**Aluminum Nitride (AlN) Film Based Acoustic Devices: Material Synthesis and Device Fabrication**

Jyoti Prakash Kar1 and Gouranga Bose2

*1Department of Electronics Engineering, University of Tor Vergata, Rome 2Department of Applied Electronics and Instrumentation Engineering, Institute of Technical Education and Research, Bhubaneswar, Orissa 1Italy* 

### **1. Introduction**

562 Acoustic Waves – From Microdevices to Helioseismology

Yang W. et al., *Nature Materials* 1, (2002) 253257

Enormous growth has taken place in electronics, especially in the field of RF communications towards the beginning of 21st century and continuously striving for better communication performance. Presently, the key concerns of RF communications is bandwidth, in the range of low/medium GHz range, to avoid frequency crowding, especially for wireless communication mobile handsets and base stations (Kim et al., 2004). In addition, reduction in signal loss, low power consumption, scaling down device size, reduction in materials and fabrication costs, and packaging of the device are main issues today. Some of these issues can be resolved, if the new generation of electroacoustic devices can be monolithically integrated with integrated circuit (IC). Conventional electroacoustic devices, used in the communication e.g. Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) based systems, are widely used for today's wireless communication. These devices are typically made on a single crystal piezoelectric substrate such as quartz, lithium niobate, and lithium tantalate (Assouar et al., 2004). Unfortunately, these substrates based electroacoustic devices are made separately and then it is wired with the signal processing chip, which has several limitations, in particular low acoustic wave velocity and high frequency device fabrication. To resolve these two core issues, thin film materials based electroacoustic devices are actively under consideration [Bender et al., 2003]. Where, a crystalline film is grown on a particular substrate, especially silicon wafer and electroacoustic device is made out of crystalline film. Thus, the electroacoustic device can be integrated with the signal processing circuit. Apart from the silicon wafer as a base material for crystalline film deposition, a variety of other substrates are also explored for academic and technology interests. Furthermore, to get electroacoustic devices of better quality in terms of high frequency and high quality factor (Q), the piezoelectric property of the film is also exploited with different type of device concept called "Micro-Electro-Mechanical Systems" (MEMS). Thin film bulk acoustic resonators (TFBAR) comes under this MEMS devices, where the crystalline film is made to resonate at RF frequency. These MEMS

Aluminum Nitride (AlN)

Table 1. Properties of AlN

**3. Synthesis of AlN film** 

Film Based Acoustic Devices: Material Synthesis and Device Fabrication 565

piezoelectricity among all other orientations of its crystal structure (Naik et al., 1999). Furthermore, its lattice matching is near to that of silicon and thus less stress is expected to be generated at the AlN/silicon interface. Owing to these properties, AlN films have received great interest as an electronic material for thermal dissipation, dielectric and

passivation layers for ICs, acoustic devices, resonators and optoelectronic devices.

Bandgap 6.2 eV, direct Thermal conductivity 2.85 Wcm-1K-1 Coefficient of thermal expansion 4-5×10-6 K-1 Refractive index 1.8-2.2 Dielectric constant 8.5

Electrical resistivity 1011-1013 Ω.cm SAW velocity 6000 m/sec Melting point 2490 ºC Hardness 9 Mhos

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,

devices have smaller size, lower insertion loss and higher-power handling capabilities than conventional SAW devices (Lee et al., 2004).

Generally, thin piezoelectric films, such as aluminum nitride (AlN), zinc oxide (ZnO) and lead zirconium titanate (PZT) are used for high frequency acoustic devices (Loebl et al., 2003; Yamada et al., 2004; Schreiter et al. 2004). AlN has higher SAW velocity, lower propagation loss, and higher thermal stability in comparison to ZnO; whereas, PZT thin films need selective substrates for deposition and thereafter, needs post-deposition poling to get specific cystal orientation. Thus, AlN seems to have edge over the ZnO and PZT films for electroacoustic devices. The critical factor of piezoelectric AlN thin film is its crystal orientation and morphology. Furthermore, to integrate with the signal processing chip, it is also essential that AlN film should be compatible to the complementary metal oxide semiconductor (CMOS) fabrication processes. In addition, AlN being a dielectric material, it can be used as an insulating material in integrated circuits as well as a piezoelectric material in electroacoustic device. Thus, it is imperative to study the presence of electrical charges and the nature of generation of defects in the AlN film along with its morphology. Usually, there are four types of electric charges present in the insulating film; namely, bulk charges (Qin) and interface (Dit) charges, fixed charges (Q) and mobile charges (Qm). In present IC processing, the presence of fixed charges (Q) and mobile charges (Qm) are eliminated upto a large extent. Furthermore, the bulk charges (Qin) and interface (Dit) charges are reduced further by the optimization of growth parameter and the post-deposition treatments. Reduction in the bulk charge (Qin) and interface charge (Dit) density is most essential in cantilever beam based MEMS resonator, otherwise the electrostatic force produced by the these charges may stuck cantilever beam on the substrate (Luo et al., 2006). Most of the MEMS are made out of single crystal silicon substrate utilizing well-matured IC fabrication technology. This poses a challenge to be compatible with a new generation of functional materials. Apart from the electrical charges, the selective etching of piezoelectric materials and silicon for electroacoustic device fabrication is a key technology.

