Katsuhiko Tokushige

Stress Analyses during Chemical Mechanical Planarization Processing with Cu/Porous Low-k Structures of LSI Devices

Porous low-k materials are required for the construction of 45-nm-node LSI devices. However, the extremely low Youngs modulus values of these materials result in the stress corrosion cracking (SCC) of the Cu interconnects during chemical mechanical planarization (CMP). We performed finite element method analyses of the stress at each step during the CMP. The results showed that the horizontal tensile stress was especially concentrated at the edges of the isolated fine wiring, and that higher tensile stresses appeared at the step of the barrier CMP. Moreover, the maximum values of the tensile stress increased with a decrease in Youngs modulus in the low-k films. The cause of the horizontal tensile stress was the downward CMP pressure, which indented the low-k films. These results suggest that CMP with a lower downward pressure and an LSI structure with a Cu dummy pattern were effective for avoiding SCC.

Electrochemical Reactions During Ru Chemical Mechanical Planarization and Safety Considerations

We analyzed electrochemical reactions during ruthenium (Ru) chemical mechanical planarization (CMP) using a potentiostat and a quartz crystal microbalance, and considered the potential safety issues. We evaluated the valence number derived from Faradays law using the dissolution mass change of Ru and total coulomb consumption in the electrochemical reactions for Ru in acidic solution and slurry. The valence numbers of dissolved Ru ions were distributed in the range of 2 to 3.5. As toxic ruthenium tetroxide (RuO4) has a valence number of 8, we were able to conclude that no toxic RuO4 was produced in the actual Ru CMP.

Analysis on Copper Photocorrosion Induced by Illuminance in Chemical Mechanical Planarization Equipment Using Photodiode and Quartz Crystal Microbalance

Photoassisted corrosion of copper (Cu) was evaluated using a photodiode and a quartz crystal microbalance (QCM). A chip-type silicon (Si) photodiode with a large junction area was used in place of actual Si devices. When the illuminated photodiode was connected to the anode and cathode electrodes in an electrolyte, it worked as a voltage source between the two electrodes, and the corrosion rate was governed by the current between the electrodes. The corrosion rate is nearly proportional to the illuminance at less than 100 lx, and corrosion initiates at an illuminance as low as 1 lx. In the geometrical aspect of the photoassisted corrosion system, the corrosion rate is proportional to the square root of the area ratio of a P-connected Cu line to an N line, and is proportional to the illuminated area of the junction in a photodiode. The wavelength of the illuminating light markedly affects the photoassisted corrosion.