- Email: [email protected]
diamond SAW devices
Diamond and Related Materials 13 (2004) 581–584
Freestanding CVD diamond elaborated by pulsed-microwave-plasma for ZnOydiamond SAW devices ´ T. Lamara, M. Belmahi*, O. Elmazria, L. Le Brizoual, J. Bougdira, M. Remy, P. Alnot ´ et Applications (CNRS-UMR 7040) Universite´ Henri Poincare´ Nancy I, Laboratoire de Physique des Milieux Ionises ´ Bld des Aiguillettes-BP 239, 54506 Vandoeuvre-Les-Nancy, Cedex, France
Abstract In previous work, the feasibility of developing high performance AlNydiamond surface acoustic wave (SAW) devices has been demonstrated using the unpolished nucleation side of freestanding CVD diamond. This process shows the advantage of avoiding the diamond polishing, which is a tedious and a time-consuming technological step. As alternative, we propose the use of a diamond substrate obtained by two stages of growth in pulsed MPACVD to constitute two superposed layers. The first one was deposited at optimal conditions with 2% CH4 in H2 gas mixture, leading to a high diamond quality. The second diamond layer is used as a support in order to get a freestanding diamond. It has a lower quality because it’s obtained by increasing the CH4 content, thus the growth rate. The first diamond layer combined with piezoelectric film defines the SAW properties. Its thickness is chosen as a function of the acoustic wave penetration depth in order to limit their propagation only in the first diamond layer. Aluminium inter-digital transducers (IDTs) were deposited by conventional contact UV photolithography on the nucleation side of the freestanding diamond. Piezoelectric ZnO film was then deposited by reactive magnetron sputtering to obtain the ZnOyIDT ydiamond structure. Electrical characterization of the SAW device exhibits high filtering properties. The following parameters are obtained with 32 mm wavelength, 3 mm ZnO and 25 mm diamond first layer thickness: acoustic phase velocity: vw s9696 mys, electromechanical coupling coefficient: K 2s0.75%, temperature coefficient of frequency: TCFs29 ppmy8C. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Plasma CVD; Diamond film; Zinc Oxide; Surface roughness; Surface acoustic wave devices
1. Introduction Surface acoustic wave (SAW) devices are critical components for wireless communication systems w1x. The increasing demand for large volume data transmission requires more and more devices operating in high frequencies. The signal processing frequency (f 0) of a SAW device is determined by the phase velocity of the acoustic wave vw and the spatial period l of the interdigital transducers (IDT): f0svw yl. In this sense, specific materials with high acoustic velocities are needed, for high frequency SAW devices manufacturing with superior performances and reduced cost of fabrication. Thanks to its highest surface acoustic *Corresponding author. Tel.: q33-3-83-684-924; fax: q33-3-83684-933. E-mail address: [email protected] (M. Belmahi).
wave phase velocity among the known materials, CVD diamond is one of the most attractive materials for such an application w2x. However, as the SAW is excited in the diamond by electromechanical conversion, a piezoelectric layer such as ZnO or AlN is required and metallic IDT are deposited on the diamond surface. The use of diamond film as substrate for a SAW device requires two important properties: i. The physicochemical quality of the diamond film must be as good as possible to obtain a high acoustic velocity. ii. The surface roughness must be as low as possible to allow the development, by photolithography technique, of the metallic IDT with sub-micronic resolution and to improve the waves propagation. In fact, the rough surface induces wave scattering and high propagation losses w3,4x. The first requirement is already fulfilled in our MPACVD reactor w5x. The second requirement is not easily fulfilled, as the roughness of a CVD diamond
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.075
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conditions have been already optimised as a function of frequency and duty cycle of the pulsed microwave power w5x. ● The second layer is a poor quality diamond layer. The experimental conditions of growth have been chosen in order to increase the growth rate.
