Method for forming a composite substrate

Abstract

A method of forming a composite substrate is provided that includes a mixed gas plasma activation process using oxygen and nitrogen that includes exposing a surface of the wafer and a surface of the piezoelectric material wafer in a plasma system, exposing the plasma activated surface of the wafer and the plasma activated surface of the piezoelectric material wafer to ultraviolet radiation in an oxygen and nitrogen atmosphere, placing the surface of the wafer on the surface of the piezoelectric material wafer, applying a mechanical load to the wafer and the piezoelectric material wafer, and thermal annealing the bonded wafer and piezoelectric material wafer, where the wafer is configured to support the piezoelectric material wafer.

Description

[Technical Field] 【0001】 This disclosure generally relates to the field of forming composite substrates, including wafer bonding processes. [Background technology] 【0002】 A surface acoustic wave (SAW) device is known, which is a device in which a support substrate and a piezoelectric substrate that propagates surface acoustic waves are joined together. It is known that by attaching a support substrate with a smaller coefficient of thermal expansion than the piezoelectric substrate, changes in the dimensions of the piezoelectric substrate during temperature changes are suppressed, and changes in the frequency characteristics of the surface acoustic wave device are reduced. 【0003】 Patent Document 1 discloses a surface acoustic wave device having a structure in which a piezoelectric substrate and a silicon substrate are joined together using a multi-step procedure, in which, in the first step, a so-called bonding layer is deposited on at least one of the bonding partners from a different specific group of materials; in the second step, the surface of this layer and the other bonding partners is activated by a plasma process, followed by a direct bonding process, and then thermal annealing. 【0004】 Patent Document 2 discloses a surface acoustic wave device for bonding piezoelectric wafers of limited wafer sizes from 0.051 m to 0.102 m to different substrate materials using a wet chemical wafer bonding surface procedure followed by Ar / O2 mixed gas plasma activation. 【0005】 Patent Document 3 discloses a two-step surface activation process prior to the direct bonding process. Here, the first activation step involves placing the lithium niobate wafer and silicon wafer to be bonded under a vacuum ultraviolet light source and activating them under humidity conditions of 20% to 80%. The second activation step involves placing the wafers activated with vacuum ultraviolet light into an N2 plasma under a pressure of 10 Pa to 80 Pa. 【0006】 Known wafer bonding methods for piezoelectric materials onto support wafers often present one or more technical challenges and / or challenges in the ultimately manufactured high-frequency filter device, including passband shift of the SAW device due to substrate temperature during annealing and thinning processes, thermal microvoids (gas release, etc.), fracture due to high electrostatic charge (e.g., warping and large bow), insufficient bonding strength against further processing of the bonded wafer pair (e.g., thinning, dicing), and increased SAW propagation loss. [Prior art documents] [Patent Documents] 【0007】 [Patent Document 1] International Publication No. 2017163722 [Patent Document 2] Chinese Patent No. 109786229 [Patent Document 3] Chinese Patent No. 109166793 [Overview of the project] [Problems that the invention aims to solve] 【0008】 The object of the present invention is to provide a method for forming a composite substrate, including a wafer bonding process having improved performance. [Means for solving the problem] 【0009】 A method for forming a composite substrate may include the features of claim 1. Further embodiments are described in the dependent claims. This method provides efficient manufacturing of thin-film surface acoustic wave (TF-SAW) devices and other related devices by efficiently forming a stable bond between a piezoelectric wafer and a support wafer. 【0010】 As an example, this method considers a process flow for improving wafer bonding onto a support wafer material of a piezoelectric crystal, as seen in known wafer bonding processes and the manufacture of SAW devices. For example, by this method, a shift in the passband of a SAW device due to the temperature of the substrate during annealing and thinning processes, thermal microvoids (e.g., gas evolution), destruction due to high electrostatic charging (e.g., warping and high bow), and insufficient bonding strength for further processing of the bonded wafer pair (e.g., thinning, dicing), as well as the possibility of improving the characteristics of SAW propagation loss. By this method, a piezoelectric material with a large electromechanical coupling coefficient and excellent stability against temperature can be provided. 【0011】 In the drawings, like reference numerals generally refer to the same parts throughout different views. The drawings are not necessarily to scale; instead, emphasis is generally placed on illustrating the principles of the invention. In the following description, various aspects of the invention are described with reference to the following drawings. 【Brief Description of the Drawings】 【0012】 [Figure 1] It is a diagram showing a schematic cross-sectional view of a composite substrate. [Figure 2] It is a diagram showing a flowchart of a method for manufacturing a composite substrate. [Figure 3A] It is a diagram showing a schematic view of a method for manufacturing a composite substrate. [Figure 3B] It is a diagram showing a schematic view of a method for manufacturing a composite substrate. [Figure 3C] It is a diagram showing a schematic view of a method for manufacturing a composite substrate. [Figure 3D] It is a diagram showing a schematic view of a method for manufacturing a composite substrate. [Figure 3E] It is a diagram showing a schematic view of a method for manufacturing a composite substrate. [Figure 3F] It is a diagram showing a schematic view of a method for manufacturing a composite substrate. [Figure 4] It is a diagram showing a schematic plasma system for a method of manufacturing a composite substrate. [Figure 5A]It is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 5B] It is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 5C] It is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 5D] It is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 6] It is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 7] It is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 8] It is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 9] It is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 10A] It is a diagram showing an infrared transmission image (IR) of the surface of a substrate constituting a composite substrate. [Figure 10B] It is a diagram showing an infrared transmission image (IR) of the surface of a substrate constituting a composite substrate. [Figure 11A] It is a diagram showing a schematic cross-sectional view of a composite substrate in a substrate thinning process. [Figure 11B] It is a diagram showing a schematic cross-sectional view of a composite substrate in a substrate thinning process. [Figure 12A] It is a diagram showing a schematic cross-sectional view of a composite substrate in a substrate thinning process. [Figure 12B] It is a diagram showing a schematic cross-sectional view of a composite substrate in a substrate thinning process. 