Process Challenges in Fully Aligned Via Integration for sub 32 nm Pitch BEOL
As BEOL pitch continues to aggressively scale, contributions from pattern dimension and edge placement constrict the available geometry of interconnects. In particular, the critical minimum insulator spacing which defines a technologies max operating voltage is now limited by Vx to Mx spacing. This spacing has historically been a challenge since the introduction of self-aligned vias due to the loss of CD and chamfer control in the non-self-aligned direction. As pitch continued to shrink from self-aligned via introduction around the 22 nm node, the fraction of via CD control and edge placement compared to the dielectric spacing between interconnects has continued to grow. Alone this trend could be combated by increasing the dielectric spacing, however, the exponential increase in Cu resistivity (under scaling) has forced BEOL technologies into strong line/space asymmetry to keep line resistance under control. At pitches below 32 nm these factors reach a tipping point, either design to exponentially increasing line resistance or lower the technology Vmax. Both approaches cause performance degradation to achieve pitch scaling.