US 20050191495 A1
Nanotube films and articles and methods of making the same are disclosed. A conductive article or a substrate comprises at least two unaligned nanotubes extending substantially parallel to the substrate and each contacting end points of the article but each unaligned relative to the other, the nanotubes providing a conductive pathway within a predefined space.
1. A conductive article or a substrate comprising:
at least two unaligned nanotubes extending substantially parallel to the substrate and each contacting end points of the article but each unaligned relative to the other, the nanotubes providing a conductive pathway within a predefined space.
2. The article of
3. The article of
4. The article of
5. The article of
6. A conductive article comprising:
at least two unaligned nanotubes extending substantially parallel to a substrate and each contacting end points of the article but each unaligned relative to the other, wherein the nanotubes conform to a non-planar surface of a substrate.
7. The conductive article of
8. The article of
9. The article of
10. The article of
11. The article of
12. The article of
13. A method of making a conductive article on a substrate comprising:
providing a layer of nanotubes on a substrate, said nanotube layer comprising at least two unaligned nanotubes, the nanotubes being substantially parallel to a surface of the substrate; and
selectively patterning the layer to define an electrically conductive element having the unaligned nanotubes.
14. The method of
15. The method of
16. The method of
17. The method of
18. The method of
This application is a continuation of U.S. patent application Ser. No. 10/776,573 filed on Feb. 11, 2004, entitled NANOTUBE FILMS AND ARTICLES, which is a continuation of U.S. patent application Ser. No. 10/128,118, filed on Apr. 23, 2002, entitled NANOTUBE FILMS AND ARTICLES, which is a continuation-in-part and claims priority under 35 U.S.C. § 120 to the following applications, and all of which are incorporated herein by reference in their entirety:
1. Technical Field
This invention relates in general to carbon nanotube films, fabrics, layers, and articles and in particular to making electrically conductive articles from carbon nanotube films, fabrics, or layers for a variety of uses in circuits or the like.
2. Discussion of Related Art
The reliable fabrication of electrically conductive, ultra-thin metallic layers and electrodes in the sub-10 nm regime is problematic, see, e.g., S. Wolf, Silicon Processing for the VLSI era; Volume 2—Process Integration (Lattice Press, Sunset Beach, 1990). Metal films in this size regime are usually non-continuous and not conductive over macroscopic distances. Furthermore, these sub-10 nm films are prone to thermal damage by electrical current making them unsuitable for applications such as electrical interconnects in semiconductor devices. Thermal damage of thin metal interconnects caused by their low heat conductivities is one of the main factors inhibiting dramatic miniaturization and performance improvements of highly integrated semiconductor devices.
Conventional interconnect technologies have a tendency to suffer from thermal damage and metal diffusion eroding the performance of the semiconductor devices especially from degradation of the electrical properties. These effects become even more pronounced with size reduction in current generation 0.18 um and 0.13 um structures, e.g. by metal diffusion through ultra-thin gate oxide layers.
There is therefore a need in the art for conductive elements that may operate well in contexts having high current densities or in extreme thermal conditions. This includes circuit contexts with very small feature sizes but includes other high current density extreme thermal environment contexts as well. There is also a need for conductive elements that will be less likely to diffuse undesirable amounts of contaminants into other circuit elements.
The invention provides nanotube films and articles and methods of making the same. Under one aspect of the invention, a conductive article or a substrate comprises at least two unaligned nanotubes extending substantially parallel to the substrate and each contacting end points of the article but each unaligned relative to the other, the nanotubes providing a conductive pathway within a predefined space.
Under other aspects of the invention, the nanotube segments may be single walled carbon nanotubes, or multi-walled carbon nanotubes. The various segments may have different lengths and may include segments having a length shorter than the length of the article.
Under other aspects of the invention, the nanotubes have a controllable density.
Under another aspect of the invention, a conductive article comprises at least two unaligned nanotubes extending substantially parallel to a substrate and each contacting end points of the article but each unaligned relative to the other, wherein the nanotubes conform to a non-planar surface of a substrate.
Under another aspect of the invention, a method of making a conductive article on a substrate includes providing a layer of nanotubes on a substrate. The nanotube layer comprises at least two unaligned nanotubes, the nanotubes being substantially parallel to a surface of the substrate. The layer is selectively patterned to define an electrically conductive element having the unaligned nanotubes.
