Accuracy Improvement in Cyclic Voltammetry Stripping Analysis of Thiourea Concentration in Copper Plating Baths

Abstract

Cyclic voltammetry stripping (CVS) has been regarded as a powerful tool for monitoring the concentrations of organic additives in plating baths. In this study, the dilution titration (DT) method of CVS was modified to improve the measurement accuracy of thiourea (TU) concentrations in Cu plating baths. The conventional DT-CVS method cannot guarantee high accuracy because the electrochemical behavior of TU is concentration and potential dependent, which can cause disturbances in the response signals. In this study, by adding polyethylene glycol (PEG) as an organic additive, the undesirable electrochemical behavior of TU was suppressed and the accuracy of DT-CVS was greatly improved. Using the improved method, the concentrations of TU and its derivatives in the plating bath were measured. The errors between the real and measured concentrations were reduced from 15.0%, 36.0%, and 15.0% using the conventional DT-CVS method to within 3.00%, 6.00%, and 6.00% for TU, N-ethyl thiourea, and N ,N-diethyl thiourea, respectively, using the improved method. Organic additives are important constituents of the electroplating baths owing to their ability to control the morphology and the film properties of the plated material.1-17 The organic additives are typically grouped into suppressors, accelerators, and levelers, based on their roles and functionality.1-17 Examples of organic additives in electroplating baths include bis(3-sulfopropyl) disulfide, polyethylene glycol (PEG), janus green b (JGB), polyethylene imine (PEI), 1,2,3-benzotriazole (BTA), and thiourea (TU). 1,2,[6][7][8][9][10][11][12][13][18][19][20][21][22][23][24][25][26][27][28][29] TU is a common additive in Cu and Ag electrodeposition baths and induces leveling and grain refining properties.18-26 TU has been used as an additive either solely or in combination with Cl − (TU-Cl − ) in baths used for electrodeposition processes involving various current/potential waveforms such as direct current, pulse-, or pulsereverse waveforms. 20,21,23,24 Various applications of TU as an additive have been studied, including the formation of the Cu twin, Ag superfilling, and the fabrication of CuS nanowire structures. 18,19,[23][24][25][26] TU is known to form thiolate complexes such as [Cu-(TU) ions. [27][28][29] Owing to the various derivatives and their different electrochemical responses, TU shows unique electrochemical behavior unlike typical suppressors or levelers. Although TU commonly reduces the Cu deposition rate, it often acts in an opposite role as an accelerator under specific conditions such as low concentrations and low overpotentials. [27][28][29] Several mechanisms explaining the accelerating effect of TU have been proposed. [27][28][29] Generally, the reduction of Cu 2+ occurs in two steps, namely the reduction of Cu 2+ to Cu + , followed by the reduction of Cu + to Cu 0 . The first step has been known to be the rate-determining step. In the presence of TU, however, additional reactions occur, as described in Eq. 1. 29 They explained that the chain reaction...

Monitoring of SPS Concentration by the Ring Current Using a Rotating Ring-Disk Electrode with Dissolving Disk Copper to Refresh a Void Free Solution

“…However, S. Choe pointed out that the enhancement of the electrochemical signal by SPS is interfered with the signal of MPS, the amount of SPS interfered with MPS may affect the behavior of electrodeposition solution. 21 Unfortunately, ECI did not show any void free evidences in their publications. On the other hand, S. Choe et al developed a two-step CVS analysis in which the total concentration and conversion ratio of SPS and MPS were separately determined.…”

“…The concentration of the accelerators, especially SPS is typically determined by CVS method [16][17][18][19][20][21][22] in combination with linear approximation technique (LAT) or modified linear approximation technique (MLAT). 20,22 G. Chalyt et al of ECI Technology published the method for determining the concentration of three organic additives.…”

“…On the other hand, S. Choe et al developed a two-step CVS analysis in which the total concentration and conversion ratio of SPS and MPS were separately determined. 21 Their modified method enabled the accurate prediction of the filling performance of testing solutions, successfully achieved void-free filling. However, S. Choe only experimented with two organic additives, did not include leveler in his testing solutions.…”

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

“…In addition, in order to monitor the concentration change, various methods for analyzing the concentration of decomposed organic additives were suggested. [32][33][34][35][36] As a result of SPS decomposition via chemical and electrochemical pathways, it has been suggested that PDS are produced as final breakdown products, whereas MPS, sulfopropanol, sulfoxide, SPS(O), and SPS(O 2 ) are formed as intermediates. 24 Once MPS is produced, the cathodes becomes depolarized, because MPS has a greater acceleration effect than does SPS.…”

Cyclic Voltammetry Stripping Analysis to Determine Iodide Ion Concentration in Cu Plating Bath

“…35,36 Accurate monitoring of these species followed by precise feedback ensures the solution reliability during the long-term plating operation. [37][38][39][40][41][42] We proposed novel electrochemical methods that enable the in-situ and real-time monitoring of the concentration of SPS/MPS couple, 39 PEG molecular weight, 42 and the thiourea concentration 37 in a real-scale Cu plating bath by using cyclic voltammetry stripping (CVS). Determination of I − has been widely researched in many fields.…”