# Patent

## Title
Integrated circuits with resistors and methods of forming the same

## Abstract

A method of forming an integrated circuit includes forming at least one transistor over a substrate. The at least one transistor includes a first gate dielectric structure disposed over a substrate. A work-function metallic layer is disposed over the first gate dielectric structure. A conductive layer is disposed over the work-function metallic layer. A source/drain (S/D) region is disposed adjacent to each sidewall of the first gate dielectric structure. At least one resistor structure is formed over the substrate. The at least one resistor structure includes a first doped semiconductor layer disposed over the substrate. The at least one resistor structure does not include any work-function metallic layer between the first doped semiconductor layer and the substrate.

## Background
TECHNICAL FIELD
The present disclosure relates generally to the field of semiconductor devices, and more particularly, to integrated circuits with resistors and methods of forming the same.
BACKGROUND
The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed.
In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling-down also produces a relatively high power dissipation value, which may be addressed by using low power dissipation devices such as complementary metal-oxide-semiconductor (CMOS) devices.
During the scaling trend, various materials have been implemented for the gate electrode and gate dielectric for CMOS devices. Metal-oxide semiconductor (MOS) transistors have typically been formed with silicon gate electrodes. There has been a desire to fabricate these devices with a metal material for the gate electrode and a high-k dielectric for the gate dielectric.



BRIEF DESCRIPTION OF THE DRAWINGS
The present disclosure is 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 and are used for illustration purposes only. In fact, the numbers and dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
FIG. 1 is a flowchart of an exemplary method of forming an integrated circuit.
FIGS. 2A-2K are schematic cross-sectional views of an integrated circuit during various gate-last high-K metal gate fabrication stages.
FIG. 2L is a schematic cross-sectional view of another exemplary integrated circuit.
FIGS. 3A-3H are schematic cross-sectional views of an integrated circuit during other gate-last high-K metal gate fabrication stages.
FIGS. 4A-4F are schematic cross-sectional views of an integrated circuit during various gate-first high-K metal gate fabrication stages.



