# Patent

## Title
Transistor with self-aligned source and drain contacts and method of making same

## Abstract

A transistor includes an active region supported by a substrate and having a source region, a channel region and a drain region. A gate stack extends over the channel region and a first sidewall surrounds the gate stack. A raised source region and a raised drain region are provided over the source and drain regions, respectively, of the active region adjacent the first sidewall. A second sidewall peripherally surrounds each of the raised source region and raised drain region. The second sidewall extends above a top surface of the raised source region and raised drain region to define regions laterally delimited by the first and second sidewalls. A conductive material fills the regions to form a source contact and a drain contact to the raised source region and raised drain region, respectively.

## Background
TECHNICAL FIELD
The present invention relates to the fabrication of integrated circuits and, more particularly, to a process for the formation of integrated circuit transistor devices with self-aligned source and drain contacts.
BACKGROUND
As the technology node continues to shrink to produce smaller and smaller integrated circuit transistor devices, it is becoming increasingly challenging to fabricate and make electrical contact to the terminals of each transistor device. One concern is preventing the inadvertent formation of a short between the transistor gate and the transistor source-drain regions when using raised source-drain epitaxial regions. Another concern is preventing the inadvertent formation of a short between adjacent active regions. Circuit designers must include sufficient spacing between devices to avoid the risk of shorting, but this solution comes at the expense of increased surface area. This area penalty can be especially problematic in designs which utilize unmerged source-drain structures for adjacent transistor devices.
A need accordingly exists in the art for an improved process for fabricating transistor devices that can address concerns with source-drain epitaxial shorting, self-alignment of source-drain contacts while maintaining reduced contact resistance, and the formation of source-drain contacts self-aligned to the gate.
SUMMARY
In an embodiment, a method comprises: forming an active region supported by a substrate and including a source region, a channel region and a drain region; forming a gate stack over the channel region; epitaxially growing a raised source region and a raised drain region over the source and drain regions, respectively, of the active region; depositing a sacrificial layer over the raised source region and raised drain region to form a source stack and a drain stack, respectively; forming a sidewall spacer that peripherally surrounds each of the gate stack, source stack and drain stack; selectively removing the sacrificial layer to form openings above the raised source region and raised drain region that are each delimited by the sidewall spacer; and filling said openings with conductive material to form a source contact and a drain contact to the raised source region and raised drain region, respectively.
In an embodiment, a method comprises: forming an active region supported by a substrate and including a source region, a channel region and a drain region; forming a gate stack over the channel region; forming a first sidewall spacer on the gate stack; epitaxially growing a raised source region and a raised drain region over the source and drain regions, respectively, of the active region adjacent the first sidewall spacer; depositing a sacrificial layer over the raised source region and raised drain region; forming a first mask over the gate stack; forming a second mask at a same level as the first mask over the sacrificial layer and the raised source region and raised drain region; etching using the first and second masks to define a source stack and drain stack covered by said second mask; forming a second sidewall spacer on the source stack and drain stack; removing the second mask; etching using the first mask to remove the sacrificial layer and form openings above the raised source region and raised drain region which are delimited by the first and second sidewall spacers; and filling said openings with conductive material to form a source contact and a drain contact to the raised source region and raised drain region, respectively.
In an embodiment, an integrated circuit comprises: an active region supported by a substrate and including a source region, a channel region and a drain region; a gate stack extending over the channel region; a first sidewall surrounding the gate stack; a raised source region and a raised drain region over the source and drain regions, respectively, of the active region adjacent the first sidewall; a second sidewall peripherally surrounding each of the raised source region and raised drain region, said second sidewall extending above a top surface of the raised source region and raised drain region to define regions laterally delimited by the first and second sidewalls; and a conductive material filling said regions to form a source contact and a drain contact to the raised source region and raised drain region, respectively.



BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the embodiments, reference will now be made by way of example only to the accompanying figures in which:
FIGS. 1-15 show process steps for the formation of an integrated circuit transistor device.



