Complete Bandgap SAW Phononic Resonant Topologies Ventsislav Yantchev SmartCom Bulgaria, AD and Uppsala University Sofia, Bulgaria and Uppsala, Sweden
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[email protected] Abstract— Resonant surface acoustic wave (SAW) structures employing 2D phononic gratings with hexagonal symmetry are micro-fabricated and tested in a proof of principle study. The proposed structures have the advantage of being compatible with the planar SAW technology, while ensuring operation within the complete frequency bandgap of the phononic grating. More specifically, 2D phononic gratings exhibiting a complete frequency bandgap are employed in the design of three distinct types of resonant test structures. The proposed test structures are fabricated on to a 128° YX LiNbO3 substrate by means of low resolution photolithography. Frequency response measurements of the SAW phononic resonators confirmed operation within the complete frequency bandgap of each grating structure. The experimental results obtained demonstrate that surface phononic gratings can be successfully used in the design of SAW resonators. The proposed research paves the way for subsequent implementation of the phononic bandgap technology in high performance micro-acoustic RF components.
achieving sufficiently low loss and steep skirt RF filters. Performance needs a significant boost to meet the requirements of future applications where loss and selectivity are critical. Such applications are expected to increase their market share in the near future due to the increase of communication bands. A closer look at the state-of art technology indicates that the dominating losses in SAW filters are extrinsic stemming from the 1D design constraints imposed by the use of IDTs.
Keywords— SAW, Phononic, Resonator, RF, Grating
I. INTRODUCTION Currently, the mobile market has a significant demand for low insertion loss filters and duplexers with high selectivity and excellent isolation [1]. Reduction of filter losses translates directly into efficient use of bandwidth, higher bit rates, reliable communications, and extended battery life since signal amplification within the communication system is minimized. On the other hand, as more communication bands are standardized for national or global use, and the requirements on filter performance are continuously “tightened,” the demand for filters with improved out-of-band rejection and steeper skirts is unremitting. The current stateof-the-art RF filter technology is based on the microwave electro-acoustic technology due to its small form factor and low fabrication cost while providing excellent performance. The dominating RF filter technology nowadays is the surface acoustic wave (SAW) technology owing to its robustness and design flexibility as well as low cost. The SAW technology employs single crystalline low loss substrates such as Quartz, LiTaO3, or LiNbO3. The main building elements of a SAW resonator (Fig. 1) are the inter-digital transducer (IDT), the reflector gratings (the end gratings in Fig. 1), and the electrical busbars. SAW components have increased their performance in recent years, although the Q factors are still insufficient for
Fig. 1. Sketch view of SAW resonator
Fig. 2. Loss mechanism in 1D designed SAW resonator
Figure 2 illustrates the major loss mechanism in SAW devices caused by the unavoidable acoustic energy radiation through diffraction and other wave phenomena [2]. Increased device aperture, as noted below, reduces such losses but results in increased ohmic losses due to increased finger length. In a classical SAW device, the narrow but long fingers of the IDT conduct the electrical current and are accessible
from one side only. At a typical metallization ratio of 50%, this results in high electrode resistance and hence in deterioration of device performance. In order to reduce the effect of series resistance, a typical SAW transducer must have short fingers which results in an increased IDT length due to impedance requirements. On the other hand, diffractive acoustic leakage (Fig. 2) occurring from the sides of the transducer [2] is proportional to the aspect ratio (length/aperture) of the IDT. Thus, practical designs make a trade-off between aperture (finger length) and transducer length, i.e., between electrical and acoustic losses, respectively. Further, the busbars (see Fig. 1) are also piezoelectrically active but in the transverse direction (i.e., along the device aperture), where generated waves cannot be confined and result in additional acoustic losses [3]. In addition, the shift of the acoustic impedance between the periodic strips and busbars areas, respectively, promotes wave coupling to the busbars as well as losses related to SAW scattering. One possible solution to the above mentioned problems is the use of phononic structures with omnidirectional bandgaps which are thought to suppress wave diffraction as well as to limit the transversal parasitic excitation of the busbars [4]. It is to be noted in advance that employing a 2D reflective structure not only provides new degrees of freedom for the device design [5] but also introduces new effects and problems [6]. Here we show first experimental results on SAW resonators employing the omnidirectional bandgap features of a properly designed phononic grating inherently integrated with the IDT and the reflector structures [7]. Theoretical grounds will be omitted since they have been thoroughly discussed in a recently published study [4]. II. DESIGN AND FABRICATION OF THE RESONANT TOPOLOGIES Three distinct types of resonant structures employing hexagonal phononic gratings are designed and subsequently fabricated and tested.