DETAILED DESCRIPTION
It is understood that the following descriptions provide many different embodiments, or examples, for implementing different features of the disclosure. 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 feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one feature's relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features.
Illustrated in FIG. 1 is a flowchart of an exemplary method of forming an integrated circuit. The integrated circuit may include various passive and active microelectronic devices, such as resistors, capacitors, inductors, diodes, metal-oxide-semiconductor field effect transistors (MOSFET), complementary MOS (CMOS) transistors, bipolar junction transistors (BJTs), laterally diffused MOS (LDMOS) transistors, high power MOS transistors, FinFET transistors, or other types of transistors. In some embodiments, the integrated circuit can include a mixed-signal circuit, a radio frequency (RF) circuit, other high-speed circuits, or any combinations thereof.
Referring now to FIG. 1, the method 100 of forming an integrated circuit can include forming at least one transistor over a substrate (block 110). The at least one transistor includes a first gate dielectric structure disposed over a substrate. A work-function metallic layer is disposed over the first gate dielectric structure. A metallic layer is disposed over the work-function metallic layer. A source/drain (S/D) region is disposed adjacent to each sidewall of the first gate dielectric structure. The method 100 also includes forming at least one resistor structure over the substrate (block 120). The at least one resistor structure includes a first doped silicon layer disposed over the substrate. The at least one resistor structure does not include any work-function metallic layer between the first doped silicon layer and the substrate.
In some embodiments, the method 100 can include forming a second gate dielectric structure between the substrate and the first doped silicon layer. The second gate dielectric structure directly contacts the first doped silicon layer. In other embodiments, the method 100 can include forming a silicide region disposed over the first doped silicon layer. In still other embodiments, the work-function metallic layer continuously extends along sidewalls of the conductive layer. In yet still other embodiments, the work-function metallic layer has a length which is substantially equal to that of the first gate dielectric structure. In still other embodiments, the method 100 can include forming a second doped silicon layer disposed over the conductive layer.
It is understood that FIG. 1 has been simplified for a better understanding of the concepts of the present disclosure. Accordingly, it should be noted that additional processes may be provided before, during, and after the method 100 of FIG. 1, and that some other processes may only be briefly described herein. It is also noted that though FIG. 1 shows the order of the blocks 110 and 120, the order shown in FIG. 1 is merely exemplary. In some embodiments, at least one process step can be shared for forming portions of the transistor and the resistor structure.
FIGS. 2A-2K are schematic cross-sectional views of an integrated circuit during various gate-last high-K metal gate fabrication stages. It is understood that FIGS. 2A-2K have been simplified for a better understanding of the concepts of the present disclosure. Accordingly, it should be noted that additional processes may be provided before, during, and after FIGS. 2A-2K, and that some other processes may only be briefly described herein.
Referring now to FIGS. 2A-2K, an integrated circuit 200 can be fabricated over a substrate 201. The integrated circuit 200 can include a transistor region 213 and a resistor region 215. The substrate 201 can be a silicon substrate doped with a P-type dopant, such as boron (a P-type substrate). Alternatively, the substrate 201 could be another suitable semiconductor material. For example, the substrate 201 may be a silicon substrate that is doped with an N-type dopant, such as phosphorous or arsenic (an N-type substrate). The substrate 201 may alternatively be made of some other suitable elementary semiconductor, such as diamond or germanium; a suitable compound semiconductor, such as silicon carbide, silicon germanium, indium arsenide, or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. Further, the substrate 201 could include an epitaxial layer (epi layer), may be strained for performance enhancement, and may include a silicon-on-insulator (SOI) structure.
Referring to FIG. 2A, an isolation structure, e.g., a shallow trench isolation (STI) feature 210, can be formed in the substrate 201. In some embodiments, the STI feature 210 can be formed by etching recesses (or trenches) in the substrate 201 and filling the recesses with a dielectric material. In some embodiments, the dielectric material of the STI feature 210 can include silicon oxide. In alternative embodiments, the dielectric material of the STI feature 210 may include silicon nitride, silicon oxy-nitride, fluoride-doped silicate (FSG), and/or a low-k dielectric material.
