A roughening processing method for the back surface of an ultrathin lithium tantalate wafer

Abstract

The present application relates to the technical field of piezoelectric crystal, and particularly relates to a roughening processing method for the back surface of an ultrathin lithium tantalate wafer, which comprises the following steps: a) protecting the front surface of the lithium tantalate wafer by pasting a film; b) heating and sandblasting abrasive, and baking the sandblasting abrasive; c) starting the rotation of a sandblasting platform, adjusting the distance between the sandblasting gun and the lithium tantalate wafer, adjusting the sandblasting pressure to 0.2-0.3 Mpa, and performing first roughening processing on the back surface of the wafer; d) after the first roughening processing is completed, removing the sandblasting particle residues attached to the surface of the wafer, readjusting the sandblasting pressure to 0.1-0.15 Mpa, and performing second roughening processing on the back surface of the wafer; e) after the second roughening processing is completed, immersing the wafer in a mixed solution tank of hydrofluoric acid and nitric acid, then immersing the wafer in a warm water tank for ultrasonic cleaning, then immersing the wafer in a normal temperature tank for ultrasonic cleaning, and finally immersing the wafer in an overflow tank to complete wafer cleaning.

Description

Technical Field

[0001] This invention relates to the field of piezoelectric crystals, specifically to a method for roughening the back surface of an ultrathin lithium tantalate wafer. Background Technology

[0002] In recent years, the widespread use of high-frequency and multi-band communication in wireless communications, exemplified by mobile phones, has led to a significant increase in demand for surface acoustic wave (SAW) filters, with higher quality requirements. Consequently, the requirements for TTV and PLTV of lithium tantalate wafers are becoming increasingly stringent. Traditional wax-coating or single-sided polishing methods are no longer sufficient to meet customer needs; a double-sided polishing process is now necessary. This involves polishing both sides of the wafer simultaneously. After polishing, a protective film is applied to the front side, while the back side undergoes roughening to achieve high flatness. Furthermore, due to the low hardness of ultra-thin lithium tantalate wafers, they are prone to significant deformation under sandblasting pressure, making it difficult to achieve uniform roughness and high flatness.

[0003] During the back roughening process of ultrathin lithium tantalate wafers, the thinner the wafer, the more difficult it is to control the roughening process, making the wafer extremely fragile. Currently, the thickness of ordinary lithium tantalate wafers is above 0.25mm, while the industry requires a roughness of 0.1-0.2μm for lithium tantalate wafers, and also requires higher wafer quality. This places higher demands on the back roughening process of ultrathin lithium tantalate wafers, and the existing roughening process cannot well meet the needs of wafers with a thickness of less than 0.25mm. The roughness of the back surface of the wafer is controlled by the sandblasting pressure and the number of reciprocating strokes. If the sandblasting pressure is too high or the number of reciprocating strokes is too many, the wafer is prone to breakage. If the sandblasting pressure is too low or the number of reciprocating strokes is too few, the roughness will be low and the sandblasting uniformity will be poor. In addition, due to the large electrostatic adsorption capacity of lithium tantalate wafers, the sandblasting abrasive is prone to agglomerate on the wafer surface, thus affecting the uniformity of sandblasting. At the same time, the overall flatness of the lithium tantalate wafer will be worse after sandblasting due to the sandblasting pressure, and acid soaking is required to repair the flatness. However, conventional acid soaking process is prone to problems such as the protective film on the wafer peeling off and the flatness index not meeting the standard.