DETAILED DESCRIPTION OF THE DRAWINGS
Reference is now made to FIGS. 1-15 showing process steps for the formation of an integrated circuit transistor device. It will be understood that the illustrations provided do not necessarily show the features drawn to scale.
The process starts with a silicon on insulator (SOI) substrate 10 wafer of a conventional type (comprising, for example, an extremely thin silicon on insulator (ETSOI) or an ultra-thin body and buried oxide (UTBB) silicon on insulator (SOI) known to those skilled in the art). The top semiconductor layer 12 (thickness 1-80 nm) of the SOI substrate 10 may be silicon doped as appropriate for the integrated circuit application. In an embodiment, the top semiconductor layer may be of the fully depleted (FD) configuration. The top semiconductor layer 12 is supported by a buried oxide layer 14 (thickness 2-200 nm), with the buried oxide layer supported by a semiconductor substrate layer 16.
FIG. 1 shows a top view of a portion of the wafer, with FIG. 1A showing a cross-section taken along line A-A and FIG. 1B showing a cross-section taken along line B-B. This relationship between the top and cross-sectional views is consistent among the figures.
Using patterned etch and fill techniques well known in the art, shallow trench isolation (STI) structures 20 are formed in the top semiconductor layer 12 to delimit active regions 22 formed from the semiconductor material of the top semiconductor layer 12. The active regions 22 may, for example, have a width of 5-40 nm and be arranged with a pitch of 10-80 nm. Any suitable length for the active regions dependent on application may be supported. In an alternative embodiment, the active regions 22 may comprise fin structures such as those known in the art for use in fabricating finFET transistors. The fin structures may be separated from each other by the STI structures, but such fin structures will typically have a height that extends above the top surface of the STI structures.
A layer of a high-K dielectric material (thickness 2-20 nm) and a layer of a polysilicon material (thickness 5-120 nm) are deposited over the wafer. Using conventional lithographic processing techniques, these layers are patterned to form a gate stack 30 comprising a gate dielectric 32 and a gate electrode 34 that crosses over the active region 22 at the location of a channel region (C) for the transistor being formed. The source region (S) and drain region (D) for the transistor are provided in the active region 22 on either side of the channel region. Although polysilicon is shown as the material for the gate electrode 34, it will be understood that this is by example only and that the gate electrode may comprise other conductive or semiconductive materials and may be formed of plural layers of conductive or semiconductive materials. The gate stack 30 may, for example, have a width that extends across multiple active regions 22 and a length of 5-40 nm.
In situations where the gate stack 30 is a “dummy” gate of the type known for use in replacement metal gate fabrication techniques, the layers 32 and 34 may be replaced by a single layer of polysilicon or other suitable material that is patterned to define the gate region.
In the event that the active region 22 is formed as a fin structure, the gate stack 30 will straddle over the active region on three sides thereof at the channel region as shown in FIGS. 1C and 1D, rather than only the top side as shown in FIGS. 1A and 1B. The remaining process steps shown herein are equally applicable to the formation of a planar-type MOSFET device (as shown) or a finFET device, and thus the active region shall include configuration as either a planar active region (FIGS. 1A and 1B) or a fin structure active region (FIGS. 1C and 1D). Further illustration and description is made in the context of the structures shown in FIGS. 1A and 1B, but all further steps are equally applicable to the structures shown in FIGS. 1C and 1D.
In this context, it will be understood that the terms “width” and “length” are taken relative to the width and length of the channel of the transistor device being formed.
A sidewall spacer 38 is then formed on the sidewalls of the gate stack 30. The sidewall spacer 38 may, for example, be formed by a conformal deposit of an insulating material on the wafer followed by an etch which preferentially removes material on the horizontal surfaces of the wafer while leaving material on the vertical surfaces of the wafer. The insulating material for the sidewall spacer 38 may comprise: SiN, SiBCN or SiOCN. The sidewall spacer 38 may have a thickness of 1-20 nm. The process for spacer formation may, for example, use a Lam mixed mode pulsing (MMP) deposition/etch/O2 flash approach. The result is shown in FIGS. 2, 2A and 2B. In this context, the sidewall spacer 38 is formed on first opposite sides of the gate stack 30.
