Patent Publication Number: US-2022238350-A1

Title: Performing Annealing Process To Improve Fin Quality Of A Finfet Semiconductor

Description:
PRIORITY DATA 
     This application is a divisional patent application of U.S. patent application Ser. No. 16/158,802, filed on Oct. 12, 2018, entitled “Performing Annealing Process to Improve Fin Quality of a FinFET Semiconductor”, which is a utility U.S. pat. app. of U.S. Provisional Patent Application No. 62/690,614, entitled “Performing Annealing Process to Improve Fin Quality of a FinFET Semiconductor” and filed on Jun. 27, 2018, the disclosures of each which are incorporated herein in their entireties. 
    
    
     BACKGROUND 
     The semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs. As this progression takes place, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as fin-like field effect transistor (FinFET) device. A typical FinFET device is fabricated with a thin “fin” (or fin-like structure) extending from a substrate. The fin usually includes silicon and forms the body of the transistor device. The channel of the transistor is formed in this vertical fin. A gate is provided over (e.g., wrapping around) the fin. This type of gate allows greater control of the channel. Other advantages of FinFET devices include reduced short channel effect and higher current flow. 
     However, conventional FinFET devices may still have certain drawbacks. For example, conventional methods of fabricating fin structures may form fins with roughened surfaces and/or excessive germanium oxide content. This may lead to problems such as line width roughness, line edge roughness, high resistivity, low carrier mobility, defect of interface traps (DIT) defects, etc. 
     Therefore, although existing FinFET devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. It is also emphasized that the drawings appended illustrate only typical embodiments of this invention and are therefore not to be considered limiting in scope, for the invention may apply equally well to other embodiments. 
         FIG. 1  is a perspective view of an example FinFET device. 
         FIGS. 2-6, 8, 10, 12, and 14  illustrate fragmentary 3-dimensional perspective views of a portion of a FinFET device at various stages of fabrication according to an embodiment of the present disclosure. 
         FIGS. 7, 9, 11, and 13  illustrate fragmentary cross-sectional side views of a portion of a FinFET device at various stages of fabrication according to an embodiment of the present disclosure. 
         FIG. 15A  illustrates a plot of a spike annealing process according to an embodiment of the present disclosure. 
         FIG. 15B  illustrates a plot of a soak annealing process according to an embodiment of the present disclosure. 
         FIG. 16  illustrates top views of a fin structure fabricated according to conventional methods and a fin structure fabricated according to an embodiment of the present disclosure. 
         FIG. 17  illustrates a graph showing a mobility improvement and defect reduction according to an embodiment of the present disclosure 
         FIG. 18  is a flowchart illustrating a method of fabricating a FinFET device according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. 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. 
     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&#39;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. 
     Still further, when a number or a range of numbers is described with “about,” “approximate,” and the like, the term is intended to encompass numbers that are within a reasonable range including the number described, such as within +/−10% of the number described or other values as understood by person skilled in the art. For example, the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm. 
     The present disclosure is directed to, but not otherwise limited to, a method of annealing a semiconductor device to enhance the quality of the semiconductor device. To illustrate the various aspects of the present disclosure, a FinFET fabrication process is discussed below as an example. In that regard, a FinFET device is a fin-like field-effect transistor device, which has been gaining popularity in the semiconductor industry. The FinFET device may be a complementary metal-oxide-semiconductor (CMOS) device including a P-type metal-oxide-semiconductor (PMOS) FinFET device and an N-type metal-oxide-semiconductor (NMOS) FinFET device. The following disclosure will continue with one or more FinFET examples to illustrate various embodiments of the present disclosure, but it is understood that the application is not limited to the FinFET device, except as specifically claimed. 
     Referring to  FIG. 1 , a perspective view of an example FinFET device  10  is illustrated. The FinFET device structure  10  includes an N-type FinFET device structure (NMOS)  15  and a P-type FinFET device structure (PMOS)  25 . The FinFET device structure  10  includes a substrate  102 . The substrate  102  may be made of silicon or other semiconductor materials. Alternatively or additionally, the substrate  102  may include other elementary semiconductor materials such as germanium. In some embodiments, the substrate  102  is made of a compound semiconductor such as silicon carbide, gallium arsenic, indium arsenide, or indium phosphide. In some embodiments, the substrate  102  is made of an alloy semiconductor such as silicon germanium, silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. In some embodiments, the substrate  102  includes an epitaxial layer. For example, the substrate  102  may include an epitaxial layer overlying a bulk semiconductor. 
     The FinFET device structure  10  also includes one or more fin structures  104  (e.g., Si fins) that extend from the substrate  102  in the Z-direction and surrounded by spacers  105  in the Y-direction. The fin structure  104  is elongated in the X-direction and may optionally include germanium (Ge). The fin structure  104  may be formed by using suitable processes such as photolithography and etching processes. In some embodiments, the fin structure  104  is etched from the substrate  102  using dry etch or plasma processes. In some other embodiments, the fin structure  104  can be formed by a double-patterning lithography (DPL) process. DPL is a method of constructing a pattern on a substrate by dividing the pattern into two interleaved patterns. DPL allows enhanced feature (e.g., fin) density. The fin structure  104  also includes an epi-grown material  12 , which may (along with portions of the fin structure  104 ) serve as the source/drain of the FinFET device structure  10 . 
     An isolation structure  108 , such as a shallow trench isolation (STI) structure, is formed to surround the fin structure  104 . In some embodiments, a lower portion of the fin structure  104  is surrounded by the isolation structure  108 , and an upper portion of the fin structure  104  protrudes from the isolation structure  108 , as shown in  FIG. 1 . In other words, a portion of the fin structure  104  is embedded in the isolation structure  108 . The isolation structure  108  prevents electrical interference or crosstalk. 
