Method for manufacturing composite substrate, composite substrate, and acoustic wave device

Abstract

According to the present invention, the bonding strength between a silicon substrate and a piezoelectric substrate is increased, and in the event that a slip occurs during device formation heat treatment, elongation of the slip is suppressed.　This method for manufacturing a composite substrate comprises: a step for preparing a piezoelectric substrate; a step for preparing a silicon substrate; a heat treatment step for subjecting the silicon substrate to heat treatment in which the temperature is increased at a predetermined temperature increase rate, the maximum attained temperature is maintained at 1100°C or higher and 1400°C or lower for a predetermined time, and the temperature is decreased at a predetermined temperature decrease rate; a bonding step for bonding the silicon substrate and the piezoelectric substrate together after the heat treatment step; and a thinning step for thinning the piezoelectric substrate.

Description

Composite substrate manufacturing method, composite substrate, and acoustic wave device

[0001] The present invention relates to a method for manufacturing a composite substrate, a composite substrate, and an acoustic wave device.

[0002] In recent years, piezoelectric materials such as lithium niobate (hereinafter referred to as "LN") and lithium tantalate (hereinafter referred to as "LT") have been used as materials for acoustic wave elements (hereinafter referred to as "acoustic wave devices"), such as SAW (Surface Acoustic Wave) filters and thin-film resonators. Furthermore, piezoelectric substrates made of these piezoelectric materials are bonded to support substrates with smaller thermal expansion coefficients than the piezoelectric substrate, such as silicon, quartz glass, quartz crystal, and borosilicate glass, to form composite substrates, which are then used as substrates for acoustic wave devices, such as SAW filters and thin-film resonators. Bonding the composite substrate to a support substrate with a smaller thermal expansion coefficient than the piezoelectric substrate reduces the size change of the composite substrate when the temperature changes, thereby reducing the change in the frequency characteristics of the acoustic wave device.

[0003] Patent Document 1 discloses a method for manufacturing a composite substrate in which a silicon substrate with an interstitial oxygen concentration of 2 to 10 ppma is used as a support substrate, this silicon substrate is bonded to a piezoelectric substrate, and then the piezoelectric substrate is thinned. It is described that this method allows the composite substrate to maintain high resistance even after a heat treatment process at 300°C.

[0004] Patent Document 2 discloses a composite substrate in which the thickness of the piezoelectric substrate is 5 to 100 μm, the support substrate is made of silicon with a resistivity of 2000 Ω cm or more, and both surface layers are oxidized to a thickness of 0.1 to 20 μm. It is stated that this results in a composite substrate with little warping when the temperature changes and excellent heat resistance.

[0005] In Patent Document 3, a bonding layer provided between a support substrate and a piezoelectric substrate is made of Si （1-x） O x The document discloses a composite substrate having a composition of (0.008≦x≦0.408), which is said to enhance the insulating properties of the bonding layer while increasing the bonding strength.

[0006] JP 2018-117030 A JP 2005-347295 A Japanese Patent No. 6375471 A

[0007] The disclosures of the above-mentioned prior art documents are incorporated herein by reference. The following analysis has been carried out by the present inventors.

[0008] However, because the difference in thermal expansion coefficient between a piezoelectric substrate and a silicon substrate is very large, if the composite substrate is bonded together and then heat treated during device formation, the bonding surfaces of the silicon substrate and the piezoelectric substrate may peel off, or slip may form in the silicon substrate, which may then expand and degrade the device characteristics.

[0009] In view of the above-mentioned problems, the object of the present invention is to provide a method for manufacturing a composite substrate, a composite substrate, and an acoustic wave device that improve the bonding strength between a silicon substrate and a piezoelectric substrate and contribute to suppressing the elongation of slips even if slips occur during the heat treatment for device formation.

[0010] In order to solve the above problems, the manufacturing method of the composite substrate of the present invention is characterized by comprising: a step of preparing a piezoelectric substrate; a step of preparing a silicon substrate; a heat treatment step of heating the silicon substrate at a predetermined heating rate, maintaining the maximum temperature at 1100°C or higher and 1400°C or lower for a predetermined time, and then cooling it at a predetermined cooling rate; a bonding step of bonding the silicon substrate and the piezoelectric substrate together after the heat treatment step; and a thinning step of thinning the piezoelectric substrate.

[0011] In the manufacturing method of the composite substrate of the present invention, oxygen precipitate nuclei and oxygen precipitates are formed on the silicon substrate by a heat treatment step in which the silicon substrate is subjected to a heat treatment with a maximum temperature of 1100°C or higher and 1400°C or lower. These oxygen precipitate nuclei and oxygen precipitates grow during the heat treatment at a relatively low temperature (e.g., 200-400°C) during device formation, so that even if slips enter the silicon substrate, the oxygen precipitates (BMD) can prevent the slip extension. In addition, silicon substrates subjected to the heat treatment at the above-mentioned high temperature can also improve bonding strength. The heat treatment step is preferably performed in a non-oxidizing atmosphere.

