**1. Introduction**

578 Acoustic Waves – From Microdevices to Helioseismology

Lee, H.C., Park, J.Y., Lee, K.H. & Bu, J.U. (2004). Preparation of highly textured Mo and AlN

Lee, S.H., Yoon, K.H., Cheong, D. S. & Lee, J.K. (2003). Relationship between residual stress

Loebl, H.P., Klee, M., Metzmacher, C., Brand, W., Milsom, R. & Lok, P. (2003). Piezoelectric

Luo, J.K., Lin, M., Fu, Y.Q., Wang, L., Flewitt, A.J., Spearing, S.M., Fleck, N.A. & Milne, W.I.

Naik, R.S., Reif, R., Lutsky J.J. & Sodini, C.G. (1999). Low-Temperature Deposition of Highly

Ni, H., Lee H.J. & Ramirez, A.G. (2005). A robust two-step etching process for large-scale

Olivares, J., Iborra E., Clement M., Vergara, L., Sangrador, J. & Sanz-Herv´as, A. (2005).

Quandt, E. & Ludwig, A. (2000). Magnetostrictive actuation in microsystems, *Sensors and* 

Schreiter, M., Gabl, R., Pitzer, D., Primig, R. & Wersing, W. (2004). Electro-acoustic

Strite, S. & Morko, H. (1992). *GaN, AlN and InN: A review*, *J. Vac. Sci. Technol. B,* Vol. 10(4),

Wang, H.H. (2000). Properties and preparation of AlN thin films by reactive laser ablation

Wang, X.D., Jiang, W., Norton, M.G. & Hipps, K.W. (1994). Morphology and orientation of nanocrystalline AlN thin films, *Thin Solid Films*, Vol. 251(2), pp. 121-126. Xu, X.H., Wu, H.S., Zhang, C.J. & Jin Z.H. (2001). Morphological properties of AlN

Yamada, H., Ushimi, Y., Takeuchi, M., Yoshino, Y., Makino, T. & Arai, S. (2004).

Yim, W.M., Stofko, E.J., Zanzucchi, P.J., Pankov, J.I., Ettenberg, M. & Gilbert, S.L. (1973).

with nitrogen discharge, *Modern Physics Letters B*, Vol. 14, 523-530.

*Vac. Sci. Technol. B*, Vol. 22(3), pp. 1127-1133.

*Sensors and Actuators A*, Vol. 132 (1), pp. 139-146.

*Solid Films*, Vol. 435, pp. 193-198.

*Physics*, Vol. 79, pp. 143-146.

691-696.

119, pp. 553-558.

pp. 1237-1266.

296.

*Films*, Vol. 388, pp. 62-67.

123–124, pp. 590-595.

*Actuators A,* Vol. 81, pp. 275-280.

*Ceramic Society*, Vol. 24, pp. 1589-1592.

films using a Ti seed layer for integrated high-*Q* film bulk acoustic resonators, *J.* 

and structural properties of AlN films deposited by r.f. reactive sputtering, *Thin* 

thin AlN films for bulk acoustic wave (BAW) resonators, *Materials Chemistry and* 

(2006).MEMS based digital variable capacitors with a high-*k* dielectric insulator,

Textured Aluminum Nitride by Direct Current Magnetron Sputtering for Applications in Thin-Film Resonators, *J. the Electrochemical Society*, Vol. 146(2), pp.

microfabricated SiO2 and Si3N4 MEMS membranes, *Sensors and Actuators A*, Vol.

Piezoelectric actuation of microbridges using AlN, *Sensors and Actuators A*, Vol.

hysteresis behaviour of PZT thin film bulk acoustic resonators, *J. the European* 

piezoelectric thin films deposited by DC reactive magnetron sputtering, *Thin Solid* 

Improvement of crystallinity of ZnO thin film and electrical characteristics of film bulk acoustic wave resonator by using Pt buffer layer, *Vacuum*, Vol. 74, pp. 689-692.

