Patent Publication Number: US-11664357-B2

Title: Techniques for joining dissimilar materials in microelectronics

Description:
RELATED APPLICATIONS 
     This nonprovisional patent application claims the benefit of priority to U.S. Provisional Patent Application No. 62/693,671 to Fountain et al., filed Jul. 3, 2018 and incorporated by reference herein, in its entirety. 
    
    
     BACKGROUND 
     Certain types of microelectronic devices are conventionally discouraged from manufacture because they would require joining of different materials that have not yet been bonded together with very great success. The bonding surfaces in these micro-devices are sometimes only a few microns across. Abbreviations for units used herein include “μm” for micron or micrometer (1 micron=one-thousandth of a millimeter); and “nm” for nanometer (1000 nanometers=1 micron). 
     There are several difficulties that arise in attempts to form useful surface bonds between different materials to be used in semiconductor device fabrication and microelectronic packaging. First, the multi-layer dielectric and metal layers that are deposited on the various substrates often cause stress which manifests as an overall wafer bow and local warpage of the substrates. One challenge of bonding these highly warped substrates is the need to place them under vacuum to force the surfaces to be flat during bonding. 
     Second, the different materials have different crystal lattice properties. Traditional bonding techniques use elevated temperature and pressure to join materials. However, after bonding, the composite system is cooled to room temperature for subsequent processing and then operation temperature (generally well below the bond temperature). Direct bonding offers a way to reduce the overall stress and strain and join at lower temperatures. Both metals and nonmetals possess crystal lattice unit cells, the basic structural building blocks of each material at or near the atomic level. The crystal lattice units of the different materials may differ in geometry, or they may have similar geometries, but differ in the scale of these similar units. Either way, direct face-to-face bonding between materials that have different crystal-lattice-unit cell geometries can cause strain problems at the interface. With direct bonding, the thermal fluctuation between the two materials during bonding and subsequent rising temperatures in anneal or when the device is in electrical operation, can cause two materials that are bonded together to expand at different rates as the temperature rises, due to differences in their respective coefficients of thermal expansion (CTEs). 
     Ideally, a first material on one side of a face-to-face bond should possess a crystalline nature that has at least one well-defined orientation with respect to the internal crystal structure of the second material, and this orientation is sometimes called single-domain epitaxy. A lattice constant is a physical dimension of the unit cells in a crystal lattice of one of the materials. Lattices in three dimensions generally have three lattice constants: a, b, and c. Matching the lattice constants between materials to be bonded at small microelectronic scales is desirable for avoiding weak and defective bonds between the two different materials. 
     Bonding together of different materials can also have electronic effects that are important in microelectronic devices. For example, matching the lattice structures between two different semiconductor materials can form a region of band gap change without altering crystal structure. This enables the existence of some types of optical devices, such as light-emitting diodes, and lasers. Band gap is an energy range in a solid or two bonded solids where there are no electron states, and the gap can be characterized as the energy difference between a top of the valence band and a bottom of the conduction band in semiconductors and insulators, or from another view, the energy needed to free a bound (valence) electron to become a conduction electron for conducting electrical current. 
     Conventional bonding together of substances with similar lattice constants, as such gallium arsenide, aluminum gallium arsenide, and aluminum arsenide has provided many breakthrough optical devices, such as LEDs, and lasers, for example. The ability to usefully bond together more diverse materials is expected to provide even more new devices of microelectronic scale. 
     Sometimes fabrication of a microelectronic device would only need two different materials to be reliably direct-bonded across a very thin bonding interface, without regard for the materials&#39; electronic effects on each other, but the respective differences in CTEs and differences in their lattice constants (lattice unit cell geometries) have conventionally made joining of these materials impossible or impractical. 
     SUMMARY 
     Techniques for joining dissimilar materials in microelectronics are provided. Example techniques include direct-bonding of dissimilar materials to each other at room temperature using a thin amorphous layer of material added to either one or both bonding surfaces of the dissimilar materials, which may be in the form of two different substrates to be joined together. Silicon oxide, silicon nitride, or silicon carbide, silicon carbonitride, silicon oxynitride, and mixtures of these dielectric materials, are examples of materials for making the thin amorphous layer, which constitutes a thin film approximately 100-1000 nm thick, for example. The dissimilar substrate materials may have significantly different coefficients of thermal expansion (CTEs) and/or significantly different crystal-lattice unit cell geometries or dimensions, conventionally resulting in too much strain to make direct-bonding feasible or reliable. Preparation of the substrates prior to direct-bonding can include adding dielectric layers to the backside of the substrates to reduce warpage, in order to render these substrates flat without a need for vacuum, thereby reducing the stress and strain at the bonding interface. These warpage reduction layers may be removed in the standard post-bond processing, after serving their purpose. After bonding, an annealing sequence achieves a well-bonded interface. The process includes a curing period at room temperature after the direct-bonding of dissimilar materials, which can strengthen the direct bonds and the resulting direct-bond energies by over 200%. A relatively low temperature anneal applied slowly at a rate of 1° C. per minute further strengthens the direct bonds. The example techniques can be utilized to direct-bond III-V semiconductors, lithium tantalate LiTaO 3 , or other non-silicon materials to silicon or other materials that previously presented bonding challenges, to various conventional substrates in wafer-to-wafer (W2 W), die-to-wafer (D2 W), and die-to-die (D2D) processes thereby enabling various novel optical, electrical, and acoustic devices. 
