Hybrid bonding mechanisms for semiconductor wafers

Abstract

A method of forming a hybrid bonding structure includes depositing an etch stop layer over surface of a substrate, wherein the substrate comprises a conductive structure, and the etch stop layer contacts the conductive structure. The method further includes depositing a dielectric material over the etch stop layer. The method further includes depositing a first diffusion barrier layer over the dielectric material. The method further includes forming an opening extending through the etch stop layer, the dielectric material and the diffusion barrier layer. The method further includes lining the opening with a second diffusion barrier layer. The method further includes depositing a conductive pad on the second diffusion barrier layer in the opening, wherein a surface of the first diffusion barrier layer is aligned with a surface of the conductive pad.

Claims

1. A method of forming a hybrid bonding structure, the method comprising: depositing a first etch stop layer over a surface of a substrate, wherein the substrate comprises a conductive structure, and the first etch stop layer contacts the conductive structure; depositing a dielectric material over the first etch stop layer; depositing a planarization stop layer over the dielectric material; forming an opening extending through the first etch stop layer, the dielectric material and the planarization stop layer; performing a deposition process to line the opening with a first diffusion barrier layer such that the first diffusion barrier layer is disposed over a top surface of the planarization stop layer and a length of sidewalls of the opening; etching the first diffusion barrier layer, wherein the etching removes the first diffusion barrier layer from the top surface of the planarization stop layer and tapers the first diffusion barrier layer to form a tapered corner of the first diffusion barrier layer near a corner of the opening wherein the tapered corner after the etching extends from the top surface of the planarization stop layer to a point coplanar with a bottom surface of the planarization stop layer; and depositing a conductive pad on the etched first diffusion barrier layer in the opening, wherein a surface of the planarization stop layer is aligned with a surface of the conductive pad.

2. The method of claim 1, wherein depositing the planarization stop layer comprises depositing SiN or SiON.

3. A method of forming a hybrid bonding structure, the method comprising: depositing an etch stop layer over a substrate, wherein the substrate comprises a conductive structure; depositing a dielectric material over the etch stop layer; forming an opening in the dielectric material, wherein the opening exposes the conductive structure; depositing a first barrier layer to line the opening; etching a first portion of the first barrier layer outside the opening and at a bottom surface of the opening, wherein the etching exposes the conductive structure and removes the first barrier layer from over a top surface of the dielectric material, wherein a second portion of the first barrier layer covers sidewalls of the opening and has a tapered corner after the etching; wherein the tapered corner after the etching extends from a top surface of a planarization stop layer deposited over the dielectric material to a point coplanar with a bottom surface of the planarization stop layer; depositing a second barrier layer, wherein the second barrier layer covers exposed surfaces of the second portion of the first barrier layer and covers the tapered corner of the first barrier layer; depositing a conductive material to fill space in the opening; and removing the conductive material and the second barrier layer outside the opening to form a conductive pad with the first barrier layer surrounding the second barrier layer and the conductive pad.

4. The method of claim 3, wherein depositing the first barrier layer comprises depositing the first barrier layer having a thickness in a range from about 0.001 m to about 10 m.

5. The method of claim 3, further comprising: depositing the planarization stop layer over the dielectric material prior to forming the opening in the dielectric material.

6. The method of claim 3, wherein depositing the first barrier layer comprises depositing the first barrier layer comprising SiN, SiON, TaN, TiN, or AN.

7. A method of forming a hybrid bonding structure, the method comprising: depositing an etch stop layer over a substrate, wherein the substrate comprises a conductive structure having a top surface; depositing a dielectric material and a planarization stop layer over the etch stop layer; forming an opening in the dielectric material, wherein the opening exposes the top surface of the conductive structure; depositing an underlying barrier layer in the opening; etching the underlying barrier layer to remove the underlying barrier layer from a bottom surface of the opening and to provide a tapered corner of the underlying barrier layer, wherein the tapered corner extends from a top surface of the planarization stop layer to a point in the opening coplanar with a bottom surface of the planarization stop layer; depositing another barrier layer to line the opening, wherein the another barrier layer forms an interface with the top surface of the conductive structure; filling the opening with a conductive material, wherein the conductive material is disposed over the interface of the another barrier layer and the top surface of the conductive structure; and removing the conductive material outside the opening, wherein removing the conductive material outside the opening comprises defining a top surface of the conductive material above a top surface of the dielectric material.

8. The method of claim 7, further comprising depositing the underlying barrier layer having a width in a direction parallel to a top surface of the substrate greater than about 0.01 microns (m).

9. The method of claim 8, wherein depositing the underlying barrier layer comprises depositing the underlying barrier layer covering a sidewall of the etch stop layer.

