# Wireless Surface Acoustic Wave Sensors at 2.45 GHz Based on AlN/Sapphire and Operable up to 600 °C

Ulrich Youbi, Sami Hage-Ali, Florian Bartoli, Jaafar Ghanbaja, Eric Frayssinet, Benjamin Damilano, Pierre-Marie Coulon, Demba Ba, Hamid M'jahed, Michel Vilasi, et al.

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## To cite this version:

Ulrich Youbi, Sami Hage-Ali, Florian Bartoli, Jaafar Ghanbaja, Eric Frayssinet, et al. Wireless Surface Acoustic Wave Sensors at 2.45 GHz Based on AlN/Sapphire and Operable up to 600 °C. Sensors and Actuators A: Physical, 2026, 401, pp.117569. 10.1016/j.sna.2026.117569. hal-05537947

HAL Id: hal-05537947

https://hal.univ-lorraine.fr/hal-05537947v1

Submitted on 5 Mar 2026

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# Wireless Surface Acoustic Wave Sensors at 2.45 GHz Based on AlN/Sapphire and Operable up to 600 °C

Ulrich Youbi<sup>a</sup>, Sami Hage-Ali<sup>a</sup>, Florian Bartoli<sup>b</sup>, Jaafar Ghanbaja<sup>a</sup>, Eric Frayssinet<sup>b</sup>, Benjamin Damilano<sup>b</sup>, Pierre-Marie Coulon<sup>b</sup>, Demba Ba<sup>a</sup>, Hamid M'Jahed<sup>a</sup>, Michel Vilasi<sup>a</sup>, Thierry Aubert<sup>a</sup>, Maxime Hugues<sup>b</sup>, Omar Elmazria<sup>a</sup>.

**Abstract**— This study reports significant progress in the field of high temperature wireless surface acoustic wave (SAW) sensors in the 2.45 GHz ISM band, using AlN/Sapphire based resonators. AlN thin films with a thickness of 0.6 µm, synthesized on (0001) sapphire substrates by Metal Organic Chemical Vapor Deposition (MOCVD), demonstrated high crystalline quality and nanometric surface roughness, essential properties for SAW applications. Initially, SAW devices operating in the 2.45 GHz ISM band were tested up to 500°C in both wired and wireless configurations using aluminium electrodes. Wireless interrogation was carried out successfully at a distance of 3 meters up to 500°C, with a temperature coefficient of frequency (TCF) measured at -58 ppm/°C. Critically, when the NiAl alloy was used for the thin film electrodes, the operating temperature was extended to 600°C while maintaining a TCF of -43 ppm/°C and stable wireless performance at 3 meters, with potential up to 10 m. These results exceed the state-of-the-art for high temperature SAW sensors in the wide 2.45 GHz ISM band. A long-duration annealing test has confirmed the self-passivation properties of the NiAl electrodes and the robustness of the NiAl/AlN/Sapphire structure, with a minimal degradation after 176 hours at 500°C, thus demonstrating its potential for long term use in extreme environments.

**Keywords:** Sensors, Wireless, Batteryless, Surface acoustic waves, AlN, Sapphire, High temperature

Corresponding author: Omar Elmazria; email: omar.elmazria@univ-lorraine.fr

## 1. INTRODUCTION

Surface acoustic wave (SAW) sensors are increasingly recognized as a promising technology for industrial monitoring of physical parameters [1], particularly in harsh environments including high temperature processes, energy systems and aerospace applications [2]. Their resonance frequency is highly sensitive to changes in environmental conditions, enabling accurate detection and tracking of various physical quantities such as temperature [3], pressure [4], and magnetic field [5]. These sensors offer several key advantages: they are compact, passive (batteryless). They can be interrogated remotely (wireless), making them suitable for environments where wired connections are difficult or impossible to implement, or where maintenance access is limited [6]. Several companies, including Transense, Sensideon, SenSanna and SAW Components, already market SAW-based solutions for harsh industrial environments, with applications ranging from torque and temperature monitoring in aerospace and electric drives to high-temperature sensing in smart grids and metallurgy [7–10]. In the steelmaking industry in particular, precise high temperature sensing is key: in furnaces/ovens an incorrect evaluation of the temperature can lead to huge energy losses and process discrepancies. A proper multipoint temperature monitoring of complex metallic parts during the cooling phase can help avoid tolerance issues due to twisting. In many cases measurement areas are restricted for workers due to safety concerns and wireless SAW sensors can help provide a safe remote query while still offering a reliable, contact-based measurement.

