# Patent

## Title
Interface for metal gate integration

## Abstract

A metal oxide semiconductor field effect transistor (MOSFET) includes a semiconductor substrate and an interlayer dielectric (ILD) over the semiconductor substrate. A gate structure is formed within the ILD and disposed on the semiconductor substrate, wherein the gate structure includes a high-k dielectric material layer and a metal gate stack. One or more portions of a protection layer are formed over the gate stack, and a contact etch stop layer is formed over the ILD and over the one or more portions of the protection layer. The metal gate stack includes aluminum and the protection layer includes aluminum oxide.

## Background
This patent is a continuation-in-part of U.S. Ser. No. 13/796,576 filed Mar. 12, 2013, which is hereby incorporated by reference.


BACKGROUND
In advanced technology nodes of integrated circuit industry, high k dielectric material and metal are adopted to form a gate stack of a field-effect transistor (FET) such as a metal-oxide-semiconductor field-effect transistors (MOSFETs). Metal gate stacks are often planarized, such as by chemical-mechanical polishing (CMP), and then covered with a contact etch-stop layer (CESL). Additional processing operations are then performed, including forming an interlayer dielectric (ILD) above the metal gate stack, and then patterning and forming a contact through the ILD. As a result of these additional processing operations, it has been found that damage sometimes occurs to the metal gate stacks. For example, an acid solvent can reach the metal gate stack and adversely affect the gate stack, such as by forming a void or other defect. It is desired to maintain the integrity of the metal gate stack throughout the various processes.
It is also desired to maintain high production efficiency, adding to maintain or reduce the number of processing operations and processing tools used to fabricate devices such as MOSFETs. Therefore, an improved interface for a metal gate stack and a method making the same are needed to address the issues identified above.



BRIEF DESCRIPTION OF THE DRAWINGS
Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
FIGS. 1 and 3 are a flowchart of a method for making a semiconductor device having a metal gate stack constructed according to various aspects of the present disclosure.
FIGS. 2, 4, and 5 are sectional views of a semiconductor device having a metal gate stack at various fabrication stages constructed according to various aspects of the present disclosure in one or more embodiments.



