Page 170 - Wire Bonding in Microelectronics

Gold-Aluminum Intermetallic Compounds 147

diffusion into the Au. Since 95% of current devices are plastic encap-
sulated, such a procedure is not an option, and even for hermetic devices
one would have to assure hermeticity or the H could leak out.
2
In the past, Al bonds to thick-film Au metallization were more
subject to failure by Kirkendall voiding than Al bonds to thin films.
Presumably this was because thick films contain more grain bound-
aries, vacancies, and impurities, all of which enhance diffusion. In
the late 1970s, Pd was added to Au thick-films for use in Al ultra-
sonic bonding [5-20, 5-21]. This resulted in either a relatively stable
Au-Al-Pd ternary compound or a concentration of Pd at the inter-
face ∗ that slowed both the Au and Al diffusion and lengthened the
life of Al wire bonds. Several applications requiring Al-wire bond-
ing currently employ such Pd-doped Au thick films. As an
example, 1 and 2% Pd, Au wires have been used for the ball

bumping of Al bond pads for flip-chip [5-22] and TAB [5-23] bond-
ing. Reliability and screening tests were run, and the long-term
reliability was calculated to be 100,000 h at 85°C for the TAB devices.
Thus, the Pd additive in Au ball bonds apparently serves as a dif-
fusion or reaction inhibiter, as it does in thick-film Au. Palladium
has recently been added to Au bonding wire (at about 1%) to slow
intermetallic growth in fine pitch wire bonds, as well as other
(proprietary) dopants, see Chaps. 3 and 9.
Titanium-tungsten metallurgical barriers have long been used to
prevent Au-bump diffusion into Al bond pads to protect the integrity
of TAB bonds. For this purpose, the Ti-W is “pumped” (diffused)
with nitrogen to improve its resistance to Au and Al-diffusion [5-24],
Problems arise when the barrier is penetrated by defects or cracked
during the TAB-bonding process. The Au and Al interdiffuse, swell-
ing the interface and further cracking the barrier [5-25, 5-26]. The
reliability then becomes worse than that of a Au ball bond directly
on the Al.
There have been cases when Ti, Ti-W, Ti-N, or Ta films are sand-
wiched between layers of Al metallization to inhibit electromigration
or for other purposes [5-27]. If Au ball bonds are welded to such pads,
then it is very important that care is taken during bonding-machine
setup to prevent cracking of the barrier layer to prevent swelling as
above. Also, there is a possibility that the top Al (when converted into
intermetallic) can dewet from the barrier and result in a lifted-off
bond. This is sometimes referred to as “pad lift,” but technically that
term refers to a separation of the entire bond pad from the chip’s sur-
face. Aluminum is frequently deposited on Cu pads in Cu/Lo-k chips
for easy wire bonding. To prevent interdiffusion of Al into the Cu, Ta,
Ti, or other diffusion barrier is first deposited on the Cu pads, fol-
lowed by the Al bonding layer.

∗ The literature is not clear on which occurs.