10. The method of claim 8, wherein depositing the underlying barrier layer comprises depositing the underlying barrier layer having a vertical portion and a horizontal portion.

11. The method of claim 7, wherein removing the conductive material outside the opening comprises exposing a top surface of the planarization stop layer co-planar with a top surface of the conductive material.

12. The method of claim 7, wherein filling the opening with the conductive material comprises filling the opening with a copper-containing material.

13. The method of claim 8, wherein depositing the underlying barrier layer comprises depositing a second dielectric material.

14. The method of claim 8, wherein depositing the underlying barrier layer comprises depositing TaN, TiN or AlN.

15. The method of claim 7, wherein depositing the another barrier layer comprises depositing the another barrier layer to have a thickness ranging from about 50 angstroms to about 1000 angstroms.

16. The method of claim 7, wherein depositing the etch stop layer comprises depositing a silicon carbide layer.

17. The method of claim 1, further comprising: after etching the first diffusion barrier layer and prior to depositing the conductive pad, depositing a second diffusion layer, wherein the second diffusion layer interposes the conductive pad and the conductive structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a cross-sectional view of a portion of a semiconductor wafer in accordance with an embodiment of the present disclosure;

(3) FIG. 2A shows a cross-sectional view of a bonded structure in accordance with some embodiments.

(4) FIG. 2B shows a cross-sectional view of a bonded structure with a mis-alignment, in accordance with some embodiments.

(5) FIG. 3 shows a cross-sectional view of a bonded structure, in accordance with some embodiments.

(6) FIGS. 4A-4E show cross-sectional views of a sequential process flow for forming a diffusion barrier layer encircling a top portion of a conductive pad, in accordance with some embodiments.

(7) FIGS. 5A and 5B show cross-sectional views of two wafers bonded together, in accordance with some embodiments.

(8) FIGS. 6A-6G show cross-sectional views of a sequential process flow for forming a diffusion barrier layer encircling a top portion of a conductive pad, in accordance with some embodiments.

(9) FIGS. 7A and 7B show top views of two neighboring conductive pads, in accordance with some embodiments.

(10) FIGS. 8A and 8B show cross-sectional views of two wafers bonded together, in accordance with some embodiments.

(11) FIGS. 9A-9F show cross-sectional views of a sequential process flow for forming a diffusion barrier layer encircling a conductive pad, in accordance with some embodiments.

(12) FIG. 10A shows a top view of a region of a wafer, in accordance with some embodiments.

(13) FIG. 10B shows a cross-sectional view of the region of the wafer in FIG. 10A, in accordance with some embodiments.

(14) FIG. 10C shows a top view of a region of the wafer in FIGS. 10A and 10B, in accordance with some embodiments.

(15) The figures illustrate aspects of the embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(16) The making and using of the embodiments of the present disclosure are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosure, and do not limit the scope of the disclosure.

(17) Hybrid bonding is one type of bonding procedure for 3DICs, wherein two semiconductor wafers are bonded together using a hybrid bonding technique. Some methods and structures of 3DICs formed by hybrid bonding are described in patent applications: U.S. Ser. No. 13/488,745, filed on Jun. 5, 2012, entitled, Three Dimensional Integrated Circuit Structures and Hybrid Bonding Methods for Semiconductor Wafers, and U.S. Ser. No. 13/542,507, filed on Jul. 5, 2012, entitled Hybrid Bonding Systems and Methods for Semiconductor Wafers. Both above-mentioned patent applications are incorporated herein by reference in their entireties.

(18) Referring to FIG. 1, a cross-sectional view of a portion of a semiconductor wafer 100 is shown in accordance with some embodiments. Two or more semiconductor wafers similar to wafer 100 illustrated may be coupled together vertically to form a 3DIC structure. The semiconductor wafer 100 includes a workpiece 102. The workpiece 102 includes a semiconductor substrate comprising silicon or other semiconductor materials and may be covered by an insulating layer, for example. The workpiece 102 may include silicon oxide over single-crystal silicon, for example Compound semiconductors, GaAs, InP, Si/Ge, or SiC, as examples, may be used in place of silicon. The workpiece 102 may include a silicon-on-insulator (SOI) or a germanium-on-insulator (GOI) substrate, as examples.

(19) The workpiece 102 may include a device region 104 formed proximate a top surface of the workpiece 102. The device region 104 includes active components or circuits, such as conductive features, implantation regions, resistors, capacitors and other semiconductor elements, e.g., transistors, diodes, etc. The device region 104 is formed over the workpiece 102 in a front-end-of-line (FEOL) process in some embodiments, for example. The workpiece 102 includes through-substrate vias (TSVs) 105 filled with a conductive material that provide connections from a bottom side to a top side of the workpiece 102, as shown in accordance with some embodiments.

