Yutaka Okinaka

Some recent topics in gold plating for electronics applications

A review is presented of selected recent topics in electrolytic and electroless gold plating for electronics applications. The topics covered include developments of non-cyanide electroplating baths for plating soft gold suitable for fabricating microbumps on silicon wafers, electroplating of hard gold and alternative materials with thermally stable electrical contact resistance and wear resistance for use on connectors exposed to elevated temperatures, and neutral, non-cyanide electroless processes for plating pure, soft gold on isolated areas of circuit boards. The development of the new electroless processes has been a subject of great interest and activity, and therefore an extensive survey of the progress in this field is included.

Effects of Saccharin and Thiourea on Sulfur Inclusion and Coercivity of Electroplated Soft Magnetic CoNiFe Film

During the course of our recent work performed to develop an electroplated CoNiFe temary alloy with high saturation magnetic flux density and low coercivity for use in magnetic recording heads, it was observed that two common sulfur-containing additives, saccharin and thiourea, behave differently with respect to the dependence of sulfur inclusion and coercivity of the alloy film on the additive concentration in the plating bath. To understand the cause of this difference, scanning tunneling microscopy (STM) was performed, using Au(111) as the substrate, to examine the structure of the adsorbed layers of the additive molecules. The result revealed that the nature of adsorption is fundamentally different for the two different additives; i.e., the adsorption of saccharin is physical and reversible, whereas thiourea undergoes irreversible chemisorption. This finding is consistent with the known behaviors of the two additives in the electroplating of nickel. In this paper the different effects of saccharin and thiourea in the electrodeposition of CoNiFe alloy are interpreted based on the STM results and relevant information available in the literature on the electrodeposition of nickel.

Electrodeposition of soft gold from a thiosulfate-sulfite bath for electronics applications

The possibility of electrodeposition of soft gold from a thiosulfate-sulfite bath was explored for electronics applications. The bath does not contain cyanide, and it is operated at a near neutral pH and a mildly elevated temperature. The bath is stable, does not undergo spontaneous decomposition without the addition of any stabilizer, and yields gold deposits with a hardness sufficiently low for use as gold bumps on semiconductor devices. Factors affecting the hardness were investigated in detail. It is shown that the use of high concentrations of the complexing agents and/or the addition of thallium(I) ions decreases both hardness and sulfur content of the deposit. The lowest Vickers hardness values achieved were approximately 80 kg mm -2 in the as-deposited state and 50 kg mm -2 after annealing at 350°C for 30 min in air. The relationship between hardness and microstructure of the deposit was also examined.

Some recent developments in non-cyanide gold plating for electronics applications

The plated gold being used by the electronics industry can be broadly classified into two categories: soft gold and hard gold. Soft gold is used for circuit metallization and for bonding semiconductor chips, while hard gold is indispensable as the contact material on electrical connectors, electromechanical relays, and printed circuit boards. The traditional baths from which to plate soft gold as well as hard gold contain the cyanide complex, [Au(CN)2]-, as the source of gold, which liberates free cyanide ions during the plating. The free cyanide is not only highly toxic but also attacks photoresists used to delineate circuit patterns and bonding pads. For these reasons, noncyanide baths are in use to plate soft gold, whereas hard gold can be plated only from cyanide baths at present. In this presentation, the current status of both electrolytic and electroless non-cyanide processes for plating soft gold will be reviewed.

Void-free trench-filling by electroless copper deposition using the combination of accelerating and inhibiting additives

Electroless copper deposition was performed on submicrometer-trench patterned substrates with a bath containing 8-hydroxy-7-iodo-5-quinoline sulfonic acid (HIQSA) as an accelerating additive and polyethylene glycol (PEG) as an inhibiting additive. Void-free copper filling of trenches was achieved by the addition of both HIQSA and PEG at specific concentrations. Copper deposition rate measurements revealed that HIQSA accelerated the deposition only when it was added together with a very low concentration of PEG. The void-free filling is considered to have resulted from the significant acceleration brought about by HIQSA at the trench bottom, where the concentration of PEG is low.

Evidence for “superfilling” of submicrometer trenches with electroless copper deposit

Void-free copper filling of trenches for ultralarge scale integrated interconnect structure was demonstrated by electroless deposition technique using polyethylene glycol as an inhibiting bath additive. With this electroless plating bath, the authors succeeded in demonstrating superfilling. Of particular interest is that the deposition at trench opening was nil during the filling process, while that at the bottom was very fast. This letter presents a demonstration and a proof of superfilling of trenches by electroless copper deposition.

Mechanism of Sulfur Inclusion in Soft Gold Electrodeposited from the Thiosulfate-Sulfite Bath

The hardness of the soft gold electrodeposited from the thiosulfate-sulfite mixed ligand bath which we developed recently, has previously been found to increase with increasing sulfur content of the gold deposit. In the present study the mechanism of the sulfur inclusion was investigated by analyzing for sulfur gold deposits produced under various experimental conditions and also by observing weight gains resulting from the adsorption of sulfur-containing species on gold using the quartz crystal microbalance (QCM) technique. The results led us to conclude that the origin of the included sulfur is primarily the adsorbed Au(I)-thiosulfate species. (Au 2 S 2 O 3 ) ads , formed as an intermediate in the gold deposition reaction.

Substrate (Ni)-catalyzed electroless gold deposition from a noncyanide bath containing thiosulfate and sulfite I. Reaction mechanism

Electroless gold deposition is known to take place on Ni-based substrates even when the conventional reducing agent is excluded from the autocatalytic bath containing thiosulfate and sulfite as ligands for gold ions. It is shown in this paper that the reaction of gold deposition from this bath on electroless Ni-B substrate is not autocatalytic but, instead, proceeds primarily by the mechanism of substrate catalysis with galvanic displacement playing a secondary role. It was established that sulfite serves as the reducing agent for the substrate-catalyzed gold deposition reaction.

Enhancement of the Ductility of Electrodeposited Copper Films by Room-Temperature Recrystallization

In conjunction with a study of the copper electrodeposition process from the acid copper sulfate bath for the fabrication of interconnections of printed circuit boards and semiconductor devices, an investigation was performed of the effect of bath additives on the relationship between the ductility of the copper deposit and its crystallographic structure and electrical resistivity. Room-temperature recrystallization, or so-called self-annealing, is known to occur in copper electrodeposits obtained from baths containing Cl - , polyethylene glycol, and bis(3-sulfopropyl)disulfide as additives. Variation with time of the crystallographic orientation, grain size, and resistivity of the deposit was followed over a period of several weeks after the deposition. During the period of self-annealing, ductility was found to increase by a factor of 1.5. The increase in ductility is shown to be related to a change in microstructure of the copper deposit.

Significance of inclusions in electroplated gold films for electronics applications

Minute amounts of metallic as well as non-metallic inclusions significantly affect both bulk and surface properties of the electroplated gold films being used by the electronics industry. Surface effects are caused by the presence of extraneous materials diffused out from the interior to the surface, while bulk effects are brought about by the influence of inclusions on the microstructure of the gold films. This article reviews the current status of understanding of such inclusion effects in both soft and hard gold: these play essential roles in the fabrication of todays microelectronic devices.