[0012] The heat treatment step is preferably performed in an oxygen atmosphere with an oxygen partial pressure of 1 to 100%. This allows the density and size of oxygen precipitate nuclei and oxygen precipitates to be controlled. Therefore, the density and size of oxygen precipitate nuclei and oxygen precipitates can be optimized to match the heat treatment used in forming the acoustic wave device.

[0013] The heat treatment step is preferably a batch heat treatment in which the predetermined heating rate is 1 to 20°C / min, the predetermined cooling rate is 1 to 20°C / min, and the maximum temperature is maintained at 1100°C or higher and 1250°C or lower for 0.5 to 20 hours. Because this relatively high-temperature heat treatment process is carried out for a long period of time, many oxygen precipitates are formed. Furthermore, the oxygen precipitate nuclei and oxygen precipitates are also formed in relatively large sizes compared to the rapid heating and cooling heat treatment, which is a short-term heat treatment process. Therefore, slip elongation can be further suppressed.

[0014] It is preferable that the method further includes a bonding layer forming step of forming a bonding layer on the surface of the silicon substrate before the bonding step. It is also preferable that the bonding layer is a silicon oxide film formed on the silicon substrate during the heat treatment step. Furthermore, the bonding layer can be configured to include one or more layers selected from the group consisting of a silicon oxide film layer, a silicon nitride film layer, an amorphous silicon layer, a polysilicon layer, an organic adhesive layer, a titanium layer, and a titanium oxide film layer.

[0015] The silicon substrate is sliced ​​from a vacancy-rich silicon single crystal produced by the Czochralski method, and has an interstitial oxygen concentration of 2.0E17 to 1.3E18 atoms / cm 3 By "silicon substrates sliced ​​from vacancy-rich (V-rich) silicon single crystals produced by the Czochralski method," it is possible to increase productivity while facilitating the formation of oxygen precipitate nuclei and oxygen precipitates. Furthermore, "interstitial oxygen concentration is 2E17 to 1.3E18 atoms / cm" 3 By making the oxygen concentration higher than that of the FZ substrate, the strength of the silicon substrate itself is increased, and by facilitating the growth of oxygen precipitate nuclei and oxygen precipitates during device heat treatment, slip extension is made more difficult.

[0016] The heat treatment process is a rapid temperature increase / decrease (RTP) process performed in an oxygen atmosphere (RTO) with an oxygen partial pressure of 1 to 100%, a maximum temperature of 1300°C or higher, and a predetermined time period of 1 to 60 seconds. While fewer oxygen precipitates are formed than in batch annealing, they grow even at relatively low temperatures during device fabrication, preventing slip extension even if slip occurs in the silicon substrate. Additionally, by using a silicon oxide film formed in an oxygen atmosphere as a bonding layer, the heat treatment process and the bonding layer formation process can be performed simultaneously, improving bonding strength. It is preferable that the silicon substrate is not polished after the heat treatment process and before the thinning process.

[0017] In order to achieve the above object, the present invention provides a composite substrate comprising a silicon substrate and a piezoelectric substrate, wherein the silicon substrate and the piezoelectric substrate are bonded together via a bonding layer, the bonding layer including at least a silicon oxide film layer, the silicon substrate being vacancy-rich, and the interstitial oxygen concentration of the bulk layer being 2.0E17 to 1.3E18 atoms / cm. 3 and the bulk layer has oxygen precipitates of 20 nm or less at a density of 1.0E2 / cm 3The oxygen precipitate nuclei and oxygen precipitates can stop the extension of slips by forming BMDs even if slips enter the silicon substrate.

[0018] The silicon substrate preferably has a DZ layer on the piezoelectric substrate side and the bulk layer on the opposite side to the piezoelectric substrate side, in which oxygen precipitates are formed. Alternatively, the silicon substrate may have a bulk layer on the piezoelectric substrate side, in which oxygen precipitate nuclei or oxygen precipitates are formed, and a DZ layer on the opposite side to the piezoelectric substrate side. The bonding layer may be configured to include one or more of a silicon oxide layer, a silicon nitride layer, an amorphous silicon layer, a polysilicon layer, an organic adhesive layer, a titanium layer, and a titanium oxide layer.

[0019] The silicon substrate has a lower interstitial oxygen concentration than the bulk layer, and the interstitial oxygen concentration is 4.0E14 to 6.0E17 atoms / cm 3 It is preferable that the DZ layer is located on the piezoelectric substrate side.

[0020] In order to achieve the above object, an acoustic wave device according to the present invention includes the above-described composite substrate and an electrode provided on a piezoelectric substrate.

[0021] According to each aspect of the present invention, it is possible to provide a method for manufacturing a composite substrate, a composite substrate, and an acoustic wave device that improve the bonding strength between a silicon substrate and a piezoelectric substrate and contribute to suppressing the elongation of slips even if slips occur during the heat treatment for device formation.

