Patent Publication Number: US-2023140107-A1

Title: Direct bonding methods and structures

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 157. 
     This application claims the benefit under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 63/263209, entitled “DIRECT BONDING METHODS AND STRUCTURES,” filed Oct. 28, 2021, the entirety of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     Field 
     The field relates to direct bonding methods and structures 
     Description of the Related Art 
     Microelectronic elements often comprise a thin piece of a semiconductor material, such as silicon or gallium arsenide or others. Chips and dies are commonly provided as individual, prepackaged units. In some unit designs, the die is mounted to a substrate or a chip carrier, which is in turn mounted on a circuit panel, such as a printed circuit board (PCB). Dies can be provided in packages that facilitate handling of the die during manufacture and during mounting of the die on the external substrate. For example, many dies are provided in packages suitable for surface mounting. The die can be mounted to the substrate by way of intervening solder balls. 
     Semiconductor dies can also be provided in “stacked” arrangements, wherein one die is provided on a carrier, for example, and another die is mounted on top of the first die. These arrangements can allow a number of different dies to be mounted within a single footprint on a circuit board and can further facilitate high-speed operation by providing a short interconnection between the dies. Often, this interconnect distance can be only slightly larger than the thickness of the die itself. For interconnection to be achieved within a stack of die packages, interconnection structures for mechanical and electrical connection may be provided on both sides (e.g., faces) of each die package (except for the topmost package). This has been done, for example, by providing contact pads or lands on both sides of the substrate to which the die is mounted, the pads being connected through the substrate by conductive vias or the like. 
     Dies or wafers may also be stacked in other three-dimensional arrangements as part of various microelectronic packaging schemes. This can include stacking layers of one or more dies or wafers on a larger base die or wafer, stacking multiple dies or wafers in vertical or horizontal arrangements, or stacking similar or dissimilar substrates, where one or more of the substrates may contain electrical or non-electrical elements, optical or mechanical elements, and/or various combinations of these. Dies or wafers may be bonded in a stacked arrangement using various bonding techniques, including direct dielectric bonding, non-adhesive techniques, such as ZiBond® or a hybrid direct bonding technique, such as DBI®, both available from Adeia (see for example, U.S. Pat. Nos. 6,864,585 and 7,485,968, which are incorporated herein in their entirety). When bonding stacked dies using a direct bonding technique, it is usually desirable that the surfaces of the dies to be bonded be extremely flat and smooth. For instance, in general, the surfaces should have a very low variance in surface topology, so that the surfaces can be closely mated to form a lasting bond. For example, it is generally preferable that the variation in roughness of the bonding surfaces be less than 3 nm and preferably less than 1.0 nm. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a bonding method can include: providing a first element having a first bonding surface, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; providing a second element having a second bonding surface, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; slightly etching the first bonding surface of the first element with a liquid etchant to activate the first bonding surface; after the slightly etching, treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another without an intervening adhesive. 
     In some embodiments, the first bonding surface of the first element is not exposed to an oxygen plasma before the direct bonding. In some embodiments, the first bonding surface of the first element is not exposed to a nitrogen plasma before the direct bonding. In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after forming the first bonding surface and before the direct bonding. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF) (e.g., a chemical formulation comprising very dilute HF, buffered hydrofluoric acid, ammonium fluoride, or very dilute HF moieties). In some embodiments, exposing the first bonding surface to HF comprises exposing the first bonding surface to a dilute HF solution having a concentration of HF in a range of 15 to 500 ppm. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with tetramethylammonium hydroxide (TMAH). In some embodiments, treating the first bonding surface with TMAH comprises exposing the first bonding surface to a TMAH solution having a concentration of TMAH of less than 100 ppm. In some embodiments, exposing the first bonding surface to the TMAH solution comprises exposing the first bonding surface to the TMAH solution having a concentration of TMAH in a range of 5 ppm to 100 ppm. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a boron-containing solution. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with boric acid. In some embodiments, treating the first bonding surface with boric acid comprises treating the first bonding surface with a boric acid solution having a concentration of boric acid in a range of 0.5 ppm to 100 ppm. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with dimethylamine borane. In some embodiments, treating the first bonding surface with boric acid comprises treating the first bonding surface with a dimethylamine borane solution having a concentration of dimethylamine borane in a range of 0.5 ppm to 100 ppm. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a dilute solution containing ammonium and fluoride ions. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a glass-forming solution. In some embodiments, treating the first bonding surface with the glass-forming solution comprises treating the first bonding surface with a solution that comprises at least one of phosphorus, arsenic, boron, fluorine, carbon, or manganese. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a solution having a pH in a range of 6 to 10. In some embodiments, the first bonding surface is not rinsed with water after the treating and before the directly bonding. In some embodiments, directly bonding comprises directly bonding the first and second nonconductive field regions at room temperature and, subsequently, annealing the first and second elements to increase a bond strength between the first and second bonding surfaces. In some embodiments, after the annealing, the bond strength between the first and second bonding surface is in a range of 400 mJ/mm2 to 2000 mJ/mm2. In some embodiments, the bond strength between the first and second bonding surface is in a range of 400 mJ/mm2 to 1800 mJ/mm2. In some embodiments, the method includes, after the annealing, further annealing the first and second elements to form electrical connections between the first and second pluralities of contact features. In some embodiments, the bonding method includes slightly etching the second bonding surface of the second element with a liquid etchant to activate the second bonding surface; after the slightly etching the second bonding surface, treating the second bonding surface with a terminating liquid treatment to terminate the second bonding surface with a terminating species. In some embodiments, the bonding method includes polishing the first element to form the first bonding surface. In some embodiments, the bonding method includes, before the slightly etching, providing a protective layer over a bonding surface of a wafer that includes the first element, the bonding surface including the first bonding surface. In some embodiments, the bonding method includes, after providing the protective layer, singulating the wafer into a plurality of elements including the first element. In some embodiments, the bonding method includes removing the protective layer from the first element and cleaning the plurality of elements without exposing the first element to a plasma. 
     In another embodiment, a bonding method includes: providing a first element having a first bonding surface, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; providing a second element having a second bonding surface, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another without an intervening adhesive, wherein the first bonding surface of the first element is not exposed to an oxygen-containing plasma before the direct bonding. 
     In some embodiments, the first bonding surface of the first element is not exposed to a nitrogen-containing plasma before the direct bonding. In some embodiments, the bonding method of includes, before the treating, slightly etching the first bonding surface of the first element with a liquid etchant to activate the first bonding surface. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF) (e.g., very dilute HF or very dilute buffered hydrofluoric acid). In some embodiments, slightly etching comprises exposing the first bonding surface to very dilute ammonium fluoride, or other very dilute substrate etching chemical species. In some embodiments, the bonding method includes, before the slightly etching, providing a protective layer over a bonding surface of a wafer that includes the first element, the bonding surface including the first bonding surface. In some embodiments, the bonding method includes, after providing the protective layer, singulating the wafer into a plurality of elements including the first element. In some embodiments, the bonding method includes removing the protective layer from the first element and cleaning the plurality of elements without exposing the first element to a plasma. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with tetramethylammonium hydroxide (TMAH). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a boron-containing solution. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with boric acid. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with dimethylamine borane. In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after forming the first bonding surface and before the direct bonding. 
     In another embodiment, a bonding method includes: providing a first element having a first bonding surface, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; providing a second element having a second bonding surface, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another (e.g., at a temperature higher than room temperature) without an intervening adhesive, wherein the first bonding surface of the first element is not exposed to a nitrogen-containing plasma before the direct bonding. 
     In some embodiments, the first bonding surface of the first element is not exposed to an oxygen-containing plasma before the direct bonding. In some embodiments, the bonding method includes, before the treating, slightly etching the first bonding surface of the first element with a liquid etchant to activate the first bonding surface. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with tetramethylammonium hydroxide (TMAH). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a boron-containing solution. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with boric acid. In some embodiments, the first bonding surface with the boron-containing solution comprises treating the first bonding surface with dimethylamine borane (or some other very dilute borane bearing solution). In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after forming the first bonding surface and before the direct bonding. 
     In another embodiment, a bonding method includes: polishing a first bonding surface of a first element, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; after the polishing, cleaning the first bonding surface of the first element without exposing the first bonding surface to a plasma; and after the cleaning, treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species. 
     In some embodiments, the bonding method includes polishing a second bonding surface of a second element, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another without an intervening adhesive. In some embodiments, cleaning the first bonding surface comprises slightly etching the first bonding surface. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with at least one of tetramethylammonium hydroxide (TMAH), boric acid, and dimethylamine borane. In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after polishing the first bonding surface and before the direct bonding. 
     In another embodiment, a bonding method includes: providing a protective layer to a bonding surface of a substrate; singulating the substrate and protective layer into a plurality of dies; removing the protective layer to expose a bonding surface of each die, the bonding surface of each die having a plurality of contact pads at least partially embedded in a nonconductive field region; and preparing the bonding surface of one or more dies for direct bonding without exposing the bonding surface of the one or more dies to a plasma. 
     In some embodiments, the die comprises a semiconductor integrated device die having active circuitry. 
