Method for improving bonding strength of lithium tantalate wafer

Abstract

The invention relates to the field of semiconductors and piezoelectric materials, in particular to a method for improving bonding strength of a lithium tantalate wafer, which comprises the following steps: a) thinning, polishing and cleaning the lithium tantalate wafer; b) cleaning the substrate slice with a weakly alkaline solution and then immediately cleaning the substrate slice with a cooling water bath; c) carrying out step-by-step cooling treatment on the substrate slice and then storing at constant temperature; d) carrying out surface activation on the lithium tantalate wafer and the substrate slice by using plasma; e) aligning the lithium tantalate wafer and the substrate slice, and pressurizing and bonding to obtain a bonded slice; and f) inspecting the bonding sheet. By optimizing the lithium tantalate wafer thinning and polishing process and the substrate slice low-temperature treatment process, the problems of edge tilting and surface roughness of the lithium tantalate wafer, self defects of the substrate slice and poor bonding caused by difference of thermal expansion coefficients of a functional layer and a substrate layer during bonding are effectively solved, and high-strength, high-precision and high-stability bonding is realized.

Description

Technical Field

[0001] The present invention relates to the fields of semiconductors and piezoelectric materials, and in particular to a method for improving the bonding strength of lithium tantalate wafers. Background Art

[0002] Lithium tantalate (LiTaO3, LT) crystals exhibit excellent piezoelectric, acousto-optic, ferroelectric, and pyroelectric effects, making them essential functional materials in surface acoustic wave filters, optical communications, lasers, and optoelectronics. Bonding lithium tantalate wafers to substrates is an emerging heterogeneous integration technology that aims to tightly integrate lithium tantalate crystals with silicon-based integrated circuits to realize novel optoelectronic devices. The development of this technology stems from the urgent need for novel optoelectronic devices, primarily used in fields such as quantum computing, high-speed optical communications, and precision sensing. While heterogeneous integration technology involving lithium tantalate wafers and silicon wafers offers numerous advantages, the bonding process also presents numerous challenges. Firstly, the mismatch in thermal expansion coefficients between the heterogeneous wafers can easily generate unnecessary stress during the bonding process, leading to a loose fit between the substrate layer and the functional layer. This in turn indirectly reduces the bond strength, causing bubbles to form and even leading to lattice mismatch, ultimately resulting in a degradation of the device's electrical and optical performance.

[0003] Improving the bonding strength between lithium tantalate wafers and silicon wafers is the key to commercializing this technology. Continuous innovation in material engineering, surface treatment, bonding processes, and other aspects is needed to break through current technical bottlenecks and ultimately achieve high-quality, high-reliability heterogeneous integration. Currently, in a high vacuum environment (<10 -5 Pa), the use of surface activated bonding (SAB) technology can effectively reduce such adverse effects. This technology relies on the action of chemical bonds to achieve strong bonding 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 mismatch of thermal expansion coefficients.

[0004] Invention publication number CN115125618A discloses a process for improving the stability of lithium niobate thin film wafers. This invention utilizes hydrogen silsesquioxane polymer photoresist as a wafer bonding buffer and, through high-temperature oxidation, forms a silicon dioxide buffer layer to enhance the high-temperature stability of lithium niobate thin film wafers. However, while this bonding method offers high bond strength, it requires high-temperature treatment, which can create unnecessary thermal stress in the functional layer of the bond sheet, impacting the warpage of the bond sheet and subsequent thinning and polishing processes.

[0005] The invention with announcement number CN109166793B discloses a method for directly bonding lithium niobate and silicon wafers using a two-step activation process of vacuum ultraviolet light followed by nitrogen plasma. This invention provides a bonding process that does not require chemical reagents to clean the surface of the wafer to be bonded, has fewer bonding processes, and can achieve stable, reliable, high-strength direct bonding between lithium niobate and silicon at low temperatures, effectively avoiding the cracking of the bonding interface and the fracture of the bonding material due to the huge difference in thermal expansion coefficients between the two. However, the warpage of the wafer after bonding in this invention is highly dependent on the warpage of the raw wafer itself, and the low temperature described is not room temperature. After pre-bonding, it still needs to be placed in an environment of 150°C for 12 hours to strengthen the bonding strength. Therefore, this invention cannot effectively reduce the poor bonding caused by the different thermal expansion coefficients of the two bonded wafers.

