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

Abstract

This improves the bonding strength between the silicon substrate and the piezoelectric substrate, and also suppresses the extension of slip even if slip occurs during the heat treatment for device formation. [Solution] A method for manufacturing a composite substrate comprises the steps of: preparing a piezoelectric substrate; preparing a silicon substrate; performing a heat treatment on the silicon substrate, which involves raising the temperature at a predetermined rate, maintaining the temperature at a maximum of 1100°C or more and 1400°C or less for a predetermined time, and then lowering the temperature at a predetermined rate; bonding the silicon substrate and the piezoelectric substrate together after the heat treatment step; and thinning the piezoelectric substrate.

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing a composite substrate, a composite substrate, and an elastic wave device. [Background technology]

[0002] In recent years, piezoelectric materials such as lithium niobate (hereinafter also referred to as "LN") and lithium tantalate (hereinafter also referred to as "LT") have been used as materials for elastic wave elements (hereinafter also referred to as "elastic wave devices") such as SAW (Surface Acoustic Wave) filters and thin-film resonators. Furthermore, piezoelectric substrates made from these piezoelectric materials are bonded to support substrates such as silicon, quartz glass, crystal, and borosilicate glass, which have a smaller coefficient of thermal expansion than the piezoelectric substrate, to form composite substrates which are then used as substrates for elastic wave devices such as SAW filters and thin-film resonators. By bonding the piezoelectric substrate to a support substrate with a smaller coefficient of thermal expansion than the piezoelectric substrate in this way, changes in the size of the composite substrate when the temperature changes are suppressed, thereby suppressing changes in the frequency characteristics of the elastic 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 ppm is used as a support substrate, this silicon substrate is bonded to a piezoelectric substrate, and then the piezoelectric substrate is thinned. It is stated that this method allows high resistance to be maintained even after a heat treatment process at 300°C.

[0004] Patent Document 2 discloses a composite substrate in which the piezoelectric substrate is 5 to 100 μm thick, 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 allows for a composite substrate with less warping when subjected to temperature changes and excellent heat resistance.

[0005] In Patent Document 3, the bonding layer provided between the support substrate and the piezoelectric substrate is made of Si (1-x) O xA composite substrate having a composition of (0.008 ≤ x ≤ 0.408) is disclosed. It is stated that this allows for increased bonding strength while improving the insulating properties of the bonding layer. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2018-117030 [Patent Document 2] Japanese Patent Publication No. 2005-347295 [Patent Document 3] Patent No. 6375471 [Overview of the project] [Problems that the invention aims to solve]

[0007] Furthermore, the disclosures in the above-mentioned prior art documents are incorporated into this book by reference. The following analysis was conducted by the inventors.

[0008] Incidentally, in composite substrates of piezoelectric substrates and silicon substrates, the difference in thermal expansion coefficients is very large. Therefore, if heat treatment is performed during device formation after bonding the substrates together, the bonding surfaces of the silicon substrate and piezoelectric substrate may peel off, or slip may occur in the silicon substrate, and this slip may expand, degrading the device characteristics.

[0009] The object of the present invention is to provide a method for manufacturing a composite substrate, a composite substrate, and an elastic wave device that improve the bonding strength between a silicon substrate and a piezoelectric substrate, and contribute to suppressing the extension of slip even if slip occurs during the heat treatment for device formation, in view of the above-mentioned problems. [Means for solving the problem]

[0010] To solve the above problems, the present invention provides a method for manufacturing a composite substrate, comprising the steps of: preparing a piezoelectric substrate; preparing a silicon substrate; performing a heat treatment on the silicon substrate, which involves raising the temperature at a predetermined rate, maintaining the temperature at a maximum of 1100°C or more and 1400°C or less for a predetermined time, and then lowering the temperature at a predetermined rate; bonding the silicon substrate and the piezoelectric substrate together after the heat treatment step; and thinning the piezoelectric substrate.

