Is Nanoelectronics the Future?

Transcription

1 Is Nanoelectronics the Future? By Anurag Srivastava Semiconductor Physics

2 Nano World

3

4 Nano: From the Greek nanos - meaning "dwarf, this prefix is used in the metric system to mean 10-9 or 1/1,000,000,000.

5 ww.mathworks.com Nanoscale? Fullerenes C 60 12,756 Km m 22 cm 0.7 nm 0.22 m m 10 millions times smaller 1 billion times smaller

6 Nano is Different: Size Matters Bulk Gold = Yellow Nano Gold = Red Quantum Dots for Imaging and Diagnostics Optical properties change with size. Depending on their size CdSe particles can appear green or red in colour. nanocrystals absorb all energies higher than their band gap, they can also be used as color converters. Sizes of biological molecules are also on the order of a few nanometers. 2 nm 5 nm Quantum dot size can be controlled during their synthesis

7 Why Is Nanotechnology So Cool? Bulk Gold mp = 1064 C Color = gold 1 nm gold particles mp = 700 C l max = 420 nm Color = brown-yellow 20 nm gold particles mp = ~1000 C l max = 521 nm Color = red 100 nm gold particles mp = ~1000 C l max = 575 nm Color = purple-pink

8 Nano- and Micro-domains

9 Soft x-ray Nanoworld Microworld Ultraviolet Visible Infrared Microwave The Scale of Things Nanometers and More Things Natural m cm 10 mm 10-3 m 1,000,000 nm = 1 mm Things Man made Head of a pin 1-2 mm Challenges Dust mite 200 mm Ant ~ 5 mm 10-4 m 0.1 mm 100 mm MicroElectroMechanical (MEMS) devices mm wide Human hair ~ mm wide Fly ash ~ mm 10-5 m 0.01 mm 10 mm O P O O O O O O O O O O O O O O Red blood cells (~7-8 mm) 10-6 m 1,000 nm = 1 (mm) Pollen grain Red blood cells Zone plate x-ray lens Outer ring spacing ~35 nm O S O S O S O S O S O S O S O S 10-7 m 0.1 mm 100 nm Fabricate and combine nanoscale building blocks to make useful devices, e.g., a photosynthetic reaction center with integral semiconductor storage. ~10 nm diameter ATP synthase 10-8 m 0.01 mm 10 nm Self-assembled, Nature-inspired structure Many 10s of nm Nanotube electrode DNA ~2-1/2 nm diameter Atoms of silicon spacing ~tenths of nm 10-9 m 1 nm ABV- m IIITM-Gwalior 0.1 nm (MP) Quantum India corral of 48 iron atoms on copper surface positioned one at a time with an STM tip Corral diameter 14 nm Carbon buckyball ~1 nm diameter Carbon nanotube ~1.3 nm diameter

10 Nanoscience is the study of phenomena and manipulations of materials at atomic(~ 0.5 nm), molecular (~1-5nm) and macromolecular (~5-100 nm) scales, where properties differ significantly from those of the bulk materials. Nanotechnology concerns design, characterization, production and application of structures, devices and systems by controlling shape and size at nanometer scale. Nanotechnology is the creation of functional materials, devices, and systems through control of matter on the nanometer (1 to 100 nm) length scale and the exploitation of novel properties and phenomena developed at that scale. Source: Nanoscience and nanotechnologies: opportunities and uncertainties, The Royal Society & the Royal Academy of Engineering, London-2004.

11 Nanoscience and Nanotechnology are truly interdisciplinary, where physicists, chemists, engineers, biologists, computer scientists, environmentalists, industrialists, and policy makers have to work together.

