Carbon nanotube synthesis for integrated circuit interconnects

Abstract

Based on their properties, carbon nanotubes (CNTs) have been identified as ideal replacements for copper interconnects in integrated circuits given their higher current density, inertness, and higher resistance to electromigration. Although at the laboratory level CNTs have proven their technical viability as interconnects, fabrication issues such as growing the desired type of CNTs in selected positions, at temperatures compatible with CMOS processing (below 500"C), and with the appropriate electrical connections, remain challenges that are hindering their introduction into industry. The purpose of this study was to develop the processes and understanding needed to establish CNTs as viable replacements for metal-based integrated circuit (IC) interconnects. Through over a thousand synthesis experiments using a dedicated thermal Chemical Vapor Deposition (CVD) system, a systematic approach was developed starting with growth of CNTs on insulating substrates, then moving to conducting substrates, and finally integrating CNT growth into insulating scaffolds with regularly spaced pores. The following results were achieved: Control of the type of carbon nanotubes grown using simple process parameter variations: By focusing on controlling catalyst morphology evolution to obtain dense and tall carpets of vertically-aligned CNTs on insulating substrates, we were able to tune the diameter and number of walls, by simply timing the introduction of a reducing agent (hydrogen) into the thermal process.

(cont.) Growth of dense carpets of vertically-aligned CNTs on conductive substrates below 5000C: By focusing on the material properties of the catalyst and underlayer, we discovered important requirements for the underlayer grain structure evolution, as well as by preheating the incoming hydrocarbon gas, growth of dense and vertically aligned carpets of nanotubes on conductive underlayers at growth temperatures below 5000C could be achieved. Electrical characterization showed that we obtained ohmic contact between the CNTs and the substrate. Control of CNT crystallinity via gas preheating : We discovered that the time and temperature of gas preheating was critical for the crystallinity of the resulting CNTs. This was done by comparing the output gases from varying gas preheat treatments to the corresponding CNT structures. This allowed a discussion of the critical gas compounds responsible for growth of crystalline CNTs. Growth of CNTs into periodic insulating scaffolds on conductive substrates: We have grown CNTs on conductive substrates and in regularly-spaced pores of an insulating anodized alumina scaffold. This allowed simulation of an interconnect via system for future measurement of the electrical properties of CNTs. This structure can also serve as a starting point for future development of dense arrays of CNT-based devices.