SAW RFID Devices Using Connected IDTs as an Alternative to Conventional Reflectors for Harsh Environments

Abstract

Remote interrogation of surface acoustic wave IDtags imposes a high signal amplitude which is related to a high coupling coefficient value (K 2) and low propagation losses (α). In this paper, we propose and discuss an alternative configuration to the standard one. Here, we replaced the conventional configuration, i.e. one interdigital transducer (IDT) and several reflectors, by a series of electrically connected IDTs. The goal is to increase the amplitude of the detected signal using direct transmission between IDTs instead of the reflection from passive reflectors. This concept can therefore increase the interrogation scope of ID-tags made on conventional substrate with high K 2 value. Moreover, it can also be extended to suitable substrates for harsh environments such as high temperature environments: the materials used exhibit limited performances (low K 2 value and relatively high propagation losses) and are therefore rarely used for identification applications. The concept was first tested and validated using the lithium niobate 128°Y-X cut substrate, which is commonly used in ID-tags. A good agreement between experimental and numerical results was obtained for the promising concept of connected IDTs. The interesting features of the structure were also validated using a langasite substrate, which is well-known to operate at very high temperatures. Performances of both substrates (lithium niobate and langasite) were tested with an in-situ RF characterization up to 600°C. Unexpected results regarding the resilience of devices based on congruent lithium niobate were obtained.

Passive Resonant Sensors: Trends and Future Prospects

“…In addition of being small, simple and robust, SAW devices can be batteryless, possibly wirelessly interrogated [28] and packageless [29,30], with multi-tagging (IDTAG) capability [31,32]. SAW devices being widely used as standard components of RF communication, SAW sensors and their reader units can be inexpensive and several solutions are commercially available [33,34,35,36].…”

“…Their lifetime is estimated at least 10 days at 350 °C, but only a few hours at 400 °C, due to the degradation of the aluminum IDTs. Finally, it was shown very recently that stoichiometric LiNbO3 (sLN) substrates allowed slightly upper temperatures [114], and that SAW devices based on cLN could be operated up to 600°C for a few hours [32] and even up to 4 days [115]. Ultimately, very few piezoelectric media can be considered for applications above 400 °C.…”

AlN/Pt/LN-Y128 Packageless Acoustic Wave Temperature Sensor

“…This bilayer structure is advantageous over the AlN/ZnO/LN-Y128 trilayer [9] as only one additional layer has to be deposited over the LN-Y128 substrate and optimized. Moreover, ZnO has poor high temperature properties, whereas all the materials in AlN/IDT(Pt)/LN-Y128 can withstand high temperature operation at least up to 600°C [12][13][14].…”

Langasite as Piezoelectric Substrate for Sensors in Harsh Environments: Investigation of Surface Degradation under High-Temperature Air Atmosphere

“…At higher temperatures, a segregation process occurs in lithium niobate crystals, leading to the apparition of lithium triniobate LiNb 3 O 8 at the substrate surface [ 3 ]. Recent work shows that LiNbO 3 could be considered to operate up to 600 °C but for limited durations [ 4 ], and the use of stoichiometric LiNbO 3 (s-LN) substrate makes it possible to exceed this limit for even longer durations [ 5 ]. Nevertheless, s-LN substrates remain expensive, and the cuts of interest for SAW applications are not commercially available.…”