Patent Publication Number: US-11658220-B2

Title: Drain side recess for back-side power rail device

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application claims priority to U.S. Provisional Application Ser. No. 63/014,880, filed Apr. 24, 2020, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs. 
    
    
     
       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. 
         FIG.  1    illustrates a perspective view of some embodiments of a semiconductor transistor device having a recessed source/drain region. 
         FIG.  2    illustrates a cross-sectional view of some additional embodiments of a semiconductor transistor device taken along line A-A′ of  FIG.  1   . 
         FIGS.  3 A- 3 B  are cross-sectional views of various embodiments of a semiconductor transistor device taken along line B-B′ of  FIG.  1   . 
         FIGS.  4 A- 4 B  are cross-sectional views of various embodiments of a semiconductor transistor device taken along line C-C′ of  FIG.  1   . 
         FIG.  5    is a cross-sectional view of some embodiments of a semiconductor transistor device taken along line D-D′ of  FIG.  1   . 
         FIGS.  6 - 29 B  illustrate various views of some embodiments of a method of forming a semiconductor transistor device having a recessed source/drain region at various stages. 
         FIG.  30    illustrates a flow diagram of some embodiments of a method corresponding to  FIGS.  6 - 29 B . 
     
    
    
     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. 
     As used herein, “around,” “about,” “approximately,” or “substantially” shall generally mean within 20 percent, or within 10 percent, or within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around,” “about,” “approximately,” or “substantially” can be inferred if not expressly stated. 
     Gate all around (GAA) transistor structures may be patterned by any suitable method. For example, the structures may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers may then be used to pattern the GAA transistor structures. After forming the GAA transistor structures, an interconnect structure may be formed thereover including power rails and signal lines disposed within interlayer dielectric (ILD) layers. 
     Current power rail design will suffer a complex metal layer routing in back-end-of-line (BEOL) when semiconductor process continues to shrink, for example, beyond 3 nm. As a result of complex metal layer routing, more masks are needed, and voltage drop (also referred as IR drop) suffers when metal wires become thinner. 
     In view of the above, the present disclosure is related to a semiconductor transistor device having a back-side power rail and manufacturing methods thereof. By moving a power rail from a front side to a back side of the semiconductor transistor device, the metal layer routing is relaxed in BEOL. Thus, fewer masks are needed, IR drop is improved, and both power rail area and active region can be enlarged. More particularly, the present disclosure is related to a semiconductor transistor device with a recessed source/drain region. In some embodiments, the semiconductor transistor device comprises a channel structure, a gate structure wrapping around the channel structure, a first source/drain epitaxial structure and a second source/drain epitaxial structure disposed on opposite endings of the channel structure, and a gate contact disposed on the gate structure. The semiconductor transistor device further comprises a back-side source/drain contact landing on a recessed bottom surface of the first source/drain epitaxial structure, and a back-side power rail disposed under and connecting the back-side source/drain contact. The back-side source/drain contact and the back-side power rail may comprise metal materials for example. In some embodiments, a bottom surface of the first source/drain epitaxial structure may be recessed to a location vertically deeper than a bottom surface of the gate structure or the channel structure. A bottom surface of the second source/drain epitaxial structure may also be recessed to a location vertically deeper than the bottom surface of the gate structure or the channel structure. In some further embodiments, a back-side dielectric cap may replace original semiconductor body material and contact the bottom surfaces of the gate structure and the second source/drain epitaxial structure. The back-side dielectric cap may comprise an oxide, nitride, carbon nitride, or low-κ dielectric materials. Thus, cell capacitance can be reduced, and current leakage problems such as a leakage between the gate structure and the back-side source/drain contact can be eliminated. 
     The semiconductor transistor devices presented herein may include a p-type GAA device or an n-type GAA device. Further, the semiconductor transistor devices may have one or more channel regions, such as semiconductor fins, nanosheets, nanowires, nanodots, etc., associated with a single, contiguous gate structure, or multiple gate structures. One of ordinary skill may recognize other examples of semiconductor transistor devices that may benefit from aspects of the present disclosure. The semiconductor transistor devices may be a portion of an integrated circuit (IC) that may include static random access memory (SRAM), logic circuits, passive components, such as resistors, capacitors, and inductors, and/or active components, such as p-type field effect transistors (PFETs), n-type FETs (NFETs), multi-gate FETs, metal-oxide semiconductor field effect transistors (MOSFETs), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof. 
       FIG.  1    shows a perspective view of a semiconductor transistor device  100  according to some embodiments.  FIG.  2    shows a cross-sectional view taken along line A-A′ of the x-direction of  FIG.  1    according to some embodiments.  FIGS.  3 A- 5    show cross-sectional views taken respectively along line B-B′, C-C′, and D-D′ of the y-direction in a first source/drain region, a gate region, and a second source/drain region of  FIG.  1    according to some embodiments. Alternatively,  FIGS.  2 - 5   , and other cross-sectional figures hereafter, can also stand alone to show varies embodiments. Also, for illustration purposes, some components are removed, shown as transparent, or only shown with boundary lines. Also, features discussed associated with one figure may be omitted in another figure but may be incorporated to embodiments shown in that figure when applicable. 
     As shown in  FIGS.  1 ,  2 , and  5   , the semiconductor transistor device  100  comprises a channel structure  102  and a gate structure  104  wrapping around the channel structure  102 . The channel structure  102  may comprise a stack of semiconductor layers separated and surrounded by a stack of metal components of the gate structure  104 . A first source/drain epitaxial structure  106  and a second source/drain epitaxial structure  108  are disposed on opposite endings of the channel structure  102 . As an example, the channel structure  102  may be pure silicon layers not doped with p-type and n-type impurities. A thickness of the channel structure  102  may be in a range between about 3 nm and about 15 nm. A width of the channel structure  102  may be in a range between about 6 nm and about 40 nm. As an example, the gate structure  104  may comprise a gate dielectric material such as high-κ materials (κ is greater than 7), a work function metal material, and a filling metal material such as tungsten or aluminum. A thickness of the gate structure  104  may be in a range between about 2 nm and about 10 nm. In some embodiments, the first and second source/drain epitaxial structures  106 ,  108  comprise a semiconductor material such as silicon, germanium, or silicon germanium. The first and second source/drain epitaxial structures  106 ,  108  may be hexagonal or diamond-like shapes. The first and second source/drain epitaxial structures  106 ,  108  may respectively be a source region and a drain region of the semiconductor transistor device  100 . 
     As shown in  FIG.  2   , on a front side of the semiconductor transistor device  100 , a front-side interconnect structure  114  may be disposed over the gate structure  104  and the first and second source/drain epitaxial structures  106 ,  108 . The front-side interconnect structure  114  may comprise a plurality of front-side metal layers  116  disposed within and surrounded by a front-side interlayer dielectric layer  112 . The front-side metal layers  116  includes vertical interconnects, such as vias or contacts, and horizontal interconnects, such as metal lines. The front-side interconnect structure  114  electrically connects various features or structures of the semiconductor transistor device. For example, a gate contact  110  may be disposed on the gate structure  104  and connected to external circuits through the front-side metal layers  116 . In some embodiments, epitaxial tips  107 ′ are disposed on opposite endings of the channel structure  102 . The epitaxial tips  107 ′ may comprise boron doped silicon germanium (SiGeB). The epitaxial tips  107 ′ may have less germanium than that of the first and second source/drain epitaxial structures  106 ,  108 . 