Referring again to FIG. 2A, a gate dielectric stack 220 can be formed over the substrate 201. In some embodiments, the gate dielectric stack 220 can include at least one interfacial dielectric layer and/or at least one high dielectric constant (high-k) layer. The interfacial dielectric layer may include a material such as silicon oxide, silicon nitride, silicon oxynitride, other gate dielectric materials, and/or combinations thereof. The high-k dielectric layer can have a material having a dielectric constant that is greater than a dielectric constant of SiO2, which is approximately 4. The high-k gate dielectric layer can have a thickness that is greater than the interfacial dielectric layer. In some embodiments, the high-k gate dielectric layer may include at least one of Al2O3, HfO, ZrO, ZrO2, ZrSiO, YO, Y2O3, LaO, La2O5, GdO, Gd2O5, TiO, TiO2, TiSiO, TaO, Ta2O5, TaSiO, HfErO, HfLaO, HfYO, HfGdO, HfAlO, HfZrO, HfTiO, HfTaO, HfSiO, SrTiO, ZrSiON, HfZrTiO, HfZrSiON, HfZrLaO, HfZrAlO, or any combinations thereof. In some embodiments, the interfacial dielectric layer and/or the high-k gate dielectric layer can be formed by, for example, a chemical vapor deposition (CVD) process, an atomic layer deposition (ALD) process, a thermal oxidation process, a furnace process, other deposition processes, or any combinations thereof.
Referring to FIG. 2B, at least one work-function metallic material 240 can be formed over the gate dielectric stack 220. In some embodiments, the work-function metallic material 240 can include at least one P-type work-function metallic material and/or at least one N-type work-function metallic material. In some embodiments, the P-type work-function metallic layer can include compositions such as ruthenium, palladium, platinum, cobalt, nickel, and conductive metal oxides, and/or other suitable materials. In other embodiments, the N-type metal material can include compositions such as hafnium, zirconium, titanium, tantalum, aluminum, metal carbides (e.g., hafnium carbide, zirconium carbide, titanium carbide, aluminum carbide), aluminides, and/or other suitable materials. In some embodiments, the work-function metallic material 240 can be formed, for example, by any suitable process, such as an ALD process, CVD process, a reduced-pressure CVD (RPCVD) process, a plasma enhanced CVD (PECVD) process, a metal organic CVD (MOCVD) process, or any combinations thereof.
In some embodiments, at least one diffusion barrier (not shown) can be formed between the gate dielectric stack 220 and the work-function metallic material 240. The diffusion barrier can be configured to prevent metallic ions of the work-function metal material 240 from diffusing into the gate dielectric stack 220. The diffusion barrier may comprise at least one material such as aluminum oxide, aluminum, aluminum nitride, titanium, titanium nitride (TiN), tantalum, tantalum nitride, other suitable material, and/or combinations thereof. In some embodiments, the diffusion barrier can be formed, for example, by any suitable process, such as an ALD process, CVD process, a physical vapor deposition (PVD) process, an RPCVD process, a PECVD process, an MOCVD process, a plating process, a sputtering process, or any combinations thereof.
Referring to FIG. 2C, a mask layer 245 can be formed over the work-function metallic material 240. The mask layer 245 can cover the transistor region 213 and have an opening 245a exposing at least a portion of the work-function metallic material 240 in the resistor region 215. In some embodiments, the mask layer 245 can be made of, for example, a photoresist material and/or a dielectric material, e.g., silicon oxide, silicon oxynitride, silicon nitride, silicon carbide, other dielectric materials, or any combinations thereof.
Referring to FIG. 2D, an etch process 246 uses the mask layer 245 as an etch mask to remove the exposed work-function metallic material 240 (shown in FIG. 2C), exposing a portion of the gate dielectric stack 220. The etch process 246 can include a dry etch process, a wet etch process, or any combinations thereof. After the mask layer 245 is removed, the remaining work-function metallic material 240a is exposed as shown in FIG. 2E. In some embodiments, the etch process can also remove a portion of the gate dielectric stack 220, such that a portion of the substrate 201 is exposed.
Referring to FIG. 2F, a semiconductor material 250 can be disposed over the remaining work-function metallic material 240a and the exposed portion of the gate dielectric stack 220. In some embodiments, the semiconductor material 250 can be made of, for example, silicon, amorphous silicon, crystalline silicon, or any combinations thereof. The semiconductor material 250 may be formed by chemical vapor deposition (CVD) or any suitable technique. In some embodiments, the semiconductor material 250 can be doped, e.g., in-situ doped, or undoped.
Referring to FIG. 2G, a patterning process can be performed to pattern the semiconductor material 250, the remaining work-function metallic material 240a and the gate dielectric stack 220 (shown in FIG. 2F). By the patterning process, gate dielectric structures 220a and 220b can be formed over the substrate 201, which are disposed in the transistor region 213 and the resistor region 215, respectively. A work-function metallic layer 240b can be disposed over the gate dielectric structure 220a. A semiconductor (e.g. poly-silicon) layer 250a can be disposed over the work-function metallic layer 240b. A semiconductor layer 250b can directly contact the gate dielectric structure 220b. 
Referring to FIG. 2H, at least one implantation process 255 can be performed to implant dopants into the substrate 201 to form a source/drain (S/D) region 270 adjacent to each sidewall of the gate dielectric structure 220a. In some embodiments, the implantation process 255 can include a lightly doped drain (LDD) implantation and/or a heavy S/D implantation. In other embodiments, a thermal process and/or a rapid thermal process (RTP) can be performed to activate dopants in the S/D regions 270.
