Wear 257 (2004) 863–868 Effects of oxidants on the removal of tungsten in CMP process Geonja Lim a , Jong-Ho Lee b , Joosun Kim b , Hae-Weon Lee b,∗ , Sang-Hoon Hyun a a Department of Ceramic Engineering, Yonsei University, Seoul 129-790, South Korea b Nano-Materials Research Center, Korea Institute of Science and Technollogy, Seoul 136-791, South Korea Received 12 August 2003; received in revised form 12 February 2004; accepted 12 February 2004 Abstract The effects of oxidants on tungsten chemical mechanical planarization (CMP) process were investigated using two different oxidants, hydrogen peroxide and ferric nitrate. The electrochemical redox properties of surface layer were characterized with potentiodynamic polarization test and resulting microstructural and chemical states of the surface layer were characterized with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) during CMP under different slurry chemicals. According to the analysis, the oxidation state and microstructure of surface layer were strongly influenced by the chemical composition of slurry. Moreover, the oxidation kinetics and resulting chemical state of oxide layer played critical roles in determining the overall CMP performance. © 2004 Elsevier B.V. All rights reserved. Keywords: Tungsten; CMP; Slurry chemicals; Surface oxide layer 1. Introduction Tungsten has been widely used materials for the for- mation of contact via and holes for connecting inter-layer metal lines in IC fabrication. In this application, the tung- sten deposited by CVD above the insulator layer should be planarized by chemical mechanical planarization (CMP) process for higher degree of local and global planarities [1]. The basic planarization mechanism of tungsten during CMP has been proposed to consist of two concurrent processes: the formation of a passive oxide layer by slurry chemicals and subsequent abrasion by slurry abrasives. The passivation reaction in this metal CMP process is predominantly con- trolled by the chemical components of the slurry. The passi- vating layer facilitates the removal of higher features due to the softness and brittleness of tungsten oxide whereas that of lower features retards the chemical etching of the tung- sten layer, consequently resulting in the global planarization [2]. Because oxidants play a crucial role in passivation reac- tions, e.g. conversion of tungsten to tungsten-oxide, various oxidants, such as H 2 O 2 [3], KIO 3 , Fe(NO 3 ) 3 [4,5], KMnO 4 , KNO 3 [5], have been investigated for tungsten CMP. Although Fe(NO 3 ) 3 has high oxidation potential, its ap- plication is normally restricted to the very low pH region due to its limited stability in aqueous solution. In compari- ∗ Corresponding author. Tel.: +82-2-958-5523; fax: +82-2-958-5529. E-mail address: hwlee@kist.re.kr (H.-W. Lee). son, H 2 O 2 , another well-known and economical oxidant has relatively low reactivity in terms of spontaneous oxidation. However, it can be readily activated in the presence of Fe 2+ ions, so-called Fenton process [6,7]. In this process, H 2 O 2 is decomposed by ferrous ion (Fe 2+ ) to produce hydroxyl radical ( • OH), and/or by ferric ion (Fe 3+ ) as well to form perhydroxyl radical ( • OOH) which is a very powerful oxi- dant. The related chemical reactions are as follows. Fe 2+ + HOOH H + -→ Fe 3+ + H 2 O + • OH (1) Fe 3+ + HOOH → Fe 2+ + H + + • OOH (2) The decomposition kinetics of H 2 O 2 depends on the experi- mental conditions such as concentrations of reactants, com- plexing agent, slurry pH and so on. According to the results of Chen and Pignatello [8], the reaction rates of Eqs. (1) and (2) are 74 and 0.02mol/s, respectively, which are ex- tremely fast compared with the overall duration required for whole CMP process including slurry preparation, supplying and planarization process. In contrast, ferric ion forms various hydrates which are dissolved at low pH. The stability of ferric ion can be varied with its ion concentration: for example, when it is above 10 -2 M, the solution is stable below pH 2. However, once ferric ion is complexed with organics present in the solution, it becomes stable even at higher pH [9]. The complexed ferric ion may reduce the decomposition rate of H 2 O 2 by limiting the contact probability between H 2 O 2 and ferric ion 0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.02.007