Integration Technology for Wafer-Level LiNbO3 Single-Crystal Thin Film on Silicon by Polyimide Adhesive Bonding and Chemical Mechanical Polishing

An integration technology for wafer-level LiNbO3 single-crystal thin film on Si has been achieved. The optimized spin-coating speed of PI (polyimide) adhesive is 3500 rad/min. According to Fourier infrared analysis of the chemical state of the film baked under different conditions, a high-quality PI film that can be used for wafer-level bonding is obtained. A high bonding strength of 11.38 MPa is obtained by a tensile machine. The bonding interface is uniform, completed and non-porous. After the PI adhesive bonding process, the LiNbO3 single-crystal was lapped by chemical mechanical polishing. The thickness of the 100 mm diameter LiNbO3 can be decreased from 500 to 10 μm without generating serious cracks. A defect-free and tight bonding interface was confirmed by scanning electron microscopy. X-ray diffraction results show that the prepared LiNbO3 single-crystal thin film has a highly crystalline quality. Heterogeneous integration of LiNbO3 single-crystal thin film on Si is of great significance to the fabrication of MEMS devices for in-situ measurement of space-sensing signals.


Introduction
Lithium niobate (LiNbO 3 ) crystal is widely used for sensors, detectors, electro-optical modulators, information storage and other micro-electromechanical systems (MEMS) devices because of its excellent piezoelectric, ferroelectric, acoustic-optical, non-linear optical effect and electromechanical coupling factors. In recent years, with the rapid development of space technology and micro-electromechanical systems, an increasing number of MEMS devices are used in the aerospace field [1][2][3][4]. The harsh environment such as low temperature and irradiation in outer space puts a huge test on the stability and lifetime of these MEMS devices [5][6][7][8][9]. LiNbO 3 single-crystal has received extensive attention in the aerospace field owing to its low-temperature resistance and intrinsic radiation resistance [10][11][12]. Silicon (Si) is the most commonly used substrate material with excellent mechanical, electrical, and thermal properties, and its processing technology is compatible with the CMOS process [13][14][15]. Under the development trend of low power consumption, miniaturization and integration of space devices, it is vital to realize the integration of large-area, high-quality LiNbO 3 single-crystal thin films on silicon.
There is a large lattice mismatch between LiNbO 3 and Si, which makes the epitaxial growth of LiNbO 3 thin films on Si substrates extremely challenging. The traditional growth methods of LiNbO 3 thin films include RF magnetron sputtering, sol-gel, chemical vapour deposition and pulsed laser deposition [16][17][18][19][20]. It is difficult to prepare LiNbO 3 a hot plate at 80 °C for 30 min and at 150 °C for 30 min; D group: baked on a hot plate at 80 °C for 30 min, at 150 °C for 30 min, and at 250 °C for 30 min) were compared. In order to fully volatilize the solvent and achieve a certain degree of solidification of PI, so that the by-product H2O produced in the subsequent bonding process is very small, we used gradient baking in the C and D groups of experiments for soft baking.
As a next step, all the samples were activated by using O2 plasma sputtering samples surface in the PVA-Tepla-loN40 plasma system. The power, pressure, and activation time of O2 plasma are 250 W, 150 mTorr, and 90 s, respectively. The flow rate of O2 is 500 sccm. After that, the two wafers were taken out of the plasma chamber and brought into contact at room temperature in air to complete the pre-bonding. After storing for 24 h in air at room temperature to saturate the pre-bonding strength, the pre-bonded wafer pair was annealed at 100 °C for 15 h to complete the bonding. Then, the bonded wafer pairs were bonded to a flat and parallel glass support disc to fit the CMP system. The bonded wafer pairs were thinned by the chemical mechanical thinning system (Logitech LP50, Glasgow, England, UK). In order to obtain an LN surface with a low TTV (total thickness variation) value, we use abrasive grains of different sizes for the grinding process. Finally, LN was polished to about 10 μm by the chemical mechanical polishing system (Logitech LP50, UK). The preparation flow chart of lithium niobate single-crystal film is shown in Figure  1.

