Tomo Murakami

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.

Atomic level analysis of carbon and silicon by a scanning atom probe

Carbon nanotubes, chemical vapor deposition (CVD) diamond, high-temperature-high-pressure (HPHT) diamond, vitreous carbon, graphite and silicon are analyzed by a scanning atom probe (SAP). All specimens contain a large amount of hydrogen. The CVD diamond grown in hydrogen exhibits the highest hydrogen concentration with a ratio of hydrogen to carbon atoms (H/C) of 2.02, and vitreous carbon has the lowest hydrogen concentration with a H/C ratio of 0.25. Significant difference is noticed in the hydrogen desorption energy among analyzed CNTs. The [111]-oriented silicon wafers are chemically etched by HF and NH4F. Although a few analyzed areas are hydrogen-covered and stay clean even when exposed to air, most areas are contaminated with hydrogen, oxygen and carbon. The carbon concentration in the HF-treated silicon is found to be slightly higher than that in the NH4F-treated silicon. An unexpected finding is the shift of the most abundant Si-H clusters from Si4H+ to Si2H+ with the depth of the analyzed area of the NH4F-treated silicon.

Long Length, High-Density Carbon Nanotube Film Grown by Slope Control of Temperature Profile for Applications in Heat Dissipation

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).

Low-temperature catalyst activator: mechanism of dense carbon nanotube forest growth studied using synchrotron radiation.

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.

Evaluation of thermal resistance of carbon nanotube film fabricated using an improved slope control of temperature profile growth

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.

Improved thermal conductivity by vertical graphene contact formation for thermal TSV

This paper reports the tailoring thermal conductivity of novel dense vertical and horizontal graphene (DVHG) structures, which we previously discovered. By removing horizontal graphene layers, resulting in forming vertical graphene contacts to the electrode, we not only improved the thermal conductivity by a factor of 10 but also improved the electrical conductivity by a factor of 100. The pyrolytic graphite, grown at a higher temperature than the DVHG, showed a high thermal conductivity of 1426 W/mK by forming vertical graphene contacts. Although the DVHG showed poor thermal properties at this point, we found that the vertical graphene contact formation can be an important technology to realize high thermal conductivity for carbon-based thermal through-silicon-vias (TSV).

CNT/graphene technologies for future carbon-based interconnects

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 SiO2 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.

Hard x-ray photoemission study of oxidation states of Ti underlayer in Fe/Ti film system

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/...

Multi-layer graphene wire grown by annealing sputtered amorphous carbon

We have fabricated multi-layer graphene directly on SiO2 by annealing sputtered amorphous carbon with a Co catalyst without use of complicated transfer processes. Structural analysis revealed that the graphene sheets formed an epitaxial structure, aligned to the Co (111) surface. An MLG wire can sustain a high current density of 107 A/cm2 which is over one order of magnitude higher than that of a Cu wire.