An Introduction to Microelectromechanical Systems() Bing-Feng Ju, Professor Institute of Mechatronic Control Engineering, College of Mechanical and Energy Engineering, Zhejiang University P.R.China, 310027 Email: mbfju@zju.edu.cn Tel: 86-571-8795-1730 Fax: 86-571-8795-1941 Presented to Graduate students at College of Mechanical and Energy Engineering Zhejiang University February to May 2010 Lecture 6 Introduction to Nanoscale Engineering A Futuristic Nanoscale Engineering Product: an artistic view of a step-shaft built with atoms If I were asked for an area of science and engineering that will most likely produce the breakthroughs of tomorrow, I would point to nanoscale science and engineering. -Neal Lane Former Assistant to President Clinton for Science and Technology Nanotechnology has given us the tools to play with the ultimate toy box of nature atoms and molecules. Everything is made from it..the possibilities to create new things appear limitless. by Horst Stormer, a Nobel Laureate in physics the way of ingeniously controlling the building of small and large structures, with intricate properties; it is the way of the future, a way of precise, controlled building, with incidentally, environmental benignness built in by design. by Ronald Hoffmann, a Nobel Laureate in chemistry. 1 3 CONTENT Self-introduction 1. Overview of and Microsystems Working Principles of Microsystems 2. The Scaling Laws Electromechanical Design of and Microsystems 3. Material for and Microsystems Part 1: Silicon and silicon compounds Part 2: Piezoelectric and polymers 4. Microfabrication Processes Part 1: Photolithography, doping with ion implantation and diffusion Part 2: Etching Part 3: Depositions: physical, chemical and epitaxy 5. Micromanufacturing Assembly, Packaging and Testing to Nanoscale Engineering Part 1: Microassembly Part 2: Packaging with surface and wire bonding Part 3: Reliability and testing 6. Introduction to Nanoscale Engineering Part 1: Overview of nanoscale engineering Part 2: Material characterization and measurements Because both nanotechnology and nanoscale engineering are rapidly expanding emerging technologies of the New Century, it is necessary to split this lecture into the following two parts just to barely cover the current state of the development: Lecture 6(Part 1) Overview of Nanoscle Engineering Nanotechnology-What is it and why? Evolution of nanotechnology Nanotechnology and nanoscale engineering Fundamentals of nanomanufacturing Applications of nanotechnology Future outlook EMS Lecture 6(Part II) Nanoscale Engineering Analysis and Measurements of Selected Material Properties 2 4 1
What is Nanotechnology? Lecture 9A Overview of Nanoscle Engineering Nanotechnology is the creation of: USEFUL/FUNCTIONAL materials, devices and systems through control of matter on the nanometer length (nm) scale, and exploitation of novel phenomena and properties (physical, chemical, biological) at that length scale to satisfy human needs. Why nanotechnology? All matter that exist in universe are made of atoms and molecules. The way molecules of various shapes and surface features organize into patterns on nanoscales determines important material properties (e.g. electrical conductivity, optical properties, mechanical strength, etc.) Specifically, two factors can determine properties of matter: The atomic structures (e.g., No. of electrons, as in the periodic table) Example: Change electric resistivity of silicon by doping. The arrangement of atoms (e.g., molecular structures) If only we can manipulate the atomic structures and control the arrangement of the atoms in matter, can we then create materials with desirable properties e.g., super strength, super electric and thermal conductivities, healthy genomes and biochemical substances, etc. 5 7 atto The Perception of Length Scale - The nanometer (nm) Typical electron radius: 2.8x10-15 m femto pico DNA < 3 nm Protein: 2-5 nm Nano** 0 10-18 10-15 10-12 10-9 (nm) Micro* 10-6 (µm) Virus: 10-7 m Hydrogen atom: 0.1 nm, or one angstrom *1 μm = 10-6 m one-tenth of human hair ** 1 nm = 10-9 m span of 10 H 2 atoms Human hair:10-4 m Active R&D in Nanotechnology: inspired by Richard Feynman s speech in 1959 milli Feynman, R., There s Plenty of Room at the Bottom: An invitation to enter a new field of physics, (miniaturization) first presented at the American Physical Society at California Institute of Technology on December 29, 1959. Subsequent publication in Engineering and Science, Caltech, February 1960. 10-3 Length (m) EMS (1918-1988) A visionary and a Nobel Laureate in Physics, 1965 6 8 2
