Electrochemical Behavior of Copper in Tetramethyl Ammonium Hydroxide Based Solutions
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experiments were 1.5 cm x 1.5 cm squares, which were diced from 200 mm wafers. To ensure electrical contact to the backside, all diced samples were coated with nickel print on the backside and along the edge. The tetramethyl ammonium hydroxide (25% TMAH) used was provided by Mallinckrodt Baker Inc. Both ammonium hydroxide (29% NH 4OH) and hydrogen peroxide (30% H202) were donated by Olin Microelectronic Materials. In experiments that were conducted in the pH range of 8 to 10, 103 M potassium nitrate (KNO 3) was added to increase the conductivity of the solution to overcome any IR drop that might occur between the working electrode and the reference electrode. The electrochemical behavior of copper and tantalum in different solutions was investigated using DC polarization experiments. From the polarization data, the corrosion current 4 density and hence the corrosion rate of copper were calculated using the Stem-Geary Equation () 8c,. ) ý_cf - 1 where fla is the anodic Tafel slope, 03.is the cathodic Tafel slope, and Rp is the polarization resistance. The R,, values were calculated from linear polarization data near the corrosion potential. In cases where one of the Tafel slopes was not well-defined (typically 0',), the corrosion current density was calculated by the intersection of the well-defined Tafel slope (typically Pj)at the corrosion potential. All electrochemical measurements were performed on EG&G Princeton Applied Research 273A Potentiostat/Galvanostat. The Tafel polarization experiments were carried out at a scan rate of 1 mV/sec. The construction of the electrochemical cell used to measure the corrosion rate of copper while being abraded by PVA brush has been described elsewhere 5 (The PVA brush used in this experiment was donated by Cupps Industries, Phoenix, AZ). During the polishing experiments, the down force was controlled at 1.5 psi and the PVA brush was rotated at 100 rpm. The polarization data during abrasion were obtained at a scan rate of 5 mV/sec. The galvanic corrosion experiments were carried out in a flat cell made from PFA and the copper sample was always connected to the working electrode lead of the potentiostat. RESULTS Figure 1 displays the corrosion current density (icon) and corrosion rate of copper in TMAH and NH 4 OH solutions as a function of solution pH. In the pH range of 8 to 10, the corrosion of copper is about the same in both TMAH and NH4 OH solutions. At pH values greater than 10, the i,-, of copper in NH 4OH solutions increases sharply with increasing pH; the icorr increases from 1 pA/cm 2 (0.22 A/min, calculated based on Cu --+ Cu 2÷) at pH-I 0 to 38 pA/cm 2 (8.4 A/min) at pH-1 1.6. The io, of copper in TMAH increases only moderately, from 1 tA/cm 2 (0.22 A/min) to 9 ptA/cm 2 (2 A/min), when the pH is increased from 10 to 13.2. In Figure 2, the effect of H 20 2 addition to NH 4OH and TMAH solutions on copper corrosion is displayed. The addition of H202 to the alkaline solutions resulted in a decrease of solution pH. Without any H202 added to the alkaline solutions at a
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