Mullite-containing sintered body, method for its production and use of the mullite-containing sintered body as a support substrate in a composite substrate

Abstract

Mullite-containing sintered body comprising mullite, silicon nitride and sialon, where the coefficient of thermal expansion is 4.1 ppm / °C or less at temperatures from 40 °C to 400 °C, the open porosity is less than 0.1%, the mean grain size of sintered particles is 1.2 µm or less, and mullite is a component that is present in the largest quantity in the sintered body.

Description

TECHNICAL AREA

[0001] The present invention relates to a mullite-containing sintered body, a method for producing the same, and the use of the mullite-containing sintered body as a support substrate in a composite substrate. TECHNICAL BACKGROUND

[0002] In general, a mullite sintered body is a material produced by sintering an aluminum oxide (Al₂O₃) and a silicon oxide (SiO₂) in a ratio of 3 to 2, exhibiting excellent thermal shock resistance and represented by 3Al₂O₃·2SiO₂. Regarding such a mullite sintered body, a material is known that was produced by forming a powder in which 30 wt% yttrium oxide-stabilized zirconium oxide (YSZ) powder is mixed into a mullite powder and sintering the resulting pellet, as disclosed, for example, in PTL 1. In PTL 1, a base substrate to be bonded to a GaN substrate is produced by cutting a mullite substrate from the mullite sintered body and polishing a major surface of the mullite substrate. The coefficient of thermal expansion of GaN is specified as 6.0 ppm / K in the range from room temperature to 1 000°C, and the coefficient of thermal expansion of Mullite is specified as 5.2 ppm / K.Therefore, considering the two substrates to be bonded together, it is preferable that the coefficient of thermal expansion of mullite be increased to approach that of the GaN substrate. Consequently, the YSZ powder is mixed into the mullite powder and sintering is carried out.

[0003] PTL 2 describes an example where a composite substrate, produced by directly bonding a functional substrate consisting of lithium tantalate, lithium niobate, or the like to a support substrate consisting of a mullite sintered body, is used for acoustic wave devices, e.g., a surface acoustic wave element. In such an acoustic wave device, the mullite substrate serving as the support substrate exhibits a low coefficient of thermal expansion of approximately 4.4 ppm / °C (40°C to 400°C) and a high Young's modulus of 220 GPa or more. Therefore, expansion and contraction due to temperature changes of the acoustic wave device itself can be reduced, and as a result, the temperature dependence of the frequency is greatly improved.To bond the functional substrate directly to the support substrate, it is necessary that each surface to be bonded has a high degree of smoothness. For example, PTL 2 mentions that the average centerline roughness Ra is preferably 3 nm or less. Furthermore, PTL 3 discloses a reaction-bonded mullite-containing ceramic as well as its fabrication and use. PTL 4 describes a dense sintered body based on cordierite, and PTL 5 discloses a mullite-based sintered body and its fabrication. LIST OF QUOTES, PATENT LITERATURE PTL 1: JP 5 585 570 B2 PTL 2: JP 5 861 016 B1 & WO 2015 / 186 560 A1 PTL 3: EP 0 531 378 B1 PTL 4: EP 1 582 509 A2 PTL 5: JP H01 - 96 062 A REVELATION OF THE INVENTION

[0004] A mullite sintered body with a coefficient of thermal expansion increased by the addition of a large quantity of another component to the mullite is described in PTL 1, and a mullite sintered body with high mullite purity is described in PTL 2. However, a mullite sintered body with a reduced coefficient of thermal expansion is not described. Furthermore, a mullite sintered body with low thermal expansion, as described above, in which the surface smoothness of a polished finished surface is high, is not known. In this respect, if a mullite sintered body exhibiting low thermal expansion but low stiffness is used as a support substrate for a composite substrate, the composite substrate may warp due to a slight temperature difference.

[0005] The present invention was carried out to solve the problems described above, and the main objective is to provide a mullite-containing sintered body with a low coefficient of thermal expansion, high stiffness, and high smoothness of a polished surface, compared with mullite alone.

