Ultrasonic Bonding: Understanding How Process Parameters Determine the Strength of Au-Al Bonds

The physics describing ultrasonic wire bonding is only partly known. A better understanding how bond strength depends on process parameters could increase productivity by speeding up process ramp-up and increasing process reliability. In this work, the influence of normal force and ultrasonic amplitude on the ball bond quality is investigated. The determination of the ultrasonic friction energy dissipated at the bond interface during bonding is reported. The analysis is based on real-time measurements of the ultrasonic tangential force FT(t) obtained in situ at the bond zone. A state-of-the-art test chip containing integrated piezoresistive xyz-force microsensors is used for the measurements. Considering the stick-slip behavior of a harmonically driven friction oscillator, several characteristic coefficients were identified to determine the friction power during bonding at the interface: the ultrasonic frequency f, the normal clamping force FN, the ultrasonic amplitude of the capillary tip measured in air A, and the compliance of the bonding system c. The time dependent friction power P(t) is found to be P(t) = 4 f FT(t) [A c FT(t) ]. The integral of P(t) over time equals to the friction energy E. Experimental values of the friction energy density E / S are reported, where S is the interface area. The friction energy density does not correlate well with the bond strength. Obviously, a varying part of E is dissipated as heat and does not directly contribute to the bond. A better correlation with the bond strength is obtained with the maximum γmax of a newly introduced parameter γ(t) = [ FT(t) min( FT(t) ) ] / [ σ S min( FT(t) ) ] where σ is a measure for the yield shear stress and FT(t) is taken during friction only. The parameter varies from 0 for no bond to 1 for a 100% complete metallic interconnection. The findings of this work support efforts towards automatic process optimization on test chips and towards control methods suitable for the production floor. Proc. Intl. Symposium on Microelectronics, IMAPS, Denver, CO, USA, pp. 626-631, 2002.