Quotations from of Richard Feynman s Speech: Feynman s vision on a new field of research: A field, in which little has been done, but in which an enormous amount can be done in principle. This field is Miniaturization. Feynman s view on The marvelous biological systems : In the tiniest cell, all information for the organization of a complex creature such as ourselves can be stored. All this information - whether we have brown eyes, or whether we think at all, or that in the embryo the jawbone should first develop with a little hole in the side so that later a nerve can grow through it - - all this information is contained in a very tiny fraction of the cell in the form of long-chain DNA molecules in which approximately 50 atoms are used for one bit of information about the cell The Very First Man-Made Nanostructure - The Buckyball Created in 1985 by a chemistry professor, Richard Smalley from Rice University - a Nobel Laureate in 1996. It contained 60 carbon atoms in the shape of a soccer ball with a diameter of 0.7 nanometer. Made from the Buckminsterfullerene - a third form of pure carbon molecule. (after the name of a futurist, R. Buckminster Fuller) 9 11 Quotations from Richard Feynman s Speech (Cont d): Feynman s view on The marvelous biological systems - Cont d: Biology is not simply writing information; it is doing something about it. Many of the cells are very tiny, but they are active; they manufacture various substances; they walk around; they wiggle; and they do all kinds of marvelous things - all on a very small scale. Also they store information. Consider the possibility that we too can make a thing very small which does what we want - that we can manufacture an object that maneuvers at that level! Feynman s vision on Nanotechnology : So, ultimately, when our computer get faster and faster and more and more elaborate, we will have to make them smaller and smaller. But there is plenty of room to make them smaller. There is nothing that I can see in the physical laws that say the computer elements cannot be made enormously smaller than they are now Major Impacts of Nanotechnology Nanotechnology is an enabling technology (source: Meyya Meyyappan, NASA Ames) Electronics, Computing and Data Storage Materials and Manufacturing Health and Medicine EMS Energy and Environment Transportation National Security Space exploration 10 12 3
Transistor Size Nanotechnology Benefits in Electronics and Computing (source: Meyya Meyyappan, NASA Ames) Processors using molecular electronics with declining energy use and cost per gate, thus increasing efficiency of computer by 10 6. Small mass storage devices: multi-tera (10 12 ) bit levels. boost speed limit of the integrated circuits Integrated nanosensors: collecting, processing and communicating massive amounts of data with minimal size, weight, and power consumption. Gates Higher transmission frequencies and more efficient utilization of optical spectrum to provide at least 10 times the bandwidth now. The Nanochip Nano transistors Nanotechnology Benefits in Electronics and Computing-Cont d Quantum computing, e.g., spinning single electron transistor. Heat = Horrendous challenge!! 13 14 SiO 2 film Display technologies, e.g., TFT. Silicon substrate (thin pure silicon film) Nanotechnology Benefits in Electronics and Computing-Cont d Intel roadmap on nano transistors using micro technology: Silicon-based 90 nm μ-technology 65 Length: 50 nm Gate oxide: 1.2 nm L = 30 Possible materials? Nanotechnology? (Likely technology) Equivalent to 2000 wires in a human hair! 45 Increase Leakage-Another major challenge! L = 20 Gate oxide: 0.3 nm 32 L = 15 2003 2005 2007 2009 2011? Year 22 L = 10 Single- Electron Transistor 15 Advantages: (1) Low unit cost. (2) Narrow gates for faster on-off Nanotechnology Benefits in Data Storage-1 The ever-increasing demand for high density information storage: 10,000 Mbits/in 2 EMS 2007 16 4
Nanotechnology Benefits in Data Storage-2 Data Storage Requirements: Density Error rate Re-writability Tracking Data rate Overall reliability Data retention Cost Source: Scanning Probes Microscope & Their Potential for Data Storage, John Mamin, IBM Almaden Research Center, San Jose, CA. (Private communication) Nanotechnology Benefits in Data Storage-4 Encouraging initial results of the Millipede development: 40 nm bit size 17 19 Nanotechnology Benefits in Data Storage-3 The continuous demands for high density data storage has passed the limit of traditional electromagnetic means. A new concept of Read-Write is being developed the Millepede project by IBM, San Jose and Zurich, Switzerland. Working principle involving indenting the surface of polymer film using AFM. Expanding ability to characterize genetic makeup will revolutionize the specificity of diagnostics and therapeutics - Nanodevices can make gene sequencing more efficient. Effective and less expensive health care using remote and in-vivo devices. Nanotube-based biosensor for cancer diagnostics Health and Medicine Medical testing, diagnosis; Drug discovery and delivery EMS New formulations and routes for drug delivery, optimal drug usage. More durable, rejection-resistant artificial tissues and organs. Sensors for early detection and prevention. 18 20 (Source: Meyya Meyya[[an, NASA Ames) 5
