Transistor with superposed bars and double-gate structure

Abstract

A method is provided for fabricating a double gate structure for transistors with superposed bars, including: providing, on a support, a stack including an alternation of one or several first bars based on a first semiconducting material, and one or several second bars based on a second semiconducting material; removing lateral portions of the second bars; forming insulating plugs in contact with lateral regions of the second bars; removing the first bars; and forming a gate electrode facing an upper face and a lower face of the second bars, the insulating plugs being arranged in contact with the lateral regions of the second bars when the gate electrode is being formed.

Claims

1. A method of making a structure for transistors, comprising the following steps: a) providing, on a support, a stack comprising an alternation of one or several first bars made of a sacrificial material based on a first semiconducting material, and one or several second bars, configured to form a channel region of a transistor and based on a second semiconducting material; b) removing lateral regions of the second bars by partial and selective etching of the second semiconducting material, so as to make the second bars narrower and to release spaces on each side of lateral faces of the second bars; c) forming insulating plugs in the spaces in contact with the lateral faces of the second bars; d) removing a central portion of the first bars by selective etching of the first semiconducting material so as to expose an upper face and a lower face of the second bars; and e) forming a double-gate electrode facing the upper face and the lower face of the second bars, the insulating plugs being arranged in contact with the lateral regions of the second bars when the double-gate electrode is being formed.

2. The method according to claim 1, further comprising, between step d) and step e), growing a third semiconducting material, with a lattice parameter different from the second semiconducting material, on the upper face and the lower face of the second bars, wherein the insulating plugs are configured to prevent growth of the third semiconducting material on the lateral faces of the second bars.

3. The method according to claim 2, wherein the third semiconducting material is based on germanium, and further comprising, before the double-gate electrode is formed, performing at least one thermal annealing or at least one oxidation so as to diffuse germanium into the second bars.

4. The method according to claim 2, wherein the third semiconducting material is based on germanium, and wherein the double-gate electrode is formed on portions of the third semiconducting material located on the upper face and on the lower face of the second bars.

5. The method according to claim 2, wherein the second bars are thinned prior to growth and by a thickness that is equal to a thickness of the third semiconducting material that is grown.

6. The method according to claim 1, further comprising, between step c) and step d), the following additional steps: forming a dummy gate; forming isolating spacers on each side of the dummy gate; selectively removing end portions of the first bars so as to reduce a length of the first bars and to release spaces on each side of end regions of the first bars; and forming internal insulating spacers within said spaces.

7. The method according to claim 6, further comprising, after the forming of the internal insulating spacers and before step d), the following additional steps: forming source and drain regions; forming at least one encapsulation layer on the source and drain regions, the encapsulation layer being arranged so as to expose the dummy gate; and removing the dummy gate.

8. The method according to claim 1, the insulating plugs being made of a dielectric material with a dielectric constant less than 7, and/or chosen from among the following dielectric materials: SiBCN, SiO.sub.2, SiN, and Si.sub.3N.sub.4.

9. The method according to claim 1, the insulating plugs being made of a dielectric material, and the selective etching of the first semiconducting material in step d) being selective with regard to the dielectric material.

10. The method according to claim 1, wherein the second bars are made of silicon, and wherein the upper face and the lower face of the second bars have an orientation corresponding to a same crystallographic plane.

11. A transistor structure comprising, on a support: a stack comprising an alternation of one or several bars, spaced apart from each other, and based on a semiconducting material, central portions of the bars forming a channel region of a transistor; a gate dielectric covering an upper face and a lower face of the central portions of the bars; a double-gate electrode in front of the upper face and the lower face of the central portions of the bars; and insulating plugs arranged in contact with lateral faces of the central portions of the bars, the insulating plugs being further arranged between the double-gate electrode and the lateral faces of the central portions of the bars so as to isolate the lateral faces of the central portions of the bars from the double-gate electrode.

12. The transistor structure according to claim 11, wherein insulating spacers arranged on opposing sides of the double-gate electrode and extending continuously from an upper face of an uppermost bar of the stack to a same level as an upper face of the double-gate electrode.

