US 20060281306 A1
A method for forming an interconnect on a semiconductor substrate comprises providing at least one carbon nanotube within a trench, etching at least one portion of the carbon nanotube to create an opening, conformally depositing a metal layer on the carbon nanotube through the opening, and forming a metallized contact at the opening that is substantially coupled to the carbon nanotube. The metal layer may be conformally deposited on the carbon nanotube using an atomic layer deposition process or an electroless plating process. Multiple metal layers may be deposited to substantially fill voids within the carbon nanotube. The electroless plating process may use a supercritical liquid as the medium for the plating solution. The wetting behavior of the carbon nanotube may be modified prior to the electroless plating process to increase the hydrophilicity of the carbon nanotube.
1. A method comprising:
providing at least one carbon nanotube within a trench;
etching at least one portion of the carbon nanotube to create an opening;
conformally depositing a metal layer on the carbon nanotube through the opening; and
forming a metallized contact at the opening that is substantially coupled to the carbon nanotube.
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15. A method comprising:
providing a bundle of carbon nanotubes within a trench;
etching a first end of the bundle of carbon nanotubes to create a first opening;
etching a second end of the bundle of carbon nanotubes to create a second opening;
conformally depositing multiple metal layers on each of the carbon nanotubes of the bundle through the openings; and
forming metallized contacts in the first and second openings that are substantially coupled to all of the carbon nanotubes of the bundle.
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22. A method comprising:
providing at least one carbon nanotube within a trench;
etching at least one portion of the carbon nanotube to create an opening;
modifying the wetting behavior of a surface of the carbon nanotube to increase its hydrophilicity; and
performing an electroless plating process on the carbon nanotube using an electroless plating bath that comprises a supercritical liquid.
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depositing a photoresist layer;
patterning the photoresist layer;
developing the photoresist layer;
etching the carbon nanotube; and
removing the developed photoresist layer.
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31. An apparatus comprising:
a bundle of carbon nanotubes mounted within a trench;
a metallized contact mounted at an end of the bundle of carbon nanotubes, wherein the metallized contact is directly coupled to substantially all of the carbon nanotubes of the bundle; and
at least one metal layer conformally deposited on a surface of each carbon nanotube, wherein each metal layer covers substantially the entire surface of each carbon nanotube.
32. The apparatus of
33. The apparatus of
Carbon nanotubes are graphene cylinders whose ends are often closed by caps including pentagonal rings. The nanotube is a hexagonal network of carbon atoms forming a seamless cylinder. These cylinders can be as little as a nanometer in diameter with lengths of tens of microns or more in some cases. Depending on how they are made, the carbon nanotubes can be single walled or multiple walled.
Carbon nanotubes may exhibit various electrical properties. Depending on the configuration, carbon nanotubes may either act as semiconductors or as conductors. For example, certain types of carbon nanotubes may exhibit a number of metallic characteristics. Among these metallic characteristics, a number of properties are of particular interest with respect to the use of carbon nanotubes as an addition to, or as a replacement for, copper metal in the interconnect structures of semiconductor chips. Carbon nanotubes have been shown to have higher electrical and thermal conductivity than copper. Carbon nanotubes have also been shown to have higher electromigration resistance than copper, and electromigration has become a larger problem as copper interconnects have become narrower. Composite materials made of carbon nanotubes and copper metal have also been shown to have higher electrical conductivity and higher electromigration resistance than copper alone.
Unfortunately, conventional interconnect structures formed using carbon nanotubes do not completely utilize the full current carrying capacity of the grapheme sheets that form the nanotubes.
Described herein are systems and methods of realizing a greater portion of the current-carrying potential of carbon nanotubes used in an interconnect. In the following description, various aspects of the illustrative implementations will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some of the described aspects. For purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the illustrative implementations. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative implementations.
Various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present invention, however, the order of description should not be construed to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation.
