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The mechanism of dense vertically aligned carbon nanotube growth achieved by a recently developed thermal chemical vapor deposition method was studied using synchrotron radiation spectroscopic techniques., The mechanism of the one-order-of-magnitude increase in the density of vertically aligned carbon nanotubes (CNTs) achieved by a recently developed thermal chemical vapor deposition process was studied using synchrotron radiation spectroscopic techniques. In the developed process, a Ti film is used as the underlayer for an Fe catalyst film. A characteristic point of this process is that C2H2 feeding for the catalyst starts at a low temperature of 450°C, whereas conventional feeding temperatures are ∼800°C. Photoemission spectroscopy using soft and hard X-rays revealed that the Ti underlayer reduced the initially oxidized Fe layer at 450°C. A photoemission intensity analysis also suggested that the oxidized Ti layer at 450°C behaved as a support for nanoparticle formation of the reduced Fe, which is required for dense CNT growth. In fact, a CNT growth experiment, where the catalyst chemical state was monitored in situ by X-ray absorption spectroscopy, showed that the reduced Fe yielded a CNT forest at 450°C. Contrarily, an Fe layer without the Ti underlayer did not yield such a CNT forest at 450°C. Photoemission electron microscopy showed that catalyst annealing at the conventional feeding temperature of 800°C caused excess catalyst agglomeration, which should lead to sparse CNTs. In conclusion, in the developed growth process, the low-temperature catalyst activation by the Ti underlayer before the excess Fe agglomeration realised the CNT densification.

Carbon-based materials, such as carbon nanotubes and graphene nanoribbons, have been studied as interconnect materials because of their lower resistivity, higher thermal conductivity, and intrinsically higher current carrying capacity compared to Cu. We have succeeded in growing networked-nanographite (NNG) films on dielectric layers without any metal catalysts, and fabricated NNG wires as the first step toward multi-layer graphene interconnects. In this study, we investigated the electrical properties and reliability, such as current tolerance and thermal stability, of these NNG wires. In addition, we investigated the mechanism of resistivity decreases in NNG. Our results show that the NNG wires can sustain a higher current density than Cu wires. However, their resistivity gradually decreases due to thermal stress. Raman and X-ray photoelectron spectroscopy revealed that the decrease in resistivity can be attributed to an increase in sp2 bonding corresponding to the formation of a graphene sheet. It is important to clarify the mechanism of the decrease in resistivity, not only to improve thermal stability but also to obtain lower resistivity in carbon interconnects.

Selective synthesis of carbon nanotubes (CNTs) and graphene was achieved by hot-filament chemical vapor deposition (CVD) using iron films as a catalyst at a temperature of 620 °C. The selective synthesis was performed in a very simple way, just by changing the catalyst thickness under the same CVD conditions. Only CNTs were obtained with Fe films of 5 nm and less, while multi-layer graphene was formed with the catalyst films of 20 nm and more. The thickness of multi-layer graphene can also be controlled by controlling the thickness of the catalyst further.

We are trying to employ nano-carbon materials, such as graphene and carbon nanotubes (CNTs), as channel and interconnect materials to realize low-power-consumption large-scale integrated circuits (LSIs). In this paper, we first explain our recent progress on the application of graphene to transistor channels. Graphene synthesis on a 200-mm Si wafer by chemical vapor deposition (CVD) and electrical properties of CVD-graphene transistors are described. Especially, details of graphene growth on Cu film, such as nucleation behavior of graphene islands depending on the growth condition, are addressed. Efforts to realize CNT interconnects at MIRAI-Selete are also reviewed. Special emphases are placed on the fabrication process of CNT vertical interconnects and their reliability.

Multiwalled carbon nanotubes (MWCNTs) were synthesized at 390 °C in a hydrogen-free atmosphere by graphite antenna chemical vapor deposition, which provides carbon radicals by the etching of the antenna itself in a He- or Ar-based noble gas plasma. An increase in the number of defects in the graphite layers of the CNTs was observed with increasing hydrogen partial pressure. The results of X-ray photoelectron spectroscopy analysis suggested that these defects were caused by the transformation from an sp 2 to an sp 3 structure in the graphite layers of CNTs due to hydrogen radicals.

