Method for manufacturing bonded body

Abstract

[PROBLEM] To suppress peeling in a bonded body, which is manufactured by providing an intermediate layer (2) on a rough surface (1a) of a piezoelectric material substate (1), subjecting the piezoelectric material substate (1) and the intermediate layer (2) to double-sided polishing, and then bonding a bonding surface of the intermediate layer (2) to a support substrate (3). [Solution] The intermediate layer (2) is provided on the first main surface (1a) of the piezoelectric material substrate (1) having the first main surface (1a) and a second main surface (1b) to obtain a laminate (10). The first main surface (1a) is roughened. The laminate (10) is subjected to double-sided polishing to polish the second main surface (1b) of the piezoelectric material substrate (1) and the bonding surface (2a) of the intermediate layer (2). The bonding surface (2b) of the intermediate layer (2A) is bonded to the support substrate (3). In the polishing step, the ratio of the average polishing amount on the outer periphery of the intermediate layer (2) to the average polishing amount on the inner periphery of the intermediate layer (the average polishing amount on the outer periphery / the average polishing amount on the inner periphery) is 1.1-1.2.

Description

Manufacturing method of the bonded body

[0001] The present invention relates to a method for producing a bonded body that can be suitably used for an acoustic wave device or the like.

[0002] Known acoustic wave devices include surface acoustic wave devices that can function as filter elements or oscillators used in mobile phones, etc., Lamb wave elements using piezoelectric thin films, thin film resonators (FBARs), etc. Known surface acoustic wave devices include those that have a support substrate and a piezoelectric material substrate that propagates surface acoustic waves bonded together, and have comb-shaped electrodes on the surface of the piezoelectric material substrate that can excite surface acoustic waves.

[0003] It has been reported that spurious signals can be reduced by roughening the surface of a piezoelectric material substrate in bonded substrates intended for surface acoustic wave devices (Patent Documents 1 and 2). Another known method is to roughen the surface of a piezoelectric substrate, provide a filler layer on the roughened surface to flatten it, and then bond this filler layer to a silicon substrate via an adhesive layer. In this method, epoxy or acrylic resins are used for the filler layer and adhesive layer, and roughening the bonding surface of the piezoelectric material substrate suppresses reflection of bulk waves and reduces spurious signals. Furthermore, because the roughened surface of the piezoelectric material substrate is filled and flattened before bonding, air bubbles are less likely to be trapped in the adhesive layer.

[0004] Patent No. 5814727 Patent No. 6427712 Patent No. 6747599

[0005] Conventionally, in order to directly bond a support substrate and an intermediate layer formed on a rough piezoelectric substrate, a single-sided polishing machine was used to polish the bonding surface of the intermediate layer to a mirror finish. However, single-sided polishing had problems such as the limited number of wafers that could be processed at one time and the wafers being easily broken when being attached to and detached from the polishing jig.

[0006] Therefore, the inventors focused on double-sided polishing, which enables the processing of a large number of piezoelectric material substrates at one time and does not require the piezoelectric material substrates to be fixed to a jig. Double-sided polishing is a commonly used method for polishing semiconductor silicon wafers, in which the wafers are placed in a carrier that holds the wafers and polished from both sides (Patent Document 3).

[0007] However, as shown in Figure 3(a), for example, when an intermediate layer 2 is formed on the rough surface of a piezoelectric material substrate of a bonded body 5 to obtain a laminate, and the laminate is then polished using a double-sided polisher, and the intermediate layer 2 is then directly bonded to a separate support substrate, it has been found that peeling tends to occur mainly at the outer peripheral edge of the intermediate layer 2. That is, in Figure 3(a), peeling sometimes occurs at the outer peripheral edge C of the intermediate layer 2. D is the bonded portion.

[0008] The object of the present invention is to provide an intermediate layer on a rough surface of a piezoelectric material substrate, subject the piezoelectric material substrate and the intermediate layer to double-side polishing processing, and then suppress peeling of the bonded body when bonding the bonding surface of the intermediate layer to a support substrate.

