Patent Publication Number: US-2023154784-A1

Title: Bottom dielectric isolation integration with buried power rail

Description:
BACKGROUND 
     The present invention generally relates to nanosheet devices having bottom dielectric isolation, and more particularly to nanosheet devices having bottom dielectric isolation and a buried power rail. 
     A Field Effect Transistor (FET) typically has a source, a channel, and a drain, where current flows from the source to the drain, and a gate that controls the flow of current through the device channel. Field Effect Transistors (FETs) can have a variety of different structures, for example, FETs have been fabricated with the source, channel, and drain formed in the substrate material itself, where the current flows horizontally (i.e., in the plane of the substrate), and FinFETs have been formed with the channel extending outward from the substrate, but where the current also flows from a source to a drain. Depending on the doping of the source and drain, an n-FET or a p-FET can be formed. Two FETs also can be coupled to form a complementary metal oxide semiconductor (CMOS) device, where a p-channel MOSFET and n-channel MOSFET are coupled together. 
     SUMMARY 
     In accordance with an embodiment of the present invention, a semiconductor device is provided. The semiconductor device includes a protective liner, and a buried power rail on a first portion of the protective liner, wherein the protective liner is on opposite sides of the buried power rail. The semiconductor device further includes a source/drain on a second portion of the protective liner, wherein the source/drain is offset from the buried power rail, and a source/drain contact on the source/drain and in electrical communication with the buried power rail. 
     In accordance with another embodiment of the present invention, a semiconductor device is provided. The semiconductor device includes a protective liner on a support pillar, and a buried power rail on a first portion of the protective liner, wherein the protective liner is on opposite sides of the buried power rail. The semiconductor device further includes a source/drain on a second portion of the protective liner, wherein the support pillar is on an opposite side of the second portion of the protective liner from the source/drain, and a plurality of nanochannels on the second portion of the protective liner, wherein the source/drain is in electrical contact with the plurality of nanochannels. The semiconductor device further includes a source/drain contact on the source/drain and in electrical communication with the buried power rail. 
     In accordance with yet another embodiment of the present invention, a method of forming a semiconductor device is provided. The method includes forming a separation trench through a stack of alternating nanosheet sections and sacrificial sections, a first sacrificial section, and into an underlying substrate mesa to form a stack of alternating nanosheet channels and sacrificial segments on a first sacrificial segment and a support pillar having a sidewall, and removing the first sacrificial section to form a first cavity. The method further includes forming a protective liner in the first cavity and on the sidewall of the support pillar, and forming a buried power rail on a first portion of the protective liner. The method further includes removing the sacrificial sections, and forming a source/drain on a second portion of the protective liner. The method further includes forming a source/drain contact electrically connecting the source/drain to the buried power rail. 
     These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description will provide details of preferred embodiments with reference to the following figures wherein: 
         FIG.  1    is a cross-sectional side view showing a stack of alternating nanosheet layers and sacrificial layers on a substrate, in accordance with an embodiment of the present invention; 
         FIG.  2    is a cross-sectional side view showing a plurality of isolation trenches formed that divide the stack layers into nanosheet sections and sacrificial sections on substrate mesas, in accordance with an embodiment of the present invention; 
         FIG.  3    is a cross-sectional side view showing an isolation region formed in the plurality of trenches adjacent to the nanosheet sections, sacrificial sections, and mesas, in accordance with an embodiment of the present invention; 
         FIG.  4    is a cross-sectional side view showing a plurality of separation trenches through the nanosheet sections, sacrificial sections, and mesas to form a stack of alternating nanosheet channels and sacrificial segments on a support pillar, in accordance with an embodiment of the present invention; 
         FIG.  5    is a cross-sectional side view showing removal of a first sacrificial segment from the stack of alternating nanosheet channels and sacrificial segments, in accordance with an embodiment of the present invention; 
         FIG.  6    is a cross-sectional side view showing formation of a protective liner in the cavity formed by removal of the first sacrificial segments and along the sidewalls of the separation trench, and a disposable fill on the protective liner, in accordance with an embodiment of the present invention; 
         FIG.  7    is a cross-sectional side view showing replacement of the disposable fill with a buried power rail on the protective liner, in accordance with an embodiment of the present invention; 
         FIG.  8    is a cross-sectional side view showing formation of a power rail cap on the buried power rail and protective liner, in accordance with an embodiment of the present invention; 
         FIG.  9    is a cross-sectional side view showing removal of the sacrificial segments, in accordance with an embodiment of the present invention; 
         FIG.  10    is a cross-sectional side view showing formation of a gate structure on the nanosheet channels, and gate isolators between device regions, in accordance with an embodiment of the present invention; 
         FIG.  11    is a cross-sectional side view showing formation of source/drains adjoining the nanosheet channels and adjacent to the gate structures, and source/drain contacts formed between the source/drain and the buried power rail, in accordance with an embodiment of the present invention; 
         FIG.  12    is a cross-sectional side view showing a stack of alternating nanosheet layers and sacrificial layers on a substrate with a buried sacrificial layer, in accordance with an embodiment of the present invention; 
         FIG.  13    is a cross-sectional side view showing formation of a plurality of isolation trenches that divide the stack layers and buried sacrificial layer into nanosheet sections and sacrificial sections on substrate mesas with a buried sacrificial section, in accordance with an embodiment of the present invention; 
         FIG.  14    is a cross-sectional side view showing an isolation region formed in the plurality of trenches adjacent to the nanosheet sections, sacrificial sections, mesas, and buried sacrificial sections, in accordance with an embodiment of the present invention; 
         FIG.  15    is a cross-sectional side view showing a plurality of separation trenches formed through the nanosheet sections, sacrificial sections, buried sacrificial sections, and mesas to form a stack of alternating nanosheet channels and sacrificial segments on a support pillar with a sacrificial slab, in accordance with an embodiment of the present invention; 
         FIG.  16    is a cross-sectional side view showing removal of a first sacrificial segment from the stack of alternating nanosheet channels and sacrificial segments, and removal of the sacrificial slabs from the support pillars, in accordance with an embodiment of the present invention; 
         FIG.  17    is a cross-sectional side view showing formation of a protective liner in the gaps formed by removal of the first sacrificial segments and sacrificial slabs, and along the sidewalls of the support pillars, and a disposable fill on the protective liner, in accordance with an embodiment of the present invention; 
         FIG.  18    is a cross-sectional side view showing replacement of the disposable fill with a buried power rail on the protective liner, in accordance with an embodiment of the present invention; 
