# Patent

## Title
Cobalt-carbon eutectic metal alloy ohmic contact for carbon nanotube field effect transistors

## Abstract

A cobalt-carbon (Co—C) eutectic metal alloy ohmic contact for a radio-frequency (RF) carbon nanotube (CNT) field effect transistor (FET) device and a method of manufacturing same are disclosed. Embodiments of a method include providing a graphite crucible, placing Co and a C source within the graphite crucible, heating the graphite crucible containing the Co and C source such that the Co and C source combine with graphite from the graphite crucible to thereby form a Co—C eutectic metal alloy, and creating an ohmic contact by depositing the Co—C eutectic metal alloy directly on top surfaces of CNTs of a RF CNT FET device such that the Co—C eutectic metal alloy is in direct contact with the CNTs. The Co—C eutectic metal alloy ohmic contact formed in this manner is consistently stabile and uniform and functions as a high work function layer that also serves as an adhesion layer to the CNTs.

## Background
FIELD
Embodiments are in the technical field of field effect transistors (FETs), particularly carbon nanotube (CNT) FETs. More particularly, embodiments disclosed herein relate to cobalt-carbon (Co—C) eutectic metal alloy ohmic contacts for CNT FETs devices and methods for making same which, inter alia, fosters improved radio frequency (RF) performance of FETs.
BACKGROUND
In the push to scale CNT FET electronics from long channel optical lithography to short channel deep ultraviolet (DUV) lithography of 0.5 microns, the device contact length needs to be reduced to less than 1 micron. In the state of the art CNT FET, a thin adhesion layer of titanium (Ti) is capped with palladium (Pd) metal as an ohmic contact to a CNT array. This Ti layer is necessary for adhesion of the Pd to the CNTs, but the Ti layer has a lower work function then the CNT array, thus resulting in a barrier at each contact point. In the state of the art, it has been shown that the Ti layer hinders channel scaling of less than 1 micron. If the Ti layer is removed from the stack, channel scaling of less than 1 micron can be measured but the lack of adhesion between the Pd and the CNT array (e.g., on quartz) results in a yield of less than 10%.
A Co—C eutectic metal alloy as a ohmic contact fixes both of these problems. A Co—C eutectic metal alloy adheres to the wafer without a need for an adhesion layer. In contrast to the use of the high work function metal Pd, the use of a a Co—C eutectic metal alloy comprising the high work function metal Co allows the Co—C eutectic metal alloy to be in direct contact with the CNTs and thus reduce the barrier on the contact. Thus, the Co—C eutectic metal alloy enables contact scaling of less than 1 micron with great adhesion resulting in yields above 75%.
While the work function of Co has been catalogued, it has not been applied to CNT FET's as an ohmic contact or part of the composition of an ohmic contact due to the difficulty in integrating the process. In embodiments herein, Co—C eutectic metal alloy ohmic contacts utilize the solubility of C into Co as a way to stabilize the deposition technique allowing for easy integration via electron beam evaporation.
SUMMARY
Embodiments are directed to a method for depositing a cobalt-carbon (Co—C) eutectic metal alloy as an ohmic contact for a radio-frequency (RF) carbon nanotube (CNT) field effect transistor (FET) device comprising: providing a graphite crucible; placing Co and a C source within the graphite crucible; heating the graphite crucible containing the Co and C source such that the Co and C source combine with graphite from the graphite crucible to thereby form a Co—C eutectic metal alloy; and creating an ohmic contact by depositing the Co—C eutectic metal alloy directly on top surfaces of CNTs of a RF CNT FET device such that the Co—C eutectic metal alloy is in direct contact with the CNTs.
In an embodiment, the depositing of the Co—C eutectic metal alloy is performed using electron beam evaporation.
In an embodiment, the C source may be graphite. The placing of the Co and graphite within the graphite crucible prior to the heating results in a ratio of Co to graphite of approximately 88:12.
In an embodiment, the deposited Co—C eutectic metal alloy may be within a range of 30-150 Å in height.
Embodiments are also directed to a Co—C eutectic metal alloy ohmic contact for a RF CNT FET device manufactured by the method above.
Embodiments are further directed to a RF CNT FET device. The RF CNT FET device comprises a plurality of CNTs, wherein the CNTs have top surfaces. The RF CNT PET device also comprises an ohmic contact deposited directly on the top surfaces of the CNTs such that the ohmic contact is in direct contact with the CNTs, wherein the ohmic contact comprises a Co—C eutectic metal alloy.
In an embodiment, the ohmic contact consists of Co and C.
In an embodiment, the deposited Co—C eutectic metal alloy may be within a range of 30-150 Å in height.
In an embodiment, the RF CNT FET device may further comprise a resistivity layer deposited directly on a top surface of the ohmic contact. The resistivity layer may comprise palladium (Pd). The RF CNT FET device may further comprise a scratch resistant layer deposited directly on a top surface of the resistivity layer. The scratch resistant layer may comprise platinum (Pt).