     The FinFET device structure  10  further includes a gate stack structure including a gate electrode  110  and a gate dielectric layer (not shown) below the gate electrode  110 . The gate electrode  110  may include polysilicon or metal. Metal includes tantalum nitride (TaN), nickel silicon (NiSi), cobalt silicon (CoSi), molybdenum (Mo), copper (Cu), tungsten (W), aluminum (Al), cobalt (Co), zirconium (Zr), platinum (Pt), or other applicable materials. Gate electrode  110  may be formed in a gate last process (or gate replacement process). Hard mask layers  112  and  114  may be used to define the gate electrode  110 . A dielectric layer  115  may also be formed on the sidewalls of the gate electrode  110  and over the hard mask layers  112  and  114 . 
     The gate dielectric layer (not shown) may include dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, dielectric material(s) with high dielectric constant (high-k), or combinations thereof. Examples of high-k dielectric materials include hafnium oxide, zirconium oxide, aluminum oxide, hafnium dioxide-alumina alloy, hafnium silicon oxide, hafnium silicon oxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafnium zirconium oxide, the like, or combinations thereof. 
     In some embodiments, the gate stack structure includes additional layers, such as interfacial layers, capping layers, diffusion/barrier layers, or other applicable layers. In some embodiments, the gate stack structure is formed over a central portion of the fin structure  104 . In some other embodiments, multiple gate stack structures are formed over the fin structure  104 . In some other embodiments, the gate stack structure includes a dummy gate stack and is replaced later by a metal gate (MG) after high thermal budget processes are performed. 
     The gate stack structure is formed by a deposition process, a photolithography process and an etching process. The deposition process include chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD), metal organic CVD (MOCVD), remote plasma CVD (RPCVD), plasma enhanced CVD (PECVD), plating, other suitable methods, and/or combinations thereof. The photolithography processes 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). The etching process includes a dry etching process or a wet etching process. Alternatively, the photolithography process is implemented or replaced by other proper methods such as maskless photolithography, electron-beam writing, and ion-beam writing. 
     FinFET devices offer several advantages over traditional Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) devices (also referred to as planar transistor devices). These advantages may include better chip area efficiency, improved carrier mobility, and fabrication processing that is compatible with the fabrication processing of planar devices. Thus, it may be desirable to design an integrated circuit (IC) chip using FinFET devices for a portion of, or the entire IC chip. 
     However, FinFET fabrication may still have shortcomings. For example, the formation of the fin structures may involve one or more etching processes, which may result in rough fin surfaces. In addition, for PFETs with SiGe fin structures, a silicon cap layer may be formed on the SiGe fin structures. However, conventional FinFET devices may not have a sufficiently thick silicon cap layer. These problems may result in high line width roughness (LWR) and/or high line edge roughness (LER), as well as high resistivity and poor channel mobility. These problems are exacerbated as the semiconductor feature sizes continue to shrink—resulting in smaller critical dimensions. Consequently, semiconductor device performance may not be satisfactory. 
     To overcome the problems discussed above, the present disclosure performs an annealing process to repair fin surfaces in order to enhance the quality of the fin structures, as discussed below in more detail with reference to  FIGS. 2-15 . In that regard,  FIGS. 7, 9, 11, and 13  illustrate fragmentary cross-sectional side views of a portion of a semiconductor device  200  that includes a FinFET device at various stages of fabrication, and  FIG. 15  is a flowchart illustrating a method of fabricating a semiconductor device according to embodiments of the present disclosure. 
     Referring now to  FIG. 2 , a semiconductor device  200  is illustrated. The semiconductor device  200  includes FinFET transistors in the illustrated embodiment and may be referred to as a FinFET device interchangeably hereinafter. The semiconductor device  200  includes a substrate  210 . The substrate  210  may be implemented as an embodiment of the substrate  102  discussed above with reference to  FIG. 1 . In some embodiments, the substrate  210  includes a semiconductive material such as a crystal silicon material. An ION implantation process may be performed to implant a plurality of dopant ions to the substrate  210 . The dopant ions may include an n-type material for NMOS devices (NFETs), for example arsenic (As) or phosphorous (P), or the dopant ions may include a p-type material for PMOS devices (PFETs), for example boron (B). After the implantation process is performed, a doping concentration level in the substrate  210  may be in a range from about 1×10 17  ions/cm 3  to about 5×10 19  ions/cm 3 . 
     The semiconductor device  200  includes a plurality of fin structures, such as fin structures  250 - 255  (note that the fin structure  252  is not directly visible in  FIG. 2  but will be shown in later Figures). The fin structures  250 - 255  may be implemented as an embodiment of the fin structures  104  discussed above with reference to  FIG. 1 . The fin structures  250 - 255  protrude vertically upwards out of the substrate  210  in the Z-direction. The fin structures  250 - 255  also each extend in an elongated manner in the X-direction and are separated from one another in the Y-direction. In the illustrated embodiment, the fin structures  250 - 251  and  254 - 255  are fin structures for NFETs and include silicon (Si), whereas the fin structures  252 - 253  are fin structures for PFETs and include silicon germanium (SiGe). Note that the SiGe fin structures  252 - 253  are disposed over portions of the substrate  210  (e.g., containing Si) that also protrude upwards. It can be seen in  FIG. 2  that the fin structures  252 - 253  are “recessed” (or not “justified”) from the fin structures  260 - 261  and  264 - 265 . In other words, the fin structures  252 - 253  have shorter dimensions in the X-direction than the fin structures  260 - 261  and  264 - 265  in  FIG. 2 . However, this is merely due to the semiconductor device  200  being a part of a static random access memory (SRAM). Stated differently, it is common for SRAM layout/design to have PFET fins that are recessed from the NFET fins. However, this is not intended to be limiting. In other embodiments or different circuit applications, the PFET fins and the NFET fins may be “justified” or “edge-aligned”, rather than being recessed from one another. 
     The fin structures  250 - 255  may be defined by mask layers  260 - 265 . In some embodiments, the mask layers  260 - 265  include a dielectric material such as silicon nitride. In some embodiments, the mask layers  260 - 265  may each include two dielectric layers (e.g., a layer of silicon nitride and a layer of silicon oxide). For example, in the embodiment shown in  FIG. 2 , the mask layers  260 - 265  include sub-layers  260 A- 265 A, respectively, where the sub-layers  260 A- 265 A contain silicon oxide, while the rest of the mask layers  260 - 265  (located over the sub-layers  260 A- 265 A) contain silicon nitride. The mask layers  260 - 265  may be formed by patterning a dielectric material with a patterned photoresist layer. The mask layers  260 - 265  are then used to pattern a semiconductor material (e.g., Si or SiGe) below to define the fin structures  250 - 255 . 