         FIG.  19    is a cross-sectional side view showing removal of the nanosheet stack template and formation of a power rail cap on the buried power rail and protective liner, in accordance with an embodiment of the present invention; 
         FIG.  20    is a cross-sectional side view showing formation of source/drains adjoining the nanosheet channels and adjacent to the gate structures, source/drain contacts formed between the source/drain and the buried power rail, and metallization layers formed on the source/drain contacts, in accordance with an embodiment of the present invention; 
         FIG.  21    is a cross-sectional side view showing devices and metallization layers flipped, in accordance with an embodiment of the present invention; 
         FIG.  22    is a cross-sectional side view showing removal of the substrate to expose the protective liner, in accordance with an embodiment of the present invention; 
         FIG.  23    is a cross-sectional side view showing formation of a backside interlayer dielectric (BILD) layer on the protective liner and semiconductor devices, and back-side interconnect features formed in the BILD layer, in accordance with an embodiment of the present invention; 
         FIG.  24    is a cross-sectional side view showing an isolation region formed in the plurality of trenches adjacent to the nanosheet sections, sacrificial sections, mesas, and buried sacrificial sections, in accordance with an embodiment of the present invention; 
         FIG.  25    is a cross-sectional side view showing a plurality of separation trenches formed through the nanosheet sections, sacrificial sections, and a portion of the mesas to form a stack of alternating nanosheet channels and sacrificial segments on a support pillar with a sacrificial section below the support pillars, in accordance with an embodiment of the present invention; 
         FIG.  26    is a cross-sectional side view showing formation of a shield layer on the sidewalls of the stack of alternating nanosheet channels, sacrificial segments, and support pillars, extension of the separation trenches through the buried sacrificial sections to form buried sacrificial segments, and removal of the buried sacrificial segments from the support pillars, in accordance with an embodiment of the present invention; 
         FIG.  27    is a cross-sectional side view showing formation of an etch-stop layer in the cavities formed by removing the buried sacrificial segments, in accordance with an embodiment of the present invention; 
         FIG.  28    is a cross-sectional side view showing formation of a protective liner in the gaps formed by removal of the first sacrificial segments, and along the sidewalls of the support pillars, in accordance with an embodiment of the present invention; and 
         FIG.  29    is a cross-sectional side view showing formation of an backside interlayer dielectric (BILD) layer on the etch-stop layer, protective liner, and semiconductor devices, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Devices and fabrication method are provided relating to nanosheet channels separated from buried power rails by a dielectric liner. In various embodiments, a buried power rail can be formed in a substrate adjacent to a nanosheet stack. 
     Embodiments of the present invention provide etch selectivity that allows a device chip to be flipped and the backside removed through well controlled thinning. In various embodiments, a protective liner or etch-stop layer can provide etch selectivity without utilizing a semiconductor-on-insulator (SeOI) substrate. 
     Exemplary applications/uses to which the present invention can be applied include, but are not limited to: nanosheet devices. 
     It is to be understood that aspects of the present invention will be described in terms of a given illustrative architecture; however, other architectures, structures, substrate materials and process features and steps can be varied within the scope of aspects of the present invention. 
     Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG.  1   ,  FIG.  1    is a cross-sectional side view showing a stack of alternating nanosheet layers and sacrificial layers on a substrate, in accordance with an embodiment of the present invention; 
     In one or more embodiments, a plurality of alternating nanosheet layers  140  and sacrificial layers  130  can be formed on a substrate  110 , where the alternating nanosheet layers  140  and sacrificial layers  130  can be formed by an epitaxial growth process on the surface of the substrate  110 . 
     In various embodiments, the substrate  110  can be a semiconductor material, including, but not limited to, group IV semiconductors, for example, silicon (Si) and germanium (Ge), IV-IV compound semiconductors, for example, silicon carbide (Si) and silicon-germanium (SiGe), III-V semiconductors, for example, gallium nitride (GaN), gallium arsenide (GaAs), and indium phosphide (InP). The semiconductor substrate can be selected to provide a surface suitable for epitaxial growth of the intended nanosheet layers  140  and sacrificial layers  130 , for example, having a crystalline structure that minimizes stress and/or defect formation in the epitaxial layers. In various embodiments, the substrate can be a single crystal wafer of the semiconductor material. 
     In one or more embodiments, the nanosheet layers  140  and sacrificial layers  130  can each be a semiconductor material, including, but not limited to, group IV semiconductors, for example, silicon (Si) and germanium (Ge), IV-IV compound semiconductors, for example, silicon carbide (Si) and silicon-germanium (SiGe), III-V semiconductors, for example, gallium nitride (GaN), gallium arsenide (GaAs), and indium phosphide (InP), where the materials of the nanosheet layers  140  and sacrificial layers  130  can be different from each other and from the substrate  110  to provide suitable etch selectivity. For example, an HNA etch can be specific for etching highly doped silicon (Si), but not lightly doped silicon (Si). Whereas, TEAH etchant can be specific for etching lightly doped silicon (Si), but not highly doped silicon (Si). Selecting appropriate materials and etchants can leverage the etch rate differences between the materials. In various embodiments, a selective etch rate can be an etch rate ratio of at least 100:1. 
     In various embodiments, the nanosheet layers  140  can have a thickness in a range of about 3 nanometers (nm) to about 15 nm, or about 4 nm to about 9 nm, although other thicknesses are also contemplated. 
     In various embodiments, the sacrificial layers  130  can have a thickness in a range of about 6 nanometers (nm) to about 20 nm, or about 8 nm to about 15 nm, although other thicknesses are also contemplated. 
     In various embodiments, the material of the nanosheet layers  140  can be chosen to provide semiconductor device channels, and the material of the sacrificial layers  130  can chosen to be selectively removable relative to the nanosheet layers  140 . 
     In one or more embodiments, a first sacrificial layer  120  can be formed on the substrate  110  between the substrate and the stack of alternating nanosheet layers  140  and sacrificial layers  130 , where the first sacrificial layer  120  can be selectively removed relative to the nanosheet layers  140  and sacrificial layers  130  and the substrate. The first sacrificial layer  120  can be formed by epitaxial growth on the substrate, where the first sacrificial layer  120  can be a material different from nanosheet layers  140  and sacrificial layers  130  and the substrate. The first sacrificial layer  120  can be made of a semiconductor material epitaxially formed on the substrate  110 . 
     In various embodiments, the first sacrificial layer  120  can have a thickness in a range of about 4 nanometers (nm) to about 15 nm, or about 6 nm to about 10 nm, although other thicknesses are also contemplated. 
     In a non-limiting exemplary embodiment, the substrate  110  can be single crystal silicon (Si), the first sacrificial layer  120  can be silicon-germanium (SiGe) with a germanium concentration greater than 50 atomic percent (at. %), the sacrificial layers  130  can be silicon-germanium (SiGe) with a germanium concentration less than 40 at. %, and the nanosheet layers  140  can be silicon (Si). In various embodiments, the first sacrificial layer  120  can be, for example, silicon-germanium (SiGe) with a germanium concentration of about 60 at. % and the sacrificial layers  130  can be, for example, silicon-germanium (SiGe) with a germanium concentration of about 30 at. %. 
