GATE STRUCTURE HAVING DESIGNED PROFILE

Abstract

Semiconductor structures are provided. The semiconductor structure includes a substrate and a gate structure formed over the substrate. In addition, a sidewall of the gate structure has a top portion having a first inclination, a middle portion having a second inclination, and a bottom portion having a third inclination, and the first inclination, the second inclination, and the third inclination are different from one another.

Claims

1. A semiconductor structure, comprising: a substrate; and a gate structure formed over the substrate; wherein a sidewall of the gate structure has a top portion having a first inclination, a middle portion having a second inclination, and a bottom portion having a third inclination, and the first inclination, the second inclination, and the third inclination are different from one another.

2. The semiconductor structure as claimed in claim 1, wherein the gate structure has a first width, a second width, and a third width, and wherein the first width is a width measured from an intersection of the top portion and the middle portion of the sidewall of the gate structure, the second width is a width measured from an intersection of the middle portion and the bottom portion of the sidewall of the gate structure, a third width is a width measured from a bottom surface of the gate structure, and the second width is larger than the first width.

3. The semiconductor structure as claimed in claim 2, wherein the second width is larger than the third width.

4. The semiconductor structure as claimed in claim 2, wherein a ratio of the second width to the first width is in a range from about 1.01 to about 1.51.

5. The semiconductor structure as claimed in claim 2, wherein a ratio of the second width to the third width is in a range from about 1.01 to about 1.44.

6. The semiconductor structure as claimed in claim 1, wherein an angle between middle portion and the bottom portion measured from an inner side of the gate structure is smaller than 180.

7. A semiconductor structure, comprising: a substrate; a metal gate structure formed over the substrate; and spacers formed on sidewalls of the metal gate structure, wherein the metal gate structure has a top portion, a middle portion, and a bottom portion, and the middle portion is wider than both the top portion and the bottom portion.

8. The semiconductor structure as claimed in claim 7, wherein the sidewall of the metal gate structure has a first inclination at the top portion, a second inclination at the middle portion, and a third inclination at the bottom portion, and the first inclination, the second inclination, and the third inclination are different from one another.

9. The semiconductor structure as claimed in claim 7, wherein a ratio of a height of the middle portion to a height of the bottom portion of the metal gate structure is in a range from about 1.5 to about 3.

10. The semiconductor structure as claimed in claim 7, wherein an angle between a bottom surface of the gate structure and a sidewall of the bottom portion of the gate structure is larger than 90.

11. The semiconductor structure as claimed in claim 7, wherein an angle between a sidewall of the middle portion and a sidewall of the bottom portion of the gate structure is in a range from about 120 to about 170.

12. The semiconductor structure as claimed in claim 7, wherein a ratio of a width of the middle portion to a width of the bottom portion is in a range from about 1.01 to about 1.44.

13. A semiconductor structure, comprising: a substrate; and a gate structure having curved sidewalls formed over the substrate; wherein each curved sidewall has a top portion, a middle portion, and a bottom portion, and an angle between the middle portion and the bottom portion of the curved sidewall measured from an inner side of the gate structure is smaller than 180.

14. The semiconductor structure as claimed in claim 13, wherein the top portion of the curved sidewall has a first inclination, the middle portion of the curved sidewall has a second inclination, and a bottom portion of the curved sidewall has a third inclination, and the third inclination is greater than the first inclination.

15. The semiconductor structure as claimed in claim 14, wherein the first inclination is greater than the second inclination.

16. The semiconductor structure as claimed in claim 13, wherein the gate structure has a first width at a top surface of the gate structure, a second width at a middle portion of the gate structure, and a third with at a bottom surface of the gate structure, and the second width is greater than both the first width and the third width.

17. The semiconductor structure as claimed in claim 16, wherein a ratio of the second width to the first width is in a range from about 1.01 to about 1.51.

18. The semiconductor structure as claimed in claim 16, wherein a ratio of the second width to the third width is in a range from about 1.01 to about 1.44.

19. The semiconductor structure as claimed in claim 13, wherein the gate structure comprises a gate dielectric layer, a work function metal layer, and a gate electrode layer.