In the Drawing,
FIGS. 2A-B illustrate two states of a memory cell according to certain embodiments of the invention;
New electromechanical memory arrays and methods for making same are disclosed in which electromechanical memory cells are created that operate analogously to the NTWCM devices disclosed in WO 01/03208, which is hereby incorporated by reference in its entirety. However, unlike the NTWCM devices disclosed in WO 01/03208, new ribbons or belts made from a matted layer of nanotubes or a non-woven fabric of nanotubes are used as an electrically conductive element. At points in this disclosure, the ribbons are referred to as traces or electrically conductive articles. In some instances, the ribbons are suspended, and in other instances they are disposed on a substrate. In some instances, they are used to deflect to certain states under electrical control, and in other instances they do not move and instead are used simply to carry an electrical current or voltage. The new nanotube belt structures are believed to be easier to build at the desired levels of integration and scale (in number of devices made) and the geometries are more easily controlled. The new nanotube ribbons are believed to be able to more easily carry high current densities without suffering the above-outlined problems experienced or expected with metal traces.
Under certain embodiments of the invention, electrically conductive articles may be made from a nanotube fabric, layer, or film. Carbon nanotubes with tube diameters as little as 1 nm are electrical conductors that are able to carry extremely high current densities, see, e.g., Z. Yao, C. L. Kane, C. Dekker, Phys. Rev. Lett. 84, 2941 (2000). They also have the highest known heat conductivity, see, e.g., S. Berber, Y.-K. Kwon, D. Tomanek, Phys. Rev. Lett. 84, 4613 (2000), and are thermally and chemically stable, see, e.g., P. M. Ajayan, T. W. Ebbesen, Rep. Prog. Phys. 60, 1025 (1997). However, using individual nanotubes is problematic because of difficulties in growing them with suitably controlled orientation, length, and the like. Creating traces from nanotube fabrics allows the traces to retain many if not all of the benefits of individual nanotubes. Moreover, traces made from nanotube fabric have benefits not found in individual nanotubes. For example, since the traces are composed of many nanotubes in aggregation, the trace will not fail as the result of a failure or break of an individual nanotube. Instead, there are many alternate paths through which electrons may travel within a given trace. In effect, a trace made from nanotube fabric creates its own electrical network of individual nanotubes within the defined trace, each of which may conduct electrons. Moreover, by using nanotube fabrics, layers, or films, current technology may be used to create such traces.
Nanotube Ribbon Crossbar Memories (NTRCM)
Because the new nanotube belt crossbar memory devices operate analogously to NTWCM, the description of their architecture and principles of operation is brief. Reference may be made to WO 01/03208 for fuller description and background.
The array has a plurality of non-volatile memory cells 103 which can be in an “on” state 105 or “off” state 106. The actual number of such cells is immaterial to understanding the invention but the technology may support devices having information storage capacities equivalent to or larger than modern non-volatile circuit devices.
Each memory cell 103 includes a nanotube ribbon 101 suspended by one or more supports 102 over electrical traces or wires, e.g., 104.
Each crossing of a ribbon 101 and a wire, e.g., 104 forms a crossbar junction and defines a memory cell. Under certain embodiments, each cell may be read or written by applying currents and or voltages to electrodes 112 which are in electrical communication with ribbons 101 or through electrodes (not shown) in communication with traces or wires 104. The supports 102 are made from a layer 108 of silicon nitride (Si3N4). Below layer 108 is a gate oxide layer 109 separating the n-doped silicon traces 104 from an underlying silicon wafer 110.
Referring conjointly to
Under certain embodiments, the nanotube ribbon 101 may be held in position at the supports by friction. In other embodiments the ribbon may be held by other means, such as by anchoring the ribbons to the supports using any of a variety of techniques. This friction can be increased through the use of chemical interactions including covalent bonding through the use of carbon compounds such as pyrenes or other chemically reactive species. Evaporated or spin-coated material such as metals, semiconductors or insulators especially silicon, titanium, silicon oxide or polyimide could also be added to increase the pinning strength. The nanotube ribbons or individual nanotubes can also be pinned through the use wafer bonding to the surface. See R. J. Chen et al., “Noncovalent Sidewall Functionalization of Single-Walled Carbon Nanotubes for Protein Immobilization,” J. Am. Chem. Soc., 123, 2001, 3838-39 and Dai et al., Appl. Phys. Lett., 77, 2000, 3015-17 for exemplary techniques for pinning and coating nanotubes by metals. See also WO01/03208 for techniques.