     In another embodiment, a bonding method includes: singulating a substrate into a plurality of dies; and preparing a bonding surface of one or more dies for bonding without exposing the bonding surface of the dies to a plasma, the bonding surface of the one or more dies having a plurality of contact pads at least partially embedded in a nonconductive field region. 
     In some embodiments, the bonding method includes coating the bonding surface of the one or more dies with a protective layer before the singulating and removing the protective layer after the singulating and before the preparing for bonding. In some embodiments, the protective layer comprises of a photosensitive material. 
     In another embodiment, a device includes: a singulated substrate having a bonding surface formed without exposing the bonding surface to a plasma, wherein the bonding surface includes a plurality of contact pads at least partially embedded in a nonconductive field region. 
     In some embodiments, the bonding surface comprises at least one of phosphorus, arsenic, boron, fluorine, manganese nitrogen, carbon. In some embodiments, a concentration of at least one of boron and carbon at the bonding surface is higher than a nominal concentration at a back-end-of-line (BEOL) interface by at least 5%. In some embodiments, a concentration of boron at the bonding surface is greater than 1×1013 atoms/cm2. In some embodiments, the concentration of boron at the bonding surface is greater than 2×1013 atoms/cm2. 
     In another embodiment, a bonded device includes: a first substrate having a first bonding surface including a plurality of contact pads at least partially embedded in a first nonconductive field region; and a second substrate having a second bonding surface directly bonded to the first bonding surface of the first substrate, wherein a bonding interface between the first substrate and the second substrate comprises at least one of phosphorus, arsenic, boron, fluorine, manganese nitrogen, and carbon, and wherein a concentration of at least one of boron and carbon along the bonding interface is higher than the nominal concentration at a BEOL interface by at least 5%. 
     In some embodiments, the second substrate comprises a plurality of contact pads at least partially embedded in a second nonconductive field region. In some embodiments, the concentration of boron along the bonding interface is greater than 1×1013 atoms/cm2. In some embodiments, the concentration of boron along the bonding interface is greater than 2×1013 atoms/cm2 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are flow diagrams of example wafer-to-wafer direct bonding processes. 
         FIGS.  2 A and  2 B  are flow diagrams of example die-to-wafer direct bonding processes. 
         FIG.  3    is a flow diagram of a wafer-to-wafer direct bonding process according to some embodiments. 
         FIGS.  4 A- 4 E  are schematic side sectional views showing various steps of the wafer-to-wafer direct bonding process shown in  FIG.  3   . 
         FIGS.  5 A and  5 B  are flow diagrams of die-to-wafer direct bonding processes according to some embodiments. 
         FIGS.  6 A- 6 H  are schematic side sectional views showing various steps of the die-to-wafer direct bonding process shown in  FIG.  5 A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Various embodiments disclosed herein relate to directly bonded structures in which two or more elements can be directly bonded to one another without an intervening adhesive. A bonded structure comprises two elements that can be directly bonded to one another without an intervening adhesive. Two or more semiconductor elements (such as integrated device dies, wafers, etc.) may be stacked on or bonded to one another to form the bonded structure. Conductive features (e.g., contact pads, exposed ends of vias (e.g., TSVs), or a through substrate electrodes) of a first element may be electrically connected to corresponding conductive features of a second element. Any suitable number of elements can be stacked in the bonded structure. For example, a third element can be stacked on the second element, a fourth element can be stacked on the third element, and so forth. Additionally or alternatively, one or more additional elements (not shown) can be stacked laterally adjacent one another along the first element. In some embodiments, the laterally stacked additional element may be smaller than the second element. In some embodiments, the laterally stacked additional element may be two times smaller than the second element. 
     In some embodiments, the elements are directly bonded to one another without an adhesive. In various embodiments, a non-conductive field region that includes a non-conductive or dielectric material can serve as a first bonding layer of the first element which can be directly bonded to a corresponding non-conductive field region that includes a non-conductive or dielectric material serving as a second bonding layer of the second element without an adhesive. The non-conductive bonding layers can be disposed on respective front sides of device portions, such as a semiconductor (e.g., silicon) portion of the elements. Active devices and/or circuitry can be patterned and/or otherwise disposed in or on the device portions. Active devices and/or circuitry can be disposed at or near the front sides of the device portions, and/or at or near opposite backsides of the device portions. The non-conductive material can be referred to as a non-conductive bonding region or bonding layer of the first element. In some embodiments, the non-conductive bonding layer of the first element can be directly bonded to the corresponding non-conductive bonding layer of the second element using dielectric-to-dielectric bonding techniques. For example, non-conductive or dielectric-to-dielectric bonds may be formed without an adhesive using the direct bonding techniques disclosed at least in U.S. Pat. Nos. 9,564,414; 9,391,143; and 10,434,749, the entire contents of each of which are incorporated by reference herein in their entirety and for all purposes. It should be appreciated that in various embodiments, the bonding layers can comprise a non-conductive material such as a dielectric material, such as silicon oxide, or an undoped semiconductor material, such as undoped silicon. Suitable dielectric bonding surface or materials for direct bonding include but are not limited to inorganic dielectrics, such as silicon oxide, silicon nitride, or silicon oxynitride, or can include carbon, such as silicon carbide, silicon oxycarbonitride, low K dielectric materials, SICOH dielectrics, silicon carbonitride or diamond-like carbon or a material comprising of a diamond surface. Such carbon-containing ceramic materials can be considered inorganic, despite the inclusion of carbon. In some embodiments, the dielectric materials may not comprise polymer materials, such as epoxy, resin or molding materials. 
     In various embodiments, direct hybrid bonds can be formed without an intervening adhesive. For example, nonconductive bonding surfaces can be polished to a high degree of smoothness. The bonding surfaces can be cleaned and exposed to a plasma and/or etchants to activate the surfaces. In some embodiments, the surfaces can be terminated with a species after activation or during activation (e.g., during the plasma and/or etch processes). Without being limited by theory, in some embodiments, the activation process can be performed to break chemical bonds at the bonding surfaces, and the termination process can provide additional chemical species at the bonding surfaces that improves the bonding energy during direct bonding. In some embodiments, the activation and termination are provided in the same step, e.g., a plasma to activate and terminate the surfaces. In other embodiments, the bonding surfaces can be terminated in a separate treatment to provide the additional species for direct bonding. In various embodiments, the terminating species can comprise nitrogen. For example, in some embodiments, the terminating species can comprise a nitrogen-bearing fluid or liquid. In some embodiments, the bonding surface(s) can be exposed to a nitrogen-containing plasma. Further, in some embodiments, the bonding surfaces can be exposed to fluorine. For example, there may be one or multiple fluorine peaks at or near a bonding interface between the first and second elements. Thus, in the directly bonded structure, the bonding interface between two non-conductive materials (e.g., the bonding layers) can comprise a very smooth interface with higher nitrogen content and/or fluorine peaks at the bonding interface. Additional examples of activation and/or termination treatments may be found throughout U.S. Pat. Nos. 9,564,414; 9,391,143; and 10,434,749, the entire contents of each of which are incorporated by reference herein in their entirety and for all purposes. 
     In various embodiments, conductive features of the first element can also be directly bonded to corresponding conductive features of the second element. For example, a hybrid bonding technique can be used to provide conductor-to-conductor direct bonds along the bond interface that includes covalently direct bonded non-conductive-to-non-conductive (e.g., dielectric-to-dielectric) surfaces, prepared as described above. In various embodiments, the conductor-to-conductor (e.g., conductive feature to conductive feature) direct bonds and the dielectric-to-dielectric hybrid bonds can be formed using the direct bonding techniques disclosed at least in U.S. Pat. Nos. 9,716,033 and 9,852,988, the entire contents of each of which are incorporated by reference herein in their entirety and for all purposes. 
     For example, non-conductive (e.g., dielectric) bonding surfaces (for example, inorganic dielectric surfaces) can be prepared and directly bonded to one another without an intervening adhesive as explained above. Conductive contact features (e.g., conductive features which may be at least partially surrounded by non-conductive dielectric field regions within the bonding layers) may also directly bond to one another without an intervening adhesive. In various embodiments, the conductive features can comprise discrete pads at least partially embedded in the non-conductive field regions. In some embodiments, the conductive contact features can comprise exposed contact surfaces of through substrate vias (TSVs). In some embodiments, the respective conductive features can be recessed below exterior (e.g., upper) surfaces (non-conductive bonding surfaces) of the dielectric field region or non-conductive bonding layers, for example, recessed by less than 30 nm, less than 20 nm, less than 15 nm, or less than 10 nm, for example, recessed in a range of 2 nm to 20 nm, or in a range of 4 nm to 10 nm. In various embodiments, prior to direct bonding, the recesses in the opposing elements can be sized such that the total gap between opposing contact pads is less than 15 nm, or less than 10 nm. The non-conductive bonding layers can be directly bonded to one another without an adhesive at room temperature in some embodiments and, subsequently, the bonded structure can be annealed. Upon annealing, the conductive features can expand and contact one another to form a metal-to-metal direct bond. Beneficially, the use of Direct Bond Interconnect, or DBI®, techniques commercially available from Adeia of San Jose, Calif., can enable high density of conductive features to be connected across the direct bond interface (e.g., small or fine pitches for regular arrays). In some embodiments, the pitch of the conductive features, such as conductive traces embedded in the bonding surface of one of the bonded elements, may be less than 40 microns or less than 10 microns or even less than 2 microns. For some applications, the ratio of the pitch of the conductive features to one of the dimensions (e.g., a diameter) of the bonding pad is less than 5, or less than 3 and sometimes desirably less than 2. In other applications, the width of the conductive traces embedded in the bonding surface of one of the bonded elements may range between 0.3 to 20 microns, e.g., in a range of 0.3 to 3 microns. In various embodiments, the conductive features and/or traces can comprise copper, although other metals may be suitable. 
