扫描电子显微镜SEM入门（英文）资料.pdf

Scanning Electron Microscopy Primer Bob Hafner This primer is intended as background for the Introductory Scanning Electron Microscopy training offered by the University of Minnesotas Characterization Facility(CharFac).The primer addresses concepts fundamental to any scanning electron microscope(SEM);it also,where possible,informs the reader concerning specifics of the facilitys four SEMs:JEOL 6500;JEOL 6700;Hitachi S-4700;and Hitachi S-900.You must learn this material prior to the hands-on training and you will be required to pass a test on it in order to become an independent SEM user at CharFac.A good source for further information is:“Scanning Electron Microscopy and X-Ray Microanalysis”by Joseph Goldstein et al.Characterization Facility,University of MinnesotaTwin Cities 4/16/2007 1mage.A look inside the black box 1 reveals a source(electron gun)of the ted ndenser and e (x,y,z-veral e maintained at high er The Big Picture To the right is a picture of our Hitachi S-4700.The microscope column,specimen chamber,and vacuum system are on the left;the computer,monitor,and many of the instrument controls on the right.As an operator you will need to understand what is happening inside the“black box”(microscope column and specimen chamber)when an instrument control is manipulated to produce a change in the monitor i quite a bit of complexity;however,we can simplify at this point.We have:electron beam which is acceleradown the column;a series of lenses(coobjective)which act to control the diameter of the beam as well as to focus the beam on the specimen;a series of apertures(micron-scalholes in metal film)which the beam passes through and which affect properties of that beam;controls for specimen positionheight)and orientation(tilt,rotation);an area of beam/specimen interaction that generates setypes of signals that can be detectedand processed to produce an image orspectra;and all of the abovvacuum levels(the value of the uppcolumn being greater than the specimen chamber).Characterization Facility,University of MinnesotaTwin Cities 4/16/2007 2 we take a closer look at see tor e also see a pair of led r rastering that focused beam across the specimen surface.The size of the he beam is rastered from left to n e at he red dot within each pixel on collected by the detector and subsequently processed to generate the ith the beam focused on the specimen surface,all we need to do to change magnification is to change ation=Ifthe lower column and specimen chamber,wethe objective lens which focuses the electron beamon the specimen surface.Asignal is generated from the specimen,acquired by the detector,and processed to produce an image or spectrum on the monidisplay.Wdeflector coils,controlby the Scan Generator,which are responsible forastering pattern is under Magnification Control.Tright and top to bottom.There is a one-to-one correspondence between the rastering pattern othe specimen and the rastering pattern used to produce the image on the monitor.The resolution we choose to imagwill obviously affect the number of pixels per row as well as the number of rows that constitute the scanned area.Tthe specimen represents an area of beam-specimen interaction from which the signal is derived(more on this later).The signal isimage.That processing takes the intensity of the signal coming from a pixel on the specimen and converts it to a grayscale value of the corresponding monitor pixel.The monitor image is a two dimensional rastered pattern of grayscale values.Wthe size of the rastered area on the specimen.The size of the monitor raster pattern is constant.Magnification will increase if we reduce the size of the area scanned on the specimen.Magnificarea scanned on the monitor/area scanned on the specimen.Characterization Facility,University of MinnesotaTwin Cities 4/16/2007 3 knowledgeable SEM operator should have a basic command of the following content areas:nteractions;cteristics;nd order to better understand the first two of these content areas,it will Beam accelerating voltage(kV):the voltage with which the 2.le of the cone of 3.ges upon the specimen 4.of the final beam ooking at the diagram it would seem that all we would have to do to maintain adequate probe current in lectron Optics lectron Guns electron gun is to provide e um e he FEG cathode consists of a sharp metal A electron optics;beam-specimen i signal types and detector chara signal quality/feature visibility relations;a signal/imaging processing.Inbe useful at this point to define the major parameters associated with the electron beam(probe)at the specimen surface.These parameters are ones that we can control as an operator and define the major modes of imaging in the SEM.They are:1.electrons are accelerated down the column;Probe convergence angle(p):the half-angelectrons converging onto the specimen;Probe current(ip):the current that impinand generates the various imaging signals;and Probe diameter or spot size(dp):the diameter at the surface of the specimen.Lin a small probe diameter would be to increase the probe convergence angle.But this is not the case dueto aberrations in the optic system(more on this later).A small probe diameter always comes with a decrease in probe current.These parameters are interrelated in other ways.For example,a decrease accelerating voltage will result in a decrease in probe current as well as an increase in probe size.