# Patent

## Title
Method and apparatus for increase strain effect in a transistor channel

## Abstract

Method of enhancing stress in a semiconductor device having a gate stack disposed on a substrate. The method utilizes depositing a nitride film along a surface of the substrate and the gate stack. The nitride film is thicker over a surface of the substrate and thinner over a portion of the gate stack.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation of U.S. application Ser. No. 10/707,842 filed Jan. 16, 2004, now U.S. Pat. No. 7,118,999 issued on Oct. 10, 2006, the disclosure of which is expressly incorporated by reference herein in its entirety.


BACKGROUND OF INVENTION
1. Field of the Invention
The invention generally relates to the enhancement of transistor performance, and more particularly to the enhancement of transistor performance by creating a desired stress in the transistor channel region.
2. Background of the Invention
As semiconductor devices shrink, carrier mobility has become a roadblock for improved device speed. Studies have shown that electron mobility can be increased significantly by creating tensile stress in a transistor channel, and hole mobility can be improved by creating compressive stress. Thus, to improve the characteristics of a semiconductor device, tensile and/or compressive stresses are created in the channel of the n-type devices (e.g., nFETs) and/or p-type devices (e.g., pFETs). Additionally, the higher the stress in the channel, the higher mobility improvement which may be realized.
The channel stress may be induced by a Si3N4 or nitride film deposited on top of the transistor. The induced stress in the channel has the same sign (tensile or compressive) with that of the nitride film. However, the induced channel stress is only a fraction of the nitride film in the magnitude. Typical stresses are about 12 GPa for tensile stresses, and about 12.5 GPa for compressive stresses, hence, the maximum strain effect is limited.
For example, a related art transistor having a stressed transistor channel includes a silicon substrate having a gate oxide formed on its upper surface. Next, a polysilicon gate is deposited on the gate oxide. Offset spacers or gate sidewalls are formed adjacent both sides of the polysilicon gate. The gate sidewalls may be used to form a proper ion implanted extension structure within the silicon substrate. Additionally, Si3N4, spacers are formed fully on each sidewall of the polysilicon gate. It should be noted that the Si3N4, spacers are typically attached to the gate sidewalls prior to ion implantation. Next, ion implantation is used to form source/drain regions within the silicon substrate on both sides of the polysilicon gate.
Further processing includes forming substrate salicide regions proximate to the source/drain regions and a gate salicide region formed on the polysilicon gate. The substrate and gate salicide regions may include CoSi or NiSi. The substrate and gate salicide regions are formed self-aligned to the source/drain regions and polysilicon gate regions.
Next, a highly stressed Si3N4 or nitride film is uniformly deposited over the silicon substrate, source and drain regions, offset spacers, and polysilicon gate with a same thickness. The stressed nitride film is deposited as a conformal layer, including being deposited over the sidewall spacers. The nitride film causes stress in the silicon substrate, including the transistor channel region through a mismatch in the crystal lattice structures of the silicon substrate and the nitride film. The induced stress in the transistor channel region is proportional and of the same sign (tensile or compressive) with the stress in the nitride film. The magnitude of the induced stress is a fraction of the stress in the nitride film.
It should be noted that in the device described above, the induced stress in the transistor channel region is a small fraction of the stress of the nitride film. Because the improvement in carrier mobility increases with increased stress magnitude in the transistor channel region, a method to produce higher stress in the transistor channel region would further improve transistor performance.
SUMMARY OF INVENTION
The invention includes a method of enhancing stress in a semiconductor device having a gate stack disposed on a substrate, including depositing a nitride film along a surface of the substrate and the gate stack, wherein the nitride film is thicker over a surface of the substrate and thinner over a portion of the gate stack.
The invention also includes a method of enhancing stress in a semiconductor device having a gate stack disposed on a substrate, including depositing a layer of nitride film over the gate stack and a surface of the substrate, and removing a portion of the gate stack and the nitride film deposited thereon.
The invention also includes a semiconductor device, having a silicon substrate, a gate stack disposed on the silicon substrate, a stressed nitride film disposed on the silicon substrate and the gate stack, wherein the stressed nitride film is thicker over the silicon substrate and thinner over a portion of the gate stack.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 2 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 3 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 4 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 5 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 6 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 7 illustrates a table showing stress vs. distance in a semiconductor device formed in accordance with the invention;
FIG. 8 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 9 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 10 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 11 illustrates a step in forming an embodiment in accordance with the invention;
FIG. 12 illustrates a step in forming an embodiment in accordance with the invention; and;
FIG. 13 illustrates a step in forming an embodiment in accordance with the invention.