     Thus, in direct bonding processes, a first element can be directly bonded to a second element without an intervening adhesive. In some arrangements, the first element can comprise a singulated element, such as a singulated integrated device die. In other arrangements, the first element can comprise a carrier or substrate (e.g., a wafer) that includes a plurality (e.g., tens, hundreds, or more) of device regions that, when singulated, form a plurality of integrated device dies. Similarly, the second element can comprise a singulated element, such as a singulated integrated device die. In other arrangements, the second element can comprise a carrier or substrate (e.g., a wafer). The embodiments disclosed herein can accordingly apply to wafer-to-wafer, die-to-die, or die-to-wafer bonding processes. In wafer-to-wafer (W2W) processes, two or more wafers can be directly bonded to one another (e.g., direct hybrid bonded) and singulated using a suitable singulation process. After singulation, side edges of the singulated structure (e.g., the side edges of the two bonded elements) may be substantially flush and may include markings indicative of the singulation process (e.g., saw markings if a saw singulation process is used). 
     As explained herein, the first and second elements can be directly bonded to one another without an adhesive, which is different from a deposition process. In one application, a width of the first element in the bonded structure is similar to a width of the second element. In some other embodiments, a width of the first element in the bonded structure is different from a width of the second element. Similarly, the width or area of the larger element in the bonded structure may be at least 10% larger than the width or area of the smaller element. The first and second elements can accordingly comprise non-deposited elements. Further, directly bonded structures, unlike deposited layers, can include a defect region along the bond interface in which nanometer-scale voids (nanovoids) are present. The nanovoids may be formed due to activation of the bonding surfaces (e.g., exposure to a plasma). As explained above, the bond interface can include concentration of materials from the activation and/or last chemical treatment processes. For example, in embodiments that utilize a nitrogen plasma for activation, a nitrogen peak can be formed at the bond interface. The nitrogen peak can be detectable using secondary ion mass spectroscopy (SIMS) techniques. In various embodiments, for example, a nitrogen termination treatment (e.g., exposing the bonding surface to a nitrogen-containing plasma) can replace some of the OH groups of a hydrolized (OH-terminated) surface with N-H moieties or species, yielding a nitrogen-terminated surface. In embodiments that utilize an oxygen plasma for activation, an oxygen peak can be formed at the bond interface. In some embodiments, the bond interface can comprise silicon oxynitride, silicon oxycarbonitride, or silicon carbonitride. As explained herein, the direct bond can comprise a covalent bond, which is stronger than van Der Waals bonds. The bonding layers can also comprise polished surfaces that are planarized to a high degree of smoothness. 
     In various embodiments, the metal-to-metal bonds between the contact pads can be joined such that copper grains grow into each other across the bond interface. In some embodiments, the copper grains grow into each other across the bond interface after annealing the bonded structures or substrates, e.g., at temperatures between 50 and 400° C. or at temperatures between 80 and 300 C. In some embodiments, the copper can have grains oriented along the 111 crystal plane for improved copper diffusion across the bond interface. The bond interface can extend substantially entirely to at least a portion of the bonded conductive features, such that there is substantially no gap between the non-conductive bonding layers at or near the bonded conductive features. In some embodiments, a barrier layer may be provided under the conductive features (e.g., which may include copper). In other embodiments, however, there may be no barrier layer under the conductive features, for example, as described in U.S. Pat. No. 11,195,748, which is incorporated by reference herein in its entirety and for all purposes. 
     Beneficially, the use of the hybrid bonding techniques described herein can enable extremely fine pitch between adjacent contact pads, and/or small pad sizes. For example, in various embodiments, the pitch p (i.e., the distance from edge-to-edge or center-to-center between adjacent conductive features can be in a range of 0.5 microns to 50 microns, in a range of 0.75 microns to 25 microns, in a range of 1 micron to 25 microns, in a range of 1 micron to 10 microns, or in a range of 1 micron to 5 microns. Further, a major lateral dimension (e.g., a pad diameter) can be small as well, e.g., in a range of 0.25 microns to 30 microns, in a range of 0.25 microns to 5 microns, or in a range of 0.5 microns to 5 microns. 
       FIGS.  1 A,  1 B,  2 A and  2 B  are flow diagrams illustrating conventional direct bonding techniques, where  FIGS.  1 A and  1 B  illustrate conventional wafer-to-wafer (W2W) direct bonding processes while  FIGS.  2 A and  2 B  illustrate conventional die-to-wafer (D2W) direct bonding processes. 
       FIG.  1 A  depicts a first wafer-to-wafer direct bonding process  100 . At step  102 , first and second semiconductor wafers are provided, where the first semiconductor wafer has a first bonding surface and the second semiconductor wafer has a second bonding surface. 
     At step  104 , the first and second bonding surfaces are polished. In some embodiments, the bonding surfaces are polished using a chemical-mechanical polishing (CMP) technique in order to planarize the surface of the wafers. The CMP process may also help to clean off some particles remaining on the planarized surface. 
     After polishing, debris, such as organic or inorganic particles, may be present on the bonding surfaces. If such debris remains on the bonding surfaces throughout the bonding process, the debris can create voids in the bonds, which can reduce the strength of the direct bonds. Accordingly, at step  106 , the first and second bonding surfaces can be cleaned using respective ashing processes, in which the first and second wafers are exposed to oxygen plasma to remove residual organic materials on the bonding surface. In some embodiments, a megasonic cleaning process may also be used to remove additional particulates from the substrate. 
     At step  108 , in some embodiments, the first and second bonding surfaces can be rinsed with deionized water (DI) to remove any additional debris. In alternative embodiments, however, rinsing the first and second bonding surfaces with DI to remove additional debris may not be performed, or may be performed to only one of the first and second bonding surfaces. Accordingly, in some alternative embodiments, only one of the bonding surfaces (e.g., the first bonding surface) is rinsed with DI while the other bonding surface (e.g., the second bonding surface) is not rinsed with DI. In still other embodiments, step  108  is not performed at all such that neither of the bonding surfaces is rinsed with DI. 
     At step  110 , the first and second bonding surfaces are activated and terminated by exposing the bonding surfaces to a plasma, such as a nitrogen plasma. The nitrogen plasma can bombard the bonding surfaces so as to break bonds at the bonding surface and to terminate the surface with nitrogen, which can increase the bond energy during bonding. In the illustrated embodiment, both the first and second bonding surfaces are exposed to the plasma to activate the surfaces. In other embodiments, however, only one of the bonding surfaces may be exposed to the plasma while the other surface is not exposed. In these embodiments, the plasma only activates one of the bonding surfaces while the other bonding surface is not activated or terminated. 
     At step  112 , the bonding surfaces can be rinsed with DI water and subsequently thoroughly dried. 
     At step  114 , the first and second bonding surfaces can be directly bonded to one another at room temperature without an intervening adhesive, thereby causing the first and second semiconductor wafers to be directly bonded to each other. During the direct bonding process, the first and second semiconductor wafers can be aligned such nonconductive regions on the first bonding surface are aligned with nonconductive regions on the second bonding and conductive contact pads on the first bonding surface are aligned with and disposed opposite from corresponding conductive contacts on the second bonding surface. In some embodiments, the direct bond can comprise a direct nonconductive bond in which nonconductive regions on the first bonding surface are directly bonded to corresponding nonconductive regions on the second bonding surface. In other embodiments, the direct bond can comprise a hybrid direct bond in which nonconductive regions on the first bonding surface are directly bonded to corresponding nonconductive regions on the second bonding surface and conductive contacts on the first bonding surface are directly bonded to corresponding conductive contacts on the second bonding surface. sIn some embodiments, the assembled or bonded structure can be annealed at a higher temperature to form metallurgical bonds between the opposing conductive contacts and to further increase the bond energy or bond strength between the bonded surfaces as explained herein. 