20. The semiconductor structure as claimed in claim 13, wherein an angle between a bottom surface of the gate structure and a bottom portion of the curved sidewall is in a range from about 91 to about 91.8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted 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.

[0007] FIGS. 1A to 1M are cross-sectional representations of various stages of forming a semiconductor structure in accordance with some embodiments.

DETAILED DESCRIPTION

[0008] The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. 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. For example, 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 between the first and second features, such that the first and second features may not be in direct contact. 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.

[0009] Further, spatially relative terms, such as beneath, below, lower, above, upper and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0010] Embodiments of a semiconductor structure are provided in accordance with some embodiments of the disclosure. The semiconductor structure includes a metal gate structure. The metal gate is formed by a gate-last process, in which a dummy gate structure is formed and replaced by the metal gate structure afterwards. The profile of the dummy gate structure is designed to improve the electrical performance of the resulting metal gate structure.

[0011] FIGS. 1A to 1M are cross-sectional representations of various stages of forming a semiconductor structure in accordance with some embodiments. As shown in FIG. 1A, a substrate 102 is provided in accordance with some embodiments. Substrate 102 may be a semiconductor wafer such as a silicon wafer. Alternatively or additionally, substrate 102 may include elementary semiconductor materials, compound semiconductor materials, and/or alloy semiconductor materials. Examples of the elementary semiconductor materials may be, but are not limited to, crystal silicon, polycrystalline silicon, amorphous silicon, germanium, and/or diamond. Examples of the compound semiconductor materials may be, but are not limited to, silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide. Examples of the alloy semiconductor materials may be, but are not limited to, SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP.

[0012] In addition, substrate 102 may include structures such as doped regions, interlayer dielectric (ILD) layers, and/or conductive features. Furthermore, substrate 102 may further include single or multiple material layers to be patterned. For example, the material layers may include a silicon layer, a dielectric layer, and/or a doped poly-silicon layer.

[0013] In some embodiments, substrate 102 includes a plurality of isolation features 104, such as shallow trench isolation (STI) features or local oxidation of silicon (LOCOS) features. Isolation features 104 are configured to isolate various elements formed in and/or upon substrate 102.

[0014] A dummy gate dielectric layer 106, a dummy gate electrode layer 108, and a hard mask layer 110 are formed over substrate 102, as shown in FIG. 1B in accordance with some embodiments.

[0015] In some embodiments, dummy gate dielectric layer 106 is made of high-k dielectric materials, such as metal oxides, metal nitrides, metal silicates, transition metal-oxides, transition metal-nitrides, transition metal-silicates, or oxynitrides of metals. Examples of the high-k dielectric material include, but are not limited to, hafnium oxide (HfO.sub.2), hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), silicon nitride, silicon oxynitride, zirconium oxide, titanium oxide, aluminum oxide, hafnium dioxide-alumina (HfO.sub.2Al.sub.2O.sub.3) alloy, or other applicable dielectric materials. Dummy gate dielectric layer 106 may be formed by chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD), metal organic CVD (MOCVD), or plasma enhanced CVD (PECVD).

[0016] In some embodiments, dummy gate electrode layer 108 is a polysilicon layer. In some embodiments, hard mask layer 110 is made of silicon oxide, silicon nitride, silicon oxynitride, or silicon carbide. Hard mask layer 110 may be formed by CVD, PVD), ALD, HDPCVD, MOCVD, or PECVD.

[0017] Next, hard mask layer 110 is patterned to form a hard mask structure 112, as shown in FIG. 1C in accordance with some embodiments. Hard mask layer 110 may be patterned by a photolithography patterning processes. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. The etching processes may include dry etching, wet etching, and/or other etching methods (e.g., reactive ion etching).

[0018] After hard mask structure 112 is formed over dummy gate electrode layer 108, dummy gate electrode layer 108 and dummy gate dielectric layer 106 are etched to form a dummy gate structure 114 below hard mask structure 112. In some embodiments, a first etching process 116 is performed to form an upper portion 118 of dummy gate structure 114, and a second etching process 120 is performed to form a bottom portion 122 of dummy gate structure 114, as shown in FIGS. 1D and 1E. As shown in FIG. 1E, dummy gate structure 114 includes dummy gate electrode layer 108 and dummy gate dielectric layer 106 in accordance with some embodiments.

