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Monolithic Integrated Multiband Acoustic Devices on Heterogeneous Substrate for Sub-6 GHz RF-FEMs

Monolithic integration of multiband (1.4~ 6.0 GHz) RF acoustic devices were successfully demonstrated within the same process flow by using the lithium niobate (LN) thin film on silicon carbide (LNOSiC) substrate. A novel surface mode with sinking energy distribution was proposed, exhibiting reduced propagation loss. Surface wave and Lamb wave resonators with suppressed transverse modes and leaky modes were demonstrated, showing scalable resonances from 1.4 to 5.7 GHz, electromechanical coupling coefficients (k2) between 7.9% and 29.3%, and maximum Bode-Q (Qmax) larger than 3200. Arrayed filters with a small footprint (4.0 x 2.5 mm2) but diverse center frequencies (fc) and 3-dB fractional bandwidths (FBW) were achieved, showing fc from 1.4 to 6.0 GHz, FBW between 3.3% and 13.3%, and insertion loss (IL) between 0.59 and 2.10 dB. These results may promote the progress of hundred-filter sub-6 GHz RF front-end modules (RF-FEMs).

Potential material platforms for monolithic integration of SH0 mode and S0 mode devices for sub-6 GHz RF-FEMs have been comparatively studied through finite element analysis. Fig. 4 shows the simulated displacement mode shapes of the S0 mode, where the resonant energy of the S0 mode can be well confined in the LNOSiC surface while almost totally leaks into the underneath silicon of LNOSi. Fig. 5 shows the simulated temperature increment (∆T) of different substrates when the same boundary heat sources of 10 5 W/m 2 were applied on their top surfaces. By comparison, integrating LiNbO3 thin films onto substrates with higher thermal conductivity can decrease the ∆T effectively, while the SiO2 buried layer will increase the interface thermal resistance, resulting in a higher ∆T. Therefore, LNOSiC is an excellent substrate possessing the advantages of both acoustic energy confinement and heat dissipation.

A. Dispersion of S0 mode and SH0 mode
Based on LNOSiC, dispersion curves of the f, Vp, and k 2 of the SH0 and S0 modes were simulated and shown in Fig. 6, in which λ is the wavelength of the intended mode. The thickness of the LiNbO3 thin film (h) and the Al electrode are set to be 450 nm and 120 nm, respectively. Both modes show large 2 , which is proportional to filter bandwidth. The Vp of the S0 mode is about 1.5 times greater than that of the SH0 mode, so as the f. Therefore, the S0 mode devices will be preferentially applied to bands above 3 GHz, while the SH0 mode devices to bands below 3 GHz. As an example, Figs. 7(a)-(f) show the simulated admittance curves and the corresponding displacement mode shapes of one-port resonators with resonance frequencies (fr) lower than 2 GHz, 3 GHz, and higher than 3 GHz.

B. SH0 mode with sinking energy distribution (SH0-SED)
Due to the dispersion of acoustic modes, a heterogeneous substrate with thickness matched layers usually corresponds to SAW devices (e.g., IHP-SAW) of a specific band. Thanks to the extremely high velocity of the slow shear bulk wave (VSSB, ~7150 m/s) and the excellent f· Q product of SiC, the SH0 mode devices on LNOSiC with scalable resonances from 1.3 to 3.0 GHz (h/λ<0.25) all maintain large admittance ratios (see examples in Figs. 7(a) and (b)). Especially, when h/λ is less than 0.15, the SH0 mode with sinking energy distribution (SH0-SED) exhibits even lower propagating loss. As an example, Fig. 8(a) shows the schematic of the delay line structure used for comparing the propagating loss of the SH0-SED (h/λ=0.09) and the SH0 (h/λ=0.18) modes, where the damping loss (Λ) of the LiNbO3 is set to be 0.002. Figs. 8(b) and (c) show the simulated S21 parameters, from which the loss factors (δ) were extracted and plotted in the insets. The smaller h/λ gets the lower δ, which will contribute to high-Q SH0-SED mode devices.

