Method for Forming a Semiconductor Device Structure

Abstract

A method for forming a semiconductor device structure includes forming a layer stack comprising alternating sacrificial layers of a first semiconductor material and channel layers of a second semiconductor material. The method includes forming over the layer stack a plurality of parallel and regularly spaced core lines and forming spacer lines on side surfaces of the core lines. The method includes forming first trenches extending through the layer stack by etching the layer stack while using the core lines and the spacer lines as an etch mask and forming insulating walls in the first trenches and in the gaps by filling the first trenches and the gaps with insulating wall material. The method also includes forming second trenches extending through the layer stack by etching the layer stack while using the spacer lines and the insulating walls as an etch mask, thereby forming a plurality of pairs of fin structures.

Claims

1. A method for forming a semiconductor device structure, the method comprising: forming a layer stack on a substrate, the layer stack comprising sacrificial layers of a first semiconductor material and channel layers of a second semiconductor material, the channel layers alternating the sacrificial layers; forming over the layer stack a plurality of parallel and regularly spaced core lines; forming spacer lines on side surfaces of the core lines, wherein a width of the spacer lines is such that gaps are formed between spacer lines formed on neighboring core lines; forming first trenches extending through the layer stack by etching the layer stack while using the core lines and the spacer lines as an etch mask; forming insulating walls in the first trenches and in the gaps by filling the first trenches and the gaps with insulating wall material; subsequent to forming the insulating walls, removing the core lines selectively to the spacer lines and the insulating walls; and subsequent to removing the core lines, forming second trenches extending through the layer stack by etching the layer stack while using the spacer lines and the insulating walls as an etch mask, thereby forming a plurality of pairs of fin structures, each pair of fin structures comprising a first device layer stack and a second device layer stack separated by a respective insulating wall.

2. The method according to claim 1, wherein the first trenches are formed to extend into the substrate.

3. The method according to claim 1, wherein the second trenches are formed to extend into the substrate.

4. The method according to claim 1, wherein the first trenches are formed to extend to a first depth in the substrate and the second trenches are formed to extend to a second depth in the substrate different from the first depth.

5. The method according to claim 4, further comprising forming a shallow trench isolation layer in the second trenches by depositing an insulating material in the second trenches and etching back the insulating material to a level below a bottom-most channel layer of each pair of fin structures.

6. The method according to claim 1, wherein the insulating wall material is conformally deposited and the method further comprises exposing an upper surface of the core lines by subjecting the insulating wall material to a planarization and/or an etch back prior to removing the core lines.

7. The method according to claim 1, wherein the first semiconductor material is Si.sub.1-yGe.sub.y and the second semiconductor material is Si.sub.1-xGe.sub.x, wherein 0≤x<y.

8. The method according to claim 1, wherein the layer stack further comprises a bottom sacrificial layer of a third semiconductor material underneath the sacrificial layers and the channel layers, and the method further comprises, subsequent to forming the second trenches: removing the bottom sacrificial layer of the first and second device layer stacks of each pair of fin structures by selective etching of the third semiconductor material, thereby forming a respective cavity in the first and second device layer stacks on opposite sides of the insulating wall; and depositing a bottom insulating material in the cavities, wherein during the acts of removing and depositing, the sacrificial layers and the channel layers of the first and second device layer stacks are supported by the respective insulating walls.

9. The method according to claim 8, wherein the bottom insulating material is conformally deposited with a thickness such that the cavities are filled with the bottom insulating material, and the method further comprises removing the bottom insulating material from each first and second device layer stack above a level of the cavities.

10. The method according to claim 8, wherein a bottom-most one of the sacrificial layers is formed on the bottom sacrificial layer.

11. The method according to claim 8, wherein the first semiconductor material is Si.sub.1-yGe.sub.y and the second semiconductor material is Si.sub.1-xGe.sub.x, wherein 0≤x<y, wherein the third semiconductor material is Si.sub.1-zGe.sub.z, wherein y<z.

12. The method according to claim 1, further comprising, processing the first and second layer stacks of each of at least a subset of the pairs of fin structures to form a first transistor device at the first device layer stack and a second transistor device at the second device layer stack, the processing comprising forming source and drain regions and forming gate stacks.

