**6. AlN film based acoustic device**

SAW and BAW are two import types of acoustic wave devices used in RF comunication. These devices can be realized either on a solid substrate/film or through micromachined suspended beam structures. In SAW devices, an elastic wave travels on the surface of a piezoelectric material and displaces the atoms about their equilibrium positions at the interface of piezoelectric film and solid substrate. The neighboring atoms at the interface then produce restoring forces to bring the displaced atoms back to their original positions. SAW can be generated by placing two inter digital transducers (IDT) either sides of the substrate. These IDTs have alternating periodic fingers (Fig. 7). RF signal is applied one of these alternating polarity fingers (IDT) that produces elastic mechanical wave in the substrate. This wave travels along the substrate and also collected by placing another IDT on the piezoelectric material, some distance away from the first IDT. The second IDT collects the RF signal, which can be retransformed into the electric signal. As the elastic-mechanical wave has the speed of acoustic wave, it introduces the delay of signal by 103 order. This is the prime use of SAW device. The periodicity p, (centre-to-centre spacing between neighbouring IDT fingers of same polarity) becomes the wavelength of the acoustic wave, and dictates its frequency f = v/p, where f and v represent acoustic wave frequency and acoustic propagation velocity, respectively.

Fig. 7. Schematic diagram of SAW device

Thin Film Bulk Acoustic Resonator (FBAR) device consisting of a piezoelectric material sandwiched between two electrodes and is acoustically de-coupled from the surrounding medium. FBAR devices, using AlN piezoelectric with thickness ranging from tenth of micrometers to several micrometers, resonate in the cellular bands of cell phones and other wireless applications. On applying voltage across the electrodes, the piezoelectric thin film undergoes a shear deformation, and a BAW resonance occurs in the AlN film due to coherent reflection at the top and bottom boundaries of the metal film or plate electrodes. The frequency of resonance is dependent on the physical structures; hence, desired resonant frequency can be obtained by tailoring physical dimension of the structure. For RF frequency, physical dimension of resonators can be realized by using MEMS technology.

SAW and BAW are two import types of acoustic wave devices used in RF comunication. These devices can be realized either on a solid substrate/film or through micromachined suspended beam structures. In SAW devices, an elastic wave travels on the surface of a piezoelectric material and displaces the atoms about their equilibrium positions at the interface of piezoelectric film and solid substrate. The neighboring atoms at the interface then produce restoring forces to bring the displaced atoms back to their original positions. SAW can be generated by placing two inter digital transducers (IDT) either sides of the substrate. These IDTs have alternating periodic fingers (Fig. 7). RF signal is applied one of these alternating polarity fingers (IDT) that produces elastic mechanical wave in the substrate. This wave travels along the substrate and also collected by placing another IDT on the piezoelectric material, some distance away from the first IDT. The second IDT collects the RF signal, which can be retransformed into the electric signal. As the elastic-mechanical wave has the speed of acoustic wave, it introduces the delay of signal by 103 order. This is the prime use of SAW device. The periodicity p, (centre-to-centre spacing between neighbouring IDT fingers of same polarity) becomes the wavelength of the acoustic wave, and dictates its frequency f = v/p, where f and v represent acoustic wave frequency and

Thin Film Bulk Acoustic Resonator (FBAR) device consisting of a piezoelectric material sandwiched between two electrodes and is acoustically de-coupled from the surrounding medium. FBAR devices, using AlN piezoelectric with thickness ranging from tenth of micrometers to several micrometers, resonate in the cellular bands of cell phones and other wireless applications. On applying voltage across the electrodes, the piezoelectric thin film undergoes a shear deformation, and a BAW resonance occurs in the AlN film due to coherent reflection at the top and bottom boundaries of the metal film or plate electrodes. The frequency of resonance is dependent on the physical structures; hence, desired resonant frequency can be obtained by tailoring physical dimension of the structure. For RF frequency, physical dimension of resonators can be realized by using MEMS technology.

**6. AlN film based acoustic device** 

acoustic propagation velocity, respectively.

Fig. 7. Schematic diagram of SAW device

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 resonator (De Los Santos, 1999).

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

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

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%.

Aluminum Nitride (AlN)

2001)

**(a)** 

suspended microstructures

**SiO2**

Film Based Acoustic Devices: Material Synthesis and Device Fabrication 575

**Property KOH EDP TMAH**  Si etch rate (100), μm/h 150 30-35 40-60 Etch quality high high medium Selectivity (111)/(100) 1:30-100 1:20 1:10-50 Under-etch rate 0.5-1.5 1.4-1.5 0.2-1.7 CMOS compatible no yes yes

Selectivity PECVD SiO2/Si 1:100-300 1:10,000 1:100-1000 Selectivity PECVD SIN/Si 1:10,000 ---------- 1:150-200 Attack of aluminum high medium low with Si Etch stop boron dope boron dope boron dope

**Al** 

**(b)** 

Cavity depth = 125 μm

Toxicity low high low Long-term stability high low medium Cost low high medium Table 2. Characteristics of important wet etchants used for silicon micromachining (French,

**AlN** 

**(c) (d)**

Cavity depth = 75 μm

Cavity depth = 150 μm

Fig. 10. Micrographs of etched silicon (a) AlN/Si, (b) Al/Si, (c) SiO2/Si, (d) AlN based

Time response, due to acoustic frequency alone, is found after the gating out the response due to electromagnetic feed through. For the centre to centre distance between the two IDT's (d) = 7 mm, the main SAW signal centred at time (delay) = d/ VSAW = 7/5058 = 1.384 μs is obtained. The central acoustic frequency (f0) response after the gating out is observed at 84.304 MHz (Fig. 9).