Fig. 1. SEM micrographs of diamond surface, (a) nucleation side, (b) growth side.
film is usually as high as many hundred nanometers to several micrometers and increases with the film thickness. We can see on the Fig. 1b a SEM micrograph of the growth side of diamond layer, and as a consequence the dramatically bad deposited IDTs on such a surface (Fig. 2b). Several methods have been proposed to overcome this difficulty: – Mechanical polishing of the diamond surface, this method is currently used but tedious and time consuming w6x. Nevertheless, very interesting results with a central frequency up to 5 or 10 GHz, and a good TCF value are reported when using this technique in case of SiO2 yIDTyZnOydiamondySi structure for instance w7x. – The use of nano-crystalline diamond (NCD) was recently reported by Bi et al. w8x. This possibility seems to be an interesting alternative for very high frequency SAW devices, but the surface resistivity is not suitable for IDT development on the NCD surface directly. – Planarisation, which has been already studied w9x but leads to a modification of the microstructural properties of the film surface. – The use of the nucleation side of a freestanding film. This method is expensive, as the growth of a freestanding diamond film is a long duration process w10,11x.
Knowing that the penetration depth of the acoustic waves is lower than l w8x, a thickness of 25 mm is chosen for the first layer. The second layer permits a self-supported diamond film to be obtained. It acts as a physical support and has no consequence on the acoustic performances for the device. The polycrystalline CVD diamond film is grown on a Si (100) substrate. Seeding the silicon surface with diamond powder in an ultrasonic bath activates the generation of nucleation sites on silicon surface. The microwave plasma reactor is working under the following conditions: average pulsed microwave power: 2.5 kW; pressure: 120 Torr; gas flow: 500 sccm, frequency 715 Hz and duty cycle of 0.7 w5x. In this working condition, the dissociation rate of methane and hydrogen molecules is high enough to enhance the growth rate and ensure a good quality of diamond film. The duration of the first step is 14 h for a 25 mm layer, working with the (H2 yCH4) gas mixture containing 2% of methane. In the second step, the methane content is increased in the gas mixture to 4% inducing a lowering of the diamond quality, but an improvement of the growth rate is f3.6 mmyh. We finally obtain a freestanding bi-layer, whose thickness is higher than 125 mm. In order to use the nucleation side of the film for the deposition of the IDTs and the ZnO layer we have to remove the Si substrate. This is done by chemical etching of silicon in a KOHyH2Oyn-propanol mixture at 80 8C. As can be seen in Fig. 1a, the nucleation side exhibits a flat surface (Rmss40 nm), which is suitable for SAW device requirements (see Fig. 2a). As comparison, image of IDTs developed on the growth side is shown in Fig. 2b. One can note the discontinuity of the fingers due to the rough growth side surface.
We propose in this paper an alternative method, which has the advantages of using the low roughness of the nucleation side and the growth of a freestanding pulsed MPACVD diamond films with a high quality for surface acoustic waves application and a high growth rate to obtain a self supported diamond film. 2. Experimental process In the proposed method the freestanding diamond film is constituted of a double layer: ● The first one deposited on the silicon substrate is a high quality diamond film. The experimental growth
Fig. 2. Optical photography of IDTs (x50) realised (a) on the nucleation side, (b) on the growth side.
T. Lamara et al. / Diamond and Related Materials 13 (2004) 581–584
Fig. 3. Schematic diagram of ZnOyIDTydiamond layered structure; (left) bi-layer diamond on silicon substrate; (right) piezoelectric ZnO film on diamond layer obtained after silicon substrate etching. IDTs are developed at ZnO–diamond interface.
A first SAW device based on layered structure ZnOy IDTydiamond has been realized on the nucleation side of the freestanding diamond film (see Fig. 3). A thin aluminium layer (150 nm) was deposited on the nucleation side by magnetron sputtering. The IDTs with a spatial period ls32 mm were then prepared by a conventional photolithography and aluminium wet etching. The piezoelectric ZnO film with a thickness of 3 mm is deposited by DC reactive magnetron sputtering, using a Zn target (purity 99.99%) and a 70% O2 –30% Ar gas mixture. These parameters have been optimised on silicon and extrapolated to diamond substrate. The diffraction diagram of ZnO deposited on Silicon exhibits the characteristic line of hexagonal ZnO with a (002) orientation perpendicular to the substrate. 3. SAW device characterization The frequency response (insertion loss) of a two-port ZnOyAlydiamond SAW filter, measured by a network analyser (HP8752A), is shown in Fig. 4. One can observe the practical filter performance of this device based on unpolished freestanding diamond substrate: a quite low insertion loss (less than y25 dB) at the central frequency (f 0s303 MHz) and 27 dB of suppression. Note that no particular design of IDT was performed to optimise the frequency response. The present resolution of the IDTs is 8 mm and can be easily reduced to 1 mm; in this case the central frequency of the filter will be in the range of 2.5 GHz. Taking into account the wavelength (ls32 mm) and the central frequency value, the calculated acoustic phase velocity is vwsf 0.ls9696 msy1 which is three times higher than the surface acoustic velocity obtained with SAW based on quartz. The electromechanical coupling coefficient (K 2) was determined by a network analyser from the Smith chart using the method described in w11,12x and the obtained value is K 2s0.75%. The temperature coefficient of frequency (TCF) was also
583
measured and the obtained value is TCFs29 ppmy8C. The results show that the studied structure ZnOypulsed diamond exhibits a good compromise between vw, K 2 and TCF. The extracted velocities are consistent with those obtained by calculations w13x, and commonly used w7,8x. At higher frequencies, the effect of the nucleation surface will have more influence on the SAW response, and consequences on the propagation losses of the acoustic velocities have to be studied. However, the results obtained on nano-crystalline diamond w8x and on very low grain size polished diamond w4x leads to expectation of an improvement of the SAW characteristics. The use of the bias enhanced nucleation method for the surface nucleation leads to limit the contribution of the low quality diamond generally obtained on the first stages of the growth when the seeding method is used to nucleate the silicon surface w14x. Fig. 5 shows the wide frequency band attenuation characteristics from 200 to 700 MHz of the same device. In addition to the previously described peak at 303 MHz, related to mode 1 (the so-called Sezawa mode), we can observe a second intense peak at 237 MHz. The corresponding velocity of this peak attributed to mode 0 is vws7584 msy1. Measured values of velocities are in good agreement with those predicted by simulation w7,15x. In fact, from the depressive curves of velocity as a function of the normalized thickness of ZnO film, the calculated velocities are, respectively, for mode 0 and mode 1. Considering the technological parameters used for our device, the normalized thickness values 2p k=hZnOs0.588 (where ks is the wave number and l hZnO is the ZnO film thickness). Additional peaks with low intensities are also observed at higher frequencies. These peaks could be attributed to higher modes or to superior harmonics.
Fig. 4. Insertion loss vs. ZnOyIDTydiamond SAW filter.
frequency
of
layered
structure
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T. Lamara et al. / Diamond and Related Materials 13 (2004) 581–584
Acknowledgments The authors wish to acknowledge Laurent Bouvot ´ et from Laboratoire de Physique des Milieux Ionises Applications for his helpful contribution during photolithography processing. References
Fig. 5. Wide band attenuation characteristics of ZnOyIDTydiamond filter.
4. Conclusion In this paper an original method is presented for the elaboration of surface acoustic waves devices based on diamond-ZnO layers. The original points are: – The use of the nucleation side of a freestanding diamond film. This method avoids the tedious task of diamond polishing. – The deposition of a thin film of high diamond quality. The thickness of this layer has to be higher than the penetration depth of the acoustic waves. – The deposition of a thick diamond film, with a lower quality, but a higher deposition rate in order to limit the duration and the cost of the process. This layer acts only as a support for the device. We have shown that this rather simple technology can lead to the elaboration of high quality SAW devices combining high SAW velocity and practical electromechanical coupling coefficient.
w1x C.K. Campbell (Ed.), Surface Acoustic Wave Devices for Mobile and Wireless Communications, Academic Press, 1998. w2x S.I. Shikata, Springer series in materials processing, in: B. Dischler, C. Wild (Eds.), Low-Pressure Synthetic Diamond, Springer-Verlag, Berlin Heidelberg, 1998, p. 261. w3x M.B. Assouar., O. Elmazria, L. Le Brizoual, M. Belmahi, P. Alnot, Proceedings of IEEE International Frequency Control Symposium and PDA Exhibition New Orleans, USA, 29–31 May 2002, p. 333. w4x T. Uemura, S. Fujii, H. Kitabayashi, K. Itakura, A. Hachigo, H. Nakahata, S. Shikata, K. Ishibashi, T. Imai, Proceedings of 2002 IEEE Ultrasonic Symposium, 2002, p. 416. w5x T. Lamara, M. Belmahi, J. Bougdira, F. Benedic, ´ ´ G. Henrion, ´ M. Remy, Surf. Coat. Technol. 174–175 (2003) 784. w6x S. Fujii, Y. Seki, K. Yoshida, H. Nakahata, K. Higaki, H. Kitabayashi, S. Shikita, ‘Diamond Wafer for SAW Application’ 1997 IEEE Ultrasonics Symposium (1997) p. 183. w7x H. Nakahata, S. Fujii, K. Higaki, A. Hachigo, H. Kitabayashi, S. Shikata, et al., Semicond. Sci. Technol. 18 (2003) S96–S104. w8x B. Bi, W.-S. Huang, J. Asmussen, B. Golding, Diamond Relat. Mater. 11 (2002) 677. w9x A.P. Malshe, B.S. Park, W.D. Brown, H.A. Naseem, Diamond Relat. Mater. 8y7 (1999) 1198. w10x V. Mortet, O. Elmazria, M. Nesladek, M.B. Assouar, G. Vanhoyland, J. D’Haen, et al., Appl. Phys. Lett. 81 (2002) 1720. w11x O. Elmazria, V. Mortet, M. El Hakiki, M. Nesladek, P. Alnot, IEEE Trans. Ultrason. Ferroelect. Freq. Control 50 (2003) 710. w12x H. Nakahata, K. Higaki, A. Hachigo, S. Shikata, N. Fujimori, Y. Takahashi, T. Kajihara, Y. Yamamoto, Jpn. J. Appl. Phys. 33 (1994) 324. w13x H. Nakahata, K. Higaki, S. Fujii, A. Hachigo, H. Kitabayashi, K. Tanabe, et al., IEEE Ultrason. Symp. (1995) 361. w14x V. Mortet, O. Elmazria, M. Nesladek, ´ M. Elhakiki, G. Vanhoyland, J. D’Haen, et al., Phys. Stat. Solidi A 199 (2003) 145. w15x F. Jungnickel, H.-J. Frohlich, ¨ Proceedings of the 12th European Conference Eurosensors XII, Sep. 1998, pp. 89–92.
Freestanding CVD diamond elaborated by pulsed-microwave-plasma for ZnOydiamond SAW devices ´ T. Lamara, M. Belmahi*, O. Elmazria, L. Le Brizoual, J. Bougdira, M. Remy, P. Alnot ´ et Applications (CNRS-UMR 7040) Universite´ Henri Poincare´ Nancy I, Laboratoire de Physique des Milieux Ionises ´ Bld des Aiguillettes-BP 239, 54506 Vandoeuvre-Les-Nancy, Cedex, France
Abstract In previous work, the feasibility of developing high performance AlNydiamond surface acoustic wave (SAW) devices has been demonstrated using the unpolished nucleation side of freestanding CVD diamond. This process shows the advantage of avoiding the diamond polishing, which is a tedious and a time-consuming technological step. As alternative, we propose the use of a diamond substrate obtained by two stages of growth in pulsed MPACVD to constitute two superposed layers. The first one was deposited at optimal conditions with 2% CH4 in H2 gas mixture, leading to a high diamond quality. The second diamond layer is used as a support in order to get a freestanding diamond. It has a lower quality because it’s obtained by increasing the CH4 content, thus the growth rate. The first diamond layer combined with piezoelectric film defines the SAW properties. Its thickness is chosen as a function of the acoustic wave penetration depth in order to limit their propagation only in the first diamond layer. Aluminium inter-digital transducers (IDTs) were deposited by conventional contact UV photolithography on the nucleation side of the freestanding diamond. Piezoelectric ZnO film was then deposited by reactive magnetron sputtering to obtain the ZnOyIDT ydiamond structure. Electrical characterization of the SAW device exhibits high filtering properties. The following parameters are obtained with 32 mm wavelength, 3 mm ZnO and 25 mm diamond first layer thickness: acoustic phase velocity: vw s9696 mys, electromechanical coupling coefficient: K 2s0.75%, temperature coefficient of frequency: TCFs29 ppmy8C. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Plasma CVD; Diamond film; Zinc Oxide; Surface roughness; Surface acoustic wave devices
1. Introduction Surface acoustic wave (SAW) devices are critical components for wireless communication systems w1x. The increasing demand for large volume data transmission requires more and more devices operating in high frequencies. The signal processing frequency (f 0) of a SAW device is determined by the phase velocity of the acoustic wave vw and the spatial period l of the interdigital transducers (IDT): f0svw yl. In this sense, specific materials with high acoustic velocities are needed, for high frequency SAW devices manufacturing with superior performances and reduced cost of fabrication. Thanks to its highest surface acoustic *Corresponding author. Tel.: q33-3-83-684-924; fax: q33-3-83684-933. E-mail address: [email protected] (M. Belmahi).
wave phase velocity among the known materials, CVD diamond is one of the most attractive materials for such an application w2x. However, as the SAW is excited in the diamond by electromechanical conversion, a piezoelectric layer such as ZnO or AlN is required and metallic IDT are deposited on the diamond surface. The use of diamond film as substrate for a SAW device requires two important properties: i. The physicochemical quality of the diamond film must be as good as possible to obtain a high acoustic velocity. ii. The surface roughness must be as low as possible to allow the development, by photolithography technique, of the metallic IDT with sub-micronic resolution and to improve the waves propagation. In fact, the rough surface induces wave scattering and high propagation losses w3,4x. The first requirement is already fulfilled in our MPACVD reactor w5x. The second requirement is not easily fulfilled, as the roughness of a CVD diamond
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.075
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T. Lamara et al. / Diamond and Related Materials 13 (2004) 581–584
conditions have been already optimised as a function of frequency and duty cycle of the pulsed microwave power w5x. ● The second layer is a poor quality diamond layer. The experimental conditions of growth have been chosen in order to increase the growth rate.
Fig. 1. SEM micrographs of diamond surface, (a) nucleation side, (b) growth side.
film is usually as high as many hundred nanometers to several micrometers and increases with the film thickness. We can see on the Fig. 1b a SEM micrograph of the growth side of diamond layer, and as a consequence the dramatically bad deposited IDTs on such a surface (Fig. 2b). Several methods have been proposed to overcome this difficulty: – Mechanical polishing of the diamond surface, this method is currently used but tedious and time consuming w6x. Nevertheless, very interesting results with a central frequency up to 5 or 10 GHz, and a good TCF value are reported when using this technique in case of SiO2 yIDTyZnOydiamondySi structure for instance w7x. – The use of nano-crystalline diamond (NCD) was recently reported by Bi et al. w8x. This possibility seems to be an interesting alternative for very high frequency SAW devices, but the surface resistivity is not suitable for IDT development on the NCD surface directly. – Planarisation, which has been already studied w9x but leads to a modification of the microstructural properties of the film surface. – The use of the nucleation side of a freestanding film. This method is expensive, as the growth of a freestanding diamond film is a long duration process w10,11x.
Knowing that the penetration depth of the acoustic waves is lower than l w8x, a thickness of 25 mm is chosen for the first layer. The second layer permits a self-supported diamond film to be obtained. It acts as a physical support and has no consequence on the acoustic performances for the device. The polycrystalline CVD diamond film is grown on a Si (100) substrate. Seeding the silicon surface with diamond powder in an ultrasonic bath activates the generation of nucleation sites on silicon surface. The microwave plasma reactor is working under the following conditions: average pulsed microwave power: 2.5 kW; pressure: 120 Torr; gas flow: 500 sccm, frequency 715 Hz and duty cycle of 0.7 w5x. In this working condition, the dissociation rate of methane and hydrogen molecules is high enough to enhance the growth rate and ensure a good quality of diamond film. The duration of the first step is 14 h for a 25 mm layer, working with the (H2 yCH4) gas mixture containing 2% of methane. In the second step, the methane content is increased in the gas mixture to 4% inducing a lowering of the diamond quality, but an improvement of the growth rate is f3.6 mmyh. We finally obtain a freestanding bi-layer, whose thickness is higher than 125 mm. In order to use the nucleation side of the film for the deposition of the IDTs and the ZnO layer we have to remove the Si substrate. This is done by chemical etching of silicon in a KOHyH2Oyn-propanol mixture at 80 8C. As can be seen in Fig. 1a, the nucleation side exhibits a flat surface (Rmss40 nm), which is suitable for SAW device requirements (see Fig. 2a). As comparison, image of IDTs developed on the growth side is shown in Fig. 2b. One can note the discontinuity of the fingers due to the rough growth side surface.
We propose in this paper an alternative method, which has the advantages of using the low roughness of the nucleation side and the growth of a freestanding pulsed MPACVD diamond films with a high quality for surface acoustic waves application and a high growth rate to obtain a self supported diamond film. 2. Experimental process In the proposed method the freestanding diamond film is constituted of a double layer: ● The first one deposited on the silicon substrate is a high quality diamond film. The experimental growth
Fig. 2. Optical photography of IDTs (x50) realised (a) on the nucleation side, (b) on the growth side.
T. Lamara et al. / Diamond and Related Materials 13 (2004) 581–584
Fig. 3. Schematic diagram of ZnOyIDTydiamond layered structure; (left) bi-layer diamond on silicon substrate; (right) piezoelectric ZnO film on diamond layer obtained after silicon substrate etching. IDTs are developed at ZnO–diamond interface.
A first SAW device based on layered structure ZnOy IDTydiamond has been realized on the nucleation side of the freestanding diamond film (see Fig. 3). A thin aluminium layer (150 nm) was deposited on the nucleation side by magnetron sputtering. The IDTs with a spatial period ls32 mm were then prepared by a conventional photolithography and aluminium wet etching. The piezoelectric ZnO film with a thickness of 3 mm is deposited by DC reactive magnetron sputtering, using a Zn target (purity 99.99%) and a 70% O2 –30% Ar gas mixture. These parameters have been optimised on silicon and extrapolated to diamond substrate. The diffraction diagram of ZnO deposited on Silicon exhibits the characteristic line of hexagonal ZnO with a (002) orientation perpendicular to the substrate. 3. SAW device characterization The frequency response (insertion loss) of a two-port ZnOyAlydiamond SAW filter, measured by a network analyser (HP8752A), is shown in Fig. 4. One can observe the practical filter performance of this device based on unpolished freestanding diamond substrate: a quite low insertion loss (less than y25 dB) at the central frequency (f 0s303 MHz) and 27 dB of suppression. Note that no particular design of IDT was performed to optimise the frequency response. The present resolution of the IDTs is 8 mm and can be easily reduced to 1 mm; in this case the central frequency of the filter will be in the range of 2.5 GHz. Taking into account the wavelength (ls32 mm) and the central frequency value, the calculated acoustic phase velocity is vwsf 0.ls9696 msy1 which is three times higher than the surface acoustic velocity obtained with SAW based on quartz. The electromechanical coupling coefficient (K 2) was determined by a network analyser from the Smith chart using the method described in w11,12x and the obtained value is K 2s0.75%. The temperature coefficient of frequency (TCF) was also
583
measured and the obtained value is TCFs29 ppmy8C. The results show that the studied structure ZnOypulsed diamond exhibits a good compromise between vw, K 2 and TCF. The extracted velocities are consistent with those obtained by calculations w13x, and commonly used w7,8x. At higher frequencies, the effect of the nucleation surface will have more influence on the SAW response, and consequences on the propagation losses of the acoustic velocities have to be studied. However, the results obtained on nano-crystalline diamond w8x and on very low grain size polished diamond w4x leads to expectation of an improvement of the SAW characteristics. The use of the bias enhanced nucleation method for the surface nucleation leads to limit the contribution of the low quality diamond generally obtained on the first stages of the growth when the seeding method is used to nucleate the silicon surface w14x. Fig. 5 shows the wide frequency band attenuation characteristics from 200 to 700 MHz of the same device. In addition to the previously described peak at 303 MHz, related to mode 1 (the so-called Sezawa mode), we can observe a second intense peak at 237 MHz. The corresponding velocity of this peak attributed to mode 0 is vws7584 msy1. Measured values of velocities are in good agreement with those predicted by simulation w7,15x. In fact, from the depressive curves of velocity as a function of the normalized thickness of ZnO film, the calculated velocities are, respectively, for mode 0 and mode 1. Considering the technological parameters used for our device, the normalized thickness values 2p k=hZnOs0.588 (where ks is the wave number and l hZnO is the ZnO film thickness). Additional peaks with low intensities are also observed at higher frequencies. These peaks could be attributed to higher modes or to superior harmonics.
Fig. 4. Insertion loss vs. ZnOyIDTydiamond SAW filter.
frequency
of
layered
structure
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T. Lamara et al. / Diamond and Related Materials 13 (2004) 581–584
Acknowledgments The authors wish to acknowledge Laurent Bouvot ´ et from Laboratoire de Physique des Milieux Ionises Applications for his helpful contribution during photolithography processing. References
Fig. 5. Wide band attenuation characteristics of ZnOyIDTydiamond filter.
4. Conclusion In this paper an original method is presented for the elaboration of surface acoustic waves devices based on diamond-ZnO layers. The original points are: – The use of the nucleation side of a freestanding diamond film. This method avoids the tedious task of diamond polishing. – The deposition of a thin film of high diamond quality. The thickness of this layer has to be higher than the penetration depth of the acoustic waves. – The deposition of a thick diamond film, with a lower quality, but a higher deposition rate in order to limit the duration and the cost of the process. This layer acts only as a support for the device. We have shown that this rather simple technology can lead to the elaboration of high quality SAW devices combining high SAW velocity and practical electromechanical coupling coefficient.
w1x C.K. Campbell (Ed.), Surface Acoustic Wave Devices for Mobile and Wireless Communications, Academic Press, 1998. w2x S.I. Shikata, Springer series in materials processing, in: B. Dischler, C. Wild (Eds.), Low-Pressure Synthetic Diamond, Springer-Verlag, Berlin Heidelberg, 1998, p. 261. w3x M.B. Assouar., O. Elmazria, L. Le Brizoual, M. Belmahi, P. Alnot, Proceedings of IEEE International Frequency Control Symposium and PDA Exhibition New Orleans, USA, 29–31 May 2002, p. 333. w4x T. Uemura, S. Fujii, H. Kitabayashi, K. Itakura, A. Hachigo, H. Nakahata, S. Shikata, K. Ishibashi, T. Imai, Proceedings of 2002 IEEE Ultrasonic Symposium, 2002, p. 416. w5x T. Lamara, M. Belmahi, J. Bougdira, F. Benedic, ´ ´ G. Henrion, ´ M. Remy, Surf. Coat. Technol. 174–175 (2003) 784. w6x S. Fujii, Y. Seki, K. Yoshida, H. Nakahata, K. Higaki, H. Kitabayashi, S. Shikita, ‘Diamond Wafer for SAW Application’ 1997 IEEE Ultrasonics Symposium (1997) p. 183. w7x H. Nakahata, S. Fujii, K. Higaki, A. Hachigo, H. Kitabayashi, S. Shikata, et al., Semicond. Sci. Technol. 18 (2003) S96–S104. w8x B. Bi, W.-S. Huang, J. Asmussen, B. Golding, Diamond Relat. Mater. 11 (2002) 677. w9x A.P. Malshe, B.S. Park, W.D. Brown, H.A. Naseem, Diamond Relat. Mater. 8y7 (1999) 1198. w10x V. Mortet, O. Elmazria, M. Nesladek, M.B. Assouar, G. Vanhoyland, J. D’Haen, et al., Appl. Phys. Lett. 81 (2002) 1720. w11x O. Elmazria, V. Mortet, M. El Hakiki, M. Nesladek, P. Alnot, IEEE Trans. Ultrason. Ferroelect. Freq. Control 50 (2003) 710. w12x H. Nakahata, K. Higaki, A. Hachigo, S. Shikata, N. Fujimori, Y. Takahashi, T. Kajihara, Y. Yamamoto, Jpn. J. Appl. Phys. 33 (1994) 324. w13x H. Nakahata, K. Higaki, S. Fujii, A. Hachigo, H. Kitabayashi, K. Tanabe, et al., IEEE Ultrason. Symp. (1995) 361. w14x V. Mortet, O. Elmazria, M. Nesladek, ´ M. Elhakiki, G. Vanhoyland, J. D’Haen, et al., Phys. Stat. Solidi A 199 (2003) 145. w15x F. Jungnickel, H.-J. Frohlich, ¨ Proceedings of the 12th European Conference Eurosensors XII, Sep. 1998, pp. 89–92.