Contains reports on one research project.Joint Services Electronics Program (Contract DAAB07-71-C-0300

Coleman, John W.

King, John G.

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II. ELECTRON OPTICSAcademic and Research StaffProf. John G. KingDr. John W. ColemanDr. Edward H. JacobsenGraduate StudentsSteven R. JostNorman D. PunskyA. HIGH-RESOLUTION HIGH-CONTRAST ELECTRON MICROSCOPY JSJoint Services Electronics Program (Contract DAAB07-71-C-0300)John G. King, John W. ColemanI. RESEARCH SUMMARYThe purpose of our work is to develop a new type of lens that will increase resolvingpower, image contrast, or both, for all kinds of electron optical scientific instruments.We shall also provide support to the Molecule Microscope project by helping to solvevarious problems in electron and ion optics.Auger Emission Microscope (AEM)We are now testing AEM-1, the first of three planned prototypes of the Auger micro-scope; this represents the first step in the development of an electron microscopecapable of resolving both positions and types of individual atoms in complex moleculesor on surfaces, without the use of heavy metal stains. Our ultimate goal is the directdemonstration of low-Z atoms such as nitrogen, oxygen, and carbon in biological spec-imens.Our results with AEM-1 indicate that the scaled-up mirror lens works qualitativelyas anticipated from the results of our original computer studies, but that refinementsare needed in alignment capability, optical spacing, tilt control, and voltage ratios beforethe AEM-1 goal of 1000 A resolution can be achieved (see Sec. II-A. 2). We expect tohave enough data from AEM-1 by October 1974 to proceed with the design and construc-tion of AEM-2.Spherical Aberration Corrector Module (SACM) and Multioptical BenchDuring this period, the development of SACM has continued principally in vacuuminterfacing between the module and the elements of the multioptical bench. In partic-ular, problems of traversing the several elements have required special attention, in JSQPR No. 114(II. ELECTRON OPTICS)JS order not to disturb the vacuum differential between the two systems.We are now ready to begin quantitative testing on this foil-type lens for correctingspherical aberration. We expect that the new lens will improve the resolution of existingmicroscopes from 8 A to 4-5 A and for probe-type instruments will allow twofold orthreefold more current into a given focused spot.2. FIRST RESULTS WITH THE AUGER ELECTRONMICROSCOPE (AEM-1)John W. Coleman, Steven R. JostIntroductionThe AEM-1 is the first prototype of an Auger electron microscope, an emission-typeinstrument capable of identifying the species and resolving the positions of individualatoms in complex molecules and on surfaces. Because Auger electrons have energiesdependent only upon the structure of the atoms from which they are ejected, they act asatom signatures if their energies are measured. If they can be collected by high-qualityimaging optics, the sites of the emitting atoms within the specimen can also be estab-lished. With such an energy-analyzing optical-imaging system, there is no need for heavymetal staining or isomorphic replacement for low-Z atoms, and thus our ultimate goalwith AEM is the direct observation of atomic carbon, oxygen, and nitrogen in biologicalspecimens. The use of AEM is not fundamentally limited, however, to any particularkind of specimen. When fully developed the instrument will have wide application, espe-cially in materials science.Program OrganizationThe AEM development program as proposed, in 1967, by E. H. Jacobsen to J. G.King was concerned at first with theoretical research in several pertinent areas andcomputer studies of wide-angle achromatic electron lenses. This research continueduntil 1972 when a formally planned program was initiated with emphasis on developinga prototype which was based initially upon the preceding theoretical studies. The pro-gram was organized to be carried out in three phases: Phase 1 (1972-1974), design, con-struction, and