**4. Post-deposition annealing effect**

AlN film may see high temperature, if AlN film is monolithically integrated during IC fabrication. Post-deposition heat treatment significantly affects the morphology and electric

Aluminum Nitride (AlN)

Film Based Acoustic Devices: Material Synthesis and Device Fabrication 571

Fig. 6. SEM micrograph of AlN films annealed at (a) as-deposited, and (b) 800 ºC

In both types of annealing, the Qin is increased with temperature. The probable reasons for the rise in the Qin may be due to the generation of trap centres with annealing in the nitrogen ambient. In RTA, the Dit is found to be strongly dependent on the annealing temperature and it significantly reduced at 600ºC. With furnace annealing, the Dit decreases

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 c-

axis oriented, whereas the films deposited on Al and Cu are rough with larger grains.

**4.3 Effect of annealing on electrical properties** 

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

with increase in annealing temperature.

charges of AlN film. The post-deposition thermal treatments (annealing) of AlN film are generally carried out by two distinguished modes; namely, Rapid Thermal Annealing (RTA) and conventional furnace annealing.

### **4.1 Rapid thermal annealing (RTA) process**

The XRD studies show that the intensity of c-axis (002) orientation increases upto annealing temperature of 800 ºC in nitrogen ambient and then it marginally decreases at 1000 ºC (Kar et al., 2005). The shift in XRD diffraction peaks is reported at higher temperatures, which may be due to the generation of stress. Granular worm-like nanostructures are found in asdeposited AlN films (Fig. 5), whereas cracks are observed for annealing at 1000 ºC. A short duration of heat pulse by RTA is barely sufficient to modulate the film surface, but not enough to activate the grains of the AlN films to merge themselves to form bigger grains. Appearances of cracks are due to the stress developed in the film. The thermal coefficient mismatch between the AlN and silicon substrate may be generated from the fast ramp up and ramp down annealing heat cycle during rapid thermal annealing. The position and density of cracks depend strongly on the defects, dislocations and the structural relaxation of grain boundaries. The surface roughness is considerably increased for the film annealed at higher temperatures due to the surface modulation.

Fig. 5. SEM micrograph of AlN films RTA processed at (a) as-deposited, and (b) 1000 ºC

### **4.2 Furnace annealing**

The intensity of the (002) peak increases with furnace annealing temperature, where the atoms acquire adequate activation energy to become (002) oriented. Sometimes, many of the atoms may not be at the crystal lattice site in the as-deposited AlN film, which causes the lattice strain and the formation of microvoids. During conventional furnace annealing, atoms get enough time to acquire sufficient kinetic energy and occupy relative equilibrium positions that minimize the lattice strain and microvoids, which results in a better crystalline film. Furthermore, the furnace annealing process minimizes the dislocations and the other structural defects and forms a better stoichiometric material. From the SEM micrographs, it is observed that the granular worm-like textures grow bigger in size with increased surface roughness as a result of annealing (Fig. 6). The possible reasons for increase in the grain size and the surface roughness may be due to atomic migration in the film towards the lower surface energy with annealing temperature (Kar et al., 2009).

charges of AlN film. The post-deposition thermal treatments (annealing) of AlN film are generally carried out by two distinguished modes; namely, Rapid Thermal Annealing (RTA)

The XRD studies show that the intensity of c-axis (002) orientation increases upto annealing temperature of 800 ºC in nitrogen ambient and then it marginally decreases at 1000 ºC (Kar et al., 2005). The shift in XRD diffraction peaks is reported at higher temperatures, which may be due to the generation of stress. Granular worm-like nanostructures are found in asdeposited AlN films (Fig. 5), whereas cracks are observed for annealing at 1000 ºC. A short duration of heat pulse by RTA is barely sufficient to modulate the film surface, but not enough to activate the grains of the AlN films to merge themselves to form bigger grains. Appearances of cracks are due to the stress developed in the film. The thermal coefficient mismatch between the AlN and silicon substrate may be generated from the fast ramp up and ramp down annealing heat cycle during rapid thermal annealing. The position and density of cracks depend strongly on the defects, dislocations and the structural relaxation of grain boundaries. The surface roughness is considerably increased for the film annealed

Fig. 5. SEM micrograph of AlN films RTA processed at (a) as-deposited, and (b) 1000 ºC

The intensity of the (002) peak increases with furnace annealing temperature, where the atoms acquire adequate activation energy to become (002) oriented. Sometimes, many of the atoms may not be at the crystal lattice site in the as-deposited AlN film, which causes the lattice strain and the formation of microvoids. During conventional furnace annealing, atoms get enough time to acquire sufficient kinetic energy and occupy relative equilibrium positions that minimize the lattice strain and microvoids, which results in a better crystalline film. Furthermore, the furnace annealing process minimizes the dislocations and the other structural defects and forms a better stoichiometric material. From the SEM micrographs, it is observed that the granular worm-like textures grow bigger in size with increased surface roughness as a result of annealing (Fig. 6). The possible reasons for increase in the grain size and the surface roughness may be due to atomic migration in the film towards the lower

and conventional furnace annealing.

**4.1 Rapid thermal annealing (RTA) process** 

at higher temperatures due to the surface modulation.

surface energy with annealing temperature (Kar et al., 2009).

**4.2 Furnace annealing** 

Fig. 6. SEM micrograph of AlN films annealed at (a) as-deposited, and (b) 800 ºC