Materials and Manufacturing Ability to synthesize nanoscale building blocks with control on size, composition etc. further assembling into larger structures with designed properties will revolutionize materials manufacturing: - Manufacturing metals, ceramics, polymers, etc. at exact shapes without machining. - Lighter, stronger and programmable materials. - Lower failure rates and reduced life-cycle costs. - Bio-inspired materials. - Multifunctional, adaptive materials. - Self-healing materials. Thermal barrier and wear resistant coatings. 21 (Source: Meyya Meyya[[an, NASA Ames) High strength, light weight composites for increasing fuel efficiency. High temperature sensors for under the hood. Improved displays. Battery technology. Wear-resistant tires.* Automated highways. (Source: Meyya Meyyappan, NASA Ames) 23 Energy and Environment (Source: Meyya Meyya[[an, NASA Ames) Energy Production - Clean, less expensive sources enabled by novel nanomaterials and processes. Energy Utilization - High efficiency and durable home and industrial lighting. - Solid state lighting can reduce total electricity consumption by 10% and cut carbon emission by the equivalent of 28 million tons/year. (Source: Al Romig, Sandia Lab) Materials of construction, such as paints, sensing changing conditions, e.g., ambient air temperature, and in response, altering their inner structure for optimal property in thermal conductivity. Step 1 Major Paradigm Shift in Nanomanufacturing - an overview of molecular manufacturing Isolation of atoms or molecules: Laser source 1. Mechanical Means- e.g. by Atomic Force Microscope (AFM): 2. Electromechanical Means e.g. by using scanning tunneling microscope: Repulsive Force Fine metal needle tip Light detector Scanning probe 22 Scanning of AFM probe EMS V Narrow gap, d 3. High energy physical chemical means: - plasma sputtering or CVD. 24 6
Step 2 Major Paradigm Shift in Manufacturing -Overview of molecular manufacturing-cont d Assembly of loose atoms or molecules: Mechanical means by using special equipment, e.g. AFM-guided nanomachining system (AGN), or by special controlled environment. Assembled loose gold atoms using STM at IBM: Biochemical means for self assembler and replication. Shaft: Bearing: Shaft (axial view): Bearing (axial view): (Nano machine components.nanotechnology-3)hsu.4/14/01 Final assembled atoms in IBM 25 Step 3 Prospective Nano-scale Device Components - Building solid structures atom by atom, or molecule by molecule: 27 Major Paradigm Shift in Manufacturing -Overview of molecular manufacturing-cont d Re-bonding free atoms and molecules (Synthesis): Mechanical synthesis, e.g. vaporizing free atoms at high temperature in high vacuum, followed by condensations. (Synthesized Chinese words of atom made by atoms) Chemical synthesis by diffusion + chemical reactions, e.g. special CVD. Bio-chemical synthesis, such as the natural molecular machines for growing proteins, enzymes and antibodies. There are other methods for producing nanoscale products, e.g. epitaxial growth for nanoparticles and nanowires. Prospective Nano-scale Device Components Cont d A step-shaft EMS A pair of matching gears 26 28 7
Nanotechnology Products and Market Prospects Prevalent nano scale products are limited to the basic rudimentary types of: Nanodots (Quantum dots or Nanoparticles) For smart coatings and paints, cosmetic products, etc. Nanowires For gates and switches in molecular electronics, sensors, smart composites, fabrics, etc. Nanotubes For structure supports, nano switches, nanofluidics, etc. Markets and Projected Revenues for Nanotechnology: $50 million in Year 2001 $26.5 billion in Year 2003 (if include products involving parts produced by nanotechnology) $1 trillion by Year 2015 (US National Science Foundation) An enormous opportunity for manufacturing industry!! The strongest and most flexible molecular material because of C-C covalent bonding and seamless hexagonal network architecture Young s modulus of over 1 TPa vs 70 GPa for Aluminum, 700 GPA for C-fiber - strength to weight ratio 500 time > for Al; similar improvements over steel and titanium; one order of magnitude improvement over graphite/epoxy Maximum strain ~10% much higher than any material Thermal conductivity ~ 3000 W/mK in the axial direction with small values in the radial direction 29 31 (source: Meyya Meyyappan, NASA Ames) Carbon Nano Tubes (CNT) (Source: Meyya Meyyappan, NASA Ames) ) CNT is a tubular form of carbon with diameter as small as 1 nm. Length: few nm to microns. CNT is configurationally equivalent to a two dimensional graphene sheet rolled into a tube. CNT exhibits extraordinary mechanical properties: Young s modulus over 1 Tera Pascal, as stiff as diamond, and tensile strength ~ 200 GPa. CNT can be metallic or semiconducting, depending on chirality. 