To investigate the effect of surface cleanness upon ultrasonic bondability, ultrasonic micro-bonding apparatus was developed, which consisted of three vacuum chambers for bonding, surface analysis and gas analysis. The bonding chamber has an ultrasonic ball bonding system with ultrasonic power source oscillating at the range from 10 kHz to 300 kHz, a surface cleaning system of ion bombardment and laser irradiation, a heating system, and a pure-gas-introduction system. The surface analysis system consisted of an Auger electron spectroscopy analyzer, an ion gun with 500 um in ion beam diameter, and a mass spectroscopy analyzer. Gas analysis was done by a quadrupole mass spectrometer.
For this apparatus, mechanical vibration characteristics of the bonding system was measured. Then, using this apparatus, ultrasonic bondability of Au wire to Au thin film deposited by physical vapor deposition was investigated. Bonding procedure was that at first Au thin film was cleaned in vacuum, next pure N2 gas was introducted about 1×105 Pa and Au ball was formed at the tip of a bonding wire by arc discharge, and then Au ball was ultrasonic-bonded to the cleaned surface with a ceramic capillary tool in pure N2 gas atmosphere. An initial surface, a cleaned surface, and a exposed surface in pure N2 gas were analyzed by AES. Main results were as follows;
(1) Au surface, which was coated with an organic resist film for preventing plating and cleaned by acetone and water, is covered with two layers of a surface carbon-rich layer and an under layer including more water.
(2) An organic material layer of two or three nanometers in thickness including water lubricates between Au film and Au ball in ultrasonic bonding, and so it deteriolates ultrasonic bondability.
(3) Ultrasonic ball-bonding of Au wire 30 μm in diameter to Au surface cleaned by ion bombardment makes failure strength of bond more than wire strength under conditions of 0.38 um : vibration amplitude, 6 mW:ultrasonic power, 10 ms:bonding time, 0.5N: bonding load and 20°C: bonding temperature in pure N2 atmosphere.