ELECTRODEPOSITION OF NICKEL–COPPER ALLOYS AND NICKEL-COPPER-ALUMINA NANOCOMPOSITES INTO DEEP RECESSES FOR MEMS A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Chemical Engineering By Amrit Panda B.S., Anna University, 1997 May, 2003 To Shwetha… ii ACKNOWLEDGEMENTS I would like to thank Dr. E. J. Podlaha, my major professor. It is her guidance, continued inspiration and patience throughout the duration of the project, which made this work possible. My thanks to Dr. K. T. Valsaraj, Dr. R. McCarley, Dr. M. Murphy, Dr. J. Chan and Dr. R. Svoboda, for accepting to be on my graduate committee. I would like to thank the assistance provided by Dr. X. Xie from the Geology department at LSU for his assistance with the SEM and WDS, Mr. Ling at CAMD for his help in the x-ray exposures, Dr. J. Jiang from Mechanical Engineering for his assistance with TEM analysis, Mr. P. Rodriguez and Mr. F. McKenzie from the Chemical Engineering workshop. I would like to acknowledge my colleagues in the Electrochemical Engineering Lab, Mr. Q. Huang, Dr. Y. Zhuang, Mr. L. Namburi, and Mr. A. Mukherjee for many stimulating technical discussions. I also thank Mr. V. Singh (Mechanical Engineering) for his assistance and advice. The assistance provided by undergraduate student workers Jill Fitzgerald, Patricio Harrington, Gabriela Ponce, Dwight Bordelon and Jay Stephenson at various stages is greatly appreciated. The funding for this work was provided by the National Science Foundation. iii TABLE OF CONTENTS DEDICATION………………………………………………………………………….ii ACKNOWLEDGEMENTS……………………………………………………………iii ABSTRACT……………………………………………………………………………vi CHAPTER 1. INTRODUCTION……………………………………………………….1 CHAPTER 2. LITERATURE REVIEW………………………………………………..6 2.1 Ni-Cu Alloy Electrodeposition…..………………………… ………………7 2.2 Composite Electrodeposition………………………………………………15 2.3 Electrodeposition in Recessed Geometries………….……………………..27 2.4 Composite Deposition in Recesses…………………………………….. …30 2.5 LIGA Microdevices………………………………………………………..31 2.6 Materials for LIGA………...………………………………………………35 CHAPTER 3. EXPERIMENTAL DESIGN…………………………………………..37 3.1 Recessed Electrode………………………………………………………..38 3.1.1 Substrate Preparation……………………………………………38 3.1.2 Bonding…………………………......................……………….. 38 3.1.3 Lithography of PMMA…………………………………….……39 3.1.4 Development and Rinsing……………………………………….40 3.1.5 Etching…………………………………………………………..43 3.2 Holder Design…………………………………………………….......…...44 3.3 Plating Cell Setup…………………………………………………………46 3.4 Rotating Cylinder Electrode………………………………………………47 3.5 Instrumentation……………………………………………………………49 3.6 Electrolytes………………………………………………………………..49 3.7 Characterization…………………………………………………………...50 CHAPTER 4.EXPERIMENTAL RESULTS….…………………………….………..52 4.1 Deep Recess Deposition………………………………………….……….52 4.1.1 Pulse Parameter Selection……………………………….………53 4.1.2 Impedance Spectroscopy……………………………….……….58 4.1.3 Ni-Cu Alloy Deposition………………………………….……...61 4.1.3.1 Low pH Deposition…………………………….……...61 4.1.3.2 High pH Deposition……………………………….…..65 4.1.4 Ni-Cu- γ Al O Composite Deposition…………………….……70 2 3 4.1.4.1 Effect of pH…………………………………….……..70 4.1.4.2 Effect of Applied Current Density……………….…...80 4.1.4.3 Effect of Alumina Concentration……………….…….85 4.1.5 Growth Rates………………………….…………………….….88 4.2 RCE Deposition…………………………………..……………………....90 iv 4.2.1 Ni-Cu Alloy Polarization Curves………………………….……91 4.2.1.1 Low pH Deposition….…………………………….…….91 4.2.1.2 High pH Deposition……...………….…………….…….93 4.2.2 Ni-Cu- γ Al O Composite Deposition...………………….……96 2 3 4.2.3 Effect of Al O on Partial Currents and Efficiencies…………. 98 2 3 4.3 TEM Analysis…………………………………………………..….….…...113 4.4 Microhardness………………………………………….…………………..116 CHAPTER 5. MODEL DEVELOPMENT……………………………………….…..117 5.1 Steady State Simulation ………………………………………………….117 5.1.1 Homogeneous Chemical Equilibrium…………………………..117 5.1.2 Reaction Mechanisms……………………………………….…..121 5.1.3 Transport Equations and Boundary Conditions………………...125 5.2 Steady State Simulation Results…………………………………………..128 5.2.1 Ni-Cu Alloy Simulation…….…………………………………..128 5.2.1.1 Low pH Simulations.………………………………….128 5.2.1.2 High pH Simulations.…………………………………132 5.2.2 Ni-Cu-γ Al O Composite Simulation.