[0006] Therefore, the prior art lacks a lithium tantalate wafer bonding method that has high bonding strength, is not easy to break, has small warping after bonding, and is easy to subsequently process. Summary of the Invention

[0007] The present invention provides a method for improving the bonding strength of lithium tantalate wafers to solve the defects of the prior art. The method has a stable processing process and solves the problems of low bonding strength and easy occurrence of poor bonding after the existing lithium tantalate wafer bonding is completed.

[0008] The technical solution adopted by the present 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 plate shape of the thinning machine platform, the lithium tantalate wafer is thinned, and then the lithium tantalate wafer is double-sided polished and cleaned;

[0010] b) Clean the substrate using a weak alkaline solution and then immediately rinse in a cooling water bath;

[0011] c) storing the substrate at a constant temperature after gradually cooling it down;

[0012] d) Using plasma to perform surface activation treatment on the surfaces of the lithium tantalate wafer and the substrate to be bonded in a vacuum environment;

[0013] e) Laminating the lithium tantalate wafer and the substrate wafer to be bonded together, aligning the edges of the lithium tantalate wafer and the substrate wafer, placing them in a bonding chamber and applying pressure to bond them to obtain a bonded wafer;

[0014] f) Inspect the bonded sheet;

[0015] The size of the lithium tantalate wafer is 4 to 8 inches, the substrate is a silicon wafer, and the shape and size of the substrate are consistent with those of the lithium tantalate wafer.

[0016] In the above step a), the plate shape of the thinning machine platform is adjusted, and the center of the thinning machine platform is trimmed to a 1-3 μm convexity, wherein the thinned surface of the lithium tantalate wafer is the surface to be bonded;

[0017] In step a), the thinned lithium tantalate wafer is double-sided polished, wherein the polishing cloth attached to the upper surface of the double-sided polishing process is a polyester fiber or polyurethane polishing cloth, and the polishing cloth attached to the lower surface of the double-sided polishing process is a non-woven fabric. The double-sided polishing machine program needs to be adjusted to achieve a double-sided polishing ratio of the lower surface removal rate to the upper surface removal rate of 7:3 to 6:4. During the polishing process, the surface to be bonded needs to be placed downward, and the amount of removal on the surface to be bonded needs to be at least 3 μm.

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

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

[0020] In step a), the cleaning of the lithium tantalate wafer after polishing needs to be sequentially carried out through ultrasonic cleaning and RCA cleaning. The RCA cleaning needs to be carried out through sulfuric acid, SC1, SC2, and RO water cleaning respectively, and finally a double-sided scrubber is used for scrubbing for more than 20 minutes. The bonding surface should always be placed downward during scrubbing, and the final cleaning particle size (>0.3μm) should be ≤50;

[0021] In step b) above, the weak alkaline solution comprises ammonia water, hydrogen peroxide, and water in a volume ratio of 1:1:(8-10), the cleaning temperature is 50±5°C, and the cleaning time is 20±5 minutes;

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

[0023] In step b) above, after the substrate is cleaned with a weak alkaline solution, to prevent the weak alkaline solution from further corroding the substrate and thereby exposing the COP defects of the substrate itself, which may cause bubbles to form on the surface to be bonded, it is necessary to immediately perform a cooling water bath cleaning after cleaning with the weak alkaline solution. The cooling water temperature is 5±2°C 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 the above step c), the substrate is stored at a temperature of 3-5°C, with each temperature being kept for 10-15 minutes, and the substrate is stored at a temperature of 8-10°C for at least 30 minutes. For example, when the wafer surface is activated, the temperature rises by about 10°C.