In some embodiments, the implantation process 255 can dope the S/D regions 270 as well as the semiconductor layer 250b. In other embodiments, doping the S/D regions 270 and the semiconductor layer 250b can be separated. For example, the semiconductor layer 250b can be in-situ doped as described above in conjunction with FIG. 2F. In other embodiments, the top surface of the semiconductor layer 250b can be covered by a mask layer (not shown) while the implantation process 255 is performed. Thereafter, a separate implantation process can be performed to implant dopants into the semiconductor layer 250b. 
In some embodiments forming LDD regions, a lightly doped drain implantation is performed to form a LDD region (not shown) in portions of the substrate 201 adjacent to each side of the gate dielectric structure 220a. The LDD regions may be formed by an ion implantation process and/or a diffusion process. N-type dopants, such as phosphorus or arsenic, may be used to form an NMOS device, and P-type dopants, such as boron, may be used to form a PMOS device.
After the formation of the LDD regions, spacers 265 and 267 can be formed on sidewalls of the silicon layer 250a and 250b, respectively. The spacers 265 and 267 can be formed by using a deposition process and an etching process (for example, an anisotropic etching process). The spacers 265 and 267 can include a suitable dielectric material, such as silicon nitride, silicon oxide, silicon carbide, silicon oxynitride, or combinations thereof. Thereafter, the S/D regions 270 can be formed by an ion implantation process and/or a diffusion process. N-type dopants, such as phosphorus or arsenic, can be used to form an NMOS device, and P-type dopants, such as boron, can be used to form a PMOS device.
In some embodiments, the integrated circuit 200 can include at least one stressor (not shown) in each the S/D region 270. The stressor can provide a stress to a channel (not labeled) of the transistor. The stressed channel can modify a mobility of carriers therein so as to change electrical characteristics, e.g., electrical current, of the transistor. In some embodiments, the stressor in each S/D region 270 can be referred to as a raised S/D region. In some embodiments, the stressor can be formed by an epitaxial process, a reduced-pressure CVD (RPCVD) process, a molecular beam epitaxy (MBE) process, a chemical vapor deposition (CVD) process, a metal-organic chemical vapor deposition (MOCVD) process, an atomic layer deposition (ALD) process, a multi-layer epitaxial process, or any combinations thereof.
In some embodiments forming a P-type transistor, the stressor can include a single SiGe or SixGe1-x layer, a multi-layer SiGe or SixGe1-x structure, an epitaxial structure, a compound material structure, other materials that can modify the carrier mobility of the transistor, or any combinations thereof. In other embodiments forming an N-type transistor, the stressor can include a single SiC or SixC1-x layer, a multi-layer SiC or SixC1-x structure, an epitaxial structure, a compound material structure, other materials that can modify the carrier mobility of the transistor, or any combinations thereof.
In some embodiments, a silicide region (not shown) can be formed on each of the S/D regions 270, the semiconductor layers 250a and/or the semiconductor layer 250b. the silicide region may comprise at least one material such as nickel silicide (NiSi), nickel-platinum silicide (NiPtSi), nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridium silicide (IrSi), erbium silicide (ErSi), cobalt silicide (CoSi), other suitable materials, or any combinations thereof.
It is noted that the formation of the silicide region (not shown) on the semiconductor layer 250b can be optional. In some embodiments, the formation of the silicide region on the semiconductor layer 250b can be carried out by the process for forming the silicide regions in the S/D regions 270. In other embodiments, the silicide region is not formed on the top surface of the semiconductor layer 250b. For example, the top surface of the semiconductor layer 250b can be covered with a dielectric layer, e.g., a nitride layer, while the salicidation process is performed. By optionally forming the silicide region on the semiconductor layer 250b, the resistance of the silicon layer 250b can be adjusted.
Referring to FIG. 2I, at least one dielectric layer, e.g., a dielectric layer 275, can be disposed over the substrate 201 and around the spacers 265 and 267. In some embodiments, the dielectric layer 275 can include at least one material such as phosphorous-doped silicate glass (PSG), boron-doped silicate glass (BSG), boron-phosphorous-doped silicate glass (BPSG), undoped silicate glass (USG), silicon oxide, silicon nitride, silicon oxynitride, low-k dielectric material, other dielectric materials, or combinations thereof.
In some embodiments, the top surface of the semiconductor layer 250a can be substantially leveled with the dielectric layer 275. In other embodiments, the top surface of the semiconductor layer 250a can be exposed and not covered by the dielectric layer 275.
Referring to FIG. 2J, a mask layer 278, e.g., oxide, nitride, oxynitride, or any combinations thereof, can be formed, covering the top surface of the semiconductor layer 250b. The mask layer 278 can prevent the semiconductor layer 250b from being removed, while the semiconductor layer 250a (shown in FIG. 2I) is removed. After removing the semiconductor layer 250a, an opening 277 is formed. The removing process can include, for example, a wet etch process, a dry etch process, or any combinations thereof. In some embodiments, the semiconductor layer 250a can be referred to as a dummy gate that is removed during a gate-last high-k metal-gate process.