Under certain preferred embodiments as shown in FIGS. 2A-B, a nanotube ribbon 101 has a width of about 180 nm and is pinned to a support 102 preferably fabricated of silicon nitride. The local area of trace 104 under ribbon 101 forms an n-doped silicon electrode and is positioned close to the supports 102 and preferably is no wider than the belt, e.g., 180 nm. The relative separation 208 from the top of the support 102 to the deflected position where the belt 101 attaches to electrode 206 (see
The nanotube ribbon 101 of certain embodiments is formed from a non-woven fabric of entangled or matted nanotubes (more below). The switching parameters of the ribbon resemble those of individual nanotubes. Thus, the predicted switching times and voltages of the ribbon should approximate the same times and voltages of nanotubes. Unlike the prior art which relies on directed growth or chemical self-assembly of individual nanotubes, preferred embodiments of the present invention utilize fabrication techniques involving thin films and lithography. This method of fabrication lends itself to generation over large surfaces especially wafers of at least six inches. (In contrast, growing individual nanotubes over a distance beyond sub millimeter distances is currently unfeasible.) The ribbons should exhibit improved fault tolerances over individual nanotubes, by providing redundancy of conduction pathways contained with the ribbons. (If an individual nanotube breaks other tubes within the rib provide conductive paths, whereas if a sole nanotube were used the cell would be faulty.) Moreover, the resistances of the ribbons should be significantly lower than that for an individual nanotubes, thus, decreasing its impedance, since the ribbons may be made to have larger cross-sectional areas than individual nanotubes.
Once such a structure 302 is created or provided, the upper surface 306 receives a catalyst 308. For example, under certain embodiments, a catalyst metal 308, containing iron (Fe), molybdenum (Mo), cobalt or other metals, is applied by spin-coating or other application techniques to create a second intermediate structure 310.
A matted layer 312 of nanotubes is then grown into a non-woven fabric of single-walled carbon nanotubes (SWNTs) to form a third intermediate structure 314. For example, the second intermediate structure 310 may be placed into an oven and heated to a high temperature (for example, about 800-1200° C.) while gases containing a carbon source, hydrogen and inert gas, such as argon or nitrogen, are flowed over the upper surface. This environment facilitates the generation or growth of the matted layer or film 312 of single-walled carbon nanotubes. The layer 312 is primarily one nanotube thick and the various tubes adhere to one another via Van der Waals forces. Occasionally, one nanotube grows over the top of another, though this growth is relatively infrequent due to the growth tendencies of the material. Under some embodiments (not shown), the catalyst 308 may be patterned to assist in growing the nanotubes with specific densities either more or less dense as is desired. When conditions of catalyst composition and density, growth environment, and time are properly controlled, nanotubes can be made to evenly distribute over a given field that is primarily a monolayer of nanotubes. Proper growth requires control of parameters including but not limited to catalyst composition and concentration, functionalization of the underlying surface, spin coating parameters (length and RPM), growth time, temperature and gas concentrations.
A photoresist may then be applied to the layer 312 and patterned to define ribbons in the matted layer of nanotubes 312. The ribbon patterns cross (for example, perpendicularly) the underlying traces 104. The photoresist is removed to leave ribbons 101 of non-woven nanotube fabric lying on planar surface 306 to form fourth intermediate structure 318.
The fourth intermediate structure 318 has portions 320 of its underlying sacrificial layer 304 exposed as shown. The structure 318 is then treated with an acid, such as HF, to remove the sacrificial layer 304, including the portion under the ribbons 101, thus forming an array 322 of ribbons 101 suspended over traces 104 and supported by supports 102.
Subsequent metalization may be used to form addressing electrodes, e.g., 112 shown in
One aspect of the above technique is that the various growth, patterning, and etching operations may use conventional techniques, such as lithographic patterning. Currently, this may entail feature sizes (e.g., width of ribbon 101) of about 180 nm to as low as 130 nm, but the physical characteristics of the components are amenable to even smaller feature sizes if manufacturing capabilities permit.