[0006] A mullite-containing sintered body according to the present invention comprises mullite, silicon nitride, and sialon, wherein the mullite-containing sintered body has a coefficient of thermal expansion of 4.1 ppm / °C or less from 40°C to 400°C, an open porosity of less than 0.1%, and a mean particle size of sintered particles of 1.2 µm or less, with mullite being the component present in the largest quantity in the sintered body. The mullite-containing sintered body exhibits a low coefficient of thermal expansion and high stiffness compared to mullite alone. Additionally, a high degree of smoothness can be achieved by polishing the surface.

[0007] A method for producing the mullite-containing sintered body according to the present invention comprises (a) producing a mixed raw material powder by mixing 70 to 90 volume percent mullite powder with a mean particle diameter of 1.5 µm or less and 10 to 30 volume percent silicon nitride powder with a mean particle diameter of 1 µm or less, such that the sum is 100 volume percent, and (b) producing the mullite-containing sintered body by forming the mixed raw material powder into a compact with a predetermined shape and subjecting the compact to hot pressing at a pressure of 2.0 to 29.4 MPa (20 to 300 kgf / cm²). 2) and a firing temperature of 1525 °C to 1700 °C. This manufacturing process is suitable for producing the mullite-containing sintered body described above according to the present invention. In this respect, the mean particle diameter of the powder is a value measured by applying a laser diffraction method (the same applies to the following).

[0008] According to the present invention, the mullite-containing sintered body described above is used as a support substrate in a composite substrate. A composite substrate is a composite material substrate in which a functional substrate is bonded to the support substrate. This support substrate is advantageously bonded to the functional substrate because the smoothness of the polished surface of the mullite-containing sintered body, which serves as the support substrate, is high. Furthermore, when the composite substrate is used for a surface acoustic wave (SAW) device, the temperature dependence of the frequency is significantly improved. The performance of an optical waveguide device, an LED device, or a switch device is also increased because the coefficient of thermal expansion of the support substrate is low. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a diagram of the manufacturing steps of a mullite-containing sintered body. Fig. Figure 2 is a perspective view of a composite substrate 10. Fig. Figure 3 is a perspective view of an electronic device 30, manufactured by applying the composite substrate 10. BEST EXECUTION OF THE INVENTION

[0009] The embodiments of the present invention are described in more detail below; however, the present invention is not limited to the embodiments described below. It should be understood that modifications and improvements based on the usual knowledge of a person skilled in the art may be appropriately implemented without departing from the inventive concept.

[0010] A mullite-containing sintered body according to the present embodiment contains mullite, silicon nitride, and sialon. Mullite is the component present in the largest quantity (primary component) of the sintered body. The mullite-containing sintered body exhibits a coefficient of thermal expansion of 4.1 ppm / °C or less from 40°C to 400°C, an open porosity of less than 0.1%, and a mean particle size of sintered particles of 1.2 µm or less. The mullite-containing sintered body exhibits a low coefficient of thermal expansion and a high Young's modulus (stiffness) compared to mullite alone. Additionally, the smoothness of a polished finished surface (polished surface) of the mullite-containing sintered body is high because the open porosity is less than 0.1%, pores are negligibly enclosed, and the mean grain size of sintered particles is as small as 1.2 µm or less.

[0011] Considering the mullite-containing sintered body according to the present embodiment, the number of pores having a maximum length of 1 µm or more, present in an area of ​​100 µm × 100 µm of a polished surface, is preferably 10 or less. In the case where the number of pores is 10 or less, the smoothness of the polished finished surface is increased. The number of such pores is preferably 3 or less and further preferably zero.

[0012] The mullite-containing sintered body according to the present embodiment preferably has a Young's modulus of 240 GPa or more and preferably a four-point flexural strength of 300 MPa or more. The Young's modulus and the strength of silicon nitride and derivatives thereof are higher than those of mullite. Therefore, the Young's modulus of the mullite-containing sintered body can be 240 GPa or more, and the four-point flexural strength can be increased to 300 MPa or more by adjusting the proportion of silicon nitride added to the mullite. In this respect, the four-point flexural strength is preferably 320 MPa or more.