30 High strength composites. Cables, tethers, beams. Multifunctional materials. Functionalize and use as polymer back bone - plastics with enhanced properties like blow molded steel. Heat exchangers, radiators, thermal barriers, cryotanks. Radiation shielding. Filter membranes, supports. Body armor, space suits. (Source: Meyya Meyyappan, NASA Ames) Challenges - Control of properties, characterization - Dispersion of CNT homogeneously in host materials - Large scale production 32 - Application development EMS 8
CNT based microscopy: AFM, STM Nanotube sensors: force, pressure, chemical Biosensors Molecular gears, motors, actuators Batteries, Fuel Cells: H 2, Li storage Nanoscale reactors, ion channels Biomedical - in vivo real time crew health monitoring - Lab on a chip - Drug delivery - DNA sequencing - Artificial muscles, bone replacement, bionic eye, ear... Nanoscale Engineering- Production and Manufacturing Challenges Controlled growth Functionalization with probe molecules, robustness Integration, signal processing Fabrication techniques 33 (Source: Meyya Meyyappan, NASA Ames) There are several issues in transforming NT from scientific discoveries in the laboratory to commercial production by industry: Facility for volume production: - Self-assembly technology. - Reel-to-reel fabrication facility. - Robust tools and processes. - Low capital and operating costs. Production systems e.g. Flexible process-oriented manufacturing systems for nanoscale products of: high volume-low variety; medium volume-medium variety, or low volume-high variety of high value. Reliability and testing techniques and standards. Integration of nanoscale structures and devices into micro-, mesoand macroscale products packaging and interconnect. Production management. 35 Comparison of Micro- and NanosScale Technologies Nanotechnology is NOT a natural outgrowth of technology!! Microsystems (MST) Technology Top-down approach Miniaturization with micrometer and submicrometer tolerances Builds on solid-state physics Evolved from IC fabrication technology Established techniques for producing electromechanical functions Established fabrication techniques Established manufacturing techniques Proven devices and engineering systems in marketplace Success in commercialization of a few products Future Outlook Nanotechnology Bottom-up approach Miniaturization with atomic accuracy Builds on quantum physics and quantum mechanics, e.g. molecular solid and gas dynamics, heat transfer Undefined Far from being developed Fabrication techniques are in experimental stage Limited to manipulation of atoms only Only a few staionary machine components of simple geometry are produced A long way to go Before we look forward to what the ultimate benefits nanotechnology can bring to humankind, let us look back what are great technological breakthroughs in the past Century of human civilization: Greatest Engineering Achievements of the 20th Century As selected by the US Academy of Engineering 1. Electrification* 11. Highways 2. Automobile* 12. Spacecraft* 3. Airplane* 13. Internet 4. Water supply and distribution 14. Imaging 5. Electronics 15. Household appliances* 6. Radio and television 16. Health technologies 7. Agriculture mechanization* 17. Petroleum and petrochemical technologies 8. Computers 18. Laser and fiber optics 9. Telephone 19. Nuclear technology* 10. Air conditioning and refrigeration* 20. High performance materials EMS * with significant mechanical engineering involvement 34 36 9