13. The method according to claim 12, wherein the processing further comprises, for each of the at least a subset of the pairs fin structures: forming a sacrificial gate structure extending across the pair of fin structures and the insulating walls; etching through the first and second device layer stacks of the pair of fin structures while using the sacrificial gate structure as an etch mask such that portions of sacrificial and channel layers of the first and second device layer stack are preserved underneath the sacrificial gate structure, forming source and drain regions by epitaxially growing semiconductor material on end surfaces of the respective channel layers of the first and second device layer stacks, at opposite sides of the sacrificial gate structure; subsequently, removing the sacrificial gate body and thereafter removing the sacrificial layers of the first and second device layer stacks by selectively etching the first sacrificial semiconductor material; and subsequently forming a gate stack on the channel layers of the first and second device layer stacks.

14. The method according to claim 13, wherein the layer stack further comprises a bottom sacrificial layer of a third semiconductor material underneath the sacrificial layers and the channel layers, and the method further comprises, subsequent to forming the second trenches: removing the bottom sacrificial layer of the first and second device layer stacks of each pair of fin structures by selective etching of the third semiconductor material, thereby forming a respective cavity in the first and second device layer stacks on opposite sides of the insulating wall; and depositing a bottom insulating material in the cavities, wherein during the acts of removing and depositing, the sacrificial layers and the channel layers of the first and second device layer stacks are supported by the respective insulating walls, wherein, subsequent to the processing, the bottom insulating material forms a bottom insulating layer underneath the source region, the drain region and the channels, on either side of the insulating wall.

15. The method according to claim 14, wherein the bottom insulating material is conformally deposited with a thickness such that the cavities are filled with the bottom insulating material, and the method further comprises removing the bottom insulating material from each first and second device layer stack above a level of the cavities.

16. The method according to claim 14, wherein a bottom-most one of the sacrificial layers is formed on the bottom sacrificial layer.

17. The method according to claim 14, wherein the first semiconductor material is Si.sub.1-yGe.sub.y and the second semiconductor material is Si.sub.1-xGe.sub.x, wherein 0≤x<y, wherein the third semiconductor material is Si.sub.1-zGe.sub.z, wherein y<z.

18. The method according to claim 12, wherein the first trenches are formed to extend into the substrate.

19. The method according to claim 12, wherein the second trenches are formed to extend into the substrate.

20. The method according to claim 12, wherein the first trenches are formed to extend to a first depth in the substrate and the second trenches are formed to extend to a second depth in the substrate different from the first depth.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0052] The above, as well as additional, features will be better understood through the following illustrative and non-limiting detailed description of example embodiments, with reference to the appended drawings.

[0053] FIG. 1 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0054] FIG. 2 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0055] FIG. 3 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0056] FIG. 4 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0057] FIG. 5 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0058] FIG. 6 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0059] FIG. 7 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0060] FIG. 8 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0061] FIG. 9 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0062] FIG. 10 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0063] FIG. 11 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0064] FIG. 12 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0065] FIG. 13 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0066] FIG. 14 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0067] FIG. 15 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0068] FIG. 16 illustrates a method for forming a semiconductor device structure, according to some embodiments.

[0069] FIG. 17 is a flow-chart of a method for forming transistor devices, according to some embodiments.

[0070] FIG. 18 is a schematic view of a forksheet device, according to some embodiments.

[0071] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary to elucidate example embodiments, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

[0072] Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings. That which is encompassed by the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example. Furthermore, like numbers refer to the same or similar elements or components throughout.

[0073] In the following, and with reference to FIGS. 1-16, embodiments of a method for forming a semiconductor device structure will be described. It is noted that the method to be described is related to a specific part of a method for forming a semiconductor device structure. However, the method may comprise preceding steps such as preparing the substrate and subsequent steps of processing the semiconductor device structure to form transistor devices, e.g. comprising forming a source/drain formation and gate stack deposition etc.

[0074] FIG. 1 depicts a semiconductor device structure 100 at an initial stage of the method.

[0075] Axes X, Y and Z indicate a first horizontal direction, a second horizontal direction transverse to the first direction, and a vertical or bottom-up direction, respectively. The X- and Y-direction may in particular be referred to as lateral or horizontal directions in that they are parallel to a main plane of a substrate 102. The Z-direction is parallel to a normal direction to the substrate 102.

[0076] FIG. 1 depicts a cross-sectional view of the structure 100 taken along the YZ plane. The cross-sectional views of the subsequent figures correspond to those in FIG. 1 unless stated otherwise.

