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] FIELD OF THE DISCLOSURE The present disclosure relates generally to the field of composite substrate formation, 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 bonded together. By attaching a support substrate with a smaller thermal expansion coefficient than the piezoelectric substrate, it is known that the change in the dimensions of the piezoelectric substrate due to temperature changes is suppressed, and the change in the frequency characteristics of the surface acoustic wave device is reduced.

[0003] US Patent No. 5,399,633 discloses a surface acoustic wave device having a structure in which a piezoelectric substrate and a silicon substrate are bonded together using a multi-step procedure, in which in a first step a so-called bonding layer from a group of different specific materials is deposited on at least one of the bonding partners, and in a second step this layer and the surface of the other bonding partner are activated by a plasma process, followed by a direct bonding step, followed by thermal annealing.

[0004] Patent Document 2 discloses a surface acoustic wave device in which a piezoelectric wafer with a limited wafer size of 0.051 m to 0.102 m is bonded to a different substrate material 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 prior to the direct bonding process. Here, the first activation step is to place the lithium niobate wafer and the silicon wafer to be bonded under a vacuum ultraviolet light source and activate them under humidity conditions of 20% to 80%. The second activation step is to place the wafers activated by the vacuum ultraviolet light in N2 plasma under a pressure of 10 Pa to 80 Pa.

[0006] Known methods of wafer bonding of piezoelectric material onto a support wafer often exhibit one or more of the following technical challenges: SAW device passband shift due to substrate temperature during annealing and thinning processes, thermal microvoids (e.g., outgassing), destruction due to high electrostatic charging (e.g., warping and large bow) and insufficient bond strength for further processing (e.g., thinning, dicing) of the bonded wafer pair, as well as increased SAW propagation losses in the final manufactured high frequency filter device. [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 Summary of the Invention [Problem to be solved by the invention]

[0008] It is an object of the present invention to provide a method for forming a composite substrate including a wafer bonding process with 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. The method provides for efficient fabrication of thin film surface acoustic wave (also called TF-SAW) devices and other related devices by efficiently forming a stable bond between the piezoelectric wafer and the support wafer.

[0010] By way of example, the method considers known wafer bonding processes and process flows for improving wafer bonding of piezoelectric crystals onto supporting wafer materials found in the manufacture of SAW devices. For example, the method may improve the characteristics of shifting the passband of the SAW device due to substrate temperature during annealing and thinning processes, thermal microvoids (e.g., outgassing), high electrostatic charging failure (e.g., warping and high bow) and insufficient bond strength for further processing of the bonded wafer pair (e.g., thinning, dicing), and SAW propagation loss. The method may provide piezoelectric materials with high electromechanical coupling coefficients and good temperature stability.

[0011] In the drawings, like reference characters typically refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various aspects of the invention are described with reference to the following drawings, in which: [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic cross-sectional view of a composite substrate. [Diagram 2] FIG. 1 shows a flow diagram of a method for manufacturing a composite substrate. [Figure 3A] FIG. 1 shows a schematic diagram of a method for manufacturing a composite substrate. [Figure 3B] FIG. 1 shows a schematic diagram of a method for manufacturing a composite substrate. [Figure 3C] FIG. 1 shows a schematic diagram of a method for manufacturing a composite substrate. [Figure 3D] FIG. 1 shows a schematic diagram of a method for manufacturing a composite substrate. [Figure 3E] FIG. 1 shows a schematic diagram of a method for manufacturing a composite substrate. [Figure 3F] FIG. 1 shows a schematic diagram of a method for manufacturing a composite substrate. [Figure 4] FIG. 1 shows a schematic plasma system for a method of manufacturing a composite substrate. [Figure 5A]FIG. 2 is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 5B] FIG. 2 is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 5C] FIG. 2 is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 5D] FIG. 2 is a diagram showing an atomic force microscope image of the surface of a substrate constituting a composite substrate. [Figure 6] FIG. 2 is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 7] FIG. 2 is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 8] FIG. 2 is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 9] FIG. 2 is a diagram showing a surface acoustic microscope image (SAM) of the surface of a substrate constituting a composite substrate. [Figure 10A] FIG. 2 is a diagram showing an infrared transmission image (IR) of the surface of a substrate constituting a composite substrate. [Figure 10B] FIG. 2 is a diagram showing an infrared transmission image (IR) of the surface of a substrate constituting a composite substrate. [Figure 11A] 1A-1D show schematic cross-sectional views of a composite substrate during a substrate thinning process. [Figure 11B] 1A-1D show schematic cross-sectional views of a composite substrate during a substrate thinning process. [Figure 12A] 1A-1D show schematic cross-sectional views of a composite substrate during a substrate thinning process. [Figure 12B] 1A-1D show schematic cross-sectional views of a composite substrate during a substrate thinning process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The following detailed description refers to the accompanying drawings, which illustrate specific details and manners in which the invention may be practiced.

