Mark J. Willey

SPS Adsorption and Desorption during Copper Electrodeposition and Its Impact on PEG Adsorption

The adsorption and activation of bis (3-sulfopropyl)-disulfide (SPS) has been studied using a microfluidic electrochemical device. The device provides for accurate transitioning of solution over the working electrode allowing for addition or removal of additives from a base plating electrolyte. The transition from a Cl - plating bath to a polyethylene glycol (PEG)/SPS/Cl - bath shows a quick ( < 2 s) suppression of copper deposition similar to that seen with a PEG/Cl - bath. Acceleration at longer times, presumably by the activation of adsorbed SPS species, is a function of SPS concentration and applied cathodic potential. When copper is plated in the presence of SPS and Cl - for times around 100 s before the introduction of PEG, the activated SPS appears to inhibit the adsorption of PEG to the surface. System response to chloride ion addition or removal is relatively fast. Desorption of an activated SPS/PEG/Cl - layer occurs slowly, even at high cathodic potentials of -0.275 V, where current densities are over 50 mA cm -2 . Numerical simulations show that the time needed for PEG to saturate the bottom of a sub-100 nm feature is small relative to SPS activation times.

Acceleration Kinetics of PEG, PPG, and a Triblock Copolymer by SPS during Copper Electroplating

Three copper-plating suppressors are examined in three-additive baths: a polyethylene glycol (PEG), a polypropylene glycol (PPG), and a triblock copolymer of the two. Bis(3-sulfopropyl)-disulfide (SPS) is found to transition each to a state of nonsuppression, i.e., accelerate each, at a rate dependent on the suppressor molecule, the SPS concentration, and, to a lesser extent, the suppressor concentration. Using a planar microscale working electrode (d = 100 μm), the kinetic currents of the plating reactions are observed without the influence of ohmic resistance, revealing far higher current densities than previously reported. Potentiostatic and galvanostatic experiments of additive adsorption at short times, t < 20 s, are compared quantitatively using a surface-blocking model to transform the data to effective surface coverage, θ EFF , vs time. A major difference is found in SPS acceleration between galvanostatic and potentiostatic experiments, with the rate of change in suppression being proportional to the current density. This results in a constant rate of change in θ EFF under constant current but a self-reinforcing rate of change in θ EFF at constant potential. A simple additive model is introduced to characterize the results.

Microfluidic Studies of Adsorption and Desorption of Polyethylene Glycol during Copper Electrodeposition

A previously described microfluidic electrochemical cell [M. J. Willey and A. C. West, Electrochem. Solid-State Lett., 9, El 7 (2006)] has been used to characterize the rates of adsorption and desorption of polyethylene glycol (PEG) onto copper (Cu) during electrodeposition. Galvanostatic and potentiostatic response to the introduction or removal of PEG from the bulk solution is measured. Adsorption time constants are ∼ 0.5 s or less for bulk PEG concentrations greater than 50 ppm. For lower concentrations, adsorption is mass-transfer controlled for the conditions prevailing within the microfluidic cell, where the average diffusion layer thickness is ∼2 μm. PEG desorption rates are much slower, typically between 10 and 100 s. For most conditions, the electrochemical signal during desorption consists of two regions, where the first stage shows a slow linear variation and the second stage a fast exponential-type transition. In the first stage, it is hypothesized that the PEG layer blankets the electrode surface. Once the polymer-layer thins sufficiently, the layer becomes patchy, and the second desorption stage commences.

Copper Filling of 100 nm Trenches Using PEG, PPG, and a Triblock Copolymer as Plating Suppressors

Patterned 100 nm trenches with an aspect ratio of 3.5 are filled at a nominal current density of -6.6 mA/cm 2 . A low acid copper plating bath is used with the accelerator bis(3-sulfopropyl)-disulfide (SPS) and one of three suppressor molecules: Poly(ethylene glycol) (PEG) 3350, poly(propylene glycol) (PPG) 725, and an ethylene--propylene-ethylene (EPE) oxide triblock copolymer at a molecular weight of 2000 g/mol. All suppressors result in superconformal filling, although the filling rates vary widely. EPE 2000 results in the most rapid filling, metallizing the features without voids in 5 s under some conditions. The superior performance of EPE 2000 is attributed to its high suppression strength, which is greater than either PEG 3350 or PPG 725 at the relevant plating potentials. EPE 2000 filling becomes more rapid as suppressor concentration decreases, down to 100 ppm, which is attributed to a strong correlation between EPE 2000 concentration and adsorption time. EPE 2000 performance is also improved as SPS concentration decreases, a result in contrast to literature observations on larger (500 nm) features. A simple expression is developed to demonstrate that the time scales of suppression, acceleration, and filling can account for this result.