……………………….…139 2 3 5.2.2.1 Reaction Mechanism……….…………………………139 5.2.2.2 Ni-Cu Alumina Simulation Results…….…….……….142 5.3 Non-Steady State Simulation....…………………….……….…………….147 5.3.1 Transport Equations and Boundary Conditions…….…….……..148 5.3.2 Non-Steady State Simulation Results………………..…….….....151 5.3.2.1 Simulation of Surface pH Rise……………………......151 5.3.2.2 Effect of Bulk pH and Buffer Strength…….…….…....153 5.3.2.3 Effect of Boundary Layer Thickness………………….156 5.3.2.4 Effect of Alumina……………………………………..159 CHAPTER 6. DISCUSSIONS AND CONCLUSIONS …………………………….163 6.1 Discussions………………………………………………………………..163 6.2 Conclusions……………………………………………………………….170 REFERENCES…….….………………………………………………………………173 NOMEMCLATURE………….………………………………………………………188 VITA………………………………………………………………………………….191 v ABSTRACT Electrodeposition is an important component in the fabrication of micro electro mechanical systems (MEMS). Nickel is the most commonly used material to produce three dimensional microstructures and few material alternatives have been demonstrated. In this dissertation, electrodeposited Ni-Cu alloys and nanocomposites are investigated as possible replacements for nickel in microsystems. Ni-Cu alloys are attractive for their corrosion resistance, magnetic and thermophysical properties. Alumina nanoparticulates included into metal matrices improve hardness and tribology of deposits. The Ni-Cu alloys and Ni-Cu-γ-Al O nanocomposites were 2 3 electrodeposited from a citrate electrolyte, both at low and high pH. Electrodeposition experiments were performed in recessed microelectrodes 500 µm thick and also on rotating cylinder electrodes. Recessed electrodes were produced by x-ray synchrotron radiation at the Center for Advanced Microstructures and Devices (CAMD). The concentration of copper in the electrolyte was much lower than the nickel concentration to ensure diffusional control. In the microstructure, the copper concentration in the deposit increased along the height, leading to a graded microstructure. This is indicative of a changing boundary layer and a transient process. The addition of alumina nanoparticles in the electrolyte led to an enhancement of copper concentration in the deposit, resulting from an enhancement of its mass transport rate. Two numerical models were developed to describe the steady state and non-steady state deposition processes. The effect of alumina on the metal deposition partial currents and side reactions is simulated by using a surface coverage model. Rotating cylinder experiments and simulation are used to extract kinetic and diffusional parameters of the nickel and vi copper species. On the recessed electrodes a transient model taking into account the time dependence of concentration is developed. The rise of surface pH, concentration gradients and buffering effects of the complexing agents are explained by the non- steady state model. vii CHAPTER 1. INTRODUCTION Electrodeposition or electroplating is the process by which an applied current or potential is used to deposit a film of metal or alloy by the reduction of metallic ions onto a conductive substrate. The electrodeposition of particulates along with a metal or alloy leads to the formation of a composite. Electroplating finds numerous applications as thin films and as three-dimensional, thick structures in microdevices. The properties of interest for both thick and thin films cover a broad range, including corrosion resistance, wear resistance, thermal, magnetic and optical characteristics. Applications include materials for printed-wiring boards, contacts, connectors, and magnetic- recording heads.1,2 High-resolution lithographic techniques has enabled electronic device integration, with electrodeposits several hundred microns thick. The importance of electrodeposition as a fabrication technology in the microelectronics industry is growing. For some of the fabrication processes involved, electrodeposition offers advantages over competing technologies such as physical and chemical vapor deposition, in that it requires simpler instrumentation and operating conditions.3 With the trend toward