     In one or more embodiments, one or more etch templates  150  can be formed on the stack of alternating nanosheet layers  140  and sacrificial layers  130 , where the etch templates  150  can be formed by lithographic and etching processes. The etch templates  150  can be patterned from a hardmask layer. In various embodiments, the etch templates  150  can be made of a hardmask material, including, but not limited to, silicon oxide (SiO), silicon nitride (SiN), silicon oxynitride (SiON), silicon carbide (SiC), silicon carbonitride (SiCN), silicon boro carbonitride (SiBCN), and combinations thereof. In various embodiments, the etch templates  150  can have multiple layers. 
     In various embodiments, adjacent pairs of etch templates  150  can be separated by a distance, for example, in a range of about 15 nm to about 120 nm, or about 35 nm to about 70 nm, although other distances are also contemplated. 
       FIG.  2    is a cross-sectional side view showing a plurality of isolation trenches formed that divide the stack layers into nanosheet sections and sacrificial sections on substrate mesas, in accordance with an embodiment of the present invention. 
     In one or more embodiments, an etch mask  160  can be formed on two or more etch templates  150 , where the etch mask  160  can cover the portion of a top layer, for example, a top nanosheet layer  140  (although the top layer may also be a sacrificial layer  130 ) exposed between the two adjacent etch templates  150 . The etch mask  160  can be formed by a blanket deposition, for example, spin-on coating of an organic planarization layer (OPL), and patterned by lithography and etching. 
     In one or more embodiments, a plurality of isolation trenches  170  can be formed through the stack of alternating nanosheet layers  140  and sacrificial layers  130 , a first sacrificial layer  120 , and into the underlying substrate  110  to form a stack of alternating nanosheet sections  142  and sacrificial sections  132  on a first sacrificial section  122  and a substrate mesa  112 . The substrate mesas  112  have a flat topped feature with steep slopes, which captures the aspects of the raised features beneath the stacks. The isolation trenches  170  can be formed in the region between adjacent etch templates  150  that are not covered by an etch mask  160 , where the isolation trenches  170  can be formed by a selective direction etch, for example, a reactive ion etch (RIE). Formation of the isolation trenches  170  can expose the sidewalls of the nanosheet layers  140  and sacrificial layers  130 , and the substrate mesas  112 . 
     In various embodiments, the isolation trenches  170  can extend a depth into the substrate  110  in a range of about 15 nm to about 200 nm, or about 50 nm to about 100 nm, although other depths are also contemplated. The isolation trenches  170  can have a width essentially equal to the distance between the etch templates  150  given the etching process and process tolerances. 
       FIG.  3    is a cross-sectional side view showing an isolation region formed in the plurality of trenches adjacent to the nanosheet sections, sacrificial sections, and mesas, in accordance with an embodiment of the present invention. 
     In one or more embodiments, the etch masks  160  can be removed to expose the surface of the top layer between the etch templates  150 . The etch masks  160  can be removed through ashing an OPL or selectively etching a dielectric material. 
     In one or more embodiments, an isolation region can be formed in the plurality of trenches  170  adjacent to the nanosheet sections  142 , sacrificial sections  132 , and mesas  112 , where the isolation region can include a trench liner  180  and a trench fill  190 . In various embodiments, the trench liner  180  can be formed by a conformal deposition, for example, atomic layer deposition (ALD), plasma enhanced ALD (PEALD), or a combination thereof. In various embodiments, the trench fill  190  can be formed by a conformal deposition (e.g., ALD, PEALD), a blanket deposition (e.g., CVD, PECVD), or a combination thereof. 
     In various embodiments, the trench liner  180  can be made of silicon oxide (SiO), silicon nitride (SiN), silicon oxynitride (SiON), and combinations thereof. In various embodiments, the trench fill  190  can be made of silicon oxide (SiO), silicon nitride (SiN), silicon oxynitride (SiON), and combinations thereof. In various embodiments, the isolation trenches  170  can be filled with a single dielectric material to form the isolation region. 
     In various embodiments, portions of the trench fill  190  and trench liner  180  can be removed from the etch templates  150  and surface of the stack by chemical-mechanical polishing (CMP) and selective etching, where the trench fill  190  and trench liner  180  can be recessed below the top surface of the top layer, for example, the surface of the top nanosheet section  142 . 
     In various embodiments, the trench liner  180  can have a thickness in a range of about 0.5 nm to about 10 nm, or about 2 nm to about 6 nm, although other thicknesses are also contemplated. 
       FIG.  4    is a cross-sectional side view showing a plurality of separation trenches through the nanosheet sections, sacrificial sections, and mesas to form a stack of alternating nanosheet channels and sacrificial segments on a support pillar, in accordance with an embodiment of the present invention. 
     In one or more embodiments, one or more separation trenches  200  can be formed through each of the stacks of alternating nanosheet sections  142  and sacrificial sections  132 , the first sacrificial section  122 , and into the underlying substrate mesas  112  to form a stack of alternating nanosheet channels  145  and sacrificial segments  135  on a first sacrificial segment  125  and support pillar  115 . The separation trenches  200  can be formed in the region between adjacent etch templates  150  that was previously covered by an etch mask  160 , where the separation trenches  200  can be formed by a selective direction etch, for example, a reactive ion etch (RIE). Formation of the separation trenches  200  can expose the sidewalls of the nanosheet channels  145  and sacrificial segments  135 , and the support pillars  115 . 
     In various embodiments, the separation trenches  200  can extend a depth into the substrate  110  in a range of about 50 nm to about 200 nm, or about 40 nm to about 150 nm, or about 40 nm to about 200 nm, although other depths are also contemplated. In various embodiments, the separation trenches  200  can extend further into the substrate  110  than the adjacent isolation trenches  170 , where the lengths of opposing sidewalls of the support pillars  115  can be different. The separation trenches  200  can have a width essentially equal to the distance between the etch templates  150  given the etching process and process tolerances. 
       FIG.  5    is a cross-sectional side view showing removal of a first sacrificial segment from the stack of alternating nanosheet channels and sacrificial segments, in accordance with an embodiment of the present invention. 
     In one or more embodiments, a first sacrificial segment  125  can be removed from between the support pillars  115  and the stack of alternating nanosheet channels  145  and sacrificial segments  135 , where the first sacrificial segment  125  can be removed through a selective isotropic etch, for example, a wet chemical etch, dry plasma etch, or combination thereof. Removal of the first sacrificial segment  125  from below each of the stacks can form a cavity between the substrate and the stack with a face of the cavity open to the separation trench  200 . The cavity can have a height about the same as the thickness of the first sacrificial segment  125 . 
