Joseph A. Levert

Contact Mechanics and Lubrication Hydrodynamics of Chemical Mechanical Polishing

A preliminary model for the contact mechanics and fluid mechanics of the chemical mechanical polishing process is presented. Only the basic equations of elastic contact surface mechanics and hydrodynamic lubrication are required. Although the model is highly idealized, no ad hoc assumptions or adjustable parameters are required. Some new experimental results are presented, reinforcing the counterintuitive experimental determination of suction fluid pressure below the pad. The model correctly predicts the magnitude of the suction pressure and the effect of load, speed, and roughness.

Interfacial Fluid Mechanics and Pressure Prediction in Chemical Mechanical Polishing

This paper reports on the measurement of fluid (water) pressure distribution at a soft (polyurethane) pad/steel interface. The distribution of the interfacial fluid pressure has been measured with a specially.-designed fixture over the typical range of normal loads and velocities used in the chemical mechanical polishing/planarization of silicon wafers. The results show that, for most cases, the leading two-thirds of the fixture exhibits a subambient pressure, and the trailing third a positive pressure. The average pressure is sub-ambient and may be of the order of 50∼100% of the normal load applied. An analytical model has been developed to predict the magnitude and distribution of the interfacial fluid pressure. The predictions of this model fit the experimental results reasonably well, especially for low sliding velocities.

Mechanism for Subambient Interfacial Pressures While Polishing With Liquids

This paper reports the results of a model for predicting the development of subambient pressures during the polishing of flat hard substrates by sliding against a compliant pad in the presence of a slurry (liquid). This work is an extension of our prior experimental work on the polishing of single crystal silicon wafers with polyurethane pads and high pH slurries containing silica particles. Subambient pressures have important implications in the polishing rate and uniformity of silicon and, therefore, in the manufacture of large-scale integrated circuits. The subambient pressure is the result of pad asperity compression at the wafer leading edge followed by elastic reexpansion beneath the wafer due to the nommiform wafer/pad contact stress. Liquid is expelled from interasperity voids where high leading edge contact stress causes asperities to be compressed. Lower contact stress behind the leading edge causes asperity reexpansion leading to recreation of interasperity voids and subambient liquid pressures. A Poiseuille like in-flow of liquid from the sides of the wafer limits the value of the subambient pressure. Numerical simulations predict subambient pressures as a function of liquid viscosity and relative velocity of the pad and wafer and the pad and wafer mechanics which follow the same trend as the experimental data.

Planarization of Copper Damascene Interconnects by Spin-Etch Process: A Chemical Approach

The present work describes the process principles of “Spin-Etch Planarization” (SEP), an emerging method of planarization of dual damascene copper interconnects. The process involves a uniform removal of copper and the planarization of surface topography of copper interconnects by dispensing abrasive free etchants to a rotating wafer. The primary process parameters comprise of (a) Physics and chemistry of etchants, and (b) Nature of fluid flow on a spinning wafer. It is evident, that unlike conventional chemical-mechanical planarization, which has a large number of variables due to the presence of pads, normal load, and abrasives, SEP has a smaller number of process parameters and most of them are primary in nature. Based on our preliminary works, we have presented the basic technical parameters that contribute to the process and satisfy the basic requirements of planarization such as (a) Uniformity of removal (b) Removal rate (c) Degree of Planarization (d) Selectivity. The anticipated advantages and some inherent limitations are discussed in the context of process principles. We believe that when fully developed, SEP will be a simple, predictable and controllable process.

The Variation of CMP Pad Asperity Friction as a Function of Speed and Normal Load

CMP (chemical mechanical polishing) is a vital IC (integrated circuit) manufacturing process. CMP is performed by rubbing the IC surface with a roughened polyurethane polishing pad in the presence of a chemically active, abrasive containing slurry. Friction is a natural consequence of CMP, and this friction can damage next generation, mechanically weak, porous dielectric materials on the IC surface. Earlier research has been done to characterize this friction, but the data is confounded with hydrodynamic effects. More recent experiments have measured the coefficient of friction with varying slurry characteristics but at low speeds and constant normal load which could decouple hydrodynamic effects. This new research measures the coefficient of friction with varying low speeds and varying loads. The coefficient of friction had a very small, statistically significant decrease with increasing normal load. This effect was attributed to elastic Hertzian pad asperity deflection with friction force being proportional to the asperity contact area. At lower normal loads, there was not statistically significant change in the coefficient as a function of speed. At the highest normal load, there was a very small increase in the coefficient of friction as the speed increased. It is proposed that counter-intuitive trend was a result of the following mechanism. Time dependent surface hydration film reduces the coefficient of friction. This surface film is damaged by rubbing, and the lower speed permitted more time for the film to regenerate.© 2008 ASME

