Analysis of Correlation between Pad Temperature and Asperity Angle in Chemical Mechanical Planarization

Chemical mechanical planarization (CMP) is a technology widely employed in device integration and planarization processes used in semiconductor fabrication. In CMP, the polishing pad plays a key role both mechanically and chemically. The surface of the pad, consisting of asperities and pores, undergoes repeated cycles of glazing induced by polishing followed by the recovery of roughness by a conditioning process applied during CMP. As a polymer material, the pad also experiences thermal expansion from changes in temperature. Such changes can be expressed in terms of surface roughness values, but these do not fully capture the actual changes to the pad surface. In this study, the change in pad temperature occurring during CMP was analyzed with regard to its effect on the asperity angle, and the influence on CMP outcome was assessed. The changes in the surface asperities according to the steady-state pad temperature were evaluated using various measurement methods. The change in pad roughness was characterized in terms of the asperity angle, and the contact state predicted according to temperature were validated by measuring the contact perimeter, the number of contact points, and related values. Through Scanning Electron Microscope (SEM) and micro-CT analysis, it was confirmed that in the continuous polishing process and the conditioning process, the changes in asperity angle due to changes in pad temperature affect the polishing outcome.

A Simple Model Linking Surface Roughness with Friction Coefficient and Manufacturing Cost

A simple theoretical model linking surface micro geometry, friction and manufacturing cost is presented. Combining a basic geometrical relationship of plastic deformation of workpiece surface asperities by a hard tool with an assumption of adhesive friction, the friction coefficient of a soft, rough surface sliding against a hard, smooth tool surface can be calculated, linking surface roughness with friction coefficient. The simple model can also link the cost related to manufacturing with a surface characterized by a given friction coefficient value. Results are presented from tests carried out to verify the simple model. Several test pieces were manufactured by turning, or grooving, an aluminum alloy and brass using different feeds, tool nose radii, and tool nose angles, achieving different surface profiles. The surfaces were characterized using a stylus profilometer and a digital microscope. The static friction coefficient was determined in terms of angle of repose using a rotary table. The experimentally determined values of the friction coefficient were compared with those predicted from feed, tool radius, and asperity angle. The tests have shown a good reproducibility, and a clear determination of the friction coefficient was possible. However, due to the low normal loads involved in this set up, the influence from the surface roughness was not clear. Further investigations are therefore proposed.

Surface Texturing for Energy Efficiency and Sustainability

Precise control of friction is very important for energy efficiency and sustainability in manufacturing processes. In the present investigation, various surface textures have been employed to vary the frictional conditions. More specifically, textures were varied from unidirectional to criss-cross to unidirectional by grinding the steel surfaces against emery papers for various numbers of cycles. Sliding experiments were conducted using an inclined pin-on-plate apparatus against the prepared steel surfaces under dry and lubricated conditions. In the experiments, it was observed that the coefficient of friction and transfer layer formation on the harder surfaces were controlled by the textures of the harder surfaces under both dry and lubricated conditions. The asperity angle of the harder surface plays a dominant role in controlling the friction and transfer layer at the sliding interface. Thus, by understanding appropriate roughness parameters, the friction and wear performance can be accurately controlled to enhance energy efficiency and the quality of the finished products in manufacturing process.

Surface Roughening and Contact Behavior in Forming of Aluminum Sheet

The evolution of the surface topography and tool-workpiece contact behavior of an aluminum sheet during plastic deformation has been investigated by a series of indentation tests for various loads, strain rates, and bulk strains. A simple formulation for estimating the roughness of the indented surface and a model of asperity flattening have been proposed. Dynamic balance between the flattening and roughening processes, such as changes of asperity angle and spacing, is considered. Theoretical predictions of the variation of real area of contact with strain show good agreement with experiments. It is found that the very different frictional behavior of aluminum compared with steel sheets is due to the serious change of roughness, including asperity angle and spacing. When the angle is small, the increases in asperity angle and spacing have strong effect on increasing the contact area ratio. However, when the asperity angle is large, increased asperity angle has a significant contribution to the resistance to flattening. The asperity flattening velocity and the increasing rate of contact area ratio arise with the prestrain; however, they are not affected by the strain rate significantly. From FFT analysis, low frequency components (below 10 cycle/mm) due to the rotation of single grains or grain groups are observed on free surface during plastic straining. While in the contact area, in all cases, the low frequency components generated by plastic deformation are compressed.

Experimental Study of Interfacial Contacting Process Controlled by Power Law Creep

Interfacial contacting processes under a high temperature and a high bonding pressure (T = 973 K, P = 30 MPa) are experimentally studied, using oxygen free copper. The faying surfaces were machined by lathe, resulting in controlled regular surface asperities. The asperity angle of surface ridges was changed from 10 to 60 deg. The change in the interfacial deformation mode with the asperity angle has been investigated. Results show the interfacial contact process is strongly influenced by the asperity angle (shape of surface ridge). The bonding tests were carried out in high vacuum atmosphere (10−4 Pa) so that the surface oxide film need not be considered. Experimental results are in good agreement with the results calculated by a finite element model, in which the interfacial contact is assumed to be produced by power law creep alone. It was thus suggested that void coalescence is governed by power law creep under the present test conditions (T = 973 K and P = 30 MPa) except for the final stage of bonding. Experimental results also suggest that the elementary rate process of interfacial contact due to power law creep is classified into two types; surface folding and interfacial expansion. Here, the surface folding is the phenomenon that two faying surfaces are overlapped to each other and the interfacial expansion means that the bonded interface area is extended along the bond-interface.

Effect of Bulk Deformation on Viscoplastic Adhering Process—A Numerical Study of Solid State Pressure Welding

Viscoplastic intimate contact process of uneven surfaces is numerically studied by using the finite element model proposed in our previous paper. The model treats only the case that the interfacial contact is the rate determining step of the solid state bonding process. The distribution of the equivalent strain rate around the void surface is strongly influenced by the bulk constraint conditions, i.e., the interfacial deformation is greatly affected by the bulk deformation. The strain rate at the void tip is strikingly increased by the bulk deformation, which accelerates the void shrinkage on the bond interface. If the bulk is deformed, the contacting process is also affected by the asperity angle α0 due to surface waviness. When α0 < 30 deg, the bonded area growth is mainly produced by the folding phenomena of the faying surfaces.