Akira Fukunaga

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.

Electroplating copper filling for 3D packaging

Electroplating technology for via filling with copper for three-dimensional packaging is one of the strong candidates. The Cu electroplating technology for perfect via filling is discussed in this paper. A plating solution was prepared, in which plating deposition potential was found to be influential on its solution flow rate. The characteristics of organic additives in the plating solution were electrochemically measured with a rotary electrode so as to evaluate the influence of the solution flow and agitation on the organic additive functions. Optimization of organic additive concentrations and ratios in the plating solution was tried in order to establish the bottom-up growth in electroplating for perfect via filling. The selected plating solutions were applied to a dip-type plating system and evaluated by using the paddle agitation of the plating system. The combination of the plating system and plating solution showed the perfect filling of the high-aspect ratio via on a 300mm wafer.

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.

Effect of Viscoelasticity of PVA Brush on Friction during Post-CMP Cleaning: A Guideline for Nodule Configuration

The application of new materials and three-dimensional structures are being used to achieve next-generation semiconductor devices. Hence, the role of the chemical-mechanical planarization (CMP) process has gained importance. Polyvinyl acetal (PVA) brushes are widely used as scrubbers during post-CMP cleaning. However, the mechanisms of brush scrubber cleaning are still a matter of debate because direct observation is difficult. Many researchers have proposed the removal mechanisms that operate during brush cleaning based on investigations of the forces acting on particles, friction, and the lubrication characteristics. Hydrodynamic drag forces and direct contact between the brush and the particles have been proposed as cleaning mechanisms [1–5]. Philipossian et al. [6] and Sun et al. [7] focused on the brush design and suggested that the existence of nodules or eccentricity of the roller brush had a significant effect on the friction. The friction system between the roller brush and the surface is complex because of the collision of nodules with the surface. Hence, the shapes of the nodules have been designed through trial and error. In this study, we focus on the friction of a single nodule to investigate the role of nodules in roller brush cleaning. The normal and friction forces are measured during brush sliding. In particular, we focus on the viscoelastic properties of a PVA brush and discuss its effects on friction.

Frictional Analysis of PVA Brush for Post CMP Cleaning: Effects of Rotation Speed, Compression Distance, and Fluid Viscosity

Chemical mechanical polishing (CMP) is widely used in the surface planarization process of semiconductor fabrication. The planarization process produces a large amount of surface residue that must be removed before the next process is initiated. Typically, a combination of roll-and pen-type polyvinyl acetal (PVA) brushes is used for removing such contaminants. In the subsequent 450-mm wafer processing step, PVA brush cleaning is a viable candidate for post-CMP cleaning. However, given that the mechanisms of nanoscale particle cleaning are still under investigation [1-, the designs and operating conditions for brush cleaning are determined by trial and error. In this study, as a first step toward understanding the cleaning mechanisms, the tribological contact condition of the PVA roll brush (i.e., hydrodynamic, mixed, or boundary lubrication) is considered by measuring the frictional coefficients. In particular, the effects of rotation speed, compression distance, brush length, and fluid viscosity on the friction force are investigated.

Influence of Wafer Edge Geometry on Removal Rate Profile in Chemical Mechanical Polishing: Wafer Edge Roll-Off and Notch

In the chemical mechanical polishing (CMP) process, uniform polishing up to near the wafer edge is essential to reduce edge exclusion and improve yield. In this study, we examine the influences of inherent wafer edge geometries, i.e., wafer edge roll-off and notch, on the CMP removal rate profile. We clarify the areas in which the removal rate profile is affected by the wafer edge roll-off and the notch, as well as the intensity of their effects on the removal rate profile. In addition, we propose the use of a small notch to reduce the influence of the wafer notch and present the results of an examination by finite element method (FEM) analysis.

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.

Measurement of the Frictional Force between PVA Roller Brushes and Semiconductor Wafers with Various Films Immersed in Chemicals

To better understanding the lubrication condition between a polyvinyl acetal (PVA) brush and cleaning surfaces, we measured the frictional force between the PVA roller brushes and the semiconductor wafers with various films immersed in chemicals. The frictional force showed different tendencies depending on the combination of chemicals and surface films. Especially, the large fluctuation of friction force was observed under the combination of Cu and NH4OH.

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.