(20) A metallization structure 106 is formed over the workpiece 102, e.g., over the device region 104 of the workpiece 102. The metallization structure 106 is formed over the workpiece 102 in a back-end-of-line (BEOL) process in some embodiments, for example. The metallization structure 106 includes conductive features, such as conductive lines 108, vias 110, and conductive pads 112 formed in an insulating material 114, which is a dielectric material. In some embodiments, the insulating material 114 is made of silicon oxide. In some embodiments, the insulating material 114 include multiple dielectric layers of dielectric materials. One or more of the multiple dielectric layers are made of low dielectric constant (low-k) material(s). In some embodiments, a top dielectric layer of the multiple dielectric layer is made of SiO.sub.2. The conductive pads 112 are contact pads or bond pads formed on a top surface of the semiconductor wafer 100, as shown in FIG. 1. Some vias 110 couple conductive pads 112 to conductive lines 108 in the metallization structure 106, and other vias 110, along with conductive metal lines 108, couple conductive pads 112 to the device region 104 of the workpiece 102. Vias 110 may also connect conductive lines 108 together in different metallization layers (not shown). The conductive features may include conductive materials typically used in BEOL processes, such as Cu, Al, W, Ti, TiN, Ta, TaN, or multiple layers or combinations thereof.

(21) In accordance with an embodiment, the conductive pads 112 are disposed proximate a top surface of the metallization structure 106 comprising Cu or a copper alloy, which is insulated from the insulating material 114 by a diffusion barrier (not shown). The metallization structure 106 may also include interconnect structures. The metallization structure 106 shown is merely for illustrative purposes. The metallization structure 106 may comprise other configurations and may include one or more conductive line and via layers, for example. Some semiconductor wafers 100 may have three conductive line and via layers, or four or more conductive line and via layers, as other examples.

(22) As mentioned above, two or more semiconductor wafers similar to wafer 100 illustrated are coupled together vertically to form a 3DIC structure. The semiconductor wafer 100 includes a workpiece 102. Region M of FIG. 1 will be used to illustrate various embodiments of hybrid bonding mechanisms. Region M includes a conductive pad 112 disposed above a via 110. The conductive pad 112 and the via 110 are surrounded by an insulating material 114.

(23) FIG. 2A shows a cross-sectional view of a bonded structure in accordance with some embodiments. The bonded structure is near region M of wafer 100. As described above, region M includes a conductive pad 112 disposed above a via 110. The conductive pad 112 and the via 110 are surrounded by an insulating material 114. The conductive pad 112 is filled with copper or copper alloy. According to one or more embodiments, the insulating material 114 is made of SiO.sub.2. FIG. 2A shows that conductive pads 112 and 152 are bonded to each and insulating materials 114 and 154 are bonded to each other, forming the bonded structure. Since the bonding involves dielectric material to dielectric material, and conductive material to conductive material, the wafer to wafer bonding is a hybrid bonding.

(24) Due to the concern of copper diffusion in SiO.sub.2, a barrier layer 113 is deposited to line the opening 111. The opening 111 is filled to form conductive pad 112. The barrier layer 113 separates the copper-containing conductive pad 112 from the insulating material 114. According to one or more embodiments, the barrier layer 113 is made of a copper diffusion barrier material. In some embodiments, barrier layer 113 is made of TaN. In some embodiments, barrier layer 113 has thickness in a range from about 10 to about 1000 .

(25) FIG. 2A shows that wafer 100 is bonded to wafer 150 with conductive pad 112 of wafer 100 being bonded to a conductive pad 152 of wafer 150. Conductive pad 152 is also made of copper or copper alloy. A barrier layer 153 is also used to separate conductive pad 152 from insulating material 154, which surrounds conductive pad 152 and via 156. The bonding of wafers 100 and 150 involves aligning the structures, such as conductive pads 112, and 152, on these two wafers to have conductive regions bonded to conductive regions, such as conductive pad 112 bonded to pad 152, and insulator regions bonded to insulator regions, such as insulating material 114 to insulating material 154. After wafer alignment, the wafers 100 and 150 are pressed together and temperature is raised to allow bonds to be formed between conductive layers and between insulating layers of these wafers. As mentioned above, detailed description of hybrid bonding is described in patent applications: U.S. Ser. No. 13/488,745, filed on Jun. 5, 2012, entitled, Three Dimensional Integrated Circuit Structures and Hybrid Bonding Methods for Semiconductor Wafers, and U.S. Ser. No. 13/542,507, filed on Jul. 5, 2012, entitled Hybrid Bonding Systems and Methods for Semiconductor Wafers. Both above-mentioned patent applications are incorporated herein by reference in their entireties.