DETAILED DESCRIPTION
It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact.
FIG. 1 is a flowchart of one embodiment of a dummy-gate method 100 for use in making a semiconductor device having a gate stack constructed according to aspects of the present disclosure. FIG. 2 provides a sectional view of one embodiment of a semiconductor structure 200 having a gate stack at various fabrication stages. The semiconductor structure 200 and the method 100 of making the same are collectively described with reference to FIGS. 1 and 2. It is understood that a dummy gate (or gate-last) process is described as an exemplary embodiment, which is not intended to be limiting except and unless as explicitly recited in the claims.
The method 100 begins at step 102 by providing a semiconductor substrate 202. The semiconductor substrate 202 includes silicon. Alternatively, the substrate 202 includes germanium or silicon germanium. In other embodiments, the substrate 202 may use another semiconductor material such as diamond, silicon carbide, gallium arsenic, GaAsP, AlInAs, AlGaAs, GaInP, or other proper combination thereof.
The semiconductor substrate also includes various doped regions such as n-well and p-wells formed by a proper technique, such as ion implantation. The semiconductor substrate 202 also includes various isolation features, such as shallow trench isolation (STI) 204, formed in the substrate to separate various devices. The formation of the STI may include etching a trench in a substrate and filling the trench by insulator materials such as silicon oxide, silicon nitride, or silicon oxynitride. The filled trench may have a multi-layer structure such as a thermal oxide liner layer with silicon nitride filling the trench. In one embodiment, the STI structure may be created using a process sequence such as: growing a pad oxide, forming a low pressure chemical vapor deposition (LPCVD) nitride layer, patterning an STI opening using photoresist and masking, etching a trench in the substrate, optionally growing a thermal oxide trench liner to improve the trench interface, filling the trench with CVD oxide, and using chemical mechanical planarization (CMP) to polish and planarize.
Continuing with the present embodiment, a high k dielectric material layer 212 is formed on the semiconductor substrate 202. The high-k dielectric layer 212 includes a dielectric material having the dielectric constant higher than that of thermal silicon oxide, about 3.9. The high k dielectric material layer 212 is formed by a suitable process such as ALD. Other methods to form the high k dielectric material layer include metal organic chemical vapor deposition (MOCVD), physical vapor deposition (PVD), UV-Ozone Oxidation or molecular beam epitaxy (MBE). In one embodiment, the high k dielectric material includes HfO2. Alternatively, the high k dielectric material layer includes metal nitrides, metal silicates or other metal oxides. In addition, an interfacial layer may be formed between the high-k dielectric material layer 212 and the substrate 202.
A polysilicon layer 220 is formed above the high-k dielectric material layer 212. The polysilicon layer 220 is formed by a proper technique, such as CVD. In one example, the polysilicon layer 220 is non-doped. In another example, the polysilicon layer 220 has a thickness between about 500 angstrom and about 1000 angstrom.
The polysilicon layer 220 is patterned, as shown in FIG. 2. Patterning the polysilicon layer 220 can be performed in various manners, such as by using a patterned mask, followed by an etch process. In the present embodiment, the gate dielectric 212 is also patterned. As a result, a dummy gate is formed, as shown in FIG. 2.
Execution proceeds to step 104, in which source and drain features 206 are formed in the substrate 202. In the present embodiment, a gate spacer 214 is formed on the sidewalls of the dummy gate 220. The gate spacer 214 includes one or more dielectric material, such as silicon oxide, silicon nitride, silicon oxynitride or combinations thereof. The source and drain 206 include doping species introduced to the semiconductor substrate 202 by a proper technique, such as ion implantation. In one embodiment, the gate stack is configured in the active region for an n-type field effect transistor (nFET), and the dopant of the source and drain is n-type dopant, such as phosphorus or arsenic. In another embodiment, the gate stack is configured in the active region for a p-type field effect transistor (pFET), and the dopant of the source and drain is p-type dopant, such as boron or gallium. In yet another embodiment, the source and drain 206 include light doped drain (LDD) and heavily doped features, as shown in FIG. 2.
The method 100 proceeds to step 106 by forming an interlayer dielectric (ILD) 230 on the substrate. The ILD 230 is deposited by a proper technique, such as CVD. The ILD 230 includes a dielectric material, such as silicon oxide, low k dielectric material or a combination. A chemical mechanical polishing (CMP) process may be applied thereafter to planarize the surface of the ILD 230.
The method 100 proceeds to step 108 by removing the polysilicon layer 220, resulting a gate trench. The polysilicon is removed by a suitable etching process, such as wet etch. At step 110, a metal gate stack is formed in the gate trench, which is discussed in greater detail below with reference to FIGS. 3-5.
Referring to FIG. 3, at step 112 a first metal layer 402 is formed having a proper work function. The first metal is also referred to as work function metal. For an nFET, the first metal 402 has a work function substantially equals to or less than about 4.4 eV, referred to as n metal. For a pFET, the first metal 402 has a work function substantially equals to or greater than about 4.8 eV, referred to as p metal. In the present example, an n metal work function (NWF) layer is formed, and includes tantalum or titanium aluminum. In another example, the p metal includes titanium nitride or tantalum nitride. The work function metal 402 is deposited by a suitable technique. In the present embodiment, the NWF is deposited by PVD.
At step 114, a blocking layer 404 is formed above the work function metal. Continuing with the example above, the blocking layer is a TiN layer. In the present embodiment, the TiN is deposited by PVD.
At step 116, a fill metal material 420 is deposited in the trench, above the blocking layer 404. In the present embodiment, the fill metal material 420 includes aluminum, although it is understood that other embodiments may be provided, such as aluminum copper alloy. The fill metal material 420 is deposited by a suitable technique, such as PVD or plating.
At step 118, the metal gate is planarized. In one embodiment, a CMP process is applied to remove the excessive metals, including any metals disposed on the ILD 230. The combination of the layers 402, 404, and 420 form a replacement metal gate for the trench left over from the polysilicon dummy gate 220 (FIG. 2). It is understood that additional layers may also be included in the replacement metal gate.
