Metal alloys and Gate Stack Engineering for CMOS Gate Electrode Application

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Title: Metal alloys and Gate Stack Engineering for CMOS Gate Electrode Application
Author: Chen, Bei
Advisors: D. W. Barlage, Committee Member
Mark Johnson, Committee Member
Veena Misra, Committee Chair
Carl Osburn, Committee Member
Abstract: The purpose of this research has been to search for proper metallic gate electrodes for CMOS devices. This dissertation covers several binary alloy metal gate research topics. First, intermetallic binary alloy RuY was investigated. From C-V analysis we obtained the effective work function of Ru-Y thin films to range from 5.0eV to 3.9eV which is suitable for dual metal gate CMOS. The rich Y film was found to be not stable on SiO2 dielectrics because of the high oxygen affinity of Y. RuxYy thin film may still be a candidate for low temperature process, especially due to its large range of work function. More over, RuY has smaller grain size than Ru which demonstrates one of the advantages of alloy by reducing grain size to achieve more uniform gate film and more uniform effective work function for the nano-size device applications. Chapter 3 presents MoxTay as a potential candidate for dual metal CMOS applications. The electrical characterization results of MoTa alloy indicates that the effective work function can be controlled to around 4.3 eV on SiO2 and is suitable for NMOS gate electrode application. The MoTa alloy forms a solid solution instead of an intermetallic compound. We report that the MoTa solid solution can achieve low work function values and is stable up to 900°C. X-ray diffraction results indicated only a single MoTa alloy phase. Moreover, from Auger electron spectroscopy and Rutherford backscattering spectroscopy analysis, MoTa was found to be stable on SiO2 under high temperature anneals and no metal diffusion into substrate Si channel was detected. This indicates that MoxTay is a good candidate for CMOS metal gate applications. Chapter 4 evaluates Ru and W capping layer for MoTa metal gate electrodes in Metal Oxide Semiconductor capacitor applications. We report that the oxygen diffusion from the capping layer plays an important role in determining the MoTa alloy effective work function value on SiO2. MoTa alloy metal gate with Ru capping exhibit stable effective work function up to 900°C anneal but is not stable with W capping. Auger electron spectroscopy and Rutherford backscattering spectroscopy analysis shows minimal oxygen diffusion into the MoTa gate stacks with Ru capping while severe oxygen diffusion into the gate is observed with W capping metal after 900°C anneal. In chapter 5, We have studied the φm behavior of AlTa alloys with varying compositions ranging from pure Al to pure Ta. The effective work function of AlTa alloy increased up to 4.45 eV as compared to pure Al work function (~4.1eV) or pure Ta work function (~4.2eV) on SiO2 at 400°C FGA. We ascribe the φm increase due to an interface dipole originating from a thin negative charged reaction layer formed between the AlTa alloy and dielectric layer. In order to further increase the stability of the AlTa alloy while still obtaining φm tuning, N was added to make AlTaN. These alloy electrodes resulted in effective work function values of ~5.13 eV after a 1000°C anneal making them suitable candidates for PMOS electrodes. Chapter 6 shows a new method that can tune the effective work function utilizing dipole layers has been demonstrated. Continued work function values can be expected by modifying the dipole strengths. This routine can potentially provide a new method for the metal gate work function research for the future wide gape semiconductor device.
Date: 2006-10-24
Degree: PhD
Discipline: Electrical Engineering

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