[0019] More specifically, an upper portion of dummy gate electrode layer 108 is etched during first etching process 116, as shown in FIG. 1D in accordance with some embodiments. In some embodiments, first etching process 116 is a dry etching process. In some embodiments, first etching process 116 includes using a first etching gas and a second etching gas. In some embodiments, the first etching gas is CF.sub.4, CH.sub.2F.sub.2, or CHF.sub.3. In some embodiments, the second etching gas is Cl.sub.2, HBr, or SF.sub.6. In some embodiments, the first etching gas is CF, and the second etching gas is Cl.sub.2.

[0020] In some embodiments, the flow rate of the first etching gas used in first etching process 116 is in a range from about 5 sccm to about 500 sccm. In some embodiments, the flow rate of the second etching gas used in first etching process 116 is in a range from about 5 sccm to about 150 sccm. In some embodiments, the volume ratio of the first etching gas to the second etching gas used in first etching process 116 is in a range of about 1.3 to about 8.3.

[0021] After upper portion 118 of dummy gate structure 114 is formed, a bottom portion of dummy gate electrode layer 108 and dummy gate dielectric layer 106 are etched during second etching process 120, as shown in FIG. 1E in accordance with some embodiments. In some embodiments, second etching process 120 is a dry etching process. In some embodiments, second etching process 120 includes using the same first etching gas and the second etching gas as those used in first etching process 116. In some embodiments, the flow rate of the first etching gas used in second etching process 120 is in a range from about 50 sccm to about 500 sccm. In some embodiments, the flow rate of the second etching gas used in second etching process 120 is in a range from about 10 sccm to about 50 sccm. In some embodiments, the volume ratio of the first etching gas to the second etching gas used in second etching process 120 is in a range of about 1.5 to about 8.5.

[0022] In some embodiments, the volume ratio of the first etching gas to the second etching gas used in first etching process 116 is smaller than the volume ratio of the first etching gas to the second etching gas used in second etching process 120. By adjusting the volume ratio of the first etching gas to the second etching gas used in first etching process 116 and second etching process 120, the profile of the resulting dummy gate structure 114 can be controlled.

[0023] For example, when the volume ratio of the first etching gas to the second etching gas used is relatively small (such as those used in first etching process 116), the inclination of the sidewall (e.g. the sidewall of upper portion 118) with respect to the top surface of substrate 102 will be relatively small. On the other hand, when the volume ratio of the first etching gas to the second etching gas used is relatively large (such as those used in second etching process 120), the inclination of the sidewall (e.g. the sidewall of bottom portion 122) with respect to the top surface of substrate 102 will be relatively large.

[0024] In addition, dummy gate structure 114 has a top width W.sub.t, a neck width W.sub.n, and a bottom width W.sub.b, as shown in FIG. 1E in accordance with some embodiments. Top width W.sub.t may be defined as the width of the top surface of dummy gate structure 114. Bottom width W.sub.b may be defined as the width of the bottom surface of dummy gate structure 114. Neck width W.sub.n may be defined as the largest width of dummy gate structure 114 measured from a position located between the top surface and the bottom surface of dummy gate structure 114.

[0025] In some embodiments, neck width W.sub.n is larger than both top width W.sub.t and bottom width W.sub.b. Dummy gate structure 114 having relatively large neck width W.sub.n enables the improvement of the electrical performance of the metal gate structure formed in the subsequent processes, and the details will be described later.

[0026] In some embodiments, a ratio of neck width W.sub.n to the top width W.sub.t is in a range from about 1.01 to about 1.51. When the ratio of neck width W.sub.n to the top width W.sub.t is too large, the relatively stronger adhesion between the metal gate and the sidewalls will result in positive device shift due to lowered resistance. When the ratio of neck width W.sub.n to the top width W.sub.t is too small, the relatively weaker adhesion between the metal gate and the sidewall will result in negative device shift due to higher resistance. In some embodiments, a ratio of neck width W.sub.n to the bottom width W.sub.b is in a range from about 1.01 to about 1.44. Similarly, when the ratio of neck width W.sub.n to the bottom width W.sub.b is too large, the resulting device will not perform as designed. When the ratio of neck width W.sub.n to the bottom width W.sub.b is too small, device becomes slower due to looser metal gate adhesion.

