**3. Synthesis of AlN film**

Depending on the intended application, various techniques have been implemented for synthesizing AlN films; namely, molecular beam epitaxy (MBE), reactive evaporation, pulsed laser deposition (PLD), chemical vapour deposition (CVD) and sputtering. Among these techniques, sputtering has the advantage of low-temperature deposition, ease of synthesis, less expensive, non-toxic, good quality films with a fairly smooth surface [Kar et al., 2006; Kar et al., 2007]. In addition, sputtering technique has also CMOS process compatibility. In sputtering technique, plasma is created between the two electrodes by applying high voltage in low pressure. The plasma region contains, positive ions, electrons and neutral sputtering gas, thus the plasma behaves like a conducting medium. Usually, argon gas is used as a sputtering gas. The material that is to be sputtered is called target and it is fixed to the negatively charged electrode. The other electrode is called anode, which is grounded so that the ratio of the target to anode area is significantly reduced. This electric configuration of the sputtering system makes high electric field at the target and that enhances the rate of sputtering. During sputtering process, the energetic ions strike the target and dislodge (sputter) the target atoms. These dislodged atoms travel through the plasma in a vapour state and stick to the surface of wafers, where they condense and form the film. AlN film can be deposited either by directly using the AlN target or by sputtering of aluminum metal in presence of argon and nitrogen gas. The sputtered aluminum atoms react with the nitrogen gas and form AlN film. This process of film deposition is called "reactive sputtering deposition". The sputtering parameters are required to be optimized for desired morphological and electrical properties. These deposition parameters are mainly sputtering pressure, wafer to target distance, sputtering power and wafer temperature. AlN film deposition by reactive sputter deposition technique requires nitrogen as a reactive gas,

Aluminum Nitride (AlN)

**3.2 Substrate temperature** 

**3.3 Sputtering pressure** 

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The structural and morphological properties of the deposited AlN films are strongly dependent on the kinetics of the sputtered atoms arrived at the substrate. The kinetics of sputtered atoms depends on the sputtering parameters. For instance, substrate temperature increases the ad-atom mobility and changes the film morphology significantly. One such illustrations of morphological change with temperature are seen from the X-Ray diffraction (XRD) studies. It is clearly seen from the XRD studies that the c-axis oriented AlN (002) peaks become prominent at moderate temperature range (200–300 ºC), but degrades significantly at 400 ºC (Kar et al., 2006). This can be attributed to the structural disorderness resulting from the incorporation of impurity atoms at higher temperature (Wang, 2000). The amount of contamination depends on the sputtering deposition system and process related factors, such as base pressure, temperature, gas purity and the partial pressure of moisture, etc. (Naik et al., 1999). Furthermore, smaller grain size with smoother surfaces is observed at lower deposition temperature, and that increases with temperature (Fig. 2). A possible reason may be that the smaller grains grow and merge to form bigger grain, due to the higher thermal energy.

Fig. 2. SEM micrograph of AlN films deposited at (a) 100 ºC, and (b) 400 ºC

The variation in crystal orientation with different sputtering pressure are observed from the XRD studies, where the intensity of (002) orientation increases with the deposition pressure and attained a maximum value at 6×10-3 mbar. On further increase to a deposition pressure of 8×10-3 mbar, the (002) crystal orientation of the AlN film is changed abruptly to the (100) orientation with lesser intensity. The deposited atoms may have altered their direction, energy, momentum and mobility due to the decrease in mean free path of the atoms with sputtering pressure. The hexagonal wurtzite structure of AlN has two kinds of Al–N bond named as B1 and B2. These bonds B1 and B2 together correspond to (110) and (002) planes, where B1 corresponds to (100) plane. The formation energy of B2 is relatively larger that of B1 (Cheng et al., 2003). Hence, the energy required for sputtering species to orient along *c*-axis is larger than the other possible planes. At low pressure, sputtering species possess enough energy to form hexagonal wurtzite crystalline structure on substrate surface. It is also reported that the surface roughness of the film increases with the increase in deposition pressure. The grain size is increased till 6×10-3 mbar deposition pressure and then it reduced to 80 nm at 8×10-3 mbar (Kar et al., 2006). In addition, inhomogeneous patterns on the surface are also observed at this higher pressure (Fig. 3). It is also observed that the AlN film

