A method for improving the bonding strength of lithium tantalate wafers

Abstract

This invention relates to the field of semiconductors and piezoelectric materials, specifically to a method for improving the bonding strength of lithium tantalate wafers, comprising the following steps: a) thinning, polishing, and cleaning the lithium tantalate wafer; b) cleaning the substrate with a weakly alkaline solution followed by immediate cooling water bath cleaning; c) subjecting the substrate to step-by-step cooling treatment and then storing it at a constant temperature; d) performing surface activation on the lithium tantalate wafer and the substrate using plasma; e) aligning the lithium tantalate wafer and the substrate and applying pressure for bonding to obtain a bonded sheet; f) inspecting the bonded sheet. This invention effectively solves the bonding problems caused by edge lifting and surface roughness of the lithium tantalate wafer, defects in the substrate itself, and differences in the thermal expansion coefficients of the functional layer and the substrate layer during bonding, by optimizing the lithium tantalate wafer thinning and polishing process and the low-temperature substrate treatment process, thus achieving high-strength, high-precision, and high-stability bonding.

Description

Technical Field

[0001] This invention relates to the fields of semiconductors and piezoelectric materials, and more specifically to a method for improving the bonding strength of lithium tantalate wafers. Background Technology

[0002] Lithium tantalate (LiTaO3, LT) crystals possess excellent piezoelectric, acousto-optic, ferroelectric, and pyroelectric effects, making them fundamental functional materials in surface acoustic wave (SAW) filters, optical communications, lasers, and optoelectronics. Bonding lithium tantalate wafers to substrates is an emerging heterogeneous integration technology aimed at tightly integrating lithium tantalate crystals with silicon-based integrated circuits to realize novel optoelectronic devices. This technology's development stems from the urgent need for novel optoelectronic devices, primarily for applications in quantum computing, high-speed optical communication, and precision sensing. While heterogeneous integration technology involving lithium tantalate wafers and silicon wafers offers numerous advantages, the bonding process also faces several challenges. Firstly, the mismatch in thermal expansion coefficients between the two wafers easily generates unnecessary stress during bonding, leading to poor adhesion between the substrate and functional layers. This indirectly reduces bonding strength, causing bubble formation and even lattice mismatch, ultimately resulting in a decline in the electrical and optical performance of the device.

[0003] Improving the bonding strength between lithium tantalate wafers and silicon wafers is key to the commercial application of this technology. Future advancements require continuous innovation in materials engineering, surface treatment, and bonding processes to overcome current technological bottlenecks and ultimately achieve high-quality, high-reliability heterogeneous integration. Currently, in high vacuum environments (<10... -5 Surface activated bonding (SAB) technology can effectively reduce such adverse effects. This technology relies on chemical bonds to achieve a strong bond at the atomic scale. It can achieve good bond strength at room temperature (about 25°C) without the need for subsequent annealing, effectively reducing the problem of thermal stress caused by the mismatch of thermal expansion coefficients.

[0004] The invention disclosed in CN115125618A is a process method for improving the stability of lithium niobate thin film wafers. This invention utilizes a hydrogen silsesquioxane polymer photoresist as a wafer bonding buffer medium and forms a silicon dioxide buffer layer through high-temperature oxidation to improve the high-temperature stability of the lithium niobate thin film wafer. However, although this bonding method has high bonding strength, it requires high-temperature treatment, which can create unnecessary thermal stress in the functional layer of the bonded wafer, affecting the warpage of the bonded wafer and subsequent thinning and polishing processes.

[0005] The invention disclosed in CN109166793B is a method for directly bonding lithium niobate and silicon wafers using a two-step activation process: vacuum ultraviolet light followed by nitrogen plasma. This invention provides a bonding process that eliminates the need for chemical cleaning of the wafer surface, reduces the number of bonding steps, and achieves stable, reliable, and high-strength direct bonding between lithium niobate and silicon at low temperatures. This effectively avoids interface cracking and material breakage caused by the significant difference in thermal expansion coefficients between the two materials. However, the warpage of the bonded wafer is highly dependent on the warpage of the raw material wafer itself. Furthermore, the described low temperature is not room temperature; after pre-bonding, the wafer must be placed in an environment of 150°C for 12 hours to enhance bonding strength. Therefore, this invention cannot effectively reduce bonding defects caused by the difference in thermal expansion coefficients between the two types of wafers.

