In addition, it should be noted that designing filters on POI substrates requires very similar skills to those required for designing SAW filters on bulk piezo wafers and manufacturing devices on POI substrates is straightforward (standard metal layer deposition for the main part) using a small number of manufacturing process steps.

SAW Resonator and Filter Designs on POI

Measured performance of SAW resonators built on lithium tantalate wafers and SAW resonators built on thin film POI were prepared and characterized. The results demonstrate the performance improvement of the POI substrate. For this experiment, a single port resonator of dipoles using 120 finger pairs and 20 electrodes on each side acting as mirrors was manufactured. The acoustic aperture was set at 40 λ and the distance between fingers and electrodes set at 1.2 μm with a ratio metal/spacing of 0.5 (see Figure 2).

A 1.6 GHz central frequency was targeted for those resonators and used tip probing to measure their characteristics. The POI substrates used had the following characteristics: 600 nm thick (YX)/42° LiTaO₃ on a 500 nm thick SiO₂ on a high resistivity Si (100 crystal).

Figure 3 Resonator k² measurement of bulk and POI substrates.

Coupling k²

Figure 4 POI-based resonator response - max. Q at anti-resonance.

Figure 5 Sensitivity of velocity vs. temperature.

The coupling k² of the POI reached 8.13 percent when the bulk LiTaO₃ wafers used for conventional TC-SAW devices was limited to 5.98 percent (see Figure 3). k² is calculated as 1-fr²/fa² (fr is the resonance frequency and fa is the anti-resonance frequency). The benefits of the higher k² provided by the POI substrate enable design of larger bandwidth filters to address some of the new 5G bands (up to 6 percent bandwidth of the center frequency).

Q-factor

Another significant performance improvement of the POI substrate appears on the Bode Q factor at anti-resonance. Under the same conditions, the Q-factor of the bulk LiTaO₃ reached 935 compared to 2200 on the POI substrate (see Figure 4). This value should enable SAW filters to compete against BAW filters in the L- and C-Bands.

Temperature Compensated Factor (TCF)

The TCF factor (cubic polynomial fit) on the POI substrate is also significantly reduced. We are able to achieve much less than 20 ppm/K (typically below 10 ppm/K) while the bulk LiTaO₃ would be around 40 ppm/K. Figure 5 shows the quasi-compensation of temperature effect on a 1.4 GHz resonator - second order effect is notable (TCF1 = −1.93 ppm/K, TCF2 = 403.5 ppb/K)

Based on our characterization work on the resonators, a ladder architecture was designed and simulated for a SAW filter at 2 GHz. The filter resonantors were implemented on POI wafers and performance measured. There was no target frequency band, but rather the filter was designed to take advantage of the modes of propagation that the POI substrate can enable.

The resulting extrapolated filter had 80 MHz bandwidth (1 dB band), less than 2 dB insertion loss, rejection greater than 40 dB and a group delay variation of about 50 ns or better. The absolute TCF was under 10 ppm/K over the entire operating range. Bandpass characteristics could be further optimized with filter design, but these results illustrate what can be achieved on this new platform (see Figure 6).

Figure 6 Transmission and group delay of the 2 GHz SAW filter fabricated on POI: wideband (a) and passband (b) performance.

Summary

The 5G roll-out is bringing a set of new challenges to front-end module devices, including form factor, thermal dissipation and performance. Improved performance is needed in order to achieve a successful roll-out. Filters are playing a key role since their numbers are increasing dramatically to support the new bands requirements in front-end modules.

Soitec has developed a new type of engineered substrate consisting of a very thin and uniform single-crystal lithium tantalate layer on a thin silicon oxide layer on high resistivity handle substrate using its Smart Cut™technology. The proposed solution provides resonators and filters with figures of merit in line with 5G filter requirements, particularly regarding the quality and coupling factors that are both greatly improved compared to standard SAWs on bulk piezoelectric material.

Designing, integrating and manufacturing filters on POI wafers remains straightforward as it relies on similar techniques used for SAW devices fabrication. SAW filters on POI can also compete with BAW filters for required frequencies in L- and S-Bands as they also bring the required performance.

The piezoelectric material expertise associated with Smart Cut™ technology allows Soitec to manufacture large volumes of uniform engineered substrates in their dedicated production line and is available to support the stringent filter requirements of 5G.