[0014] By way of example, a method is described for providing wafer bonding of piezoelectric material onto a support substrate fabricated using mixed gas plasma activation, which includes oxygen (O2) and nitrogen (N2) mixed gas plasma activation, surface modification with a mixture of O2 and N2 gases containing ultraviolet (UV) light, e.g., by using an excimer lamp, bonding process in low vacuum, and thermal annealing at low temperatures below 200°C.

[0015] The method achieves high bonding quality on silicon (Si), silicon oxide, silicon carbide (SiC), cubic boron nitride (c-BN) and diamond for piezoelectric substrates such as lithium tantalate (LT) and lithium niobate (LN). The resulting bonded material (also called composite substrate) may be free or substantially free of macroscopic and microscopic voids throughout the bonded interface. Alternatively or additionally, the composite substrate may be free of creaking and warping of the bonded wafers. The initial pre-bond state (also called composite substrate precursor) may exhibit higher bond strength than using known bonding techniques. The composite structure may 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 mixers in fluid processing devices.

[0016] Piezoelectric materials from the material group, such as lithium tantalate (LT), lithium niobate (LN), etc., have a large electromechanical coupling coefficient that is advantageous for achieving wideband filter characteristics. However, these materials have the disadvantage of low stability to temperature. 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 with strong bonding between the wafer and the piezoelectric material (also called cohesive composite substrate) or a composite substrate with strong adhesion. The composite substrate may be free of microvoids 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] The method may be configured such that the plasma activation process accompanied by the UV modification process forms an intermediate film (also called a bonding layer) at a predetermined level. The properties of the intermediate film may be controlled by the parameters of the plasma activation process and the parameters of the UV modification process that affect the bonding surface of 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., the wafer and the piezoelectric material wafer.

[0018] The intermediate layer may include one or more amorphous oxides and one or more amorphous nitrides, where the amorphous oxides may improve bond quality and the amorphous nitrides may improve bond strength.

[0019] The plasma activation process and mixed gas UV surface modification process can be configured to remove impurities and organic contaminants from the surfaces of the substrates to clean the surfaces to be bonded, thereby improving the bonding quality and strength between the piezoelectric substrate and the support substrate at low temperatures.

[0020] The bonding process may be carried out in low vacuum, preferably up to 0.1 Pa. In this way, the residues of the water film are removed from both substrate surfaces.

[0021] As an example, the bonding process is direct wafer bonding of different material combinations. With this method, efficient bonding with high bond strength of Si / / Si, Si / / SiO2, LT / / Silicon Dioxide, LT / / Si, LT / / Quartz, and LT / / Sapphire (Al2O3) can be achieved at room temperature through material-related optimization of plasma activation parameters. Plasma activation parameters may include plasma power, plasma gas pressure, plasma duration, gas flow ratio (N2 to O2), pre-bonding process conditions (e.g., vacuum atmosphere and mechanical pressure), as well as 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, e.g., the piezoelectric wafer (also called the second substrate) and the support substrate wafer (also called the first substrate).

[0023] The first substrate (support substrate) includes or is formed of, for example, silicon (Si), silicon oxide (SiO2), sapphire (Al2O3), silicon carbide (SiC), cubic boron nitride (c-BN), and diamond. The second substrate (piezoelectric substrate) includes or is formed of, for example, lithium niobate (LN), lithium tantalate (LT), and quartz (SiO2). After the plasma activation process and the UV modification process, the first substrate is directly bonded to the second substrate. The mixed gas plasma surface activation process with the mixed gas UV irradiation modification process can bond the piezoelectric lithium niobate (LN) surface or the piezoelectric lithium tantalate (LT) surface to the support substrate. The bonding process can be used to bond the dissimilar materials of the LN wafer or LT wafer to any one of the wafers of Si, SiO2, c-BN, SiC, and diamond at room temperature in vacuum ambient conditions.

[0024] The plasma activation process may be carried out in a plasma chamber having a first electrode and a second electrode enclosed in a housing and capable of adjusting the gas flow between the first electrode and the second electrode. The power of the first electrode and the second electrode may be adjusted independently (also referred to as being separated) from each other. The potential between the first electrode and the second electrode may form a plasma from the gas molecules flowing between the first electrode and the second electrode. Thus, the power provided to the first electrode and the second electrode may also be referred to as plasma power. The plasma power to the first electrode, which may be the upper electrode of the plasma chamber, may range from 10W to 150W, for example. The plasma power to the second electrode, which may be the lower electrode of the plasma chamber, may range from 10W to 75W, for example.

