by Ed Korczynski, senior technical editor, Solid State Technology
While the recently concluded annual International Interconnect Technology Conference (IITC) will continue to cover process integration topics, there is renewed interest in novel “unit processes” which may be used for many applications. As an example, chemical-mechanical planarization (CMP) processes are used throughout modern IC manufacturing — to minimize the needed lithographic depth-of-focus, to use remove damascene overfill, etc. — and new approaches are pursued to handle the ever shrinking process windows at the leading edge of production. Japanese researchers are pursuing a production-worthy way to add electro-planarization to a CMP tool to provide a new degree of freedom in processing.
Selete worked with Roki Techno Co., Ltd. on a hybrid electro-CMP technology based on a complex patterned pad to provide electrical contacts in a standard Accretech CMP tool, using 2mm high pad structures with radially symmetric alternating arrays of anode and cathode cells with top surfaces covered by traditional CMP pad material. The sidewalls of the cells are exposed metal to make contact to the electrolyte in anode cells 1.3mm and cathodes cells 1.8mm deep. Since the wafer contacts only the patterned polymer pad material, this technique may be considered as “non-contact” to the physical anode/cathode. Lacking direct electrical contact, the resistance of the electrolyte needed to be decreased to suppress voltage drop, which increased the current flow and the corresponding removal-rate up to a point.
The first target application for this new unit process is the formation of global interconnects using Cu and low-k dielectrics. Since low-k films lack mechanical strength, planarization processes cannot use a lot of downforce and must instead rely upon chemical removal effects which may be accelerated by electrochemistry. Selete started working on e-CMP four years ago, first working with carbon as an anode material in direct contact with the wafer, and finding 71% removal efficiency compared to traditional CMP. The initial efficiency of non-contact e-CMP was 45%, but optimizing the anode and cathode cell geometries has allowed for an e-CMP process with efficiency of 80%. Using this process, 1.4μm/min of Cu has been removed with no corrosion, and with 6% within-wafer non-uniformity on 300mm wafers. On a 75mm diameter wafer they showed >6μm/min removal-rate, so it’s possible that an innovation in the pad could provide for faster removal on 300mm wafers. — E.K.