Some Practical Aspects of Electroless Gold Plating

Abstract

The electroless gold plating process using potassium borohydride as the reducing agent has been investigated for impurity effects, material compatibility, bath agitation effects, and thickness uniformity and line resolution in selective plating of patterned substrates. Impurities may cause a decrease in plating rate [Ni(II) ], bath instability [Ni(II), Co(II), Fe(II) ], thickness nonuniformity (polyethylene, organics in deionized water), and nodule formation (some surfactants). Bath agitation is beneficial: it increases plating rate, minimizes porosity of thin deposits, and eliminates nodule formation. Edge build-up generally occurs in selective pattern plating but, with proper selection of bath compositions and agitation conditions, it can be maintained below 10% in the thickness range of 1-12 ~m. The rate of lateral growth of electroless gold deposits is about 60% of that of perpendicular growth under optimum plating conditions. Also considered in this paper are certain aspects of the scale-up and waste disposal problems associated with electroless gold plating.Electroless gold plating has been found to be useful in a variety of applications, especially for selective plating on patterned substrates for electronics applications. Such applications generally require pure soft gold with a thickness in the range of 1-15 ~m. A bath developed in this laboratory (1) has been found to be quite suitable for forming such deposits. Previous papers described the general bath characteristics (1), physical properties of deposits (2), bath operation with replenishment (3), reaction mechanism (4), and the nucleation and growth of deposits (5). The purpose of this paper is to describe several other aspects of the process which are important from the practical viewpoint and which have hitherto not been discussed. The topics covered include impurity effects, material compatibility, bath agitation effects, deposit thickness uniformity, and line resolution in selective plating of fine line patterned substrates. General recommendations are made as a guide for users of this process. Specific applications will be described in separate communications. Solution Preparation and Plating ProcedureCompositions of three electroless gold plating baths used are listed in Table I. Bath A was used often in our earlier studies (1-3) including those of impurity effects and porosity described in this paper. More recently, baths B and C have been used exclusively. These two baths contain less KCN and KBH4 and, therefore, are more preferable than bath A for practical reasons. Bath B gives the highest deposition rate (5-7 ~m/hr at 70~176 with vigorous agitation), but deposits with acceptable physical properties can be obtained only when plated with agitation. Bath C is slower plating (2 ~m/hr at 70~ with agitation) but gives better thickness uniformity on thin deposits in fine line plating. Details will be described in subsequent sections. It is convenient to prepare the baths by dilution of 5• concentrated stock solutions. These solutions can...

Electroless Deposition of Gold

“…Decomposition of the borohydride and DMAB baths can also occur when they are contaminated with certain organics FIGURE 21.5 Effect of bath contamination with Ni 2 þ and Cu 2 þ ions on plating rate of borohydride bath [5]. IMPROVED BOROHYDRIDE AND DMAB BATHS such as polyethylene [5].…”

“…IMPROVED BOROHYDRIDE AND DMAB BATHS such as polyethylene [5]. One should be aware of possibilities of contamination with trace amounts of organic materials from storage bottles and vessels as well as from the water used to prepare the solutions.…”

“…The original baths did not contain stabilizing additives, and they were sensitive to minute amounts of impurities, which tended to lead to spontaneous decomposition. Nevertheless, those baths were employed successfully with sufficient stability under carefully controlled conditions [2][3][4][5], yielding highly pure, soft gold useful for the purpose of bonding semiconductor devices [6]. Other successful applications documented in the open literature include (1) metallization of GaAs microwave field-effect transistors [7], (2) formation of ohmic contacts consisting of Pd/Sn/Au films on n-GaAs [8], (3) metallization of polyvinylidene fluoride (PVDF) films used for making piezoelectric devices [9], (4) deposition of a conducting layer on the interior surface of waveguide tubes made of an aluminum alloy [10], and (5) deposition of a gold layer on tungsten-metallized ceramics for packaging semiconductor devices [11].…”

Electroless Deposition

“…There have been laboratory studies of the electroless plating of the platinum group metals, including palladium, palladium phosphorus, palladium boron, platinum, and ruthenium and rhodium alloys (17). While these may have some …”

Electroless Gold Plating on III–V Compound Crystals

“…The bath composition is given in Appendix 1 (2). We have found that it is convenient to prepare a 5• concentrated solution for storage.…”