AlN/Pt/LN structure for SAW sensors capable of operating at high temperature

Abstract

There is a need for wireless sensors able to operate in the intermediate temperature range (ITR) between 300 °C and 600 °C. Surface acoustic wave (SAW) sensors are promising candidates to solve this issue. However, existing SAW sensors most often fail in the ITR, due to the quick degradation of the sensor housing in extreme conditions. A promising way to circumvent the issue is to use “package-less” devices, where the acoustic waves are guided in a multilayered structure where they are intrinsically protected from adverse environmental effects. We present here an innovative multilayered structure that fulfills all the basic requirements, to achieve a wireless and “package-less” SAW Sensor for the ITR. The structure is made of a thin AlN layer deposited on top of a Y + 128°LN substrate and equipped with buried Pt electrodes. Numerical simulations of the acoustic waves propagating in SAW resonators built on this structure reveal the existence of a useful Rayleigh-type SAW that propagates at the AlN/LN interface with a velocity up to 4500 m/s and a high electromechanical coupling k2=5.6%, without leakage into the substrate. The existence of this mode is due to specific properties of the Y + 128°LN cut, which are analyzed in detail in this paper. The performances of an optimized AlN/Pt/LN structure are also compared to the ones of previously suggested “package-less” structures, including AlN/ZnO/Sapphire. It is shown that better device characteristics can be expected from the AlN/Pt/LN structure in the ITR.

FEM Modeling of the Temperature Influence on the Performance of SAW Sensors Operating at GigaHertz Frequency Range and at High Temperature Up to 500 °C

“…However, it has a number of limitations, the most important being the excessive damping (propagation losses) of surface waves when the operating frequency exceeds 1 GHz [15]. Recently, efforts have been devoted to the development of a SAW sensor heterostructure for high and very high temperature applications such as AlN/Pt/LiNbO 3 [16] and AlN/Sapphire [17][18][19][20][21]. The AlN/Sapphire SAW structure can also operate at very high frequencies [18].…”

Optimization of AIN Composite Structure Based Surface Acoustic Wave Device for Potential Sensing at Extremely High Temperature

“…LGS also has many restrictions such as low surface acoustic velocity and increasing acoustic propagation losses with the temperature at higher frequency [9]. Compared with the former two, aluminum nitride (AlN) exhibits higher surface wave velocity (5607 m/s) [10].…”

“…However, YCOB suffers from the pyroelectric effect, and its surface is easy to absorb impurities in the environment at high temperature, which causes severe degradation of device performance. LGS also has many restrictions such as low surface acoustic velocity and increasing acoustic propagation losses with the temperature at higher frequency [ 9 ]. Compared with the former two, aluminum nitride (AlN) exhibits higher surface wave velocity (5607 m/s) [ 10 ].…”

AlN/ZnO/LiNbO3Packageless Structure as a Low-Profile Sensor for Potential On-Body Applications

“…the human body. To eliminate this bottleneck, some authors have suggested a bilayer structure with AlN and LiNbO3 128° Y-cut [8]. The confinement of the energy is possible due to the use of heavy metal electrodes (Pt) and occurs mostly owing to specific properties of LiNbO3 128° Y-cut which shows a decoupling between Rayleigh modes and Shear Horizontal modes.…”