Method for processing filter substrate, substrate and TC-SAW filter

Abstract

The present application provides a method for processing a filter substrate, a filter substrate, and a TC-SAW filter. A method for fabricating a filter substrate includes fabricating a support layer substrate having a convex concentric circular structure, the convex concentric circular structure having a protruding center and at least two ring structures, the ring structures gradually decreasing in height from the center to the outside, bonding the side of the support layer substrate having the convex concentric circular structure to a piezoelectric wafer to obtain a composite substrate, and reducing the piezoelectric layer in the composite substrate to a predetermined thickness range and then polishing its surface.The use of a support layer substrate having a convex concentric circular shape significantly reduces the difficulty of polishing the piezoelectric layer, resulting in a piezoelectric layer substrate with a basically consistent thickness and a piezoelectric layer film with good uniformity, significantly improving the yield of piezoelectric layer film filter chips.

Description

[Technical field]

[0001] The present application relates to the field of filter technology, and more particularly to a method for processing a substrate for a filter, a substrate, and a TC-SAW filter. [Background technology]

[0002] Traditional SAW (surface acoustic wave) filter technology has a low quality factor Q value (<1000) and the frequency shifts with the change of working temperature, so it is difficult to meet the requirements for filters of radio frequency terminals in the 5G era where the frequency band is becoming more and more crowded, so traditional SAW filters need to develop into temperature compensated SAW filters (TC-SAW) with high frequency and stable temperature characteristics. The big problem is that SAW devices are sensitive to temperature changes, and as the temperature increases, the rigidity of the substrate material decreases and the sound speed also decreases, so the filter working frequency will shift to a certain extent with the change of external temperature. A currently available alternative is to adopt a composite substrate, which mainly uses a room temperature bonding method to bond a piezoelectric layer (lithium niobate (LN) / lithium tantalate (LT) wafer) to a substrate (spinel, polysapphire (Poly-SA), single crystal sapphire (SA), silicon slice, etc.) under high vacuum and pressure, and then reduces the thickness of the piezoelectric layer on the substrate to 15-30 μm by a thickness reduction polishing technique to form a composite substrate required for TC-SAW. TC-SAW devices manufactured using this type of substrate have the characteristics of high Q value and low temperature coefficient of frequency (TCF), which greatly improves the performance of the filter.

[0003] As the demand for communications continues to move toward higher frequencies, the thickness of the piezoelectric layer becomes thinner. However, when the thickness of the piezoelectric layer is less than 5 μm, the difference in thickness of the piezoelectric layer on the substrate becomes large due to processing technology issues. As shown in FIG. 1, after processing, the thickness ranges from 2.5 μm to 3.4 μm with obvious differences, which results in very large differences in the filter characteristics. Therefore, importance is being attached to the thickness uniformity of the piezoelectric layer.

[0004] Currently, the thickness uniformity of piezoelectric layers on substrates manufactured in the industry is about 20-40% (assuming the target thickness is 3 μm, the actual finished thickness range is 2.1 μm-3.6 μm, which is 70% and 120% of 3 μm, which are differences of 30% and 20%, respectively, so the thickness uniformity is 20%-30%), and the effect of frequency shift in 900 MHz filters exceeds ±1000 ppm, and in 1800 MHz filters the effect of frequency shift exceeds ±2000 ppm, which leads to very low yields of filter chips. Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, how to improve the thickness uniformity of the piezoelectric layer has become a focus of this research field. [Means for solving the problem]

[0006] Therefore, an objective of the embodiments of the present application is to provide a processing method for improving the thickness uniformity of the piezoelectric layer, for use in increasing the yield of filter chips.

[0007] According to a first aspect, a method for processing a support layer substrate having a convex concentric structure, the convex concentric structure having a protruding center and at least two layers of a ring structure, the ring structure having a height gradually decreasing from the center to the outside, bonding the side of the support layer substrate having the convex concentric circle structure to a piezoelectric wafer to obtain a composite substrate; and The present invention provides a method for processing a filter substrate, which includes reducing the thickness of the piezoelectric layer in the composite substrate to a predetermined thickness range and then polishing the surface.