testing of AEM-1 and design of AEM-2; Phase 2 (1975), construction andtesting of AEM-2 and design of AEM-3; Phase 3 (1976), construction and testing of AEM-3and completion of the program.The specific purpose of constructing AEM-1 was to enable study of the computer-designed electron optics of the imaging system, in order to exclude constraints that wouldbe placed on the electron optics by the subsequent optics of the energy-analyzing system.JS In particular, we are using a simulated Auger electron source, injected into optics scaledQPR No. 114HEMISPHERICALBOSSELECTRODEj~7 ciiK\TYPICALTRAJECTORYnIELECTRON BEAM FOR SECONDARYELECTRON EXCITATIONHOUSINGRING ELECTRODE (E6) -(- 18.0kV)RING ELECTRODE (E5)(-22.0 kV)RING ELECTRODE (E3)(TEMPORARILY GROUND)RING ELECTRODE ( l)(-18.0 kV)RING ELECTRODE ( O)Fig. II-1."Most promising" geometry.BOSS ELECTRODE WITH APERTURE (E8)(-20.0 kV)RING ELECTRODE (E7)( -22.1 kV)I amSPEC MEN HOLDER (SH)(-20.0 kV)SIMULATED SOURCE ELECTRONOR SECONDARY ELECTRONRING ELECTRODE (E4)RING ELECTRODE (E2)Fig. II-2. Mirror accelerator objective system. Voltages are for 300 eVelectrons from the simulated source situated in the specimenholder (SH).QPR No. 114f-' RINGS-  ELECTRODESI S IS\\ czMESPECIMEN,-- 6 " oC1(II. ELECTRON OPTICS)up to facilitate study of geometric tolerances and alignment. Energy analysis in AEM-1is limited to that which is chromatically inherent in the imaging optics as a function ofinput electron energy and lens voltages.Construction of AEM-1With the exclusion of the vacuum and electrical systems, the construction of AEM-1has been implemented with the following design.a. Mirror Accelerator Objective SystemThe geometry of this system is based upon the computer studies of Jacobsen and1 2Ofsevit I and of Thomson which are related to the design of achromatic lenses withextremely wide acceptance angles by the standards of conventional electron optics. Theselenses are wide cones of the order of 10* that have been seen heretofore only in visibleoptics. The cylindrically symmetric geometry was selected as "most promising" forthe starting point, and the dimensions that we used (see Figs. II-1 and II-2) furnishimprovement by a factor of 4 over the dimensions given by computer. Because of thescale factor and uncompensated spherical aberration, we expect resolution of only~1000 A with AEM-1. This is not important in this phase of the program.b. Simulated Source of Auger ElectronsThe simulated source (Fig. II-3) is a coil-form thermionic emitter used in a3mmSIMULATED-,SOURCEFig. II-3.I Simulated source (325 eV max)JS of Auger electrons.Fig. II-4.Specimen holder.QPR No. 114(II. ELECTRON OPTICS)miniature triode gun to send a spray of electrons of controllable energy into the mirror JSaccelerator objective system: The source rests in the specimen holder (SH) (Figs. II-2and 11-4).c. Projection Recording SystemThe projection recording system (Fig. II-5) has two sets of magnetic deflectors andtwo magnetic lenses, with a final throw of 15 cm. This system, with a pick-up objectplane 6 cm in front of the gap of the first lens, allows variable magnification fromapproximately 40-1970 X; further magnification ranges are possible if the position of thepick-up plane is other than at 6 cm. The deflectors are for alignment, and the final imageis caught on a fluorescent screen on a glass prism. Photographs of the image are madewith an MP-4 Polaroid camera exterior to the vacuum.d. Secondary Electron Excitation SystemThe secondary electron excitation system in AEM-1 (Fig. II-6) is the forerunner ofthe Auger electron excitation system that will be used in AEM-2 and AEM-3. In AEM-1the whole system is electrostatic. It is built from a Braucks-type electron gun, an Einzel(unipotential) lens, and two sets of electrostatic deflectors. The excitation beam entersthe mirror accelerator objective system (see Fig. H1-2) by means of an aperture (E8) andis stopped by the specimen holder (SH). In this way, secondaries with a wide range ofenergy may