[0011] In the present invention's method for manufacturing a composite substrate, a heat treatment step is performed on a silicon substrate to achieve a maximum temperature of 1100°C to 1400°C, thereby forming oxygen precipitation nuclei and oxygen precipitates on the silicon substrate. These oxygen precipitation nuclei and oxygen precipitates grow during the relatively low-temperature heat treatment (e.g., 200-400°C) during device formation, so even if slip enters the silicon substrate, the oxygen precipitates (BMD) can stop the extension of the slip. In addition, if the silicon substrate has undergone the high-temperature heat treatment described above, the bonding strength can also be improved. The heat treatment step is preferably performed in a non-oxidizing atmosphere.

[0012] The above heat treatment process is preferably carried out under an oxygen atmosphere with an oxygen partial pressure of 1 to 100%. This allows for control of the density and size of oxygen nuclei and oxygen precipitates. Therefore, the density and size of oxygen nuclei and oxygen precipitates can be optimized to match the heat treatment during elastic wave element formation.

[0013] The above heat treatment process 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 reached is 1100°C to 1250°C, held for 0.5 to 20 hours. Because this relatively high-temperature heat treatment process is carried out for a long period of time, a large amount of oxygen precipitates are formed. In addition, the oxygen precipitate nuclei and oxygen precipitates are produced in a relatively larger size compared to rapid heating and cooling heat treatment, which is a short-time heat treatment process. Therefore, the elongation of slip can be further suppressed.

[0014] It is preferable to further include a bonding layer formation step in which a bonding layer is formed on the surface of the silicon substrate before the bonding step described above. It is also preferable to use a silicon oxide film formed on the silicon substrate during the heat treatment step described above as the bonding layer. Furthermore, the bonding layer can be configured to include one or more layers selected from 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 above silicon substrate was sliced ​​from a vacancy-rich silicon single crystal manufactured by the Czochralski method, with an interstitial oxygen concentration of 2.0E17 to 1.3E18 atoms / cm³. 3 It is preferable that the silicon substrate is sliced ​​from a vacancy-rich (V-rich) silicon single crystal produced by the Czochralski method, thereby increasing productivity and facilitating the formation of oxygen nuclei and oxygen precipitates. Furthermore, the interstitial oxygen concentration is 2E17~1.3E18 atoms / cm³. 3 By doing so, the oxygen concentration can be increased relatively compared to FZ substrates, thereby increasing the strength of the silicon substrate itself and making it easier for oxygen nuclei and oxygen precipitates to grow during device heat treatment, which makes it more difficult for slip to elongate.

[0016] The above heat treatment process is a rapid heating and cooling process performed in an oxygen atmosphere (hereinafter, rapid heating and cooling will also be referred to as "RTP," and in particular, rapid heating and cooling in an oxygen atmosphere will also be referred to as "RTO"), where the oxygen partial pressure is preferably 1 to 100%, the maximum temperature reached is 1300°C or higher, and the predetermined period is preferably 1 to 60 s. Although fewer oxygen precipitates are formed compared to batch annealing, they grow even at relatively low temperatures during device formation, and even if slip occurs in the silicon substrate, the extension of the slip is suppressed. In addition, by using the silicon oxide film formed in an oxygen atmosphere as the bonding layer, the heat treatment process and the bonding layer formation process can be performed simultaneously, and the bonding strength can be improved. It is preferable that the silicon substrate is not polished after the heat treatment process but before the thinning process.

[0017] In order to solve the above problems, the composite substrate of the present invention has a silicon substrate and a piezoelectric substrate, the silicon substrate and the piezoelectric substrate are joined via a joining layer, the joining layer includes at least a silicon oxide film layer, the silicon substrate is void-rich, and the oxygen concentration between lattice points in the bulk layer is 2.0E17 to 1.3E18 atoms / cm 3 and the bulk layer has 1.0E2 or more oxygen precipitates with a size of 20 nm or less per cm 3 This oxygen precipitation nucleus and oxygen precipitate can stop the elongation of the slip by the oxygen precipitate (BMD) even if a slip enters the silicon substrate.

