Key features of the Semitool CFD plating reactor are discussed and illustrated
using numerical modeling. ... Aided by simulation using the CFD-ACE analysis.
Design and Simulation of the Semitool CFD Plating Reactor Greg Wilson, Paul McHugh, Kyle Hanson, John Klocke, Ken Gibbons, Tom Ritzdorf. Applied Materials, Inc., 655 W. Reserve Drive, Kalispell, MT 59901, USA February 2010
Introduction Key features of the Semitool CFD plating reactor are discussed and illustrated using numerical modeling. The reactor provides substantially uniform mass transfer to the wafer; electric field shaping capability for a variety of plating baths, metals, and seed layers; and time dependent control of the current density across the wafer to compensate for changing metal sheet resistance as the wafer is plated. The numerical modeling was an important component of the chamber design process.
Simulation Aided by simulation using the CFD-ACE analysis program, the flow of electrolyte and the electric field were modeled to design a flexible plating reactor applicable for a variety of metals. Accurate modeling capability allowed many virtual design iterations to arrive at important features of the reactor that have proven successful in the field. These key features are discussed below:
Electric Field Concentric multiple anodes are used to shape the electric field for a control of the plated film for a variety of bath conductivities and seed layer sheet resistance. The outer anodes are smaller in width and are placed closer to the wafer so that the terminal effect at the edge of the wafer can be more effectively controlled. The anode shapes "seen" by the wafer are virtual so that the actual anodes can be placed farther away from the wafer. In this way the electric field is not influenced by electrode shape/erosion. This remote anode placement also allows insertion of an ionic membrane (Figure 1).
62 mS/cm Catholyte
33mS/cm Anolyte
Figure 1. Axisymmetric electric-field model of the 4-anode CFD reactor including a membrane cartridge. The anolyte and catholyte have different conductivities.
Figure 2. Axisymmetric flow model of the CFD reactor.
Electrolyte flow
A combination of flow provided through the center anode path along with wafer rotation yields a substantially uniform flow of ions to the wafer. The inflow is delivered in a radially inward and horizontal direction so that any "jets" created by diffusers are not pointed at the wafer. The inflow impinges on itself and turns upward to the wafer. A significant advantage of the chamber design is the decoupling of the electric field and flow. Each can be changed independently in
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Voltage Drop f (Time, CurrentDensity , Radius ) 0.8
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contrast to more traditional reactors with diffuser plates where changing a hole in the diffuser influences both the flow and electric field at the same time (Figure 2).
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Film Thickness Control Models of the chamber were used to create and test algorithms to determine optimal electric current settings for the multiple anodes. Model-based software for both final film shape and transient control were developed.
i f (Time, Current Density, Anode Position )
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Evaluation of Results
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wafer Figure 3 shows simulations illustrating the capability of the multiple anode design to change the electric field within the reactor to plate uniformly on seed layers with different sheet resistances. For uniform plating, the radial voltage drop in the electrolyte must match the voltage drop within the seed layer i f (Time, Current Density, Anode Position) (caused by the current flowing between the edge contacts and plating surface). The Figure 3. Example of electric field shaping to reactor electric field is adjusted by changing adjust for different seed layer sheet thickness: the relative current passed though the various 400Ǻ (top) and 3000Ǻ (bottom). virtual anode channels. The thin seed layer 1.5 in Figure 3 (top) is characterized a high voltage 1.4 Profile after 5 seconds drop across the wafer and by low current to the 1.3 outer anode. A low sheet resistance case is 1.2 Final profile depicted in Figure 3 (bottom).
Conclusion The extensive use of virtual design iterations has enabled faster, more cost effective mechanical and process development. Simulations are also vital to future development of emerging applications for Cu damascene and packaging.
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Figure 4 illustrates the capability of changing the current distribution to the anodes while a wafer is plated. With static currents to the wafer (top), the final film is flat but the initial film is very edge thick. In copper damascene plating, this initial period is when the features are filled. Flat profiles are attained throughout the recipe with a schedule of varying anode currents (bottom).
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Figure 4. Plating results without (top) and with (bottom) dynamic current adjustment.
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