[0027] It should be noted that, although dummy gate structure 114 shown in FIG. 1E is divided into upper portion 118 and bottom portion 122, there is no actual interface between upper portion 118 and bottom portion 122. That is, the dotted line shown in FIG. 1E is merely for better understanding the concept of the disclosure, and the scope of the disclosure is not intended to be limiting.

[0028] Furthermore, a first distance D.sub.1 is measured from the top surface to the bottom surface of dummy gate structure 114, and first distance D.sub.1 may also be seen as the height of dummy gate structure 114. A second distance D.sub.2 is measured from the bottom surface of dummy gate structure 114 to the position from which neck width W.sub.n is measured. In some embodiments, a ratio of second distance D.sub.2 to first distance D.sub.1 is in a range from about 0.26 to about 0.39.

[0029] After dummy gate structure 114 is formed on substrate 102, a dielectric layer 116 is formed to cover dummy gate structure 114 over substrate 102, as shown in FIG. 1F in accordance with some embodiments. Dielectric layer 116 may be made of silicon nitride, silicon oxide, silicon carbide, silicon oxynitride, or other applicable materials. Dielectric layer 116 may be formed by chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD), metal organic CVD (MOCVD), or plasma enhanced CVD (PECVD).

[0030] Next, a dry etching process is performed to form spacers 119, as shown in FIG. 1G in accordance with some embodiments. In some embodiments, the dry etching process includes using an etching gas, and the etching gas comprises helium (He), argon (Ar), hydrogen bromide (HBr), nitrogen (N.sub.2), methane (CH.sub.4), tetrafluoromethane (CF.sub.4), monofluoromethane (CH.sub.3F), difluoromethane (CH.sub.2F.sub.2), trifluoromethane (CHF.sub.3), oxygen (O.sub.2), or a combination thereof.

[0031] After spacers 119 are formed, source and drain regions are formed in substrate 102 in accordance with some embodiment. More specifically, substrate 102 is recessed to form recesses 121, as shown in FIG. 1H in accordance with some embodiments. In some embodiments, recesses 121 are aligned with spacers 119. Recesses 121 may be formed by a dry etching process.

[0032] Afterwards, recesses 121 are further etched to form enlarged recesses 122, as shown in FIG. 1I in accordance with some embodiments. In some embodiments, enlarged recesses 122 are aligned with spacers 119. Enlarged recesses 122 may be formed by a wet etching process.

[0033] Next, a semiconductor material is filled in enlarged recesses 122 to form strained source and drain (SSD) structures 124, as shown in FIG. 1J in accordance with some embodiments. In some embodiments, silicon germanium (SiGe) is formed in enlarged recesses 122 of substrate 102 to form SiGe source and drain structures. Strained source and drain structures 124 may alternatively be referred to as raised source and drain regions.

[0034] In some embodiments, strained source and drain structure 124 are formed by an epitaxial (epi) process. The epi process may include a selective epitaxy growth (SEG) process, CVD deposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy, or other applicable epi processes. The epi process may use gaseous and/or liquid precursors, which may interact with the composition of substrate 102. The deposited semiconductor material may be different from substrate 102. Accordingly, a channel region is strained or stressed to improve carrier mobility of the device and enhance device performance.

[0035] In addition, strained source and drain structures 124 may be in-situ doped or undoped during the epi process. If strained source and drain structures 124 are undoped during the epi process, they may be doped in subsequent processes. For example, strained source and drain structure 124 may be doped by an ion implantation process, plasma immersion ion implantation (PIII) process, gas and/or solid source diffusion process, or other applicable processes. In addition, an annealing process may further be processed to strained source and drain structure 124, such as a rapid thermal annealing process. However, it should be noted that, although strained source and drain structures 124 are shown in FIGS. 1H to 1M, they are merely examples, and the source and drain regions of the disclosure are not limited to strained source and drain structures.