【Mode for Carrying Out the Invention】 【0013】 The following detailed description refers to the accompanying drawings which illustrate specific details and embodiments in which the present invention may be implemented. 【0014】 As an example, a method for providing a piezoelectric material wafer bond on a support substrate manufactured using mixed gas plasma activation is described. For example, this method includes surface modification with a mixed gas of O2 and N2, including oxygen (O2) and nitrogen (N2) plasma activation, for example, ultraviolet (UV) light using an excimer lamp, a bonding process in a low vacuum, and thermal annealing at a low temperature of less than 200°C. 【0015】 This method achieves high bonding quality to silicon (Si), silicon oxide, silicon carbide (SiC), cubic boron nitride (c-BN), and diamond, in the case of piezoelectric substrates such as lithium tantalate (LT) and lithium niobate (LN). The resulting bonded material (also called composite substrate) may be completely free of macroscopic and microscopic voids throughout the bond interface. Alternatively or additionally, the composite substrate may be completely free of creeking and warping of the bonded wafer. The initial pre-bonded state (also called composite substrate precursor) may exhibit higher bond strength than when using known bonding techniques. The composite structure can be used to achieve thin-film surface acoustic wave (also called TF-SAW) devices for various applications, for example, as filtering devices in communications or pumps or mixing devices in fluid processing devices. 【0016】 Piezoelectric materials from material groups such as lithium tantalate (LT) and lithium niobate (LN) have a large electromechanical coupling coefficient, which is advantageous for achieving broadband filtering characteristics. However, these materials have the disadvantage of low temperature stability. This method provides a permanent wafer direct bonding method (also called fusion bonding) by using a mixed gas plasma activation process and UV surface modification, providing a composite substrate (also called an aggregated composite substrate) or a composite substrate with strong adhesion between the wafer and the piezoelectric material. In the composite substrate, microvoids may not be present at the bonding interface of various material combinations such as LT / / Si, LN / / Si, LN / / SiO2, LT / / SiO2, quartz / / Si, LN / SiC, LT / / SiC, LN / / BN, LT / / BN, LT / / Diamond, and LN / / Diamond. 【0017】 This method may be configured such that a plasma activation process accompanied by a UV modification process forms an interlayer (also called a bonding layer) at a predetermined level. The properties of the interlayer can be controlled by parameters of the plasma activation process and the UV modification process that affect the bonding surface between the piezoelectric substrate and the support substrate. The bonding layer may contain the same components as the plasma gas used, e.g., oxygen and nitrogen, and the materials of the two substrates to be bonded, e.g., a wafer and a piezoelectric material wafer. 【0018】 The intermediate layer may contain one or more amorphous oxides and one or more amorphous nitrides. The amorphous oxides may improve bonding quality, and the amorphous nitrides may improve bonding strength. 【0019】 The plasma activation process and the mixed gas UV surface modification process can be configured to clean the surfaces to be bonded by removing impurities and organic contaminants from the substrate surface. This improves the bonding quality and bonding strength between the piezoelectric substrate and the support substrate at low temperatures. 【0020】 The bonding process may preferably be carried out under a low vacuum of up to 0.1 Pa. In this way, any water film residue is removed from both substrate surfaces. 【0021】 As an example, the bonding process is the direct wafer bonding of combinations of different materials. This method enables efficient bonding at room temperature with high bonding strength for Si / / Si, Si / / SiO2, LT / / Silicon dioxide, LT / / Si, LT / / Quartz, and LT / / Sapphire (Al2O3) through material-related optimization of plasma activation parameters. Plasma activation parameters may include plasma power, plasma gas pressure, plasma duration, gas flow rate ratio (N2 to O2), pre-bonding process conditions (e.g., vacuum atmosphere and mechanical pressure), and mixed gas UV surface modification and (post) thermal annealing processes. 【0022】 This method provides surface activation using a gas mixture of O2 and N2 for plasma surface activation and mixed gas UV surface modification of both wafers to be bonded, for example, a piezoelectric wafer (also called a second substrate) and a support substrate wafer (also called a first substrate). 【0023】 The first substrate (support substrate) includes, for example, silicon (Si), silicon oxide (SiO2), sapphire (Al2O3), silicon carbide (SiC), cubic boron nitride (c-BN), and diamond, or is formed from them. The second substrate (piezoelectric substrate) includes, for example, lithium niobate (LN), lithium tantalate (LT), and quartz (SiO2), or is formed from them. After a plasma activation process and a UV modification process, the first substrate is directly bonded to the second substrate. A mixed-gas plasma surface activation process accompanied by a mixed-gas UV irradiation modification process can bond a piezoelectric lithium niobate (LN) surface or a piezoelectric lithium tantalate (LT) surface to the support substrate. The bonding process can be used to bond a dissimilar material, such as an LN wafer or an LT wafer, to one of the Si, SiO2, c-BN, SiC, and diamond wafers at room temperature under vacuum conditions. 【0024】 The plasma activation process can be carried out in a plasma chamber in which a first electrode and a second electrode are enclosed within a housing, and the gas flow between the first electrode and the second electrode can be adjusted. The powers of the first electrode and the second electrode can be adjusted independently of each other (also referred to as being separated). The potential between the first electrode and the second electrode can form plasma from gas molecules flowing between the first electrode and the second electrode. Therefore, the power supplied to the first electrode and the second electrode may also be referred to as plasma power. The plasma power for the first electrode, which can be the upper electrode of the plasma chamber, can be in the range of, for example, 10 W to 150 W. The plasma power for the second electrode, which can be the lower electrode of the plasma chamber, can be in the range of, for example, 10 W to 75 W. 