# Patent

## Title
Narrow body field-effect transistor structures with free-standing extension regions

## Abstract

Narrow-body FETs, such as, FinFETs and trigates, exhibit superior short-channel characteristics compared to thick-body devices, such as planar bulk Si FETs and planar partially-depleted SOI (PDSOI) FETs. A common problem, however, with narrow-body devices is high series resistance that often negates the short-channel benefits. The high series resistance is due to either dopant pile-up at the SOI/BOX interface or dopant diffusion into the BOX. This disclosure describes a novel narrow-body device geometry that is expected to overcome the high series resistance problem.

## Background
TECHNICAL FIELD
The present disclosure relates generally to complementary metal-oxide semiconductor field-effect transistors, and more particularly, to a narrow-body FET geometry.
BACKGROUND
Complementary metal-oxide semiconductor (“CMOS”) field-effect transistors (“FETs”) are employed in almost every electronic circuit application, such as signal processing, computing, and wireless communications. CMOS FETs have a gate that controls the flow of electrical current between the source and drain. Scaling down the gate length of both N-channel FETs (NFETs) and P-channel FETs (PFETs) in CMOS circuits to shorter dimensions leads to increased CMOS circuit speed. However, detrimental short-channel effects lead to high off-state leakage currents in CMOS devices, thereby increasing the power consumption. In case of extreme short-channel effects, CMOS circuits fail to operate.
FETs with multiple gates built on narrow bodies, such as, fin-shaped field effect transistors (“FinFETs”) and trigates have better electrostatic integrity than thick-body partially-depleted silicon-on-insulator (“PDSOI”) devices. However, narrow body devices on silicon-on-insulator (“SOI”) suffer from high series resistance due to loss of doping from thin extension regions into the buried oxide (“BOX”).
A known method to solve the doping loss problem is the use of advanced anneal techniques, such as millisecond laser spike anneal (“LSA”) and flash anneal (“FLA”). These anneal techniques have a very small time scale to avoid dopant diffusion, and therefore, doping loss, and yet achieve high temperatures to electrically activate the dopants. Since very low energy implants are needed for thin body devices to avoid amorphization of the thin extension regions, the lack of any implant diffusion during LSA and FLA leads to very steep extension doping profiles with high chemical concentration exceeding the solid solubility limit near the surface of the thin extension region. Therefore, a significant fraction of the implanted dose, although present in the thin extension region, is not electrically active. Even though the thin-extension sheet resistance is significantly lowered upon using LSA/FLA instead of conventional spike rapid thermal annealing (“RTA”), it is not low enough and makes the FET series resistance high compared to that of PDSOI devices.
Therefore, a new method is desired where one can create an extension doping profile that is uniform in the thin extension region and does not suffer from doping loss to the BOX.
BRIEF SUMMARY
The present disclosure provides a method of fabricating narrow-body FET devices that have a uniform extension doping profile as well as novel narrow-body device geometry. The narrow-body FET devices of the present disclosure overcome the high series resistance problems that often outweigh the short-channel benefits of most narrow-body devices because they do not suffer from dopant piling at the SOI/BOX interface, which is generally the case for n-type dopants such as As for NFETS, or encounter dopant diffusion into the BOX region, which is generally the case for p-type dopants such as B, for PFETs.
More particularly, the present disclosure relates to a method of fabricating a narrow-body SOI device comprising uniformly doped extension regions comprising:
(A) processing a narrow-body FET until extension implant regions are formed on both sides of a gate electrode;
(B) depositing a layer of oxide over the extension implant regions and a source and drain region of the narrow-body FET;
(C) forming a nitride spacer on both sides of the gate electrode by depositing a layer of nitride followed by a nitride RIE process that is selective to oxide;
(D) dipping the narrow-body FET into HF acid to create air gaps underneath the extension regions; and
(E) performing spike rapid thermal annealing on the narrow-body FET.
The present disclosure also relates to a narrow-body SOI devices comprising:
(A) a substrate;
(B) uniformly doped extension regions;
(C) a gate electrode, wherein the gate electrode is separated from the substrate and uniformly doped extension regions by a first nitride space;
(D) air gaps in the substrate underneath the uniformly doped extension regions;
(E) a second nitride spacer; and
(F) a silicide layer.
Still other objects and advantages of the present disclosure will become readily apparent to those skilled in the art from the following detailed description, where it is shown and described only the preferred embodiment(s), simply by way of illustration of the best mode. As will be realized, the disclosure is capable of other and different embodiments, and its several details are capable of modification in various obvious respects, without departing from the disclosure. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a narrow-body FET after extension implants are formed.
FIG. 2 is a cross-sectional view of a narrow-body FET depicted in FIG. 1 after the oxide liner is deposited and the nitride spacer formed.
FIG. 3 is a cross-sectional view of a narrow-body FET depicted in FIG. 2 after being dipped in HF acid to release the extension regions.
FIG. 4 is a cross-sectional view of a narrow-body FET depicted in FIG. 3 after rapid thermal annealing to create uniformly doped extension regions.
FIG. 5 is a cross-sectional view of a narrow-body FET depicted in FIG. 4 after the final nitride spacer is formed.
FIG. 6 is a cross-sectional view of a narrow-body FET depicted in FIG. 5 after the self-aligned silicide process.
FIG. 7 is a cross-sectional view of an embodiment of the device of the present disclosure.