[0036] After strained source and drain structures 124 are formed, a contact etch stop layer (CESL) 126 is formed to cover dummy gate structure 114 over substrate 102, as shown in FIG. 1K in accordance with some embodiments. In some embodiments, contact etch stop layer 126 is made of silicon nitride, silicon oxynitride, and/or other applicable materials. Contact etch stop layer 126 may be formed by plasma enhanced CVD, low pressure CVD, ALD, or other applicable processes.

[0037] After contact etch stop layer 126 is formed, an inter-layer dielectric (ILD) layer 128 is formed on contact etch stop layer 126 over substrate 102 in accordance with some embodiments. Inter-layer dielectric layer 128 may include multilayers made of multiple dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, tetraethoxysilane (TEOS), phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), low-k dielectric material, and/or other applicable dielectric materials. Examples of low-k dielectric materials include, but are not limited to, fluorinated silica glass (FSG), carbon doped silicon oxide, amorphous fluorinated carbon, parylene, bis-benzocyclobutenes (BCB), or polyimide. Inter-layer dielectric layer 128 may be formed by chemical vapor deposition (CVD), physical vapor deposition, (PVD), atomic layer deposition (ALD), spin-on coating, or other applicable processes.

[0038] Afterwards, a polishing process is performed to inter-layer dielectric layer 128, as shown in FIG. 1K in accordance with some embodiments. In some embodiments, inter-layer dielectric layer 128 is planarized by a chemical mechanical polishing (CMP) process until the top surfaces of dummy gate structure 114 is exposed.

[0039] After the polishing process is performed, dummy gate structure 114 is replaced by a metal gate structure 130. More specifically, dummy gate structure 114 is removed to form trench 132, as shown in FIG. 1L in accordance with some embodiments. As described previously, dummy gate structure 114 has a relatively large neck width W.sub.n, compared to top width W.sub.t and bottom width W.sub.b. Therefore, trench 132 formed by removing dummy gate structure 114 also has a relatively large neck width, which is larger than the top width and the bottom width of trench 132.

[0040] Hard mask structure 112, dummy gate electrode layer 108, and dummy gate dielectric layer 106 may be removed by various etching processes. After hard mask structure 112, dummy gate electrode layer 108, and dummy gate dielectric layer 106 are removed, metal gate structure 130 is formed in trench 132, as shown in FIG. 1M in accordance with some embodiments.

[0041] In some embodiments, metal gate structure 130 includes a high-k dielectric layer 134, a work function metal layer 136, and a metal gate electrode layer 138. In some embodiments, high-k dielectric layer 134 is made of high k dielectric materials such as hafnium oxide (HfO.sub.2), hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), metal oxides, metal nitrides, metal silicates, transition metal oxides, transition metal nitrides, transition metal silicates, oxynitrides of metals, metal aluminates, zirconium silicate, zirconium aluminate, silicon oxide, silicon nitride, silicon oxynitride, zirconium oxide, titanium oxide, aluminum oxide, or hafnium dioxide-alumina (HfO.sub.2Al.sub.2O.sub.3) alloy.

[0042] Work function metal layer 136 is formed over high-k dielectric layer 134 in accordance with some embodiments. Work function metal layer 136 is tuned to have a proper work function. For example, if a P-type work function metal (P-metal) for a PMOS device is desired, P-type work function materials may be used. Examples of the P-type work function materials include, but are not limited to, titanium nitride (TiN), tungsten nitride (WN), tungsten (W), ruthenium (Ru), palladium (Pd), platinum (Pt), cobalt (Co), nickel (Ni), conductive metal oxides, and/or other applicable materials.

[0043] On the other hand, if an N-type work function metal (N-metal) for NMOS devices is desired, N-type metal materials may be used. Examples of the N-type work function materials include, but are not limited to, titanium aluminide (TiAl), titanium aluminium nitride (TiAlN), carbo-nitride tantalum (TaCN), hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta), aluminum (Al), metal carbides (e.g., hafnium carbide (HfC), zirconium carbide (ZrC), titanium carbide (TiC), aluminum carbide (AlC)), aluminides, and/or other applicable materials.

