Chapter 2 Instrumentation for Analytical Electron Microscopy Lecture 6. Chapter 2 CHEM 793, 2011 Fall

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1 Chapte Instumentation fo Analytical Electon Micoscopy Lectue 6 Chapte CHEM 793, 011 Fall

2 Outline Electon Souces (Electon Guns) Themionic: La 6 o W Field emission gun: cold o Schottky Lenses Focusing Abeation Pobe size Detectos electon detectos X-ay detectos enegy loss spectometes Imaging Chapte CHEM 793, 011 Fall Diffaction ight field imaging (F) Dak field imaging (DF) Phase contast imaging (HRTEM) mode Scanning tansmission electon micoscopy (STEM) mode Hologaphy mode (not coveed) etc.

3 Chaacteistic X-ays fo EDX 100KV Absobed electons Electonhole pais emsstahlung X-ays Electon diffaction HRTEM image Imaging + electon enegy loss spectoscopy (EELS) Chapte CHEM 793, 011 Fall

4 Illumination system: SEM and TEM ae simila Objective lens and stage Obsevation /imaging system Chapte CHEM 793, 011 Fall

5 Chapte CHEM 793, 011 Fall TEM usually have two condense lenses

6 E-gun Chapte CHEM 793, 011 Fall Schematic lens configuation of a TEM system

7 Electon Illumination Souces Majo electon beam paametes Electon pobe diamete, d p electon pobe cuent, i p V 0 i p α p electon pobe convegence, α p acceleating voltage, V 0 Fo AEM we equie: d p a stable souce of electon A lage cuent in a small spot Thee ae thee souce types: a tungsten themionic souce a La6 themionic souce a field emission souce Chapte CHEM 793, 011 Fall

8 Final Pobe size A geneal expession fo the beam size, d p : p g d = d + d + d + s c d d Fo best esolution in many applications we need to use the smallest beam diamete. Fo STEM mode, the smallest pobe size detemines the best esolution. Since d p includes all the disks of least confusion fom all lens abeations, image esolution can be obtained fom above equation. Chapte CHEM 793, 011 Fall

9 Chapte CHEM 793, 011 Fall Final Pobe Size (cont ) d c s g p d d d d d = () )...( min a C C C C d C C s s s optimum = = α Fo a themionic gun, C 0 >>λ, the contibution of d d and d c can be neglected. 1/ 0 4 = βπ p i C

10 Final Pobe Size (cont ) p d = d + d + d + g s c d d C 0 4i p = βπ 1/ Fo a FEG gun, C 0 <<λ, the contibution of d g and d c can be neglected. α d optimum min λ = 0.9 C s 3 4 = 0.8λ C 1 4 s (b) These expessions can be used to estimate the optimum apetue angle and the esolution limit of a high esolution TEM/STEM. Equation (b) is especially impotant fo evaluating the capabilities of diffeent TEM instuments. Note that fo esolution depends moe stongly λ than Cs. This encouages the use of high acceleating voltage (small λ). Cs can be coected by cuent technology. Chapte CHEM 793, 011 Fall

11 Pobe V0=00 KV and λ=.5 pm d s d g d d α p FEG La6 W Chapte CHEM 793, 011 Fall

12 Focusing of Electons: Magnetic Lenses All moden analytical electon micoscopes use magnetic lenses Magnetic lenses ae poo compaed to glass lenses suffe many abeations (distotions) that educe image quality In analytical electon micoscope, magnetic lenses ae used to poduce a demagnified image of the electon souce coss ove at the specimen (condense lens) A themionic spot size lies between µm and the nomal desied spot size is 1-10 nm. This equies a demagnification of 10,000x. Tip vibation is also educed by 10,000x, so some vibation may be toleated. The spot size of a field emission is 5nm, so fo the same 1nm spot size a demagnification of only 5x is equied. Vibation of the tip is also demagnified but by 5x so a vey stable platfom is needed!! Chapte CHEM 793, 011 Fall

13 Lens A lens is an optical component which is used to focus beams of adiation. Lenses fo light ae usually made of a glassy mateial, wheeas nonunifom electomagnetic fields ae used as lens fo electons. Magnetic field, Cuent, I, in coil Chapte CHEM 793, 011 Fall

14 Magnetic Lenses Electon ay paths A magnetic lens consists of a wie winding aound a soft ion coe The magnetic field in the lens deflects the electon path, esulting in a focusing of the beam Right hand ule of vecto poduct to detemine the foce on the electon F d = m dt = ( ) ( ) q E + v = e v Chapte CHEM 793, 011 Fall

15 z=0 =0 Assumption z->max.@ z=0 fo lage IzI and modeate, > z z ө = 0 by cylindical symmety Cuent out ө z I Cuent in z z Magnetic field in and aound a shot solenoid, as shown the z and component Chapte CHEM 793, 011 Fall z

16 Chapte CHEM 793, 011 Fall z=0 =0 z->max.@ z=0 fo lage IzI and modeate, > z ө = 0 by cylindical symmety The Loentz foce poduces the focusing action to the moving electon ( ) z z z z z ev F v v e F ev F So z z v v v v e v F θ θ θ θ θ θ θ = = = + + = + + = = ^ ^ ^ ^ ^ ^ F V

17 Tajectoy otates out of plane Foce towads optic axis V ө F z Tajectoy otates out of plane by ө (image otation) ө F The electon tavels in nealy helical path in a homogeneous field Fө Vz Vz F = e v F F F z θ = e = ev θ = ev >z ( v v ) θ z z >z Spial out Spial in <z Chapte CHEM 793, 011 Fall O Paaxial tajectoy