[0009] The present invention provides a method for manufacturing a bonded body, comprising: an intermediate layer growing step of providing an intermediate layer on a first main surface of a piezoelectric material substrate having a first main surface and a second main surface to obtain a laminate, wherein the first main surface is roughened; a polishing step of polishing the second main surface of the piezoelectric material substrate and a bonding surface of the intermediate layer by subjecting the laminate to double-side polishing; and a bonding step of bonding the bonding surface of the intermediate layer to a support substrate, wherein in the polishing step, the ratio of the average amount of polishing of the outer periphery of the intermediate layer to the average amount of polishing of the inner periphery of the intermediate layer (average amount of polishing of the outer periphery / average amount of polishing of the inner periphery) is 1.1 or more and 1.2 or less.

[0010] The inventors of the present invention have investigated the cause of peeling of the intermediate layer, mainly along the periphery, when bonding the bonding surface of the intermediate layer to a support substrate after providing an intermediate layer on the rough surface of a piezoelectric substrate and subjecting the piezoelectric substrate and intermediate layer to double-side polishing. This phenomenon does not occur when double-side polishing a silicon substrate.

[0011] During this research, we confirmed that when an intermediate layer was placed on the rough surface of a piezoelectric substrate and the piezoelectric substrate and intermediate layer were subjected to double-side polishing, the amount of polishing at the center of the intermediate layer was greater than that at the periphery. This opposite phenomenon was unexpected, since when double-side polishing a silicon wafer, the amount of polishing at the periphery is generally greater than the amount of polishing at the inner periphery.

[0012] In this way, when a piezoelectric material substrate having an intermediate layer provided on a rough surface is subjected to double-side polishing, polishing of the central portion tends to proceed faster than polishing of the peripheral portion. Therefore, when the central portion is polished to a mirror finish, a rough surface remains on the peripheral portion, which is thought to be why peeling remains on the peripheral portion after direct bonding.

[0013] To find a solution to this problem, the inventor further investigated the pressure distribution applied from the polishing pad to the piezoelectric material substrate and intermediate layer. Typically, in double-sided polishing, the polishing pad is elastic, so it tends to sink toward the periphery of the silicon wafer during processing. This pad sinking results in greater abrasion at the periphery than at the center. However, the opposite phenomenon has been observed when an intermediate layer is formed on a roughened piezoelectric material substrate. Therefore, the inventor investigated the pressure distribution applied to the laminate during processing. Specifically, with the laminate held on a carrier, a piezoelectric pressure-sensitive sheet was sandwiched between the laminate and the polishing platen, and pressure was applied to the laminate in a stationary state. The results showed that the pressure at the periphery of the intermediate layer was lower than at the center. This is thought to be due to membrane stress generated by the formation of the intermediate layer on the piezoelectric material substrate, which warped the resulting laminate, resulting in an unusual pressure distribution. It was also confirmed that increasing the processing load reduced the pressure ratio between the center and periphery. This is presumably because the warpage of the laminate was corrected by the increase in processing pressure, and the pressure difference between the center and the outer periphery was reduced.

[0014] Based on the above findings, the inventors have conceived the idea of ​​making the average amount of polishing of the outer periphery T of the intermediate layer 2 greater than the average amount of polishing of the inner periphery I when double-side polishing the laminate, as shown in Fig. 3(b) . Specifically, they have found that peeling after bonding to the support substrate can be suppressed by setting the ratio of the average amount of polishing of the outer periphery T of the intermediate layer 2 to the average amount of polishing of the inner periphery I of the intermediate layer 2 (average amount of polishing of the outer periphery T / average amount of polishing of the inner periphery I) to be 1.1 or more and 1.2 or less, thereby arriving at the present invention.