[0008] In one embodiment, the processing of the support layer substrate having the convex concentric circle structure includes: Processing the support layer substrate to a predetermined thickness or using a support layer substrate having a predetermined thickness; performing a first thickness reduction operation from the front surface of the support layer substrate downward and from the side surface of the support layer substrate toward the center to obtain a central cylindrical structure; carrying out multiple thinning operations using the same method as the first thinning operation to obtain a ring structure having a predetermined number of layers; and The method includes performing double-side polishing on the front and back surfaces of the support layer substrate to obtain an ultra-flat substrate having a total thickness deviation smaller than a predetermined value.

[0009] In one embodiment, the first thickness reduction is performed from the front surface of the support substrate downward and from the side surface of the support substrate toward the center, The thickness reduction operation is performed by selecting and using a grinding wheel having a grinding wheel diameter of 2000# to 6000# and an angle between the central axis and the central axis of the support layer substrate of 0.5° to 2°; In the above-mentioned thickness reduction operation, the processing damage layer on the substrate surface is controlled to 4 to 10 μm.

[0010] In one embodiment, performing double-sided polishing on the front and back surfaces of the support layer substrate includes using a double-sided polishing machine having an upper plate, an internal gear, and a lower plate to polish the support layer substrate, the upper plate having a polishing pad and used for polishing the front surface of the support layer substrate, and the lower plate having a polishing pad and used for polishing the back surface of the support layer substrate; The rotation speed of the upper platen is 15-25 rpm / min, the rotation speed of the internal gear is 15-25 rpm / min, the rotation speed of the lower platen is 30-50 rpm / min, and the polishing pressure is 60-200 g / cm 3 It is.

[0011] In this application, the thickness uniformity of the composite substrate is 4% to 10%.

[0012] In one embodiment, the height difference between the annular structures in the convex concentric ring structure is less than 0.3 μm, and the maximum height value of the support layer substrate is less than 1 μm.

[0013] In one embodiment, the step of reducing the thickness of the piezoelectric layer in the composite substrate to a predetermined thickness range and then polishing the surface of the piezoelectric layer includes: The method includes reducing the thickness of the piezoelectric layer to 10-20 μm using a thickness reduction device, and then polishing the piezoelectric layer to a thickness of less than 5 μm using an adjustable air pad polisher.

[0014] In one embodiment, the material of the support layer substrate is any one of spinel, polysapphire, single crystal sapphire, high resistivity silicon, SiC, ALN, and quartz.

[0015] According to a second aspect of the present application, there is provided a filter substrate manufactured using any of the processing methods described above.

[0016] According to a third aspect of the present application, there is provided a TC-SAW filter comprising a substrate manufactured using any of the above processing methods, the substrate having interdigital transducers mounted on a surface thereof, the substrate having a thickness uniformity of less than 10%. Effect of the Invention

[0017] By utilizing the beneficial effects of the method for processing a filter substrate of the present application, 1. The thickness uniformity of the piezoelectric layer can be controlled to 4%~10% so that the total thickness deviation is THK_max-THK_min<0.3μm; 2. Filters manufactured using the substrate can have a uniform temperature coefficient of frequency (TCF) with upper and lower limits varying by less than 2 ppm / °C; 3. The filter has good frequency shift, the frequency shift at 900MHz can be controlled less than ±500ppm, and the frequency shift at 1800MHz can be controlled less than ±1000ppm. [Brief description of the drawings]

