Pad Conditioning and Pad Surface Characterization in Oxide Chemical Mechanical Polishing
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Pad Conditioning and Pad Surface Characterization in Oxide Chemical Mechanical Polishing A. Scott Lawing Rodel, Inc., 3804 East Watkins Street, Phoenix, AZ 85034 Abstract An understanding of the pad surface state and its effect on the polishing process is critical to a more fundamental understanding of Chemical Mechanical Polishing (CMP). Vertical Scanning Interferometry has been shown to be a useful tool to monitor the pad surface state. In this paper, various techniques for analyzing pad surface data obtained using interferometry are described. The pad surface state can be modified through the manipulation of process and pad conditioner design parameters. Many of the parameters extracted from interferometry data show strong correlations with corresponding polish data. A careful analysis of these complementary data can yield significant insight into the mechanisms of CMP processes. Introduction A quantitative understanding of the pad-wafer interface is fundamental to a more complete working model of CMP [1,2,3,4]. The nature of pad-wafer contact and pad surface morphology in general have an enormous impact on the polishing process, influencing both contact mechanics and fluid flow. In previous work [5,6], it was demonstrated that manipulation of padwafer contact mechanics can have a dramatic effect on oxide polishing with both fumed silica and colloidal polishing slurries. By adjusting process conditions and pad conditioner aggressiveness, polishing can be driven from a contact dominated to a hydrodynamically influenced regime. An ability to obtain quantitative measures of the pad surface state was central to the development of the models that have been proposed previously. In this paper, the analytical methods that have been developed to extract pertinent quantitative statistics from pad surface interferometer images will be discussed in greater detail. Recent studies have highlighted the importance of near-surface pad statistics on polishing behavior. Oliver et al [7] showed a strong correlation between the average asperity height of the pad near-surface region with polish rate decay when conditioning was suspended. Borucki [8] has proposed a model that describes the time evolution of the pad height probability distribution function (pdf) due to abrasive wear and successfully captures the polish rate decay based on the data observed by Stein [2] and Oliver [7]. Lawing [5,6] showed that deviations from Prestonian polish behavior can be explained based on changes in the pad height distribution statistics arising from deformation of the pad surface and the influence of hydrodynamic effects. In ex situ rate decay experiments, the experimental pad height distribution evolution measured by Lawing [6] can be accurately captured by the Borucki [8] model, providing strong evidence that the mechanism of pad surface deformation is the abrasive wear of pad surface asperities[9]. Various methods have been utilized to study pad surfaces, including optical interferometry [2,3,5,6,7], optical microscopy [4,10] and stylus profilo
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