At step 119, a protection layer 422 is formed over the replacement metal gate. In the present embodiment, the protection layer 422 is a metal oxide layer, formed by combining metal from the replacement metal gate with oxygen. The layer 422 also can be formed by adding H2O2 or an oxidizing agent into the post-CMP cleaning solution (after, or part of, step 118). Yet another approach of forming layer 422 is to adjust the PH value of the post-CMP cleaning solution to less than 7. In other embodiments, the protection layer 422 may be another suitable layer, such as metal nitride.
Continuing with the present embodiment, the metal oxide protection layer 422 is formed in the same processing chamber (in-situ) as a later-formed contact etch stop layer (CESL), discussed below. In this way, forming the protection layer 422 is a pre-treatment to forming the CESL, and the processing steps used in the pre-treatment are consistent or applicable to those used when forming the CESL. It is understood that in other embodiments, the protection layer 422 can be performed in a different processing chamber (ex-situ) as the later-formed CESL.
For example, the device 400 is heated to a temperature of about 400° C., which in the present example, is the same temperature that the device would be preheated to for forming the CESL. Before the CESL is formed, one or more oxidation gases are introduced into the processing chamber. Example oxidation gases include O2, N2O, and O3. The oxidation gas reacts with the above-described layers 402, 404, and 420 that form the replacement metal gate. The result of the reaction is the metal oxide layer protection layer 422. As mentioned above, in one example, the replacement gate stack includes aluminum. In this example, the protection layer includes aluminum oxide (Al2O3).
At step 120, a capping layer is formed over the protection layer 422. In the present embodiment, a non-silane oxide layer 425 is deposited on the replacement metal gate, such as by chemical vapor deposition (CVD) or atomic layer deposition (ALD).
At step 122, a second ILD 430 is formed on the substrate. The ILD 430 is deposited by a proper technique, such as CVD or spin-on glass. The ILD 430 includes a dielectric material, such as silicon oxide, low k dielectric material or a combination. A chemical mechanical polishing (CMP) process may be applied thereafter to planarize the surface of the ILD 430.
At step 124, a contact 440 is formed in the second ILD 430, to the replacement metal gate. Contacts 442 are also formed in the second ILD 430 that extend down to the source/drain regions 206. In the present embodiment, the contacts 440 and 442 are formed by first forming a contact opening in the second ILD 430 by lithography and etching. For the contact 440, the etching proceeds to the CESL 425 and proceeds through the protection layer 422. However, and as shown in FIG. 5, portions of the protection layer exist, designated as portions 422a and 422b, on either side of the opening for contact 440. The openings for contacts 440, 442 are then filled, such as with a barrier layer (e.g., TiN), and a conductive plug (e.g., W).
Other processing steps may follow to form a functional circuit. For example, a silicide process is used for the contacts 442. Also, an interconnect structure is formed on the substrate and is designed to couple various transistors and other devices to form a functional circuit. The interconnect structure includes various conductive features, such as metal lines for horizontal connections and contacts/vias for vertical connections. The various interconnect features may implement various conductive materials including copper, tungsten and silicide. In one example, a damascene process is used to form copper-based multilayer interconnect structure.
The present disclosure is not limited to applications in which the semiconductor structure includes a field effect transistor, such as a metal-oxide-silicon (MOS) transistor, and may be extended to other integrated circuit having a metal gate stack. For example, the semiconductor structure 400 may include a dynamic random access memory (DRAM) cell, a single electron transistor (SET), and/or other microelectronic devices (collectively referred to herein as microelectronic devices). In another embodiment, the semiconductor structure 200 includes FinFET transistors. Of course, aspects of the present disclosure are also applicable and/or readily adaptable to other type of transistor, including single-gate transistors, double-gate transistors and other multiple-gate transistors, and may be employed in many different applications, including sensor cells, memory cells, logic cells, and others.
The present disclosure provides a structure, such as a metal oxide semiconductor field effect transistor, and methods of forming the same. According to one embodiment, a structure includes a semiconductor substrate and a first interlayer dielectric (ILD) over the semiconductor substrate. A gate structure is formed within the ILD and disposed on the semiconductor substrate, wherein the gate structure includes a high-k dielectric material layer and a metal gate stack. One or more portions of a protection layer are formed over the gate stack, and a contact etch stop layer is formed over the first ILD and over the one or more portions of the protection layer. In some embodiments, the metal gate stack includes aluminum and the protection layer includes aluminum oxide.
In one embodiment, a method of forming a semiconductor structure includes forming a metal gate stack over a substrate and forming a protection layer over the metal gate stack. An etch stop layer is deposited over the protection layer, and an interlayer dielectric is deposited over the etch stop layer. The method further includes forming a contact through the interlayer dielectric, through the etch stop layer, through the protection layer, and to the metal gate stack.
In another embodiment, a method of making a metal oxide semiconductor field effect transistor includes forming a high-k dielectric over a substrate and forming a polysilicon gate over the high-k dielectric. A first interlayer dielectric (ILD1) is deposited over the substrate and around the polysilicon gate. After depositing the ILD1, the polysilicon gate is removed to leave a gate trench. A metal gate stack is then formed in the gate trench, the metal gate stack including a plurality of metal layers. The ILD1 and the metal gate stack are planarized, and a protection layer is formed over the planarized metal gate stack. An etch stop layer is then formed over the ILD1 and the protection layer.
The foregoing has outlined features of several embodiments. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.

