Page 157 - Wire Bonding in Microelectronics

Gold-Aluminum Intermetallic Compounds 135

temperature (in Kelvins). ∗ The value of K changes for each intermetal-
lic phase, and is also dependent upon the neighboring phases, which
supply additional Au and Al for continued compound formation.
Because of this, Philofsky lists nine different rate constants for the
five Au-Al compounds. Figure 5-2 shows the relative rates of inter-
metallic formation. From this, it is apparent that Au Al grows much
5 2
faster than the other phases. (Because of this, it is also the phase most
often cited as resulting in Kirkendall voiding and bond failures.)
The mechanical properties of the five compounds differ among
themselves and vary widely from those of the Au and Al. The crystal-
lographic lattice constants are larger (see Table 5-1), so they occupy a
larger volume, and thus, plagued bonds often appear to be lifted up.
The thermal expansion coefficients are considerably lower for the
compounds than for either Au or Al, and some reliability implica-
tions for both of these differences are discussed in Noolu’s App. 5B
of this chapter. Temperature cycling can be used to reveal potential
†
failures resulting from these property differences [5-7].
The compounds are also much harder (and more brittle), so, plagued
bonds can crack during temperature cycling or other stresses. Some
detailed properties of these Au-Al compounds are given in Table 5-1.
The rate of diffusion of one metal into the other (or into itself) is
dependent on the number of defects in the crystal lattice. Defects can
be vacancies, dislocations, and grain boundaries. During diffusion,
one atom moves into an empty lattice position (vacancy), and another
atom moves into the empty position of the first. Grain boundaries
and surfaces, because the lattice has more open structures, have many
vacancies and they increase the diffusion rate by orders of magnitude
compared to diffusion in the bulk or a single crystal. Poorly welded
bonds consist of numerous isolated microwelds which contain large
surface area-to-volume ratios, as well as mechanical stresses that
result in numerous lattice defects. Thick-film metallizations also con-
tain many grain boundaries, stresses, and impurities, all of which
result in lattice defects. Thus, it is not surprising that poorly welded
bonds or Al-wire bonds to thick films fail rapidly, see App. 5A for a
discussion on this problem.
A generic activation energy, E (one that combines the effects of all
five compounds and/or Kirkendall voids), for various bond failures
is often measured by workers. As a result, the literature abounds with
different values of (E) for various properties thought to be related to

∗ In chemical or metallurgical literature, one often sees the equation written as:
K = C exp (−Q /RT), where Q is the activation energy in kilocalories/mole (1 eV ≈
23 K). Cal/mole), R is the gas constant (1.98), and T = temperature in Kelvins.
† lntermetallic problems were revealed in plastic encapsulated devices in several
hundred temperature cycles from −40 to 140°C (1% cumulative failures occurred
at 300 cycles, and 2% at 800). Four times as many cycles were required for 0 to
125°C testing.