As will be explained below, there are many possible ways of creating the intermediate structures or analogous structures described above.
A silicon wafer 400 is provided with an oxide layer 402. The oxide layer is preferably a few nanometers in thickness but could be as much 1 □m. A silicon nitride (Si3N4) layer 404 is deposited on top of the oxide surface 402. The silicon nitride layer is preferably at least 30 nm thick.
The silicon nitride layer 404 is then patterned and etched to generate cavities 406 to form support structure 407. With modern techniques the cavity width may be about 180 nm wide or perhaps smaller. The remaining silicon nitride material defines the supports 102 (e.g., as row, or perhaps columns).
A covering 408 of n-doped silicon is then deposited to fill the cavities 406. The covering 408 for exemplary embodiments may be about 1 □m thick but may be as thin as 30 nm.
The covering 408 is then processed, for example by self-flattening of thick silicon layers or by annealing, to produce a planar surface 306, discussed above, to form structure 411. In the case of self-flattening, reactive ion etching (RIE) with end-point detection (EPD) may be utilized until the upper surface 410 of the etched silicon nitride is reached.
The structure 411 is then oxidized to form and define sacrificial layers 304 of SiO2 about 10-20 nm deep into planar surface 306.
The unconverted, remaining portions of silicon form traces 104.
After the layer 514 is added, an annealing step is performed to yield a planarized surface 306 to form a structure 411 like that described above. The annealing step causes the silicon of layer 514 to flow into the cavities 406.
Like that described in connection with
The silicon nitride layer 602 is then patterned and etched to generate spacings 606 and to defined supports 102. The etching process exposes a portion 608 of the surface of silicon substrate 600.
The exposed silicon surface 608 is oxidized to generate a silicon dioxide (SiO2) layer 610 having a thickness of a few nm. These layers 610 eventually insulate traces 104 analogously to the way insulating layer 109 did for the above-described structures 302.
Once the insulating layers 610 have been created, the traces 104 may be created in any of a variety of manner.
Reactive ion etching (RIE) or the like may then be used to etch the Si3N4 layer 704 to form cavities 708 and to define supports 102.
Afterwards, n-doped silicon 710 may be deposited in the cavities 708. Under certain embodiments silicon is deposited to a height about equal to the height 712 of the Si3N4 supports 102.
The photoresist 706 and silicon 710 on top of the photoresist 706 are then stripped away to form an intermediate structure 411 like that described above.
The structure 411 is then oxidized to generate the sacrificial SiO2 layers 304.
The top silicon dioxide (SiO2) layer 808 is patterned by photolithography to create an RIE mask 810. The mask is used to etch the exposed portions 812 of second silicon layer 806 down to the first silicon dioxide layer 804. This etching creates cavities 814 and defines traces 104.
The cavities 814 are filled and covered with silicon nitride (Si3N4) 816.
The Si3N4 covering 816 is backetched with RIE to the same height 818 as the remaining portions of the SiO2 layer 806 covering the n-doped silicon electrodes 104 (which form the sacrificial layer 304).
The exposed metal electrodes 902 may then be removed by wet chemical methods or dry chemical methods. This forms an intermediate structure 411′ like the structure 411 described above, but with a buried electrode 903, as an artifact of the silicon growing process.
The structure 411′ is then oxidized to form sacrificial layers 304 at the exposed portions of silicon, as described above. For example, the layers 304 may be grown to a thickness of about 10 nm.
Using nitridization techniques, exposed portions 1010 of n-doped silicon layer 1006 are chemically converted to Si3N4 supports 102. The unconverted portions of layer 1006 form traces 104.
The mask 1008 is removed forming a structure 411 like that described above.
The exposed portions 1012 of silicon surface are then oxidized to form the SiO2 sacrificial layers 304.
The surfaces of traces 104 are oxidized to form the SiO2 layer 1106 which acts as an alternative form of sacrificial layer 304′.