     This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein. 
         FIG.  1    is a diagram of example substrates, such dies and/or wafers, made of dissimilar materials direct-bonded together via deposited oxide layer or native oxide film using the example techniques described herein. 
         FIG.  2    is a diagram of direct-bond energy versus curing time with respect to example techniques described herein. 
         FIG.  3    is a flow diagram of an example process for direct-bonding LiTaO 3  with silicon. 
         FIG.  4    is a flow diagram of an example process for direct-bonding LiTaO 3  with sapphire. 
         FIG.  5    is a flow diagram of an example process for direct-bonding LiTaO 3  with fused silica glass. 
         FIG.  6    is a flow diagram of an example process for direct-bonding LiTaO 3  with silicon by creating a native oxide of the LiTaO 3  and/or the silicon. 
         FIG.  7    is a flow diagram of an example process for direct-bonding LiTaO 3  with sapphire by creating a native oxide of the LiTaO 3  and/or the sapphire. 
         FIG.  8    is a flow diagram of an example process for direct-bonding LiTaO 3  with fused silica glass by creating a native oxide of the LiTaO 3  and/or the fused silica glass. 
         FIG.  9    is a diagram of example direct-bond energy increases at different anneal durations and different temperatures, with slow temperature increase during anneal. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     This disclosure describes example techniques, processes, and methods for successfully joining diverse and dissimilar materials to each other in microelectronics, and describes resulting layer structures and devices. The example processes and structures apply to materials already routinely used in semiconductor fabrication and in the microelectronic packaging arts, and to combinations of materials not conventionally used in these arts. The example techniques, processes, and methods described herein are provided for fabricating novel semiconductor and microelectronic devices, and also for fabricating conventional devices with more efficient and practical designs. 
     Although the example processes described herein can theoretically be applied and used between many and almost all solid, amorphous, and/or crystalline materials, a few materials are now described as representative examples, for the sake of description and illustration of the example processes and resulting structures. 
     Lithium tantalate (LiTaO 3  or simply “LiT”) is a mineral and a compound of lithium, tantalum, and oxygen that possesses useful optical and electronic properties in the microelectronic arts. As a useful component of potential microelectronic devices, LiTaO 3  has optical, piezoelectric, and pyroelectric properties that make it useful for nonlinear optics, passive infrared sensors and motion detectors, terahertz generation and detection, surface acoustic wave applications, cell phone applications, cooling, heating, and small scale production of neutrons (atomic particles) for the nuclear industry, for example. LiTaO 3  has a relatively large thermal expansion coefficient, with thermal expansion on the order of 12 ppm that varies along different crystalline axis directions. If LiTaO 3  is warped during heating it develops an internal charge imbalance, which can result in residual warpage after cooling. In a pyroelectric effect, LiTaO 3  generates a temporary voltage within itself, when heated or cooled, without any imposed physical warping. Changes in temperature slightly modify positions of atoms within its crystal structure changing the polarization of the material. In a different but related piezoelectric effect, LiTaO 3  also generates an electric charge in response to applied mechanical stress (the imposed warpage). These pyroelectric and piezoelectric effects of LiTaO 3  should be taken into account when bonding this material to another substrate made of different material. 
     Analogously, lithium niobate (LiNbO 3 ) is a compound of lithium, niobium, and oxygen. Crystals of LiNbO 3  are useful as optical waveguides, in mobile phones, as piezoelectric sensors, optical modulators and in various other linear and non-linear optical applications. LiNbO 3  is considered an artificially made dielectric material that does not exist in nature. 
     Example techniques described herein enable direct-bonding of LiTaO 3  (or LiNbO 3 ) to other semiconductor, dielectric, and insulator materials for purposes of making practical microelectronic devices. In the description which follows, LiTaO 3  is described representatively for both LiTaO 3  and LiNbO 3  and for many other similar materials too. As above, the example techniques, processes, and methods herein can be applied between almost any solid materials, but LiTaO 3  is used in the descriptions representatively, as an example of a material to be joined to other materials that are dissimilar in CTEs, lattice unit cell geometries, or other properties. 