       FIG.  6    is a cross-sectional side view showing formation of a protective liner in the cavity formed by removal of the first sacrificial segments and along the sidewalls of the separation trench, and a disposable fill on the protective liner, in accordance with an embodiment of the present invention. 
     In one or more embodiments, a protective liner  210  can be formed in the cavity formed by removal of the first sacrificial segments  125  and along the sidewalls of the separation trench  200 , where the protective liner  210  can be formed by a conformal deposition (e.g., ALD, PEALD). The protective liner  210  can cover the exposed surfaces of the support pillars  115  and the bottom surface of the separation trench  200 . 
     In various embodiments, the protective liner  210  can be an electrically insulating dielectric material, including, but not limited to, silicon nitride (SiN), silicon carbide (SiC), silicon oxy carbide (SiOC), silicon oxy carbonitride (SiOCN), silicon boro carbonitride (SiBCN), and combinations thereof. 
     In various embodiments, the protective liner  210  can have a thickness in a range of about 3 nm to about 20 nm, or about 5 nm to about 10 nm, although other thicknesses are also contemplated. The protective liner  210  can have a thickness of about half the height of the cavity formed by removing the first sacrificial segments  125 , where the protective liner  210  can fill in the cavity. In various embodiments, the liner thickness is at least half of the cavity height, although the protective liner  210  can be thicker. 
     In one or more embodiments, a disposable fill  220  can be formed on the protective liner  210  in the separation trenches  200 , where the disposable fill  220  can be formed by a spin-on coating (e.g., OPL) to fill in the trench space between the sidewalls of the protective liner  210 . 
     In various embodiments, the disposable fill  220  can be an OPL. 
     In various embodiments, the disposable fill  220  can be etched back below the bottom most nanosheet channel  145 , followed by removing the exposed dielectric liner  210 , to expose the sides of the nanosheet channels  145  and sacrificial segments  135 , where the top surfaces of the disposable fill  220  and protective liner  210  can be between the top and bottom surfaces of the bottom layer, for example, the lower most sacrificial segment  135 . 
       FIG.  7    is a cross-sectional side view showing replacement of the disposable fill with a buried power rail on the protective liner, in accordance with an embodiment of the present invention. 
     In one or more embodiments, the disposable fill  220  can be removed using a selective etch, for example, RIE, wet etch, dry plasma etch, or a combination thereof. 
     In one or more embodiments, a buried power rail  230  can be formed on the protective liner  210  in the space left by removing the disposable fill  220 . A conductive material can be formed in the separation trenches  200  to form the buried power rail  230  can be formed on the protective liner  210 , and the conductive material can be etched back to a predetermined height. The top surface of the conductive material can be below the bottom surface of the portion of the protective liner  210  formed in the cavities. The protective liner  210  can form a U-shaped channel around the buried power rail  230 , where sidewalls of the protective liner  210  are on opposite sides of the buried power rail  230 . 
     In various embodiments, the buried power rail  230  can be made of a conductive material, for example, a metal, including, but not limited to, tungsten (W), ruthenium (Ru), cobalt (Co), molybdenum (Mo), and combinations thereof. In various embodiments, a thin adhesion layer, for example, titanium nitride (TiN), tantalum nitride (TaN), etc., may be used. 
       FIG.  8    is a cross-sectional side view showing formation of a power rail cap on the buried power rail and protective liner, in accordance with an embodiment of the present invention. 
     In one or more embodiments, a power rail cap  240  can be formed on the buried power rail  230  and protective liner  210 . The power rail cap  240  can be an electrically insulating dielectric material that covers the buried power rail  230 . 
     In various embodiments, the power rail cap  240  can be an electrically insulating dielectric material, including, but not limited to, silicon oxide (SiO), silicon nitride (SiN), silicon carbide (SiC), silicon oxy carbide (SiOC), silicon oxy carbonitride (SiOCN), and combinations thereof. The power rail cap  240  can be a different material from the protective liner  210 . The power rail cap  240  can be the same material as trench fill  190 . The power rail cap  240  can be formed by overfilling the dielectric material, followed by CMP to stop on the etch templates  150 , followed by dielectric recess so the active nanosheet stack layers are revealed for device fabrication. 
       FIG.  9    is a cross-sectional side view showing removal of the sacrificial segments, in accordance with an embodiment of the present invention. 
     In one or more embodiments, additional process steps are done, including removing the etch templates  150 , forming the sacrificial gate over the nanosheet stack, forming spacer, inner spacer, and S/D epi, followed by ILD formation and ILD CMP to exposed the sacrificial gate. After that, the sacrificial gates are removed, and the sacrificial segments  135  can be removed from between the nanosheet channels  145 , where the sacrificial segments  135  can be removed using a selective isotropic etch (e.g., wet chemical etch, dry plasma etch). In various embodiments, removal of a bottom sacrificial segment  135  can expose the top surface of the protective liner  210  in the cavity. 
       FIG.  10    is a cross-sectional side view showing formation of a gate structure on the nanosheet channels, and gate isolators between device regions, in accordance with an embodiment of the present invention. 
     In one or more embodiments, a gate structure  250  can be formed on a portion of the nanosheet channels  145 , where the gate structure  250  can be a gate all-around structure that fills in the spaces between the nanosheet channels  145 . In various embodiments, the gate structures  250  can include a gate dielectric layer on the nanosheet channels  145 , a work function material layer on the gate dielectric layer, and a conductive gate fill on the work function material layer. 
     In one or more embodiments, a portion of the gate structure can be removed from between adjacent stacks of the nanosheet channels  145  to form a trough. The gate structure can be removed down to the surface of the power rail caps  240 . 
     In one or more embodiments, a gate cut slab  260  can be formed in the trough in the gate structure to form a plurality of electrically isolated gate structures on different stack(s) of nanosheet channels  145 . In various embodiments, the gate cut slab  260  can be an electrically insulating dielectric material, including, but not limited to, silicon oxide (SiO), silicon nitride (SiN), silicon oxy nitride (SiON), silicon boronitride (SiBN), silicon carbo nitride (SiCN), silicon boro carbonitride (SiBCN), and combinations thereof. 
       FIG.  11    is a cross-sectional side view showing formation of source/drains adjoining the nanosheet channels and adjacent to the gate structures, and source/drain contacts formed between the source/drain and the buried power rail, in accordance with an embodiment of the present invention. 
     In one or more embodiments, source/drains  270  can be formed on the protective liner  210 , such that the protective liner  210  electrically insulates the source/drains from the support pillar  115 , substrate  110 , and buried power rail  230 . The source/drains  270  can be formed on the protective liner  210  by a lateral epitaxial growth from the exposed faces of the nanosheet channels  145 , where a source/drain  270  can be formed on each of opposite sides of the nanosheet channels  145 . The source/drains  270  on the protective liner  210  can be above and laterally offset from the buried power rail  230  with the protective liner  210  between the source/drain  270  and the buried power rail  230 . 