[0015] (a) shows a state in which an intermediate layer 2 is provided on a first main surface 1a of a piezoelectric material substrate 1, (b) shows the state after double-side polishing of the intermediate layer and the piezoelectric material substrate, (c) shows a state in which a neutralizing atomic beam A is irradiated onto a bonding surface 2b of the intermediate layer 2A, and (d) shows a state in which a neutralizing atomic beam B is irradiated onto a bonding surface 3a of a support substrate 3. (a) shows a bonded body 5, (b) shows a state in which the piezoelectric material substrate of the bonded body has been polished, and (c) shows an acoustic wave element 7. (a) shows a peeling pattern in the bonded body 5, and (b) shows the outer and inner peripheral portions of the intermediate layer 2.

[0016] The present invention will be described in detail below with reference to the accompanying drawings. As shown in Fig. 1(a), a piezoelectric material substrate 1 having a first principal surface 1a and a second principal surface 1b is prepared. The first principal surface 1a is roughened. Next, an intermediate layer 2 is provided on the principal surface 1a of the piezoelectric material substrate, thereby producing a laminate 10.

[0017] Next, the laminate 10 is subjected to double-side polishing processing. As a result, the second main surface 1b of the piezoelectric material substrate 1 is polished, resulting in a piezoelectric material substrate 1A having a polished surface 1c (see FIG. 1(b)). At the same time, the surface 2a of the intermediate layer is polished, resulting in an intermediate layer 2A having a polished bonding surface 2b.

[0018] Next, in a preferred embodiment, a neutralization beam is irradiated onto the bonding surface 2b of the intermediate layer 2 as shown by arrow A to activate the bonding surface 2b. Meanwhile, as shown in Fig. 1(d), a neutralization beam is irradiated onto the bonding surface 3a of the support substrate 3 as shown by arrow B to activate the bonding surface 3a. Then, as shown in Fig. 2(a), the bonding surface 3a of the support substrate 3 and the bonding surface 2b of the intermediate layer 2A are directly bonded to obtain a bonded body 5.

[0019] In a preferred embodiment, the polished surface 1c of the piezoelectric material substrate 1A of the bonded body is further polished to reduce the thickness of the piezoelectric material substrate 1B as shown in Fig. 2(b), thereby obtaining a bonded body 6. Reference numeral 1d denotes the polished surface. In Fig. 2(c), a surface acoustic wave element 7 is fabricated by forming a predetermined electrode 8 on the polished surface 1d of the piezoelectric material substrate 1B.

[0020] According to the present invention, in the double-side polishing process, the ratio of the average polishing amount of the outer peripheral portion of the intermediate layer to the average polishing amount of the inner peripheral portion of the intermediate layer (average polishing amount of the outer peripheral portion / average polishing amount of the inner peripheral portion) is set to 1.1 or more and 1.2 or less, where each average polishing amount is measured as follows.

[0021] First, the outer and inner circumferential portions of the intermediate layer are defined as follows. That is, as shown in FIG. 3(b), the width (radius) of the intermediate layer 2 is defined as L. In the example of FIG. 3(b), the intermediate layer 2 is not a perfect circle but has an orientation flat. In such a case, the radius of an imaginary circle that includes the entire outer contour of the intermediate layer 2 is defined as L. Here, the region of width (radius) i as viewed from the center O of the imaginary circle is defined as the inner circumferential portion, and the approximately ring-shaped region of width t outside of that is defined as the outer circumferential portion T. Here, the relationship between i and L is as follows: i = 0.93 × L. The film thicknesses of the outer circumferential portion T and the inner circumferential portion I before and after processing were measured using a microspectrophotometer (OPTM manufactured by Otsuka Electronics Co., Ltd.). However, because it is difficult to define the film thickness on a rough surface, measurements were taken at 80 points, and the average value was used as the film thickness.

[0022] Each of the constituent elements of the present invention will be further described below. The use of the bonded body of the present invention is not particularly limited, and it can be suitably applied to, for example, an acoustic wave element or an optical element.

[0023] Known acoustic wave elements include surface acoustic wave devices, Lamb wave elements, and thin film resonators (FBARs). For example, a surface acoustic wave device has an input-side IDT (interdigital transducer) electrode (also called a comb-shaped electrode or interdigital transducer) that excites surface acoustic waves and an output-side IDT electrode that receives the surface acoustic waves, both of which are provided on the surface of a piezoelectric material substrate. When a high-frequency signal is applied to the input-side IDT electrode, an electric field is generated between the electrodes, exciting surface acoustic waves that propagate across the piezoelectric substrate. The propagated surface acoustic waves can then be extracted as an electrical signal from the output-side IDT electrode provided in the propagation direction.