DETAILED DESCRIPTION
The present disclosure, which is directed to a novel narrow-body device geometry that overcomes the high series resistance problems afflicting all narrow-body devices, and a method of fabricating these narrow-body FET devices, will now be described in greater detail by referring to the drawings that accompany the present application. It is noted that in the accompanying drawings, like reference numerals are used for describing like and/or corresponding elements.
The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “having” or “including” and not in the exclusive sense of “consisting only of.” The terms “a” and “the” are used herein are understood to encompass the plural as well as the singular.
FIGS. 1-7 depict the steps used to fabricate a FinFET (i.e., a double gate metal-oxide-semiconductor FET (“MOSFET”)) device in accordance with one embodiment of the invention. Using conventional and known FinFET or trigate processing techniques, a narrow body FET is processed until extension implants, as is depicted in FIG. 1. FIG. 1 is a cross-sectional view through the SOI fin region. In FIG. 1, a substrate, such as a silicon-on-insulator Separation by Implantation of Oxygen (“SIMOX”) structure, is provided including a silicon substrate 1, a buried oxide (“BOX”) layer 2 on top of the silicon substrate, and a silicon layer 3 on top of the buried oxide. The oxide layer 4 underneath the gate electrode is a hardmask in the case of FinFETs and is a gate oxide in the case of trigates. When the oxide underneath the gate in a two-dimensional figure, such as FIG. 1, is approximately twice as thick or more than the equivalent oxide thickness (“EOT”) of the gate dielectric on the sidewalls, then the oxide underneath the gate is considered to be a hardmask. An oxide offset spacer 5 further surrounds the gate electrode.
As depicted in FIG. 2, a layer 6 of oxide (about 3-5 nanometers thick) is then deposited and followed by a nitride spacer 7 formation. The nitride spacer 7 formation includes the deposition of a thin layer of nitride (about 3-10 nanometers thick) followed by reactive ion etching (“RIE”) that is selective to oxide and therefore stops on the oxide layer 6. The RIE is a nitride RIE that is selective to oxide and therefore stops on oxide. The formation of the nitride spacer 7 encapsulates the oxide offset spacer 5 as well as the oxide layer 4 underneath the gate electrode.
Next, the entire wafer is then dipped in HF acid to create air gaps 8 underneath the extension regions 9, as is depicted in FIG. 3. The concentration of the HF acid used is approximately 100:1 (water:HF). The extension regions 9 are then released from the BOX layer 2 and are thus free-standing. The extension regions are released when the BOX layer 2 underneath the SOI region is etched away and an air gap remains under the extension regions 9.
The wafer is then annealed using spike RTA, which creates uniformly-doped extension regions 10 due to diffusion, as showing in FIG. 4. For spike RTA, the temperature is approximately 1000-1100° C. and the time is approximately 1-5 seconds. Spike RTA is preferred over other annealing techniques, such as LSA and FLA. LSA and FLA lead to abrupt doping profiles, and therefore most dopants, while still in the SOI extension regions, are not electrically active using LSA and FLA. Spike RTA, however, provides uniformly-doped extension regions so that the resistivity of the extension is low and thus a low series resistance is obtained.
Since the extension regions 9 are not in contact with the BOX 2, no doping loss occurs from the extension regions 9 to the BOX 2. Normally, when thin body FETs built on SOI are annealed, extension doping, especially boron, will move out of the silicon and into the oxide because boron diffuses more quickly in oxide than in silicon. The nitride spacer 7 encapsulating the thin body prevents the dopants that are in the silicon from moving out of the silicon and into the oxide.
The wafer is then processed like any conventional narrow-body FET using known methods beginning with the spacer formation for a self-aligned silicide process, as is showing in FIG. 6. A key difference, however, between the FinFETs or trigates formed by the method of the instant disclosure from conventional devices is that the nitride spacer 11 will fill up the air-gap underneath the extensions as shown in FIG. 5.
A cross-sectional view of the resulting device is depicted in FIG. 7.
The foregoing description illustrates and describes the disclosure. Additionally, the disclosure shows and describes only the preferred embodiment(s) but, as mentioned above, it is to be understood that it is capable to use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the invention concepts as expressed herein, commensurate with the above teachings and/or skill or knowledge of the relevant art. The embodiments described herein above are further intended to explain best modes known by the applicants and to enable others skilled in the art to utilize the disclosure in such, or other, embodiments and with the various modifications required by the particular applications or uses thereof. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended to the appended claims be construed to include alternative embodiments.
All publications and patent applications cited in this specification are herein incorporated by reference, and for any and all purposes, as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. In the event of an inconsistency between the present disclosure and any publications or patent applications incorporated herein by references, the present disclosure controls.