One of the main challenges in developing SAW sensors for high-temperature industrial applications lies in the choice of piezoelectric materials and sensor electrodes (IDTs), which must withstand the high temperature conditions [11]. Very few piezoelectric substrates can operate at high temperatures (> 400°C). Commonly used materials, such as quartz and LiNbO₃, are subject to various physical phenomena that limit their performance above 400°C. For example, at this temperature quartz experiences structural disorder which reduces the quality factor of the devices [12]. LiNbO₃ undergoes a decrease in electrical resistivity with increasing temperature (reaching 10⁵ Ω.cm at 600°C); this reduction leads to electrical losses that decrease the sensor performance [11,13]. Alternatively, langasite (LGS) is currently the most suitable piezoelectric crystal for high temperatures, as it remains stable up to its melting temperature (1470°C). Studies have shown that a SAW signal remains above 1000°C [14] and, in a wireless configuration, SAW sensors based on LGS can operate up to 600°C [15]. However, its propagation losses at high temperatures increase drastically over 1 GHz. Consequently, its use at high temperatures is mostly restricted to the 433 MHz ISM band [16,17]. This band is less advantageous than the 2.45 GHz ISM band since the relative bandwidth is much narrower. So, it limits the overall temperature excursion and the potential for multi-sensors operation. In this context, aluminium nitride (AlN) thin films deposited on sapphire substrates have been identified as a promising alternative to LGS [18–22]. AlN

<footnote><sup>a</sup>Université de Lorraine, CNRS, IJL UMR 7198, F-54000 Nancy, France</footnote>

<footnote><sup>b</sup>Université Côte d'Azur, CNRS, CRHEA UMR 7073, F-06560 Valbonne, France</footnote>
films do not oxidize in air up to 600°C and their oxidation remains very limited at 700°C thanks to a native passivation layer of Al₂O₃ [18]. In addition, its SAW velocity is the highest among piezoelectric materials (5700 m/s), and its low propagation losses make AlN particularly suitable for high-frequency wireless SAW sensors in the 2.45 GHz ISM band and even beyond [23]. In our previous work, we demonstrated the potential of the Al/AlN/Sapphire bilayer structure for wireless SAW sensors, demonstrating that these sensors could be effectively interrogated at a distance of 1 meter for temperatures up to 500°C [24]. **Table 1** summarizes the state of the art of wireless SAW sensors developed for high-temperature operation. Several attempts have been made using LGS substrates with different metallizations (Pt, Ti, Rh, ZrO₂), which enabled operation up to 650–700°C. However, the wireless range reported in these cases remained very limited, typically below 0.5 m, and most of the devices operate at relatively low frequencies (200–433 MHz).

The present study has two main objectives: first, to enhance the interrogation distance range at elevated temperatures by optimizing the thickness of the AlN layer, and second, to expand the operating temperature of the wireless SAW sensors. To do so, we investigated the combination of interdigital transducers (IDTs) made with NiAl alloy with the AlN/Sapphire piezoelectric structure for operation up to 600°C in the 2.45 GHz ISM band. The NiAl alloy is expected to offer a good compromise between acoustic performance and high-temperature stability [25].

First, we present the AlN film synthesis on sapphire and the microfabrication of resonators with either Al and NiAl electrodes. Then, the electrical characterizations of SAW sensors in wired and wireless configurations from room temperature up to 600°C are thoroughly discussed.