[0013] The mullite-containing sintered body according to the present embodiment preferably has an average centerline roughness Ra of the polished surface of 1.5 nm or less. A composite substrate produced by bonding a functional substrate to a support substrate is known as a composite substrate used for an acoustic wave device and the like. The bonding ability between the support substrate and the functional substrate is good by applying the mullite-containing sintered body described above, wherein the Ra of the polished surface has a roughness of 1.5 nm or less than that of the support substrate. For example, the proportion of the bonding interface that is actually bonded (bonding area fraction) is 80% or more (preferably 90% or more). The average centerline roughness Ra of the polished surface is preferably 1.1 nm or less and further preferably 1.0 nm or less.

[0014] The mullite-containing sintered body according to the present embodiment preferably has a coefficient of thermal expansion of 3.8 ppm / °C or less at temperatures from 40°C to 400°C. When a composite substrate in which the support substrate is such a mullite-containing sintered body is used for an acoustic wave device, the thermal expansion of the functional substrate is reduced when the temperature of the acoustic wave device increases, compared to its inherent thermal expansion. This improves the temperature dependence of the frequency of the acoustic wave device. The coefficient of thermal expansion is further preferably 3.5 ppm / °C or less at temperatures from 40°C to 400°C.

[0015] An embodiment of a method for producing the mullite-containing sintered body according to the present invention is now described. As in Fig. Figure 1 shows a manufacturing process for the mullite-containing sintered body that includes (a) a step to produce a mixed raw material powder and (b) a step to produce the mullite-containing sintered body. ▪Step (a): Production of mixed raw material powder

[0016] The mixed raw material powder is produced by mixing a mullite powder and a silicon nitride powder. Preferably, a powder with high purity and a small mean particle diameter is used as the mullite raw material. The purity is preferably 99.0% or more, more preferably 99.5% or more, and further preferably 99.8% or more. The unit of purity is mass percent. Furthermore, the mean particle diameter (D50) is 1.5 µm or less, and preferably 0.1 to 1.5 µm. Regarding the mullite raw material, a commercially available product may be used, or a material produced by applying a high-purity aluminum oxide or silicon dioxide powder may be used. Examples of methods for producing the mullite raw material include one described in PTL 2.Considering the silicon nitride raw material, it is preferred that a powder with a small mean particle diameter can be used. The mean particle diameter is 1 µm or less, and preferably 0.1 to 1 µm. The mullite raw material and the silicon nitride raw material are mixed. The mixed raw material powder is produced by mixing 70 to 90 volume percent mullite raw material and 10 to 30 volume percent silicon nitride raw material, such that the total is 100 volume percent. The mixing can be carried out by using a mixer, e.g., a pot mill, and by performing drying, if necessary, by using a spray dryer.

[0017] Step (b): Production of Mullite-Containing Sintered Body. The mixed raw material powder produced in step (a) is formed into a compact with a predetermined shape. There are no particular limitations regarding the forming process, and a conventional forming method can be used. For example, the mixed raw material powder can be press-molded by applying a mold. In the case of press-molding, the formability is increased by spray-drying the mixed raw material powder into granules. Additionally, a body for extrusion can be produced by adding an organic binder, or a slurry can be prepared for forming into a film or sheet. In these processes, organic binder components must be removed before or during a firing step. High-pressure forming by CIP (isostatic cold pressing) can also be carried out.

[0018] The resulting compact is then fired to produce the mullite-containing sintered body. To increase the surface smoothness of the mullite-containing sintered body, it is preferable at this stage to keep the sintered particles fine and to remove gas during sintering. A very useful technique for this is a hot-pressing process. When the hot-pressing process is used, compaction occurs in the state of fine granules at a lower temperature compared to pressureless sintering, and the retention of coarse pores, which is often observed in pressureless sintering, can be suppressed. The firing temperature (maximum temperature) during hot pressing is 1525 °C to 1700 °C. The pressing pressure during hot pressing is also set to between 2.0 and 29.4 MPa (20 to 300 kgf / cm²). 2). In particular, a lower pressing pressure is preferred because the size of the hot pressing device can be reduced and its service life increased. A suitable dwell time at the firing temperature can be selected taking into account the shape and size of the pressing, the properties of the furnace, and the like. In particular, the dwell time is, for example, preferably 1 to 12 hours and more preferably 2 to 8 hours. There is no particular limitation when considering the firing atmosphere. In general, the atmosphere during hot pressing is an inert atmosphere, e.g., nitrogen, argon, or the like. The rate of temperature rise and the rate of temperature fall can be suitably adjusted taking into account the shape and size of the pressing, the properties of the furnace, and the like, and can be set in the range, for example, from 50°C / h to 300°C / h.