Referring to FIG. 2K, a conductive layer 280 can be formed in the opening 277 (shown in FIG. 2J). As shown, a transistor 213a and a resistor structure 215a can be formed over the substrate 201. In some embodiments, the conductive layer 280 can be made of at least one material, such as aluminum, copper, Ti, TiN, TaN, Ta, TaC, TaSiN, W, WN, MoN, MoON, RuO2, and/or other suitable materials. In some embodiments, the conductive layer 280 can be formed, for example, by any suitable process, such as an ALD process, a CVD process, a physical vapor deposition (PVD) process, a reduced-pressure CVD (RPCVD) process, a PECVD process, an MOCVD process, a plating process, a sputtering process, or any combinations thereof.
Referring again to FIG. 2K, the work-function metallic layer 240b can be disposed between the conductive layer 280 and the gate dielectric structure 220a. The work-function metallic layer 240b can have a length that is substantially equal to the length of the gate dielectric structure 220a. The semiconductor layer 250b can directly contact the gate dielectric structure 220b. 
In some embodiments, the integrated circuit 200 can include a mixed-signal circuit and/or a RF circuit. The transistor 213a can be a high-k/metal gate (HK/MG) transistor of the integrated circuit 200. The semiconductor layer 250b disposed in the resistor region 215 can serve as a resistor in the integrated circuit 200. A more precisely controlled resistance of the resistor in the mixed-signal and RF circuits is desired.
As noted, the resistance of the resistor can be adjusted by in-situ and/or implantation doping of the semiconductor layer 250b. Because the semiconductor layer 250b has a substantive thickness, a minor thickness variation of the semiconductor layer 250b resulting from a deposition process does not substantially affect the thickness of the semiconductor layer 250b. The resistance variation of the semiconductor layer 250b resulting from the thickness variation can be reduced. A desired resistance precision of the semiconductor layer 250b can be achieved.
It is also noted, the processes and/or structures described above in conjunction with FIGS. 2A-2K are merely exemplary. The scope of this application is not limited thereto. For example, as described in conjunction with FIG. 2D, the etch process 246 can remove portions of the work-function metallic material 240 and the gate dielectric stack 220, such that a portion of the substrate 201 is exposed. In at least this embodiment, the semiconductor layer 250b formed in FIG. 2K can directly contact the STI feature 210.
FIG. 2L is a schematic cross-sectional view of another exemplary integrated circuit. In FIG. 2L, the resistor region 215b includes another resistor structure 215b. The resistor structure 215b can include a gate dielectric structure 220c disposed over the substrate 201. A semiconductor layer 250c can directly contact the gate dielectric structure 220c. Spacers 269 can be formed on sidewalls of the semiconductor layer 250c. A silicide region 268 can be formed on the semiconductor layer 250c. The formation of the resistor structure 215b can be carried out by the process of forming the resistor 215a described above in conjunction with FIGS. 2A-2K.
Referring again to FIG. 2L, the resistor structure 215a does not have a silicide region on the top of the semiconductor layer 250b, whereas the resistor structure 215b does have a silicide region on top of the semiconductor layer 250c. The formation of the silicide region 268 can include, for example, blocking the top surface of the semiconductor layer 250 with a dielectric layer while the salicidation process for forming the silicide regions in S/D regions 270 is performed. It is noted that because the silicide region can be optionally formed in the resistor structures 215a and 215b, the resistance of the resistor structures 215a and 215b, in series or parallel, can be adjusted.
It is understood that additional processes may be performed to complete the fabrication of the integrated circuit 200. For example, these additional processes may include forming dielectric materials, contact or via plugs, metallic regions, and/or metallic lines (not shown) over the conductive layer 280 and the semiconductor layers 250b/250c for interconnection. The dielectric layers may include materials such as silicon oxide, silicon nitride, silicon oxynitride, low-k dielectric material, ultra low-k dielectric material, or any combinations thereof. The contact/via plugs, metallic regions, and/or metallic lines can include materials such as tungsten, aluminum, copper, titanium, tantalum, titanium nitride, tantalum nitride, nickel silicide, cobalt silicide, other proper conductive materials, and/or combinations thereof. The contact/via plugs, metallic regions, and/or metallic lines can be formed by any suitable processes, such as deposition, photolithography, and etching processes, and/or combinations thereof. For the sake of simplicity, these additional processes are not described herein.
FIGS. 3A-3H are schematic cross-sectional views of an integrated circuit during another gate-last high-k metal-gate fabrication stages. Items of FIGS. 3A-3H that are the same or similar items in FIGS. 2A-2K are indicated by the same reference numerals, increased by 100. It is understood that FIGS. 3A-3H have been simplified for a better understanding of the concepts of the present disclosure. Accordingly, it should be noted that additional processes may be provided before, during, and after FIGS. 3A-3H, and that some other processes may only be briefly described herein.
