Advances in Gold Metallization at Motorola's Compound Semiconductor Fab (CS1)
Motorola Compound Semiconductor One: CS-1
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Keywords: Gold, Plating, Cost Savings, Motorola
The use of Au(I)-sulfite complex bath for gold metallization in the compound semiconductor industry is gradually increasing. However, manufacturing costs have limited its widespread application since such baths decompose rapidly and have to be replaced frequently at a high cost. This work centers on the setting up of a cost effective pilot laboratory for routine bath analysis, the development of analytical tools used in the evaluation of the bath variables, and effects on statistical process metrics such as gold reflectivity and film stress. Atomic absorption spectroscopy determines concentration of Au and Tl, titrimetric methods are used to evaluate EDTA and free sulfite concentrations in the bath, while Au conductivity and bath pH are determined using a combination meter. Results from routine analysis and consequent bath adjustments have led to a fourfold retention in bath-life, while improving process integrity. Annual maintenance hours have been reduced by more than 400 hours at an annual savings of $90,000.† This article will begin by introducing the reasons that prompted the origination of an in-house lab. It will discuss the current baths on the market and why the sodium-gold-sulfite bath is chosen over other alternatives. Plating thicknesses, mask types, and current density will be discussed along with a brief overview of the current process. The primary chemicals and their roles in the chemistry will be detailed as well as the pertinent physical data. Details will be discussed about what procedures are performed in the lab as well as what equipment and reagents are required to perform those analyses. Several groundbreaking results have stemmed from the analysis lab. These results include better process control, improved plating step coverage in backside vias, longer bath life, less maintenance and a highly cost efficient process. All of these improvements will be detailed. The paper concludes with a discussion about the labís future contributions to the plating process as well as photo and wet etch processes.