[0022] FIG. 1 is a schematic cross-sectional view showing an example of a composite substrate according to embodiment 1 of the present invention. FIG. 2 is a flowchart showing a method for manufacturing a composite substrate according to embodiment 1. FIG. 3 is a schematic view showing a batch-type heat treatment apparatus used in the heat treatment step of embodiment 1. FIG. 4 is a schematic view showing a heat treatment process in the heat treatment step of embodiment 1. FIG. 5 is a schematic view showing a rapid temperature increase / decrease heat treatment apparatus used in the heat treatment of embodiment 2. FIG. 6 is a flowchart showing a method for manufacturing a composite substrate according to embodiment 2. FIG. 7 is a schematic cross-sectional view showing an example of a composite substrate according to embodiment 3 of the present invention.

[0023] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiments described below. In addition, the same or corresponding elements in each drawing are appropriately designated by the same reference numerals. Furthermore, it should be noted that the drawings are schematic, and the dimensional relationships and ratios of each element may differ from those in reality. There may also be parts in which the dimensional relationships and ratios differ between the drawings.

[0024] 1 is a schematic cross-sectional view showing an example of a composite substrate 5 according to a first embodiment of the present invention. As shown in Fig. 1, the composite substrate 5 is fabricated by bonding a piezoelectric substrate 1 and a silicon substrate 2, which is a support substrate having a smaller thermal expansion coefficient than the piezoelectric substrate 1, via an insulating bonding layer 3. With this configuration, when stress is generated in the piezoelectric substrate 1 and the silicon substrate 2 due to a temperature change, the thermal expansion of the piezoelectric substrate 1 can be suppressed, thereby improving the frequency-temperature characteristics compared to those of a single piezoelectric substrate.

[0025] The composite substrate 5 can be fabricated by, for example, forming an oxide film on one or both of the piezoelectric substrate 1 and the silicon substrate 2, bonding them together under a load. The bonding layer 3 may be a multi-layer structure including one or more layers selected from the group consisting of a silicon oxide layer, a silicon nitride layer, an amorphous silicon layer, a polysilicon layer, an organic adhesive layer, a titanium layer, and a titanium oxide layer. The thickness of the bonding layer 3 is preferably 0.05 to 50 μm. A thickness thinner than 0.05 μm is insufficient to reduce the warpage of the composite substrate 5, while a thickness thicker than 50 μm is undesirable because cracks may occur in the piezoelectric substrate 1. In addition, the surface may be hydrophilized using an ammonia / hydrogen peroxide solution or plasma activation treatment to enhance adhesion. The size of the substrate is not particularly limited; for example, diameters of 125 mm, 150 mm, or 200 mm may be used, or may be larger or smaller.

[0026] The piezoelectric substrate 1 in this embodiment has a thickness of 0.5 to 150 μm. Although the thickness varies depending on the application, it is preferably 30 to 120 μm, more preferably 50 to 100 μm. Furthermore, for composite substrates 5 used in high-frequency acoustic wave devices, the thickness of the piezoelectric substrate 1 is preferably 0.5 to 10 μm, more preferably 0.5 to 3.0 μm. To achieve the desired thickness of the piezoelectric substrate 1, for example, the piezoelectric substrate 1 may be ground and polished after the composite substrate 5 is formed. Note that a thickness of less than 0.5 μm is undesirable because cracks may occur due to processing strain caused by polishing. Furthermore, a thickness of more than 150 μm is undesirable because the piezoelectric substrate 1 may crack when the composite substrate 5 is heat-treated during device formation. The piezoelectric substrate 1 may also be thinned using a so-called ion implantation separation method. The ion implantation separation method is a method in which ions of a light element such as hydrogen or helium are implanted into an active layer wafer at a predetermined depth to form an ion-implanted layer, the active layer wafer is then bonded to a support substrate wafer, and the active layer wafer is then delaminated at the ion-implanted layer to reduce its thickness.

[0027] The piezoelectric substrate 1 can be made of any piezoelectric crystal material, but lithium tantalate or lithium niobate is preferable because it has a large electromechanical coupling coefficient, a wide bandwidth as a frequency selective filter, and allows the production of an acoustic wave element with low insertion loss. The substrate orientation can be selected appropriately depending on the type of piezoelectric crystal material and the application of the acoustic wave element, such as 36° rotated Y-cut, 41° rotated Y-cut, or 45° rotated Y-cut.

[0028] The support substrate is a silicon substrate 2 which has been heated at a predetermined heating rate, heat-treated at a maximum temperature of 1100° C. to 1400° C. for a predetermined time, and cooled at a predetermined cooling rate to form oxygen precipitate nuclei 4 and oxygen precipitates 4. In this specification, particles with a size of less than 1 nm are referred to as oxygen precipitate nuclei 4, and particles with a size of 1 nm or more are referred to as oxygen precipitate nuclei 4, and particles with a size of 1 nm or more are referred to as oxygen precipitate 4.

