Page 30 - Wire Bonding in Microelectronics

The Technical Intr oduction to the Thir d Edition 9

treatments, bond parameters of 90 gf (880 mN), plus normal
power and time produced good strong bonds. As in this
example, a great deal of ingenuity may be called for, but in
most cases, good bonds can be made.
2. Are there any potential bondability or handling problems for
high-volume production? Does the metallization form a soft
oxide during long storage or normal chemical treatment? As
an example, Ni and Cu are bondable by Al and Au, but each
has a soft oxide (covering a hard metal) that reduces the
bondability and must be removed or else special bond sched-
ules or techniques developed. Aluminum is soft, but has a
hard-brittle oxide which easily shatters and is pushed aside
into debris zones during bonding. (See Fig. 2-7 that pictori-
ally shows this process.) This type of oxide does not present a
bonding problem.
3. Is the new wire harder than Al or Au (e.g., Cu)? If so, when
bonding to pads over Si, GaAs, or most other low-fracture
toughness semiconductor chips, the cratering probability is
increased and may require special bonding techniques, sched-
ules, or modification of the under-pad structure by adding
hard barriers such as Ti, Ti-W, Ti-N, or Ta (see Sec. 5.1.5).
4. Are there numerous intermetallic compounds that may form
and affect reliability? Look up the phase diagram. If such
intermetallic compounds exist and have high melting points
(e.g., >1000°C), then they are stable and should not signifi-
cantly affect reliability. If low (500°C), then they are less sta-
ble, and their constituents will continue to diffuse, and their
reliability is in question. The activation energy of individual
compounds can be approximated from the following empiri-
cal equation:

melting point (K) (kcal/mmol)
Activation energy ~
A

where A ≈ 35 for face-centered cubic metals but A ≈ 50 for
Au-Al intermetallic compounds (Note: 1 eV ≈ 23 kcal/mol).
There are many complications in using such a formula
because the nucleation and other properties of different com-
pounds vary, but it is a beginning. As an example, Ni-Al
compounds have a very high melting point. They are refrac-
tory and are stable at jet turbine blade temperatures: Ni-Al
bonds do not fail from intermetallic problems.
Even if there are no intermetallics in a bimetallic bond sys-
tem there may still be interdiffusion which could lead to