where it is introduced into the sputtering chamber along with inert argon gas. Argon ions produced in the plasma due to sputtering power and thereafter they strike to the aluminum target and sputter aluminum atoms. These aluminum atoms react with nitrogen and form AlN compound and that deposit on the wafer. Hence, the gas flow ratios need to be optimized. To increase sputtering rate, magnets are placed under the aluminum target, so that magnetic field and the electric field are perpendicular to each other. This configuration of sputtering system is called "magnetron sputtering technique". In the magnetron sputtering, electrons travel in spiral motion in the plasma region. This increases the collision of electrons to neutral argon atoms significantly and that increases argon ions in many folds, thus sputtering rate becomes high.

AlN film can be deposited by DC (direct current) and RF (radio frequency, 13.56 MHz) magnetron sputtering modes. In the DC mode of sputter deposition, the target material must be conductive, so that plasma can sustain. If trace of impurity is present in the system, the surface of the aluminum target becomes contaminated and target poisoning takes place. On the other hand, RF sputtering has the major advantages to produce good quality film, high deposition rate and less chance of target poisoning. For these reasons, RF sputtering technique is preferred than the DC sputtering technique. To obtain well oriented crystalline AlN films for SAW and MEMS structures, the RF sputtering parameters need to be optimized. The sputtering parameters are: RF power, substrate temperature, sputtering pressure, nitrogen concentration and target-substrate distances (Dts). AlN films are deposited on CMOS IC compatibility silicon (100) wafer by the RF reactive magnetron sputtering. The change in morphological and electrical properties of the AlN films with the growth parameters are reported in following section.

### **3.1 RF power**

Amorphous AlN film is found at lower RF sputtering power (100 W), but films became (002) oriented at a sputtering power of 200 W. Further increase of RF power to 400 W, a significant increase in (002) orientation has taken place. This is due to the increase of kinetic energy of atoms that leads to atomic movements on the substrate surface as a result of higher RF power. These newly arrived surface atoms are called "ad-atom". Higher sputtering power increases the AlN grain size that leads to increase in surface roughness as shown in scanning electron microscope (SEM) images (Fig. 1) (Kar et al., 2009).

Fig. 1. SEM micrographs of AlN films deposited at (a) 200 W, and (b) 300 W

## **3.2 Substrate temperature**

566 Acoustic Waves – From Microdevices to Helioseismology

where it is introduced into the sputtering chamber along with inert argon gas. Argon ions produced in the plasma due to sputtering power and thereafter they strike to the aluminum target and sputter aluminum atoms. These aluminum atoms react with nitrogen and form AlN compound and that deposit on the wafer. Hence, the gas flow ratios need to be optimized. To increase sputtering rate, magnets are placed under the aluminum target, so that magnetic field and the electric field are perpendicular to each other. This configuration of sputtering system is called "magnetron sputtering technique". In the magnetron sputtering, electrons travel in spiral motion in the plasma region. This increases the collision of electrons to neutral argon atoms significantly and that increases

AlN film can be deposited by DC (direct current) and RF (radio frequency, 13.56 MHz) magnetron sputtering modes. In the DC mode of sputter deposition, the target material must be conductive, so that plasma can sustain. If trace of impurity is present in the system, the surface of the aluminum target becomes contaminated and target poisoning takes place. On the other hand, RF sputtering has the major advantages to produce good quality film, high deposition rate and less chance of target poisoning. For these reasons, RF sputtering technique is preferred than the DC sputtering technique. To obtain well oriented crystalline AlN films for SAW and MEMS structures, the RF sputtering parameters need to be optimized. The sputtering parameters are: RF power, substrate temperature, sputtering pressure, nitrogen concentration and target-substrate distances (Dts). AlN films are deposited on CMOS IC compatibility silicon (100) wafer by the RF reactive magnetron sputtering. The change in morphological and electrical properties of the AlN films with the

Amorphous AlN film is found at lower RF sputtering power (100 W), but films became (002) oriented at a sputtering power of 200 W. Further increase of RF power to 400 W, a significant increase in (002) orientation has taken place. This is due to the increase of kinetic energy of atoms that leads to atomic movements on the substrate surface as a result of higher RF power. These newly arrived surface atoms are called "ad-atom". Higher sputtering power increases the AlN grain size that leads to increase in surface roughness as

shown in scanning electron microscope (SEM) images (Fig. 1) (Kar et al., 2009).

