High Accuracy Concentration Analysis of Accelerator Components in Acidic Cu Superfilling Bath

Abstract

We have devised a modified cyclic voltammetry stripping (CVS) method to measure the concentrations of bis-(sulfopropyl) disulfide (SPS) and 3-mercapto-1-propane sulfonate (MPS) in Cu plating solutions. Though MPS, a breakdown product of SPS, enhances the Cu deposition rate on flat electrodes, it is not a superfilling-capable accelerator for the damascene structure, unlike SPS. Therefore, accurate measurement of SPS in damascene Cu plating baths is important. However, enhancement of the Cu deposition rate by MPS interferes with the electrochemical signal of SPS, leading to a significant error when using the modified linear approximation technique (MLAT)-CVS analysis method. To evaluate their concentrations individually, a two-step CVS analysis was performed in which the total accelerator concentration ([SPS] + 1/2[MPS]) and conversion ratio were separately determined. All MPS species in the bath were oxidized to SPS by controlling the plating solution pH. Subsequent MLAT-CVS analysis successfully revealed the total accelerator concentration in the Cu plating solution. Individual SPS and MPS concentrations were thereby calculated using the conversion ratio evaluated from the difference in their relative accelerating abilities. This modified method enabled determination of the SPS concentration with <10% error, suggesting a reliable and high accuracy tool to predict pattern filling capabilities of plating solutions. Owing to the merits of high throughput, low process cost, and good film quality, Cu electroplating has a wide application, from traditional Cu foil production to state-of-the-art semiconductor metallization processes. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The Cu plating solution typically contains small amounts of organic additives, 1-16 which enhance the uniformity, 13 brightness, 14 and mechanical properties of the Cu film. 15 In addition, for particular application to damascene Cu plating, the additives enable bottom-up filling at trenches or vias by controlling the relative deposition rates of Cu at the top and bottom of the features.1-12 For this application, the organic additives are grouped as the accelerator, suppressor, and leveler based on their electrochemical behaviors and their roles in pattern filling.One of the most widely used accelerators is bis-(sulfopropyl) disulfide (SPS), a dimer of 3-mercapto-1-propane sulfonate (MPS). Acceleration by SPS in Cu superfilling has been explained by the competitive adsorption theory [17][18][19][20][21] or by the catalytic action caused by the reduction of cupric ions. 22,23 In the competitive adsorption theory, acceleration is regarded as a recovery of the deposition rate previously suppressed by the polyethylene glycol (PEG)-Cl inhibition layer, and recovery occurs through the displacement of the PEG-Cl layer with SPS.17-19 A recent study revealed that the acceleration is a result of the dissociative adsorption of SPS with displacement of the well-ordered Cl − layer. 20,21 MPS, a dissociated product of SPS, reconverts to SPS with re...

Observation of Bis-(3-sulfopropyl) Disulfide (SPS) Breakdown at the Cu Cathode and Insoluble Anode under Open-Circuit, Unpowered Closed-Circuit, and Electrolysis Conditions

“…The modified linear approximation technique-cyclic voltammetry stripping (MLAT-CVS) was used for analyzing the concentration of SPS in the target Cu bath. 37 The Pt rotating disk electrode (RDE, 0.0314 cm 2 ), Pt rod, and Ag/AgCl [3 M KCl] were used as working, counter, and reference electrodes, respectively. As an intercept solution (Cu plating solution with excessive suppressor), 37 a Cu bath with pH 2, consisting of 0.125 M CuSO 4 , 0.5 M H 2 SO 4 , 0.86 M NaOH, 1 mM NaCl, and 70 mM PEG-3400, was used.…”

“…37 The Pt rotating disk electrode (RDE, 0.0314 cm 2 ), Pt rod, and Ag/AgCl [3 M KCl] were used as working, counter, and reference electrodes, respectively. As an intercept solution (Cu plating solution with excessive suppressor), 37 a Cu bath with pH 2, consisting of 0.125 M CuSO 4 , 0.5 M H 2 SO 4 , 0.86 M NaOH, 1 mM NaCl, and 70 mM PEG-3400, was used. Before analyzing, the pH of the target bath was adjusted to 2 by adding 1.73 M NaOH and 1 mM NaCl.…”

“…24 Once MPS is produced, the cathodes becomes depolarized, because MPS has a greater acceleration effect than does SPS. 25,37 Under the open-circuit condition, two molecules of MPS spontaneously oxidized to one molecule of SPS through the reaction with Cu 2+ ion. 16 On the other hand, PDS is very stable, both chemically and electrochemically, so it does not react with any species.…”

Detection of Organic Additives in Copper Plating Bath Using Voltammetric Methods That Involve a Screen-printed Nano-Au Electrode

“…This study respectively predicts the concentrations of accelerator, suppressor and leveler to within 0.5%, 2.7% and 0.12% [19]. Choe et al [20] devised a modified CVS method to measure the concentration of bis-(sulfopropyl) disulfide (SPS) and 3-mercapoto-1-propane sulfonate (MPS) in Cu plating solutions. A twostep CVS analysis determines the total accelerator concentration and conversion ratio.…”

“…A twostep CVS analysis determines the total accelerator concentration and conversion ratio. The modified method allows measurement of the SPS concentration with less than 10% error [20]. Iodine ions are used as a leveling agent in the copper plating process.…”