Thermal conductivity of a multi-walled carbon nanotube (MWNT) was evaluated using the ΔVgs method for the first time. MWNTs were prepared in pillars, and the pillars were placed on two aluminum nitride (AlN) substrates. An AlGaN/GaN high electron mobility transistor (HEMT) chip, which was set on an AlN substrate, was used as a thermometer and as a thermal source. The thermal resistance of the sample was measured using the ΔVgs method. From the thermal resistance of MWNT pillars, we succeeded in evaluating the thermal conductivity of a MWNT pillar and that of a MWNT as high as 74.2 and 950 W/(mK), respectively. The thermal conductivity of MWNTs evaluated in this study is much higher than that of usual metals. We confirmed that MWNTs are promising materials for heat dissipation problems.

We propose a new approach to fabricating carbon nanotube (CNT) via interconnects for future LSIs, which uses preformed catalyst nanoparticles to grow the CNTs. A newly designed impactor provided size-classified catalyst particles. For the new approach, we employ a TiN contact layer which is more resistant to oxidation and enables us to form a lower resistance ohmic contact between CNTs and wiring layers. The resultant CNT–via resistance was 0.59 Ω for 2-µm vias, which is the smallest ever reported. We also study the CNT nucleation process using molecular dynamics and discuss how to remove amorphous carbon from the nucleation process. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We developed a fast dry oxidation process for chemical vapor deposition (CVD) grown multiwalled carbon nanotubes (MWNTs) using short-wavelength UV [vacuum ultraviolet (VUV) light (λmax=172 nm)] irradiation. We introduced carboxyl groups as many as 3% of carbon atoms within 1 min at room temperature using this process. MWNTs with the carboxyl groups were analyzed by attenuated total reflection (ATR) Fourier transform infrared absorption spectroscopy (FT-IR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The attachment of the carboxyl groups was further confirmed by reacting ferritin molecules with the MWNTs. The developed dry process with a high reaction rate is a powerful tool for the applications of carbon nanotubes in nanoelectronics.

Bimetallic particles of Ti and Co have been used to grow multi-walled carbon nanotubes (MWNTs) to study the effects of Ti on the growth. The particles were generated by laser ablation and size-classified with a differential mobility analyzer. Diameter-controlled MWNTs were grown on a silicon substrate from 5-nm particles by thermal chemical vapor deposition using acetylene at 510 and 610 °C. Titanium has been found to enhance the MWNT growth significantly. There is an optimum Ti fraction for the growth, as confirmed by evaluation of the growth probabilities of MWNTs. A possible mechanism of growth enhancement by Ti is discussed.

We have successfully developed control of the growth mode of carbon nanotubes (CNTs) by using titanium film and its oxidized form, and measured physical properties. The growth mode of CNTs can be controlled by the under layers of catalysts. Obtained CNT films have the site density of 1010 cm-2 for both the base- and tip-growth modes. Measured electrical resistance of 2-µm-diameter CNT-via, mechanical strength, and thermal conductivity for the base-growth mode CNT films were 91.1 Ω, 670 kPa and 291 W/mK, respectively. Better physical properties were observed for base-growth mode CNT films, compared with the tip-growth mode CNT films. In the base-growth mode, one CNT with a TiC contact had an estimated low electrical resistance of 100 kΩ, a high mechanical strength of 5.5 MPa, and a high thermal conductivity of 2400 W/mK.

By using plasma-enhanced chemical vapor deposition (p-CVD), we grew vertically aligned multi-walled carbon nanotubes (CNTs) directly on a nickel-monosilicide layer, without any catalysts other than a metal-silicide layer. The results we obtained are quite important for practical application of CNT vias formed directly on nickel-monosilicide (NiSi) layers, which form the source, drain, and gate region of metal-oxide-semiconductor field-effect transistors (MOSFETs). By using a nickel-silicide layer as a catalyst, the nanotube diameter became smaller than that obtained with a nickel film catalyst. We have also clarified the possibility that CNT diameter can be controlled by forming various Ni-silicide phases such as Ni2Si, NiSi and NiSi2. We suggest that Ni-silicide composition plays an important role in controlling the diameter of the nanotubes. These results will enable us to fabricate CNT vias for future LSI interconnects.