“Doing SEM”involves understanding the trade offs that necessarily occur when we choose operational parameters.E EThe purpose of the a stable beam of electrons of adjustablenergy.There are three main types ofelectron guns:Tungsten hairpin;Lanthanhexaboride(LaB6);and Field emission.Wwill concentrate on the latter since the CharFacs four SEMs are all field emission gun(FEG).T(usually Tungsten)tip with a radius of less than 100 nm.A potential difference(V1=extraction voltage)is established between the first anode and the tip.The result is anelectric field,concentrated at the tip,which facilitates electron emission(emission current).Characterization Facility,University of MinnesotaTwin Cities 4/16/2007 4he potential difference between the tip and the second grounded anode determines the accelerating here are two types of FEGs:-4700;Hitachi S-900),and oth types of field emission require that the tip remain free of contaminants and oxide and thus they d lthough the FEG has a moderate emission current,its“Brightness”value is orders of magnitude t n.Tungsten LaB6Thermal FEG Cold FEG Tvoltage(V0)of the gun.The higher the accelerating voltage the faster the electrons travel down the column and the more penetrating power they have.T Cold(JEOL 6700;Hitachi S Thermally assisted(JEOL 6500).Brequire Ultra High Vacuum conditions(10-10 to 10-11 Torr).In the cold FEG the electric field produceby the extraction voltage lowers the work function barrier and allows electrons to directly tunnel throughitthus facilitating emission.The cold FEGS must have their tip“flashed”(briefly heated)periodically to free absorbed gas molecules.The thermally assisted FEG(Schottky field emitter)uses heat and chemistry(nitride coating)in addition to voltage to overcome the potential barrier level.Agreater than the thermionic Tungsten and LaB6 sources(table below).Brightness is the beam currenper unit area per solid angle =4ip /(dp p)2 and,unlike current,it is conserved down the columBrightness increases linearly with accelerating voltage.Brightness(A/cm2str)105106108108Lifetime(hrs)40-100-1000 00 00 2001010Source Size 30-100 um 5-50 um 5 nm 5 nm Energy Spread(eV)1-3 1-2 1 0.3 Current Stability(%hr)1 1 5 5 Vacuum(Torr)10-5-7-11-11101010 he high brightness value is due to the fact that a given emission current occurs within a very small ther es.A tter hese enhanced FEGSEM capabilities come with a cost(literally).Very expensive vacuum systems a Tsource size as the beam exits the gun.This“Source Size”for FEGs is on the order of nanometers rathan microns for the other emission sources.The ability to have enough probe current(and thus potential signal)in a probe of small diameter allows the FEGSEM to obtain the resolution it doThe ability to achieve a small probe diameter is directly related to the source size or the diameter of the electron beam exiting the gun.An electron beam emanating from a small source size is said to have high spatial coherency.Electron beams can also be characterized in terms of temporal coherency.beam with high temporal coherency will have electrons of the same wavelength.In reality there is a certain“Energy Spread”associated with the beam.As we will see,lower energy spreads result in beresolution and are particularly important in low accelerating voltage imaging.Tmust be attached to these microscopes to achieve the vacuum levels they require.The advantages of coherent beam source will be negated if the beam is interacting with molecules on its path down the column.The vacuum at the gun level of the column is kept at 10-10 to 10-11 Torr;the vacuum in the specimen chamber is in the 10-5 to 10-6 Torr range 1 Torr=133 Pa=1.33 mbar.The table below isprovided simply to give you a feeling for what these vacuum levels translate to inside the microscope.Characterization Facility,University of MinnesotaTwin Cities 4/16/2007 5acuum Atoms/cm3 Distance between atoms Mean Free Path Time to monolayer V1 Atm(760 Torr)10195 x 10-9 meters 10-7 meters 10-9 seconds 10-2 Torr 10142 x 10-7 meters 10-2 meters 10-4 seconds 10-7 Torr 1091 x 10-5 meters 103 meters 10 seconds 10-10 Torr 1061 x 10-4 meters 106 meters 104 seconds We wont spend time here talking about the various kinds of pumps and gauges associated with the y the microscope is at good vacuum level when you begin your session;scope is closed during levels are achieved prior to engaging the high tension;and lectron Lenses es are used to demagnify the image of the beam source exiting the electron gun and all is useful to reason by analogy with glass lenses used for focusing light to help one