     In various embodiments, the source/drains  270  on each of opposite sides of the nanosheet channels  145  can be doped to form an n-type or a p-type semiconductor transistor device. 
     In one or more embodiments, a dielectric cover layer  290  can be formed on the source/drains  270 , where the dielectric cover layer  290  can be formed by a conformal deposition, a blanket deposition, or a combination thereof. 
     In various embodiments, the dielectric cover layer  290  can be made of silicon oxide (SiO), silicon nitride (SiN), or a combination thereof. 
     In one or more embodiments, a source/drain contact  280  can be formed in the dielectric cover layer  290  and between one of the source/drains  270  and the buried power rail  230 , where a portion of the power rail cap  240  can be removed to allow electrical connection between the source/drain contact  280  and the buried power rail  230 , such that the source/drain contact  280  passes through the power rail cap  240 . In various embodiments, a source/drain contact  280  can be formed to the source/drains  270  of a transistor device without forming an electrical connection to the buried power rail  230 . 
       FIG.  12    is a cross-sectional side view showing a stack of alternating nanosheet layers and sacrificial layers on a substrate with a buried sacrificial layer, in accordance with an embodiment of the present invention. 
     In various embodiments, a buried sacrificial layer  117  can be formed on the substrate  110 , and a upper substrate layer  111  can be formed on the buried sacrificial layer  117 , where the buried sacrificial layer  117  can be formed by epitaxial growth on the substrate, and the upper substrate layer  111  can be formed by epitaxial growth on the buried sacrificial layer  117 . The upper substrate layer  111  can be the same semiconductor material as the substrate  110 , or the upper substrate layer  111  can be a different semiconductor material from the substrate  110 . 
     In one or more embodiments, a first sacrificial layer  120  can be formed on the upper substrate layer  111 , where the first sacrificial layer  120  can be formed by epitaxial growth on the upper substrate layer  111 . The first sacrificial layer  120  can be made of a material different from the upper substrate layer  111 . 
     In one or more embodiments, a plurality of alternating nanosheet layers  140  and sacrificial layers  130  can be formed on the first sacrificial layer  120 , where the alternating nanosheet layers  140  and sacrificial layers  130  can be formed by an epitaxial growth process on the surface of the first sacrificial layer  120 . The nanosheet layers  140  can be the same semiconductor material as the substrate  110  and/or the upper substrate layer  111 , or the nanosheet layers  140  can be a different semiconductor material from the substrate  110  and/or the upper substrate layer  111 . 
     The first sacrificial layer  120  can be made of a material different from nanosheet layers  140  and sacrificial layers  130 , where the first sacrificial layer  120  can be selectively removed relative to the nanosheet layers  140 , the sacrificial layers  130 , and the upper substrate layer  111 . The first sacrificial layer  120  can be between the upper substrate layer  111  and the stack of alternating nanosheet layers  140  and sacrificial layers  130 . 
     In various embodiments, the buried sacrificial layer  117  can be the same material as the first sacrificial layer  120 , so the buried sacrificial layer  117  and the first sacrificial layer  120  can be removed using the same etch process. 
     In one or more embodiments, one or more etch templates  150  can be formed on the stack of alternating nanosheet layers  140  and sacrificial layers  130 , where the etch templates  150  can be formed by lithographic and etching processes. The etch templates  150  can be patterned from a hardmask layer. 
       FIG.  13    is a cross-sectional side view showing formation of a plurality of isolation trenches that divide the stack layers and buried sacrificial layer into nanosheet sections and sacrificial sections on substrate mesas with a buried sacrificial section, in accordance with an embodiment of the present invention. 
     In one or more embodiments, an etch mask  160  can be formed on two or more etch templates  150 , where the etch mask  160  can cover the portion of a top layer, for example, a top nanosheet layer  140  (although the top layer may also be a sacrificial layer  130 ) exposed between the two adjacent etch templates  150 . The etch mask  160  can be formed by a blanket deposition, for example, a spin-on coating of an organic planarization layer (OPL) or other dielectric material resistant to etching. 
     In one or more embodiments, a plurality of isolation trenches  170  can be formed through the stack of alternating nanosheet layers  140  and sacrificial layers  130 , a first sacrificial layer  120 , the upper substrate layer  111 , and the buried sacrificial layer  117  into the underlying substrate  110  to form a stack of alternating nanosheet sections  142  and sacrificial sections  132  on a first sacrificial section  122  and a substrate mesa  112 , where a buried sacrificial section  118  is between the substrate mesa  112  and the underlying substrate  110 . In various embodiments, a buried sacrificial section  118  is between a top and a bottom surface of each substrate mesa  112 , where the isolation trenches  170  extend into the substrate  110 . The isolation trenches  170  can be formed between adjacent etch templates  150  that are not covered by an etch mask  160 , where the isolation trenches  170  can be formed by a selective direction etch, for example, a reactive ion etch (RIE). Formation of the isolation trenches  170  can expose the sidewalls of the nanosheet layers  140  and sacrificial layers  130 , the substrate mesas  112 , and the buried sacrificial sections  118 . 
     In various embodiments, the isolation trenches  170  can extend a depth from the bottom surface of the first sacrificial section  122  in a range of about 70 nm to about 400 nm, or about 100 nm to about 250 nm, although other depths are also contemplated. The isolation trenches  170  can extend below the bottom surface of the buried sacrificial layer  117 , so the sidewall faces of the buried sacrificial layer  117  are exposed. The isolation trenches  170  can have a width essentially equal to the distance between the etch templates  150  given the etching process and process tolerances. 
       FIG.  14    is a cross-sectional side view showing an isolation region formed in the plurality of trenches adjacent to the nanosheet sections, sacrificial sections, mesas, and buried sacrificial sections, in accordance with an embodiment of the present invention. 
     In one or more embodiments, an isolation region can be formed in the plurality of trenches  170  adjacent to the nanosheet sections  142 , sacrificial sections  132 , first sacrificial sections  122 , substrate mesa  112 , and buried sacrificial section  118 , where the isolation region can include a trench liner  180  and a trench fill  190 . In various embodiments, the trench liner  180  can be formed by a conformal deposition, for example, atomic layer deposition (ALD), plasma enhanced ALD (PEALD), or a combination thereof. In various embodiments, the trench fill  190  can be formed by a conformal deposition (e.g., ALD, PEALD), a blanket deposition (e.g., CVD, PECVD), or a combination thereof. 
     In various embodiments, the trench liner  180  can be made of silicon nitride (SiN), silicon oxynitride (SiON), and combinations thereof. In various embodiments, the trench fill  190  can be made of silicon oxide (SiO), silicon nitride (SiN), silicon oxynitride (SiON), and combinations thereof. In various embodiments, the isolation trenches  170  can be filled with a single dielectric material to form the isolation region. 