Chemical Mechanical Polishing Friction Measurements With Silica Atomic Force Microscope Tips

The feature sizes on Integrated Circuits (ICs) continue to decrease to provide higher device densities and smaller chip designs. To accomplish this, current fabrication and processing technology must be advanced to achieve these goals. In particular, Chemical Mechanical Polishing (CMP), which is used for planarization of wafers and logic circuit components during IC fabrication, can cause severe surface damage to components in the form of delamination or distortion of surface features. CMP utilizes polishing particles suspended between a polymeric pad and the substrate to be polished. To control the process with higher precision the fundamentals of friction between CMP surfaces need to be analyzed. To investigate the friction contributions of the polishing particles in the CMP process, individual CMP abrasive particles are modeled by a silica atomic force microscope (AFM) probe with a radius of curvature on the order of 200 nm that is utilized in a scanning probe microscope (SPM). Lateral forces are measured that occur in simulated polishing of silica substrates and polyurethane pad material in a liquid environment. Results are obtained as a function of pH and environment and are compared with macroscopic friction results obtained using a high precision tribometer with a glass ball.Copyright

CMP Friction as a Function of Slurry Silica Nanoparticle Concentration and Diameter

Next generation integrated circuits (IC’s) will require the use of porous dielectric materials with low shear strengths and at the same time still require processing with chemical mechanical polishing (CMP). CMP polishes the substrate material (e.g. SiO2 ) by rubbing the surface with nanometer scale SiO2 particles. The particles are suspended in an aqueous slurry and are rubbed on the substrate by a porous polyurethane pad. The high friction of CMP can damage porous dielectric materials. This research is defining the source of this friction to enable development of CMP which will not damage porous (low dielectric constant) materials. Experiments were done to determine the contributions of the SiO2 particles and bare pad asperities to the total friction with an SiO2 substrate. Further experiments were done to determine the change in friction for various SiO2 particle sizes. This system level friction data was obtained using a bench-scale tribometer at contact stresses near that of commercial CMP. Very low speeds (∼ 10−3 m/s) were used to eliminate confounding hydrodynamic effects. AFM (Atomic Force Microscope) and instrumented indentation experiments will be used to determine the friction contribution of a single SiO2 particle/substrate contact. The bench-scale experiments showed that the coefficient of friction increased by as much as 10% with the increasing weight percent of SiO2 particles in the slurry. This data suggests that a constant real area of contact is distributed between bare pad asperity contacts and higher friction SiO2 particle contacts. As more SiO2 particles are added, their contact area increases as the pad asperity contacts decrease thereby increasing the coefficient of friction of the system. Further experiments revealed a trend of increasing coefficient of friction with smaller particle diameter. This friction increase is consistent with theories which suggest that particle real contact area is a function of the particle cross-sectional area as opposed to a purely Hertzian model. The results demonstrate a possible route to controlling CMP friction by varying the relative combinations of the pad friction, the percentage of abrasives in the slurry, and the abrasive particle size.© 2006 ASME

The Relative Friction Force Contributions of Polishing Pads and Slurries During Chemical Mechanical Polishing

Next generation integrated circuits (IC’s) will require the use of porous dielectric materials with shear strengths much lower than the currently used dense silicon dioxide. The high friction of CMP (chemical mechanical polishing) may damage these porous dielectric materials. This research is being performed to define the nanoscale source of this poorly understood CMP friction to enable development of less damaging CMP processes. It is proposed that the nanoscale friction on the IC from CMP is a variable combination of two-body pad nanoasperity to IC contact and three-body nanocontact of the slurry particle between the pad nanoasperity and the IC surface. This research uses a combination of individual nanoscale friction measurements for CMP of SiO2 , an analytical model to sum these effects, and bench scale CMP experiments to guide the research and validate the model.Copyright