Characterization and Measurements
The residual strain of the PI thin film was evaluated by Raman scattering (Renishaw navia) excited by a 532 nm laser. The chemical state of PI thin film was characterized by a Fourier transform infrared spectrometer (FT-IR, Nicolet iS50, Waltham, Massachusetts, USA) and the surface morphology of PI thin film was analyzed by an atomic force microscope (AFM, MFP-3D, Santa Barbara, California, USA). The tensile tester (GOTECH AI-3000S Extensograph, Taiwan, China) was applied to test the bonding strength. In this test, a 1.023 mm-wide bonded sample was pulled at the rate of 200 mm/min, and the average force was calculated to yield the bonding strength. The interfaces of the samples were identified by scanning electron microscopy (SEM, ZEISS SUPRA-55, Oberkochen, Baden-Wurttemberg, Germany). The crystalline quality of LN thin films was examined by X-ray diffraction (XRD, DX-2700B, Dandong, Liaoning Province, China).

Results and Discussion
The thickness of the polyimide intermediate layer is different under different spinning speeds. The stress accumulation of polyimide films of different thicknesses is different during curing, which will affect the subsequent wafer bonding and LiNbO3 thin film applications. Excessive stress accumulation will cause bonded samples to fracture during the heat treatment. Due to the residual stress, the materials on both sides of the PI intermediate layer will be subjected to stresses of equal magnitude and opposite directions.

Characterization and Measurements
The residual strain of the PI thin film was evaluated by Raman scattering (Renishaw navia) excited by a 532 nm laser. The chemical state of PI thin film was characterized by a Fourier transform infrared spectrometer (FT-IR, Nicolet iS50, Waltham, Massachusetts, USA) and the surface morphology of PI thin film was analyzed by an atomic force microscope (AFM, MFP-3D, Santa Barbara, California, USA). The tensile tester (GOTECH AI-3000S Extensograph, Taiwan, China) was applied to test the bonding strength. In this test, a 1.023 mm-wide bonded sample was pulled at the rate of 200 mm/min, and the average force was calculated to yield the bonding strength. The interfaces of the samples were identified by scanning electron microscopy (SEM, ZEISS SUPRA-55, Oberkochen, Baden-Wurttemberg, Germany). The crystalline quality of LN thin films was examined by X-ray diffraction (XRD, DX-2700B, Dandong, Liaoning Province, China).

Results and Discussion
The thickness of the polyimide intermediate layer is different under different spinning speeds. The stress accumulation of polyimide films of different thicknesses is different during curing, which will affect the subsequent wafer bonding and LiNbO 3 thin film applications. Excessive stress accumulation will cause bonded samples to fracture during the heat treatment. Due to the residual stress, the materials on both sides of the PI intermediate layer will be subjected to stresses of equal magnitude and opposite directions. The internal lattice of the LiNbO 3 single-crystal will be distorted and the interplanar spacing will change, which will affect the quality of the LiNbO 3 single-crystal. We prepared polyimide films of different thicknesses on the substrate by spin coating at 2000 rad/min, 3000 rad/min, 3500 rad/min, 4000 rad/min and 5000 rad/min, and baked them to make them fully cured. The residual strain of the PI film with different spin-coating conditions is characterized by Raman spectroscopy. The Raman scattering spectrum is related to the vibration of solid molecules, and the frequency of peak is equal to the vibration frequency of material atoms [27,28]. If there is stress in the material, some stress-sensitive bands will move and deform [29]. As shown in Figure 2, three vibration peaks were observed in all samples, located at~1385.918 cm −1 ,~1609.211 cm −1 and 1788.400 cm −1 . From the Raman spectrum, we can understand that as the thickness of the PI film increases, the Raman peak has a very small frequency shift trend to the lower frequencies, which indicates that there is very weak tensile stress in the film [30]. When the spin-coating speed is 4000 rad/min, the Raman peak has a slight frequency shift, moving from 1385.918 cm −1 to 1384.252 cm −1 , which indicates that an extremely weak tensile stress exists in the PI film during the growth process. In general, the extremely weak tensile stress has a negligible effect on the LiNbO 3 /Si bonded structure. increases, the Raman peak has a very small frequency shift trend to the lower frequencies, which indicates that there is very weak tensile stress in the film [30]. When the spincoating speed is 4000 rad/min, the Raman peak has a slight frequency shift, moving from 1385.918 cm −1 to 1384.252 cm −1 , which indicates that an extremely weak tensile stress exists in the PI film during