12 Nanoscience? When people talk about Nanoscience, they start by describing things in their own way Physicists and Material Scientists point to things like new nanocarbon materials: They effuse about nanocarbon s strength and electrical properties Graphene Carbon Nanotube C60 Buckminster Fullerene

13 Nanotechnology is Not just new products a new means of production Manufacturing systems that make more manufacturing systems exponential proliferation Vastly accelerated product improvement cheap rapid prototyping Affects all industries and economic sectors general-purpose technology Inexpensive raw materials, potentially negligible capital cost economic discontinuity Portable, desktop-size factories social disruption Impacts will cross borders global transformation

14 Nano-bio Biologists counter that nanocarbon is a recent discovery THEY VE been studying DNA and RNA for much longer (And are already using it to transform our world)

15 What is Nanoelectronics? Nanoelectronics refer to the use of nanotechnology on electronic components, especially transistors. Although the term nanotechnology is generally defined as utilizing technology less than 100 nm in size, nanoelectronics often refer to transistor devices that are so small that inter-atomic interactions and quantum mechanical properties need to be studied extensively. As a result, present transistors do not fall under this category, even though these devices are manufactured with 45 nm, 32 nm, or 22 nm technology. Semiconductor Physics

16 Is this technology new? In one sense there is nothing new Whether we knew it or not, every piece of technology has involved the manipulation of atoms at some level. Many existing technologies depend crucially on processes that take place on the nanometer scale. Ex: Photography & Catalysis Nanotechnology, like any other branch of science, is primarily concerned with understanding how nature works.

17 Why is this length scale so important? There are five reasons: 1. The wavelike properties of electrons inside matter are influenced by variations on the nanometer scale. By patterning matter on the nanometer length, it is possible to vary fundamental properties of materials (for instance, melting temperature, magnetization, charge capacity) without changing the chemical composition. 2. The systematic organization of matter on the nanometer length scale is a key feature of biological systems. Nanotechnology promises to allow us to place artificial components and assemblies inside cells, and to make new materials using the self-assembly methods of nature.

18 Why is this length scale so important? 3. Nanoscale components have very high surface areas, making them ideal for use in composite materials, reacting systems, drug delivery, and energy storage. 4. The finite size of material entities, as compared to the molecular scale, determine an increase of the relative importance of surface tension and local electromagnetic effects, making nanostructured materials harder and less brittle. 5. The interaction wavelength scales of various external wave phenomena become comparable to the material entity size, making materials suitable for various opto-electronic applications.

19 How Small We can make the grains? Because of high surface areas conventional powders methods reach their limits at 10-6 m (1 micron) Smaller particles can be made but special methods are needed!

20 Working at the nanoscale Working in the nanoworld was first proposed by Richard Feynman back in But it's only true in the last decade. The world of the ultra small, in practical terms, is a distant place. We can't see or touch it. Because, optical microscopes can't provide images of anything smaller than the wavelength of visible light (ie, nothing smaller than 380 nanometres).

21 Some Nano definitions Cluster A collection of units (atoms or reactive molecules) of up to about 50 units Colloids A stable liquid phase containing particles in the nm range. A colloid particle is one such nm particle. Nanoparticle A solid particle in the nm range that could be nonocrystalline, an aggregate of crystallites or a single crystallite Nanocrystal A solid particle that is a single crystal in the nanometer range

22 What is so special about nanoscale Atoms and molecules are generally less than a nm. Size-dependent properties Surface to volume ratio A 3 nm iron particle has 50% atoms on the surface A 10 nm particle 20% on the surface A 30 nm particle only 5% on the surface Not just size reduction but phenomena intrinsic to nanoscale Size confinement Dominance of interfacial phenomena Quantum mechanics New behavior at nanoscale is not necessarily predictable from what we know at macroscales

23 Nanostructures Noble Metal Nanoparticles Carbon Nanotubes Sun, Y.; Xia, Y. Science 2002, 298, Courtesy of the Van Duyne group CdSe Quantum Dots Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787 Courtesy of Liza Babayon Vigolo, B; Penicuad, A.; Coulon, C.; Sauder, C.; Pailler, R Journey, C.; Bernier, P. Poulin, P. Science 2000, 290, 1331

24 Size Matters As the size of an object becomes smaller and smaller, approaching nanoscale, the surface molecules become increasingly important relative to internal molecules Because of the increasing proportion of surface molecules relative to internal molecules Thus, the surface properties of materials of nanoscale objects become more influential in determining the behavior of the objects And the influence of bulk properties is reduced

25 Properties vary with the size of the material (Bulk) Gold is a shiny yellow metal Nanoscopic gold, i.e. clusters of gold atoms measuring 1 nm across, appears red Bulk gold does not exhibit catalytic properties Au nanocrystal is an excellent low temperature catalyst. Therefore, if we can control the processes that make a nanoscopic material, then we can control the material s properties.

