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 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 understanding the planarization process that occurs in the case of patterned wafers. Professor Subramanian is also 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 doctoral student Qingjun Qin have been studying the mechanical removal of a polymer using an alumina slurry as a function of relative velocity, applied pressure, and particle concentration using a Struers Benchtop polisher. Experiments have been performed using IC-1000 pads, and additional experiments using Suba 500 pads are under way. The results clearly demonstrate the inadequacy of the Preston model in describing mechanical removal rates over a wide range of velocities. The removal rates do increase linearly with increasing pressure, but the dependence on velocity appears to be nonlinear. Also, similar experiments have been performed on the removal of copper, and it has been found that even when abrasive particles in de-ionized water with no added chemical are used, the removal of copper is not purely mechanical. Rather, a film is formed on the surface that appears to be removed by mechanical action. Work in under way on developing an understanding of the behavior of the removal rate in these experiments, and on the design of new experiments.

Professor Subramanian is also working with doctoral student Qingjun Qin, on developing theoretical descriptions of various aspects of polishing in an orbital polishing tool. A SpeedFam/IPEC 676 orbital tool is available at Clarkson for testing predictions from these modeling efforts. Experiments have been performed on the polishing of blanket copper films deposited on 200 mm silicon wafers, using an abrasive-free chemical solution for material removal. In these experiments, the concentration of the chemical and the orbital speed were both varied, and the variation of material removal rate was measured as a function of radial position on the wafer in each case. One of the objectives of the model is to predict these radial variations of removal rate. The pad is constructed with a rectangular grid of grooves, and the slurry is introduced from underneath the pad through a set of 61 holes that are located at the intersections of selected grooves. The model involves describing the flow behavior in the groove structure of the pad, and combining this description with a model of the relative motion of each point on the wafer relative to the pad.

In addition to his CMP research, Professor Subramanian is collaborating with Clarkson University Professor John McLaughlin (with support from NASA) on the motion of a liquid drop on a solid surface because of the action of wettability gradients. Information about this work is available at his website (http://www.clarkson.edu/subramanian/solid.htm).

Smart Particles for Copper CMP

Important issues in CMP today are the control of the polishing rate and selectivity among different materials on the surface, depending upon their relative topographical locations. To meet such challenges, Professor Yuzhuo Li and his graduate students are evaluating polishing particles with "tunable" surface functionalities as part of a collaboration with Dr. Stuart Hellring of PPG. The results have been very encouraging. More recently in collaboration with Professor Devon Shipp, a team including postdoctoral associate Li Liu has also explored some innovative methods to synthesize smart particles. A full evaluation of these particles is under way. If successful, the smart particles will provide unique polishing characteristics and superior CMP performance.

Abrasive Free Systems for Copper CMP

CMP has become an enabling technology for the semiconductor industry. While continuous innovation in slurry development is still in demand, some attention has been placed on the development of abrasive free CMP technology . Professor Yuzhuo Li and a group of students led by Jason Keleher have been actively investigating a wide range of possible abrasive free formulations. The results have been very encouraging. Due to its very nature, an abrasive free system can eliminate such defect problems as severe scratching, particle contamination, and slurry instability like particle aggregation or settling. Using carefully selected passivation and complexation agents in the presence of an oxidizer , excellent surface quality has been obtained on copper/tantalum/oxide/low k surfaces. The research is a collaboration between Professor Li's group and SACHEM, Inc.

Fundamentals of CMP, Self-Organization of Nanoporous Colloids, and Biophysics

CAMP Professor Igor Sokolov, from the Department of Physics at Clarkson University , uses Scanning Probe Microscopy (SPM) for a variety of research topics. He has used SPM to study the fundamentals of copper CMP. The SPM tip was used to mimic a single abrasive silica particle, typical of those used in CMP slurry. He is collaborating with CAMP Professor Subramanian on a project involving the measurement of particle-wafer and particle-pad interactions using an Atomic Force Microscope.

Professor Sokolov is working with CAMP Professor Ian Suni, in studying a new architecture of biosensors. He is also synthesizing nanoporous glasses.

In addition Professor Sokolov is investigating the force interaction in complex biological systems, including epithelial human cells and various bacteria. In collaboration with Professor Craig Woodworth (of Clarkson's Department of Biology), he studies the mechanical properties of ageing human cells. He is also collaborating with CAMP Professor Anja Mueller and Professor Stefan Grimberg (of Clarkson's Civil and Environmental Engineering Department) to study molecular mechanisms of bacterial interaction with various oils during the process of bioremediation.