[0006] Therefore, there is a lack of a bonding method for lithium tantalate wafers that has high bonding strength, is not prone to breakage, has low warpage after bonding, and is easy to process in subsequent processes. Summary of the Invention

[0007] This invention provides a method for improving the bonding strength of lithium tantalate wafers to overcome the shortcomings of existing technologies. This method has a stable processing procedure and solves the problems of low bonding strength and poor bonding after the existing lithium tantalate wafers are bonded.

[0008] The technical solution adopted by this invention to solve the problem is: a method for improving the bonding strength of lithium tantalate wafers, comprising the following steps:

[0009] a) After adjusting the platform shape of the thinning machine, the lithium tantalate wafer is thinned, and then the lithium tantalate wafer is polished on both sides and cleaned.

[0010] b) Clean the substrate with a weak alkaline solution, and then immediately perform a cooling water bath cleaning;

[0011] c) After the substrate is cooled down step by step, it is stored at a constant temperature.

[0012] d) In a vacuum environment, plasma is used to perform surface activation treatment on the bonding surfaces of the lithium tantalate wafer and the substrate.

[0013] e) The lithium tantalate wafer and the substrate are bonded together. After aligning the edges of the lithium tantalate wafer and the substrate, they are placed in the bonding cavity and pressure bonded to obtain the bonded wafer.

[0014] f) Inspect the bonded wafers;

[0015] The lithium tantalate wafer is 4 to 8 inches in size, and the substrate is a silicon wafer with the same shape and size as the lithium tantalate wafer.

[0016] In step a) above, the shape of the thinning machine platform is adjusted so that the center of the thinning machine platform is raised by 1~3μm, wherein the thinning surface of the lithium tantalate wafer is the bonding surface;

[0017] In step a) above, the thinned lithium tantalate wafer is subjected to double-sided polishing. The polishing cloth on the upper disk is made of polyester fiber and polyurethane, while the polishing cloth on the lower disk is made of non-woven fabric. The double-sided polishing machine program needs to be adjusted to achieve a ratio of 7:3 to 6:4 between the removal rate of the lower disk and the removal rate of the upper disk. During the polishing process, the bonding surface needs to be placed downwards, and the removal amount of the bonding surface needs to be at least 3μm.

[0018] As a preferred embodiment, the polishing cloth made of polyester fiber and polyurethane is SUBA800, and the non-woven fabric is one of polyester fiber, nylon staple fiber, and polyester staple fiber.

[0019] In step a) above, the thickness of the polished lithium tantalate wafer is 150~250μm, the TTV is ≤1μm, and the surface must be free of scratches, defects, and dirt.

[0020] In step a) above, the cleaning of the polished lithium tantalate wafers needs to be carried out by ultrasonic cleaning and RCA cleaning in sequence. The RCA cleaning needs to be carried out by sulfuric acid, SC1, SC2 and RO water respectively. Finally, a double-sided brushing machine is used for brushing, and the brushing time is more than 20 minutes. During brushing, the bonding surface must always be placed downwards. The final cleaning particle size (>0.3μm) ≤50.

[0021] In step b) above, the weak alkaline solution consists of ammonia, hydrogen peroxide, and water, with a volume ratio of 1:1:(8~10). The cleaning temperature is 50±5℃ and the cleaning time is 20±5min.

[0022] As a preferred embodiment, the ratio of the above-mentioned weakly alkaline solution is ammonia:hydrogen peroxide:water = 1:1:10;

[0023] In step b) above, after cleaning the substrate with a weak alkaline solution, in order to prevent the weak alkaline solution from further corroding the substrate and exposing the COP defects of the substrate itself, which would lead to the formation of bubbles on the bonding surface, a cooling water bath cleaning should be performed immediately after cleaning with the weak alkaline solution. The cooling water temperature is 5±2℃ and the cooling water cleaning time is 5±1min. The subsequent pure water ultrasonic cleaning time is 20±5min, the ultrasonic current intensity is 1~2A, the ultrasonic frequency is 50~100kHz, and the pure water overflow cleaning time is 20±5min.