(a)
1D periodic strip grating. More specifically the distance of the W masses is chosen to be a=10μm determining Al grating pitch of p=a*Sin(60) and corresponding central wavelength λ=2p=17.32μm. The diameter of the W masses is chosen to be d=5.0μm (d/p=0.58), while their thickness H=720nm (H/λ=4.15%). The Al strip thickness is h=380nm (h/λ=2.2%) with 0.4 metallization ratio. The characteristic frequencies of the resonant eigen-modes of this structure are shown in Table I bellow, as calculated by means of COMSOL finite element analysis [4]. RSAW and SH-SAW are used to denote the SAW modes originating from the Reyleigh and shear SAW propagating within the ΓM region of the Brillouin zone [4], respectively. The complete bandgap is here formed between the lower frequency branch of the SH-SAW mode at K-point (199MHz) and the upper branch of the RSAW mode at Mpoint (204.9 MHz). Further, frequency response FEA suggested that phononic transducer employing this primitive cell can efficiently excite at the upper edge of the complete bandgap (204.5 MHz). The specific mode of excitation is shown on Fig. 3b. Further a weak coupling to the lower branch of the SH-SAW at point M (192.8MHz) also occurs due to the specific energy localization under the masses as has been recently explained [5]. TABLE I.
RESONANT PHONONIC EIGEN-MODES
Borders of the Brillouin zone
RSAW – Lower Branch
M-point K-point
SAWModes SH-SAW Lower branch
RSAW – Upper Branch
178.6 MHz
192.8MHz
204.9MHz
184.82 MHz
199.0 MHz
219.2MHz
The above proposed structure was implemented in the design of a 17.32μm wavelength SAW resonator operating at the upper branch of the RSAW mode in M-point of the Brillouin zone (i.e on the upper edge of the complete bandgap). These devices have been fabricated by means of low resolution photolithography. 380nm Al followed by a 720nm W were deposited by sputter deposition. W masses with diameters of 5μm were initially etched in CF4 plasma using a 2μm photoresist. Subsequently 4μm wide Al electrodes were etched in Cl2/BCl plasma while the W masses have been serving as a hard mask. The as fabricated resonant structure is shown in Figure 4.
(b)
Fig. 3. M-mode SAW resonator a) Primitive cell b)Main resonant mode
Figure 3a represent sketch view of the primitive cell constructing the SAW phononic resonator operating at the M point of the Brillouin zone. This cell consist of 2D hexagonal array of tungsten (W) masses superimposed on aluminum (Al)
(a)
(b)
Fig. 4. M-mode SAW resonator a) As Fabricated b)Close view
It consists of 51 strips in the transducer, 31 strips in each reflector and aperture of 600μm (~35λ).
COMSOL calculations of the as proposed structure proved the existence of a complete bandgap (see Fig. 6).
Closer look to the properties of the phononic grating reveals also a possibility to extend the diversity of SAW transduction. Using the structure proposed in Fig. 5 one can excite either SAW mode in Y-direction of the grating (here coinciding with the Z-direction of the crystal) or SAW mode towards the X-direction of the grating. These SAWs are respectively at the K and the M point of the Brillouin zone. The difference is in the way the voltage is applied. A 3 strip per wavelength grating would excite at the K-point [4], while a 2 strip per wavelength grating would excite at the M-point of the Brillouin zone [5]. Thus IDTs employing this topology are referred to as either a K-mode SAW transducer or M-mode transversely-coupled SAW transducer [4,5].
The K-mode SAW resonator employing the proposed grating is shown as fabricated in Fig. 7. As discussed splitted finger transducer is used for the excitation of the SAW at the K-point of the Brillouin zone.
(a)
(b)
Fig. 7. K-mode SAW resonator a) As Fabricated b)Close view
Fig. 5. Topology of the transveresly-coupled transducer a) the primittive cell b) spatial electric field distribution.
Devices employing the above primitive cell employs the same W and Al layer as for the M-mode SAW resonator. The piezoelectric base substrate is chosen to be 128° Y-cut LiNbO3 with wave propagation along the X axis of the crystal which is further chosen to coincide with the X axis of the primitive transducer cell. In this configuration, the tungsten (W) masses are with major diameters d1=5.2μm and d2=2.8μm in X and Y directions, respectively. The distance between masses is 10μm.
Frequency [MHz]
230
The K-mode SAW test structure consist of 74 strips in the transducer and 33 strips in each reflector. The aperture is 30λ, where λ=15μm is the wavelength of the K-mode SAW. The strip grating has pitch of about 5um, while each strip is 2.4μm wide. Using the same composite grating the M-mode transversely-coupled test structure is shown as fabricated in Fig. 8. Here SAW mode is excited in X-direction of the phononic grating through a regular strip transducer. It consists of 104 strips in the transducer, Al grating pitch of 5μm and strip width (in Y direction) of 2.4μm. The device aperture (in X direction) is 180μm. SAW aperture (in Y-direction) is 30λ, where λ=17.32μm.