BRIEF DESCRIPTION OF THE DRAWINGS
The detailed description will refer to the following drawings in which like numerals refer to like items, and in which:
FIG. 1 is a plan view of an embodiment of a RF CNT FET device.
FIG. 2 is a cross-sectional side view of a source portion of the RF CNT FET device taken along line 2-2 shown in FIG. 1.
FIG. 3 is a cross-sectional side view of a gate portion of the RF CNT FET device taken along line 3-3 shown in FIG. 1.
FIG. 4 is a cross-sectional side view of a drain portion of the RF CNT FET device taken along line 4-4 shown in FIG. 1.
FIG. 5 is a flowchart illustrating an embodiment of a method for depositing a Co—C eutectic metal alloy as an ohmic contact for a RF CNT FET device.



DETAILED DESCRIPTION
It is to be understood that the figures and descriptions of the present invention may have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements found in a typical RF CNT FET or typical method for making a RF CNT FET. Those of ordinary skill in the art will recognize that other elements may be desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. It is also to be understood that the drawings included herewith only provide diagrammatic representations of the presently preferred structures of the present invention and that structures falling within the scope of the present invention may include structures different than those shown in the drawings. Reference will now be made to the drawings wherein like structures are provided with like reference designations.
Contact to RF CNTs is dependent on the ability to deposit a metal with a high work function to match the band structure of the array of CNTs. In embodiments disclosed herein, the evaporation of a Co—C eutectic metal alloy in a graphite crucible is used in the method for making an ohmic contact for RF CNT FET's. The ability to deposit the Co—C eutectic metal alloy is based on the use of a graphite crucible and is achieved via incorporation of C (e.g., graphite) in the Co source to be placed in the graphite crucible from small amounts up to the eutectic weight percent of Co Carbide to improve contacts.
The initial test for CNT contacts is to determine for a known CNT array the current per tube that is obtained from a two terminal ID-VD measurement at 1 Volt. Next, a scanning electron microscope (SEM) is used on the CNT array to determine the number of tubes present in the CNT array. In an embodiment, a CNT FET utilizing a Co—C eutectic metal alloy/Pd/Pt ohmic contact stack exhibits a current per tube of 3-4 micro amps per CNT. Whereas, a state of the art CNT FET utilizing a Ti/PD/Pt ohmic contact stack exhibits a current per tube of 4-5 micro amps per CNT. Next, CNT FETs were fabricated to the gate to check channel scaling. For the CNT FET utilizing a Co—C eutectic metal alloy/Pd/Pt ohmic contact stack, channel scaling is achieved for at least the characteristics from 8 microns to 2 microns. The CNT FET utilizing a Co—C eutectic metal alloy/Pd/Pt ohmic contact stack enables a CNT FET to be fabricated through the gate with Source-Drain spacing of less than 1 micron.
CNT arrays are prepared/patterned for source and drain electrode deposition. Co source is either pre-mixed with a C source before placing the pre-mix in a graphite crucible, or the Co is mixed with C within the graphite crucible for in-situ mixing.
The composition of the crucible is graphite. When a source of Co placed within the crucible is melted, the Co source absorbs some of the graphite from the crucible. However, to achieve a clean reaction having uniformity in the melt, it is necessary to additionally utilize graphite in the source placed within the crucible. With Co alone, over a period of time, thin films produced would having undesirably inconsistent compositions depending on the age of the Co source. Graphite, therefore, is necessary for reproducibility, stability, and uniformity purposes. Thus, a premixture source containing Co and C such as graphite achieves a consistently stable and uniform meld as compared to using a source consisting of only Co.
Co in the form of, for example, chunks or pellets, and graphite powder is added to the graphite crucible. The crucible and mixture therein are heated at a temperature in a range of approximately 1600° C.-1700° C. for a period of at least approximately 3 hours. During the melting process, the Co is reacted with C from the crucible. The melting process results in the Co and graphite from the premixture, along with graphite from the crucible, being melded together to form a Co—C eutectic metal alloy.
The crucible containing the Co—C eutectic metal alloy is allowed to cool into a solid source and is then loaded into an electron beam evaporation machine. The Co—C eutectic metal alloy is hit with an electron beam to locally melt sections of the Co—C eutectic metal alloy to vaporize it. The vapor will then rise up and condense onto a semiconductor wafer as a thin film having the Co—C eutectic metal alloy composition. The deposited thin film Co—C eutectic metal alloy serves as an adhesion layer and works as a high work-function match (i.e. dominant to carry holes).
In one exemplary embodiment, the method utilizes a crucible consisting of graphite having a volume of 7 cc and the premixed Co:graphite ratio that is initially added to the pre-melted crucible is approximately 88:12.
Other high work-function elements such as Pd in place of (or in addition to) Co in the above process were attempted. But in practice, use of these other high work-function materials required a separate adhesion layer to adhere the metal to the CNTs. As such, Co is required as the high work-function element in the above process.