[0029] The thickness of the silicon substrate 2 is preferably 50 to 500 μm. The silicon substrate 2 has a smaller thermal expansion coefficient than the piezoelectric substrate 1 and is also suitable for mass production. The resistivity of the silicon substrate 2 is preferably high, preferably 1000 Ω·cm or more, and more preferably 4000 Ω·cm or more. In this embodiment, the silicon substrate 2 is preferably manufactured using the Czochralski method (hereinafter also referred to as the "CZ method") to form oxygen precipitate nuclei 4 and oxygen precipitates 4, and is preferably a silicon substrate sliced ​​from a so-called vacancy-rich (V-rich: vacancy-dominant) silicon single crystal. The interstitial oxygen concentration of the silicon substrate 2 before heat treatment is preferably 2.0E17 to 1.3E18 atoms / cm. 3 is desirable, and 5.0E17 to 9.0E17 atoms / cm 3 It is more desirable that:

[0030] In addition, a polysilicon layer may be formed on the silicon substrate 2 as a charge trapping layer, and in this embodiment, the silicon substrate on which the polysilicon layer is formed will also be simply referred to as a ``silicon substrate'' and will be included in the silicon substrate 2.

[0031] The silicon substrate 2 of this embodiment has the bulk layer 7 containing the oxygen precipitates 4 of 20 nm or less at a density of 1.0E2 / cm. 3 The size and density of the oxygen precipitates 4 were measured using a transmission electron microscope (TEM).

[0032] The silicon substrate 2 of this embodiment is configured so that the DZ layer 6 is located on the side that is bonded to the bonding layer 3 (the piezoelectric substrate 1 side), and the bulk layer 7, in which oxygen precipitate nuclei 4 and oxygen precipitates 4 are formed, is located on the side that is not bonded to the bonding layer 3 (the side opposite the piezoelectric substrate 1 side). With this configuration, the bonding strength between the piezoelectric substrate 1 and the silicon substrate 2 is increased, and even if slips are introduced into the silicon substrate 2 during the heat treatment for forming the device, the oxygen precipitates 4 suppress the extension of the slips.

[0033] Alternatively, the DZ layer 6 may be located on the side not bonded to the bonding layer 3 (the side opposite to the piezoelectric substrate 1), and the bulk layer 7 in which oxygen precipitate nuclei 4 and oxygen precipitates 4 are formed may be located on the side bonded to the bonding layer 3 (the side facing the piezoelectric substrate 1). With this configuration, even if slip occurs near the bonding surface of the silicon substrate 2, it is possible to prevent the slip from extending to the bonding surface. In particular, if the slip extends to the bonding surface, cracks or peeling of the bonding surface may occur, which is undesirable. Therefore, with the above configuration, it is possible to primarily prevent the slip from extending near the bonding surface.

[0034] 2 is a flowchart showing a method for manufacturing the composite substrate 5 of embodiment 1. As shown in FIG. 2, the method for manufacturing the composite substrate 5 of embodiment 1 includes a step (S1) of preparing a silicon substrate 2, a heat treatment step (S2), a bonding layer forming step (S3), a step (S4) of preparing a piezoelectric substrate 1, a bonding step (S5), and a thinning step (S6).

[0035] In the step (S1) of preparing a silicon substrate 2, a silicon substrate 2 is prepared to be used as a support substrate for the composite substrate 5. The thickness of the silicon substrate 2 is preferably 50 to 500 μm. The silicon substrate 2 may be one sliced ​​from a silicon single crystal manufactured by the well-known FZ method or CZ method, but it is preferable that the silicon substrate 2 is a silicon substrate sliced ​​from a silicon single crystal manufactured by the CZ method. In addition, the interstitial oxygen concentration of the silicon substrate 2 before heat treatment is 2.0E17 to 1.3E18 atoms / cm. 3 is desirable, and 5.0E17 to 9.0E17 atoms / cm 3 It is more desirable that:

[0036] In the heat treatment step (S2), the silicon substrate 2 is subjected to a heat treatment in which the temperature is increased at a predetermined rate, the maximum temperature is maintained at 1100°C or higher and 1400°C or lower for a predetermined time, and the temperature is decreased at a predetermined rate. Here, it is preferable that the predetermined rate of temperature increase is 1 to 20°C / min, the predetermined rate of temperature decrease is 1 to 20°C / min, and a batch heat treatment is performed in which the maximum temperature is maintained at 1100°C or higher and 1250°C or lower for 0.5 to 20 hours. By this heat treatment step (S2), oxygen precipitate nuclei 4 and oxygen precipitates 4 are formed in the silicon substrate 2.

[0037] In the bonding layer formation step (S3), a bonding layer 3 is formed on the surface of the silicon substrate 2. The bonding layer 3 may include one or more layers selected from the group consisting of a silicon oxide layer, a silicon nitride layer, an amorphous silicon layer, a polysilicon layer, an organic adhesive layer, a titanium layer, and a titanium oxide layer, and may have a multilayer structure including multiple layers. The thickness of the bonding layer 3 is preferably 0.05 to 50 μm.