<table><caption>Table 1. State of the art Wireless SAW sensor for high temperatures</caption><thead><tr><th>Type</th><th>K² at RT</th><th>Q at RT</th><th>FOM at RT</th><th>Wireless range</th><th>Frequency</th><th>Max. Temperature</th><th>Ref.</th></tr></thead><tbody><tr><td>Pt/Rh/ZrO₂/LGS</td><td>-</td><td>-</td><td>-</td><td>A few cm</td><td>200 MHz</td><td>650°C</td><td>[17], 2011</td></tr><tr><td>Pt/Ti/LGS</td><td>-</td><td>-</td><td>-</td><td>0.5 m</td><td>340 MHz</td><td>700°C</td><td>[26], 2012</td></tr><tr><td>Pt/Ti/LGS</td><td>-</td><td>-</td><td>-</td><td>-</td><td>433 MHz</td><td>700°C</td><td>[27], 2012</td></tr><tr><td>Cu-doped Al/Quartz</td><td>0.2%</td><td>7450</td><td>1.49</td><td>0.5 m</td><td>433 MHz</td><td>480°C</td><td>[28], 2015</td></tr><tr><td>SiO₂/Pt/41°Y-LiNbO₃</td><td>-</td><td>-</td><td>-</td><td>A few cm</td><td>400MHz</td><td>400°C</td><td>[29], 2020</td></tr><tr><td>Ti/Al/LiNbO₃ (Delay line)</td><td>-</td><td>-</td><td>-</td><td>0.5 m</td><td>2.45 GHz</td><td>450°C</td><td>[30], 2006</td></tr><tr><td>Pt-Al₂O₃/Zr/LGS</td><td>-</td><td>-</td><td>-</td><td>5 mm</td><td>2.45 GHz</td><td>600°C</td><td>[31], 2007</td></tr><tr><td>Pt/AlN/Sapphire</td><td>1.85%</td><td>130</td><td>2.4</td><td>-</td><td>2.45 GHz</td><td>500°C</td><td>[32], 2013, our group</td></tr><tr><td>Al/AlN(1µm)/Sapphire</td><td>0.048%</td><td>3500</td><td>1.68</td><td>1 m</td><td>2.45 GHz</td><td>500°C</td><td>[24], 2024, our group</td></tr><tr><td>Commercial SAW</td><td>-</td><td>13500</td><td>-</td><td>-</td><td>433 MHz</td><td>350°C</td><td>[10]</td></tr><tr><td>Commercial SAW</td><td>-</td><td>6800</td><td>-</td><td>-</td><td>2.45 GHz</td><td>275°C</td><td>[10]</td></tr><tr><td>Al/AlN(600nm)/Sapphire</td><td>0.048%</td><td>6000</td><td>2.88</td><td>3 m</td><td>2.45 GHz</td><td>500°C</td><td>This work</td></tr><tr><td>NiAl/AlN(600nm)/Sapphire</td><td>0.048%</td><td>-</td><td>-</td><td>3 m</td><td>2.45 GHz</td><td>600°C</td><td>This work</td></tr></tbody></table>

## 2. EXPERIMENTAL METHODS

### 2.1. Substrate preparation

Thin AlN films with a thickness of 0.6 µm were grown by metal-organic chemical vapor deposition (MOCVD) on (0006) sapphire substrates. The growth parameters summarized in **Table 2** were optimized to obtain high-quality AlN films for SAW applications, characterized by highly (002)-orientated grains and surface roughness in the nanometer range. The microstructure and surface morphology of the films were analyzed using X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively.

<caption>Table 2. Growth parameters of AlN thin films</caption>
<table><thead><tr><th>Precursor for Al</th><th>Precursor for N</th><th>Setpoint temperature</th><th>Temperature at substrate surface</th><th>Growth rate [nm/min]</th></tr></thead><tbody><tr><td>TMA(Al(CH<sub>3</sub>)<sub>3</sub>), 78 sccm</td><td>NH<sub>3</sub>, 50 sccm</td><td>1350°C</td><td>1125°C</td><td>13</td></tr></tbody></table>

XRD measurements of the 0.6 µm-thick AlN films show three main peaks at 2θ positions of 36.3°, 42°, and 76.7°, corresponding to the AlN (002), Al₂O₃ (0006), and AlN (004) planes, respectively (Figure 1). The strong intensity of the AlN (002) peak indicates a preferential orientation of the films along the (002) plane. The rocking curve of the AlN (002) peak exhibits a FWHM of 0.42°, indicating a low dispersion in grain orientation (inset Figure 1).

<figure><img src="image_3.png" /><figcaption>Figure 1. XRD diagram of AlN(002) layers.</figcaption></figure>

AFM measurements were performed at different locations on the sample. A typical 10x10 µm² AFM image is shown in Figure 2. The root mean square (RMS) roughness across the wafer is ranging from 0.3 to 1.2nm. Note that the central region has a higher roughness (2nm) than the peripheral regions. This variation could be attributed to thermal gradients during MOCVD growth. However, the roughness values still meet the requirements of SAW devices.