【0025】 The processing time (also referred to as plasma activation time) of the plasma activation process for the first substrate and / or the second substrate can be in the range of about 5 seconds (s) to about 180 seconds, for example, in the range of about 10 seconds to about 120 seconds. 【0026】 The plasma operating pressure of the plasma activation process for the first substrate and / or the second substrate can be in the range of about 10 Pa to about 300 Pa, for example, in the range of about 20 Pa to about 100 Pa. 【0027】 The thermal annealing of the bonded composite substrate precursor can be carried out at a temperature in the temperature range of about 80 °C to about 190 °C, for example, in the temperature range of about 100 °C to about 175 °C, in a forming gas atmosphere. 【0028】 The bonding by direct covalent bonding between the crystalline piezoelectric material of the second substrate and the material of the first substrate is in the range of about 0.5 J / m 2 ~ about 2.5 J / m 2 For example, 0.5 J / m 2 , 0.8 J / m 2 , 1.0 J / m 2 [[ID=·27]]1.2 J / m 2 1.5 J / m 2 1.8 J / m 2 or 2.0 J / m 2Adjustable joint strength can be achieved. 【0029】 The composite substrate is bonded by covalent bonds and may be one of various combinations of materials having high bonding strength, such as LT / / Si, LT / / SiO2, LT / / SiC, LT / / c-BN, and LT / / Diamond, LN / / Si, LN / / SiO2, LN / / SiC, LN / / c-BN, and LN / / Diamond, or any combination thereof. 【0030】 The bonding interface of the composite substrate may be a high-quality bond, for example, it may be free of microvoids, gas emissions, and damage to the surface structure. 【0031】 The bonding interface of the composite substrate can exhibit excellent thermal stability at high temperatures up to 200°C. 【0032】 This method makes it possible to bond a first substrate to a second substrate of various commercially available wafer formats (e.g., 0.051m, 0.102m, 0.153m, and 0.204m, and even larger support wafer sizes). [Examples] 【0033】 Figure 1 shows a schematic cross-sectional view of a composite substrate 100 in which a first substrate 102 is bonded to a second substrate 106 via a bonding layer 104. The second substrate 106 may contain a piezoelectric material as described above, or may be formed from a piezoelectric material. The first substrate 102 and the second substrate 106 are directly bonded. Therefore, the bonding layer 104 may contain a combination of materials: the first substrate, the second substrate 106, oxygen, and / or nitrogen. 【0034】 For example, the bonding layer 104 can be divided into oxide amorphous domains (multiple), nitride amorphous domains (multiple), and metallic amorphous domains (multiple) at the direct bonding interface (there may be multiple) and sub-bonding interface (there may be multiple). 【0035】 The composite substrate 100 can be used, for example, to form a TF-SAW device. 【0036】 The method for forming the composite substrate 100 can be improved or further optimized by varying the plasma conditions, namely the plasma power, plasma duration, plasma gas pressure being less than 200 Pa, the mixed gas flow rate and the ratio of the two gas flow rates, and exposure to low-power UV light (e.g., in the range of approximately 150 nm to 500 nm) in a mixed gas (O2 + N2) atmosphere. 【0037】 The composite substrate 100, made of various material combinations, includes high adhesive strength, no or substantially no thermal gas release, and low electrostatic charge (for example, the bow of the composite substrate 100 may be less than 20 μm), as well as the bonded first and second substrates not being damaged during thermal annealing at temperatures up to 200°C and during any mechanical thinning of the LT / LN bonded wafer portion to a film thickness of approximately 250 μm to approximately 2 μm (no damage occurs with various wafer sizes, for example, when the wafers of the first substrate 102 and the second substrate 106 are up to 0.153 m). 【0038】 Figure 2 shows a flow diagram of a method 200 for forming the composite substrate 100 shown in Figure 1. Method 200 may include a plasma activation process 202, which includes exposing a first surface of the first substrate into a plasma system. The plasma is a mixed gas plasma of oxygen and nitrogen at a first ambient pressure. The first ambient pressure may be lower than atmospheric pressure. 【0039】 Method 200 may include a plasma activation process 204, which includes exposing a second surface of a second substrate. The plasma system may be a high-frequency system having plasma between a first electrode and a second electrode (e.g., a plasma chamber including a high-frequency generator or microwave generator that generates gas plasma from a gas flowing through the plasma chamber). The first and second electrodes may be configured to be powered and controlled separately. The plasma is a mixed gas plasma of oxygen and nitrogen at a first ambient pressure. The first ambient pressure may be lower than atmospheric pressure. 【0040】 The plasma system is shown in more detail in Figure 4. 【0041】 Method 200 may further include a surface modification process comprising 206 exposing a plasma-activated first surface to ultraviolet irradiation in an atmosphere of oxygen and nitrogen at atmospheric pressure. Method 200 may further include a surface modification process comprising 208 exposing a plasma-activated second surface 208 to ultraviolet irradiation in an atmosphere of oxygen and nitrogen at atmospheric pressure. 【0042】 Method 200 may further include a process 210 for forming a composite substrate precursor by placing the first surface of a first substrate on the second surface of a second substrate. 【0043】 Method 200 may further include a process 212 for bonding a first substrate to a second substrate by applying a mechanical load to the composite substrate precursor with a second ambient pressure. 【0044】 This method may further include a process 214 of thermal annealing the bonded composite substrate precursor. 【0045】 Here, the first substrate is a wafer configured to support a second substrate, which is a single crystal and an amorphous material. The second substrate is a piezoelectric material wafer. 【0046】 Figures 3A to 3F show schematic diagrams of the method of Figure 2 for manufacturing composite substrates. As an example, this method includes a general process flow for directly bonding a second substrate 106 to a first substrate 102. This method may generally include a process of exposing the first substrate 102 and the second substrate 106 to excimer light 306, 308 emitted by excimer lamps 302, 304 in an ambient air atmosphere, as shown in Figure 3A. The excimer lamps 302, 304 may provide electromagnetic radiation (also called light) with wavelengths or wavelength spectra (also called ultraviolet (UV) light) in the wavelength range of about 150 nm to about 500 nm. The excimer light 306, 308 may be the same or different for the first substrate 102 and the second substrate 106. Thus, organic and non-organic contaminants may be reduced or even eliminated. 