[0077] The structure 100 comprises a substrate 102. The substrate 102 may be a conventional semiconductor substrate suitable for CMOS device processing. The substrate 102 may be a single-layered semiconductor substrate, for instance formed by a bulk substrate such as a Si substrate, a germanium (Ge) substrate or a silicon-germanium (SiGe) substrate. A multi-layered/composite substrate is however also possible, such as an epitaxially grown semiconductor layer on a bulk substrate, or a semiconductor-on-insulator (SOI) substrate, such as a Si-on-insulator substrate, a Ge-on-insulator substrate, or a SiGe-on-insulator substrate.

[0078] In FIG. 1 a layer stack 110 has been formed on the substrate 102. The layer stack 110 comprises first sacrificial layers 114 of a first semiconductor material and channel layers 116 of a second semiconductor material. The channel layers 116 are arranged alternatingly with the first sacrificial layers 114.

[0079] The layer stack 110 may as shown further comprise a bottom sacrificial layer 112 of a third semiconductor material underneath the first sacrificial layers 114 and the channel layers 116. As will be further described below, presence of a bottom sacrificial layer 112 may facilitate forming of a bottom insulating layer underneath the first sacrificial layers 114 and the channel layers 116. However, bottom isolation may also be provided in other ways, such as by an insulating layer of a SOI-substrate, and may hence be omitted. In embodiments comprising the bottom sacrificial layer 112, a bottom-most one of the sacrificial first layers 114 may be formed on the bottom sacrificial layer 112.

[0080] For example, the first and second semiconductor materials may be Si.sub.1-yGe.sub.y and Si.sub.1-xGe.sub.x respectively. The third semiconductor material (if present in the layer stack 110) may be Si.sub.1-zGe.sub.z, wherein 0≤x<y<z. In a more specific example, the second semiconductor material may be a Si, the first semiconductor material may be SiGe.sub.0.25, and the third semiconductor material may be SiGe.sub.0.5 or SiGe.sub.0.65. These relative differences in Ge-content facilitate a selective processing (e.g. selective etching) of the different sacrificial layers and the channel layers of the layer stack 110. For example, a SiGe layer with a greater concentration of Ge than another Si or SiGe layer may be etched selectively (i.e. at a greater rate) using an HCl-based dry etch may be used. A further example is ammonia peroxide mixture (APM). However, other appropriate etching processes (wet or dry) allowing selective etching of higher Ge-content SiGe layers with respect to lower Ge-content SiGe (or Si) layers are per se known in the art and may also be employed for this purpose.

[0081] The layers of the device layer stack 110 may each be epitaxial layers, e.g. epitaxially grown using deposition techniques which per se are known, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). This enables high quality material layers with a potential degree of control of composition and dimensions.

[0082] The first sacrificial layers 114 (and also the bottom sacrificial layer 112 if present) may be of a uniform thickness. Correspondingly, the channel layers 116 may be of a uniform thickness. The first sacrificial layers 114 may for example have a thickness of 5-15 nm, such as 7 nm. The channel layers may for example have a thickness of 5-15 nm, such as 10 nm.

[0083] The layer stack 110 may as shown further optionally comprise a top sacrificial layer 118 of the first semiconductor material. The top sacrificial layer 118 may be formed with a greater thickness than each one of the first sacrificial layers 114. As may be understood from the following, this may facilitate forming the insulating wall with an increased height above a top-most channel layer.

[0084] As further shown in FIG. 1, a plurality of parallel and regularly spaced core lines 120 have been formed on the layer stack 110. The core lines 120 extend in the X-direction. The core lines 120 are spaced apart along the Y-direction. The core lines 120 may be formed of a hard mask material, e.g. a nitride material such as SiN, SiCN, SiON, SiCON or SiBCN. The core lines 120 may be formed by depositing the hard mask material over the layer stack 110. The hard mask material may subsequently be patterned to form the pattern of regularly and parallel spaced core lines 120. Examples of patterning techniques include single patterning techniques such as lithography and etching (“litho-etch”) or multiple-patterning techniques such as (litho-etch)′, self-aligned double or quadruple patterning (SADP or SAQP). Prior to depositing the hard mask material layer, an etch stop layer (e.g. of a dielectric hard mask material different from the material of the core lines 120) may optionally be deposited on the layer stack 110 wherein the hard mask material layer may be deposited on the etch stop layer. An etch stop layer between the hard mask material layer and the layer stack 110 may serve as protection for the layer stack 110 during the core line patterning. Each core line 120 may accordingly as shown be formed on a respective etch stop layer portion 119.