<figure><img src="image_4.png" /><figcaption>Figure 2. 2D topography of AlN layers</figcaption></figure>

## 2.2. Design and fabrication

Based on a down-scale of the optimized design of our previous work [22], SAW resonators operating in the 2.45 GHz ISM band were fabricated on the AlN (0.6 µm)/sapphire structure. The SAW propagation was chosen along the [1100] direction of the sapphire substrate. Each resonator consists of an IDT with 100 pairs of fingers and 300 reflectors on each side. The spatial period (λ) of the IDT is 2.3 µm, while the one of the reflectors is slightly smaller at 2.29 µm. The metallization ratio is 70% for the reflectors and 60% for the IDT. The Al and NiAl metallizations are 80 nm thick. This thickness is selected considering a tradeoff between ohmic losses and mechanical damping. The device aperture and the spacing between the IDT and reflector are set at 41.5 λ<sub>IDT</sub> and 0.21 λ<sub>IDT</sub>, respectively. Using a slightly asynchronous resonator configuration maximizes the reflectors'
reflection coefficient and eliminates certain transverse modes. This leads to reduced losses and, consequently, a higher quality factor, thereby enhancing wireless detection and resolution capabilities [22,33].

Two types of metallization were selected for the fabrication of interdigital transducers (IDTs). First, aluminium was used due to its low density (2.7 g/cm³) and excellent electrical conductivity, making it suitable for SAW applications at high frequency and intermediate temperatures (i.e. below 500°C). The fabrication process began with the spin coating of a bilayer of positive PMMA (Polymethyl Methacrylate) resist on the AlN/Sapphire substrate. This was followed by exposure using electron-beam lithography (Raith 150-II, Raith GmbH, Dortmund, Germany) and Methyl IsoButhyl Ketone (MIBK) developing. An 80 nm-thick aluminium layer was then deposited by electron-beam evaporation. The final structure is obtained by a lift-off process (Figure 3.a). For higher temperature applications, NiAl alloy was selected due to its superior high-temperature resistance compared to aluminium, particularly its high melting point (1640°C) and excellent oxidation resistance [34]. These properties enable SAW measurements at temperatures up to at least 600°C. As electron-beam evaporation was not available for NiAl deposition, the fabrication of NiAl IDTs relied on a combination of magnetron sputtering, electron-beam lithography and etching. A thin NiAl layer (80 nm) was first deposited onto the AlN/Sapphire substrate by sputtering, using a NiAl target with 99.99% purity and a 50/50 stoichiometric ratio. The sample was then coated with a negative resist (ma-N 2405), exposed using electron beam lithography, and followed by MF 319 development. The etching step was carried out chemically using a solution of H₃PO₄, HNO₃, and H₂O. The final structure is obtained after removal of the resist in acetone (Figure 3.b). The different steps of both processes are illustrated in Figure 4 and 5.

<figure><img src="image_5.png" /><figcaption>Figure 3. Scanning electron microscopy image of IDTs: (a) Al IDT (b) NiAl IDT</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 4. Process for the fabrication of SAW devices by lift-off process using a positive PMMA bilayer resist</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 5. Process for the fabrication of SAW devices by etching process using a negative Ma-N 2405 resist.</figcaption></figure>

## 2.3. Characterizations

Impedance measurements were performed under air atmosphere, using a specific experimental setup, including a vector network analyzer (VNA, model Agilent E5061B) and an RF probe station equipped with thermal probes (Signatone S-1060 series). Two types of probes were used. First, GSG probes with 150 µm pitch were used for the initial measurement to assess the room temperature electrical properties of resonators. Then, for high temperature measurements, 1500 µm GS probes (Z Probes from Süss Microtech, Garching, Germany) were used. These RF probes have been adapted with a water-cooling system to withstand temperatures of up to 600°C.

Admittance measurements over the entire temperature range allowed the extraction of quality factors Q and electromechanical coupling coefficients K². The quality factor Q was calculated from the conductance (real part of the resonator admittance Y₁₁) and expressed as the ratio of the peak frequency (f₀) to the full width at half maximum (Δf):

$$Q = \frac{f_0}{\Delta f} \qquad (1)$$
The electromechanical coupling coefficient ($K^2$) is determined using the following expression:

$$K^2 = 2 \frac{f_a - f_r}{f_r} \quad (2)$$

Where $f_a$ and $f_r$ are respectively the resonance and anti-resonance frequencies extracted from the extrema of the modulus of the admittance $|Y_{11}|$ in the frequency domain.