26 Size Dependent Properties Chemical properties reactivity, catalysis Thermal properties melting temperature Mechanical properties adhesion, capillary forces Optical properties absorption and scattering of light Electrical properties tunneling current Magnetic properties superparamagnetic effect

27 Some Size Effects: Atomic Bonding Two types of atomic bonds: 1. Primary bonds combining atoms into molecules 2. Secondary bonds attraction between molecules to form bulk materials Secondary bonds become more important for nanoscale objects because their shapes and properties depend on these secondary bonding forces Thus, material properties and behavior of nanoscale objects are different from those of much larger objects

28 Size Effects: Quantum Mechanics Branch of physics concerned with the notion that all forms of energy occur in discrete units when observed on a small enough scale Example: electricity is conducted in units of electrons Quantum mechanics are significant for nanoscale entities One implication: As microelectronic devices reach nanoscale, we approach the limits of technological feasibility of current fabrication processes for integrated circuits Properties of nanostructured materials are size dependant. Properties can be tuned simply by adjusting the size, shape or extent of agglomeration.

29 Unique Properties of Nanoscale Materials Quantum size effects result in unique mechanical, electronic, photonic, and magnetic properties of nanoscale materials Chemical reactivity of nanoscale materials greatly different from more macroscopic form, e.g., gold Vastly increased surface area per unit mass, e.g., upwards of 1000 m 2 per gram New chemical forms of common chemical elements, e.g., fullerenes, nanotubes of carbon, titanium oxide, zinc oxide, other layered compounds

30 Nanofabrication Top-down Approach Bottom-up Approach

31 Top-down vs Bottom-up Top-down techniques take a bulk material, machine it, modify it into the desired shape and product classic example is manufacturing of integrated circuits using a sequence of steps sush as crystal growth, lithography, deposition, etching, Chemical Mechanical Planarization, ion implantation (Microelectronic/Nanoelectronics Fabrication Approach) Bottom-up techniques build something from basic materials assembling from the atoms/molecules up not completely proven in manufacturing yet Examples: Self-assembly Sol-gel technology Deposition (old but is used to obtain nanotubes, nanowires, nanoscale films ) Manipulators (AFM, STM,.)

32 Top-down From large items to smaller ones. The most common method are electron beam lithography (EBL) and scanning probe lithography (SPL). The approach involves molding or etching materials into smaller components. Making IC? Starting with a thin sheet Si wafer, cleaned, coated, preferentially etched using highly focused optics in as many as 100 separate operations before the final IC is complete.

33 Bottom-up A general approach going from small items to bigger ones. Building larger, more complex objects by integration of smaller building blocks or components. The sketch shows the essence of bottom-up manufacturing. Self-assembly from the gaseous phase. Two principle vapor-phase technologies that are useful and widely practiced: molecular beam epitaxy (MBE) and vapor-deposition (PVD, CVD).

34 History of Nanotechnology ~ 2000 Years Ago Sulfide nanocrystals used by Greeks and Romans to dye hair ~ 1000 Years Ago (Middle Ages) Gold nanoparticles of different sizes used to produce different colors in stained glass windows 1959 There is plenty of room at the bottom by R. Feynman 1974 Nanotechnology - Taniguchi uses the term nanotechnology for the first time 1981 IBM develops Scanning Tunneling Microscope 1985 Buckyball - Scientists at Rice University and University of Sussex discover C Engines of Creation - First book on nanotechnology by K. Eric Drexler. Atomic Force Microscope invented by Binnig, Quate and Gerbe 1989 IBM logo made with individual atoms 1993 Carbon nanotube discovered by S. Iijima 1999 Nanomedicine 1st nanomedicine book by R. Freitas 2000 National Nanotechnology Initiative launched 2010 Noble Prize to Graphene

