ScAlN Thin Films Research Articles

Influence of underlayer roughness on the properties of Sc0.4Al0.6N thin films prepared via sputter deposition

Abstract A ScAlN thin film is one of the key materials of MEMS and high-frequency filters used in new-generation communication devices. Piezoelectricity can be improved by increasing Sc concentration. However, abnormal grains often appear at high Sc concentrations, degrading crystallinity and piezoelectricity. Herein, we demonstrated that underlayer roughness considerably affects the emergence of abnormal grains in a Sc0.4Al0.6N thin film formed via reactive DC sputtering. Dry etching with Ar plasma can effectively reduce the surface roughness of amorphous SiN and polycrystalline Si. Sc0.4Al0.6N thin films deposited on amorphous SiN and polycrystalline Si with sufficient flat surfaces exhibited a low density of abnormal grains, high crystallinity and piezoelectricity, and low loss tangent. Moreover, such high-quality thin films were obtained on a borophosphosilicate glass flattened using a reflow process without Ar etching. Therefore, underlayer roughness played an important role. The findings can help enable the large-scale production of highly doped ScAlN thin films.

Emerging ferroelectric materials ScAlN: applications and prospects in memristors.

The research found that after doping with rare earth elements, a large number of electrons and holes will be produced on the surface of AlN, which makes the material have the characteristics of spontaneous polarization. A new type of ferroelectric material has made a new breakthrough in the application of nitride-materials in the field of integrated devices. In this paper, the application prospects and development trends of ferroelectric material ScAlN in memristors are reviewed. Firstly, various fabrication processes and structures of the current ScAlN thin films are described in detail to explore the implementation of their applications in synaptic devices. Secondly, a series of electrical properties of ScAlN films, such as the current switching ratio and long-term cycle durability, were tested to explore whether their electrical properties could meet the basic needs of memristor device materials. Finally, a series of summaries on the current research studies of ScAlN thin films in the synaptic simulation are made, and the working state of ScAlN thin films as a synaptic device is observed. The results show that the ScAlN ferroelectric material has high residual polarization, no wake-up function, excellent stability and obvious STDP behavior, which indicates that the modified material has wide application prospects in the research and development of memristors.

Oblique incidence ultrasonic reflectometry device based on c-axis tilted ScAlN films for evaluating viscoelastic properties of liquids above 100 MHz

The ultrasonic complex reflectometry is a powerful tool for evaluating the viscoelastic properties of small liquid samples. However, the shear mode reflectometry device has not been used at frequencies above 100 MHz because shear-mode piezoelectric films are difficult to obtain at such frequencies. Here, the oblique incidence reflectometry with 143 MHz quasi-shear waves excited by c-axis tilted ScAlN thin films was proposed to realize high-sensitivity evaluation of the viscoelastic properties of liquids. Using the prism type reflectometry MEMS, the shear elasticity and shear viscosity of glycerin solutions were estimated from their complex reflection coefficients.

Impact of dislocation density on the ferroelectric properties of ScAlN grown by molecular beam epitaxy

We report on the effect of dislocation density on the ferroelectric properties of single-crystalline ScAlN thin films grown by molecular beam epitaxy. Wurtzite phase and atomically smooth ScAlN films have been grown on bulk GaN, GaN on sapphire, and GaN on Si substrates with dislocation densities ranging from ∼107 to 1010 cm−2. Despite the significant difference in dislocation density, ferroelectricity is observed in all three samples. The presence of high densities of dislocations, however, results in enhanced asymmetric P–E loops and overestimated remnant polarization values. Further measurements show that the leakage current and breakdown strength can be improved with decreasing dislocation density. Detailed studies suggest that trapping/detrapping assisted transport is the main leakage mechanism in epitaxial ferroelectric ScAlN films. This work sheds light on the essential material quality considerations for tuning the ferroelectric property of ScAlN toward integration with mainstream semiconductor platforms, e.g., Si, and paves the way for next-generation electronics, optoelectronics, and piezoelectronics.

High-fidelity patterning of AlN and ScAlN thin films with wet chemical etching

We report on the anisotropic wet etching of sputtered AlN and Sc0.2Al0.8N thin films. With tetramethyl ammonium hydroxide at 80 °C, the etch rates along the c-axis were 330 and 30 nm/s for AlN and Sc0.2Al0.8N, respectively. Although the etching was anisotropic, significant lateral etching below the mask occurred, perpendicular to the c-axis. With a 1 µm Sc0.2Al0.8N film, it could be up to 1800 nm. We studied the lateral etching with molybdenum, SiO2, SiNx and TiO2 masks, and found the leading cause for the lateral etching to be modification of the AlN or Sc0.2Al0.8N surface caused by ion bombardment and surface oxidation by ambient air. The lateral etching was reduced by optimizing the mask deposition and with thermal annealing. With Sc0.2Al0.8N, the lateral etching was reduced down to 35–220 nm depending on the mask, while with AlN, it was reduced to negligible. These results can be used for developing optimised mask deposition processes for better etch characteristics and for microfabrication of AlN and ScxAl1‑xN thin-film structures.