The structure is overgrown with Si3N4 1108 and back etched to form a planar surface 306 and to form alternative first intermediate structure 302′″. As will be evident to those skilled in the art, under this approach, when the sacrificial layer is subsequently removed, traces 104 will be separated from supports 102. Other variations of this technique may be employed to create alternative transverse cross-sections of trace 104. For example, the traces 104 may be created to have a rounded top, or to have a triangular or trapezoidal cross section. In addition, the cross section may have other forms, such as a triangle with tapered sides.
As was explained above, once a first intermediate structure is formed, e.g., 302, a matted nanotube layer 312 is provided over the planar surface 306 of the structure 302. In preferred embodiments, the non-woven fabric layer 312 is grown over the structure through the use of a catalyst 308 and through the control of a growth environment. Other embodiments may provide the matted nanotube layer 312 separately and apply it directly over the structure 302. Though structure 302 under this approach preferably includes the sacrificial layer to provide a planar surface to receive the independently grown fabric, the sacrificial layer may not be necessary under such an approach.
Because the growth process causes the underside of such nanotubes to be in contact with planar surface 306 of intermediate structure 302, they exhibit a “self-assembly” trait as is suggested by
In certain embodiments, nanotubes grow substantially unrestrained in the x- and y-axis directions, but are substantially restricted in the z-axis (perpendicular to page of
A plan view of the matted nanotube layer 312 with underlying silicon traces 104 is shown in
As explained above, once the matted nanotube layer 312 is provided over the surface 306, the layer 312 is patterned and etched to define ribbons 101 of nanotube fabric that cross the supports 102. The sacrificial layer is then removed (e.g., with acid) forming the array 322 described above in connection with
Subsequent metalization may be used to form addressing electrodes, e.g., 112 shown in
More specifically, under certain embodiments described above, nanotubes are used to form NTRCM arrays. Certain embodiments use nanotube technology, whether in individual wire or belt form, to implement addressing logic to select the memory cell(s) for reading or writing operations. This approach furthers the integration of nanotube technology into system design and may provide beneficial functionality to higher-level system design. For example, under this approach the memory architecture will not only store memory contents in non-volatile manner but will inherently store the last memory address.
The nanotube-based memory cells have bistability characterized by a high ratio of resistance between “0” and “1” states. Switching between these states is accomplished by the application of specific voltages across the nanotube belt or wire and the underlying trace, in which at least one of the memory cell elements is a nanotube or a nanotube ribbon. In one approach, a “readout current” is applied and the voltage across this junction is determined with a “sense amplifier.” Reads are non-destructive, meaning that the cell retains its state, and no write-back operations are needed as is done with DRAM.
A perpendicular intersection of two lines 1404 and 1406 depicted as a dot 1402 indicates a junction of two nanotubes or nanotube ribbons. In this regard, the interaction is analogous to a “pass transistor” found in CMOS and other technology, in which the intersection may be opened or closed.
Locations such as 1420 where one nanotube or nanotube ribbon may cross another but which are not intended to create a crossbar junction may be insulated from one another with a lithographically patterned insulator between the components.
For the sake of clarity, the decoder illustrated is for a 3-bit binary address carried on addressing lines 1408. Depending on the value of the encoding the intersections (dots) will be switched to create only one path through which sensing current I may pass to select lines 1418.
To use this technique, a “dual rail” representation 1408 of each bit of the binary address is fashioned externally so that each of the address bits 1410 is presented in true and complementary form. Thus, line 1406 may be the logical true version of address line 1408 a and line 1407 may be the logical complement of address line 1408 a. The voltage values of the representation 1408 are consistent with that needed to switch a crossbar junction to the “1” or “0” state as described above.
In this fashion an address 1408 may be used to supply a sense current I to a bit or row of bits in an array, e.g., to nanotubes or nanotube ribbons. Likewise, the same approach may be used to sense a given trace, for example, selecting specific array column(s) to read sense from in conjunction with selecting a row. Thus this approach may be used for X and/or Y decoding both for reading and for writing operations.
Certain embodiments of the invention provide a hybrid technology circuit 1500, shown in
In other embodiments, the X and Y address decoders 1504 and 1506 may be substituted with the nanotube wire or belt addressing technique discussed above. In these embodiments the core would include memory cells and addressing logic.