[0018] [[ID=IO]]Preferably, the silicon substrate has a DZ layer on the piezoelectric substrate side and the bulk layer with oxygen precipitates formed on the side opposite to the piezoelectric substrate side. Further, the silicon substrate may have a bulk layer with oxygen precipitation nuclei or oxygen precipitates formed on the piezoelectric substrate side and a DZ layer on the side opposite to the piezoelectric substrate side. The joining layer can be configured to include any one or more layers 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.

[0019] The silicon substrate has a DZ layer with an oxygen concentration between lattice points lower than that of the bulk layer and an oxygen concentration between lattice points of 4.0E14 to 6.0E17 atoms / cm 3 and the DZ layer is preferably located on the piezoelectric substrate side.

[0020] In order to solve the above problems, the elastic wave device of the present invention is characterized by having the composite substrate described above and an electrode provided on the piezoelectric substrate.

Advantages of the Invention

[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 elastic wave device that improve the bonding strength between a silicon substrate and a piezoelectric substrate, and that contribute to suppressing the extension of slip even if slip occurs during the heat treatment for device formation. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 is a schematic cross-sectional view showing an example of a composite substrate according to Embodiment 1 of the present invention. [Figure 2] Figure 2 is a flowchart showing the manufacturing method of the composite substrate according to Embodiment 1. [Figure 3] Figure 3 is a schematic diagram showing a batch-type heat treatment apparatus used in the heat treatment process of Embodiment 1. [Figure 4] Figure 4 is a schematic diagram showing the heat treatment process in the heat treatment step of Embodiment 1. [Figure 5] Figure 5 is a schematic diagram showing the rapid heating and cooling heat treatment apparatus used in the heat treatment of Embodiment 2. [Figure 6] Figure 6 is a flowchart showing the manufacturing method of the composite substrate according to Embodiment 2. [Figure 7] Figure 7 is a schematic cross-sectional view showing an example of a composite substrate according to Embodiment 3 of the present invention. [Modes for carrying out the invention]

[0023] Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the embodiments described below. In each drawing, the same or corresponding elements are appropriately denoted 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 of reality. Even between drawings, there may be parts where the dimensional relationships and ratios differ from each other.

[0024] (Embodiment 1) Figure 1 is a schematic cross-sectional view showing an example of a composite substrate according to Embodiment 1 of the present invention. As shown in Figure 1, the composite substrate 5 is manufactured by bonding a piezoelectric substrate 1 and a silicon substrate 2, which is a support substrate having a smaller coefficient of thermal expansion, via an insulating bonding layer 3. With this configuration, when stress is generated in the piezoelectric substrate 1 and the silicon substrate 2 in response to temperature changes, the thermal expansion of the piezoelectric substrate 1 can be suppressed, thereby improving the frequency-temperature characteristics compared to a piezoelectric substrate alone.

[0025] The composite substrate 5 can be manufactured, for example, by forming an oxide film on one or both of the piezoelectric substrate 1 and the silicon substrate 2, bonding them together, and applying a load to create a bond. The bonding layer 3 may include one or more layers from among 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 also be a multilayer structure. The thickness of the bonding layer 3 is preferably 0.05 μm to 50 μm. If it is thinner than 0.05 μm, the effect of reducing the warping of the composite substrate is small, and if it is thicker than 50 μm, cracks may occur in the piezoelectric substrate 1, which is undesirable. In addition, the adhesive strength may be enhanced by hydrophilizing the surface with an ammonia / hydrogen peroxide aqueous solution or by plasma activation treatment. The size of the substrate is not particularly limited; for example, substrates with diameters of 125 mm, 150 mm, or 200 mm may be used, or they may be larger or smaller.