Futuristic Industrial Products in the 21 st Century by Nanotechnology Near-term Products New vaccines and medicines that cure many incurable diseases. Synthetic antibody-like nanoscale drugs and devices seeking out to destroy malignant cells in human or animal bodies. In-vivo medical diagnostic and drug delivery systems. Smart surface coating materials with selfadjusting thermal conductance for buildings and refrigeration systems. Smart fabrics for self-cleaning clothe. Super-strong materials for light weight airplanes, vehicles and structures. Clean energy conversion systems and superlong life batteries. New breed of crops and domestic animals that can feed entire world population. Dream Products Dust sized super-intelligent computers. Needle-tip sized robots for biomedical applications and for search and rescue. Spacecraft weighing less than today s family cars. Biomedicine, e.g. in-vivo systems and surgery that can sustain human lives to 150 years and longer. Robots with artificial human intelligence becoming the mainstream workforce in our Society. Unlimited supply of clean renewable energies that replace all fossil fuel produced energies on the Earth. Tele-transportation systems that can transport human anywhere on Earth in seconds. Spacecraft for human/cargo inter-planet traveling. 37 Nanoscale Engineering Analysis Any device or engineering system, regardless of its size, is an assembly of components. Nanoscale engineering systems are no exception. Each component is expected to perform a specific function, e.g. structural support, or as connecting components, or as signal sensing, or as actuating mechanisms. Each component must have proper configuration and sufficient physical strength to carry out the intended function(s). Components should sustain externally applied forces, heat, chemical and biological attacks. Reliable techniques must be available to ensure components ability to perform the intended functions and survive in hostile environmental conditions. Techniques in design, plus reliable fabrication, packaging and assembly techniques for quality and volume production are called nanoscale engineering. Computational nanoscale engineering analysis that ensure the proper design of these components is thus an essential part of nanoscale engineering. 39 Lecture 6 Nanoscale Engineering Analysis and Measurements of Selected Material Properties Part I Nanoscale Engineering Analysis Part II Measurements of Selected Material Properties Engineering Analyses in Micro- and Macroscale For structural components in micro- and macroscale (i.e., with linear size > 1 μm): The laws of physics based on the behavior of continuum apply in these scales. A continuum = Aggregations of a great many atoms or molecules. EMS Behavior of a continuum = Behavior of aggregation of atoms or molecules There are differences in behavior of individual atoms or molecules, but the aggregation of large number of these atoms and molecules average these differences to give phenomenological properties. Hence, behavior of individual atoms does not affect the overall behavior of the continuum. 38 40 10
Engineering Analyses in Micro- and Macroscale Cont d Linear theory of elasticity for stress analysis; Newton s 2nd law for rigid-body dynamic analysis; Fourier law for heat conduction analysis; Newton s cooling law for heat convection analysis; Fick s law for diffusion analysis; Navier-Stokes s equations for fluid dynamics analysis Engineering Analyses in Nanoscale Cont d Inter-molecular physical-chemical actions are another unique element of nanoscale engineering analyses. Elastic bonds : Atoms Because the way molecules of various shapes and surface features organize into patterns on nanoscales determines important material properties Size-dependent material properties must be incorporated in nanoscale engineering analysis. 41 43 Engineering Analyses in Nanoscale For structural components in nanoscale (i.e., with linear size < 1 μm): The minute size of the component contains many fewer atoms or molecules than a continuum. These few atoms or molecules in a nanoscale substance are interconnected chemical bonds (simulated by elastic springs): (a) A solid plane (c) Elastic bonds of atoms Attraction force Repulsion force Typical inter-molecular forces: d o Inter-molecular distance,d (b) Aggregated atoms Elastic bonds : (c) Elastic bonds of atoms Consequently, individual atomic behavior plays dominant role in the overall behavior of the structure. Theories derived for continua cannot be used to model nanoscaled substance. Phenomenological models based on quantum physics and quantum mechanics are used as bases for engineering analyses of components of nanoscale. Mechanical strength, e.g., in maximum tensile strength and Young s modulus. Electrical properties, e.g., electrical resistivity. Atoms Size-dependent Properties of Nanoscale Materials Chemical properties, e.g., reactivity to chemical reactions. Optical properties, e.g., reactivity to light same material with different colors depending on its size a value to drug delivery and medical diagnosis. EMS Thermophysical properties, e.g., thermal conductivity, thermal diffusivity, and coefficient of thermal expansion. Melting points and solubility. Magnetic properties. Catalytic properties, e.g., ability to enhance chemical reactions - a vital value to drug discovery processes. Viscosity for liquid and surface tension. 42 44 11