[0025] The treatment time of the plasma activation process (also referred to as the plasma activation time) of the first substrate and / or the second substrate may 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, respectively.

[0026] The plasma operating pressure of the plasma activation process of the first substrate and / or the second substrate may 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, respectively.

[0027] Thermal annealing of the bonded composite substrate precursor may be carried out in a forming gas atmosphere at a temperature ranging from about 80° C. to about 190° C., such as from about 100° C. to about 175° C.

[0028] The direct covalent bond between the crystalline piezoelectric material of the second substrate and the material of the first substrate has a thermal conductivity of about 0.5 J / m 2 ~about 2.5J / m 2 range, e.g. 0.5 J / m 2 , 0.8J / m 2 , 1.0J / m 2 , 1.2J / m 2 , 1.5J / m 2 , 1.8J / m 2 , or 2.0 J / m 2An adjustable bond strength of can be achieved.

[0029] The composite substrate may be one of a variety of combinations of materials that are covalently bonded together and have high bond strength, for example, 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, e.g., the interface may be free of microvoids, free of outgassing, and free of damage to the surface structure.

[0031] The bonding interface of the composite substrate can have excellent thermal stability to high temperatures up to 200°C.

[0032] This method may enable bonding of first and second substrates in a variety of commercially available wafer formats (eg, 0.051 m, 0.102 m, 0.153 m, and 0.204 m, as well as supporting larger wafer sizes). EXAMPLES

[0033] 1 shows a schematic cross-sectional view of a composite substrate 100 having a first substrate 102 bonded to a second substrate 106 via a bonding layer 104. The second substrate 106 may include or be formed from a piezoelectric material as described above. The first substrate 102 and the second substrate 106 are directly bonded. Thus, the bonding layer 104 may include a combination of materials from the first substrate, the second substrate 106, oxygen, and / or nitrogen.

[0034] By way of example, the bonding layer 104 may be divided into oxide amorphous domain(s), nitride amorphous domain(s), and metal amorphous domain(s) at the direct bond interface(s) and sub-bond interface(s).

[0035] The composite substrate 100 may be used, for example, to form a TF-SAW device.

[0036] The method for forming the composite substrate 100 can be improved or even optimized by varying the plasma conditions, i.e., plasma power, plasma duration, plasma gas pressure less than 200 Pa, mixed gas flow rate and the ratio of both gas flow rates, and exposure to UV light (e.g., in the range of about 150 nm to 500 nm) at low power under mixed gas (O2+N2) atmosphere.

[0037] The composite substrate 100 of various material combinations includes high adhesive strength, no or substantially no thermal outgassing, and low static charging (e.g., the bow of the composite substrate 100 can be less than 20 μm), as well as no damage to the bonded first and second substrates during thermal annealing at temperatures up to 200° C. and optional mechanical thinning of the LT / LN bonded wafer portions to a thickness of about 250 μm to about 2 μm (without any damage for various wafer sizes, e.g., first substrate 102 and second substrate 106 wafers up to 0.153 m).

[0038] Figure 2 shows a flow diagram of a method 200 of forming the composite substrate 100 shown in Figure 1. The method 200 may include a plasma activation process 202 that includes exposing a first surface of a first substrate in 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 less than atmospheric pressure.

[0039] The method 200 may include a plasma activation process 204 that includes exposing a second surface of the second substrate. The plasma system may be a radio frequency system (e.g., a plasma chamber including a radio frequency or microwave generator that creates a gas plasma from a gas flowing through the plasma chamber) having a plasma between a first electrode and a second electrode. The first electrode and the second electrode may be configured to be separately powered and controlled. The plasma is a mixed gas plasma of oxygen and nitrogen at a first ambient pressure. The first ambient pressure may be less than atmospheric pressure.

[0040] The plasma system is shown in more detail in FIG.

[0041] The method 200 may further include a surface modification process including exposing 206 the plasma activated first surface to ultraviolet radiation in an atmosphere of oxygen and nitrogen at atmospheric pressure. The method 200 may further include a surface modification process including exposing 208 the plasma activated second surface to ultraviolet radiation in an atmosphere of oxygen and nitrogen at atmospheric pressure.

[0042] The method 200 may further include a process 210 of forming a composite substrate precursor by disposing a first surface of a first substrate on a second surface of a second substrate.

[0043] The method 200 may further include a process 212 of bonding the first substrate to the second substrate by applying a mechanical load to the composite substrate precursor at a second ambient pressure.