13. The transistor structure according to claim 12, wherein the insulating spacers also extend at least along the upper face of the uppermost bar of the stack in a direction orthogonal to a direction of the insulating plugs extending along the lateral faces of the central portions of the bars.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) This invention will be better understood after reading the description of example embodiments given purely for information and that are in no way limitative, with reference to the appended drawings on which:

(2) FIGS. 1A-1D illustrate an example of a method of fabricating a structure for a transistor channel and with semiconducting bars located one above the others, insulating plugs being arranged on each side of the bars;

(3) FIGS. 2A-2G illustrate the use of a double gate for the channel structure with semiconducting bars;

(4) FIGS. 3A-3B illustrate a variant embodiment for which a semiconducting material is grown on the upper and lower faces of the bars, lateral growth being inhibited by the insulating plugs;

(5) FIG. 4 illustrates a step of germanium enrichment of the semiconducting bars;

(6) FIG. 5 illustrates a transistor structure with bars located one above the others and strained, and a gate electrode with a double-gate type layout;

(7) FIGS. 6A-6E illustrate a variant of the method of making a double gate structure for a channel structure with superposed semiconducting bars;

(8) Identical, similar or equivalent parts of the different figures have the same numeric references to facilitate the comparison between the different figures.

(9) The different parts shown on the figures are not necessarily all at the same scale to make the figures more easily understandable.

(10) Furthermore, in the following description, terms that are dependent on the orientation such as “on”, “above”, “top”, “bottom”, “lateral”, etc. of a structure should be understood assuming that the structure is oriented as shown on the figures.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

(11) Refer to FIG. 1A that gives an example of a semiconducting structure with superposed semiconducting bars from which a transistor channel structure can be formed.

(12) The structure 10 can be made on a substrate 1 of the type commonly called “bulk” formed from a semiconducting layer or may be a semiconductor on insulator type substrate, for example of the SOI type comprising an semiconducting support layer covered by an insulating layer based for example on SiO.sub.2, itself covered with a thin surface semiconducting layer, for example made of silicon.

(13) The structure 10 comprises a stack with an alternation of layers based on a sacrificial material 6, typically a first semiconducting material, and layers based on a second material 8 that is semiconducting and is different from the first material 6. The material 6 can be etched selectively with regard to the second semiconducting material 8. The second semiconducting material 8 can also be etched selectively relative to the first semiconducting material 6. For example, the first material 6 is based on silicon germanium, while the second material 8 is made of silicon. Such a stack can be made by successive epitaxies of semiconducting layers.

(14) The stacked layers structure represented on FIG. 1A in this case is a structure obtained after etching, the etched layers typically comprising portions in the form of bars. The structure is thus formed from an alternation of bars 4a, 4b, 4c, 4d based on the first material 6 and bars 5a, 5b, 5c based on the second material 8. The stacked bars 4a, 5a, 4b, 5b, 4c, 5c, 4d that are indifferently called “nanowires”, “nanosheets” may for example be in the form of a parallelepiped or approximately a parallelepiped. The thickness of the bars 5a, 5b, 5c may typically be between 4 and 12 nm, for example between 6 nm and 8 nm.

(15) The width of the bars is typically more than their thickness (dimension measured along a direction parallel to the principal plane of the substrate and particularly to the y axis of an [O; x; y; z] orthogonal coordinate system, the principal plane of the substrate being a plane of the substrate defined on FIG. 1A as a plane parallel to the [O; x; y] plane). The thickness e is typically between 6 nm and 10 nm while the width can for example be between 6 nm and 100 nm.

(16) The next step is partial etching of portions of the stacked structure that can be isotropic and selective, so as to partially remove some of the bars 5a, 5b, 5c in the stack. In this example, the bars based on the second material 8 are removed in preference 8. Etching is preferably selective isotropic etching of the second semiconducting material 8 with regard to first material 6. For example, in the case in which the bars 5a, 5b, 5c are based on Si while the bars 4a, 4b, 4c, 4d are based on SiGe, etching can be an isotropic etching of Si, selective with regard to SiGe, such etching can be made for example by wet etching using ammonia (NH.sub.3).

(17) This is how the width of the bars 5a, 5b, 5c is reduced. This width reduction can be between one and several nanometres, for example of the order 1 to 3 nm. Once the reduction has been made, the bars 5a, 5b, 5c may have a width W for example more than 20 nm and less than 100 nm, typically between 15 nm and 50 nm.

(18) The result thus obtained is a stack comprising an alternation of bars 4a, 4b, 4c, 4d based on the material 6 and bars based on the semiconducting material 8 with smaller lateral dimensions that those of bars 4a, 4b, 4c, 4d.

(19) Due to the difference in lateral dimensions firstly between bars 4a, 4b, 4c, 4d and secondly bars 5a, 5b, 5c, the structure comprises a toothed profile, with notches 12 or grooves 12 at its lateral flanks.