     LiTaO 3  has conventionally proven incompatible for direct-bonding with important semiconductor and dielectric materials such as silicon (Si), glass (amorphous silicon dioxide SiO 2  and other ingredients), fused silica (glass made of pure silicon dioxide SiO 2 ), sapphire, and other common and uncommon substrates used, or that could be used, in the semiconductor and microelectronic fabrication and packaging arts. 
     Example techniques described below enable practical use of LiTaO 3  in microelectronic devices, especially in wafer-level fabrication of microelectronic devices, wherein the fabrication can be greatly optimized by using direct-bonding and/or direct hybrid bonding between the surfaces of the different or incompatible materials (between substrates, such as those of dies and/or wafers made of different materials being joined to make the novel or conventional microelectronic devices). 
     Example Techniques 
     In an example process, a low temperature bonding process enables heterogeneous integration of diverse materials within a microelectronic device. This low temperature bonding process can be especially useful in fabrication processes that use substrates, such as those of dies or wafers made of two different materials that are conventionally incompatible with each other with respect to conventional direct-bonding processes. 
       FIG.  1    shows example structures in which a first wafer  100  of a first material is to be direct-bonded to a second wafer  102  of a second material at room temperature. Room temperature is defined in its usual way as a humanly comfortable ambient temperature, generally around 70° F. or around 21.1° C. The first material of the first wafer  100  possesses a first coefficient of thermal expansion (CTE) and a first set of physical dimensions of respective unit cells of a first crystal lattice of the first material, while the second material of the second wafer  102  possesses a second CTE and a second set of physical dimensions of respective unit cells of a second crystal lattice of the second material. The second CTE of the second wafer  102  is different from the first CTE of the first material and/or the second set of physical dimensions of the unit cells of the second wafer  102  is different from the first physical dimensions of the unit cells of the first material of the first wafer  100 . 
     Respective thin oxide, carbide, nitride, carbonitride, oxynitride layers  104  &amp;  106 , or combinations thereof, are deposited on one or both bonding surfaces of the first wafer  100  and the second wafer  102 . The deposited oxide, carbide, nitride, carbonitride, or oxynitride layers  104  &amp;  106  may be only 100-1000 nm thick, and serve as direct-bonding intermediaries between the wafers  100  &amp;  102 . The deposited oxide, carbide, nitride, carbonitride, or oxynitride layers  104  &amp;  106  are a different compound than either the first material of the first wafer  100  or the second material of the second wafer  102 . So the direct-bonding described herein is different than conventional direct-bonding, which is often described as molecular bonding directly between two surfaces made of the same atoms or molecules: the same material being bonded to itself across an interface. 
     The first wafer  100  and the second wafer  102  are then direct-bonded together at room temperature to make a joined wafer, with the thin amorphous layer of oxide, carbide, nitride, carbonitride, or oxynitride intervening at the molecular bonding interface. The amorphous layer may comprise silicon. 
     The joined wafer is held for approximately 48 hours to strengthen direct bonds in a passive curing stage, wherein solid crystal bonds or other bonds at the solid bonding interface are allowed to spontaneously consolidate and seek their most stable energy levels under normal molecular kinetic motion at the room temperature level. 
     The joined wafer is then annealed by slowly raising the ambient temperature from room temperature level to a relatively low annealing temperature of 50° C. at a temperature increase rate of about 1° C. per minute or less, to anneal the direct bonds of the joined wafer. 
     Additional Example Processes 
     The direct-bonding of dissimilar materials at room temperature to make a microelectronic device can also be accomplished by creating one or more native oxide (or carbide, nitride, carbonitride, oxynitride, etc.) films  108  &amp;  110  on one or both bonding surfaces of the first wafer  100  and second wafer  102 . This can be accomplished by exposing the bonding surface(s) to a strong oxidizing solution, or oxygen plasma, for example, or other reactive agent in the cases of native carbide generation or native nitride generation. 
     As introduced above, challenges in heterogeneous integration of different materials within a microelectronic device are related to the differential CTEs, incompatibility of lattice unit cells at the bonding interface, as often indicated by respective lattice constants that vary greatly between materials, and differences in general material properties. 
     The example direct-bonding process that utilizes a step of creating a native oxide layer (or carbide layer, nitride layer, carbonitride, oxynitride, etc.) aims to take advantage of two or more substrates made of different materials that have specific advantages provided by the properties of devices that can be fabricated on each respective different type of substrate, now joined in one device by the example direct-bonding process. The example low temperature bonding technique enables multiple integration possibilities for a broad field of electronic devices. 
     In an implementation, the example process begins by thermally equilibrating the different materials to be direct-bonded together with each other and with the environment. In other words, the direct-bonding process begins with all materials and the environment at the same temperature, providing an equilibrium of molecular kinetic energy among the materials being used. This equilibrium controls/prevents uncontrolled flows of heat energy between materials or across the bonding interface during the initial contact of bonding surfaces during the direct-bonding process, resulting in exquisite homogeneity of the direct-bonds formed at small scale, and results in exquisite control of the example process. In other words, the careful equilibrium of temperature at the start of this example direct-bonding process can ultimately ensure a very uniform bonded interface, at the molecular level. In one implementation, this uniformity of the direct-bonded interface is limited only by the degree of flatness achievable on the surfaces to be direct-bonded together prior to contact between the two surfaces. 