(a)
Experiment
(b)
Fig. 8. M-mode transversely –coupled SAW resonator a) as Fabricated b)close view
220 210
III. RESULTS AND DISCUSSION ?
200 190 180 170 (0,0)
Γ
(1,0)
Μ
(1,3-0.5)
Κ
Brillouin Zone Fig. 6. SAW propagation characteristics.
(0,0)
Γ
The proposed here structures representing the M-mode resonator, the K-mode resonator and the transversely coupled M-mode resonator are subsequently characterized by means of VNA measurements of the S11 scattering parameter. The results are then transformed in electrical admittance. Measurements are done in the vicinity of the frequency bandgap. Thus the frequency positions of the resonances can directly be compared to the theoretical predictions deduced by FEA simulations. In Fig. 9 the close to resonance response of the M-mode test resonant structure (shown on Fig. 4) is shown as measured.
Admittance [1/Ω]
0.008
Magnitude
0.007
0.006
0.005
0.004 190
Fig. 9. Close to resonance response of the M-mode SAW resonator
Two resonance peaks are observed and identified with the SHSAW lower branch mode at about 193MHz and the Rayleigh SAW upper branch mode at about 204MHz. these frequencies are in very good agreement with the theoretically predicted by FEA demonstrated in Table I above. This in turn makes us more confident to claim that the as designed device operate at the upper edge of the omnidirectional SAW bandgap. It is noted that a strict observation of the complete bandgap edges is not possible by this structure but is possible by measuring the K-mode and the transversely coupled M-mode devices in a complementary manner. The latter stems from the fact that both devices excite waves propagating in orthogonal directions.
195
200
205
210
215
Frequency [MHz] Fig. 11. Measured admittance of 1-port LWR with regular busbar
In Fig. 11 the close to resonance response of the transversely coupled M-mode test resonant structure (shown on Fig. 8) is shown as measured. A lower branch SH-SAW at about 189 MHz and a upper branch of Rayleigh SAW at about 211MHz are observed in good agreement with the calculated bandgap structure (see Fig. 6). All experimental points are shown on Fig. 6 thus providing a more complete description of the complete bandgap structure. IV. CONCLUSIONS
Admittance [S]
Three distinct types SAW resonant structures operating in the vicinity of the complete SAW bandgap are experimentally demonstrated in a prove of principle study. REFERENCES [1]
[2]
[3] Fig. 10. Close to resonance response of the K-mode SAW resonator
In Fig. 10 the close to resonance response of the K-mode test resonant structure (shown on Fig. 7) is shown as measured. Both resonances are splitted which is in good agreement to the theoretical predictions. To avoid this splitting effect a zig-zag geometry of the transducer has earlier been proposed [4]. A lower branch SH-SAW at about 188 MHz and a upper branch of Rayleigh SAW at about 222MHz are observed in good agreement with the calculated bandgap structure (see Fig. 6).
[4]
[5] [6]
[7]
R. Ruby, in Proc. 2011 Joint Conference of the IEEE International Freq. Control (FCS) and the European Freq. and Time Forum (EFTF), San Francisco, California, 1 - 5 May, pp 1-10. J. Koskela, J. Knuuttila, T. Makkonen, V. Plessky and M. Salomaa, "Side Radiation of Rayleigh Waves from Synchronous SAW Resonators", IEEE Trans. Ultrason. Ferrolectr. Freq. Contr., vol. 54, no. 4, pp. 861 - 869, 2007 J. Meltaus, S. Hong and V. Plessky, "Acoustic Losses in Busbars" in Proc. 2006 IEEE Ultrason. Symp., Vancouver, Canada, 3 - 6 October, pp. 1859-1862, 2006. V. Yantchev and V. Plessky, "Analysis of two dimensional composite surface grating structures with applications to low loss microacoustic resonators", J. Appl. Phys., Vol. 114, Issue 7, art.no. 074902, 2013. V. Yantchev, “A transversely coupled phononic surface acoustic wave transducer”, Appl. Phys. Lett., vol 104, pp. 103503 - 103503-3, 2014 V. Yantchev, “Complete Bandgap SAW Phononic Resonators”, In Proc. 2014 European Frequency and time Forum (EFTF), Neuchatel, Switzerland, 23 – 26 June, 2014, In Print M. Solal, J. Gratier, T. Kook, "A SAW resonator with twodimensionalreflectors," IEEE Trans. On UFFC, vol. 57, pp. 30-37, 2010