DETAILED DESCRIPTION
Transistor performance may be enhanced by creating stress in the transistor channel region. Accordingly, a gate structure may be made so that an enhanced stress is induced in the transistor channel from a stressed nitride film. To achieve this enhanced strain effect, one embodiment of the invention includes removing the gate spacer from the wafer after salicide is formed and then depositing a non-conformal Si3N4 film including forming little or no nitride on gate sidewalls at the top of the gate stack. In this manner, enhancement of transistor performance is provided by creating a desired stress in the transistor channel region of the device. It should be noted that this method and apparatus to increase strain effect in a channel of a transistor works for both n-type or p-type devices. As such, the nitride may be made either tensile or compressive by suitably adjusting the deposition parameters as is well known in the art.
In other embodiments, this structure may be obtained by coating the wafer with spin-on material after a stressed nitride film is deposited. The stressed nitride film at the lower region on either side of the gate is protected by the spin-on material during subsequent material removal steps. In still another embodiment of the method, the enhanced stress structure may be achieved by using a CMP (chemical mechanical planarization) process to remove a nitride stack from the gate top region after a spin-on material has been applied. The latter technique may also be applicable to replacement gate processes.
Referring now to the figures, FIG. 1 shows a silicon substrate 12 having a gate oxide 14 formed on its upper surface. Next, a polysilicon gate 16 is formed on the gate oxide 14. Offset spacers or gate sidewalls 18 are formed on the side of the polysilicon gate 16. The offset spacers 18 are used to form a proper ion implanted extension structure within the silicon substrate 12.
FIG. 2 shows Si3N4 spacers 20 formed on each side of the polysilicon gate 16. The Si3N4 spacers 20 are attached to the offset spacers 18 prior to ion implantation. Next, ion implantation is used to form source/drain regions 22 within the silicon substrate 12 at either side of the polysilicon gate 16. For n-type transistors, a shallow and high-dose of arsenic ions, for example, may be used to form the source/drain regions. For the p-type transistors, a shallow and high dose of BF2 ions, for example, may be used to form the source/drain regions.
FIG. 3 shows the gate oxide 14 removed on either side of the Si3N4 spacers 20. Also shown are substrate salicide regions 24 formed proximate to the source/drain regions 22 and a gate salicide region 26 formed on the polysilicon gate 16. The substrate and gate salicide regions, 24 and 26, may include CoSi or NiSi. The substrate and gate salicide regions, 24 and 26, are formed self-aligned to the source/drain regions 22 and polysilicon gate regions 16.
FIG. 4 shows a semiconductor device before forming a stressed transistor channel having a silicon substrate 12 with a doped salicide region 24. On top of the silicon substrate 12 is a gate oxide 14, and on top of the gate oxide 14 is a polysilicon gate 16. Gate sidewalls 18 are formed on the sides of the polysilicon gate 16. A gate salicide region 26 is formed on the polysilicon gate 16 to complete the gate stack 19. Together, the polysilicon gate 16 and gate sidewalls 18 form the gate stack 19. For purposes of this description, the gate stack may also include, for example, the polysilicon gate 16, gate sidewalls 18, and gate salicide region 26. The structure thus described is formed in any conventional manner known to those of skill in the art.