[0044] The method may further include a process 214 of thermally annealing the bonded composite substrate precursor.

[0045] Here, the first substrate is a wafer configured to support a second substrate that is monocrystalline and amorphous, the second substrate being a piezoelectric material wafer.

[0046] 3A-3F show schematic diagrams of the method of FIG. 2 for manufacturing a composite substrate. As an example, the method includes a general process flow for direct wafer bonding of the second substrate 106 to the first substrate 102. The 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 lamp(s) 302, 304 in an ambient air atmosphere, as shown in FIG. 3A. The excimer lamps 302, 304 may provide electromagnetic radiation (also called light) having a wavelength or wavelength spectrum (also called ultraviolet (UV) light) in a 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] 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, which can reduce surface species of the first substrate material and the second substrate material, respectively, and form a bonding layer 104 on the first surface and the second surface.

[0048] The 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. The rinsing processes 314, 316 may include megasonic assisted deionized (DI) water rinsing and drying of the rinsed first and second substrates. In this manner, particles and organic contaminants may 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 using, for example, a megasonic cleaning nozzle (wherein megasonic may correspond to an acoustic signal having a frequency in the range of 700 kHz to 5 MHz) and a DI water cleaning nozzle, for example, in that order thereafter. The cleaned substrates 102, 106 may be placed in a spinner. The spinner may spin the cleaned substrates to dry them. Cleaning using the megasonic cleaning nozzle, the DI water cleaning nozzle, and the spinner may be integrated into a single wafer cleaner system.

[0050] The surface modification process of the method shown in FIG. 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 atomic 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 a low temperature (e.g., less than 100° C.) during UV light exposure. This method may remove any contaminants from the first and second surfaces. Furthermore, the surface energy and polarity of the first and second surfaces can be tailored for atomic or molecular level chemical reactions, thus strengthening the functional groups on the dangling bonds, which may improve the adhesive strength at room temperature.

[0051] The process of forming the composite substrate precursor 330 may include, for example, a plane-to-plane or notch-to-notch mechanical alignment of the first and second surfaces, as shown in FIG. 3E. The bonding process may be performed in a vacuum atmosphere at room temperature with a process gas pressure of less than 0.1 Pa. The bonding process may be performed under a mechanical pressure of up to 5 kN. The mechanical pressure may be 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 a 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 in face-to-face mechanical alignment 326 (e.g., a wafer plane of the first substrate and a wafer plane of the second substrate) in a standard wafer bonder system. A standard wafer bonder system may utilize mechanical force pressures of up to 5 kN in a vacuum ambient environment of around 0.1 Pa. Thus, van der Waals forces attract the first substrate and the second substrate into contact, with a force of up to 0.8 J / m at room temperature. 2 The adhesive strength is 1.2 J / m 2 For even greater enhancement, thermal annealing may be applied at a preselected temperature(s) between 100° C. and 190° C. for several hours.

[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 hotplate, as shown in Figure 3F, for one or more temperature periods at one or more temperatures up to 200°C. In this way, the bonding strength, e.g., cohesion, of the bonding layer 104 (see Figure 1) can be improved.

[0054] A surface modification process including ambient UV light exposure can be an additional process applied to clean most organic contaminants from the first and second surfaces. The UV light exposure can include, for example, excimer light having a wavelength of less than 400 nm.

[0055] The UV light exposure may be performed under normal air atmosphere. The UV light exposure may be capable of or may be configured to break molecular bonds of organic materials. In this manner, the UV light exposure may reduce various hydrocarbons on the first surface and / or the second surface, for example, prior to plasma activation.

[0056] FIG. 4 shows a schematic diagram of a dual RF capacitively coupled plasma system 400 that can be used for mixed gas plasma activation of a wafer surface.

[0057] The plasma system 400 may have a conventional horizontal capacitive 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 sources 408 and 410 and one or more controllers (not shown). The RF power sources 408 and 410 may provide RF power at different frequencies in the kHz range to the electrodes 404, 406. The plasma power may range from about 10 W to about 300 W for the first electrode and the second electrode. The plasma power may have slight variations depending on the process gas mixture.

[0058] As an example, single wafers of the first substrate 102 and second substrate 106 may be placed on a second electrode 406, e.g., a bottom electrode 406 in a plasma chamber 402 of the plasma system 400. A mixture 418 of gaseous oxygen 412 and gaseous nitrogen 414 is provided to the plasma chamber via an adjustable valve system 416 that provides flow control of the oxygen 412 and nitrogen 414, respectively. RF power at the electrodes 404, 406 adjustably forms a plasma 310 and / or 312 from the gas mixture 418. The surfaces of the substrates 102, 106 to be bonded (e.g., a first surface of the first substrate or a 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 period may have a duration ranging from about 5 seconds to about 120 seconds. The duration depends on the best known method (BKM) recipe.