       FIG.  1 B  depicts a second wafer-to-wafer direct bonding process  100 ′. At step  104 ′, first and semiconductor wafers are polished using a CMP technique to planarize the surface of the wafers. At step  106 ′, the first and second bonding surfaces are ashed. As described above in connection with step  106  of  FIG.  1 A , the ashing process is used to clean the bonding surfaces. In some embodiments, the first and second bonding surfaces are ashed by exposing the surfaces to a plasma, such as an oxygen plasma. At step  108 ′, the first and second bonding surfaces are rinsed with deionized water to remove any additional debris from the bonding surfaces. At step  110 ′ the bonding surfaces are activated. In some embodiments, the bonding surfaces are activated by exposing the surfaces to a plasma, as described above in connection with step  110  of  FIG.  1 A . At step  112 ′, the first and second bonding surfaces are rinsed with deionized water and then thoroughly dried. At step  114 ′, the first and second bonding surfaces are directly bonded together. In some embodiments, the first and second bonding surfaces are directly bonded together without an intervening adhesive and then annealed, as described above in connection with step  114  of  FIG.  1 A . 
       FIG.  2 A  depicts a conventional D2W direct bonding process  200 , which may be generally similar to the W2W process shown in  FIGS.  1 A and  1 B . At step  202 , as explained above, first and second semiconductor wafers are provided, where the first semiconductor wafer has a first bonding surface and the semiconductor wafer has a second bonding surface. 
     At step  204 , the first and second bonding surfaces are polished. In some embodiments, the bonding surfaces are polished using a CMP process. 
     At step  206 , to protect the first bonding surface during singulation of the first semiconductor wafer, a protective layer (such as a photosensitive resist layer, which can comprise an organic layer) is provided over the first bonding surface. The wafer with the protected surface can be supported on a dicing sheet, with the protective layer facing up for the singulation operation. 
     At step  208 , the first semiconductor wafer is singulated into a plurality of integrated device dies, with the protective layer protecting the bonding surface during singulation. Any suitable singulation process may be used, such as sawing, laswer singulation, etching, etc. 
     At step  210 , the protective layer is stripped from at least one of the singulated dies to expose the first bonding surface. In some embodiments, a suitable solvent is used to remove the protective layer. The suitable solvent may include a resist stripping solvent. In some embodiments, the now-exposed first bonding surface of the singulated die may also be rinsed with DI and dried. 
     At step  212 , the first bonding surface of the singulated integrated device die and the second bonding surface of the second semiconductor wafer are cleaned using an ashing process. For example, the first and second bonding surfaces can be exposed to oxygen plasma to remove undesirable organic debris from the surfaces. 
     At step  214 , in some embodiments, the first and second bonding surfaces may be rinsed with DI and dried to remove any remaining debris or organic material. 
     At step  216 , the first and second bonding surfaces are activated by exposing the bonding surface to a plasma, such as a nitrogen-containing plasma. 
     At step  218 , in some embodiments, the first and second bonding surfaces are then rinsed with DI water and subsequently thoroughly dried. 
     At step  220 , the first and second bonding surfaces are directly bonded to one another at room temperature without an intervening adhesive, thereby causing the singulated integrated device die to be directly bonded to each other. In some embodiments, the direct bond can comprise a direct nonconductive bond in which nonconductive regions on the first bonding surface are directly bonded to corresponding nonconductive regions on the second bonding surface. In other embodiments, the direct bond can comprise a hybrid direct bond in which nonconductive regions on the first bonding surface are directly bonded to corresponding nonconductive regions on the second bonding surface and conductive contacts on the first bonding surface are directly bonded to corresponding conductive contacts on the second bonding surface. To cause the conductive contact pads on the first bonding surface to bond to the corresponding contact pads on the second bonding surface, the bonded structure can be annealed at a higher temperature after the nonconductive regions are bonded together, which result in the formation of a metallurgical joint between the opposing conductive contacts on the first and second surfaces. Annealing the bonded structure can also increase increase the bond energy or bond strength between the nonconductive regions on the bonded first and second bonding surfaces. 
       FIG.  2 B  depicts a second die-to-wafer direct bonding process  200 ′. At step  210 ′ singulated dies are cleaned. At step  204 ′, a semiconductor wafer is polished using a CMP technique to form a host bonding surface. At step  212 ′ the bonding surfaces of the singulated dies and the host bonding surface are ashed. As described above in connection with step  106  of  FIG.  1 A , the ashing process is used to clean the bonding surfaces. In some embodiments, the bonding surfaces of the singulated dies and the host bonding surface are ashed by exposing the surfaces to a plasma, such as an oxygen plasma. At step  214 ′ the bonding surfaces of the singulated dies and the host bonding surface are rinsed with deionized water. At step  216 ′, the bonding surfaces of the singulated dies are ashed while the host bonding surface are activated, as described above in connection with step  110  of  FIG.  1 A . At step  218 ′, the bonding surfaces of the singulated dies and the host bonding surface are rinsed with deionized water. At step  220 ′, the bonding surfaces of the singulated dies are directly bonded to the host bonding surface. In some embodiments, the bonding surfaces of the singulated dies are bonded to the host bonding surface without an intervening adhesive and then annealed at a higher temperature, as described above in connection step  114  of  FIG.  1 A . 
     Various embodiments disclosed herein can beneficially obviate the use of a plasma for direct bonding processes. The use of one or multiple plasma exposure or treatment steps (e.g., an oxygen ashing plasma and/or an activating nitrogen and/or oxygen plasma) increases the manufacturing costs of direct bonding processes. Moreover, for elements singulated on a dicing frame, excessive plasma bombardment of the bonding surface of the semiconductor elements may increase the surface roughness by excessively oxidizing the conductive contacts at the bonding surfaces, which can cause the contacts to protrude over the bonding surface. Accordingly, excessive plasma bombardment can reduce the bonding strength between the bonding surfaces or even prevent the surfaces from bonding together. Additionally, plasma bombardment of the exposed surfaces of the dicing tape surface may transfer water insoluble organic residues from the dicing tape to the bonding surface of the die. The bonding surface of dies with organic dicing residues tend to exhibit low bonding energies when bonded to the prepared surface of a host substrate. In addition, the organic residues from the nitrogen activation step may contaminate the bonding surfaces of the dies with water insoluble organic residues and can also contaminate the vacuum chamber of the plasma tool. This tool contamination can result in contamination of all bonding surfaces processed in the chamber. The plasma chamber can be decontaminated by an extensive oxygen chamber ashing step, followed by wiping the interior of the plasma chamber. The plasma chamber may be requalified by a designated method. Cleaning and requalifying the plasma chamber results in higher operating and process ownership costs. 
     Also, some low dielectric constant materials and some spin-on dielectric materials (for example Silk TM  from Dow Chemicals), may be degraded by direct exposure to a very reactive plasma, such as oxygen or nitrogen plasma. The exposure may render the surface of the dielectric highly hydrophilic and, in some instances, may degrade and increase the dielectric constant K of the material. 
     Accordingly, due to the unwanted cost associated with device bonding surface contamination and chamber requalification when a dicing frame is exposed to nitrogen plasma, there remains a continuing demand for improved direct bonding processes that do not use a plasma process (e.g., in which the bonding processes do not utilize any plasma processes for one or more of the elements to be bonded). 
       FIG.  3    is a flow diagram of a wafer-to-wafer (W2W) direct bonding process  300 , according to various embodiments.  FIGS.  4 A- 4 E  are schematic side sectional views showing various steps of the W2W process  300  of  FIG.  3   . As shown in  FIG.  4 A , at step  302 , first and second wafers  400  are provided. Each wafer  400  includes a plurality of elements  404  formed therein and a plurality of saw streets S such that, when singulated along the saw streets S, the wafer  400  is separated into a plurality of singulated elements  404  or dies. Each wafer  400  includes a bonding surface  402 . Each of the plurality of elements  404  comprises a base portion  406  and a nonconductive field or bonding region  408  over the base portion  406 . In some embodiments, the nonconductive field region  408  is formed from a dielectric material and the base portion  406  is formed from a semiconductor material. In some embodiments, the nonconductive field region  408  comprises a nonconductive bonding layer (e.g., a dielectric bonding layer) provided over a semiconductor base portion. The base portion of one or both the first and second elements can comprise active circuitry (e.g., transistors) therein. Each of the elements  400  includes one or more conductive contact features  410  within the nonconductive field region  408 . In some embodiments, the contact features  410  are at least partially embedded in the nonconductive field region  408  such that the contact features  410  are surrounded by a dielectric material (such as silicon oxide). The conductive features can comprise contact pads in various embodiments. In other embodiments, the conductive features can comprise exposed ends of conductive vias or any other suitable contact structure configured to electrically connect and bond to another device. Although not shown due to scale, as explained above, the conductive contact features can be recessed below the bonding surfaces prior to direct bonding. 
     In some embodiments, the base portions  406  of one or more of the elements  404  comprises active circuitry (e.g., transistors) therein and the active circuitry can be coupled to the contact features  410  in the nonconductive field region  408 , for example, by conductive traces formed in the field region  408 . In some embodiments, the base portions  406  for all the elements  404  in one of the wafers  400  can comprise active circuitry therein. In other embodiments, however, the base portions  406  for the elements  404  in a given wafer  404  do not include active circuitry. In these embodiments, the elements  404  having base portions  406  that do not comprise active circuitry can be bonded to elements  404  having base portions  406  that do comprise active circuitry therein. In other embodiments, the elements  404  having base portions  406  comprising active circuitry therein can be bonded to other elements  404  having base portions comprising active circuitry therein. 