(20) These notches or grooves 12 form spaces on each side of the lateral faces of the bars 5a, 5b, 5c that are filled by a dielectric material (or on which this material is formed) for example such as silicon nitride (SiN, Si.sub.3N.sub.4, Si.sub.xN.sub.y), or silicon oxide (SiO.sub.2) or SiBCN.

(21) The dielectric material, deposited for example in a conforming manner, is then etched so as to make insulating plugs 15 on each side of the bars based on a semiconducting material 8 (FIG. 1C giving a perspective view and FIG. 1D giving a cross-sectional view). The insulating plugs 15 have a width Dc that can be several nanometres and that correspond to the depth of the notches or grooves 12.

(22) The insulating plugs 15 can subsequently be used as masking elements preventing epitaxial growth on the second bars 5a, 5b, 5c made of a semiconducting material 8. The insulating plugs 15 may also form a lateral electrical insulation between the stacked semiconducting structure and a gate electrode. In this case, plugs are preferably chosen made of a dielectric material with a low dielectric constant, particularly less than 7.

(23) The next step (FIG. 2A) is to form a dummy gate 21, for example made of polysilicon and insulating spacers 23a, 23b called “external” spacers on each side of the dummy gate 21. The insulating spacers 23a, 23b are based for example on silicon nitride (SiN) or SiCN. The insulating spacers 23a, 23b can advantageously be formed based on a dielectric material different from that formed by insulating plugs 15. The insulating spacers 23a, 23b and the sacrificial gate 21 thus cover at least a central zone of the stack of semiconducting bars (FIG. 1B).

(24) Typically, external spacers 23a, 23b are etched so as to further etch the ends of the structure and expose its flanks.

(25) The next step can then be to make insulating spacers called “internal” spacers. This is done by firstly selective etching of portions of the first material 6, in this case end portions of the first bars 4a, 4b, 4c, 4d (FIG. 2B). Removal of the first material 6 exposes spaces 31 forming cavities, that are then filled with a dielectric material 33 for example based on silicon nitride (FIG. 2C).

(26) A thickness of this dielectric material 33 can then be removed in zones on each side of the spacers 23a, 23b. An anisotropic method is chosen in preference so as to keep the dielectric in the cavities and remove excess outside these cavities. This removal can be done for example by means of a wet process based on phosphoric acid so as to keep regions made of dielectric material 33 around the ends of the bars 4a, 4b, 4c, 4d. These regions made of dielectric material 33, also called “internal spacers” preferably have an external face aligned with the face of external spacers 23a, 23b, as in the example illustrated in FIG. 2C.

(27) Source and drain semiconducting blocks 45a, 45b can then be formed. These blocks 45a, 45b can then be made for example by epitaxial growth and initiate at least on one portion of the bars 5a, 5b, 5c of semiconducting material 8. The semiconducting blocks 45a, 45b are made, for example, from silicon. The semiconducting material 8 can be doped, particularly during its growth.

(28) A layer 57 also called an “encapsulation” layer 57 is then formed so as to cover the structure (FIG. 2D). The encapsulation layer 57 is preferably insulating and for example may be based on silicon oxide. A step to remove a thickness from this mask layer 57, for example by CMP (Chemical Mechanical Planarisation), can then be performed stopping at the summit or the top face of the sacrificial gate 21.

(29) An opening 59 is then made so as to once again expose a central region of the stack of semiconducting bars (FIG. 2D). This opening 59 is formed by removing the sacrificial gate 21. When the sacrificial gate 21 is made of polysilicon, it can be removed for example by wet etching using a solution based on ammonia, stopping on the sacrificial gate dielectric, that can then be removed in the opening 59, for example by etching using hydrofluoric acid for the typical case of a dielectric based on silicon oxide.

(30) The first material 6 is then removed in the opening 59.

(31) In particular, selective etching of the first material 6 with regard to the second material 8 is then made. Etching is also selectively with regard to the material of the insulating plugs 15. Central portions of the first bars 4a, 4b, 4c, 4d located in the opening 59 are thus removed.

(32) Preferably, etching is also selective with regard to the material(s) of the internal spacers 33 and external spacers 23a, 23b. A selective removal can be made, for example by chemical vapour phase etching, for example using HCl.

(33) In the case in which the removed material 6 is Si.sub.1-aGe.sub.a. The result obtained in the opening 59, is thus suspended bars 5a, 5b, 5c based on the semiconducting material 8, in this example silicon, and covered on their lateral faces by insulating plugs 15.