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

[0027] The piezoelectric substrate 1 can be any piezoelectric crystalline material, but lithium tantalate or lithium niobate is preferable because its large electromechanical coupling coefficient allows for the manufacture of elastic wave elements with a wide bandwidth and low insertion loss as frequency-selective filters. The substrate orientation can be appropriately selected depending on the type of piezoelectric crystalline material and the application of the elastic 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 that is heated at a predetermined heating rate, heat-treated at a maximum temperature of 1100°C to 1400°C for a predetermined time, and then cooled at a predetermined cooling rate to form oxygen precipitation nuclei and oxygen precipitates 4. In this specification, elements smaller than 1 nm in size are referred to as oxygen precipitation nuclei, and elements larger than 1 nm in size are referred to as oxygen precipitates.

[0029] The thickness of the silicon substrate 2 is preferably 50 to 500 μm. If it is a silicon substrate 2, the thermal expansion coefficient is smaller than that of the piezoelectric substrate 1, and it is also suitable for mass production. Note that the resistivity of the silicon substrate 2 is preferably high resistance, preferably 1000 Ω·cm or more, and more preferably 4000 Ω·cm or more. The silicon substrate of the present embodiment is manufactured using the Czochralski method (hereinafter, also referred to as the "CZ method") in order to form oxygen precipitation nuclei and oxygen precipitates 4, and is preferably a silicon substrate sliced from a so-called vacancy-rich (V-rich: Vacancy dominant) silicon single crystal. Also, the inter-lattice 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 is more desirable.

[0030] Note that a polysilicon layer may be formed as a charge trap layer on the silicon substrate 2. In the present embodiment, a silicon substrate on which a polysilicon layer is formed is also simply referred to as a "silicon substrate" and is included in the silicon substrate.

[0031] The silicon substrate 2 of the present embodiment has oxygen precipitates 4 with a size of 20 nm or less at 1.0E2 pieces / cm 3 or more in the bulk layer 7. The size and density of these oxygen precipitates were measured using the transmission electron microscopy method (Transmission Electron Microscopy: TEM).

[0032] The silicon substrate 2 of the present embodiment is configured such that the DZ layer 6 is located on the side that joins the bonding layer 3 (the piezoelectric substrate side), and a bulk layer in which oxygen precipitation nuclei and oxygen precipitates 4 are formed is located on the side that does not join the bonding layer 3 (the side opposite to the piezoelectric substrate side). By adopting such a configuration, while increasing the bonding strength between the piezoelectric substrate 1 and the silicon substrate 2, even when slip occurs in the silicon substrate 2 during the device formation heat treatment, the elongation of the slip is suppressed by the oxygen precipitates 4.

[0033] Alternatively, the DZ layer 6 may be located on the side that does not bond with the bonding layer 3 (opposite the piezoelectric substrate side), and the bulk layer 7, on which oxygen precipitation nuclei and oxygen precipitates 4 are formed, may be located on the side that bonds with the bonding layer 3 (piezoelectric substrate side). With this configuration, even if slip occurs near the bonding surface of the silicon substrate 2, it is possible to suppress the extension of the slip to the bonding surface. In particular, since it is undesirable for the slip to extend to the bonding surface, as this can cause cracks or delamination at the bonding surface, the above configuration can be used to specifically suppress the extension of slip near the bonding surface.

[0034] Figure 2 is a flowchart showing the manufacturing method of the composite substrate according to Embodiment 1. As shown in Figure 2, the manufacturing method of the composite substrate according to Embodiment 1 includes a step of preparing a silicon substrate (S1), a heat treatment step (S2), a bonding layer formation step (S3), a step of preparing a piezoelectric substrate (S4), a bonding step (S5), and a thinning step (S6).

[0035] In the step of preparing the silicon substrate 2 (S1), a silicon substrate 2 to be used as a support substrate for the composite substrate 5 is prepared. The thickness of the silicon substrate 2 is preferably 50 to 500 μm. The silicon substrate 2 can be made from a silicon single crystal sliced ​​using the well-known FZ method or CZ method, but it is preferable to use a silicon substrate sliced ​​from a silicon single crystal manufactured using the CZ method. Furthermore, the interstitial oxygen concentration of the silicon substrate 2 before heat treatment should be 2.0E17 to 1.3E18 atoms / cm³. 3 This is preferable, with a concentration of 5.0°E17 to 9.0°E17 atoms / cm². 3 It would be even more desirable if that were the case.