In certain embodiments, the hybrid circuit 1500 may be formed by using a nanotube core (having either just memory cells or memory cells and addressing logic) and by implementing the surrounding circuitry using a field programmable gate array. The core and gate array circuitry may be contained in a single physical package if desired. Or, they may be packaged separately. For example, a hermitically packaged nanotube circuit (having memory or memory and addressing logic) may be combined with a PLD/FPGA/ASIC in which the I/O interfacing logic is contained. The resulting compact chipset provides access to the benefits of the NT memory for the user of the product, while maximizing the use of “off-the-shelf” technologies, which may be utilized on an as-needed basis by the manufacturer.
This particular embodiment is suggested to conform to the PCI bus standard, typical of today's personal computers. Other passive circuitry, such as capacitors, resistors, transformers, etc. (not pictured) would also be necessary to conform to the PCI standard. A front-side bus speed of 200 MHz-400 MHz is annotated, suggesting the kinds of external clock speeds such a chipset might run at. This speed is limited by the PCB interconnects and FPGA/PLD/ASIC speed, and also the chip packages, not the NT memory cell speed.
Carbon Nanotube Films, Layers, Fabrics, and Articles
The above embodiments of NTRCM and addressing lines use traces or electrically conductive articles made from nanotube layers 312, such as those shown in
The traces and electrically conductive articles may be used in other forms of circuits. For example, nanotube traces may be used for their ability to withstand high current densities, normally found in very small sized traces (e.g., sub 10 nm regimes). They may also be used to reduce the likelihood of contaminating other circuit features.
If the film is to be grown, a catalyst may be used, as described above. However, the catalyst (shown as 308 in
If the film 312 is to be deposited, pre-grown nanotubes may be used. For example, under certain embodiments of the invention, nanotubes may be suspended in a solvent in a soluble or insoluble form and spin-coated over the surface to generate the nanotube film 312. In such an arrangement the film may be one or more nanotubes thick, depending on the spin profile and other process parameters. Appropriate solvents include dimethylformamide, n-methylpyrollidinone, n-methyl formamide, orthodichlorobenzene, paradichlorobenzene, 1,2, dichloroethane, alcohols, water with appropriate surfactants such as sodium dodecylsulfate or TRITON X-100 or others. The nanotube concentration and deposition parameters such as surface functionalization, spin-coating speed, temperature, pH and time can be adjusted for controlled deposition of monolayers or multilayers of nanotubes as required.
The nanotube film 312 could also be deposited by dipping the wafer or substrate in a solution of soluble or suspended nanotubes. The film could also be formed by spraying the nanotubes in the form of an aerosol onto a surface.
When conditions of catalyst composition and density, growth environment, and time are properly controlled, nanotubes can be made to evenly distribute over a given field that is primarily a monolayer of nanotubes.
Upon formation of the nanotube matte 312, a photoresist layer may be spin-coated on the nanotube film 312 and patterned by exposure or the like to define conductive traces. In the example of
Once so defined, the exposed photoresist may be processed to remove some of the layer but to leave the traces 101. Subsequent metallization may be used to form addressing electrodes or a fanned interconnect structure, e.g., 1706 shown in
With reference to
While these interconnects may primarily be formed of a monolayer of nanotubes, multilayer ribbons and mattes can also be envisioned using proper growth conditions. This requires control of parameters including but not limited to catalyst composition and concentration, functionalization of the underlying surface, spin coating parameters (length and RPM, for example 40 seconds, 50-5000 rpm), growth time, temperature and gas concentrations.
One aspect of the above technique is that the various growth, deposition, patterning, and etching operations may use conventional techniques, such as lithographic patterning. With current technology, traces may be made to have widths of about 180 nm to as low as 130 nm. However, the physical characteristics of the traces 101 are amenable to even smaller feature sizes if manufacturing capabilities permit.
Conventional interconnect technologies have a tendency to suffer from thermal damage and metal diffusion eroding the performance of the semiconductor devices especially from degradation of the electrical properties. These effects become even more pronounced with size reduction in current generation 0.18 um and 0.13 um structures, e.g. by metal diffusion through ultra-thin gate oxide layers. In contrast, carbon nanotube ribbons 101 are not beset with these problems. They are substantially more robust having the highest known thermal conductivities and are not prone to thermal failure. Furthermore, no metal or dopant diffusion can occur since they are constructed entirely of covalently bound carbon atoms.