Ion-beam-induced in-plane a-axis oriented (0001) AlN and ScAlN thin film BAW resonators

High-Q RF bulk acoustic wave filters are still required for mobile communication applications. However, the in-plane orientation of commercial AlN films is random, despite their excellent out-of-plane c-axis orientation. We proposed combining two different orientation control techniques: self-texture for out-of-plane alignment and ion beam irradiation for in-plane alignment. To demonstrate the effectiveness of this approach, both in-plane a-axis and out-of-plane c-axis aligned AlN and ScAlN films were grown on Ti/silica glass substrates using 0.3–0.5 keV grazing ion-beam-assisted RF sputtering technique without using epitaxial growth. The clear six symmetry in the pole-figure observed in AlN and ScAlN thin films indicates the single-crystal-like in-plane a-axis orientation. The ion beam direction corresponded to the in-plane 〈101̄0〉 axis. In-plane x-ray diffraction ϕ-scan curve and ω-scan curve FWHM were measured to be 25° and 2.2°.

Bulk acoustic wave transformer based on the combination of the high-ε epitaxial PbTiO3 and low-ε ScAlN thin films

A rectifier antenna (called a rectenna) is useful for wireless power charging of electronic devices. To enhance the rectifier efficiency, we propose sub-GHz bulk acoustic wave (BAW) piezoelectric transformers based on the combination of high and low dielectric constant (ε) thin films. The input layers are sputter-epitaxial lead titanate (PbTiO3) films, which possess both high electromechanical coupling coefficient kt and high ε, and the output layers are ScAlN films with high kt but low ε. The voltage gain of 12.3 in 50 MHz ranges and 40.9 in 400 MHz ranges was experimentally observed in the thickness-extensional mode BAW transformer. The experimental results are in good agreement with the theoretical predictions of the electromechanical transmission line model. The BAW transformer is promising for a high voltage gain transformer for wireless power transfer.

Highly coupled leaky surface acoustic wave on hetero acoustic layer structures based on ScAlN thin films with a c-axis tilt angle

In this paper, we present hetero acoustic layer (HAL) structures based on ScAlN thin films with a c-axis tilt angle. The surface acoustic wave (SAW) excitation modes and their propagation characteristics for different Euler angles (0°, θ, ψ) of the ScAlN thin film are analyzed using the finite element method. We find that the acoustic field distribution of HAL structures can be adjusted by rotating their Euler angles and that normal SAWs are suppressed; however, leaky SAWs (LSAWs) with significantly improved electromechanical coupling coefficients (K 2) are excited. A maximum K 2 of 14% can be obtained for shear horizontal (SH)-type LSAWs, which is almost 7 times larger than that of the previously reported normal SAW Sezawa mode. Moreover, the optimized HAL structure consisting of (0°, 90°, 60°) ScAlN thin film/SiO2/AlN/Si simultaneously possesses a larger K 2, a wider bandwidth, and a higher Q value, which is promising for the design and application of wideband SAW devices with high performance.

High-frequency surface acoustic wave resonator with ScAlN/hetero-epitaxial diamond

High-frequency and wideband surface acoustic wave (SAW) devices have become an important topic in 5G communication systems and beyond. For this purpose, piezoelectric ScAlN thin films deposited on high SAW velocity diamond are studied in SAW resonators on the polycrystalline diamond (PCD) and hetero-epitaxial diamond (HED) substrates. A very strong c-axis orientation of ScAlN on HED was confirmed. Interdigital transducers of 0.8 μm and 0.5 μm were fabricated to realize 2.2 to 3.5 GHz high-frequency resonators. High Q values were obtained for the 2.3–2.5 GHz device on both PCD and HED. Additionally, a high electro-mechanical coupling coefficient (K2) of 5.40–5.52% and high SAW velocities of 7400–7766 m/s were obtained in the 2.3–2.5 GHz devices. In comparison to the simulated coupling coefficient results of a previous report, ScAlN/diamond might have higher K2 for the Sezawa wave.

Microwave Irradiation Process for Al\u2013Sc Alloy Production

Scandium is being explored as an alloying element for aluminium alloys, which are gaining importance as high-performance lightweight structural alloys in the transportation industry. Sc-rich ScAlN thin films show strong piezoelectricity and can be fabricated on a hard substrate for use as wideband surface acoustic wave filters in next-generation wireless mobile communication systems. However, the use of ScAlN thin films in microelectromechanical system devices is limited by the high cost of metallic Sc, which is due to the difficulty in smelting of this material. Here, we propose a novel microwave irradiation process for producing Al-Sc alloys, with Mg ions as a reducing agent. Although scandium oxide is thermodynamically stable, intermetallic Al3Sc is obtained in high yield (69.8%) via a low-temperature (660 °C) reduction reaction under microwave irradiation. Optical spectroscopy results and thermodynamic considerations suggest a non-thermal equilibrium reaction with the univalent magnesium ions excited by microwave irradiation.