[0004] To address this issue, patent CN109015394A discloses a dry processing method for the back surface roughness of lithium tantalate substrates. This patent mainly controls the back surface roughness by adjusting the spray gun pressure, the distance between the spray gun and the lithium tantalate wafer, and the angle between the spray gun and the lithium tantalate. This patent targets lithium tantalate substrates with a thickness between 0.5-1 mm, but it does not specifically address issues such as fragmentation and missed spraying that occur during the process of achieving the target roughness. Patent CN107932340A discloses a single-sided polishing method for planarizing ultrathin wafers. It uses a sandblasting mechanism and a wafer adsorption platform with shock absorption. The sandblasting mechanism reciprocates and rotates, spraying abrasive particles onto the wafer surface at a specific angle until the surface is uniformly and completely roughened. This patent avoids wafer breakage due to excessive instantaneous force when abrasive particles hit the wafer surface by adsorbing the wafer onto the shock absorption platform. However, if the wafer is fixed using a shock absorption platform, it will continuously vibrate during the sandblasting process under the sandblasting pressure. While this prevents wafer breakage, it reduces wafer planarity. Patent CN212907651U discloses a device for timely removal of surface deposits after silicon wafer sandblasting. It uses a 2-5mm thick layer of EVA material to adsorb the slurry on the silicon wafer surface without damaging it. This invention mainly focuses on post-processing adsorption cleaning of the wafer surface and is only applicable to wet sandblasting processes. It also fails to address the problem of abrasive particle agglomeration caused by electrostatic adsorption on the wafer. Patent CN106392895A discloses a novel method for roughening the surface of polycrystalline silicon wafers with diamond wire. By roughening the surface of polycrystalline silicon wafers with sandblasting, a single-sided rough surface with a thickness of 2μm-6μm is formed on the surface of the polycrystalline silicon wafer. This effectively reduces the problem of color difference and high reflectivity caused by uneven texture on the surface of the diamond wire polycrystalline wafer. The purpose of this patent is significantly different from that of this patent. The process does not involve key control measures such as wafer flatness control and sandblasting uniformity control. Summary of the Invention

[0005] The purpose of this invention is to provide a method for roughening the back surface of an ultrathin lithium tantalate wafer.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for roughening the back surface of an ultrathin lithium tantalate wafer includes the following steps:

[0008] a) Apply a protective film to the front of the lithium tantalate wafer, and then vacuum-adsorb the lithium tantalate wafer onto the sandblasting platform;

[0009] b) Bake the sandblasting abrasive at 80-85℃ for 30-40 minutes;

[0010] c) Turn on the sandblasting platform and rotate it at 30-40 rpm. Adjust the distance between the sandblasting gun nozzle and the lithium tantalate wafer to 50-80 mm and adjust the sandblasting pressure to 0.2-0.3 MPa to perform the first roughening process on the back of the wafer.

[0011] d) After completing the first roughening process, turn off the sandblasting platform rotation and sandblasting pressure to remove the sandblasting particles adhering to the wafer surface. Then restore the sandblasting platform rotation speed to 30-40 rpm and readjust the sandblasting pressure to 0.1-0.15 MPa to perform the second roughening process on the back of the wafer.

[0012] e) After the second roughening process is completed, remove the wafer and place it in a mixed solution of hydrofluoric acid and nitric acid. Heat and soak for 30-50 minutes, then place it in a warm water bath for ultrasonic cleaning for 10-20 minutes, then place it in a room temperature bath for ultrasonic cleaning for 10-20 minutes, and finally place it in an overflow tank.

[0013] In step a), the thickness of the lithium tantalate wafer is 0.18-0.25mm. Due to its thinness and softness, the lithium tantalate wafer is prone to adsorption damage and even fragmentation during sandblasting. Therefore, precise control of the film thickness and platform adsorption pressure is necessary. The film applied to the lithium tantalate wafer is either a blue film or a UV film, with a thickness of 0.16-0.2mm. If the film is too thin, it cannot protect the front of the wafer, allowing particles to easily penetrate and scratch the wafer. However, if it is too thick, the wafer will undergo greater elastic deformation after adsorption, resulting in a higher BOW value after sandblasting. The adsorption pressure of the sandblasting platform is 0.3-0.5MPa. Too low a platform adsorption pressure cannot secure the wafer, but too high a pressure can easily leave suction cup marks due to the thinness of the wafer. In step b), the sandblasting abrasive is silicon carbide, alumina, or ceramic powder with a mesh size of #1000-2000.

[0014] In step c), during the first roughening process, the spray gun reciprocates 1-2 times, the reciprocating speed is 2-3 min / time, and the sandblasting pressure is 0.2-0.3 MPa.

[0015] In step d), during the second roughening process, the number of reciprocating strokes of the spray gun is 3-5, the reciprocating speed is 4-5 min / stroke, and the sandblasting pressure is 0.1-0.15 MPa.

[0016] In step d), to achieve the target roughness, excessive sandblasting pressure and too many reciprocating strokes can easily cause wafer breakage, while insufficient sandblasting pressure and too few reciprocating strokes will result in low roughness and poor sandblasting uniformity. Therefore, a segmented sandblasting method is adopted. The first sandblasting controls the wafer roughness, and the second sandblasting compensates for minor missed spots on the wafer surface.