As shown in FIG. 4, any Si3N4 spacer which may exist (referenced as numeral 20 in FIG. 2) is removed from gate sidewalls 18 using isotropic Si3N4 RIE (reactive ion etching) process, which is selective to CoSi and oxide. The Si3N4 spacer may be removed completely, reduced in size, or simply recessed using the RIE method. It should be noted that any process which removes or reduces the Si3N4 spacer from the top of the gate stack may be used, as should be known to those of skill in the art.
Referring to FIG. 5, a non-conformal Si3N4 or nitride film 32 is deposited on the transistor gate using PECVD (plasma enhanced chemical vapor deposition) process. A non-conformal Si3N4 film can be formed using a PECVD deposition method at relatively low temperature, which is desirable at this stage of wafer processing. Although PECVD is used as an example, it should be noted that any process capable of depositing a non-conformal nitride film may be used. In one implementation, the nitride film 32 should be deposited so that there is little or no Si3N4 on gate sidewalls 18. Because the nitride film 32 is absent or reduced on the gate sidewalls 18, stress concentrators or raisers are created due to the abrupt termination of the material, causing a higher stress to be induced in the transistor channel 34, as further described below.
FIG. 6 is a graph of the results of a numerical simulation of the stress, Sxx, in a transistor channel as a function of horizontal distance in microns along a device substrate for both before (solid line) and after (dashed line) removing a nitride cap in a region 5 nm below the gate oxide for a polysilicon gate 60 nm wide, where the nitride film has a stress, Sxx, of 2.0 GPa formed in the transistor channel which is greater than approximately 4.5×109 dynes/cm2 at about 5 nm below a gate oxide, and a 50 nm thick spacer, which may be one implementing feature of the invention. In another implementation, the enhanced stress in the transistor channel is greater than approximately 5.5×109 dynes/cm2 at about 5 nm below a gate oxide.
As shown by the graph of FIG. 6, the numerical simulation shows that stress in the corresponding transistor channel increases 39% when the Si3N4 stack is removed from the top region of the gate, especially when the Si3N4 is removed from gate sidewalls near the top of the gate stack.
Referring to FIGS. 7-8, another embodiment of transistor channel stress enhancement is shown where FIG. 7 illustrates a semiconductor device having a silicon substrate 12 with a gate oxide 14 thereon. The silicon substrate 12 has salicide region 24. A polysilicon gate 16 with gate sidewalls 18 is located on the gate oxide 14. A gate salicide region 26 is formed on top of the polysilicon gate 16.
In a variation to the device of FIG. 4, a Si3N4 spacer 36 is added to the device shown in FIG. 7. The spacer 36 of FIG. 7 occupies a small region on the surface of the silicon substrate 12 and gate sidewalls 18, as well as being formed on the lower region of the gate stack. That is, the spacer is not provided along the entire sidewall. Such a configuration minimizes the amount of Si3N4 formed near the top of the gate stack. The Si3N4 spacer 36 may be formed by various processes, such as, for example, recessing the Si3N4 spacer 36 using RIE process, which is selective to SiO.sub.2 and suicide. Any other process which may form a Si3N4 spacer towards the bottom of the gate stack may be used, as can be implemented by those of skill in the art.
Referring to FIG. 8, a non-conformal Si3N4 or nitride layer 38 is deposited on the device using, for example, a PECVD process so that there is little or no Si3N4 deposited on the gate sidewalls 18. Once again, by depositing a minimum amount of nitride on the gate sidewalls 18, the nitride structure terminates more abruptly creating stress concentrators or raisers, which enhance the stress formed in a transistor channel 40.
Although a PECVD process is used in the example, any process, such as, for example, high density plasma deposition, which may deposit a non-conformal nitride film may be used. Furthermore, any process which preferentially reduces the amount of nitride deposited on the gate sidewalls 18 may be used.
