# Patent

## Title
Etch stop layer for silicon (Si) via etch in three-dimensional (3-D) wafer-to-wafer vertical stack

## Abstract

A method of forming a silicon (Si) via in vertically stacked wafers is provided with a contact plug extending from selected metallic lines of a top wafer and an etch stop layer formed prior to the contact plug. Such a method comprises selectively etching through the silicon (Si) of the top wafer until stopped by the etch stop layer to form the Si via; depositing an oxide layer to insulate a sidewall of the Si via; forming a barrier layer on the oxide layer and on the bottom of the Si via; and depositing a conduction metal into the Si via to provide electrical connection between active IC devices located on vertically stacked wafers and an external interconnect.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of and claims the benefit of application Ser. No. 10/443,831 filed May 23, 2003 now U.S. Pat. No. 7,148,565, which is a divisional of application Ser. No. 10/077,822, filed Feb. 20, 2002, which issued as U.S. Pat. No. 6,645,832 on Nov. 11, 2003, the contents of which are incorporated herein by reference in their entirety.


TECHNICAL FIELD
The present invention relates to a semiconductor process and, more specifically, relates to a method of using Nickel Silicide (NiSi) as an etch stop layer for the silicon (Si) via etch in vertically stacked wafer processing in a three-dimensional (3-D) wafer-to-wafer vertical stack.
BACKGROUND
Integrated circuits (ICs) form the basis for many electronic systems. Essentially, an integrated circuit (IC) includes a vast number of transistors and other circuit elements that are formed on a single semiconductor wafer or chip and are interconnected to implement a desired function. The complexity of these integrated circuits (ICs) requires the use of an ever increasing number of linked transistors and other circuit elements.
Many modern electronic systems are created through the use of a variety of different integrated circuits; each integrated circuit (IC) performing one or more specific functions. For example, computer systems include at least one microprocessor and a number of memory chips. Conventionally, each of these integrated circuits (ICs) is formed on a separate chip, packaged independently and interconnected on, for example, a printed circuit board (PCB).
As integrated circuit (IC) technology progresses, there is a growing desire for a “system on a chip” in which the functionality of all of the IC devices of the system are packaged together without a conventional PCB. Ideally, a computing system should be fabricated with all the necessary IC devices on a single chip. In practice, however, it is very difficult to implement a truly high-performance “system on a chip” because of vastly different fabrication processes and different manufacturing yields for the logic and memory circuits.
As a compromise, various “system modules” have been introduced that electrically connect and package integrated circuit (IC) devices which are fabricated on the same or on different semiconductor wafers. Initially, system modules have been created by simply stacking two chips, e.g., a logic and memory chip, one on top of the other in an arrangement commonly referred to as chip-on-chip structure. Subsequently, multi-chip module (MCM) technology has been utilized to stack a number of chips on a common substrate to reduce the overall size and weight of the package, which directly translates into reduced system size.
Existing multi-chip module (MCM) technology is known to provide performance enhancements over single chip or chip-on-chip (COC) packaging approaches. For example, when several semiconductor chips are mounted and interconnected on a common substrate through very high density interconnects, higher silicon packaging density and shorter chip-to-chip interconnections can be achieved. In addition, low dielectric constant materials and higher wiring density can also be obtained which lead to the increased system speed and reliability, and the reduced weight, volume, power consumption and heat to be dissipated for the same level of performance. However, MCM approaches still suffer from additional problems, such as bulky package, wire length and wire bonding that gives rise to stray inductances that interfere with the operation of the system module.
An advanced three-dimensional (3D) wafer-to-wafer vertical stack technology has been recently proposed by researchers to realize the ideal high-performance “system on a chip”. In contrast to the existing multi-chip module (MCM) technology which seeks to stack multiple chips on a common substrate, 3-D wafer-to-wafer vertical stack technology seeks to achieve the long-awaited goal of vertically stacking many layers of active IC devices such as processors, programmable devices and memory devices inside a single chip to shorten average wire lengths, thereby reducing interconnect RC delay and increasing system performance.
One major challenge of 3-D wafer-to-wafer vertical stack integration technology is the bonding between wafers and between die in a single chip. In general, a dielectric layer is used to bond respective wafers. In the dielectric bonding method, high aspect ratio silicon (Si) via etch is required to provide electrical conductivity between vertically stacked wafers. Therefore, a metal bonding method is preferred to stack wafers. However, existing etch stop layers for silicon (Si) via etch in wafer stacking with metal bonding method can cause oxidation or metal recess which may negatively impact the power delivery system. Therefore, a need exists for an improved etch stop layer for the Si via etch in vertically stacked wafer processing (i.e., 3-D interconnect processing) which provides electrical conductivity between vertically stacked wafers.



BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of exemplary embodiments of the present invention, and many of the attendant advantages of the present invention, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:
FIG. 1 illustrates an example three-dimensional (3-D) wafer-to-wafer vertical stack forming a single chip;
FIG. 2 illustrates an example three-dimensional (3-D) wafer-to-wafer vertical stack according to an embodiment of the present invention;
FIG. 3 illustrates an example silicon (Si) via etch in an example three-dimensional (3-D) wafer-to-wafer vertical stack according to an embodiment of the present invention; and
FIG. 4 illustrates an example process of forming one or more silicon (Si) vias in an example three-dimensional (3-D) wafer-to-wafer vertical stack according to an embodiment of the present invention.



DETAILED DESCRIPTION
The present invention is applicable for use with all types of semiconductor wafers and integrated circuit (IC) devices, including, for example, MOS transistors, CMOS devices, MOSFETs, and new memory devices and communication devices such as smart card, cellular phone, electronic tags, gaming devices which may become available as semiconductor technology develops in the future. However, for the sake of simplicity, discussions will concentrate mainly on exemplary use of a simple three-dimensional (3-D) wafer-to-wafer vertical stack and an etch stop layer for metallic wafer-to-wafer and die-to-die bonding in such a three-dimensional (3-D) wafer-to-wafer vertical stack, although the scope of the present invention is not limited thereto.
Attention now is directed to the drawings and particularly to FIG. 1, an example three-dimensional (3-D) wafer-to-wafer vertical stack is illustrated. As shown in FIG. 1, the 3-D vertical stack (chip) 100 may comprise any number of active device polysilicon (Si) wafers, such as wafer #1 110 including, for example, one or more microprocessors; wafer #2 120 including one or more memory devices; and wafer #3 130 including one or more radio-frequency (RF) or optical communication devices. The bottom wafer 110 is typically thick to support the stacking of the top wafers 120 and 130, while the top wafers 120 and 130 are thinned to minimize interconnection lengths between wafers 110, 120 and 130.
In a typical 3-D vertical stack 100 shown in FIG. 1, the active device wafers 110, 120 and 130 are bonded using a dielectric layer 102, while all active layers on wafers 110, 120 and 130 are electrically connected using interwafer vias 104. However, the dielectric layer 102 can be cost-prohibitive for mass production. In addition, the interwafer vias 104 between adjacent wafers 110, 120 and 130 can be lengthy which can lead to interconnect RC delays in active IC devices.
As a result, a metal to metal bond can be used to stack wafers 110, 120 and 130 to form the vertical stack 100 shown in FIG. 1. This metal to metal bond method can serve not only as electrical connections to active IC devices on the vertically stacked wafers 110, 120 and 130 on a 3-D wafer-to-wafer vertical stack 100 but also bond adjacent wafers 110, 120 and 130. Dummy metal bonding pads can also be made to increase the surface area for wafer bonding and serve as auxiliary structures such as ground planes or heat conduits for the active IC devices. In addition, improved etch stop layers for the Si via etch can be used in vertically stacked wafer processing (i.e., 3-D interconnect processing) which provide more efficient electrical conductivity between vertically stacked wafers 110, 120 and 130.