[0025] In the above step d), 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 the above step d), the surface activation time is 15 to 35 seconds, the activation power is 200 to 350 W, the ion gun standard voltage is 1500 ± 100 kV, the current is 100 ± 10 mA, the initial argon gas flow rate is 33 ± 3 sccm, and the subsequent argon gas flow rate is 35 ± 2 sccm;

[0027] In step e), 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 the above step e), the aligned wafer is placed in the bonding chamber and pressurized to ensure close contact between the bonded wafers, eliminate bubbles, and improve the bonding quality. The bonding chamber pressure is 1000~3000N, and the pressurization speed conversion point is when the interval between the substrate layer and the lithium tantalate layer is 0.5mm. The fast pressurization speed is 20±5mm / s, the slow pressurization speed is 0.15±0.05mm / s, and the pressing time is 40±10s.

[0029] In step f) above, the inspection includes bubble detection and bond strength measurement. Bubbles are detected using an infrared microscope, and bond strength is measured using a scalpel method. In the scalpel method, the blade thickness must be 75-100 μm, the crack length must be greater than 115 ± 5 mm, the Young's modulus of the functional 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 / m 2 .

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

[0031] First, the present invention optimizes the thinning process of the lithium tantalate wafer before bonding to ensure that the surface of the lithium tantalate wafer is uniformly concave, effectively offsetting the edge warping during the bonding pressurization process, and avoiding defects such as poor edge bonding caused by the edge warping;

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

[0033] Thirdly, compared to conventional bonding methods, the wafer bonding method employed by the present invention addresses the problem of silicon-based substrates being oxidized after cleaning with alkaline solutions such as SC1 and potentially exposing crystal-of-origin defects (COPs). 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, because the functional layer and substrate layer are made of different materials and have different thermal expansion coefficients, their thermal expansion varies in different wafer directions. The resulting wafer warpage directly affects the degree of adhesion at the wafer bonding interface and causes unnecessary stress concentration. The present invention effectively reduces the generation of thermal stress by cooling the substrate, thereby improving the surface quality of the bonding surface. The thermal expansion coefficient of silicon is 2.6ppm / °C, while the thermal expansion coefficient of lithium tantalate wafers in the a1 and a2 directions is 1.61ppm / °C, and the thermal expansion coefficient in the a3 direction is 4.1ppm / °C. Combining the two wafer thicknesses allows the constant temperature of the silicon-based wafer to be adjusted, improving the accuracy and stability of the bonding, reducing offset and error during the bonding process, and thereby enhancing the bond strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Flow chart of the method of the present invention;

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

[0037] Figure 3 This is a picture of wafer chipping when measuring the bonding strength MAX using the wafer inserter method of the present invention;

[0038] Figure 4 Infrared micrographs of the wafer processed in Example 1 of the present invention and images of the measurement by the slotting method;

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

[0040] Figure 6 This is an infrared micrograph of the wafer processed in Comparative Example 1 of the present invention and a picture of the measurement by the slotting method;

[0041] Figure 7 This is an infrared micrograph of poor bonding of the wafer processed in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0042] The present invention will be further described below with reference to the following examples, but the scope of protection of the present invention should not be limited thereto.

[0043] Example 1

[0044] Prepare a 4-inch lithium tantalate wafer with a thickness of 350μm, trim the thinning machine platform to a central protrusion of 3μm, ensure that the wafer surface is concave after thinning, and adjust the ratio of the removal rate of the lower disk surface to the removal rate of the upper disk surface during double-sided polishing to 7:3. During the polishing process, the surface to be bonded is placed downward, and the removal amount of the surface to be bonded is 3μm; after polishing, the lithium tantalate wafer TTV is ≤1μm, and the surface is free of scratches, defects, and dirt; after polishing, the lithium tantalate wafer needs to be cleaned by ultrasonic cleaning first, and then by RCA cleaning, which requires sulfuric acid, SC1, SC2, and RO water cleaning respectively, with each cleaning time of 15 minutes, and finally scrubbed with a double-sided scrubber for more than 20 minutes. The surface to be bonded should always be placed downward during scrubbing, and the final cleaning particle size (>0.3μm) is ≤50;

