Patent Publication Number: US-2022231153-A1

Title: CMOS Fabrication Methods for Back-Gate Transistor

Description:
BACKGROUND 
     In recent development of integrated circuits, two dimensional (2D) semiconductor electronic devices were studied. A 2D transistor may include a 2D channel, which includes a channel having the thickness in atomic scale, with the channel formed between two insulator layers. 
    
    
     
       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 noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS. 1A, 1B, 2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 9A, 9B, 10A, 10B, 11A ,  11 B,  12 A,  12 B,  13 A,  13 B,  14 A,  14 B,  15 A,  15 B,  16 A,  16 B,  17 A,  17 B,  18 A, and  18 B illustrate the top views and cross-sectional views of intermediate stages in the formation of a Complementary Metal-Oxide-Semiconductor (CMOS) device in accordance with some embodiments. 
         FIG. 19  illustrates a cross-sectional view of a CMOS device in accordance with some embodiments. 
         FIGS. 20 through 24  illustrate a PFET-first process in the formation of a dielectric doping layer and a passivation layer in accordance with some embodiments. 
         FIGS. 25 through 28  illustrate the cycles of some atomic layer deposition processes in accordance with some embodiments. 
         FIG. 29  illustrates a process flow for forming a CMOS device in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “underlying,” “below,” “lower,” “overlying,” “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. 
     A Complementary Metal-Oxide-Semiconductor (CMOS) device and the method of forming the same are provided in accordance with some embodiments. The CMOS device includes a p-type MOS (PMOS) device and an n-type MOS (PMOS) device, which are formed based on two-dimensional channels such as Carbon Nanotubes (CNT) as channel materials. Appropriate channel doping is made to improve the NMOS device in order to achieve NMOS behavior rather than PMOS behavior. The metals of source/drain contact plugs of the PMOS device and the NMOS device are selected to reduce contact tunneling barrier. The Embodiments discussed herein are to provide examples to enable making or using the subject matter of this disclosure, and a person having ordinary skill in the art will readily understand modifications that can be made while remaining within contemplated scopes of different embodiments. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. Although method embodiments may be discussed as being performed in a particular order, other method embodiments may be performed in any logical order. 
       FIGS. 1A, 1B, 2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 9A, 9B, 10A, 10B, 11A ,  11 B,  12 A,  12 B,  13 A,  13 B,  14 A,  14 B,  15 A,  15 B,  16 A,  16 B,  17 A,  17 B,  18 A, and  18 B illustrate the top views and cross-sectional views of intermediate stages in the formation of a CMOS device in accordance with some embodiments of the present disclosure. The corresponding processes are also reflected schematically in the process flow shown in  FIG. 29 . 
       FIGS. 1A and 1B  illustrate a top view and a cross-sectional view, respectively, of a portion of wafer  10 , which includes substrate  20 . The substrate  20  may be a semiconductor substrate, such as a bulk semiconductor substrate, a Semiconductor-On-Insulator (SOI) substrate, or the like. Substrate  20  may be doped (e.g., with a p-type or an n-type dopant) or undoped. The semiconductor substrate  20  may be a part of wafer  10 , such as a silicon wafer. Generally, an SOI substrate is a layer of a semiconductor material formed on an insulator layer. The insulator layer may be, for example, a Buried Oxide (BOX) layer, a silicon oxide layer, or the like. The insulator layer is provided on a substrate, typically a silicon or glass substrate. Other substrates such as a multi-layered or gradient substrate may also be used. In accordance with some embodiments, the semiconductor material of semiconductor substrate  20  may include silicon; germanium; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, carbon-doped silicon, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. Substrate  20  may also be formed of other materials such as sapphire, Indium tin oxide (ITO), or the like. 
     In accordance with some embodiments, active devices  22 , which may include transistors (such as planar transistors, Fin Field Effect Transistors (FinFETs), or the like) may be formed at the surface of semiconductor substrate  20 . The transistors may have semiconductor materials such as silicon, silicon germanium, or the like as channel materials. In accordance with alternative embodiments, active devices  22  are not formed. Accordingly, active devices  22  are illustrated as being dashed to show that they may, or may not, be formed. 
     Over substrate  20  may reside interconnect structure  24 , which may include dielectric layers such as Inter-Layer Dielectric (ILD), and may or may not include Inter-Metal Dielectrics (IMDs) over the ILD. In accordance with some embodiments, the ILD is formed of or comprises silicon oxide, phospho-silicate glass (PSG), boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG), fluorine-doped silicate glass (FSG), or the like. The ILD may be formed using spin coating, Flowable Chemical Vapor Deposition (FCVD), or the like. In accordance with alternative embodiments of the present disclosure, the ILD is formed using a deposition method such as Plasma Enhanced Chemical Vapor Deposition (PECVD), Low Pressure Chemical Vapor Deposition (LPCVD), or the like. Contact plugs (not shown) such as source/drain contact plugs, gate contact plugs, etc., may be formed in the ILD. 
