Volume 107, Number 6, November–December 2002Journal of Research of the National Institute of Standards and Technology[J. Res. Natl. Inst. Stand. Technol. 107, 567–603 (2002)]X-Ray Microanalysis in the Variable Pressure(Environmental) Scanning Electron MicroscopeVolume 107 Number 6 November–December 2002Dale E. NewburyNational Institute of Standards andTechnology,Gaithersburg, MD [email protected] x-ray microanalysis per-formed in the variable pressure and envi-ronmental scanning electron microscopes issubject to additional artifacts beyondthose encountered in the conventional scan-ning electron microscope. Gas scatteringleads to direct cont ributions to the spec-trum from the environmental gas, as wellas remote generation of x rays by electronsscattered out of the focussed beam. Theanalyst can exert some degree of controlover these artifacts, but depending on theexact situation, spurious elements can ap-pear at the trace (< 0.01 mass fraction),minor (0.01 mass fraction to 0.1 mass frac-tion), or even major (> 0.1 mass fraction)levels. Dispersed particle samples give theleast compromised results, while finescale microstructures are the most severelycompromised. Procedures to optimize thesituation based upon specimen preparationas well as spectral processing are de-scribed.Key words: energy dispersive x-ray spec-trometry; environmental scanning elec-tron microscopy (ESEM); variable pressurescanning electron microscopy (VP-SEM);x-ray mapping; x-ray microanalysis; x-rayspectrometry.Accepted: August 22, 2002Available online: http://www.nist.gov/jresContents1. Introduction ............................ 5682. X-Ray Spectrometry in the VP-ESEM....... 5682.1 Extraneous X-Ray Peak(s) Due toEnvironmental Gases ................. 5692.2 Primary Beam Gas Scattering: RemoteExcitation of X Rays ................. 5712.3 Charging Effects ..................... 5743. Strategies for Dealing With Gas Scattering . . . 5763.1 Selection of Instrumental Parameters toMinimize Gas Scattering .............. 5763.2 Pressure Variation Method............. 5773.3 Intercepting the Unscattered Beam ...... 5833.4 Bremsstrahlung Normalization Method . . . 5833.5 X-Ray Focusing Optics ............... 5834. X-Ray Microanalysis in VPSEM-ESEM:Influence of the Specimen Configuration..... 5844.1 The Analytical Blank ................. 5844.1.1 The Analytical Blank for ParticleAnalysis: Conventional SEM ...... 5854.1.2 The Analytical Blank for ParticleAnalysis: VP-ESEM ............. 5854.2 Isolated Particles..................... 5854.2.1 Particles Dispersed on Carbon-Containing Substrates ............ 5854.2.2 Reducing the Background: Use ofThin Foil Substrates ............. 5904.2.3 Alternatives to Carbon Substrates . . 5914.3 Fibers.............................. 5914.4 Bulk Specimen ...................... 5934.4.1 Homogeneous, Single Phase ...... 5934.4.2 Heterogeneous, MultiphaseMicrostructures ................. 5985. X-Ray Mapping in the VP-ESEM .......... 5986. Conclusions ............................ 6027. References ............................. 602567Volume 107, Number 6, November–December 2002Journal of Research of the National Institute of Standards and Technology1. IntroductionCharacterization of chemical microstructure is one ofthe most important applications of the conventionalscanning electron microscope (SEM) equipped with anenergy dispersive x-ray spectrometer (EDS) [1]. Interestin electron-excited x-ray microanalysis is potentiallyeven greater in the variable pressure (VPSEM) and envi-ronmental scanning electron microscopes (ESEM)where dynamic chemical experiments can be con-ducted. The distinction between the VP-SEM andESEM is made on the basis of achieving gas pressuresthat can maintain an equilibrium between water vaporand liquid water. With sample cooling to 2 ⬚C, this equi-librium can be established at approximately 660 Pa. Anarbitrary division between ESEM and VPSEM can bemade at 100 Pa. X-ray microanalysis performed in theVPSEM-ESEM is subject to additional conditions andconstraints that arise from the presence of the environ-mental gas and its influence on the primary electronbeam. Figure 1 shows schematically the effects of elas-tic and inelastic scattering of the beam electrons by thegas atoms. The major consequence of inelastic scatter-ing is the generation of characteristic and continuum(bremsstrahlung) x rays from the gas atoms that con-tribute to the measured Si-EDS spectrum. X-ray pro-duction is a relatively rare event, suffered by one in 106electrons or fewer. Most electrons do not suffer signifi-cant energy loss from inelastic interactions.The consequences of elastic scattering are the reduc-tion of beam current within the focused probe and redis-tribution of this current to form a wide “skirt” aroundthe beam, significantly degrading the spatial resolutionFig 1. Schematic diagram illustrating formation of electron scattering“skirt” around the unscattered beam in a VPSEM-ESEM. Elasticscattering leads to t ransfer of electrons from the focused beam to theskirt. Inelastic scattering leads to inner shell ionization and subsequentemission of characteristic x rays from the gas, which will be collectedby the EDS if emitted into the solid angle defined by the collimator.of x-ray microanalysis. These gas-scattering effects cangreatly alter the results achieved with x-ray microanaly-sis in the VPSEM-ESEM compared to performing asimilar x-ray measurement in a conventional SEM underhigh vacuum conditions, given that the specimen wouldbe compatible with high vacuum. This paper will con-sider the special aspects of x-ray spectrometry and mi-croanalysis performed in the VPSEM-ESEM, especiallythe impact of gas scattering on spectrum quality, meth-ods of specimen preparation to minimize the effects ofgas scattering, practical aspects of qualitative and quan-titative x-ray microanalysis, and prospects for future im-provements in this area.2. X-Ray Spectrometry in theVPSEM-ESEMElectron-excited x-ray spectrometry performed withwavelength dispersive spectrometry (WDS) and/orsemiconductor energy dispersive spectrometry (Si-EDS) in the SEM is a mature technique that is widelyemployed across many of the sciences [1]. Specimenexcitation with a focussed electron beam at a fixed posi-tion can achieve lateral spatial resolution down to ap-proximately 1 m or less, depending on the beam en-ergy and the exact composition of the specimen at thebeam location. WDS and
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