Diameter-controlled multiwalled carbon nanotubes (MWNTs) have been grown by hot-filament chemical vapor deposition using nickel nanoparticles as catalyst. The nanoparticles were generated by laser ablation, classified with a differential mobility analyzer, and deposited onto silicon substrate. The particle size is tunable down to 2–3 nm, and particles with a geometric mean diameter of 5.1 nm (geometric standard deviation: 1.1) were used for carbon-nanotube growth. MWNTs were grown on the substrate using acetylene at 550 °C. The particles did not coalesce during growth, and the MWNTs had outer diameters matching the particle sizes, indicating that the current method can produce diameter-controlled MWNTs.

We have succeeded in making peapod structures with vertically aligned multi-walled carbon nanotubes (MWNTs), grown on a silicon substrate using hot filament chemical vapor deposition. Opening the tips of the MWNTs was performed at 600 ∘C using oxygen and then C60 molecules were deposited on them to fill their inner spaces. We confirmed that the MWNTs were open at the tips and filled with C60 molecules using transmission electron microscopy. In addition, it has been determined that MWNTs can be filled with other materials, such as molybdenum oxide.

We have developed a method of directing or bending carbon nanotubes (CNTs) in the desired direction along a structure. The new method utilizes careful positioning of a catalyst with respect to the structure, so that the CNTs growing from the catalyst experience an effective van der Waals force from the structure. The force causes a CNT-bundle to grow in the desired direction. For instance, a CNT-bundle coming out of a hole can be bent to the right or left depending on the catalyst patterning at the bottom. This approach is promising for achieving three-dimensional CNT wiring for future electronic devices.

We have developed a novel process, the implantation of carbon nanotubes (CNTs) into holes like plugs, vias, and through-silicon vias (TSVs) for the first time. The developed low-temperature process is suitable for back-end-of-line (BEOL) processes of LSI. With this approach, we can use high-quality CNTs grown at high temperature on a different substrate and densify CNTs. Consequently, the implanted CNT plugs had a resistance one order of magnitude lower than the directly grown CNT plugs. In addition, we successfully integrated the implanted CNT plug with transferred multilayer graphene (MLG) wires for all carbon interconnects.

We report numerical simulations of heat dissipation properties of nano-carbon through silicon via (TSV), thermal interface material (TIM), and chip package towards a high heat dissipation LSI 3-D packaging. By using vertically aligned multi-walled CNTs (MWNTs) as both TSV and TIM materials and graphite as chip package, a boundary temperature just under a heat source decreased 40.8K in total, comparing to that using conventional materials. This result suggests superior heat dissipation properties of nano-carbon 3-D packaging.

The Fe/Ti catalyst system that was recently found to be effective for the growth of dense carbon nanotube (CNT) forests was studied using hard x-ray photoemission spectroscopy (HAXPES). It was previously found that the Ti support layer was partially oxidized at room temperature (RT) and absorbed oxygen from the Fe overlayer at higher temperatures, which gave rise to the dense CNT forest growth. The aim of the present study was to elucidate the reason for the initial oxidation of the Ti layer at RT, which remained unclear from the results of the previous study. The control of the initial Ti oxidation is important because it could affect the reduction and activation of the Fe layer at higher temperatures. Depth-dependent HAXPES measurements using different x-ray incidence angles revealed that the degree of oxidation of the 1-nm-thick Ti layer in a sample that had been aged at RT for approximately three months varied depending on the depth. This suggests that oxidation of the Ti layer proceeded after the Fe/...

By using plasma-enhanced chemical vapor deposition (p-CVD), we grew vertically aligned multiwall carbon nanotubes (CNTs) directly on a nickel-silicide layer, which can be used as electrodes for metal-oxide-semiconductor field-effect transistors (MOSFETs). By using a nickel-silicide layer as a catalyst, the nanotube diameter became smaller than that possible with a nickel film catalyst. We suggest that Ni-silicide composition plays an important role in controlling the diameter of the nanotubes. To our knowledge, this is the first report on diameter-controlled vertically aligned CNT growth on catalytic metal-silicide substrates.