the growth process. In general, the extremely weak tensile stress has a negligible effect on the LiNbO3/Si bonded structure. Polyimide adhesive has excellent adhesion properties. The solute in PI-5100 is polyamide acid, which exists in the organic solvent in the form of a complex. It is difficult to completely remove the solvent, which requires a long period of baking at a higher temperature. The adhesion performance of the PI films gradually decreases during the process of baking to remove the solvent. Moreover, PI films produce by-products during the curing process (polyamide acid forms polyimide through a polycondensation Polyimide adhesive has excellent adhesion properties. The solute in PI-5100 is polyamide acid, which exists in the organic solvent in the form of a complex. It is difficult to completely remove the solvent, which requires a long period of baking at a higher temperature. The adhesion performance of the PI films gradually decreases during the process of baking to remove the solvent. Moreover, PI films produce by-products during the curing process (polyamide acid forms polyimide through a polycondensation reaction). In order to obtain high-quality PI films that can be used for wafer bonding, PI films pre-baked under different conditions were characterized by FT-IR, as shown in Figure 3. According to Figure 3b, it can be seen that the characteristic peaks of N-H (NH 2 ) and O-H (COOH) appear in 2900~3200 cm −1 , and strong absorption peaks of C=0 (CONH) and C-NH (CONH) appear at 1654 cm −1 and 1538 cm −1 respectively, indicating that the main component of the film at this time is polyamide acid. With the increase of baking temperature and baking time, the characteristic peaks above disappeared gradually. After baking at 250 • C for 2 h, all the characteristic peaks above disappeared, and asymmetric and symmetrical stretching vibration of C=0 appeared at 1774 cm −1 and 1712 cm −1 , and characteristic peak of C-N appeared at 1367 cm −1 , indicating that polyamide acid had been completely converted into polyimide. After baking under condition (d), the above peaks exist simultaneously, indicating that the film has achieved a certain degree of curing, but the cross-linking between molecules is not obvious, which lays a good foundation for the success of subsequent bonding.  Although polyimide has excellent performance, it will produce by-products (H2O) during the curing process, which will cause holes in the bonding interface [31]. Therefore, we need to soft bake the polyimide to make the solvent fully volatilize and reach a certain Although polyimide has excellent performance, it will produce by-products (H 2 O) during the curing process, which will cause holes in the bonding interface [31]. Therefore, we need to soft bake the polyimide to make the solvent fully volatilize and reach a certain degree of curing [32]. In this way, during the curing process of polyimide, the amount Nanomaterials 2021, 11, 2554 6 of 10 of by-products is very small, so that no voids appear in the bonding interface [33]. In order to research the effect of pre-baking conditions on the wafer bonding, and since the temperature of polyimide curing is above 150 • C, we used 100 mm BF33 wafer and 100 mm Si wafer to perform the aforementioned comparative experiments. Put the bonded wafer pairs into the wafer bonding machine, and heat up to 250 • C for 2 h. As shown in Figure 4a,b, the PI peptizer is not sufficiently volatilized, causing a large number of bubbles and even PI modification at the bonding interface. Due to insufficient pre-curing of PI film, there are a large number of water molecules in the bonding interface, which affects the bonding quality, as shown in Figure 4c. Figure 4d shows that after baking at 80 • C, 150 • C, and 250 • C for 30 min, the peptizer is fully volatilized, the pre-curing degree meets the bonding conditions, and the final uniform, complete, non-porous bonding interface. Based on the above analysis, the polyimide adhesive layer is pre-baking at 80 • C, 150 • C, and 250 • C for 30 min. Figure 4e presents the bonded pairs of LiNbO 3 wafer and Si wafer after indirect bonding. Due to the transparency of the LiNbO 3 , the resulting bonding interface can be seen by looking through the LiNbO 3 layer. There are almost no bubbles and voids at the bonded interface, and no cracks can be observed at the interface. The bonding strength of the LiNbO 3 /Si indirect bonded sample is measured to be 11.379 MPa. The high bonding strength can make the bonded samples withstand the stress caused by the harsh mechanical grinding process. Bonded samples maintain high bond strength at low temperatures. Compared with the PI adhesive bonding method proposed in this paper, the bonding method based on the BCB adhesive reported in the published article cannot achieve Si-LiNbO 3 wafer-level bonding [21,23].
Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 12 degree of curing [32]. In this way, during the curing process of polyimide, the amount of by-products is very small, so that no voids appear in the bonding interface [33]. In order to research the effect of pre-baking conditions on the wafer bonding, and since the temperature of polyimide curing is above 150 °C, we used 100 mm BF33 wafer and 100 mm Si wafer to perform the aforementioned comparative experiments. Put the bonded wafer pairs into the wafer bonding machine, and heat up to 250 °C for 2 h. As shown in Figure  4a,b, the PI peptizer is not sufficiently volatilized, causing a large number of bubbles and even PI modification at the bonding interface. Due to insufficient pre-curing of PI film, there are a large number of water molecules in the bonding interface, which affects the bonding quality, as shown in Figure 4c. Figure 4d shows that after baking at 80 °C, 150 °C, and 250 °C for 30 min, the peptizer is fully volatilized, the pre-curing degree meets the bonding conditions, and the final uniform, complete, non-porous bonding interface. Based on the above analysis, the polyimide adhesive layer is pre-baking at 80 °C, 150 °C, and 250 °C for 30 min. Figure   After the wafer bonding is completed, the LiNbO3 single-crystal thin film is prepared by chemical mechanical thinning and polishing. In the lapping 1 process, a Al2O3 slurry with 20 μm particle size is used to reduce the thickness of the LiNbO3 wafer. As shown in Figure 5a, the Al2O3 slurry with 20 μm particle size has a faster rate of thinning wafer, so thinner LN wafers can be quickly obtained. Black line shows when the speed of the grinding disc was increased from 20 rad/min to 70 rad/min, the removal rate gradually increased from 7.76 μm/min to 13.63 μm/min. Red line shows the TTV value decreases first and then increases with the increase of the rotating speed of the grinding disc, and the minimum value (5.9 μm) is obtained when the rotating speed of the grinding disc is 40 rad/min. The same trend was found in the lapping 2 process. Because the larger particle size of alumina has a greater cutting effect on the LiNbO3 wafer, it produces deeper scratches on the surface of the wafer, which can only be repaired by polishing for a long time. The embedded volume of A2O3 abrasive with 9 μm particle size on the surface of the wafer is smaller than that of A2O3 abrasive with 20 μm particle size. Therefore, the rate of thinning the wafer by A2O3 slurry with 9 μm particle size is slower, but the damage on the wafer surface is greatly reduced. When the thickness of the LiNbO3 is about 70 μm, the Al2O3 slurry with 9 μm particle size is used in the lapping 2 process to reduce the thickness of the LiNbO3 wafer to about 13 μm. In Figure 5b, black line shows that when the speed of the grinding disc is increased from 20 rad/min to 70 rad/min, the removal rate increases After the wafer bonding is completed, the LiNbO 3 single-crystal thin film is prepared by chemical mechanical thinning and polishing. In the lapping 1 process, a Al 2 O 3 slurry with 20 µm particle size is used to reduce the thickness of the LiNbO 3 wafer. As shown in Figure 5a, the Al 2 O 3 slurry with 20 µm particle size has a faster rate of thinning wafer, so thinner LN wafers can be quickly obtained. Black line shows when the speed of the grinding disc was increased from 20 rad/min to 70 rad/min, the removal rate gradually increased from 7.76 µm/min to 13.63 µm/min. Red line shows the TTV value decreases first and then increases with the increase of the rotating speed of the grinding disc, and the minimum value (5.9 µm) is obtained when the rotating speed of the grinding disc is 40 rad/min. The same trend was found in the lapping 2 process. Because the larger particle size of alumina has a greater cutting effect on the LiNbO 3 wafer, it produces deeper scratches on the surface of the wafer, which can only be repaired by polishing for a long time. The embedded volume of A 2 O 3 abrasive with 9 µm particle size on the surface of the wafer is smaller than that of A 2 O 3 abrasive with 20 µm particle size. Therefore, the rate of thinning the wafer by A 2 O 3 slurry with 9 µm particle size is slower, but the damage on the wafer surface is greatly reduced. When