understand the:a perfect optical lens.The the:a lens of different strength nd lens n :a hypothetical fixed position lens of variable strength(symbolized by the dashed lines)that achieves vacuum system since those are maintained by the staff.However,as an operator,you should be veraware of the vacuum state of the microscope and ensure that:the“Gun Valve”separating the upper column from the rest of the microsample exchange;appropriate vacuum you use gloves when mounting samples and transferring them to the column your samples are dry and free of excessive outgassing.EElectromagnetic lensto focus the beam on the specimen.Condenser lenses are involved in demagnification;the objective lens focuses on the specimen as well as demagnifies.The source size of the FEG is comparatively smso that the amount of demagnification necessary to produce small probe sizes is less than that of other electron sources.Itoperation of electron lenses.Analogies also have regions of non-correspondence which are equally important to understand.Arays emanating from a point in the object plane come to one common well defined point inimage plane.The optical lens has a fixed focal point and the object is in focus at the image plane.B(represented as a thicker lens)athus focal point.Focusing(changing the height of the along the optic axis)the object othe image plane results in a change in magnification.Cthe same magnification change as in B.Characterization Fachere are a number of points to emphasize here when thinking about scanning electron microscopy.The object being imaged is the source diameter(Gaussian intensity distribution)of the electron beam 2.demagnifying(not magnifying)this beam source diameter.The amount of 3.represented in C.SEMs have stationary electromagnetic lenses 4.ane of e he objective lens is used to focus the beam on the by e;o:specimen in focus.ds to be decreased for specimen he figure below shows a simplified column with one condenser lens,an objective lens,and an aperture he amount of of the o at nd likewise,the amount T 1.as it exits the gun.We are interested indemagnification is simply p/q.The type of lens used in SEMs iswhich we can vary the strength of by altering the amount of current running through them.SEMs will have more than one electromagnetic lens.Under this circumstance the image plthe first lens becomes the object plane of the second.The total demagnification is the product of thdemagnification of lens one with lens two.Tspecimen.Coarse focusing of the specimen is done choosing the working distance(WD=distance betweenthe bottom of the objective lens and the specimen);focusing the objective lens to coincide with this valuand then changing the physical height of the specimen tbring it into focus.Fine focusing is subsequently done solely with the objective lens.AB:working distance neeto be in focus Tfor each lens.The portion of the electron beam blocked by the apertures is represented with black lines.Both A and B show the source diameter of the electron beam exiting the gun(dG)being focused to an electron probe(dP)of a given diameter at the specimen.The diameter of the probe on the right is smaller.Why is this?Tility,University of MinnesotaTwin Cities 4/16/2007 6demagnificationsource diameter to the beam at dB is p1/q1.Sthe diameter of the beam dB=dG/(p1/q1).Aof demagnification of the beam at dB to the electron probe(dP)is p2/WD.So the diameter of the electronprobe at the specimen dP=dB/(p2/WD).Characterization Facility,University of MinnesotaTwin Cities 4/16/2007 he strength of the condenser lens is stronger(thicker)in B than A.This results in a smaller diameter o be n electromagnetic lens 2 consists of a coil of copper wires inside an to hen an electron passes through an electromagnetic lens it is subjected e e s o far weve mentioned that electromagnetic lenses are unlike optical lensnce)inally,it is important to keep in mind that electron lenses,compared to glass lenses,perform much a ens Aberrations ur representations depict a perfect lens.That is,we pherical aberration(dS):The further off the optical axis(the closer to k TdB and thus a smaller probe diameter on the specimen.A smaller probe diameter will enable better resolution but it comes at a cost.The stronger condenser lens setting in B causes more of the beam tstopped by the objective aperture and thus a reduction in probe current occurs.Beam current increases to the 8/3 power as probe diameter increases.Adequate current is essential to produce images with the necessary contrast and signal to noise ratio.Airon pole piece.A current through the coils creates a magnetic field(symbolized by red lines)in the bore of the pole pieces which is usedconverge the electron beam.Wto two vector forces at any particular moment:a force(HZ)parallel to the core(Z axis)of the lens;and a force(HR)parallel to the radius of thlens.These two forces are responsible for two different actions on the electrons,spiraling and focusing,as they pass through the lens.An electron pass