Analysis of Copper to Tantalum Transition in Copper CMP
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ANALYSIS OF COPPER TO TANTALUM TRANSITION IN COPPER CMP
J.M. KANG, S. WU, T. SELVARAJ, P.D. FOO Institute of Microelectronics, DSIC-MD, 11 Science Park Road, Singapore ABSTRACT The evolution from Cu to Ta surface during Cu CMP has been investigated. As a first step of Cu CMP, removing excess Cu on field area to Ta often gives rise to topography issues such as dishing or erosion. The exposure of Ta is detected by in-situ reflectance measurement, where the downward slope-start signifies the beginning of Ta exposure. For blanket wafers, the maximum remaining Cu thickness at slope-start increases with initial Cu thickness. Applying lower polishing pressure and table speed, so-called soft landing, near the Cu-Ta transition stage results in thicker remaining Cu. In patterned wafers, Ta exposure starts near high pattern density areas such as wide lines, and contrary to blanket wafers, remaining Cu thickness at slope-start cannot be correlated simply with initial Cu thickness. In this experiment, with increasing initial Cu thickness, remaining Cu thickness at slope-start decreased. The amount of dishing is largely proportional to the initial Cu thickness.
INTRODUCTION One of the first questions to be considered in Cu chemical-mechanical polishing (CMP) is what the optimum Cu deposition thickness is for a good CMP result. Once deposition thickness is decided, then the next question is how much over polishing is needed after Ta exposure to achieve good planarity. For a given polishing property, the first step CMP result is greatly influenced by these variables. Especially, when a slurry of low Cu:Ta:dielectric selectivity is used for second step, the planarity after first step is decisive for the final planarity and dielectric thickness uniformity. In this work, Cu to Ta transition has been analyzed by correlating in-situ reflectance change during CMP with the post-CMP Cu thickness and surface profile. The relation between the planarity expressed as total-indicated-range (TIR) and the deposition thickness was obtained and compared with the experimental results. The influence of over polishing on TIR according to the polishing condition is discussed.
EXPERIMENTAL DETAILS Blanket films of 0.8, 1.2 and 2.2 µm Cu were prepared by varying electroplating time after 250 Å Ta barrier and 2000 Å Cu seed layer formation. For patterned wafers, 0.6, 0.8 and 1.2 µm Cu films were made by the same method on the substrates with 0.55 µm deep trenches. Also, Sematech-931 wafers were used for the study of over-plated bumps. Wafers were polished using AMAT Mirra polisher equipped with an in-situ reflectance measurement tool. Cu thickness and step height were measured with Four Dimension 4-point probe and Tencor P-30 profiler, respectively. Commercially available alumina based slurry was used through this experiment. For main polishing, 90 rpm - 3 psi condition was used as the table speed and polishing pressure. M3.5.1
Near the final stage, 45 rpm - 2 psi condition was used for soft landing. Through this experiment, the Cu removal rate of ma
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