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THE SIMULTANEOUS DETERMINATION OF SULFITE AND SULFATE IONS IN ENGLEHARD GOLD PLATING BATHS USING ION CHROMATOGRAPHY

IP.com Disclosure Number: IPCOM000006579D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2002-Jan-16
Document File: 3 page(s) / 145K

Publishing Venue

Motorola

Related People

Cary B. Powell: AUTHOR [+2]

Abstract

The APDAC TAB organization is concerned with the bumping of wafers using an Englehard gold sufite system where 22pm ofgold are plated electrolyticalIy in about 22 minutes. The Englehard system consists of gold complexed with sulfite plus borate and hydroxide as elec- trolytes. As the bath is used the sulfite is oxidized to sulfate during electrolysis and/or contact with air. Ini- tially we analyzed the bath only for gold content by inductively coupled plasma spectroscopy (ICP) and for sulfite concentration by iodometric titration. However we now know that bath performance is a function of the buildup of sulfate in the system causing an increase in conductivity and "spikes" (single crystal growth) in the gold finish, therefore it became necessary to develop a procedure to determine the concentration of the sulfate in the bath.

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MOIOROLA INC. Technical Developments Volume 16 August 1662

THE SIMULTANEOUS DETERMINATION OF SULFITE AND SULFATE IONS IN ENGLEHARD GOLD PLATING BATHS USING ION CHROMATOGRAPHY

by Cary B. Powell and Eric J. Woolsey

INTRODUCTION

   The APDAC TAB organization is concerned with the bumping of wafers using an Englehard gold sufite system where 22pm ofgold are plated electrolyticalIy in about 22 minutes. The Englehard system consists of gold complexed with sulfite plus borate and hydroxide as elec- trolytes. As the bath is used the sulfite is oxidized to sulfate during electrolysis and/or contact with air. Ini- tially we analyzed the bath only for gold content by inductively coupled plasma spectroscopy (ICP) and for sulfite concentration by iodometric titration. However we now know that bath performance is a function of the buildup of sulfate in the system causing an increase in conductivity and "spikes" (single crystal growth) in the gold finish, therefore it became necessary to develop a procedure to determine the concentration of the sulfate in the bath.

  The first procedure tried was to analyze the total sulfur content using an ICP spectrometer. Once the total sulfur content was determined and the sulfite concen- tration was found (by titration), the sulfate content was deduced by back calculation. The spectroscopic tech- nique was not reproducible or repeatable on a day to day basis possibly due to interferences from large gold and alkali metal concentrations.

  The next procedure tried was to analyze both sul- fate and sulfite by ion chromatography (IQ. The mobile phase was a 20mM phosphate buffer using an ion exclu- sion column (Waters ICPak A) to perform the separa- tions. The concentrations of the species were determined using standard peak area integration techniques. The results gave us both sulfate and sulfite concentrations but also indicated the presence of sulfide and thiosulfate ions as well.

  After some experiments in trying to better resolve the sulfate and sulfite peaks it was concluded that the phosphate buffer is not the ideal buffer for sulfur speciation. Other researchers doing IC separation of sul- fur species have used both borate and napthalenetrisul-

0 Mofarola. Inc. 1992

fonate buffer systems for their separations. The napthalenetrisulfonate buffer is best suited for the sepa- ration of thiosulfate and polythionates and not for sul- fate and sulfite therefore we decided to try a borate buffer.

  The new mobile phase is a IOmM borate butfer using the same ion exclusion column, and concentrations were again determined from peak areas. The results so far have been encouraging since we have reduced the anal- ysis time, reduced the background noise, and improved the sensitivity. Efforts to baseline resolve sulfite from sulfate are still in process but in running separate stand- ards we have learned that dilute aqueous sulfite solu- tions are highly unstable and oxidize to sulfate rapidly. Therefore we have adopted to de...