Maximizing Selectivity During Wet-Chemical Gold Etching

K.C. Su,a H.H. Lu,a C.C. Chen,a and J.C. Mooreb
aLee Chang Yung Electronic Materials (LCYEM), 4F, No. 83, Sec. 4, Bade Road, Taipei, 105, Taiwan R.O.C.
bDAETEC, LLC, 1227 Flynn Rd., Unit 310, Camarillo, CA 93012, USA
a886 (2) 2763-1611,, b(805) 484-5546,

Keywords: Au bumping, UBM, etch rate, inhibitor, iodine/iodide.

Gold metallization is well known for its preferred properties of conductivity and corrosion resistance in GaAs wafer manufacturing for patterned metal lines and backside thermal sinks [1-3]. In flip-chip processing, electroplated gold bumps are used to establish the main input/output (I/O) terminals between chip and board contact [4-5]. During manufacturing, it becomes necessary to remove unwanted thin seed layers of gold in order to isolate the feature and adjacent insulation. Etching is normally conducted with a chemical mixture of iodide and iodine along with additives which aid in processing. These mixtures are known to be aggressive to other metals such as aluminum. Chemistries have been developed which protect these metals. Typical etch rates for gold in such solutions are approach 5000Å/min and provide a poor selectivity ratio of <20:1. Inhibited systems can improve selectivity by more than 100-fold. Further discussion of the gold tech chemistry variants, inhibitors, and processing results are discussed.
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