     The example direct-bonding is performed at room temperature, and then held at room temperature for up to 48 hours or more for curing, even before an annealing step, to greatly increase bond energy: the bond strength that will be present after a subsequent annealing step is completed. 
     For example, with wafers that have large differences from each other with respect to CTEs, the wafers are kept at room temperature for a period of time after direct-bonding for a curing that allows bond energy to increase even before elevating the temperature during annealing to speed up the bond energy increase. Bond strengths of 1000 mJ/m 2  or greater have been measured with this example room temperature curing technique. 
       FIG.  2    shows example increase in bond energy versus curing time at room temperature after direct-bonding of dissimilar wafer materials. One example technique uses TEOS (tetraethoxysilane) in TEOS-to-TEOS direct oxide bonding with nitrogen plasma surface activation followed by an example 29% NH 4 OH (aqueous ammonia) rinse. Bond strengths of 1000 mJ/m 2  or greater have been measured with this example room temperature curing of the bond, before the annealing stage of direct-bonding. 
     Example Room Temperature Direct-Bonding Techniques Using Thin Amorphous Intermediary Bonding Layer to Join Dissimilar Materials 
     Some example processes for direct-bonding dissimilar materials at room temperature use a thin intermediary layer less than 1 micron thick (between approximately 100-1000 nm) as an “adapter” layer between diverse oxides, carbides, nitrides, carbonitrides, or oxynitrides for the direct-bonding. This thin intermediary layer may be an amorphous oxide layer, such as silicon dioxide (SiO 2 ), with a thickness dimension within the range above, less than 1 micron thick. Silicon dioxide films can be grown by a plasma enhanced chemical vapor deposition (PECVD) system using liquid tetraethoxysilane (TEOS) as the source of Si, or by TEOS photochemical decomposition, for example. 
       FIG.  3    shows a first example process  300 , in which a LiTaO 3  wafer, substrate, or surface is direct-bonded at room temperature to a silicon wafer, substrate, or surface via an intervening thin amorphous bonding layer made of an oxide material, such as SiO 2 , or another thin amorphous dielectric intermediary for direct-bonding, such as a nitride, carbide, carbonitride, or oxynitride. In the flow diagram of  FIG.  3   , operations of the example process  300  are shown in individual blocks. 
     At block  302 , a material such as TEOS-derived amorphous SiO 2  is deposited onto one or both of the LiTaO 3  wafer, substrate, or surface and/or the silicon wafer, substrate, or surface. The layer of oxide material may have a deposited thickness of approximately 100-1000 nm (0.1-1.0 microns). 
     At block  304 , the respective surfaces of LiTaO 3  and SiO 2 , one or both of these surfaces covered with the thin amorphous film of oxide, carbide, nitride, carbonitride, and/or oxynitride, are planarized by chemical mechanical planarization (CMP) to a smooth, flat surface. Asperities including defects and rough spots that would cause bonding voids can be removed at this step. 
     At block  306 , the respective wafers, substrates, or surfaces may be cleaned with scrubbing by PVA brush, and a deionized (DI) water rinse. 
     At block  308 , the respective wafers, substrates, or surfaces may be cleaned with a Megasonic SC1 process, rinsed with DI water and spin-dried on a Goldfinger® processing tool, for example, for megasonic wave particle removal without etching. 
     At block  310 , the wafers, substrates, or surfaces are plasma-activated with nitrogen plasma in RIE mode, for example with a −200 to −300 volt bias. This nitrogen plasma activation with such parameters may be unique when applied to LiTaO 3  direct-bonding. 
     At block  312 , the wafers, substrates, or surfaces are spray-rinsed with 29% NH 4 OH (aqueous ammonia). This rinse with 29% NH 4 OH may also be a unique process when applied to the LiTaO 3  direct-bonding. The NH 4 OH rinsing helps to remove particles from the wafers, substrates, or surfaces. In some cases, depending on materials, the NH 4 OH rinsing can help to increase bond energy. The wafers, substrates, or surfaces may be spin-dried on a spin bonder tool, for example. 
     At block  314 , the wafers, substrates, or surfaces are contacted with each other for direct-bonding at room temperature, with the LiTaO 3  wafer, for example, loaded as the top wafer. Infrared (IR) drying lamps are not used to avoid excessive bowing and warping upon bonding. The room temperature bonding also avoids excessive bowing of the wafers. Likewise, heating during spin drying could also cause bowing of the LiTaO 3 , which does not fully relax after cooling due to pyroelectric charge build up in the LiTaO 3  material, so such heated spin-drying after direct bonding may be avoided. 
