Ru-based Gate Electrodes for Advanced Dual-Metal Gate CMOS Devices

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Title: Ru-based Gate Electrodes for Advanced Dual-Metal Gate CMOS Devices
Author: Zhong, Huicai
Advisors: G. Parsons, Committee Member
G. Lucovsky, Committee Member
V. Misra, Committee Chair
J. R. Hauser, Committee Member
C. M. Osburn, Committee Member
Abstract: The purpose of this research has been to search for proper metallic gate electrodes for CMOS devices. The most important criteria for an alternative metal gate electrode are (a) appropriate work function for NMOS or PMOS (b) excellent thermal/chemical stability under high temperature (c) low resistivity and high active electrical concentration and (d) feasibility for integration. This dissertation covers the following research areas. The properties of reactive sputtering deposited Ru and RuO2 films were systematically studied. Techniques such as XRD, AFM, XPS and TEM were used to characterize the films. High temperature stability was observed for both Ru and RuO2 films on SiO2 and ZrO2. The films also exhibited low stress, good oxygen barrier properties and were easy to dry etch. Electrical properties were evaluated on MOS capacitors with SiO2 and high-K dielectrics including ZrO2, Al2O3, and Y2O3. Excellent stability of oxide thickness and flatband voltage was observed for SiO2 dielectric capacitors. For capacitors with high-K dielectrics, equivalent oxide thickness (EOT) change and flatband voltage shift as a function of annealing temperature were observed. This was attributed to excess oxygen in the high-K dielectrics that was causing the oxidation of the Si substrate. Next, Ru-Ta alloy films prepared by reactive sputtering were evaluated as gate electrode materials. The alloy film resistivities had a strong dependence on film composition and annealing temperature. XRD and XPS measurements were used to evaluate the phases for alloy films. Ru1Ta1 phase was formed in alloy film with at. % Ta between 40% and 60%. Excellent thermal stability of alloy films with Ta at.% < 60% was observed up to RTA anneal temperatures of 1000C. A non-linear relationship between work function and Ru-Ta alloy composition was measured. The values ranged from 4.2eV to 5eV, making it possible for this alloy to be used as gate electrode for both N- and P-MOSFET device. Finally, NMOSFETs and PMOSFETs using the best gate electrodes, namely, Ru-Ta alloy for NMOS and Ru for PMOS, were fabricated using SiO2 as the gate dielectric. A comprehensive analysis of device performance was made with metal gate electrodes and poly-Silicon controls. Excellent hole mobility for Ru gate P-MOSFET was observed. The mobility of N-MOSFETs with Ru-Ta alloys gate electrode were found to be slightly lower than the universal curve and this was attributed to the presence of interface states arising from the sputtering process. This suggests the need for optimizing the deposition process. In conclusion, Ru-based alloys were evaluated as gate electrodes and were found to exhibit desirable properties to be considered as proper candidates for CMOS devices.
Date: 2002-08-19
Degree: PhD
Discipline: Electrical Engineering

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