Both Q and $K^2$ are critical factors in qualifying a SAW resonator and assessing its performance in wireless configuration. The figure of merit (FoM) value, defined as the product of Q and $K^2$, is commonly used as metric for evaluating wireless SAW resonators:

$$FoM = Q \cdot K^2 \quad (3)$$

In a second time, wireless measurements were carried out to measure backscattered power as a function of the temperature. These measurements were obtained using a wireless interrogator from SAW Components Company (Dresden, Germany), connected to a patch antenna array with a gain of 19 dB. The SAW sensor is connected to a separate patch antenna with a gain of 20 dB. The wireless interrogation principle is sketched in Figure 6.

<figure><img src="image_8.png" /><figcaption>Figure 6. Experimental setup for wireless measurements of SAW resonators at 3 m</figcaption></figure>

# 3. IMPEDANCE MEASUREMENT AND WIRELESS INTERROGATION

## 3.1. Aluminum electrodes

The reflection coefficient $S_{11}$ measured at room temperature with the probe station is shown in Figure 7. For these initial impedance measurements, short GSG probes with 150 µm pitch were used. Results are compared with those of the previously published Al(80nm)/AlN(1µm)/Sapphire structure using commercially available AlN/Sapphire wafer from MTI Corp [24]. The measured center frequency is 2.482 GHz while the MTI Corp structure exhibits a lower center frequency of 2.463 GHz. The measured phase velocity is then 5716 m/s, then slightly higher than the 5672 m/s of the 1µm-thick AlN from MTI Corp.

This difference is mainly due to the difference in AlN layer thickness. The values obtained in the [1100] direction are in very good agreement with those predicted by Tsubouchi's study [35], for relative thicknesses $kh_{AlN}$ ($kh_{AlN} = 2\pi h_{AlN}/\lambda_{IDT}$) of 1.6 and 2.7 respectively.

<figure><img src="image_9.png" /><figcaption>Figure 7. Comparison of the reflection coefficient $S_{11}$ measured for the achieved resonator on optimized AlN layer Al(80nm)/AlN(0.6µm)/Sapphire and on the commercially available one Al(80nm)/AlN(1µm)/Sapphire, [36] in wired configuration.</figcaption></figure>

Characterizations of these two resonators at room temperature show that the 600 nm-thick AlN structure ($kh_{AlN} = 1.6$) has a quality factor Q around 6000 and an electromechanical coupling coefficient $K^2$ of 0.048% (Figure 8), higher than the Q = 4500
and K² = 0.04% values obtained for the 1µm thick AlN structure (khAlN = 2.7) (Figure 9). These results show that reducing the thickness of the AlN layer improves the resonator selectivity, because the khAlN now fits the design optimization performed in [22].

Then the resonator with 600 nm AlN thickness was investigated at medium-high temperatures. To allow characterizations compatible with our water-cooled 1500 µm GS probes, 240 nm thick Pt GS access pads were added to the resonator, with some additional resistive losses and a small impact on the quality factor. Figure 10 shows the evolution of the S₁₁ parameter of the Al(80nm)/AlN(0.6µm)/Sapphire resonator at temperature ranging from 23°C to 400°C. A clear shift in the resonance frequency is observed, decreasing from 2.481 GHz at 23 °C to 2.428 GHz at 400°C. This indicates a negative temperature coefficient of frequency (TCF).

The TCF of the SAW sensors is defined by equation (4):

$$TCF = \frac{1}{f(T_0)} \cdot \frac{f(T)-f(T_0)}{T-T_0} \quad (4)$$

where f(T) and f(T₀) represent the resonance frequencies at temperatures T and T₀ = 23 °C respectively.

<figure><img src="image_10.png" /><figcaption>Figure 8. Characteristic parameters at room temperature of the Al(80nm)/AlN(600nm)/Sapphire SAW resonator.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 9. Characteristic parameters at room temperature of the Al(80nm)/AlN(1µm)/Sapphire SAW resonator.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Figure 10. Reflection coefficient S₁₁ of the Al(80nm)/AlN(600nm)/Sapphire SAW resonator for various temperatures, in wired configuration.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Figure 11. Evolution of the operating frequency as a function of temperature of the Al(80nm)/AlN(600nm)/Sapphire SAW resonator in wired configuration.</figcaption></figure>

As shown in Figure 11, the variation of frequency with temperature exhibits a good linearity and a TCF value of -58 ppm/°C is obtained for this structure, which is in agreement with the literature [35].