【0047】 Figure 3B shows that the first substrate 102 can be exposed to mixed gas plasma activation 310 at room temperature, and the second substrate 106 can be exposed to mixed gas plasma activation 312 at room temperature. This can reduce the surface species of the materials of the first and second substrates, respectively, and allow a bonding layer 104 to be formed on the first and second surfaces. 【0048】 This method may include rinsing processes 314, 316 of the plasma-activated first substrate 102 and the plasma-activated second substrate 106, as shown in Figure 3C. Rinsing processes 314, 316 may include rinsing and drying of the rinsed first and second substrates with megasonic-assisted deionized (DI) water. In this way, particulate matter and organic contaminants can be removed from the first and second surfaces. 【0049】 In other words, after surface activation, the first substrate 102 and the second substrate 106 may be cleaned, for example, thereafter in this order, using a megasonic cleaning nozzle (where megasonic can correspond to an acoustic signal having a frequency in the range of 700 kHz to 5 MHz) and a DI water cleaning nozzle. The cleaned substrates 102 and 106 may be placed in a spinner. The spinner can rotate the cleaned substrates to dry them. Cleaning using a megasonic cleaning nozzle, a DI water cleaning nozzle, and a spinner can be integrated into a single wafer cleaner system. 【0050】 The surface modification process of the method shown in Figure 3D may include exposing the first substrate 102 and the second substrate 106 to light 318, 320 emitted by one or more excimer lamps 322, 324. The excimer light 318, 320 may be the same or different for the first substrate 102 and the second substrate 106. The first surface of the first substrate 102 and the second surface of the second substrate 106 may be exposed to a mixture 334 of N2 and O2 radicals and other reactive atoms or molecular particles during UV light exposure. The mixture 332, 334 may be the same or different for the first substrate 102 and the second substrate 106. The first surface of the first substrate 102 and the second surface of the second substrate 106 may be exposed to low temperatures (e.g., below 100°C) during UV light exposure. By this method, any contaminants can be removed from the first and second surfaces. Furthermore, the surface energy and polarity of the first and second surfaces can be adjusted to match chemical reactions at the atomic or molecular level. In this way, the functional groups on the dangling bond are strengthened, which may improve the adhesive strength at room temperature. 【0051】 The process for forming the composite substrate precursor 330 may include, for example, mechanical alignment of the planes or notches of the first and second surfaces, as shown in Figure 3E. The bonding process may be carried out in a vacuum atmosphere at room temperature with a processing gas pressure of less than 0.1 Pa. The bonding process may be carried out under a mechanical pressure of up to 5 kN. The mechanical pressure is directed to compress the first and second substrates. In other words, the mechanical pressure may be directed to press the first substrate against the second substrate, or vice versa. 【0052】 In other words, in the process of forming a composite substrate precursor, a first substrate 102 having a first surface and a second substrate 106 having a second surface may be placed facing each other (e.g., the wafer plane of the first substrate and the wafer plane of the second substrate) in a standard wafer bonder system by mechanical alignment 326. A standard wafer bonder system can utilize mechanical pressure of up to 5 kN in a vacuum environment of around 0.1 Pa. Thus, the first and second substrates attract and contact each other due to van der Waals forces, with a maximum pressure of 0.8 J / m at room temperature. 2 A bonding strength of 1.2 J / m is obtained. 2 For further improvement, thermal annealing may be applied for several hours at a pre-selected temperature (sometimes multiple temperatures) between 100°C and 190°C. 【0053】 The thermal annealing process 328 (also called post-thermal annealing) itself can be carried out in a standard forming gas atmosphere furnace and / or on a hot plate for one or more temperature periods at one or more temperatures up to 200°C, as shown in Figure 3F. In this way, the bonding strength, such as cohesive force, of the bonded layer 104 (see Figure 1) can be improved. 【0054】 A surface modification process, including exposure to ambient UV light, may be an additional process applied to clean most organic contaminants from the first and second surfaces. UV light exposure may include, for example, excimer light with a wavelength less than 400 nm. 【0055】 UV light exposure can be carried out in a normal air atmosphere. UV light exposure can, or may be configured to, break the molecular bonds of organic materials. In this way, UV light exposure can reduce various hydrocarbons on a first and / or second surface, for example, before plasma activation. 【0056】 Figure 4 shows a schematic diagram of a dual RF capacitively coupled plasma system 400 that can be used for mixed gas plasma activation on a wafer surface. 【0057】 The plasma system 400 may have a conventional horizontal capacitance parallel dual electrode including a first electrode 404 and a second electrode 406. The first electrode 404 and the second electrode 406 may be separately powered and / or controlled by one or more RF power supplies 408 and 410 and one or more controllers (not shown). The RF power supplies 408 and 410 may provide RF power to electrodes 404 and 406 at different frequencies in the kHz range. The plasma power may range from about 10W to about 300W for the first and second electrodes. The plasma power may have slight variations depending on the mixture of process gases. 【0058】 As an example, a single wafer of a first substrate 102 and a second substrate 106 may be placed on a second electrode 406, for example, a lower electrode 406 in a plasma chamber 402 of a plasma system 400. A mixture 418 of gaseous oxygen 412 and gaseous nitrogen 414 is supplied to the plasma chamber via an adjustable valve system 416 that provides flow control of oxygen 412 and nitrogen 414, respectively. RF power from electrodes 404, 406 adjustably forms plasma 310 and / or 312 from the gas mixture 418. The surfaces of the substrates 102, 106 to be bonded (e.g., the first surface of the first substrate or the second surface of the second substrate) may be briefly exposed to the plasma mixture 310 and / or 312 of oxygen 412 and nitrogen 414. The duration can range from about 5 seconds to about 120 seconds. The duration depends on the recipe of the best known method (BKM). 