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The cyanoaurate (III) ion has been found to form and accumulate in acidic or neutral gold plating baths containing cyanoaurate(I) as the source of gold. This trivalent gold species brings about a decrease in plating current efficiency because its reduction to gold metal requires three faradays per mole, whereas only one faraday is consumed to reduce one mole of Au (I). The accumulation of Au (III) should therefore be minimized to produce gold deposits with a thickness unaffected by bath age. This paper describes various methods for achieving this objective, which include the use of the so‐called "dimensionally stable anode" and a techniqe of chemically reducing Au (III) to Au (I).

A new photoelectrochemical (PEC) technique for filling via holes in GaAs FET's with a solid deposit of metal, such as gold, has been developed. Photogenerated electrons reduce solvated Au(CN) 2 -, directly on the FET source pads, allowing narrow straight-walled plasma-etched via holes to be filled without forming voids. The photogenerated holes cause the decomposition of a small amount of the semi-insulating (SI) substrate. With illumination from a 1200-W tungsten-halogen lamp, plating rates of 0.8 µm/min over a 2-in-diam wafer are achieved. The plating rate is insensitive to Au(CN) 2 - concentration in the range 0.01 to 0.15 M. The resulting GaAs FET's show improved mechanical stability and thermal resistance.

The electroless gold plating process using potassium borohydride as the reducing agent has been investigated for impur i ty effects, material compatibili ty, bath agitat ion effects, and thickness uni formi ty and line resolution in selective plating of pat terned substrates. Impuri t ies may cause a decrease in plat ing rate [Ni(II) ], bath instabi l i ty [Ni( I I ) , Co(II) , Fe ( I I ) ], thickness nonuni formi ty (polyethylene, organics in deionized water) , and nodule format ion (some surfactants) . Bath agitat ion is beneficial: it increases plat ing rate, minimizes porosity of thin deposits, and eliminates nodule formation. Edge bui ld-up general ly occurs in selective pat tern plating but, with proper selection of bath compositions and agitation conditions, it can be main ta ined 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 opt imum 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 plat ing has been found to be useful in a variety of applications, especially for selective plat ing on pat terned substrates for electronics applications. Such applications general ly 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 impur i ty effects, mater ial compatibility, bath agitation effects, deposit thickness uniformity, and line resolution in selective plat ing of fine line pat terned 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 Procedure Compositions of three electroless gold plat ing baths used are listed in Table I. Bath A was used often in our earlier studies (1-3) including those of impur i ty 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 plat ing (2 ~m/hr at 70~ with agitation) but gives bet ter thickness uni formi ty on th in deposits in fine l ine plating. Details wil l be described in subsequent sections. It is convenient to prepare the baths by di lut ion of 5• concentrated stock solutions. These solutions can be stored at room temperature for at least three months without noticeable decrease in pla t ing rate. Since a very small amount of hydrogen gas " Electrochemical Society Life Member. "" Electrochemical Society Act ive Member.