Computational Nanoscale Engineering Analysis Computational nano-engineering technology aims at the development of nanometer scale modeling and simulation methods to enable the design and construction of realistic nanometer scale devices and systems. Computational nanoscale engineering design encompasses: Stress Analysis Molecular Dynamics Molecular Heat Transmission Molecular Gas Dynamics Modifications for finite element analysis of nanoscale solids: The constitutive equations for incremental stress and strain components for a conventional elastoplastic stress analysis of solids with temperature and strain rate-dependent properties: ( The Finite Element Method for Thermomechanics, T.R. Hsu, 1986) * T & ε 1 F F { dσ } = [ C ]{ dε } [ C ]{ ( α } dt + { D } dt + { D } d & ε ) [ C ] σ dt + d & ep ep e { } ε 6 σ * where S = * * (, ) + 6 * σ σ C T & ε σ [ C e ] [C e ] = elasticity matrix; [C ep ] = elastic-plasticity matrix; {D T } and { D ε& } S T 45 & ε 47 Young s Modulus of Silicon: Miller index for Young s modulus of bulk orientation material <100> 129.5 Unit: GPa [Ce} = respective temperature-dependent and strain rate-dependent elasticity matrix. α = coefficient of thermal expansion; F = plastic yield function; T = temperature and {D T } = respective coefficient matrices relating to the strain rate and temperature T * σ = translated deviatoric stress following von-mises plastic potential function Significant modifications on the above constitutive equations are required to accommodate size-dependant material properties and the inter- molecular mechanistic behavior. <110> <111> Stress Analysis of Solids in Nanoscale 168.0 186.5 Young s modulus of nanocantilever at 3 nm thick 75 Significant size-dependent Young s modulus of nanoscale materials has invalidated the linear generalized Hooke s law: { σ } = [ C( E, υ )]{ ε } in which {σ} = stress tensor, {ε} = strain tensor, [ C ( E, υ )] = the elasticity matrix with E = Young s modulus and ν = Poisson s ratio. Significant modifications of the constitutive relations of materials at this scale b/c the values of both E and vary with the size of the structure. ν Overview of Molecular Dynamics Deformation of any matter, whether it is solid or fluid, result in movement of atoms or molecules within the matter. Displacement vector Atoms with elastic bonds: 110 130 % Change EMS (a) Initial equilibrium position (b) Position at time t 1 (c) Position at time t 2 Oscillatory motions of an atom subject to external disturbance At molecular level, i.e. nanoscale, such movement of atoms or molecules may contribute significantly to the overall physical consequences. One thus needs to treat EVERY problem in nanoscale as a TRANSIENT problem. The name static mechanics does not exist in nano-mechanics. Every mechanics analysis is thus related to Molecular Dynamics. 42.1 34.5 30.3 46 48 12
Principle of Molecular Dynamic Simulation It computes the MOTION of individual molecules in solids, liquids and gases. Motion of molecules includes: position, velocities and orientations with time (trajectories) All molecules are subject to inter-molecular force field and thus the potential energy that includes: static force, kinetic energy, electromagnetic, thermal, etc. Stochastic Metropolis Monte Carlo Force-biased Monte Carlo Simulation methods covers the spectrum of: Brownian dynamics General Langevin dynamics The Schrodinger Equation for Nonrelativistic Quantum Mechanics (1926) The position of a charged particle, φ ( r, t) in a force field of N-charged particles with masses, m 0, m 1, m 2,, m N-1 can be obtained from the following equation: 2 2 r r h 1 φ (, t) r r φ (, t) + U (, t) φ (, t) = ih 2 2 t where m r j r = position vector, or the coordinates U ( r, t) j j Deterministic = potential energy function h = the Planck constant The inter-molecular force on molecule i caused by N-1 other molecules is: F i ( r, t ) Molecular dynamics 49 51 Molecular Dynamics in Engineering Design of Nanoscale Structures It determines: (1) The required forces (or energy) to move single atoms (or molecules) in a molecular force field. (a) Initial equilibrium position r φ r = r Displacement vector (2) The motion of individual atoms (or molecules): Trajectories (Positions, Velocities and Orientations) when subject to external force field. (3) The Schrodinger s equation (1926) is used as the basic governing equation for the deterministic molecular dynamics. Principle-molecular dynamics.nanotechnology-3)hsu.4/14/01 Attraction force Repulsion force Atoms with elastic bonds: (b) Position at time t 1 (c) Position at time t 2 Molecular Dynamics Inter-Molecular Forces with Separation: EMS d o F d Inter-molecular distance,d 50 52 (Inter-molecular forces.nems-1)hsu.4/5/01 13