     At block  316 , the wafers (or bonded substrates or bonded surfaces) are held at room temperature for 48 hours, as an example duration, to allow bond strength to increase before slowly elevating temperature for the subsequent annealing step. The bonding strength increases in relation to the time duration allowed for the room temperature curing, as shown in  FIG.  2   . 
     At block  318 , the temperature of the direct-bonded wafers, substrates, or surfaces are ramped slowly up slowly to 50° C. in an oven, in a temperature elevation process of less than 1 degree ° C. per minute. This relatively low temperature annealing step avoids slippage of the bonds that are being strengthened, and minimizes bowing of the wafers (substrates or surfaces) being annealed. 
       FIG.  4    shows a second example process  400 , in which a LiTaO 3  wafer, substrate, or surface is direct-bonded at room temperature to a sapphire wafer, substrate, or surface via an intervening thin amorphous bonding layer made of an oxide material, such as SiO 2 , or another thin amorphous dielectric intermediary for direct-bonding, such as a nitride, carbide, carbonitride, and/or oxynitride. In the flow diagram of  FIG.  4   , operations of the example process  400  are shown in individual blocks. 
     At block  402 , a representative material such as TEOS-derived amorphous SiO 2 , or a thin amorphous layer of a carbide, nitride, carbonitride, or oxynitride dielectric, is deposited onto one or both of the LiTaO 3  wafer, substrate, or surface and/or the sapphire wafer, substrate, or surface. The thin layer of the amorphous material, such as the oxide, carbide, nitride, carbonitride, or oxynitride may have a deposited thickness of approximately 100-1000 nm. 
     At block  404 , the respective surfaces of oxide-covered LiTaO 3  and/or oxide-covered SiO 2  (or nitride, carbide, carbonitride, oxynitride, etc.), are planarized by chemical mechanical planarization (CMP) to a smooth, flat surface. Asperities, such as defects and rough spots, that would cause bonding voids can be removed at this step. 
     At block  406 , the respective wafers, substrates, or surfaces may be cleaned with PVA brush scrubbing and deionized (DI) water. 
     At block  408 , the respective wafers, substrates, or surfaces may be cleaned with a Megasonic SC1 process, rinsed with DI water and spin-dried on a Goldfinger® processing tool, for example, for megasonic wave particle removal without etching. 
     At block  410 , the wafers, substrates, or surfaces are plasma-activated with nitrogen plasma in RIE mode, for example with a −200 to −300 volt bias. 
     At block  412 , the wafers, substrates, or surfaces are spray-rinsed with 29% NH 4 OH (aqueous ammonia, or ammonium hydroxide). This rinse with 29% NH 4 OH may be a unique process when applied to the LiTaO 3  direct-bonding. The NH 4 OH rinsing removes particles from the wafers, substrates, or surfaces. In some cases, depending on materials, the NH 4 OH rinsing increases bond energy. The wafers, substrates, or surfaces may be spin-dried on a spin bonder tool, for example. 
     At block  414 , the wafers, substrates, or surfaces are direct-bonded to each other at room temperature, with the LiTaO 3  wafer, for example, loaded as the top wafer. Infrared (IR) drying lamps are not used to avoid excessive bowing and warpage upon bonding. The room temperature bonding also avoids excessive bowing of the wafers. Likewise, heating during spin drying may cause bowing of the LiTaO 3 , which does not fully relax after cooling due to pyroelectric charge build up in the LiTaO 3  material, and such heating may be avoided at this step. 
     At block  416 , the wafers (or substrates or surfaces) are held at room temperature for at least 48 hours, as an example duration, to allow bond strength to increase and consolidate before elevating temperature for the subsequent annealing step. The bonding strength during the passive room temperature cure increases in relation to the time duration allowed, as shown in  FIG.  2   . 
     At block  418 , the temperature of the direct-bonded wafers, substrates, or surfaces are ramped up to 50° C. in an oven, at a rate of temperature increase of about 1 degree ° C. per minute, or less. Sapphire is an extremely stiff material and if the wafers are not bonded strongly enough before increasing the temperature, wafer expansion and resulting warpage can cause the wafers to separate. So this relatively low temperature annealing step avoids slippage of the bond being strengthened, and minimizes bowing of the wafers being direct-bonded together. 
       FIG.  5    shows a third example process  500 , in which a LiTaO 3  wafer, substrate, or surface is direct-bonded at room temperature to a fused silica glass wafer, substrate, or surface via an intervening thin amorphous bonding layer made of an oxide material, such as SiO 2 , or made of a nitride, carbide, carbonitride, or oxynitride dielectric. In the flow diagram of  FIG.  5   , operations of the example process  500  are shown in individual blocks. 