     At step  304 , the bonding surfaces  402  of the first and second wafers  400  can be planarized and polished by a chemical-mechanical polishing (CMP) process. 
     As shown in  FIG.  4 B , at step  306 , the bonding surfaces  402  of one or both of the first and second wafers  400  can be cleaned or very slightly etched with a liquid or vapor cleaning chemical  414 . In some embodiments, the cleaning chemical  414  is dispersed over the bonding surface  402  with a liquid dispersion device  412 . The cleaning chemical  414  can be formulated to remove debris from the bonding surface  402  of the nonconductive field region  408  and, depending on the nature of the dielectric material that forms the nonconductive field region  408 , may slightly etch the bonding surface  402  and the contact features  410 . In some embodiments, the cleaning chemical  414  is configured to etch the bonding surface  402  without degrading the smoothness of the bonding surface  402 . For example, the cleaning chemical  414  may remove less than 10 nm from the bonding surface  402 , less than 5 nm from the bonding surface  402 , or less than 0.5 nm from the bonding surface  402 . In various embodiments, for example, the cleaning chemical  414  comprises a very dilute buffered hydrofluoric acid (BHF) solution having a concentration of less than 1%, e.g., in a range of 10 ppm to 1000 ppm or in a range of 50 ppm to 500 ppm. 
     The cleaning chemical  414  may comprise one or more suitable surfactants to improve uniformity of cleaning across the bonding surface  402 . In some embodiment, the activity of the cleaning chemical  414  may be moderated by the incorporated alcohols, hydroxyalcohols (e.g., alcohols containing hydroxyl functional groups or radicals), etc. Beneficially, using cleaning chemical  414  to clean the bonding surface  402  can obviate the use of a plasma cleaning step (e.g., an ashing step) shown in  FIG.  1   , such that an oxygen-containing plasma is not used to clean the wafer(s)  400 . 
     As shown in  FIG.  4 B , a liquid dispersion device  412  can be used to disperse the cleaning chemical  414  over the element  404 . For example, the liquid dispersion device  412  can be configured to spray the cleaning chemical  414  over the bonding surface  402  by activating a fan nozzle or a non-fan nozzle. However, in some embodiments a liquid dispersion device  412  to apply the cleaning chemical  414  to the bonding surface may not be utilized and the cleaning chemical  414  can be applied to the bonding surface  402  using other techniques. For example, in some embodiments, the cleaning chemical  414  can be applied to the bonding surface  402  by at least partially immersing the elements  404  in the cleaning chemical  414  or by dipping the wafer  400  in the cleaning chemical  414 . In some embodiments, the bonding surface  402  may be cleaned with the assistance of a megasonic cleaning device. 
     In some embodiments, more than one cleaning solution is used at step  306  to clean the bonding surfaces  402 . For example, in some embodiments, water can be used in conjunction with the cleaning chemical  414  to clean the bonding surfaces  402 . Specifically, the bonding surfaces  402  can be rinsed with DI to remove unwanted debris from the bonding surface. After rinsing the bonding surfaces  402  with DI, the bonding surfaces  402  may then be dried before the cleaning chemical  414  is applied to the bonding surface  402  to prepare the bonding surface  402  for the chemical treatment step. In some embodiments, the step  306  may incorporate a DI rinse and drying the substrate after the DI rinsing step. In other embodiments, there may be no DI rinse between steps  304  and  306 . In still other embodiments, very slight etching can be omitted, particularly if a prior cleaning step (e.g., developing solution) leaves a suitable surface for subsequent processing. 
     In some embodiments, the wafers  400  can be bonded after the cleaning and drying step of step  306 . After the bonding operation at room temperature, the bonded wafers  400  may be annealed at a higher temperature, e.g., in a range of 80° C. to 400° C. for a time ranging between 15 minutes to 60 minutes, depending on the process recipe. In general, the bond energy of the interface of the bonded wafers 400 or substrate may be less than 1000 mJ/mm 2 , and may not be adequate for a subsequent operation, for example, the grinding of the back surface of wafer to thin the wafer for subsequent operations. 
     As shown in  FIG.  4 C , and at step  308  of  FIG.  3   , the bonding surface  402  of one or both wafers  400  is treated with a terminating fluid treatment  418  dispersed by a dispersion device  416 . The terminating fluid treatment is performed to terminate the bonding surface  402  of the wafer(s)  400  with a terminating species  420 , which can enhance bonding between the two wafers  400 . The terminating fluid treatment  418  can comprise liquid or vapor. Beneficially, the use of a fluid (liquid or vapor) terminating species  420  can obviate the use of a plasma in the direct bonding process. For example, in the disclosed embodiment, no nitrogen plasma process may be used before direct bonding. In various embodiments, the bonding surface  402  of one or both wafers  400  may not be exposed to any plasma before direct bonding. 
     In various embodiments, terminating liquid treatment  418  can comprise a chemical species with low alkalinity and low acidity. For example, the terminating fluid treatment  418  can comprise a solution having a pH in a range of 5 to 11, or in a range of 6 to 10. In some embodiments, the pH is less than about 9. 
     In some embodiments, the terminating fluid treatment (e.g., terminating treatment) leaves residues of nitrogen, boron, phosphorus, or their various combinations on the bonding surface  402  (and at the bonding interface of the bonded structure), which can increase covalent bond strength after direct bonding. In some embodiments, the terminating fluid treatment  418  can comprise ammonium hydroxide (NH 4 OH). For example, in some embodiments, the terminating fluid treatment  418  comprises tetramethylammonium hydroxide (TMAH). The TMAH solution can have a concentration of TMAH of less than 100 ppm, e.g., in a range of 0.5 ppm to 100 ppm, in a range of 1 ppm to 100 ppm, or 5 ppm to 100 ppm in DI water. Beneficially, the TMAH in the liquid treatment  418  can terminate the bonding surface  402  sufficiently for direct bonding, while avoiding excessive damage of the conductive contact features  410 . Other analogues of TMAH may be applied for the terminating treatment  418 , including, for example, tert-Butylammonium hydroxide, tetraethylammonium hydroxide, ethyltrimethylammonium hydroxide and the like. In some embodiments, the carrier or solvent for the terminating compound  420  may comprise lower molecular weight solvents, for example, alcohols (methyl alcohol, ethyl alcohol) or ketones. In some embodiments, the carrier for the terminating compound  420  may be an inert gas, for example nitrogen, argon or helium. For example, an Atomic Layer Deposition (ALD) method may be used to form the terminating treatment. In various embodiments, for example, the conductive contact features  410  comprise copper pads or vias. In contrast to ammonium hydroxide, very low concentration TMAH will not excessively etch the copper contact features, which would alter the pre-bonding pad recess and, accordingly, increase the temperature required for the metallic conductive contact pads  410  on the opposing bonding surfaces to bond together. TMAH or other, lower alkalinity chemicals can beneficially reduce the degree of etching of the copper features, which can improve the yield of the direct metallic bonds. In various embodiments, the element(s) may not be exposed to an ozone treatment. 
     In other embodiments, the terminating fluid treatment  418  can comprise a boron-containing solution. For example, in some embodiments, the terminating fluid treatment  418  can comprise boric acid. The boric acid can comprise a boric acid solution having a concentration of boric acid in a range of 0.5 ppm to 200 ppm. Other sources of boron containing solutions or gases (for example, boric anhydride, borane-ammonia, borane trimethyl complex, tetrahydroxydiboron, etc.) may be applied for terminal treatment on the bonding surface  402  of the substrate. 
     In some embodiments, the terminating fluid treatment  418  includes both boron and nitrogen. For example, the terminating fluid treatment  418  can comprise a solution of dimethylamine borane (DMAB). The DMAB solution can have a concentration of dimethylamine borane in a range of 0.5 ppm to 500 ppm, for example, in a range of 0.5 ppm to 200 ppm. 
     In other embodiments, the terminating fluid treatment  418  can comprise a fluoride containing solution. For example, in some embodiments, the terminating fluid treatment  418  can comprise a dilute solution of ammonium fluoride, ammonium bifluoride, a combination of ammonium fluoride and ammonium bifluoride, or another ammonium-fluoride complexes. The terminating solution can comprise, for example, an ammonium-fluoride solution having a concentration in a range of 0.5 ppm to 200 ppm. In some embodiments, ammonium ions and fluoride ions from other sources (e.g., ammonium hydrogen fluoride) may be used to terminate the bonding surface  402 . 
     In some embodiments, the terminating fluid treatment  418  comprises a glass-forming solution that can aid in strengthening direct bonds. The glass-forming solution can comprise a relatively low molecular weight compound including elements from one or more of Groups III and/or V. The glass-forming solution can have a molecular weight of less than 600 Daltons (Da), less than 500 Da, less than 200 Da, or less than 150 Da. The glass-forming solution can have a molecular weight in a range of 5 Da to 150 Da. For example, the glass-forming solution can comprise at least one of phosphorus, arsenic, boron, fluorine, carbon, and manganese (for example, less than 1000 ppm of these elements). The amounts of boron, nitrogen and carbon can exceed the nominal threshold concentration found between the interface of two dielectric layers beneath the bonding surface. The glass-forming species listed above can originate from residues on the surface from the chemical treatment. The concentration of the glass forming element at the bonding surface can be higher than the concentration of glass forming elements found at the interface of two coated dielectric layers beneath the bonding surface of the back-end-of-line (BEOL) layers. In some embodiment, the concentration of the glass forming element can be at least 3%, 5% 10% or 20% higher than the concentration of the same elements in a BEOL dielectric interface in the device. 