[0018] In order to clearly explain the technical solutions of the embodiments of the present application, the drawings used in the embodiments are briefly described below. The drawings below are only for illustrating some embodiments of the present application, and are not intended to limit the scope, and it should be understood that those with ordinary skill in the art can also obtain other related drawings according to these drawings without any creative effort. [Figure 1] FIG. 1 is a schematic diagram showing the structure of a TC-SAW filter. [Diagram 2] 1 is a diagram showing a flow of a conventional method for processing a composite substrate. [Diagram 3] FIG. 2 is a distribution diagram of the film thickness of a LiTaO3 (LT) lithium tantalate piezoelectric layer manufactured using a conventional processing method. [Figure 4] FIG. 2 is a flow diagram of a method for processing a filter substrate according to an embodiment of the present application. [Diagram 5] FIG. 2 is a schematic diagram showing a cross section of a support layer substrate according to an embodiment of the present application. [Figure 6] FIG. 2 is a polishing area distribution diagram of an adjustable air pad polishing apparatus according to an embodiment of the present application. [Figure 7] FIG. 1 is a diagram showing the appearance of sapphire in a comparative example. [Figure 8] FIG. 13 is a distribution diagram of the LT film thickness after polishing in the comparative example. [Figure 9] FIG. 13 is a distribution diagram of the LT film thickness after polishing in an example. [Figure 10] FIG. 11 is a diagram comparing the distribution of frequency temperature coefficients between a comparative example (left) and an embodiment (right). [Figure 11] FIG. 5 is a schematic diagram showing a processing flow corresponding to the method of FIG. 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In order to more clearly explain the objectives, technical solutions and advantages of the embodiments of the present application, the following will clearly and completely explain the technical solutions in the embodiments of the present application in combination with the accompanying drawings of the embodiments of the present invention. It should be apparent that the described embodiments are only some of the embodiments of the present application, and not all of the embodiments. Generally, the components of the embodiments of the present application depicted and shown in the accompanying drawings can be laid out and designed in various different configurations.

[0020] Therefore, the detailed description of the embodiments of the present application provided below in the accompanying drawings does not constitute any limitation on the protection scope of the present application, but merely represents selected embodiments of the present application. All other embodiments obtained based on the embodiments of the present application without the need for a person skilled in the art to perform creative work belong to the scope of the claims of the present application.

[0021] Fig. 1 shows a schematic diagram showing the structure of a TC-SAW filter. Referring to Fig. 1, the TC-SAW filter includes a piezoelectric layer 10, a support layer 20, and an interdigital transducer 30. The piezoelectric layer 10 and the support layer 20 form a composite substrate.

[0022] Fig. 2 shows the flow of a conventional method for processing a composite substrate. Referring to Fig. 2, the method for processing a composite substrate includes: (1) separately manufacturing a support layer 20 and a piezoelectric layer 10; (2) bonding the support layer 20 and the piezoelectric layer 10 at room temperature; (3) reducing the thickness of the piezoelectric layer 10; (4) performing adjustable air pad polishing on the support layer 20 to reduce the thickness to a required thickness for production; and (5) obtaining a finished product.

[0023] The composite substrate obtained by the above processing method has a difference in height between the high and low points on the top and bottom surfaces of 1 μm or more.

[0024] 3 shows the thickness distribution of the LiTaO3 (LT) lithium tantalate piezoelectric layer 10 manufactured using a conventional processing method, and according to FIG. 3, the thinnest point of the LT piezoelectric layer 10 is 2.54 μm, and the thickest point is 3.51 μm. The target thickness of the LT piezoelectric layer 10 is 3 μm, and the actually completed thickness range is 2.54 μm to 3.51 μm, which is 84.7% and 117% of 3 μm, with respective differences of 15.3% and 17%, and therefore the thickness uniformity is 15.3% to 17%. The thickness deviation is more than 15%, and in view of the required target thickness deviation being less than 10%, there is obvious non-uniformity in the thickness.

[0025] The composite substrate processing method used in this application can make the thickness deviation of the composite substrate less than 10%. The composite substrate processing method in this application will be described in detail below.

[0026] Fig. 4 is a flow diagram of a processing method for a filter substrate shown in an embodiment of the present application, and Fig. 11 is a schematic diagram showing a processing flow corresponding to the method of Fig. 4. Referring to Fig. 4 and Fig. 11, the method includes the following steps.

[0027] S101: A support layer substrate 100 having a convex concentric circle structure is processed.

[0028] In the processing method of the present application, the feature of the convex concentric structure in the support layer substrate 100 is that it has a protruding center, has at least two layers of a circular ring structure, and the height of the circular ring structure gradually decreases from the center to the outside, forming a multi-layer cake shape. The side of the support layer substrate 100 that is processed into the convex concentric structure is defined as the front side of the support layer substrate 100, and the opposite side is defined as the back side of the support layer substrate 100.

