**3.5 Target-substrate distance**

568 Acoustic Waves – From Microdevices to Helioseismology

has changed its orientation with less Al-N bond density and reduction of grain size at 8×10-3 mbar sputtering pressure. Hence, it is inferred that the structural disorder and/or the change in the Al-N bond density/angles must have taken place at this particular sputtering pressure.

Fig. 3. SEM micrograph of the AlN films deposited at (a) 2×10-3 mbar, and (b) 8×10-3 mbar

Fig. 4. SEM micrograph of AlN films deposited at (a) 20 % N2, and (b) SEM image of AlN

At lower nitrogen concentration, the intensity of (100) peak is relatively more prominent than (002), but the trend reverses with higher nitrogen concentration (Kar et al., 2006). At 80% N2, a highly oriented (002) peak is observed without trace of (100) orientation. Lower argon and higher nitrogen gas concentration results slower aluminum sputtering rate. If the time interval for the arrival of Al species at the wafer surface is slower, the Al atom gets enough time to react with N2. This increases the probability of Al-N bond formation and bonded Al-N molecules get more time to adjust themselves along (002) orientation on the substrate. On the other hand, at higher argon concentration, Al does not get enough time for complete nitridation due to higher sputtering rate. In addition, faster arrival of the Al at the substrate surface results not only in a poor AlN bond, but also provides less time for the newly formed AlN to arrange itself along c-axis. A surface texture of smaller grain size, smoother, homogeneous and dense granular microstructures has been observed at higher concentrations of nitrogen. This indicates a low surface mobility of the ad-atoms at high

film for Dts of 5 cm

**3.4 Gas flow ratio** 

The kinetics of the sputtered species arriving at the substrate controls the ad-atom mobility and atomic rearrangement that governs the microstructure of the film. From the XRD studies, it is observed that the intensity of c-axis orientation of the film decreases with increase in target to substrate distance Dts (Kar et al., 2008). At shorter Dts, the Ar ions travel almost normal to the target due to the high electrical field and knock out Al atoms around perpendicular to the target. Because of short deposition path, the probability of collisions of the Al atom with gas atoms is low. Therefore, a good quality film is obtained at lower Dts (5 cm). On the other hand, at larger Dts, the chances of Al collision with gas molecules is increased. In this process Al atoms lose its kinetic energy significantly as well as alter deposition angles. These randomly arriving Al atoms, with lesser energy, cause selfshadowing effects and reduce atomic migration that leads to generation of voids in the film (Lee et al., 2003). The grain size of the AlN film increases with Dts. For lower Dts, smaller grain with minimum surface roughness is observed (Fig. 4(b)) and a coarser grain is found at the highest Dts (8 cm). Surface roughness of the synthesized AlN films are also increases with Dts. The kinetic energy of deposited species is considered to be a major factor for the grain size and the surface roughness of the film.

### **3.6 Variation of electrical properties with sputtering parameter**

The AlN film can be used as a dielectric layer in IC; hence, the electric charges are essential to study with the sputter deposition parameters. Electric charges like Qin and Dit are highly governed by the sputter deposition parameters. A decrease in the Qin is observed with sputtering power, where as Dit is found to be minimum at moderate RF power. At higher temperature, better electrical properties in the bulk as well as the interface of sputtered AlN films are reported; this is mainly due to the formation of bigger grain size and its associated effects. It is reported that the defects produced by stress, voids and incorporation of gases are main responsible cause for the monotonic increase in Qin. The Dit has a minimum value at 6×10-3 mbar sputtering pressure. The Qin and Dit increases with nitrogen concentration. This will have a deleterious effect for silicon-based devices at higher nitrogen concentration. Rise in the Qin and Dit with the increase in Dts is also reported. It is seen that at larger Dts, the morphological as well as the electrical properties of the AlN films deteriorates, whereas, at shorter Dts the quality of the film comes out to be better (Kar et al., 2007). Apart from the electric charges, it is observed that better crystallinity posses AlN films of higher dielectric constant.