Fig. 9. (a) Optical image of AlN based SAW devices, and (b) Measured response of the SAW device

### **6.2 Fabrication of AlN film based MEMS**

Anisotropic etching of silicon is a key technology for fabrication of various threedimensional structures such as thin membranes and microbridges for MEMS. Generally, anisotropic silicon etching is done by potassium hydroxide (KOH) or ethylenediamine pyrochatechol (EDP) etchant (French, 2001; Ni et al., 2005). Another technique, which is more versatile, CMOS process compatible and nontoxic, provides better selective etching using doped tetramethylammonium hydroxide (TMAH) etchant (Biswas et al., 2006). The characteristics of these three etchants are listed in Table 2. Many AlN and Al based MEMS structures are also isolated from silicon by silicon dioxide. Hence, their selective etching is very important. Diluted tetramethylammonium hydroxide (TMAH, 5 wt %), doped with silicic acid (30.5 g/l) and ammonium persulphate (5.5 g/l), is suitable for CMOS silicon microprocessing. To protect Al and AlN, silicic acid has been chosen instead of pure silicon powder, because silicic acid dissolves quickly in TMAH solution. Ammonium persulphate (AP) is also added to the above-mentioned solution to reduce the surface roughness of etched silicon. The etch rate of silicon in doped TMAH is found to be 50 μm/hour. During silicon etching the Al, AlN and SiO2 films are used as mask layers (Fig. 10). Low etch rates of Al and AlN (18-30 nm/hour) as well as SiO2 (2.5 nm/hour) are found to be suitable for MEMS applications. Probable reason for low etch rate of Al may be the formation of a passivating layer during TMAH etching (Fujitsuka et al., 2004). Dilute TMAH is a well known etchant for AlN film (Kim et al., 2004) and Al as well. But doped TMAH shows significantly lower etch rates for AlN and Al, which are exploited for AlN based suspended microstructures. Fig. 10 (d) depicts suspended Cr/AlN/Cr/SiO2 cantilevers fixed at one end, where in one of the microstructures is lifted up because of the stress (Kar et al., 2009).

Time response, due to acoustic frequency alone, is found after the gating out the response due to electromagnetic feed through. For the centre to centre distance between the two IDT's (d) = 7 mm, the main SAW signal centred at time (delay) = d/ VSAW = 7/5058 = 1.384 μs is obtained. The central acoustic frequency (f0) response after the gating out is observed

Fig. 9. (a) Optical image of AlN based SAW devices, and (b) Measured response of the SAW

Anisotropic etching of silicon is a key technology for fabrication of various threedimensional structures such as thin membranes and microbridges for MEMS. Generally, anisotropic silicon etching is done by potassium hydroxide (KOH) or ethylenediamine pyrochatechol (EDP) etchant (French, 2001; Ni et al., 2005). Another technique, which is more versatile, CMOS process compatible and nontoxic, provides better selective etching using doped tetramethylammonium hydroxide (TMAH) etchant (Biswas et al., 2006). The characteristics of these three etchants are listed in Table 2. Many AlN and Al based MEMS structures are also isolated from silicon by silicon dioxide. Hence, their selective etching is very important. Diluted tetramethylammonium hydroxide (TMAH, 5 wt %), doped with silicic acid (30.5 g/l) and ammonium persulphate (5.5 g/l), is suitable for CMOS silicon microprocessing. To protect Al and AlN, silicic acid has been chosen instead of pure silicon powder, because silicic acid dissolves quickly in TMAH solution. Ammonium persulphate (AP) is also added to the above-mentioned solution to reduce the surface roughness of etched silicon. The etch rate of silicon in doped TMAH is found to be 50 μm/hour. During silicon etching the Al, AlN and SiO2 films are used as mask layers (Fig. 10). Low etch rates of Al and AlN (18-30 nm/hour) as well as SiO2 (2.5 nm/hour) are found to be suitable for MEMS applications. Probable reason for low etch rate of Al may be the formation of a passivating layer during TMAH etching (Fujitsuka et al., 2004). Dilute TMAH is a well known etchant for AlN film (Kim et al., 2004) and Al as well. But doped TMAH shows significantly lower etch rates for AlN and Al, which are exploited for AlN based suspended microstructures. Fig. 10 (d) depicts suspended Cr/AlN/Cr/SiO2 cantilevers fixed at one end, where in one of the microstructures is lifted up because of the stress (Kar et al., 2009).

at 84.304 MHz (Fig. 9).

device

**6.2 Fabrication of AlN film based MEMS** 


Table 2. Characteristics of important wet etchants used for silicon micromachining (French, 2001)

Cavity depth = 75 μm

Cavity depth = 125 μm

Fig. 10. Micrographs of etched silicon (a) AlN/Si, (b) Al/Si, (c) SiO2/Si, (d) AlN based suspended microstructures

Aluminum Nitride (AlN)

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