With the aim of achieving low-power consumption, high-performance LSIs, our work focuses on the development of low-resistance carbon nanotube (CNT)/graphene interconnect technologies. For vertical interconnects, we report on a previously unseen dense vertical and horizontal graphene (DVHG) structure, which is expected to lead to a low electrical resistance. For horizontal interconnects, we have succeeded in forming multi-layer graphene (MLG) directly on SiO 2 by annealing sputtered amorphous carbon. Furthermore, achieving low-resistance contacts between the vertical CNTs and horizontal graphene represents a critical issue for 3D interconnects. We grew vertically aligned CNTs on MLG, and analyzed the contact structure using cross-sectional TEM-EELS measurements. Migration of Ti and Co into the graphene layers was clearly observed. This may contribute to the reduction of contact resistance between CNTs and graphene.

Carbon nanotubes (CNTs) offer unique properties such as the highest current density, ballistic transport, ultrahigh thermal conductivity, and extremely high mechanical strength. Because of these remarkable properties, they have been expected for use as wiring materials and as alternate channel materials for extending complementary metal-oxide-semiconductor (CMOS) performance in future very large scale integration (VLSI) technologies. In this paper, we report the present status of CNT growth technologies and the applications for via interconnects (vertical wiring) and field-effect transistors (FETs). We fabricated CNT via and evaluated its robustness over a high-density current. In our technology, multiwalled carbon nanotubes (MWNTs) were successfully grown at temperatures as low as 365°C using Co catalyst nanoparticles, which were formed and deposited by a custom-designed particle generation and deposition system. The density of MWNTs grown at 450°C reaches more than 1×1012/cm2. MWNTs were grown in via holes with a diameter as small as 40 nm. The resistance of CNT vias with a diameter of 160 nm was found to be of the same order as that of tungsten plugs. The CNT via was able to sustain a current density as high as 5.0×106A/cm2 at 105°C for 100 h without any deterioration in its properties. We propose a Si-process compatible technique to control carrier polarity of CNFETs by utilizing fixed charges introduced by the gate oxide. High-performance p- and n-type CNFETs and CMOS inverters with stability in air have been realized.

Carbon nanotubes (CNTs) are attractive as nanosize structural elements from which devices can be constructed by bottom-up fabrication. A CNT is a macromolecule of carbon and is made by rolling a sheet of graphite into a cylindrical shape. CNTs exhibit excellent electrical properties that include current densities exceeding 109 A/cm2 and ballistic transport along the tube. Because of these factors, with their large electro-migration tolerance and low electrical resistance, CNTs can be used as nano-size wiring materials, and are thus becoming potential candidates for future LSI interconnects. Much effort has been made to produce CNT vias, which use bundles of MWNTs (multi-walled carbon nanotubes), as vertical wiring materials as shown in Figure 1. Sato et al. demonstrated low-resistance CNT vias employing a novel metallization technology, which used preformed catalyst metal particles, to grow dense MWNT-bundles by thermal chemical vapor deposition (CVD).

We have improved the high-current reliability of carbon nanotube (CNT) via interconnects by chemical mechanical polishing (CMP) and vacuum in situ metal deposition processes. These processes enable us to decrease the contact resistance of a CNT via to the upper and lower Cu lines, and also increase the number of CNTs contributing to current flow. Consequently, the current density per CNT was decreased, and current tolerance properties were improved. As a result, the CNTs via interconnects were able to withstand a high current density of 4×107 A/cm2 per via, i.e., 1.7×108 A/cm2 per CNT. In addition, we found that the failure mode of Cu-line/CNT-via/Cu-line interconnects with a CNT density of 3×1011 tubes/cm2 was the slit void formation at the Cu line under the via, which is similar to that of Cu via interconnects. Furthermore, we discussed how to further increase the tolerance of electromigration (EM), taking advantage of their high thermal conductivity.

We have succeeded in fabricating ultrafine carbon nanotube (CNT) via interconnects with SiOC interlayer dielectrics. High-quality multiwalled CNTs are grown in via holes with a diameter of 70 nm using pulse-exited remote plasma-enhanced chemical vapor deposition at 450 °C. The resistance of a 70-nm-diameter CNT via is 52 Ω, which is the lowest ever reported for CNT via interconnects. The CNT via interconnect has the capability to sustain current density as high as 1×108 A/cm2.