### **2. Properties of AlN film**

AlN is a III-V family compound having hexagonal wurtzite crystal structure with lattice constants a = 3.112 Å and c = 4.982 Å (Yim et al., 1973). In this structure, each Al atom is surrounded by four N atoms, forming a distorted tetrahedron with three Al---N(i) (i = 1, 2,3) bonds named B1 and one Al---N0 bond in the direction of the c-axis, named B2. The bond lengths of B1 and B2 are 1.885 Å and 1.917 Å, respectively. The bond angle N0---Al---Ni is 107.7º and that for N1---Al---N2 is 110.5 º (Xu et al., 2001).

AlN has gained ground in semiconductor industry because of its unique electrical, mechanical, piezoelectric and other properties (Table 1). Some of these noteworthy properties are wide bandgap, high thermal conductivity, high SAW velocity, moderately high electromechanical coupling coefficient, high temperature stability, chemical stability to atmospheric gases below 700 ºC, high resistivity, low coefficient of thermal expansion (close to Si), high dielectric constant and mechanical hardness (Xu et al., 2001; Strite et al., 1992; Wang et al., 1994). Its high thermal conductivity (about 100 times that of SiO2 and roughly equal to that of silicon) and electrical insulating property can prove to be a good dielectric layer for a new generation of integrated circuit devices, particularly in metal insulator semiconductor (MIS) devices. High heat dissipation of AlN can significantly enhance device lifetime and efficiency. AlN film with (002) preferred orientation (c-axis) has maximum piezoelectricity among all other orientations of its crystal structure (Naik et al., 1999). Furthermore, its lattice matching is near to that of silicon and thus less stress is expected to be generated at the AlN/silicon interface. Owing to these properties, AlN films have received great interest as an electronic material for thermal dissipation, dielectric and passivation layers for ICs, acoustic devices, resonators and optoelectronic devices.


Table 1. Properties of AlN

564 Acoustic Waves – From Microdevices to Helioseismology

devices have smaller size, lower insertion loss and higher-power handling capabilities than

Generally, thin piezoelectric films, such as aluminum nitride (AlN), zinc oxide (ZnO) and lead zirconium titanate (PZT) are used for high frequency acoustic devices (Loebl et al., 2003; Yamada et al., 2004; Schreiter et al. 2004). AlN has higher SAW velocity, lower propagation loss, and higher thermal stability in comparison to ZnO; whereas, PZT thin films need selective substrates for deposition and thereafter, needs post-deposition poling to get specific cystal orientation. Thus, AlN seems to have edge over the ZnO and PZT films for electroacoustic devices. The critical factor of piezoelectric AlN thin film is its crystal orientation and morphology. Furthermore, to integrate with the signal processing chip, it is also essential that AlN film should be compatible to the complementary metal oxide semiconductor (CMOS) fabrication processes. In addition, AlN being a dielectric material, it can be used as an insulating material in integrated circuits as well as a piezoelectric material in electroacoustic device. Thus, it is imperative to study the presence of electrical charges and the nature of generation of defects in the AlN film along with its morphology. Usually, there are four types of electric charges present in the insulating film; namely, bulk charges (Qin) and interface (Dit) charges, fixed charges (Q) and mobile charges (Qm). In present IC processing, the presence of fixed charges (Q) and mobile charges (Qm) are eliminated upto a large extent. Furthermore, the bulk charges (Qin) and interface (Dit) charges are reduced further by the optimization of growth parameter and the post-deposition treatments. Reduction in the bulk charge (Qin) and interface charge (Dit) density is most essential in cantilever beam based MEMS resonator, otherwise the electrostatic force produced by the these charges may stuck cantilever beam on the substrate (Luo et al., 2006). Most of the MEMS are made out of single crystal silicon substrate utilizing well-matured IC fabrication technology. This poses a challenge to be compatible with a new generation of functional materials. Apart from the electrical charges, the selective etching of piezoelectric materials

and silicon for electroacoustic device fabrication is a key technology.

107.7º and that for N1---Al---N2 is 110.5 º (Xu et al., 2001).

AlN is a III-V family compound having hexagonal wurtzite crystal structure with lattice constants a = 3.112 Å and c = 4.982 Å (Yim et al., 1973). In this structure, each Al atom is surrounded by four N atoms, forming a distorted tetrahedron with three Al---N(i) (i = 1, 2,3) bonds named B1 and one Al---N0 bond in the direction of the c-axis, named B2. The bond lengths of B1 and B2 are 1.885 Å and 1.917 Å, respectively. The bond angle N0---Al---Ni is

AlN has gained ground in semiconductor industry because of its unique electrical, mechanical, piezoelectric and other properties (Table 1). Some of these noteworthy properties are wide bandgap, high thermal conductivity, high SAW velocity, moderately high electromechanical coupling coefficient, high temperature stability, chemical stability to atmospheric gases below 700 ºC, high resistivity, low coefficient of thermal expansion (close to Si), high dielectric constant and mechanical hardness (Xu et al., 2001; Strite et al., 1992; Wang et al., 1994). Its high thermal conductivity (about 100 times that of SiO2 and roughly equal to that of silicon) and electrical insulating property can prove to be a good dielectric layer for a new generation of integrated circuit devices, particularly in metal insulator semiconductor (MIS) devices. High heat dissipation of AlN can significantly enhance device lifetime and efficiency. AlN film with (002) preferred orientation (c-axis) has maximum

**2. Properties of AlN film** 

conventional SAW devices (Lee et al., 2004).
