GENERAL CHEMISTRY CHM 2045
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This 19 page Class Notes was uploaded by Brigitte Wyman on Friday September 18, 2015. The Class Notes belongs to CHM 2045 at University of Florida taught by Staff in Fall. Since its upload, it has received 5 views. For similar materials see /class/207017/chm-2045-university-of-florida in Chemistry at University of Florida.
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Date Created: 09/18/15
LECTURE 2 SEM Components Electron Gun Filament Wehnelt cylinder or grid cap Anode Lens System Condenser and Objective Lenses Detection System electron photon collector Visual and recording cathode ray tubes CRTs or Computer Screen and the electronics associated with them Scanning Electron Microscopy Eledron Eledron Gun leam 4 AM Illllllh M agnearc rIIIIIIIIIIIII 4 Lens Scanning cons Secondary Elec1ron Deaecror Scanning Electron Microscopy Slocum gun tummy N Sigma Famimomn 00pm has Secondary Baducaltmod jl cm electrons m decade Spea mm bout http micro magnetfsuedu primerjavaelectronmicroscopy magnify1 Fncus Cnmrzst Brluhlness Mznm cmmn O m 6 Chnnsenszmnle n n Geck r m c Electron Beam Parameters Optical Axis Convergence Angle Probe Current amp Energy Accelerating Electron Probe Voltage SIze Probe Diameter Scanning Electron Microscopy Each of these four beam parameters dominates one of the four major SEM imaging modes Resolution Mode dp Highcurrent mode ip Depthof focus mode ocp Lowvoltage mode Vo SEM Imaging modes the operating conditions affect the information provided about a specimen Four important beam parameters are set by the operating conditions the electron probe size diameter dP is the diameter of the final beam at the surface of the specimen the electron probe current iP is the current that impinges upon the specimen and generates the various imaging signals the electron probe convergence angle ocp is the half angle of the cone of electrons converging onto the specimen the electron beam accelerating voltage V0 Each of these four beam parameters dominates one of the four major SEM imaging modes Resolution Mode dp Highcurrent mode ip Depthoffocus mode ocp Lowvoltage mode V0 Vacuum System rotary pump diffusion pump turbo pumps EMA 5108 Vacuum Science and Technology 3 Prereq CHM 2045 PHY 3101 MAP 2302 or equivalEents or consent of instructor Introduction to the generation and use of vacuum for scientific research and industrial production Kinetic theory of gases discussed as necessary to understand vacuum phenomena Description of components and materials vacuum systems design and uses in metallurgy electronics physics and chemistry Rough Vacuum 105 102 Pa Medium Vacuum101 10391 Pa High Vacuum 10391 10395 Pa Ultra High Vacuum lt 10395 Pa Rotary Pump allows for rough to medium vacuum Operates from atmospheric pressure down to as low as 2 x 10392 Torr Diffusion Pump High Vacuum Operating pressure range of 10392 to less than 10399 Torr Turbo Pumps Ultra High Vacuum Operating Pressure Range 10393 to less than 103910 Torr UltraHigh Vacuum FEI SEM UHV TEM 10 Turbom Diff High Vacuum SEM TEM usion Pump Medium Vacuum LVSEM Rough Vacuum 1Stage Rotary 2Stage Rotary olecular Pump 10m 10 10quot 10quot 10s 10s 10quot mbar Pa 10quot 1o 10391 105 Mi l ln d High vacuum P39U MP Specimen chamber 1 Hnughj vacuum Pump SEM ChamberAlmosphere 1x 10 torr Enrdl Atmosphere 760 tort Moon Atmosphere 1 x 10 15 lorr Electron Gun is where the beam of electrons is generated Electron Gun Components Filament source Wehnelt Cylinder or Grid Cap Anode Types of Sources Thermoionic Tungsten and LaB Field Emission Tungsten and LaB Tungsten Filament source of electron stream by thermionic or field emission L336 Cathode negative electrode FEG V G Cap is maintained at a negative potential It is usually set at a volta e Filament Heating Supply slightly more negative than the filament WWW to provide a focusing effect on the beam Electrons move toward positive 1233 equipotentials and are repelled by negative equipotentials Electrons leave the filament only where the positive electrostatic field meets the surface of the filament Emmmf The grip cap produces a strong focusing A action forcing the electrons to crossover of diameter d nude Plate Anode is positive and attradsaccelerates the electrons down the column towards the lenses Accelerating Voltage voltage difference between the filament an the anode At zoxv the filament will be placed at zoooo Vwith respect to the anode which is at ground potential Electrostatic potentials in SEM electron gun Jo oom Filament Grid Unwind An0de Fig 2 eiecfrosfa c poten ais in SEM electron gun Thermionic Emission occurs when enough heat is supplied to the emitter so that electrons can overcome the workfunction energy barrier EW of the material and escape from the material Electrons have a range of energies The highest energy state in the metal is called the Fermi Level EF Melal Escaping HUI Ew Electron lnleriace When the emitter material filament is heated resistive heating to a high temperature a small fraction of the electrons at the Fermi level acquire enough energy to overcome EW and escape into the vacuum Vacuum Represents the energy work necessaw to place an electron in the vacuum from the lowest energy state in the meta Constant for all thermionic emitters Cathode Current Density minor effect Boltzmann s Constant JC AC T2expEWkT 86x105eVK TK emission temperature Work function of the filament material EW has a strong effect on the emission current It is desirable to operate the electron gun at the lowest possible temperature to reduce evaporation of the filament Materials with a low work function are desired Tungsten Wire W has a low EW 45 eV Typical Emission Temperature 2700K JC 34 Acm2 Two important parameters for any electron gun are Amount of current it produces of e39 interacting with the sample Current stability information is recorded as a function of time Emission Current total current emitted from the filament Beam Current portion of electron current that leaves the gun through the hole in the anode at each lens and aperture along the column the beam current becomes smaller Probe Current electron current measured at the specimen It is several orders of magnitude smaller than the beam current Filament Saturation to ensure a stable beam current a condition of saturation must be established This means that small increases or decreases in the filament heating current do not change the electron beam current Operallng Polnl False Peak Beam Currenl lb gt Filament Current if gt False Peak appears when some other part of the filament surface reaches emission temperature before the filament tip The beam current rises and then falls before the saturation condition is established Saturation Point Operating Point is the level of heating current for which no further increase in beam current can be obtained At saturation electrons are only emitted from the tip Undersaturation No stable operation condition Oversaturation filament burns
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