     Interconnect structure  24  may also include IMD(s). In accordance with some embodiments of the present disclosure, the IMDs are formed of low-k dielectric materials having dielectric constants (k-values) lower than about 3.0. The IMDs may comprise Black Diamond (a registered trademark of Applied Materials), a carbon-containing low-k dielectric material, Hydrogen SilsesQuioxane (HSQ), MethylSilsesQuioxane (MSQ), or the like. In accordance with some embodiments of the present disclosure, the formation of the IMDs includes depositing a porogen-containing dielectric material and then performing a curing process to drive out the porogen, and hence the remaining IMDs are porous. Metal lines and vias (not shown) may be formed in the IMDs. The IMDs may have metal lines and vias therein. An example of the metal lines and vias is represented by metal line  30  and via  28 , which are in dielectric layer  26 B. The dielectric layer(s) underlying dielectric layer  26 B are collectively shown as  26 A. 
     In accordance with some embodiments, the subsequently formed CMOS devices are over layers  26 A and  26 B. Layer  26 A may be an ILD having contact plugs (not shown) therein. Layer  26 B may be an IMD. In accordance with alternative embodiments, the to-be-formed CMOS devices are over ILD layer  26 A, and no IMD is underlying the CMOS devices. In accordance with yet alternative embodiments, the CMOS devices and the subsequently formed isolation layer  32  are formed on substrate  20  directly, with no ILD and IMD underlying the CMOS devices and etch stop layer  32 . 
     In accordance with some embodiments, metal lines  30  and the underlying vias  28  are formed using a damascene process, which includes etching dielectric layer  26 B to form a via opening and a trench over and joined with the via opening, filling the via opening and the trench with conductive materials, and performing a planarization process such as a Chemical Mechanical Polish (CMP) process or a mechanical grinding process to remove excess portions of the conductive materials. Metal lines  30  and vias  28  may include diffusion barrier layer  29  and copper-containing material  31  over diffusion barrier layers  29 . Diffusion barrier layer  29  may include titanium, titanium nitride, tantalum, tantalum nitride, or the like. Diffusion barrier layer  29  has the function of preventing the copper in copper-containing material  31  from diffusing into dielectric layer  26 B. Diffusion barrier layer  29  may also act as a glue layer. 
     As shown in  FIG. 1B , isolation layer  32  is formed. Isolation layer  32 , which may also act as an etch stop layer, may be formed over interconnect structure  24  or in physical contact with substrate  20 . In accordance with some embodiments of the present disclosure, isolation layer  32  is formed of or comprises an oxide such as silicon oxide, a nitride such as silicon nitride, a high-k dielectric material such as aluminum oxide, hafnium oxide, zirconium oxide, lanthanum oxide, or the like. 
     As also shown in  FIG. 1B , wafer  10  includes PMOS region  100 P and NMOS region  100 N, in which a PMOS device and an NMOS device, respectively, are to be formed, which collectively form a CMOS device. Furthermore, device region  1001  is also shown, which will not have PMOS and NMOS devices formed therein. It is appreciated that although metal line  30  and via  28  are shown in device region  1001 , the metal lines and vias may also be formed in device regions  100 P and  100 N, and may be directly underlying the PMOS device and the NMOS device that are to be formed in subsequent processes. In some of the subsequent figures, device region  1001  is not shown. In the subsequent processes starting from  FIGS. 1A and 1B  and ending with  FIGS. 18A and 18B , each of the figure numbers includes letter “A,” or “B.” The letter “A” indicates that the respective figure shows a top view. The letter “B” indicates that the respective figure shows the reference cross-section “B-B” in the respective top view. 
       FIGS. 2A and 2B  illustrate a top view and a cross-sectional view, respectively, in the formation of p-type work-function layer  34 P and an n-type work-function layer  34 N. The respective process is illustrated as process  202  in the process flow  200  as shown in  FIG. 29 . The formation of each of p-type work-function layer  34 P and n-type work-function layer  34 N may include depositing a blanket metal layer, and then patterning the blanket metal layer through a photolithography process. In accordance with some embodiments, the p-type work-function layer  34 P may be formed of or comprises a metal such as platinum (Pt), palladium (Pd), gold (Au), or the alloys thereof. The n-type work-function layer  34 N may be formed of or comprises a metal such as Al, Ti, or the alloys thereof. The thicknesses of work-function layers  34 P and  34 N may be in the range between about 1 nm and about 10 nm. The formation method may include Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), or the like. Work-function layers  34 P and  34 N act as the gate electrodes of the respective PMOS and NMOS devices, and are referred to as gate electrodes  34 P and  34 N, respectively. 
