Low-Temperature Side Contact to Carbon Nanotube Transistors: Resistance Distributions Down to 10 nm Contact Length.

Carbon nanotube field-effect transistors (CNFETs) promise to improve the energy efficiency, speed, and transistor density of very large scale integration circuits owing to the intrinsic thin channel body and excellent charge transport properties of carbon nanotubes. Low-temperature fabrication (e.g., <400 °C) is a key enabler for the monolithic three-dimensional (3D) integration of CNFET digital logic into a device technology platform that overcomes memory bandwidth bottlenecks for data-abundant applications such as big-data analytics and machine learning. However, high contact resistance for short CNFET contacts has been a major roadblock to establishing CNFETs as a viable technology because the contact resistance, in series with the channel resistance, reduces the on-state current of CNFETs. Additionally, the variation in contact resistance remains unstudied for short contacts and will further degrade the energy efficiency and speed of CNFET circuits. In this work, we investigate by experiments the contact resistance and statistical variation of room-temperature fabricated CNFET contacts down to 10 nm contact lengths. These CNFET contacts are ∼15 nm shorter than the state-of-the-art Si CMOS "7 nm node" contact length, allowing for multiple generations of future scaling of the transistor-contacted gate pitch. For the 10 nm contacts, we report contact resistance values down to 6.5 kΩ per source/drain contact for a single carbon nanotube (CNT) with a median contact resistance of 18.2 kΩ. The 10 nm contacts reduce the CNFET current by as little as 13% at VDS = 0.7 V compared with the best reported 200 nm contacts to date, corroborated by results in this work. Our analysis of RC from 232 single-CNT CNFETs between the long-contact (e.g., 200 nm) and short-contact (e.g., 10 nm) regimes quantifies the resistance variation and projects the impact on CNFET current variability versus the number of CNT in the transistor. The resistance distribution reveals contact-length-dependent RC variations become significant below 20 nm contact length. However, a larger source of CNFET resistance variation is apparent at all contact lengths used in this work. To further investigate the origins of this contact-length-independent resistance variation, we analyze the variation of RC in arrays of identical CNFETs along a single CNT of constant diameter and observe the random occurrence of high  RC, even on correlated CNFETs.