     Further, on a back side of the semiconductor transistor device  100 , in some embodiments, a back-side source/drain contact  120  is disposed underlying the first source/drain epitaxial structure  106  and connects the first source/drain epitaxial structure  106  to a back-side power rail  122  disposed under the back-side source/drain contact  120 . A back-side interconnect structure  124  may be formed to be electrically coupled to the back-side source/drain contact  120 . The back-side interconnect structure  124  may comprise a plurality of back-side metal lines  216  and metal vias  218  disposed within and surrounded by a back-side interlayer dielectric layer  212 . The back-side interconnect structure  124  electrically connects various features or structures of the semiconductor transistor device. For example, back-side interconnect structure  124  may be disposed on the back-side power rail  122  and connect external circuits to the back-side source/drain contact  120 . The back-side source/drain contact  120  and the back-side power rail  122  may comprise metal materials for example. For example, the back-side source/drain contact  120  may comprise metal, such as tungsten (W), cobalt (Co), ruthenium (Ru), aluminum (Al), copper (Cu), or other suitable materials. As an example, the back-side source/drain contact  120  may have a thickness between about 5 nm to about 50 nm and a width between about 20 nm to about 40 nm. Thus, the first source/drain epitaxial structure  106  can be connected to external circuits from the back side of the semiconductor transistor device  100  through the back-side source/drain contact  120 . Thereby, more metal routing flexibility is provided, and the cell capacitance can be reduced. In some embodiments, a first dielectric liner  118  is disposed along a sidewall of the back-side source/drain contact  120  and separates the back-side source/drain contact  120  from the back-side dielectric cap  126 . As an example, the first dielectric liner  118  may have a thickness less than about 5 nm. 
     As shown in  FIGS.  1 - 3 B , the back-side source/drain contact  120  may land on a recessed bottom surface  106   b  of the first source/drain epitaxial structure  106 . In some embodiments, the bottom surface  106   b  of the first source/drain epitaxial structure  106  may be recessed as a convex shape reaching a location vertically deeper than a bottom surface  104   b  of the gate structure  104 . In some embodiments, the bottom surface  106   b  of the first source/drain epitaxial structure  106  may have a convex shape along x-direction from the first source/drain epitaxial structure  106  to the second source/drain epitaxial structure  108  as shown in  FIG.  2    and also have a convex shape along y-direction as shown in  FIG.  3 A / 3 B. The y-direction may be perpendicular to the x-direction. In some embodiments, the bottom surface  106   b  of the first source/drain epitaxial structure  106  is vertically about 10 nm to 20 nm deeper than the bottom surface  104   b  of the gate structure  104 . In some embodiments, a low temperature epitaxial layer  119  may be disposed between the recessed bottom surface  106   b  of the first source/drain epitaxial structure  106  and the back-side source/drain contact  120 , and the metal alloy layer  121  can be formed on the low temperature epitaxial layer  119 . The low temperature epitaxial layer  119  may have a doping concentration greater than that of the first source/drain epitaxial structure  106 , such that a better metal alloy layer  121  can be formed subsequently to gain performance. As an example, the low temperature epitaxial layer  119  may have a thickness less than about 20 nm. A metal alloy layer  121  may be disposed on the first source/drain epitaxial structure  106  or the low temperature epitaxial layer  119  for contact landing. The metal alloy layer  121  may be a silicide layers formed by a self-aligned salicide process. The metal alloy layer  121  may include a material selected from titanium silicide, cobalt silicide, nickel silicide, platinum silicide, nickel platinum silicide, erbium silicide, palladium silicide, combinations thereof, or other suitable materials. In some embodiments, the metal alloy layer  121  may include germanium. 
     As shown in  FIGS.  1  and  2   , a bottom surface  108   b  of the second source/drain epitaxial structure  108  may be recessed to a location vertically deeper than the bottom surface  104   b  of the gate structure  104 . The bottom surface  108   b  of the second source/drain epitaxial structure  108  may be recessed even deeper to a location vertically exceeding a bottom surface  102   b  of the channel structure  102 . In some embodiments, the bottom surface  108   b  of the second source/drain epitaxial structure  108  may have a convex shape along x-direction from the first source/drain epitaxial structure  106  to the second source/drain epitaxial structure  108  as shown in  FIG.  2   . The bottom surface  108   b  of the second source/drain epitaxial structure  108  may also have a convex shape along y-direction as shown in  FIG.  4   . The y-direction may be perpendicular to the x-direction. In some embodiments, the bottom surface  108   b  of the second source/drain epitaxial structure  108  is vertically about 10 nm to 20 nm deeper than the bottom surface  104   b  of the gate structure  104 . The cell capacitance is further reduced compared to the embodiments where the bottom surface  108   b  of the second source/drain epitaxial structure  108  is below the bottommost of the channel structure  102 . 
     As shown in  FIGS.  1 - 2  and  4 - 5   , on the back side of the semiconductor transistor device  100 , in some embodiments, a back-side dielectric cap  126  is disposed under the gate structure  104  and may also extend under the second source/drain epitaxial structure  108 . The back-side dielectric cap  126  replaces original semiconductor body material, helps to separate and insulate the gate structure  104  and the back-side source/drain contact  120 , and thus reduces cell capacitance and eliminates current leakage problems such as a leakage between the gate structure  104  and the back-side source/drain contact  120 . The back-side dielectric cap  126  may comprise an oxide, nitride, carbon nitride, or low-κ dielectric materials. 
     As shown in  FIGS.  1 - 2   , inner spacers  128  are disposed on opposite endings of the metal components of the gate structure  104  to isolate the gate structure  104  from the first and second source/drain epitaxial structures  106 ,  108 . In some embodiments, gate spacers  134  are disposed along opposite sidewalls of an upper portion of the gate structure  104 . The outer surfaces of the inner spacers  128  may be substantial coplanar with outer surfaces of the channel structure  102  and/or the gate spacers  134 . In some embodiments, an upper isolation structure  220  is disposed in trenches between the gate spacers  134 . The upper isolation structure  220  provides electrical insulation between the gate structures  104 . 
     As shown in  FIG.  5   , in some embodiments, a gate structure  104  comprises a gate dielectric layer  232  and a gate electrode  230 . The gate electrode  230  includes one or more work function metal layer (s) and a filling metal. The gate dielectric layer  232  may be conformally formed lining outer surfaces of the gate electrode  230 . The gate dielectric layer  232  may be in contact with the channel structure  102 . In some embodiments, the gate dielectric layer  232  includes a high-κ material (κ is greater than 7) such as hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), lanthanum oxide (La 2 O 3 ), hafnium aluminum oxide (HfAlO 2 ), hafnium silicon oxide (HfSiO 2 ), aluminum oxide (Al 2 O 3 ), or other suitable materials. 
