Shohei Shima

Localized Oxidation of the Cu Surface after Chemical Mechanical Planarization Processing

After chemical mechanical planarization (CMP) processing of a Cu/low-k structure device, defects are often observed and some of them induce problems in manufacturing very large scale integrated circuit (VLSI) devices. As an example of defects, watermarks and protrusions on the Cu are detected. We found that the number of watermarks or protrusions is strongly affected by the cleaning conditions. The energy dispersive X-ray analysis (EDX) showed that these protrusions were composed of Cu and O. Moreover, atomic force microscopy (AFM) observations revealed that these protrusions grew during the storage time after the postcleaning. Electrochemical measurements also indicated that the protrusions were oxidized copper formed in the cleaning solutions due to the difference in corrosion current densities for various conditions of the Cu surface. Therefore, optimization of the post-CMP cleaning processing is a key issue for the reduction of defects such as protrusions.

Visualizing Nanoscale Distribution of Corrosion Cells by Open-Loop Electric Potential Microscopy.

Corrosion is a traditional problem but still one of the most serious problems in industry. To reduce the huge economic loss caused by corrosion, tremendous effort has been made to understand, predict and prevent it. Corrosion phenomena are generally explained by the formation of corrosion cells at a metal-electrolyte interface. However, experimental verification of their nanoscale distribution has been a major challenge owing to the lack of a method able to visualize the local potential distribution in an electrolytic solution. In this study, we have investigated the nanoscale corrosion behavior of Cu fine wires and a duplex stainless steel by in situ imaging of local corrosion cells by open-loop electric potential microscopy (OL-EPM). For both materials, potential images obtained by OL-EPM show nanoscale contrasts, where areas of higher and lower potential correspond to anodic areas (i.e., corrosion sites) and cathodic areas, respectively. This imaging capability allows us to investigate the real-time transition of local corrosion sites even when surface structures show little change. This is particularly useful for investigating reactions under surface oxide layers or highly corrosion-resistant materials as demonstrated here. The proposed technique should be applicable to the study of other redox reactions on a battery electrode or a catalytic material. The results presented here open up such future applications of OL-EPM in nanoscale electrochemistry.

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.

Structures of Cu surfaces developing in benzotriazole solutions: Effect of pH

The effect of pH on layer formation onto clean Cu surfaces in benzotriazole (BTA) aqueous solutions was studied by in situ spectroscopic ellipsometry. The effect of H2O2 addition was also investigated. Time changes in the ellipsometric parameters Ψ and Δ, which correspond to the structural changes of the layers on Cu, were discussed. In acidic solutions, a BTA or a Cu–BTA complex layer grows directly on Cu. The out-diffusion of Cu is suppressed at the Cu layer interface. When H2O2 was mixed, the Cu surface is eroded in acidic solutions. In alkaline solutions, the BTA layer grows on the oxidized Cu layer, or no growth occurs, depending on the composition of the solutions. In neutral solutions, the Cu–BTA complex layer forms on Cu, and the uncovered part is oxidized in the presence of H2O2.

Metrologies of abrasive behaviors for understanding and upgrading CMP process

Laser scanning confocal fluorescent microscopy and the friction mode of Atomic Force Microscopy were adopted as metrologies of abrasive behaviors and removal force during polishing and cleaning processes As small as a diameter of 50nm silica particle can be individually detected and spatial distribution of particles in three dimensional space also clearly observed using the fluorescent microscopy. The fine particle removal force was able to measure using the friction mode of AFM. Accurate force was quantified by calibrating torsion spring constant of an AFM cantilever using a MEMS micro-force sensor with the particles attached to the sensor probe.

In situ ellipsometry of Cu surfaces immersed in benzotriazole–hydrogen peroxide solutions

In the chemical mechanical polishing (CMP) of Cu, the Cu surface is oxidized and is concurrently removed by the mechanical function of an abrasive. Surface oxidation can lead to severe surface corrosion, and to prevent this, a corrosion inhibitor is added to slurries. Accurate understanding of the competition between oxidation and passivation is essential for advanced Cu CMP technologies. In this work, layer formation on clean Cu surfaces in benzotriazole (BTA), H2O2, and BTA–H2O2 aqueous solutions was studied by in situ spectroscopic ellipsometry. Time changes of ellipsometric parameters are discussed with respect to BTA and H2O2 concentrations. It was found that the BTA adsorbs onto the Cu surface and the adsorbed BTA transforms into a Cu–BTA complex in about 3 min after the onset of adsorption. The BTA/complex layer passivates the Cu surface against oxidation by H2O2.

Effect of pH and chemical mechanical planarization process conditions on the copper–benzotriazole complex formation

Benzotriazole (BTA) has been used to protect copper (Cu) from corrosion during Cu chemical mechanical planarization (CMP) processes. However, an undesirable Cu–BTA complex is deposited after Cu CMP processes and it should be completely removed at post-Cu CMP cleaning for next fabrication process. Therefore, it is very important to understand of Cu–BTA complex formation behavior for its applications such as Cu CMP and post-Cu CMP cleaning. The present study investigated the effect of pH and polisher conditions on the formation of Cu–BTA complex layers using electrochemical techniques (potentiodynamic polarization and electrochemical impedance spectroscopy) and the surface contact angle. The wettability was not a significant factor for the polishing interface, as no difference in the contact angles was observed for these processes. Both electrochemical techniques revealed that BTA had a unique advantage of long-term protection for Cu corrosion in an acidic condition (pH 3).

Characterization of Cu-BTA Organic Complexes on Cu during Cu CMP and Post Cu Cleaning

Although copper have better electrical properties than aluminum such as low resistivity and high electro-migration resistivity, aluminum has been used as an interconnect material due to the difficulty in Cu dry etching. Since CMP process has been adapted to the semiconductor fabrication, Cu became the choice of materials for interconnection. However, copper CMP process introduces new defects on the surface such as slurry particle, organic residue, scratch and corrosion [1].

Surface potential change with droplet formation and Cu watermark growth by electrochemical oxidation resulting therefrom

To reveal the mechanism of watermark formation on a Cu film, the Volta potential in a droplet area on Cu was evaluated using the scanning Kelvin-probe method. The droplet area on Cu exhibited an upward convex potential profile, indicating that the electrochemical reactions were more active in the areas around the droplet than at the center of the droplet. On the other hand, Si exhibited a profile converse to that of Cu: electrochemical reactions were more active at the center of the droplet area than in the areas around it. These evaluations revealed that the pinning and ring-shaped watermark formation at the droplet area on a Cu film resulted from the said Volta potential profile.