Page 51 - Wire Bonding in Microelectronics

30 Cha pte r T w o

surface contaminant or oxide on a given bond-pad metal to under-
stand its effect on bondability and reliability.

2.4 Bonding with High(er) Frequency Ultrasonic Energy
The original reasons for choosing 60 kHz are obscure, but fine-wire
bonding machines have used that frequency for bonding from the
1960s to the present. This resonance frequency resulted in transducers
and tools that were appropriate to microelectronic dimensions and
were stable during the bonding load. However, other frequencies
(e.g., 25 kHz) have been used for bonding large-diameter Al wires in
power devices. The possibility of US welding over a wide frequency
range has been known for some time. The Welding Handbook [2-33]
cites ultrasonic welding frequencies as ranging from 0.1 up to 300 kHz.
Also, the ultrasonic softening process was verified up to 1 MHz by
Langenecker, so it is not surprising that higher frequencies would
find use in microelectronics bonding if an advantage could be
demonstrated. The present interest in using high frequency (HF) for
microelectronics wire bonding was started by Ramsey and Alfaro
[2-34]. They studied thermosonic Au ball bonding on IC pads using
US energy in the range of 90 to 120 kHz and reported that such fre-
quencies produced better welding at lower temperatures and in
shorter bonding times. This also resulted in more complete Au-Al
intermetallic formation and thus, more complete weld formation.
More recently, several papers have given additional advantages for
using HF (>60 kHz) for both ball and wedge bonding [2-35], as dis-
cussed in the following paragraph.
There have been no large-scale (published) implementations and
design of experiment (DOE) comparisons between different frequen-
cies, and for the many possible bonding problem situations (cratering,
contamination, metallic oxides, soft substrates, etc.). Nevertheless, all
small-diameter-wire autobonders since approximately the year 2000,
have incorporated transducer frequencies between ~100 and 140 kHz
drive for the transducers. The lower mass (inertia) of the small HF
transducers allows bonding machines to run faster. The other advan-
tage is that strong HF wedge bonds can be made with low deforma-
tion and with shorter welding time [2-34]. All of these are desirable
for high-speed and fine-pitch bonding. However, one study [2-36]
compared Al wedge bonding at 60 and 120 kHz in a DOE study and
found little low-deformation advantage. The authors did find that
less metal splash occurred around the bond perimeter at 120 kHz,
and higher yields were achieved in shorter bonding times. In other
studies [2-37], a range of higher frequencies (100, 140, 250 kHz) were
reported to produce good Au ball-bonds at low temperature (50°C)
compared to 60 kHz. The best shear strength and the shortest bond-
ing time were obtained at 250 kHz. However, the bonding window