     As shown in  FIGS.  1  and  3 A- 5   , in some embodiments, a lower isolation structure  160 , a middle isolation structure  132 , and a hard mask  136  can collectively function as an insulating structure separating two semiconductor transistor devices  100   a,    100   b  along the y-direction. The back-side dielectric cap  126  may be surrounded by the lower isolation structure  160 . As shown in  FIGS.  3 A and  4 A , in some embodiments, the back-side source/drain contact  120  and the surrounding first dielectric liner  118  and the back-side dielectric cap  126  and the surrounding second dielectric liner  127  may extend along surfaces of a lower isolation structure  160  and the middle isolation structure  132  and extend on upper surfaces of the first and second source/drain epitaxial structures  106 ,  108 . As shown in  FIGS.  3 B and  4 B , in some alternative embodiments, air gaps  192  may be formed surrounding lower portions of the first source/drain epitaxial structure  106  and the second source/drain epitaxial structure  108 . In some alternative embodiments, air gaps  192  may be formed between the middle isolation structure  132  and surrounding lower portions of the first source/drain epitaxial structure  106  and the second source/drain epitaxial structure  108 . The back-side source/drain contact  120  and the surrounding first dielectric liner  118  and the back-side dielectric cap  126  and the surrounding second dielectric liner  127  may extend downwardly in recesses of the first and second source/drain epitaxial structures  106 ,  108 . 
       FIGS.  6 - 29 B  illustrate a method for manufacturing a semiconductor transistor device at various stages in accordance with some embodiments of the present disclosure. In some embodiments, the semiconductor transistor device shown in  FIGS.  6 - 29 B  may be intermediate devices fabricated during processing of an integrated circuit (IC), or a portion thereof, that may include static random access memory (SRAM), logic circuits, passive components, such as resistors, capacitors, and inductors, and/or active components, such as p-type field effect transistors (PFETs), n-type FETs (NFETs), multi-gate FETs, metal-oxide semiconductor field effect transistors (MOSFETs), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof. 
     As shown in a perspective view of  FIG.  6   , a substrate  140  is provided. In some embodiments, the substrate  140  may be a part of a wafer, and may comprise silicon (Si), germanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs) or other appropriate semiconductor materials. In some embodiments, the substrate  140  is a semiconductor-on-insulator (SOI) structure comprising a bulk substrate  142 , an insulator substrate layer  144  on the bulk substrate  142 , and a semiconductor substrate layer  146  on the insulator substrate layer  144 . In various embodiments, the substrate  140  may include any of a variety of substrate structures and materials. 
     As shown in a perspective view of  FIG.  7   , in some embodiments, a stacked structure  150  is formed over the substrate  140 . The stacked structure  150  includes first semiconductor layers  152  and second semiconductor layers  154  stacked alternately. The first semiconductor layers  152  will serve as channel regions of the semiconductor transistor device. The second semiconductor layers  154  are sacrificial layers which will be subsequently removed and replaced with a gate material. The first semiconductor layers  152  and the second semiconductor layers  154  are made of materials having different lattice constants, and may include one or more layers of Si, Ge, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP, GaAsSb or InP. In some embodiments, the first semiconductor layers  152  and the second semiconductor layers  154  are made of Si, a Si compound, SiGe, Ge or a Ge compound. The stacked structure  150  may be formed on the substrate  140  through epitaxy, such that the stacked structure  150  forms crystalline layers. Though  FIG.  7    shows four layers of the first semiconductor layer  152  and three layers of the second semiconductor layer  154 , the number of the layers are not so limited, and may be as small as 1 for each layer. In some embodiments, 2-10 layers of each of the first and second semiconductor layers are formed. By adjusting the numbers of the stacked layers, a driving current of the semiconductor transistor device can be adjusted. 
     In some embodiments, the first semiconductor layers  152  may be pure silicon layers that are free from germanium. The first semiconductor layers  152  may also be substantially pure silicon layers, for example, with a germanium atomic percentage lower than about 1 percent. Furthermore, the first semiconductor layers  152  may be intrinsic, which are not doped with p-type and n-type impurities. In some embodiments, a thickness of the first semiconductor layers  152  is in a range between about 3 nm and about 15 nm. 
     In some embodiments, the second semiconductor layers  154  can be SiGe layers having a germanium atomic percentage greater than zero. In some embodiments, the germanium percentage of the second semiconductor layers  154  is in a range between about 10 percent and about 50 percent. In some embodiments, a thickness of the second semiconductor layers  154  is in a range between about 2 nm and about 10 nm. 
     As shown in a perspective view of  FIG.  8   , in some embodiments, the stacked structure  150  (see  FIG.  7   ) is patterned to form fin structures  156  and trenches  158  extending in the X direction. In some embodiments, the stacked structure  150  is patterned by an etching process using a patterned mask layer  157  as an etch mask, such that portions of the stacked structure  150  not covered by the mask layer  157  are removed. The semiconductor substrate layer  146  may also be partially or fully removed in this process. The mask layer  157  may include a first mask layer and a second mask layer. The first mask layer may be a pad oxide layer made of a silicon oxide, which can be formed by a thermal oxidation process. The second mask layer may be made of a silicon nitride (SiN), which is formed by chemical vapor deposition (CVD), including low pressure CVD (LPCVD) and plasma enhanced CVD (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or other suitable process. The mask layer  157  may be patterned using varies multiple patterning techniques, such as self-aligned double patterning (SADP), self-aligned quadruple patterning (SAQP), and the like.  FIG.  8    shows two fin structures  156  arranged in the Y direction and parallel to each other, but the number of the fin structures is not limited to, and may be as small as one and three or more. In some embodiments, one or more dummy fin structures are formed on both sides of the fin structures  156  to improve pattern fidelity in the patterning operations. 
     As shown in a perspective view of  FIG.  9   , in some embodiments, a lower isolation structure  160  is formed over the insulator substrate layer  144  in lower portions of the trenches  158 , which is also referred to as a shallow trench isolation (STI) structure. Upper portions of the fin structures  156  are exposed from the lower isolation structure  160 . The lower isolation structure  160  may be formed by forming an insulating material over the insulator substrate layer  144  followed by a planarization operation. The insulating material is then recessed to form the lower isolation structure  160  so that the upper portions of the fin structures  156  are exposed. The insulating material may comprise a dielectric material such as, for example, a nitride (e.g., silicon nitride, silicon oxynitride, silicon oxygen carbon nitride, silicon carbon nitride), a carbide (e.g., silicon carbide, silicon oxygen carbide), an oxide (e.g., silicon oxide), borosilicate glass (BSG), phosphoric silicate glass (PSG), borophosphosilicate glass (BPSG), a low-κ dielectric material with a dielectric constant less than 7 (e.g., a carbon doped oxide, SiCOH), or the like. In some embodiments, the lower isolation structures  160  are formed through various steps comprising a thermal oxidation or deposition process (e.g., physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), sputtering, etc.), and removal processes (e.g., wet etching, dry etching, chemical mechanical planarization (CMP), etc.). 
