Determining Crystallographic Orientation in Wafer-Bonded LTO-on-Si POI Using SDCOM

Wafer bonding has become an essential technology for advanced electronic and acoustic devices, enabling the integration of materials with complementary mechanical, thermal, and piezoelectric properties [1]. In radiofrequency (RF) applications, piezoelectric-on-insulator (POI) substrates are particularly attractive, as they combine high-performance piezoelectric thin films with the manufacturability and thermal properties of silicon [2].

When a thin film of lithium tantalate (LiTaO₃, LTO) is bonded onto silicon via an intermediate oxide layer [3], the resulting structure enables high-quality, low-loss acoustic resonators suitable for surface acoustic wave (SAW) filters used in modern communication systems, including 5G and emerging 6G technologies. These performance improvements arise from enhanced acoustic confinement, reduced substrate losses, improved thermal stability, and tunable electromechanical coupling [4].

A defining characteristic of LTO is its strong crystallographic anisotropy [2]. Key parameters such as acoustic velocity, electromechanical coupling coefficient (k²), and temperature coefficient of frequency (TCF) vary significantly depending on both the out-of-plane cut and the in-plane azimuthal orientation of the wafer. Precise orientation control is therefore critical, as it directly influences wave propagation, coupling efficiency, suppression of spurious modes, and thermal behavior in SAW devices. Consequently, POI substrates are often engineered with specific crystal cuts and orientations tailored to target RF frequency bands.

Accurate characterization of bonded POI structures requires a measurement technique capable of resolving the crystallographic orientation of each layer independently. For wafers consisting of a ~600 nm Y42° LiTaO₃ film bonded to a Si(111) substrate, the SDCOM crystal orientation tool provides an effective solution (Figure 1). At this film thickness, the silicon substrate remains accessible within the diffraction geometry and the layer produces enough intensity to be measured, allowing both materials to be probed from the same surface.

During an azimuthal scan, the diffractometer determines the orientation of each crystal by measuring both the out-of-plane tilt (offcut magnitude) and the in-plane angular position relative to the wafer flat (offcut direction). By comparing the angular positions of diffraction features from the LTO film and the silicon substrate, the relative rotational offset between the two layers, commonly referred to as the twist, can be determined. This measurement principle is illustrated schematically in Figure 2, where the crystallographic planes of the bonded materials intersect along the surface normal, and their angular separation defines the twist.

To demonstrate the capabilities of SDCOM, we evaluated a series of wafers that were fabricated in an attempt to achieve twist values in the range from –4° to +4° from a target value T. This twist offset was achieved via manual bonding for practical reasons. Consequently, these results do not reflect the capabilities of automated industrial bonding; however, they allowed us to easily explore a broad twisting range. The orientation of a single layer can be measured in approximately 25 seconds, enabling full characterization of a bonded wafer in about 50 seconds. Each wafer was measured repeatedly over ten runs under identical conditions.

The results, summarized in Table 1, show tight clustering of both offcut and twist values, demonstrating high measurement repeatability. The standard deviations are on the order of 0.001° for offcut and 0.01° for twist, indicating excellent stability and precision of the method.

Table 1. Offcut and twist measurement results for LTO-on-Si wafers, including repeatability statistics and deviation from target values.

Figure 3 shows that the measured twist values follow the nominal twist values targeted during fabrication. Although a moderate systematic positive bias is present (mean signed error +0.69°, mean absolute error 0.86°), we can still observe the close match between these angles. The trend supports the use of the measurement approach for process evaluation and highlights opportunities to refine bonding alignment.

Overall, these results confirm that SDCOM provides a fast, reliable, and accurate approach for characterizing crystallographic relationships in LTO-on-Si POI structures. Since even small misalignments can significantly affect acoustic performance, thermal drift, and RF device characteristics, precise orientation metrology is essential for process control. With twist determination precision at the 10⁻² degree level and similarly tight offcut standard deviations of only a few thousandths of a degree, SDCOM enables reliable monitoring of wafer-bond alignment. By offering dual-layer orientation analysis without sample inversion and completing measurements in under one minute, SDCOM represents a robust metrology solution for both research and high-volume manufacturing environments, where accuracy and throughput directly impact device yield.

References

1. K. Geshi et al, “Wafer Bonding of Polycrystalline Spinel with LiNbO₃/LiTaO₃ for Temperature Compensation of RF Surface Acoustic Wave Devices”, SEI Technical Review, no. 75, pp. 116-119, 2012.
2. Y. Yan et al, “Wafer-Scale Fabrication of 42° Rotated Y-Cut LiTaO₃-on-Insulator (LTOI) Substrate for a SAW Resonator”, ACS Applied Electronic Materials, vol. 1, no. 8, pp. 1660–1666, 2019.
3. J. Utsumi, “Direct bonding of lithium tantalate to silicon at room temperature by using a self-sputtered bonding method”, Japanese Journal of Applied Physics, vol. 58, SGGC06, 2019.
4. B. Baker et al, “Surface energy engineering for LiTaO3 and α-quartz SiO₂ for low temperature (<220 °C) wafer bonding”, Journal of Vacuum Science & Technology A, vol. 37, 041101, 2019.