The variations of Q, K², and FoM with temperature are summarized in Figure 12. The quality factor (Q) decreases throughout the investigated temperature range. At room temperature, with the added GS platinum pads, Q now reaches 5000, while at 400°C it remains at a significant 2000 value. At the same time K² increases with temperature, from 0.04% at room temperature up to 0.09% at 400°C. The decrease in Q with the increasing temperature can be attributed a) to the increase of the K² (discussed below) since Q and K² generally follow opposite trends, and b) to increased loss mechanisms in the SAW resonator. Concerning the loss
mechanisms, at high temperatures, intrinsic acoustic attenuation increases due to phonon-phonon interactions and thermoelastic losses. At the same time, the electrical resistivity of the metallic electrodes increases with temperature, leading to higher ohmic losses in the IDTs. Additional effects such as electrode oxidation and surface state evolution may also contribute to increased acoustic energy dissipation, leading to the observed decrease in Q.

In contrast, the observed increase of the electromechanical coupling coefficient K² with temperature can be explained by the different temperature dependencies of the material parameters governing K². In fact, K² depends on the piezoelectricity properties of the structure, and on its stiffness. If the piezoelectric properties improve, or if the structure softens, the K² increases. Both effects occur when the temperature rises. The softening is a well-known effect which partly explains the decrease of Q. Concerning piezoelectricity, this is partly related to the dilatation effect. Furthermore, the FoM value remains very close to 2 over the entire investigated temperature range, demonstrating the potential of the device for wireless interrogation.

<figure><img src="image_14.png" /><figcaption>Figure 12. Evolution of quality factor (Q), electromechanical coupling coefficient (K²), figure of merit (FoM) with the temperature.</figcaption></figure>

Wireless measurements at a distance of 3 m were carried out up to 580°C by measuring the backscattered power using a wireless interrogator, as described previously in Figure 6. This long heating-cooling cycle measurement takes 4 hours. As shown on Figure 13, the sensor can be remotely interrogated up to 580°C with a signal-to-noise ratio (SNR) over 35 dB from RT up to 500°C. At 580°C the signal level drops strongly, most likely due to an irreversible oxidation of the Al electrodes.

The sensor remained over 500°C for around 20 minutes. Despite the oxidation of the Al electrodes, the relative frequency variation Δf/f remains almost unchanged on cooling. The measured TCF of -59 ppm/°C is stable throughout the heating and cooling phase, and in good agreement with the value obtained in the wired configuration (Figure 14).

<figure><img src="image_15.png" /><figcaption>Figure 13. Backscattered power of the Al(80nm)/AlN(60nm)/Sapphire SAW resonator wirelessly interrogated at a distance of 3 m as a function of temperature.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Figure 14. Temperature dependence of the resonance frequency of the Al(80nm)/AlN(600nm)/Sapphire SAW resonator measured between RT and 580°C in wireless configuration.</figcaption></figure>

Furthermore, as mentioned previously, the signal amplitude at 500°C remains well above the noise level corresponding to a large signal to noise ratio (SNR), thus enabling wireless measurements at greater distances than 3m. This distance can be estimated using the Friis equation [37], which describes the link budget between two antennas separated by a distance D in free space:

$$ \left(\frac{P_r}{P_e}\right)_{dB} = G_{r,dB} + G_{e,dB} - 20.\log\left(\frac{4\pi D}{\lambda}\right) \quad (5) $$

where:

- P_e and P_r are respectively the emission and reception power,
- $G_e$ and $G_r$ are the gain of the emission and reception antennas,

- D is the distance between antennas,

- and $\lambda$ the electromagnetic wavelength in free space.

For this calculation, we assume that the minimal signal amplitude for a successful detection needs to be 10 dB above the noise level at 500°C, corresponding to a signal level of around -48 dB (see **Figure 13**, yellow curve).

The difference in the link budget as a function of distance D considering a round-trip transmission is given by: $N=2 \times 20 \log(D/3)$, where N represents the path loss difference between the reference distance of 3 m (with a signal level of -27 dB) and the maximum distance D (with a target level of -48 dB. Equating this difference to the signal level margin, we obtain:

$$N = 2 \times 20 \log \left( \frac{D}{3} \right) = -27 \text{dB} - (-48 \text{dB}) = 21 \text{dB}$$

Solving this equation, at 500°C, the maximum wireless measurement distance would therefore be around D = 10 m with a 10 dB signal to noise ratio margin.