[0036] In the heat treatment step (S2), the silicon substrate 2 is subjected to a heat treatment in which the temperature is raised at a predetermined rate, the maximum temperature reached is between 1100°C and 1400°C and held for a predetermined time, and the temperature is cooled at a predetermined rate. Here, it is preferable to perform a batch heat treatment in which the predetermined rate of raising the temperature is 1 to 20°C / min, the predetermined rate of cooling is 1 to 20°C / min, and the maximum temperature reached is between 1100°C and 1250°C and held for 0.5 to 20 hours. Through this heat treatment step (S2), oxygen precipitation nuclei and oxygen precipitates are formed on 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 includes one or more layers from among 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 also be a multilayer structure including multiple layers. The thickness of the bonding layer 3 is preferably 0.05 μm to 50 μm.

[0038] In the step of preparing the piezoelectric substrate 1 (S4), the piezoelectric substrate 1 to be used in the composite substrate 5 is prepared. As mentioned above, the piezoelectric substrate 1 can be any material made of piezoelectric crystalline material. The thickness of the piezoelectric substrate is preferably 30 to 120 μm, and more preferably 50 to 100 μm. Furthermore, if it is a composite substrate used in a high-frequency elastic 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 after being appropriately processed according to the application of the composite substrate.

[0039] In the bonding process (S5), the piezoelectric substrate 1 and the silicon substrate 2 are bonded together via the bonding layer 3 by applying a load. Heat may be applied during bonding. Alternatively, plasma treatment may be performed before bonding. Subsequently, in the thinning process (S6), both sides of the composite substrate 5 are polished to obtain the desired thickness of the piezoelectric substrate 1 and the silicon substrate 2. After double-sided polishing, single-sided polishing may be performed to obtain the desired thickness of the piezoelectric substrate 1 and the silicon substrate 2.

[0040] Figure 3 is a schematic diagram showing a batch-type heat treatment apparatus used in the heat treatment process of Embodiment 1. As shown in Figure 3, the batch-type heat treatment apparatus 10 has a heat treatment board 11, and multiple silicon substrates are housed in the batch-type heat treatment apparatus 10 with multiple silicon substrates stored on the heat treatment board 11. The batch-type heat treatment apparatus 10 is provided with a heat treatment gas inlet 12, 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] Figure 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 under 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 process of Embodiment 1, a silicon substrate sliced ​​from a void-rich silicon single crystal ingot grown by the CZ method is subjected to a heat treatment as shown in Figure 4, in which the temperature is raised from T2 to T1 at a predetermined heating rate, the temperature is held at the highest reached temperature T1 from t1 to t2 for a time t3, and the temperature is lowered from T1 to T2 at a predetermined heating rate.

[0043] In the case of batch heat treatment, as in Embodiment 1, the heating rate is preferably 1 to 20°C / min, and more preferably 5 to 15°C / min. Similarly, the cooling rate is preferably 1 to 20°C / min, and more preferably 5 to 15°C / min. The maximum temperature T1 is preferably between 1100°C and 1250°C, and more preferably between 1150°C and 1250°C. Furthermore, the holding time t3 at the maximum temperature is preferably 0.5 to 20 hours. However, from the viewpoint of productivity, it is even more desirable to hold it for 0.5 to 10 hours.

[0044] As a result of the batch heat treatment described above, a DZ layer and a bulk layer are formed on the silicon substrate, and the bulk layer contains oxygen precipitates with a size of 20 nm or less at a rate of 1.0E4 to 1.0E11 particles / cm². 3Furthermore, by using a non-oxidizing Ar atmosphere, oxygen precipitation nuclei are formed in the bulk layer, while the interstitial oxygen concentration in the DZ layer is significantly reduced compared to the bulk layer to 4.0E14~6.0E17 atoms / cm². 3 Therefore, the composite substrate 5 has a lower interstitial oxygen concentration compared to the bulk layer, ranging from 4.0E14 to 6.0E17 atoms / cm³. 3 It has a DZ layer. Therefore, with the composite substrate 5 using the silicon substrate 2 of this embodiment, it is possible to suppress the extension of slip 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 heating and cooling heat treatment under an oxygen atmosphere, and the bonding layer formation process is also incorporated into the heat treatment process. The other components are substantially the same as those of Embodiment 1, so their descriptions are omitted.