Epitaxially grown AlN and its optical band gap, *J. Appl. Phys.*, Vol. 44(1), pp. 292-

There is an increasing demand of electronic components for aerospace, aircraft industries, sensors, automotive, chemical and material processing applications, to name just a few, able to operate reliably and for long time at high-temperature. Measurements reliability requires the electronic components to be placed directly inside the extreme environment, and to withstand temperatures of several centigrade degrees with lifetimes of several hours. The device mounting and packaging, but first of all the device materials must be stable with the working temperature, otherwise temperature-induced stress may result in device's failures. Electroacoustic devices based on surface and bulk acoustic wave (SAW and BAW) technology must satisfy the requirements of low cost, high frequency, high-Q, low loss, large piezoelectric coupling and zero temperature coefficient of delay (TCD) to be key devices in the communication and sensor fields. The temperature stability of the piezoelectric crystal is an essential characteristic because of its direct link with the temperature sensitivity of the electroacoustic device operation frequency. The high operation frequency is an essential characteristic for SAW and BAW devices to be used in mobile phones, cordless headphones, alarm and security systems, military equipment, sensors, etc. The temperature stability and the high operation frequency demands can be met through a proper choice of the piezoelectric substrate crystal cut, new piezoelectric materials and/or multilayer configurations. The use of temperature stable cuts of single crystal bulk piezoelectric materials or temperature compensated multilayers represents two possible solutions to the temperature stability requirement. The use of high-resolution lithography techniques and/or of high SAW velocity materials is required in order to extend the upper limit of the electroacoustic device frequency range. Submicron feature sized interdigital transducers (IDTs) are required to implement GHz range SAW devices on *slow* piezoelectric materials, while micron feature sized IDTs can still be used on *fast* materials, since the SAW device centre frequency, f = v/λ, depends on both the phase velocity of the propagating medium, *v*, and on the acoustic wavelength λ, being the IDT's period *p =* λ*/2*. Conventional piezoelectric substrates, such as quartz, lithium niobate (LiNbO3), and lithium tantalate (LiTa03) crystals, cannot be used above 500°C. Quartz ST cut is a temperature stable material but it shows an alpha-beta transition at 573°C, which causes the loss of piezoelectricity, and results in a non-operable device. SAW devices implemented on LiNb03 have been studied for a temporary usage at 400°C [1]; however the LiNbO3 acoustic wave properties are highly dependent on temperature since it is a pyroelectric

Surface Acoustic Wave Devices for Harsh Environment 581

900°C, thus allowing the realization of high frequency, temperature compensated dispersive electroacoustic devices for high temperature applications. Electroacoustic devices implemented on Si substrates offer the opportunity to integrate the device with the surrounding electronic circuitry on the same chip. Moreover, the opposite TCD of Si ( ∼30 ppm/°C) and AlN (∼ -30 ppm/°C) allows the realisation of zero-temperature-coefficient acoustic devices at the proper film thickness to be used as sensors and actuators where low loss, low thermal drift, high sensitivity and high signal-to-noise ratio are demanded [7, 8]. Pt is the material of choice for metallic components that have to withstand oxidation, thank to its high temperature coefficient of resistance: Pt can be grown in thin film form and both the

In the present chapter the sustainability of Pt and AlN films on sapphire and Si substrates

Bulk piezoelectric single crystals, such as LGS and GaPO4, can be used for the implementation of non dispersive SAW devices, such as delay lines, filters and resonators, and the SAW propagation characteristics, such as phase velocity, electroacoustic coupling efficiency K2 and TCD, depend on the crystal cut and SAW propagation direction, as well as on the geometry of the IDTs . The SAW propagation is excited by IDTs located at the free surface of the piezoelectric substrate and directly exposed to the surrounding environment,

AlN can be grown in thin film form onto non piezoelectric substrates, such as silicon or sapphire, thus allowing the realization of dispersive electroacoustic devices. Moreover, if the AlN film is sandwiched between the IDTs and the ground electrode, four piezoelectric coupling configurations can be obtained by placing the IDTs at the substrate/film interface or at the film surface, with and without the floating electrode opposite the IDTs. These four structures will be mentioned hereafter as substrate/film/IDT (SFT), substrate/IDT/film/metal (STFM)*,* substrate/IDT/film (STF) andsubstrate/metal/film/IDT (SMFT), respectively. Figure 2a shows the top view of a dispersive SAW delay line, and figure

**Piezoelectric substrate** 

When the IDTs are located at the substrate/film interface, the piezoelectric film plays the role of both the acoustic wave transductor and protective layer of the underlying IDTs.

IDTs and ground electrodes can be easily defined by lift off technique.

for high temperature applications is assessed.