[0017] In step d), after the wafer completes the first roughening process, due to the electrostatic adsorption between the wafer and the sandblasting abrasive, abrasive agglomerates will form on the wafer surface, causing some areas of the wafer to be unable to be sandblasted evenly. Therefore, it is necessary to use a lint-free cloth soaked in anhydrous ethanol to wipe the residual particles on the wafer surface, and supplement it with deionized air blowing to reduce the electrostatic adsorption capacity of the wafer.

[0018] In step e), The ratio of hydrofluoric acid to nitric acid is 1:2, which provides the best ability to repair flatness. The temperature is 40-45℃. If the acid temperature is too high, it will evaporate too quickly and the cost will be high. If the acid temperature is too low, the repair ability will be weak. The soaking time is 30-50 minutes. Within this time range, the BOW value can be stabilized within 20μm. The temperature of the warm water bath is 35-40℃, and the ultrasonic frequency is 28Hz. The temperature of the room temperature bath is 20-30℃, and the ultrasonic frequency is 40Hz. The particulate impurities after sandblasting are larger than 2μm, and the temperature change adaptability of the lithium tantalate wafer is within 10-20℃. This cleaning process can ensure the cleanliness of the wafer and reduce the risk of wafer breakage. In step e), the wafer is cleaned in sequence through the mixed acid etching tank, the warm water tank, the room temperature tank, and the overflow tank. Since the flatness of the wafer after sandblasting is poor and the wafer is coated with a film on the front side, in order to prevent the film from falling off and to ensure that the sandblasting abrasive is completely removed, the acid ratio, soaking temperature, time, and cleaning frequency must be precisely controlled.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] (1) This process solves the problems of adsorption damage and adsorption fragments of ultrathin lithium tantalate wafers by controlling the film thickness and platform adsorption pressure;

[0021] (2) This process achieves back surface roughness control while ensuring the uniformity of the wafer sandblasting surface through secondary sandblasting, and also reduces the fragmentation rate of wafer sandblasting.

[0022] (3) This process removes static electricity by cleaning with a brush during the sandblasting process, which solves the problem of agglomeration of sandblasting abrasive and the problem of missing sandblasting caused by the sandblasting abrasive not being able to act on the wafer surface.

[0023] (4) By controlling the wafer flatness index repair process, this process ensures the repair of wafer flatness index, avoids the peeling of wafer protective film and the adhesion of sandblasting abrasive on the wafer, and reduces the risk of fragmentation during wafer cleaning. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1:

[0026] (1) Film application and sandblasting: A film application machine is used to apply a protective film to the front of the 6-inch ultra-thin lithium tantalate wafer, with a film thickness of 0.18mm. Open the sandblasting machine's operating window, wipe the sandblasting platform with alcohol, and place the coated lithium tantalate wafer, back side down, onto the sandblasting platform at an adsorption pressure of 0.4 MPa. Ensure the wafer and the coating platform are completely aligned. After placing the wafer, close the operating window, turn on the sandblasting platform and rotate it at 35 rpm. Adjust the distance between the sandblasting gun nozzle and the lithium tantalate wafer to 60 mm, adjust the sandblasting pressure to 0.25 MPa, adjust the sandblasting gun's reciprocating stroke to 152 mm, the number of reciprocations to 2, and the running speed to 3 min / cycle. After adjusting the parameters, run the program to perform the first roughening process on the wafer. After completing the first roughening process, turn off the sandblasting platform rotation and sandblasting pressure, and clean the agglomerated sandblasting particles on the wafer surface. After cleaning, restore the sandblasting platform rotation to 35 rpm, readjust the sandblasting pressure to 0.12 MPa, adjust the sandblasting gun's reciprocating stroke to 152 mm, the number of reciprocations to 4, and the running speed to 4 min / cycle. Run the program to complete the second roughening process on the wafer.

[0027] (2) Cleaning: After the second roughening process is completed, the wafer is removed and placed in a mixed solution bath of hydrofluoric acid and nitric acid with a ratio of 1:2 and a temperature of 40°C for 40 minutes. Then it is placed in a warm water bath for ultrasonic cleaning for 15 minutes with a temperature of 35°C and an ultrasonic frequency of 28Hz. Then it is placed in a room temperature bath for ultrasonic cleaning for 15 minutes with a temperature of 30°C and an ultrasonic frequency of 40Hz. Finally, it is placed in an overflow tank.

[0028] The lithium tantalate wafer processed in this embodiment has a back surface roughness of 0.12μm and an overall flatness of 15μm. Microscopic observation shows that there are no defects such as missed spraying on the back surface and no scratches or marks on the front surface, which meets the product quality target requirements.

