Growth of Al-Cu Thin Films on LiNbO₃ Substrates for Surface Acoustic Wave Devices Based on Combinatorial Radio Frequency Magnetron Sputtering

Abstract

Al-Cu thin films were fabricated by RF magnetron sputtering from aluminum (Al) and copper (Cu) metal targets to improve the acoustic performance of SAW devices on LiNbO₃ substrates. To optimize the electrode material for SAW devices, Al-Cu films with various compositions were fabricated and their electrical, mechanical, and acoustic properties were comprehensively evaluated. The Al-Cu films exhibited a gradual decrease in resistivity with increasing Al content. The double-electrode SAW devices composed of Al-Cu films demonstrated a resonant frequency of 70 MHz and an average insertion loss of −16.1 dB, which was significantly lower than that of devices made with traditional Au or Al electrodes. Additionally, the SAW devices showed an increase in the FWHM values of the resonant frequency and a decrease in the insertion loss as the Al content in the IDT electrode decreased. These findings indicate that improving the performance of SAW devices can be achieved by reducing the density of the IDT electrodes, rather than focusing solely on their electrical characteristics.

1. Introduction

Surface acoustic waves (SAWs) are elastic waves that travel along the surface of a piezoelectric material and were first discovered by Rayleigh in 1885 [1]. The propagation phenomenon of SAWs was clarified by White and Voltmer, and the interdigital transducer (IDT) was first made in 1965 [2]. The IDT consists of two interlocking comb-shaped metal electrodes deposited on a piezoelectric material. When a sinusoidal wave is applied to the SAW device, the piezoelectric material beneath the IDT vibrates due to the inverse piezoelectric effect and, at this time, the electrical energy is converted into acoustic energy, generating an acoustic wave perpendicular to the IDT [3,4,5,6]. SAW technology has been used in a wide range of applications, including wireless radio transmission [7,8] (filters, spectral duplexers, radio frequency identification (RFID) tags), mesoscopic systems [9,10], and a variety of sensors such as those for chemical vapor [11], humidity [12], biology [13,14], gas [15], pressure [16], and contaminants [17].

The characteristics of the IDT material, a core component of SAW sensors, determine the performance of the SAW sensor. Traditionally, noble metals have been used as IDT materials [18], but their high density limits the generation of the SAWs from the piezoelectric substrate, resulting in decreased sensor sensitivity and frequency response. Additionally, while aluminum [19] is suitable for generating high-frequency SAWs, its low electrical conductivity leads to signal losses. These limitations of IDT materials have been a major obstacle to improving the performance of SAW sensors. As a result, researchers have been working to develop new IDT materials using various materials and fabrication techniques such as thin films, microstructures, and composites. The development of new IDT materials is expected to enhance the performance of SAW sensors and expand their applications.

Of the many physical vapor deposition (PVD) methods, including sputtering [20], electron beam (e-beam) evaporation [21], ion beam deposition [22], and sputter ion plating [23], electron beam evaporation and sputtering have been the most commonly employed techniques for producing thin films. The e-beam evaporation exhibits several limitations when applied to the deposition of multi-component thin films. Firstly, precise composition control can be challenging. Simultaneously evaporating multiple elements requires meticulous regulation of each element’s evaporation rate to achieve the desired compositional profile. This is particularly difficult when dealing with materials with significantly different melting points, as compositional gradients can arise, compromising film uniformity. Secondly, contamination risks are elevated. Residual gasses can still be present in high-vacuum environments, and sequential evaporation of multiple materials may introduce cross-contamination, degrading film purity. Thirdly, complex equipment and substantial costs are involved. Depositing multiple materials simultaneously necessitates intricate multi-source evaporation systems, and precise control over temperature and current for each source demands expensive equipment and maintenance.

RF sputtering offers excellent film uniformity by delivering uniform energy across the substrate through plasma generation. Its versatility enables the deposition of complex multi-component thin films by utilizing a wide range of target materials [24,25,26,27,28,29,30]. Moreover, the high-energy particles generated during sputtering enhance the adhesion between the film and substrate. The precise control over the microstructure, achievable through RF sputtering, makes it a valuable tool for fabricating various functional thin films.

The pursuit of enhanced SAW device performance has been hindered by the trade-offs inherent in traditional single-element electrodes. To address this issue and enhance SAW device performance, this study proposes the use of aluminum–copper (Al-Cu) composite thin films deposited by radio frequency (RF) magnetron sputtering as a novel electrode material. By combining the high electrical conductivity of Cu with the lightweight properties of Al, we expect to achieve both high electrical conductivity and low film density.

