Modeling and Mapping of Nanotopography Interactions with CMP

Abstract

As the demand for planarity increases with advanced IC technologies, nanotopography has arisen as an important concern in shallow trench isolation (STI) chemical mechanical polishing (CMP) processes. Previous work has shown that nanotopography, or small surface height variations on raw wafers 20 to 50 nm in amplitude extending across millimeter scale lateral distances, can result in substantial CMP-induced localized thinning of surface films such as oxides or nitrides used in STI [1]. This interaction with CMP depends both on characteristics of the wafer such as heights and spatial wavelengths of the nanotopography, and characteristics of the CMP process including the planarization length or pad stiffness.

Nanotopography Issues in Shallow Trench Isolation CMP

“…Depending on the pad and process used, typical extracted pad moduli range from 20 MPa to 900 MPa, with rms fitting errors of 1-2.8 nm. 6 An example set of cross sections is shown in Figure 5 with an extracted E of 147 MPa, illustrating both the "transmission" of nanotopography into oxide removal and the good accuracy achieved with the fitted model.…”

“…Keywords: chemical-mechanical planarization, chemical-mechanical polishing, CMP, contact-wear simulation, nanotopography, shallow trench isolation (STI), silicon wafers. ing models, 6 and filter-based approaches. 4,6 While these simplified methods can predict the results of CMP on a single-material oxide over nanotopography, it is not clear how to extend these to the polishing of STI stacked films, where both oxide and nitride are polished simultaneously with different removal rates.…”

“…ing models, 6 and filter-based approaches. 4,6 While these simplified methods can predict the results of CMP on a single-material oxide over nanotopography, it is not clear how to extend these to the polishing of STI stacked films, where both oxide and nitride are polished simultaneously with different removal rates. Instead of these approaches, we have found it best to use a contactwear model for nanotopography that can naturally account for film stacks.…”

“…Instead of these approaches, we have found it best to use a contactwear model for nanotopography that can naturally account for film stacks. 6 We begin with a contact-mechanics model, as described by Chekina and Keer 7 and Yoshida. 8 The concept is to relate local pressures on the wafer surface to the polishing-pad displacement and use the local pressures to derive local removal rates, assuming that the polish rate is linearly proportional to the pressure.…”

“…The simulation surface is sectioned into discrete elements, and a time-stepped algorithm is used to determine the final post-CMP film surface. 6 Treating the pad as a linear elastic material, the relationship between pad displacement and pad pressure is given by…”