[0044] It should be noted that work function metal layer 136 may be a single layer or may include multiple layers made of various work function materials, although only one layer is shown in FIG. 1M, and the scope of the disclosure is not intended to be limiting.

[0045] Metal gate electrode layer 138 is formed over work function metal layer 136 in accordance with some embodiments. In some embodiments, metal gate electrode layer 138 is made of a conductive material, such as aluminum, copper, tungsten, titanium, tantulum, titanium nitride, tantalum nitride, nickel silicide, cobalt silicide, TaC, TaSiN, TaCN, TiAl, TiAlN, or other applicable materials. Gate dielectric layer 134, work function metal layer 136, and metal gate electrode layer 138 may be formed by any applicable process to any applicable thickness.

[0046] It should be noted that additional layers may be formed above and/or below gate dielectric layer 134, work function metal layer 136, and metal gate electrode layer 138, such as liner layers, interface layers, seed layers, adhesion layers, barrier layers, or the like. In addition, gate dielectric layer 134, work function metal layer 136, and metal gate electrode layer 138 may include one or more materials and/or one or more layers.

[0047] As described above, dummy gate structure 114 has a relatively large neck width W.sub.n, and therefore, trench 132 also has a relatively large neck width. The relatively large neck width of trench 132 enable the improvement of the adhesion between metal gate structure 130 and spacers 119. In addition, metal gate structure 130 also has a relatively large neck width in accordance with some embodiments.

[0048] As shown in FIG. 1M, metal gate structure 130 has a top surface, a bottom surface, and a curved sidewall. In some embodiments, an angle .sub.1 between the bottom surface and the curved sidewall is larger than 90. In some embodiments, angle .sub.1 between the bottom surface and the curved sidewall is in a range from about 91 to about 91.8. When angle .sub.1 between the bottom surface and the curved sidewall is too large, hot carrier effect may happen. When angle .sub.1 between the bottom surface and the curved sidewall is too small, metal void may be formed.

[0049] In addition, metal gate structure 130 has a top portion 130a, a middle portion 130b, and a bottom portion 130c, as shown in FIG. 1M in accordance with some embodiments. Top portion 130a of metal gate structure 130 may have a height H.sub.t. Middle portion 130b of metal gate structure 130 may have a height H.sub.m. Bottom portion 130c of metal gate structure 130 may have a height H.sub.b.

[0050] In some embodiments, height H.sub.t is substantially equal to the thickness of hard mask structure 112. In some embodiments, height H.sub.m is substantially equal to the thickness of upper portion 118 of dummy gate structure 114. In some embodiments, height H.sub.b is substantially equal to the thickness of bottom portion 122 of dummy gate structure 114. In some embodiments, a ratio of height H.sub.m of middle portion 130b to height H.sub.b of bottom portion 130c of metal gate structure 130 is in a range from about 1.5 to about 3. When the ratio of height H.sub.m to height H.sub.b is too large, metal void may be formed. When the ratio of height H.sub.m to height H.sub.b is too small, hot carrier effect may happen.

[0051] Furthermore, since metal gate structure 130 has top portion 130a, middle portion 130b, and bottom portion 130c, each curved sidewall of metal gate structure 130 also has a top portion 140a, a middle portion 140b, and a bottom portion 140c. In some embodiments, an angle .sub.2 between middle portion 140b and bottom portion 140c of the curved sidewall of metal gate structure 130 is smaller than 180. In some embodiments, angle .sub.2 between middle portion 140b and bottom portion 140c of the curved sidewall of metal gate structure 130 is in a range from about 120 to about 170.

[0052] As shown in FIG. 1M, top portion 140a of the curved sidewall, which may be seen as the curved sidewall of metal gate structure 130 at top portion 130a, has a first inclination. In addition, middle portion 140b of the curved sidewall, which may be seen as the curved sidewall of metal gate structure 130 at middle portion 130b, has a second inclination, and bottom portion 140c of the curved sidewall, which may be seen as the curved sidewall of metal gate structure 130 at bottom portion 130c, has a third inclination. In some embodiments, the first inclination, the second inclination, and the third inclination of the curved sidewall are different from one another.

