MOS TRANSISTOR AND METHOD OF MANUFACTURING THE SAME

Abstract

A MOS transistor includes a semiconductor layer resting on an insulator and having a substantially planar upper surface. The semiconductor layer extends down to a first depth in the channel region, and down to a second depth, greater than the first depth, in the source and drain regions. In the channel region, the semiconductor layer is formed from a portion of an upper semiconductor layer of a silicon on insulator substrate. In the source and drain regions, the semiconductor layer is formed by epitaxially grown semiconductor material.

Claims

1. An integrated circuit, comprising: a silicon on insulator substrate having a semiconductor substrate coated with an insulating layer and with a semiconductor layer; a MOS transistor gate supported by a portion of the semiconductor layer forming a channel region and a portion of the insulating layer which insulates the channel region from the semiconductor substrate; an epitaxial source region and epitaxial drain region on opposite sides of the channel region, wherein the epitaxial source region and epitaxial drain region have a depth from a bottom of the MOS transistor gate that is deeper than a depth of the channel region from the bottom of the MOS transistor gate; and an insulating region insulating a bottom of the epitaxial source region and epitaxial drain region from the semiconductor substrate.

2. The integrated circuit of claim 1, wherein the channel region and the epitaxial source region and epitaxial drain region have a coplanar upper surface.

3. The integrated circuit of claim 1, wherein the insulating region comprises an insulating liner that lines the bottom of the epitaxial source region and epitaxial drain region, a side of the portion of the insulating layer under the channel region and an upper surface of the semiconductor substrate.

4. The integrated circuit of claim 3, wherein the insulating liner is also present on side walls of the MOS transistor gate.

5. The integrated circuit of claim 3, wherein the insulating region further comprises an insulating fill material in contact with the insulating liner.

6. The integrated circuit of claim 5, wherein the insulating liner and insulating fill material are also present on side walls of the MOS transistor gate.

7. An integrated circuit, comprising: a silicon on insulator substrate having a semiconductor substrate coated with an insulating layer and with a semiconductor layer; a MOS transistor gate supported by a portion of the semiconductor layer forming a channel region and a portion of the insulating layer which insulates the channel region from the semiconductor substrate; an epitaxial source region and epitaxial drain region on opposite sides of the channel region, wherein the epitaxial source region and epitaxial drain region have a depth from a bottom of the MOS transistor gate that is deeper than a depth of the channel region from the bottom of the MOS transistor gate; a first cavity between the epitaxial source region and the semiconductor substrate; a second cavity between the epitaxial drain region and the semiconductor substrate; and an insulating liner that lines surfaces of the first and second cavities to insulate the epitaxial source region and epitaxial drain region from the semiconductor substrate.

8. The integrated circuit of claim 7, further comprising an insulating fill material that fills said first and second cavities and is in contact with the insulating liner.

9. The integrated circuit of claim 7, wherein the insulating fill material is also present adjacent side walls of the MOS transistor gate.

10. The integrated circuit of claim 7, wherein the channel region and the epitaxial source region and epitaxial drain region have a coplanar upper surface.

11. The integrated circuit of claim 7, wherein the insulating liner a bottom surface of the epitaxial source and drain regions, a side wall of the portion of the insulating layer which insulates the channel region from the semiconductor substrate and an upper surface of the semiconductor substrate.

12. The integrated circuit of claim 11, wherein the insulating liner further extends on side walls of the MOS transistor gate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

[0023] FIGS. 1 and 2, previously described, are simplified cross-section views, each showing an example of a MOS transistor formed from an SOI-type structure;

[0024] FIG. 3 is a simplified cross-section view of an example of a transistor formed from a SOI-type structure;

[0025] FIG. 4 is a simplified cross-section view of an embodiment of a MOS transistor formed from a SOI-type structure; and

[0026] FIGS. 5A to 5H are simplified cross-section views of a structure at successive steps of an embodiment of a method of manufacturing a transistor of the type in FIG. 4.

