Copper-Platinum Deposition by Pulse Plating Kanchan Mondal a and Shashi B. Lalvani a,z Paper and Chemical Engineering, Miami University, Oxford, Ohio 45056 USA In order to improve the mechanical properties of copper, pulse plating techniques were investigated for deposition of Cu-Pt alloys employing a pyrophosphate bath containing chloroplatinic acid as a source of platinum. Cyclic voltammetry experiments showed that the copper reduction is a two-step process and that platinum reduction occurs at potentials close to that observed for copper deposition. Forward peak current densities ranging from 2.5 to 7.5 A dm -2 were employed for the bulk electrodeposition experi- ments. Bright, shiny, and crack-free deposits were obtained at low current densities. The amount of platinum observed in the deposits was found to increase with the current impressed for both forward pulse and pulse reverse techniques. The Knoop hardness was found to increase with the platinum content of the deposits. The corrosion rate of the deposits measured in a solution of NaCl was found to decrease with platinum content. The data show that deposits containing up to 5.6 and 6.5 wt % of platinum can be obtained by forward pulse only and pulse reverse plating, respectively. As compared to a copper sample, the Cu-Pt deposits obtained by forward pulse and reverse pulse techniques exhibited a 31 and 55.4% increase in Knoop hardness, respec- tively. As compared to copper, up to a 45.4% increase in corrosion resistance was observed for deposits produced by the application of a forward pulse. The deposits obtained by pulse reverse exhibited a 35.6% improvement in corrosion resistance over those obtained by the forward pulse technique under identical forward peak current density. © 2006 The Electrochemical Society. DOI: 10.1149/1.2186760 All rights reserved. Manuscript submitted July 26, 2005; revised manuscript received January 26, 2006. Available electronically April 10, 2006. Copper has a very high thermal conductivity and can easily be electroplated, hence it is used to form complex shapes and seamless objects free of stresses. In addition, copper exhibits good corrosion resistance. However, it suffers from relatively low mechanical strength as compared to nickel, which also possesses comparable, albeit somewhat lower thermal conductivity. Previous research 1 has shown that the incorporation of platinum in the electroplated copper enhanced its mechanical strength. A pyrophosphate bath Table I employing chloroplatinic acid as a source of platinum was investigated. 1 Bright, shiny, and crack-free deposits were obtained at low current densities i.e., 1–2 A dm -2 . The Knoop hardness was found to increase with platinum content of the deposits. As com- pared to electrodeposited copper from the acid bath, the Cu-Pt de- posits exhibited a 17% increase in Knoop hardness and a 21% in- crease in corrosion resistance. 1,2 Previous research 1 was an investigation of dc methods for Cu-Pt deposition. However, in this research pulse plating methods were employed as they offer certain advantages. 3 Pulsing current has been used in the past to improve the quality of the deposits that cannot normally be achieved by dc. For example, low-frequency unipolar pulses have been used for the manufacture of copper foils. 4 Pulsing technology is also employed to increase the brightness of the deposit by planarizing and is extensively used in plating elec- tronic interconnects. 5 Nickel electrodeposits showed significant im- provements by the use of pulsed electrodeposition. 6-8 In a recent paper, 9 it was demonstrated that electrodeposit microdistribution from uniform to strongly antileveling can be achieved via the appli- cation of different types of galvanostatic and potentiostatic current pulses. Though pulse plating was initially used to plate copper and gold, extensive work on other metals such as nickel, chromium, palladium, zinc, silver, etc. has also been reported in the literature. 10-12 Apart from single metals, the pulse plating techniques are also extended to plating numerous alloys. 13-15 Some of the other advantages observed include improved throwing power, selective deposits, improved plating over difficult-to-plate geometries, re- duced consumption or elimination of additives, increased through- put, increased repeatability, reduced cost, and improved alloy plating. 16,17 However, these techniques have its own disadvantages, such as high initial investment of time and capital to identify the optimal conditions. Reviews of the applications of pulse plating and the limitations have been presented by Devraj et al. 18 and Pearson and Dennis. 19 The pulse reversal method involves reversal of the current flow for a specified time. By doing this, the deposit is essentially polished during the reverse period. 20 Pulse reverse is an excellent way to equalize the plating thickness distribution. Areas exposed to current densities are plated during the cathodic cycle and metal is dissolved and/or passivated during the anodic cycle. This procedure is em- ployed to polish areas that tend to overplate during the cathodic part of the cycle. As with the pulse plating method, the magnitude of the current density and reverse time can be manipulated very easily. 21 Advantages of this technique include improvements in porosity, ductility, hardness, electrical conductivity, wear resistance, and roughness, as well as enhancement of plating thickness distribution, the composition and structure of which are not easily achievable. Periodic pulse reverse PPR plating was introduced as an improved processing technique for acid-copper plating. 22 The technology im- proved plating distribution at relatively high current densities, thereby enhancing the productivity of high-technology products, de- fined as high-aspect-ratio through-hole bias. The use of PPR, as an alternative to the use of additives for electrochemical deposition of nickel was studied by Tang. 6 With optimized pulse plating param- eters, and in some cases in combination with additives, substantial improvement of the deposit properties was achieved. The deposits obtained were smooth with low residual stress. In this paper, cyclic voltammetry experiments were conducted to understand the mechanism of Cu and Pt deposition. The results from forward and pulse reverse electrodeposition of Cu-Pt are also pre- sented. Platinum content in the deposits was correlated with peak forward and reverse pulse current densities. Corrosion rates of the deposits in NaCl solutions were determined. In addition, the Knoop hardness was measured as a guide for evaluating the mechanical strength of the electrodeposits. Experimental Cyclic voltammetry.— The voltammograms in this study were recorded using a conventional three-electrode configuration. The reference electrode was a saturated calomel electrode SCE. A plati- num microelectrode was used as the working electrode while a plati- num wire was employed as the counter electrode. The experiments were conducted using a Gamry Instruments P3 potentiostat. The data acquisition system used was Gamry Instruments CorrWare 3.0 with CV 130 module. Electrodeposition.— Copper platinum alloy was electrodeposited onto rectangular stainless steel sheets of 15  50 mm. The substrate was polished using 240, 320, 480, and 600 grit wet SiC polishing paper successively. The anode was made of a rectangular sheet of copper measuring 30  50 mm. Both anode and cathode were a Present address: Department of Mechanical Engineering and Energy Processes, Southern Illinois. z E-mail: lalvansb@muohio.edu Journal of The Electrochemical Society, 153 6 C393-C399 2006 0013-4651/2006/1536/C393/7/$20.00 © The Electrochemical Society C393 Downloaded 27 Jun 2009 to 131.230.196.14. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp