Super-High-Frequency SAW Resonators on AlN/Diamond (original) (raw)
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Optimization of AlN thin layers on diamond substrates for high frequency SAW resonators
Materials Letters, 2012
AlN/diamond heterostructures are very promising for high frequency surface acoustic wave (SAW) resonators. In their design, the thickness of the piezoelectric film is one of the key parameters. On the other hand, the film material quality and, hence, the device performance, also depend on that thickness. In this work, polished microcrystalline diamond substrates have been used to deposit AlN films by reactive sputtering, from 150 nm up to 3 μm thick. A high degree of the c-axis orientation has been obtained in all cases. SAW one port resonators at high frequency have been fabricated on these films with a proper combination of the film thickness and transducer size.
Microelectronic Engineering, 2013
Due to its mechanical properties, diamond is very attractive as an active material for the fabrication of SAW resonators for high frequency applications. In this work, the synthesis of piezoelectric AlN films by reactive sputtering at room temperature has been optimized on thick diamond layers grown on Si and alumina substrates in order to process high frequency devices. The effect of diamond underlayer microstructure is evaluated by TEM. Two sets of samples are studied, AlN/NCD/Alumina and AlN/MCD/ Si. The orientation of the AlN grains is shown to improve with the film thickness and the diamond grain size. For NCD underlayer, the AlN deposited on top is more oriented. Moreover, above 1 lm from the AlN/ diamond interface, a high degree of the c-axis orientation (perpendicular to the AlN/diamond interface) is demonstrated even though two different grain lattice orientations are shown to coexist: one with the ð2 1 1 0Þ planes remaining parallel to the TEM-preparation lamella and the other with ð0 1 1 0Þ planes. The AlN/diamond interface is smooth down to the nm-scale.
Very high frequency surface acoustic wave SAW devices based on AlN/diamond layered structures were fabricated by direct writing using e-beam lithography on the nucleation side of chemical vapor deposition diamond. The interdigital transducers made in aluminum with resolutions down to 500 nm were patterned on AlN/diamond layered structure with an adapted technological process. Experimental results show that the Rayleigh wave and the higher modes are generated. The fundamental frequency around 5 GHz was obtained for this layered structure SAW device and agrees well with calculated results from dispersion curves of propagation velocity and electromechanical coupling coefficient. Because of the increasing volume of information, the demand for larger band pass surface acoustic wave SAW filters at high frequency has become important. 1 The highest frequency devices are expected from the diamond substrate/ piezoelectric material thin film layered structures because diamond exhibits the highest acoustic wave velocity among all materials. 2 Aluminum nitride is one of the most promising materials for high frequency SAW devices because of its very high acoustic velocity among piezoelectric materials and its fairly large piezoelectric coupling coefficient along the c axis. 3 In previous work, we have demonstrated experimentally that the structure combining AlN piezoelectric film and freestanding diamond layer exhibits a phase velocity around 12 km/ s three times higher than that of conventional piezoelectric material such as quartz. 4,5 However, the operating frequencies of the realized devices remain under 2 GHz due the limitation of photolithography resolution. In this work, the high velocity of diamond is combined with the fine resolution of the electron beam lithography EBL to achieve SAW devices operating at 5 GHz. We have used electron beam lithography for rapid and versatile prototyping on small substrates to obtain interdigital transducer IDT structures in the submicron and nanometer ranges. We present here the very high frequency SAW devices based on AIN/ diamond structures realized using e-beam lithography combined with lift-off technique. The achievement of such device is not trivial. In fact, AIN/diamond layered structures present a high electrical resistivity, involving charge accumulation on the top surface by electrons injected during the e-beam process, and consequently, the quasi-impossibility to pattern the IDTs. The solution to this problem will be presented. Polycrystalline diamond films were grown on silicon wafers by plasma enhanced chemical vapor deposition PECVD, using an ASTeX reactor operating at a microwave power of 6000 W and a pressure of 163 103 Pa, with 3% volume mixture of CH4 in H 2. 6 After deposition, the poly-crystalline diamond layer was removed from the silicon sub-strate by a wet chemical etching in a HNA solution HF : HNO 3 :CH 3 COOH, leading to a freestanding diamond layer with a flat surface at the nucleation side. The nucleation side of the freestanding diamond surface is smooth with an average root-mean-square roughness R rms of 15 nm, determined by the nucleation density. AlN thin films were prepared by reactive rf magnetron sputtering on CVD diamond substrates. The growth experimental conditions were published elsewhere. 7,8 The growth rate is 0.5 m / h, and the thickness of films was measured by scanning electron microscopy SEM from the cross section of structure. For the device considered in this study, the AlN thickness was fixed to 1 m. The present sub-micron fabrication process was applied to the fabrication of high frequency SAW devices. As the diamond substrate is electrically rather insulating and in order to overcome the charge accumulation on the top surface of the substrate, a 10 nm layer of aluminum was deposited on the top surface before the resist coating. The electrosen-sitive resist used is a double layer consisting of two different electrosensitive resists: a copolymer consisting of a mixture of polymethyl methacrylate and methacrylic MMA and 950 K polymethyl methacrylate PMMA from Micro Chem. Both resists are spun with the same experimental conditions. The double layer of resist is used to facilitate the lift-off process step afterwards. The lithography was done at an acceleration voltage of 30 kV and a current of 55 pA. The exposure dose for each structure strongly depends on its size a
High Frequency Surface Acoustic Wave Resonators on Silicon
Sensors and Microsystems, 2001
High-frequency and wideband surface acoustic wave (SAW) devices have become an important topic in 5G communication systems and beyond. For this purpose, piezoelectric ScAlN thin films deposited on high SAW velocity diamond are studied in SAW resonators on the polycrystalline diamond (PCD) and hetero-epitaxial diamond (HED) substrates. A very strong c-axis orientation of ScAlN on HED was confirmed. Interdigital transducers of 0.8 μm and 0.5 μm were fabricated to realize 2.2 to 3.5 GHz high-frequency resonators. High Q values were obtained for the 2.3-2.5 GHz device on both PCD and HED. Additionally, a high electro-mechanical coupling coefficient (K 2) of 5.40-5.52% and high SAW velocities of 7400-7766 m/s were obtained in the 2.3-2.5 GHz devices. In comparison to the simulated coupling coefficient results of a previous report, ScAlN/diamond might have higher K 2 for the Sezawa wave.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2013
Diamond has the highest known SAW phase velocity, sufficient for applications in the gigahertz range. However, although numerous studies have demonstrated SAW devices on polycrystalline diamond thin films, all have had much larger propagation loss than single-crystal materials such as LiNbO 3. Hence, in this study, we fabricated and characterized one-port SAW resonators on single-crystal diamond substrates synthesized using a high-pressure and high-temperature method to identify and minimize sources of propagation loss. A series of one-port resonators were fabricated with the interdigital transducer/AlN/diamond structure and their characteristics were measured. The device with the best performance exhibited a resonance frequency f of 5.3 GHz, and the equivalent circuit model gave a quality factor Q of 5509. Thus, a large fQ product of approximately 2.9 × 10 13 was obtained, and the propagation loss was found to be only 0.006 dB/wavelength. These excellent properties are attributed mainly to the reduction of scattering loss in a substrate using a single-crystal diamond, which originated from the grain boundary of diamond and the surface roughness of the AlN thin film and the diamond substrate. These results show that single-crystal diamond SAW resonators have great potential for use in low-noise super-highfrequency oscillators.
Sputter optimization of AlN on diamond substrates for high frequency SAW resonators
Proceedings of the 8th Spanish Conference on Electron Devices, CDE'2011, 2011
The AIN/diamond structure is an attractive combination for SAW devices and its application at high frequencies. In this work, the synthesis of A1N thin films by reactive sputtering has been optimized on diamond substrates in order to process high frequency devices. Polished microcrystalline and as-grown nanocrystalline diamond substrates have been used to deposit A1N of different thickness under equal sputtering conditions. For the smoother substrates, the FWHM of the rocking curve of the (002) A1N peak varies from 3.8° to 2.7° with increasing power. SAW one port resonators have been fabricated on these films, whose electrical characterization (in terms of Sll parameters) is reported.
Very high frequency surface acoustic wave (SAW) devices based on the AlN/diamond layered structure are fabricated by direct writing using e-beam lithography on the nucleation side of nanocrystalline diamond (NCD) films deposited by microwave plasma assisted chemical vapor deposition process. The NCD nucleation side is characterized from the point of view of microstructure, morphology and surface topography. Surface roughness as low as 6 nm is reached, which enhances the deposition of AlN film on this flat surface. The interdigital transducers IDTs made in aluminum with lateral resolution down to 600 nm are successfully patterned on the AlN/NCD layered structure with an adapted technological process. Experimental results show that the Rayleigh wave and the higher mode are generated. A high frequency around 4 GHz (mode 1) is obtained for the considered layered structure SAW device, exhibiting a phase velocity of 9200 m/s taking into account the wavelength of 2.4 μm. This value agrees well with calculated values determined from dispersion curves of phase velocity.
—In this work, we report on the fabrication results of surface acoustic wave (SAW) devices operating at frequencies up to 8 GHz. In previous work, we have shown that high acoustic velocities (9 to 12 km/s) are obtained from the layered AlN/diamond structure. The interdigital transducers (IDTs) made of aluminium with resolutions up to 250 nm were successfully patterned on AlN/diamond-layered structures with an adapted technological process. The uniformity and periodicity of IDTs were confirmed by field emission scanning electron mi-croscopy and atomic force microscopy analyses. A highly oriented (002) piezoelectric aluminum nitride thin film was deposited on the nucleation side of the CVD diamond by magnetron sputtering technique. The X-ray diffraction effectuated on the AlN/diamond-layered structure exhibits high intensity peaks related to the (002) AlN and (111) diamond orientations. According to the calculated dispersion curves of velocity and the electromechanical coupling coefficient (K 2), the AlN layer thickness was chosen in order to combine high velocity and high K 2. Experimental data extracted from the fabricated SAW devices match with theoretical values quite well.
Piezoelectrically excited surface acoustic waves in diamond-based components
2012 International Conference on Mathematical Methods in Electromagnetic Theory, 2012
Polycrystalline diamond film layers are considered as attractive substrates for surface acoustic wave (SAW) devices operating at GHz frequencies because the diamond produces the highest acoustical wave velocities among all other materials. To enable SAW excitation by interdigital transducers, non-piezoelectric diamond layers are covered with thin piezoelectric coatings. In addition to SAW modes, such structures support pseudosurface acoustic waves (PSAWs). Their velocities are even greater than that of any SAW one, however, high attenuation caused by leakage losses (LL) obstructs their effective application. Nevertheless, at certain film-thickness-to-wavelength ratios h/λ the LL might become so small that the PSAW, in fact, degenerates into a high-velocity SAW propagating with practically negligible attenuation. Earlier such optimal ratios were discovered and experimentally verified for the first pseudo-surface (Sezawa) mode excited in a two-layer structure. The present work is focused on the revealing and studying such effects for higher modes as well as for three-layer structures with different diamond-to-piezocoating thickness ratios H/h.