[0024] The piezoelectric material substrate may have a metal film on its bottom surface. The metal film serves to increase the electromechanical coupling coefficient near the back surface of the piezoelectric substrate when a Lamb wave element is manufactured as an acoustic wave device. In this case, the Lamb wave element has a structure in which a comb-shaped electrode is formed on the surface of the piezoelectric substrate and the metal film of the piezoelectric substrate is exposed by a cavity provided in the support substrate. Examples of materials for such a metal film include aluminum, aluminum alloy, copper, and gold. When manufacturing a Lamb wave element, a composite substrate including a piezoelectric substrate without a metal film on its bottom surface may also be used.

[0025] The bottom surface of the piezoelectric material substrate may also have a metal film and an insulating film. The metal film serves as an electrode when a thin-film resonator is manufactured as an acoustic wave device. In this case, the thin-film resonator has electrodes formed on the front and back surfaces of the piezoelectric substrate, and the insulating film has a cavity in which the metal film of the piezoelectric substrate is exposed. Examples of materials for such metal films include molybdenum, ruthenium, tungsten, chromium, and aluminum. Examples of materials for the insulating film include silicon dioxide, phosphorus silica glass, and boron phosphorus silica glass.

[0026] Examples of optical elements include optical switching elements, wavelength conversion elements, and optical modulation elements. A periodically poled structure can be formed in a piezoelectric material substrate.

[0027] The piezoelectric material substrate used in the present invention may be single crystal or polycrystalline. Specific examples of the material of the piezoelectric material substrate include lithium tantalate (LT) single crystal, lithium niobate (LN) single crystal, lithium niobate-lithium tantalate solid solution single crystal, quartz, and lithium borate. Of these, LT or LN is more preferable. The normal direction of the main surface of the piezoelectric material substrate is not particularly limited. For example, when the piezoelectric material substrate is made of LT, it is preferable to use a direction rotated 32 to 55 degrees from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, or (180°, 58 to 35°, 180° in Euler angle representation) because this reduces propagation loss. When the piezoelectric material substrate is made of LN, (a) a direction rotated 37.8° from the Z axis to the -Y axis around the X axis, which is the propagation direction of the surface acoustic wave, and using Euler angles of (0°, 37.8°, 0°) is preferred because it results in a large electromechanical coupling coefficient, or (b) a direction rotated 40 to 65° from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, and using Euler angles of (180°, 50 to 25°, 180°) is preferred because it results in a high sound velocity. Furthermore, the size of the piezoelectric material substrate is not particularly limited, but for example, it may have a diameter of 100 to 200 mm and a thickness of 0.15 to 1 μm.

[0028] The material of the support substrate is preferably silicon, sapphire, or quartz crystal.

[0029] In a preferred embodiment, the intermediate layer is made of one or more materials selected from the group consisting of silicon oxide, silicon nitride, aluminum nitride, alumina, tantalum pentoxide, mullite, niobium pentoxide, and titanium oxide. The method for forming the intermediate layer is not limited, but examples include sputtering, chemical vapor deposition (CVD), and evaporation.

[0030] In the present invention, one main surface of a piezoelectric material substrate is processed to form a roughened surface. This roughened surface is a surface with uniformly periodic asperities formed within the surface, with an arithmetic mean roughness of 0.05 μm≦Ra≦0.5 μm and a height Ry from the lowest valley to the highest peak in the range of 0.5 μm≦Ry≦5 μm. The appropriate roughness depends on the wavelength of the elastic wave and is appropriately selected so as to suppress the reflection of bulk waves. Methods for roughening the surface include grinding, polishing, etching, and sandblasting.

