Effect of Dissolved Oxygen on the Removal of BTA from Cu By Tetra Methyl Ammonium Hydroxide
Authors: Srinivasan Ramanathan , Raghavan Srini , MS Amrutha 
Published: February 2020

In the fabrication of copper interconnects, chemical mechanical polishing (CMP) is used to remove excess copper and barrier layers. Because of its ability to form a protective layer, Benzotriazole (BTA) is used as Cu corrosion inhibitor during CMP as well as post-CMP cleaning [1, 2]. It is desirable to remove this protective layer between multiple copper deposition and CMP steps. Investigations on the removal of chemisorbed BTA by tetra methyl ammonium hydroxide (TMAH), hydroxylamine and organic acid have been reported [2-4]. These studies have used 0.1 wt % and lower concentration of BTA at natural pH ( < 7) or alkaline pH (11) to form adsorbed layers [3-5]. The effect of dissolved oxygen content on the adsorption of BTA and subsequent removal by TMAH is not yet reported.
In this work, removal of BTA adsorbed on Cu at a pH value of ~10, which is close to the conditions that exist during barrier CMP, using a quartz crystal microbalance (QCM) technique. Since the study focussed only on the BTA removal, particles were not included in the test solution. A thin layer of Cu was first electrodeposited on Au electrodes of a quartz crystal in an electrolyte containing 10 mM CuSO4 and 100 mM H2SO4. The electrodeposition was carried out at – 150 mV vs. Ag/AgCl for 300 s. An SRS QCM-200 model quartz crystal microbalance (QCM) was used to monitor the changes in mass of the Cu coated crystal, during BTA adsorption and removal. A freshly prepared surface was used for each run. The dissolved oxygen level was controlled by bubbling either nitrogen or air before and during the experiments. During the adsorption and removal experiments, the temperature was maintained at 30 ± 0.1 °C using a circulating bath. After the completion of each experimental run, the Cu layer was stripped by maintaining the electrode at +600 mV vs.Ag/AgCl in the same solution.
In the first part of investigation, a Cu sample was immersed in KOH solution of pH 10 without BTA for 200 s. After this conditioning time, the solution was switched to 1 wt% TMAH. The changes in the crystal resonance frequency were converted to changes in mass per unit area, using Sauerbrey equation as Δm = -17.67(ng/cm2/Hz) Δf and reported in Figure 1. Figure 1(a) shows that when air is bubbled in the KOH solution, the mass increases over time and this is probably due to the formation of oxides and hydroxides of Cu. Upon switching the solution to TMAH, the mass decreases and goes below the original value. This indicates that TMAH removes Cu oxides, hydroxides and Cu. When the sample is immersed in KOH solution with nitrogen bubbling, the mass remains relatively constant, but switching the solution to TMAH causes a reduction in mass. This confirms that TMAH etches bare Cu.
In the second part of the investigation, a Cu sample was immersed in KOH solution of same pH but containing 0.1 wt% BTA for 200 sec. The results are presented in Figure 1 (b). The mass increases to roughly the same extent regardless of whether air or nitrogen is bubbled in the solution. When the solution was switched to aerated TMAH solution, the mass decreases rapidly indicating that BTA and possibly the oxides of Cu are removed. On the other hand, when nitrogen deaerated (nitrogen saturated) TMAH was used, the mass continues to increase suggesting that BTA is not removed. These results indicate that BTA adsorbed on oxide free Cu surface is not attacked by TMAH to the same extent as that adsorbed on oxide free surface.
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Figure 1