Assessment of Planarization length variation by the Step-Polish-Response (SPR) Method
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Assessment of Planarization length variation by the Step-Polish-Response (SPR) Method Johann. W. Bartha, Tilo Bormann, Kathrin Estel, Dieter Zeidler Dresden University of Technology, Institute of Semiconductor and Microsystems Technology 01062 Dresden, Germany ABSTRACT In spite of the fact, that the main purpose of CMP is the planarization of surfaces, most processes are optimized with respect to the removal rate. This might be due to a lack in techniques for the determination of the planarization behavior. The commonly used expression “Planarization Length” implies a maximum lateral extension over which the planarization is obtained and which can not be improved. Several attempts have been made to determine the planarization length by studying the CMP on different test patterns. The general problem in the interpretation of the data is the interaction of neighboring pattern with different pattern densities. This problem does not apply to the most simple pattern possible, which is a single step with an extension of the up and down areas much larger than the planarization length. In this case the spatial derivative of the resulting contour after CMP is directly the demanding transfer function to be used for the convolution. This concept, initially proposed by Boning et. al., was applied to evaluate the polish of copper and silicon oxide. Wafers with concentrically steps in Cu or SiO2 films with an extension of 1 to 5 cm have been prepared. During polish, the initially infinite steep step widens up, yielding the planarization length as a function of the removal respective the polishing time. Significant differences in the evaluation of the planarization length could be quantified depending on the pad, slurry and tool parameters. Our first experiments revealed a decrease in planarization length by the addition of BTA in a copper slurry. We believe that the SPR method enables a unambiguous, pattern independent determination of the planarization capability in CMP. INTRODUCTION Modern chip manufacturing requires very flat surfaces for each subsequent process step. A shrinking of the Critical Dimension (CD) is possible by using shorter wavelengths (λ) or higher numerical apertures (NA) of the lithographic system. However this strategy decreases the Depth Of Focus (DOF) and increases requirements for planarization respectively for the Chemical Mechanical Polishing. CD = K1
λ NA
DOF = K 2
(1)
λ
NA2
(2)
This fact creates a demand for a well defined characterization of the CMP process. The most prominent parameter for the CMP characterization is the removal rate and its uniformity. However the goal of CMP is not the fast remove of the film material, but its planarization. Therefore the determination of the planarization length (PL) is most important.
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A commonly accepted understanding of planarization length is the definition as the lateral distance between the up and down areas at a potential equilibrium state. A further improvement should not be possible after this state has been reached.
Figure 1. A
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