     Still referring to  FIG. 2 , isolation structures  270  are formed to electrically isolate the fins  250 - 255 . The isolation structures  270  may also be referred to as shallow trench isolation (STI) structures. In some embodiments, the isolation structures  270  contain a dielectric material such as silicon oxide. Other suitable dielectric materials are envisioned in other embodiments, as long as the isolation structures  270  have a different material composition than the mask layers  260 - 265 . For example, when the mask layers  260 - 265  contain silicon nitride, the isolation structures  270  may contain silicon oxide, so that there may be etching selectivity between the mask layers  260 - 265  and the isolation structures  270 . The isolation structures  270  may be formed by depositing the dielectric material to fill the openings that are formed as a result of the fin structures  255 - 255  being patterned by the hard mask layers  260 - 265 , and then performing a polishing process (such as chemical mechanical polishing) to planarize the surface of the dielectric material. 
     A liner layer  280  may also be formed between on the fin structures  250 - 255 . The liner layer  280  may include a dielectric material, such as silicon nitride. The liner layer  280  is formed before the isolation structures  270 , and thus the liner layer  280  may be located between the fin structures  250 - 255  and the isolation structures  270 . 
     Referring now to  FIG. 3 , an etch-back process  300  is performed to the semiconductor device  200  to etch away a portion of the isolation structures  270 . For example, an upper portion of the isolation structures  270  is etched away, so that the sidewalls of the mask layers  260 - 265  are exposed. Due to the etching selectivity between the isolation structures  270  and the mask layers  260 - 265 , the portions of the isolation structures  270  may be removed without substantially affecting the mask layers  260 - 265 . 
     Referring now to  FIG. 4 , a mask layer removal process  320  may be performed to the semiconductor device  200  to remove the mask layers  260 - 265 . In some embodiments, the mask layer removal process  320  may include an etching process. Again, due to the etching selectivity between the isolation structures  270  and the mask layers  260 - 265 , the mask layers  260 - 265  may be removed without substantially affecting the isolation structures  270 , or the fin structures  250 - 255  underneath the mask layers  260 - 265 . In embodiments (such as the illustrated embodiment) where the mask layers  260 - 265  include the sub-layers  260 A- 265 A, the mask layer removal process  320  may remove the main portion of the mask layers  260 - 265  (e.g., containing silicon nitride) but may not remove the sub-layers  260 A- 265 A (e.g., containing silicon oxide). These sub-layers  260 A- 265 A may be removed in another etching process performed after the process  320 , which is not specifically illustrated herein for reasons of simplicity. 
     Referring now to  FIG. 5 , after the mask layers  260 - 265  (including the sub-layers  260 A- 265 A) are completely removed, a fin recess process  350  is performed to the semiconductor device  200  to etch away portions of the isolation structures  270 , thereby forming recesses that are collectively defined by the fin structures  250 - 255  (e.g., defining the sidewalls of the recesses) and the isolation structures  270  (e.g., defining the bottom of the recess). In some embodiments, the fin recess process  350  includes an etching process, for example a dry etching process. In such an etching process, an etching selectivity exists between the fin structures  250 - 255  and the isolation structures  270 . As such, the isolation structures  270  may be etched away without substantially affecting the fin structures  250 - 255 . As a result of the fin recess process  350 , an upper segment of each of the fins structures  250 - 255  becomes exposed (e.g., their sidewall surfaces are not covered by the isolation structures  270 ). At this stage of fabrication, the fin structures  250 - 255  may have a lateral dimension  360  measured in the Y-direction. The lateral dimension  360  may still be greater than what is desired, and thus it will be reduced in a trimming process discussed below. 
     Referring now to  FIGS. 6-7  (where  FIG. 7  illustrates a cross-sectional view of the fin structures  252 - 253  (SiGe fins for the PFETs)), a fin trim process  400  is performed to the semiconductor device  200  to reduce a lateral dimension of each of the fin structures  250 - 255 , for example the lateral dimension measured in the Y-direction. In some embodiments, the fin trim process  400  is performed in a first semiconductor fabrication tool having a first chamber, for example a semiconductor fabrication tool that includes a wet cleaning chamber or a dry etching chamber. The fin trim process  400  effectively shrinks a critical dimension (CD) of the FinFET devices herein, since the CD is associated with the lateral dimension of the fin structures  250 - 255 . 
     In some embodiments, the fin trim process  400  includes an oxidation process in which the surface portions of the fin structures  250 - 255  are oxidized, followed by applying a hydrofluoric (HF) acid to the fin structures  250 - 255  to remove the oxidized portions thereof. As a result of the fin trim process  400 , the “trimmed” fin structures  250 - 255  now have a smaller lateral dimension  410  in the Y-direction. In other words, the lateral dimension  410  is smaller than the lateral dimension  360  shown in  FIG. 5 . In some embodiments, the lateral dimension  410  is in a range from about 5 nanometers (nm) to about 12 nm. 
     As shown in  FIG. 7 , the fin recess process  350  and the fin trim process  400  are performed to completely expose the sidewall surfaces of the fin structures, such as the surfaces  430  of the fin structures  252 - 253 . In some embodiments, in order to ensure there is sufficient margin to expose the surfaces  430  of the fin structures  252 - 253 , sidewall surfaces  440  of a portion of the semiconductor layer (e.g., protruding portions of the substrate  210 ) underneath the fin structures  252 - 253  may also become exposed. In addition, the side surfaces of a small portion of the liner layer  280  may become exposed as well. 