     As shown in a perspective view of  FIG.  10   , in some embodiments, a cladding semiconductor layer  161  is formed over outer surfaces of the fin structures  156 . In some embodiments, the cladding semiconductor layer  161  comprises a semiconductor material, such as germanium, silicon germanium, or the like. In some embodiments, the cladding semiconductor layer  161  comprises the same material as the second semiconductor layers  154 . Further, in some embodiments, the cladding semiconductor layer  161  may be formed by an epitaxy growth process or a deposition process (e.g., PVD, CVD, PECVD, ALD, sputtering, etc.). 
     As shown in a perspective view of  FIG.  11   , in some embodiments, a middle isolation structure  132  is formed over the lower isolation structure  160  between the fin structures  156 . A dielectric liner  130  may be formed between the middle isolation structure  132  and the lower isolation structure  160  along sidewalls of the cladding semiconductor layer  161  and the lower isolation structure  160 . A hard mask  136  may then be formed on top of the middle isolation structure  132  and the dielectric liner  130 . The middle isolation structure  132  and the dielectric liner  130  provide electrical insulation between the fin structures  156 , and the hard mask  136  prevents loss of the middle isolation structure  132  during future patterning steps. 
     In some embodiments, the dielectric liner  130 , the middle isolation structure  132 , and the hard mask  136  are formed by deposition (e.g., PVD, CVD, PECVD, ALD, sputtering, etc.) and removal (e.g., etching, chemical mechanical planarization (CMP), etc.) processes. The middle isolation structure  132  may have a top surface below that of the fin structures  156 . In some embodiments not shown in  FIG.  11   , the planarization process of the hard mask  136  may also remove the cladding semiconductor layer  161  from above the fin structures  156 . The hard mask  136  may have a top surface coplanar with that of the fin structures  156 . In some embodiments, the middle isolation structure  132  and the lower isolation structures  160  may each comprise a low-κ dielectric material, wherein the dielectric constant is less than 7, such as, for example, silicon oxynitride, silicon carbon nitride, silicon oxygen carbide, silicon oxygen carbon nitride, silicon nitride, or some other suitable low-κ dielectric material. The dielectric liner  130  may comprise a different material than the middle isolation structure  132  for selective removal processes. The hard mask  136  may comprise a high-κ dielectric material, wherein the dielectric constant is greater than 7, such as, for example, hafnium oxide, zirconium oxide, hafnium aluminum oxide, hafnium silicon oxide, aluminum oxide, or some other suitable high-κ dielectric material. 
     As shown in the perspective view of  FIG.  12   , in some embodiments, the cladding semiconductor layer  161  and the mask layer  157  (see  FIG.  8   ) are etched from top of the fin structures  156 . Top surfaces of the first semiconductor layer  152  and the cladding semiconductor layer  161  may be exposed from the removal process. In some embodiments, the hard mask  136  is selectively etched by a dry etching process and/or a wet etching process, for example. 
     As shown in the perspective view of  FIG.  13   , in some embodiments, dummy gate structures  170  are formed over the fin structures  156  along the y-direction spaced apart from one another in the x-direction. In some embodiments, the dummy gate structures  170  may comprise a sacrificial gate dielectric layer  162 , a sacrificial gate electrode layer  164 , a pad layer  166 , and a mask layer  168  one stacked over another in the order stated. Though two dummy gate structures  170  are shown in  FIG.  13   , but the number of the dummy gate structures  170  are not limited to, and may be more or fewer than two. In some embodiments, the sacrificial gate dielectric layer  162  may comprise, for example, a dielectric material such as a nitride (e.g., silicon nitride, silicon oxynitride), a carbide (e.g., silicon carbide), an oxide (e.g., silicon oxide), or some other suitable material. The sacrificial gate electrode layer  164  may comprise, for example, polysilicon. The pad layer  166  and the mask layer  168  may comprise thermal oxide, nitride, and/or other hard mask materials and are formed by way of photolithography processes. 
     Subsequently, gate spacers  134  are formed along opposite sidewalls of the dummy gate structures  170 . For example, a blanket layer of an insulating material for sidewall spacers is conformally formed to cover the dummy gate structures  170  by using plasma enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), sub-atmospheric chemical vapor deposition (SACVD), or the like. The blanket layer is deposited in a conformal manner so that it is formed to have substantially equal thicknesses on vertical surfaces, such as the sidewalls, horizontal surfaces, and the top of the dummy gate structures  170 . In some embodiments, the insulating material of the blanket layer may comprise a silicon nitride-based material. The blanket layer is then etched using an anisotropic process to form the gate spacers  134  on opposite sidewalls of the dummy gate structures  170 . 
     As shown in the perspective view of  FIG.  14 A , the x-direction cross-sectional view of  FIG.  14 B , the y-direction cross-sectional view of  FIG.  14 C  in a gate region, and the y-direction cross-sectional view of  FIG.  14 D  in a source region or a drain region, in some embodiments, a removal process is performed to remove fin structures  156  from a first source/drain region  176  and a second source/drain region  178  according to the dummy gate structures  170 . As a result, the first semiconductor layers  152  and the second semiconductor layers  154  are shortened along x-direction and may be vertically aligned with the gate spacers  134  (See  FIG.  14 B ). As an example, the exposed portions of the fin structures  156  are removed by using a strained source/drain (SSD) etching process. The SSD etching process may be performed in a variety of ways. In some embodiments, the SSD etching process may be performed by a dry chemical etch with a plasma source and a reaction gas. The plasma source may be an inductively coupled plasma (ICR) etch, a transformer coupled plasma (TCP) etch, an electron cyclotron resonance (ECR) etch, a reactive ion etch (RIE), or the like and the reaction gas may be a fluorine-based gas, chloride (Cl2), hydrogen bromide (HBr), oxygen (O 2 ), the like, or combinations thereof. In some other embodiments, the SSD etching process may be performed by a wet chemical etch, such as ammonium peroxide mixture (APM), ammonium hydroxide (NH 4 OH), tetramethylammonium hydroxide (TMAH), combinations thereof, or the like. In yet some other embodiments, the SSD etch step may be performed by a combination of a dry chemical etch and a wet chemical etch. Further, in some embodiments, the removal process may also remove an upper portion of the semiconductor substrate layer  146  between the dummy gate structures  170  after removing the bottommost first semiconductor layer  152 . The semiconductor substrate layer  146  or the bottommost first semiconductor layer  152  may have a concave top surface along the x-direction in the first source/drain region  176  and the second source/drain region  178 . The top surface may be recessed between the lower isolation structure  160 . 
     In addition, the removal process may also comprise an isotropic etchant to further remove ending portions of the second semiconductor layers  154  under the gate spacers  134  and/or the dummy gate structures  170 . Thus, after the removal process, the first semiconductor layers  152  are wider than the second semiconductor layers  154  in the x-direction. The first semiconductor layers  152  may be formed as the channel structure of the transistor device after the removal process. It will be appreciated that the channel structure may exhibit stacked rectangle-like shapes as illustrated in the cross-sectional view of  FIG.  14 B  and other figures, whereas in other embodiments, the channel structure may exhibit other shapes such as circles, octagons, ovals, diamonds, or the like. 