     At block  502 , a material such as TEOS-derived amorphous SiO 2  (or a nitride, carbide, carbonitride, or oxynitride) is deposited onto one or both of the LiTaO 3  wafer, substrate, or surface and/or the fused silica glass wafer, substrate, or surface. The thin layer of material, such as the oxide, nitride, carbide, carbonitride, or oxynitride may have a deposited thickness of approximately 100-1000 nm. 
     At block  504 , the respective surfaces of oxide-covered LiTaO 3  and/or oxide-covered SiO 2 , for example, are planarized by chemical mechanical planarization (CMP) to a smooth, flat surface. Asperities including defects and rough spots that would cause bonding voids can be removed at this step. 
     At block  506 , the respective wafers, substrates, or surfaces may be cleaned with PVA brush scrubbing and deionized (DI) water. 
     At block  508 , the respective wafers, substrates, or surfaces may be cleaned with a Megasonic SC1 process, rinsed with DI water and spin-dried on a Goldfinger® processing tool, for example, for megasonic wave particle removal without etching. 
     At block  510 , the wafers, substrates, or surfaces are plasma-activated with nitrogen plasma in RIE mode, for example with a −200 to −300 volt bias. This nitrogen plasma activation with these parameters may be unique when applied to LiTaO 3  direct-bonding. 
     At block  512 , the wafers, substrates, or surfaces are spray-rinsed with 29% NH 4 OH (aqueous ammonia or ammonium hydroxide). This rinse with 29% NH 4 OH may be a unique process when applied to the LiTaO 3  direct-bonding. The NH 4 OH rinsing removes particles from the wafers, substrates, or surfaces. In some cases, depending on materials, the NH 4 OH rinsing increases bond energy. The wafers, substrates, or surfaces may be spin-dried on a spin bonder tool, for example. 
     At block  514 , the wafers, substrates, or surfaces are direct-bonded to each other at room temperature, with the LiTaO 3  wafer, for example, loaded as the top wafer. Infrared (IR) drying lamps are not used to avoid excessive bowing and warpage upon bonding. The room temperature bonding also avoids excessive bowing of the wafers, for example. Likewise, heating during spin drying may also cause bowing of the LiTaO 3 , which does not fully relax after cooling due to pyroelectric charge build up in the LiTaO 3  material, and such heating may be avoided. 
     At block  516 , the wafers (or substrates or surfaces) are held at room temperature for at least 48 hours, for example, to allow bond strength to increase before elevating temperature for the subsequent annealing step. The bonding strength increases in relation to the time duration allowed for room temperature curing, as shown in  FIG.  2   . 
     At block  518 , the temperature of the direct-bonded wafers, substrates, or surfaces is ramped up to 50° C. in an oven, at a rate of temperature rise that is about 1 degree per minute, or preferably less. This relatively low-temperature annealing step avoids slippage of the bonds being strengthened, and minimizes bowing of the wafers being direct-bonded together. 
     Example Room Temperature Direct-Bonding Techniques for Joining Dissimilar Materials without a Discrete Intervening Bonding Layer 
       FIG.  6    shows a fourth example process  600 , for direct-bonding a representative LiTaO 3  wafer at room temperature to a silicon wafer, without depositing a discrete layer of an oxide, carbide, nitride, carbonitride, or oxynitride material as a bonding intermediary. In the flow diagram of  FIG.  6   , operations of the example process  600  are shown in individual blocks. 
     At block  602 , a native oxide (of silicon) or oxide patina of silicon is created on the silicon wafer. In one implementation, the native oxide may be created by first cleaning the wafer with a piranha solution of hydrogen peroxide-sulfuric acid (H 2 O 2 :H 2 SO 4 ) in a ratio of 1:3, and then rinsing with water. The 1:3 H 2 O 2 :H 2 SO 4  solution grows or creates a native oxide film on the silicon, which is conducive to forming high strength bonds in oxide-to-oxide direct-bonding. 
     At block  604 , the surface of the silicon wafer with native oxide is rinsed in a Megasonic deionized water process and can be spun and rinsed dry on a Goldfinger® processing tool, for example. 
     At block  606 , the surface of the LiTaO 3  wafer (only) is plasma-activated with nitrogen plasma in RIE mode with a −200 to −300 volt bias. 
     At block  608 , both wafers are spray-rinsed with deionized water. The silicon wafer is rinsed with water to avoid pitting the silicon and native oxide surface, although the native oxide may be enough to protect the silicon surface, in which case use of NH 4 OH solution to enhance direct-bonding may be preferred. Although the LiTaO 3  wafer is usually cleaned with water, cleaning with NH 4 OH solution can also be performed. In some cases, rinsing with the NH 4 OH increases direct-bond energies to more complete potential. The wafers are then spin-dried, for example on a bonder tool. 