Turning now to FIG. 2, an example three-dimensional (3-D) wafer-to-wafer vertical stack according to an embodiment of the present invention is illustrated. Specifically, FIG. 2 illustrates an example 2-wafer vertical stack 200 according to an embodiment of the present invention. However, the number of wafers in a vertical stack is not limited thereto. Through 3-D interconnect structure, wiring between vertically stacked wafers can be shortened resulting in a faster signal and minimal interconnect RC delays. In addition, the vertical stack can effectively integrate diverse process technologies on a single wafer process, such as, for example, logic/memory stacking, processor stacking, optical interconnect, system-on-chip, and RF interconnect.
As shown in FIG. 2, the bottom silicon (Si) wafer 210 contains an active device layer 212 supporting one or more active IC devices (not shown). Likewise, the top Si wafer 220 also contains an active device layer 222 supporting one or more active IC devices (not shown). The wafers 210 and 220 may be aligned using a standard alignment tool and bonded, via a metal bonding layer 106 deposited on opposing surfaces of the bottom wafer 210 and the top wafer 220 at designated bonding areas to establish electrical connections between active IC devices on vertically stacked wafers 210 and 220 and to bond adjacent wafers 210 and 220, while maintaining electrical isolation between bonding areas via an ILD layer 108. The top wafer 220 can also be thinned by either a Chemical Mechanical Polish (CMP), grinding, or Silicon (Si) wet etch process so as to be much more pliable than those of standard thickness and to allow for greater thickness variations across the vertically stacked wafers 210 and 220 for the same applied bonding pressure.
In the example 2-wafer vertical stack 200 shown in FIG. 2, the metal bonding process between adjacent wafers 210 and 220 may be performed in a vacuum or an inert gas environment, and a dielectric recess can be made surrounding the metal bonding areas to facilitate direct metal bonding between adjacent wafers 210 and 220 to ensure that the adjacent wafers 210 and 220 are bonded, while maintaining electrical isolation between the metal bonding areas. The metal bonding layer 106 may include a plurality of Copper (Cu) lines on opposing surfaces of the vertically stacked wafers 210 and 220 that can be used for metal diffusion bonding and serve as electrical contacts between active IC devices on the vertically stacked wafers 210 and 220. Copper (Cu) may be selected because of its low electrical resistivity, high electro-migration resistance and high diffusivity. However, other metallic materials can also be used, including, for example, tin, indium, gold, nickel, silver, palladium, palladium-nickel alloy, titanium, or any combination thereof.
After the wafer bonding and silicon (Si) thinning processes are completed, one or more interwafer vias 224 may be etched, via the top wafer 220, to establish electrical connections of active IC devices between vertically stacked wafers 210 and 220 and an external interconnect, via a C4 bump 226. Typically a tungsten “W” plug (metal contact layer) is formed during a standard W contact process to establish an interconnect with the copper (Cu) lines (the metal bonding layer 106) of the vertically stacked wafers 210 and 220. The “W” plug may then serve as an etch stop to stop the silicon (Si) via etch. In addition, existing barrier layers such as TaN/Ta (Tantalum Nitride/Tantalum), which form a barrier layer for the copper (Cu) interconnect, or TiN/Ti (Titanium Nitride/Titanium), which form an adhesion layer for the W plug, can be used to stop the silicon (Si) via etch during the Si via etching process. However, both the TaN/Ta and the TiN/Ti layers do not constitute a good etch stop layer because Si etch chemistry can easily etch them. As a result, the W plug can be recessed and Cu lines can be oxidized, which may negatively impact the power delivery system. Accordingly, NiSi (Nickel Silicide) has been advantageously utilized to serve as an etch stop layer for Si via etch in the vertically stacked wafers 210 and 220 to provide electrical conductivity between vertically stacked wafers 210 and 220. NiSi (Nickel Silicide) may be selected because of its high selectivity with the Si via etch and its low resistance to prevent any negative impact on the power delivery system. A several micron deep Si via etch can stop at a thin NiSi etch stop layer very easily. Another advantage of using NiSi as an etch stop layer is that, if NiSi is used as a Silicide material in transistors of IC active devices, then no additional process steps are required to form a NiSi layer.