DETAILED DESCRIPTION

[0027] The same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and are detailed. In the following description, terms such as on, upper, lower refer to the orientation of the concerned elements in the corresponding drawings. Unless otherwise specified, term substantially means to within 10%, preferably to within 5%, when it concerns thickness values, and to within 5 nm, preferably to within 1 nm, when it concerns a level. Further, unless otherwise specified, a first element resting on a second element means that the first and second elements are in contact with each other.

[0028] FIG. 3 shows an example of MOS transistor enabling to avoid at least some of the disadvantages of the transistors of FIGS. 1 and 2.

[0029] In transistor 31 of FIG. 3, semiconductor layer 11 and insulating layer 13 have been etched all the way to substrate 15, between insulating walls 17, by using gate stack 7 and spacers 9 as an etch mask. As a result, channel region 5 rests on an insulating region 33 corresponding to a portion of insulating layer 13. Deep source and drain regions 35 have then been formed by epitaxy from substrate 15 all the way to the upper level of semiconductor layer 11.

[0030] A disadvantage of such a transistor is that the deep source and drain regions are not insulated from substrate 15.

[0031] It would thus be desirable to have a transistor having the advantages of transistors 1, 21, and 31 without their disadvantages. FIG. 4 is a cross-section view schematically showing an embodiment of a MOS transistor.

[0032] MOS transistor 41 of FIG. 4 is similar to transistor 31 of FIG. 3 with the difference that the deep source and drain regions of transistor 43 are insulated from the substrate. More specifically, in transistor 41, deep source and drain regions 43 rest on at least one insulating layer 45 arranged on substrate 15. Each of regions 43 extends upwards from an intermediate level of insulating region 33 all the way to the upper level of channel region 5. Thus, the source, drain, and channel regions of the transistor are formed in a semiconductor layer having a substantially planar upper surface, this semiconductor layer being thicker at the level of the source and drain regions than at the level of the channel region.

[0033] In such a transistor, the small thickness of the channel region enables, in operation, to obtain a fully depleted channel region. Further, the transistor has a low source and drain access resistance due to the fact that the deep source and drain regions are thick, as well as a low stray capacitance between the gate stack and the source and drain regions due to the fact that the deep source and drain regions do not border the gate spacers.

[0034] FIGS. 5A to 5H are simplified cross-section views illustrating a structure at successive steps of an embodiment of a method of manufacturing a transistor of the type in FIG. 4.

[0035] In FIG. 5A, the structure comprises a semiconductor layer 51, doped with a first conductivity type, resting on an insulating layer 53, itself resting on a semiconductor substrate 55, layers 51 and 53 forming an SOI-type structure. Insulating walls 57 have been formed through layers 51 and 53 and penetrate into substrate 55. Insulating walls 57 laterally delimit the location of the transistor to be formed. A gate stack 59 comprising a gate insulator 59A topped with a gate electrode 59B has been formed on semiconductor layer 51. An insulating layer 61 coats gate stack 59 and forms spacers 63 on the sides thereof.

[0036] As an example, semiconductor layer 51 is a silicon layer. The thickness of semiconductor layer 51 may be in the range from 3 to 30 nm. Insulating layer 53 for example is silicon oxide. The thickness of layer 53 is for example in the range from 10 to 30 nm.

[0037] Substrate 55 is for example made of single-crystal silicon. Insulating walls 57 are for example made of silicon oxide. The material of layer 61, for example, silicon nitride, is selected so that layers 51 and 53 are selectively etchable over layer 61.

[0038] FIG. 5B shows the structure of FIG. 5A after the removal by etching, all the way to substrate 55, of semiconductor layer 51 and of insulating layer 53, gate stack 59 coated with insulating layer 61 being used as an etch mask. Thus, under gate stack 59 coated with layer 61, there remains a portion 51A of semiconductor layer 51 and a portion, or region, 53A of insulating layer 53, semiconductor portion 51A resting on insulating region 53A and corresponding to the channel region of the transistor.

[0039] As shown herein, when insulating walls 57 are made of the same material as insulating layer 53, an upper portion of the walls is also removed during the etching of layer 53.

