Integrated temperature microsensors for characterization and optimization of thermosonic ball bonding process

A novel ball bond process optimization method based on the thermal response of an integrated aluminum microsensor is reported. The in situ temperature during ball bonding is measured and analyzed. The ultrasonic period shows distinct stages corresponding to scrubbing of the ball on the pad, intermet allic bond growth, and ball deformation by ultrasonic softening. A peak of the signal indicates the end of interconnection growth. This can be used fo r bond time optimization. When optimizing bonding force, the sensor signal correlates with ball shear strength, Using this method, bonding force proce ss windows can be determined by on-line measurements. A test measurement sh ows that at a chip temperature of 34 degrees C, the bonding force optimized by the microsensor method is 260 mN whereas it is 252 mN when using conven tional shear testing for optimization. In summary, the method produces a we alth of new insights in transient thermal phenomena of the ball bonding pro cess and promises to simplify the evaluation of process windows.