[0019] An embodiment of a composite substrate is now described. The composite substrate according to the present embodiment is produced by bonding a functional substrate to a support substrate composed of the mullite-containing sintered body described above. In the composite substrate, the bonding area between the two substrates is large to ensure good bonding strength. There is no particular limitation regarding the functional substrate, and examples include lithium tantalate, lithium niobate, gallium nitride, and silicon. The bonding method is preferably direct bonding. In the case of direct bonding, the surfaces to be bonded on each functional substrate and the support substrate are polished and then activated. The two substrates are pressed together while the two surfaces to be bonded face each other.Activation of the surface to be bonded is carried out, for example, by irradiation with ion beams of inert gas (argon or the like) or irradiation with plasma or neutral atomic beams. The ratio of the thickness of the functional substrate to the thickness of the support substrate (thickness of functional substrate / thickness of support substrate) is preferably 0.1 or less. Fig. Figure 2 shows an example of a composite substrate. A composite substrate 10 is produced by directly bonding a piezoelectric substrate 12, which serves as the functional substrate for a support substrate 14.

[0020] The composite substrate according to the present embodiment can be used for an acoustic device and the like. Examples of such electronic devices include acoustic wave devices (a surface acoustic wave device, a Lamb wave element, a thin-film resonator (FBAR), and the like), as well as LED devices, optical waveguide devices, and switching devices. When the composite substrate described above is used for the acoustic wave device, the temperature dependence of the frequency is greatly improved because the coefficient of thermal expansion of the mullite-containing sintered body, which serves as the support substrate, is 4.1 ppm / °C or less (40°C to 400°C) and is low. Fig.Figure 3 shows an example of an electronic device 30, fabricated using the composite substrate 10. The electronic device 30 is a 1-port SAW resonator, that is, a surface acoustic wave device. Initially, the piezoelectric substrate 12 of the composite substrate 10 is provided with a pattern of many electronic devices 30 by applying conventional photolithography technology. Subsequently, each of the electronic devices 30 is cut by cold granulation. In the electronic device 30, IDT (interdigital transducer) electrodes 32 and 34 and reflection electrodes 36 are formed on the surface of the piezoelectric substrate 12 by applying photolithography technology.

[0021] The present invention is not limited to the embodiment described above and can be implemented in various ways, as long as they fall within the technical scope of the invention. EXAMPLES 1. Production of mixed raw material powder

[0022] A commercially available mullite powder with a purity of 99.9% or greater and a mean particle diameter of 1.5 µm was used as a mullite raw material, and a commercially available silicon nitride powder with a purity of 97% or greater and a mean particle diameter of 0.8 µm was used as a silicon nitride raw material. The mullite and silicon nitride raw materials were weighed out at the ratios shown in Experimental Examples 1 to 3 in Table 1 and mixed in a pot mill by applying 5 mm diameter aluminum oxide pellets. A mixed raw material powder was produced by spray drying. 2. Production of mullite-containing sintered bodies