18 Chapte CHEM 793, 011 Fall Paaxial tajectoy govening equation Φ: potential Z ө m e dz d dz d = Φ = = Φ + η η θ η ev dt d m = P.W. Hawkes, Electon optics and electon micoscopy, 1989, Taylo & Fancis Ltd., ISN

19 Vz Uppe pole-piece z FP f lowe pole-piece Focusing action of magnetic lens Chapte CHEM 793, 011 Fall Optical Axis

20 Focusing action of magnetic lens Off-axis electons inteact with these finge fields and begin to spial though the lens and move towad the optic axis The distance fom the point whee an electon stats to bend towad the axis to the point whee it cosses the axis is known as the focal length, f The focal length of the lens can be continuously vaiedly alteing the stength of the magnetic field which is contolled by the excitation cuent passing though the lens coil Chapte CHEM 793, 011 Fall

21 Summay: The magnetic lens poduces a stong magnetic field by passing a cuent though a set of windings (coppe coil). In TEM, this field act as a convex lens, binging off axis ays back to focus. Fo magnetic lens, the image is otated to a degee depending on the stength of the lens. Focal length also can be changed by changing the stength of the cuent, and theeby of. Convex lens A convex lens (fo light) is thicke in the cente than at its peiphey. All electomagnetic lenses used in electon micoscopy act as if they wee convex lenses. Image is otated Convex lens Chapte CHEM 793, 011 Fall

22 Chapte CHEM 793, 011 Fall

23 Definition of Some Impotant Angles Apetue α, beam-convegence semi-angle Specimen β, collection semi-angle θ, geneal scatteing angle Apetue Optical axis Chapte CHEM 793, 011 Fall

24 Summay : All electonmagnetic lenses act like thin convex lenses. So thei thickness can be ignoed. Equations fo convex lens ae applicable, like lens fomula 1/f= 1/v+ 1/u Magnification: M=v/u Chapte CHEM 793, 011 Fall

25 E-gun Chapte CHEM 793, 011 Fall Schematic lens configuation of a TEM system

26 Condense Lens In electon micoscope, the condense lenses ae used to demagnify the diamete of the beam. Electon souce Light souce Light micoscope Chapte CHEM 793, 011 Fall

27 In electon micoscope, double condense (C1 and C) system is used to adjust the illumination condition. The double condense system o illumination system consists of two o moe lenses and an apetue. It is used in both SEM and TEM. Its function is to contol spot size (C1) and beam convegence and intensity (C). Two o moe lenses can act togethe and thei ay diagams can be constucted using the thin lens appoximation fo each of them. The diagam opposite shows the ay diagam fo the double condense system. The black dots epesent the focal point of each lens. Chapte CHEM 793, 011 Fall

28 Fist condense lens, C1 C1 the fist condense lens is shown highlighted in the diagam. Its function is to ceate a demagnified image of the gun cossove, which acts as the object fo the illumination system contol the minimum spot size obtainable in the est of the condense system. Chapte CHEM 793, 011 Fall

29 Fist condense lens C1 the fist condense lens is shown highlighted in the diagam. Its function is to ceate a demagnified image of the gun cossove, which acts as the object fo the illumination system contol the minimum spot size obtainable in the est of the condense system. Chapte CHEM 793, 011 Fall

30 Second condense lens C C is adjusted the focus to poduce an image of gun cossove at the font focal plane of the uppe objective polepiece, and then geneate a boad paallel beam of electon incident on the specimen. C can affects the convegence of the beam at the specimen, and the diamete of the illuminated aea of the specimen. Question: Sitting at the micoscope you can only see the image of the specimen on the fluoescent sceen. How would you know when the condense lens (C) is focused on the specimen? If you now go away fom this condition how could you tell whethe the beam was ovefocused o undefocused?" Chapte CHEM 793, 011 Fall

31 C is focused The illuminated aea is at a minimum. The beam is pobe (mico o nano pobe). The beam is at its least coheent and most convegent. The intensity of illumination on the viewing sceen is the geatest. Image contast will be educed. Fo outine TEM wok, neve opeate in such beam condition. Fo thick poo-tansmission sample, focusing C will compensate fo poo-tansmission. The convegent-beam mode, focusing C, is used fo CED (convegent beam electon diffaction) and STEM (scanning TEM). Since convegence destoys the coheency and image contast, the beam has to be scanned to fom an image, i.e. STEM image. Chapte CHEM 793, 011 Fall

32 C is undefocused The illuminated aea inceases The beam is paallel and coheent The paallel illumination is essential to get the shapest diffaction pattens and the best image contast. The small apetue educe the electon cuent falling on sample, and deceases the angle of beam convegence, and theefoe incease the coheence of the beam. The paallel-beam mode, undefocusing C, is used fo SAD (selection aea and conventional TEM mode. A highe magnification means stengthening C, so the beam illuminates less of the specimen ( it is not eally paallel, just not vey convegent) To undefocus C, just simply incease the illumination aea on the specimen. Chapte CHEM 793, 011 Fall

33 C is ovefocused The beam convegence decease, and the electon come fom the cossove only. The cossove image is above the image plane. Ovefocusing C can check the beam astigmatism. Chapte CHEM 793, 011 Fall

34 Condense apetue The condense apetue contols the faction of the beam which is allowed to hit the specimen. It theefoe helps to contol the intensity of illumination, and in the SEM, the depth of field. Chapte CHEM 793, 011 Fall

35 Paallel beam with lage apetue, i.e. lage convegence. Paallel beam with small apetue, i.e. small convegence. Chapte CHEM 793, 011 Fall

36 Next lectue Objective lens Imaging mode Sample pepaation Lab tou time: 10:00-10:30 AM, Thusday, 09//11. I will meet you at 145, HRC Chapte CHEM 793, 011 Fall