Page 50 - Wire Bonding in Microelectronics

Ultrasonic Bonding Systems and Technologies 29

interfacial motion ceases. ∗ Ultrasonic energy is then absorbed into the
entire weldment area (wire, interface, and bond pad). As a consequence
of deformation cleaning, the center of the bond area is left relatively
unwelded in wedge bonds. This may not occur if there are orbital or
sideways tool vibration modes present during bonding causing some
center welding to occur.
Extensive microstructural details of bond interfaces were obtained
with a transmission electron microscope [2-19] as well as some using
a SEM [2-1]. Those taken along the interface of monometallic US
welds showed, variously, grain boundaries, no grain boundaries,
debris zones of oxides, and contaminants, as well as numerous crys-
tallographic defects. However, in general, the monometallic bonding
process results in interface formation that is similar to grain bound-
aries in polycrystalline materials, but continuous along the interface.
Au-Al US welds, made at room temperature, show debris zones as
well as clear metallic boundaries (similar to grain boundaries). There
are also intermetallics along this boundary, which are a normal part
of such Au-Al bond formation [2-31]. As a consequence of deforma-
tion cleaning, the center of the bond area is left unwelded (minimum
mass motion occurs in the center of a compressed, deforming ball/
wire). This may not occur if there are orbital or sideways tool (capil-
lary) vibration modes present during bonding. Such modes can be
easily revealed by modern laser vibrometer measurements as was
shown in Fig 2-9.
Contaminants in the bond interface can inhibit weld formation by
preventing the deforming metal surfaces from coming into intimate
contact. A thin, hard oxide on a soft metal, such as 0.5 to 1 nm (50–100 A)
of Al O on Al, will break up and be thinly dispersed or pushed
2 3
into “debris” zones with little effect on the average welded area.
However, soft oxides on harder metals, such as NiO on Ni (which is
used in large-wire power device packages), appear to serve as a lubri-
cant during initial weldment contact and deformation, remaining on
the surface and preventing weld formation. This holds true for soft
oxides (e.g., Cu O ) on soft metals (e.g., whereas Au does not have an
x y
oxide, diffused-to-the-surface Cu will oxidize and has been shown to
significantly increase the activation energy required for Au-to-Au TC
bonding [2-32]). Also, as little as 0.2 nm (20 A) of a carbonaceous con-
taminant has been shown to reduce bondability on any bonding sur-
face (see Chap. 7). Thus, it is important to understand the nature of a

∗ Joshi [2-1] used a laser interferometer and observed that the tool, the bond, the
pad, and the laminated polymer substrate moved in unison during most of the
bonding process. This substrate motion would not occur on hard, brittle substrates,
such as silicon or ceramic. Here the final motion must be between the tool and
the wire, as was found in [2-9]. Ultrasonic energy is then absorbed into the entire
weldment volume (wire, interface, and bond pad).