     As shown in the perspective view of  FIG.  15 A  and the x-direction cross-sectional view of  FIG.  15 B , in some embodiments, inner spacers  128  are formed on the endings of the second semiconductor layers  154  in the x-direction. Outer surfaces of the inner spacers  128  may be substantial coplanar with outer surfaces of the first semiconductor layers  152  and/or the gate spacers  134 . In some embodiments, the inner spacers  128  are formed by a deposition process (e.g., CVD, PVD, PECVD, ALD, sputtering, etc.) followed by a selective removal process. For example, in some embodiments, a continuous layer may first be formed along sidewalls and over the dummy gate structures  170 . Then, a vertical etching process can be conducted to remove portions of the continuous layer not vertically covered by the gate spacers  134  to form the inner spacers  128 . Further, in some embodiments, the inner spacers  128  comprise a dielectric material such as, for example, silicon oxynitride, silicon carbon nitride, silicon oxygen carbide, silicon oxygen carbon nitride, silicon nitride or some other suitable material. 
     As shown in the perspective view of  FIG.  16 A , the x-direction cross-sectional view of  FIG.  16 B , and the y-direction cross-sectional view of  FIG.  16 C  in the first source/drain region, in some embodiments, a first sacrificial source/drain contact  180  is formed under the first source/drain region  176  with a hard mask layer  182  covering the second source/drain region  178 . In some embodiments, the first sacrificial source/drain contact  180  extends deep in the semiconductor substrate layer  146 . As an example, the first sacrificial source/drain contact  180  may have a thickness of about 50 nm. In some embodiments, a trench is formed firstly by etching the bottommost first semiconductor layer  152  and/or at least a portion of the semiconductor substrate layer  146  directly under the first source/drain region  176 . Then, a sacrificial material is filled in the trench to form the first sacrificial source/drain contact  180 . In some embodiments, the first sacrificial source/drain contact  180  may comprise intrinsic SiGe material having a germanium atomic percentage greater than zero. In some embodiments, the germanium percentage of the first sacrificial source/drain contact  180  is in a range between about 10 percent and about 50 percent. In some embodiments, the first sacrificial source/drain contact  180  comprises the same material as the second semiconductor layers  154 . Further, in some embodiments, the first sacrificial source/drain contact  180  may be formed by an epitaxy growth process or a deposition process (e.g., PVD, CVD, PECVD, ALD, sputtering, etc.). By forming the trench and the first sacrificial source/drain contact  180  therein, a source/drain contact can be formed self-aligned later by replacing the first sacrificial source/drain contact  180 , such that an overlay shift of contact landing is eliminated. 
     As shown in the perspective view of  FIG.  17 A , the x-direction cross-sectional view of  FIG.  17 B , the y-direction cross-sectional view of  FIG.  17 C  in the first source/drain region, and the y-direction cross-sectional view of  FIG.  17 D  in the second source/drain region, in some embodiments, a first source/drain epitaxial structure  106  and a second source/drain epitaxial structure  108  are formed respectively in the first source/drain region  176  and the second source/drain region  178  on opposite sides of the dummy gate structure  170  (See  FIG.  16 A ). In some embodiments, the first source/drain epitaxial structure  106  may be formed on the first sacrificial source/drain contact  180  (See  FIG.  17 C ). The second source/drain epitaxial structure  108  may be formed on the semiconductor substrate layer  146  (See  FIG.  17 D ). The first and second source/drain epitaxial structures  106 ,  108  may respectively be a source and a drain of the semiconductor transistor device. In some embodiments, the first and second source/drain epitaxial structures  106 ,  108  comprise a semiconductor material. For example, the first and second source/drain epitaxial structures  106 ,  108  may comprise doped silicon, germanium, or silicon germanium such as boron doped silicon germanium (SiGeB). In some embodiments, the first and second source/drain epitaxial structures  106 ,  108  are formed by way of an epitaxy growth process. The first and second source/drain epitaxial structures  106 ,  108  may be hexagonal or diamond-like shapes. Air gaps  192  may be formed surrounding lower portions of the first source/drain epitaxial structure  106  and the second source/drain epitaxial structure  108 . 
     In some embodiments, an intermediate source/drain layer  107  is formed underneath the first and second source/drain epitaxial structures  106 ,  108  prior to forming the first and second source/drain epitaxial structures  106 ,  108 . The intermediate source/drain layer  107  may comprise boron doped silicon germanium (SiGeB). The intermediate source/drain layer  107  may have less germanium than that of the first and second source/drain epitaxial structures  106 ,  108  and function as a buffer layer for device design. As an example, the intermediate source/drain layer  107  may have a thickness of about 20 nm. In some embodiments, the intermediate source/drain layer  107  is formed by an epitaxial process, and an epitaxial tip  107 ′ is concurrently formed on opposite endings of the first semiconductor layers  152  by the same epitaxial process. Thus, the epitaxial tip  107 ′ may have the same composition as the intermediate source/drain layer  107 . 
     As shown in the perspective view of  FIG.  18 A , the x-direction cross-sectional view of  FIG.  18 B , the y-direction cross-sectional view of  FIG.  18 C  in the first source/drain region, and the y-direction cross-sectional view of  FIG.  18 D  in the second source/drain region, in some embodiments, an upper isolation structure  220  is formed over the structure previously formed covering the first and second source/drain epitaxial structures  106 ,  108 . A planarization process is subsequently performed to lower the gate spacers  134  and expose the sacrificial gate dielectric layer  162  and the sacrificial gate electrode layer  164  on a same horizontal plane. In some embodiments, an etch stop liner  210  may be conformally formed lining the structure previously formed prior to forming the upper isolation structure  220 . In some embodiments, the etch stop liner  210  may comprise silicon nitride. In some other embodiments, the etch stop liner  210  may comprise other dielectric materials such as silicon dioxide, silicon oxynitride, or the like. The etch stop liner  210  can be formed using plasma enhanced CVD (PECVD), however, other suitable methods, such as low pressure CVD (LPCVD), atomic layer deposition (ALD), and the like, can also be used. The upper isolation structure  220  may be formed by chemical vapor deposition (CVD), high-density plasma CVD, spin-on, sputtering, or other suitable methods. In some embodiments, the upper isolation structure  220  may comprise silicon dioxide. In some other embodiments, the upper isolation structure  220  may comprise other dielectric materials such as carbon doped oxide dielectrics including Si, O, C and/or H (SiCOH or SiOC), a low-κ material, or organic materials (e.g., polymers). The planarization operation may comprise a chemical-mechanical process (CMP). 
     As shown in the perspective view of  FIG.  19 A , the x-direction cross-sectional view of  FIG.  19 B , the y-direction cross-sectional view of  FIG.  19 C  in the gate region, in some embodiments, a replacement gate process is performed to form a gate structure  104 . In some embodiments, the gate structure  104  is formed by firstly removing the sacrificial gate dielectric layer  162  and the sacrificial gate electrode layer  164 , thereby exposing the first and second semiconductor layers  152 ,  154  (see  FIG.  18 B ). The upper isolation structure  220  protects the first and second source/drain epitaxial structures  106 ,  108  during the removal of the sacrificial gate dielectric layer  162  and the sacrificial gate electrode layer  164 . The sacrificial gate electrode layer  164  can be removed using plasma dry etching and/or wet etching. When the sacrificial gate electrode layer  164  is polysilicon and the upper isolation structure  220  is silicon oxide, a wet etchant such as a TMAH solution can be used to selectively remove the sacrificial gate electrode layer  164 . The sacrificial gate electrode layer  164  can be removed using plasma dry etching and/or wet etching. Subsequently, the sacrificial gate dielectric layer  162  is removed as well. As such, the first and second semiconductor layers  152 ,  154  are exposed. 
