Abstract
This paper presents the design, simulation and fabrication process of a phase array ultrasonic transducer based on 1mm-thick lead zirconate titanate (PZT) substrate with 128 independently accessible electrodes through flip chip bonding with indium solder bump. The reflow processes of indium have been explored under air, nitrogen, hydrogen, liquid metal flux, and formic acid (H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ). Among these, formic acid is found to be the best ambient for forming indium bump, as it removes indium oxide and clean out residuals most effectively. Also, different sizes of under bump metallization (UBM) and indium pattern have been tested, and the measured heights of indium bumps after a reflow show that the lateral dimension ratio of 1:2.5 between the UBM and indium patterns offers the best wettability over the UBM with the smallest residual.
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