We fabricated a carbon nanotube (CNT) via interconnect and evaluated its electrical properties. We found that the CNT via resistance was independent of temperatures, which suggests that the carrier transport is ballistic. From the via height dependence of the resistance, the electron mean free path was estimated to be about 80 nm, which is similar to the via height predicted for hp32-nm technology node. This indicates that it will be possible to realize CNT vias with ballistic transport for hp32-nm technology node and below. It was also found that a CNT via was able to sustain a current density as high as 5.0 × 106 A/cm2 at 105 °C for 100 hours without any deterioration.

We fabricated a carbon nanotube (CNT) via interconnect and evaluated its robustness over a high-density current. CNTs were synthesized at temperatures as low as 365 °C, which is probably the lowest for this application, without degrading the ultra low-k interlayer dielectrics (k = 2.6). We measured the electrical properties of CNT vias as small as 160 nm in diameter and found that a CNT via was able to sustain a current density as high as 5.0×106 A/cm2 at 105 °C for 100 hours without any deterioration in its properties.

We studied the electrical properties of a carbon nanotube (CNT) via interconnect fabricated by a novel damascene process which is mostly compatible with conventional Cu interconnects. It was found that the resistance of 60-nm-height vias was independent of temperatures as high as 423 K, which suggests that the carrier transport is ballistic. The obtained resistance of 0.05 Omega for 2.8-mum-diameter vias is the lowest value ever reported. From the via height dependence of the resistance, the electron mean free path was estimated to be about 80 nm, which is similar to the via height predicted for 32-nm technology node (year 2013). This indicates that it will be possible to realize CNT vias with ballistic transport for 32-nm technology node and below.

Multiwalled carbon nanotubes (CNTs) have been grown by remote plasma-enhanced chemical vapor deposition at temperatures as low as 400 °C. In via formation, the selective growth of CNT bundles in via holes at 430 °C and chemical mechanical polishing for planarization have been performed. The electrical evaluation of CNT single vias with various diameters reveals that the via resistance is inversely proportional to the via area. This result indicates that the CNTs are grown with uniform quality and density in the via holes with various diameters and stable contact formations are obtained. Moreover, the resistances of single vias are approximately equivalent to the via resistances estimated from the resistances of via chains, demonstrating the via-to-via uniformity of the CNT vias obtained by the remote plasma-enhanced chemical vapor deposition.

To achieve denser and taller carbon nanotube (CNT) arrays on Cu foils, catalyst and chemical vapor deposition (CVD) conditions were carefully engineered. CNTs were grown to ∼50 µm using Fe/TiN/Ta catalysts in which Ta and TiN acted as diffusion barriers for Cu and Ta, respectively. A tradeoff was found between the mass density and height of the CNT arrays, and CNT arrays with a mass density of 0.30 g cm−3 and height of 45 µm were achieved under optimized conditions. Thermal interface materials (TIMs) with CNT array/Cu foil/CNT array structures showed decreasing thermal resistance from 86 to 24 mm2 K W−1 with increasing CNT array mass densities from 0.07–0.08 to 0.19–0.26 g cm−3 for Cu and Al blocks with surfaces as rough as 20–30 µm. The best CNT/Cu/CNT TIMs showed thermal resistance values comparable to that of a typical indium sheet TIM.

We have successfully improved the weight density of a 40-µm-long carbon nanotube (CNT) to 0.27 g/cm3 by improving the slope control of temperature profile (STEP) growth. It was found that the CNT growth is reaction-limited in the early stage of STEP growth and supply-limited in the late stage. We succeeded in doubling the CNT density over the conventional method by optimizing the raw material supply and temperature slope during the reaction-limited and supply-limited periods. A CNT thermal interface material was fabricated using a CNT film prepared by this method. Thermal resistance was 11% below a conventional indium thermal interface material. Owing to the above reasons, CNTs show promise as heat dissipation materials.