[0024] In step c) above, the substrate is cooled down in increments of 3 to 5°C and stored at a constant temperature for 10 to 15 minutes for each increment. The substrate is stored at a constant temperature of 8 to 10°C for at least 30 minutes. For example, in this invention, the temperature rises by about 10°C during wafer surface activation.

[0025] In step d) above, before performing the surface activation treatment, the bonding machine cavity is purged with nitrogen gas, wherein the purity of the nitrogen gas used is at least 99.999%, and the purging time is 2 to 3 minutes.

[0026] In step d) above, the surface activation time is 15-35s, the activation power is 200-350W, the standard voltage of the ion gun is 1500±100kV, the current is 100±10mA, the initial argon flow rate is 33±3sccm, and the subsequent argon flow rate is 35±2sccm.

[0027] In step e) above, the lithium tantalate wafer and the substrate are edge aligned, wherein the coarse alignment rotation angle is -3° to 3°, the visual recognition diameter range is 90 to 110 μm, the judgment point error range is 1 μm, and the compression ratio is 3.

[0028] In step e) above, the aligned wafer is placed in the bonding cavity and pressurized to ensure tight contact between the bonding wafers, eliminate air bubbles, and improve bonding quality. The bonding cavity pressure is 1000~3000N, and the pressure rate switching point is when the substrate layer and the lithium tantalate layer are 0.5mm apart. The fast pressure rate is 20±5mm / s, the slow pressure rate is 0.15±0.05mm / s, and the pressing time is 40±10s.

[0029] In step f) above, the inspection includes bubble inspection and bond strength measurement. Bubbles are inspected using an infrared microscope, and bond strength is measured using the blade insertion method. In the blade insertion method, the blade thickness is 75~100μm, the crack length must be greater than 115±5mm, the Young's modulus of the functional layer must be 130GPa, the Young's modulus of the substrate layer must be 130GPa, and the surface energy must reach Δγ=1.01J / m². 2 .

[0030] Compared with the prior art, the present invention has the following beneficial technical effects:

[0031] In the first aspect, the present invention optimizes the thinning process of lithium tantalate wafers before bonding, ensuring that the surface of the lithium tantalate wafers is uniformly concave, effectively offsetting the edge lifting during the bonding pressurization process, and avoiding defects such as poor edge bonding caused by edge lifting.

[0032] Secondly, relevant research results show that surface roughness will significantly affect bonding strength. When the surface roughness exceeds 1 nm, the bonding strength will drop sharply. Therefore, this invention optimizes the polishing process of lithium tantalate wafers before bonding and reduces the surface roughness of the bonding surface to below 0.2 nm by AFM measurement, which effectively improves the bonding strength when the bonding surfaces are in contact.

[0033] Thirdly, compared with conventional bonding methods, the wafer bonding method adopted in this invention addresses the problem that silicon-based substrates may expose crystal-native defects (COP) after oxidation following cleaning with alkaline solutions such as SC1. By promptly cooling with cooling water, further corrosion of the silicon wafer surface is stopped, thus avoiding poor bonding caused by defects in the silicon-based substrate itself during bonding.

[0034] Fourthly, due to the different materials of the functional layer and the substrate layer, their coefficients of thermal expansion differ, resulting in varying thermal expansion in different wafer directions. This wafer warpage directly affects the bonding interface's adhesion and causes unnecessary stress concentration. This invention, by cooling the substrate, effectively reduces thermal stress, thereby improving the surface quality of the bonding surface. Silicon has a coefficient of thermal expansion of 2.6 ppm / ℃, while lithium tantalate wafers have a coefficient of thermal expansion of 1.61 ppm / ℃ in the a1 and a2 directions and 4.1 ppm / ℃ in the a3 direction. Combining these two wafer thicknesses allows for adjustment of the isothermal temperature of the silicon-based wafer, improving bonding accuracy and stability, reducing offset and errors during bonding, and thus enhancing bonding strength. Attached Figure Description