Potential Energy Function in Schrodinger Equation Molecule i Inter-molecular forces q i q j U ( r ) = i< j 4π ε 0 d where q i, q j = respective charge intensities of Particle i and j d ij = distance between Particle i and j ε o = permittivity of the free space. (Potential energy function.nanotechnology-3)hsu.4/14/01 Overview of Molecule j Molecular Heat Transmission in nanoscale Any input of thermal energy to a substance from an external source can cause a disturbance to the energy equilibrium in its natural state. Input thermal energy (a) Initial state (b) Lattice movement at time t 1 (c) Lattice movement at time t 2 This input energy can cause the LATTICES that connect atoms to deform (extend or contract) depending on the form of the input energy. Because lattices are considered as elastic bonds, any extension or contraction of any lattice will result in a series of vibrations of lattices. Vibration of lattices are always associated with energy. ij 53 Elements of Molecular Dynamic Simulation Principal Modules: Equation of Motion Environmental Interactions Molecular Interactions Sub-modules: Dynamic equilibrium with applied forces and energies Dynamic and static properties Molecular trajectories (soft or hard sphere collision) Phase-space trajectories for vibrating molecules Distribution functions of sample molecules in 2-D and 3-D (velocities, properties, etc.) Periodic boundary conditions Molecular Heat Transmission A. Energy Carrier: Heat conduction in solids requires carriers. Heat transportation in solids of in sub-micrometeer and nanoscales is predominantly done by the flow of phonons. There are collisions and scattering of free-phonons take place at all times during heat transmission: EMS P 2 P 1 P (t 4 2 ) (t P 3 1 ) (t 4 ) (t 3 ) The energy associated with local lattice vibration is called ENERGY CARRIER 55 x 56 z d 1 Plane A Plane B d2 d 3 Thin film thickness, H y The average mean free path (MFP): d 1 + d 2 + d 3 λ = 3 The average mean free time (MFT): ( t t1) + ( t 3 t 2) + ( t 4 t 3) t 4 τ = = 3 3 2 t1 54 14
The thermal conductivity, Molecular Heat Transmission Cont d (For solid size, H < 7λ) B. Size-dependent Thermophysical Properties 1 k = CV λ 3 Parameters for Thermal Conductivity of Thin Films [Flik et.al. 1992 and Tien and Chen 1994] Materials Dielectric and semiconductors Specific heats, C Specific heat of electrons, Ce Specific heat of phonons, Cs Molecular velocity, V Electron Fermi velocity, V e 1.4x10 6 m/sec Velocity of phonons (sound velocity), Vs 10 3 m/sec Average mean free path, λ Electron mean free path, λe 10-8 m Phonon mean free path, λs from 10-7 m and up Example: With an average value of λ = 10-7 m for phonons, the k for 200 nm thick silicon film is only 83% of the same property of silicon in macroscale. 57 Overview of Molecular Gas Dynamics Gas dynamics derived from continuum theory breaks down at scale less than 1 µm (or < 1000 nm). Molecular physics governs. Gas flow in this scale is rarefied. Mean free path (MFP) and Mean free time (MFT) dominate in energy transportation. Knudson number (Kn) and Mach number (Ma) characterize the flow. Kn = MFP (λ)/characteristic length ( L); λ 65 nm Ma = Speed of sound in the rarefied gas/ Speed of sound at standard conditions (Ma < 0.3) 59 Molecular Heat Transmission Cont d C. Modified Heat Conduction Equation The heat conduction equation carries additional term for energy carrier s movement: r Q 1 T ( r, t) τ 2T r t 2 (, ) T ( r, t) + = + k α t α t 2 where the relaxation time, τ is: λ τ = V in which V is the average velocity of the heat carrier. The value of τ 10-10 seconds is used for semiconducting materials. Modeling of Molecular Gas Dynamics K n = MFP (λ)/characteristic length ( L); λ 65 nm for most gases M a = Speed of sound in the rarefied gas/speed of sound at standard conditions Gas Flow Navier-Stoke Equation Non-slip boundaries Slip boundaries K n 0 0.01 0.1 1.0 10 Continuum Continuum Transition Free theories theories with slip boundaries molecular flow EMS Modified Navier-Stokes equation can be used for 0.01 < Kn < 0.13. New gas dynamics theory and formulations need to be developed for nano scale gas flow with Kn > 0.13. 60 r L r 58 15
