UNPRECEDENTED FIGURE OF MERIT IN EXCESS OF 108 IN 920 MHZ ALUMINUM NITRIDE CROSS-SECTIONAL LAMÉ MODE RESONATORS SHOWING KT2 IN EXCESS OF 6.2%

In this work we demonstrate a 900 MHz cross-sectional Lamémode resonator showing a kt2 in excess of 6.2% and a loaded quality factor in excess of 1750. Such a high kt2, which closely matches that Finite-Element-Methods (FEM) predicted value, is attained through the use of a 3-finger CLMR and a thickness-fieldexcitation (TFE) scheme. This device not only shows a high kt2 comparable to the one attained by film-bulk-acoustic resonators, FBARs, but is also characterized by an unprecedented figure-ofmerit (FoM∽108) that, to the best of the authors’ knowledge, is the highest demonstrated in AlN resonators operating at ~1 GHz. INTRODUCTION icro Electro Mechanical (MEM) resonators are key enablers for the development of miniaturized and low-power multi-band radiofrequency (RF) systems capable of operating in the extremely crowded modern commercial and military spectral environment. For years, MEM resonators have been researched for their ability to attain high quality factors (Q) and large electromechanical coupling coefficients (kt2) in small volumes. Recently, the complete maturation of the Aluminum Nitride (AlN) Film-Bulk-Acoustic resonator (FBAR) technology [1] has allowed the replacement of off-chip surface acoustic wave (SAW) devices in commercial products, hence enabling better performance in a miniaturized form-factor. The AlN FBAR technology relies mainly on the e33 piezoelectric coefficient of AlN to transduce resonant vibration along the thickness of an AlN plate. As the device resonance frequency (fr) is set by the thickness of the AlN plate (TAlN), it cannot be tuned lithographically. As a consequence, the integration of multi-frequency FBAR-based filters on the same chip can only be attained through an increase of the fabrication complexity (i.e. by mass-loading or trimming). Such a limitation has been overcome by the AlN contour-mode resonator (CMR) technology [2]. In fact, AlN CMRs rely mainly on the e31 piezoelectric coefficient of AlN to transduce resonant vibration along an inplane direction of an AlN plate (e.g., width extensional or length extensional motion). Therefore, the lithographically set lateral dimension of the device determines its resonance frequency, enabling the fabrication of CMRs operating in the Ultra-High(UHF), Very-High(VHF) and Super-High(SHF) frequency ranges on the same chip [3],[4]. Although multi-frequency AlN CMRs can be readily integrated on the same chip, their kt2 is lower than that of FBARs, due to the intrinsically lower amplitude of the e31 compared to the e33. For this reason FBAR-based filters have been preferred to the CMR-based ones for the implementation of low insertion loss and wideband passive filtering networks. More recently, AlN cross-sectional Lamé-mode resonators were demonstrated (CLMRs) [5],[6]. CLMRs are piezoelectric resonators formed by two metallic IDTs sandwiching an AlN plate. They rely on the combined use of both the e31 and the e33 piezoelectric coefficients of AlN to transduce a Lamé-mode in the cross-section of an AlN plate through. Thanks to this special feature, CLMRs can simultaneously achieve high-kt2 and a lithographic definition of their resonance frequency (fres), enabling the implementation of lithographically defined integrated contiguous and not-contiguous pre-select filters for platforms adopting carrier-aggregation (CA). In this work we demonstrate 1-port CLMRs simultaneously showing a record-high kt2 in excess of 6.2%, a quality factor (Q) in excess of 1750 and a Figure of Merit (i.e. FoM=Q kt2) in excess of 108. To the best of the authors’ knowledge, such a high FoM-value is the highest ever demonstrated in AlN resonators operating in the same frequency range. CROSS-SECTIONAL LAMÉ MODE RESONATORS AlN CLMRs are piezoelectric resonators capable of transducing a Lamé-mode in the cross-section of AlN plates through the coherent combination of the e31 and e33 piezoelectric coefficients of AlN (Fig. 1). Thanks to the opposite sign of these two piezoelectric coefficients, in-phase charge components are generated by vibration along both the cross-sectional directions (thickness and width) of the plate. As a consequence, the kt2 attained by AlN CLMRs is function of both the e31 and e33 piezoelectric coefficients of AlN, and it is higher than the kt2 achieved by conventional laterally vibrating AlN resonators such as CMRs. Figure 1: schematic-view of a 3-finger CLMR. The device is formed by two IDTs sandwiching an AlN film. The pitch of the IDTs (W) is selected to be similar to the thickness of the AlN layer (TAlN). Such choice enables the excitation of high-kt2 degenerate or nondegenerate Lamé-modes in plates. The mode-shape relative to the total displacement of the same device, when exciting a nondegenerate Lamé mode, is also reported. Similarly to CMRs, CLMRs can be excited through either a Lateral-Field-Excitation (LFE) [7] or a thickness-field-excitation (TFE) approach [6]. LFE CLMRs are formed by one set of IDT patterned on either the top or the bottom surface of an AlN layer. In contrast, TFE CLMRs are formed by two interdigital-metal electrodes sandwiching an AlN film. The IDTs, in both TFE and LFE CLMRs, are needed to produce the excitation of the electric field in the cross-section of the AlN layer. As demonstrated in [5], CLMRs achieve maximum kt2-value when the pitch of the IDTs (W) is set to a specific value (Wopt) similar to thickness (TAlN) of the AlN plate. In fact, in this scenario, a nondegenerate Lamé-mode is excited in the crosssection of the AlN plate. However, due to the capability of exciting M 978-1-940470-02-3/HH2016/$25©2016TRF 94 Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, South Carolina, June 5-9, 2016 high-kt2 degenerate cross-sectional Lamé modes [8] in plates, CLMRs can attain high kt2 also when W is slightly different from Wopt. In addition, since fres depends on W, the transduction of such degenerate modes also enables a significant lithographic tunability of the device operating frequency. This feature is crucial for the implementation of multi-frequency resonators and filters monolithically integrated on the same chip with minimal fabrication complexity. In this work, a non-degenerate CLMR (adopting the optimum IDT geometry, i.e. W=Wopt) showing a record kt2 in excess of 6.2% is experimentally demonstrated for the first time. Fabrication Process The CLMRs presented in this work are formed by a 4 μm thick AlN layer and two 0.1 μm thick platinum IDTs placed on the top and bottom surfaces of the AlN film. The choice of using platinum for the bottom IDT was dictated by the need of growing a high quality AlN film. Platinum was also used for the top IDT in order to preserve high acoustic symmetry in the device cross-section. Figure 2: Microfabrication process of TFE CLMRs: (a) Pt film was deposited on top of Silicon, through a 10 nm thick Ti layer used as adhesion layer, and patterned through lift-off process; (b) AlN film was deposited on top of the Pt film and vias in the AlN were formed; (c) Pt film was deposited on top of Si and patterned through lift-off process; (d) AlN film was etched through the use of a SiO2 hard mask that was preferred to traditional photoresist mask to attain steeper AlN sidewalls; (e) Si substrate was released by XeF2 isotropic etching. The devices were fabricated using a four-mask microfabrication process (Figure 2): 10/100 nm of Ti/Pt was deposited and patterned on top of a high resistivity silicon wafer to form the bottom IDT. Next, a 4 μm thick AlN film was sputter-deposited. Then, we etched AlN through wet etching to form the vias. Next, 10/100 nm of Ti/Pt was deposited and patterned to form the top IDT. Then, the AlN film was etched by ICP in Cl2 based chemistry to define the width of the AlN plate. This was done through the use of a hard mask made out of 2 μm of SiO2 so as to attain steep AlN sidewall angles (>75o). Finally, the Silicon substrate underneath the resonator was released through XeF2 isotropic etching. High-FoM exceeding 108 in AlN CLMRs 3-fingers 920 MHz TFE CLMRs were fabricated. These devices are formed by two 100-nm thick platinum IDTs sandwiching a 4μm thick AlN-plate. The pitch of their IDTs was optimized (i.e. W was set to 5 μm), through simulation, in order to maximize the simulated kt2 value. A Scanned Electron Microscope (SEM) picture of one of the fabricated CLMRs is shown in Figure 3. Two quarter-wave acoustic transformers were designed at the edges of the device [9] in order to minimize the loss of acoustic energy through the anchors (i.e. known as anchor-losses) [10],[11]. As verified through 3D-FEA (Figure 4), this technique, which was originally developed for AlN CMRs, is also effective in minimizing the displacement in the inactive-regions of CLMRs, thus enabling a reduction of anchor-losses and, consequently, a larger mechanical quality factor (Qm). Both the 3D-FEA simulated and measured admittance (Y) curves relative to the best fabricated CLMRs (Figure 3) are reported in Figure 5. As evident, a kt2 value in excess of 6.2% was extracted through MBVD-fitting [12] of the measured response. Such value matches closely the predicted value (∽6.6%) found through FEA, thus confirming its validity. In addition, the measured response showed a loaded quality-factor, Qload, (extracted from the measured device 3dBbandwidth), in excess of 1750. Such high Qload and kt2 values allowed to achieve a measured FoM in excess of 108. To the best of the authors’ knowledge, such a large FoM is the highest ever demonstrated in AlN resonators operating in the same frequency range. Figure 3: Scanned-Electron-Microscope picture of a fabricated CLMR. Figure 4: left) Simulated modal-distribution of the device displacement, through 3D-FEA. As evident, the use of the acoustic quarter-wave transformers enables a significant reduction of the magnitude of the total-displacement reaching the resonator inactive-regions; right) 3D-FEA simulated cro