**11. Acknowledgments**

544 Acoustic Waves – From Microdevices to Helioseismology

 **Au TPR - PCFV coating data points Al CRF - PCFV coating data points**

0 2 4 6 8 10 12 14 16 18

**Coating time in minutes** Fig. 19. Frequency downshift vs. deposition time for Au and Al devices electro spray coated

> **Octane 1100 ppm concentration**

**Tetrachloroethylene 1000 ppm concentration** 

**9.4 Gas probing behaviour of PCFV coated Au and Al TPR and CRF RSAW sensors**  To compare the gas sensitivities of Au vs. Al sensors, pairs of devices of the same type according to Fig. 12 were PCFV coated in the same electro spray deposition method and probed with cooling agent, octane and tetrachloroethylene at different concentrations. Cooling agent and octane were among those gases that the sensors were intended to operate with in a specific application. After coating, the Au/Al pairs were selected to have nearly the same loss increase as a result of PCFV deposition to simulate identical mass loading. The gas probing results for 4 different loss increase values (thicknesses) are summarized in Table 9. As expected from the mass sensitivity data in Fig. 19, the Au devices demonstrate higher gas sensitivity than their Al counterparts. Another important conclusion evident from Table 9 is that soft polymer coating also requires an optimum film thickness for maximum gas sensitivity. In this case the optimum PCFV thickness is achieved when both the Au and Al

PCFV coated device→ Al CRF Au TPR Al CRF Au TPR Al CRF Au TPR Al\_8/Au\_8,7 dB 12,5 kHz 15 kHz 13 kHz 15 kHz 23 kHz 25 kHz **Al\_6,2/Au\_5,5 dB 11 kHz 21 kHz 14 kHz 19 kHz 26 kHz 28 kHz**  Al\_4,6/Au\_4,6 dB 8 kHz 11 kHz 9 kHz 18 kHz 17 kHz 27 kHz Al\_3,5/Au\_3,3 dB 6 kHz 8 kHz 11 kHz 13 kHz 17 kHz 17 kHz Table 9. Summary of the gas probing performance of Au vs. Al device pairs PCFV coated to

This chapter has highlighted important practical aspects for the design and operation of chemical gas detection systems using STW and RSAW resonant devices. It has been shown that both acoustic wave modes provide excellent gas sensitivity and low detection limits,

devices are coated to about 6 dB loss increase, (numbers in bold in Table 9).

nearly identical loss increase values in the same electro spray deposition process

**Cooling agent 40000 ppm concentration** 


with soft PCFV film

**Probing gas** → **Loss increase values for each device pair** ↓

**10. Summary and conclusions** 




**Frequency shift in MHz**



0

The author wishes to gratefully acknowledge Dr. E. Radeva from the Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences in Sofia, Bulgaria for expert preparation of the HMDSO films used in this study as well Professor Shigeru Kurosawa and his research associates from the National Institute of Materials Chemistry in Tsukuba, Japan for the deposition of the semisolid ST and AA films. Special thanks are directed to Dr. Michael Rapp and his research team at the Research Centre Karlsruhe in Germany for the opportunity to perform a substantial part of this work at those laboratories.

### **12. References**


**24** 

*Germany* 

**Ultrananocrystalline Diamond as Material for** 

Diamond is one of the most promising materials for SAW applications due to the highest sound velocity and thermal conductivity. Unfortunately single crystals and epitaxial CVDfilms are expensive and beyond that conventional CVD grown microcrystalline diamond films feature large facet structures with high roughness inapplicable for this application. Ultra-Nanocrystalline diamond (UNCD) films grown in a microwave plasma enhanced chemical vapor deposition (MPECVD) system on Si substrates possess a smooth surface making it an ideal material for SAW applications. Moreover, due to its nanocrystalline structure, the film properties of the UNCD material can be tailored in a wide range to adjust them to the specific needs of a SAW filter. For this task a profound understanding of the growth process of UNCD and the dependency of the film performance from the film properties is needed. In addition, a simple and quick method to characterize the properties of the UNCD films is necessary. Laser-induced SAW pulse method, which is fast and accurate, is demonstrated to measure the mechanical and structural properties of the UNCD films. AFM measurements were done to correlate the SAW pulse method results with the

Another advantage of the UNCD films is, that highly C-axis textured aluminum nitride (AlN) films can be grown directly on UNCD films by DC-sputtering. Using this technique a

SAW devices are electromechanical products commonly used in high frequency applications such as filters, oscillators and transformers and are based on the transduction of acoustic waves. SAW filters are now widely used in mobile telephones applications for filtering and provide significant advantages in performance, cost, and size over other filter technologies such as quartz crystals (based on bulk waves), LC filters, and waveguide filters by offering a high degree of frequency selectivity with low insertion loss in compact size (Campbell, 1989). In SAW filters an Interdigital Transducer (IDT) that is attached to a piezoelectric material converts electrical signals to a mechanical wave. The piezoelectric effect and the electric field generated by the IDT are distorting the crystal close to its

feasibility study for SAW devices has been successfully performed.

**1. Introduction** 

**2. Saw filters** 

surface.

surface roughness of the deposited films.

**Surface Acoustic Wave Devices** 

*University Duisburg-Essen and CeNIDE, Duisburg* 

Nicolas Woehrl and Volker Buck