the thickness of the LiNbO 3 is about 70 µm, the Al 2 O 3 slurry with 9 µm particle size is used in the lapping 2 process to reduce the thickness of the LiNbO 3 wafer to about 13 µm. In Figure 5b, black line shows that when the speed of the grinding disc is increased from 20 rad/min to 70 rad/min, the removal rate increases from 0.30 µm/min to 2.70 µm/min. As the speed of the grinding disc increases, red line shows that the TTV value first decreases and then increases. When the speed of the grinding disc is 20 rad/min, the TTV value is 2.70 µm. When the speed of the grinding disc was increased to 40 rad/min, the TTV value reached the minimum, which was 1.7 µm. Subsequently, as the rotation speed of the grinding disc increases, the TTV value also increases. When the speed of the grinding disc is increased to 70 rad/min, the TTV value is 2.5 µm. According to the result of the lapping process, the TTV value will not always decrease as the rotation speed of the grinding disc increases. Too small or too large a rotation speed will cause the fixture rotation speed to be inconsistent with the grinding disc rotation speed. The uneven distribution of the slurry on the bottom of the wafer will result in uneven surface removal rates. Therefore, choosing suitable abrasive grains instead of increasing the speed of the disc to increase the removal rate can minimize the probability of damage. from 0.30 μm/min to 2.70 μm/min. As the speed of the grinding disc increases, red line shows that the TTV value first decreases and then increases. When the speed of the grinding disc is 20 rad/min, the TTV value is 2.70 μm. When the speed of the grinding disc was increased to 40 rad/min, the TTV value reached the minimum, which was 1.7 μm. Subsequently, as the rotation speed of the grinding disc increases, the TTV value also increases. When the speed of the grinding disc is increased to 70 rad/min, the TTV value is 2.5 μm.
According to the result of the lapping process, the TTV value will not always decrease as the rotation speed of the grinding disc increases. Too small or too large a rotation speed will cause the fixture rotation speed to be inconsistent with the grinding disc rotation speed. The uneven distribution of the slurry on the bottom of the wafer will result in uneven surface removal rates. Therefore, choosing suitable abrasive grains instead of increasing the speed of the disc to increase the removal rate can minimize the probability of damage. After the lapping process is completed, the damaged layer on the wafer surface needs to be removed by a chemical mechanical polishing process [34][35][36]. Amorphous silica with an abrasive grain size of 50 nm was used for polishing. During the polishing process, the polishing liquid drops on a part of the polishing disc. The centrifugal force caused by the rotation of the polishing disc will evenly distribute the polishing liquid on the surface of the running disc. The polishing liquid is mainly composed of chemical substances and abrasive grains. Chemical substances have a chemical interaction with the polished surface, and abrasive grains have a mechanical (physical) interaction with the polished surface. As shown in Figure 6a, when the rotating speed of the polishing disc is 70 r/min, the wafer surface roughness of the LiNbO3 wafer first decreases and then slowly increases with the increase of the slurry flow rate. When the slurry flow rate is 100 mL/min, the wafer surface roughness is 7.13 nm. At this time, the mechanical polishing rate is greater than the chemical corrosion rate. When the slurry flow rate increases, more effective abrasives and solutions participate in the polishing action, resulting in enhanced mechanical grinding and chemical corrosion. However, the chemical corrosion rate is faster than the mechanical grinding rate. When chemical corrosion and mechanical grinding are in balance, the smallest wafer surface roughness can be obtained. When the slurry flow rate is 150 mL/min, the surface roughness reaches the minimum value (2.55 nm). After that, with the increase of the slurry flow rate, the chemical corrosion rate is greater than the mechanical polishing rate. At this time, the wafer surface is corroded severely and the surface roughness increases. When the slurry flow rate is 