Active devices under CMOS I/O pads

Active devices, including electrostatic discharge protection devices and ring-oscillator circuits, under CMOS I/O pads are investigated in a 130 nm full eight-level copper metal complementary metal-oxide-semiconductor process, using fluorinated silicate glass (FSG) low-k inter-metal dielectric. The high current I-V curve measured in the second breakdown trigger point (V_t2, I_t2) of ESD protection devices under various metal level stack structures, shows that i) I_t2 depends very weakly on the number of metal levels used, as expected given specific junction power dissipation criteria; and ii) V_t2 increases with the number of metal level stacks of I/O pads because of increased dynamic impedance due to the presence of more metal levels, as clarified by a simple RC model. Moreover, no noticeable degradation in the speed of the ring-oscillator circuit, as measured for a variety of test structures subjected to bonding mechanical stress, thermal stress by temperature cycling and DC electrical stress by transmission line pulse, as well as AC electrical stress by capacitive-coupling experiments. Accordingly, active devices under CMOS I/O pads are independent of bonding pad metal level structures.