Fig. 1. SAW delay line on a piezoelectric substrate

2b shows the cross sections of the four coupling configurations.

**2. AlN-based SAW devices** 

as shown in figure 1.

**IDTs**

material and has a TCD as high as ∼75 ppm/°C. LiTaO3 shows properties, such as a low Curie temperature (607°C), sensitivity to temperature variations (TCD = 22 ppm/°C for the X-112°Y cut) and a strong pyroelectricity, which limit its operation at elevated temperatures. Piezoelectric bulk single crystals such as GaPO4, LGS (La5Ga3SiO14) and its isomorphs (called LGX family group) substrates are widely investigated for the realization of SAWbased devices able to work at high temperature. LGS belongs to the trigonal class 32 group as quartz but it has no αβ transitions and can operate up to its melting temperature of 1470°. It shows zero or very low TCD cuts with zero power flow angle and higher electromechanical coupling coefficient than that of quartz [2]. Langasite based SAW devices are not suitable for operation in the GHz range as a consequence of their low phase velocity and high acoustic losses (from 1 to 0.01 dB/wavelength [3]). GaPO4 has twice the sensitivity of quartz and many its physical constants are stable up to about 900°C, but the accessible frequencies are limited to values of 1 GHz as a consequence of quite high acoustic losses.

The technology of thin piezoelectric films (such as AlN) offers the opportunity of combining the properties of the substrate with those of the film: thus a composite arrangement of fast materials with opposite sign *TCD*s and a proper design of the electroacoustic configuration enable achieving a thermally stable SAW device operating in the GHz range.

Aluminium nitride (AlN) is a piezoelectric material that shows interesting properties, such as excellent thermal conductivity (180W/mK), low coefficient of thermal expansion (CTE, 4.1 x 10-6 °C-1), and good resistance to thermal shock and caustic chemicals [4], that make it useful as protective coating and guarantee the stability of the AlN-based devices when they are in contact with extreme environments. It is currently being investigated due to its promising potentialities for high-temperature, high-power, and high-frequency electronics. It has demonstrated to be an ideal candidate for packaging SiC-devices for high-temperature applications [5] thanks to its CTE that closely matches those of Si (3.5 x 10-6 °C-1) and SiC (3.7 x 10-6 °C-1), high electrical resistivity, high mechanical strength, and chemical inertness. Reactively sputtered AlN films have been used as an effective encapsulant for GaN [6] at an annealing temperature of 1100°C substituting the standard dielectric encapsulants, such as SiO2 and Si3N4, that are not viable at so high temperatures. AlN maintains its piezoelectricity up to 1200°C in vacuum and shows very high BAW and SAW velocities (∼6000 m/s and 11300 m/s for transversal and longitudinal BAWs propagating along the z direction, 5607 m/s for SAWs propagating in the *z* plane) that make it the ideal candidate for microwave electroacoustic devices implementation. Furthermore, AlN can be grown in thin film form onto non piezoelectric substrates by techniques as simple as the rf reactive sputtering. Both the structural properties of the substrate and the experimental sputtering parameters (such as the reactive gas flow rate, the partial pressure of reactive and inert gasses, the substrate temperature, the rf power, and the substrate-target distance) affect the morphological and structural properties of the sputtered thin films. The requirements for a suitable substrate include also a thermal coefficient-of-expansion compatible with that of the film, hightemperature stability, machinability, good adherence of the AlN film: among the available substrates, silicon, platinum and sapphire satisfy these requirements. Al2O3, substrates have a wide range of industrial applications as structural ceramic and optical materials. Al2O3 is extensively used as a high temperature, corrosion resistant refractory material due to its hardness, chemical durability, abrasive resistance, mechanical strength, and good electrical insulation. Al2O3 shows good thermal conductivity (24 W/mK), high SAW velocity (in the range 5555 to 5706 m/s in the c-plane), positive TCD (∼70 ppm/°C) and a CTE that closely matches that of AlN. The AlN/Al2O3 –based multilayers can withstand temperatures up to

material and has a TCD as high as ∼75 ppm/°C. LiTaO3 shows properties, such as a low Curie temperature (607°C), sensitivity to temperature variations (TCD = 22 ppm/°C for the X-112°Y cut) and a strong pyroelectricity, which limit its operation at elevated temperatures. Piezoelectric bulk single crystals such as GaPO4, LGS (La5Ga3SiO14) and its isomorphs (called LGX family group) substrates are widely investigated for the realization of SAWbased devices able to work at high temperature. LGS belongs to the trigonal class 32 group