C. Suppression of the in-band spurious modes
As shown in Figs. 7(a)-(c), both modes are suffered from the transverse mode spurious responses (transverse standing waves) while the S0 mode is also affected by the leaky mode and the overtone modes. Fig. 9(a) shows the simulated mode shapes of the main mode and the first three-order transverse modes of a SH0 mode resonator, and Fig. 9(b) shows the corresponding admittance curve and conductance curve. By adding small border regions (piston IDT) at the tip and the root of the IDT fingers, the transverse modes can be effectively mitigated, as shown in Fig. 9(c). Fig. 10(a) shows the simulated mode shapes of the leaky mode near the anti-resonance (fa) of the S0 mode. In our opinion, the leaky mode is mainly caused by the reflectors with insufficient reflection. By appropriately reducing the pitch of the reflectors, the leaky mode can be well suppressed, as shown in Figs. 10(b)-(c). Based on the above optimized resonators, ladder-type filters with mitigated in-band spurious responses were designed, and the simulated S21 parameters were plotted in Fig. 11, showing fc from 1.4 to 4.4 GHz and FBW between 4.2% and15.5%. Fig. 12(a) presents the image of the 4-inch LNOSiC wafer, which was fabricated by the ion-cutting process. Fig. 12(b) shows the cross-sectional scanning electron microscope (SEM) image of the LNOSiC, in which the thickness of the LiNbO3 layer is about 470 nm. The resonators and filters were prepared through electron beam lithography, metal (Ti/Al) evaporation and lift-off process. Fig. 13 shows the optical microscope image of the fabricated filters with a footprint of 4.0×2.5 mm 2 , and the insets show the zoom-in resonators as well as the optimized IDTs and reflectors.

IV. MEASUREMENT RESULTS
The frequency responses of the demonstrated devices were characterized using a VNA (E5071C) at room temperature in air. Figs. 14 and 15 show the measured admittance and Bode-Q curves of the low-band (SH0-SED mode) and the mid-band (SH0 mode) resonators with and without the piston IDTs, in which the transverse modes were effectively mitigated while the Qmax is almost unaffected. As predicted in Fig. 8, the SH0-SED mode resonator exhibits an excellent Qmax of 3680 at 1.45 GHz, resulting in a high figure-of-merit (k 2 ·Q max) of above 319. Figs. 16(a) and (b) show the measured admittance curves of the high-band (S0 mode) resonators with fr of 3.7 GHz and 4.4 GHz, respectively. When the pitch of the reflector ( ) is 0.9 times the pitch of the IDTs ( ), the leaky modes of both S0 mode resonators were effectively suppressed. By comparison, it is found that the experimental results shown in Figs. 14-16 and the simulations in Figs. 9 and 10 are in great agreement. Figs. 17(a) and (b) show the admittance curves and the extracted k 2 of the prepared SH0 mode and S0 mode resonators, respectively, showing broadband resonances (1.4~5.7 GHz) and large k 2 . Fig. 18 shows the measured S21 parameters of the prepared monolithically integrated ladder-type filters (labeled as F1~F8), and the table below lists the corresponding IL and the 3-dB bandwidth, showing fc from 1.4 to 6.0 GHz, the minimum IL of 0.59 dB and the maximum bandwidth of 488 MHz. With optimized designs, the in-band spurious responses of the arrayed filters have been effectively mitigated. The insets in Fig. 18 serve as examples, where the zoomed-in in-band responses of the filters with and without the optimized designs are compared. The filters with SH0 mode resonators are preferred to 4G-LTE bands, while those with S0 mode resonators are suitable for 5G-FR1 and WiFi-6 bands. Note that, the spurious passband of the S0 mode filter (marked by a black dashed box) is mainly caused by the overtone modes (labeled in Fig. 7(c)) and can be mitigated by using the embedded IDTs, as the simulations shown in Fig. 19(a). The SEM image of the preliminarily prepared embedded IDTs is shown in Fig. 19(b), in which the electrode deposition process needs to be optimized.

V. CONCLUSIONS
The monolithic integration of surface wave and Lamb wave filters for 4G-LTE, 5G-FR1 and WiFi-6 bands were firstly demonstrated on a single LNOSiC substrate. In-band spurious responses of the filters were effectively mitigated. The prepared resonators show scalable resonances from 1.4 to 5.7 GHz, and k 2 between 7.9% and 29.3%. The newly proposed SH0-SED mode exhibits an excellent Qmax of 3680. Arrayed filters with a footprint of 4.0×2.5 mm 2 show fc from 1.4 to 6.0 GHz, FBW between 3.3% and 13.3%, and IL between 0.59 and 2.10 dB. Further optimization of this technology may promote the progress of hundredfilter sub-6 GHz RF-FEMs.