     In one or more embodiments, the etch masks  160  can be removed to expose the surface of the top layer between the etch templates  150 . The etch masks  160  can be removed through ashing an OPL or selectively etching a dielectric material. 
       FIG.  15    is a cross-sectional side view showing a plurality of separation trenches formed through the nanosheet sections, sacrificial sections, buried sacrificial sections, and mesas to form a stack of alternating nanosheet channels and sacrificial segments on a support pillar with a sacrificial slab, in accordance with an embodiment of the present invention. 
     In one or more embodiments, one or more separation trenches  200  can be formed through each of the stacks of alternating nanosheet sections  142  and sacrificial sections  132 , the first sacrificial section  122 , through the underlying substrate mesas  112  and buried sacrificial section  118  to form a stack of alternating nanosheet channels  145  and sacrificial segments  135  on a first sacrificial segment  125 , support pillar  115 , and buried sacrificial segment  119 . The separation trenches  200  can be formed in the region between adjacent etch templates  150  that was previously covered by an etch mask  160 , where the separation trenches  200  can be formed by a selective direction etch, for example, a reactive ion etch (RIE). Formation of the separation trenches  200  can expose the sidewalls of the nanosheet channels  145  and sacrificial segments  135 , first sacrificial segment  125 , support pillar  115 , and buried sacrificial segment  119 . 
     In various embodiments, the separation trenches  200  can extend a depth into the substrate  110  in a range of about 70 nm to about 400 nm, or about 100 nm to about 250 nm, although other depths are also contemplated. In various embodiments, the separation trenches  200  can extend further into the substrate  110  than the adjacent isolation trenches  170 , where the distance to the bottom surface of the trenches on opposite sides of the buried sacrificial segment  119  can be different. The separation trenches  200  can have a width essentially equal to the distance between the etch templates  150  given the etching process and process tolerances. 
       FIG.  16    is a cross-sectional side view showing removal of a first sacrificial segment from the stack of alternating nanosheet channels and sacrificial segments, and removal of the sacrificial slabs from the support pillars, in accordance with an embodiment of the present invention. 
     In one or more embodiments, the first sacrificial segment  125  and the buried sacrificial segment  119  can each be removed, where the first sacrificial segment  125  and the buried sacrificial segment  119  can be removed through a selective isotropic etch, for example, a wet chemical etch, dry plasma etch, or combination thereof. Removal of the first sacrificial segment  125  from below each of the stacks can form a cavity between the substrate and the stack with a face of the cavity open to the separation trench  200 . The cavity can have a height about the same as the thickness of the first sacrificial segment  125 . Removal of the buried sacrificial segment  119  can form a second cavity between the substrate  110  and the support pillar  115 . Removal of the first sacrificial segment  125  and/or buried sacrificial segment  119  can expose portions of the trench liner  180  and/or trench fill  190 . 
       FIG.  17    is a cross-sectional side view showing formation of a protective liner in the gaps formed by removal of the first sacrificial segments and sacrificial slabs, and along the sidewalls of the support pillars, and a disposable fill on the protective liner, in accordance with an embodiment of the present invention. 
     In one or more embodiments, a protective liner  210  can be formed in the first cavity formed by removal of the first sacrificial segments  125  and the second cavity formed by removal of the buried sacrificial segment  119 , as well as along the sidewalls of the separation trench  200 , where the protective liner  210  can be formed by a conformal deposition (e.g., ALD, PEALD). The protective liner  210  can cover the exposed surfaces of the support pillars  115  and the bottom surface of the separation trench  200 . 
     In various embodiments, the protective liner  210  can be an electrically insulating dielectric material, including, but not limited to, silicon nitride (SiN), silicon carbide (SiC), silicon oxy carbide (SiOC), silicon oxy carbonitride (SiOCN), silicon boro carbonitride (SiBCN), and combinations thereof. 
     In various embodiments, the protective liner  210  can have a thickness in a range of about 2 nm to about 20 nm, or about 5 nm to about 10 nm, although other thicknesses are also contemplated. The protective liner  210  can have a thickness of about half the height of the cavity formed by removing the first sacrificial segments  125 , where the protective liner  210  can fill in the cavity. 
     In one or more embodiments, a disposable fill  220  can be formed on the protective liner  210  in the separation trenches  200 , where the disposable fill  220  can be formed by a spin-on coating (e.g., OPL) to fill in the trench space between the sidewalls of the protective liner  210 . 
     In various embodiments, the disposable fill  220  can be an OPL. 
     In various embodiments, the disposable fill  220  and protective liner  210  can be etched back to expose the sides of the nanosheet channels  145  and sacrificial segments  135 , where the top surfaces of the disposable fill  220  and protective liner  210  can be between the top and bottom surfaces of the bottom layer, for example, the lower most sacrificial segment  135 . 
       FIG.  18    is a cross-sectional side view showing replacement of the disposable fill with a buried power rail on the protective liner, in accordance with an embodiment of the present invention. 
     In one or more embodiments, the disposable fill  220  can be removed using a selective etch, for example, RIE, wet etch, dry plasma etch, or a combination thereof. 
     In one or more embodiments, a buried power rail  230  can be formed on the protective liner  210  in the space left by removing the disposable fill  220 . A conductive material can be formed in the separation trenches  200  to form the buried power rail  230  can be formed on the protective liner  210 , and the conductive material can be etched back to a predetermined height. The top surface of the conductive material can be below the bottom surface of the portion of the protective liner  210  formed in the cavities. 
     In various embodiments, the buried power rail  230  can be made of a conductive material, for example, a metal, including, but not limited to, tungsten (W), ruthenium (Ru), cobalt (Co), molybdenum (Mo), and combinations thereof. In various embodiments, a thin adhesion layer, for example, titanium nitride (TiN), tantalum nitride (TaN), etc., may be used. 
       FIG.  19    is a cross-sectional side view showing removal of the nanosheet stack template and formation of a power rail cap on the buried power rail and protective liner, in accordance with an embodiment of the present invention. 
     In one or more embodiments, a power rail cap  240  can be formed on the buried power rail  230  and protective liner  210 . The power rail cap  240  can be an electrically insulating dielectric material that covers the buried power rail  230 . 
     In various embodiments, the power rail cap  240  can be an electrically insulating dielectric material, including, but not limited to, silicon oxide (SiO), silicon nitride (SiN), silicon carbide (SiC), silicon oxy carbide (SiOC), silicon oxy carbonitride (SiOCN), and combinations thereof. The power rail cap  240  can be a different material from the protective liner  210 . The power rail cap can be the same dielectric material as trench fill  190 . It can be formed by overfilling the dielectric, followed by CMP and dielectric recess. After that, the nanosheet stack template (hardmask) can be removed. 