     One downside of the fin trim process  400  is that it may roughen the surfaces of the fin structures  250 - 255 . This is shown in more detail in the cross-sectional view of  FIG. 7 , where it can be seen that the exposed surfaces  430  of the fin structures  252 - 253  are rough. For example, the surfaces  430  may exhibit a significant amount of topography variation, such as protrusions and dips, and are therefore not as flat or smooth as they are desired to be. It is understood that the fin trim process  400  is not necessarily the sole culprit for the surface roughness of the fin structures. Other processes discussed above such as the processes for defining the shape of the fin structures  250 - 255  (e.g., using the mask layers  260 - 265 ), or even the fin recess process  350  (discussed above with reference to  FIG. 5 ), may also contribute to the surface roughness of the fin structures  250 - 255 . Excessive surface roughness of the fin structures  250 - 255  may be undesirable, as it may result in a high resistivity and/or low carrier mobility, since it may more difficult for carriers such as electrons or holes to move in or through rough surfaces. 
     In some embodiments, the surface roughness of the surfaces  430  may be represented by a line width roughness (LWR), or by a line edge roughness (LER). It may be said that at this stage of fabrication, the fin structures (such as the fin structures  252 - 253 ) have a first surface roughness, for example a LWR in a range between about 2.1 nm and about 2.3 nm, or a LER in a range between about 1.8 nm and about 2.0 nm. 
     Referring now to  FIGS. 8-9 , a silicon cap layer  500  is formed on the fin structures  250 - 255 , including the sidewall surfaces of the fin structures. The silicon cap layer  500  is formed using a silicon cap formation process  510 . In some embodiments, the silicon cap formation process  510  may include an epitaxial growth process, such that silicon (i.e., the silicon cap layer  500 ) is grown on the fin structures  252 - 253 . The epitaxial growth process may also cause an amorphous silicon layer  505  to be grown on the upper surface of the isolation structure  270 . In some embodiments, the silicon cap formation process  510  is performed in a second semiconductor fabrication tool that includes a second chamber and a third chamber. The second semiconductor tool is different from the first semiconductor fabrication tool in which the fin trim process  400  is performed. As such, the wafer on which the semiconductor device  200  is formed is transferred from the first semiconductor fabrication tool to the second semiconductor fabrication tool. During this transfer process, the wafer may be exposed to ambient air, which contains oxygen that may oxidize surfaces of the wafer (e.g., on both the NFETs and the PFETs). The resulting surface oxide may be removed in the second chamber of the second semiconductor fabrication tool, which may be a dry etching chamber, for example. Thereafter, the wafer is transferred internally within the second semiconductor fabrication tool from the second chamber to the third chamber, which may be a chamber for performing epitaxial growth. The internal transfer from the second chamber to the third chamber is performed under a vacuum (or substantially oxygen-free) environment, which prevents surface oxides from being formed on the wafer again. 
     One reason for forming a silicon cap layer  500  on the SiGe fin structures  252 - 253  is that SiGe is not a desirable candidate for oxides. That is, an oxidation process will need to be performed later (discussed below with reference to  FIGS. 12-13 ) to form a dielectric layer on the fin structures. In some embodiments, such a dielectric layer may serve as an interfacial layer for a PFET transistor. For optimal device performance, silicon oxide is a more desirable candidate than germanium oxide for this dielectric layer. As such, it would be undesirable to oxidize the SiGe material of the fin structures  252 - 253  directly. Instead, it would be better to oxidize a silicon material, for example the silicon material of the silicon cap layer  500 . This is one reason for forming the silicon cap layer  500  over the fin structures  252 - 253 . 
     It is also understood that while it is not necessary to form this silicon cap layer  500  over the fin structures  250 - 251  and  254 - 255  of the NFETs—since the fin structures  250 - 251  and  254 - 255  already are made of silicon—the silicon cap layer  500  may still be formed on the fin structures  250 - 251  and  254 - 255  in the illustrated embodiment. This is because it may have been more complicated and more expensive to not form the silicon cap layer for the NFETs, which may involve forming a protective layer over the fin structures  250 - 251  and  254 - 255  for the sole purpose of preventing the formation of the silicon cap layer  500  thereon, and then removing such a protective layer later after the silicon layer  500  has been formed over the fin structures  252 - 253  for the PFETs. Nevertheless, it is understood that in some embodiments, the silicon cap layer  500  may be formed on the fin structures  252 - 253  but not on the fin structures  250 - 251  and  254 - 255 . 
     At this stage of fabrication shown in  FIG. 9 , the silicon cap layer  500  is formed to have a thickness  520 . In some embodiments, the thickness  520  is in a range between about 5 angstroms and about 11 angstroms, with a median value of about 8 angstroms. This range of the thickness  520  may not be optimal for the subsequent I/O oxide formation, as it may be too thin. If the thickness  520  is too thin, it may mean that the subsequent I/O oxide formation will consume (e.g., oxidize) not only all of the silicon cap layer  500 , but also a portion of the SiGe material of the fin structures  252 - 253 . As discussed above, germanium oxide is not an ideal material for the interfacial layer. Thus, it is desirable to have a silicon cap layer  500  that is sufficiently thick, so that the subsequent oxidation process will not “eat into” the SiGe material of the fin structures  252 - 253 . While conventional FinFET fabrication methods have not sufficiently addressed this issue, the present disclosure will thicken the silicon cap layer via an annealing process, as discussed in more detail below. 
     Referring now to  FIGS. 10-11 , an annealing process  550  is performed to the semiconductor device  200 . In some embodiments, the annealing process  550  includes a spike annealing process. The spike annealing process may be performed using the following process parameters or conditions: a peak annealing temperature that is maintained to be in a range between about 800 degrees Celsius and about 900 degrees Celsius, an annealing time (or duration) (during which the peak temperature is maintained) that is in a range between about 1 second and about 10 seconds, and an annealing pressure that is in a range between about 50 torrs and about 760 torrs. In other embodiments, the annealing process  550  includes a soak annealing process. The soak annealing process may be performed using the following process parameters or conditions: a peak annealing temperature that is maintained to be in a range between about 300 degrees Celsius and about 450 degrees Celsius, an annealing time (or duration) (during which the peak temperature is maintained) that is in a range between about 50 seconds and about 200 seconds, and an annealing pressure that is in a range between about 50 torrs and about 760 torrs. In some embodiments, the annealing process  550  may include a combination of a spike annealing process and a soak annealing process. In some embodiments, an inert gas (e.g., N 2 ) is used for the spike annealing process and/or the soak annealing process. In some embodiments, a flow rate of the inert gas in the spike annealing process and/or the soak annealing process is in a range between about 5 standard liters per minute (SLM) and about 45 SLM. 