     In some embodiments, the bonding surface(s)  402  may not be rinsed with water after being treated with the terminating fluid treatment  418  and direct bonding occurs. In other embodiments, however, the bonding surface(s)  402  may be rinsed with water after the terminating treatment  418  and before direct bonding occurs. 
     In embodiments where the bonding surfaces  402  are exposed to a terminating fluid treatment  418  that include at least one of boron and carbon, the concentration of at least one of boron and carbon at the bonding surface (and bonding interface of the bonded structure) can be higher than a nominal concentration at a back-end-of-line (BEOL) interface by at least 5%. For example, a concentration of boron at the bonding surface (or bond interface) can be greater than 1×10 13  atoms/cm 2 , or greater than 2×10 13  atoms/cm 2 . 
     In the illustrated embodiment, the terminating fluid treatment  418  is depicted as being sprayed onto the wafer  400  with dispersion device  416 . However, this is merely an example and the skilled artisan will appreciate that the terminating fluid treatment  418  can be applied in other suitable methods. For example, in some embodiments, the wafers  400  can instead be dipped into the terminating fluid to expose the wafer(s)  400  to the terminating fluid vapor or fluid. 
     In some embodiments, after performing step  308 , the treated wafer  400  can be dried without being rinsed by DI. The drying step may comprise spin drying the wafer  400  in a range of 1000 rpm to 4000 rpm and for a time in a range of 30 seconds to 240 seconds. Other drying processes and times may be used. In other embodiments, the treated substrate may be rinsed with DI before drying the substrate. 
     At step  310 , after being treated with the terminating fluid treatment  418 , the treated wafers  400  are directly bonded to each other to form a bonded device  422  without using an intervening adhesive. As shown in  FIG.  4 D , a first wafer  400   a  is positioned over a second wafer  400   b.  The first wafer  400   a  includes a first bonding surface  402   a  and a plurality of elements  404   a,  where each element  404   a  includes a base portion  406   a,  a nonconductive field region  408   a,  and a plurality of nonconductive contact features  410   a  at least partially embedded in the respective nonconductive field region  408   a.  Similarly, the second wafer includes a second bonding surface  402   b  and a plurality of elements  404   b,  where each element  404   b  includes a base portion  406   b,  a nonconductive field region  408   b,  and a plurality of conductive contact features  410   b  at least partially embedded in the nonconductive field region  408   b.  In the illustrated embodiment, both wafers  400   a,    400   b  underwent the polishing process described above in connection with step  304 , the etching/cleaning process described above in connection with step  306  and  FIG.  4 B , and the treating process described above in connection with step  308  and  FIG.  4 C . Accordingly, both first and second bonding surfaces  402   a,    402   b  are terminated with the terminating species  420 . In other embodiments, however, only one of the bonding surfaces  402   a,    402   b  is terminated with the terminating species  420 . 
     To bond the first and second wafers  400   a,    400   b  together, the first and second wafers  400   a,    400   b  can be positioned such that the first and second bonding surfaces  402   a,    402   b  face each other and a first element  404   a  of the first wafer  400   a  is aligned with a second element  404   b  of the second wafer  400   b.  Specifically, the elements  404   a  and  404   b  can be aligned such that the conductive contact features  410   a  for the first element  404   a  are positioned directly over the conductive contact features  410   b  for the second element  404   b  and such that the nonconductive field region  408   a  for the first element  404   a  is aligned with the nonconductive field region  408   b  for the second element  404   b.  In this way, when the first and second wafers  400   a,    400   b  are brought together to be directly bonded to each other, as shown in  FIG.  4 E , the nonconductive field regions  408   a  and  408   b  are in direct contact with each other and the conductive contact features  410   a  and  410   b  are directly aligned with each other. 
     Once the wafers  400   a,    400   b  are aligned, the first and second elements  404   a ,  404   b  can be directly bonded to each other to form bonded device  422 . The first and second nonconductive field regions  408   a,    408   b  of the first and second elements  404   a,    404   b  can be directly bonded to one another at room temperature using various direct bonding techniques such as those disclosed herein. 
     At step  312 , after directly bonding the nonconductive field regions  408   a ,  408   b  together, in some embodiments the bonded device  422  can be annealed to increase a bond strength between the first and second bonding surfaces  402   a,    402   b.  The annealing can be performed at a temperature in a range of 80° C. to 400° C., or in a range of 150° C. to 300° C. to increase the bond strength. The annealing can be performed for a time period in a range of 5 minutes to 120 minutes or in a range of 15 minutes to 45 minutes. After this anneal, the bond strength between the first and second bonding surfaces  402   a,    402   b  can be in a range of 1000 mJ/mm 2  to 2500 mJ/mm 2 , e.g., in a range of 1200 mJ/mm 2  to 2400 mJ/mm 2 . The bond strength after the annealing step in the disclosed embodiments using the chemical treatment can be sufficiently strong for subsequent operations, for example, the grinding of the back surface of a wafer to thin the wafer from the backside, or thinning the backside of the wafer to reveal TSV for die stacking. Additionally or alternatively, the first and second elements  404   a,    404   b  can be annealed in a subsequent anneal to cause the conductive contact features  410   a,    410   b  to expand so as to form electrical connections between first and second pluralities of contact features. 
       FIG.  5 A  is a flow diagram of a first die-to-wafer (D2W) direct bonding process  500 , according to various embodiments.  FIGS.  6 A- 6 H  are schematic side sectional views showing various steps of the D2W process  500  of  FIG.  5 A . Unless otherwise noted, the features and steps of  FIGS.  5 A and  6 A- 6 H  may be the same as or generally similar to the features and steps shown in  FIGS.  3  and  4 A- 4 E . As shown in  FIG.  6 A , at step  502  of the process  500 , a wafer  600  can be provided. The wafer  600  has a bonding surface  602  and includes a plurality of elements or dies  604 . Each element  604  in the wafer  600  includes a base portion  606 , a nonconductive field region  608 , and a plurality of conductive contact features at least partially embedded in the nonconductive field region. The wafer  600  and elements  604  may be the same or generally similar to the wafer  400  and elements  404  shown in  FIG.  4 A . 
     At step  504 , the bonding surface  602  of the singulated element  604  can be planarized and polished by a CMP process. 
     As shown in  FIG.  6 B , at step  506 , a protective layer  614  is provided over the bonding surface  602 . The protective layer  614  protects the bonding surface  602  and/or the conductive contact features  610  during singulation. In some embodiments, the protective layer  614  comprises a photosensitive resist, which can comprise an organic material. In other embodiments, the protective layer  614  comprises a non-photosensitive material which can be removed using chemicals such as Futurrex PC6 16000 or PC43-1500, sold by Futurrex, Inc. of Franklin, N.J. The wafer  600  with the protective layer  614  can be mounted on a dicing frame  612  with the protective layer  614  facing up, such that the backside of the wafer  600  opposite the bonding surface  602  is contacting the adhesive of the dicing frame  612 . 
     As shown in  FIG.  6 B , at step  508 , after providing the protective layer  614 , the wafer  600  can be singulated into a plurality of elements or dies  604 . Singulation can be performed using various singulation techniques and methods. Each singulated element  604  includes a base portion  606 , a nonconductive field region  608 , and at least one conductive contact features  610  at least partially embedded in the nonconductive field region  608 . After singulation, the protective layer  614  may also still be on the bonding surfaces  602  of the elements  604 . 
     As shown in  FIG.  6 C , at step  510 , the protective layer  614  is stripped from the bonding surface  602  of at least one of the singulated elements  604 . In some embodiments, a suitable solvent is used to remove the protective layer  614 . The suitable solvent may include a resist stripping solvent. In some embodiments, the now-exposed bonding surface  602  may also be rinsed with DI and dried. In the disclosed embodiments, the protective layer is removed from the dies or elements without exposing the element  604  to a plasma (e.g., without exposing the dies to an ashing or oxygen-containing plasma). For example, protective layers  614  can be removed by exposure to a liquid alkaline developer or by other protective layer stripping chemicals. 
     After singulating and polishing the element  604 , any remaining protective layer residue and unwanted organic and inorganic debris (protective layer residues and particulates) can be cleaned from the surfaces (e.g., bonding surface and side walls) of the singulated element  604 . As shown in  FIG.  6 E , at step  512 , the bonding surface  602  of the element  604  is cleaned or very slightly etched with a liquid or vapor cleaning chemical  614 . In some embodiments, the cleaning chemical  614  is dispersed over the bonding surface with a liquid dispersion device  616 . As explained above in connection with  FIG.  4 B  and step  306 , a cleaning chemical  618  may be applied to prepare the bonding surface  602  for the chemical treatment step. The cleaning chemical  618  can be formulated to remove debris from the bonding surface  602  of the element  604  and, depending on the nature of the dielectric material that forms the nonconductive field region  608 , the chemical  618  may slightly etch the surface of the nonconductive field region  608  and the conductive contact features  610 . The cleaning process may not significantly degrade the smoothness of the bonding surface  602 . 28or example, the cleaning step may remove less than 10 nm from the surface of the substrate, less than 5 nm, or less than 0.5 nm. 