       FIG.  20    is a cross-sectional side view showing formation of source/drains adjoining the nanosheet channels and adjacent to the gate structures, source/drain contacts formed between the source/drain and the buried power rail, and metallization layers formed on the source/drain contacts, in accordance with an embodiment of the present invention. 
     In one or more embodiments, source/drains  270  can be formed on the protective liner  210 , such that the protective liner  210  electrically insulates the source/drains from the support pillar  115 , substrate  110 , and buried power rail  230 . The source/drains  270  can be formed on the protective liner  210  by a lateral epitaxial growth from the exposed faces of the nanosheet channels  145 , where a source/drain  270  can be formed on each of opposite sides of the nanosheet channels  145 . 
     In various embodiments, the source/drains  270  on each of opposite sides of the nanosheet channels  145  can be doped to form an n-type or a p-type semiconductor transistor device. 
     In one or more embodiments, a dielectric cover layer  290  can be formed on the source/drains  270 , where the dielectric cover layer  290  can be formed by a conformal deposition, a blanket deposition, or a combination thereof. 
     In one or more embodiments, a source/drain contact  280  can be formed in the dielectric cover layer  290  and between one of the source/drains  270  and the buried power rail  230 , where a portion of the power rail cap  240  can be removed to allow electrical connection between the source/drain contact  280  and the buried power rail  230 . In various embodiments, a source/drain contact  280  can be formed to the source/drains  270  of a transistor device without forming an electrical connection to the buried power rail  230 . 
     In one or more embodiments, one or more source/drain vias  300  can be formed in the dielectric cover layer  290 , where the source/drain vias  300  can be formed on the source/drain contact(s)  280  that are not electrically connected to the buried power rail  230 . 
     In various embodiments, one or more first level metal lines  310  can be formed in the dielectric cover layer  290 , where the first level metal lines  310  can be in electrical communication with one or more of the source/drain vias  300 . The one or more first level metal lines  310  and one or more source/drain vias  300  can form a first metallization layer in the dielectric cover layer  290 . The dielectric cover layer  290  can be with different dielectrics with different dielectric constant between the MOL contacts  280  and first level metal lines  310  of BEOL interconnects (e.g. in the MOL, the ILD layer is SiO 2  based with dielectric constant 3.9, and in the BEOL, the ILD layer is low-k dielectric with dielectric constant less than 3.9). 
     In various embodiments, one or more second level metal lines  320  can be formed over the dielectric cover layer  290  and M1 lines  310 , where the second level metal lines  320  can form one or more metallization layer(s) above (e.g. more than 10 levels of BEOL interconnects). After that, a carrier wafer  410  is bonded to the upper surface of the BEOL layer to facilitate the wafer flipping and backside processing. 
       FIG.  21    is a cross-sectional side view showing devices and metallization layers flipped, in accordance with an embodiment of the present invention. 
     In one or more embodiments, the substrate  110 , devices, dielectric cover layer  290 , and metallization layers  300 ,  310  can be flipped to allow access to the bottom surface of the substrate  110 . In various embodiments, a carrier wafer  410  is attached to the upper surface of the BEOL by wafer bonding process and serves as handler wafer for the wafer flipping. In various embodiments, the carrier wafer  410  can be a semiconductor wafer. The backside of the substrate  110  is ready for further processing. 
       FIG.  22    is a cross-sectional side view showing removal of the substrate to expose the protective liner, in accordance with an embodiment of the present invention. 
     In one or more embodiments, the substrate  110  can be removed to expose a bottom surface of the protective liner  210  and trench liner  180 . The substrate  110  can be removed using grinding, CMP (wet or dry), etching, or a combination thereof. The presence of the protective liner  210  and the trench liner  180  can provide a reference level with etch-stop properties that can provide an etch-stop surface, so that substrate removal process(es) won&#39;t damage the frontside devices. 
       FIG.  23    is a cross-sectional side view showing formation of a backside interlayer dielectric (BILD) layer on the protective liner and semiconductor devices, and back-side interconnect features formed in the BILD layer, in accordance with an embodiment of the present invention. 
     In one or more embodiments, an backside interlayer dielectric (BILD) layer  340  can be formed on the protective liner  210  and the trench liner  180 , where the BILD layer  340  can be formed by a blanket deposition (e.g., CVD, PECVD). 
     In one or more embodiments, the backside interconnect  350  can be formed with lithography and etching process, followed by metallization, such as Cu metallization. A backside interconnect  350  can be formed in the BILD to wire the buried power rail  230  to backside power supplies. 
       FIG.  24    is a cross-sectional side view showing an isolation region formed in the plurality of trenches adjacent to the nanosheet sections, sacrificial sections, mesas, and buried sacrificial sections, in accordance with an embodiment of the present invention. 
     In various embodiments, the buried sacrificial layer  117  can be the same material as the sacrificial layers  130 , so the buried sacrificial layer  117  can be selectively removed relative to the and the first sacrificial layer  120 . 
     In one or more embodiments, one or more etch templates  150  can be formed on the stack of alternating nanosheet layers  140  and sacrificial layers  130 , where the etch templates  150  can be formed by lithographic and etching processes. The etch templates  150  can be patterned from a hardmask layer. 
       FIG.  25    is a cross-sectional side view showing a plurality of separation trenches formed through the nanosheet sections, sacrificial sections, and a portion of the mesas to form a stack of alternating nanosheet channels and sacrificial segments on a support pillar with a sacrificial section below the support pillars, in accordance with an embodiment of the present invention. 
     In one or more embodiments, one or more separation trenches  200  can be formed through each of the stacks of alternating nanosheet sections  142  and sacrificial sections  132 , the first sacrificial section  122 , through the underlying substrate mesas  112  to form a stack of alternating nanosheet channels  145  and sacrificial segments  135  on a first sacrificial segment  125 , and support pillar  115 . The separation trenches  200  can stop before reaching the and buried sacrificial section  118 . The separation trenches  200  can be formed in the region between adjacent etch templates  150  that was previously covered by an etch mask  160 , where the separation trenches  200  can be formed by a selective direction etch, for example, a reactive ion etch (RIE). Formation of the separation trenches  200  can expose the sidewalls of the nanosheet channels  145  and sacrificial segments  135 , first sacrificial segment  125 , and support pillar  115 . The support pillars  115  can be on and joined by a portion of the substrate mesa  112  between the bottom of the separation trench  200  and the buried sacrificial section  118 . 
     In various embodiments, the separation trenches  200  can extend a depth into the substrate  110  in a range of about 50 nm to about 400 nm, or about 100 nm to about 250 nm, although other depths are also contemplated. In various embodiments, the separation trenches  200  can extend further into the substrate  110  than the adjacent isolation trenches  170 , where the distance to the bottom surface of the trenches on opposite sides of the buried sacrificial segment  119  can be different. The separation trenches  200  can have a width essentially equal to the distance between the etch templates  150  given the etching process and process tolerances. 