Afterwards, n-doped silicon or metal such as molybdenum, tungsten or tantalum 1910 and a sacrificial layer 1912 such as aluminum oxide may be deposited in the cavities 1906, also forming corresponding features 1914 and 1916.
The photoresist 1912, material 1914 and aluminum oxide (Al2O3) 1916 on top of the photoresist 1912 are then stripped away to form an intermediate structure 1918 with electrodes 104 and sacrificial layer 304. A spin-on-glass (SOG) such as flowable oxide (FOX) is spin-coated over the structure 1918 and annealed using a ramped temperature protocol at 600° C. using standard techniques forming a SiO2 layer 1920 at a height of from 200-2000 nm above the top of the sacrificial layer 1912.
Reactive ion etching (RIE) or the like may then be used to etch the SiO2 layer 1920 to form a structure 302 with supports 102.
The choice of electrode material is limited by the method by which the nanotubes are placed upon the substrate surface. The three above methods include spin-coated catalyst-based growth, gas-phase catalyst-assisted CVD and spin-coating or direct deposition of nanotubes. In the case of the catalyst-based growth as has been described above the catalyst is distributed on the surface either by spin-coating, or dipping the substrate in the catalyst material followed by standard washing protocols. In each of these cases the nanotubes are then grown via a CVD process at 800° C. using a combination of hydrogen and carbon-containing precursor gas as has been described above. Thus, electrode materials which are sufficiently robust to survive these temperatures would be preferred including molybdenum, tungsten, tantalum, germanium, copper and alloys thereof. The electrode material can be constructed of a single or stacked structure of materials including silicon, tungsten, molybdenum, tantalum, copper and others. The stacked electrode structure may assist with or be sufficient in creating a Schottky barrier sufficient for rectification of each memory bit.
In the event that the nanotubes are grown using a gas-phase catalyst such as ferrocene, it is possible to envision substantially lower temperatures being required for growth allowing the use of electrode materials that melt at a substantially lower temperature less than 800° C. and as low as 400° C. Some gas-phase catalysts of interest may include cobalt, tungsten, molybdenum or rhenium metallocenes containing five of six-membered rings. These compounds can with the proper knowledge of inorganic chemistry be synthesized and brought by the use of a bubbler into the gas-phase to act as nucleation sites on substrates for nanotube growth. Of course these materials would be substantively compatible with the typical CMOS processes known in the literature and used by standard industrial fabrication facilities.
In the event that nanotubes are deposited on a surface at room temperature by spin-coating of a solution or suspension of nanotubes then the choice of electrode materials is expanded substantially. In this case there is no high temperature step and any metal typically compatible with standard CMOS metallization conditions would be acceptable especially, aluminum, and alloys thereof.
The sacrificial layer 304 can be constructed of Al2O3, metal oxides, salts, metals and other materials. The intermediate structure 302 can be formed using a variety of materials to form the supports 102 including SOG, SiO2 and others. In the event that a low temperature spin-coating of nanotube protocol is chosen the materials suitable to be sacrificial layers expands substantially. This could include materials such as PMMA or other polymers, metals such tungsten, chromium, aluminum, bismuth and other transition and main group metals. Also other semiconductors such as germanium and insulators such as salts, oxides and other chalcogenides.
The choice of materials for the support layer greatly depends upon the method chosen for nanotube growth and other factors. In the even that a low-temperature process is chosen for placing nanotubes on the surface, one can envision utilizing such materials as Al2O3, silicon monoxide, semiconductors, insulators and polymers such as polyimide.
The materials selection process is confined to those materials that are compatible with the fabrication process described above. It is understood by those sufficiently skilled in the art that upon selection of a particular electrode material, the sacrificial layer and support materials naturally become limited based upon typical processing steps available in semiconductor fabrication. Likewise if a particular sacrificial layer is chosen the choices of electrode and sacrificial layer materials is suitably limited. Furthermore, upon selection of a particular support material it follows that the electrode and sacrificial layer materials choice is likewise limited.
Though most of the disclosure above is written as if the fabric were made of nanotubes of the same type, e.g., all single walled, the fabrics may be composed of all multi-walled structures or of a combination of single- and multi-walled structures.