       FIGS. 3A and 3B  illustrate a top view and a cross-sectional view, respectively, in the formation of gate dielectric layer  38 . The respective process is illustrated as process  204  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments, the gate dielectric layer comprises a high-k dielectric material such as HfO 2 , Al 2 O 3 , ZrO 2 , or the like, while other materials such as silicon oxide may also be used. The deposition method may include Atomic Layer Deposition (ALD), CVD, or the like. Gate dielectric layer  38  is formed as a conformal layer extending on the top surface and the sidewalls of p-type work-function layer  34 P and n-type work-function layer  34 N. For example, the difference between the horizontal thickness T 1  of the horizontal portions and the thickness T 2  of the vertical portions of gate dielectric layer  38  may be smaller than 20 percent or 10 percent of both of thicknesses T 1  and T 2 . An ALD cycle of an example ALD process for forming HfO 2  is illustrated in  FIG. 25 , which is discussed in detail in subsequent paragraphs. In accordance with some embodiments, the thickness of gate dielectric layer  38  is in the range between about 1 nm and about 15 nm. 
       FIGS. 4A and 4B  illustrate a top view and a cross-sectional view, respectively, in the formation of gate contact openings  40 P and  40 N in gate dielectric layer  38 . The respective process is illustrated as process  206  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments, the formation of gate contact openings  40 P and  40 N includes an etching process, with a patterned photo resist formed to define the positions of via openings  40 P and  40 N. As shown in  FIG. 4A , gate electrodes  34 P and  34 N are revealed through gate contact openings  40 P and  40 N, respectively. The etching may be performed through a wet etching process or a dry etching process. The formation of gate contact openings  40 P and  40 N is for the easier formation of gate contact openings as shown in  FIGS. 15A and 15B , so that fewer layers need to be etched. In accordance with alternative embodiments of the present disclosure, the process shown in  FIGS. 4A and 4B  is skipped. 
     Referring to  FIGS. 5A and 5B , semiconductor layer  42  is formed through deposition. The respective process is illustrated as process  208  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments of the present disclosure, semiconductor layer  42  is formed of a low-dimensional material, which may include carbon nanotube networks, aligned carbon nanotubes, two-dimensional (2D) materials such as Transition Metal Dichalcogenides (TMDs), or the like. The carbon nanotube networks and aligned carbon nanotubes may be formed using immersion, drop-casting, or the like methods. The TMD material may be the compound of a transition metal and a group-VIA element. The transition metal may include W, Mo, Ti, V, Co, Ni, Zr, Tc, Rh, Pd, Hf, Ta, Re, Ir, Pt, or the like. The group-VIA element may be sulfur (S), selenium (Se), tellurium (Te), or the like. For example, semiconductor layer  42  may be formed of or comprise MoS 2 , MoSe 2 , WS 2 , WSe 2 , or the like. The formation of the TMD material may include CVD, for example, with MoO 3  powder and sulfur (s) (or Se) powder being used as precursors, and nitrogen (N 2 ) being used as a carrier gas. In accordance with alternative embodiments of the present disclosure, PECVD or another applicable method may be used to form the TMD material. In accordance with some embodiments of the present disclosure, semiconductor layer  42  has a thickness in the range between about 0.7 nm and about 5 nm. Semiconductor layer  42  is also formed as a conformal layer, for example, with the variation of the thicknesses of the horizontal portions and vertical portions being smaller than about 20 percent or 10 percent. 
       FIGS. 6A and 6B  illustrate a top view and a cross-sectional view, respectively, in the patterning of semiconductor layer  42  into portions  42 P and  42 N in device regions  100 P and  100 N, respectively. Portion  42 P and portion  42 N of semiconductor layer  42  will act as the channel material and some parts of source/drain regions of the respective PMOS device and the NMOS device. The respective process is illustrated as process  210  in the process flow  200  as shown in  FIG. 29 . The patterning may be performed by forming a photo resist, and then performing an etching process to remove some portions of semiconductor layer  42 , so that the remaining portions  42 P and  42 N are disconnected from each other. Furthermore, the portions of semiconductor layer  42  extending into via openings  40 P and  40 N are also etched, revealing via openings  40 P and  40 N again, as shown in  FIG. 6A . An appropriate etchant is selected corresponding to the material of semiconductor layer  42 . For example, when semiconductor layer  42  is formed of carbon nanotube networks and aligned carbon nanotubes, an oxygen-based etching gas such as O 2 , O 3 , or combinations thereof may be used as the etching gas. 
     Referring to  FIGS. 7A and 7B , an annealing process  44  (marked in  FIG. 7B ) is performed. The respective process is illustrated as process  212  in the process flow  200  as shown in  FIG. 29 . The corresponding process gas used for the annealing process may include forming gas, which includes hydrogen (H 2 ) and nitrogen (N 2 ). In accordance with some embodiments, the annealing process  44  is performed at a temperature in the range between about 250° C. and about 400° C. Carrier gas such as Ar, He, and/or Ne may be added. The annealing duration may be in the range between about 30 minutes and about 60 minutes. The gas flow of hydrogen in the forming gas may be in the range between about 13 percent and about 15 percent of the flow rate of the carrier gas. The ambient pressure during the annealing process may be in the range between about 100 torr and one atmosphere. The annealing process may cure the semiconductor layer  42  and gate dielectric layer  38  to improve their quality. For example, the density of gate dielectric layer  38  may be less porous after the annealing process  44 . 