(26) FIG. 2A shows that the surface of conductive pad 152 matches the surface conductive pad 112 and the copper-containing material(s) in both conductive pad 112 and 152 are enclosed by barrier layers 113 and 153. Conductive pads 112 and 152 do not come in contact with insulating materials 114 and 154.

(27) However, the alignment of wafer 100 to wafer 150 could be offset due to process variation. FIG. 2B a bonding structure similar to FIG. 2A with a misalignment of the bonding structure, in accordance with some embodiments. FIG. 2B shows that conductive pad 112 is shifted slightly in one direction (e.g., to the right) relative to conductive pad 152 due to alignment and/or process variations. As a result, portion of conductive pad 152 is exposed to insulating material 114 in region N and portion of conductive 112 is exposed to insulating material 154 in region O. Such exposure would result in copper diffusion, which could degrade device performance.

(28) FIG. 3 shows a cross-sectional view of a region of wafer 150.sub.1 being bonded to wafer 100.sub.1, in accordance with some embodiments. FIG. 3 shows the width W.sub.top1 of conductive pad 152.sub.1 is larger than the width W.sub.bot1 of conductive pad 112.sub.1. Although a center C.sub.top of conductive pad 152.sub.1 is aligned with a center C.sub.bot of conductive pad 112.sub.1 in FIG. 3, mis-alignment can occur due to alignment and/or process variation(s). To prevent copper diffusion, a diffusion barrier layer 160.sub.1 is formed on a surface of wafer 100.sub.1 and encircles a top portion of the conductive pad 112.sub.1. FIG. 3 shows that the diffusion barrier layer 160.sub.1 separates a surface of conductive pad 152.sub.1 not covered by conductive pad 112.sub.1 from the insulating material 114 to prevent copper diffusion.

(29) The diffusion barrier layer 160.sub.1 may be made of any type of material that blocks the diffusion of copper, such as SiN, SiON, TaN, TiN, AlN, etc. In some embodiments, diffusion barrier layer 160.sub.1 is made of polymers, such as benzocyclobutene (BCB) polymer dielectric, which can block copper diffusion. The materials for the function of copper diffusion barrier described above may be conductive, such as TaN, TiN, and AlN, or dielectric, such as SiN and SiON. For the embodiments shown in FIG. 3, the diffusion barrier layer 160.sub.1 should be dielectric to prevent shorting between neighboring conductive features. The diffusion barrier layer 160.sub.1 is bonded to the insulating material 154. The diffusion barrier layer used, such as layer 160.sub.1, should bond with the material on the other wafer. For example, diffusion barrier layer 160.sub.1 should bond with insulating material 154. SiN or SiON bonds with SiO.sub.2, which can be used of insulating material 154. TaN, TiN, and AlN also bond with SiO.sub.2. FIG. 3 also show an optional etch stop layer 121 in wafer 100.sub.1 and an optional etch stop layer 151 in wafer 150.sub.1.

(30) Since the diffusion barrier layer 160.sub.1 is only formed on wafer 100.sub.1, whose conductive pad 112.sub.1 is smaller than the opposing conductive pad 152.sub.1, there is a risk of copper diffusion if a portion of the smaller conductive pad 112.sub.1 is shifted to be outside a boundary of conductive pad 152.sub.1 due to mis-alignment. For a perfect alignment, the center C.sub.bot of conductive pad 112.sub.1 is aligned with the center C.sub.top of conductive pad 152.sub.1, as shown in FIG. 3. A distance from an edge to center of conductive pad 152.sub.1 is W.sub.top1/2. A distance from an edge to center of conductive pad 112.sub.1 is W.sub.bot1/2. If either of the wafers 100.sub.1 or 150.sub.1 shift by an amount greater than W.sub.top1/2W.sub.bot1/2, a portion of conductive pad 112.sub.1 is shifted to be outside the boundary of conductive pad 152.sub.1, which would result in copper diffusion. Therefore, W.sub.top1/2W.sub.bot1/2 should be greater than the alignment control limit. In some embodiments, W.sub.top1/2W.sub.bot1/2 is greater than about 0.01 m. In some embodiments, a diffusion barrier layer similar to diffusion barrier layer 160.sub.1 is also formed on the surface of wafer 150.sub.1. In such embodiments, diffusion barrier layer 160.sub.1 of wafer 100.sub.1 is bonded to another diffusion barrier layer on wafer 150.sub.1.