[0029] In one possible method, the method for processing the support layer substrate 100 having a convex concentric circle structure includes: a. Process the support layer substrate 100 to a predetermined thickness, or use a support layer substrate 100 with a predetermined thickness. The thickness of the support layer substrate 100 is the maximum height of the cross section of the convex concentric circle structure. Refer to FIG. 5, which shows the cross section of the support layer substrate 100. In the figure, 2 is the thickness of the support layer substrate 100, that is, the maximum height of the cross section of the convex concentric circle structure.

[0030] b. A first thickness reduction operation is performed from the front side of the support layer substrate 100 downward and from the side of the support layer substrate 100 toward the center to obtain a central cylindrical structure.

[0031] In this step, as one implementation method, a grinding wheel having a diameter of 2000# to 6000# and an angle of 0.5° to 2° between the central axis and the central axis of the support layer substrate 100 is selected and used to perform the thickness reduction. In the thickness reduction, the processing damage layer on the substrate surface is controlled to 4 to 10 μm.

[0032] c. Perform multiple thickness reductions using the same method as the first thickness reduction to obtain a ring structure with a predetermined number of layers. Referring to Figure 5, 1 shows the height difference between the ring structures of each layer in the convex concentric ring structure, and Figure 5 also shows the distance difference between the ring structures of each layer and the central axis of the support layer substrate 100.

[0033] After step C is completed, the convex concentric circle structure in the support layer substrate 100 has a primarily center-symmetric structure, and the TTV (total thickness deviation) is less than 1.5 μm.

[0034] d. The front and back surfaces of the support layer substrate 100 are double-sided polished to obtain an ultra-flat substrate with a total thickness deviation smaller than a predetermined value.

[0035] In this step, one implementation method is to perform double-side polishing on the front and back surfaces of the support layer substrate 100, The method includes polishing the support layer substrate 100 using a double-sided polishing machine having an upper surface, an internal gear, and a lower surface. A polishing pad is provided on the upper surface, which is used to polish the front side of the support layer substrate 100. A polishing pad is provided on the lower surface, which is used to polish the back side of the support layer substrate 100.

[0036] The rotation speed of the upper plate is 15-25 rpm / min, the rotation speed of the internal gear is 15-25 rpm / min, the rotation speed of the lower plate is 30-50 rpm / min, and the grinding pressure is 60-200 g / cm 3 It is.

[0037] The TTV of the support layer substrate 100 is further optimized by double-sided polishing, and the concentric circle structure is processed into a concentric circle convex structure.

[0038] According to the above processing method of the support layer substrate 100, the support layer substrate 100 in this application needs to have a height difference between layers of less than 0.3 μm, and the maximum thickness value of the support layer substrate 100 needs to be less than 1 μm.

[0039] In the present application, the material of the support layer substrate 100 can be selected from any of spinel, polysapphire (Poly-SA), single crystal sapphire, high resistivity silicon, SiC, ALN, quartz, and the like.

[0040] S102: The side of the support layer substrate 100 having the convex concentric circle structure is bonded to the piezoelectric wafer 200 to obtain a composite substrate.

[0041] The support layer substrate 100 on which the convex concentric circle structure is manufactured is bonded to a piezoelectric wafer 200. In one embodiment, the initial thickness of the piezoelectric wafer 200 is 150 μm. After bonding, the piezoelectric wafer 200 constitutes a piezoelectric layer.

[0042] S103: After reducing the thickness of the piezoelectric layer in the composite substrate to a predetermined thickness range, the surface is polished.

[0043] In this step, the piezoelectric layer is reduced in thickness to 10-20 μm by a thickness reduction device, and then the piezoelectric layer is polished to a thickness of less than 5 μm by an adjustable air pad polisher.

[0044] Fig. 6 shows the distribution of the polishing zones of the adjustable air pad polishing machine. Referring to Fig. 6, there are three zones 601, 602, and 603 distributed in the form of concentric circles, and different polishing pressures can be set in each zone, and the polishing operation can be realized in the convex concentric circle structure.