Carbon nanotubes may be used for interconnections on an integrated circuit, replacing or being used in conjunction with traditional copper metal. Carbon nanotubes conduct electrons ballistically, in other words, without the scattering that gives copper its resistance. Dielectric material with a low dielectric constant (low-k), such as amorphous, carbon based insulation or fluorine doped silicon dioxide, may be used to insulate the carbon nanotubes. For instance, carbon-doped oxide (CDO) is a low-k dielectric material that may be used as the carbon based insulation.
With reference to
The CDO layer 100 is planarized using chemical mechanical polishing (CMP), as is well known by those of ordinary skill in the art. The planarized CDO layer 100 may be patterned using conventional photolithography and etching techniques to create a patterned layer. In one implementation, a trench 104 results from the etching process. Carbon based precursor material may then be deposited into the trench 104 within the CDO layer 100. A carbon nanotube 106 may be created from the carbon based precursor material and functions as an electrical interconnection between electrical contacts within the integrated circuit structure 102. This process may be repeated to create multiple layers of chip level interconnections using carbon nanotubes 106 and CDO layers 100.
As shown, conventional interconnect structures formed using carbon nanotubes do not utilize the full current-carrying capacity of the graphene sheets of the carbon nanotubes. This is partly due to voids 206 that exist within a bundle of carbon nanotubes and voids 206 that exist between the shells of multi-walled carbon nanotubes, as demonstrated in
As such, in accordance with implementations of the invention, a novel carbon nanotube interconnect structure may be formed through a conformal and substantially complete deposition of metal on all of the graphene sheets constituting the carbon nanotube interconnect structure. Novel contacts may also be formed on the ends of the carbon nanotube interconnect structure that are physically coupled to substantially all of the graphene sheets constituting the carbon nanotube interconnect structure. Interconnect structures formed in accordance with the invention may realize a greater portion of the current-carrying potential of the carbon nanotubes.
An interconnect structure may be formed within the trench 302 using one or more carbon nanotubes 304.
In accordance with an implementation of the invention, a metal 306 may be conformally deposited onto each of the graphene sheets that constitute the carbon nanotubes 304. The metal 306 may be used to fill voids that exist within each carbon nanotube 304 and voids that exist between the carbon nanotubes 304. The metal 306 may be deposited as multiple thin, conformal layers using processes such as atomic layer deposition (ALD), physical vapor deposition (PVD), and electroless plating. In implementations of the invention, metals that may be used to conformally fill the carbon nanotubes 304 include, but are not limited to, copper (Cu), aluminum (Al), gold (Au), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os), silver (Ag), iridium (Ir), titanium (Ti), and alloys of any or all of these metals. In some implementations, the metal or metals used may undergo chemical surface modification to provide improved electronic coupling.
Metallized contacts 308 may be formed at each end of the bundle of carbon nanotubes 304, thereby capping the ends of the interconnect structure and providing electrical contacts to the interconnect. Unlike the conventional contacts described with reference to
Similar to what is shown in
Metallized contacts 308 may again be formed at each end of the multi-walled carbon nanotube 400, thereby capping the ends of the interconnect structure and providing electrical contacts to the interconnect. The metallized contacts 308 shown in
In accordance with this implementation, one or more carbon nanotubes 304, including but not limited to single-walled, double-walled or multi-walled nanotubes, may be grown using conventional methods (502 of
One or more of the carbon nanotubes are then placed into the trench 302 within the dielectric layer 300 to form an interconnect structure (504 of
To form contacts at specific areas along the length of the carbon nanotube bundle, common lithographic methods may be used to create openings into the interconnect structure (506 of
The plasma etching process forms openings 602 in the carbon nanotubes 304 that generally extend all the way down to the bottom surface of the trench 302, as shown in
After the openings 602 have been etched, the mask 600 is removed and the method 500 utilizes atomic layer deposition (ALD) of metal 306 to conformally fill the carbon nanotubes 304 and to form the metallized contacts 308 (508 of
Known ALD precursor chemistries may be utilized that are appropriate for the metal chosen to conformally fill the carbon nanotubes. For example, in one implementation of the invention, platinum metal may be chosen to conformally fill the carbon nanotubes and to form the metallized contacts. In this implementation, known precursors chemistries for platinum metals, including but not limited to beta-diketonates, cyclopentadienyl, arenes, allyls, and carbonyls, may be used with an appropriate co-reactant such as oxygen or hydrogen. Again, complete surface conformality and coverage is expected with ALD as it is a surface limited deposition method.