     These process parameters above of the annealing process  550  are not arbitrarily chosen but rather are carefully tuned to improve the quality of the fin structures  252  and  253 . In that regard, the annealing process  550  is tuned to repair the fin structures  252 - 253  and reduce the surface roughness of the fin structures  252  and  253 , to increase a purity of the silicon cap layer  500 , and to thickens the silicon cap layer  500 . 
     For example, the annealing process  550  provides enough energy to allow the atoms on the surface of the fin structures  252 - 253  to rearrange themselves to have a more crystal-like quality (e.g., smooth and orderly). The result is smoother fin surfaces  560 . In other words, the fin surfaces  560  are smoother or have less roughness than the fin surfaces  430 . In some embodiments, the surface roughness of the surfaces  560  may also be represented by a line width roughness (LWR), or by a line edge roughness (LER). 
     LWR and LER are discussed in more detail with reference to  FIG. 16 . In more detail,  FIG. 16  illustrates top views of a fin structure  1010  and a fin structure  1020 . The fin structure  1010  may represent a fin structure that is a result of being fabricated according to conventional processes, whereas the fin structure  1020  may represent the fin structure fabricated according to the present disclosure, for example the fin structures  252 - 253 . In some embodiments, LWR and LER are measured as follows: a plurality of (e.g., n) points are taken on opposite sides of the fin structure. For example, a plurality of points A1-An are taken on the “left” boundary of the fin structure  1010 , and a plurality of points B1-Bn are taken on the “right” boundary of the in structure. These points A1-An and B1-Bn may be obtained by a scanning electron microscope (SEM) image, for example. The horizontal distance between each set of points A1-B1 through An-Bn measures the “width” or critical dimension (CD) of the fin structure at a different segment. For example, the distance between A1-B1 measures the width or CD of the fin structure  1010  at a top end portion of the fin structure  1010 , the distance between An-Bn measures the width or CD of the fin structure  1010  at a bottom end portion of the fin structure  1010 , and the distances between the rest of the point sets between A1 and An and B1 and Bn measure the widths or CDs of the fin structure  1010  at various segments between the top end and the bottom end. When all these distances are measured, a variation among them—for example a three sigma (where sigma is a standard deviation) value—may be used to define the LWR. The LWR measures how uniform the width or CD of the fin structure  1010  is throughout the fin structure  1010 . As such, a high LWR value may indicate a fin structure that has some portions that are significantly wider than other portions. 
     The definition of LER is still based on the points A1-An and B1-Bn, but LER is defined differently from LWR. In some embodiments, LER is defined as follows: a horizontal distance is measured between a fixed point on one side (e.g., A1) and the rest of the points on the opposite side (e.g., B1-Bn). Note that the horizontal distance is not the same as the diagonal distance. For example, the horizontal distance between A1 and B2 is not the diagonal distance that would connect A1 to B2. Rather, the horizontal distance would be a distance between B2 and a point corresponding to A1&#39;s horizontal position (e.g., as if A1 is shifted downwards until it is horizontally aligned with B2). In any case, once the horizontal distances between the fixed point and all the points on the opposite side are obtained, a variation among them (e.g., a three sigma value) may be used to define the LER. The LER measures how “straight” or “linear” the fin structure  1010  is overall. As such, a high LER value may indicate a fin structure that may be too “wiggly” or “wavy”. 
     The LWR and LER are defined the same way for the fin structure  1020 , but the points A1-An and B1-Bn are not specifically shown for the fin structure  1020  for ease of illustration. It can be seen based on  FIG. 16  that the fin structure  1010  (not fabricated according to the present disclosure) has significantly greater variations in its width compared to the fin structure  1020 , and as such it has a greater LWR. Furthermore, the fin structure  1010  is more “wiggly” than the fin structure  1020 , which indicates that the fin structure  1010  has a greater LER than the fin structure  1020 . 
     Thus, it may be said that at the stage of fabrication after the performance of the annealing process  550 , the fin structures (such as the fin structures  252 - 253 ) have a second surface roughness that is less than the first surface roughness associated with the surfaces  430 , for example a LWR in a range between about 1.7 nm and about 1.9 nm, or a LER in a range between about 1.5 nm and about 1.7 nm. It is understood that the reduced surface roughness of the fin structures  252 - 253  fabricated according to the present disclosure may also be represented by a ratio of the LWR versus the average fin width, or a ratio of the LER versus the average fin width. For example, with the average fin width of the fin structure  252  or  253  being represented by Fin_width_average, a ratio of LWR:Fin_width_average is in a range between about 1:4 and about 1:5, and a ratio of LER:Fin_width_average is also in a range between about 1:4 and about 1:5. In comparison, fin structures not fabricated according to the present disclosure typically has much ratios of LWR:Fin_width_average or LER:Fin_width_average that are much greater than 1:4 or 1:5 (e.g., a ratio of 1:3 or 1:2). The annealing process  550  also reduces the presence of germanium oxide in the fin structures  252 - 253 . In that regard, after the formation of the fin structures  252 - 253 , native oxides in the form of germanium oxide (GeO x ) may be naturally formed on the fin structures  252 - 253 . After the formation of the silicon cap layer  500 , the germanium oxide material may be located at an interface between the fin structures  252 - 253  and the silicon cap layer  500 . The presence of the germanium oxide material herein lowers a purity of the silicon cap layer and/or the purity of the SiGe material of the fin structures and may have a negative impact with respect to density of interface trap (DIT). 
