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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 graduate students Lirong Guo and Qingjun Qin, 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 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 in the case of the IC-1000 pad. The removal rates initially increase with increasing pressure or velocity, but tend to level off at sufficiently large values of these parameters. This does not appear to be the case with the soft Suba 500 pads. Work is 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 work, Professor Subramanian is collaborating on a project 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.

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 Lirong Guo and Suresh Govindaswamy are evaluating polishing particles with "tunable" surface functionalities as a part of collaboration with Drs. Stuart Hellring and Colin McCann of PPG. The results have been very encouraging. More recently in collaboration with Professor Devon Shipp, a team including postdoctoral associates Ho-Cheol Kang and Li Liu have also explored some innovative methods to synthesize smart particles. A full evaluation of these particles is underway. 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 undergraduate students (Ken Rushing and Sarah Kenny supervised by senior graduate student Jason Keleher) have been actively investigating a wide range of possible abrasive free formulations. The results have been exceptional. 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 Li's group and Drs. Charles Little, Bill Wojtczak, and Mark Stasney of SACHEM, Inc.

Fundamentals of Copper CMP, Self-Organization, and Biophysics

CAMP Professor Igor Sokolov, from the Department of Physics at Clarkson University, has organized a laboratory for Scanning Probe Microscopy (SPM). The SPM has been used by Professor Sokolov to study fundamentals of copper CMP. An SPM tip was used to mimic a single abrasive silica particle, typical of those used in CMP slurry. The friction forces acting between such an abrasive particle and the surface during the polishing process are being studied. This work promises to shed light on the actual forces acting on slurry particles during the CMP process. 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 also modeling the shaping mechanism of nanoporous colloidal silica particles and the atomic structures of crystalline surfaces. In addition he is investigating the force interaction in complex biological systems, including epithelial human cells and various bacteria.