[0046] Figure 5 is a schematic diagram showing a rapid heating and cooling heat treatment apparatus used for heat treatment in Embodiment 2. As shown in Figure 5, the rapid heating and cooling heat treatment apparatus 20 includes a chamber (reaction tube) 21 equipped with an atmosphere gas inlet 20a and an atmosphere gas outlet 20b, a plurality of lamps 30 spaced apart at the top of the chamber 21, and a substrate support section 40 that supports the silicon substrate 2 in the reaction space 25 inside the chamber 21. Although not shown, it also includes a rotating means for rotating the silicon substrate 2 around its central axis at a predetermined speed.

[0047] The substrate support section 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 lamp 30 is made of, for example, a halogen lamp. The stage 40a is made of, for example, quartz. This rapid heating and cooling heat treatment apparatus 20 can uniformly heat and treat the entire silicon substrate 2 with a heating and cooling temperature gradient of 10 to 300°C / second.

[0048] The heat treatment process in the heat treatment step of Embodiment 2 will be described with reference to Figure 4.

[0049] In the heat treatment process of Embodiment 2, a silicon substrate is placed in a chamber maintained at a desired temperature T2 (for example, 500°C). Then, O2 gas is introduced into the chamber 21 as the atmospheric gas from the atmospheric gas inlet 20a.

[0050] Next, the silicon substrate 2 is heated by the lamp 30 and rapidly heated at a predetermined heating rate until it reaches a maximum temperature T1 between 1100°C and 1400°C. At this point, the silicon substrate 2 is controlled to be held at the maximum temperature T1 for a predetermined time (t3). After holding the silicon substrate 2 at the maximum temperature T1 for the predetermined time, it is cooled down to T2 at a predetermined cooling rate, and the RTP is terminated.

[0051] In this embodiment, O2 gas was used as an example of the atmospheric gas, but argon gas, nitrogen gas, or a gas mixed with argon gas with a partial pressure of 1 to less than 100% may also be used.

[0052] Furthermore, while the maximum temperature T1 of this embodiment may be between 1100°C and 1400°C, it is preferable that it be above 1300°C, and more preferably between 1300°C and 1350°C. The holding time is preferably between 1 and 60 seconds, and more preferably between 5 and 30 seconds. The heating rate is preferably between 5 and 150°C / min, and more preferably between 10 and 80°C / min. Similarly, the cooling rate is preferably between 5 and 150°C / min, and more preferably between 10 and 80°C / min.

[0053] This heat treatment process forms a silicon oxide film on the silicon substrate 2, and also creates oxygen precipitates with a size of 20 nm or less at a rate of 1.0E2 to 1.0E4 particles / cm² in the bulk layer. 3A layer is formed. At the same time, numerous oxygen precipitation nuclei are also formed. Compared to batch annealing, fewer oxygen precipitates are formed, which is due to the very fast heating and cooling rates and the short holding time at the highest temperature. These oxygen precipitates grow even at the relatively low-temperature heat treatment during device formation, and suppress the extension of slips even if slips occur in the silicon substrate. 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 by performing a two-stage heat treatment as a heat treatment process, thinning the LT by a so-called ion implantation separation method, and making the bonding layer 3 a multilayer structure of a silicon oxide film layer and an organic adhesive layer.