(31) FIGS. 4A-4E show cross-sectional views of a sequential process flow for forming a diffusion barrier layer 160.sub.1 over the surface of wafer 100.sub.1 and encircling the top portion of the conductive pad 112.sub.1, in accordance with some embodiments. FIG. 4A shows a portion of wafer 100.sub.1 with a via 110, which is embedded in a dielectric material 114.sub.L. As described above in FIG. 1, device region 104, TSVs 105, and other interconnection layers have been formed. To keep the description simplified, only via 110 is shown in FIG. 4A. After wafer 100.sub.1 of FIG. 4A is provided, an etch stop layer 121, a dielectric material (or layer) 114.sub.T, and a protective layer 160.sub.1 are sequentially deposited over the surface of wafer 100.sub.1, as shown in FIG. 4B in accordance with some embodiments. The etch stop layer 121 is a dielectric film and is used as an etch stop during an etching process for patterning an opening 111 of conductive pad 112.sub.1. In some embodiments, the etch stop layer 121 is made of SiC with a thickness in a range from about 10 to about 5000 . In some embodiments, the insulating material 114.sub.T is made of SiO.sub.2 with a thickness in a range from about 50 to about 10,000 . The protective layer 160.sub.1 is made of SiN or SiON with a thickness in a range from about 5 to about 1000 , in accordance with some embodiments.

(32) After the dielectric layers 121, 114.sub.T and 160.sub.1 are deposited, wafer 100.sub.1 is patterned and etched to form opening 111.sub.1, as shown in FIG. 4C in accordance with some embodiments. Opening 111.sub.1 has a width W.sub.1 in a range from about 0.1 m to about 50 m, in accordance with some embodiments. Opening 111.sub.1 exposes a top of via 110. After opening 111.sub.1 is formed, a barrier layer 113 is deposited to line opening 111.sub.1 and a conductive material 115, such as copper or copper alloy, for conductive pad 112.sub.1 is deposited to fill opening 111.sub.1. In some embodiments, the barrier layer 113 is made of TaN with a thickness in a range from about 50 to about 1000 . The TaN may be formed by physical vapor deposition (PVD) or other applicable methods. A thin copper seed layer (not shown) may be deposited on the barrier layer 113 to assist the formation of the conductive material (layer) of conductive pad 112.sub.1. The thin copper seed layer is deposited by PVD with a thickness in a range from about 10 to about 500 . The copper-containing conductive material 115 for pad 112.sub.1 is then deposited to fill opening 111.sub.1, as shown in FIG. 4D in accordance with some embodiments. The copper-containing conductive material 115 for pad 112.sub.1 is deposited by a plating process, in accordance with some embodiments. The copper seed layer is not shown in FIG. 4D because the copper seed layer merges with the copper-containing conductive material 115 that fills opening 111.sub.1.

(33) The copper-containing conductive material 115 and barrier layer 113 outside opening 111.sub.1 is then removed, such as by a chemical-mechanical polishing process, or an etching process. FIG. 4E shows a cross-sectional view of wafer 100.sub.1 after the removal process, in accordance with some embodiment. FIG. 4E shows that diffusion barrier layer 160.sub.1 is exposed after the removal process. The diffusion barrier layer 160.sub.1 can block diffusive copper, from wafer 150.sub.1 as described above, from reaching the device region (not shown) in wafer 100.sub.1.

(34) FIG. 5A shows a cross-sectional view of wafer 150.sub.2 being bonded to wafer 100.sub.2, in accordance with some embodiments. FIG. 5A shows the width W.sub.top2 of conductive pad 152.sub.2 is about the same as the width W.sub.bot2 of conductive pad 112.sub.2. Diffusion barrier layer 160.sub.T is formed on wafer 150.sub.2 and diffusion barrier layer 160.sub.B is formed on wafer 100.sub.2. Diffusion barrier layer 160.sub.B encircles and surrounds the conductive pad 112.sub.2. A portion of diffusion barrier layer 160.sub.B is deposited next to the etching stop layer 121.sub.B. Diffusion barrier layer 160.sub.T also encircles and surrounds the conductive pad 152.sub.2. A portion of diffusion barrier layer 160.sub.T is deposited next to the etching stop layer 121.sub.T. The diffusion barrier layers 160.sub.T and 160.sub.B are dielectric and may be made of materials chosen from the list of materials described for diffusion barrier layer 160.sub.1.

(35) Diffusion barrier layers 160.sub.T and 160.sub.B prevent copper diffusion resulting from misalignment of conductive pads 112.sub.2 and 152.sub.2. The thickness T of the diffusion barrier layer should be large enough to cover alignment variation. In some embodiments, the thickness T is greater than about 0.01 m, which is an alignment control limit for conductive pads 112.sub.2 and 152.sub.2.

(36) FIG. 5B shows a cross-sectional view of wafer 150.sub.2 being bonded to wafer 100.sub.2, in accordance with some embodiments. FIG. 5B is similar to FIG. 5A with the exception that there is no diffusion barrier layer 160.sub.T surrounding conductive pad 152.sub.2. Diffusion barrier layer 160.sub.B encircles and surrounds the conductive pad 112.sub.2. Diffusion barrier layer 160.sub.B prevents copper diffusion resulting from mis-alignment of conductive pads 112.sub.2 and 152.sub.2. The thickness T of the diffusion barrier layer 160.sub.B should be large enough to cover alignment variation. In some embodiments, the thickness T is greater than about 0.01 m, which is an alignment control limit for conductive pads 112.sub.2 and 152.sub.2.

