Integrated Temperature Microsensors for Characterization and Optimization of Thermosonic Ball Bonding Process

A novel ball bond process optimization method based on a thermal signal from an integrated aluminum microsensor is reported. The in-situ temperature during the ball bonding is measured and analyzed. The ultrasonic period shows distinct features corresponding to the scrubbing of the ball on the pad and the intermetallic bond growth, and the ball deformation by ultrasonic softening. A peak of the signal indicates the end of interconnection growth. This can be used for bond time optimization. When optimizing bonding force, the sensor signal correlates with ball shear strength. Using this method, bonding force process windows can be determined by on-line measurements. Maximum shear strength and maximum microsensor signal were within 20 mN at 34°C. In summary, the method produces a wealth of new insights in transient thermal phenomena of the ball bonding process and promises to simplify the evaluation of ball bonding process windows. Introduction Wire bonding technology connects the majority of todays integrated circuits to the external world [1]. It is a flexible technique evolving rapidly to meet new requirements. There are many factors determining the quality of a wire bond process and being therefore subject to optimization. The quality is assessed by measurable outputs of the manufacturing process (responses). For ball bonding, responses include the height and diameter of the deformed ball, ball shear strength and percentage of intermetallic compound (PIC) values for metallurgical bond qualification [2-5]. They are influenced by machine input