     At block  610 , the wafers are direct-bonded together at room temperature with the LiTaO 3  wafer loaded as the top wafer, for example. Infrared drying lamps are generally avoided to prevent excessive bowing and warpage upon direct-bonding. Room temperature direct-bonding also avoids excessive bowing of the wafers. Heating during the spin-drying could cause bowing of the LiTaO 3  wafer, which does not fully relax after cooling due to pyroelectric charge build-up in the LiTaO 3  material, so can be avoided at this step. 
     A block  612 , the joined and direct-bonded wafers are held at room temperature for at least 48 hours or more to allow bond strength to increase before elevating the temperature in a subsequent annealing step. The bond strength increases in relation to the duration of the room temperature cure, as shown in  FIG.  2   . 
     At block  614 , the joined and direct-bonded wafers are warmed and heated in an oven up to a temperature of approximately 50° C. to anneal, at a rate of temperature increase that is 1 degree ° C. per minute, or less. This relatively low-temperature anneal applied gradually avoids slippage of the direct-bonds and minimizes bowing and warpage. 
       FIG.  7    shows a fourth example process  700 , for direct-bonding a LiTaO 3  wafer at room temperature to a sapphire wafer without depositing a discrete layer of an oxide, carbide, nitride, carbonitride, or oxynitride material as a bonding intermediary. In the flow diagram of  FIG.  7   , operations of the example process  700  are shown in individual blocks. 
     At block  702 , the LiTaO 3  wafer and a sapphire wafer are cleaned with a piranha solution of hydrogen peroxide and sulfuric acid (H 2 O 2 :H 2 SO 4 ) in a ratio of 1:3, and then rinsed with water. 
     At block  704 , the surfaces of the wafers are rinsed in a Megasonic deionized water process and can be spun and rinsed dry on a Goldfinger® processing tool, for example. 
     At block  706 , the surface of sapphire wafer is plasma-activated with oxygen in RIE mode with a −200 to −300 volt bias, and the surface of the LiTaO 3  wafer is plasma-activated with oxygen or nitrogen plasma in RIE mode with a −200 to −300 volt bias. 
     At block  708 , the wafers are spray-rinsed with 29% NH 4 OH solution to remove particles and to enhance the bond energies possible in the direct-bonding process. The wafers may be spun dry. 
     At block  710 , the wafers are contacted together for the direct-bonded at room temperature with the LiTaO 3  wafer loaded as the top wafer, for example. Infrared drying lamps may be avoided to prevent excessive bowing and warpage upon direct-bonding. The room temperature direct-bonding avoids excessive bowing of the wafers as well as allows stronger direct-bonds to form across the dissimilar materials. Heating during the spin drying would cause bowing of the LiTaO 3  wafer, which does not fully relax after cooling due to pyroelectric charge build-up in the LiTaO 3  material, so this heating may be omitted. However, the built-up pyroelectric charge may be discharged or dissipated by connecting an electrical circuit to parts of the LiTaO 3  wafer, or by electrically shorting, shunting, or grounding the LiTaO 3  wafer with an external conductor. Discharging the built-up pyroelectric charge may relieve bowing or warping of the LiTaO 3  wafer. 
     A block  712 , the joined and direct-bonded wafers are held at room temperature for at least 48 hours or more to allow bond strength to increase before elevating temperature in a subsequent annealing step. The bond strength increases with greater time at room temperature, as shown in  FIG.  2   . 
     At block  714 , the joined and direct-bonded wafers are warmed and heated in an oven up to 50° C. to anneal, at a rate of temperature increase of about 1 degree ° C. per minute, or less. This relatively low-temperature anneal applied gradually avoids slippage of the direct-bonds and minimizes bowing and warpage. 
       FIG.  8    shows a fourth example process  800 , for direct-bonding a LiTaO 3  wafer at room temperature to a fused silica glass wafer without depositing a discrete layer of an oxide, carbide, nitride, carbonitride, or oxynitride material as a bonding intermediary. In the flow diagram of  FIG.  8   , operations of the example process  800  are shown in individual blocks. 
     At block  802 , the LiTaO 3  wafer and a fused silica glass wafer are cleaned with a piranha solution of hydrogen peroxide and sulfuric acid (H 2 O 2 :H 2 SO 4 ) in a ratio of 1:3, and then rinsed with water. 
     At block  804 , the surfaces of the wafers are rinsed in a Megasonic deionized water process and can be spun and rinsed dry on a Goldfinger® processing tool, for example. 
     At block  806 , the bonding surfaces of fused silica glass wafer and the LiTaO 3  wafer are plasma-activated with oxygen plasma or nitrogen plasma in RIE mode with a −200 to −300 volt bias. 
     At block  808 , the bonding surfaces of the wafers are spray-rinsed with 29% NH 4 OH solution to remove particles and to enhance bond energy of direct-bonding. The wafers may be spun dry. 