(37) FIGS. 6A-6G show cross-sectional views of a sequential process flow for forming a diffusion barrier layer 160.sub.B over the surface of wafer 100.sub.2 and encircling the top portion of the conductive pad 112.sub.2, in accordance with some embodiments. FIG. 6A shows a portion of wafer 100.sub.2 with a via 110, which is embedded in an insulating material 114.sub.L. After wafer 100.sub.2 of FIG. 6A is provided, an etch stop layer 121 and an insulating material (or layer) 114.sub.T are sequentially deposited over the surface of wafer 100.sub.2, as shown in FIG. 6B in accordance with some embodiments.

(38) After the dielectric layers 121 and 114.sub.T are deposited, wafer 100.sub.2 is patterned and etched to form opening 111.sub.2, as shown in FIG. 6C in accordance with some embodiments. Opening 111.sub.2 has a width W.sub.2 in a range from about 0.1 m to about 50 m, in accordance with some embodiments. Opening 111.sub.2 exposes the top of via 110. After opening 111.sub.2 is formed, a barrier layer 113 is deposited to line opening 111.sub.2 and a conductive material, such as copper or copper alloy, for conductive pad 112.sub.2 is deposited to fill opening 111.sub.2. A thin copper seed layer (not shown) may be deposited on the barrier layer 113 to assist the formation of the conductive material (layer) of conductive pad 112.sub.2. The copper-containing conductive material for pad 112.sub.2 is then deposited to fill opening 111.sub.2. The copper-containing conductive material and barrier layer 113 outside opening 111.sub.2 is then removed, as shown in FIG. 6D in accordance with some embodiments.

(39) Afterwards, at least a portion of insulating material 114.sub.T surrounding conductive pad 112.sub.2 is removed, as shown in FIG. 6E, in accordance with some embodiments. In some embodiments, the entire insulating material 114.sub.T on wafer 100.sub.2 is removed. In some other embodiments, only the insulating material 114.sub.T surrounding conductive pad 112.sub.2 is removed. The insulating material 114.sub.T is removed by an etching process, which could be a dry process, such as a plasma etching process, or a wet etching process. In some embodiments, wafer 100.sub.2 is patterned to expose the areas of insulating material 114.sub.T to be removed prior to the removal process to protect (or cover) conductive pad(s) 112.sub.2 and the portion of insulating material 114.sub.T not intended to be removed. Openings 131.sub.2 are formed after the insulating material 114.sub.T or insulating material surrounding conductive pas 112.sub.2 is removed, as shown in FIG. 6E, in accordance with some embodiments.

(40) Afterwards, diffusion barrier layer 160.sub.B and insulating material 114.sub.T are deposited sequentially to fill openings 131.sub.2, as shown in FIG. 6F in accordance with some embodiments. The excess insulating material 114.sub.T and diffusion barrier layer 160.sub.T outside openings 131.sub.2 are moved, as shown in FIG. 6G in accordance with some embodiments.

(41) FIG. 7A shows a top view of two neighboring conductive pads 112.sub.2, in accordance with some embodiments. FIG. 7A shows that each conductive pad 112.sub.2 is surrounded by a barrier layer 113 and a ring of diffusion barrier layer 160.sub.B. The thickness of diffusion barrier layer 160.sub.B is T or T, as described above in FIGS. 5A and 5B respectively. The areas surrounding diffusion barrier layer 160.sub.B, such as region P, is made of insulating material 114.sub.T with diffusion barrier layer 160.sub.B underneath, as shown in FIG. 6G, in accordance with some embodiments. In some other embodiments, the diffusion barrier layer 160.sub.B underneath the insulating material 114.sub.T does not extend all the way to the next structure, such as to a neighboring conductive pad structure. FIG. 7B shows that the diffusion barrier layer 160.sub.B surrounding conductive pad 112.sub.2A only extend to region Q, which does not connect to the diffusion barrier layer 160.sub.B surrounding conductive pad 112.sub.2B, which also only extend to region R, in accordance with some embodiments. The width of the diffusion barrier layer 160B under insulating material 114T is T*. In some embodiments, the width T* is in a range from about 0.001 m to about 10 m.