       FIGS. 8A and 8B  illustrate a top view and a cross-sectional view, respectively, in the formation of the source/drain contacts  46 P. The respective process is illustrated as process  214  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments, the formation process includes depositing a conductive material, which may include a metal such as Pt, Pd, or the like, or an alloy thereof, and then performing an etching process to pattern the conductive material. The deposition method may include PVD, CVD, or the like. The thickness of the conductive material may be in the range between about 10 nm and about 50 nm. The patterned conductive material form source/drain contacts  46 P, which are in physical contact with the opposing end portions of semiconductor layer  42 P.  FIGS. 8A and 8B  illustrate that each of the source/drain contacts  46 P includes a portion overlapping a portion of gate electrode  34 P, with a portion of the semiconductor layer  42 P being exposed through the source/drain contacts  46 P to act as the channel of the PMOS device. 
       FIGS. 9A and 9B  illustrate a top view and a cross-sectional view, respectively, in the formation of the source/drain contacts  46 N. The respective process is illustrated as process  216  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments, the formation process includes depositing a conductive material, which may include a metal such as Ti, Al, or the like, or alloys thereof, and then performing an etching process to pattern the conductive material. Source/drain contacts  46 N may also include a gold (Au) layer over the Ti or Al layer to improve the contact. The deposition method may include PVD, CVD, or the like. The thickness of the lower conductive material such as the Ti layer or the Al layer may be in the range between about 10 nm and about 50 nm, and the thickness of the upper conductive material such as the Au layer may be in the range between about 10 nm and about 30 nm. The patterned conductive layers form source/drain contacts  46 N, which are in physical contact with some portions of semiconductor layer  42 N.  FIGS. 9A and 9B  illustrate that each of the source/drain contacts  46 N includes a portion overlapping an end portion of gate electrode  34 N, with a portion of the semiconductor layer  42 N being exposed through the source/drain contacts  46 N to act as the channel of the NMOS device. 
     In accordance with some embodiments of the present disclosure, the source/drain contacts  46 P of the PMOS device and the source/drain contacts  46 N of the NMOS device use different metals, for example, with source/drain contacts  46 P using Pt and/or Pd, and source/drain contacts  46 N using Ti and/or Al. This may reduce the contact tunneling barriers for both of the PMOS and NMOS devices, wherein the contact tunneling barriers include the barrier between semiconductor layer  42 P and source/drain contacts  46 P, and the barrier between semiconductor layer  42 N and source/drain contacts  46 N. 
     In subsequent processes shown in  FIGS. 10A, 10B, 11A, 11B, 12A, 12B, 13A , and  13 B, a channel-doping capping layer for the NMOS device and a passivation capping layer for the PMOS device are formed. The illustrated example is an NFET-first and PFET-last process, in which the formation of the channel-doping capping layer for the NMOS device is performed before the formation of the passivation capping layer for the PMOS device. 
     Referring to  FIGS. 10A and 10B , dielectric doping layer  48 N is formed. The respective process is illustrated as process  218  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments, dielectric doping layer  48 N is or comprises aluminum oxide (Al 2 O 3 ). In accordance with some embodiments, aluminum oxide layer  48 N includes lower layer  48 N 1 , and upper layer  48 N 2  over lower layer  48 N 1 . The formation of the lower layer  48 N 1  may include depositing an aluminum layer, for example, using PVD, and then oxidizing the aluminum layer to form an aluminum oxide layer. The deposited aluminum layer may have a very small thickness, for example, in the range between about 1 nm and about 2 nm. In the oxidation process, the deposited aluminum layer may be oxidized in open air (hence through nature oxidation), or in an oxygen (O 2 ) containing environment with a low oxygen partial pressure, for example, with the oxygen partial pressure being in the range between about 10 torr and about 500 torr. The oxidation may be performed at room temperature (for example, between about 20° C. and about 25° C.), or at a moderately elevated temperature, for example, in a temperature range between about 20° C. and about 150° C., or in a temperature range between about 100° C. and about 120° C. With the nature oxidation or the low partial pressure or the low temperature oxidation, the oxidation rate is reduced, resulting in a higher-quality aluminum oxide  48 N 1 . 
     By forming an aluminum layer and then oxidizing the aluminum layer to form aluminum oxide layer  48 N 1 , aluminum oxide layer  48 N 2  may be deposited on aluminum oxide layer  48 N 1  through ALD (such as Plasma-Enhanced ALD (PEALD) or thermal ALD as shown in  FIGS. 27 and 28 , respectively). The thickness of aluminum oxide layer  48 N 2  may be in the range between about 10 nm and about 50 nm. By forming aluminum oxide layer  48 N 1  as being the base on which aluminum oxide layer  48 N 2  is deposited, the uniformity of aluminum oxide layer  48 N is improved than depositing aluminum oxide layer  48 N 2  directly on semiconductor layer  42 . Furthermore, with the aluminum oxide layer  48 N 1  formed through deposition and oxidation, the water steam that may be used in the ALD process for forming aluminum oxide layer  48 N 2  is separated from the channel material of the NMOS device, and the sensitivity of the threshold voltage of the resulting NMOS to the doping of aluminum into channel layer  42 B is improved. In addition, aluminum oxide layer  48 N 2  is less porous. 