In order to facilitate the growth of interconnects or electrode materials it may become useful to first form a pattern using standard lithographic methods to define regions where the nanotubes are intended to grow in a horizontal fashion over the surface. Such an approach has been used to pattern SiO2 structures to grow thick multiwalled vertical nanotubes. In a similar approach patterned SiO2 can be used for the purpose of growing horizontal nanotube films with a thickness of 1-1000 nm to create structures of the form described above such as 101. Other materials which provide a support for nanotube growth and nucleation such as insulators and metal oxides may be useful when used in concert with properly chosen gas-phase metallocenes or other vaporizable metallic precursors to yield patterned nanotube ribbons. This underlying patterned layer could also act as a sacrificial layer which upon removal would form a suspended nanotubes. This method of growth represents a form of “positive” growth whereby the nanotubes use the prepatterned surface as a nucleation site.
In a further embodiment one can envision using a “negative” growth method whereby the lithographically patterned substrate contains a metallic or other material which does not support nanotube growth. When a proper gas-phase precursor such as a metallocene or similar compound is supplied the nanotubes would substantively grow only in the regions without the patterned material. The removal of an underlying material could provide suspended nanotubes 101 or interconnect structures upon the removal of the patterned metallic species.
In yet another embodiment, instead of using wet-chemical removal of sacrificial layer to suspend nanotubes at specific height over electrodes, a controlled etch of the electrode (i.e. 15 nm etch of 0.18 um wide electrode) can be used; e.g. metal (e.g. copper) and semiconductor (e.g. silicon) electrodes can be etched at etch rates of a few nanometer per second.
In another embodiment pinning of nanotubes onto the supports using an overlayed thin coating to prevent slipping of tubes during operation. This would open “windows” just over the memory cell itself.
The electrical properties of the layers and electrically conductive articles can be tuned by controlling the cross section of the nanotube ribbons. For example, the ribbon thickness may be increased at a given width and nanotube density. The higher the cross section, the greater the number of conduction channels leading to enhanced electrical properties.
The method of preparing of the nanotube ribbons allows continuous conductivity even over rough surface topologies. In contrast, typical evaporation of metallic electrodes would suffer from structural and thus, electrical defects.
Besides carbon nanotubes other materials with electronic and mechanical properties suitable for electromechanical switching could be envisioned. These materials would have properties similar to carbon nanotubes but with different and likely reduced tensile strength. The tensile strain and adhesion energies of the material must fall within a range to allow bistability of the junction and electromechanical switching properties to exist within acceptable tolerances.
For the purpose of integrating CMOS logic for addressing two approaches can be envisioned. In the first embodiment the nanotube array will be integrated before metallization but after ion implantation and planarization of the CMOS logic devices. A second method involves growth of the nanotube arrays before fabrication of the CMOS devices involving ion implementation and high temperature annealing steps. Upon completion of these steps the final metallization of both the nanotube ribbons and the CMOS devices will proceed using standard and widely used protocols.
Electrodes consisting of n-doped silicon on top of some metal or semiconductor line can also be envisioned. This will still provide rectifying junctions in the ON state so that no multiple current pathways exist.
In addition to rectifying junctions, there are other widely accepted and used methods to prevent the occurrence of electrical crosstalk (i.e. multiple current pathways) in crossbar arrays. Tunnel barriers on top of the static, lithographically fabricated electrodes prevent the formation of ohmic ON states. No leakage currents at zero bias voltage will occur but a small bias voltage has to be applied for the charge carriers to overcome this barrier and tunnel between the crossing lines.
Methods to increase the adhesion energies through the use of ionic, covalent or other forces can be envisioned to alter the interactions with the electrode surfaces. These methods can be used to extend the range of bistability with these junctions.
Nanotubes can be functionalized with planar conjugated hydrocarbons such as pyrenes which may then aid in enhancing the internal adhesion between nanotubes within the ribbons.
Certain of the above aspects, such as the hybrid circuits and the nanotube technology for addressing, are applicable to individual nanotubes (e.g., using directed growth techniques, etc.) or to nanotube ribbons.
It will be further appreciated that the scope of the present invention is not limited to the above-described embodiments but rather is defined by the appended claims, and that these claims will encompass modifications of and improvements to what has been described.