[0059] Mixing of gases, such as oxygen 412 and nitrogen 414, in the plasma chamber 402 can be performed by controlling the flow rates of the respective gases 412, 414 forming the gas mix 418 via a valve system 416. The flow rates of oxygen 412 and nitrogen 414 can range from about 5 standard cubic centimeters per minute (sccm) to about 100 sccm. At the same time, the gas pressure in the plasma chamber 402 during plasma activation can be controlled, for example, by a suction volume throttle valve in a vacuum pump system (not shown). The gas pressure can range from about 10 Pa to about 300 Pa. The N2 gas concentration to O2 gas flow rate ratio can range from about 5% to about 95%, for example, 10%, 15%, 20%, 25%, 30%, 50%, 75%, 95%, depending on the materials of the first substrate and the second substrate, respectively.

[0060] UV light exposure 318 in a mixed gas environment may be used to improve the adhesive strength at room temperature and reduce the annealing temperature. UV light exposure (e.g., having a wavelength in the range of about 150 nm to about 500 nm) using, for example, an excimer lamp may be performed under standard pressure and mixed gas N2 / O2 (e.g., 50% / 50%, but note that the ratio may vary depending on the situation) atmosphere. In this case, the mixed gas may provide oxygen and nitrogen as reactants in the reaction between the piezoelectric material of the second substrate and the material of the first substrate. Thus, activated groups, such as hydroxyl groups (-OH), may be generated on the surfaces of the first substrate and the second substrate. 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 performed in a process chamber at atmospheric pressure.

[0061] Infrared analysis and scanning ultrasonic microscopy of bonded composite substrate precursors can show bond interfaces (e.g., the bond layers described above) that are generally free of detectable voids (also called microvoids), typically caused by thermal outgassing.

[0062] 5A-5D show AFM images 500, 502, 508, 510 with linear scan height profiles 504, 506, 512, 514 of the materials shown after mixed gas (O2+N2) plasma activation 310 or 312 of FIG. 3B, e.g., images of the surfaces of silicon (Si) shown in FIG. 5A, thermal silicon dioxide (SiO2) film (0.5 μm thick) on a Si wafer shown in FIG. 5B, lithium tantalate shown in FIG. 5C, and quartz shown in FIG. 5D, after mixed gas (O2+N2) plasma activation using the BKM recipe for each material. 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, 514. Smooth surfaces with root mean square (RMS, Rq) roughness of about 0.5 nm or less may be preferred for bonding.

[0063] FIG. 6 shows a scanning acoustic microscope (SAM) image of a first example of a composite substrate. The first example includes Si / / Si wafer bonding of 0.204 m wafers. Here, the Si / / Si wafer bonding is performed using plasma activation with mixed gas concentration using BKM recipe, gas flow less than 100 sccm, gas pressure up to 100 Pa, and plasma power of first and second electrodes less than 50 W. The process before the bonding process (also called pre-bonding process) is performed in vacuum at room temperature under identical conditions of mechanical pressure 5 kN (piston down), followed by post-bonding annealing at 300 °C for 2 hours. FIG. 6 shows a SAM image of the composite wafer pre-bonded in vacuum. In contrast to pre-bonding in atmospheric pressure environment, this is a high quality bonded interface without microvoids or thermal outgassing across the bonded interface, with a bond strength of 2.0 J / m 2 may increase to.

[0064] FIG. 7 shows a scanning acoustic microscope (SAM) image 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 treatment gas pressure less than 50 Pa, low plasma power less than 100 W for the first and second electrodes, respectively, and a mixed gas plasma activation process with a pre-bonding process at room temperature in vacuum under the same conditions, followed by a post-annealing at 300° C. for 2 hours. The interface of both bonded wafers (e.g., first and second substrates) is substantially free of microvoids or heat emission, except for a few small handling particles across the edge bonded area. After post-thermal annealing, the bond strength is 1.6 J / m 2 may increase to.

[0065] FIG. 8 shows a scanning acoustic microscope (SAM) image of a third example of a composite substrate. The composite substrate includes LiTaO3(LT) as the second substrate and Si as the first substrate. The image shown shows the composite substrate after thermal annealing at 175°C. The composite substrate can be fabricated and evaluated following 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 area of ​​LiTaO3(LT) can be completely bonded. 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 adhesion strength is 1.27 J / m 2 The second substrate may be thinned from a thickness of about 250 μm to about 400 μm to a thinned thickness in the range of about 1 μm to about 250 μm, such as in the range of about 2 μm to about 10 μm.

