10.1109/ULTSYM.2014.0377
AlN Solidly Mounted Resonators for High Temperature Applications T. Mirea*, M. DeMiguel-Ramos, M. Clement, J. Olivares and E. Iborra
V. Yantchev and I. Katardjiev Department of Engineering Sciences, Solid State Electronics Uppsala University Uppsala,Sweden
GMME-CEMDATIC-ETSIT Universidad Politécnica de Madrid Madrid, Spain *Corresponding author:
[email protected]
Abstract— Monitoring under harsh environments, particularly high temperatures (> 600°C), is on high demand nowadays. Applications such as gas control in propulsion systems or fire detection in early stages are good examples. During the last decades, materials and devices have been extensively investigated for these applications. Few have proven thermal and chemical stability. Among the most used devices are surface acoustic wave (SAW) resonators using a langasite (LGS) substrate. Their main disadvantage is related to their transducer topology. Their long and narrow electrode strips are subjected to destructive agglomeration. In order to solve this problem, solidly mounted bulk resonators (SMR) are here proposed as an alternative. They provide rigidity, high performance and large electrodes. Here we investigate the performance of SMR devices after annealing under vacuum condition at 700°C for a cumulative time of 24h. SMRs are composed of porous-SiO2/Mo Bragg mirrors and Ir or Mo electrodes. Their performance shows and initial overall improvement with subsequent stabilization. Qp×keff2 in the order of 72 are achieved. Further investigations on full dielectric Bragg mirrors will be performed. With this initial study we demonstrate that SMRs can be a good alternative to SAWs for high temperature applications. Keywords—Solidly Mounted Resonator, AlN, sensor, high temperature.
I. INTRODUCTION Monitoring or sensing under harsh environmental conditions is now a challenging issue. A need for real time monitoring of different magnitudes, in particular at very high temperatures (> 600°C), can be found in different sectors. For example, it can be found in the aerospace sector, where it is important to control gases in propulsion jet engines. Also in the automotive industry, where engines, tires or brakes need to be monitored. Chemical analysis and measurement of temperatures and pressures at high temperature are also of great interest for industrial processes control. Among these, fire detection in the early stages and all applications that present inaccessible and hard-conditions environments can be also included. This wide range of applications has promoted intensive studies in this field the last decades. Within the different sensing techniques [1], piezoelectric acoustic sensors appear to be the most promising ones [2].
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In particular, one of the most explored approaches is that of a surface acoustic wave (SAW) resonator [3-5], although high temperature applications have been also proven for other devices such as Lamb wave resonators [6]. A SAW device is based on an interdigital transducer (IDT) placed on a piezoelectric single crystal. The numerous studies on this device can be justified by the main advantage it presents. It can be passively interrogated and hence used as wireless sensor [3]. Concerning the materials needed for such critical application, many studies have been focused on their selection [7,8]. Regarding the piezoelectric substrates, their main limitations are phase transitions. These can transform them into non-piezoelectric materials. Single-crystal langasite (LGS) has been placed as the most appropriate candidate for SAW resonators [4]. As for the electrodes, platinum (Pt) with different adhesion layers or as part of composites has proven to be a suitable material [9, 10]. One disadvantage when using SAW devices for high temperature applications is related to their transducer topology. Namely, the IDTs are constructed by long and narrow electrode strips, which are subjected to destructive agglomeration. For that reason, solidly mounted bulk resonators (SMR) based on thin AlN piezoelectric films can be a good alternative [11]. These types of devices are mechanically rigid structures that have proven relatively high electromechanical coupling (keff2) factors (more than 6%). Besides, they have large electrodes that can be designed relatively thick at the expense of their high keff2. This topology allows avoiding electrode destructive agglomeration. As an alternative to LGS crystals, polycrystalline AlN has also proven to be chemically and thermally stable material [8,12]. Single-port resonators have been proven to be suitable for high temperature wireless interrogation [13]. So far, one-port SAW Langasite and Quartz resonators have been demonstrated in this scheme, but SMRs seem also suitable since they can provide comparable and even stronger response owing to their higher electromechanical coupling. However, this is still not a mature technology and hence needs further investigation. Here we present initial studies on the effect of high temperatures (~700°C) on the integrity and performance of SMRs after more than 24 hours of cumulative time.
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2014 IEEE International Ultrasonics Symposium Proceedings
II. EXPERIMENTAL
temperature and exposed to ambient. At that moment their performance was assessed and their appearance monitored.
Two types of SMRs with different top electrode material were used for the annealing tests. Both consisted of acoustic reflectors, or Bragg mirrors, made of five alternating layers of low acoustic impedance porous-SiO2 and high acoustic impedance molybdenum (Mo). Both materials were deposited by pulsed-DC sputtering. The thicknesses of these layers were of 535 nm and 629 nm, respectively, which correspond to λ/4 for a frequency of 2.5 GHz. Between each SiO2 and Mo layer, a thin 10 nm-thick titanium (Ti) film was deposited to promote adherence. To act as bottom electrode, a 120 nm-thick iridium (Ir) film was evaporated on top of the Bragg mirror, precoated with a thin Ti adhesion layer. Onto the Ir bottom electrode, a c-axis oriented AlN film with thickness ranging from 1 µm to 1.2 µm was deposited in an ultra-high vacuum sputtering system. The system, pumped with by a cryogenic pump to a base pressure of less than 10-8 Torr is fitted with a sample transfer system to avoid oxygen contamination in the deposition chamber. The high purity Al target was sputtered in 40:60 Ar/N2 admixtures with a pulsed DC power supply operating at 250 kHz and 1200 W. Substrate temperature was 400ºC and RF substrate polarization during the deposition allowed to control the stress to low compressive values (