[0023] The mixed raw material powder from each of experimental examples 1 to 3 was placed in a mold with a diameter of approximately 125 mm and was formed into the shape of a disc with a thickness of approximately 10 to 15 mm by a pressure of 19.6 MPa (200 kgf / cm²). 2 The material was shaped to obtain a mullite-containing pellet. This pellet was then placed in a hot-press graphite mold with an inner diameter of approximately 125 mm, and a mullite-containing sintered body was produced in a hot-press furnace to achieve a diameter of approximately 125 mm and a thickness of 5 to 8 mm. The maximum firing temperature was set to 1650 °C, the residence time at firing temperature was set to 5 hours, and both the temperature rise and fall rates were set to 100 °C / h. The pressing force was set to 19.6 MPa (200 kgf / cm²). 2The temperature was set to 900 °C or higher during the temperature rise. The atmosphere in the furnace was set to vacuum until 900 °C was reached, and once 900 °C was reached, nitrogen (N2) was introduced to perform sintering in N2. After the firing temperature was held for a predetermined time, the temperature was reduced to 1200 °C, the pressure and atmosphere in the furnace were stopped, and natural cooling to room temperature was allowed. Additionally, in Experiment 4, a compact and a sintered body were also produced, except that a mullite powder was used alone. 3. Evaluation of characteristics

[0024] Test pieces (bendable rods measuring 4 × 3 × 40 mm and similar) were cut from the sintered bodies of test examples 1 to 4, and various properties were evaluated. Additionally, to achieve a polished surface, a test piece measuring approximately 4 × 3 × 10 mm was polished to a high-gloss finish. Polishing was performed using diamond abrasive grains of 3 µm and 0.5 µm, and lapping with diamond abrasive grains of 0.1 µm or less was carried out as the final finishing step. The evaluated properties were described below. (1) Crystal phase

[0025] The sintered body was pulverized and a crystal phase was identified by applying an X-ray spectrograph. The measurement conditions were set to CuK. α, 50 kV, 300 mA, and 2θ = 5° to 70° were set and a rotating anode X-ray diffractometer (RINT manufactured by Rigaku Industrial Corp.) was used. (2) ratio of the crystal phase

[0026] The peak-area ratio of each crystal phase was calculated from the XRD profile of the one described above (1). The peak area of ​​mullite (210) (2θ = 26.2°) was assumed to be 1, and the peak area of ​​each crystal phase relative to that was expressed as the crystal phase ratio. Here, the representative peak of each crystal phase is considered: the (101) area (2θ = 20.6°) of silicon nitride, the (3-20) area (2θ = 24.6°) of Si₂Al₃O₇N, and the (200) area (2θ = 26.9°) of Si₅Al₅ON₇N of sialon were used. (3) Bulk density and open porosity

[0027] A bending rod was used and the bulk density and open porosity were measured by applying an Archimedes method in which pure water was used. (4) Young's module

[0028] The measurement was performed using a static deflection method in accordance with JIS R1602. The test piece was set to have the shape of a bending bar measuring 3 mm × 4 mm × 40 mm. (5) Flexural strength

[0029] The four-point bending strength was measured in accordance with JIS R1601. The test piece was set to have the shape of a 3 mm × 4 mm × 40 mm bending bar or half that size. (6) Coefficient of thermal expansion (40°C to 400°C)

[0030] The measurement was performed using a differential-type pushrod in accordance with JIS R1618. The shape of the test piece was set to 3 mm × 4 mm × 20 mm. (7) Number of pores

[0031] The polished surface of the sintered body completed as described above was observed by applying a SEM, and the number of pores having a maximum length of 1 µm or more, present in a 100 µm × 100 µm area, was counted. (8) Surface smoothness (Ra)

[0032] The average centerline roughness Ra of the polished surface of the sintered body finished as described above was measured using an AFM. The measuring range was set to 10 µm × 10 µm. (9) Mean grain size of sintered particles

[0033] The polished surface of the sintered body, finished as described above, was chemically etched using phosphoric acid. The dimensions of 200 or more of the sintered particles were measured using SEM, and the mean grain size of the sintered particles was calculated using linear analysis. The coefficient of linear analysis was set to 1.5, and the value obtained by multiplying the length actually measured using SEM by 1.5 was taken as the mean grain size of the sintered particles. (10) capacity for attachment