1J2-3 Effect of negative ion bombardment increased in low-pressure sputtering deposition on piezoelectric properties of ScAlN thin films

1J2-3 Effect of negative ion bombardment increased in low-pressure sputtering deposition on piezoelectric properties of ScAlN thin films

Film stress dependence on deposition temperature in scandium aluminium nitride thin film

This paper reports investigation of stress in ScAlN thin films deposited on 4-in silicon wafers at various substrate temperature. ScAlN has a 5 times higher piezoelectric constant, but simultaneously ScAlN thin film has a large film stress. To relax the stress, the substrate temperature was changed between 170 and 250°C. Film stress, crystal quality, piezoelectric constant, and Sc concentration was investigated for the deposited ScAlN films. The film stress was relaxed as the substrate temperature increases.

Synthesis of ScAlN thin films on Si (100) substrates at room temperature

Scandium aluminum nitride alloy (ScAlN) thin films have been synthesized using reactive sputtering of a scandium aluminum alloy (Sc0.40Al0.60) target on Si (100) substrates. We have investigated the effects of two sputtering control factors, namely sputtering process pressure and discharge power, on the c-axis growth of the piezoelectric layers. According to our X-ray diffraction analysis, the films are highly textured in the [001] direction at low process pressures and moderate to high discharge powers. X-ray energy dispersive spectroscopy data reveal that the Sc content in the synthesized ScAlN thin films is proportional to the Sc content in Sc0.40Al0.60 alloy target. Finally, surface acoustic wave resonators have been manufactured to demonstrate the feasibility of this material for micro electro mechanical systems applications based on ScAlN/Si bilayer systems.

Smelting of Scandium by Microwave Irradiation

Scandium is being explored as an alloying element for aluminum alloys, which are gaining importance as high-performance lightweight structural alloys in the transportation industry. A few years ago, Sc was also found to be suitable for use in electrical devices. High-Sc-content ScAlN thin films have attracted significant attention because of their strong piezoelectricity. The piezoelectric response of ScAlN suggests that ScAlN thin films formed on a hard substrate would be suitable surface acoustic wave wideband filters for next-generation wireless communication systems. However, it is often difficult to use ScAlN thin films in MEMS devices—including acoustic ones—because of the extremely high price of metallic Sc, given the difficulty associated with smelting it. Here, we propose a novel process for smelting Sc metal by microwave irradiation. Sc metal was able to be obtained successfully from ScF3 through a microwave-irradiation-based carbon reduction reaction. The reaction temperature for this reduction process was approximately 880°C, which is half of that for the conventional smelting process involving reduction with Ca. Thus, the proposed microwave irradiation process has significant potential for use in the smelting of Sc metal.

ScAlN MEMS Cantilevers for Vibrational Energy Harvesting Purposes

Piezoelectric energy harvesting offers the possibility to make use of ambient vibrations most beneficially to feed low power sensor nodes. This paper demonstrates the fabrication and characterization of piezoelectric cantilevers with AlN and Sc x Al 1-x N (x = 27%) thin films for the evaluations of their energy harvesting performance. The characterization mainly focuses on the measurement of the output power at variable load resistance to achieve maximum power output. Superior properties of ScAlN thin films for energy harvesting compared with AlN films are confirmed. Furthermore, an analytical model is employed to extract material parameters for ScAlN. High piezoelectric coefficients e 31 = 1.6 C/m 2 and d 31 = 6.9 pm/V are determined for ScAlN. Finally, a proof mass is attached to further increase the output power at optimized load resistance.

Preparation of ScAlN film on Hastelloy alloys under different sputtering power

Piezoelectric film based on flexible substrate, which is conductive and heat-proof, is very promising for cantilever beams to constitute vibration energy collectors. Sc-doped AlN thin films based on Hastelloy alloys flexible substrate were prepared by DC reactive magnetron sputtering under sputtering power from 110W to 200W. The crystal quality of ScAlN films was investigated. Results show that the sputtering power greatly influences the preparation of c-axis-oriented ScAlN thin films. The crystal quality first increases and then decreases with increase of sputtering power, reaching the best crystalline state at 170W, presenting full width at half maximum of 2.3°.

