AC conductivity analysis as a measure of low k dielectric capacitor reliability degradation due to moisture ingress

Abstract

In this paper, we will show that AC conductivity analysis can be used as an extremely sensitive, non-destructive indicator to determine the level of degradation in low k dielectric capacitors caused by moisture ingress. We found that an increase in AC conductivity (AC loss) correlates with a decreasing capacitor failure voltage, as determined by Ramp Voltage Stress (RVS) measurements. Exposure of the low-k dielectric layers to a relative humidity (RH) as low as 9.2% for a day, was enough to cause substantial degradation in failure voltage (about 45% decrease). Comb-comb capacitor structures used as the test vehicles were fabricated in 14 nm Complementary Metal Oxide Semiconductor (CMOS) technology using low k dielectrics in the Back-End-of-the-Line (BEOL). An ultraviolet (UV) laser was used to ablate trenches through the BEOL all the way down to the silicon level in the substrate, thus creating controlled breaches in the chip moisture barrier. In this manner, moisture could diffuse in along interfaces and through the porous dielectric. The capacitors were held at constant relative humidity levels from 0 to 90% RH, over various time intervals. The AC loss percent change correlated well with measured failure voltage. It was also shown that high-temperature forming gas (FG) anneals could reverse most, if not all the effects of the moisture, demonstrating subsequent failure voltages comparable to as manufactured, unablated low k dielectric capacitor structures.