Turning now to FIG. 3, an example silicon (Si) via etch in an example three-dimensional (3-D) wafer-to-wafer vertical stack 200 according to an embodiment of the present invention is illustrated. As shown in FIG. 3, a tungsten “W” contact plug 310 is formed during a standard W contact process. A nickel silicide (NiSi) etch stop layer 320 is formed before the W contact plug 310 is established and before the wafers 210 and 220 are bonded. The Si via etch can be stopped at the NiSi etch stop layer 320 to prevent etching through the W contact plug 310. Each of the interwafer vias 224 may contain an oxide layer 332 deposited on a sidewall, a copper (Cu) barrier/seed 334 deposited on the oxide layer 332 and the NiSi etch stop layer 320, and copper (Cu) 336 electroplated on the Si via 224 to serve as an electrical connection between active IC devices located on the vertically stacked wafers 210 and 220 and an external interconnect.
FIG. 4 illustrates an example process of forming one or more silicon (Si) vias in an example three-dimensional (3-D) wafer-to-wafer vertical stack according to an embodiment of the present invention. After the tungsten “W” contact plug 310 and NiSi etch stop layer 320 are formed at the top Si wafer 220, and the wafer-to-wafer bonding and silicon (Si) thinning processes are completed, interwafer vias 224 (see FIGS. 2-3) are patterned by conventional lithography and the silicon (Si) is etched using an etch mask until stopped by the NiSi etch stop layer 320 at block 410. After the interwafer vias 224 are etched, an oxide layer 332 as shown in FIG. 3 can be deposited in the Si vias 224 at block 420. Then anisotropic oxide etch is done to remove the oxide at the bottom of the Si vias 224 at block 430. After the anisotropic oxide etch, a barrier/seed layer 334 is then deposited on the oxide layer 332 and the bottom of the Si vias 224 at block 440. After the barrier/seed layer 334, copper (Cu) can then be deposited in the Si vias 224, via electroplating and Chemical Mechanical Polish (CMP), to establish electrical connections between active IC devices located on the vertically stacked wafers 210 and 220 and an external interconnect, via the C4 bump 226 shown in FIG. 2.
As described in this invention, the NiSi etch stop layer according to an embodiment of the present invention can effectively enable the silicon (Si) via etch to provide electrical conductivity between vertically stacked wafers without undesirable copper (Cu) oxidation or tungsten recess. If NiSi is already used as a silicide material in transistors of IC active devices, then no additional process steps may be required to form NiSi. The existing silicide material can be effectively used as an etch stop during the formation of one or more interwafer vias.
While there have been illustrated and described what are considered to be exemplary embodiments of the present invention, it will be understood by those skilled in the art and as technology develops that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. For example, other types of metallic materials that exhibit a high selectivity with the silicon (Si) via etch may also be used in lieu of NiSi (Nickel Silicide) to serve as an etch stop layer. Many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from the scope thereof. Therefore, it is intended that the present invention not be limited to the various exemplary embodiments disclosed, but that the present invention includes all embodiments falling within the scope of the appended claims.