[0035] Figure 1 This is a flowchart of the method of the present invention;

[0036] Figure 2 This is a schematic diagram of the inserting method of the present invention;

[0037] Figure 3 Images showing wafer chipping at maximum bonding strength measured using the wafer insertion method of this invention;

[0038] Figure 4 Infrared micrographs and images measured by the blade insertion method of the processed wafer in Embodiment 1 of the present invention;

[0039] Figure 5 This is an image of a substrate sheet from Comparative Example 1 of the present invention that has not been cleaned in a cooling water bath;

[0040] Figure 6 The images shown are infrared micrographs and measurement images of the wafer processed by the insert method in Comparative Example 1 of this invention.

[0041] Figure 7 This is an infrared micrograph of a wafer with poor bonding, which is shown in Comparative Example 1 of this invention. Detailed Implementation

[0042] The present invention will be further described below with reference to embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0043] Example 1

[0044] Prepare a 4-inch lithium tantalate wafer with a thickness of 350μm. Adjust the thinning machine platform to have a 3μm central bulge to ensure that the wafer surface is concave after thinning. Adjust the ratio of the removal rate of the lower disk to the upper disk to 7:3 during double-sided polishing. During polishing, place the wafer with the bonding surface facing down, and remove 3μm of the bonding surface. After polishing, the lithium tantalate wafer has a TTV ≤ 1μm and no scratches, defects, or dirt on the surface. After polishing, the lithium tantalate wafer needs to be cleaned by ultrasonic cleaning first, followed by RCA cleaning, which requires cleaning with sulfuric acid, SC1, SC2, and RO water respectively, with each cleaning time being 15 minutes. Finally, use a double-sided brushing machine to brush for more than 20 minutes. During brushing, the bonding surface must always be placed facing down. The final cleaning particle size (>0.3μm) ≤ 50.

[0045] Prepare a 4-inch silicon wafer with a thickness of 500μm. Polish the silicon wafer and clean it with an alkaline solution of ammonia, hydrogen peroxide and water in a volume ratio of 1:1:10 for 20 minutes at a cleaning temperature of 50℃. After cleaning, place it in a water bath at a cooling temperature of 5℃ for 5 minutes. Then, perform pure water ultrasonic cleaning for 20 minutes at an ultrasonic current intensity of 2A and an ultrasonic frequency of 100kHz. Perform pure water overflow cleaning for 20 minutes. After cleaning, spin dry the wafer.

[0046] The silicon wafer was placed in a constant temperature chamber at 20℃ for 10 minutes, then in a constant temperature chamber at 15℃ for 10 minutes, and then in a constant temperature chamber at 10℃ for 30 minutes.

[0047] The bonding machine chamber was purged with nitrogen gas to a vacuum level of 9.99 × 10⁻⁶. -6 Pa, fill the chamber with argon gas until the pressure reaches 0.1 MPa, start the ion gun, and use plasma to perform surface activation treatment on lithium tantalate and silicon wafer to improve the surface activity of the wafer. The surface activation time is 30s and the activation power is 200W.

[0048] A vision-based approach is used to align the lithium tantalate with the main reference edge of the silicon wafer. After alignment, the wafer is placed in the bonding cavity and pressure is applied to ensure tight contact between the bonding wafers, eliminating air bubbles and poor edge bonding. The bonding cavity pressure is 2000N, the rapid pressure application speed is 20mm / s, the slow pressure application speed is 0.15mm / s, and the pressing time is 40s.

[0049] f) Use an infrared microscope to measure bonding defects such as air bubbles in the bonded sheet, and use the insert knife method to measure the bonding strength of the bonded sheet;

[0050] The lithium tantalate bonded sheets prepared in this embodiment were tested and found to have a bonding yield of 97.2%, with few defects such as bonding bubbles. Using the insert method, 95% of the bond strengths exceeded 2.56 J / m. 2 The remaining portion exhibits low edge bond strength in the bonded sheets, but no debonding occurs. The edge bond strength of the remaining 5% of the bonded sheets is not less than 1.6 J / m. 2 .