miniaturization, electrodeposition has established itself as the manufacturing technology of choice.1,4 Electrodeposition also holds great promise to conformably deposit metals and alloys for microelectronic fabrication and HARMs (High Aspect Ratio Microdevices). Fabrication of high aspect ratio microstructures by electrodeposition is of increasing interest for a multitude of applications in microelectromechanical systems (MEMS). Conventional anisotropic silicon etching used to create recesses for 1 subsequent metal deposition is restricted to crystallographic orientations. Traditional ultraviolet (UV) lithography is limited by the depth of the pattern as a result of the lower energy of the UV beams, resist sensitivity and contrast.5 To produce tall, three dimensional structures, deep x-ray lithography, which uses high energy x-rays produced by a synchrotron, is utilized to transfer a pattern onto thick x-ray resists with lateral dimensions on the order of a few microns and with heights of up to 2 mm. Lithography comprises one step of the LIGA process. LIGA is the German abbreviation of the three major process steps, lithography (Lithographie), electroplating (Galvanoformung) and molding (Abformung).6 The use of the LIGA process overcomes many drawbacks of other traditional fabrication methods and pattern transfer techniques such as chemical etching, sputter etching, reactive ion etching. Electrodeposition is an integral part of the LIGA process. It is employed after exposure and development of the resist and is used to build up a complementary pattern onto a metal substrate by filling the empty spaces of the electrically non-conductive resist starting from the electrically conducting base. The metal pattern produced in such a way can then be used as the microdevice itself or can serve as a mold for plastic replication.7 X-ray lithography combined with electroforming and molding techniques thus allow the fabrication of microstructures, several hundred micrometers in depth, with any cross sectional shape with nearly atomic resolution and vertical side-walls.5 Applications of LIGA microstructures are in the manufacture of a wide variety of devices, including microelectronic circuits, sensors, actuators and microoptical microstructures. Several prototypes of micro heat-exchangers, microturbines with partly 2 movable parts, micropumps with valve openings and a pumping chamber have also been demonstrated.2 Electrodeposited elemental nickel has been the material of choice to demonstrate a variety of microdevice prototypes, including micro-gears, and micro- cantilevers.6 Ehrfeld et al. 8 have reviewed several elemental deposition systems such as Ni, Cu, Au, Fe and also a few alloy systems like NiFe and NiCo for their suitability in the fabrication of HARMs. At present, materials electroformed as LIGA microstructures and devices are limited to only a few elemental metals and alloys such as Ni, Cu, Au, Zn, Ni-Co, and Ni-Fe etc. A paucity of available materials limits the utilization of LIGA products in a number of potential applications where materials, with specifically better properties such as corrosion resistance and mechanical strength are required. A much wider selection of materials with properties that can be tailored to a specific application have to be developed to provide improved and novel applications for future microdevices. Electrodeposited alloys and composites are of particular interest because of their unique and often superior properties compared with their metal counterparts.3, 19, 64 As thin films, the electrodeposited alloys of nickel have been of interest for their various properties such as low internal stress,9 magnetic properties,10 hardness,11 malleability, ductility12 and resistance to corrosion.13 Nickel copper alloys have great potential in industrial applications where, resistance to corrosion,7 and thermo-electric characteristics are required.11 Its corrosion resistance makes it suitable for use as a protective coating in marine and corrosive environments.9 Past studies on Ni-Cu alloys have involved work primarily on planar14-25 electrodes. Relatively few studies exist on 3