Lithium niobate (LiNbO₃) is a key component in integrated and guided-wave optics. This material has a trigonal crystal structure and exhibits high electro-optic, pyroelectric, photo-elastic, and piezoelectric properties. LiNbO₃ is inherently birefringent. It has good acoustic wave characteristics and a moderately high acousto-optic figure-of-merit. Because of its abundance of large-magnitude physical effects, LiNbO₃ is widely used in a variety of applications, including acoustic delay lines, acoustic wave transducers, optical amplitude modulators, acoustic filters, second-harmonic generators, optical phase modulators, phase conjugators, beam deflectors, Q-switches, holographic data processing devices, dielectric waveguides, memory elements, and more. In this study, Al-Cu composite thin films with various compositions will be fabricated using combinatorial RF sputtering on LiNbO₃ substrates and applied to SAW devices to comprehensively evaluate their electrical, mechanical, and acoustic properties.

2. Materials and Methods

The Al-Cu thin films with different composition, thickness, and resistivity on a 4-inch silicon substrate were grown by RF sputtering, and then the various physical properties of materials suitable for SAW-IDT electrodes were systematically investigated. Before the deposition, the Si substrate was primarily cleaned using a soap bath process to remove organic contaminants. The substrates were immersed in a cleaning solution at 60 °C for 15 min, followed by an ultrasonic cleaning process to enhance the cleaning efficiency. After the soap bath, the substrates were rinsed sequentially with IPA and DI water to remove any residual contaminants. Finally, the substrates were dried with a nitrogen blow dryer.

To obtain the Al-Cu thin films, single metal Al and Cu targets were used, and the Al-Cu thin film was grown in a fixed state without rotation of the substrate to reach the compositional gradient. The sputtering conditions for the compositionally graded Al-Cu thin films were as follows: RF power of 100 W (Al and Cu); base pressure of 3 × 10⁻⁶ Pa; working pressure of 0.21 Pa; gas flow of Ar (99.99%); and the substrates were not heated prior to deposition. The Al-Cu thin films were cut into 6 pieces, and the structural/chemical/electrical properties of each sample were evaluated. Al-Cu thin films for SAW devices were fabricated by rotational growth during sputtering, and three kinds of AC thin films with uniform compositions were classified into Al-rich thin films, thin films with similar Al/Cu composition ratios, and Al-poor thin films. To grow these thin films, the base pressure of 3 × 10⁻⁶ Pa, the working pressure of 0.21 Pa, the gas flow of Ar (30 sccm), and the growth temperature of R.T were fixed, and the RF powers applied to the Al and Cu targets were varied. For the growth of Al-rich thin films, the RF powers applied to the Al and Cu targets were 300 and 75 W, respectively. For the Al-Cu thin films with an Al/Cu composition ratio of about 1, the RF powers for the Al and Cu targets were 200 W. In cases of the Al-Cu thin films with Al-poor characteristics, RF powers of 75 and 300 W were applied to the Al and Cu targets, respectively.

The SAW-IDT pattern with a straight configuration was designed to have a resonant frequency of 70 MHz, as shown in Figure 1. This pattern consisted of an Al-Cu thin film thickness of 200 nm, a pattern width of 14 μm, and 90 electrode pairs. The IDT electrode was patterned onto a 4-inch-diameter LiNbO₃ (LN) substrate with a crystal orientation rotated 128 degrees from the +y axis through the +z axis about the x axis. Before depositing the IDT electrodes, the LN wafer was cleaned using Piranha solution, and then IDT patterns were formed using photolithography with a positive photoresist (AZ GXR 601, Merck, Kenilworth, NJ, USA), as shown in Figure 2.

The compositional distribution and morphological properties of the Al-Cu thin films were examined using field emission scanning microscopy (FE-SEM, Quanta 200, OR, Hillsboro, USA). Electrical resistivity was determined using the Hall Effect Measurement System with van der Pauw geometry (Model 7707, Lake Shore Cryotronics, Westerville, OH, USA) at a constant magnetic field of 4 kG. The IDT patterns were observed using a 3D laser optical microscope (Model OLS4100-SAA, OLYMPUS, Center Valley, PA, USA). The resonant frequency of the SAW device was analyzed using a Vector Network Analyzer (Model E5080B, Keysight, Santa Rosa, CA, USA).

