Alan C. West

Copper Deposition in the Presence of Polyethylene Glycol I. Quartz Crystal Microbalance Study

The addition of polyethylene glycol (PEG) and Cl - to an acid copper electrolyte inhibits the deposition reaction for cathodic overpotentials of up to about 150 mV. Adding Cl - only promotes the deposition reaction, while adding PEG alone has a relatively small effect on electrode kinetics. Frequency shifts of an electrochemical quartz crystal microbalance suggest the adsorption of a monolayer of PEG molecules that are collapsed into spheres provided chloride ions are present, with little adsorption occurring when Cl - is absent. This behavior is the same for gold and copper surfaces. Transient current measurements suggest that chloride ions affect the PEG adsorption equilibrium rather than adsorption kinetics alone.

Leveling and Microstructural Effects of Additives for Copper Electrodeposition

The roles of two model additives, bis(3‐sulfopropyl) disulfide (SPS) and Janus Green B (JGB), in the deposition of copper from an acid‐copper sulfate electrolyte containing polyethylene glycol (PEG) and were studied by deposition on microprofiled electrodes, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that leveling occurs only when all four additives are present, suggesting that additive‐additive interactions are important to the leveling mechanism. Moreover, an optimal flux of the active leveling agent exists, an effect that may be explained by the classical diffusion‐adsorption theory of leveling. SEM and TEM micrographs show that the additive SPS removes the columnar structure of the deposit and effects micron‐sized, unoriented grains provided PEG and are present; the subsequent addition of JGB decreases the grain size of the film significantly.

Copper Deposition in the Presence of Polyethylene Glycol II. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) results are shown and interpreted with regard to other recent experimental studies for copper deposition in the presence of polyethylene glycol and Cl - . The model assumes the adsorption of a nearly complete monolayer of PEG in the presence of chloride ions and no adsorption without Cl - . The primary effect of PEG adsorption which does not appear to vary with time during an EIS measurement, is a blocking of available surface sites for charge transfer. The model effectively predicts changes in steady-state and EIS results with changes in Cl - concentration and rotation rate on a rotating disk electrode.

Electrochemical and Fill Studies of a Multicomponent Additive Package for Copper Deposition

An acid-copper plating bath containing chloride ions polyethylene glycol (PEG), bis(3-sulfopropyl) disulfide (SPS), and Janus Green B (JGB) has been characterized by electrochemical methods and by fill studies on electrode surfaces patterned with trenches ranging in size from 200 to 600 nm, with aspect ratios between 2 and 4. The electrochemical methods employed include linear sweep voltammetry (LSV), chronoamperometry (CA), and cyclic voltammetric stripping (CVS) on a rotating disk electrode. Comparison of the methods shows that in all the cases an analysis of LSV and CA gives consistent estimates of an effective surface coverage of additives. CVS results are consistent with the other results only in baths that are free of JGB. The ability to produce void-free deposits of a hath containing all of the additives can be understood in part with LSV and CA results However a very good superfilling can also be obtained from a plating bath containing chloride ions, PEG, SPS, but no JGB. This observation cannot be easily interpreted with measurements within the context of established leveling theories.

Effect of Electrolyte Composition on Lithium Dendrite Growth

Lithium deposition is observed in situ using a microfluidic test cell. The microfluidic device rapidly sets up a steady concentration gradient and minimizes ohmic potential loss, minimizes electrolyte usage, and shows good repeatability. Dendrite growth is observed at different current densities for electrolytes containing lithium hexafluorophosphate or lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) in mixtures of propylene carbonate (PC) and dimethyl carbonate. Dendrites are formed at shorter times in electrolytes containing LiTFSI and high amounts of PC. The time to first observed dendrites increases linearly (for all electrolyte compositions) with a resistance given by the Tafel slope of the lithium reduction polarization curve.

Three-Additive Model of Superfilling of Copper

A model describing the effect of an accelerator bis(3-sulfopropyl)disulfide (SPS) and the two inhibitors poly(ethylene glycol) and Janus Green B (PEG and JGB) on the leveling efficacy on submicrometer trenches of an acid-copper plating bath is described, simulated, and compared to experimental till studies. All parameters of the model arc estimated from electrochemical measurements on a nonpatterned, rotating disk electrode A multicomponent version of a Frumkin isotherm is employed to account for the interaction between SPS and PEG, and the interaction between PEG and JGB are described by a competitive adsorption, Langmuir model. Shape-change simulations employing these descriptions of the surface phenomena are consistent with fill studies over a large range of SPS and JGB concentrations; however, simulations do not predict the result that void-free deposits are achieved when the concentration of JGB is zero.

Theory of Filling of High‐Aspect Ratio Trenches and Vias in Presence of Additives

Numerical simulations are employed to estimate the importance of parameters such as leveling-additive mobility and concentration on a processs capability of filling trenches or vias of a given size. Geometries relevant to ultralarge scale integration-copper technologies are considered. Simulation results are presented within the context of a suggested experimental protocol. The importance of controlling the operating conditions is demonstrated. For example, simulations indicate that a poorly controlled additive concentration can lead to an increase in void size over that which would be obtained in an additive-free bath. Finally, the design of robust processes is discussed in terms of the need to account for apparently random variations in the process.

Influence of rib spacing in proton-exchange membrane electrode assemblies

A two-dimensional design analysis of a membrane-electrode assembly for a proton-exchange membrane fuel cell is presented. Specifically, the ribs of the bipolar plates restrict the access of fuel and oxidant gases to the catalyst layer. The expected change in cell performance that results from the partial blocking of the substrate layer is studied by numerical simulation of the oxygen electrode and the membrane separator. The effects of rib sizing and the thickness of the gas-diffusion electrode on the current and water distributions within the cell are presented. For all of the cases considered, the two-dimensional effect only slightly alters the half-cell potential for a given applied current but has a significant influence on water management.Concentrated solution theory with variable transport properties is used in the membrane electrolyte to solve for the electrical potential and local water content. The Stefan-Maxwell equations are used in the gas-diffusion electrode to determine the local mole fractions of nitrogen, oxygen and water vapour.A control-volume formulation is used for the resolution of the coupled nonlinear differential equations. One advantage of the control-volume approach over finite-difference methods is the relative ease in which internal boundary points in fuel-cell and battery models are handled. This and other advantages are briefly discussed.

Pulse reverse copper electrodeposition in high aspect ratio trenches and vias

A theoretical model, based on a one-dimensional approximation, for the filling of high-aspect ratio trenches and vias with copper by pulse reverse electrodeposition is presented. The dependence of void size on the pulse waveform is demonstrated for submicron features. It is shown that the off-time should be on the order of the diffusion-time constant and that the deposition time should be smaller than the diffusion time constant. Reverse current, i.e., dissolution, is shown to play an important role in diminishing void size. Comparison with two-dimensional calculations indicates that the theory is an adequate yet efficient means of simulating shape change.

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.