Referring now to FIG. 3A, an integrated circuit 300 can be fabricated over a substrate 301. The integrated circuit 300 can include a transistor region 313 and a resistor region 315. The integrated circuit 300 can include an isolation structure, e.g., a shallow trench isolation feature 310 that can be formed in the substrate 301. A gate dielectric stack 320 can be formed over the substrate 301.
Referring to FIG. 3B, a semiconductor material 350 can be formed over the gate dielectric stack 320. In some embodiments, the semiconductor material 350 can continuously extend from the transistor region 313 to the resistor region 315. In other embodiments, the semiconductor material 350 can be undoped or in-situ doped.
Referring to FIG. 3C, a patterning process can be performed to pattern the semiconductor material 350 and the gate dielectric stack 320 (shown in FIG. 3B). By the patterning process, gate dielectric structures 320a and 320b can be formed over the substrate 301, which are disposed in the transistor region 313 and the resistor region 315, respectively. A semiconductor layer 350a can be disposed over the gate dielectric structure 320a. A semiconductor layer 350b can directly contact the gate dielectric structure 320b. 
Referring to FIG. 3D, at least one implantation process 355 can be performed to implant dopants into the substrate 301 to form a source/drain (S/D) region 370 adjacent to each sidewall of the gate dielectric structure 320a. Spacers 365 and 367 can be disposed on sidewalls of the semiconductor layers 350a and 350b, respectively. It is noted that the process of forming the S/D regions 370 can optionally include the processes of forming the LDD regions, stressors and/or silicide regions described above in conjunction with FIG. 2H.
Referring to FIG. 3E, at least one dielectric layer, e.g., a dielectric layer 375, can be disposed over the substrate 301 and around the spacers 365 and 367. In some embodiments, the top surface of the semiconductor layer 350a can be substantially level with the dielectric layer 375. In other embodiments, the top surface of the semiconductor layer 350a can be exposed and not covered by the dielectric layer 375.
Referring to FIG. 3F, a mask layer 378, e.g., oxide, nitride, oxynitride, or any combinations thereof, can be formed, covering the top surface of the semiconductor layer 350b. The mask layer 378 can prevent the semiconductor layer 350b from being removed, while the semiconductor layer 350a (shown in FIG. 3E) is removed. After removing the semiconductor layer 350a, an opening 377 is formed. The removal process can include, for example, a wet etch process, a dry etch process, or any combinations thereof.
Referring to FIG. 3G, a work-function metallic layer 340b can be disposed in the opening 377 and along sidewalls of the spacers 365 and the top surface of the gate dielectric structure 320a. In some embodiments, at least one diffusion barrier (not shown) can be formed between the gate dielectric structure 320a and the work-function metallic layer 340b. 
Referring to FIG. 3H, a conductive layer 380 can be formed in the opening 377 (shown in FIG. 3G). As shown, a transistor 313a and a resistor structure 315a can be formed over the substrate 301. In some embodiments, the conductive layer 380 can have a top surface that is substantially level with the top surface of the dielectric layer 375.
Referring again to FIG. 3H, the work-function metallic layer 340 can be disposed between the conductive layer 380 and the gate dielectric structure 320a. The work-function metallic layer 340 can continuously extend around the sidewalls and bottom of the conductive layer 380. The semiconductor layer 350b can directly contact the gate dielectric structure 320b. 
As noted, the resistance of the resistor can be adjusted by in-situ and/or implantation doping of the semiconductor layer 350b. Because the semiconductor layer 350b has a substantive thickness, a minor thickness variation of the semiconductor layer 350b resulting from a deposition process does not substantially affect the thickness of the semiconductor layer 350b. The resistance variation of the semiconductor layer 350b resulting from the thickness variation can be reduced. A desired resistance precision of the semiconductor layer 350b can be achieved. In some embodiments, the process and/or structure described above in conjunction with FIG. 2L can be applied to the process and/or structure of FIGS. 3A-3H to desirably adjust the resistance of the resistors.
FIGS. 4A-4F are schematic cross-sectional views of an integrated circuit during various gate-first high-k metal-gate fabrication stages. It is understood that FIGS. 4A-4F have been simplified for a better understanding of the concepts of the present disclosure. Accordingly, it should be noted that additional processes may be provided before, during, and after FIGS. 4A-4F, and that some other processes may only be briefly described herein.
Referring now to FIGS. 4A-4F, an integrated circuit 400 can be fabricated over a substrate 401. Items of FIGS. 4A-4F that are the same or similar items in FIGS. 2A-2K are indicated by the same reference numerals, increased by 200. The integrated circuit 400 can include a transistor region 413 and a resistor region 415. In FIG. 4A, an isolation structure, e.g., a shallow trench isolation feature 410, can be formed in the substrate 401.
Referring again to FIG. 4A, a gate dielectric stack 420, a work-function metallic material 440 and a metallic layer 443 can be sequentially formed over the substrate 401. In some embodiments, the metallic layer 443 may be made of at least one material such as aluminum oxide, aluminum, aluminum nitride, titanium, titanium nitride (TiN), tantalum, tantalum nitride, other suitable material, and/or combinations thereof. In some embodiments, the metallic layer 443 can be referred to as a cap metallic layer.