[0038] In the step (S4) of preparing a piezoelectric substrate 1, the piezoelectric substrate 1 to be used in the composite substrate 5 is prepared. As described above, the piezoelectric substrate 1 may be made of any piezoelectric crystal material. The thickness of the piezoelectric substrate 1 is preferably 30 to 120 μm, and more preferably 50 to 100 μm. Furthermore, in the case of a composite substrate 5 used in a high-frequency acoustic wave device, the thickness of the piezoelectric substrate 1 is preferably 0.5 to 10 μm, and more preferably 0.5 to 3.0 μm. The piezoelectric substrate 1 can be prepared by processing it appropriately according to the application of the composite substrate 5.

[0039] In the bonding step (S5), the piezoelectric substrate 1 and the silicon substrate 2 are bonded together via the bonding layer 3 and a load is applied to bond them. At this time, the bonding may be performed while applying heat. Alternatively, the bonding may be performed after plasma treatment. Thereafter, in the thinning step (S6), double-sided polishing of the composite substrate 5 is performed to adjust the thicknesses of the piezoelectric substrate 1 and the silicon substrate 2 to the desired values. Note that after double-sided polishing, single-sided polishing may be performed to adjust the thicknesses of the piezoelectric substrate 1 and the silicon substrate 2 to the desired values.

[0040] Fig. 3 is a schematic diagram showing a batch-type heat treatment apparatus 10 used in the heat treatment step of embodiment 1. As shown in Fig. 3, the batch-type heat treatment apparatus 10 has a heat treatment board 11, and a plurality of silicon substrates 2 are housed in the batch-type heat treatment apparatus 10 while being stored on the heat treatment board 11. The batch-type heat treatment apparatus 10 is provided with an inlet 12 for a heat treatment gas, and the heat treatment gas is introduced into the reaction chamber of the batch-type heat treatment apparatus 10 to heat-treat the silicon substrates 2.

[0041] 4 is a schematic diagram showing the heat treatment process in the heat treatment step of embodiment 1. The heat treatment step of embodiment 1 is a batch heat treatment in an Ar atmosphere. The heat treatment atmosphere may be a non-oxidizing atmosphere such as Ar or nitrogen, or an oxygen atmosphere.

[0042] In the heat treatment step of the first embodiment, a silicon substrate 2 sliced ​​from a vacancy-rich silicon single crystal ingot grown by the CZ method is subjected to a heat treatment in which the temperature is increased from T2 to T1 at a predetermined temperature increase rate, the maximum temperature reached at T1 is maintained for a time period of t3 from t1 to t2, and the temperature is decreased from T1 to T2 at a predetermined temperature increase rate, as shown in FIG.

[0043] In the case of batch heat treatment such as the heat treatment of embodiment 1, the temperature rise rate is preferably 1 to 20°C / min, more preferably 5 to 15°C / min. The temperature drop rate is also preferably 1 to 20°C / min, more preferably 5 to 15°C / min. The maximum temperature T1 is preferably 1100°C or higher and 1250°C or lower, more preferably 1150°C or higher and 1250°C or lower. The holding time t3 at the maximum temperature is preferably 0.5 to 20 hours. However, from the viewpoint of productivity, it is more preferable to hold the temperature for 0.5 to 10 hours.

[0044] By the above-mentioned batch heat treatment, a DZ layer 6 and a bulk layer 7 are formed on the silicon substrate 2, and the bulk layer 7 contains oxygen precipitates 4 of 20 nm or less at a density of 1.0E4 to 1.0E11 / cm. 3In addition, by forming oxygen precipitation nuclei 4 in the bulk layer 7 under a non-oxidizing Ar atmosphere, the interstitial oxygen concentration in the DZ layer 6 is significantly reduced compared to that in the bulk layer 7, to 4.0E14 to 6.0E17 atoms / cm 3 Therefore, the composite substrate 5 has a lower interstitial oxygen concentration than the bulk layer 7, and is 4.0E14 to 6.0E17 atoms / cm 3 Therefore, the composite substrate 5 using the silicon substrate 2 of this embodiment can suppress the extension of slips while suppressing the decrease in resistivity due to thermal donors.

[0045] (Embodiment 2) Embodiment 2 is characterized in that the heat treatment process is a rapid temperature increase / decrease heat treatment in an oxygen atmosphere, and the heat treatment process also serves as the bonding layer formation process. Since the other configurations are substantially the same as those of embodiment 1, the description thereof will be omitted.

[0046] 5 is a schematic diagram showing a rapid heating / cooling heat treatment apparatus 20 used in the heat treatment of embodiment 2. As shown in Fig. 5, the rapid heating / cooling heat treatment apparatus 20 includes a chamber (reaction tube) 21 having an atmospheric gas inlet 20a and an atmospheric gas outlet 20b, a plurality of lamps 30 spaced apart above the chamber 21, and a substrate support 40 that supports a silicon substrate 2 in a reaction space 25 within the chamber 21. Furthermore, although not shown, the apparatus includes a rotation means for rotating the silicon substrate 2 around its central axis at a predetermined speed.