An epitaxial process is then used to grow an epitaxial layer 40 of semiconductor material on the wafer. The layer 40 may, for example, have a thickness of 20-60 nm, where that thickness is preferably less than the height of the gate stack 30, and more preferably about 30-40% of the height of the gate stack 30. The result is shown in FIGS. 3, 3A and 3B. If the transistors being produced are of the nMOS type, the material of the epitaxial layer 40 may, for example, comprise SiP/Si with a Phosphorus dopant concentration of 1×1019 at/cm3 to 1×1021 at/cm3. The material may include substitutional Carbon at 0.5% to 5%. If the transistors being produced are of the pMOS type, the material of the epitaxial layer 40 may, for example, comprise SiGeB with a Boron dopant concentration of 1×1019 at/cm3 to 2×1021 at/cm3. The material may include substitutional Germanium at 25% to 75%.
A sacrificial layer 44 (thickness 400-600 nm) of a dielectric material is then conformally deposited on the wafer to a thickness sufficient to cover the gate stack 30. A chemical mechanical polishing is then performed to remove the portions of the layer 44 above the top of the gate stack 30. The result is shown in FIGS. 4, 4A and 4B. The dielectric material of the layer 44 may, for example, comprise low temperature oxide or a flowable oxide deposited using chemical vapor deposition (CVD) or plasma-enhanced chemical vapor deposition (PECVD).
A layer of hard mask material is then deposited over the wafer. The hard mask material may, for example, comprise SiON deposited using PECVD. Using conventional lithographic techniques, the layer of hard mask material is patterned to leave a gate mask 50 over each region of the wafer where the gate stack 30 crosses (intersects) the active region 22. The result is shown in FIGS. 5, 5A and 5B.
An additional layer 52 (thickness about 80 nm) of hard mask material is then conformally deposited on wafer to a thickness sufficient to cover the gate mask 50. A chemical mechanical polishing is then performed to remove the portions of the layer 52 above the top of the gate mask 50. The result is shown in FIGS. 6, 6A and 6B. The additional hard mask material may, for example, comprise amorphous carbon.
Using lithographic techniques known in the art, the additional layer 52 is patterned to define a mask 54 that covers the active regions 22 and areas of the wafer adjacent the active regions. The result is shown in FIGS. 7, 7A and 7B.
An etch is then performed using the masks 50 and 54 to block material removal. The etch extends through the various layers 40 and 44 to reach the top of the STI structures 20 and form openings 56. The result is shown in FIGS. 8, 8A and 8B. The etch process may, for example, comprise carbonyl sulfide (COS) added to oxygen as the additive etch gas etching of the amorphous carbon layers selective to SiON in the reactive ion etch (RIE). Florin-based anisoptropic RIE can be used to stop on the STI structures 20. The result of the etch leaves a separate gate stack 60 over each active region 22 and a source-drain stack 62 over each active region 22 on either side of the separate gate stack 60.
A sidewall spacer 68 is then formed on the sidewalls of the separate gate stack 60 and the newly exposed sidewalls of each separate source-drain stack 62. The sidewall spacer 68 may, for example, be formed by a conformal deposit of an insulating material on the wafer followed by an etch which preferentially removes material on the horizontal surfaces while leaving material on the vertical surface. The insulating material for the sidewall spacer 68 may comprise: SiN, SiBCN or SiOCN. The sidewall spacer 68 may have a thickness of 1-20 nm. The process for spacer formation may, for example, use a Lam mixed mode pulsing (MMP) deposition/etch/O2 flash approach. The result is shown in FIGS. 9, 9A, 9B and 9C (wherein FIG. 9C is a cross-section taken along line C-C of FIG. 9). In this context, the sidewall spacer 68 is formed on second opposite sides of the separate gate stack 60, with the sidewall spacer 38 formed on the first opposite sides of the gate stack 30, and the sidewall spacers 38 and 68 peripherally surrounding the separate gate stack 60 and each source-drain stack 62.
