Electrochemical Investigation of Surface Reactions for Chemical Mechanical Planarization of Tantalum in Oxalic Acid Solutions

Abstract

Oxalic acid ͑OA͒-based nonalkaline solutions of H 2 O 2 are found to support chemically mediated removal of Ta-oxide surface films on Ta. The associated surface reactions are critical for chemical mechanical planarization ͑CMP͒ of Ta barrier layers in the fabrication of interconnect structures. In the present work, we probe the underlying mechanisms of these reactions using dc and ac electrochemical techniques. A Ta coupon electrode is used as a model system in abrasive-free solutions of 1 wt % OA + x wt % H 2 O 2 at pH 3.0 and 6.0 ͑x = 0, 1, and 5͒, where the chemical component of CMP is selectively examined in the absence of mechanical abrasion. These nonalkaline solutions are essential for preventing chemical damage to low-k dielectrics underlying Ta barriers in interconnects. The electrochemical data presented here explain the recently reported findings of Ta CMP experiments using OA solutions, and provide the necessary framework to put forward a reaction scheme for the chemically mediated disintegration of Ta 2 O 5 films formed on Ta in such solutions. A comparison of the dc and ac electrochemical data helps to identify the relative roles of the different chemical additives in these CMP solutions.

Electrochemical investigation of the roles of oxyanions in chemical–mechanical planarization of tantalum and tantalum nitride

“…1 with plots (a), (b), and (c) representing KNO 3 , K 2 SO 4 , and KH 2 PO 4 solutions, respectively. The native Ta 2 O 5 layer of Ta in this case is not only formed by reaction (1), but also by H 2 O 2 [8,10]:…”

“…2 in front of the Ta substrate represents the native oxide of electrochemically untreated Ta, or, a mixture of native and electro-generated Ta 2 O 5 of anodically treated Ta. The native oxides are formed by air (and moisture) [10]:…”

“…Electro-generation of Ta 2 O 5 is a multi-step process [8,10,16,17], where e -is the electron charge. Reactions (2) and (3) occur at the oxide-solution and the metal-oxide interfaces, respectively.…”

“…6 often is replaced by the applied overpotential, g = (E -E oc ) [8,10]. As oxyanions from the solution adsorb onto the electrode, they compete with the O 2-anions to react with the Ta 5?…”

“…Linear polarization resistances (LPRs) are determined to check if and how the solutiondependent polish rates and the surface activity of Ta are correlated. Fourier transform electrochemical impedance spectroscopy (FT-EIS) is employed to obtain electrode equivalent circuit (EEC) models of the reactive interfaces [9,10], which clarify further the chemical mechanism of oxyanion-induced CMP of Ta and TaN.…”