【0059】 The mixing of gases in the plasma chamber 402, such as oxygen 412 and nitrogen 414, can be achieved by controlling the flow rates of each gas 412 and 414 that form the gas mix 418 via a valve system 416. The flow rates of oxygen 412 and nitrogen 414 may range from about 5 standard cubic centimeters per minute (sccm) to about 100 sccm. Simultaneously, the gas pressure in the plasma chamber 402 during plasma activation may be controlled, for example, by a suction volume throttle valve in a vacuum pump system (not shown). The gas pressure may range from about 10 Pa to about 300 Pa. The N2 gas concentration to O2 gas flow rate ratio may range from about 5% to about 95%, respectively, depending on the materials of the first and second substrates, for example, 10%, 15%, 20%, 25%, 30%, 50%, 75%, and 95%. 【0060】 UV light exposure in a mixed gas environment 318 may be used to improve adhesion strength at room temperature and lower the annealing temperature. For example, exposure to UV light using an excimer lamp (e.g., having wavelengths in the range of approximately 150 nm to approximately 500 nm) may be carried out under standard pressure and a mixed gas N2 / O2 atmosphere (e.g., 50% / 50%, although it should be noted that the ratio may vary depending on the circumstances). In this case, the mixed gas may provide oxygen and nitrogen as working agents in the reaction between the piezoelectric material of the second substrate and the material of the first substrate. Thus, activating groups, such as hydroxyl groups (-OH), may be generated on the surfaces of the first and second substrates. This may increase the polarity and surface energy of the first surface of the first substrate and the second surface of the second substrate. UV light exposure may be carried out in a process chamber at atmospheric pressure. 【0061】 Infrared analysis and scanning ultrasonic microscopy of bonded composite substrate precursors reveal the bonding interface (e.g., the bonding layer described above), and generally there may be very few detectable voids (also called microvoids) that are typically caused by thermal gas emission. 【0062】 Figures 5A–5D show AFM images 500, 502, 508, and 510, including linear scan height profiles 504, 506, 512, and 514 of the materials shown after mixed gas (O2+N2) plasma activation 310 or 312 of Figure 3B, for example, images of the silicon (Si) shown in Figure 5A, the thermal silicon dioxide (SiO2) film (0.5 μm thick) on the Si wafer shown in Figure 5B, the lithium tantalate shown in Figure 5C, and the quartz shown in Figure 5D after mixed gas (O2+N2) plasma activation using individual material-specific BKM recipes. Despite individual scratches, the RMS roughness of all substrates may be much lower than 0.5 nm, as shown in the linear scan height profiles 504, 506, 512, and 514. For bonding, a smooth surface with a root mean square (RMS, Rq) roughness of about 0.5 nm or less may be preferred. 【0063】 Figure 6 shows a scanning ultrasonic microscope (SAM) image of the first example of a composite substrate. The first example involves a Si / / Si wafer bonding of a 0.204 m wafer. Here, the Si / / Si wafer bonding is performed using plasma activation with a mixed gas concentration using a BKM recipe, a gas flow of less than 100 sccm, a gas pressure of up to 100 Pa, and plasma power of less than 50 W for the first and second electrodes. The pre-bonding process (also called the pre-bonding process) is performed in a vacuum, at room temperature, under identical conditions of mechanical pressure of 5 kN (piston descent), followed by post-bonding annealing at 300°C for 2 hours. Figure 6 shows a SAM image of the composite wafer pre-bonded in a vacuum. This is a high-quality bond interface with no microvoids or thermal gas release across the bond interface, in contrast to pre-bonding in an atmospheric pressure environment, and a bond strength of 2.0 J / m 2 It could increase to that extent. 【0064】 Figure 7 shows scanning ultrasonic microscope (SAM) images of a second example of a composite substrate. The second example includes Si / / SiO2 wafers (both 0.204 m in diameter) bonded by a BKM recipe, plasma processing gas pressure of less than 50 Pa, low plasma power of less than 100 W for each of the first and second electrodes, and a mixed gas plasma activation process consisting of a pre-bonding process at room temperature in vacuum under identical conditions, followed by post-annealing at 300°C for 2 hours. There are virtually no microvoids or heat emissions at the interface of both bonded wafers (e.g., the first and second substrates), except for a few small handling particles crossing the edge bonding region. After post-thermal annealing, the bond strength is 1.6 J / m 2 It may increase up to that point. 【0065】 Figure 8 shows scanning ultrasonic microscope (SAM) images of a third example of a composite substrate. The composite substrate contains LiTaO3(LT) as the second substrate and Si as the first substrate. The images shown show the composite substrate after thermal annealing at 175°C. The composite substrate can be manufactured and evaluated using the same procedure as in Example 1. The composite substrate precursor can be bonded at room temperature under a mechanical pressure of 2.0 kN in a vacuum ambient environment of 0.1 Pa. The entire LiTaO3(LT) can be bonded completely. After annealing at 175°C for 2 hours, a void-free interface can be observed. After post-annealing at 175°C for 5 hours, the adhesive strength is 1.27 J / m 2 This is possible. The second substrate may be thinned from a thickness of approximately 250 μm to approximately 400 μm to a thinned thickness in the range of approximately 1 μm to approximately 250 μm, for example, in the range of approximately 2 μm to approximately 10 μm. 【0066】 Figure 9 shows a scanning ultrasonic microscope (SAM) image of a fourth example of the composite substrate 900. The composite substrate includes a silicon wafer as a first substrate having a thermal oxide (e.g., 500 nm thick) after thermal annealing at 150°C, and an LT wafer as a second substrate. In the fourth example, a 0.153 m wafer with the first and second substrates bonded together can be manufactured and evaluated using the same procedure as in Example 2. Here, the material of the piezoelectric single crystal substrate (second substrate) is lithium tantalate (LT), and the Si handle wafer can be covered with a 500 nm thermal oxide (first substrate) and prepared for use as an RF velocity layer as a support material. Figure 9 shows a scanning ultrasonic microscope (SAM) image of the LT / / SiO2 bond at room temperature (RT) and the bond interface after thermal annealing at 150°C for 2 hours. The entire wafer pair can be completely bonded. The measured bond strength is 1.1 J / m 2 This can occur. Examination of this bonding interface by SAM measurement reveals numerous small microbubbles, and the same phenomenon can occur even when thermal oxide wafer bonding is performed in a vacuum. However, in the case of a Si wafer with sputter-deposited oxide (thickness 100 nm to 1000 nm), no gas release or gas bubbles can be observed after 5 hours of thermal annealing at 150°C. The second substrate may be thinned from a thickness of approximately 250 μm to 400 μm to a thinned thickness in the range of approximately 1 μm to 250 μm, for example, in the range of approximately 2 μm to 10 μm. 