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

[0056] The steps of preparing the silicon substrate (S7) and the piezoelectric substrate (S9) are the same as in Embodiment 1 and are therefore 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. However, RTP may be performed after RTP, or batch annealing may be performed after batch annealing, but it is desirable to include one or more batch annealing stages. This is because if one or more batch annealing stages are included, the oxygen precipitates will be larger and formed at a higher density. In particular, a two-stage heat treatment of batch annealing after RTP is more desirable because the oxygen precipitate nuclei formed in RTP can be grown into oxygen precipitates in the subsequent batch annealing. The RTP conditions and batch annealing conditions can be substantially the same as in Embodiments 1 and 2.

[0058] In the implantation process (S10), ion implantation is performed on the piezoelectric substrate 1 to pre-form an embrittlement layer at a predetermined depth on the piezoelectric substrate. As will be described in more detail later, this embrittlement layer is used to thin the piezoelectric substrate after bonding.

[0059] In the organic adhesive layer formation step (S11), an organic adhesive layer is formed, which serves as a bonding layer for joining the piezoelectric substrate and the silicon substrate. Examples of materials for the organic adhesive layer include epoxy resin and acrylic resin.

[0060] In the bonding process (S12), the piezoelectric substrate and the silicon substrate are bonded together by applying a load via a bonding layer. In the thinning process (S13), the piezoelectric substrate 1 is thinned by peeling off the embrittlement layer that was previously formed to a predetermined depth in the implantation process (S10) in order to achieve the desired thickness of the piezoelectric substrate and the silicon substrate.

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

[0062] (Examples) Here, we will describe the verification experiment of the effectiveness of implementing the present invention. In this verification experiment, a 36° rotated Y-cut lithium tantalate substrate was used as the piezoelectric substrate 1, and a composite substrate 5 was used, which was manufactured by the CZ method or FZ method as the silicon substrate 2, with a main surface being a (100) plane, a resistivity of approximately 1500 Ω·cm, and an interstitial oxygen concentration as shown in Table 1 below, and bonded together. Furthermore, for the silicon substrate 2 in each example, a silicon single crystal manufactured by the CZ method or FZ method was sliced, and then polished and cleaned to give the surface a mirror finish. As shown in Figure 2, for each example of the composite substrate 5, the prepared silicon substrate 2 was subjected to the heat treatment process shown in Table 1, a SiO2 film was formed as a bonding layer 3 by thermal oxidation of the silicon substrate 2, bonded to the 36° rotated Y-cut lithium tantalate substrate, and then thinned by double-sided polishing. Note that both the piezoelectric substrate 1 and the silicon substrate 2 used had a diameter of 150 mm. The manufacturing method, interstitial oxygen concentration, and heat treatment conditions for the silicon substrate 2 in each example are as shown in Table 1 below. (Comparative example) As a comparative example, a composite substrate 5 is obtained by bonding a silicon substrate 2 and a piezoelectric substrate 1, similar to those in the above example, via a bonding layer 3. However, the silicon substrate 2 is either not heat-treated or heat-treated under different conditions than those in the example. The manufacturing method, interstitial oxygen concentration, and heat treatment conditions for the silicon substrate 2 in 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 the slip evaluation, the composite substrate 5 was heated to 300°C, held at 300°C for 120 seconds, and then cooled to 30°C, a heat treatment that was repeated 20 times. When slip occurred, the extent of the slip was observed using X-ray topography (XRT) (for each example and comparative example, five composite substrates 5 were used for each condition in the verification experiment). Furthermore, batch annealing in this verification experiment was performed under three conditions: oxygen partial pressure of 1%, 5%, and 100% in an oxygen atmosphere, and under four conditions: an Ar atmosphere, and it was confirmed that the same results could be obtained regardless of the oxygen partial pressure. The heating rate and cooling rate at this time were both 3°C / min. At least one slip occurred in each silicon substrate 2, and the evaluation below was based on whether the slip was suppressed before it extended to the bonding surface, etc. ○: Suppresses most of the slip elongation. △: Suppresses nearly half of the slip elongation. ×: Slip elongation is not being suppressed.

[0064] [Table 1]

[0065] As described above, in the composite substrates 5 (Examples 1-14) using silicon substrates 2 that underwent heat treatment by heating at a predetermined heating rate, holding at a maximum temperature of 1100°C to 1400°C for a predetermined time, and then cooling at a predetermined cooling rate, the slip evaluation was ○ or △, confirming the effect of suppressing slip elongation.