[0040] FIG. 5C shows the structure of FIG. 5B after the forming of a sacrificial semiconductor layer 65 by epitaxy from substrate 55 all the way to an intermediate level (i.e., between its upper and lower surfaces) of insulating region 53A. Further, a semiconductor layer 67 has been formed by epitaxy from sacrificial layer 65 all the way to a level sub-stantially identical to the level of the upper surface of channel region 51A.

[0041] The material of sacrificial layer 65, for example, SiGe, is selected to be able to grow in single-crystal form on single-crystal substrate 55 and so that semiconductor layer 67 can grow in single-crystal form on layer 65. Further, the material of sacrificial layer 65 is selected to be selectively etchable over that of layer 67 and of substrate 55. The thickness of sacrificial layer 65 is for example in the range from one quarter of to half that of insulating region 53A. The material of semiconductor layer 67 may be the same as that of channel region 51A, in this example, silicon. Semiconductor layer 67 may be doped, on forming thereof by epitaxy, with the conductivity type opposite to that of channel region 51A. The thickness of layer 67 is for example in the range from 8 to 75 nm. FIG. 5D shows the structure of FIG. 5C after the removal by etching of an upper portion of insulating walls 57 to laterally expose sacrificial layer 65. The sacrificial layer is then removed by selective etching over substrate 55, over semiconductor layer 67, and over insulating layer 53A to form, on either side of insulating region 53A, cavities 69 extending between substrate 55 and layer 67. As shown herein, doped regions 71 of the second conductivity type may be formed by implantation into layer 67. Regions 71 partly diffuse in region 51A, under gate spacers 63, to form source and drain, or LDD (Lightly Doped Drain) extensions.

[0042] FIG. 5E shows the structure of FIG. 5D after the forming of an insulating layer 73 over the entire exposed surface of the structure, layer 73 being removed from the upper surface of semiconductor layer 67. The portions of layer 73 remaining on the sides of gate stack 59 bordered with spacers 63 are designated with reference numeral 75. As an example, layer 73 is a silicon oxide layer. The thickness of layer 73 is for example in the range from 1 to 20 nm. Layer 73 is for example formed by chemical vapor deposition.

[0043] FIG. 5F shows the structure of FIG. 5E after the deposition of an insulating layer 77, over the entire exposed surface of the structure. The deposition is performed so as, in particular, to fill cavities 69. Thus, semiconductor layer 67 is insulated from substrate 55 by the materials of layers 73 and 77, layers 73 and 77 also enabling to laterally insulate layer 67.

[0044] As an example, insulating layer 77 is made of silicon nitride and may be formed by chemical vapor deposition or by the so-called ALD or Atomic Layer Deposition technique.

[0045] FIG. 5G shows the structure of FIG. 5F after the removal by etching of portions of layer 77 resting on the upper surface of layer 67, of a portion of layer 77 resting on layer 61, and of a portion of layer 61 resting on the top of gate stack 59. Thus, gate stack 59 is bordered by a spacer resulting from the assembly of layers, or layer portions, 63, 75 and 77. A step of doping layer 67 is then carried out to form therein doped source and drain regions 83 of the second conductivity type.

[0046] A transistor of the same type as transistor 41 of FIG. 4 is thus obtained.

[0047] FIG. 5H shows the structure of FIG. 5G after siliciding of the exposed surface of source and drain regions 83, and possibly of the exposed surface of gate 59, to form silicided regions 93 at the level of the exposed surface of source and drain regions 83 and possibly a silicided region 95 at the top of gate stack 59.

[0048] Since the sides of gate stack 59 are successively bordered with spacers 63, 75, and 77, silicided regions 93 do not penetrate into channel region 51A.

[0049] Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, the order and the number of the steps of the previously-described manufacturing method may be modified. For example, it is possible not to carry out the deposition of layer 73, the forming of spacers 75, and/or the forming of doped regions 71 in layer 67.

[0050] Similarly, the gate stack has not been described in detail. Different types of stacks may be used: insulators of high permittivity may be provided as a gate insulator, and the gate electrode and conductive gate stack may comprise metal layers.

[0051] Further, the materials and the thicknesses of the previously-described layers may be adapted by those skilled in the art.

[0052] Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.