[0029] To highlight the beneficial effects of the present invention, the following comparative experiments were also conducted.

[0030] Comparative Example 1:

[0031] (1) Film application and sandblasting: Use a film application machine to apply a protective film to the front of the 6-inch ultra-thin lithium tantalate wafer. The film thickness is 0.18mm. Open the sandblasting machine's operating window, wipe the sandblasting platform with alcohol, and place the lithium tantalate wafer with the back side down on the sandblasting platform. Make sure the wafer and the film application platform are completely aligned. After placing the wafer, close the operating window, turn on the sandblasting platform and rotate it at 35rpm. Adjust the distance between the sandblasting gun nozzle and the lithium tantalate wafer to 60mm, adjust the sandblasting pressure to 0.25Mpa, adjust the reciprocating stroke of the sandblasting gun to 152mm, the number of reciprocations to 6 times, and the running speed to 3min / time. After completing the parameter adjustment, run the program to roughen the wafer.

[0032] (2) Cleaning: Same as in Example 1.

[0033] The back surface roughness of the lithium tantalate wafer processed in this comparative example is 0.16μm, but the wafer has cracked, which is judged to be caused by excessive sandblasting pressure and excessive sandblasting reciprocating number.

[0034] Comparative Example 2:

[0035] (1) Film application and sandblasting: Use a film application machine to apply a protective film to the front of the 6-inch ultra-thin lithium tantalate wafer. The film thickness is 0.18mm. Open the sandblasting machine's operating window, wipe the sandblasting platform with alcohol, and place the lithium tantalate wafer with the back side down on the sandblasting platform. Make sure the wafer and the film application platform are completely aligned. After placing the wafer, close the operating window, turn on the sandblasting platform and rotate it at 35rpm. Adjust the distance between the sandblasting gun nozzle and the lithium tantalate wafer to 60mm, adjust the sandblasting pressure to 0.12Mpa, adjust the reciprocating stroke of the sandblasting gun to 152mm, the number of reciprocations to 6 times, and the running speed to 3min / time. After completing the parameter adjustment, run the program to roughen the wafer.

[0036] (2) Cleaning: Same as in Example 1.

[0037] The lithium tantalate wafers processed in this comparative example did not show any cracks, but the roughness on the back side was only 0.05-0.08μm, which did not meet the requirements. This was determined to be caused by insufficient sandblasting pressure.

[0038] Comparative Example 3:

[0039] (1) Film application and sandblasting: Use a film application machine to apply a protective film to the front of the 6-inch ultra-thin lithium tantalate wafer. The film thickness is 0.18mm. Open the sandblasting machine's operating window, wipe the sandblasting platform with alcohol, and place the lithium tantalate wafer with the back side down on the sandblasting platform. Make sure the wafer and the film application platform are completely aligned. After placing the wafer, close the operating window, turn on the sandblasting platform and rotate it at 35rpm. Adjust the distance between the sandblasting gun nozzle and the lithium tantalate wafer to 60mm, adjust the sandblasting pressure to 0.25Mpa, adjust the reciprocating stroke of the sandblasting gun to 152mm, the number of reciprocations to 2, and the running speed to 3min / time. After completing the parameter adjustment, run the program to roughen the wafer.

[0040] (2) Cleaning: Same as in Example 1.

[0041] The back surface roughness of the lithium tantalate wafer processed in this comparative example is 0.13 μm, which meets the requirements. However, under a microscope, the uniformity of the back surface of the wafer is poor, and there are missed spraying points. It is judged that this is caused by insufficient sandblasting reciprocating number and failure to use secondary sandblasting.

[0042] Comparative Example 4:

[0043] (1) Film application and sandblasting: Use a film application machine to apply a protective film to the front of the 6-inch ultra-thin lithium tantalate wafer. The film thickness is 0.18mm. Open the sandblasting machine's operating window, wipe the sandblasting platform with alcohol, and place the film-applied lithium tantalate wafer with its back side down on the sandblasting platform. Confirm that the wafer and the film application platform are completely aligned. After placing the wafer, close the operating window, turn on the sandblasting platform and rotate it at 35rpm. Adjust the distance between the sandblasting gun nozzle and the lithium tantalate wafer to 60mm, adjust the sandblasting pressure to 0.3MPa, adjust the reciprocating stroke of the sandblasting gun to 152mm, the number of reciprocations to 2, and the running speed to 3min / time. After completing the parameter adjustment, run the program to perform the first roughening process on the wafer. Then, readjust the sandblasting pressure to 0.12MPa, the number of reciprocations to 4, and the running speed to 4min / time to complete the second roughening process on the wafer.