Referring to FIG. 4B, a mask layer 445 can be formed over the metallic layer 443. An etch process 446 uses the mask layer 445 to remove portions of the metallic layer 443 and the work-function metallic material 440, exposing a portion of the gate dielectric stack 420. The etch process 446 can include a dry etch process, a wet etch process, or any combinations thereof. After the mask layer 445 is removed, the remaining metallic layer 443a is exposed as shown in FIG. 4C. In some embodiments, the etch process 446 can also remove a portion of the gate dielectric stack 420, such that a portion of the substrate 401 is exposed.
Referring to FIG. 4D, a semiconductor material 450 can be disposed over the remaining metallic layer 443a and the exposed gate dielectric stack 420. In some embodiments, the semiconductor material 450 can continuously extend from the transistor region 413 to the resistor region 415. In other embodiments, the semiconductor material 450 can be undoped or in-situ doped.
Referring to FIG. 4E, a patterning process can be performed to pattern the semiconductor material 450, the remaining metallic layer 443a, the remaining work-function metallic material 440a and the gate dielectric stack 420 (shown in FIG. 4D). By the patterning process, gate dielectric structures 420a and 420b can be formed over the substrate 401, which are disposed in the transistor region 413 and the resistor region 415, respectively. A work-function metallic layer 440b can be disposed over the gate dielectric structure 420a. A metallic layer 443b can be disposed over the work-function metallic layer 440b. A silicon layer 450a can be disposed over the metallic layer 443b. A silicon layer 450b can directly contact the gate dielectric structure 420b. 
Referring to FIG. 4F, at least one implantation process 455 can be performed to implant dopants into the substrate 401 to form a source/drain (S/D) region 470 adjacent to each sidewall of the gate dielectric structure 420a. Spacers 465 and 467 can be disposed on sidewalls of the semiconductor layers 450a and 450b, respectively. It is noted that the process of forming the S/D regions 470 can optionally include the processes of forming the LDD regions, stressors and/or silicide regions described above in conjunction with FIG. 2H.
As noted, the resistance of the resistor can be adjusted by in-situ and/or implantation doping the semiconductor layer 450b. Because the semiconductor layer 450b has a substantive thickness, a minor thickness variation of the semiconductor layer 450b resulting from a deposition process does not substantially affect the thickness of the semiconductor layer 450b. The resistance variation of the semiconductor layer 450b resulting from the thickness variation can be reduced. A desired resistance precision of the semiconductor layer 450b can be achieved. In some embodiments, the process and/or structure described above in conjunction with FIG. 2L can be applied to the process and/or structure of FIGS. 4A-4F to desirably adjust the resistance of the resistors.
In a first embodiment of this application, a method of forming an integrated circuit includes forming at least one transistor over a substrate. The at least one transistor includes a first gate dielectric structure disposed over a substrate. A work-function metallic layer is disposed over the first gate dielectric structure. A conductive layer is disposed over the work-function metallic layer. A source/drain (S/D) region is disposed adjacent to each sidewall of the first gate dielectric structure. At least one resistor structure is formed over the substrate. The at least one resistor structure includes a first doped semiconductor layer disposed over the substrate. The at least one resistor structure does not include any work-function metallic layer between the first doped semiconductor layer and the substrate.
In a second embodiment of this application, a method of forming an integrated circuit includes forming a first gate dielectric structure and a second gate dielectric structure over a substrate. The first gate dielectric structure is in a transistor region and the second gate dielectric structure is in a resistor region. A first semiconductor layer is formed over the first gate dielectric structure and a second semiconductor layer directly contacts the second gate dielectric structure. Dopants are implanted into source/drain (S/D) regions adjacent to sidewalls of the first gate dielectric structure. Thereafter, the first semiconductor layer is removed. A conductive layer is formed over the first gate dielectric structure.
In a third embodiment of this application, an integrated circuit includes at least one transistor and at least one resistor structure. The transistor includes a first gate dielectric structure disposed over a substrate. A work-function metallic layer is disposed over the first gate dielectric structure. A conductive layer is disposed over the work-function metallic layer. A source/drain (S/D) region is disposed adjacent to each sidewall of the first gate dielectric structure. The resistor structure is disposed over the substrate. The at least one resistor structure includes a first doped semiconductor layer disposed over the substrate. The at least one resistor structure does not include any work-function metallic layer between the first doped semiconductor layer and the substrate.
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.