【0067】 Figures 10A and 10B show infrared (IR) transmission images of bonded composite substrates, including a LiTaO3 wafer as a second substrate bonded to a quartz wafer (Figure 10A) and a sapphire wafer (Figure 10B) as the first substrates, respectively. The bonded 0.102 m wafers can be manufactured and evaluated according to the same procedure as in Example 3. In this example of the present invention, the piezoelectric material of the second substrate may be a 0.204 m single-crystal lithium tantalate (LT) wafer, and the first substrates may be 0.204 m quartz (fused silica) and 0.204 m sapphire (Al2O3), respectively. The composite substrates are pre-bonded at room temperature under a mechanical pressure of 2 kN in a vacuum environment, followed by thermal annealing at less than 200°C for 5 hours. The bonded composite substrates exhibit high bonding quality, with no macrovoids or thermal microvoids formed before and after annealing. The adhesive strength is 1.3 J / m 2 This is possible. The second substrate may be thinned from a thickness of approximately 250 μm to approximately 400 μm to a thinned thickness in the range of approximately 1 μm to approximately 250 μm, for example, in the range of approximately 2 μm to approximately 10 μm. 【0068】 Figures 11A and 11B show schematic diagrams of a method for manufacturing piezoelectric material on a SiC wafer composite substrate before the thinning and polishing process 1120 (Figure 11A). Here, the second substrate 106 has a first thickness 1110. The thickness 1110 of the second substrate 106 can be reduced to a second thickness 1112 by the thinning and polishing process 1120, as shown in Figure 11B. The second thickness 1112 may be in the range of about 1 μm to about 250 μm, for example, in the range of about 2 μm to about 10 μm. The thin composite substrate 1100 may contain piezoelectric material to silicon carbide (SiC), and its bonding surface may include a Si surface. The wafer bonding can be manufactured and evaluated in the same procedure as in Example 3 above. However, the material of the piezoelectric single crystal substrate may include lithium tantalate (LT) single crystal, and the material of the first substrate 102 may include a 4H-SiC substrate. 【0069】 Figures 12A and 12B show schematic diagrams of a method for manufacturing piezoelectric material on a thin film c-BN or diamond wafer composite substrate before a thinning and polishing process 1120 (Figure 12A) (for example, including a first substrate made of silicon and a thin layer of c-BN or diamond). Here, the second substrate 106 has a first thickness 1210. The thickness 1210 of the second substrate 106 can be reduced to a second thickness 1212 using the thinning and polishing process 1120, as shown in Figure 12B. The second thickness 1212 may be in the range of about 1 μm to about 250 μm, for example, in the range of about 2 μm to about 10 μm. 【0070】 The composite substrate 100 may include a piezoelectric material of a second substrate 106 bonded to a first substrate further having, for example, a c-BN layer 1204 or a thin diamond layer 1204 (e.g., less than 1 μm thick). In other words, the composite substrate 100 may include a thin film of c-boron nitride layer 1204 or diamond layer 1204. The c-BN layer 1204 can also be deposited using physical vapor deposition (PVD), in which case the c-BN layer 1204 may be deposited by radio-frequency magnetron sputtering (RFMS). A c-BN film with a thickness of about 1.1 μm may be deposited on a first substrate 102 having a very smooth surface, for example, a Si substrate. The composite substrate 100 may be manufactured and evaluated according to the same procedure as in Example 2, and the mixed gas plasma may be activated based on high plasma power in a gas mixture with a total gas flow rate of less than 50 sccm and a nitrogen / oxygen gas ratio (O2:N2) of 1:1. The bonding layer 104 is very uniform and its thickness may be in the range of 10.0 nm. The bonding layer 104 may contain a large proportion of amorphous nitride from the mixed gas activation process and mixed gas UV surface modification, and achieves 0.8 J / m without using any post-bonding heating. 2 The joint strength can be improved within this range. Here, post-thermal annealing at temperatures above 200°C can increase the joint strength to a maximum of 1.8 J / m 2 It has the potential to raise the level to that point. 【0071】 The diamond layer 1204 can also be deposited on the silicon substrate 102 by PVD. The diamond film 1204 may have a maximum thickness of approximately 2.0 μm. The diamond layer 1204 can be polished using ion beam trimming to achieve a surface roughness (RMS) of less than 0.5 nm. Wafer bonding can be manufactured and evaluated using the same procedure as in the above embodiments. The mixed gas plasma can be activated based on high plasma power, and in the case of long-duration plasma activation, it can be activated by mixed gas UV surface modification for a long period of less than 120 seconds in a nitrogen / oxygen gas mixture in a 1:1 (O2:N2) ratio with a total gas flow rate of less than 75 sccm. The bonded layer 104 is very uniform and may have a maximum thickness of approximately 15.0 nm. The bonded layer 104 may contain a large proportion of amorphous nitride depending on the mixed gas plasma activation of the first and second surfaces to be bonded, in order to improve bond strength without using heating. In this case, post-thermal annealing at temperatures exceeding 200°C may be used to increase the joint strength. 【0072】 For example, single-crystal substrates of lithium tantalate and lithium niobate can be used in elastic wave filters that may have low thermal conductivity. A surface activation process and a method for manufacturing thin-film bonded wafers are provided that can produce an excellent bonding layer between bonded wafers of composite substrates by using a mixed-gas (O2+N2) plasma and UV surface modification under a mixed-gas atmosphere (O2+N2). The plasma activation method and the UV surface modification method can be used when directly bonding a first substrate such as silicon (Si), silicon oxide (SiO2), sapphire (Al2O3), silicon carbide (SiC), cubic boron nitride (c-BN), and diamond to a second substrate such as lithium niobate (LN), lithium tantalate (LT), and quartz (SiO2). 【0073】 Plasma surface activation processes can be used to bond lithium tantalate (LT) and lithium niobate (LN) surfaces to various substrates. 【0074】 This bonding method can be used to bond dissimilar materials, such as LN wafers and LT wafers, to substrates of Si, SiO2, sapphire, c-BN, and SiC (for example, having diameters of 0.051 m, 0.102 m, 0.153 m, and 0.204 m) in a vacuum environment at room temperature. 【0075】 The mixed gas plasma process can be used for surface activation of all second and first substrates, provided that the plasma power of the first electrode of the plasma system is in the range of 10W to 150W and the plasma power of the second electrode of the plasma system is in the range of 10W to 150W. 