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Open Phd Thesis - Final Draft.Pdf The Pennsylvania State University The Graduate School College of Earth and Mineral Sciences DEPOSITION, CHARACTERIZATION, AND THERMOMECHANICAL FATIGUE OF NICKEL ALUMINIDE AND RUTHENIUM ALUMINIDE THIN FILMS A Dissertation in Materials Science and Engineering by Jane A. Howell © 2010 Jane A. Howell Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2010 The dissertation of Jane A. Howell was reviewed and approved* by the following: Suzanne E. Mohney Professor of Materials Science and Engineering Dissertation Co-Advisor Co-Chair of Committee Christopher L. Muhlstein Associate Professor of Materials Science and Engineering Dissertation Co-Advisor Co-Chair of Committee Joseph R. Flemish Senior Scientist and Professor of Materials Science and Engineering Clifford J. Lissenden Professor of Engineering Science and Mechanics Joan M. Redwing Professor of Materials Science and Engineering Chair, Intercollege Graduate Degree Program in Materials Science and Engineering *Signatures are on file in the Graduate School ii ABSTRACT Intermetallic thin films have properties that make them attractive for applications such as metallizations, high temperature coatings, microelectromechanical systems, and diffusion barrier layers. In this study B2 aluminide films (NiAl and RuAl) have been deposited and characterized. Both intermetallics could be deposited at temperatures near room temperature using co- sputtering with an as-deposited resistivity of 45.5 ± 1.5 μΩcm for NiAl and 157 ± 4 μΩcm for RuAl. Ni/Al multilayers with a wavelength of 30 nm and below were fully reacted to form NiAl after annealing for 2 h at 400°C. These films had a resistivity of 15.5-26.7 μΩcm (wavelengths from 15.4-30 nm) after a 4 h anneal at 400°C, and the lower values of resistivity correspond to films with larger wavelengths. In order for Ru/Al multilayers to be fully reacted at 400°C, wavelengths of less than 10 nm were required as well as longer annealing times (more than 11 h). RuAl from the 400°C reaction of Ru/Al multilayers had a resistivity of 71.6-123 μΩcm. The lowest resistivity obtained for the B2 thin films was 11 ± 0.1 μΩcm, which was obtained for NiAl after a 20 min anneal at 800°C. Both NiAl and RuAl have excellent bulk oxidation resistance, and in this study it was found that the oxidation resistance of the intermetallic thin films was superior to Al, Ni, and Ru metal films. The intermetallic films showed no observable surface changes (light microscopy) up to 500ºC in flowing oxygen and were conductive to higher oxidation temperatures (800°C for RuAl, 850ºC for NiAl) than Ni (500ºC), Al (600ºC), and Ru (800ºC, although vaporization may have begun at ~700ºC). The intermetallics NiAl and RuAl along with Ru and Au have been patterned into thin line structures and then tested using an alternating current (100 Hz) to induce thermomechanical fatigue (TMF) with 200 thermal cycles per second. RuAl samples were able to withstand higher cyclic values of ΔT than NiAl for comparable times to failure. Both NiAl and RuAl were able to withstand higher values of ΔT than gold. The ΔT for tests on NiAl ranged from ~300-520°C (Tmax: 400-600°C), with a time to failure of 100’s of hours when ΔT was near 300°C (Tmax: ~400°C). Samples of NiAl that had a lower resistivity were able to withstand higher current densities due to reduced Joule heating. RuAl had a longer time to failure than NiAl at high testing temperatures (ΔT > ~350°C), but trends in the data indicate that the time to failure at lower temperature may be higher for NiAl. Curves of the resistance as a function of time were plotted for all of the samples during the course of the AC tests. These curves showed three distinct regions, which in the case of the intermetallic films can be ascribed to heating, the production of defects (dislocations and vacancies), and crack growth (and possibly voids). It was determined that by measuring the slope of the R(t) curve at the beginning of the test, the time to failure and the length of time spent in each of the three regions could be estimated. This will enable future tests to be paused at specific locations along the R(t) curves so that damage formation and crack growth may be studied. The use of Ni- and Ru-aluminide films combined with 2 different etchants for the fabrication of MEMS has also been investigated. Using a conventional wet etch for SiO2 sacrificial layers (HF) led to cracking and/or buckling depending on the stress state in the film (tensile/compressive). However, using a gas phase etchant for silicon sacrificial layers, XeF2, free-standing regions could be formed that were crack free. Resonators were fabricated from co-sputtered NiAl and iii RuAl, and annealed multilayers of Ni/Al and Ru/Al, and first out-of-plane bending mode resonance was observed by using XeF2 etching. The best results were obtained for as-deposited NiAl that was co-sputtered at 1.5 mTorr and was under a compressive stress of ~0.83 GPa. While the devices were not completely flat, they were free-standing, and improvements are expected by decreasing the stress in the co-sputtered films by increasing the sputtering pressure. The results for RuAl resonators are also promising as the films can withstand high compressive stresses (~1.5 GPa, calculated from edge buckling), and improved performance from co- sputtered RuAl is expected by increasing the sputtering pressure in order to decrease the film stress. Thermal actuators fabricated from as-deposited co-sputtered NiAl showed ~18 m of motion with an applied current of 70 mA, and very little curvature/buckling was noted. The effect of the high compressive stress is less pronounced in the doubly supported actuators than in the singly supported resonators. Improvements in processing should lead to improvements in the device yield and curvature, but the results presented plus the high strength, high electrical conductivity, good oxidation/corrosion resistance, and moderate toughness make NiAl and RuAl suitable materials for MEMS. iv TABLE OF CONTENTS LIST OF FIGURES ....................................................................................................................... ix LIST OF TABLES ..................................................................................................................... xxvi ACKNOWLEDGEMENTS ....................................................................................................... xxix CHAPTER 1 ................................................................................................................................... 1 INTRODUCTION AND BACKGROUND ................................................................................... 1 I. INTRODUCTION ................................................................................................................. 2 II. INTERMETALLIC COMPOUNDS ..................................................................................... 3 A. Selection of Intermetallics ............................................................................................ 4 III. OVERVIEW OF NiAl ........................................................................................................... 6 A. Mechanical Properties of NiAl ..................................................................................... 7 B. Electrical Properties of NiAl ....................................................................................... 11 IV. OVERVIEW OF RuAl ........................................................................................................ 13 A. Mechanical Properties of RuAl ................................................................................... 14 B. Electrical Properties of RuAl ...................................................................................... 15 V. DISCUSSION ...................................................................................................................... 16 REFERENCES ............................................................................................................................. 17 CHAPTER 2 ................................................................................................................................. 27 NICKEL ALUMINIDE FILM FABRICATION .......................................................................... 27 I. INTRODUCTION ............................................................................................................... 28 A. Ni-Al Films from the Reaction of Ni/Al Multilayers ................................................. 28 B. Ni-Al Films from Co-Deposition and Alloyed Sources/Targets ................................ 31 II. EXPERIMENTAL PROCEDURE ...................................................................................... 35 III. RESULTS AND DISCUSSION .......................................................................................... 37 A. Ni-Al Films from the Reaction of Ni/Al Multilayers ................................................. 39 B. Co-Sputtered Ni-Al Films ........................................................................................... 49 IV. CONCLUSIONS.................................................................................................................. 57 REFERENCES ............................................................................................................................. 59 CHAPTER 3 ................................................................................................................................
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