     The annealing process  550 —with the specifically tuned process parameters discussed above-provides an energy boost to help break the bonds between germanium and oxygen in germanium oxides. Germanium and oxygen may be diffused outwards. In this diffusion process, the oxygen component may oxidize the silicon atoms of the silicon cap layer  500  to form silicon oxide at an outer surface of the silicon cap layer. The germanium component may escape the semiconductor device  200  in the form of a gaseous product. Again, the process parameters of the annealing process  550  are carefully tuned to facilitate the mechanism discussed above. For example, if the annealing temperature is too high, and/or when the annealing duration is too long, and/or when the annealing pressure is too low, then silicon and germanium may reflow instead of being diffused outwards, and this may undesirably alter the shape of the fin structures  252 - 253 . For example, the fin structures  252 - 253  may each have a ball-like shape due to the reflow of silicon and/or germanium, rather than the rectangular or trapezoidal shape that is more desirable. On the other hand, if the annealing temperature is too low, and/or when the annealing duration is too short, and/or when the annealing pressure is too high, then the annealing process  550  may not be able to provide enough of an energy boost to facilitate the diffusion of the germanium and oxygen, and the fin structures  252 - 253  may not be repaired sufficiently. 
     Here, as a result of the carefully configured annealing process  550  (e.g., with the specific process parameters discussed above to reduce the fin surface roughness and to thicken the silicon cap layer), the fin structures  252 - 253  may be repaired and may be substantially free of germanium oxide, and the silicon cap layer  500  is also thickened (through the elimination or reduction of the germanium content). For example, the silicon cap layer  500  now has a thickness  580  that is greater than the thickness  520  (corresponding to before the annealing process  550  is performed). In some embodiments, the thickness  580  is in a range between about 7 angstroms and about 15 angstroms, with a median value of about 11 angstroms. Comparing the values of the thickness  580  to the thickness  520  (e.g., between about 5 angstroms and 11 angstroms with a media value of about 8 angstroms) of the silicon cap layer  500  before the annealing process  550 , it can be seen that the annealing process  550  thickens the silicon cap layer  500  by at least a few angstroms. 
     The range of thickness  580  is also specifically configured to optimize the subsequent I/O oxidation process. If the value of the thickness  580  is too low, then as discussed above the subsequent I/O oxidation process may consume all of the silicon cap layer  500  (even though it has been thickened) and could potentially “eat into” the SiGe material of the fin structures  252 - 253 . On the other hand, if the value of the thickness  580  is too high, then the subsequent I/O oxidation process may leave too much of the silicon material of the silicon cap layer  500  un-oxidized. For example, an outer portion of the silicon cap layer  500  may be oxidized into silicon oxide, while an inner portion of the silicon cap layer  500  may remain silicon. When this occurs, the channel (and/or source/drain) material for the PFET becomes a mixture of SiGe and silicon, rather than just SiGe (which is desired for PFETs). As the thickness  580  increases, this problem may become exacerbated (e.g., a greater percentage of the PFET channel and/or source/drain is now made of silicon, rather than pure SiGe). As such, it may be desirable to have the silicon cap layer  500  sufficiently thickened to an extent such that it may be mostly oxidized in the subsequent oxidation process, without risking the underlying SiGe material of the fin structures  252 - 253  being oxidized. The thickness  580 —tuned as a result of the carefully chosen annealing process parameters discussed above—can achieve this goal. 
     Referring now to  FIGS. 12-13 , a dielectric layer  600  is formed over the semiconductor device  200 . In some embodiments, the dielectric layer  600  is formed using an oxidation process and an ALD process  620 . The oxidation process oxidizes a portion of the silicon cap layer  500  and the amorphous silicon layer  505  near their outside surfaces. The oxidized portions form a part of the dielectric layer  600 , which may contain silicon oxide. The rest of the dielectric layer  600  is formed by the ALD process. As discussed above, since the annealing process  550  effectively removes germanium oxide from the interface of the silicon cap layer  500  and the fin structures  252 - 253 , the oxidation of the silicon cap layer  500  can form a better quality silicon oxide layer as the dielectric layer  600 . The dielectric layer  600  may serve as an interfacial layer in a gate structure for the PFETs and may be interchangeably referred to as an interfacial layer hereinafter. It is understood that in embodiments where the isolation structures  270  contain silicon oxide, the dielectric layer  600  and the isolation structures  270  may have substantially similar (or the same) material compositions. For example, they may both have a silicon oxide composition, though the silicon oxide of the isolation structures  270  may include slightly more impurities than the silicon oxide of the dielectric layer  600 . 
     Also as discussed above, since the annealing process  550  thickens the silicon cap layer  500 , the oxidation process  610  can be performed with minimal risk of inadvertently oxidizing the SiGe material of the fin structures  252 - 253 . In the embodiment shown in  FIG. 13 , a small portion of the silicon cap layer  500  may still remain un-oxidized even after the performance of the oxidation process  610 . In some embodiments, a thickness of the remaining portion of the silicon cap layer  500  is in a range between about 3 angstroms and about 11 angstroms. In other embodiments, however, substantially all of the silicon cap layer  500  may be oxidized. 
     Referring now to  FIG. 14 , a plurality of gate structures, such as gate structures  700  and  701 , may be formed over the fin structures  250 - 255 . For example, the gate structures  700 - 701  may each include a high-k gate dielectric and a metal gate electrode. A high-k dielectric material is a material having a dielectric constant that is greater than a dielectric constant of SiO2, which is approximately 4. In an embodiment, the high-k gate dielectric includes hafnium oxide (HfO2), which has a dielectric constant that is in a range from approximately 18 to approximately 40. In alternative embodiments, the high-k gate dielectric may include ZrO2, Y2O3, La2O5, Gd2O5, TiO2, Ta2O5, HfErO, HfLaO, HfYO, HfGdO, HfAlO, HfZrO, HfTiO, HfTaO, or SrTiO. The dielectric layer  600 , serving as the interfacial layer herein, is disposed between the gate dielectric and the fin structures  250 - 255 . Note that while portions of the dielectric layer  600  remain on the sidewalls of the fin structures  250 - 255  in  FIG. 14 , the portion of the dielectric layer  600  located over/above the fin structures  250 - 255  are removed during a dummy gate etch process, and thus no portions of the dielectric layer  600  are shown above the fin structures in  FIG. 14 . 
