**8. SAW pulse technique**

554 Acoustic Waves – From Microdevices to Helioseismology

Fig. 4. RMS-roughness measurements as a function of hydrogen admixture in process gas

An especially appealing field of application for UNCD is nitrogen doped semiconducting films. UNCD films are usually insulating, but n-doping is easily possible by admixture of

To investigate the influence of the nitrogen admixture in the plasma on the film properties, more films were deposited at a pressure of 200 mbar with admixtures of nitrogen from 0 %

Fig. 5. High resolution SEM measurement of a UNCD film deposited with 2.5 % hydrogen

and 2.5 % nitrogen admixture. The scale bar shown corresponds to 1 µm

**7. Influence of nitrogen admixture on morphology** 

nitrogen to the process gas. (Gruen, 2004)

to 7.5 %.

The low surface roughness of UNCD films on the one hand and the high speed of sound in single crystalline diamond on the other hand are making UNCD a very promising material for SAW application. Yet the decisive question is whether the abundance of grain boundaries in the films or the amorphous matrix surrounding the grains will change this picture by e. g. damping the excellent propagation characteristics of surface acoustics waves. The laser-induced SAW pulse method is capable of measuring the SAW-related (i.e. mechanical and structural) properties of thin films (Weihnacht et. al, 1997) (Schenk et. al., 2001) and was used in this work. The applicability of this method for investigating the film properties of polycrystalline diamond films was demonstrated in previous publications (Lehmann et. al., 2001). This method allows measuring all necessary material constants and the wave excitation and propagation parameters decisive for the performance of the SAW material. The biggest advantage of this method is, that it is not necessary to prepare a piezoelectric layer or patterning an interdigital transducer (IDT) structure on the surface, and that rather thin films can also be measured without being disturbed by effects from the Si substrate. The method is a fast and accurate way to measure acoustic wave propagation

Ultrananocrystalline Diamond as Material for Surface Acoustic Wave Devices 557

Beyond that the damping of the amplitude spectra with increasing propagation length can deliver an estimation of SAW propagation losses due to scattering at thin film imperfections.

As expected the elastic modulus is higher (the material is stiffer) for higher admixtures of hydrogen (Fig. 9). This can be explained by the larger diamond crystals and a smaller

Fig. 8. Measured velocity dispersion and fitted data

Fig. 9. E-modulus as a function of hydrogen admixture

effects in thin film systems (Schneider et. al. 1997). Pioneering work on utilizing surface acoustic waves as a tool in material science has been done by P. Hess, a general overview can be found in (Hess, 2002).

A somewhat different setup has been used in this work and is schematically shown in Fig. 7. This setup is commercially available at Fraunhofer IWS Dresden1.

A pulsed laser beam (N2-laser at 337.1 nm, 0.5 ns pulse duration) is focused on the substrate by a cylindrical lens to excite a line-shaped broadband SAW pulse via a thermo-elastic mechanism. A piezoelectric PVDF polymer foil, pressed onto the sample surface by a sharp steel wedge (width around 5 µm), is used as a broadband sensor for detecting the SAW pulse propagated along the surface of the thin film system. SAW propagation measurements are performed for different propagation lengths between a few mm and some cm. The signal will then be amplified, digitized by an oscilloscope and converted to complex valued spectra (i.e. amplitude and phase spectra) by a fast Fourier transform algorithm. By doing so for different well-known propagation lengths on the one hand the SAW phase velocity dispersion can be determined accurately from the accompanying phase spectra. Knowledge of the velocity dispersion of a film system is decisive, because it gives the possibility to recover the materials parameters (e.g. elastic constants, mass density and film thickness). To derive the elastic properties, a theoretical approach, modeling the films as an isotropic layer but taking into account the anisotropy of the silicon substrate, was fitted to the measured dispersion data. The fact that we have a specimen that consists of a film on top of a substrate introduces a length scale, and thus generates the observed dispersion effect from that the elastic and mechanical properties can be derived.

### Fig. 7. Principle of SAW pulse technique

A measurement of the SAW phase velocity as a function of frequency as well as the fitted data is shown in Fig. 8. The phase velocity increases with frequency in the case of diamond on silicon substrate ('anomalous dispersion' or 'stiffening case'), because the smaller wavelengths, propagating predominantly in the film, have higher phase velocity.

<sup>1</sup> LAWave**®** (http://www.iws.fhg.de/projekte/062/e\_pro062.html)

effects in thin film systems (Schneider et. al. 1997). Pioneering work on utilizing surface acoustic waves as a tool in material science has been done by P. Hess, a general overview

A somewhat different setup has been used in this work and is schematically shown in Fig. 7.

A pulsed laser beam (N2-laser at 337.1 nm, 0.5 ns pulse duration) is focused on the substrate by a cylindrical lens to excite a line-shaped broadband SAW pulse via a thermo-elastic mechanism. A piezoelectric PVDF polymer foil, pressed onto the sample surface by a sharp steel wedge (width around 5 µm), is used as a broadband sensor for detecting the SAW pulse propagated along the surface of the thin film system. SAW propagation measurements are performed for different propagation lengths between a few mm and some cm. The signal will then be amplified, digitized by an oscilloscope and converted to complex valued spectra (i.e. amplitude and phase spectra) by a fast Fourier transform algorithm. By doing so for different well-known propagation lengths on the one hand the SAW phase velocity dispersion can be determined accurately from the accompanying phase spectra. Knowledge of the velocity dispersion of a film system is decisive, because it gives the possibility to recover the materials parameters (e.g. elastic constants, mass density and film thickness). To derive the elastic properties, a theoretical approach, modeling the films as an isotropic layer but taking into account the anisotropy of the silicon substrate, was fitted to the measured dispersion data. The fact that we have a specimen that consists of a film on top of a substrate introduces a length scale, and thus generates the observed dispersion effect from that the

A measurement of the SAW phase velocity as a function of frequency as well as the fitted data is shown in Fig. 8. The phase velocity increases with frequency in the case of diamond on silicon substrate ('anomalous dispersion' or 'stiffening case'), because the smaller

wavelengths, propagating predominantly in the film, have higher phase velocity.

1 LAWave**®** (http://www.iws.fhg.de/projekte/062/e\_pro062.html)

This setup is commercially available at Fraunhofer IWS Dresden1.

elastic and mechanical properties can be derived.

Fig. 7. Principle of SAW pulse technique

can be found in (Hess, 2002).

Fig. 8. Measured velocity dispersion and fitted data

Beyond that the damping of the amplitude spectra with increasing propagation length can deliver an estimation of SAW propagation losses due to scattering at thin film imperfections.

Fig. 9. E-modulus as a function of hydrogen admixture

As expected the elastic modulus is higher (the material is stiffer) for higher admixtures of hydrogen (Fig. 9). This can be explained by the larger diamond crystals and a smaller

Ultrananocrystalline Diamond as Material for Surface Acoustic Wave Devices 559

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

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

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

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.

Pressure 240 mbar Gasflow 400 sccm H2 fraction 2 % CH4 fraction 0,8 % Ar fraction 97,2 % MW-power 1 kW

Table 2. Deposition conditions

before (Chalker et. al., 1999).

each side (Fig. 11).

common and reliable industrial process.

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 can reach ca. 65 % of this value.