3. Results and Discussion

3.1. Exploration of SAW-IDT Electrode Materials Based on Combinatorial RF Sputtering

Figure 3 schematically illustrates the combinatorial sputtering system designed to efficiently vary the composition of Al-Cu thin films. By simultaneously employing Al and Cu metal targets to generate the plasma on the substrate, and subsequently adjusting the RF power applied to each target while keeping the substrate stationary, a compositional gradient is formed on the substrate. The shadow mask is utilized to confine the deposition area and ensure uniform film thickness. The compositional analysis reveals a linear variation in the Al and Cu content across the sample, depending on the sample position. This combinatorial approach enables the rapid fabrication of thin films with various compositions in a single process, facilitating the efficient investigation of the correlation between properties and composition, ultimately aiming to identify the optimal thin film for SAW-IDT electrodes.

Figure 4 shows the cross-section and surface images of the Al-Cu (AC) thin films grown on the Si substrate with the compositional gradient. The AC thin films show excellent adhesion properties without any cracks or voids at the interface between the substrate and thin film. Additionally, it can be observed that the AC thin film has a uniform film thickness of 120 nm regardless of the sample position, indicating that there is no difference in the deposition rates of the Al and Cu thin films. The AC1 film is predicted to have a dominant Al content because it is close to the Al target, whereas the AC6 film is expected to have a high Cu content because it is adjacent to the Cu target. In the thin films between AC1 and AC6, the Al and Cu contents alternate in proportion to the distance between the substrate and the target, so that Al-Cu thin films with different composition ratios can be grown in a one-pot sputtering process.

Figure 5 shows changes in the chemical composition and film thickness of Al and Cu depending on the sample position of the AC thin films. As expected, the composition gradient of the AC thin film changed consistently depending on the sample position. As shown in Figure 5, as the sample number increased, the Al content of the AC thin films gradually decreased from 64.9 to 38.4 at%, and the Cu content was correspondingly distributed from 35.1 to 61.6 at%. In other words, the Al and Cu contents in the films change linearly depending on the distance between the target and substrate, which means that the AC thin films with different Al-Cu composition ratios can be formed with the one-pot deposition process.

Figure 6 shows the electrical resistivity and calculated density of AC thin films with different Al-Cu composition ratios. Since the electrical resistivity of the IDT electrode material has a significant impact on SAW-IDT performance by lowering the resistance of the alternating current signal applied to generate the surface acoustic wave, changes in electrical properties according to the composition ratio of the AC thin films should be closely investigated. In addition, since the density of the material also has a significant effect on the SAW-IDT performance, the density change according to the composition of the AC thin film is shown in Figure 3. The density of mixed films can be calculated from the following equation:

$${\rho }_{Al\text{-}Cu}=({\rho }_{Al}\times V_{Al}+{\rho }_{Cu}\times V_{Cu})/100$$

(1)

where ρ is the theoretical density and V is the volume percent. As the sample number of the AC thin film increased (as the Al content decreased), the resistivity gradually decreased, which means that the electrical properties of the AC thin film improved as the content of Cu, which has excellent electrical properties, increased. However, it was confirmed that the density of the AC thin film gradually increased as the sample number increased. The performance of the SAW-IDT device is determined not only by the excellent conductivity of the electrode material, but also by the condition that the acoustic wave generated on the surface of the piezoelectric substrate is not disturbed by the high electrode density.

3.2. Optimization of SAW-IDT Electrode Materials

So far, the AC thin films with composition gradients have been closely analyzed for their chemical/electrical behavior, such as composition distribution and electrical resistivity. In particular, we mainly focused on whether it was possible to form metal composite thin films that could minimize electrical resistance while maintaining low IDT thin film density. As is well known, the theoretical densities of Al and Cu are 2.7 and 8.94 g/cm³, and the electrical resistivities are 2.82 × 10⁻⁶ ohm-cm and 1.724 × 10⁻⁶ ohm-cm, respectively. In this way, the composite thin films were fabricated using materials with different physical/electrical properties through the PVD process, and it was demonstrated that the films suitable for the SAW-IDT device can be fabricated. Gold, which is generally used as the SAW-IDT electrode material, shows excellent electrical properties, but its high density hinders the generation of sound waves from the piezoelectric substrate. Therefore, we investigated the chemical/electrical properties of the Al-Cu thin films with different Al composition ratios and confirmed the acoustic properties of SAW-IDT devices fabricated with these Al-Cu thin films.