Referring to FIG. 9, another method of enhancing stress in a transistor channel is shown where a nitride film 28 deposited over a silicon substrate 12 having a gate oxide 14, Si3N4 spacers 20, sidewall spacers 18 and polysilicon gate 16 with a gate salicide region 26 on top. After the nitride film 28 has been deposited, a spin-on material 42, such as, for example, ARC (antireflection coating), oxide, or SILKTM (a combination of silicon and low k dielectric material) is applied.
By applying a material using a spin-on technique, the material is preferentially deposited in the low spots and leaves the high spots uncoated. Additionally, any material which may act as a resist when the nitride film 28 is subsequently removed may be a suitable spin-on material. Thus, the spin-on coating method leaves the high points of the nitride film 28 exposed for further processing. Other methods which preferentially deposit material in the low regions and leave the high region exposed may used to form film on top of the nitride film 28.
Referring to FIG. 10, the exposed portion of the nitride film 28 is removed through a Si3N4 RIE process so that the nitride film 28 is preferentially removed only from the top of the gate stack. By removing the exposed portions of the nitride film 28, the upper portion of the gate stack is released from the nitride film 28 while the top of the silicon substrate remains covered. Such selective removal of the nitride film 28 forms a transistor channel 44 having enhanced stress therein due to the silicon substrate 12 remaining covered. The nitride film 28 may be removed by any process which preferentially removes the nitride film 28 and leaves the spin-on material 42.
FIGS. 11-13 illustrate another embodiment by which a transistor channel having enhanced stress may be formed. Referring to FIG. 11, a semiconductor device like that shown in FIG. 9 is illustrated having a silicon substrate 12 with a gate oxide 14 thereon. The silicon substrate 12 has salicide regions. A polysilicon gate 16 with gate sidewalls 18 is located on the gate oxide 14. A gate salicide region 26 is formed on top of the polysilicon gate 16, all of which is known by those of skill in the art and Implemented in accordance with known practices. Additionally, a nitride coating 28 with exposed high spots and spin-on material 42 is shown. Alternatively, an oxide layer (HDP (high density plasma) oxide or BPSG (borophosphorosilicate glass)) may be applied leaving the high spots of the nitride film 28 exposed. Additionally, any material which may act as a resist when the nitride film 28 is subsequently removed may be a suitable spin-on material.
As shown in FIG. 12, the wafer is planarized using CMP to remove the exposed portion of the nitride film 28 and the top region of the gate stack. Accordingly, the silicon substrate 12 remains covered with nitride film 28 while the top of the gate stack is removed. By removing the top of the gate stack along with any nitride film 28 formed thereon, an enhanced stress transistor channel 46 is formed within the silicon substrate 12. Any other known method to planarize the wafer may be used to remove the top of the gate stack.
Finally, as shown in FIG. 13, the spin-on material 42 is removed, and a salicide gate region 48 is formed on the top of the gate stack. Any method which preferentially removes the spin-on material may used for this step. Once again, the resulting structure includes a substrate covered with a nitride film, while avoiding nitride film on an upper portion of a gate stack. It should be noted that the method illustrated in FIGS. 11-13 may be readily applicable to a replacement gate process.
While the invention has been described in terms of embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. For example, the invention can be readily applicable to bulk substrates.