     The second semiconductor layers  154  and the cladding semiconductor layer  161  (see  FIG.  14 C ) are then removed or etched using an etchant that can selectively etch the second semiconductor layers  154  and the cladding semiconductor layer  161  at a faster etching rate than etching the first semiconductor layers  152 . The inner spacers  128  protect the first and second source/drain epitaxial structures  106 ,  108  from the etchant used in etching the second semiconductor layers  154  and the cladding semiconductor layer  161  since the inner spacers  128  is made of a material that has etching selectivity to that of the second semiconductor layers  154  and the cladding semiconductor layer  161 . 
     A gate structure  104  is then formed and/or filled between the gate spacers  134  and the inner spacers  128 . That is, the gate structure  104  encircles (or surrounds or wraps) the first semiconductor layers  152 , in which the first semiconductor layers  152  are referred to as channels of the semiconductor transistor device. The gate spacers  134  are disposed on opposite sides of the gate structure  104 . The gate structure  104  comprises a gate dielectric layer  232  and a gate electrode  230 . The gate electrode  230  includes one or more work function metal layer (s) and a filling metal. The gate dielectric layer  232  may be conformally formed. That is, the gate dielectric layer  232  is in contact with the lower isolation structure  160  and the first semiconductor layers  152  (See  FIG.  19 C ). In some embodiments, the gate dielectric layer  232  includes a high-κ material (κ is greater than 7) such as hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), lanthanum oxide (La 2 O 3 ), hafnium aluminum oxide (HfAlO 2 ), hafnium silicon oxide (HfSiO 2 ), aluminum oxide (Al 2 O 3 ), or other suitable materials. In some embodiments, the gate dielectric layer  232  may be formed by performing an ALD process or other suitable process. 
     The work function metal layer of the gate electrode  230  is formed on the gate dielectric layer  232 , and the work function metal layer surrounds the first semiconductor layers  152  in some embodiments. The work function metal layer may include materials such as titanium nitride (TiN), tantalum (TaN), titanium aluminum silicon (TiAlSi), titanium silicon nitride (TiSiN), titanium aluminum (TiAl), tantalum aluminum (TaAl), or other suitable materials. In some embodiments, the work function metal layer may be formed by performing an ALD process or other suitable process. The filling metal of the gate electrode  230  fills the remained space between the gate spacers  134  and between the inner spacers  128 . That is, the work function metal layer(s) is in contact with and between the gate dielectric layer  232  and the filling metal. The filling metal may include material such as tungsten or aluminum. After the deposition of the gate dielectric layer  232  and the gate electrode  230 , a planarization process, such as a CMP process, may be then performed to remove excess portions of the gate dielectric layer  232  and the gate electrode  230  to form the gate structure  104 . 
     In some embodiments, an interfacial layer (not shown) is optionally formed prior to forming the gate structure  104  to surround exposed surfaces of the first semiconductor layers  152  and exposed surfaces of the semiconductor substrate layer  146  (See  FIG.  19 B ,  FIG.  19 C ). In various embodiments, the interfacial layer may include a dielectric material such as silicon oxide (SiO 2 ) or silicon oxynitride (SiON), and may be formed by chemical oxidation, thermal oxidation, atomic layer deposition (ALD), chemical vapor deposition (CVD), and/or other suitable methods. 
     As shown in the perspective view of  FIG.  20   , in some embodiments, a front-side interconnect structure  114  is formed over the gate structure  104  and the first and second source/drain epitaxial structures  106 ,  108  (see  FIG.  22 B ). The front-side interconnect structure  114  may comprise a plurality of front-side metal layers  116  disposed within and surrounded by a front-side interlayer dielectric layer  112 . The front-side interconnect structure  114  electrically connects various features or structures (e.g., a gate contact  110  and/or other contacts) of the semiconductor transistor device. The front-side metal layers  116  includes vertical interconnects, such as vias or contacts, and horizontal interconnects, such as metal lines. The various interconnection features may implement various conductive materials including copper, tungsten and silicide. In some examples, a damascene process is used to form copper multilayer interconnection structure. Subsequently, a carrier substrate  240  is formed above the front-side interconnect structure  114 . For example, the carrier substrate  240  is bond to the front-side interconnect structure  114 . In some embodiments, carrier substrate  240  is sapphire. In some other embodiments, the carrier substrate  240  is silicon, a thermoplastic polymer, oxide, carbide, or other suitable material. 
     As shown in the perspective view of  FIG.  21   , in some embodiments, the workpiece is “flipped” upside down and thinned to expose the first sacrificial source/drain contact  180  and the semiconductor substrate layer  146  from a back-side. The bulk substrate  142 , the insulator substrate layer  144 , and at least an upper portion of the lower isolation structure  160  are removed. The bulk substrate  142 , the insulator substrate layer  144 , and the lower isolation structure  160  may be removed in a plurality of process operations, for example, firstly removing the bulk substrate  142  followed by removal of the insulator substrate layer  144  and the lower isolation structure  160 . In some embodiments, the removal processes include removal of the bulk substrate  142 , the insulator substrate layer  144 , and the lower isolation structure  160  using, for example, CMP, and/or TMAH etching. 
     As shown in the perspective view of  FIG.  22 A , the x-direction cross-sectional view of  FIG.  22 B , and the y-direction cross-sectional view of  FIG.  22 C  in the first source/drain region, in some embodiments, the first sacrificial source/drain contact  180  is removed, and the underlying first source/drain epitaxial structure  106  is recessed from the back-side thereof to form a back-side source/drain contact trench  234  recessed into an upper portion of the first source/drain epitaxial structure  106 . The first source/drain epitaxial structure  106  can be recessed or etched using an etchant that can selectively etch the first source/drain epitaxial structure  106  at a faster etching rate than etching surrounding dielectric materials. In some embodiments, the etching process is isotropic or includes an isotropic etching process, and a bottom surface  106   b  of the first source/drain epitaxial structure  106  may be recessed as a convex shape both along the x-direction (see  FIG.  22 B ) and along the y-direction (see  FIG.  22 C ) reaching a location vertically deeper than a bottom surface  104   b  of the gate structure  104  (see  FIG.  22 B ). The airgaps  192  may be exposed. In some alternative embodiments, the etching process comprises an anisotropic etch such as a vertical etch or a combination of anisotropic etch and isotropic etch, and the first source/drain epitaxial structure  106  may be vertically or slantly recessed and a remaining upper sidewall of the first source/drain epitaxial structure  106  separates the airgaps  192  from the formed recess (see  FIG.  3 B ). In some embodiments, the bottom surface  106   b  of the first source/drain epitaxial structure  106  is vertically about 10 nm to 20 nm deeper than the bottom surface  104   b  of the gate structure  104 . 
