Page 96 - Wire Bonding in Microelectronics

Bonding W ir e Metallur gy and Characteristics 75

Copper Gold
FIGURE 3B-1

Copper work hardens easily, gold does not (see copper in Fig. 3B-1).

Issues: Some of the most important ones are cratering, oxidation
and long-term reliability.
The Bonding Process: The copper wire bonding process has to be
tailored around the physical properties of the material, the hardness.
The significant differences between copper and gold require much
higher bonding parameters in order to achieve similar quality factors
(pull and shear strength, ball size, and shape). The increase in bond-
ing parameters such as ultrasonic power and force signifies a signifi-
cant increase in capillary wear therefore reducing effective life of the
tool. This is the reason Gaiser Tool Co. recommends special ceramic
material for this application.
The capillary material acoustical and wear resistance are key to
maximize life and reduce ultrasonic power requirements.
The impact of the copper hardness is most significant during the
ball bond process, when copper alloys with the aluminum pad. This
can result in bond pad subsurface damage (cratering, chip outs). The
most common solution to this problem is either increase bond pad
thickness or use a protective under layer (TiW most common). Many
times the solution is to implement both, metal thickness and a protec-
tive under layer.
Copper oxidizes rapidly when exposed to oxygen, therefore spe-
cial care must be taken to protect it. The use of enclosed containers in
the wire bonder are encourage to keep the wire spool free of oxidation
as well as to the use of inert gases such as argon (Ar) or nitrogen (N).
Copper oxide is a layer that prevents the pure copper from alloy-
ing or diffusing or bonding with the surfaces in contact.
Long-term reliability, especially when devices are exposed to
high-pressure high-humidity temperature cycling, has yielded mixed
results. Some claim no problems while others reported multiple fail-
ures beyond 500 cycles.
One of the failure modes associated to copper’s work-hardening
ability is neck breaks, commonly seen during temperature cycling
test (see Fig. 3B-2).