Adsorption Kinetics of Polyvinylpyrrolidone during Copper Electrodeposition

Adsorption studies of polyvinylpyrrolidone (PVP) during potentiostatic copper electrodeposition in the presence of common plating additives are presented. Experiments were conducted using a flow-through microfluidic electrochemical cell. Results show that in the presence of chloride ions, PVP adsorption characteristics are similar to those of polyethylene glycol (PEG), with very fast suppression achieved at PVP concentrations above 200 ppm. In contrast to PEG, PVP inhibits deposition rates even on electrodes in which Cu has been predeposited in the presence of bis(sulfopropyl)disulfide.

A Microfluidic Device to Measure Electrode Response to Changes in Electrolyte Composition

A microfluidic device is used to measure electrode response to rapid changes in electrolyte composition. These changes in composition are achieved by switching the electrolyte flowing through an electrolyte channel containing a working and counter electrode. In the present investigation, the time response of the device was characterized by reducing tri-iodide ions at a rate controlled by mass-transfer to the electrode surface. Results are compared with finite-element simulations that assume an ideal two-dimensional parabolic flow. Simulated, theoretical, and experimental steady-state limiting current densities are all in excellent agreement. Experimental time constants are also in accord with simulations.

Influence of Aromatic Functionality on Quaternary Ammonium Levelers for Cu Plating

Electrochemical and spectroscopic methods are utilized to study the interaction of three levelers ― Dodecyltrimethyl ammonium bromide (DTAB), benzyldimethylhexadecyl ammonium chloride (BDAC), and thonzonium bromide (ThonB) ― with Cu electrode surfaces. Electroplating Cu in the presence of 0, 10, 25, and 50 ppm concentrations of each of the levelers showed that each of the additives inhibit the onset of Cu deposition. Surface Enhanced Raman Spectroscopy (SERS) showed that DTAB exhibits no potential dependent behavior while both BDAC and ThonB exhibit potential dependent behavior. The stability of DTAB, evidenced by the gradual onset Cu deposition and lack of potential dependence in the SERS data, is possibly due to the formation of hemi or pre-micellar structures on the Cu surface. Stabilization of BDAC and ThonB is attained by the electrostatic interactions with the surface as evidenced by the potential dependent behavior exhibited in the SERS. One contributing factor to the performance of amphiphilic cationic surfactants as levelers is the concentration at which micellization occurs (cmc).

Uniformity Effects when Electrodepositing Cu onto Resistive Substrates in the Presence of Organic Additives

Cu electrodeposition on thin-film, highly resistive, Pt electrodes was studied experimentally. Electroplating additives can have a large impact on thickness uniformity across long resistive electrodes. Results are compared to simulations, and it is shown that nucleation influences thin-film uniformity and that electroplating additives may also cause an increase in the nonuniformity due to the spatial variation in their acceleration or inhibition across the electrode. The spatial variations in suppression are particularly strong in the presence of bis(3-sulfopropyl disulfide), whose deleterious effect can be reduced by a leveler.

Adsorption and Desorption Kinetics of a Block Copolymer Wetting Agent Used in Copper Electroplating

Current development of copper electroplating additives has led to the development of suppressors that not only allow for proper bottom-up fill but also that wet the surface properly for defect-free entry of the copper-seeded wafer into the plating bath. The current work analyzes the adsorption and desorption characteristics of a common block copolymer wetting agent using an electrochemical flow-through cell. Adsorption characteristics are similar to that of polyethylene glycol (PEG) during electroplating, where the adsorption process becomes a strong function of polymer bulk concentration under 200 ppm and where increasing applied potential or current causes the surface suppression of the polymer to decrease. Desorption experiments showed similar results to PEG as well, where a two-stage desorption is seen when the applied potential or current becomes large enough. Computation fluid dynamic modeling of the flow-through cell allowed for the determination of the saturated surface coverage at -0.1 V vs Ag/AgCl, which was 0.041 μg/cm 2 . Additionally, the adsorption rate constant was modeled to be 6.02, 2.045, and 1.22 X 10 5 m 3 /kmol s for applied potentials of -0.1, -0.15, and -0.2 V vs Ag/AgCl, assuming a constant saturated surface coverage of 0.041 μg/cm 2 .