     As shown in the perspective view of  FIG.  23 A , the x-direction cross-sectional view of  FIG.  23 B , and the y-direction cross-sectional view of  FIG.  23 C  in the first source/drain region, in some embodiments, an opening of the back-side source/drain contact trench  234  is enlarged and rounded (now labeled as  234 ′) for better filling in subsequent processing steps. In some embodiments, the semiconductor substrate layer  146  and the lower isolation structure  160  are etched from top with a protective coating layer  235  covering a lower portion of the back-side source/drain contact trench  234 . In some embodiments, the protective coating layer  235  is made of a bottom anti-reflective coating (BARC) material such as an organic material and is formed in the back-side source/drain contact trench  234  by spin-on or other filling techniques. An etch-back process is then performed to remove the protective coating layer  235  from an upper portion of the back-side source/drain contact trench  234 , such that the upper portion can be enlarged. From top view, the enlarged back-side source/drain contact trench  234 ′ may have varies shapes, such as rectangular, square shape, circle, or other applicable shapes. In some embodiments, a top lateral dimension Li of the enlarged back-side source/drain contact trench  234 ′ may be as much as 30 nm greater than a lower lateral dimension Li of the enlarged back-side source/drain contact trench  234 ′. 
     As shown in the perspective view of  FIG.  24 A , the x-direction cross-sectional view of  FIG.  24 B , and the y-direction cross-sectional view of  FIG.  24 C  in the first source/drain region, in some embodiments, a second sacrificial source/drain contact  236  is filled in the enlarged back-side source/drain contact trench  234 ′. In some embodiments, the second sacrificial source/drain contact  236  is formed by depositing a dielectric material such as silicon nitride in the back-side source/drain contact trench  234  followed by a planarization process to remove excessive portions, such that the second sacrificial source/drain contact  236  may be coplanar with the lower isolation structure  160  and the semiconductor substrate layer  146 . A first dielectric liner  118  may be formed between the enlarged back-side source/drain contact trench  234 ′ and the second sacrificial source/drain contact  236  prior to forming the second sacrificial source/drain contact  236 . The first dielectric liner  118  may act as a diffusion barrier protecting later formed source/drain contact from diffusion. The first dielectric liner  118  also protects the inner spacers  128  and the channel structure  102  during subsequent removal of the second sacrificial source/drain contact  236  and other cleaning processes. As an example, the first dielectric liner  118  may have a thickness less than about 5 nm. 
     As shown in the perspective view of  FIG.  25 A , the x-direction cross-sectional view of  FIG.  25 B , and the y-direction cross-sectional view of  FIG.  25 C  in the second source/drain region, in some embodiments, the semiconductor substrate layer  146  is removed to form a back-side capping trenches  238  above the second source/drain epitaxial structure  108  and the gate structure  104 . The underlying second source/drain epitaxial structure  108  and the gate structure  104  may be exposed. In some embodiments, the semiconductor substrate layer  146  is removed first and then the second source/drain epitaxial structure  108  is recessed by an isotropic etch or a combination of isotropic and anisotropic etch. A bottom surface  108   b  of the second source/drain epitaxial structure  108  may be recessed as a convex shape both along the x-direction (see  FIG.  25 B ) and along the y-direction (see  FIG.  25 D ) reaching a location vertically deeper than the bottom surface  104   b  of the gate structure  104 . The airgaps  192  may be exposed. In some alternative embodiments, the etching process comprises an anisotropic etch such as a vertical etch or a combination of anisotropic etch and isotropic etch, and the second source/drain epitaxial structure  108  may be vertically or slantly recessed and a remaining upper sidewall of the second source/drain epitaxial structure  108  separates the airgaps  192  from the formed recess (see  FIG.  4 B ). In some embodiments, the bottom surface  108   b  of the second source/drain epitaxial structure  108  is vertically about 10 nm to 20 nm deeper than the bottom surface  104   b  of the gate structure  104 . 
     As shown in the perspective view of  FIG.  26 A , the x-direction cross-sectional view of  FIG.  26 B , the y-direction cross-sectional view of  FIG.  26 C  in the gate region, and the y-direction cross-sectional view of  FIG.  26 D  in the second source/drain region, in some embodiments, a second dielectric liner  127  and a back-side dielectric cap  126  are formed in the back-side capping trenches  238  (see  FIG.  25 A ). The second dielectric liner  127  and the back-side dielectric cap  126  may be formed directly above the second source/drain epitaxial structure  108  and the gate structure  104 . The second dielectric liner  127  protects the second source/drain epitaxial structure  108  from oxidation, and also prevents metal gate threshold shift during subsequent manufacturing processes. The second dielectric liner  127  may be formed by for example, a conformal deposition process to deposit a dielectric material in the back-side capping trenches  238 , and the back-side dielectric cap  126  may be formed by for example, a deposition process to deposit a dielectric material on the second dielectric liner  127 , followed by a CMP process to remove excess dielectric materials outside the back-side capping trenches  238 . In some embodiments, the second dielectric liner  127  and the back-side dielectric cap  126  comprise dielectric materials different from the second sacrificial source/drain contact  236 . As an example, the second dielectric liner  127  may be made of low-κ material (κ&lt;7) such as SiO 2 , Si 3 N 4 , silicon carbonitride (SiCN), silicon oxycarbide (SiOC), silicon oxycarbonitride (SiOCN), and the like or high-κ material (κ&gt;7) such as HfO 2 , ZrO 2 , ZrAlOx, HfAlOx, HfSiOx, AlOx, and the like. In some embodiments, the back-side dielectric cap  126  have a convex top surface  126   s  contacting the second source/drain epitaxial structure  108 . As an example, the back-side dielectric cap  126  may be formed with a thickness T of about 40 nm from the bottom surface  108   b  to a top surface of the back-side dielectric cap  126  after the CMP process. As an example, the second dielectric liner  127  may have a thickness less than about 5 nm. 
     As shown in the perspective view of  FIG.  27 A , the x-direction cross-sectional view of  FIG.  27 B , and the y-direction cross-sectional view of  FIG.  27 C  in the first source/drain region, in some embodiments, the second sacrificial source/drain contact  236  (see  FIG.  26 A ) is removed. A low temperature epitaxial layer  119  may be formed on the recessed bottom surface  106   b  of the first source/drain epitaxial structure  106  in the enlarged back-side source/drain contact trench  234 ′. The low temperature epitaxial layer  119  is formed with a doping concentration greater than that of the first source/drain epitaxial structure  106 , such that a better metal alloy layer can be formed subsequently to gain performance. As an example, the low temperature epitaxial layer  119  may be formed with a thickness of about 5 nm. 
