Topography Reduction During Barrier CMP Improved Due to Tantalum Oxidation
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TOPOGRAPHY REDUCTION DURING BARRIER CMP IMPROVED DUE TO TANTALUM OXIDATION Paul Lefevre, Katsuyoshi Ina, Kenji Sakai, Kazusei Tamai, Scott Rader Fujimi Corporation, 747 Church road, Elmuhurst, IL 60126
Abstract Barrier CMP can reduce the topography generated during Cu CMP. In case of selective barrier CMP slurry like 1:10:1, it is expected that the topography reduction could not exceed the Ta thickness. Some recent observations made at Fujimi show that the topography reduction can be twice larger than the Ta thickness without dielectric loss. This paper presents the chemical mechanical phenomenon that is responsible for this 40 to 50 nm topography reduction during barrier CMP. At the end of the Cu CMP, Ta is exposed to oxidizer. This process can oxidize an important part of the Ta barrier which becomes Ta 2O5 according to Pourbaix diagram [1]. Ta2O5 can be as much as 2.3X thicker than original Ta layer. This phenomenon explains why topography reduction can be twice higher than initial Ta thickness. This mechanism explains why after Cu CMP it is likely to have more than 30 nm dishing on copper lines. The last and more important consequence is that this reduces final topography and total copper metal loss at the same time by about 30 nm. Obviously very high selectivity Cu CMP slurry (Cu:Ta ~ 1000:1) is necessary to have neither Ta erosion nor Ta 2O5 erosion during copper CMP. High selectivity slurry is required during barrier CMP in order to reduce the loss of copper and Dielectric during Barrier CMP.
Introduction There are two conventional approaches to reduce topography during barrier CMP. First approach is using very selective barrier CMP slurry. If the barrier CMP removes only Ta the topography reduction is limited to the thickness of the Ta. This method has the merit to keep high metal thickness but is limited in terms of topography performance. The second common approach is using low selectivity slurry so that all materials Cu, Ta and dielectric are removed at similar rates. In this case the CMP can continue to planarize and reach very flat surface. The topography correction then happens while thinning dielectric and copper line. This method enables to reach flat topography but at the expense of dielectric and metal loss. However, Fujimi has observed experimental results where selective barrier CMP gives important dishing correction as much as twice the barrier thickness without dielectric loss
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or copper loss. This paper presents the observation made and gives a tentative explanation for such phenomenon.
Initial Observation: during selective Ta CMP, topography reduction is higher than Ta thickness Figure 1 shows experimental results using Fujimi copper CMP slurry DCM-C7 followed by Barrier CMP slurry DCM-B6. The first step slurry has extremely high selectivity
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