The variable pressure SEM23Scanning Microscopy Vol. 13, No. 1, 1999 (Pages 23-41) 0891-703599$5.00+.25Scanning Microscopy International, Chicago (AMF O’Hare), IL 60666 USAAbstractThe variable pressure scanning electron microscope(VPSEM) is a promising new family of techniques whichpermits the imaging of insulators without preparation (coat-ing). The elimination of specimen charging is accomplishedin the VPSEM by the introduction of air inside the speci-men chamber. Unfortunately, directing an electron beaminto a gas creates various interactions between the gas, thebeam, the signal, and the specimen. The aims of this studywere to correlate probe current measurement, Monte Carlosimulation and X-ray microanalysis is order to optimize theuse of the VPSEM. A method has been proposed to deter-mine the scattered cross section of the air gas. The experi-mental conditions to optimize imaging and X-raymicroanalysis are quite similar. However, microanalysis ofthe light element such as carbon, and oxygen is problem-atic because the conditions are not optimal for this kind ofanalysis.Key Words: Variable pressure scanning electron micro-scope (VPSEM), environmental scanning electron micro-scope, skirting, X-ray microanalysis.*Address for correspondence:C. MathieuUniversité d’Artois, Faculté Jean PerrinSP 18, 62307 Lens Cedex, FranceTelephone number: +33-3-21791710FAX number: +33-3-21791717E-mail: [email protected] electron microscopy (SEM) is traditionallyperformed in a vacuum, while the vast majority of micro-scopes operate at a pressure below 10-2 Pa. A number ofmanufacturers now offer instruments to perform SEM atrelatively high pressure. This includes a variety of tech-niques reported in the scientific and commercial literature,e.g., environmental scanning electron microscopy (ESEM),Wet-SEM, controlled-atmosphere scanning electronmicroscopy (CAT-SEM), Low Vacuum SEM and VariablePressure Scanning Electron Microscopy (VPSEM).Farley and Shah (1990a,b) introduced the term highpressure scanning electron microscopy (HPSEM) to dis-tinguish these techniques from conventional high vacuumtechniques such as regular SEM and low temperature SEM.The conventional Everhart-Thornley detector cannot beused in the HPSEM, and the following modes of detectionhave been employed:Specimen current mode and biased current modeFarley and Shah (1990a,b) reported that images of aquality comparable to those obtained by the Everhart-Thornley detector could be obtained by a new detectionmode. This new mode of detection was called the “biasspecimen current detection mode”. A biasing electrode isused above the specimen to influence the trajectories of thecharge carriers and hence the image contrast. The speci-men is connected, via the specimen stub, to the virtual earthterminal of a charge-sensitive amplifier to collect the cur-rent generated in and around the specimen. It has been suc-cessfully shown that the current can be collected from thespecimen for the purpose of image generation, for bothconducting and non-conducting specimens.Emissive modeThe emissive mode of detection has been employedusing a “gaseous detector device” [GDD, see Danilatos(1990a) for a review]. The GDD is a collecting electrodethat, under working conditions is placed in the vicinity ofthe specimen and it is positively biased. The electrode col-lects emitted electrons, along with the electrons generatedby the emitted and the primary electrons due to ionizationprocesses.THE BEAM-GAS AND SIGNAL-GAS INTERACTIONS IN THE VARIABLE PRESSURESCANNING ELECTRON MICROSCOPEC. Mathieu*Université d’Artois, Faculté Jean Perrin, Lens Cedex, France(Received for publication June 1, 1998 and in revised form May 10, 1999)C. Mathieu24Backscattered modeBackscattered electron imaging with high gas pressurein the specimen chamber has proven to be a useful tech-nique in the scanning electron microscope. For example,the specimen may be surrounded by air or nitrogen gas toinspect non-conductive surfaces (Moncrieff et al., 1978,1979; Danilatos and Robinson, 1979) or by water vapor inbiological applications (Robinson, 1974). This techniquehas been used in the Variable Pressure Scanning ElectronMicroscope (VPSEM) (Mathieu, 1996).In the next section, the different kinds of interactionswhich take place inside the specimen chamber will be de-scribed.Outline of General Interactions in the VPSEMWhen the electron beam strikes a specimen, there is ahost of reactions or interactions between the primary elec-tron beam and the specimen, and their study has consti-tuted a fundamental topic of electron microscopy. Thus, aprimary electron may undergo elastic or inelastic collisionsin the specimen resulting in the generation of secondary(SE) or backscattered electrons (BSE), X-rays etc., andchanges in the specimen by molecular scission or cross link-ing. All of these different interactions are characterized bythe fact that they occur between two entities: the beam andthe specimen.By allowing gas around the specimen, the number andtype of reactions are multiplied and it is helpful if thesereactions are classified and studied in a logical manner ac-cording to some natural distinction. Four main entities canbe distinguished which interact with each other: beam, gas,specimen and signals (Danilatos, 1990b). Therefore, thelarge number of reactions can be subdivided into six gen-eral types of interactions as will be discussed below. Thesegeneral types are not independent from each other and theymay influence each other.Beam-specimen interactionsInteractions of the electron beam with the specimencan result in:(1) Beam scattering which determines the interactionvolume,(2) Generation of signals,(3) Modification of the nature of the specimen (beamirradiation effects).Beam-specimen interactions have constituted the pri-mary objective of study in electron microscopy.Beam-gas interactionsThe electron beam and the gas interact with each otherand the result of this interaction is(1) Scattering of the beam,(2) Generation of signals such as SE, BSE, X-rays andcathodoluminescence,(3) Modification of the gas due to the creation of posi-tive and negative ions, dissociation products and excitedmolecules.The scattering of the beam will constitute the subjectof a detailed analysis. This process determines the limits ofcontrast and resolution. The generation of signals in thegas by the primary beam should be
View Full Document
Unlocking...