We propose a new approach to fabricating carbon nanotube (CNT) vias, which uses preformed catalyst nanoparticles to grow CNTs. A newly-designed impactor provided size-classified catalyst particles, and a new deposition system injected them into via holes down to 40 nm in diameter. The resultant CNT-via resistance was 0.59 Omega for 2-mum vias, which is the lowest ever reported, improved from the previous studies using catalyst films. The improvement resulted from higher-density CNTs grown in the via holes by employing the nanoparticle catalyst

We have succeeded in lowering the resistance of multi-walled carbon nanotube (MWNT) vias, using parallel channel conduction of each tube's inner shells. By optimizing the structure of the interface between MWNTs and Ti bottom contact layers, we could obtain a via resistance of 0.7 /spl Omega/ for a 2-/spl mu/m-diameter via consisting of about 1000 MWNTs. The corresponding resistance of about 0.7 k/spl Omega/ per MWNT indicates that most of the inner shells contribute to carrier conduction as an additional channel. The total resistance of the CNT vias that we fabricated is in the same order of magnitude as the theoretical value of W plugs and one order of magnitude higher than the theoretical value of Cu vias.

We have developed CNT vias consisting of about 1000 tubes by using a low-temperature CVD method with metal catalysts (Ni or Co), TiC ohmic contacts, and Ta barrier layers on Cu wiring. The total resistance of the CNT via was three orders of magnitude lower than that of one CNT, indicating that the current flows in parallel through about 1000 tubes. We believe this is the first trial demonstration of CNT vias for future LSI interconnects.

We have developed carbon nanotube (CNT) vias consisting of about 1000 tubes. The total resistance of the CNT via was measured as three orders of magnitude lower than the current flows in parallel through about 1000 tubes. There is no degradation observed for 100 hours at the via current density of 2/spl times/10/sup 6/ A/cm/sup 2/, which is favourably with Cu vias. This is the first trial demonstration of CNT vias for future LSI interconnects.

We have successfully integrated multilayer graphene (MLG) wires connected to tungsten (W) plugs with contact metal layers for carbon wire and metal plug/via hybrid interconnects for the first time. Generally, owing to the high temperature associated with the growth of high-quality MLG films, it is necessary to develop effective transfer and connection processes for MLG for back-end-of-line (BEOL) processing in LSI. In this study, using MLG formed by annealing sputtered amorphous carbon with a cobalt (Co) catalyst layer, we have successfully performed the transfer of MLG without any electrical open failures and resistance changes in the wires, despite the use of a simple transfer method without any support layers. Moreover, the connections between the transferred MLG wires and the W plugs were drastically improved using TiN/Ti as contact metal layers, as compared with the case of not using any contact metal layers. This study will be of great importance for developing contact structures not only in MLG wire and metal plug/via hybrid interconnects without carbon nanotubes (CNTs) but also in all carbon interconnects using CNT plug/via.

The resistive switching characteristics of a TiO2/Ti structure have been investigated using a conductive atomic force microscopy (AFM) system with 5-nm-diameter carbon nanotube (CNT) probes. The resistive switching showed bipolar resistive random access memory (ReRAM) behaviors with extremely low switching currents in the order of Picoamperes when voltages were applied. From transmission electron microscopy (TEM) observation, we confirmed that filament-like nanocrystals, having a diameter of about 10 nm, existed in TiO2 films at resistive switching areas after not only set operation but also reset operation. Moreover, photoemission electron microscopy (PEEM) analysis showed that the anatase-type TiO2 structure did not change after set and reset operations. From these results, we suggested that the Picoampere resistive switching occurred at the interface between the TiO2 dielectric and conductive nanocrystal without any structural changes in the TiO2 film and nanocrystal. The resistive switching mechanism we suggested is highly promising to realize extremely low-power-consumption ReRAMs with vertically contacted CNT electrodes.

We have developed a new growth method for a film of dense, vertically aligned carbon nanotubes (CNTs). We varied the slope of the growth temperature profile between 450 and 800 °C. By using the method with an Fe/Ti catalyst, the filling factor of the CNT film was measured to be 0.28, which is 20 times denser than that in the case where conventional CVD growth is utilized. We name this growth method the slope control of temperature profile (STEP) growth. Another feature of CNT films obtained by STEP growth is their mirror like surfaces. This allows for the measurement of the thermal conductivity by a pulse optical heating thermoreflectance method. The maximum thermal conductivity of the STEP-grown CNT film was 260 W m-1 K-1, which is higher than those of a solder and Si. This result suggests that STEP-grown CNT films are effective heat dissipation materials and can be used as thermal interface material (TIM) and thermal through silicon via (TSV).