(34) The bars 5a, 5b, 5c based on the semiconducting material 8 advantageously have a central portion that is not covered by another material, such that an empty space is formed above and below the central portion of the bars 5a, 5b, 5c based on the semiconducting material 8 (FIG. 2E).

(35) It is then possible to deposit at least one layer of gate dielectric, for example a stack of SiO.sub.2 and HfO.sub.2. In particular, this layer covers an upper face 5′ and a lower face 5″ of bars. At least one gate material, for example such as TiN and W, is then deposited. A gate 61 is thus formed facing the upper face 5′ and the lower face 5″ of the second bars 5a, 5b, 5c (the central portion of these second bars 5a, 5b, 5c forming the channel), while keeping plugs acting as electrical insulation in contact with the lateral faces of the second bars 5a, 5b, 5c (FIGS. 2F and 2G). The fabricated gate 61 in this case is not a fully encapsulating gate enabling electrostatic control of the semiconducting bars 5a, 5b, 5c on 3 dimensions, but rather a double gate, provided that the insulating plugs 15 isolate the lateral faces of the semiconducting bars 5a, 5b, 5c with regard to the gate 61.

(36) According to a variant of the example embodiment that has just been described, after the step to remove the first bars, it is advantageous to make epitaxial growth of the semiconducting material 75 on an upper face and on a lower face of the second bars 5a, 5b, 5c (FIGS. 3A and 3B). The insulating plugs 15 then prevent lateral growth of the semiconducting material. The material 75 is typically chosen so as to have a lattice parameter different from that of the material 8 of the second bars 5a, 5b, 5c in order to strain them.

(37) For example, the semiconducting material 75 that is grown may be based on Si.sub.1-xGe.sub.x when the second bars 5a, 5b, 5c are made of silicon. The added thickness may for example be between 1 nm and 5 nm, typically of the order 3 nm.

(38) In this case, Si.sub.1-xGe.sub.x is grown on planes (100) while preventing growth on the planes (110). A uniform growth rate and growth method can be maintained to obtain a good quality crystalline semiconducting material 75 with a uniform thickness on the bars 5a, 5b, 5c.

(39) According to a variant, a thickness of the second bars 5a, 5b, 5c made of Si can be removed for example by oxidation/deoxidation, and then replaced by Si.sub.1-xGe.sub.x with the same thickness as the removed thickness, so as to have second bars with approximately the same dimensions as before partial removal but composed of different semiconducting materials in a core-shell configuration.

(40) A double gate can then be formed on such a structure. In this case, the bars 5a, 5b, 5c are kept with a structure comprising a core and an envelope made of different semiconducting materials.

(41) As a variant (FIG. 4), germanium 75′ is diffused from the material 75 in the second bars 5a, 5b, 5c before the double gate is formed. To achieve this, one or several controlled thermal annealings or oxidations are performed. For example, at least one annealing can be done at a temperature between 400° C. and 1100° C. for a duration that can vary between 0.5 second and several seconds.

(42) Another variant (FIG. 5) includes thinning of the bars 5a, 5b, 5c, for example by etching using ammonia or a derivative, before growth of the material 75. This thinning may be of the order of several nanometres, for example between 1 and 3 nm. Once thinning is complete, epitaxial growth of the material 75 can be started possibly followed by enrichment when the material 75 contains germanium.

(43) A variant of the method includes production of insulating plugs 15 after formation of source and drain blocks 45a, 45b, a dummy gate, insulating spacers 23a, 23b and an encapsulation layer 157 to protect the source and drain blocks 45a, 45b. In the example embodiment in FIG. 6A, the sacrificial gate is removed and a cavity 159 is formed exposing a central region of the stack of first bars and second bars 4a, 5a, 4b, 5b, 4c, 5c, 4d.

(44) Then (FIG. 6B giving a cross-sectional view of the stack inside the cavity 159), partial etching of the lateral portions of the second bars 5a, 5b, 5c of the stack is made, to reduce their width with regard to the first bars 4a, 4b, 4c, 4d.

(45) Insulating plugs 15 are then made in contact with the lateral faces of the bars 5a, 5b, 5c, by deposition of a dielectric material and then etching of this dielectric material (FIG. 6C).

(46) Then, the first bars 4a, 4b, 4c, 4d are removed in the cavity 159 (FIGS. 6D and 6E).

(47) A semiconducting material with a lattice parameter different from that of the material used for the second bars can then be grown to strain the second bars, and the gate structure is then made by depositing at least one gate dielectric and at least one gate material in the cavity 159. As a variant, it is possible to make the double gate structure directly.