[0085] In FIG. 2, spacer lines 122 have been formed on each core line 120. A respective pair of spacer lines 122 has been formed on and along the opposite and vertically oriented side surfaces of each core line 120. The spacer lines 122 are formed with a width (along the Y-direction) such that longitudinal first gaps 124 (extending along the X-direction) are formed between spacer lines 122 formed on neighboring (i.e. consecutive) core lines 120.

[0086] As will be apparent from the below, a width of the spacer lines 122 defines the width of the channel layers 116 of the device layer stacks of the pairs of fin structures to be formed. Meanwhile, a width of the first gaps 124 (e.g. along the Y-direction) defines the width of the respective insulating wall between each pair of fin structures to be formed. The insulating wall may for example be formed with a width in a range from 8-20 nm.

[0087] The spacer lines 122 may be formed by conformally depositing a spacer material over the layer stack 110 and the core lines 120, and subsequently etching the spacer material using a top-down anisotropic etching process, such that portions of the spacer material remains on the side surfaces of the core lines 120 to define the spacer lines 122, and upper surface portions of the layer stack 110 are exposed between the spacer lines 122 (i.e. in the first gaps 124). The spacer lines 122 may be formed of a dielectric material, different from a material of the core lines 120. The spacer lines 122 may for example be formed of an oxide, such as SiO.sub.2 deposited using ALD. However other materials are also possible such as any of the material examples listed in connection with the core lines 120, provided the material has a sufficient etch contrast to the material of the core lines 120.

[0088] In FIG. 3, first trenches 126 extending through the layer stack 110 have been formed by etching the layer stack 110 while using the core lines 120 and the spacer lines 122 as an etch mask. The pattern defined the first gaps 124 has accordingly been transferred into the layer stack 110 by etching to form the first trenches 126. The first trenches 126 may be etched using e.g. a top-down anisotropic etching process. The first trenches 126 may as shown be formed to extend into a thickness portion of the substrate 102, e.g. to a first depth in the substrate 102. Extending the first trenches 126 into the substrate 102 allows a base portion of the insulating wall 128 to be formed to be embedded in the substrate 102. This may confer an increased structural stability mitigating a risk of collapse of the fin structures 140 to be formed. The first trenches 126 may for example extend 20-50 nm into the substrate 102, i.e. below the bottom sacrificial layers 112.

[0089] FIGS. 4-5 depict process steps for forming insulating walls 128 in the first trenches 126 and in the gaps 124.

[0090] In FIG. 4, the first trenches 126 and the gaps 124 have been filled with an insulating wall material 127. The insulating wall material 127 may be conformally deposited to fill the first trenches 126 and cover the core lines 120 and the spacer lines 122.

[0091] In FIG. 5, the insulating wall material 127 has been subjected to a planarization (e.g. chemical mechanical polishing, CMP) and/or an etch back (isotropic or anisotropic, wet or dry) to expose an upper surface of the core lines 120 and the spacer lines 122. The insulating wall material 127 has thus been separated into discrete respective insulating walls 128 in the first trenches 126 and the first gaps 124. The processing may as shown also result in a slight recess of the core lines 120 and the spacer lines 122, such that the device structure 100 may be provided with a planar upper surface.

[0092] The insulating wall material 127 may for example be an oxide, a nitride or carbide material, such as such as SiN, SiCO, SiCN or SiOCN deposited e.g. by ALD. It is however also possible to deposit the insulating wall material 127 using non-conformal deposition processes such as chemical vapor deposition (CVD) and flowable dielectric deposition. The insulating wall material 127 may in any case be different from a material of the core lines 120 and a material of the spacer lines 122, as well as of the etch stop layer portions 119 if present.

[0093] In FIG. 6, subsequent to forming the insulating walls 128, the core lines 120 have been removed selectively to the spacer lines 122 and the insulating walls 128. Any sufficiently selective etching process may be used, isotropic or anisotropic, wet or dry. By removing the core lines 120, second gaps 129 are formed between pairs of spacer lines 122. In the illustrated example, the core lines 120 have been removed selectively to, and thus stopped on, the etch stop layer portions 119, which remain to mask the layer stack 110.

