**7. Conclusion**

This chapter focuses on the study of RF sputtered AlN films in view of ICs, acoustic devices and MEMS applications. It is divided into two distinct parts; growth, characterization and optimization of device worthy AlN film (mostly on silicon), and demonstration of acoustic and MEMS device applications. The morphological and electrical properties of RF sputtered AlN films are studied with sputtering power, deposition temperature, sputtering pressure, gas flow ratio and target to substrate spacing (Dts). Higher RF power (400 W) and nitrogen concentration (80%), moderate substrate temperature (200 °C) and sputtering pressure (6×10-3 mbar), lower target-substrate distance (5 cm) is suitable for the growth of smooth, highly c-axis oriented film with better electrical properties. Post-deposition (RTA and furnace) annealing has a significant impact on the morphology as well as the electrical properties. The RTA processed AlN films have relatively high c-axis (002) orientation films at 800 ºC, where microcracks are appeared during RTA process at 1000 ºC. Bulk charge density is increased with annealing temperature both types of annealing. A significant reduction in interface charge density is found at 600ºC with RTA process, whereas it decreases with furnace annealing temperature. AlN films are deposited and characterized on different substrates by RF reactive magnetron sputtering. On SiO2/Si substrates, (002) orientation is deteriorated and surface roughness of the films is increased with the increase in oxide thickness. c-axis (002) oriented films are observed on Si and GaAs substrates, whereas AlN (100), (002) and (102) oriented peaks are seen on InP substrates. AlN films, deposited on GaAs substrate, show bump like structures. c-axis oriented AlN films are also observed on metallic films. The AlN films, deposited on Al and Cu, are found to be rough with larger grains. Piezoelectric nature of RF deposited AlN films is ascertained from the performance of a SAW device. This device is centred around a frequency of 84.3 MHz and acoustic phase velocity is inferred to be 5058 m/ sec with K2 of 0.34 %. The TMAH solution, doped with an appropriate ratio of silicic acid and ammonium persulphate, is developed for micromachining of AlN based structures. The etch rate of silicon is around 50 μm/hour. On the otherhand, the doped solution has negligible impact on Al, AlN and SiO2 films. The growth of highly (002) oriented AlN films, post deposition process and micromachining method will provide an appropriate platform for the fabrication of futuristic electronic devices.

## **8. References**


This chapter focuses on the study of RF sputtered AlN films in view of ICs, acoustic devices and MEMS applications. It is divided into two distinct parts; growth, characterization and optimization of device worthy AlN film (mostly on silicon), and demonstration of acoustic and MEMS device applications. The morphological and electrical properties of RF sputtered AlN films are studied with sputtering power, deposition temperature, sputtering pressure, gas flow ratio and target to substrate spacing (Dts). Higher RF power (400 W) and nitrogen concentration (80%), moderate substrate temperature (200 °C) and sputtering pressure (6×10-3 mbar), lower target-substrate distance (5 cm) is suitable for the growth of smooth, highly c-axis oriented film with better electrical properties. Post-deposition (RTA and furnace) annealing has a significant impact on the morphology as well as the electrical properties. The RTA processed AlN films have relatively high c-axis (002) orientation films at 800 ºC, where microcracks are appeared during RTA process at 1000 ºC. Bulk charge density is increased with annealing temperature both types of annealing. A significant reduction in interface charge density is found at 600ºC with RTA process, whereas it decreases with furnace annealing temperature. AlN films are deposited and characterized on different substrates by RF reactive magnetron sputtering. On SiO2/Si substrates, (002) orientation is deteriorated and surface roughness of the films is increased with the increase in oxide thickness. c-axis (002) oriented films are observed on Si and GaAs substrates, whereas AlN (100), (002) and (102) oriented peaks are seen on InP substrates. AlN films, deposited on GaAs substrate, show bump like structures. c-axis oriented AlN films are also observed on metallic films. The AlN films, deposited on Al and Cu, are found to be rough with larger grains. Piezoelectric nature of RF deposited AlN films is ascertained from the performance of a SAW device. This device is centred around a frequency of 84.3 MHz and acoustic phase velocity is inferred to be 5058 m/ sec with K2 of 0.34 %. The TMAH solution, doped with an appropriate ratio of silicic acid and ammonium persulphate, is developed for micromachining of AlN based structures. The etch rate of silicon is around 50 μm/hour. On the otherhand, the doped solution has negligible impact on Al, AlN and SiO2 films. The growth of highly (002) oriented AlN films, post deposition process and micromachining method will provide an appropriate platform for the fabrication of futuristic electronic

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microstructural characterisation of reactive RF magnetron sputtering AlN films for surface acoustic wave filters, *Diamond and Related Materials*, Vol. 13, pp. 1111-1115. Belyanin, A.F., Boulov, L.L., Zhirnov, V.V., Kamenev, A.I., Kovalskij, K.A. & Spitsyn, B.V.

(1999). Application of aluminum nitride films for electronic devices, *Diamond and* 

controlled monolithic integrated surface acoustic wave (SAW) gas sensors, *Sensors* 

**7. Conclusion** 

devices.

**8. References** 

1146-1151.

*Related Materials*, Vol. 8, pp. 369-372.

*and Actuators B*, Vol. 93, pp. 164-168.


**26** 

 *Italy* 

Cinzia Caliendo

**Surface Acoustic Wave Devices** 

*Istituto dei Sistemi Complessi, ISC-CNR, Area della Ricerca Roma 2, Rome* 

*/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

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

**1. Introduction** 

period *p =* 

λ

**for Harsh Environment** 

