Electrodeposition of Gold from a Thiosulfate-Sulfite Bath for Microelectronic Applications

Abstract

The feasibility of using a thiosulfate-sulfite bath to electrodeposit soft gold for microelectronic applications has been investigated. This bath is stable at near-neutral pH, shows good compatibility with positive photoresists, and does not contain any additives. It was found that the bath produced gold bumps with straight sidewalls, flat top surfaces, and a good reproduction of the resist pattern was achieved. The thickness uniformity, roughness, stress, hardness, adhesion, and shape of the plated structures were found to be compatible with the requirements for a wide range of microelectronic applications, including wafer bumping.Electrodeposition of soft gold has numerous applications in microelectronics and microsystem technology. 1-8 In the electronic packaging industry, for example, interconnects between integrated circuits ICs and external devices are performed using tape automated bonding ͑TAB͒, chip-on-glass ͑COG͒, and chip-on-flex ͑COF͒ techniques. 1-3 A key process in all of these technologies is gold wafer bumping. In addition to connecting driver ICs to flat panel displays, gold bumping is used in many other packaging applications -particularly where a high density of input/output ͑I/O͒ connections is required. 4 Electrodeposition of gold is also used in the fabrication of transmission lines and air bridges on gallium arsenide ICs. 5 The electrodeposition of soft gold has also been used to fabricate X-ray masks for the deep X-ray lithography electroforming, and plastic molding ͑LIGA͒ process. [6][7][8] All of the above technologies utilize the through-mask plating technique. 9 Typically this involves depositing a conductive seed layer on a wafer substrate, followed by patterning with a photoresist material. The wafer is then gold plated, followed by resist removal and seed layer etching. Although the requirement for each application varies slightly, usually, gold deposits are required to have high purity, low porosity, low stress, and good adhesion to the substrate. 1-8 It is also important that the fabricated features exhibit straight sidewalls, a relatively planar top surface and are uniform in height across the wafer. 1-3 For reliable thermocompression bonding ͑e.g., in TAB͒ low deposit hardness is also required. 2,3 Similarly, in transmission-line applications, 5 a high electrical conductivity and low surface roughness is desirable.For most applications requiring soft gold deposition through a photoresist mask, sulfite-based plating baths are employed. 1,2,5,7,8,10,11 Cyanide-based electrolytes can also be used to deposit soft gold, 3 but due to their toxicity and poor compatibility with many photoresists, they are rarely used. For example, the use of gold cyanide baths often leads to the delamination of the resist from the seed layer. 3,5 This leads to gold deposition underneath the resist, which is highly undesirable in most applications. In contrast, sulfitebased baths exhibit better resist compatibility, and underplating is not usually a problem. 5,11 However, as gold sulfite baths are typica...

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