[0094] In FIG. 7, subsequent to removing the core lines 120 (and then the etch stop layer portions 119 if present), second trenches 130 extending through the layer stack 110 have been formed by etching the layer stack 110 while using the spacer lines 122 and the insulating walls 128 as an etch mask. The pattern defined by the second gaps 129 has accordingly been transferred into the layer stack 110 by etching to form the second trenches 130. Accordingly, as shown, a plurality of pairs of fin structures 140 have been formed. Each pair of fin structures 140 comprises a respective fin-shaped first and second device layer stack 142, 144. The first and second device layer stacks 142, 144 of each pair of fin structures 140 are separated by a respective insulating wall 128. Spacer lines 122 (or at least portions thereof remaining after the planarization and/or etch back) may remain as a capping on each first and second device layer stack 142, 144. The second trenches 130 may be etched using a top-down anisotropic etching process. The second trenches 130 may as shown be formed to extend into a thickness portion of the substrate 102, e.g. to a second depth in the substrate 102. The second depth may as shown be greater than the first depth of the first trenches 126. The second trenches 130 may for example extend 40-70 nm into the substrate 102, i.e. below the bottom sacrificial layers 112.

[0095] FIGS. 8-10 depict optional process steps for removing selected ones of the plurality of pairs of fin structures 140. These process steps may be applied to introduce an increased spacing between neighboring pairs of fin structures 140 in desired regions of the substrate 102.

[0096] In FIG. 8, a masking layer 150 has been deposited to cover the pairs of fin structures 140 and fill the second trenches 130. The masking layer 150 may for instance comprise a planarizing layer of spin-on-carbon or another organic spin-on material. Although depicted as a single layer, the masking layer 150 may typically be formed as a mask layer stack, comprising e.g. a hard mask layer and a photoresist layer.

[0097] In FIG. 9, the masking layer 150 has been patterned to define an opening 152 exposing one or more of the pairs of fin structures 140, in FIG. 9 exemplified by the partially shown pair 140′. The opening 152 may be formed by lithography and etching.

[0098] In FIG. 10, pairs of fin structures 140 not masked by the masking layer 150 have been removed by etching. Due to the multiple different materials of the fin structures 140, the spacer lines 122 and the insulating wall 128, multiple different etching steps and etching chemistries may be employed.

[0099] In FIG. 11, the masking layer 150 has been removed from the remaining pairs of fin structures 140.

[0100] FIGS. 12-14 depict process steps for forming a bottom insulating layer 164 in each first and second layer stack 142, 144 of the pairs of fin structures 140.

[0101] In FIG. 12, the bottom sacrificial layer 112 of the first and second device layer stacks 142, 144 of each pair of fin structures 140 has been removed by selective etching of the third semiconductor material. A respective cavity 160 has thereby been formed in the first and second device layer stacks 142, 144, on opposite sides of the insulating wall 128. Any of above listed example etching processes facilitating a selective etching of e.g. Si.sub.1-zGe.sub.z to SiGe.sub.1-xGe.sub.x and SiGe.sub.1-yGe.sub.y (0≤x<y<z) may be employed. As may be appreciated, that the bottom sacrificial layer 112 may be removed along the entire longitudinal dimension of the pairs of fin structures 140, such that the respective cavities 160 may be coextensive with the fin structures 140, i.e. the remaining parts of the first and second device layer stacks 142, 144.

[0102] In FIG. 13, a bottom insulating material 162 has been deposited in the cavities 160. The bottom insulating material may as shown be conformally deposited over the pairs of fin structures 140 with a thickness such that the cavities 160 are filled with the bottom insulating material 162. The bottom insulating material 162 may for example be selected among the examples mentioned for the insulating wall material. The portion of the bottom insulating material 162 filling a respective cavity 160 may define a bottom insulating layer 164 in the cavity 160.

[0103] During the removal of the bottom sacrificial layers 112 and the subsequent deposition of the bottom insulating material 162, the first and second device layer stacks 142, 144 may be supported by the respective insulating walls 128, such that they are suspended above the respective cavities 160 until filled with the bottom insulating material 162.

[0104] In FIG. 14, an initial STI layer 166 has been formed by depositing a (second) insulating material to fill the second trenches 130. The initial STI layer 166 may accordingly as shown cover and embed the pairs of fin structures 140. The insulating material may be an oxide, such as silicon oxide deposited e.g. by CVD, for example by flowable CVD (FCVD) or another conventional inter-layer dielectric material suitable as STI.