## Claims

1. A method of forming a silicon (Si) via in vertically stacked wafers having a contact plug extending from selected metallic lines of a top wafer and an etch stop layer processed before the contact plug, comprising:
selectively etching a silicon (Si) layer of the top wafer until stopped by an etch stop layer comprised of a material selective to a silicon etch to form a Si via;
depositing an oxide layer to insulate a sidewall of the Si via;
forming a barrier/seed layer on the Si via; and
filling the Si via with a conduction metal to provide electrical connection between active IC devices located on the vertically stacked wafers and an external interconnect.

2. The method as claimed in claim 1, wherein the contact plug is comprised of tungsten (W).

3. The method as claimed in claim 1, wherein the vertically stacked wafers are bonded via respective metallic lines which serve as electrical contacts between active IC devices on the vertically stacked wafers.

4. A method, comprising:
forming an etch stop layer comprised of a material selective to a silicon etch on designated areas on a surface of a top wafer;
forming a contact plug layer on the etch stop layer on the designated areas on the surface of the top wafer;
forming vertically stacked wafers by bonding the top wafer with a bottom wafer, via respective metallic lines; and
forming an interwafer via on the top wafer by selectively etching through a silicon (Si) layer of the top wafer until stopped by the etch stop layer comprised of the material selective to the silicon etch, depositing an oxide layer to insulate a sidewall of the interwafer via, depositing a barrier/seed layer on the oxide layer and on the bottom of the interwafer via, and filling the interwafer via with a conduction metal to provide electrical connection between active IC devices located on the vertically stacked wafers and an external interconnect.

5. The method as claimed in claim 4, wherein the respective metallic lines are copper (Cu) lines deposited on opposing surfaces of the vertically stacked wafers to serve as electrical contacts between the active IC devices located on the vertically stacked wafers.

6. The method as claimed in claim 4, wherein the conduction metal deposited in the Si via is comprised of copper (Cu).

7. The method as claimed in claim 4, further comprising: thinning the top wafer so that the top wafer is thinner than the bottom wafer.

8. The method as claimed in claim 7, wherein forming an etch stop layer comprised of the material selective to the silicon etch comprises forming an etch stop layer comprised of nickel silicide (NiSi).

9. A method of forming a vertically stacked wafer structure, comprising:
providing a top wafer including an active layer and a dielectric layer, the active layer comprised of silicon;
forming an interconnect in the dielectric layer, wherein forming the interconnect comprises:
forming an etch stop layer at an end of the interconnect, the etch stop layer contacting the active layer and comprised of a material selective to a silicon etch;
providing a contact plug in contact with the etch stop layer;
forming a first metal line in contact with the contact plug;
forming a second metal line on a corresponding surface of a bottom wafer;
bonding the first metal line and the second metal line to establish metal bonding between the top wafer and the bottom wafer;
etching the active layer of the top wafer until stopped by the etch stop layer to form a Si via;
depositing an oxide layer to insulate a sidewall of the Si via; and
filling the Si via with a conduction metal to provide electrical connection between active IC devices located on the wafers and an external interconnect.

10. The method as claimed in claim 9, wherein providing the contact plug comprises providing a contact plug comprised of tungsten (W).

11. The method as claimed in claim 9, wherein the material selective to the silicon etch is NiSi.

12. The method as claimed in claim 9, further comprising:
prior to etching the active layer to form the Si via, thinning the top wafer by one of: Chemical Mechanical Polish (CMP), grinding, or a Si wet etch process.

13. The method as claimed in claim 9, further comprising:
subsequent to depositing the oxide layer and prior to filling the Si via with the conduction metal, forming a baffler/seed layer on the Si via.

14. The method as claimed in claim 13, wherein forming the baffler/seed layer on the Si via comprises depositing a copper (Cu) barrier/seed layer on the oxide layer and the etch stop layer.

15. The method as claimed in claim 9, wherein filling the Si via with the conduction metal comprises electroplating the Si via with copper (Cu).

16. The method as claimed in claim 9, wherein forming the first metal line in contact with the contact plug comprises forming a copper (Cu) metal line in contact with the contact plug.

17. The method as claimed in claim 9, wherein forming the first metal line in contact with the contact plug comprises forming a first metal line comprised of a material selected from the group consisting of: tin, indium, gold, nickel, silver, palladium, palladium-nickel alloy, and titanium.

18. The method as claimed in claim 9, further comprising:
providing a dummy metal bonding pad between the top wafer and the bottom wafer.

19. The method as claimed in claim 9, further comprising:
forming the external interconnect on the Si via.

20. The method as claimed in claim 9, wherein the active layer of the top wafer includes active devices associated with a first system, and the bottom wafer includes active devices associated with a second system, the systems differing and each system selected from the group consisting of: a logic system, a memory system, and a communications system.