     At block  810 , the wafers are direct-bonded together at room temperature with the LiTaO 3  wafer loaded as the top wafer, for example. Infrared drying lamps are avoided to prevent excessive bowing and warpage. Room temperature direct-bonding is used to avoid excessive bowing of wafers. Heating during the spin drying would cause bowing of the LiTaO 3  wafer, and can be avoided. Built-up pyroelectric charge in the LiTaO 3  wafer may be discharged or dissipated by connecting an electrical circuit to parts of the LiTaO 3  wafer, or by electrically shorting, shunting, or grounding the LiTaO 3  wafer with an external conductor. Discharging the built-up pyroelectric charge may relieve a bowing tendency or warping of the LiTaO 3  wafer due to these effects. 
     A block  812 , the joined and direct-bonded wafers are held at room temperature for at least 48 hours or more to allow bond strength to increase before elevating temperature in a subsequent annealing step. The bond strength increases with more time held at room temperature, as shown in  FIG.  2   . 
     At block  814 , the joined and direct-bonded wafers are warmed and heated in an oven up to a temperature of 50° C. to anneal, at a rate of temperature increase of about 1 degree ° C. per minute, or less. This relatively low-temperature anneal applied slowly avoids slippage of the direct-bonds and minimizes bowing and warping. 
     To further refine the example direct-bonding processes at room temperature that use no discrete deposited layer of oxide, carbide, nitride, carbonitride, or oxynitride material, an oxygen plasma activation can sometimes yield higher bond strength than nitrogen plasma activation, particularly when bonding LiTaO 3  to sapphire. However, nitrogen plasma activation may work better for bonding LiTaO 3  to silicon without the discrete deposited oxide layer intermediary between wafers. 
     Annealing of direct-bonds between LiTaO 3  and silicon may attain higher bond strength with higher temperature. Nominally, direct-bonded wafers that have cured at room temperature for up to 48 hours are heated to 50° C. in an oven at rate of temperature increase than is about 1 degree ° C. per minute, or less. In the case of forming a native oxide, nitride, carbide, carbonitride, oxynitride, on one or both bonding surfaces as an intermediary for direct-bonding the dissimilar materials, after the annealing step at a gradual temperature increase up to 50° C., the temperature of the joined wafers can be further ramped up to 100° C. at a rate of temperature increase that is about 1 degree ° C. per minute or less to increase the bond strength even further, without significant increase in bowing or warping of the wafers, as shown in  FIG.  9   . 
     Both the example processes that deposit a discrete thin oxide, carbide, nitride layer, carbonitride, and/or oxynitride on one or both wafers as an intermediary for direct-bonding between dissimilar materials of the respective wafers at room temperature, and on the other hand, the example processes that form a native oxide (or carbide, nitride, carbonitride, or oxynitride) on one or both surfaces of the wafers by oxidizing native materials of a first wafer and/or second wafer can direct-bond diverse materials together at room temperature. Curing at room temperature for up to 48 hours or even beyond greatly strengthens the direct bonds formed in either case. 
     The materials of the first wafer and second wafer to be direct-bonded together at room temperature may have significantly different CTEs and lattice constants. The dissimilar materials to be used in a given microelectronic device constructed by the example processes described herein may be managed with thin film engineering principles for balancing wafer warpage of the two wafers, due to the difference in the materials used. Likewise, different thicknesses of the different materials of the two wafers can be leveraged for the two wafers of different thicknesses of the different materials to cancel out each other&#39;s bowing and warping. In an implementation, for the materials discussed herein, the bowing or warpage in general is assumed or calculated to be in the range of 25 μm per inch of diameter down to 10 μm per inch of diameter. 
     The example processes described above can be used to make stacks of wafers of different materials for a microelectronic device or package. The various stack layers can also be used to make multiple stacks of integrated circuits combined with other optical and acoustic devices, with or without direct-bonding between all the layers present. 
     In the foregoing description and in the accompanying drawings, specific terminology and drawing symbols have been set forth to provide a thorough understanding of the disclosed embodiments. In some instances, the terminology and symbols may imply specific details that are not required to practice those embodiments. For example, any of the specific dimensions, quantities, material types, fabrication steps and the like can be different from those described above in alternative embodiments. Moreover, the description of the techniques and devices with regard to wafers could be employed in D2D, D2 W, or W2 W applications. The term “coupled” is used herein to express a direct connection as well as a connection through one or more intervening circuits or structures. The terms “example,” “embodiment,” and “implementation” are used to express an example, not a preference or requirement. Also, the terms “may” and “can” are used interchangeably to denote optional (permissible) subject matter. The absence of either term should not be construed as meaning that a given feature or technique is required. 
     Various modifications and changes can be made to the embodiments presented herein without departing from the broader spirit and scope of the disclosure. For example, features or aspects of any of the embodiments can be applied in combination with any other of the embodiments or in place of counterpart features or aspects thereof. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. 
     While the present disclosure has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations possible given the description. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the disclosure.