[0034] A disc with a diameter of approximately 100 mm and a thickness of approximately 600 µm was cut from each of the sintered bodies of experimental examples 1 to 4. This disc was high-gloss finished as described above and then cleaned to remove particles, impurities, and the like from the surface. The resulting disc was then used as a support substrate, and a composite substrate was produced by direct bonding between the support substrate and the functional substrate. Specifically, the surfaces to be bonded on each support substrate and the functional substrate were initially activated by argon ion beams, and then the two substrates were bonded together by pressing at 10 tons with the two surfaces facing each other to produce a composite substrate. A lithium niobate (LN) substrate was used as the functional substrate.Regarding the development of bonding capability, a case where the bonding area fraction in an IR transfer image was 90% or more was classified as "best", a case where the fraction was 80% or more and less than 90% was classified as "good", and a case where the fraction was less than 80% was classified as "poor". 4. Assessment result

[0035] Each of the mullite-containing sintered bodies from Experiments 1 to 3 was produced by firing the mixed raw material powder, in which the mullite raw material and the silicon nitride raw material were mixed, and a portion of the silicon nitride was converted to sialon by firing. Each of the mullite-containing sintered bodies from Experiments 1 to 3 contained silicon nitride and the like, and therefore the Young's modulus and the four-point flexural strength were improved compared to those of the mullite-only sintered body from Experiment 4. That is, the Young's modulus was improved to 240 GPa or more, and the four-point flexural strength was improved to 320 MPa or more.Additionally, the coefficient of thermal expansion of each of the mullite-containing sintered bodies from Experimental Examples 1 to 3 was 4.1 ppm / °C or less (3.5 to 4.1 ppm / °C) at 40°C to 400°C, and therefore this value was lower than that of the mullite-only sintered body from Experimental Example 4. Furthermore, considering each of the mullite-containing sintered bodies from Experimental Examples 1 to 3 and the mullite-only sintered body from Experimental Example 4, the open porosity was less than 0.1%, the mean particle size of sintered particles was 1.2 µm or less (1.0 to 1.2 µm), and therefore the average centerline roughness Ra of the polished surface was as small as 1.1 nm or less (0.9 to 1.1 nm).Consequently, in the case where a disc cut from each of the sintered bodies of experimental examples 2 to 4 was directly bonded to the functional substrate, the bonding ability was "best" because the bonded area fraction was 90% or more. Conversely, in the case where a disc cut from the sintered body of experimental example 1 was directly bonded to the functional substrate, the bonding ability was "good" because the bonded area fraction was 80% or more and less than 90%. In this respect, the number of pores, which is 3 or fewer (zero), contributed to such a reduction in the value of the average centerline roughness Ra of the polished surface.

[0036] Furthermore, experimental examples 1 to 3 correspond to the examples according to the present invention, and experimental example 4 corresponds to a comparative example. The present invention is not limited to these examples.

Claims

Mullite-containing sintered body comprising mullite, silicon nitride and sialon, wherein the coefficient of thermal expansion at 40 °C to 400 °C is 4.1 ppm / °C or less, the open porosity is less than 0.1%, the mean grain size of sintered particles is 1.2 µm or less, and mullite is a component present in the largest quantity in the sintered body. Mullite-containing sintered body according to claim 1, wherein the number of pores having a maximum length of 1 µm or more, which are located in an area of ​​100 µm × 100 µm of a polished surface, is 10 or less. Mullite-containing sintered body according to claim 1 or 2, wherein the Young's modulus is 240 GPa or more. Mullite-containing sintered body according to one of claims 1 to 3, wherein the four-point bending strength is 320 MPa or more. Mullite-containing sintered body according to one of claims 1 to 4, wherein the average centerline roughness Ra of the polished surface is 1.5 nm or less. A method for producing the mullite-containing sintered body according to any one of claims 1 to 5, comprising: (a) producing a mixed raw material powder by mixing 70 to 90 volume percent mullite powder with a mean particle diameter of 1.5 µm or less and 10 to 30 volume percent silicon nitride powder with a mean particle diameter of 1 µm or less, such that the sum is 100 volume percent, and (b) producing the mullite-containing sintered body by forming the mixed raw material powder into a compact with a predetermined shape and subjecting the compact to hot-press firing at a pressure of 2.0 to 29.4 MPa (20 to 300 kgf / cm2) and a firing temperature of 1525 °C to 1700 °C. Use of the mullite-containing sintered body according to one of claims 1 to 5 as a support substrate in a composite substrate.

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