1998 Volume 16 Issue 1 Pages 93-104
The effect of surface cleanness upon ultrasonic bondability was investigated, in order to obtain a reliable micro-bond with high strength and small deformation when bonding was done at low temperature and low energy conditions. Au, Cu and Al physical vapor deposition films on a Si wafer, which are materials with different oxidizing characteristics, were selected for bonding pads. At first, the bonding surfaces of the Au, Cu and Al films were treated for contamination by cleaning, and the surfaces were quantitatively analyzed by Auger electron spectroscopy. After that an Au wire was ultrasonic ball bonded onto each surface in air or pure N2 gas, and the fracture mode of the Au ball bond in a pull test and the bond strength of it in a shear test were investigated. Then ultrasonic bondability was evaluated from the ultrasonic power by which the pull strength of the Au ball bond rose above the Au wire strength, and from the probability of which the pull strength of it rose above the Au wire strength at a ultrasonic power. Additionally, the surface cleanness of Au, Cu and Al films was investigated for the films exposed to pure N2 gas or air after being cleaned by ion bombardment and the films with a contaminated surface obtained by immersion in water and acetone. Main results are as follows.
(1) The ultrasonic ball bondability of an Au wire onto an Au film at 373K in air is improved by cleaning the Au film surface by ion bombardment before bonding, but it onto an Al film is not improved so much by cleaning the Al film surface.
(2) The ultrasonic ball bondability of an Au wire onto Au and Al uncleaned films in pure N2 gas was better than it onto Au and Al films cleaned by ion bombardment before bonding in air. This means that the ambient gas influences the bondability much more than the cleanness of a bonding surface.
(3) Therefore, it is important to clean a bonding surface and change an ambient gas to a pure and dry gas for obtaining a reliable bond in ultrasonic bonding of an Au wire onto an Au pad at low temperature and low energy conditions, and it is important to change an ambient gas to a pure and dry gas in the case of an Al pad.
