Important issues in CMP today are control of the polishing rate and selectivity among different materials on the surface, depending upon their relative topographical locations. To accomplish these goals, a polishing particle with "tunable" surface functionalities is the most promising solution. CAMP Professor Yuzhuo Li, collaborating with Doctors Stuart Hellring and Colin McCann of PPG, graduate students Bing Du and Jason Keleher, visiting scholar Xiaojing Shi, and undergraduate student Lauren Camaione, has evaluated the effect of surface modification to achieve a desired planarity distance and surface quality

As shown in Figure 5, both surface-unmodified silica particles (714) and surface modified particles (715-717) give essentially the same Prestonian response for Cu removal. This is anticipated since the copper CMP in this case is heavily slurry-chemistry dependent, and the particle surface modification is relatively unimportant. However, the results shown in Figure 6 demonstrate a significant positive deviation from the Preston equation as the high pressure makes more hydroxyl groups available to interact with the Ta surface. For the first time, the PPG-Clarkson team has demonstrated that the surface functionality at the supramolecular level can be controlled and manipulated by varying the polishing parameters.


CAMP's Annual Technical Meeting 2001
















Figure 5

Figure 5
. Cu MRR using surface modified and unmodified particles

Figure 6

Figure 6. Ta MRR using surface modified and unmodified particles


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 reactive liquid or polishing solution technology. Professor Yuzhuo Li and his research group are involved in such studies. Reactive liquids are designed to chemically interact with the Cu surface via complexation or direct etch attack while utilizing the abrasive strength of the polishing pad. These systems have weak interaction with the barrier or ILD material and thus generate a high selectivity in removal rate. They can eliminate such defect problems as severe scratching, particle contamination, and slurry instability like particle aggregation or settling. Graduate student Jason Keleher, visiting scholar Xiaojing Shi, and undergraduate students Steve Waud and Sarah Kenney in Professor Li's lab have conducted a series of studies on the various chemistry aspects of abrasive free systems. Using carefully selected passivation and complexation agents in the presence of an oxidizer, excellent surface quality can be obtained on copper/tantalum/oxide/low k surfaces. In the presence of balanced chemistry, formulated abrasive free systems developed in Professor Li's lab have yielded excellent results in terms of material removal rate, WIWNU, dishing, and erosion with blanket/pattern wafers. The results were presented at the Sixth International CMP Symposium held in Lake Placid.


CAMP Professor Igor Sokolov, from the Department of Physics at Clarkson University, has organized a laboratory for Scanning Probe Microscopy. The laboratory is fully operational. Professor Sokolov is currently studying CMP related friction, at the nanometer level, between the material of CMP slurries and wafers. The study is being carried out in various chemical environments, typical for real CMP processes. This work promises to shed light on the possible connection between the value of friction and various CMP parameters.


With postdoctoral research associate Ashok Babel, Professors Raymond Mackay and S.V. Babu are examining the effect of surfactant additives to CMP polishing slurries based on the various outcomes of the polishing process. Work has primarily centered on the polishing of copper with alumina slurries. Results have shown that, with the proper choice of surfactant, the slurry can be stabilized over a wide pH range, polishing rates can be maintained or even increased, and surface finish significantly improved. Qualitative examinations also show that the presence of surfactant enhances particle removal (post-CMP clean). Current studies are focused on extending these results to the polishing of glass and other silica-based substrates using ceria slurry.


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