Summary on Nanoscale Engineering Analysis R&D on computational nanoscale engineering so far has been sporadic and has not received support as it deserves. It is a matter of time that industry (and thus the government funding agencies) will recognize its essential value in the design of nanoscale devices and nano-microscale systems. Computational nanoscale engineering presents enormous challenge to researchers due to the extreme complexity in the bridging the quantum and continuum mechanics. Because of this extreme complexity, the following inter-play relationship will play a major role in the overall development of nanoscale engineering design. Theory Typical Instruments Computer Simulation 1. SEM/TEM 2. AFM/STM/Stylus Profiler 3. Nano-indenter Experiment 61 63 Typical Instruments 1. SEM/TEM 2. AFM/STM/Stylus Profiler 3. Nano-indenter 电子显微镜的发展史 1938 年, 德国工程师 Max Knoll 和 Ernst Ruska 制造出了世界上第一台透射电子显微镜 (TEM) Max Knoll(1897-1969) Ernst Ruska(1906-1988) 电子显微镜的分辨率可以达到纳米级 (10-9 m) 可以用来观察很多在可见光下看不见的物体, 例如病毒 EMS 62 64 16
1952 年, 英国工程师 Charles Oatley 制造出了第一台扫描电子显微镜 (SEM) 电子显微镜下的蚊子 Charles Oatley 1. 透射电镜 (TEM) 透射电镜即透射电子显微镜 (Transmission Electron Microscope, 简称 TEM), 通常称作电子显微镜或电镜 (EM), 是使用最为广泛的一类电镜 65 67 一 电子显微镜及电子显微术概况 ( 一 ) 电子显微镜近年来, 电镜的研究和制造有了很大的发展 一方面, 电镜的分辨率不断提高, 透射电镜的点分辨率达到了 0.2-0.3nm, 晶格分辨率已经达到 0.1nm 左右, 通过电镜, 人们已经能直接观察到原子像 ; 另一方面, 除透射电镜外, 还发展了多种电镜, 如扫描电镜 分析电镜等 根据加速电压的大小分为以下 3 种 : (1) 一般 TEM 最常用的是 100KV 电镜 这种电镜分辨率高 ( 点 0.3nm, 晶格 0.14nm), 但穿透本领小, 观察样品必须很薄, 约为 30~100nm, 如细胞和组织的超薄切片 复型膜和负染样品等 相当普及 (2) 高压 TEM 目前常用的是 200KV 电镜 这种电镜对样品的穿透本领约为 100KV 电镜的 1.6 倍, 可以在观察较厚样品时获得很好的分辨本领, 从而可以对细胞结构进行三维观察 (3) 超高压 TEM 目前已有 500KV 1000KV 和 3000KV 的超高压 TEM 这类电镜具有穿透本领强 辐射损伤小 可以配备环境样品室及进行各种动态观察等优点, 分辨率也已达到或超过 100KV 电镜的水平 在超高压电镜上附加充气样品室, 使人们可以观察活细胞内的超微结构动态变化 EMS 66 68 17
2. 扫描电镜 (SEM) 扫描电镜即扫描电子显微镜 (Scanning Electron Microscope, 简称 SEM) 主要用于观察样品的表面形貌 割裂面结构 管腔内表面的结构等 工作原理 : 利用电子射线轰击样品表面, 引起二次电子等信号的发射, 经检测装置接收后成像的一类电镜 主要优点 : 景深长, 所获得的图像立体感强, 可用来观察生物样品的各种形貌特征 69 71 一些电镜图片 染色体染色体被精子包围的卵子 扫描电镜原理 EMS 骨髓细胞 SARS AIDS 70 72 18
依据性能不同主要分为 : (1) 一般 SEM 目前一般扫描电镜采用热发射电子枪, 分辨率为 6nm 左右, 若采用六硼化镧电子枪, 分辨率可提高到 4~5nm 我校有这样的设备 (2) 场发射电子枪 SEM 由于场发射电子枪具有亮度高 能量分散少, 阴极源尺寸小等优点, 这种电镜的分辨率已达到 3nm 场发射电子枪 SEM 的另一个优点是可以在低加速电压下进行高分辨率观察, 因此可以直接观察绝缘体而不发生充 放电现象 (3) 生物用 SEM 这种 SEM 备有冰冻冷热样品台, 可把含水生物样品迅速冷冻并对冰冻样品进行观察, 可以减少化学处理引起的人为变化, 使观察样品更接近于自然状态 如要观察内部结构, 还可用冷刀把样品进行切开, 加温使冰升华, 并在其上喷镀一层金属再进行观察, 所有这些过程都在 SEM 中不破坏真空的状态下进行 5. 扫描透射电镜 SEM 中电子射线作用于样品后, 其中一部分电子可透过样品成为透射电子, 将透过样品的透射电子和散射电子用检测器接收成像, 即成为扫描透射电镜 这种电镜一般用场发射电子枪, 兼有 TEM SEM 和分析电镜的特点, 能观察较厚的样品, 分辨本领和成像质量都很好, 是近年来电镜技术的最大改进之一 73 75 4. 分析电镜分析电镜是利用电子射线轰击样品所产生的 X 射线或俄歇电子对样品元素进行分析的一类电镜 其特点是能在观察超微结构的同时, 对样品中一个极微小的区域进行化学分析, 从而在超微结构水平上测定各种细胞结构的化学成分及其变化规律 (1) 分析 TEM 在 TEM 上配备 X 射线能谱仪后即成为分析 TEM, 目前很多 100KV 和 200KV TEM 都可以装上 X 射线检测附件, 进行样品的元素分析 (2) 分析 SEM 在 SEM 上配备 X 射线能谱仪后, 便可兼有电子探针分析样品化学成分的功能 (3) 扫描俄歇电镜 把 SEM 与俄歇电子能量分析仪相结合, 即成为扫描俄歇电镜, 它能对样品表面进行微区元素分析, 是一种表面微观分析电镜 ( 二 ) 电子显微术电镜具有很高的分辨本领, 能观察极微小的结构 但是电镜不能直接观察天然状态下的生物标本, 必须通过各种技术将生物标本制成特殊的电镜生物样品, 才能放入电镜进行观察, 这些电镜样品制备技术称为电子显微术 EMS 电子显微术 : 将生物标本制成特殊的电镜观察用生物样品的制备技术 74 76 19
1. 与 TEM 有关的电子显微术 (1) 超薄切片技术 超薄切片技术就是通过固定 脱水 包埋 切片和染色等步骤, 将生物标本切成薄于 0.1μm 的超薄切片的样品制备技术, 用于生物组织的内部超微结构研究 77 上图超薄切片机局部 2. 与 SEM 有关的电子显微术 (1)SEM 常规制样技术 SEM 适合于研究生物样品的表面特征, 样品制备包括样品观察面的暴露 固定 干燥和导电等步骤, 使表面特征充分暴露而不变形 这一技术是 SEM 样品制备的常规技术, 主要用于组织 细胞 寄生虫等表面形貌的研究 下图为 SEM 标本处理设备 79 下图玻璃刀制作仪 EMS 78 80 20
Typical Instruments 1. SEM/TEM 2. AFM/STM/Stylus Profiler 3. Nano-indenter 81 82 83 The Measuring Mechanism of AFM(I) EMS 84 21
The Measuring Mechanism of AFM(II) 85 86 EMS 87 88 22
The Configuration of a Typical AFM System (DI 3100 Series) X Z Y Laser signal Photodiode Specifications of the DI 3100: X-Y Scanning Area: Less than 100μm 100μm Laser Cantilever Z Scanning Distance: About 6μm Scanning method: Raster 89 90 EMS 91 92 23
The applications of AFM Typical Instruments I. Life science application II. Nanoscience application Part of these images are downloaded from website of www.vecoo.com 93 1. SEM/TEM 2. AFM/STM/Stylus Profiler 3. Nano-indenter EMS 95 96 94 24
EMS 97 98 99 100 25
π E r = S 2 a H = F max a 1 (1 υ = E E r 2 2 1 ) (1 υ 2 ) 1 + E 2 101 EMS 103 104 102 26
EMS 105 106 107 108 27
Summary There has been unprecedented interests (and expectations) expressed by the scientific community, and colossal amount of investments in R&D by governments of leading industrialized nations in the world. Current state of nanotechnology appears focused on scientific discovery with rudimentary products at laboratory scale. Near term breakthroughs in significant commercial applications of nanotechnology appear in the new superior materials and disruptive biomedical applications. R&D on creative and novel micro/nano scale products that have identified global market potentials is a sensible direction to follow. Nanoscale engineering is a necessary vehicle to reach this goal. Related soft nanotechnology, e.g. commercialization and global marketing for nano scale products, and volume production technologies should be encouraged and supported. 109 课件下载地址 : http://sklofp.zju.edu.cn/jpkc/index.htm EMS 110 28