## Claims

1. A method of enhancing stress in a semiconductor device having a gate stack disposed on a substrate, comprising:
depositing a nitride film along a surface of the substrate and the gate stack having sidewalls,
wherein the deposited nitride film is thicker over a surface of the substrate and thinner over a portion of the gate stack and is thinner over a portion of the sidewalls of the gate stack than over the portion of the same gate stack.

2. The method of claim 1, further comprising forming a spacer adjacent only a lower portion of the gate stack.

3. The method of claim 2, wherein forming the spacer comprises reducing a size of the spacer.

4. The method of claim 3, wherein the reducing of the size of the spacer comprises reactive ion etching.

5. The method of claim 1, wherein the nitride film is a non-conformal nitride film.

6. The method of claim 1, wherein the depositing of the nitride film comprises a plasma enhanced vapor deposition process.

7. The method of claim 1, wherein the deposition of the nitride film provides enhanced stress within a transistor channel.

8. A method of enhancing stress in a semiconductor device, comprising:
depositing a layer of nitride film over a gate stack and a surface of a substrate such that the nitride film is thinner over a portion of vertical sides of the gate stack than over the same gate stack; and
removing the nitride film over the gate stack to provide enhanced stress in a transistor channel under the gate stack,
wherein a gate is about 60 nm wide, a spacer is about 50 rim wide, and the nitride film provides a stress of about 2.0 GPa, the enhanced stress in the transistor channel is greater than approximately 4.5×109 dynes/cm2 at about 5 nm below a gate oxide, and
wherein, after the removing, a portion of the layer of nitride film remains on the surface of the substrate.

9. The method of claim 8, further comprising depositing a resist material on a surface of the nitride film over the substrate while leaving a surface of the nitride film proximate an upper portion of the gate stack exposed.

10. The method of claim 9, wherein the depositing of the resist material comprises depositing one of a spin-on material, an anti-reflection coating, an oxide film, and a low k material.

11. The method of claim 9, wherein the depositing of the resist material comprises depositing at least one of an oxide layer or a borophosphorosilicate glass on low spots and leaving high spots exposed.

12. The method of claim 8, wherein a gate is about 60 nm wide, a spacer is about 50 nm wide, and the nitride film provides a stress of about 2.0 GPa, the enhanced stress in the transistor channel is greater than approximately 4.5 ×109 dynes/cm2 at about 5 nm below a gate oxide.

13. The method of claim 8, further comprising depositing a salicide gate region on top of the gate stack.

14. A method of enhancing stress in a semiconductor device, comprising:
depositing a layer of nitride film over a gate stack and a surface of a substrate;
removing the nitride film on the gate stack to provide enhanced stress in a transistor channel under the gate stack;
depositing a resist material on a surface of the nitride film over the substrate while leaving a surface of the nitride film proximate an upper portion of the gate stack exposed; and
removing an upper portion of the gate stack and the nitride film disposed thereon,
wherein a gate is about 60 nm wide, a spacer is about 50 nm wide, and the nitride film provides a stress of about 2.0 GPa, the enhanced stress in the transistor channel is greater than approximately 4.5×109 dynes/cm2 at about 5 nm below a gate oxide.

15. The method of claim 14, wherein the removing of the upper portion of the gate stack and the nitride film disposed thereon comprises reactive ion etching.

16. The method of claim 14, wherein the removing of the upper portion of the gate stack and the nitride film disposed thereon comprises chemical mechanical polishing.

17. The method of claim 10, A method of enhancing stress in a semiconductor device, comprising:
depositing a layer of nitride film over a gate stack and a surface of a substrate such that the nitride film is thinner over a portion of vertical sides of the gate stack than over the same gate stack; and
removing the nitride film over the gate stack to provide enhanced stress in a transistor channel under the gate stack,
wherein a gate is about 60 nm wide, a spacer is about 50 nm wide, and the nitride film provides a stress of about 2.0 GPa, the enhanced stress in the transistor channel is greater than approximately 4.5×109 dynes/cm2 at about 5 nm below a gate oxide, and
further comprising depositing a resist material on a surface of the nitride film over the substrate while leaving a surface of the nitride film proximate an upper portion of the gate stack exposed,
wherein the depositing of the resist material comprises depositing one of a spin-on material, an anti-reflection coating, an oxide film, and a low k material, and
further comprising forming spacers at a lower portion of the sidewalls of the gate stack.

18. The method of claim 17, wherein the forming of the spacers includes forming the spacers along substantially all of the sidewall and etching the spacers to form spacers at the lower portion of the sidewalls.