[0045] Prepare a 4-inch silicon wafer with a thickness of 500 μm, polish the 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°C. After cleaning, place it in a water bath at a temperature of 5°C for 5 minutes. Subsequently, use pure water ultrasonic cleaning for 20 minutes, the ultrasonic current intensity is 2A, the ultrasonic frequency is 100 kHz, and the pure water overflow cleaning time is 20 minutes. After cleaning, the wafer is dried;

[0046] The silicon wafer was placed in a 20°C constant temperature chamber for 10 minutes, then placed in a 15°C constant temperature chamber for 10 minutes, and then placed in a 10°C constant temperature chamber for 30 minutes;

[0047] Use nitrogen to purge the bonding machine cavity and evacuate the bonding machine cavity to 9.99×10 -6 Pa, fill with argon until the chamber pressure reaches 0.1MPa, start the ion gun, use plasma to activate the surface of 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] Using a visual solution, the lithium tantalate is aligned with the main reference edge of the silicon wafer. After alignment, the wafer is placed in the bonding chamber and pressurized to ensure close contact between the bonded wafers and eliminate bubbles and poor edge bonding. The bonding chamber pressure is 2000N, the fast pressurization speed is 20mm / s, the slow pressurization speed is 0.15mm / s, and the pressing time is 40s.

[0049] f) Use infrared microscope to measure the poor bonding bubbles of the bonding sheet and use the knife method to measure the bonding strength of the bonding sheet;

[0050] The lithium tantalate bonding sheet prepared in this embodiment was tested and found to have a bonding yield of 97.2%, with few defects such as bonding bubbles. 95% of the bonding strengths measured using the plunger method exceeded 2.56 J / m 2 The remaining 5% of the bonding sheets have a low edge bonding strength, but no debonding occurs. The edge bonding strength of the remaining 5% of the bonding 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 and perform double-sided polishing on the lithium tantalate wafer. After polishing, the lithium tantalate wafer needs to be cleaned first by ultrasonic cleaning, then by RCA cleaning, which requires sulfuric acid, SC1, SC2, and RO water cleaning respectively, with each cleaning time of 15 minutes. Finally, use a double-sided scrubber for scrubbing for more than 20 minutes. The bonding surface should always be placed downward during scrubbing. The final cleaning particle size (>0.3μm) is ≤50;

[0053] b) Prepare a 4-inch silicon wafer with a thickness of 500 μm 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 50°C. Then, use pure water ultrasonic cleaning for 20 minutes at an ultrasonic current of 2A and a frequency of 100 kHz. After the cleaning is completed, spin dry the silicon wafer. Figure 5 As shown in the figure, the surface of the silicon wafer that has not been cleaned in a cooling water bath has exposed many crystal native defects;

[0054] c) Use nitrogen to purge the bonding machine cavity and evacuate the bonding machine cavity to 9.99×10 -6 Pa, fill with argon until the chamber pressure reaches 0.1MPa, start the ion gun, and use plasma to perform surface activation treatment on the lithium tantalate wafer and silicon wafer 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 chamber and pressurize to ensure close contact between the wafers and eliminate bubbles. The bonding chamber pressure is 2000N, the fast pressurization speed is 20mm / s, the slow pressurization speed is 0.15mm / s, and the pressing time is 40s.

[0056] e) Use an infrared microscope to measure the poor bonding bubbles on the bonding wafers, and use the slotting method to measure the bonding strength of the bonding wafers. Infrared microscopes are used to measure wafer defects, including poor bonding on the wafer surface, poor bubbles caused by dirty particles, and subsequent processing breakage caused by the bond edge being lifted and not bonded.

[0057] The lithium tantalate bonding sheet prepared in this comparative example was tested and found to have a bonding yield of 94.53%, with a high number of defects such as bonding bubbles. The bonding strength of 82% of the bonding sheets measured by the plunger method exceeded 2.56 J / m 2 The remaining 5% of the bonding pieces have edge bonding strengths lower than 0.9 J / m. 2 .