A layer 70 (thickness 400-600 nm) of a dielectric material is then conformally deposited on wafer within the openings 56 to a thickness sufficient to cover the masks 50 and 54. A chemical mechanical polishing is then performed to remove the portions of the layer 70 above the top of the masks 50 and 54. The result is shown in FIGS. 10, 10A and 10B. The dielectric material of the layer 70 may, for example, comprise an oxide deposited using chemical vapor deposition (CVD) or plasma-enhanced chemical vapor deposition (PECVD).
The mask 54 is then selectively removed and an etch is performed through the opening left by removal of the mask 54 to remove the oxide layer 44 stopping on the epitaxial layer 40 and forming openings 58 which are laterally delimited by the sidewall spacer 38 on the gate stack 30 and the sidewall spacer 68 surrounding the source-drain stack 62. The result is shown in FIGS. 11, 11A, 11B and 11C (wherein FIG. 11C is a cross section taken along line C-C of FIG. 11). The etch process may, for example, comprise carbonyl sulfide (COS) added to oxygen as the additive etch gas etching of the amorphous carbon layers selective to SiON in the reactive ion etch (RIE). The result of the etch leaves the separate gate stacks 60 over each active region 22 as well as the epitaxial layer 40 of the source-drain stacks 62 over each active region 22 on either side of the separate gate stack 60. In this regard, a source region 66s is provided on one side of the separate gate stack 60 and a drain region 66d is provided on the other side of the separate gate stack 60.
The top surfaces of the source region 66s and drain region 66d in each opening 58 are then converted to a silicide 70. The result is shown in FIGS. 12, 12A and 12B. Techniques for forming silicides are well known to those skilled in the art. In an implementation for a relatively large (greater than or equal to 20 nm) critical dimension process, the following silicide process may be used: a SiCoNi preclean, a deposit of a NiPt layer using RFPVD, and an anneal at 380° C. for 30 seconds. For a relatively small (less than 20 nm) critical dimension, the following silicide process may be used for nMOS transistor devices: a gas cluster ion beam (GCIB)/dHF preclean, a trench epitaxial layer of SiP with a Phosphorous amorphous implant (1×1019 at/cm3 to 1×1021 at/cm3) and anneal, a deposit of a Ti layer by PVD, and a laser anneal. For a relatively small (less than 20 nm) critical dimension, the following silicide process may be used for pMOS transistor devices: a gas cluster ion beam (GCIB)/dHF preclean, a trench epitaxial layer of GeB with a Boron amorphous implant 1×1019 at/cm3 to 2×1021 at/cm3) and anneal, a deposit of a Ti layer by PVD, and a laser anneal.
A metal contact 74 for each of the source and drain is then formed within the openings 58 over the silicide 70. The metal contact 74 is preferably formed by a barrier layer (3-5 nm ALD of Ti), a liner (2-4 nm ALD of TiN) and metal fill (200 nm CVD of W, Co, Cu, Al or alloys thereof). A chemical mechanical polishing is then performed to remove the portions of the layer, liner and fill of the metal contact 74 above the top of the separate gate stacks 60. The gate mask 50 is also removed. The result is shown in FIGS. 13, 13A and 13B.
Conventional middle of line (MOL) and back-end of line (BEOL) processes may then be used to form extensions of the metal contacts 74 for the source and drain as well as a metal contact 76 for the gate. Examples of structural configurations for the MOL/BEOL structures are shown in FIGS. 14 and 15.
In the implementation described above, the gate stack 30 may comprise a fully formed gate electrode for the transistor device in a manner consistent with the “gate-first” fabrication technique. In an alternative embodiment, the gate stack may instead comprise a “dummy” gate structure in a manner consistent with the “gate-last” (or replacement metal gate) fabrication technique. In the gate-last process, following the step of FIGS. 13, 13A and 13B, the process would next comprise removal of the materials 32/34 of the dummy gate structures to form an opening delimited by the sidewall spacer 38 and the sidewall spacer 68 that surround the dummy gate stack 30. This is followed by fabrication of the gate electrode in the opening by, for example, deposition of a high-K dielectric material to cover the channel region, deposition of a work function metal and deposition of a polysilicon or metal electrode material. In either the gate-first or gate-last technique, the gate electrode may further include a silicide region.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