[0053] Furthermore, metal gate structure 130 has a first width W.sub.1, a second width W.sub.2, and a third width W.sub.3. More specifically, an interface between top portion 130 and middle portion 130b of metal gate structure 130 has first width W.sub.1. That is, first width W.sub.1 is a width measured from an intersection of top portion 140a and middle portion 140b of the curved sidewall of metal gate structure 130.

[0054] In addition, an interface between middle portion 130b and bottom portion 130c of metal gate structure 130 has second width W.sub.2. That is, second width W.sub.2 is a width measured from an intersection of middle portion 140b and bottom portion 140c of the curved sidewall of metal gate structure 130. In addition, third width W.sub.3 is a width measured from a bottom surface of metal gate structure 130.

[0055] In some embodiments, second width W.sub.2 is larger than both first width W.sub.1 and third width W.sub.3. In some embodiments, a ratio of second width W.sub.2 to first width W.sub.1 is in a range from about 1.01 to about 1.51. When ratio of second width W.sub.2 to first width W.sub.1 is too large, a positive device shift may happen and the device will not performed as designed. When ratio of second width W.sub.2 to first width W.sub.1 is too small, a negative device shift may happen and the device will become slower. In some embodiments, a ratio of second width W.sub.2 to third width W.sub.3 is in a range from about 1.01 to about 1.44. When ratio of second width W.sub.2 to third width W.sub.3 is too large, a hot carrier effect may happen. When ratio of second width W.sub.2 to third width W.sub.3 is too small, metal void may be formed.

[0056] It should be noted that, although metal gate structure 130 shown in FIG. 1M is divided into top portion 130a, middle portion 130b, and bottom portion 130c, there is no actual interface between them. That is, the dotted lines shown in FIG. 1M are merely for better understanding the concept of the disclosure, and the scope of the disclosure is not intended to be limiting.

[0057] As described previously, dummy gate structure 114 has a relatively large neck width W.sub.n, and therefore trench 132 also has a relatively large neck width. The relatively large neck width of trench 132 enables metal gate structure 130 formed therein to have better adhesion with spacers 119. Therefore, the electrical performance of metal gate structure 130 may be improved.

[0058] In addition, when metal gate structure 130 is formed in trench 132 having the relatively large neck width, the resulting metal gate structure 130 may also have better uniformity. Therefore, the yield in forming the semiconductor structure including metal gate structure 130 is also improved.

[0059] Furthermore, the profile of dummy gate structure 114 is controlled by adjusting the amount of the first etching gas and the second etching gas used in first etching process 116 and second etching process 120 in accordance with some embodiments. That is, dummy gate structure 114 with a designed profile can be formed by in-situ adjusting the condition of the etching processes without using complicated and/or additional processes.

[0060] Embodiments for forming a semiconductor structure are provided. The semiconductor structure includes a metal gate structure. The metal gate structure is formed by forming a dummy gate structure and replacing the dummy gate structure by the metal gate structure afterwards. The profile of the dummy gate structure is designed, such that the electrical performance of the metal gate structure can be improved. In addition, no complicated processes are required.

[0061] In some embodiments, a semiconductor structure is provided. The semiconductor structure includes a substrate and a gate structure formed over the substrate. In addition, a sidewall of the gate structure has a top portion having a first inclination, a middle portion having a second inclination, and a bottom portion having a third inclination, and the first inclination, the second inclination, and the third inclination are different from one another.

[0062] In some embodiments, a semiconductor structure is provided. The semiconductor structure includes a substrate and a metal gate structure formed over the substrate. The semiconductor structure further includes spacers formed on sidewalls of the metal gate structure. In addition, the metal gate structure has a top portion, a middle portion, and a bottom portion, and the middle portion is wider than both the top portion and the bottom portion.

[0063] In some embodiments, a semiconductor structure is provided. The semiconductor structure includes a substrate and a gate structure having curved sidewalls formed over the substrate. In addition, each curved sidewall has a top portion, a middle portion, and a bottom portion, and an angle between the middle portion and the bottom portion of the curved sidewall measured from an inner side of the gate structure is smaller than 180.

[0064] The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. 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.