## 3.2. NiAl electrodes

To extend the operating temperature range of the sensors, we replaced the aluminium electrodes with NiAl alloy IDTs. **Figure 15** shows that heating the SAW device leads to a shift in the resonant frequency from 2.47 GHz at 23°C to 2.41 GHz at 600°C. It demonstrates the ability of the sensors to enlarge the temperature range operation with this new configuration while staying in the 2.45 ISM band. The average TCF extracted from these measurements is about -43 ppm/°C. The variation in TCF between both studied structures is mainly due to differences in the thermoelastic properties of aluminium and NiAl alloy, and their interaction with the substrate, which influence the propagation of elastic surface waves.

In contrast to the behaviour of aluminium IDTs (**Figure 10**) a drop of the baseline reflection coefficient $S_{11}$ was also observed (**Figure 15**). This is most likely due to the higher electrical resistivity of nickel-aluminium compared with aluminium. A further optimization of the NiAl film deposition process aiming to improve the film electrical conductivity is in progress.

<figure><img src="image_17.png" /><figcaption>Figure 15. Coefficient of reflection $S_{11}$ of the NiAl(80nm)/AlN(600nm)/Sapphire SAW resonator at different temperatures, in wired configuration.</figcaption></figure>

After the heating phase, the sensor was cooled and the $S_{11}$ parameter was measured again at room temperature. The comparative results of the $S_{11}$ parameters, before and after the temperature cycle, are shown in **Figure 16**. The reflection coefficient remains almost constant, demonstrating the stability of the structure up to 600°C, at least for short durations, the full temperature cycle lasting around 4h.
<figure><img src="image_18.png" /><figcaption>Figure 16. Reflection coefficient $S_{11}$ of the NiAl(80nm)/AlN(600nm)/Sapphire SAW resonator before heating phase and after cooling phase.</figcaption></figure>

As for the sensors with aluminium IDTs, we carried out a wireless interrogation up to 600°C. The wireless responses measured at 3 meters, show a remarkable stability in temperature and no significant degradation in amplitude. This demonstrates the strong potential of the NiAl/AlN/Sapphire structure for high-temperature wireless applications (Figure 17). The intrinsic properties of NiAl and AlN show promising potential for operation at higher temperatures. However, the experimental setup required to perform stable measurements above 600 °C is currently under development. Considering that a minimum signal to noise ratio of 10 dB is required for successful detection, we estimate, using the method described hereabove, that the maximum wireless interrogation distance at 600°C of the NiAl/AlN/Sapphire SAW sensor is approximately ten meters.

<figure><img src="image_19.png" /><figcaption>Figure 17. Backscattered power of the NiAl/AlN/Sapphire SAW resonator wirelessly interrogated at a distance of 3 m as a function of temperature.</figcaption></figure>

The variation of frequency versus temperature is plotted in Figure 18. A good linearity was observed over the entire temperature range (from 23 to 600°C). A TCF value of -43ppm/°C is measured for the NiAl/AlN/Sapphire sensor in wireless configuration. The relatively low TCF obtained for the NiAl sensor remains suitable for high-temperature applications, particularly due to bandwidth management constraints. On the 2.45 GHz ISM band, which is 83.5 MHz wide, the measured frequency drift reaches approximately 61 MHz at 600 °C, representing around 72% of the available band. A higher TCF would risk pushing the signal outside regulatory limits, making the sensor legally unusable, while the current TCF leaves sufficient margin to consider operation even higher temperatures. Furthermore, in the case of simultaneous use of several sensors on the same frequency band, a small TCF becomes an advantage. For example, a reduction in the TCF by a factor of two (approximately -20 ppm/°C) would lead to a frequency excursion of approximately 28.5 MHz over the same temperature range, allowing up to three sensors to be differentiated within the ISM band without spectral overlap.
<figure><img src="image_20.png" /><figcaption>Figure 18. Temperature dependence of the resonance frequency of NiAl(80nm)/AlN(600nm)/Sapphire SAW resonators measured between RT and 600 °C in wireless configuration.</figcaption></figure>

This result marks a significant advance in the state of the art of wireless SAW sensors operating in high-temperature environments and represents, to our knowledge, the best performance reported in the 2.45 GHz ISM band (Table 1).

**Figure 19** compares the backscattered power of Al and NiAl transducers on AlN/Sapphire. NiAl IDTs show good stability after two temperatures cycles of 4 hours up to 600°C, while aluminium-based IDTs show a significant deterioration with 8 dB attenuation after one 4-hour cycle up 580°C. This shows the superiority of NiAl/AlN/Sapphire sensors at high temperatures. However, aluminium-based sensors maintain better magnitude signals than NiAl-based sensors between room temperature and 400°C, which is due to aluminium's better electrical conductivity and smaller density. It also has to be noted that Al-based SAW sensor shows a good stability after one heating-cooling temperature cycle up to 400°C (green curve). In summary, Al/AlN/Sapphire sensors are more suitable up to 400°C, in good agreement with the literature [3], while NiAl/AlN/Sapphire sensors perform better above this temperature, because of the better resistance of NiAl alloy to oxidation phenomena in such conditions, as discussed in the next section.