[0047] The substrate support 40 includes a ring 22 that supports the outer periphery of the silicon substrate 2, and a stage 40a that supports the ring 22. The chamber 21 is made of, for example, quartz. The lamps 30 are made of, for example, halogen lamps. The stage 40a is made of, for example, quartz. This rapid heating and cooling heat treatment apparatus 20 can uniformly heat and process the entire silicon substrate 2 with a temperature gradient of 10 to 300°C / sec.

[0048] The heat treatment process in the heat treatment step of the second embodiment will be described with reference to FIG.

[0049] In the heat treatment process of the second embodiment, the silicon substrate 2 is placed in a chamber 21 maintained at a desired temperature T2 (for example, 500° C.). The chamber 21 is filled with O 2 gas from an atmospheric gas inlet 20 a. ２ The gas is introduced as an atmospheric gas.

[0050] Next, the silicon substrate 2 is heated by the lamps 30 and is rapidly heated at a predetermined temperature increase rate until it reaches a maximum temperature T1 of 1100° C. to 1400° C. Here, the silicon substrate 2 is controlled so as to be maintained at the maximum temperature T1 for a predetermined time (t3). After maintaining the silicon substrate 2 at the maximum temperature T1 for the predetermined time, the temperature is decreased to T2 at a predetermined temperature decrease rate, and the RTP is terminated.

[0051] In this embodiment, the atmospheric gas is O ２ Although gas was used as an example, argon gas, nitrogen gas, O ２ A gas having a partial pressure of 1% or more but less than 100% and mixed with argon gas may also be used.

[0052] Furthermore, although the maximum temperature T1 in this embodiment may be 1100°C or higher and 1400°C or lower, it is preferably 1300°C or higher, and more preferably 1300°C or higher and 1350°C or lower, and the holding time is preferably 1 to 60 seconds, and more preferably 5 to 30 seconds. The temperature rise rate is preferably 5 to 150°C / min, and more preferably 10 to 80°C / min. Similarly, the temperature drop rate is preferably 5 to 150°C / min, and more preferably 10 to 80°C / min.

[0053] This heat treatment step forms a silicon oxide film on the silicon substrate 2, and oxygen precipitates 4 of 20 nm or less are formed in the bulk layer 7 at a rate of 1.0E2 to 1.0E4 / cm. 3At the same time, numerous oxygen precipitate nuclei 4 are also formed. Fewer oxygen precipitates 4 are formed compared to batch annealing, but this is due to the extremely fast temperature increase and decrease rates and the short holding time at the maximum temperature. These oxygen precipitates 4 grow even during heat treatment at relatively low temperatures during device formation, and suppress the extension of slips even if slips occur in the silicon substrate 2. In addition, by using the silicon oxide film formed in an oxygen atmosphere as the bonding layer 3, the heat treatment process and the bonding layer formation process can be performed simultaneously, and the bonding strength can be improved.

[0054] (Embodiment 3) Embodiment 3 is characterized in that a two-stage heat treatment is performed as a heat treatment process, the LT is thinned by a so-called ion implantation separation method, and the bonding layer 3 has a multilayer structure of a silicon oxide film layer and an organic adhesive layer.

[0055] 6 is a flowchart showing a method for manufacturing the composite substrate 5 of this embodiment. As shown in FIG. 6, the method for manufacturing the composite substrate 5 of this embodiment includes a step (S7) of preparing the silicon substrate 2, a heat treatment step (S8), a step (S9) of preparing the piezoelectric substrate 1, an implant step (S10), an organic adhesive layer formation step (S11), a bonding step (S12), and a thinning step (S13).

[0056] The step of preparing a silicon substrate 2 (S7) and the step of preparing a piezoelectric substrate 1 (S9) are the same as those in the first embodiment, and therefore will be omitted.

[0057] In the heat treatment step (S8) of this embodiment, a two-stage heat treatment is performed. In the two-stage heat treatment, for example, RTP is performed in an oxygen atmosphere followed by batch annealing in an Ar atmosphere. While RTP may be performed after RTP, or batch annealing may be performed after batch annealing, it is preferable to include at least one batch annealing stage. This is because the oxygen precipitates 4 are large and formed at high density if batch annealing is performed at least once. In particular, a two-stage heat treatment of RTP followed by batch annealing is more preferable because the oxygen precipitate nuclei 4 formed in the RTP can grow into oxygen precipitates 4 in the subsequent batch annealing. The RTP conditions and batch annealing conditions can be substantially the same as those in the first and second embodiments.

[0058] In the implantation step (S10), ions are implanted into the piezoelectric substrate 1 to form an embrittlement layer at a predetermined depth in the piezoelectric substrate 1. As will be described in detail later, the piezoelectric substrate 1 is thinned after being bonded together using this embrittlement layer.