Figure 7 shows SEM images of the Al-Cu thin films deposited on Si wafers with different Al contents. The AC series samples (a to c) were grown with constant film thickness but different composition ratios of each sample, and the electrical/acoustic behaviors of the AC films according to the composition ratios were systematically investigated. As shown in Figure 7, all the AC series samples showed a constant film thickness of 200 nm, and no defects such as voids or cracks were observed at the substrate interface. In addition, the AC series films showed a considerably smooth surface morphology without pinholes or cracks. In this way, it was confirmed that the AC thin films were uniformly deposited over the entire 4-inch wafer, showing that all the AC thin films do not have any interface defects or thickness deviations.

Table 1. The resistivity and calculated density of three different types of AC thin films with varying Al content.

Sample	AC-a	AC-b	AC-c	Remark
Al content (at%)	71.6	48.3	19.8	Measured
Resistivity (ohm-cm)	9.1 × 10−5	4.6 × 10−5	1.2 × 10−5	Measured
Density (g/cm3)	4.1	5.4	7.3	Calculated

shows the resistivity and calculated density of the AC series samples with varying Al content. As the Al content of the AC films decreased from 71.6 to 19.8 at%, the electrical resistivity decreased from 9.1 × 10⁻⁵ to 1.2 × 10⁻⁵ ohm-cm, and the calculated density of the films increased from 4.1 to 7.2 g/cm³. The composition and resistivity of the three AC series samples showed almost constant values with no deviation, proving that the chemical/electrical properties of the AC films were uniform over the entire wafer.

3.3. Fabrication of SAW Devices and Acoustic Properties

Figure 8 shows the SAW-IDT patterns fabricated by Al-Cu thin films with different Al contents. Three kinds of SAW-IDT devices with the identical pattern width of 14 μm and film thickness of 200 nm were fabricated using Al-Cu thin films with different Al contents. These devices fabricated by RF sputtering were stably grown on the piezoelectric substrate without any cracking or interference of the electrode surface. As shown in Table 1, the AC films have different electrical properties and densities depending on the Al content, and the acoustic behaviors of these properties on the SAW-IDT performance were closely investigated. Figure 9 shows the resonant frequency of the double-electrode SAW-IDT devices with the IDT pattern width of 14 µm and 90 IDT electrode pairs. The resonant frequency of the three SAW-IDT devices was observed around 70 MHz, suggesting that there were no defects due to inaccuracy in the electrode pattern width in the SAW-IDT devices. The SAW-IDT devices composed of Al-Cu thin films with Al contents of 71.6, 48.3, and 19.8 at% showed excellent insertion losses of −18.9, −13.8, and −15.7 dB, respectively.

However, as the Al content in the IDT electrode decreased, the resonant frequency showed slightly asymmetric behavior and the FWHM values gradually increased. As is well known, the inverse piezoelectric effect is defined as the vibration frequency at which crystals of a piezoelectric material expand or contract when electrical energy is applied to the SAW device [31,32]. The increase in the density of IDT electrodes formed on a piezoelectric substrate can restrict the movement of crystals of the piezoelectric material, which results in a decrease in the resonant frequency insertion loss. Additionally, in order to compare the device performance depending on the materials of the IDT electrode, the SAW device with the Au electrode was fabricated, and the acoustic properties such as resonant frequency, FWHM value, and insertion loss are depicted in Figure 10. At the resonant frequency of 70 MHz, the insertion loss of the SAW device fabricated with the Au electrode was drastically reduced compared to that of the Al-Cu electrode. In particular, weak shoulder peaks (P1, P3) were observed near the main peak (P2), which suggests that the high density of the Au-IDT electrode caused an unstable inverse piezoelectric effect. This result demonstrates that the acoustic performance of SAW devices composed of Al-Cu thin films can be dramatically improved compared to gold, which has very high electrical conductivity but high density.

4. Conclusions

In conclusion, this study demonstrates the potential of Al-Cu thin films as a promising electrode material for SAW devices. The Al-Cu electrodes fabricated using combinatorial RF sputtering exhibited excellent performance in terms of resonant frequency, insertion loss, and selectivity. Compared to traditional Au electrodes, the Al-Cu electrodes showed a significant improvement in insertion loss, achieving an average of −16.1 dB. Although the electrical characteristics of the Al-Cu films varied slightly with Al content, the overall performance of the SAW devices remained consistent. These findings highlight the importance of developing new materials that can simultaneously satisfy the electrical and mechanical requirements of IDT electrodes to further enhance SAW device performance. Future research should explore alternative alloy compositions and fabrication techniques to optimize the properties of Al-Cu thin films and expand their use in various SAW device applications.