## Claims

1. A method, comprising:
forming an active region supported by a substrate and including a source region, a channel region and a drain region;
forming a gate stack over the channel region;
epitaxially growing a raised source region and a raised drain region over the source and drain regions, respectively, of the active region;
depositing a sacrificial layer over the raised source region and raised drain region to form a source stack and a drain stack, respectively;
forming a sidewall spacer that peripherally surrounds each of the gate stack, source stack and drain stack;
selectively removing the sacrificial layer to form openings above the raised source region and raised drain region that are each delimited by the sidewall spacer; and
filling said openings with conductive material to form a source contact and a drain contact to the raised source region and raised drain region, respectively.

2. The method of claim 1, wherein forming the active region comprises defining a fin structure and forming the gate stack comprises forming the gate stack on three sides of the fin structure.

3. The method of claim 1, wherein the substrate is a silicon on insulator (SOI) substrate.

4. The method of claim 1, wherein the gate stack is a dummy gate stack and further comprising:
removing the dummy gate stack to form another opening delimited by the first and second sidewall spacers; and
fabricating an insulated gate electrode structure in said another opening.

5. The method of claim 1, wherein forming the sidewall spacer comprises:
forming a first sidewall spacer on sidewalls of the gate stack; and
forming a second sidewall spacer on sidewalls of the source stack and drain stack.

6. The method of claim 5, wherein the first sidewall spacer is further provided on a sidewall of each of the source stack and drain stack.

7. The method of claim 5, wherein forming the first sidewall spacer comprises forming the first sidewall spacer on first opposed sidewalls of the gate stack, and wherein forming the second sidewall spacer further comprises forming the second sidewall spacer on second opposed sidewalls of the gate stack.

8. The method of claim 1, further comprising:
depositing a first mask over the gate stack;
depositing a second mask over the raised source region and raised drain region; and
etching using the first and second masks to define the source stack and the drain stack.

9. The method of claim 8, wherein etching further defines the gate stack.

10. The method of claim 8, wherein selectively removing the sacrificial layer comprises:
removing the second mask; and
etching using the first mask to remove the sacrificial layer.

11. The method of claim 8, wherein the first and second masks are formed on a same level.

12. The method of claim 1, wherein forming the active region comprises forming a plurality of active regions separated from each other by an insulating material, and wherein forming the gate stack comprises forming the gate stack to extend over the plurality of active regions.

13. The method of claim 12, wherein forming the sidewall spacer comprises:
forming a first sidewall spacer on first opposed sidewalls of the gate stack;
removing portions of the gate stack between adjacent ones of the plurality of active regions to define a corresponding plurality of separate gate stacks; and
forming a second sidewall spacer on second opposed sidewalls of each separate gate stack.

14. The method of claim 13, wherein forming the sidewall spacer further comprises forming said second sidewall spacer also on sidewalls of the source stack and drain stack.

15. The method of claim 1, wherein filling said openings comprises forming a silicide.

16. A method, comprising:
forming an active region supported by a substrate and including a source region, a channel region and a drain region;
forming a gate stack over the channel region;
forming a first sidewall spacer on the gate stack;
epitaxially growing a raised source region and a raised drain region over the source and drain regions, respectively, of the active region adjacent the first sidewall spacer;
depositing a sacrificial layer over the raised source region and raised drain region;
forming a first mask over the gate stack;
forming a second mask at a same level as the first mask over the sacrificial layer and the raised source region and raised drain region;
etching using the first and second masks to define a source stack and drain stack covered by said second mask;
forming a second sidewall spacer on the source stack and drain stack;
removing the second mask;
etching using the first mask to remove the sacrificial layer and form openings above the raised source region and raised drain region which are delimited by the first and second sidewall spacers; and
filling said openings with conductive material to form a source contact and a drain contact to the raised source region and raised drain region, respectively.

17. The method of claim 16, wherein forming the active region comprises defining a fin structure and forming the gate stack comprises forming the gate stack on three sides of the fin structure.

18. The method of claim 16, wherein the substrate is a silicon on insulator (SOI) substrate.

19. The method of claim 16, wherein the gate stack is a dummy gate stack and further comprising:
removing the dummy gate stack to form another opening delimited by the first and second sidewall spacers; and
fabricating an insulated gate electrode structure in said another opening.

20. A method, comprising:
forming a gate stack over a substrate that includes a source region, a channel region and a drain region;
forming a first sidewall spacer on opposite sides of the gate stack;
epitaxially growing a raised source region and a raised drain region over the source and drain regions, respectively, on opposite sides of the gate stack;
forming a source stack with a deposit of a sacrificial material over the raised source region;
forming a drain stack with a deposit of said sacrificial material over the raised drain region;
forming a second sidewall spacer on sides of the source stack and drain stack;
selectively removing the sacrificial material to form openings above the raised source region and raised drain region, wherein each opening is delimited by the first and second sidewall spacers; and
depositing a conductive material in said openings to form a source contact and a drain contact to the raised source region and raised drain region, respectively.

21. The method of claim 20, wherein the gate stack is a dummy gate stack and further comprising:
removal of the dummy gate stack to form another opening delimited by at least the first sidewall spacer; and
depositing a conductive material in said another opening to form an insulated gate electrode structure.

22. The method of claim 20, further comprising forming a fin of semiconductor material which includes the source region, channel region and drain region.

23. The method of claim 20, wherein forming the first sidewall spacer comprises forming the first sidewall spacer on first opposed sidewalls of the gate stack, and wherein forming the second sidewall spacer further comprises forming the second sidewall spacer on second opposed sidewalls of the gate stack.