Electrodeposition of Gold

While all that glitters may not be gold, it is the most beautiful of all the elements in their pure form. The name gold comes from the Old English Anglo-Saxon word for geolo meaning “yellow” while the symbol, Au, comes from the Latin word aurum, meaning “glowing dawn.” Historically gold was one of the first metals known. Gold has been valuable throughout the ages chiefly because of its physical properties of softness, ductility, corrosion resistance, density, and scarcity. The first gold coin dates back to about 560 BC. Gold cups and jewelry predating 3500 BC have been found in Iraq. The ancient Egyptians knew how to hammer gold into leaf as thin as 66 nm. The importance of gold as a form of currency was at a high point in the 1900s whenmost countries were on the gold standard. The bullion price of gold over the past 21 years is shown in Figure 4.1. There have been about 160 metric kilotons of gold mined since it was first discovered, and it occupies about 0.005 ppm of the earth’s crust. The world demand for gold in 2007 was approximately 80,000,000 troy ounces. Figure 4.2 shows the breakdown of the gold usage by industry. Jewelry comprises the largest fraction of the usage; however, much of that is not electroplated. For jewelry applications, gold is commonly alloyed with group IB or IIB metals, particularly copper, silver, platinum, and palladium, principally to improve its strength and wear resistance. The electrodeposition of gold is a relatively new process; it has been traced to the early work of Brugnatelli in 1805 [1]. The motives behind the use of electroplated gold changed dramatically in themid-twentieth centurywhen the emerging electronics industry required special-purpose electrical connections. The electronics industry consumed 5,330,000 troy ounces in 1994. Figure 4.3 shows the growth in the use of gold in the electronics industry by year. The use of electroplated gold in a variety of different functions in the electronics industry has led to (1) many advances in our fundamental understanding of the electrodeposition process and (2) new electroplating technologies over the past 25 years. Electrochemically deposited gold has satisfied many of the demands of the electronics industry. Gold has the third best electrical and thermal conductivity of all metals at room temperature. Also it has high ductility and excellent wear resistance, which are important for electrical contacts. The inertness of gold prevents the formation of insulating surface oxides (as compared to metals like aluminum). Group IIB metals (e.g., gold) are not good catalysts for other reactions, thus avoiding certain problems. For example, group IB metals (particularly platinum and palladium) can catalyze the polymerization of organic molecules forming insulating layers. In addition, gold is an excellent metal for wire bonding integrated circuits (ICs). Gold wires can be bonded to pure, soft gold pads by thermocompression bonding (300–400 C at high pressure to form a weld) or thermosonic bonding (150–200 Cwith ultrasonic energy to form a weld). Aluminum wires can be attached by ultrasonic bonding (ambient temperaturewith ultrasonic energy). These benefits have justified the high cost of gold in the packaging and interconnection of ICs. A geographical breakdown of the use of gold in the electronics industry is also shown in Figure 4.3. Japan is the largest consumer of gold for electronic applications, followed by North America, Western Europe, and the Pacific rim (excluding Japan). Although the use of gold for electronic interconnections has maintained steady growth over the past two decades, its use has not kept pace with the more rapid growth in the microelectronics industry, primarily because of gold’s high cost. The microelectronics industry has increased speed, performance, and packing density (number of transistors per unit area) while maintaining