<figure><img src="image_21.png" /><figcaption>Figure 19. Comparison of the backscattered power of Al(80nm)/AlN(600nm)/Sapphire and NiAl(80nm)/AlN (600nm)/Sapphire SAW resonators as a function of temperature.</figcaption></figure>

## 4. DISCUSSION

NiAl(80nm)/AlN/Sapphire sensors demonstrated stable operation up to 600°C for around 4 hours (Figure 16). In an additional experiment 150 nm-thick NiAl electrodes on AlN/Sapphire were subject to heating at 500°C in air for 176 hours (i.e. one week). Transmission electron microscopy was then used to observe the film stack at the nanoscale after this annealing process. Bright-field transmission electron microscopy (BF-TEM) micrograph shows columnar growth of the AlN layer (Figure 20). Furthermore, the electron diffraction pattern (inset Figure 20) along the [210] zone axis indicates a growth in the direction [001] in perfect agreement with the results previously obtained by X-ray diffraction (Figure1).
<figure><img src="image_22.png" /><figcaption>Figure 20. BF-TEM micrograph of NiAl IDT on AlN/Sapphire substrate after 176 h at 500°C in air atmosphere, and electron diffraction pattern of the AlN layer along the [210] zone axis (inset).</figcaption></figure>

The 150 nm-thick NiAl electrodes showed a remarkable long-term resistance: after 176 hours, only a thin, homogeneous 22 nm layer of alumina (Al₂O₃) has formed on the surface as shown in **Figure 21**. This naturally formed layer is protecting a significant underlying layer of NiAl, confirming the durability of NiAl IDTs at this temperature. Although the study involved an initial thickness of 150 nm, similar alumina formation could be expected with thinner layers. The observed Al₂O₃ thickness is consistent with the results reported by J. Maufay at 500°C, particularly after 250 hours of annealing. These results pave the way for extended SAW measurements at 500°C and 2.45 GHz over several days [25]. However, a limiting factor remains the relatively low conductivity of the NiAl layers (43 μΩ.cm), which requires improvement. A potential solution to improve conductivity would be to make multilayer NiAl deposits, as detailed in [38].

<figure><img src="image_23.png" /><figcaption>Figure 21. BF-STEM micrograph and EDS elemental maps of NiAl IDT on AlN/Sapphire substrate after 176 h at 500°C in air atmosphere.</figcaption></figure>
## 5. CONCLUSION

In this study, we demonstrated significant advances in wireless SAW measurements at 2.45 GHz, using AlN/Sapphire bilayer structure for applications in harsh environments, thanks to the optimization of the constituent materials. Firstly, we achieved wireless interrogation at a distance of 3 metres for devices based on the Al/AlN/Sapphire structure, up to 500°C. The obtained results pave the way for wireless measurements at even greater distances, up to around ten meters.

Next, NiAl alloy was used to fabricate IDT electrodes, offering better oxidation resistance than aluminium. The NiAl/AlN/Sapphire structure showed remarkable stability and a temperature sensitivity (TCF) of -43 ppm/°C. These characteristics enabled wireless interrogation up to 600°C at a distance of 3 metres (with potential up to 10 m), with stable performance over several hours. To the best of our knowledge, these results far exceed the state of the art for wireless SAW sensors in the large 2.45 ISM band. To assess the long-term durability, annealing tests were carried out on the NiAl/AlN/Sapphire structure for 176 hours at 500°C. The results were promising, as only a thin layer of 22 nm oxidised out of an initial thickness of 150 nm, confirming the potential of the structure for prolonged wireless interrogation over several days at 500°C. Finally, future work will focus on improving the conductivity of NiAl layers. Multilayer structures of this alloy will be explored to further optimise long-term device performance in extreme environments.

## ACKNOWLEDGMENT

This research was supported the CASSIOPEE Project funded by Institut Carnot ICEEL, by the french France 2030 program «Initiative d'Excellence Lorraine (LUE)», reference ANR-15-IDEX-04-LUE » and Campus France Gabon (Ulrich Youbi PhD grant).

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