       FIG. 27  illustrates an example of a PEALD cycle for depositing aluminum oxide (Al 2 O 3 ) in accordance with some embodiments. The PEALD process includes pulsing (conducting) oxygen into the respective chamber, with plasma being turned on when the oxygen is pulsed. The power of the plasma may be in the range between about 20 watts and about 60 watts, for example, with a RF power source operating at a frequency of 13.56 MHz. The oxygen is then purged through a nitrogen pulse. Next, a precursor such as trimethylaluminium (TMA, (CH 3 ) 3 Al)) is pulsed, followed by the purging of the precursor through a nitrogen pulse. The PEALD cycle may cause an atomic aluminum oxide layer of about 1.2 Å to be deposited, and a plurality of PEALD cycles may increase the thickness of the aluminum oxide to the desirable thickness. 
       FIG. 28  illustrates an example of a thermal ALD cycle for forming aluminum oxide (Al 2 O 3 ) in accordance with some embodiments. The thermal ALD process includes pulsing water steam (H 2 O) into the respective chamber. The water steam is then purged through a nitrogen pulse. Next, a precursor such as TMA is pulsed, followed by the purging of the precursor through a nitrogen pulse. During the thermal ALD, the temperature of wafer  10  may be in the range between about 150° C. and about 300° C. The thermal ALD cycle may cause an atomic aluminum oxide layer of about 1.0 Å to be deposited, and a plurality of thermal ALD cycles may increase the thickness of the aluminum oxide to the desirable thickness. It is appreciated that since water steam may be used, aluminum oxide layer  48 N 1  prevents the water steam from reaching and degrading semiconductor layer  42 . 
     In accordance with alternative embodiments, instead of forming an aluminum oxide layer as the dielectric doping layer, a hafnium oxide layer (which may be hafnium-rich) is formed as dielectric doping layer  48 N. The thickness of the hafnium oxide layer or the hafnium-rich hafnium oxide layer  48 N may be in the range between about 10 nm and about 50 nm. The hafnium oxide layer  48 N may be formed in a PEALD process as shown in  FIG. 25 , which PEALD process may also be used for forming gate dielectric layer  38 . As shown in  FIG. 25 , the PEALD process includes pulsing a precursor such as tetrakis(ethylmethylamido)hafnium (also referred to as Hf(NMeEt) 4 ) into the reaction chamber, followed by the purging of the precursor through a nitrogen pulse. Next, the oxygen is pulsed into the respective chamber, with plasma turned on when the oxygen is pulsed. The power of the plasma may be in the range between about 20 watts and about 60 watts, for example, with a RF power source operating at the frequency of 13.56 MHz. Next, the oxygen is purged through a nitrogen pulse. PEALD cycle may cause an atomic hafnium oxide layer of about 1.2 Å to be deposited, and a plurality of PEALD cycles are performed. 
       FIG. 26  illustrates an example of a thermal ALD cycle for forming hafnium-rich hafnium oxide (Al 2 O 3 ) in accordance with some embodiments. The thermal ALD process includes pulsing water steam (H 2 O) into the respective chamber. The water steam is then purged through a nitrogen pulse. Next, a precursor such as Hf(NMeEt) 4  is pulsed, followed by the purging of Hf(NMeEt) 4  through a nitrogen pulse. Next, one or more additional Hf(NMeEt) 4  pulsing and purging process is performed to increase the atomic percentage of hafnium. For example, in the hafnium-rich hafnium oxide, the hafnium atomic percentage may be greater than about 30 percent, and may be in the range between about 32 percent and about 35 percent. During the thermal ALD process, the temperature of the wafer may be in the range between about 150° C. and about 300° C. The thermal ALD cycle may cause an atomic hafnium-rich hafnium oxide layer of about 1.0 Å to be deposited, and a plurality of thermal ALD cycles are performed. 
       FIGS. 11A and 11B  illustrate a top view and a cross-sectional view, respectively, in the patterning of dielectric doping layer  48 N. The process includes the removal of dielectric doping layer  48 N from device region  100 P, which is performed in a lithography process. The respective process is illustrated as process  220  in the process flow  200  as shown in  FIG. 29 . 
       FIGS. 12A and 12B  illustrate a top view and a cross-sectional view, respectively, in the deposition of dielectric passivation layer  48 P. The respective process is illustrated as process  222  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments, passivation layer  48 P is formed of or comprises silicon oxide. The deposition method may include PVD, which may include a thermal evaporate process. The resulting passivation layer  48 P may have a thickness in the range between about 10 nm and about 50 nm. 