## Claims

1. A method of forming an integrated circuit, the method comprising:
forming at least one transistor over a substrate, the at least one transistor comprising a first gate dielectric structure disposed over the substrate, a work-function metallic layer being disposed over the first gate dielectric structure, a conductive layer being disposed over the work-function metallic layer, and a source/drain (S/D) region being disposed adjacent to each sidewall of the first gate dielectric structure;
forming at least one resistor structure over an isolation feature in the substrate, the at least one resistor structure comprising a first doped semiconductor layer disposed over the substrate, wherein a top surface of the at least one resistor is substantially level with a top surface of the at least one transistor; and
forming a second doped semiconductor layer over the conductive layer.

2. The method of claim 1, further comprising:
forming a second gate dielectric structure between the substrate and the first doped semiconductor layer, wherein the second gate dielectric structure directly contacts the first doped semiconductor layer.

3. The method of claim 1, further comprising:
forming a silicide region disposed over the first doped semiconductor layer.

4. The method of claim 1, wherein the work-function metallic layer has a length which is substantially equal to that of the first gate dielectric structure.

5. The method of claim 1, wherein the work-function metallic layer continuously extends along sidewalls of the conductive layer.

6. The method of claim 1, further comprising forming spacers on each sidewall of each of the resistor structure.

7. A method of forming an integrated circuit, the method comprising:
forming a first gate dielectric structure and a second gate dielectric structure over a substrate, wherein the first gate dielectric structure is in a transistor region and the second gate dielectric structure is in a resistor region;
forming a first semiconductor layer over the first gate dielectric structure and a second semiconductor layer directly contacting the second gate dielectric structure;
wherein forming the first gate dielectric structure and the second gate dielectric structure over the substrate and forming the first semiconductor layer over the first gate dielectric structure and the second semiconductor layer directly contacting the second gate dielectric structure comprise:
forming a gate dielectric stack over the substrate;
forming a work-function metallic material over the gate dielectric stack;
forming a mask layer over the work-function metallic material, the mask layer covering the transistor region and having an opening exposing at least a portion of the work-function metallic material in the resistor region;
removing the exposed work-function metallic material to expose a portion of the gate dielectric stack;
thereafter, removing the mask layer to expose the remaining work-function metallic material;
forming a semiconductor material over the remaining work-function metallic material and the exposed gate dielectric stack;
patterning the semiconductor material so as to form the first semiconductor layer over a work-function metallic layer over the first gate dielectric structure in the transistor region and to form the second semiconductor layer directly contacting the second gate dielectric structure;

implanting dopants into source/drain (S/D) regions adjacent to sidewalls of the first gate dielectric structure;
thereafter, removing the first semiconductor layer; and
forming a conductive layer over the first gate dielectric structure.

8. The method of claim 7, wherein implanting dopants into the S/D regions implants dopants in the second semiconductor layer.

9. The method of claim 7, further comprising:
forming a silicide region disposed over the second semiconductor layer.

10. The method of claim 7, further comprising forming a stressor in the source/drain regions.

11. The method of claim 7, further comprising forming spacers on sidewalls of the first gate dielectric structure and the second gate dielectric structure.

12. The method of claim 7, further comprising growing the source/drain regions such that a top surface of the source drain region extends above a top surface of the substrate.

13. The method of claim 7, wherein the remaining work-function metallic material has a length which is substantially equal to that of the first gate dielectric structure.

14. A method of forming an integrated circuit, the method comprising:
forming a gate dielectric layer over a substrate, the substrate having a first region and a second region, wherein an isolation feature is formed in the first region;
forming a work function layer over in contact with the gate dielectric layer, wherein the work function layer extends over an entirety of the gate dielectric layer;
removing the work function layer and the gate dielectric layer in the first region;
forming a semiconductor layer over the isolation feature in the first region and over the gate dielectric layer in the second region;
defining a transistor gate structure in the second region;
defining a first resistor structure in the first region, wherein the first resistor structure comprises the semiconductor layer in direct contact with the isolation feature, wherein a top surface of the first resistor structure is substantially coplanar with a top surface of the transistor gate structure;
forming source/drain regions adjacent to sidewall portions of the transistor gate structure in the second region; and
forming a second doped semiconductor layer over the conductive layer.

15. The method of claim 14, wherein forming the source/drain regions comprises forming a stressor in the source/drain regions.

16. The method of claim 14, further comprising defining a second resistor structure in the first region, the second resistor structure comprising the semiconductor layer in direct contact with the isolation feature, wherein a resistance of the second resistor structure is different from a resistance of the first resistor structure.

17. The method of claim 16, further comprising forming a silicide region on the semiconductor layer of the second resistor structure.

18. The method of claim 14, further comprising forming spacers on sidewalls of the transistor gate structure and the first resistor structure.

19. The method of claim 14, wherein forming the source/drain regions comprises growing the source/drain regions such that a top surface of the source drain region extends above a top surface of the substrate.

20. The method of claim 14, wherein the remaining work-function metallic layer in the second region has a length which is substantially equal to that of the gate dielectric layer in the second region.