[0105] In FIG. 15, a recess process (top-down), e.g. comprising planarization (such as CMP) and/or etch back, has been applied to the initial STI layer 166 to define a partly recessed STI layer 166′. The recess may as shown proceed to remove also the spacer lines 122 and thus expose a layer of the first and second device layer stacks 142, 144. In the illustrated embodiment the thicker top sacrificial layer 118 is exposed. However, the exposed layer may also be a top-most first sacrificial layer 114 or a top-most channel layer 116, in embodiments not comprising the top sacrificial layer 118.

[0106] In FIG. 16, a final STI layer 168 has been formed by further recessing (e.g. etching back) the partly recessed STI layer 166′ in the second trenches 130. The final STI-layer 168 may hence fill a bottom part of the second trenches 130 and embed a base portion of each pair of fin structures 140. Depending on an etch contrast between the bottom insulating material 162 and the insulating material of the STI-layer 168, the recessing may simultaneously remove portions of the bottom insulating material 162 such that the first and second layer stacks 142, 144 are exposed at a level above an upper surface of the STI layer 168. However, portions of the bottom insulating material 162 may also be removed in a separate etch step (e.g. isotropic) after defining the final STI layer 168.

[0107] In FIG. 16, the recessing has been stopped slightly above a level of the cavities 160 and the bottom insulating layers 164 therein. More specifically the recessing has been stopped at a level coinciding with a level of the bottom-most first sacrificial layer 114. This however merely represents an example and it is also possible to proceed further with the etch back, e.g. to a level falling within or below the cavities 160, as the layers of the first and second device layer stacks 142, 144 of the pairs of fin structures 140 remaining above the cavities 160 may mask the bottom insulating material 162 deposited in the cavities 160. In any case the recessing may proceed to a level below a bottom-most channel layer 116 to allow the bottom-most channel layer 116 to be accessed by subsequent processing steps.

[0108] The resulting semiconductor device structure 100 shown in FIG. 16, comprising the plurality of pairs of fin structures 140 surrounded by the STI-layer 168 may as discussed be a suitable precursor for subsequent device fabrication, e.g. to form forksheet devices.

[0109] FIG. 17 is a flow chart of an example process flow which may be applied to the pairs of fin structures 140 to form a semiconductor device structure comprising a pair of closely spaced FETs of complementary conductivity types in accordance with the forksheet design. The processing steps may be applied to each of the pairs of fin structures 140 shown in FIG. 16 or only a subset thereof.

[0110] In step S502, a number of sacrificial gate structures may be formed across the pairs of fin-structures 140 and the (respective) insulating walls 128. Each sacrificial gate structure may comprise a sacrificial gate body (e.g. of amorphous Si) and a pair of gate spacers on opposite sides of the sacrificial gate body. The sacrificial gate structures may be formed using conventional processing techniques as per se are known in the art.

[0111] In step S204, the first and second device layer stack 142, 144 of each pair of fin structures 140 may be recessed (e.g. etched back top-down) using the (respective) sacrificial gate structure as an etch mask, such that portions of sacrificial 114 (and 118) and channel layers 116 of each first and second device layer stack 142, 144 are preserved underneath the sacrificial gate structure.

[0112] In step S206, inner spacers may be formed at opposite sides of each device layer stack 142, 144. Inner spacers may be formed in a manner which per se is known in the art of NWFETs/NSHFETs. For example, inner spacer cavity formation may proceed by: forming recesses in each device layer stack 142, 144 by an isotropic etching process selective to the first semiconductor material; a conformal spacer material deposition (e.g. SiN, SiCO deposited by ALD-dielectric); followed by etching of the spacer material such that spacer material remains only in the recesses to form the inner spacers.

[0113] In step S208, source/drain regions may be formed on end surfaces of the channel layers 116 of each device layer stack 142, 144, at opposite sides of the respective sacrificial gate structures. The source/drain regions may for example be formed by selective area Si epitaxy. Techniques such as in-situ doping and/or ion implantation may be used to define n-type and p-type source/drain regions. Source/drain regions of p-type and source/drain regions of n-type may be formed sequentially on opposite sides of each insulating wall 128 by masking the device layer stack (e.g. 142 or 144) at the opposite side of the insulating wall 128. The insulating walls 128 may facilitate separation between the p- and n-type source/drain regions.