One or more of the carbon nanotubes are used to form an interconnect structure by being placed into a trench within a dielectric layer (704). If the carbon nanotubes are grown directly within the trench, then this portion of the process may be eliminated. In implementations of the invention, a bundle of carbon nanotubes are placed within the trench to form the interconnect structure. Alternately, at least one single or multi-walled carbon nanotube may be placed or grown within the trench.
Common lithographic methods may be used to create openings into the interconnect structure (706). The etching processes may remove a portion of the carbon nanotubes to form openings through which a metal may be deposited and to allow metallized contacts to be formed that are coupled to substantially all of the graphene sheets that constitute the carbon nanotubes used in the interconnect structure.
After openings into the carbon nanotubes have been etched, rather than relying on ALD, the method 700 utilizes an electroless metal deposition in supercritical carbon dioxide (scCO2) to conformally fill the carbon nanotubes with metal and to form the metallized contacts (708). Electroless metal deposition in scCO2 enables the conformal deposition of metal on all of the graphene sheets that constitute a carbon nanotube bundle. This process may substantially or completely fill the core diameter of single or multi-walled carbon nanotubes with a metal, for example platinum or palladium.
As is known in the art, electroless metal deposition involves the deposition of a metal from a solution onto a substrate by a controlled chemical reduction reaction. The metal or metal alloy being deposited generally catalyzes the controlled chemical reduction reaction. Electroless metal deposition has several advantages over electroplating, another common plating process well known in the art. For example, electroless plating requires no electrical charge applied to the substrate, electroless plating generally results in a more uniform and nonporous metal layer on the target, and electroless metal deposition is autocatalytic and continuous once the plating process is initiated.
In accordance with the invention, a supercritical liquid such as scCO2 is used as the medium for the electroless plating solution. Supercritical liquids are known to penetrate the very small voids, gaps, and inner walls of carbon nanotubes due to their negligible viscosity. Supercritical liquids also leave little or no residues behind since the supercritical liquid, for example scCO2, will evaporate as a gas (i.e., CO2) once the conditions that make it supercritical are removed. Furthermore, as will be described below, supercritical liquids such as scCO2 tend to enhance the interaction between the carbon nanotube surface and the metal ions in the electroless plating solution.
In an implementation of the invention, the electroless plating solution includes a supercritical liquid (e.g., scCO2), a compound containing the metal to be deposited (e.g., a metal salt), and a reductant. In one implementation, the metal salt may include, but is not limited to, palladium hexafluoroacetylacetonate (Pd(hfac)2), which is soluble in scCO2, and the reductant may include, but is not limited to, hydrogen (H2). Electroless metal deposition in scCO2 works similar to electroless deposition of metal in water—the metal salt and the reductant are dissolved into the scCO2 and the electroless plating process is carried out.
In another implementation, a conventional, non-supercritical, electroless plating chemistry may be used. In one such implementation, palladium may be used in the electroless plating process. In some implementations, the palladium deposition may be followed by a copper deposition. A standard electroless plating solution is similar to the solutions described above but uses a liquid such as water in lieu of a supercritical liquid.
In implementations of the invention, the electroless plating solutions described above may further include complexing agents (e.g., an organic acid or amine) that prevent chemical reduction of the metal ions in solution while permitting selective chemical reduction on a surface of the target, chemical reducing agents (e.g., hypophosphite, dimethylaminoborane (DMAB), formaldehyde, hydrazine, or borohydride) for the metal ions, buffers (e.g., boric acid, an organic acid, or an amine) for controlling the pH level of the solution, and various optional additives such as solution stabilizers (e.g., pyridine, thiourea, or molybdates) and surfactants (e.g., a glycol). It is to be understood in all of the above described electroless plating processes that the specific composition of the plating solution will vary depending on the desired plating outcome.
