Page 35 - Wire Bonding in Microelectronics

14 Cha pte r T w o

number of interconnections in microelectronics today. (Because of
price and some mechanical/electrical properties, Cu wire is replacing
Au in some applications, see Chap. 3.)
The ultrasonic transducer is the heart of all types of wire-bonding
machines and, therefore, is described first in this section. The various
wire-bonding technologies, as well as the competing interconnection
technologies, are also described and compared.

2.2 Ultrasonic Transducer and Tool Vibration Modes
The ultrasonic transducer and bonding tool (inserted to the proper
length) together form a mechanical resonant structure. The operating
frequency is chosen so that both components can be made compatible
with the size of the welding structures (i.e., generally, the higher
the frequency, the smaller the components). Small, low-mass/inertia
transducers speed up the mechanical movement. Most current
autobonder systems operate over 100 kHz and typically from 120 to
140 kHz. (Experiments have been carried out as high as 250 kHz.) An
example of a traditional older (60 kHz) transducer used in microelec-
tronics bonding, including the capillary (or bonding tool), is given in
Fig. 2-1. There are five main parts to the transducer. The first, (A), is
the electrical-to-mechanical energy transducer. This is usually a
piezoelectric element ∗ and it converts the 60 kHz (or higher frequency)
electrical energy (from the ultrasonic power supply) into mechanical
vibrations that travel to the tool. The second, (B), is the clamp. It is
located on a vibration node and is the part that is clamped or held by
the bonding machine. If it happens to be located off the node, then
part of the ultrasonic energy will be fed into the machine housing
instead of the bonding tool. The third, (C), is often referred to as the
horn, and it usually has a taper that magnifies the ultrasonic wave
amplitude as an electrical transformer that can step up voltage. The
fourth, (D), is the ultrasonic wave amplitude. The tool, or capillary,
(E), is clamped perpendicular to the axis of the horn, so it is driven in
a front-to-back vibration mode. The electrical energy from the ultra-
sonic power supply is stabilized by a phase-lock circuit to minimize
drift and keep the ultrasonic system close to its resonance during
bonding. Application of the energy to the transducer may be sharp or
slowly ramped up, depending on the particular manufacturer and
application. A modern 120 kHz autobonder transducer is shown in
Fig. 2-2. It performs the same function as in Fig. 2-1, but because of
design and higher frequency, is very small [~4 cm (~1.6 in) long] with
low mass and thus low-mechanical inertia. Both features are essential

∗
These elements generally have a high impedance, and, in some cases, the vibration
amplitude will decrease during the bonding load.