Mari Ohfuti

Novel approach to fabricating carbon nanotube via interconnects using size-controlled catalyst nanoparticles

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

Low-resistance multi-walled carbon nanotube vias with parallel channel conduction of inner shells [IC interconnect applications]

We have succeeded in lowering the resistance of multi-walled carbon nanotube (MWNT) vias, using parallel channel conduction of each tubes 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.

Robustness of CNT Via Interconnect Fabricated by Low Temperature Process over a High-Density Current

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.

Electrical Properties of Carbon Nanotube Via Interconnects Fabricated by Novel Damascene Process

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.

Performance Estimation of Graphene Field-Effect Transistors Using Semiclassical Monte Carlo Simulation

A semiclassical Monte Carlo simulation was run to estimate the performances of a monolayer and a bilayer (with vertical electric field of 1 V/nm applied) graphene-channel field-effect transistor (FET). The vertical field produces a band gap of 0.16 eV and gives semiconductive properties in the bilayer graphene. Electrons in monolayer graphene show a notable velocity overshoot of up to 7.6×107 cm/s. A sub-0.1 ps transit time is also expected in a 65-nm channel device. The performance of a bilayer graphene-channel FET is inferior to a monolayer graphene one, but comparable with that of an InP high electron mobility transistor (HEMT). This lower performance may be attributed to the electron effective mass produced by the vertical field.

Carbon nanotube via interconnects with large current carrying capacity

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.

Carbon nanotube via technologies for advanced interconnect integration

We developed planar carbon nanotube (CNT) vias and their low-temperature fabrication processes consisting of low-temperature CNT growth and chemical mechanical polishing (CMP) of the CNT bundles. We were able to lower the CNT growth temperature to 400°C, which meets the requirement to avoid thermal damage to LSIs. Not only was the CMP effective for planarization; it also lowered the via resistance by about 25% with improved distribution. Our low-temperature planar CNT via technologies are very promising for the achievement of low-resistance scaled-down CNT vias for future LSI interconnects.

Carbon Nanotube Interconnect Technologies for Future LSIs

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).