35 Nanoscale Size Effect Realization of miniaturized devices and systems while providing more functionality Attainment of high surface area to volume ratio Manifestation of novel phenomena and properties, including changes in: - Physical Properties (e.g. melting point) - Chemical Properties (e.g. reactivity) - Electrical Properties (e.g. conductivity) - Mechanical Properties (e.g. strength) - Optical Properties (e.g. light emission)

36 Some Milestones Milestone - 1 In 1959, in APS lecture at California Institute of Technology, USA, Prof. Feymann said that: In great future, we can arrange the atoms, the way we want, devices will be made on atomic scale, materials will be with intelligence, the motors smaller than pin head will control every thing. Prof. R.P. Feynman Nobel Prize Winner

37 Milestone - 2 Today, Nanomaterials and Nanotechnology are at the center stage. Nanotechnology is regarded as a key Technology.

38 Milestone Moore s Law of continuous miniaturization has been a driver towards nanotechnology. Moore s Law states that number of active transistor elements per chip doubles every 18 months.

39

40 Transistor Scaling

41 Transistor Scaling and Research Roadmap

42 Milestone G. Bennig & H, Rohrer at IBM Zurich Laboratory developed the Scanning Tunneling Microscope. For surface studies 1984 G. Bennig developed the Atomic Force Microscope times powerful than STM

43 Milestone R. Smalley and H. Kroto, Robert F. Curl discovered fullerenes C 60 and jointly got Nobel Prize in Chemistry for this discovery in 1996.

44 Milestone S. Iijima (NEC, Japan) discovered single walled carbon nanotube.

45 Milestone - 7 In the 1930s, Landau and Peierls (and Mermin, later)showed thermodynamics prevented 2-d crystals in free state. Melting temperature of thin films decreases rapidly with decreasing thickness. In 2004, experimental discovery of graphene- high quality 2-d crystals. The Nobel Price 2010 in physics has been awarded to two scientists at the Manchester University for discovering the graphene. Konstantin Novoselov and Andre Geim were born in Russia, then they moved to the Netherlands and finally joining the Manchester University.

46 Milestone 7 They came up with an experiment in 2004 when they used scotch tape and a block of carbon to strip oneatom thick layers of carbon. Their discoveries were amazing as the graphene layers were almost transparent, almost as strong as steel, great heat and electric conductors, and very stretchy. It has been considered the perfect material and nowadays it has enormous implications in electronic devices, however, they can also be used to study quantum mechanics among others.

47 Milestone 7

48 Opportunities Changed Behaviour of Nanomaterials When we decrease the size of a material its behaviour and properties changes drastically as the dimensions reach 100nm or so. The main reasons for this change in behaviour are an increased relative surface area increased chemical activity

49 Diamonds, Graphite, Fullerenes & Carbon Nanotubes

50 Carbon Nanotubes

51 Carbon Nanotubes

52 Nanowires

53 DNA Nanowires are the way out for connecting the transistors in the chip on a nanoscale. This is where DNA could come in as Metallic Nanowires.

54 Opportunities (contd.) Confining electrons within a space of a few nanometers causes them to exhibit quantum behaviour Discrete energy levels Changes in optical, magnetic and electrical properties. This provides us a basis for technologies for developing products with prescribed properties.

55 Self Organized Growth : Quantum Dot Self organized growth of semiconductor quantum dots A quantum dot A quantum dot is of semiconducting material with dimensions of the order of 10nm. A dot has an electronic energy level structure analogous to an atom and is also called an artificial atom.

56 Single Electron Transistor Quantum Dot Source Drain Conductance calculations Effect of Defects Quantum Dot Source Drain Defect

57 Type of nanomaterials

58 Why different properties at nanoscale? Quantum Confinement Surface to volume ratio

59 Quantum confinement is responsible for the increase of energy difference between energy states and band gap. A phenomenon tightly related with the optical and electronic properties of the materials. The quantum confinement effect can be observed once the diameter of the particle is of the same magnitude as the wavelength of the electron wave function. When materials are this small, their electronic and optical properties deviate substantially from those of bulk materials.

