Advances in Gold
Metallization at Motorola's Compound Semiconductor Fab (CS1)
Motorola Compound
Semiconductor One: CS-1
* Formally with
Motorola
Phone: (480)
413-8436, FAX: (480) 413-5748, Email: chad.becker@motorola.com
Keywords: Gold, Plating,
Cost Savings, Motorola
Abstract
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 lab’s future
contributions to the plating process as well as photo and wet etch processes.