[0044] (2) Cleaning: Same as in Example 1.

[0045] The back surface roughness of the lithium tantalate wafer processed in this comparative example is 0.12μm, and the overall flatness of the wafer is 17μm. However, a small number of missed spray points are still found on the back surface of the wafer under a microscope. It is determined that the wafer surface particles were not wiped with a lint-free cloth soaked in anhydrous ethanol and not purged with deionized air, resulting in excessive electrostatic adsorption capacity on the wafer surface.

[0046] Comparative Example 5:

[0047] (1) Film-coated sandblasting: Same as in Example 1;

[0048] (2) Cleaning: After the second wafer roughening process is completed, the wafer is removed and placed in a hydrofluoric acid and nitric acid mixed solution bath with a hydrofluoric acid-nitric acid ratio of 1:2 and a temperature of 40°C. The wafer is heated and soaked for 40 minutes; then it is placed in pure water for overflow.

[0049] The back surface roughness of the lithium tantalate wafer processed in this comparative example is 0.13 μm, but the overall flatness of the wafer is 48 μm, and internal cracks have appeared in the wafer. It is determined that the tension of the wafer protective film changes during the hot and cold switching process, which causes the wafer flatness to deteriorate.

[0050] 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 roughening the back surface of an ultra-thin lithium tantalate wafer, characterized by: The method comprises the following steps: a) a lithium tantalate wafer is protected by a film on the front side, and then the lithium tantalate wafer is vacuum adsorbed on a sandblasting platform, wherein the thickness of the lithium tantalate wafer is 0.18-0.25 mm; b) the sandblasting abrasive is heated, and the sandblasting abrasive is baked at 80-85 DEG C for 30-40 min; c) the sandblasting platform is started, the rotation speed of the sandblasting platform is 30-40 rpm, the distance between the sandblasting gun and the lithium tantalate wafer is adjusted to 50-80 mm, the sandblasting pressure is adjusted to 0.2-0.3 Mpa, the back side of the wafer is roughened for the first time, the reciprocating number of the sandblasting gun is 1-2 times, and the reciprocating speed is 2-3 min / time; d) after the first roughening, the rotation of the sandblasting platform and the sandblasting pressure are turned off, the surface of the wafer is wiped with anhydrous ethanol by using a dust-free cloth to remove the residual sandblasting particles, and the wafer is blown by deionized air to reduce the electrostatic adsorption capacity, after the removal of the particles, the rotation speed of the sandblasting platform is increased to 30-40 rpm, the sandblasting pressure is adjusted to 0.1-0.15 Mpa, the back side of the wafer is roughened for the second time, the reciprocating number of the sandblasting gun is 3-5 times, and the reciprocating speed is 4-5 min / time; e) after the second roughening, the wafer is taken out, immersed in a mixed solution of hydrofluoric acid and nitric acid, the temperature of the solution tank is 40-45 DEG C, then the wafer is ultrasonically cleaned in a warm water tank with a temperature of 35-40 DEG C, then the wafer is ultrasonically cleaned in a normal-temperature tank with a temperature of 20-30 DEG C, and finally the wafer is placed in an overflow tank to complete the cleaning.

2. The method for roughening the back surface of an ultra-thin lithium tantalate wafer according to claim 1, characterized in that: In step a), the film attached to the lithium tantalate wafer is one of a blue film and a UV film, and the thickness of the film is 0.16-0.2 mm.

3. The method of claim 1, wherein the method is characterized by: In step a), the adsorption pressure of the sandblasting platform is 0.3-0.5 Mpa.

4. The method of claim 1, wherein the method is characterized by: In step b), the sandblasting abrasive is silicon carbide, alumina or ceramic powder, and the mesh number is #1000-2000.

5. The method of claim 1, wherein the method is characterized by: In step e), the ratio of hydrofluoric acid to nitric acid is 1:2, the wafer is immersed in the mixed solution of hydrofluoric acid and nitric acid for 30-50 min, ultrasonically cleaned in the warm water tank for 10-20 min at a frequency of 28 Hz, and ultrasonically cleaned in the normal-temperature tank for 10-20 min at a frequency of 40 Hz.