60 In order to understand quantum confinement, we need to go back to the very basics of quantum mechanics; namely the particle-in-a-box. All we need to worry about is, that the spacings between the energy levels increase as the length of the box decreases. Quantitatively, E n = n 2 h 2 /8mL 2. In the case of semiconductors this simply means that the band gap, starting from the bulk value, increases as the size of the nanocrystal decreases. In bulk solids the energy levels are closely spaced and thus form quasi-continuous bands. Going to the nano-regime the energy level separation increases and discrete energy levels are observed. Calculations on different systems show that quantum confinement effects are observable at sizes below 10 nm for most materials (~20 nm for Pb chalcogenides). Onset of confinement depends on a number of parameters such as the dielectric constant of the semiconductor and effective masses of the charge carriers.

61 Nanowires Nanowires: One-dimensional structures with two quantum confined directions and one unconfined direction for electrical conduction, which allows them to be used in various applications due to their unique electronic density of states. Nanowires: purely 1-D structures. Having widths or diameter tens of nm.

62 Nanowires: Year wise progress Yang et al. Nano Lett (2010)

63 Nanowires Applications Nanowires are subject of active research because IT industry is approaching the physical limits of conventional CMOS technology. Nanowires of GaN, AlN etc can be used for nano-leds. Nanowires of Si and Ge can be used for P-N junction Diodes. Use of nanowires as Solar cells Tian et al.nature 449, 885 (2007) Nanowires could be used to link tiny components into extremely small circuits. Quantum confinement in nanomaterials leads to quantum mechanical effects such as ballistic transport (Ballistic conduction or Ballistic transport is the transport of electrons in a medium with negligible electrical resistivity due to scattering. ) Realization of solar cells, efficient Li-ion batteries, sensors etc.

64 Nanowires Applications Nanowire batteries now as 'small as possible,' could one day be included with nano toys. (Aug 4th 2011 ) Rice University scientists claim their miniscale wires are "as small as such devices can possibly get," because each one comes complete with its own anode, cathode and gel-like electrolyte coating Criss-crossed nanowires can compute (February 9 th 2011 Nature) Scientists have stitched together nanowires to create a microchip capable of basic computation. e-batteries-now-as-small-as-possible-couldone-day-be/ Researchers have used germanium wires to create a 'nanochip'

65 Nanoelectronic device? A very small devices to ovecome limits on scalability Examples: Single-Electron Transistors controlled electron tunneling to amplify current Resonance Tunneling Device quantum device use to control current

66 Problem of Making More Powerful Chips The number of transistors on a chip will approximately double every 18 to 24 months (Moore s Law). This law has given chip designers greater incentives to incorporate new features on silicon.

67 Problem of Making More Powerful Chips Moore's Law works largely through shrinking transistors, the circuits that carry electrical signals. By shrinking transistors, designers can squeeze more transistors into a chip.

68 Problem of Making More Powerful Chips However, more transistors means more electricity and heat compressed into a smaller space. Furthermore, smaller chips increase performance but also create the problem of complexity.

69 Problem of Making More Powerful Chips Band diagram when on A basic MOSFET

70 Problem of Making More Powerful Chips Quantum and coherence effects, high electric fields creating avalanche dielectric breakdowns, heat dissipation problems in closely packed structures as well as the nonuniformity of dopant atoms and the relevance of single atom defects are all roadblocks along the current road of miniaturization.

71 Problem of Making More Powerful Chips Problem 1: Carrier mobility decreases as channel length decrease and vertical electric fields increase.

72 Problem of Making More Powerful Chips Problem 2: Tunneling through gate oxide (off state current). E ox

73 Problem of Making More Powerful Chips Problem 3: Wattage/Area increases as density increases

74 Nanotechnology Potential Nanoparticles Nanomaterials Nanoelectronics Nanooptics Nanomagnetics Nanofluidics Nanobioelectronics