Metrological Assessment of the Coefficient of Friction of Various Types of Silica Using the Motor Current During ILD-CMP
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Metrological Assessment of the Coefficient of Friction of Various Types of Silica Using the Motor Current During ILD-CMP Harald Jacobsen1, Eric Stachowiak1, Gerfried Zwicker1, Wolfgang Lortz2, Ralph Brandes3 1
Fraunhofer Institut für Siliziumtechnologie, Itzehoe, Germany Degussa AG, Hanau, Germany 3 Degussa Corporation, Piscataway, N.J., USA 2
Abstract In the work presented the coefficient of friction (COF) was firstly determined metrologically by a systematic assessment of motor currents for different products of pressure and velocity (p ⋅ v). With all seven test slurries it could be shown that the inserted energy ECMP is proportional to the product of pressure and velocity. The COF of the parameter domain considered here does not depend on the inserted energy or the product of p ⋅ v in a first approximation. For all tested slurries it was demonstrated that, both the COF and the removal rate (RR) behave analogously (low COF→ low RR). Measurements of the viscosity η have shown that η is not a constant in the shear rate range relevant for CMP. Using the obtained viscosity values a mean slurry film thickness in the range of 2,5 – 6 µm could be calculated. 1. Introduction For various CMP applications a large number of commercial slurries are available for ILD polishing which are typically based either on colloidal silica or fumed silica. Both types of slurries show a different behaviour in respect to material removal, which can be described by Preston’s Law and modifications thereof. To create a general equation describing the removal rate (RR) for an arbitrary CMP process the removal mechanism has to be researched and understood more in its basics. In all equations describing removal rates, a coefficient Cp is used to sum up the influences in polishing behaviour deriving from variables such as the applied pad or slurry characteristics, mean slurry film thickness as well as the coefficient of friction (COF) between wafer, slurry and pad. The measured COF seems to be an interesting value to make new conclusions and to form a new equation for the RR. 2. Theoretical Considerations and Experimental Procedure 2.1 Consumables and Experimental Details Five test slurries based on fumed silica (AEROSIL® product line) with different BET surface areas ranging from 50 m2/g to 300 m2/g were investigated in ILD polishing. These noncommercial test slurries provided by Degussa were treated by a novel high-shear milling method (wet-jet mill, WJM1). Aim of the wet-jet mill treatment was to evaluate whether the modification of fumed-silica by reduction of aggregate size could lead to particles that perform more like non-
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aggregated colloidal-silica. The basic idea is the purposive transformation of the irregularly shaped fumed-silica particles to agglomerates with more spherical shape by using high milling energies. For this purpose, theses dispersions have been prepared by using two wet-jet mill cycles with a pressure of about 250MPa. As reference materials, two commercial standard slurries (one with colloidal silica
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