Stoyan Bliznakov

Factors Controlling the Less Noble Metal Retention in Nanoporous Structures Processed by Electrochemical Dealloying

A study of the dealloying behavior of Au 0.25 Ag 0.75 alloy in solutions with different pH values (0, 1, and 5.2) demonstrates that the retention of Ag during the parting process is a result of either oxide formation or kinetic trapping in the form of an alloy. Current―voltage data show that while the dealloying critical potential is generally independent of pH, the dissolution rate is strongly affected by the solution acidity. pH substantially affects Ag retention in both the oxide and alloy forms. To discriminate between the retention forms, two different approaches employed for removing the Ag oxide from freshly dealloyed samples comprise (i) oxide dissolution in 1 M HClO 4 and (ii) oxide reduction to elemental Ag followed by a subsequent electrochemical oxidation to Ag + ions. A critical analysis of the experimental results suggests that the pH and dissolution rate affect more significantly the amount of Ag retained in the form of oxide, while the dealloying potential has a stronger impact on Ag trapping. In addition, the Ag retained as oxide is determined to be predominantly AgO. Overall, the findings of this work could enable the development of strategies for structural and compositional control in electrochemically processed metallic nanoporous materials.

Improving Copper Electrodeposition in the Microelectronics Industry

Sporadic voiding within the interfacial Cu3Sn intermetallic compound (IMC) layer-sometimes referred to as ¿Kirkendall voiding¿-has been found to lead to degradation of solder joint reliability in board level, mechanical shock testing. It has been suggested that the voiding phenomenon is a result of the incorporation of organic impurities in the copper (Cu) deposit during electroplating. In the present study, Cu samples were electroplated galvanostatically from a generic solution, containing Cl- ions, as well as a suppressor [polyethylene glycol (PEG)], and a brightener (bis(3-sulfopropyl) disulfide, SPS) as additives. Overpotential transients were measured during electroplating with a range of current densities (0.5-40 mA cm-2) in baths with various compositions. Effects of the bath chemistry on the Cu surface morphology, as well as on the propensity for voiding after soldering, were also investigated. Elemental analysis of selected samples was performed by SIMS. Plating at 10-20 mA·cm-2 with an optimized bath composition led to Cu with a fine-grain structure and smooth appearance. Solder joints formed from these deposits remained void free after soldering and thermal aging. Lower current densities, ran in the same plating bath, led to a significant propensity for voiding, apparently because of incorporation of, principally, SPS and its breakdown products into the growing layer. Continuous plating at 10 mA cm-2 for up to 18 hours without replenishment revealed a strong dependence on bath aging, with Cu changing from ¿void-proof¿ to clearly ¿void-prone.¿ These trends were attributed to the different rates of consumption for PEG and SPS and changes in the contaminants being incorporated in the deposits. In general, differences in the voiding behavior of the plated Cu could be predicted by monitoring a set of characteristic overpotential transient signatures.

Electrochemical Method for Quantitative Determination of Trace Amounts of Lead

An ultrasensitive chronoamperometric method for quantitative determination of trace amounts of lead (down to 20 ppb) in acidic solutions is proposed in this paper. The method is based on observations that a complete underpotentially deposited (UPD) lead layer inhibits the electroreduction of nitrate on a bare Cu(111) electrode. To asses the limits of the method, both the electroreduction of nitrate and UPD of lead monolayer on copper single (111) and polycrystalline electrodes in perchloric acidic solution are studied by means of cyclic voltammetry, chronoamperometry, and rotating disk electrode (RDE) experiments. It is found that an inexpensive polycrystalline copper electrode is sensitive enough for analytical detection of lead traces in electrolytes down to 1 x 10(-8) M. Analytical results obtained by the proposed method in 2 orders of magnitude concentration range are compared to atomic absorption spectroscopy measurements to evaluate and assess the sensitivity of the employed experimental protocol. The excellent match between both analytical approaches validates the applicability of the proposed method.

Double-scale roughness and superhydrophobicity on metalized Toray carbon fiber paper.

A simple, inexpensive route for the fabrication of a superhydrophobic metal surface is described. Carbon-carbon composite paper (Toray TGP-H) is electroplated with copper. The copper layer is made hydrophobic by self-assembling a monolayer of dodecanethiol. The surface topography required to induce superhydrophobic behavior is achieved by varying the plating bath composition (Cl-, PEG, and SPS additives) and the time of deposition (effective thickness of the Cu layer). The surface morphology created by the original arrangement of the carbon fibers in the Toray paper (diameter 8 microm, spacing 30 microm) does not produce superhydrophobic behavior. This is true for both continuous and incomplete copper coatings. Truly superhydrophobic behavior (large contact angles, 160-165 degrees, and very small contact angle hysteresis, 2 to 3 degrees) is achieved when a continuous copper layer is deposited on the carbon fibers and also a second micrometer-range roughness is developed as a result of the formation of small copper crystallites (size approximately 1 microm).

Controlling Cu electroplating to prevent sporadic voiding in Cu 3 Sn

One reliability concern, when soldering to a Cu surface finish, is the sporadic mechanical degradation of solder joints due to the formation and growth of voids within the interfacial Cu3Sn intermetallic compound (IMC) layer and at its interface with the Cu pad structure. Excess organic impurity incorporation during Cu electroplating has been shown to cause this problem. The level of impurity incorporation is found to depend greatly on interactions between the plating additive chemistry and the plating process parameters. A general picture has been developed, based upon the parabolic adsorption behavior of organic molecules on metal electrodes in aqueous plating solutions as a function of the applied potential. Thus the propensity for voiding, in soldering and subsequent thermal aging, can be manipulated by varying a range of plating parameters. The mechanistic understanding required to devise practical control is outlined. Any remaining research required for the formulation of step-by-step process guidelines is identified.

Understanding, Controlling and Minimizing the Voiding, Sporadically Occurring in Solder Joints with Electroplated Copper

The present work offers generic remedies for a problem that is known to have cost the industry billions of dollars. When an interconnect is made between typical Sn containing solder and Cu pads, metallurgical reactions lead to the formation of Cu6Sn5 and Cu3Sn intermetallic compounds (IMC), as seen in Figure. Right after reflow the latter is usually extremely thin, but both grow subsequently by solid state reactive diffusion processes. On rare occasions the Cu3Sn intermetallic compound formed with specific electroplated Cu samples has been seen to develop voids and weaken to a catastrophic extent over time near or at room temperature. More commonly voids have been seen to grow in accelerated thermal aging and cause wear out or fatigue life degradation in board level testing [1, 2].