【0076】 The mixed gas plasma process for surface activation can be used on all second and first substrates with plasma durations (hours) ranging from 5 seconds to 180 seconds. 【0077】 The mixed gas plasma process for surface activation of the second and first substrates can be used with a gas pressure plasma of 10 Pa to 200 Pa. 【0078】 All mixed gas plasma processes for surface activation of the second and first substrates can be used in conjunction with post-thermal annealing in a forming gas atmosphere at 100°C to 200°C. 【0079】 This method provides a composite substrate having a high-quality bonding interface that is free of macrovoids and microvoids, has strong bonding, does not emit thermal gases, and does not damage the surface structure. 【0080】 The composite substrate can exhibit excellent thermal stability at high temperatures up to 250°C. Therefore, efficiently manufactured thin-film surface acoustic wave (TF-SAW) devices can be provided. 【0081】 This method can provide bonding over substantially the entire wafers of a first substrate and a second substrate having different wafer sizes. In other words, a method for forming a composite substrate includes a plasma activation process, which may include exposing a first surface of the first substrate and a second surface of the second substrate to a plasma system, the plasma system may be a high-frequency system having plasma between a first electrode and a second electrode that can be separately powered, the plasma may be a mixed gas plasma of oxygen and nitrogen at a first ambient pressure which may be below atmospheric pressure, and the surface modification process may include exposing the plasma-activated first surface and the plasma-activated second surface to ultraviolet irradiation in an oxygen and nitrogen atmosphere at atmospheric pressure, forming a composite substrate precursor by placing the first surface of the first substrate on the second surface of the second substrate, bonding the first substrate to the second substrate by applying a mechanical load to the composite substrate precursor at a second ambient pressure, and thermal annealing the bonded composite substrate precursor. The first substrate may be a wafer configured to support a second substrate, which may be single crystal or amorphous, and the second substrate may be a piezoelectric material wafer. 【0082】 The second ambient pressure may be atmospheric pressure. The second ambient pressure is approximately 0.1 Pa to approximately 10 -3 The vacuum pressure may be in the range of Pa. 【0083】 This method may further include polishing the first surface and the second surface such that each surface has a root mean square roughness of less than 0.5 nm. 【0084】 This method may include a plasma power of a first electrode in the range of approximately 10W to approximately 300W. This method may also include a plasma power of a second electrode in the range of approximately 10W to approximately 300W. The plasma activation period of one or more periods may be a period of approximately 5 seconds to approximately 180 seconds. 【0085】 The mixed gas plasma may include a gas flow rate ratio of nitrogen to oxygen in the range of about 0.01 to about 0.99. The gas flow of the mixed gas plasma may be in the range of about 5 sccm to about 100 sccm. The first ambient pressure may be in the range of about 10 Pa to about 200 Pa. The method may further include a rinsing process which may include rinsing and drying of the plasma-activated first and second surfaces with megasonic-assisted deionized (DI) water. The surface modification process may include the rinsing process. Exposure to ultraviolet irradiation may be performed before the rinsing process, or after the rinsing process. 【0086】 Ultraviolet irradiation may include electromagnetic radiation with wavelengths in the range of approximately 150 nm to 500 nm. 【0087】 During UV irradiation, the temperatures of the first and second surfaces may fall below approximately 100°C. 【0088】 The mechanical load can have a maximum mechanical pressure of 5kN when the piston is lowered. 【0089】 The thermal annealing process may involve the temperature of the bonded composite substrate precursor for one or more temperature periods at temperatures of 100°C, 120°C, 150°C, 175°C, 190°C, and 250°C. 【0090】 The second substrate piezoelectric material wafer may include a piezoelectric single crystal selected from the group which may include lithium niobate (LN), lithium tantalate (LT), quartz, and any combination thereof. 【0091】 This method may further include, in addition to the second substrate, a third substrate bonded to the first substrate, wherein the third substrate may be a piezoelectric material wafer different from the piezoelectric material of the second substrate. The piezoelectric material wafer of the third substrate may include a piezoelectric single crystal selected from the group that may include lithium niobate (LN), lithium tantalate (LT), quartz, and any combination thereof. 【0092】 The first substrate wafer may include a material substrate selected from the group that may include silicon (Si), silicon oxide (SiO2), silicon carbide (SiC), cubic boron nitride (c-BN), sapphire, diamond, and any combination thereof. 【0093】 In this specification, the term “as an example” is used to mean “serving as an example, case, or explanation.” Any embodiment or design described “as an example” in this specification is not necessarily construed as being preferable or advantageous to other embodiments or designs. In this specification, the term “exemplary” is used to mean “serving as an example, case, or explanation.” Any example or design described “exemplary” in this specification is not necessarily construed as being preferable or advantageous to other examples or designs. 【0094】 In the specification or claims, the words “plurality” and “multiple” explicitly refer to a quantity greater than one. In the specification or claims, terms such as “groups,” “sets,” “collections,” “series,” “arrangements,” and “groupings” refer to a quantity equal to or greater than one, i.e., one or more. Any plural form of a term that is not explicitly stated as “plurality” or “multiple” also refers to a quantity equal to or greater than one. 【0095】 For example, the terms “processor” or “controller” as used herein may be understood as any kind of technical entity that enables the processing of data. Data may be handled according to one or more specific functions performed by the processor or controller. Furthermore, as used herein, a processor or controller may be understood as any kind of circuit, such as any kind of analog or digital circuit. Thus, a processor or controller may be or include analog circuits, digital circuits, mixed-signal circuits, logic circuits, processors, microprocessors, central processing units (CPUs), graphics processing units (GPUs), digital signal processors (DSPs), field-programmable gate arrays (FPGAs), integrated circuits, application-specific integrated circuits (ASICs), etc., or any combination thereof. Any other kind of implementation of each function may also be understood as a processor, controller, or logic circuit. Any two (or more) of the processors, controllers, or logic circuits detailed herein may be realized as a single entity with equivalent functions, etc., and conversely, any single processor, controller, or logic circuit detailed herein may be realized as two (or more) separate entities with equivalent functions, etc. 