[0031] Next, the bonding surface of the intermediate layer and the bonding surface of the support substrate are polished to obtain flat surfaces. Each flat surface must have an Ra of 1 nm or less, and more preferably 0.3 nm or less.

[0032] Next, the bonding surfaces of the intermediate layer and the support substrate are activated by irradiating them with a neutralization beam. When performing surface activation using a neutralization beam, a saddlefield-type fast atom beam source is used as the beam source. An inert gas is introduced into the chamber, and a high voltage is applied to the electrodes from a DC power supply. This generates a saddlefield-type electric field between the electrode (positive electrode) and the housing (negative electrode), causing electrons e to move, generating a beam of atoms and ions from the inert gas. Of the beams that reach the grid, the ion beam is neutralized by the grid, and a beam of neutral atoms is emitted from the fast atom beam source. The atomic species that make up the beam are preferably inert gases (argon, nitrogen, etc.). The voltage during activation by beam irradiation is preferably 0.5 to 2.0 kV, and the current is preferably 50 to 200 mA.

[0033] Next, the activated bonding surfaces are brought into contact with each other in a vacuum atmosphere and bonded. The temperature during this process is room temperature, but specifically, 40°C or less is preferable, and 30°C or less is more preferable. The temperature during bonding is particularly preferably 20°C or more and 25°C or less. The pressure during bonding is preferably 100 to 20,000 N.

[0034] A bonded structure was fabricated according to the method described with reference to FIGS. 1 to 3 . Specifically, a lithium tantalate substrate (LT substrate) with an orientation flat (OF), a diameter of 6 inches, and a thickness of 350 μm was used as the piezoelectric material substrate 1. A silicon substrate with an OF, a diameter of 6 inches, and a thickness of 230 μm was prepared as the support substrate 3. The LT substrate was a 46° Y-cut X-propagation LT substrate, with the propagation direction of the surface acoustic wave (SAW) set to X and the cutout angle being a rotated Y-cut plate. The main surface 1 a of the piezoelectric material substrate 1 and the bonding surface 3 a of the support substrate 3 were mirror-polished to an arithmetic mean roughness Ra of 1 nm. The arithmetic mean roughness was evaluated using an atomic force microscope (AFM) in a square field of view measuring 10 μm long x 10 μm wide.

[0035] Next, the main surface 1a of the piezoelectric material substrate 1 was roughened. The roughening was performed as follows. When roughening the main surface 1a of the piezoelectric material substrate 1, lapping is preferred. Coarse abrasive grains such as GC#1000 or GC#2500 are used for lapping. When the roughened surface thus processed was measured using a New View 7300 manufactured by Zygo Corporation, it showed an Ra of 100 to 300 nm and an Rmax value of 1.4 to 4.0 μm.

[0036] Next, using a sputtering device, an intermediate layer 2 having a thickness of 6 μm was formed on the rough surface of a 6-inch, 350 μm-thick piezoelectric material substrate. When the roughness of the bonding surface 2 a of the intermediate layer 2 was measured using a white light interferometer (New View manufactured by Zygo), it was found that unevenness of 2 μm in P-V value had been formed, so the polishing amount was set to 2.5 μm. Next, a carrier for double-side polishing was prepared, and the laminate 10 was set in the carrier. A urethane pad was used as the polishing pad, and colloidal silica was used as the polishing abrasive.

[0037] After processing, the thickness of the intermediate layer was measured using a microspectrophotometric film thickness meter (OPTM manufactured by Otsuka Electronics Co., Ltd.). At that time, the radius of 70 mm was used as the boundary, and the average polishing amount in the inner periphery I and the average polishing amount in the outer periphery T and their ratio were calculated. The pressure during double-side polishing was adjusted, and the carrier thickness was adjusted within the range of 250 to 350 μm, thereby adjusting the average polishing amount in the outer periphery / average polishing amount in the inner periphery as shown in Table 1.

