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CAMP Professor Raymond Mackay Selected Maryland Chemist of the Year 2001

CAMP Professor Raymond Mackay (of Clarkson University's Department of Chemistry) has been selected by the Maryland Section of the American Chemical Society to receive the Maryland Chemist of the Year Award for 2001. The award is in recognition of his outstanding contributions to chemistry. Professor Mackay has numerous publications and many prestigious honors. He has done extensive research in the areas of micelles and the application of electrochemical methods. His current research interests include the preparation of nanolatexes, polymer/nanoparticle composites, and the effect of surfactants on polishing slurries. He served as CAMP Director for eight years and is currently on leave and serving as Acting Director of Research and Technology at the U.S. Army Edgewood Chemical Biological Center, Aberdeen Proving Ground in Maryland.

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CAMP's Annual Technical Meeting 2001
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Dr. Dana Barry and Sharon Williams Receive APEX Awards for the CAMP Newsletter

CAMP Editor/Technical Writer Dr. Dana Barry received an APEX 2001 Award of Publication Excellence, for editorial content and over-all communications excellence, for the 1999-2000 CAMP Annual Report Newsletter. Designer Sharon Williams (of Ad Workshop in Lake Placid, NY) received an APEX 2001 Award of Publication Excellence for the design and layout of the same newsletter. Both awards were given by Communications Concepts, Inc. in Springfield, VA.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

THE RESEARCH

MODELING OF THE CHEMICAL-MECHANICAL POLISHING PROCESS

Professor Ahmadi and his group are developing a model (based on mechanical contact theory) for the chemical-mechanical polishing process. The goal of their research is to provide a fundamental understanding of the parameters that control the effectiveness of CMP for surface planarization. Their current work focuses on the abrasive particle, wafer, and pad contact and the abrasive and adhesive wear mechanisms in the chemical-mechanical polishing process. They are developing a model for interactions of pad asperities with abrasive particles and the wafer. Their analysis includes the influence of abrasive particle adhesion to the surface of the wafer. Also they are looking at the CMP process using hard and soft pads and dilute and concentrated slurries. In addition Professor Ahmadi and his students are also studying the effect of abrasive particle shapes, slurry pH, and colloidal forces on the removal rate.

Their model predictions are described in detail and compared with the available semi-empirical correlations in the paper " A Model for Mechanical Wear and Abrasive Particle Adhesion During the Chemical-Mechanical Polishing Process," by G. Ahmadi and X. Xia, Journal of the Electrochemical Society , 148 (3) G99-G109 (2001).

Professor R. Shankar Subramanian is working on various aspects of modeling of chemical- mechanical polishing. He is interested in predicting overall removal rates from blanket wafers, and phenomena such as dishing and erosion in patterned wafers. Along with former graduate student Lu Zhang, he has developed a detailed transport model in which the convective and diffusive transport of chemical species in the pores of the polishing pad is accommodated using a two-dimensional repeating cell description. The appropriate conservation equations for momentum and species, along with the boundary conditions, are solved numerically in this approach. The model can predict the chemical removal rate as a function of relative speed between the wafer and the pad and geometric parameters. Abrasive removal of material can be accommodated in the transport model as well. He and graduate student Lu Zhang have recently used the model to predict copper removal rates in a Strasbaugh 6CA polishing tool by an abrasive-free solution containing hydrogen peroxide and glycine, and compared these predictions with experimentally measured rates. The model correctly predicts the trend of the removal rate plotted against the relative velocity between the wafer and the polishing pad over a range of glycine concentrations, displaying a non-linear dependence on the velocity. The removal rate is small at low relative velocities and approaches an asymptotic rate at larger values of the relative velocity that corresponds approximately to chemical removal at the incoming glycine concentration. An article based on this work will shortly appear in Thin Solid Films.

Professor Subramanian also is interested in the process by which mechanical removal of material occurs at the microscopic level. Here, the issues are the role of the mechanical properties such as the relative hardness of the wafer, abrasive particle, and the pad, the role of asperities on the pad, and the coupling of the chemistry to the mechanical removal process. He and graduate student Lirong Guo have been studying the mechanical removal of copper in an alumina slurry as a function of relative velocity, applied pressure, and particle concentration using a Struers Benchtop polisher. Experiments have been performed using both IC-1000 and Suba 500 pads. The results clearly demonstrate the inadequacy of the Preston model in describing mechanical removal rates over a wide range of velocities and pressures

Graduate student Rajesh Appat is working with Professor Subramanian on predicting the polishing rates of steps on patterned wafers. The initial modeling work is focused on fixed abrasive pads used for polishing oxide films in applications such as shallow trench isolation (STI). In fixed abrasive pads, an abrasive such as ceria is incorporated into pillars on a flat polymer substrate, and is held together by a binder. The binder disintegrates during polishing, exposing the abrasive particles. Two important reported features of fixed abrasive polishing of STI oxide films are that the rate at which a step is polished is a stronger function of pattern density than is the case when a slurry containing abrasive particles is used, and that the polish rate of blanket films is extremely low. A preliminary model has been developed that captures these two features, and an article that contains information about the model and sample predictions is scheduled for publication in the December 2001 issue of Electrochemical and Solid State Letters.

Professor Subramanian has recently begun working with a new doctoral student Qingjun Qin on developing theoretical descriptions of various aspects of polishing in an orbital polishing tool. A SpeedFam/IPEC 676 orbital tool has recently been installed at Clarkson, and the predictions from models to be developed in this project will be compared with experimental data obtained from this tool.

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