Masahiro Horibe

Electrical Properties of Carbon Nanotube Bundles for Future Via Interconnects

We have developed carbon nanotube (CNT) vias consisting of about 1000 tubes using thermal chemical vapor deposition (CVD) at a growth temperature of 450°C with cobalt catalysts, titanium carbide ohmic contacts, and tantalum barrier layers on copper wiring. The lowest resistance obtained was about 5 Ω/via. 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. No degradation was observed for 100 hours at via current densities of 2×106 A/cm2, which is favorably compared with Cu vias.

Simultaneous Formation of Multiwall Carbon Nanotubes and their End-Bonded Ohmic Contacts to Ti Electrodes for Future ULSI Interconnects

We have succeeded in growing multiwall carbon nanotubes (MWNTs) with low-resistance ohmic contacts to titanium electrodes by hot-filament chemical vapor deposition (HF-CVD) using a nickel catalyst layer on a titanium electrode. The contact resistance of the sample with nickel/titanium electrodes was two orders of magnitude smaller than that of the sample with nickel catalyst electrodes without titanium. We assumed that the low-resistance ohmic contact was achieved by forming titanium carbide (TiC) during the growth at the MWNT/titanium electrode interface. Moreover, we have demonstrated the growth of vertically aligned bundles of MWNTs, which were end-bonded to the lower titanium electrodes, selectively in via holes. We believe this is the first report of such simultaneous formation of MWNTs and their end-bonded low-resistance ohmic contacts, and its first trial application to carbon nanotube (CNT) vias for future ULSI interconnects.

Carbon nanotube 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.

Carbon Nanotube Growth Technologies Using Tantalum Barrier Layer for Future ULSIs with Cu/Low-k Interconnect Processes

We succeeded in developing carbon nanotube (CNT) vias specifically adapted for the copper interconnect process used in ultra large-scale integrated circuits. The CNTs were grown selectively on titanium films using Co catalyst films. The use of tantalum enabled CNTs to be grown on Cu lines and prevented any increase in the sheet resistance of the Cu lines. A Cu wire/CNT via/Cu wire structure was fabricated and low resistance of the via was demonstrated. In addition, tests showed that a high current density of about 106 A/cm2 flowed into the CNT via for 125 hours.

Mechanical Polishing Technique for Carbon Nanotube Interconnects in ULSIs

We examined a mechanical polishing technique for multiwalled carbon nanotube (MWNT) vias. This polishing technique involved the use of diamond particles fixing MWNT protrusions of the samples. The 1-µm-high outthrust MWNTs were polished, and then flat sample surfaces were obtained by controlling polishing pressure and polishing time. A cross-sectional image of a cut MWNT was obtained by high-resolution scanning electron microscopy. Mechanically polished MWNT interconnects with a high current density and a low resistance were developed.

Influence of Growth Mode of Carbon Nanotubes on Physical Properties for Multiwalled Carbon Nanotube Films Grown by Catalystic Chemical Vapor Deposition

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.

A high-temperature superconductor latching driver operated at 30 K for a single-flux-quantum/semiconductor interface

We report on a latching-type driver operated at 30 K for the output interface of single-flux-quantum (SFQ) circuits. Ramp-edge-type high-temperature superconductor (HTS) junctions showed only small inherent hysteresis in current–voltage characteristics at a temperature of 30 K. To have hysteresis large enough for latching operation, we chose La-doped YbBaCuO as a counter electrode of an interface modified junction, and used amorphous LaSrAlTaO to form an additional barrier to the interface modified barrier. The junction shunted with optimized capacitance showed sufficiently large hysteresis of 33%, which is as large as that of a non-shunted junction at 4.2 K. We employed 20 kA cm−2 as the critical current density (Jc) of the junction to fabricate dc/SFQ, Josephson transmission lines (JTLs) and the latching driver on the same chip. It is smaller than the optimum current density that gives the smallest punchthrough probability of the junction, which was calculated to be 10−12 for a 30 Gbps ac bias by using the junction parameters at 30 K. However, the probability for Jc of 20 kA cm−2 is still smaller than that of low-temperature semiconductor junctions because of the larger Jc of the HTS junctions. The fabricated latching driver successfully converted the SFQ pulse, which passed through several JTLs from the dc/SFQ circuit, to a 1 mV level signal at 30 K.

Carbon Nanotube Via Technologies for Future LSI Interconnects

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.

High-speed operation of HTS SQUID-array interface circuits with a cryocooler

The authors evaluated the operation of high-temperature superconducting quantum interference device (SQUID)-array interface circuits (IFCs) with normal-metal control lines. Transimpedance amplification was obtained at an operating speed of 1 Gb/s using a cryocooler. The effect of the number of SQUIDs connected in series and the number of arrays connected in parallel on the level of output from the SQUID-array IFCs was examined by Josephson circuit simulation, and then the effect of statistical spreads of junction characteristics was evaluated by Monte Carlo simulation. It was found that the configuration of two parallel SQUID arrays with 64 SQUIDs gives the highest output when the junction characteristics in the arrays have a certain spread. The authors fabricated the IFCs by using the conventional interface-engineered junction process. The process reproducibility was 100 /spl mu/A /spl plusmn/25% for junction I/sub c/, and 3.02 pH /spl plusmn/5% and 2.57 pH /spl plusmn/17% for the sheet inductance of the upper and lower electrodes, respectively. The transimpedance at low frequencies reached 20 and 4 V/A for input levels of 20 and 100 /spl mu/A, respectively. Output voltages as high as 4.4 mV at 4.2 K and 2.3 mV at 40 K were obtained. Furthermore, an output voltage of 600 /spl mu/V was obtained for a 1-Gb/s 2/sup 15/-1 pseudo-random binary signal input at 40 K.

Simultaneous formation of multi-wall carbon nanotubes and their low-resistance ohmic contacts for future ULSI via interconnects

As a bottom-up fabrication process for future ULSI interconnects, we have developed vertical growth of multi-wall carbon nanotubes (MWNTs) on titanium electrodes and simultaneous formation of their end-bonded ohmic contacts. We believe this is the first report of such simultaneous formation of MWNTs and end-bonded ohmic contacts with a contact resistance two orders of magnitude smaller than the current method.