     In various embodiments, for example, the cleaning chemical  618  may comprise a very dilute buffered hydrofluoric acid (BHF) solution having a concentration in a range of 10 to 1000 ppm and preferably between 50 to 500 ppm. The cleaning chemical  618  may include one or more suitable surfactants to improve uniformity of cleaning across the bonding surface  602 . In some embodiment, the chemical activity of the cleaning chemical  618  may be moderated by the incorporated alcohols, hydroxyalcohols, etc. Beneficially, using the cleaning chemical  618  to clean the bonding surface  602  can obviate the use of a plasma cleaning step (e.g., an ashing step) described in connection  FIGS.  1  and  2   , such that an oxygen-containing plasma is not used to clean the singulated dies on a dicing frame. 
     In some embodiments, more than one cleaning solution is used at step  512  to clean the bonding surface  602 . For example, in some embodiments, water can be used in conjunction with the cleaning chemical  618  to clean the bonding surface  602 . Specifically, the bonding surfaces  602  can be rinsed with DI. Alternatively, or in addition, step  512  may also incorporate a DI rinse and drying the substrate after the DI rinsing step. 
     As shown in  FIG.  6 F , at step  514 , the bonding surface  602  of the singulated die  604  is treated with a terminating fluid treatment  622  by a dispersion device  620  while the singulated dies  604  are still on the dicing sheet. The terminating fluid treatment  622  is performed to terminate the bonding surface  602  of the singulated dies  604  with a chemical bonding species  624 , which can enhance bonding between the die and the host wafer. The use of a fluid (liquid or vapor) chemical treating species can obviate the use of a plasma in the direct bonding process, and can result in the bonding surface  602  being very clean, without unwanted debris and with a pad recess of less than 10 nm, e.g., a recess of about 4.3 nm. 
     By contrast, for nitrogen plasma treated singulated dies on a dicing frame, the plasma bombardment of the exposed surfaces of the dicing tape surface may transfer water insoluble organic residues from the dicing tape to the bonding surface of the die. The bonding surface of dies with organic dicing residues tend to exhibit low bonding energies when bonded to the prepared surface of a host substrate. In addition, the organic residues from the nitrogen activation step may contaminate the vacuum chamber of the plasma tool. This tool contamination results in contamination of all bonding surfaces processed in the chamber. The plasma chamber may be decontaminated by an extensive oxygen chamber ashing step, followed by wiping the interior of the plasma chamber. The plasma chamber may be requalified by a suitable method. 
     In the disclosed plasma-free embodiments, no nitrogen plasma process may be used before direct bonding surface preparation steps. In various embodiments, the bonding surface  602  of the singulated die may not be exposed to any plasma before direct bonding. 
     In the illustrated embodiment, the terminating fluid treatment  622  is depicted as being sprayed onto the element  604  with dispersion device  620 . However, this is merely an example and the skilled artisan will appreciate that the fluid terminating treatment  622  can be applied to the element in other suitable methods. For example, in some embodiments, the singulated dies  604  can be dipped into the fluid treatment to expose the element  604  to the terminating vapor or fluid. In some embodiments, the terminating fluid may be applied between a megasonic transducer and the bonding surface of the element  604 , where the megasonic transducer assists in cleaning and terminating the bonding surfaces. 
     In some embodiments, after performing step  514 , the treated element  604  can be dried without being rinsed by DI. The drying step may comprise spin drying the frame or host between 1000 to 4000 rpm and for times varying from 30 seconds to 240 seconds. Other drying processes and times may be used. In other embodiments, the treated substrate may be rinsed with DI before drying the substrate. 
     After treating the singulated element  604 , the element  604  can be directly bonded to a wafer. Accordingly, at step  516 , a second semiconductor wafer is provided. The second semiconductor wafer may be prepared in a manner generally similar to that described above in connection with  FIGS.  3  and  4 A- 4 E . Specifically, the second semiconductor wafer can include a bonding surface that has been polished using a CMP process, cleaned or slightly etched, and then treated with a terminating fluid. Additionally, the second semiconductor wafer can be prepared without being exposed to plasma. 
     At step  518 , the treated element  604  is directly bonded to the prepared wafer without using an intervening adhesive to form bonded device  636 . As shown in  FIG.  6 G , the treated element  604  is positioned over the prepared wafer  626 . The prepared wafer  626  includes a bonding surface  628 , a base portion  630 , a nonconductive field region  632 , and plurality of conductive contact features  634  at least partially embedded in the nonconductive field region  632 . In the illustrated embodiment, both bonding surfaces  602  and  628  were terminated with the terminating species  624 . In other embodiments, however, only one of the bonding surfaces is terminated with the terminating species  624 . 
     To bond the element  604  and the wafer  626 , the element  604  and wafer  626  are positioned such that the bonding surfaces  602  and  628  face each other and the element  604  and wafer  626  are generally aligned with each other, although there may be some small misalignments. For example, the element  604  and wafer  626  can be generally aligned such that the conductive contact features  610  on the element  604  are positioned over the conductive contact features  634  of the wafer  626  and the nonconductive field region  608  for the element  604  is aligned with the nonconductive field region  632  for the wafer  626 . In this way, when the element  604  and the wafer  626  are brought together to be directly bonded to each other, as shown in  FIG.  6 H , the nonconductive field regions  608 ,  632  are in direct contact with each other and the conductive contact features  610 ,  634  are directly aligned with each other. 
     Once the element  604  and the wafer  626  are generally or substantially aligned, the element  604  and the wafer can be directly bonded to each other to form bonded device  636 . The nonconductive field regions  608 ,  632  of the die  604  and wafer  626  can be directly bonded to one another at room temperature using various bonding techniques. 
     At step  520 , after the nonconductive field regions  608 ,  632  have been directly bonded together (for example, at room temperature), in some embodiments the bonded device  636  can be subsequently annealed to increase a bond strength between the first and second bonding surfaces  602 ,  628 . The annealing can be performed at a temperature in a range of 100° C. to 400° C., or in a range of 150° C. to 300° C. to increase the bond strength. The annealing can be performed for a time period in a range of 15 minutes to 180 minutes. Higher temperature anneals can utilize shorter annealing times. The higher annealing temperature can induce expansion in the confined metal pads  610 ,  634  to mate and bond across a recess or gap, to form a mechanical and electrical joint, electrically interconnecting the die and the host wafer. The annealing process can result in excellent mechanical mating between the conductive layers of the element and the wafer. 
     In some embodiments, additional dies may be bonded by the disclosed methods to the wafer  626  or the backside of element  604 . In some applications, the bonded structure may be singulated in such a manner that the element  604  are smaller in cross sectional that the wafer  626 . In other embodiments the cross-sectional area of the singulated die stack may be similar to the area of the singulated host wafer. 
       FIG.  5 B  depicts a second D2W direct bonding process  500 ′, according to various embodiments. At step  502 ′, one or more singulated dies are cleaned. At step  504 ′, a semiconductor wafer is polished using a CMP technique to form a host bonding surface. At step  506 ′, the bonding surfaces of the singulated dies and the host bonding surface are cleaned or very slightly etched. As described above in connection with step  512 , the bonding surfaces are cleaned or etched with a cleaning chemical that is configured to prepare the bonding surfaces for a chemical treatment step by removing debris from the bonding surfaces and, in some embodiments, very slightly etching the bonding surfaces. At step  508 ′, in some embodiments, the bonding surfaces of the singulated dies and the host bonding surface are rinsed with deionized water. At step  510 ′, the bonding surfaces of the singulated dies and the host bonding surface are treated with a terminating liquid treatment. As described above in connection with step  514 , the terminating liquid treatment is performed to terminate the bonding surfaces with a chemical bonding species, which can enhance bonding between the bonding surfaces of the singulated dies and the host bonding surface. At step  512 ′, the bonding surfaces are rinsed with deionized water. At step  514 ′, the bonding surfaces of the singulated dies are bonded to the host bonding surface. As described above in connection with step  518 , the bonding surfaces of the singulated dies and the host bonding surface can be directly bonded together at room temperature without an adhesive and then annealed to strengthen the bond. 
     In one embodiment, a bonding method can include: providing a first element having a first bonding surface, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; providing a second element having a second bonding surface, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; slightly etching the first bonding surface of the first element with a liquid etchant to activate the first bonding surface; after the slightly etching, treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another without an intervening adhesive. 