       FIGS. 13A and 13B  illustrate a top view and a cross-sectional view, respectively, in the patterning of passivation layer  48 P. The process includes the removal of passivation layer  48 P from device region  100 N, which is performed in a lithography process. The respective process is illustrated as process  224  in the process flow  200  as shown in  FIG. 29 . 
     Referring to  FIGS. 14A and 14B , an annealing process  50  ( FIG. 14B ) is performed. The respective process is illustrated as process  226  in the process flow  200  as shown in  FIG. 29 . The process gas may include forming gas, which includes hydrogen (H 2 ) and nitrogen (N 2 ). Carrier gas such as Ar, He, and/or the like may be added. In accordance with some embodiments, the annealing process is performed at a temperature in the range between about 250° C. and about 400° C. The annealing duration may be in the range between about 30 minutes and about 60 minutes. The gas flow rate of hydrogen in the forming gas may be in the range between about 13 percent and about 15 percent of the flow rate of the carrier gas. The ambient pressure during the annealing process may be in the range between about 100 torr and one atmosphere. The annealing process may cure passivation layer  48 P and dielectric doping layer  48 N. Furthermore, the aluminum or hafnium in dielectric doping layer  48 N may diffuse into semiconductor layer  42 N, causing the shifting of the resulting MOS device from P-side to N-side. For example, if aluminum or hafnium is not doped into semiconductor layer  42 N, the resulting NMOS device in device region  100 N may demonstrate a negative threshold voltage (meaning the device is turned on when a negative Vgs voltage is applied). With the doping of aluminum or hafnium, the threshold voltage is increased as being a positive threshold voltage. The magnitude of threshold-voltage-tuning may be tuned through adjusting the thickness of dielectric doping layer  48 N, and/or the atomic percentage of hafnium when the hafnium-rich hafnium oxide is used. In accordance with some embodiments, in the annealing process, the threshold voltage of the NMOS device is increased, for example, by an amount in a range between about 0.5 volts and about 1.0 volts. On the other hand, due to the selection of the material of the passivation layer  48 P, in the anneal process, the threshold voltage of the PMOS device is unchanged. 
       FIGS. 15A and 15B  illustrate a top view and a cross-sectional view, respectively, in the re-opening of gate contact opening  40 P ( FIG. 15A ) and the formation of source/drain contact openings  51 P ( FIG. 15B ). The respective process is illustrated as process  228  in the process flow  200  as shown in  FIG. 29 . The gate contact opening  40 P as shown in  FIG. 4A  may have been filled with dielectric passivation layer  48 P, which may be formed of or comprise silicon oxide. Hence, an etching process is performed to remove dielectric passivation layer  48 P from gate contact opening  40 P, and contact opening  40 P is revealed again. 
       FIGS. 16A and 16B  illustrate a top view and a cross-sectional view, respectively, in the re-opening of gate contact opening  40 N and the formation of source/drain contact openings  51 N. The respective process is illustrated as process  230  in the process flow  200  as shown in  FIG. 29 . The gate contact opening  40 N as shown in  FIG. 4A  may have been filled with dielectric doping layer  48 N, which may be formed of HfO 2 , Hf-rich HfO 2 , Al 2 O 3 , or the like. Hence, an etching process is performed to remove dielectric doping layer  48 N from gate contact opening  40 N, and contact opening  40 N is revealed again. 
       FIGS. 17A and 17B  illustrate a top view and a cross-sectional view, respectively, in the formation of metal interconnections  54 , which include  54 A,  54 B,  54 C, and  54 D in accordance with some embodiments. The respective process is illustrated as process  232  in the process flow  200  as shown in  FIG. 29 . PMOS device  102 P and NMOS device  102 N are thus formed, which collectively form a CMOS device. In accordance with some embodiments, metal interconnections  54  are formed of or comprise copper, aluminum, tungsten, or the like. The formation process may include PVD, CVD, or the like. In accordance with some embodiments, metal interconnection  54 A interconnects gate electrodes  34 P and  34 N. Metal interconnection  54 B interconnects the drain of NMOS device  102 N and the drain of PMOS device  102 P. Metal interconnections  54 C and  54 D are connected to the source regions of PMOS device  102 P and NMOS device  102 N. It is appreciated that the illustrated example connection scheme is for an inverter. If other circuits are to be formed using PMOS device  102 P and NMOS device  102 N, different connections are adopted. 
       FIGS. 18A and 18B  illustrate a top view and a cross-sectional view, respectively, in the formation of dielectric layer  56 . The respective process is illustrated as process  234  in the process flow  200  as shown in  FIG. 29 . In accordance with some embodiments, dielectric layer  56  is formed of a dielectric material such as silicon oxide, silicon nitride, or the like, which may be formed using PECVD or the like method. In accordance with alternative embodiments, dielectric layer  56  is formed of a low-k dielectric material, which may be selected from the same group of candidate materials form forming dielectric layer  26 B. 