We report on the electrical properties of carbon nanotube (CNT) via interconnects with improvement in contact formations between the CNT via and metal electrodes. For the improvement of the bottom contact formation, a TiN/TaN multilayer on a Cu bottom wiring layer is applied to suppress formation of a highly resistive oxide layer on the TaN barrier layer. The top electrode formation with good coverage on CNTs reduces contact resistance. The current–voltage characteristics of ultrafine CNT via interconnects exhibit ohmic behavior. The resistance of the CNT via interconnect is inversely proportional to the via area, indicating that the CNT bundles are grown with uniform quality and density in various-diameter via holes.

We have improved the high-current reliability of carbon nanotube (CNT) via interconnects by chemical mechanical polishing (CMP) and vacuum in situ metal deposition processes. These processes enable us to decrease the contact resistance of a CNT via to the upper and lower Cu lines, and also increase the number of CNTs contributing to current flow. Consequently, the current density per CNT was decreased, and current tolerance properties were improved. As a result, the CNTs via interconnects were able to withstand a high current density of 4×107 A/cm2 per via, i.e., 1.7×108 A/cm2 per CNT. In addition, we found that the failure mode of Cu-line/CNT-via/Cu-line interconnects with a CNT density of 3×1011 tubes/cm2 was the slit void formation at the Cu line under the via, which is similar to that of Cu via interconnects. Furthermore, we discussed how to further increase the tolerance of electromigration (EM), taking advantage of their high thermal conductivity.

We report a synthesis of a closely packed multi-walled carbon nanotube (MWCNT) forest by a multi-step growth method, including a new approach to immobilize catalytic nanoparticles, using plasma-based chemical vapor deposition. The CNT packing density reaches one-half of the theoretical value, where the space of 30–40% is filled with MWCNTs. This value is approximately one order of magnitude larger than that of as-grown CNT forest synthesized using conventional methods. The method is applicable even at a spatially restricted region, for example, in trench or via hole, and is available at the growth temperature as low as 450 °C.

Carbon nanotubes (CNTs) have been grown by surface-wave plasma-enhanced chemical vapor deposition (CVD) using size-classified Co nanoparticles at low temperatures. A mesh grid is used in the remote plasma-enhanced CVD system for the suppression of ion bombardment damage from plasma. The control of the electric field distribution using a mesh grid with a narrower opening is effective for the CNT growth. Multiwalled CNTs are obtained at a low temperature of 400 °C under low ion-density conditions. No significant difference in the microscopic structure of the CNTs grown at temperatures between 400 and 500 °C is observed.

Carbon nanotube (CNT) growth at temperatures below 400 °C by pulse-excited remote plasma chemical vapor deposition was demonstrated. Reduction of plasma power was carried out in order to decrease the amount of all particles, ions, electrons, and radicals. In addition, a biased plate-type screening electrode was introduced to removal of charged particles, ions, and electrons. The negative bias below 50 V was most effective for growth rate. High-quality CNT growth with the growth rate of 98 nm/min was successfully obtained at 400 °C. The results suggest that both removal of charged particles and control of the amount of radicals are important for high-quality CNT growth at temperatures below 400 °C.

Vertically aligned multiwalled carbon nanotubes (MWCNTs) were synthesized by remote plasma chemical vapor deposition at a low temperature of 390°C, which meets the requirement of the large scale integration (LSI) process. For wiring application, we measured the electrical properties of MWCNT-via structures with and without chemical mechanical polishing (CMP). The via resistances were reduced using inner shells of MWCNTs whose caps were opened due to CMP. The improved resistance after annealing at 400°C was 0.6Ω for 2μm vias. Our process is suitable for LSI because the temperature never exceeds the allowable temperature of 400°C in the Si LSI process.

To clarify the effect the oxygen has on the carbon nanotube (CNT) growth mechanisms, the authors use infrared absorption spectroscopy for the monitoring of CNT growth on cobalt catalyst surfaces. CNT grew when methanol was used as a reaction gas, while they did not grow when methane was used. The authors observed spectral changes due to the formation of cobalt oxides and methoxides on the cobalt catalyst surfaces only during the growth of CNT. The results indicate that partial oxidation of the cobalt catalyst surface increases the adsorption probability of the reaction gas and ultimately induces growth of CNT.