[0066] FIG. 9 shows a scanning acoustic microscope (SAM) image of a fourth example of a composite substrate 900. The composite substrate includes a silicon wafer as the first substrate with thermal oxide (e.g., 500 nm thick) after thermal annealing at 150° C., and a LT wafer as the second substrate. In the fourth example, a bonded 0.153 m wafer of the first and second substrates can be fabricated and evaluated following 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 is covered with 500 nm thermal oxide (first substrate) and can be prepared for use as an RF velocity layer as a support material. FIG. 9 shows a scanning acoustic microscope (SAM) image of the LT / / SiO2 bond at room temperature (RT) and the bonded interface after thermal annealing at 150° C. for 2 hours. The entire area of ​​the wafer pair can be fully bonded. The measured adhesive strength is 1.1 J / m 2 From the examination of this bonding interface by SAM measurements, numerous small microbubbles are observed, and the same phenomenon can occur when thermal oxide-wafer bonding is performed in vacuum. However, for a Si wafer with a sputter-deposited oxide (thickness 100 nm to 1000 nm), it can be observed that there is no outgassing and no gas bubbles after thermal annealing at 150 °C for 5 hours. The second substrate may be thinned from a thickness of about 250 μm to about 400 μm to a post-thinning thickness in the range of about 1 μm to about 250 μm, for example in the range of about 2 μm to about 10 μm.

[0067] 10A and 10B show infrared (IR) transmission images of bonded composite substrates including a LiTaO3 wafer as the second substrate bonded to a quartz wafer (FIG. 10A) and a sapphire wafer (FIG. 10B) as the first substrate, respectively. The bonded 0.102 m wafers can be fabricated and evaluated following the same procedure as in Example 3. In the present example, the piezoelectric material of the second substrate can be a 0.204 m single crystal lithium tantalate (LT) wafer, and the first substrate can 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 ambient environment, followed by thermal annealing at less than 200° C. for 5 hours. The bonded composite substrates show high bonding quality with no macrovoids and thermal microvoids formed before and after annealing. The adhesive strength is 1.3 J / m 2 The second substrate may be thinned from a thickness of about 250 μm to about 400 μm to a thinned thickness in the range of about 1 μm to about 250 μm, such as in the range of about 2 μm to about 10 μm.

[0068] 11A and 11B show a schematic diagram of a method for fabricating a piezoelectric material on a SiC wafer composite substrate before a thinning and polishing process 1120 (FIG. 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 a thinning and polishing process 1120 as shown in FIG. 11B. The second thickness 1112 can be in a range of about 1 μm to about 250 μm, for example, in a range of about 2 μm to about 10 μm. The thin composite substrate 1100 can include a piezoelectric material to silicon carbide (SiC), and the bonding surface can include a Si face. The wafer bonding can be fabricated and evaluated in a similar procedure to Example 3 above. However, the material of the piezoelectric single crystal substrate can include a lithium tantalate (LT) single crystal, and the material of the first substrate 102 can include a 4H-SiC substrate.

[0069] 12A and 12B show a schematic diagram of a method for fabricating piezoelectric material on a thin c-BN or diamond wafer composite substrate (e.g., including a first substrate made of silicon and a thin layer of c-BN or diamond) before a thinning and polishing process 1120 (FIG. 12A). 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 a thinning and polishing process 1120, as shown in FIG. 12B. The second thickness 1212 can 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 the second substrate 106 bonded to a first substrate further comprising 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 a c-boron nitride layer 1204 or a diamond layer 1204. The c-BN layer 1204 may also be deposited using a physical vapor deposition (PVD) method, 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, e.g., a Si substrate. The composite substrate 100 can be fabricated and evaluated following the same procedure as in Example 2, where the mixed gas plasma can 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 highly uniform and can be in the range of 10.0 nm in thickness. 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 can be bonded to a thickness of 0.8 J / m without the use of any post-bonding heating. 2 where post thermal annealing at temperatures above 200°C can increase the bond strength up to 1.8 J / m 2 It is possible to increase it to .

[0071] The diamond layer 1204 can also be deposited on the silicon substrate 102 by PVD. The diamond film 1204 can have a layer thickness of up to about 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. The wafer bond can be fabricated and evaluated in a similar procedure to the above example. The mixed gas plasma can be activated based on high plasma power and activated by mixed gas UV surface modification for a long time period of less than 120 seconds in a nitrogen / oxygen gas mixture with a ratio of 1:1 (O2:N2) with a total gas flow rate of less than 75 sccm for long time plasma activation. The bonding layer 104 can be very uniform and have a thickness of up to about 15.0 nm. The bonding layer 104 can include a majority of amorphous nitride depending on the mixed gas plasma activation of the first and second surfaces to be bonded to improve the bonding strength without using heating. Here, post-thermal annealing at temperatures above 200° C. may be used to increase the bond strength.

