Page 162 - Wire Bonding in Microelectronics

Gold-Aluminum Intermetallic Compounds 139

Noolu studied these volume transformations during Au-Al phase
changes. The essence of his work on intermetallics is that there are
major differences in the five intermetallic lattice sizes. During ther-
mal stress, a specific compound may transform into other compounds.
In most cases the second compound is smaller or larger (by up to
~20%) than the first, and leaves cracks or stress behind. Once formed,
a crack can propagate during temperature cycling. Also excess Al or
Au can be released (or absorbed) as the compounds change. See the
Noolu’s App. 5B for a description of this phenomena.
Silicon may form ternary compounds with Au and Al, but, as
shown by Philofsky, these are no more detrimental to bond quality
than the pure Au-Al compounds by themselves, and, in some cases,
may be helpful.
These intermetallic compounds are not the normal cause of fail-
ure. They are mechanically strong (although brittle) and electrically
conductive. Bond failures result from the formation of Kirkendall
voids, as well as from the susceptibility of Au-Al couples to degrada-
tion by impurities or corrosion. The latter two causes are extensively
discussed in following sections. Kirkendall voids form when either
the Al or Au diffuses out of one region faster than it diffuses in from
the other side of that region. Vacancies pile up and condenses to form
voids, normally on the Au-rich side along the Au Al -to-Au interface.
5 2
The rates of diffusion vary with temperature and with different
phases and are dependent upon the adjacent phases, as well as the
number of vacancies in the original metals.
Classical Kirkendall voids require bake times greater than an
hour at temperatures greater than 300°C to occur on the Au-rich side
(Au Al ), and greater than 400°C on the Al-rich side (AuAl ), or much
5 2 2
longer times at lower temperatures [5-2, 5-3]. Such temperatures and
times are seldom reached during modern bonding or modern device
and systems packaging. Thus, it is rare that well-made bonds on
integrated circuits used in normal environments actually fail due to
the formation of classical Kirkendall voids. However, the failures
resulting from impurities (see Sec. 5.2), poor welding (see App. 5A),
hydrogen, or other defects in plated Au layers (see Chap. 6) can
appear to have resulted from classical Kirkendall voiding. Thus, it is
essential to understand the classical failure modes.

5.1.3 The Classical Au-Al Compound Failure Modes
An example of Au-Al compound formation is shown in Fig. 5-4. Here, a
poorly formed ball-bond was subjected to high temperature, and the
reaction generated considerable intermetallic compound. This particu-
lar bond was both electrically conductive and mechanically strong
(as later shown by a ball-shear test). Thus, the presence of these compounds
will not necessarily cause bonds to fail. However, even if such bonds do
not fail, the interface strength does degrade. The typical degradation of