Contains research objectives and summary of research on one research project.Joint Services Electronics Program (Contract DAAB07-75-C-1346

Coleman, John W.

Graham, Michael R.

DSpace@MIT

II. ELECTRON OPTICSAcademic and Research StaffProf. John G. KingDr. John W. ColemanDr. Edward H. JacobsenGraduate StudentsMichael R. GrahamNorman D. Wittels1. HIGH-RESOLUTION HIGH-CONTRAST ELECTRON OPTICS: FINAL JSEPRESULTS WITH THE FIRST PROTOTYPE AUGER ELECTRONMICROSCOPE (AEM-1)Joint Services Electronics Program (Contract DAAB07-75-C-1346)Michael R. Graham, John W. ColemanIntroductionAEM-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. First results with the AEM-1 werereported in Quarterly Progress Report No. 114 (page 12), and further results werepresented in 1975 at the annual meeting of the Electron Microscopy Society of America.1Final results are presented in this report.Summary of the Earlier ResultsThe AEM-1 was constructed to study problems of high-quality imaging with verywide-angle lenses, independently of the complicated problems associated with theproduction and use of Auger electrons in the system. In AEM-1 the simulated Augerelectrons are emitted in the back direction, collected throughout a wide cone (6-9 °semiangle), reversed in direction by an electrostatic mirror objective, acceleratedthrough up to 20 kV, and transferred optically to the projection system (see Fig. II-1).The earlier results showed conclusively that our mirror objective accelerator systemis a practicable approach to developing wide-angle collection and optical processing forAuger electrons.Final Results with the AEM-lThe following results, not published previously, are final for the AEM-l:1. The mirror objective accelerator system has been investigated in depth as acompound objective lens (as compared with investigation as a projection lens in theearly work) and has been found to be quantitatively and qualitatively within the expectedperformance range.2. The system has a magnification of 30 when coupled to the existing projectionsystem, and this can be extended upward, depending upon revised spacing for the pro-jection elements. JSEPPR No. 117TYPICALTRAJECTORY0=m i-HEM!EPHE I'-AEL F CTT 41 DFRINGELECTRODES7SIMUI ATED' SPECIMENFig. II-1. Geometry of the prototype.(a) (b)Fig. 11-2. (a) 100 mesh/inch stainless-steel woven grid. The weavingof the .001 wire is resolved, and indicates a resolutionof 10-15 Lm in this totally uncorrected image taken withthe mirror objective collecting through a solid cone of~9' semiangle. The emitting coil (simulated Auger source)is 3 mm below the mesh and the image is out of focus.(b) Microcoil thermionic emitter (simulated Auger source).The mirror objective has been focused on the coil, whichis below the mesh, with the mesh still in position.PR No. 117JSEPTJSEP(II. ELECTRON OPTICS)3. Optimal potential ratios have been determined for pairs of electrode voltages. JSEPThese ratios are well within the expected ranges predicted by the earlier computerstudies.4. The energy window inherent in the mirror objective accelerator is ±40 eV atvoltage settings for 300 eV electron input, and ±90 eV at voltage settings for 1000 eVinput. The relatively narrow widths of the windows over the 100-1400 eV input rangehave somewhat lessened the anticipated background illumination problem.5. It has been possible to obtain quality images of the simulated Auger source overthe 100-1400 eV range, although the resolution is a function of energy with the AEM-1system, which by design is uncorrected for aberration and unequipped for downstreamenergy filtering. The optimal voltages on the electrodes throughout this energy rangegive a consistent picture of the optical mode of operation of the compound lensingsystem.6. The attainable resolution of the uncorrected AEM-1 system is of the order oftens of microns. This was the working goal and the theoretical prediction for thisscaled-up (4X) first prototype of the final AEMI (see Fig. II- 2 a).7. The depth of field and focus is such that either the 100 mesh/inch grid((Fig. II-2a)or the electron emitter coil beneath it (Fig. II-2b) can be imaged reproducibly. Theoptimal voltage differentials for changing from imaging one to imaging the other havebeen tabulated.ConclusionsThe next prototype AEMI, which is now under construction, will use fixed-beamexcitation to produce Auger electrons that have energy characteristic of the atoms fromwhich they are emitted. The optical system, scaled-down by a factor of 4, will beessentially the same as that investigated in AEMI-1, but will incorporate the changesindicated by our studies and be fully corrected by means of a foil lens 2 for both chro-matic defects and spherical aberration. The information gained through design, con-struction, and study of AEMI-1 has given us the knowledge to make faster progress thanwe had anticipated, and with the AEAI that is now under construction, we hope to investi-gate silicon in depth profiles with spatial resolutions to a few angstroms.Referencesi. J. W. Coleman and E. H. Jacobsen, "Fixed-Beam-Excitation Auger EM with Simu-lated Specimens: AEM-1," Proc. Thirty-third Annual Mieeting, Electron Micros-copy Society of America, Las Vegas, Nevada, August 11-15, 1975, pp. 118-119.2. N. D. Wittels and E. H. Jacobsen, " The Unipotential Foil Lens," Proc. Thirty-thirdAnnual Meeting, Electron -Microscopy Society of America, Las Vegas, Nevada,August 11-15, 1975, p. 142. JSEPPR No. 117

Electron Optics

http://dspace.mit.edu/bitstream/1721.1/56588/1/RLE_PR_117_II.pdf

Electron Optics

Abstract

Similar works

Full text

Available Versions

DSpace@MIT