[0058] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention 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: The following steps are involved: a) Processing lithium tantalate wafers; b) Clean the substrate with a weak alkaline solution and immediately rinse in a cooling water bath at a temperature of 5±2°C for 5±1 min. c) After gradually cooling the substrate, store it at a constant temperature of 8-10°C for at least 30 minutes; d) using plasma to perform surface activation treatment on the surfaces to be bonded of the lithium tantalate wafer and the substrate; e) Laminating the surfaces of the lithium tantalate wafer and the substrate to be bonded and aligning their edges, and applying pressure in a bonding chamber to obtain a bonded wafer; f) Inspect the bonded wafers; Wherein, the substrate is made of silicon.

2. The method for improving the bonding strength of lithium tantalate wafers according to claim 1, wherein: In the step a), the processing includes thinning, double-side polishing and cleaning in sequence.

3. The method for improving the bonding strength of lithium tantalate wafers according to claim 2, wherein: In the step a), during thinning, the shape of the processing platform of the thinning machine is adjusted so that the center of the thinning machine platform is raised by 1 to 3 μm, wherein the thinned surface of the lithium tantalate wafer is the surface to be bonded.

4. The method for improving the bonding strength of lithium tantalate wafers according to claim 2, wherein: In step a), during double-sided polishing, the surface to be bonded is placed face down, a polishing cloth made of polyester fiber and polyurethane is attached to the upper polishing surface, and a polishing cloth made of non-woven fabric is attached to the lower polishing surface. The non-woven fabric is made of one of polyester fiber, nylon staple fiber, and polyester staple fiber. The ratio of the removal rate of the lower and upper polishing surfaces is 7:3 to 6:4, and the amount of material removed from the surface to be bonded is at least 3 μm. After polishing, the lithium tantalate wafer has a thickness of 150 to 250 μm, a TTV of ≤1 μm, and no scratches, defects, or dirt on the surface.

5. The method for improving the bonding strength of lithium tantalate wafers according to claim 2, wherein: In step a), the cleaning needs to be carried out in sequence through ultrasonic cleaning and RCA cleaning. The RCA cleaning is carried out in sequence through sulfuric acid, SC1, SC2, and RO water cleaning. Then, a double-sided scrubber is used to scrub for more than 20 minutes and the bonding surface is placed downward. The final cleaning particle size (>0.3μm) is ≤50.

6. The method for improving the bonding strength of lithium tantalate wafers according to claim 1, wherein: In step b), the weak alkaline solution is composed of ammonia water, hydrogen peroxide, and water, wherein the volume ratio of ammonia water: hydrogen peroxide: water is 1:1:(8-10), and the cleaning temperature and time of the weak alkaline solution are 50±5°C and 20±5 minutes, respectively; after cleaning in a cooling water bath, ultrasonic cleaning is performed with pure water for 20±5 minutes, the ultrasonic current intensity is 1-2A, the ultrasonic frequency is 50-100kHz, and the pure water overflow cleaning time is 20±5 minutes.

7. The method for improving the bonding strength of lithium tantalate wafers according to claim 1, wherein: In the step c), the temperature is lowered step by step by 3-5°C, and the constant temperature storage time at each step is 10-15 minutes.

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

9. The method for improving the bonding strength of lithium tantalate wafers according to claim 1, wherein: In the step e), the edges of the lithium tantalate wafer and the substrate sheet 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 rate is 3; the bonding chamber pressure is 1000 to 3000 N, the pressing speed conversion point is when the interval between the substrate layer and the lithium tantalate layer is 0.5 mm, wherein the fast pressing speed is 20±5 mm / s, the slow pressing 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 according to claim 1, wherein: In step f), the inspection includes using an infrared microscope to detect bubbles and using a plunger method to measure the bond strength. In the plunger method, the blade thickness is 75-100 μm, the crack length is required to be greater than 115±5 mm, the Young's modulus of the lithium tantalate layer is 130 GPa, the Young's modulus of the substrate layer is 130 GPa, and the surface energy is required to reach Δγ=1.01 J / m 2 .

11. The method for improving the bonding strength of lithium tantalate wafers according to any one of claims 1 to 10, characterized in that: The size of the lithium tantalate wafer is 4 to 8 inches, and the shape and size of the substrate sheet are consistent with those of the lithium tantalate wafer.

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