The Co—C eutectic metal alloy is deposited on the CNTs in a thickness/height in a range of approximately 30-150 Å, with a preferred thickness of 100 Å. As a result, the thin film of the deposited Co—C eutectic metal alloy reduces stress. A resistivity layer such as Pd is then deposited on the top surface of the Co—C eutectic metal alloy in a thickness in a range of 50-100 nm, with a preferred thickness of 50 nm. The Pd layer ensures a diffusion barrier for later processing steps and to create a stack conducive to good ohmic contact with the CNTs. Next, a scratch resistant layer such as Pt is then deposited on the top surface of the Pd layer in a thickness in a range of 20-50 nm, with a preferred thickness of 20 nm. The FET is finished through the gate dielectric using gate lithography in order to result in a 3-terminal measurement of superior ID-VD and ID-VG characteristics.
In prior art CNT FETs, Pd was used as the high work function layer, but it required Ti as an adhesion layer to the CNTs. However, for the Pd to provide the work function required, the Ti layer needed to be extremely thin, i.e., on the order of 15 Å, leading to unreliable formation of the Ti layer in the electron beam deposition process. In the embodiments described herein, the single Co—C eutectic metal alloy layer replaces both the Ti layer functioning as the adhesion layer and the Pd layer functioning as the high work function layer, although Pd is still utilized as a resistivity layer deposited on the Co—C eutectic metal alloy layer for resistivity purposes, as mentioned above.
With reference to FIG. 1, shown is a plan view of an embodiment of a RF CNT FET device 100. With reference to FIG. 2, shown is a cross-sectional side view of a source portion 200 of the RF CNT FET taken along line 2-2 shown in FIG. 1. The RF CNT FET device 100 comprises a plurality of CNTs 101, wherein the CNTs 101 have top surfaces. The RF CNT FET device 100 also comprises an ohmic contact 105 deposited on a top surface of a substrate 103 and deposited directly on the top surfaces of the CNTs 101 such that the ohmic contact 105 is in direct contact with the CNTs 101, wherein the ohmic contact 105 comprises a Co—C eutectic metal alloy.
In an embodiment, the ohmic contact 105 consists of Co and C.
In an embodiment, the deposited Co—C eutectic metal alloy may be within a range of 30-150 Å in height.
In an embodiment, the RF CNT FET device 100 may further comprise a resistivity layer 107 deposited directly on a top surface of the ohmic contact 105. The resistivity layer 107 may comprise Pd. The RF CNT FET device 100 may further comprise a scratch resistant layer 109 deposited directly on a top surface of the resistivity layer 107. The scratch resistant layer 109 may comprise Pt.
With reference to FIG. 3, shown is a cross-sectional side view of a gate portion 300 of the RF CNT FET taken along line 3-3 shown in FIG. 1. A dielectric layer 127 is deposited on the CNTs 101 and the top surface of the substrate 103, and a gate layer 129 is deposited on a top surface of the dielectric layer 127.
With reference to FIG. 4, shown is a cross-sectional side view of a drain portion 400 of the RF CNT FET taken along line 4-4 shown in FIG. 1. The RF CNT FET device 100 also comprises an ohmic contact 115 deposited on a top surface of a substrate 103 and deposited directly on the top surfaces of the CNTs 101 such that the ohmic contact 115 is in direct contact with the CNTs 101, wherein the ohmic contact 115 comprises a Co—C eutectic metal alloy.
In an embodiment, the ohmic contact 115 consists of Co and C.
In an embodiment, the deposited Co—C eutectic metal alloy may be within a range of 30-150 Å in height.
In an embodiment, the RF CNT FET device 100 may further comprise a resistivity layer 117 deposited directly on a top surface of the ohmic contact 115. The resistivity layer 117 may comprise Pd. The RF CNT FET device 100 may further comprise a scratch resistant layer 119 deposited directly on a top surface of the resistivity layer 117. The scratch resistant layer 119 may comprise Pt.
With reference to FIG. 5, shown is a flowchart illustrating an embodiment of a method 500 for depositing a Co—C eutectic metal alloy as an ohmic contact for a RF CNT FET device. Embodiments are directed to a method 500 for depositing a Co—C eutectic metal alloy as an ohmic contact for a RF CNT FET device comprising: providing a graphite crucible (block 502); placing Co and a C source within the graphite crucible (block 504); heating the graphite crucible containing the Co and C source such that the Co and C source combine with graphite from the graphite crucible to thereby form a Co—C eutectic metal alloy (block 506); and creating an ohmic contact by depositing the Co—C eutectic metal alloy directly on top surfaces of CNTs of a RF CNT FET device such that the Co—C eutectic metal alloy is in direct contact with the CNTs (block 508).
In an embodiment, the depositing of the Co—C eutectic metal alloy is performed using electron beam evaporation.
In an embodiment, the C source may be graphite. The placing of the Co and graphite within the graphite crucible prior to the heating results in a ratio of Co to graphite of approximately 88:12.
In an embodiment, the deposited Co—C eutectic metal alloy may be within a range of 30-150 Å in height.
Embodiments are also directed to a Co—C eutectic metal alloy ohmic contact for a RF CNT FET device manufactured by the method above.
For simplicity purposes, it is noted that the elements illustrated in FIGS. 1-4 are not drawn to scale.
Embodiments disclosed herein utilizing Co—C eutectic metal alloy ohmic contacts for CNT FETs devices and methods for making same fosters improved RF performance of FETs.
The contemplated modifications and variations specifically mentioned above are considered to be within the spirit and scope of the present invention.
Those of ordinary skill in the art will recognize that various modifications and variations may be made to the embodiments described above without departing from the spirit and scope of the present invention. It is therefore to be understood that the present invention is not limited to the particular embodiments disclosed above, but it is intended to cover such modifications and variations as defined by the following claims.