     The metal gate electrode may include a work function metal component and a fill metal component. The work functional metal component is configured to tune a work function of its corresponding FinFET to achieve a desired threshold voltage Vt. In various embodiments, the work function metal component may contain: TiAl, TiAlN, TaCN, TiN, WN, or W, or combinations thereof. The fill metal component is configured to serve as the main conductive portion of the functional gate structure. In various embodiments, the fill metal component may contain Aluminum (Al), Tungsten (W), Copper (Cu), or combinations thereof. 
     In some embodiments, the formation of the gate structures  700 - 701  may include a gate replacement process. In the gate replacement process, a dummy gate electrode (e.g., containing polysilicon) is formed initially and is then replaced later by the metal gate electrode. In some embodiments of the gate replacement process, a dummy gate dielectric (e.g., containing silicon oxide) may also be formed initially then then replaced later by the high-k gate dielectric. It is understood that the gate structures  700 - 701  may be surrounded by an interlayer dielectric (ILD) that is also formed over the isolation structures  270  and over the fin structures  250 - 255 . The ILD is not illustrated herein so that the gate structures  700 - 701  can be seen more clearly. It is also understood that the formation of the gate structures  700 - 701  may involve a plurality of process steps that are not the focus of the present disclosure, and hence they are not discussed in detail herein. Additional processes (e.g., interconnect structure formation, packaging, testing, etc.) may be performed to complete the fabrication of the semiconductor device  200  after the formation of the gate structures  700 - 701 , which are also not discussed herein in detail for reasons of simplicity. 
       FIG. 15A  illustrates a graph  800  that contains a plot  810  of an embodiment of the spike annealing process  550  discussed above. The X-axis of the graph  800  represents time, for example with a unit of second. The Y-axis of the graph  800  represents temperature, for example with a unit of degree Celsius. Thus, the plot  810  represents how the temperature of the spike annealing process varies over time. At time=X0, the annealing temperature is maintained at Y1, which is below the peak temperature. In some embodiments, Y1=600 degrees Celsius. At time=X1, the temperature of the annealing process begins to jump or is ramped up rapidly. In some embodiments, the rapid ascension of the temperature (vs time) has a slope in a range between about 90 degrees Celsius per second and about 110 degrees Celsius per second, for example about 100 degrees Celsius per second. The ramping up of the temperature continues until a peak annealing temperature of Y2 is reached at time=X2. In some embodiments, the peak temperature Y2 is greater than 800 degrees Celsius but is less than 900 degrees Celsius, for example between about 810 degrees Celsius and about 840 degrees Celsius. The peak temperature Y2 is mostly maintained (e.g., a minor drop off of a few degrees is permitted) until time=X3. That is, the peak temperature duration (in which the peak temperature of Y2 is maintained) is equal to X3−X2. In some embodiments, the duration of X3−X2 is in a range between about 1 second and about 10 seconds, for example between about 1.5 seconds and about 3 seconds. After X3, the temperature begins to drop, but at a rate that is slower than the rate for which it is ramped up. Again, the characteristics of the annealing process are specifically configured to achieve various goals such as repairing the fin structures  252 - 253  (e.g., by removing the germanium oxide) and/or thickening the silicon cap layer  500 . 
       FIG. 15B  illustrates a graph  900  that contains a plot  910  of an embodiment of the soak annealing process  550  discussed above. The X-axis of the graph  900  represents time, for example with a unit of second. The Y-axis of the graph  900  represents temperature, for example with a unit of degree Celsius. Thus, the plot  910  represents how the temperature of the soak annealing process varies over time. At time=X0, the annealing temperature is maintained at Y1, which is below the peak temperature. In some embodiments, Y1=300 degrees Celsius. At time=X1, the temperature of the annealing process begins to increase, but at a rate much lower than when the temperature is ramped up in the spike annealing process. In some embodiments, the increase of the temperature (vs time) has a slope in a range between about 10 degrees Celsius per second and about 20 degrees Celsius per second, for example about 14 degrees Celsius per second. The ramping up of the temperature continues until a peak annealing temperature of Y2 is reached at time=X2. In some embodiments, the peak temperature Y2 is greater than 350 degrees Celsius but is less than 450 degrees Celsius, for example between about 375 degrees Celsius and about 425 degrees Celsius. The peak temperature Y2 is mostly maintained (e.g., a minor drop off of a few degrees is permitted) until time=X3. That is, the peak temperature duration (in which the peak temperature of Y2 is maintained) is equal to X3−X2. The peak temperature duration of the soak annealing process is much longer than that of the spike annealing process. In some embodiments, the duration of X3−X2 is in a range between about 50 seconds and about 200 seconds, for example between about 100 seconds and about 150 seconds. After time X3, the temperature is ramped down gradually, until time X4 is reached, where the temperature has dipped to Y3. In some embodiments, Y3 is in a range between 175 degrees Celsius and 225 degrees Celsius. After time X4, the tool may be turned off, and the temperature drops sharply below Y3. Though the annealing profile for the soak annealing process is different from that of the spike annealing process, they both achieve the same effect, for example with respect to repairing the fin structures  252 - 253  by removing the germanium oxide and/or thickening the silicon cap layer  500 . 
       FIG. 17  is a graph  1100  that illustrates a mobility improvement and a defect reduction according to embodiments of the present disclosure. The graph  1100  includes a horizontal X-axis and a vertical Y-axis. The X-axis corresponds to wafer defects, for example density of interface trap (DIT). The Y-axis corresponds to carrier mobility. Two groups of wafer samples are illustrated in  FIG. 17 . The group of wafer samples  1110  are wafer samples where the wafers are fabricated according to the methods of the present disclosure discussed in detail above. The group of wafer samples  1120  are wafer samples where the wafers are fabricated according to conventional processes. As shown in  FIG. 17 , the group of wafer samples  1110  not only have fewer wafer defects compared to the group of wafer samples  1120 , but also higher mobility. Alternatively stated, by performing the processes of the present disclosure, the wafer samples may be “moved” from the bottom right of the graph  1100  (where defects are greater but mobility is smaller) to the top left of the graph  1100  (where defects are fewer but mobility is higher). 