[0072] As an example, lithium tantalate and lithium niobate single crystal substrates can be used for acoustic wave filters that may have low thermal conductivity. A surface activation process that can develop a good bonding layer between bonded wafers of a composite substrate and a method for manufacturing a thin film bonded wafer by using mixed gas (O2+N2) plasma and UV surface modification under mixed gas atmosphere (O2+N2) is provided. The plasma activation method and 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] The plasma surface activation process can be used to bond lithium tantalate (LT) and lithium niobate (LN) surfaces to a variety of substrates.

[0074] This bonding method can be used to bond dissimilar materials of LN and LT wafers to substrates of Si, SiO2, sapphire, c-BN, and SiC (e.g., with diameters of 0.051 m, 0.102 m, 0.153 m, and 0.204 m) in a vacuum ambient environment at room temperature.

[0075] A mixed gas plasma process may be used for surface activation of all second and first substrates, with the plasma power of the first electrode of the plasma system being in the range of 10W-150W and the plasma power of the second electrode of the plasma system being in the range of 10W-150W.

[0076] A mixed gas plasma process for surface activation may be used for every second substrate and first substrate with plasma duration (time) between 5 seconds and 180 seconds.

[0077] The mixed gas plasma process for surface activation of the second substrate and the first substrate may be used at a gas pressure of 10 Pa to 200 Pa plasma.

[0078] A mixed gas plasma process for surface activation of all second and first substrates may be used with post thermal annealing under forming gas atmosphere at 100°C-200°C.

[0079] The method provides a composite substrate with a high quality bond interface that is free of macro- and micro-voids, has strong bonding, is free of thermal outgassing, and has an undamaged surface structure.

[0080] The composite substrate may have excellent thermal stability against high temperatures up to 250° C. Therefore, an efficiently fabricated thin film surface acoustic wave (TF-SAW) device may be provided.

[0081] The method can provide bonding for substantially the entire wafer of the first and second substrates, the wafers being different in size. In other words, the method for forming the composite substrate can include a plasma activation process, which can include exposing the first surface of the first substrate and the second surface of the second substrate, respectively, in a plasma system, which can be a radio frequency system having a plasma between a first electrode and a second electrode, which can be separately powered, the plasma can be a mixed gas plasma of oxygen and nitrogen at a first ambient pressure, which can be less than atmospheric pressure, and the surface modification process can include exposing the plasma activated first surface and the plasma activated second surface to ultraviolet radiation in an atmosphere of oxygen and nitrogen 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 crystalline 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 may be between about 0.1 Pa and about 10 -3 The vacuum pressure may be in the range of Pa.

[0083] The method may further include polishing the first surface and the second surface such that the first surface and the second surface each have a root mean square roughness of less than 0.5 nm.

[0084] The method can include a first electrode plasma power in the range of about 10 W to about 300 W. The method can include a second electrode plasma power in the range of about 10 W to about 300 W. The one or more periods of plasma activation can be for a duration of about 5 seconds to about 180 seconds.

[0085] The mixed gas plasma may include a gas flow ratio of nitrogen flow rate to oxygen flow rate 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 that may include megasonic assisted rinsing and drying of the plasma activated first and second surfaces with deionized (DI) water. The surface modification process may include a rinsing process. The rinsing process may be preceded by exposure to ultraviolet radiation. Alternatively, the rinsing process may be followed by exposure to ultraviolet radiation.

[0086] Ultraviolet radiation can include electromagnetic radiation having a wavelength in the wavelength range of about 150 nm to about 500 nm.

[0087] During UV irradiation, the temperatures of the first surface and the second surface can be less than about 100°C.

[0088] The mechanical load may have a mechanical pressure on piston downward of up to 5 kN.

[0089] The thermal annealing process may include heating 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 piezoelectric material wafer of the second 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.

[0091] The method may further include a third substrate bonded to the first substrate in addition to the second substrate, and 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] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any aspect or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs. The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." An example or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other examples or designs.

[0094] The words "plurality" and "multiple" in the specification or claims expressly refer to a quantity greater than one. The terms "group", "set", "collection", "series", "array", "grouping", and the like in the specification or claims refer to a quantity equal to or greater than one, i.e., one or more. Any terms expressed in the plural form, not expressly stated as "plurality" and "multiple", also refer to a quantity equal to or greater than one.

