**5. Growth of AlN films on different substrates**

Many a times, AlN films are made on an insulator (SiO2) for isolation or it is deposited over the metallic electrodes for thin film resonators (TFR). In future, AlN film on high speed semiconductor substrates such as GaAs, InP can be exploited for high speed signal processing and Micro-Opto-Electro-Mechanical Systems (MOEMS) applications. Hence, integration of AlN films on GaAs and InP substrates for a new generation of high-speed devices/subsystems, especially for telecommunications, and radar applications are required. Growth and surface morphology of a deposited film depends not only on the kinetics of the arriving species at the substrate, but also on the nature of the substrates chosen, even if they belong to the same family. In addition, substrate orientation, thermal conductivity and thermal expansion coefficients play vital roles in film growth and its morphology. C-axis oriented AlN films are deposited on Si and SiO2/Si substrates by RF reactive magnetron sputtering, where the degree of orientation decreases with increase in oxide thickness. The surface roughness of the films deposited on SiO2/Si is higher. AlN films are also deposited on GaAs and InP substrates by reactive magnetron sputtering technique under identical deposition conditions. c-axis (002) oriented films are observed on GaAs substrates; whereas, AlN (100), (002) and (102) oriented peaks are seen in case of InP substrates. Surface morphology of the films deposited on Si and InP substrates seems to be similar, but the films on InP are little rougher with the development of nano-pores. AlN films, grown on GaAs substrates, forms bump like structures (Kar et al., 2009), which may be due to thermal and/or lattice mismatch. It is important to note that the crystallinity and stochiometry of the initial layer of AlN film also plays a significant role in the creation of defects and mismatches (Ahmed et al., 1992). Crystal orientation of AlN films is also a strong function of the bottom metal electrodes. AlN films deposited on metals (Al, Cu, Cr, Au) are caxis oriented, whereas the films deposited on Al and Cu are rough with larger grains.

Aluminum Nitride (AlN)

resonator (De Los Santos, 1999).

Film Based Acoustic Devices: Material Synthesis and Device Fabrication 573

MEMS resonators are comprised of a microscale mechanical element, which converts mechanical to electrical signal and vice versa. One of the prominent resonator structures is MEMS cantilever, which is based on thin piezoelectric films. Film resonates when an ac voltage is applied across the film. Resonator can be made without piezoelectric material (electrostatic, capacitive resonator), but it suffers with large resistance, in the range of MΩ, and depends on driving voltage. On the other hand, piezoelectric resonators have smaller resistance of the order of KΩ and are more suitable for UHF device applications. (Lakin, 1999; Quandt et al., 2000; Humad et al., 2003). In addition, the output is easier to sense in a piezoelectric resonator. Furthermore, a piezoelectric resonator has certain advantages over the electrostatic resonator (capacitive resonators), such as low current consumption and lower actuation voltages (Olivares et al.; 2005). But the quality factor (Q) of piezoelectric resonator is smaller than that of a capacitive resonator. The quality factor of any resonator is proportional to the decay time, and is inversely proportional to the bandwidth around resonance. Higher Q represents higher frequency stability and accuracy capability of the

Fig. 8. XRD pattern, and AFM image (inset) of sputtered AlN film for SAW

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 AlN film with better electrical properties. A c-axis (002) oriented peak is recorded at 2θ value of 36.1º (Fig. 8). The atomic force micrograph of the film shows dense microstructure with continuous grain growth (inset of Fig. 8). This kind of film is suitable for SAW devices. In a typical case, each IDT consisted of 25 pairs of fingers/electrodes with 30 μm centre-tocentre spacing between the two neighbouring fingers comprising a pair (p/2). The width of each finger/electrode is designed to be 15 μm (p/4) with each of 6.0 mm length and 5.0 mm overlap, producing a SAW filter with an acoustic wavelength of 60 μm (Kar et al., 2009). The SAW device parameters are: AlN film thickness = 0.92 μm, acoustic wavelength = 60 μm, SAW velocity = 5058 m/sec, electromechanical coupling coefficient (K2) = 0.34%.

**X 0.500 μm/div Z 55.000 nm/div** 

**6.1 Evaluation of AlN films through SAW devices** 
