**10. Feasibility study**

As a feasibility study SAW resonators with sputtered AlN film as piezoelectric transducer have been produced. Fig. 10 shows the concept of the fabricated AlN-UNCD layered SAW resonator.

Fig. 10. Schematic Structure of AlN-UNCD layered SAW resonator with golden IDT patterns shaped by photolithography

In the previous chapters it was shown that UNCD films are very suitable as basic material for SAW applications. It was shown that the addition of hydrogen on the one hand improves the elastic constants (towards the value of diamond single crystals), and on the other hand increases the roughness (to values of microcrystalline diamond films), which leads to large propagation loss. Thus a compromise has to be made. The process parameters used for this feasibility study are given in table 2.


Table 2. Deposition conditions

contribution of amorphous matrix and the fact that the elastic modulus of the amorphous matrix is significantly lower than the modulus of the diamond grains. While the elastic modulus for diamond is around 1220 GPa the elastic modulus of the deposited UNCD films

The influence of the nitrogen admixture on the elastic modulus of the deposited films was

The films that were deposited with additional nitrogen are less stiff compared to films where no additional nitrogen was used. The elastic modulus of the UNCD films deposited with 2.5 % nitrogen in the plasma was measured to be around 370 GPa and increasing the nitrogen admixture even higher to 7.5 % in the plasma resulted in UNCD films with values for the elastic modulus as low as 100 GPa. Thus it was shown that UNCD films deposited

An opposite trend can be found when oxygen is used as admixture to the process gas. It was shown that the Young's modulus can be increased up to 950 GPa (ca. 75 % of single crystalline diamond). The reason can be found in the effective etching of sp2-bonded carbon

As a feasibility study SAW resonators with sputtered AlN film as piezoelectric transducer have been produced. Fig. 10 shows the concept of the fabricated AlN-UNCD layered SAW

Fig. 10. Schematic Structure of AlN-UNCD layered SAW resonator with golden IDT patterns

In the previous chapters it was shown that UNCD films are very suitable as basic material for SAW applications. It was shown that the addition of hydrogen on the one hand improves the elastic constants (towards the value of diamond single crystals), and on the other hand increases the roughness (to values of microcrystalline diamond films), which leads to large propagation loss. Thus a compromise has to be made. The process parameters

**9. Influence of nitrogen and oxygen on mechanical properties** 

with additional nitrogen are unsuitable for the application as SAW device.

by the oxygen in the plasma and thus bigger diamond crystals (Shen et. al., 2006).

can reach ca. 65 % of this value.

measured by nanoindentation.

**10. Feasibility study** 

shaped by photolithography

used for this feasibility study are given in table 2.

resonator.

In order to induce a surface acoustic wave in the UNCD material, a piezoelectric layer is necessary. AlN was chosen for this feasibility study due to being the material with the highest phase velocity (6700 m/s) among piezoelectric materials (Ishihara et. al., 2002). The applicability of AlN thin films on various CVD diamond substrates was demonstrated before (Chalker et. al., 1999).

AlN is an intrinsic piezoelectric material; the wurtzite structure is thermodynamically stable. Several methods for deposition of AlN-films have been reported e.g. MOCVD (Tsubouchi & Mikoshiba, 1985), MBE (Weaver et. al., 1990) and reactive DC or RF sputtering (Akiyama et. al., 1998)(Karmann et. al., 1997). Reactive sputtering processes have the advantage of low substrate temperatures (Dubois & Muralt, 2001)(Naik et. al., 1999)(Tait & Mirfazli, 2001)(Assouar et. al., 2004). Here, magnetron sputtering processes was chosen, for being a common and reliable industrial process.

However, highly (002) oriented films with smooth surfaces are required. Thus deposition parameters (power, pressure, N2 ratio and substrate temperature) have to be systematically optimized to reach this goal. The influence of oxygen on the film structure was demonstrated before (Vergara et. al., 2004) showing that a low residual gas pressure is crucial for the desired film properties. Therefore a vacuum chamber with turbo molecular pump and a load lock system was used in this work to assure clean conditions. By that, highly oriented AlN films with very smooth surface were deposited on UNCD films that turned out to possess good piezoelectric properties. (Lee et. al., 2007). DC power was 300 W at a pressure of 0.4 Pa and 50 sccm N2 gas flow at 300°C. The film thickness of the AlN films was ca. 3.5 µm and structure, morphology and bonding structure were characterized by X-Ray diffractrometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy (Renishaw, RA100) and NEXAFS in synchrotron technique.

On top of the AlN film a gold film was deposited by sputtering which was shaped by conventional photolithography. The resonator consists of a central IDT with reflectors at each side (Fig. 11).

The produced SAW Resonators were analyzed due to their performance. Thickness of UNCD as well as AlN have been systematically varied (2 µm to 6.2 µm for UNCD, 1.4 µm to 3.5 µm for AlN). It was measured that with increasing thickness of AlN and UNCD films the resonance frequency increases as well and the resonance peak become clearer. The increase of resonance frequency and thus of SAW velocity is due to reduced influence of the low SAW velocity of the Si substrate. The clearer resonance peak means larger coupling coefficient, which is due to the relative thickness of AlN piezoelectric layer increasing.

Furthermore the influence of the IDT pair number on the SAW resonator performance was investigated (100 Pairs to 200 Pairs). It was measured that the resonance frequency and the resonance strength kept almost the same while doubling the IDT pair numbers.

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This feasibility study indicates that the SAW velocity and coupling coefficient only depend on the relative thickness of ALN and UNCD films, but are not affected by IDT pattern.

Fig. 11. Schematic Pattern design of SAW Resonator. The actual device consists of significant more lines