[0059] In the organic adhesive layer formation step (S11), an organic adhesive layer is formed to serve as the bonding layer 3 for bonding the piezoelectric substrate 1 and the silicon substrate 2. Examples of materials for the organic adhesive layer include epoxy resin and acrylic resin.

[0060] In the bonding step (S12), the piezoelectric substrate 1 and the silicon substrate 2 are bonded together with the bonding layer 3 interposed therebetween and a load is applied to bond them together. In the thinning step (S13), in order to set the thickness of the piezoelectric substrate 1 to a desired value, the embrittlement layer formed to a predetermined depth in the implant step (S10) is peeled off to thin the piezoelectric substrate 1.

[0061] 7 is a schematic cross-sectional view showing an example of a composite substrate 5 according to this embodiment. As shown in FIG. 7, the composite substrate 5 is formed by bonding a silicon substrate 2, a silicon oxide film layer 3b made of SiOx, an organic adhesive layer 3a, and a piezoelectric substrate 1 in this order. Bonding in this order enhances the adhesive strength of the composite substrate 5 according to embodiment 3. Note that the organic adhesive layer 3a may be replaced with a silicon nitride film layer, an amorphous silicon layer, a polysilicon layer, a titanium layer, or a titanium oxide film layer.

[0062] (Examples) Here, we will explain an experiment verifying the effects of the present invention. In this experiment, a composite substrate 5 was used, in which a 36° rotated Y-cut lithium tantalate substrate was used as the piezoelectric substrate 1 and a silicon substrate 2 manufactured by the CZ or FZ method, with a (100) principal surface, a resistivity of approximately 1500 Ω-cm, and an interstitial oxygen concentration as shown in Table 1 below. The silicon substrate 2 in each example was prepared by slicing from a silicon single crystal manufactured by the CZ or FZ method, polishing, and cleaning to obtain a mirror-finished surface. As shown in Figure 2, the composite substrate 5 in each example was prepared by subjecting the prepared silicon substrate 2 to the heat treatment process shown in Table 1, then thermally oxidizing the silicon substrate 2 to form a SiO2 film as the bonding layer 3. The silicon substrate 2 was then bonded to the 36° rotated Y-cut lithium tantalate substrate and then thinned by double-sided polishing. Both the piezoelectric substrate 1 and the silicon substrate 2 had a diameter of 150 mm. The manufacturing method, interstitial oxygen concentration, and heat treatment conditions for the silicon substrate 2 in each example were as shown in Table 1 below. Comparative Examples The comparative examples used were composite substrates 5 formed by bonding a silicon substrate 2 and a piezoelectric substrate 1 via a bonding layer 3, similar to those in the above-described examples, but in which the silicon substrate 2 was not heat-treated or was heat-treated under conditions different from those in the examples. The manufacturing method, interstitial oxygen concentration, and heat treatment conditions for the silicon substrate 2 of each comparative example are as shown in Table 1 below.

[0063] In this verification experiment, slip evaluation was performed using the composite substrate 5 of each example and comparative example shown in Table 1. For slip evaluation, the composite substrate 5 was heated to 300°C, held at 300°C for 120 seconds, and then cooled to 30°C. This heat treatment was repeated 20 times, and X-ray topography (XRT) was used to observe whether slip extension could be suppressed if slip occurred (for each example and comparative example, five composite substrates 5 were used per condition). Furthermore, batch annealing in this verification experiment was performed under three conditions: oxygen partial pressures of 1%, 5%, and 100%, and an Ar atmosphere. It was confirmed that the same results were obtained regardless of the oxygen partial pressure. The heating and cooling rates were both 3°C / min. At least one slip occurred in each silicon substrate 2, and the following evaluation was performed based on whether the generated slip was suppressed before it extended to the bonding surface, etc. ○: Most of the slip extension is suppressed. △: Nearly half of the slip extension is suppressed. ×: Slip extension is not suppressed.

[0064]

[0065] As described above, in the composite substrates 5 (Examples 1 to 14) using silicon substrates 2 that had been subjected to heat treatment in which the temperature was increased at a predetermined rate, the maximum temperature reached was maintained at 1100°C or higher and 1400°C or lower for a predetermined time, and then the temperature was decreased at a predetermined rate, the slip evaluation was rated as ○ or △, confirming the effect of suppressing slip elongation.

[0066] On the other hand, when composite substrate 5 was produced using silicon substrate 2 that had not been subjected to heat treatment (Comparative Examples 1, 4, and 5), it was confirmed that slip elongation could not be suppressed. Similarly, when composite substrate 5 was produced using silicon substrate 2 that had been subjected to heat treatment (batch annealing, RTO) with a maximum temperature of 1000°C (Comparative Examples 2 and 3), it was confirmed that slip elongation could hardly be suppressed.

