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The Development of High-Temperature Stable Amorphous Metal Alloy Gates for the Reduction of Threshold Voltage Variability in Short Channel Cmos Devices

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The Development of High-Temperature Stable Amorphous Metal Alloy Gates for the Reduction of Threshold Voltage Variability in Short Channel Cmos Devices THE DEVELOPMENT OF HIGH-TEMPERATURE STABLE AMORPHOUS METAL ALLOY GATES FOR THE REDUCTION OF THRESHOLD VOLTAGE VARIABILITY IN SHORT CHANNEL CMOS DEVICES A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Melody Ellen Grubbs December 2011 © 2011 by Melody Ellen Grubbs. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/mj693yp8026 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Bruce Clemens, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Yoshio Nishi, Co-Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Paul McIntyre I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Michael Deal Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Abstract Polycrystalline metal gates have replaced polycrystalline silicon gates in complementary metal oxide semiconductor (CMOS) devices for high speed performance applications. This is because the decreased capacitance caused by the gate electrode depletion layer in polycrystalline silicon gates was a pressing concern for device performance and continued scaling. Metal gates do not have this depletion issue; however, work function tuning is an issue. Consequently, a focus in the metal gate area is placed on work function engineering and the effect of processing conditions on the effective work function. The first project in this thesis studies the effect oxygen on the work function of tungsten electrodes. As expected, it was found that the presence of the meta-stable A15 phase is correlated with oxygen concentration. Tungsten films with minimal oxygen content contain only the stable bcc phase. However, after a forming gas anneal was performed on MOS capacitors made from these films, it was found that the A15 phase converts completely to a strained bcc phase. Despite this uniform transformation, it was found that the work function of the tungsten gate electrodes varies from 4.5–4.9 eV depending on the partial pressure of oxygen that the tungsten films were grown in. It is hypothesized that the observed variation is due solely to the incorporation of oxygen from the growth environment into the W layer at the SiO2/W interface. This work shows that stochastic fluctuations in processing conditions could also cause wafer-to- wafer variability, even in long channel devices. Another concern with metal gates is that due to their polycrystalline nature, device variability could become a problem as the gate dimensions are scaled down and become iv comparable to the grain size since work function can vary significantly with varying grain orientation. The main project in this thesis is to develop amorphous metal gates that have the potential to reduce work function variability with respect to polycrystalline gates in nano-scale MOS devices. Since gate-first processing is advantageous, the amorphous alloys to be characterized need to have high temperature stability (Tcrystallization > 1000˚C). Consequently, refractory transition metal-metalloid bulk metallic glasses of the form (TaxW1-x)80Si10B10 were chosen as the first gate electrode materials to be developed. These alloys have nMOS-compatible work functions and the amorphous phase is stable at 1000oC; however, secondary ion mass spectrometry showed that boron diffused into the channel region at temperatures above 800oC. The development of Ta40W40Si10C10 solved this problem. This material also has an nMOS- compatible work function and is stable at temperatures in excess of 1000˚C. When Ta40W40Si10C10 is integrated in long channel transistor devices, the effective channel mobility also appears to be enhanced with respect to crystalline tantalum and Ta47.5W47.5Si2.5C2.5 gates. Also, methods to produce pMOS compatible amorphous alloys and the ALD development of amorphous materials are recommended as future work. v Table of Contents Abstract ......................................................................................................................... iii Chapter 1: Introduction .......................................................................................... 1 1.1. History of Transistor Gates .................................................................................... 2 1.2. Ideal Gate Properties .............................................................................................. 4 1.3. Gate Work Function and Transistor Operation...................................................... 4 1.4. Gate Integration: Gate-First and Gate-Last Processes ........................................... 6 1.5. The Case for Amorphous Metal Gates................................................................... 7 1.6. Summary .............................................................................................................. 13 1.7. Thesis Objectives and Organization .................................................................... 13 References ................................................................................................................... 15 Chapter 2: Experimental Procedures ............................................................... 16 2.1. Introduction to Sputter Deposition....................................................................... 16 2.2. X-ray Diffraction of Amorphous Materials ......................................................... 17 2.3. XPS Analysis ....................................................................................................... 21 2.4. MOS Capacitor Process Flow .............................................................................. 21 2.5. MOS Capacitor Analysis ..................................................................................... 23 2.6. Ring Gate Transistor Process Flow...................................................................... 25 2.7. MOS Transistor Analysis..................................................................................... 28 References ................................................................................................................... 30 Chapter 3: The Effect of O2 on the Tungsten Work Function ................ 31 3.1. Introduction .......................................................................................................... 31 3.2. Experimental Procedure ....................................................................................... 34 3.3. Results .................................................................................................................. 34 3.4. Discussion ............................................................................................................ 40 3.5. Conclusions .......................................................................................................... 41 References ................................................................................................................... 43 Chapter 4: Amorphous Metal Alloy Gates: Initial Synthesis and Characterization of Ta-W-Si-B on SiO2 and HfO2……………...…………………….44 4.1. Previous Work on Amorphous Metal Gates ........................................................ 44 vi 4.2. Initial Material Choice..........................................................................................46 4.3. Ta-W-Si-B Synthesis ............................................................................................47 4.4. Ta-W-Si-B Alloy Characterization on SiO2 .........................................................48 4.4.1. XRD and RTA Analysis ..................................................................................... 48 4.4.2. Work Function Analysis ..................................................................................... 49 4.5. Ta-W-Si-B Alloy Characterization on HfO2 ........................................................51 4.5.1. XRD and RTA Analysis ..................................................................................... 52 4.5.2. Work Function Analysis ..................................................................................... 52 4.6 Composition vs. Thermal Stability ........................................................................54 4.7. SIMS Analysis of select Ta-W-Si-B alloys: Boron Penetration ..........................58 4.7.1. Sample Selection and Prep ................................................................................. 58 4.7.2. SIMS Analysis ..................................................................................................
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