be sent through the mirror system, in contrast to electrons of a much nar-rower range of energy which may be sent through with the simulated source.The overall assembly is shown schematically in Fig. 11-7, and a view of the instru-ment is seen in Fig. 11-8.Data CollectionWe are now taking three kinds of data with AEM-1.i. Focused and/or shadow images of grids (100-1500 mesh/inch) at selected siteswithin the mirror accelerator objective system.ii. Azimuthal intensity patterns (field energy profiles) measured from the fieldsphotographed from the fluorescent screen.iii. Specular reflection patterns recorded by photographs of the fluorescent screen.All photographs are tabulated on worksheets (Fig. II-9) and filed. In this way we areacquiring the hundreds of images that are necessary to gain statistically definitive knowl-edge of the effects in the crucial areas of beam energy, electrode voltage, and sys-tem alignment, including the effects of electrode machining tolerances, tilts, and JSQPR No. 114MAGNETICDEFLECTOR SET NO. IELECTROMAGNETICPROJECTOR LENS NO. 1MAGNETICDEFLECTOR SET NO. 2ELECTROMAGNETICPROJECTOR LENS NO. 2FLUORESCENT SCREENP-4 CAMERAGLASS PRISM WITHTRANSPARENT CONDUCTIVECOATINGFig. II-5.Projection and recording system.S------ FILAMENTANODE r - WEHNELT CYLINDERELECTROSTATIC OE. "BRAUCRKS ELECTRODEDEFLECTOR SET NO. I 1TELECENTRIC STOP -E2c nm -2222 CO (EINZEL LENS)ELECTROSTATICDEFLECTOR SET NO. 2 EFig. II-6.Secondary electron excitation system.SECONDARY ELECTRONEXCITATION SYSTEMMIRROR ACCELERATOR OBJECTIVSYSTEMPROJECTION AND RECORDINGSYSTEM0E 0V222=ZCr=£2ZZZItzz~P~cr"nFig. II-7.AEM-1 overall assembly.QPR No. 114incnnn=tEi~~~ .ile,;s; I-, i~*iliaZ~ "S~~~~is~r~ ""~/WFig. 11-8. Photograph of AEM-1. JSQPR No. 114(II. ELECTRON OPTICS)INPUTS NUMBER 1 INPUTS NUMBER 2PD -kV PD -kV PD -kV PD -kVE7-- 12 E7-E1 1.25 E7- 12.2 E7-E1 1.25E7-E8 1.15 SH-E6 1.15 E7-E8 1.1 SH-E6 1.2E7-SH 1.1 BS-E6 1.15 E7-SH 1.1 BS-E6 1.2E7-E5 10V BS-SH 0 E7-E5 20V BS-SH 0Pl (mA): 11 P2 (mA) : 6 Pl (mA): 10 P2(mA): 7.8COMMENTS:- 12 kV ON GUN-1 min EXPOSUREMESH ON E2E7 SET TO GIVEMAX BRIGHTNESSE7E6SHBSP1 ZIRESULTS NUMBER 1 RESULTS NUMBER 2Fig. 11-9. Sample AEM worksheet. (Series No. 1, May 14, 1974, S. R. Jost.)concentricities with respect to each other.Preliminary ResultsWe have used the data from AEM-1 to extract four kinds of information.i. To study the use of the mirror accelerator objective system as a compound pro-jection lens (before studying it as a compound objective lens).ii. To find the kind and, as far as possible, the amount of geometric aberration inthe system. (Spherical aberration, which will be treated independently at length, hasbeen omitted.)iii. To determine beam convergence and divergence in the preacceleration region(to determine the gross chromatic defect defining the energy window).iv. To better align the system and find the degrees of freedom that are needed.Although quantitative statements must await further statistics, our first resultsare qualitatively clear and reproducible. As shown in Fig. II-10, when the mirrorJS accelerator objective is focused as a compound projection lens it delivers an imageQPR No. 114(II. ELECTRON OPTICS)of less than unity magnification. Since the convergence/divergence characteristicof the preacceleration region is still not clear, we cannot separate the compoundfocal length into the contributing focal lengths, but we feel fairly certain that theTRAJECTORYTABLE1 300 eV2 275 eV3 325 eVo- V-------------- PrV771 07M------ --------a a777P1S~ ® czMs 0.2DFig. II-10. Mirror accelerator objective system focused as acompound projection lens.postacceleration derived unipotential lens is limiting the overall magnification, andacts, at present, primarily as a transfer lens. Since we want magnification fromthe compound system of the order of 100 when it is used as an objective, and we esti-mate from the results when it is used as a projector that as an objective it candeliver maximum magnification of an order of magnitude less, we must