[0051] Comparative Example 1

[0052] a) Prepare a 4-inch lithium tantalate wafer with a thickness of 350μm. Polish the lithium tantalate wafer on both sides. After polishing, the lithium tantalate wafer needs to be cleaned by first ultrasonic cleaning, then by RCA cleaning, which requires cleaning with sulfuric acid, SC1, SC2 and RO water respectively. Each cleaning time is 15min. Finally, use a double-sided brushing machine to brush for more than 20min. During brushing, the bonding side should always be placed downward. The final cleaning particle size (>0.3μm) ≤50.

[0053] b) Prepare a 4-inch silicon wafer with a thickness of 500μm. Clean it for 20 minutes using an alkaline solution of ammonia, hydrogen peroxide, and water in a volume ratio of 1:1:10 at 50℃. Follow this with ultrasonic cleaning in pure water for 20 minutes at a current intensity of 2A and a frequency of 100kHz. Then, perform an overflow rinse with pure water for another 20 minutes. After cleaning, spin-dry the silicon wafer. Figure 5 As shown, the surface of a silicon wafer that has not been cleaned with a cooling water bath exposes many native crystal defects.

[0054] c) Purge the bonding machine cavity with nitrogen gas to evacuate it to a vacuum level of 9.99 × 10⁻⁶. -6 Pa, fill the chamber with argon gas until the pressure reaches 0.1 MPa, start the ion gun, and use plasma to perform surface activation treatment on the lithium tantalate wafer and silicon substrate to improve the surface activity of the wafer. The surface activation time is 30s and the activation power is 200W.

[0055] d) Align the lithium tantalate wafer with the main reference edge of the silicon wafer. After alignment, place the wafer into the bonding cavity and apply pressure to ensure tight contact between the bonding wafers and eliminate air bubbles. The bonding cavity pressure is 2000N, the rapid pressure application speed is 20mm / s, the slow pressure application speed is 0.15mm / s, and the pressing time is 40s.

[0056] e) Use an infrared microscope to measure bonding bubble defects in bonded sheets and use the insert method to measure the bonding strength of bonded sheets; use an infrared microscope to measure wafer defects, including poor bonding on the wafer surface, bubble defects caused by dirt particles, and subsequent processing breakage caused by bonding edge lifting and non-bonding.

[0057] The lithium tantalate bonded sheets prepared in this comparative example were tested and found to have a bonding yield of 94.53%, with a relatively high number of defects such as bonding bubbles. Using the indentation method, the bonding strength of 82% of the bonded sheets exceeded 2.56 J / m. 2 The remaining portion exhibits low edge bond strength and debonding, with the remaining 5% of bonded sheets having edge bond strength below 0.9 J / m. 2 .

[0058] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for improving the bonding strength of lithium tantalate wafers, characterized in that, Includes the following steps: a) Processing lithium tantalate wafers; b) Clean the substrate with a weak alkaline solution and immediately perform a cooling water bath cleaning, wherein the temperature of the cooling water used in the cooling water bath is 5±2℃ and the cleaning time is 5±1min. c) After the substrate is cooled down step by step, it is stored at a constant temperature of 8-10°C for at least 30 minutes. d) Use plasma to perform surface activation treatment on the bonding surfaces of the lithium tantalate wafer and the substrate; e) The surfaces to be bonded of the lithium tantalate wafer and the substrate are bonded together and their edges are aligned. Pressure is applied in the bonding cavity to obtain the bonded sheet. f) Inspect the bonded wafers; The substrate is made of silicon.

2. The method for improving the bonding strength of lithium tantalate wafers as described in claim 1, characterized in that, In step a), the processing includes thinning, double-sided polishing, and cleaning in sequence.

3. The method for improving the bonding strength of lithium tantalate wafers as described in claim 2, characterized in that, In step a), during thinning, the shape of the processing platform of the thinning machine is adjusted so that the trimming center of the thinning machine platform protrudes by 1 to 3 μm, wherein the thinning surface of the lithium tantalate wafer is the bonding surface.