(42) FIG. 8A shows a cross-sectional view of wafer 150.sub.3 being bonded to wafer 100.sub.3, in accordance with some embodiments. FIG. 8A shows the width W.sub.top3 of conductive pad 152.sub.3 is about the same as the width W.sub.bot3 of conductive pad 112.sub.3. Diffusion barrier layer 160.sub.T is formed on wafer 150.sub.3 and diffusion barrier layer 160.sub.B is formed on wafer 100.sub.3. Diffusion barrier layer 160.sub.B encircles and surrounds the conductive pad 112.sub.3. Diffusion barrier layer 160.sub.T also encircles and surrounds the conductive pad 152.sub.3. The diffusion barrier layers 160.sub.T and 160.sub.B are dielectric and may be made of a material(s) similar to diffusion barrier layer 160.sub.1.

(43) Diffusion barrier layers 160.sub.T and 160.sub.B prevent copper diffusion resulting from mis-alignment of conductive pads 112.sub.3 and 152.sub.3. The thickness T* of the diffusion barrier layers 160.sub.T and 160.sub.B should be large enough to cover alignment variation. In some embodiments, the thickness T* is greater than about 0.01 m, which is an alignment control limit for conductive pads 112.sub.3 and 152.sub.3. Diffusion barrier layers 160.sub.T and/or 160.sub.B can be a dielectric material or a conductive material. The conductive nature of diffusion barrier layers 160.sub.T and/or 160.sub.B would not cause shorting, as long as their widths are less than half of the distance between two neighboring conductive pads on the same substrate. As mentioned above, 160.sub.T and/or 160.sub.B may be made of materials, such as SiN, SiON, TaN, TiN, AlN, etc.

(44) FIG. 8B shows a cross-sectional view of wafer 150.sub.3 being bonded to wafer 100.sub.3, in accordance with some embodiments. FIG. 7B is similar to FIG. 7A with the exception that there is no diffusion barrier layer 160.sub.T surrounding conductive pad 152.sub.3. Diffusion barrier layer 160.sub.B encircles and surrounds the conductive pad 112.sub.3. Diffusion barrier layer 160.sub.B prevents copper diffusion resulting from mis-alignment of conductive pads 112.sub.3 and 152.sub.3. The thickness T* of the diffusion barrier layer 160.sub.B should be large enough to cover alignment variation. In some embodiments, the thickness T* is greater than about 0.01 m, which is an alignment control limit for conductive pads 112.sub.3 and 152.sub.3.

(45) FIGS. 9A-9F show cross-sectional views of a sequential process flow for forming a diffusion barrier layer 160.sub.B encircling conductive pad 112.sub.3, in accordance with some embodiments. FIG. 9A shows a portion of wafer 100.sub.3 with a via 110, which is embedded in an insulating material 114.sub.L. After wafer 100.sub.3 of FIG. 9A is provided, an etch stop layer 121, an insulating material (or layer) 114.sub.T, and a planarization stop layer 161 are sequentially deposited over the surface of wafer 100.sub.3, as shown in FIG. 8B in accordance with some embodiments. The planarization layer is made of SiN or SiON with a thickness in a range from about 0.001 m to about 50 m, in accordance with some embodiments. After these dielectric layers are deposited, wafer 100.sub.3 is patterned and etched to form opening 111.sub.3, also as shown in FIG. 8B in accordance with some embodiments. Opening 111.sub.3 has a width W.sub.3 in a range from about 0.1 m to about 50 m, in accordance with some embodiments. Opening 111.sub.3 exposes the top of via 110. After opening 111.sub.3 is formed, a diffusion barrier layer 160.sub.B is deposited to cover the substrate surface and to line opening 111.sub.3, as shown in FIG. 9C in accordance with some embodiments. The thickness of the diffusion barrier layer 160.sub.B deposited is in a range from about 0.001 m to about 10 m, in accordance with some embodiments.

(46) An etch process is performed afterwards to remove the diffusion barrier layer 160.sub.B on the surface of the wafer 100.sub.3 and also to taper the diffusion barrier layer 160.sub.B near corners 164, as shown in FIG. 9D in accordance with some embodiments. The tapered corners 164 make gap-fill of opening 111.sub.3 easier. Afterwards, barrier layer 113 and the conductive material 115 for the conductive pad 112.sub.3 are deposited sequentially to fill the remaining of opening 111.sub.3, as shown in FIG. 9E in accordance with some embodiment. A planarization process is then performed to remove the excess conductive material and barrier layer 113 outside opening 111.sub.3. The planarization layer 161 is used to prevent the underlying insulating material 114.sub.T from being removed excessively. After the excess conductive layers outside the opening 111.sub.3 are removed, the exposed planarization layer 161 is removed, in accordance with some embodiments. FIG. 9F shows a cross-sectional view of conductive pad 112.sub.3 with a diffusion barrier layer 160.sub.B formed next to and surrounding it, in accordance with some embodiments.