In further implementations of the invention, the wetting behavior of the carbon nanotubes may be modified to enhance the electroless plating process. The wetting of the carbon nanotubes generally enables an improved interaction between the carbon nanotube surface and the metal ions in the plating solution. Furthermore, because the use of scCO2 as the plating solution medium also enhances the interaction of the surface of the carbon nanotubes with the metal ions, combining the use of scCO2 with a process for wetting the carbon nanotubes results in an improved and more complete metal deposition.
It is believed that the improved interaction between the carbon nanotube surface and the metal may be attributed to the surfactant-like qualities of the scCO2 and of the hydrophilic groups present when the wetting behavior of the carbon nanotubes has been modified. The scCO2 and the hydrophilic groups may also enhance the solvent, slurry, or medium effects thus leading to an enhanced interaction. It is further believed that the improved interaction between the carbon nanotube surface and the metal may lead to an improved adhesion between the carbon nanotube surface and the metal due to a temporary or permanent decrease in surface energy. This decrease in surface energy leads to the exposure of a greater portion of the carbon nanotube surface to the electroless plating solution and prevents the carbon nanotubes from balling up and minimizing their surface energy in contact with the metal.
The improved interaction between the carbon nanotube surface and the metal may also be attributed to increased capillary action that results from modifying the wetting behavior of the carbon nanotubes. The electroless plating solution, and the metal ions in particular, tend to be drawn into the carbon nanotubes by capillary action. Therefore, increasing the hydrophilicity of the carbon nanotubes increases the penetration of electroless plating solution and metal ions within the nanotubes.
In implementations of the invention, the wetting behavior of the surface of the carbon nanotubes may be attenuated through chemical modification. For example, the introduction of hydrogen-bonding functionalities may increase the hydrophilicity of the carbon nanotubes, thereby leading to enhanced water miscibility. Functionalities that favor these hydrophilic interactions include, but are not limited to, amines, amides, hydroxyls, carboxylic acids, aldehydes, and fluorides.
There are many known processes by which carbon nanotubes may be functionalized. Some of these processes include, but are not limited to, the following: (1) carboxylic acid functionalization through nitric acid oxidation; (2) carboxyl reduction to alcohols or aldehydes (e.g. NaBH4); (3) alcohol oxidation to aldehydes or carboxylic acids (e.g. pyridinium chlorochromate, Swern oxidation, etc); (4) amination of alcohols or carboxylic acids (e.g. NaN3, SOCl2/NH3, etc); (5) alkylation through the generation of alkyl radicals with alkyliodides/benzoyl peroxide; (6) 1,3-dipolar cycloadditions to the aromatic carbon nanotube framework; (7) arylation of carbon nanotubes with 4-chlorobenzenediazonium tetrafluoroborate, thus yielding a pendant aryl chloride functionality; (8) water solubilization of carbon nanotubes through reactive coating with polymers such as polyarleneethynlene; (9) attachment of metallic groups to sidewalls through [2+1]-cycloaddition attachment of gold colloids; (10) attachment of bio-molecules to carbon nanotubes (e.g., amino acids, proteins, DNA, etc.).
The aryl chlorides are prone to further functionalization including inter-carbon nanotube Heck-coupling reactions to yield covalently linked nanotubes and conversion of aryl iodides into amines, alcohols, or fluorides. This functionalization would be expected to increase carbon nanotube hydrophilicity leading to water miscibility. The methods presented herein may be employed for basement film-generation or wetting of the carbon nanotubes. It is believed that these methods for wetting carbon nanotubes may be applied for any transition metal, including but not limited to palladium, platinum, rhodium, ruthenium, gold, osmium, silver, and iridium.
The above description of illustrated implementations of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications may be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific implementations disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.