1470°. It shows zero or very low TCD cuts with zero power flow angle and higher electromechanical coupling coefficient than that of quartz [2]. Langasite based SAW devices are not suitable for operation in the GHz range as a consequence of their low phase velocity and high acoustic losses (from 1 to 0.01 dB/wavelength [3]). GaPO4 has twice the sensitivity of quartz and many its physical constants are stable up to about 900°C, but the accessible frequencies are limited to values of 1 GHz as a consequence of quite high acoustic losses. The technology of thin piezoelectric films (such as AlN) offers the opportunity of combining the properties of the substrate with those of the film: thus a composite arrangement of fast materials with opposite sign *TCD*s and a proper design of the electroacoustic configuration

Aluminium nitride (AlN) is a piezoelectric material that shows interesting properties, such as excellent thermal conductivity (180W/mK), low coefficient of thermal expansion (CTE, 4.1 x 10-6 °C-1), and good resistance to thermal shock and caustic chemicals [4], that make it useful as protective coating and guarantee the stability of the AlN-based devices when they are in contact with extreme environments. It is currently being investigated due to its promising potentialities for high-temperature, high-power, and high-frequency electronics. It has demonstrated to be an ideal candidate for packaging SiC-devices for high-temperature applications [5] thanks to its CTE that closely matches those of Si (3.5 x 10-6 °C-1) and SiC (3.7 x 10-6 °C-1), high electrical resistivity, high mechanical strength, and chemical inertness. Reactively sputtered AlN films have been used as an effective encapsulant for GaN [6] at an annealing temperature of 1100°C substituting the standard dielectric encapsulants, such as SiO2 and Si3N4, that are not viable at so high temperatures. AlN maintains its piezoelectricity up to 1200°C in vacuum and shows very high BAW and SAW velocities (∼6000 m/s and 11300 m/s for transversal and longitudinal BAWs propagating along the z direction, 5607 m/s for SAWs propagating in the *z* plane) that make it the ideal candidate for microwave electroacoustic devices implementation. Furthermore, AlN can be grown in thin film form onto non piezoelectric substrates by techniques as simple as the rf reactive sputtering. Both the structural properties of the substrate and the experimental sputtering parameters (such as the reactive gas flow rate, the partial pressure of reactive and inert gasses, the substrate temperature, the rf power, and the substrate-target distance) affect the morphological and structural properties of the sputtered thin films. The requirements for a suitable substrate include also a thermal coefficient-of-expansion compatible with that of the film, hightemperature stability, machinability, good adherence of the AlN film: among the available substrates, silicon, platinum and sapphire satisfy these requirements. Al2O3, substrates have a wide range of industrial applications as structural ceramic and optical materials. Al2O3 is extensively used as a high temperature, corrosion resistant refractory material due to its hardness, chemical durability, abrasive resistance, mechanical strength, and good electrical insulation. Al2O3 shows good thermal conductivity (24 W/mK), high SAW velocity (in the range 5555 to 5706 m/s in the c-plane), positive TCD (∼70 ppm/°C) and a CTE that closely matches that of AlN. The AlN/Al2O3 –based multilayers can withstand temperatures up to

enable achieving a thermally stable SAW device operating in the GHz range.

transitions and can operate up to its melting temperature of

as quartz but it has no

αβ 900°C, thus allowing the realization of high frequency, temperature compensated dispersive electroacoustic devices for high temperature applications. Electroacoustic devices implemented on Si substrates offer the opportunity to integrate the device with the surrounding electronic circuitry on the same chip. Moreover, the opposite TCD of Si ( ∼30 ppm/°C) and AlN (∼ -30 ppm/°C) allows the realisation of zero-temperature-coefficient acoustic devices at the proper film thickness to be used as sensors and actuators where low loss, low thermal drift, high sensitivity and high signal-to-noise ratio are demanded [7, 8]. Pt is the material of choice for metallic components that have to withstand oxidation, thank to its high temperature coefficient of resistance: Pt can be grown in thin film form and both the IDTs and ground electrodes can be easily defined by lift off technique.

In the present chapter the sustainability of Pt and AlN films on sapphire and Si substrates for high temperature applications is assessed.