19. A method of enhancing stress in a semiconductor device, comprising:
depositing a layer of nitride film over a gate stack and a surface of a substrate;
removing the nitride film over the gate stack to provide enhanced stress in a transistor channel under the gate stack; and
forming sidewalls only at a lower portion of the gate stack,
wherein, after the depositing, the deposited nitride film is thicker over an upper surface of the gate stack and thinner over a portion of side surfaces of the same gate stack.

20. A method of enhancing stress in a semiconductor device, comprising:
depositing a layer of nitride film over a gate stack and a surface of a substrate such that the nitride film is thinner over a portion of side surfaces of the gate stack than over the same gate stack; and
removing the nitride film over the gate stack to provide enhanced stress in a transistor channel under the gate stack,
wherein for a semiconductor device having a gate about 60 nm wide, a spacer about 50 nm wide, and a nitride film stress of about 2.0 GPa, the enhanced stress in the transistor channel is greater than approximately 5.5×109 dynes/cm2 at about 5 nm below a gate oxide, and
wherein, after the removing, a portion of the layer of nitride film remains on the surface of the substrate.

21. The method of claim 20, further comprising depositing a resist material on a surface of the nitride film over the substrate while leaving a surface of the nitride film proximate an upper portion of the gate stack exposed.

22. The method of claim 20, wherein the depositing of the resist material comprises depositing one of a spin-on material, an anti-reflection coating, an oxide film, and a low k material.

23. The method of claim 20, wherein the depositing of the resist material comprises depositing at least one of an oxide layer or a borophosphorosilicate glass on low spots and leaving high spots exposed.

24. A method of enhancing stress in a semiconductor device, comprising:
depositing a layer of nitride film over a gate stack and a surface of a substrate;
removing the nitride film on the gate stack to provide enhanced stress in a transistor channel under the gate stack;
depositing a resist material on a surface of the nitride film over the substrate while leaving a surface of the nitride film proximate an upper portion of the gate stack exposed; and
removing an upper portion of the gate stack and the nitride film disposed thereon,
wherein for a semiconductor device having a gate about 60 nm wide, a spacer about 50 nm wide, and a nitride film stress of about 2.0 GPa, the enhanced stress in the transistor channel is greater than approximately 5.5×109 dynes/cm2 at about 5 nm below a gate oxide.

25. The method of claim 24, wherein the removing of the upper portion of the gate stack and the nitride film disposed thereon comprises reactive ion etching.

26. The method of claim 24, wherein the removing of the upper portion of the gate stack and the nitride film disposed thereon comprises chemical mechanical polishing.

27. A method of enhancing stress in a semiconductor device, comprising:
depositing a layer of nitride film over a gate stack and a surface of a substrate such that the nitride film is thinner over a portion of side surfaces of the gate stack than over the same gate stack; and
removing the nitride film over the gate stack to provide enhanced stress in a transistor channel under the gate stack.,
wherein for a semiconductor device having a gate about 60 nm wide, a spacer about 50 nm wide, and a nitride film stress of about 2.0 GPa, the enhanced stress in the transistor channel is greater than approximately 5.5×109 dynes/cm2 at about 5 nm below a gate oxide, and
further comprising forming spacers at a lower portion of the sidewalls of the gate stack.

28. The method of claim 27, wherein the forming of the spacers includes forming the spacers along substantially all of the sidewall and etching the spacers to form spacers at the lower portion of the sidewalls.

29. A method of enhancing stress in a semiconductor device having a gate stack disposed on a substrate having a gate oxide formed on its upper surface, comprising:
forming spacers on sides of a polysilicon gate;
depositing a nitride film along a surface of the substrate and the gate stack such that the nitride film is thinner over a portion of the spacers than over the same gate stack;
applying a spin on material; and
removing a portion of the nitride film covering the gate stack and a top portion of the gate stack,
wherein the nitride film is thicker over a surface of the substrate and thinner over a portion of the same gate stack.