eventuallyexplore the possibility of changing the geometry and voltage ratio of the unipotentialelement.Geometric aberration in the system is summarized in Table II-1. We feel thatalthough there is still a great deal of aberration, we shall learn to remove itby alignment and compensate for it as our knowledge of the system grows. Spherical JSQPR No. 114(II. ELECTRON OPTICS)JS Table II-1. Mirror/Accelerator/Objective aberration.Aberration Present Probable CausePincushion Distortion Yes Lack of properaperture andtelecentric stopBarrel Distortion NoComa Yes (tentative) UnknownAstigmatism Yes Concentricitytolerancing andmisalignmentSpherical Aberration Yes Inherent in mirrorsystem design(spherical aberrationuncorrected)aberration will be corrected in AEM-2 and AEM-3 by means of foil lenses, whichare under development in another project in our laboratory. Astigmatism will becorrected eventually by conventional electron optical means.The mirror accelerator objective geometry was computed from the postulate ofachromatism, but this condition only applies for a given setting of electrode voltagesand a corresponding input beam energy. If the beam energy is changed from thathandled achromatically by the system, a new chromatic defect is introduced oper-ationally. This results in an inherent "beam energy window" at any given settingof electrode voltages. We find that this window is no wider than -50 eV at the vol-tage settings for 300 eV input, and it may be considerably narrower. This energy-selecting characteristic in no way obviates the need for additional means of energyselection in AEM-Z and AEM-3 because the Auger peaks must be isolated from thegeneral secondary electron spectrum, and hence we need windows with widths twoorders of magnitude smaller. The importance of finding this window in AEM-1 isgreat. Much earlier in the project than we had anticipated, we have the evidencethat our computer-generated system is in the known class of electrostatic mirrorlenses (for which such windows are hallmarks), and so the system can be scaled to useall results that are found in testing AEM-1 directly in the design of AEM-2.We have found definitely that system alignment is critical. Prealignment withmandrels during assembly is insufficient; the system must be capable of adjust-ment in vacuo as the operator monitors the images. At present, we are besetwith image sweep at each incremental voltage change on any electrode, and severevignetting is evident in all cases. Figure II-11 gives a summary of the alignmentJS facilities that we now have and those that we are going to need.QPR No. 114(II. ELECTRON OPTICS)NEEDED EVENTUALLY AVAILABLE S0 : l 0 - TILT NO. In-71 r=-z[zza CzzalSCI 0 - TILT NO. 2TRAVERSE W.R.T. ZCHAMBER FRAME X -TRAVERSEW.R.T. INST. FRAME ® TILT NO. 3TRAVERSEW.R.T. INST. FRAMEE ® cm TILT NO. 4Fig. II-11. Available alignment facilities (right) and alignment facilitiesthat are needed (left).ConclusionFrom our results with AEM-1 we draw the following conclusions.i. Our mirror accelerator objective system is a practicable approach to developingwide-angle collection and optical processing of Auger electrons.ii. Our data, although not yet definitive, have served to point up the severity of theproblems that we must solve to achieve the goal of atomic identification coupled with res-olution to the atomic dimensions. Nevertheless, we are encouraged to proceed because.nothing has been found to indicate that we should not do so. At this point, we foreseeonly engineering problems that can be solved without recourse to further invention.References1. E. H. Jacobsen and D. S. Ofsevit, "Electron Microscopy," Quarterly ProgressReport No. 99, Research Laboratory of Electronics, M. I. T., October 15, 1970, p. 2.2. M. G. R. Thomson, "Design of an Achromatic Combined Electron Mirror and Accel-erating Lens," Quarterly Progress Report No. 108, Research Laboratory of Elec-tronics, M.I.T., January 15, 1973, p. 17. iQPR No. 114

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