[0038] That is, when the pressure during polishing was lowered, the inner peripheral portion of the intermediate layer tended to be polished more. In contrast, by increasing the pressure during double-side polishing, the warp of the laminate was corrected, and the outer peripheral portion was polished more. Furthermore, when the thickness of the carrier was increased, the difference in thickness between the carrier and the laminate was reduced, and the polishing pad did not sink as deeply into the outer peripheral portion of the intermediate layer, resulting in a decrease in the average polishing amount of the outer peripheral portion. On the other hand, when the thickness of the carrier was reduced, the difference in thickness between the carrier and the laminate 10 was increased, and the polishing pad did sink as deeply into the outer peripheral portion of the intermediate layer, resulting in a relatively large polishing amount of the outer peripheral portion.

[0039] Next, the bonding surface 2b of the intermediate layer 2 and the bonding surface 3a of the support substrate 3 were cleaned to remove any dirt, and then introduced into a vacuum chamber. －６ After evacuation to the Pa range, the bonding surfaces of the substrates were irradiated with a fast atom beam (acceleration voltage 1 kV, Ar flow rate 27 sccm) for 120 seconds. Next, the bonding surfaces of the intermediate layer and the supporting substrate were brought into contact with each other, and then they were bonded by applying a pressure of 10,000 N for 2 minutes.

[0040] Next, the surface 1b of the piezoelectric material substrate 1 was ground and polished so that the thickness was reduced from the original 250 μm to 3 μm. Next, image processing was performed using images of the bonded body to determine the peeled portion at the interface between the intermediate layer and the support substrate, and the peeled area ratio was calculated. Specifically, the image processing was performed to identify the peeled portion between the intermediate layer and the support substrate based on the difference in contrast, and the ratio of the peeled area to the total area of ​​the piezoelectric layer was taken as the ratio of the area of ​​the peeled portion to the total area of ​​the bonded surface of the intermediate layer. The ratio (%) of the area of ​​the peeled portion to the total area of ​​the bonded surface of the intermediate layer was then measured, and the results are shown in Table 1.

[0041]

[0042] As a result, when the ratio of the average polishing amount of the outer peripheral portion to the average polishing amount of the inner peripheral portion was low and the inner peripheral portion was more heavily polished, the peeled area ratio increased mainly due to peeling at the outer peripheral edge. However, even when the ratio was 1.0, it was found that the peeled area ratio was still high. On the other hand, when the ratio of the average polishing amount of the outer peripheral portion to the average polishing amount of the inner peripheral portion was within the range of 1.1 to 1.2, that is, when the outer peripheral portion was slightly more polished than the inner peripheral portion, the peeled area ratio surprisingly decreased significantly. However, it was found that when this ratio exceeded 1.2, the peeled area ratio actually increased.

Claims

1. A method for producing a bonded body, comprising: an intermediate layer growing step of providing an intermediate layer on a first main surface of a piezoelectric material substrate having a first main surface and a second main surface to obtain a laminate, the first main surface being roughened; a polishing step of polishing the second main surface of the piezoelectric material substrate and a bonding surface of the intermediate layer by subjecting the laminate to double-sided polishing; and a bonding step of bonding the bonding surface of the intermediate layer to a support substrate, wherein in the polishing step, the ratio of the average amount of polishing of the outer periphery of the intermediate layer to the average amount of polishing of the inner periphery of the intermediate layer (average amount of polishing of the outer periphery / average amount of polishing of the inner periphery) is 1.1 or more and 1.2 or less.

2. The method of claim 1, wherein said intermediate layer is made of a material selected from the group consisting of silicon oxide, silicon nitride, aluminum nitride, alumina, tantalum pentoxide, mullite, niobium pentoxide and titanium oxide.

3. The method according to claim 1 or 2, characterized by comprising: a step of activating the bonding surface of the bonding layer and the bonding surface of the support substrate by irradiating the bonding surface of the intermediate layer and the bonding surface of the support substrate with a neutralizing beam; and a step of directly bonding the bonding surface of the bonding layer and the bonding surface of the support substrate.

4. The method according to any one of claims 1 to 3, wherein the piezoelectric material substrate is made of lithium niobate, lithium tantalate or a lithium niobate-lithium tantalate solid solution.