     In some embodiments, the first bonding surface of the first element is not exposed to an oxygen plasma before the direct bonding. In some embodiments, the first bonding surface of the first element is not exposed to a nitrogen plasma before the direct bonding. In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after forming the first bonding surface and before the direct bonding. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF). In some embodiments, exposing the first bonding surface to HF comprises exposing the first bonding surface to a dilute HF solution having a concentration of HF in a range of 15 to 500 ppm. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with tetramethylammonium hydroxide (TMAH). In some embodiments, treating the first bonding surface with TMAH comprises exposing the first bonding surface to a TMAH solution having a concentration of TMAH of less than 100 ppm. In some embodiments, exposing the first bonding surface to the TMAH solution comprises exposing the first bonding surface to the TMAH solution having a concentration of TMAH in a range of 5 ppm to 100 ppm. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a boron-containing solution. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with boric acid. In some embodiments, treating the first bonding surface with boric acid comprises treating the first bonding surface with a boric acid solution having a concentration of boric acid in a range of 0.5 ppm to 100 ppm. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with dimethylamine borane. In some embodiments, treating the first bonding surface with boric acid comprises treating the first bonding surface with a dimethylamine borane solution having a concentration of dimethylamine borane in a range of 0.5 ppm to 100 ppm. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a dilute solution containing ammonium and fluoride ions. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a glass-forming solution. In some embodiments, treating the first bonding surface with the glass-forming solution comprises treating the first bonding surface with a solution that comprises at least one of phosphorus, arsenic, boron, fluorine, carbon, or manganese. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a solution having a pH in a range of 6 to 10. In some embodiments, the first bonding surface is not rinsed with water after the treating and before the directly bonding. In some embodiments, directly bonding comprises directly bonding the first and second nonconductive field regions at room temperature and, subsequently, annealing the first and second elements to increase a bond strength between the first and second bonding surfaces. In some embodiments, after the annealing, the bond strength between the first and second bonding surface is in a range of 400 mJ/mm2 to 2000 mJ/mm2. In some embodiments, after the annealing, the bond strength between the first and second bonding surface is in a range of 400 mJ/mm2 to 1800 mJ/mm2. In some embodiments, the method can include, after the annealing, further annealing the first and second elements to form electrical connections between the first and second pluralities of contact features. In some embodiments, the method can include slightly etching the second bonding surface of the second element with a liquid etchant to activate the second bonding surface; and after the slightly etching the second bonding surface, treating the second bonding surface with a terminating liquid treatment to terminate the second bonding surface with a terminating species. In some embodiments, the method can include polishing the first element to form the first bonding surface. In some embodiments, the method can include, before the slightly etching, providing a protective layer over a bonding surface of a wafer that includes the first element, the bonding surface including the first bonding surface. In some embodiments, the method can include, after providing the protective layer, singulating the wafer into a plurality of elements including the first element. In some embodiments, the method can include comprising removing the protective layer from the first element and cleaning the plurality of elements without exposing the first element to a plasma. 
     In another embodiment, a bonding method can include: providing a first element having a first bonding surface, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; providing a second element having a second bonding surface, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another without an intervening adhesive. The first bonding surface of the first element may not be exposed to an oxygen-containing plasma before the direct bonding. 
     In some embodiments, the first bonding surface of the first element is not exposed to a nitrogen-containing plasma before the direct bonding. In some embodiments, the method can include, before the treating, slightly etching the first bonding surface of the first element with a liquid etchant to activate the first bonding surface. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF). In some embodiments, the method can include, before the slightly etching, providing a protective layer over a bonding surface of a wafer that includes the first element, the bonding surface including the first bonding surface. In some embodiments, the method can include, after providing the protective layer, singulating the wafer into a plurality of elements including the first element. In some embodiments, the method can include removing the protective layer from the first element and cleaning the plurality of elements without exposing the first element to a plasma. In some embodiments, treating the first bonding surface comprises treating the first bonding surface with tetramethylammonium hydroxide (TMAH). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a boron-containing solution. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with boric acid. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with dimethylamine borane. In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after forming the first bonding surface and before the direct bonding. 
     In another embodiment, a bonding method can include: providing a first element having a first bonding surface, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; providing a second element having a second bonding surface, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another without an intervening adhesive, wherein the first bonding surface of the first element is not exposed to a nitrogen-containing plasma before the direct bonding. 
     In some embodiments, the first bonding surface of the first element is not exposed to an oxygen-containing plasma before the direct bonding. In some embodiments, the method can include, before the treating, slightly etching the first bonding surface of the first element with a liquid etchant to activate the first bonding surface. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with tetramethylammonium hydroxide (TMAH). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with a boron-containing solution. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with boric acid. In some embodiments, treating the first bonding surface with the boron-containing solution comprises treating the first bonding surface with dimethylamine borane. In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after forming the first bonding surface and before the direct bonding. 
     In another embodiment, a bonding method can include: polishing a first bonding surface of a first element, the first bonding surface comprising a first nonconductive field region and a first plurality of contact features; after the polishing, cleaning the first bonding surface of the first element without exposing the first bonding surface to a plasma; and after the cleaning, treating the first bonding surface with a terminating liquid treatment to terminate the first bonding surface with a terminating species. 
     In some embodiments, the method can include polishing a second bonding surface of a second element, the second bonding surface comprising a second nonconductive field region and a second plurality of contact features; and after the treating, directly bonding the first bonding surface to the second bonding surface such that the first and second nonconductive field regions are directly bonded to one another without an intervening adhesive and such that the first and second pluralities of contact features are directly bonded to one another without an intervening adhesive. In some embodiments, cleaning the first bonding surface comprises slightly etching the first bonding surface. In some embodiments, slightly etching comprises exposing the first bonding surface to hydrofluoric acid (HF). In some embodiments, treating the first bonding surface comprises treating the first bonding surface with at least one of tetramethylammonium hydroxide (TMAH), boric acid, solutions of ammonium ions and fluoride ions, and dimethylamine borane. In some embodiments, the first bonding surface of the first element is not exposed to a plasma treatment after polishing the first bonding surface and before the direct bonding. 
     In another embodiment, a bonding method can include: providing a protective layer to a bonding surface of a substrate; singulating the substrate and protective layer into a plurality of dies; removing the protective layer to expose a bonding surface of each die, the bonding surface of each die having a plurality of contact pads at least partially embedded in a nonconductive field region; and preparing the bonding surface of one or more dies for direct bonding without exposing the bonding surface of the one or more dies to a plasma. 
     In some embodiments, the die comprises a semiconductor integrated device die having active circuitry. 
     In another embodiment, a bonding method can include: singulating a substrate into a plurality of dies; and preparing a bonding surface of one or more dies for bonding without exposing the bonding surface of the dies to a plasma, the bonding surface of the one or more dies having a plurality of contact pads at least partially embedded in a nonconductive field region. 
     In some embodiments, the method can include coating the bonding surface of the one or more dies with a protective layer before the singulating and removing the protective layer after the singulating and before the preparing for bonding. In some embodiments, the protective layer comprises of a photosensitive material. 
     In another embodiment, a device can comprise: a singulated substrate having a bonding surface formed without exposing the bonding surface to a plasma, wherein the bonding surface includes a plurality of contact pads at least partially embedded in a nonconductive field region. 
     In some embodiments, the bonding surface comprises at least one of phosphorus, arsenic, boron, fluorine, manganese nitrogen, carbon. In some embodiments, a concentration of at least one of boron and carbon at the bonding surface is higher than a nominal concentration at a back-end-of-line (BEOL) interface by at least 5%. In some embodiments, a concentration of boron at the bonding surface is greater than 1×10 13  atoms/cm 2 . In some embodiments, the concentration of boron at the bonding surface is greater than 2×10 13  atoms/cm 2 . 
     In another embodiment, a bonded device can include: a first substrate having a first bonding surface including a plurality of contact pads at least partially embedded in a first nonconductive field region; and a second substrate having a second bonding surface directly bonded to the first bonding surface of the first substrate, wherein a bonding interface between the first substrate and the second substrate comprises at least one of phosphorus, arsenic, boron, fluorine, manganese nitrogen, and carbon, and wherein a concentration of at least one of boron and carbon along the bonding interface is higher than the nominal concentration at a BEOL interface by at least 5%. 
     In some embodiments, the second substrate comprises a plurality of contact pads at least partially embedded in a second nonconductive field region. In some embodiments, the concentration of boron along the bonding interface is greater than 1×10 13  atoms/cm 2 . In some embodiments, the concentration of boron along the bonding interface is greater than 2×10 13  atoms/cm 2 . 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” “include,” “including” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” The word “coupled”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Likewise, the word “connected”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Moreover, as used herein, when a first element is described as being “on” or “over” a second element, the first element may be directly on or over the second element, such that the first and second elements directly contact, or the first element may be indirectly on or over the second element such that one or more elements intervene between the first and second elements. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     Moreover, conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” “for example,” “such as” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel apparatus, methods, and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. For example, while blocks are presented in a given arrangement, alternative embodiments may perform similar functionalities with different components and/or circuit topologies, and some blocks may be deleted, moved, added, subdivided, combined, and/or modified. Each of these blocks may be implemented in a variety of different ways. Any suitable combination of the elements and acts of the various embodiments described above can be combined to provide further embodiments. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.