200 mL/min, the wafer surface roughness is 4.81 nm. The polishing results of LiNbO3 at different speeds of the polishing disc After the lapping process is completed, the damaged layer on the wafer surface needs to be removed by a chemical mechanical polishing process [34][35][36]. Amorphous silica with an abrasive grain size of 50 nm was used for polishing. During the polishing process, the polishing liquid drops on a part of the polishing disc. The centrifugal force caused by the rotation of the polishing disc will evenly distribute the polishing liquid on the surface of the running disc. The polishing liquid is mainly composed of chemical substances and abrasive grains. Chemical substances have a chemical interaction with the polished surface, and abrasive grains have a mechanical (physical) interaction with the polished surface. As shown in Figure 6a, when the rotating speed of the polishing disc is 70 r/min, the wafer surface roughness of the LiNbO 3 wafer first decreases and then slowly increases with the increase of the slurry flow rate. When the slurry flow rate is 100 mL/min, the wafer surface roughness is 7.13 nm. At this time, the mechanical polishing rate is greater than the chemical corrosion rate. When the slurry flow rate increases, more effective abrasives and solutions participate in the polishing action, resulting in enhanced mechanical grinding and chemical corrosion. However, the chemical corrosion rate is faster than the mechanical grinding rate. When chemical corrosion and mechanical grinding are in balance, the smallest wafer surface roughness can be obtained. When the slurry flow rate is 150 mL/min, the surface roughness reaches the minimum value (2.55 nm). After that, with the increase of the slurry flow rate, the chemical corrosion rate is greater than the mechanical polishing rate. At this time, the wafer surface is corroded severely and the surface roughness increases. When the slurry flow rate is 200 mL/min, the wafer surface roughness is 4.81 nm. The polishing results of LiNbO 3 at different speeds of the polishing disc are shown in Figure 6b. The slurry flow rate is set to 150 mL/min, and the surface roughness decreases with the increase of the rotation speed. When the rotation speed of the polishing disc is 30 rad/min, the surface roughness is 16.54 nm. As the rotating speed of the polishing disc increases, the mechanical polishing rate increases. When the rotation speed is 70 rad/min and the polishing liquid flow rate is 150 mL/min, the chemical corrosion effect and the mechanical grinding effect are balanced, and the minimum wafer surface roughness (2.55 nm) is obtained. are shown in Figure 6b. The slurry flow rate is set to 150 mL/min, and the surface roughness decreases with the increase of the rotation speed. When the rotation speed of the polishing disc is 30 rad/min, the surface roughness is 16.54 nm. As the rotating speed of the polishing disc increases, the mechanical polishing rate increases. When the rotation speed is 70 rad/min and the polishing liquid flow rate is 150 mL/min, the chemical corrosion effect and the mechanical grinding effect are balanced, and the minimum wafer surface roughness (2.55 nm) is obtained. It is important to evaluate the bonding interface. Figure 7a illustrates the SEM crosssectional view of the integration sample of LiNbO3 single-crystal thin film on Si. From top to bottom, Si, PI and LiNbO3 single crystal films are in order, and each layer is clearly visible and of uniform thickness. The thickness of LiNbO3 single-crystal thin film was measured by SEM, and its thickness was about 10 μm. Moreover, the thickness of the PI film is about 6.16 μm. It can be seen that the bonding interfaces are tightly and clearly visible. The XRD patterns of the LiNbO3 thin film is shown in Figure 7b. A Z-cut bulk LN wafer is also measured by XRD as the reference [37,38], and it can be seen that the LiNbO3 thin film prepared in this paper is almost consistent with the bulk LN wafer in the XRD pattern. Only one diffraction peak located at 39.28° for the Z-cut LN thin film is observed according to Figure 7b, indicating the single-crystalline structure of the thin film [39]. The FWHMs deduced from the first-order diffraction peak is 0.19° for the Z-cut LiNbO3 thin film, demonstrating