[0095] For example, the term "processor" or "controller" as used herein may be understood as any kind of technical entity that allows processing of data. The data may be handled according to one or more specific functions executed by the processor or controller. Furthermore, a processor or controller as used herein may be understood as any kind of circuit, for example any kind of analog circuit or digital circuit. Thus, a processor or controller may be or include an analog circuit, a digital circuit, a mixed signal circuit, a logic circuit, a processor, a microprocessor, a central processing unit (CPU), a graphics processing unit (GPU), a digital signal processor (DSP), a field programmable gate array (FPGA), an integrated circuit, an application specific integrated circuit (ASIC), etc., or any combination thereof. Any other kind of implementation of the respective functions may also be understood as a processor, a controller, or a logic circuit. It is understood that 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" may be understood to mean, for example, a direct or indirect connection and / or interaction (e.g., mechanical and / or electrical). For example, several elements may be mechanically connected to physically hold them together (e.g., a plug connected to a socket) and electrically connected to have a conductive path (e.g., a signal path exists along a communication chain).

[0097] While the above description and associated figures may depict components of an electronic device as separate elements, those skilled in the art will appreciate various possibilities for combining or integrating 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, running separate software components on a common processor core, etc. Conversely, those skilled in the art will appreciate 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 originally provided thereon, separating a software component into two or more sections each running on a separate processor core, etc. It is also understood that the particular implementations of hardware and / or software components are merely exemplary, and that other combinations of hardware and / or software performing the methods described herein are within the scope of the present disclosure.

[0098] It is understood that the implementations of the methods detailed herein are exemplary in nature and can therefore be implemented in corresponding devices. Similarly, it is understood that the implementations of the devices detailed herein can be implemented as corresponding methods. It is therefore understood that a device corresponding to a method detailed herein may include one or more components configured to perform each aspect of the associated method.

[0099] All acronyms defined in the above description also apply throughout the claims contained herein.

[0100] Although the present disclosure has been shown and described in detail with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims. The scope of the present disclosure is therefore indicated by the appended claims, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. 1. A method of forming a composite substrate, the method comprising: The process includes an oxygen and nitrogen mixed gas plasma activation process, the process comprising: 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 atmosphere of oxygen and nitrogen; placing the first surface of the wafer on the first surface of the piezoelectric material wafer; applying a mechanical load to the wafer and to the piezoelectric material wafer; thermally annealing the bonded wafer and the piezoelectric material wafer; Including, The method wherein the wafer is configured to support the piezoelectric material wafer.

2. 10. The method of claim 1, further comprising polishing a surface of the wafer and a surface of the piezoelectric material wafer such that the surface of the wafer and the surface of the piezoelectric material wafer, respectively, have a root mean square roughness of less than 0.5 nm.

3. The method of claim 1 or 2, wherein the atmosphere comprises a gas flow ratio of nitrogen flow rate to oxygen flow rate in the range of 0.01 to 0.

99.

4. The method of any one of claims 1 to 3, wherein the gas flow rate of the atmosphere is in the range of about 5 sccm to 100 sccm.

5. The method of any one of claims 1 to 4, wherein the plasma activation comprises an ambient pressure in the range of about 10 Pa to 200 Pa.

6. The method according to any one of claims 1 to 5, further comprising a rinsing process comprising megasonic assisted rinsing of the plasma activated surface of the wafer and the plasma activated surface of the piezoelectric material wafer with deionized (DI) water and drying.

7. The method of claim 6 , wherein the surface modification process comprises a rinsing process.

8. 8. The method according to claim 6 or 7, wherein the exposure to ultraviolet radiation is carried out before the rinsing process.

9. 8. The method according to claim 6 or 7, wherein the exposure to ultraviolet radiation is carried out after the rinsing process.

10. The method of any one of claims 1 to 9, wherein the ultraviolet radiation comprises electromagnetic radiation having a wavelength in the wavelength range of about 150 nm to 500 nm.

11. The method according to any one of claims 1 to 10, wherein the temperature of the surface of the wafer and the surface of the piezoelectric material wafer during the UV irradiation is below 100°C.

12. The method according to any one of claims 1 to 11, wherein the mechanical load comprises a mechanical pressure on piston down of up to 5 kN.

13. 13. The method of any one of claims 1 to 12, 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.

14. A method according to any one of claims 1 to 13, further comprising bonding a further piezoelectric material wafer to the wafer, the piezoelectric material of the further piezoelectric material wafer being different to the piezoelectric material of the piezoelectric material wafer.

15. 15. The method of claim 14, wherein the piezoelectric material of the further 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.

16. The wafer is made of silicon (Si), silicon oxide (SiO ２ 16. The method of any one of claims 1 to 15, 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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