Immersion/Electroless Deposition of Cu on Ta

Abstract

Continuous Cu films are deposited on Ta by a two-step process; galvanic displacement of Ta by Cu from ammonium fluoride solutions and subsequent electroless Cu deposition from a formaldehyde-containing bath. The conditions necessary for good adhesion are discussed. The extent of oxide film removal in the HF pretreatment solution is studied by electrochemical impedance spectroscopy. The charge-transfer resistance of about 57 V-cm 2 is several orders of magnitude lower than that measured for the Ta Beginning in the late 1990s, Cu has been replacing Al for deep submicrometer Si device interconnects and vias. Since Cu is a deep level defect, an intervening Ta/TaN diffusion barrier has been em- ployed, and this material appears to be viable for the 90 and 65 nm technology nodes. 1 The relatively high resistivity of this diffusion barrier requires the deposition of a Cu seed layer prior to Cu electrodeposition. 2 Currently, both the barrier and seed layers are deposited in two separate tools by a mixed plasma/physical vapor deposition process known as either ionized physical vapor deposi- tion ~I-PVD! or ion metal plasma ~IMP!. 3-5 Electrochemical deposi- tion of either of these layers would provide substantial cost reduc- tions during microelectronic device fabrication and is currently an area of significant interest. Here we report electrochemical deposition of Cu thin films on Ta with good adhesion by galvanic displacement from fluoride- containing aqueous solutions and subsequent electroless deposition. Since Ta is a refractory metal, Ta electrodeposition processes gen-