[0066] On the other hand, when composite substrates 5 were fabricated using silicon substrates 2 that had not undergone heat treatment (Comparative Examples 1, 4, and 5), it was confirmed that slip elongation could not be suppressed. Similarly, when composite substrates 5 were fabricated using silicon substrates 2 that had undergone 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 embodiments, the present invention is not limited to the above embodiments. For example, the composite substrates of Embodiments 1 to 3 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 to optical devices such as optical switching elements and wavelength conversion elements. [Explanation of symbols]

[0068] 1. Piezoelectric substrate 2. Silicon substrate 2a Polysilicon layer 3 Bonding layer 3a Organic adhesive layer 3b Silicon oxide layer 4. Oxygen precipitation nuclei and oxygen precipitates 10 Batch-type heat treatment equipment 11 Heat treatment boards 12. Inlet for heat treatment gas 20 Rapid heating and cooling heat treatment apparatus 20a Atmospheric gas inlet 20b Atmospheric gas outlet 21 Chambers 22 rings 25 Reaction space 30 lamps 40 Substrate support section 40a Stage

Claims

1. The process of preparing a piezoelectric substrate, The process of preparing the silicon substrate, A heat treatment step is performed on the silicon substrate, which involves heating it at a predetermined heating rate, maintaining the maximum temperature reached at 1100°C or higher and 1400°C or lower for a predetermined time, and then cooling it at a predetermined cooling rate. After the heat treatment step, a bonding step is performed in which the silicon substrate and the piezoelectric substrate are bonded together. A thinning step for thinning the piezoelectric substrate, A method for manufacturing a composite substrate, characterized by having the following features.

2. The method for manufacturing a composite substrate according to claim 1, characterized in that the heat treatment step is performed under a non-oxidizing atmosphere.

3. The method for manufacturing a composite substrate according to claim 1, characterized in that the heat treatment step is performed 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 step is 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 reached is 1100°C or higher and 1250°C or lower, and the temperature is held at that temperature for 0.5 to 20 hours.

5. The method for manufacturing a composite substrate according to claim 1, further comprising a bonding layer formation 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, characterized in that the silicon oxide film formed on the silicon substrate during the heat treatment step is used as a bonding layer.

7. The method for manufacturing a composite substrate according to claim 5 or 6, characterized in that the bonding layer includes one or more layers selected from 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, with an interstitial oxygen concentration of 2.0E17 to 1.3E18 atoms / cm³. 3 A method for manufacturing a composite substrate according to claim 1, characterized in that it is the same as described in claim 1.

9. The method for manufacturing a composite substrate according to claim 1, characterized in that the heat treatment step is a rapid heating and cooling heat treatment performed 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 s.

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

11. It has a silicon substrate and a piezoelectric substrate, The silicon substrate and the piezoelectric substrate are joined together via a bonding layer. The bonding layer includes at least a silicon oxide film layer, The silicon substrate is pore-rich, and the interstitial oxygen concentration of the bulk layer is 2.0E17 to 1.3E18 atoms / cm³. 3 And, The bulk layer contains oxygen precipitates with a size of 20 nm or less at a rate of 1.0E2 particles / cm². 3 A composite substrate characterized by having the above features.

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 on the side opposite to the piezoelectric substrate side in which the oxygen precipitate is formed.

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

14. The composite substrate according to claim 11, characterized in that the bonding layer includes one or more layers selected from 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.

15. The silicon substrate has a lower interstitial oxygen concentration than the bulk layer, and the interstitial oxygen concentration is between 4.0E14 and 6.0E17 atoms / cm³. 3 The composite substrate according to claim 11, wherein the DZ layer is located on the piezoelectric substrate side.

16. An elastic wave device characterized by comprising a composite substrate according to any one of claims 11 to 15, and an electrode provided on the piezoelectric substrate.

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