[0045] The comparative examples and the examples will be specifically compared and described below.

[0046] Comparative Example: First, a normally processed sapphire substrate was used as a support layer, and the shape of the sapphire substrate was measured using NIDEK (FT-900). It was found that the TTV of the sapphire substrate was 1.19 μm and the shape was non-concentric. Figure 7 shows the shape of the sapphire in the comparative example. Then, the sapphire and LT wafers were bonded at room temperature and in high vacuum, and after bonding, the LT thickness was reduced to 4.5 μm using a thickness reduction device, and then polished to a thickness of 3.0 μm. After polishing, the LT thickness was measured using a Flimmetrics (F-54) film thickness measurement device. Figure 8 shows the distribution of the polished LT film thickness in the comparative example. According to the data shown in Figure 8, the LT thickness uniformity is about 35.3%.

[0047] The above manufacturing flow of the composite substrate is based on the conventional composite substrate processing flow shown in FIG. 2. It can be seen that the adjustable air pad polishing cannot deal with the eccentric shape of the sapphire substrate, and there is an obvious difference in the thickness of the piezoelectric layer of the final product.

[0048] Working Example: Taking a general sapphire wafer, the above-mentioned method of processing the support layer substrate with a convex concentric circle structure is used to process the sapphire wafer. When the sapphire wafer is reduced in thickness, the amount of the reduction is determined by the TTV before the reduction. For example, if the TTV before the reduction is 9 μm, the amount of the reduction needs to be greater than 9 μm, which means that the TTV can be improved. After the reduction, the front profile of the sapphire is measured using NIDEK (FT-900), and the TTV is 1.01 μm, and the profile is a concentric circle.

[0049] Then, sapphire was bonded to the LT (lithium tantalate, LiTaO3) piezoelectric layer at room temperature and in high vacuum. After bonding, the thickness of the LT piezoelectric layer was reduced to 4.5 μm using a thickness reduction device, and then the LT piezoelectric layer was polished to a thickness of 3.0 μm. After polishing, the thickness of the LT piezoelectric layer was measured using a Flimmetrics (F-54) film thickness measurement device. FIG. 9 shows a distribution diagram of the polished LT film thickness in the embodiment. Referring to FIG. 9, the thickness uniformity of the LT piezoelectric layer was about 9.2%.

[0050] In the composite substrate obtained by using the filter substrate processing method of the present application, the thickness uniformity of the piezoelectric layer can be less than 10% because a convex concentric structure is provided on the side of the support substrate that is bonded to the piezoelectric layer, and each layer in the convex concentric structure has central symmetry, and when the piezoelectric layer is bonded to the support substrate and the thickness reduction operation is performed, the stress of the piezoelectric layer on the support substrate is mainly concentrated on the central cylindrical structure, and the distribution of the stress is more uniform for the ring structures other than the central cylindrical structure, so that the resistance during the grinding and thickness reduction operation can be reduced, and a piezoelectric substrate with a basically consistent thickness can be obtained. The thickness deviation of the piezoelectric layer produced by the processing method of the present application is within 10%, and in some embodiments, the thickness deviation can be within 5%.

[0051] Figure 10 shows the distribution of temperature coefficient of frequency for the comparative example (left) and the embodiment (right). As shown in Figure 10, the composite substrate in the embodiment has a uniform distribution of temperature coefficient of frequency (TCF) of 17ppm / ℃. At the same time, the embodiment has better filter frequency shift and can achieve ideal performance when applied to 900MHZ and 1800MHz respectively.

[0052] Through tests on the composite substrates made of different materials using the above processing method, it can be seen that the composite substrates obtained using the above processing method have the following characteristics: 1. The thickness uniformity of the piezoelectric layer is within 4%~10%, and the total thickness deviation is THK_max-THK_min<0.3μm; 2. It has a uniform temperature coefficient of frequency (TCF), with the upper and lower limits differing by less than 2 ppm / ℃; 3. It has good filter frequency shift, the frequency shift of 900MHz can be controlled less than ±500ppm, and the frequency shift of 1800MHz can be controlled less than ±1000ppm.

