Page 154 - Wire Bonding in Microelectronics

132 Cha pte r F i v e

more properly referred to as impurity-driven or corrosion reactions.
These are discussed in Sec. 5.2, but an understanding of the classical
failures is essential to comprehend the nature of present day ones.
The most definitive early work on Au-Al compounds, oriented
toward microelectronic bonding was done by Philofsky [5-1, 5-2, 5-3],
and those interested in more details are referred to his publications.
The compounds will occur in the bonded interface between Au wire
and Al metallization, and vice versa. Such compounds begin to form
during the actual process of Au-Al thermosonic or ultrasonic bond-
ing. (It is considered to be a necessary part of the Au-Al bonding/
welding mechanism.) The compounds will continue growing during
the cure of plastic molding compounds (typically at 175°C for 3 to
5 h) and grow during qualification screening (burn-in, stabilization
bakes), or at any time when high temperatures are encountered dur-
ing the life of the device. A few monolayers of such compounds will
even form at room temperature if clean metal surfaces are brought
into intimate contact (such as evaporated films).
Even though Au-Al intermetallics have been associated with
many bond failures, and many people are “frightened to death of them,”
the fact is they are always present in Au-Al bonded interfaces. While
they are considered to be the basis for Au-Al bonding, ultrasonic
welding certainly does take place in monometallic Au-Au and Al-Al
bonds. Ramsey [5-4] has studied this phenomenon in Au-Al wire
bonding and first reported that the compounds appear during the
actual bonding process. Approximately 95% of all IC devices have
Au-Al bonding, are plastic encapsulated and, in the resin curing pro-
cess, are subjected to the above high temperatures. This curing can
drive the initial intermetallics completely through some thin Al bond
pads. Thus, the IC industry has learned to live with plague. Bond
failures usually occur because of impurities in the bond interface (or
in the plastic encapsulant), poor welding (which produces isolated
micro-welds, see App. 5A), and/or extreme thermal exposure.
Wire bonds are made both to and with non-Au or Al metals.
Depending on the possible metallic alloys, these also may develop
various intermetallic compounds in the interface, and are discussed
below (see Sec. 5.3). These include several noble metals, such as Pd,
as well as Ni and Cu.

5.1.2 Intermetallic Compound Formation
in the Au-Al System
There are five Au-Al intermetallic compounds, as shown in the phase
diagram of Fig. 5-1 [5-5]. These are Au Al , Au Al, Au Al, AuAl , and
5 2 4 2 2
AuAl. We note that recent interpretation of crystal structure has
changed the designation of Au Al used in the past, to Au Al [5-6].
5 2 8 3
This chapter has retained the older designation since the early quoted
works/figures use it, and using both in the same chapter would be