(47) FIG. 10A shows a top view of a region 1100 of a wafer 1000, in accordance with some embodiments. Region 1100 has two conductive pads 112, which are surrounded and encircled by a barrier layer 113 and a diffusion barrier layer 160. Diffusion barrier layer 160 is similar to diffusion barrier layer 160.sub.1 described above and has the function of blocking copper diffusion. In some embodiments, diffusion barrier layer 160 is made of polymers, such as benzocyclobutene (BCB) polymer dielectric, which can block copper diffusion.

(48) FIG. 10B shows a cross-sectional view of FIG. 10A cut along line XX, in accordance with some embodiments. FIG. 10B shows that conductive pads 112 are embedded in insulating material 114 and are surrounded by diffusion barrier layer 160 near the surface of wafer 1000. In some embodiments, the diffusion barrier layer 160 is originally in liquid form and is deposited by a spin-on process. The liquid-form material of diffusion barrier layer 160 undergoes polymerization to become solid-form diffusion barrier layer 160. In some embodiments, solvent of the liquid-form material of diffusion barrier layer 160 evaporates in the process of forming the solid-form diffusion barrier layer 160.

(49) FIG. 10C shows a top view of a region 1500 of wafer 1000, in accordance with some embodiments. FIG. 10C shows that the diffusion barrier layer 160 forms a band-region D surrounding conductive pads 112 that encircle a device region A. In some embodiments, region A has an array of photo sensors (or pixels). Region A is surrounded by a region B, which is made of oxide (SiO.sub.2), in accordance with some embodiments. Region C that surrounds the band-region D is also made of oxide, in accordance with some embodiments. In some embodiments, the band-region D surrounding region A follows the shape of region A. In the embodiment shown in FIG. 10C, the band-region D has a shape of rectangular band-shape. Wafer 1000 is bonded to another wafer with an identical arrangement of conductive pads, diffusion barrier layer 160 and photo sensors, in accordance with some embodiments.

(50) The conductive pads 112 shown above in FIGS. 7A, 7B, and 10A are in square and/or rectangular shapes. According to other embodiments, the conductive pads 112 are in other applicable shapes, such as circular, oval, etc.

(51) The embodiments of diffusion barrier layer described above provide mechanisms of forming a copper diffusion barrier layer to prevent device degradation for hybrid bonding of wafers. The diffusion barrier layer(s) encircles the copper-containing conductive pads used for hybrid bonding. The diffusion barrier layer can be on one of the two bonding wafers or on both bonding wafers.

(52) One aspect of this description relates to a method of forming a hybrid bonding structure. The method includes depositing an etch stop layer over surface of a substrate, wherein the substrate comprises a conductive structure, and the etch stop layer contacts the conductive structure. The method further includes depositing a dielectric material over the etch stop layer. The method further includes depositing a first diffusion barrier layer over the dielectric material. The method further includes forming an opening extending through the etch stop layer, the dielectric material and the diffusion barrier layer. The method further includes lining the opening with a second diffusion barrier layer. The method further includes depositing a conductive pad on the second diffusion barrier layer in the opening, wherein a surface of the first diffusion barrier layer is aligned with a surface of the conductive pad.

(53) Another aspect of this description relates to a method of forming a hybrid bonding structure. The method includes depositing an etch stop layer over a substrate, wherein the substrate comprises a conductive structure. The method further includes depositing a dielectric material over the etch stop layer. The method further includes forming an opening in the dielectric material, wherein the opening exposes the conductive structure. The method further includes depositing a first diffusion barrier layer to line the opening. The method further includes etching a portion of the first diffusion barrier layer outside the opening and at a bottom surface of the opening, wherein the etching exposes the conductive structure, wherein a portion of the first diffusion barrier layer covers sidewalls of the opening. The method further includes depositing a second diffusion barrier layer, wherein the second diffusion barrier layer covers exposed surfaces of the portion of the first diffusion barrier layer. The method further includes depositing a conductive material to fill space in the opening. The method further includes removing conductive material and the second diffusion barrier layer outside the opening to form a conductive pad with the first diffusion barrier layer surrounding the second diffusion barrier layer and the conductive pad.

(54) Still another aspect of this description relates to a method of forming a hybrid bonding structure. The method includes depositing an etch stop layer over a substrate, wherein the substrate comprises a conductive structure. The method further includes depositing a dielectric material over the etch stop layer. The method further includes forming an opening in the dielectric material, wherein the opening exposes the conductive structure. The method further includes depositing a first diffusion barrier layer to line the opening. The method further includes filling the opening with a conductive material. The method further includes removing the conductive material outside the opening. The method further includes removing at least a portion of the dielectric material to expose sidewalls of the first diffusion barrier layer. The method further includes depositing a second diffusion barrier layer in contact with the exposed sidewalls of the first diffusion barrier layer.

(55) Although embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present disclosure. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.