1999 Volume 17 Issue 1 Pages 139-147
The influence of ambient gas on ultrasonic bondability was investigated, in order to obtain a reliable micro-bond with high strength and small deformation when bonding is done at low temperature and low energy conditions. Au, Cu and Al physical vapor deposition films on Si wafers, which are materials with different oxidizing characteristics and different hardnesses, were selected for bonding pads. First the bonding surfaces of the Au, Cu and Al films were treated for contamination by cleaning, and then an Au wire was ultrasonic ball bonded onto each surface in various ambient gases. The surfaces were quantitatively analyzed with Auger electron spectroscopy. After that, the fracture mode of the Au ball bond in a pull test was investigated and ultrasonic bondability was evaluated from a relationship between the successful bonding rate and ultrasonic power. The results are summarised as follows.
(1) The ultrasonic ball bondability in Ar gas containing 0.1 vol% H2O or 0.2 vol% acetone for an Au wire bonded onto an Au or Al film cleaned by ion bombardment deteriorates in comparison with that bonded in a vacuum. However, in N2 gas or Ar gas containing 1 vol% O2 being the same bondability is obtained as in a vacuum. This is attributed to a vapor of H2O or acetone being adsorbed on the metal film surface which acts as a lubricant during ultrasonic bonding of the Au ball to these films and prevents formation of a fresh and cleaned surface of the bonded metal. For the physisorbed gas of N2, Ar or O2 this does not occur.
(2) The ultrasonic ball bondability on an Au film is improved by cleaning the film surface and eliminating H2O and organic vapor in ambient gas until the H2O content in ambient gas is less than 0.1 vol%.
(3) The ultrasonic ball bondability on an Al film is improved by eliminating H2O and organic vapor in bonding ambient gas even without special surface cleaning.
(4) The ultrasonic ball bondability of an Au wire onto a Cu film deteriorates in Ar gas containing H2O, acetone or O2. In particular for O2 containing gas, enough bondability to get a 100% successful bonding rate is not obtained within an ultrasonic power of 1000 mW. This is O2 gas forms an oxide film on the Cu film which is much harder than the Au ball.