[0114] In step S210, one or more inter-layer dielectric (ILD) materials may be deposited to cover the pairs of fin structures 140, the source/drain regions and the sacrificial gate structures.

[0115] In step S212, the sacrificial gate structures may be replaced by functional gates stacks. The replacement may proceed in accordance with a replacement metal gate (RMG) flow. According to an RMG flow, gate trenches are formed on opposite sides of each respective insulating wall 128 by removing the sacrificial gate bodies (e.g. using a selective amorphous Si etch). Pairs of n-side and p-side gate trenches exposing the respective device layer stacks 142, 144 of the pairs of fin structures 140 may hence be formed, each pair of p-side and n-side gate trenches being separated by a respective insulating wall 128. The RMG flow may proceed by gate dielectric deposition (e.g. high-K dielectric such as HfO.sub.2, HfSiO, LaO, AlO or ZrO), gate work function metal deposition and gate (metal) fill deposition.

[0116] The process may further comprise a step of channel release, interleaved in the RMG process: That is, subsequent to forming the gate trenches, selectively removing the first sacrificial layers 114 (and 118) of each device layer stack 142, 144 by selective etching of the first sacrificial material. Suspended channel layers 116 (e.g. nanosheets) may hence be defined in each gate trench. Due to the presence of the insulating wall 128, the channel layers 116 will be “partly released” in the sense that their upper and lower surfaces as well as outer sidewall surfaces may be laid bare while their inner sidewall surfaces abut the insulating wall 128.

[0117] For improved device performance a p-type work function metal (pWFM) may be provided in the p-type device region (e.g. in the p-side gate trench) and a n-type work function metal (nWFM) may be provided in the n-type device region (e.g. in the n-side gate trench). Step S212 may for example comprise sub-steps: S212a of pWFM deposition in the p- and n-type device regions; S212b of selective removal of the pWFM from the n-type device region; step S212c of nWFM deposition in the n-type device region, and optionally also the p-type device region; step S212d of gate fill deposition. The pWFM removal may comprise etching the pWFM in the n-type device region while masking the p-type device region. The insulating walls 128 may counteract lateral etching of the pWFM in the p-type device region. Examples of gate fill material include W, Al, Co or Ru. The nWFM and pWFM may be deposited in a conformal deposition process, such as ALD. The gate fill material may e.g. be deposited by CVD or PVD. In this sequence of sub-steps S212a-d, reference to “pWFM” may be substituted by “nWFM” and vice versa. Examples of nWFM include TiAl and TiAlC. Examples of pWFM include TiN and TaN.

[0118] Step S212 may be followed by step S214 of recessing the functional gate stacks, and optionally, gate cut formation, as per se is known in the art.

[0119] The method may further comprise forming source/drain contacts on the source/drain regions, e.g. by etching contact trenches in the ILD and depositing one of more contact metals therein.

[0120] FIG. 18 schematically shows a cross sectional view of a forksheet device 100 which may be formed at one of the pairs of fin structures 140 using the above discussed process steps. The cross section is taken across the channel layers 116, through the gate stack. The gate stack comprises a first WFM 182 (e.g. nWFM or pWFM) deposited at the channel layers 116 of the first device layer stack 142 and second WFM 184 (e.g. pWFM or nWFM) deposited at the channel layers 116 of the second device layer stack 144. The first and second WFM metals 182, 184 and the first and second device layer stack 142, 144 are separated by the insulating wall 128. The respective portions of the gate stack accordingly each has a fork-like shape, with a number of prongs extending along and between the channel layers 116 of the respective FETs. The gate stack may further comprise a gate metal fill 186. In the illustrated example the gate stack extends across the wall 128 such that the p-side gate stack and the n-side gate stack are electrically connected. However it is also possible to form the n-side and p-side gate stacks to be disconnected by recessing the gate stack to a level below the insulating wall 128.

[0121] In the above, various aspects of the disclosure have been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

[0122] While some embodiments have been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are to be considered illustrative and not restrictive. Other variations to the disclosed embodiments can be understood and effected in practicing the claims, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures or features are recited in mutually different dependent claims does not indicate that a combination of these measures or features cannot be used. Any reference signs in the claims should not be construed as limiting the scope.