       FIG.  26    is a cross-sectional side view showing formation of a shield layer on the sidewalls of the stack of alternating nanosheet channels, sacrificial segments, and support pillars, extension of the separation trenches through the buried sacrificial sections to form buried sacrificial segments, and removal of the buried sacrificial segments from the support pillars, in accordance with an embodiment of the present invention. 
     In one or more embodiments, a shield layer  360  can be formed on the sidewalls of the stack of alternating nanosheet channels  145 , sacrificial segments  135 , first sacrificial segment  125 , and support pillar  115 , where the shield layer  360  can be formed by a conformal deposition (e.g., ALD, PEALD). Portions of the shield layer  360  can be removed from the bottom surface of the separation trenches  200  using a selective, directional etch (e.g., RIE). 
     In various embodiments, the shield layer  360  can be made of silicon oxide (SiO), silicon nitride (SiN), silicon oxynitride (SiON), silicon carbide (SiC), silicon carbonitride (SiCN), silicon boro carbonitride (SiBCN), and combinations thereof. 
     In various embodiments, the shield layer  360  can have a thickness in a range of about 1 nm to about 6 nm, although other thicknesses are also contemplated. 
     In one or more embodiments, the separation trenches  200  can be extended through the buried sacrificial section  118  to form buried sacrificial segment  119 . After extension of the separation trenches  200 , the bottom of the separation trenches  200  can be below the bottom of the isolation trenches  170 . 
     In one or more embodiments, the buried sacrificial segment  119  can be removed to form cavities between the substrate  110  and the support pillar  115 . 
       FIG.  27    is a cross-sectional side view showing formation of an etch-stop layer in the cavities formed by removing the buried sacrificial segments, in accordance with an embodiment of the present invention. 
     In one or more embodiments, an etch-stop layer  370  can be formed in the cavities formed by removing the buried sacrificial segments  119 , where the etch-stop layer  370  can be formed by a conformal deposition (e.g., ALD, PEALD). A portion of the etch-stop layer  370  in the separation trenches  200  can be removed using a selective, isotropic etch. 
     In various embodiments, the etch-stop layer  370  can be made of a dielectric material, including, but not limited to, aluminum oxide (AlO), aluminum nitride (AlN), silicon carbide (SiC), and combinations thereof. 
       FIG.  28    is a cross-sectional side view showing formation of a protective liner in the gaps formed by removal of the first sacrificial segments, and along the sidewalls of the support pillars, in accordance with an embodiment of the present invention. 
     In one or more embodiments, the shield layer  360  can be removed from the sidewalls of the separation trenches  200 , where the shield layer  360  can be removed using a selective, isotropic etch. Removal of the shield layer  360  can leave a step in the support pillars  115  having a width of the thickness of the shield layer  360 . 
     In one or more embodiments, a protective liner  210  can be formed in the cavity formed by removal of the first sacrificial segments  125  and along the sidewalls of the separation trench  200 , where the protective liner  210  can be formed by a conformal deposition (e.g., ALD, PEALD). The protective liner  210  can cover the exposed surfaces of the support pillars  115 , etch-stop layer  370 , and the bottom surface of the separation trench  200 . 
     In various embodiments, the protective liner  210  can be an electrically insulating dielectric material, including, but not limited to, silicon nitride (SiN), silicon carbide (SiC), silicon oxy carbide (SiOC), silicon oxy carbonitride (SiOCN), silicon boro carbonitride (SiBCN), and combinations thereof, where the protective liner  210  can have a dielectric constant less than the dielectric constant of silicon dioxide (SiO 2 ). 
     In various embodiments, the protective liner  210  can have a thickness in a range of about 3 nm to about 20 nm, or about 5 nm to about 10 nm, although other thicknesses are also contemplated. The protective liner  210  can have a thickness of about half the height of the cavity formed by removing the first sacrificial segments  125 , where the protective liner  210  can fill in the cavity. 
     In one or more embodiments, a disposable fill  220  can be formed on the protective liner  210  in the separation trenches  200 , where the disposable fill  220  can be formed be a conformal deposition (e.g., ALD, PEALD) to fill in the trench space between the sidewalls of the protective liner  210 . 
     In various embodiments, the disposable fill  220  can be an OPL. 
     In various embodiments, the disposable fill  220  and protective liner  210  can be etched back to expose the sides of the nanosheet channels  145  and sacrificial segments  135 , where the top surfaces of the disposable fill  220  and protective liner  210  can be between the top and bottom surfaces of the bottom layer, for example, the lower most sacrificial segment  135 . 
     In various embodiments, the additional processes to form the buried power rail  230 , power rail cap  240 , gate structure  250 , source/drains  270 , source/drain contacts  280 , ILD layer  290 , metallization layers and carrier wafer bonding can be conducted. The substrate  110  and devices can be flipped and substrate  110 —can be removed stopping on the etch stop layer  370  and dielectric liner  210 . 
       FIG.  29    is a cross-sectional side view showing formation of an backside interlayer dielectric (BILD) layer on the etch-stop layer, protective liner, and semiconductor devices, in accordance with an embodiment of the present invention. 
     In one or more embodiments, an backside interlayer dielectric (BILD) layer  340  can be formed on the protective liner  210 , the etch stop layer  370 , and the trench liner  180 , where the BILD layer  340  can be formed by a blanket deposition (e.g., CVD, PECVD). 
     In one or more embodiments, the backside interconnect  350  can be formed with lithography and etching process, followed by metallization, such as Cu metallization. A backside interconnect  350  can be formed in the BILD to wire the buried power rail  230  to backside power supplies. 
     The present embodiments can include a design for an integrated circuit chip, which can be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer can transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
     Methods as described herein can be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     It should also be understood that material compounds will be described in terms of listed elements, e.g., SiGe. These compounds include different proportions of the elements within the compound, e.g., SiGe includes Si x Ge 1-x  where x is less than or equal to 1, etc. In addition, other elements can be included in the compound and still function in accordance with the present principles. The compounds with additional elements will be referred to herein as alloys. 
     Reference in the specification to “one embodiment” or “an embodiment”, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
     It is to be appreciated that the use of any of the following “/”, “and/or”, and “at least one of”, for example, in the cases of “A/B”, “A and/or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and/or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This can be extended, as readily apparent by one of ordinary skill in this and related arts, for as many items listed. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, can be used herein for ease of description to describe one element&#39;s or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the FIGS. It will be understood that 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 FIGS. For example, if the device in the FIGS. is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. The device can be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein can be interpreted accordingly. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers can also be present. 
     It will be understood that, although the terms first, second, etc. can be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the scope of the present concept. 
     It will also be understood that when an element such as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements can also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements can be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     Having described preferred embodiments of a device and method of fabricating the device (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.