       FIG. 18  is a flowchart illustrating a method  1200  according to an embodiment of the present disclosure. The method  1200  includes a step  1210 , in which a semiconductor device is provided. The semiconductor device includes a fin structure that protrudes vertically upwards. 
     The method  1200  includes a step  1220 , in which a lateral dimension of the fin structure is reduced. 
     The method  1200  includes a step  1230 , in which a semiconductor layer is formed on the fin structure. 
     The method  1200  includes a step  1240 , in which an annealing process is performed to the semiconductor device. 
     The method  1200  includes a step  1250 , in which a dielectric layer is formed over the fin structure. 
     In some embodiments, a surface of the fin structure has a first roughness after the lateral dimension is reduced in step  1220 , and the surface of the fin structure has a second roughness less than the first roughness after the annealing process is performed in step  1240 . 
     In some embodiments, germanium oxide is formed at an interface between the fin structure and the semiconductor layer before the annealing process is performed in step  1240 , and the annealing process of step  1240  reduces the germanium oxide. 
     In some embodiments, the annealing process of step  1240  thickens the semiconductor layer. 
     In some embodiments, the semiconductor device comprises a fin structure that contains silicon germanium, and a silicon layer is epitaxially grown as the semiconductor layer, and at least a portion of the semiconductor layer is oxidized into the dielectric layer. 
     In some embodiments, the annealing process comprises a spike annealing process. In some embodiments, the spike annealing process is performed with an annealing temperature in a range between about 800 degrees Celsius and about 900 degrees Celsius, with an annealing time in a range between about 1 second and about 10 seconds, and with an annealing pressure in a range between about 50 torrs and about 760 torrs. 
     In some embodiments, the annealing process comprises a soak annealing process. In some embodiments, the soak annealing process is performed with an annealing temperature in a range between about 300 degrees Celsius and about 450 degrees Celsius, with an annealing time in a range between about 50 seconds and about 200 seconds, and with an annealing pressure in a range between about 50 torrs and about 760 torrs. 
     It is understood that additional processes may be performed before, during, or after the steps  1210 - 1250  of the method  1200 . For example, the method  1200  may include a step of forming a gate structure over the fin structure, wherein the dielectric layer serves as an interfacial layer of the gate structure. For reasons of simplicity, other additional steps are not discussed herein in detail. 
     In summary, the present disclosure performs a carefully tuned annealing process after the formation of the silicon cap layer and before the oxidation of the silicon cap layer. The annealing process reduces the germanium oxide content at the interface between the PFET fin structures and the silicon cap layer by causing the germanium to diffuse out of the semiconductor device. This mechanism also effectively thickens the silicon cap layer. The annealing process also repairs the rough fin surfaces by rearranging the atoms on the fin surfaces. After the annealing process is performed, an oxidation process is performed to oxidize the (now thickened) silicon cap layer, in order to form an interfacial layer to be used as a part of a gate structure of the FinFET transistor. 
     Based on the above discussions, it can be seen that the present disclosure offers advantages over conventional FinFET fabrication. It is understood, however, that other embodiments may offer additional advantages, and not all advantages are necessarily disclosed herein, and that no particular advantage is required for all embodiments. One advantage is that the present disclosure improves the fin structure quality. For example, without the performance of the annealing process herein, the fin structures may have roughened surfaces, which may lead to high resistivity and low carrier mobility, etc. Here, the annealing process rearranges the atoms on the surfaces of the fin structures, which smoothens out the fin structures. The smoother fin structures have better carrier mobility and lower resistivity. The smoother fin structures also mean better LWR and LER. As another example, the germanium oxide formed at an interface between the fin structures and the silicon cap layer may lead to density of interface trap (DIT) defects. Here, the annealing process provides extra energy to help break up the bonds between the germanium and oxygen, and the germanium may diffuse out of the semiconductor device, and oxygen may bond with silicon at the outer surfaces of the silicon cap layer. The reduction in germanium oxide reduces the DIT defects, as well as thickens the silicon cap layer, so that the silicon cap layer can be better served as a layer to be oxidized subsequently to form the interfacial layer of a gate structure. Other advantages include compatibility with existing FinFET fabrication, so the present disclosure is therefore easy and convenient to implement. 
     One aspect of the present disclosure pertains to a method of fabricating a semiconductor device. A semiconductor device is provided. The semiconductor device has a fin structure that protrudes vertically upwards. A lateral dimension of the fin structure is reduced. A semiconductor layer is formed on the fin structure after the reducing of the lateral dimension. An annealing process is performed to the semiconductor device after the forming of the semiconductor layer. A dielectric layer is formed over the fin structure after the performing of the annealing process. 
     Another one aspect of the present disclosure pertains to a method of fabricating a semiconductor device. A semiconductor device is provided. The semiconductor device has a fin structure that protrudes vertically upwards. A fin trimming process is performed to reduce a lateral dimension of the fin structure. The fin structure has roughened surfaces after the fin trimming process. A silicon cap layer is formed on the fin structure after the fin trimming process has been performed. The semiconductor device is annealed after the silicon cap layer has been formed. The fin structure has less roughened surfaces after the annealing. At least a portion of the silicon cap layer is converted into a dielectric layer after the annealing. 
     Yet another aspect of the present disclosure pertains to a semiconductor device. The semiconductor device includes a substrate that contains a semiconductive material. A fin structure protrudes out of the substrate. The fin structure is a fin structure for a p-type FinFET transistor. A silicon cap layer is disposed on the fin structure. A dielectric layer is disposed on the silicon cap layer. The fin structure has a line width roughness (LWR) between about 1.7 nanometers (nm) and about 1.9 nm. The fin structure has a line edge roughness (LER) between about 1.5 nanometers (nm) and about 1.7 nm. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. 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. For example, by implementing different thicknesses for the bit line conductor and word line conductor, one can achieve different resistances for the conductors. However, other techniques to vary the resistances of the metal conductors may also be utilized as well.