       FIG. 18B  further illustrates the formation of metal line  58  and via  60 , which may be formed using a damascene process. It is appreciated that metal lines and vias may also be formed to connect to metal interconnections  54 , which metal lines and vias are not shown. 
       FIG. 19  illustrates a CMOS device including a PMOS device  102 P and an NMOS device  102 N in accordance with some embodiments. Unless specified otherwise, the materials and the formation processes of the components in these embodiments (and the embodiments shown in  FIGS. 20 through 23 ) are essentially the same as the like components, which are denoted by like reference numerals in the embodiments shown in the preceding figures. The details regarding the formation process and the materials of the components shown in the preceding figures may thus be found in the discussion of the preceding embodiments. 
     The CMOS device shown in  FIG. 19  is similar to the embodiments shown in  FIGS. 18A and 18B , except that gate electrodes  34 P and  34 N, instead of protruding over an underlying planar dielectric layer, are formed in dielectric layer  26 B, and may have top surfaces coplanar with the top surface of dielectric layer  26 B. The formation of gate electrodes  34 P and  34 N may be performed through damascene processes, which include forming trenches in dielectric layer  26 B, filling metallic layers into the trenches, and then performing a planarization process to level the top surfaces of gate electrodes  34 P and  34 N with the top surface of dielectric layer  26 B. Gate electrodes  34 P and  34 N may be formed of the material as discussed referring to  FIGS. 2A and 2B , and may be formed in separate processes, or may be formed of a same material (and in a same process) such as copper or a copper alloy. Gate dielectric layer  38  in accordance with these embodiments is thus a planar layer. 
     In the process shown in preceding  FIGS. 10A and 10B through 13A and 13B , an NFET-first process is performed, in which the dielectric doping layer  48 N is formed before dielectric passivation layer  48 P. In accordance with alternative embodiments, as shown in  FIGS. 20 through 24 , a PFET-first process is performed, in which the dielectric passivation layer  48 P is formed before the formation of dielectric doping layer  48 N. It is appreciated that  FIGS. 20 through 24  merely illustrate the formation of passivation layer  48 P and dielectric doping layer  48 N. Other details are not shown, and may be found referring to previous figures including  FIGS. 18A and 18B  and  FIG. 19 , and their correspond formation processes. For example, the formation of gate electrodes  34 P and  34 N and their shapes may be found referring to  FIG. 18B  or  FIG. 19 . 
       FIG. 20  illustrates the formation of gate dielectric  38 , semiconductor layers  42 P and  42 N, and source/drain contacts  46 P and  46 N. Next, referring to  FIG. 21 , dielectric passivation layer  48 P is formed. Referring to  FIG. 22 , annealing process  44  is performed. Next, as shown in  FIG. 23 , dielectric doping layer  48 N is formed, which extends into both of PMOS device region  100 P and NMOS device region  100 N. Next, as shown in  FIG. 24 , the portion of dielectric doping layer  48 N in device region  100 P is removed, leaving dielectric doping layer  48 N in device region  100 N. Since dielectric passivation layer  48 P is formed before the formation of dielectric doping layer  48 N, the respective process is referred to as a PFET-first process. In accordance with some embodiments, annealing process  50  is performed. The subsequent process may be realized through the teaching referring to  FIGS. 14A / 14 B- 18 A/ 18 B. 
     The embodiments of the present disclosure have some advantageous features. By forming a dielectric doping layer over the channel of the NMOS device and doping the channel of the NMOS device with an appropriate metal such as Al or Hf, the threshold voltage of the resulting device is increased, and the resulting NMOS device, rather than having the PMOS device behavior, demonstrates proper NMOS behavior, that is, when a positive Vgs is applied, starts conducting current. By adopting different metals for source/drain contacts of the PMOS and NMOS devices, the tunneling barriers of the source/drain contacts of both of the PMOS device and NMOS device are reduced. Also, by depositing and oxidizing aluminum before depositing aluminum oxide, or adjusting the hafnium atomic percentage in the hafnium oxide, the dielectric doping layer may be used to tune the threshold voltage of the NMOS device to desirable values. 