Sputter deposition of ScAlN using large size alloy target with high Sc content and reduction of Sc content in deposited films

This paper describes deposition of ScAlN thin films by the conventional radio frequency (RF) magnetron sputtering using large size Sc–Al alloy targets with high Sc content. Two 4-in. Sc–Al alloy targets with the Sc content of 43 and 32% were prepared by the sintering method instead of the conventional dissolution method, and deposited film qualities and uniformity were evaluated. In both cases, uniform ScAlN thin films were obtained throughout the three inch wafer. However, measured Sc content was significantly lower than that of the target. Influence of the N2 content in the sputtering gas was also investigated, and the result indicated that nitridation of the target surface is at least one of the major reasons causing the reduction of the Sc content in the deposited films. We also reports variation in film qualities observed with the accumulated sputtering time.

Effects of sputtering atmosphere on the properties of c-plane ScAlN thin films prepared on sapphire substrate

In this work, scandium aluminum nitride alloy (ScAlN) thin films were prepared on c-sapphire substrates by DC reactive magnetron sputtering with a scandium aluminum alloy (Sc0.06Al0.94) target. The crystal orientation and surface morphology were detected by XRD and AFM, respectively. The electrical properties were analyzed by a standard ferroelectric test system and piezoelectric response force microscopy. The results show that the sputtering atmosphere, including N2/Ar flow ratio and sputtering pressure, appears to be important to influence the crystal quality and electrical properties of ScAlN films. With the N2/Ar flow ratio increasing from 3.1:7 to 3.6:7, the crystal orientation and surface morphology of ScAlN films firstly improves and then gets worse. Meanwhile, the electrical qualities of the films performs a similar variation. When the sputtering pressure increases from 0.3 to 0.7 Pa, the properties of the films change obviously too, and the best sputtering pressure is determined as 0.5 Pa. Finally, highly c-axis ScAlN films can be obtained with a N2/Ar flow ratio of 3.4:7 and a sputtering pressure of 0.5 Pa, and the FWHM value of the rocking curve, the RMS roughness, the resistivity, the dielectric constant ɛr and piezoelectric constant d33 are 2.6°, 2.650 nm, 2.9 × 1012 Ω/cm, 12.2 and 8.1 pC/N, respectively.

Influence of sputtering power on crystal quality and electrical properties of Sc-doped AlN film prepared by DC magnetron sputtering

Scandium-doped aluminum nitride alloy (ScAlN) thin films were deposited on (100) silicon substrates by DC reactive magnetron sputtering method using a scandium aluminum alloy (Sc0.06Al0.94) target. The influence of sputtering power on the crystalline structure, surface morphology and electrical properties of ScAlN thin films were investigated. The XRD patterns indicated all the films showed a single pronounced hexagonal (002) peak. According to the peak intensities in θ/2θ scans and rocking curve FWHM measurements of the (002) peaks, the crystalline quality of ScAlN thin film first increased and then decreased, reaching the best crystalline state at a sputtering power of 130W. The best surface morphology of ScAlN thin film was obtained at 130W and the surface roughness reached a minimum of 2.612nm. Then the piezoelectric response of ScAlN thin films was measured and the highest value, 8.9pC/N, was achieved at the sample with the best crystal quality. The resistivity and dielectric constant change in the same rule as the crystal quality, first increasing to a maximum value of 3.35×1012Ωcm and 13.6, and then decreasing with the sputtering power increasing. In addition, when the sputtering power was 130W, the highest breakdown field strength and lowest leakage current were obtained, with values 1.12MV/cm and 3×10−8A, respectively.

Influence of N2/Ar-flow ratio on crystal quality and electrical properties of ScAlN thin film prepared by DC reactive magnetron sputtering

Scandium aluminum nitride alloy (ScAlN) thin films were prepared using DC reactive magnetron sputtering with a scandium aluminum alloy (Sc0.06Al0.94) target on n-type silicon substrates. We have investigated the influence of N2/Ar-flow ratio on the crystalline structure, the surface morphology and the electrical properties of ScAlN thin films. Consequently, it was statistically proved that the N2/Ar-flow ratio was an important control factor in the process of sputtering. According to the peak intensities in θ/2θ scans and rocking curve FWHM measurements of the (002) peaks, the crystalline quality of ScAlN thin film first increased and then decreased, reaching the best crystalline state at a N2/Ar-flow ratio of 3.3:7. The best surface morphology of ScAlN thin film was obtained at N2/Ar-flow ratios of 3.2:7, 3.3:7 and 3.4:7 and the surface roughness reached a minimum of 2.612nm at 3.3:7. The resistivity and dielectric constant first increased to maximum values of 3.35×1012Ωcm and 13.6, and then decreased with the ratio increasing. Moreover, ScAlN thin film exhibited a higher value of resistivity and dielectric constant when compared with un-doped AlN thin film. In addition, when the N2/Ar-flow ratio was 3.3:7, the highest breakdown field strength and lowest leakage current were obtained, with values 1.12MV/cm and 3×10−8A, respectively.