【0096】 The term "connected" can be understood to mean, for example, direct or indirect connections and / or interactions (mechanical and / or electrical). For example, several elements can be mechanically connected and held together physically (e.g., a plug connected to a socket), or electrically connected to have a conductive path (e.g., a signal path exists along a communication chain). 【0097】 While the above description and related diagrams may show components of electronic devices as separate elements, those skilled in the art will understand the various possibilities of combining or integrating these separate elements into a single element. This may include combining two or more circuits from a single circuit, mounting two or more circuits on a common chip or chassis to form an integrated element, or running separate software components on a common processor core. Conversely, those skilled in the art will recognize the possibility of separating a single element into two or more separate elements, such as splitting a single circuit into two or more separate circuits, separating a chip or chassis into separate elements that were originally located there, or separating software components into two or more sections, each running on a separate processor core. Furthermore, it will be understood that the specific implementations of hardware and / or software components are merely illustrative, and other combinations of hardware and / or software implementing the methods described herein are also within the scope of this disclosure. 【0098】 Implementations of the methods detailed herein are essentially illustrative and are therefore understood to be implementable in corresponding devices. Similarly, implementations of the devices detailed herein are understood to be implementable as corresponding methods. Therefore, it is understood that devices corresponding to the methods detailed herein may include one or more components configured to carry out each aspect of the relevant method. 【0099】 All acronyms defined in the above description also apply to the entire scope of the claims contained herein. 【0100】 While this disclosure is described and illustrated in detail with reference to specific embodiments, it will be understood by those skilled in the art that various modifications can be made in form and detail without departing from the spirit and scope of this disclosure as defined by the appended claims. Therefore, the scope of this disclosure is defined by the appended claims, and thus all modifications within the meaning and equivalence of the claims are intended to be encompassed.

Claims

[Claim 1] A method for forming a composite substrate, the method is The process includes a mixed gas plasma activation process of oxygen and nitrogen, and the process is Exposing the surface of the wafer and the surface of the piezoelectric material wafer within the plasma system, Exposing the plasma-activated surface of the wafer and the plasma-activated surface of the piezoelectric material wafer to ultraviolet irradiation in an oxygen and nitrogen atmosphere, Placing the surface of the wafer on the surface of the piezoelectric material wafer, A wafer is formed by applying a mechanical load to the wafer and the piezoelectric material wafer, The bonded wafer is subjected to thermal annealing, Includes, A method wherein the wafer is configured to support the piezoelectric material wafer. [Claim 2] The method according to claim 1, further comprising polishing the surface of the wafer and the surface of the piezoelectric material wafer such that each surface has a root mean square roughness of less than 0.5 nm. [Claim 3] The method according to claim 1 or 2, wherein the atmosphere includes a gas flow rate ratio of nitrogen flow rate to oxygen flow rate in the range of 0.01 to 0.

99. [Claim 4] The method according to claim 1 or 2, wherein the gas flow rate of the atmosphere is in the range of about 5 sccm to 100 sccm. [Claim 5] The method according to claim 1 or 2, wherein the plasma activation includes an ambient pressure in the range of about 10 Pa to 200 Pa. [Claim 6] The method according to claim 1, further comprising a rinsing process including rinsing and drying the plasma-activated surface of the wafer and the plasma-activated surface of the piezoelectric material wafer with megasonic-assisted deionization (DI) water. [Claim 7] The method according to claim 6, wherein the process of exposing the plasma-activated surface of the wafer and the plasma-activated surface of the piezoelectric material wafer to ultraviolet irradiation in an atmosphere of oxygen and nitrogen at atmospheric pressure includes the rinsing process. [Claim 8] The method according to claim 6, wherein the exposure to ultraviolet irradiation is performed before the rinsing process. [Claim 9] The method according to claim 6, wherein the exposure to ultraviolet irradiation is performed after the rinsing process. [Claim 10] The method according to any one of claims 1, 2, and 6 to 9, wherein the ultraviolet irradiation includes electromagnetic radiation having a wavelength in the wavelength range of approximately 150 nm to 500 nm. [Claim 11] The method according to claims 1, 2 and 6 to 9, wherein during the ultraviolet irradiation, the temperature of the surface of the wafer and the surface of the piezoelectric material wafer are less than 100°C. [Claim 12] The method according to any one of claims 1, 2, and 6 to 9, wherein the mechanical load has a maximum mechanical pressure of 5 kN when the piston is lowered. [Claim 13] The method according to any one of claims 1, 2, and 6 to 9, wherein the piezoelectric material wafer comprises a piezoelectric single crystal selected from the group consisting of lithium niobate (LN), lithium tantalate (LT), quartz, and any combination thereof. [Claim 14] The method according to any one of claims 1, 2, and 6 to 9, further comprising bonding a further piezoelectric material wafer to the wafer, wherein the piezoelectric material of the further piezoelectric material wafer is different from the piezoelectric material of the piezoelectric material wafer. [Claim 15] The method according to claim 14, wherein the piezoelectric material of the further piezoelectric material wafer includes a piezoelectric single crystal selected from the group consisting of lithium niobate (LN), lithium tantalate (LT), quartz, and any combination thereof. [Claim 16] The wafer is made of silicon (Si), silicon oxide (SiO ２ The method according to any one of claims 1, 2, and 6 to 9, comprising a material selected from the group consisting of silicon carbide (SiC), cubic boron nitride (c-BN), sapphire, diamond, and any combination thereof.

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