ALD of Al₂O₃ for Carbon Nanotube vertical interconnect and its impact on the electrical properties

Abstract

Because of their superior electronic properties and bottom-up growth mode, Carbon Nanotubes (CNT) may offer a valid alternative for high aspect ratio vertical interconnects in future generations of microchips. For being successful, though, CNT based interconnects must reach sufficiently low values of resistance to become competitive with current W or Cu based technologies. This essentially means that CMOS compatible processes are needed to produce dense CNT shells of extremely high quality with almost ideal contacts. Moreover, their electrical properties must be preserved at every process step in the integration of CNT into vertical interconnect structures. In this work this latter aspect is analyzed by studying the changes in the electrical characteristics when encapsulating CNT into different oxides. Oxide encapsulation is often exploited to hold the CNT in place and to avoid snapping during a polishing step. On the other hand, oxide encapsulation can influence the properties of the grown CNT which are directly exposed to possibly harmful oxidative conditions. Two different deposition techniques and oxides were evaluated: Chemical Vapor Deposition (CVD) of SiO₂ (reference) and Atomic Layer Deposition (ALD) of Al₂O₃ in less aggressive oxidizing conditions. The two processes were transferred to CNT interconnect test structures on 200mm wafers and electrically benchmarked. The CNT resistance was measured in function of the CNT length which allows the extraction and individual distinction of the resistive contributions of the CNT and the contacts. It is shown that the encapsulating SiO₂ deposited by CVD degrades the resistance of CNT by altering their quality. Directions for future improvements have been identified and discussed.