following etch rate was observed 200 nm min −1 in 0.6 wt.% TMAH. The thickness of the piezoelectric AlN film for the STFR is 2 μm. Cr thin film is used as the top and bottom electrode. This device is free-standing and has a resonant frequency of 1.65 GHz for the 2 μm AlN thin film, a K eff 2 of 2.4%, Q s of 91.7, Q p of 87.7.