MIRAI Researchers Deposit Low-Resistance CNTs
Kenji Tsuda, Asia Contributing Editor -- Semiconductor International, 1/3/2008 8:51:00 AM
The Japanese semiconductor research consortium MIRAI announced that its researchers have successfully formed low-resistance carbon nanotubes (CNTs) in vias with a diameter of 160 nm.
The CNTs exhibited a via resistance of 34 Ω at a formation temperature of 450°C using a thermal chemical vapor deposition (CVD) process, and 63 Ω at 400°C.
Researchers worldwide have been seeking to develop high-density CNTs with lower resistivity but, thus far, the diameters have been in the 2 µm range. The best data seen by the MIRAI researchers has been a 40 nm diameter via with a density of 9 × 10-11/cm2. However, the reproducibility was poor.
The research group, led by Yuji Awano, formed 160 nm vias using CNTs with an average density of 3 × 10-11/cm2, 30× higher densities than normal, with good reproducibility, Awano said.
The MIRAI group also developed a process flow compatible with a copper interconnect damascene process.
The MIRAI engineers improved on the nozzle of the deposition system to eject smaller catalysts. The ejected nanoparticles run through a two-stage chamber for size classification before reaching the surface of the targeted wafer. The first stage has lower pressure than the previous nozzle chamber, and the second-stage chamber is lower than the first chamber. Thus, the directivity of the nanoparticles increases linearly at the wafer surface — the linearity of the nanoparticles is important to penetrate the vias. The wafers are then transferred into a thermal CVD chamber to deposit the CNTs more densely with CH4 gas decomposition.
In an actual copper interconnect process, a <400°C temperature process is required, and the MIRAI team is now studying the use of plasma CVD to reduce the temperature. To reduce wafer damage caused by ion etching, the team used two kinds of CVD technologies.
The tip-discharge CVD generates a plasma discharge at the tip of the antenna of a microwave generator, where the electric field is maximized. The microwave power is smaller — only 60 W — and the distance between the tip to the wafer surface is longer to reduce ion generation.
The surface-excited plasma CVD technology uses a mesh grid to remove ions and pick up only the plasma radicals. The deposition temperature is 390°C for the tip-discharge radical CVD and 380°C for the surface-excited plasma CVD.
CNTs have potential advantages, including 1000× the current density of copper, higher endurance against electromigration, 10× the thermal conductivity of copper, and 100× the mechanical strength of steel. However, to use CNTs as interconnects, a key issue is to obtain lower resistances with higher-density CNTs in smaller-diameter vias using lower process temperatures. To produce higher-density tubes, a key is to deposit smaller catalyst particles at the bottom of the via to produce individual tubes with smaller diameters, Awano said.