the highly crystalline quality of the LiNbO3 film. It is important to evaluate the bonding interface. Figure 7a illustrates the SEM crosssectional view of the integration sample of LiNbO 3 single-crystal thin film on Si. From top to bottom, Si, PI and LiNbO 3 single crystal films are in order, and each layer is clearly visible and of uniform thickness. The thickness of LiNbO 3 single-crystal thin film was measured by SEM, and its thickness was about 10 µm. Moreover, the thickness of the PI film is about 6.16 µm. It can be seen that the bonding interfaces are tightly and clearly visible. The XRD patterns of the LiNbO 3 thin film is shown in Figure 7b. A Z-cut bulk LN wafer is also measured by XRD as the reference [37,38], and it can be seen that the LiNbO 3 thin film prepared in this paper is almost consistent with the bulk LN wafer in the XRD pattern. Only one diffraction peak located at 39.28 • for the Z-cut LN thin film is observed according to Figure 7b, indicating the single-crystalline structure of the thin film [39]. The FWHMs deduced from the first-order diffraction peak is 0.19 • for the Z-cut LiNbO 3 thin film, demonstrating the highly crystalline quality of the LiNbO 3 film. Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 12 are shown in Figure 6b. The slurry flow rate is set to 150 mL/min, and the surface roughness decreases with the increase of the rotation speed. When the rotation speed of the polishing disc is 30 rad/min, the surface roughness is 16.54 nm. As the rotating speed of the polishing disc increases, the mechanical polishing rate increases. When the rotation speed is 70 rad/min and the polishing liquid flow rate is 150 mL/min, the chemical corrosion effect and the mechanical grinding effect are balanced, and the minimum wafer surface roughness (2.55 nm) is obtained. It is important to evaluate the bonding interface. Figure 7a illustrates the SEM crosssectional view of the integration sample of LiNbO3 single-crystal thin film on Si. From top to bottom, Si, PI and LiNbO3 single crystal films are in order, and each layer is clearly visible and of uniform thickness. The thickness of LiNbO3 single-crystal thin film was measured by SEM, and its thickness was about 10 μm. Moreover, the thickness of the PI film is about 6.16 μm. It can be seen that the bonding interfaces are tightly and clearly visible. The XRD patterns of the LiNbO3 thin film is shown in Figure 7b. A Z-cut bulk LN wafer is also measured by XRD as the reference [37,38], and it can be seen that the LiNbO3 thin film prepared in this paper is almost consistent with the bulk LN wafer in the XRD pattern. Only one diffraction peak located at 39.28° for the Z-cut LN thin film is observed according to Figure 7b, indicating the single-crystalline structure of the thin film [39]. The FWHMs deduced from the first-order diffraction peak is 0.19° for the Z-cut LiNbO3 thin film, demonstrating the highly crystalline quality of the LiNbO3 film.

Conclusions
In this study, an integration technology for wafer-level LiNbO 3 single-crystal thin film on silicon by polyimide adhesive bonding and chemical mechanical polishing is described. The residual strain and chemical composition of the polyimide film were studied by Raman spectroscopy and FT-IR. The results show that the weak tensile stress has a negligible effect on the LiNbO 3 /Si bonded structure and 80 • C for 30 min, 150 • C for 30 min, and 250 • C for 30 min are the best pre-bake conditions, which can obtain the highest quality PI film that can be used for bonding. The great bonding strength can withstand the stress generated by the harsh chemical-mechanical thinning and polishing, and a large-area, high-quality LiNbO 3 single-crystal thin film is prepared. After the bonding process, the LiNbO 3 wafer was lapped by Al 2 O 3 with abrasive slurry sizes of 20 and 9 µm, respectively, and then polished by an amorphous silica with an abrasive slurry size of 50 nm. Finally, the thickness of the LiNbO 3 wafer with a 100 mm diameter can be reduced from 500 to 10 µm with almost no cracks. SEM results show that the bonding interface is defect-free and uniform. The LiNbO 3 single-crystal thin film prepared in this paper meets the requirements of low temperature and other harsh environments for LiNbO3-on-Si devices.