## Claims

1. A method of forming a semiconductor structure, comprising:
forming a metal gate stack over a substrate, wherein the metal gate stack includes a work function metal layer, a blocking layer, and a fill metal layer;
forming a protection layer over the metal gate stack such that the protection layer physically contacts the work function metal layer, the blocking layer, and the fill metal layer, wherein the protection layer is formed by adjusting a pH of a cleaning solution applied to the metal gate stack to be less than 7;
depositing an etch stop layer over the protection layer;
depositing an interlayer dielectric over the etch stop layer; and
forming a contact through the interlayer dielectric, through the etch stop layer, through the protection layer, and to the metal gate stack.

2. The method of claim 1, wherein the protection layer is a metal oxide.

3. The method of claim 2, wherein the protection layer is formed in-situ with the deposition of the etch stop layer.

4. The method of claim 2 further comprising:
forming source and drain regions in the substrate; and
forming source and drain contacts through the interlayer dielectric, through the etch stop layer, and to the source and drain regions, respectively.

5. The method of claim 2, wherein the metal gate includes aluminum and the protection layer includes aluminum oxide (Al2O3).

6. The method of claim 1, further comprising polishing the metal gate stack, the polishing including a post-polish clean.

7. The method of claim 1,
wherein forming the protection layer over the metal gate stack includes polishing the metal gate stack, the polishing including a post-polish clean; and
wherein the protection layer is formed by adjusting the pH of the cleaning solution during the post-polish clean to be less than 7.

8. The method of claim 1, wherein the work function metal layer is a p-type work function metal layer.

9. The method of claim 1, wherein the work function metal layer is a n-type work function metal layer.

10. The method of claim 1, further comprising:
forming a dummy gate stack over the substrate;
forming sidewall spacers on the dummy gate stack;
removing the dummy gate stack to for a trench between the sidewall spacers; and
wherein forming the metal gate stack over the substrate includes forming the metal gate stack in the trench between the sidewall spacers.

11. A method of making a metal oxide semiconductor field effect transistor, comprising:
forming a high-k dielectric over a substrate;
forming a polysilicon gate over the high-k dielectric;
depositing a first interlayer dielectric (ILD1) over the substrate and around the polysilicon gate;
after depositing the ILD1, removing the polysilicon gate to leave a gate trench;
forming a metal gate stack in the gate trench, the metal gate stack including a plurality of metal layers;
planarizing the ILD1 and the metal gate stack;
forming a protection layer over the planarized metal gate stack; and
forming an etch stop layer over the ILD1 and the protection layer
wherein the planarizing includes a post-polish clean, and
wherein the protection layer is formed by adjusting a pH of a cleaning solution during the post-polish clean to be less than 7.

12. The method of claim 11 wherein the protection layer is a metal oxide layer including oxygen and at least one metal from the plurality of metal layers.

13. The method of claim 12 wherein the at least one metal is aluminum, the protection layer includes aluminum oxide.

14. The method of claim 11, wherein the plurality of metal layers includes a work function metal layer, a blocking layer, and a fill metal layer.

15. The method of claim 14, wherein forming the protection layer over the planarized metal gate stack includes forming the protection layer directly on the work function metal layer, the blocking layer, and the fill metal layer.

16. A method comprising:
forming a metal gate stack over a substrate;
forming a protection layer over the metal gate stack, wherein forming the protection layer includes adjusting a pH of a cleaning solution applied to the metal gate stack to be less than 7 to form the protection layer;
forming an interlayer dielectric layer over the protection layer; and
forming a contact through the interlayer dielectric layer and the protection layer to the metal gate stack.

17. The method of claim 16, wherein the metal gate stack includes a work function metal layer, a blocking layer, and a fill metal layer; and
wherein forming the protection layer over the metal gate stack include forming the protection layer directly on the work function metal layer, the blocking layer, and the fill metal layer.

18. The method of claim 17, wherein the work function metal layer is a p-type work function metal layer.

19. The method of claim 17, wherein the work function metal layer is a n-type work function metal layer.

20. The method of claim 16, further comprising depositing an etch stop layer over the protection layer.