     In accordance with some embodiments of the present disclosure, a method includes forming a first gate electrode and a second gate electrode; forming a gate dielectric layer extending over the first gate electrode and the second gate electrode; and forming an n-type transistor and a p-type transistor. The forming the n-type transistor comprising forming a first low-dimensional semiconductor layer over the gate dielectric layer, wherein a first portion of the first low-dimensional semiconductor layer is directly over the first gate electrode; forming first source/drain contacts on opposing sides of the first gate electrode, wherein the first source/drain contacts are in in contact with first opposing portions of the first low-dimensional semiconductor layer; depositing a dielectric doping layer comprising aluminum oxide or hafnium oxide over and contacting the first low-dimensional semiconductor layer. The forming the p-type transistor comprises forming a second low-dimensional semiconductor layer over the gate dielectric layer, wherein a second portion of the second low-dimensional semiconductor layer is directly over the second gate electrode; forming second source/drain contacts on opposing sides of the second gate electrode, wherein the second source/drain contacts are in in contact with second opposing portions of the second low-dimensional semiconductor layer; and depositing a dielectric passivation layer over and contacting the second low-dimensional semiconductor layer, wherein the dielectric passivation layer and the dielectric doping layer are formed of different materials. In an embodiment, the method further comprises, after the dielectric doping layer and the dielectric passivation layer are deposited, performing an annealing process to drive a metal in the dielectric doping layer into the first low-dimensional semiconductor layer. In an embodiment, the dielectric doping layer comprises aluminum oxide, and the depositing the dielectric doping layer comprises depositing an aluminum layer; oxidizing the aluminum layer to form a first aluminum oxide layer; and depositing a second aluminum oxide layer on the first aluminum oxide layer. In an embodiment, the depositing the dielectric doping layer comprises a plurality of atomic layer deposition cycles to form a hafnium-rich oxide, and wherein each of the plurality of atomic layer deposition cycles comprises a water steam pulse process and a plurality of hafnium-comprising precursor pulse processes. In an embodiment, the annealing process results in a first threshold voltage of the n-type transistor to be increased, and a second threshold voltage of the p-type transistor is unchanged by the annealing process. In an embodiment, the first source/drain contacts comprise aluminum or titanium, and the second source/drain contacts comprise palladium. In an embodiment, the first low-dimensional semiconductor layer and the second low-dimensional semiconductor layer are formed simultaneously, and comprise a material selected from the group consisting of carbon nanotube networks and aligned carbon nanotubes. In an embodiment, the method further comprises forming a low-k dielectric layer, wherein the first gate electrode and the second gate electrode are over at least a portion of the low-k dielectric layer. 
     In accordance with some embodiments of the present disclosure, a device comprises an n-type transistor and a p-type transistor. The n-type transistor comprises a first gate electrode; a first gate dielectric layer over the first gate electrode; a first low-dimensional semiconductor layer comprising a first portion over the first gate dielectric layer; a dielectric doping layer comprising aluminum oxide or hafnium oxide over and contacting the first low-dimensional semiconductor layer. The p-type transistor comprises a second gate electrode; a second gate dielectric layer over the second gate electrode; a second low-dimensional semiconductor layer comprising a second portion over the second gate dielectric layer; and a dielectric passivation layer over and contacting the second low-dimensional semiconductor layer, wherein the dielectric passivation layer and the dielectric doping layer are formed of different materials. In an embodiment, the dielectric passivation layer comprises silicon oxide, and the dielectric doping layer comprises aluminum oxide. In an embodiment, the dielectric passivation layer comprises silicon oxide, and the dielectric doping layer comprises hafnium oxide. In an embodiment, the hafnium oxide is hafnium-rich. In an embodiment, the n-type transistor comprises first source/drain contacts on opposing sides of the first gate electrode, wherein the first source/drain contacts are in in contact with first opposing portions of the first low-dimensional semiconductor layer; and the p-type transistor comprises second source/drain contacts on opposing sides of the second gate electrode, wherein the second source/drain contacts are in in contact with second opposing portions of the second low-dimensional semiconductor layer, and the first source/drain contacts and the second source/drain contacts comprise different metals. In an embodiment, the first source/drain contacts comprise aluminum or titanium, and the second source/drain contacts comprise palladium. In an embodiment, the first low-dimensional semiconductor layer comprises a top portion directly over the first gate electrode; and a sidewall portion on a sidewall of the first gate electrode. 
     In accordance with some embodiments of the present disclosure, a device comprises a semiconductor substrate; a low-k dielectric layer over the semiconductor substrate; an isolation layer over the low-k dielectric layer; a first work function layer over the isolation layer, wherein the first work function layer is an n-type work function layer; a first low-dimensional semiconductor layer on a first top surface and a first sidewall of the first work function layer; first source/drain contacts contacting opposing end portions of the first low-dimensional semiconductor layer; and a dielectric doping layer over and contacting a channel portion of the first low-dimensional semiconductor layer, wherein the dielectric doping layer comprises a metal selected from aluminum and hafnium, and wherein the channel portion of the first low-dimensional semiconductor layer further comprises the metal. In an embodiment, the device further comprises an additional transistor formed at a surface of the semiconductor substrate, wherein the additional transistor comprises silicon or silicon germanium as a channel material. In an embodiment, the device further comprises a second work function layer over the isolation layer, wherein the second work function layer is a p-type work function layer; a second low-dimensional semiconductor layer on a second top surface and a second sidewall of the second work function layer; second source/drain contacts contacting opposing end portions of the second low-dimensional semiconductor layer; and a dielectric passivation layer, wherein the dielectric passivation layer and the second low-dimensional semiconductor layer are free from the metal. In an embodiment, the first source/drain contacts and the second source/drain contacts comprise different metals. In an embodiment, the metal comprises hafnium. 
     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.