Fig. 1. SEM micrographs of AlN films deposited at (a) 200 W, and (b) 300 W

argon ions in many folds, thus sputtering rate becomes high.

growth parameters are reported in following section.

**3.1 RF power** 

The structural and morphological properties of the deposited AlN films are strongly dependent on the kinetics of the sputtered atoms arrived at the substrate. The kinetics of sputtered atoms depends on the sputtering parameters. For instance, substrate temperature increases the ad-atom mobility and changes the film morphology significantly. One such illustrations of morphological change with temperature are seen from the X-Ray diffraction (XRD) studies. It is clearly seen from the XRD studies that the c-axis oriented AlN (002) peaks become prominent at moderate temperature range (200–300 ºC), but degrades significantly at 400 ºC (Kar et al., 2006). This can be attributed to the structural disorderness resulting from the incorporation of impurity atoms at higher temperature (Wang, 2000). The amount of contamination depends on the sputtering deposition system and process related factors, such as base pressure, temperature, gas purity and the partial pressure of moisture, etc. (Naik et al., 1999). Furthermore, smaller grain size with smoother surfaces is observed at lower deposition temperature, and that increases with temperature (Fig. 2). A possible reason may be that the smaller grains grow and merge to form bigger grain, due to the higher thermal energy.

Fig. 2. SEM micrograph of AlN films deposited at (a) 100 ºC, and (b) 400 ºC

### **3.3 Sputtering pressure**

The variation in crystal orientation with different sputtering pressure are observed from the XRD studies, where the intensity of (002) orientation increases with the deposition pressure and attained a maximum value at 6×10-3 mbar. On further increase to a deposition pressure of 8×10-3 mbar, the (002) crystal orientation of the AlN film is changed abruptly to the (100) orientation with lesser intensity. The deposited atoms may have altered their direction, energy, momentum and mobility due to the decrease in mean free path of the atoms with sputtering pressure. The hexagonal wurtzite structure of AlN has two kinds of Al–N bond named as B1 and B2. These bonds B1 and B2 together correspond to (110) and (002) planes, where B1 corresponds to (100) plane. The formation energy of B2 is relatively larger that of B1 (Cheng et al., 2003). Hence, the energy required for sputtering species to orient along *c*-axis is larger than the other possible planes. At low pressure, sputtering species possess enough energy to form hexagonal wurtzite crystalline structure on substrate surface. It is also reported that the surface roughness of the film increases with the increase in deposition pressure. The grain size is increased till 6×10-3 mbar deposition pressure and then it reduced to 80 nm at 8×10-3 mbar (Kar et al., 2006). In addition, inhomogeneous patterns on the surface are also observed at this higher pressure (Fig. 3). It is also observed that the AlN film

Aluminum Nitride (AlN)

**3.5 Target-substrate distance** 

grain size and the surface roughness of the film.

**4. Post-deposition annealing effect** 

constant.

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

Film Based Acoustic Devices: Material Synthesis and Device Fabrication 569

nitrogen concentration. In contrast, bigger grain size with increased roughness is observed at lower nitrogen concentration, where the newly formed smaller grain merges together with a previously formed grain and becomes bigger in size. The size and distribution of the micrograins is quite uniform at 80% nitrogen concentration. At lower nitrogen concentrations, Ar+ ions transfer more energy to the Al target during bombardment, generating more aluminum atoms that make clusters with incomplete nitridation of aluminum on the wafer surface. This

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

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

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

leads to formation of fewer bonds, a poor c-axis orientated and a rough film (Fig 4 (a)).

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 film for Dts of 5 cm

### **3.4 Gas flow ratio**

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 nitrogen concentration. In contrast, bigger grain size with increased roughness is observed at lower nitrogen concentration, where the newly formed smaller grain merges together with a previously formed grain and becomes bigger in size. The size and distribution of the micrograins is quite uniform at 80% nitrogen concentration. At lower nitrogen concentrations, Ar+ ions transfer more energy to the Al target during bombardment, generating more aluminum atoms that make clusters with incomplete nitridation of aluminum on the wafer surface. This leads to formation of fewer bonds, a poor c-axis orientated and a rough film (Fig 4 (a)).
