Design and Use of a Laser Interferometer for Ultrasonic Bonding Studies

An in-line laser interferometer was developed to study motions during ultrasonic bonding. The interferometer consists of a 2 milliwatt He-Ne laser with output mirrors at both ends, a lens with adjustment apparatus, and a photodiode detector. The detector output is displayed on an oscilloscope. The lens permits the beam to be focussed to a spot diameter less than 25 micrometers. The beam is focussed on a periodically moving target which results in a fringe pattern displayed on the oscilloscope. Peak-to-peak displacements up to 50 micrometers have been measured with an accuracy of ± 25 nanometers. The fringe pattern observed may be described in terms of a Michelson interferometer. The experimentally observed fringe pattern corresponds exactly to the theoretical plot using the Michelson interferometer. Peak-to-peak displacements are determined by counting the number of fringes on the oscilloscope.