## Claims

1. A narrow-body SOI device comprising:
(A) a substrate comprising a silicon substrate, a buried oxide layer on top of the silicon substrate, and a silicon layer on top of the buried oxide layer;
(B) uniformly doped extension regions;
(C) a gate electrode that is in direct contact with an oxide spacer that surrounds the bottom of the gate electrode and extends to the sidewalls of the gate dielectric such that it separates the gate electrode from the substrate and uniformly doped extension regions, wherein the silicone layer of (A) is in direct contact with the oxide spacer surrounding the bottom of the gate electrode;
(D) a first nitride spacer formed directly on the oxide spacer on the sidewalls of the gate electrode but not the bottom of the gate electrode;
(E) individual gaps in the buried oxide layer underneath the uniformly doped extension regions, the individual gaps being separated from each other by an intervening portion of the buried oxide layer lying beneath the gate electrode;
(F) a second nitride spacer overlaying the first nitride layer and filling the individual gaps in the substrate underneath the uniformly doped extension regions; and
(G) a silicide layer atop the gate electrode and separately overlaying at least a portion of the uniformly doped extension regions.

2. The narrow-body SOI device of claim 1, wherein rapid thermal annealing is used to create the uniformly doped extension regions.

3. The narrow-body SOI device of claim 1, wherein the individual gaps are formed by dipping the narrow-body SOI device into HF acid.

4. The narrow-body SOI device according to claim 1, wherein the oxide spacer surrounding the bottom of the electrode is a hardmask.

5. The narrow-body SOI device according to claim 4, wherein the thickness of the oxide spacer surrounding the bottom of the electrode is approximately twice as thick as the thickness of the oxide spacer on the sidewalls of the electrode.

6. The narrow-body SOI device according to claim 1, wherein the thickness of the first nitride spacer is about 3 to 10 nanometers.

7. The narrow-body SOI device according to claim 1, wherein the oxide spacer surrounding the bottom of the electrode is a hardmask.

8. The narrow-body SOI device according to claim 7, wherein the thickness of the oxide spacer surrounding the bottom of the electrode is approximately twice as thick as the thickness of the oxide spacer on the sidewalls of the electrode.

9. The narrow-body SOI device according to claim 8, wherein the thickness of the first nitride spacer is about 3 to 10 nanometers.