[0067] Although the present invention has been described above based on the embodiments, the present invention is not limited to the above embodiments. For example, the composite substrate 5 according to the first to third embodiments of the present invention can be suitably applied to acoustic wave devices such as surface acoustic wave devices, Lamb wave elements, and thin film resonators, or optical devices such as optical switching elements and wavelength conversion elements.

[0068] REFERENCE SIGNS LIST 1 Piezoelectric substrate 2 Silicon substrate 3 Bonding layer 3a Organic adhesive layer 3b Silicon oxide film layer 4 Oxygen precipitate nuclei (oxygen precipitates) 10 Batch-type heat treatment apparatus 11 Heat treatment board 12 Heat treatment gas inlet 20 Rapid temperature increase / decrease heat treatment apparatus 20a Atmospheric gas inlet 20b Atmospheric gas outlet 21 Chamber 22 Ring 25 Reaction space 30 Lamp 40 Substrate support 40a Stage

Claims

1. A method for manufacturing a composite substrate, comprising: a step of preparing a piezoelectric substrate; a step of preparing a silicon substrate; a heat treatment step of heating the silicon substrate at a predetermined heating rate, maintaining the maximum temperature at 1100°C or higher and 1400°C or lower for a predetermined time, and then cooling it at a predetermined cooling rate; a bonding step of bonding the silicon substrate and the piezoelectric substrate together after the heat treatment step; and a thinning step of thinning the piezoelectric substrate.

2. The method for producing a composite substrate according to claim 1, wherein the heat treatment step is carried out in a non-oxidizing atmosphere.

3. The method for manufacturing a composite substrate according to claim 1, wherein the heat treatment step is carried out in an oxygen atmosphere with an oxygen partial pressure of 1 to 100%.

4. The method for manufacturing a composite substrate according to claim 1, characterized in that the heat treatment process is a batch heat treatment in which the predetermined temperature increase rate is 1 to 20°C / min, the predetermined temperature decrease rate is 1 to 20°C / min, and the maximum temperature is maintained at 1100°C or higher and 1250°C or lower for 0.5 to 20 hours.

5. The method for manufacturing a composite substrate according to claim 1, further comprising a bonding layer forming step of forming a bonding layer on the surface of the silicon substrate before the bonding step.

6. The method for manufacturing a composite substrate according to claim 1, wherein a silicon oxide film formed on the silicon substrate during the heat treatment process serves as a bonding layer.

7. The method for manufacturing a composite substrate according to claim 5 or 6, wherein the bonding layer includes one or more layers selected from the group consisting of a silicon oxide layer, a silicon nitride layer, an amorphous silicon layer, a polysilicon layer, an organic adhesive layer, a titanium layer, and a titanium oxide layer.

8. The silicon substrate is sliced ​​from a vacancy-rich silicon single crystal produced by the Czochralski method, and has an interstitial oxygen concentration of 2.0E17 to 1.3E18 atoms / cm 3 2. The method for producing a composite substrate according to claim 1, wherein 9. A method for manufacturing a composite substrate according to claim 1, wherein the heat treatment process is a rapid temperature increase / decrease heat treatment carried out in an oxygen atmosphere, the oxygen partial pressure is 1 to 100%, the maximum temperature reached is 1300°C or higher, and the predetermined period is 1 to 60 seconds.

10. The method for manufacturing a composite substrate according to claim 1, wherein the silicon substrate is not polished after the heat treatment step and before the thinning step.

11. A semiconductor device comprising: a silicon substrate; and a piezoelectric substrate, wherein the silicon substrate and the piezoelectric substrate are bonded via a bonding layer, the bonding layer including at least a silicon oxide film layer, the silicon substrate being vacancy-rich, and the interstitial oxygen concentration of the bulk layer being 2.0E17 to 1.3E18 atoms / cm 3 and the bulk layer has oxygen precipitates of 20 nm or less at a density of 1.0E2 / cm. 3 A composite substrate characterized by having the above.

12. The composite substrate according to claim 11, wherein the silicon substrate has a DZ layer on the piezoelectric substrate side and the bulk layer in which the oxygen precipitates are formed on the side opposite the piezoelectric substrate side.

13. The composite substrate described in claim 11, characterized in that the silicon substrate has a bulk layer in which the oxygen precipitate nuclei or the oxygen precipitates are formed on the piezoelectric substrate side, and a DZ layer on the opposite side to the piezoelectric substrate side.

14. The composite substrate according to claim 11, wherein the bonding layer comprises one or more of a silicon oxide layer, a silicon nitride layer, an amorphous silicon layer, a polysilicon layer, an organic adhesive layer, a titanium layer, and a titanium oxide layer.

15. The silicon substrate has a lower interstitial oxygen concentration than the bulk layer, and the interstitial oxygen concentration is 4.0E14 to 6.0E17 atoms / cm 3 12. The composite substrate according to claim 11, further comprising a DZ layer, the DZ layer being located on the piezoelectric substrate side.

16. An acoustic wave device comprising the composite substrate according to any one of claims 11 to 15 and an electrode provided on the piezoelectric substrate.

Images