[0053] According to the above technical method, the present application uses a support layer substrate having a convex concentric circular shape, which significantly reduces the difficulty of polishing the piezoelectric layer and obtains a piezoelectric layer substrate with a basically consistent thickness, thereby obtaining a piezoelectric thin film with good uniformity, and thus greatly improving the yield of filter chips made of piezoelectric thin film.

[0054] In a second aspect of the present application, there is provided a filter substrate manufactured using the above processing method.

[0055] In a third aspect of the present application, a TC-SAW filter is provided. Referring to FIG. 1, the TC-SAW filter includes a substrate manufactured using the above processing method. Interdigital transducers are mounted on a surface of the substrate. The thickness uniformity of the substrate is less than 10%. The substrate includes a support layer and a piezoelectric layer, and the interdigital transducers are disposed on the piezoelectric layer.

[0056] The above are only preferred embodiments of the present application, and the present application is not limited thereto, and various modifications and variations may be made by those skilled in the art to which the present invention pertains. As long as the modifications and variations are within the spirit and principles of the broadest interpretation of the present application, they shall be included within the scope of protection of the present application.

Claims

1. Processing a support layer substrate having a convex concentric circle structure, the convex concentric circle structure having a protruding center and at least two layers of a ring structure, the ring structure having a height gradually decreasing from the center to the outside; bonding the side of the support layer substrate having the convex concentric circle structure to a piezoelectric wafer to obtain a composite substrate; and The method includes reducing the thickness of the piezoelectric layer in the composite substrate to a predetermined thickness range and then polishing the surface of the piezoelectric layer; The processing of the support layer substrate having the convex concentric circle structure includes: Processing the support layer substrate to a predetermined thickness or using a support layer substrate having a predetermined thickness; performing a first thickness reduction operation from the front surface of the support layer substrate downward and from the side surface of the support layer substrate toward the center to obtain a central cylindrical structure; performing multiple thickness reductions using the same method as the first thickness reduction to obtain a ring structure having a predetermined number of layers; and and polishing the front and back surfaces of the support layer substrate to obtain an ultra-flat substrate having a total thickness deviation smaller than a predetermined value.

2. The first thickness reduction operation is performed from the front surface of the support layer substrate downward and from the side surface of the support layer substrate toward the center, The thickness reduction operation is performed by selecting and using a grinding wheel having a grinding wheel diameter of 2000# to 6000# and an angle between the central axis and the central axis of the support layer substrate of 0.5° to 2°; 2. The processing method according to claim 1, wherein in the thickness reduction operation, a processing damage layer on the substrate surface is controlled to 4 to 10 μm.

3. The double-sided polishing of the front and back surfaces of the support layer substrate includes polishing the support layer substrate using a double-sided polishing device having an upper plate, an internal gear, and a lower plate, the upper plate having a polishing pad and used for polishing the front surface of the support layer substrate, and the lower plate having a polishing pad and used for polishing the back surface of the support layer substrate; The rotation speed of the upper plate is 15-25 rpm / min, the rotation speed of the internal gear is 15-25 rpm / min, the rotation speed of the lower plate is 30-50 rpm / min, and the polishing pressure is 60-200 g / cm ３ 2. The processing method according to claim 1, wherein

4. 4. The processing method according to claim 1, wherein the thickness uniformity of the composite substrate is 4% to 10%.

5. 5. The processing method according to claim 4, wherein the height difference between the annular structures in the convex concentric circle structure is less than 0.3 μm, and the maximum height value of the support layer substrate is less than 1 μm.

6. The step of reducing the thickness of the piezoelectric layer in the composite substrate to a predetermined thickness range and then polishing the surface of the piezoelectric layer is 6. The method of claim 5, further comprising: reducing the thickness of the piezoelectric layer to 10-20 μm with a thickness reducing device, and then polishing the piezoelectric layer to a thickness of less than 5 μm with an adjustable air pad polisher.

7. 7. The processing method according to claim 6, wherein the material of the support layer substrate is any one of spinel, polysapphire, single crystal sapphire, high-resistivity silicon, SiC, ALN, and quartz.

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