## Claims

1. A method for depositing a cobalt-carbon (Co—C) eutectic metal alloy, the method comprising:
providing a graphite crucible;
placing Co and a C source within the graphite crucible;
heating the graphite crucible containing the Co and C source such that the Co and C source combine with graphite from the graphite crucible to thereby form a Co—C eutectic metal alloy; and
creating an ohmic contact by depositing the Co—C eutectic metal alloy directly on top surfaces of carbon nanotubes (CNTs) of a radio-frequency (RF) CNT field effect transistor (FET) device such that the Co—C eutectic metal alloy is in direct contact with the CNTs.

2. The method of claim 1 wherein the depositing the Co—C eutectic metal alloy is performed using electron beam evaporation.

3. The method of claim 1 wherein the C source is graphite.

4. The method of claim 3 wherein the placing the Co and graphite within the graphite crucible prior to the heating results in a volume ratio of Co to graphite of approximately 88:12.

5. The method of claim 1 wherein the deposited Co—C eutectic metal alloy is within a range of 30-150 Å in height.

6. A cobalt-carbon (Co—C) eutectic metal alloy ohmic contact for a radio-frequency (RF) carbon nanotube (CNT) field effect transistor (FET) device manufactured by the method of:
providing a graphite crucible;
placing Co and a C source within the graphite crucible;
heating the graphite crucible containing the Co and C source such that the Co and C source combine with graphite from the graphite crucible to thereby form a Co—C eutectic metal alloy; and
creating an ohmic contact by depositing the Co—C eutectic metal alloy directly on top surfaces of CNTs of a RF CNT FET device such that the Co—C eutectic metal alloy is in direct contact with the CNTs.

7. The ohmic contact of claim 6 wherein the depositing the Co—C eutectic metal alloy is performed using electron beam evaporation.

8. The ohmic contact of claim 6 wherein the C source is graphite.

9. The ohmic contact of claim 8 wherein the placing the Co and graphite within the graphite crucible prior to the heating results in a volume ratio of Co to graphite of approximately 88:12.

10. The ohmic contact of claim 6 wherein the deposited Co—C eutectic metal alloy is within a range of 30-150 Å in height.

11. A radio-frequency (RF) carbon nanotube (CNT) field effect transistor (FET) device comprising:
a plurality of CNTs of a RF CNT FET device, wherein the CNTs have top surfaces; and
an ohmic contact deposited directly on the top surfaces of the CNTs such that the ohmic contact is in direct contact with the CNTs;
wherein the ohmic contact comprises a cobalt-carbon (Co—C) eutectic metal alloy.

12. The RF CNT FET device of claim 11 wherein the ohmic contact consists of Co and C.

13. The RF CNT FET device of claim 11 wherein the deposited Co—C eutectic metal alloy is within a range of 30-150 Å in height.

14. The RF CNT FET device of claim 11 further comprising a resistivity layer deposited directly on a top surface of the ohmic contact.

15. The RF CNT FET device of claim 14 wherein the resistivity layer comprises palladium (Pd).

16. The RF CNT FET device of claim 14 further comprising a scratch resistant layer deposited directly on a top surface of the resistivity layer.

17. The RF CNT FET device of claim 16 wherein the scratch resistant layer comprises platinum (Pt).