4. The method for improving the bonding strength of lithium tantalate wafers as described in claim 2, characterized in that, In step a), during double-sided polishing, the surface to be bonded is placed downwards. The upper polishing plate is covered with a polishing cloth made of polyester fiber and polyurethane, and the lower plate is covered with a polishing cloth made of non-woven fabric. The non-woven fabric is one of polyester fiber, nylon short fiber, or polyester short fiber. The ratio of the removal rates of the lower and upper plates is 7:3 to 6:4, and the removal amount on the surface to be bonded is at least 3 μm. After polishing, the thickness of the lithium tantalate wafer is 150 to 250 μm, the TTV is ≤1 μm, and the surface is free of scratches, defects, and dirt.

5. The method for improving the bonding strength of lithium tantalate wafers as described in claim 2, characterized in that, In step a), the cleaning process requires ultrasonic cleaning and RCA cleaning in sequence. The RCA cleaning process involves sulfuric acid, SC1, SC2, and RO water cleaning in sequence. Then, the surface to be bonded is placed face down after brushing for more than 20 minutes using a double-sided brushing machine. The final cleaning effect must meet the following requirement: the number of particles larger than 0.3μm does not exceed 50.

6. The method for improving the bonding strength of lithium tantalate wafers as described in claim 1, characterized in that, In step b), the weakly alkaline solution is composed of ammonia, hydrogen peroxide, and water, wherein the volume ratio of ammonia:hydrogen peroxide:water is 1:1:(8-10). The cleaning temperature and time of the weakly alkaline solution are 50±5℃ and 20±5min, respectively. After cleaning in a cooling water bath, the solution is ultrasonically cleaned with pure water for 20±5min. The ultrasonic current intensity is 1-2A, the ultrasonic frequency is 50-100kHz, and the pure water overflow cleaning time is 20±5min.

7. The method for improving the bonding strength of lithium tantalate wafers as described in claim 1, characterized in that, In step c), the temperature is gradually reduced in increments of 3 to 5°C, and each increment is kept at a constant temperature for 10 to 15 minutes.

8. The method for improving the bonding strength of lithium tantalate wafers as described in claim 1, characterized in that, In step d), the plasma is argon gas with a purity of not less than 99.9999%, an argon pressure of 0.1–0.2 MPa, a surface activation treatment time of 15–35 s, a power of 200–350 W, a standard voltage of 1500±100 kV for the ion gun, a current of 100±10 mA, an initial argon flow rate of 33±3 sccm, and a subsequent argon flow rate of 35±2 sccm.

9. The method for improving the bonding strength of lithium tantalate wafers as described in claim 1, characterized in that, In step e), the edges of the lithium tantalate wafer and the substrate are aligned, wherein the coarse alignment rotation angle is -3° to 3°, the visual recognition diameter range is 90 to 110 μm, the judgment point error range is 1 μm, and the compression ratio is 3; the bonding cavity pressure is 1000 to 3000 N, and the pressure speed conversion point is when the interval between the substrate layer and the lithium tantalate layer is 0.5 mm, wherein the fast pressure speed is 20±5 mm / s, the slow pressure speed is 0.15±0.05 mm / s, and the pressing time is 40±10 s.

10. The method for improving the bonding strength of lithium tantalate wafers as described in claim 1, characterized in that, In step f), the inspection includes using an infrared microscope to inspect for bubbles and using the blade insertion method to measure the bond strength. In the blade insertion method, the blade thickness is 75–100 μm, the crack length must be greater than 115 ± 5 mm, the Young's modulus of the lithium tantalate layer must be 130 GPa, the Young's modulus of the substrate layer must be 130 GPa, and the surface energy must reach Δγ = 1.01 J / m². 2 .

11. The method for improving the bonding strength of lithium tantalate wafers as described in any one of claims 1-10, characterized in that, The lithium tantalate wafer is 4 to 8 inches in size, and the substrate has the same shape and size as the lithium tantalate wafer.

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