     As shown in the perspective view of  FIG.  28 A , the x-direction cross-sectional view of  FIG.  28 B , and the y-direction cross-sectional view of  FIG.  28 C  in the first source/drain region, in some embodiments, a back-side source/drain contact  120  is formed on the low temperature epitaxial layer  119  in the enlarged back-side source/drain contact trench  234 ′. The back-side source/drain contact  120  may have sidewalls contacting inner sidewalls of the first dielectric liner  118 . In some embodiments, prior to forming the back-side source/drain contact  120 , a metal alloy layer  121  may be formed on the low temperature epitaxial layer  119  or the first source/drain epitaxial structure  106  if the low temperature epitaxial layer  119  was not formed. The metal alloy layer  121  may be a silicide layers formed by a self-aligned salicide process. The metal alloy layer  121  may include a material selected from titanium silicide, cobalt silicide, nickel silicide, platinum silicide, nickel platinum silicide, erbium silicide, palladium silicide, combinations thereof, or other suitable materials. In some embodiments, the metal alloy layer  121  may include germanium. In some embodiments, the back-side source/drain contact  120  may be made of metal, such as W, Co, Ru, Al, Cu, or other suitable materials. As an example, the metal alloy layer  121  may be formed with a thickness of about 5 nm. After the deposition of the back-side source/drain contact  120 , a planarization process, such as a chemical mechanical planarization (CMP) process, may be then performed. 
     As shown in the perspective view of  FIG.  29 A  and the x-direction cross-sectional view of  FIG.  29 B , in some embodiments, a back-side power rail  122  and a back-side interconnect structure  124  are formed to be electrically coupled to the back-side source/drain contact  120 . The back-side interconnect structure  124  may comprise a plurality of back-side metal lines  216  and metal vias  218  disposed within and surrounded by a back-side interlayer dielectric layer  212 . The back-side interconnect structure  124  electrically connects various features or structures of the semiconductor transistor device. For example, the back-side interconnect structure  124  may be disposed on a back-side power rail  122  that connects external circuits to the back-side source/drain contact  120 . 
       FIG.  30    illustrates a flow diagram of some embodiments of a method  3000  of forming an integrated chip having multiple transistor devices with a high device density due to air spacer structures and high-κ dielectric spacer structures. 
     While method  3000  is illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. 
     At act  3002 , a plurality of fin structures of stacked first and second semiconductor layers are formed on a substrate. An isolation structure is formed between the fin structures (see, e.g.,  FIGS.  6 - 12   ).  FIGS.  6 - 12    illustrate the perspective views of some embodiments corresponding to act  3002 . 
     At act  3004 , a plurality of dummy gate structures is formed overlying the fin structures.  FIG.  13    illustrates the perspective view of some embodiments corresponding to act  3004 . 
     At act  3006 , portions of the fin structures not covered by the dummy gate structures are etched and removed from opposite sides of dummy gate structure. The second semiconductor layers are horizontal recessed from the first semiconductor layers.  FIGS.  14 A- 14 C  illustrate the various views of some embodiments corresponding to act  3006 . 
     At act  3008 , inner spacers are formed on opposite endings of second semiconductor layers.  FIGS.  15 A- 15 B  illustrate the various views of some embodiments corresponding to act  3008 . 
     At act  3010 , a first dummy backside contact is formed in the substrate.  FIGS.  16 A- 16 C  illustrate the various views of some embodiments corresponding to act  3010 . 
     At act  3012 , first and second source/drain epitaxial structures are formed on opposite sides of the recessed fin structure.  FIGS.  17 A- 17 D  illustrate the various views of some embodiments corresponding to act  3012 . 
     At act  3014 , the second semiconductor layers are replaced with a metal gate structure. Then, a gate contact and a front-side interconnect structure are formed.  FIGS.  18 A- 20    illustrate the various views of some embodiments corresponding to act  3014 . 
     At act  3016 , a contact trench is formed, and a bottom surface of first source/drain epitaxial structure is recessed.  FIGS.  21 - 22 C  illustrate the various views of some embodiments corresponding to act  3016 . 
     At act  3018 , an opening at top of the contact trench is enlarged.  FIGS.  23 A- 23 C  illustrate the various views of some embodiments corresponding to act  3018 . 
     At act  3020 , a second dummy backside contact is formed reaching on the recessed bottom surface of the first source/drain epitaxial structure.  FIGS.  24 A- 24 C  illustrate the various views of some embodiments corresponding to act  3020 . 
     At act  3022 , a bottom surface of second source/drain epitaxial structure is recessed.  FIGS.  25 A- 25 D  illustrate the various views of some embodiments corresponding to act  3022 . 
     At act  3024 , a back-side dielectric cap is formed on the bottom surface of the second source/drain epitaxial structure.  FIGS.  26 A- 26 D  illustrate the various views of some embodiments corresponding to act  3024 . 
     At act  3026 , a low temperature epitaxial layer on bottom surface of first source/drain epitaxial structure.  FIGS.  27 A- 27 C  illustrate the various views of some embodiments corresponding to act  3026 . 
     At act  3028 , a back-side source/drain contact is formed reaching on bottom surface of first source/drain epitaxial structure  FIGS.  28 A- 28 C  illustrate the various views of some embodiments corresponding to act  3028 . 
     At act  3030 , a back-side power rail and a backside interconnect structure are formed.  FIGS.  29 A- 29 B  illustrate the various views of some embodiments corresponding to act  3030 . 
     Accordingly, in some embodiments, the present disclosure relates to a semiconductor transistor device. The semiconductor transistor device comprises a channel structure and a gate structure wrapping around the channel structure. The semiconductor transistor device further comprises a first source/drain epitaxial structure and a second source/drain epitaxial structure disposed on opposite endings of the channel structure and a back-side source/drain contact disposed under the first source/drain epitaxial structure. The second source/drain epitaxial structure has a concave bottom surface. 
     In other embodiments, the present disclosure relates to a semiconductor transistor device. The semiconductor transistor device comprises a channel structure and a gate structure wrapping around the channel structure. The semiconductor transistor device further comprises a first source/drain epitaxial structure and a second source/drain epitaxial structure disposed on opposite endings of the channel structure and a back-side source/drain contact disposed under and contacting the first source/drain epitaxial structure. The semiconductor transistor device further comprises a gate contact disposed on the gate structure and a back-side dielectric cap disposed under and contacting the second source/drain epitaxial structure. The second source/drain epitaxial structure has a bottom surface locating higher than a bottom surface of the gate structure. 
     In yet other embodiments, the present disclosure relates to a method of manufacturing a semiconductor transistor device. The method comprises forming a fin structure over a substrate by alternately stacking first semiconductor layers and second semiconductor layers and forming a dummy gate structure over the fin structure. The method further comprises removing a portion of the fin structure uncovered by the dummy gate structure and forming inner spacers on opposite sides of remaining portions of the first semiconductor layers. The method further comprises forming a first source/drain epitaxial structure and a second source/drain epitaxial structure on opposite endings of the fin structure. The method further comprises replacing the dummy gate structure and the first semiconductor layers with a metal gate structure. The method further comprises removing the substrate and forming a back-side capping trench to expose a bottom surfaces of the metal gate structure and a bottom surface of the second source/drain epitaxial structure and performing an isotropic etch to recess the bottom surface of the second source/drain epitaxial structure to have a concave shape. The method further comprises forming a back-side dielectric cap in the back-side capping trench and forming a back-side source/drain contact under and contacting the first source/drain epitaxial structure. 
     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.