An Aberration Corrected Photoemission Electron Microscope at the Advanced Light Source Advanced Light Source Experimental Systems Group J.Feng1, A.A.MacDowell1, R.
Download ReportTranscript An Aberration Corrected Photoemission Electron Microscope at the Advanced Light Source Advanced Light Source Experimental Systems Group J.Feng1, A.A.MacDowell1, R.
An Aberration Corrected Photoemission Electron Microscope at the Advanced Light Source Advanced Light Source Experimental Systems Group J.Feng1, A.A.MacDowell1, R. Duarte1, A.Doran1, E.Forest2, N. Kelez1, M.Marcus1, D.Munson1, H.A.Padmore1, K.Petermann1, S. Raoux3, D. Robin1, A. Scholl1, R. Schlueter1, P.Schmid1,J. Stöhr4, W.Wan1, D.H. Wei5 and Y. Wu6 1)Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA2)High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 3050810, Japan 3)IBM, Almaden Research Center, 650 Harry Road, San Jose, CA 95120 USA 4)Stanford Synchrotron Radiation Laboratory, P.O.Box 20450, Stanford, CA 94309, USA 5) SRRC, No.1 R &D Rd. VI, Hsinchu 300, Taiwan 6)Department of Physics, Duke University, Durham, NC 27708, USA Abstract A new ultrahigh-resolution photoemission electron microscope called PEEM3 is being developed at the Advanced Light Source. An electron mirror combined with a sophisticated magnetic beam separator is used to provide simultaneous correction of spherical and chromatic aberrations. PEEM3 electron mirror has four rotationally symmetric electrons and gives three free knobs to adjust the focal length, the chromatic and the spherical aberrations so that a wide aberration region can be covered for all the operation modes of objective lens. PEEM3 magnetic separator has double mirror symmetry configuration and images its entrance plane 1:1 in its exit plane. A further enormous advantage of the aberration correction is the increase of electron transmission. The goal of the PEEM3 project is to achieve the highest possible transmission of the system at resolutions comparable to our present PEEM2 system and to enable significantly higher resolution, albeit at the sacrifice of intensity. We have left open the possibility to add an energy filter at a later date, if it becomes necessary driven by scientific need to improve the resolution further. The instrument will be installed on an elliptically polarized undulator beamline and will be used for the study of complex materials at high spatial and spectral resolution. PEEM3 Layout PEEM3 Concept Objective lens K-B X-ray mirror Magnetic Beam separator Electric dodecapole Transfer optics Transfer lens Table1. Specification of PEEM3 separator Projector / Detector 5 axis sample support heatablecoolable 10-10 torr Deflector Separator cooling water feedthrough Size of magnet 28cm Size of gap 7mm Width of groove 3mm, 3mm, 3mm Dispersion at 45 degree 0 Mirror reflection 22.50, 450 Total bend angle 900 Current in coils 72.39A, -144.78A, 72.39A Electron path Bending electron energy 20KeV Mirror Field 259.6Gauss Magnetic coil In-vacuum vibration isolated optical table CCD Sample Immersion Field lens lens Electron Mirror Deflector Intermediate lens Apertures Electric-magnetic dodecapole Projector lens Imaging CCD detector Electron tetrode mirror Projector/Detector Projector lens CCD Diagnostic CCD detector Fig.2 Mechanical 3D model of PEEM3 layout Fig.1 Concept of a X-ray photoemission electron microscope using electron mirror corrector at the ALS PEEM3 microscope consists of objective lens, electric-magnetic dodecapole, magnetic beam separator, electron mirror, transfer and projector lens. Variable beam sizes on sample from 3micron to 50 micron are carried out by a pair of bendable K-B mirror. Additionally, a UV-lamps and a laser system will be mounted to the sample chamber. The critical components of PEEM3 are the electron mirror aberration corrector and aberration-free magnetic beam separator. Chromatic aberration X -84mm Y -306mm Spherical Aberration X 71mm X -362mm Y -374mm Y 5900mm The beam separator is of so-called double mirror symmetry for each quadrant of the magnet to cancel all the second-order geometric aberrations. The imaging property of the beam separator is equivalent to that of a telescopic four round lens. The object side focal plane of the first lens is transferred with unit magnification into the image side focal plane of the fourth lens. The specification of the separator is given in table 1. PEEM3 Performance PEEM3 Endstation PEEM2 –PEEM3 comparison PEEM3 VLS monchromator PEEM2 PEEM3 microscope Sample transfer Optics Electrostatic lens movable pinhole, alignment PEEM Corrector Octopole Electromagnetic dodacapole Resolution 20nm Beamline An elliptically polarized undulator (EPU) at the straight sector 11 of the ALS will be used to produced in-plan linear, perpendicular linear, left and right handed circularly polarized radiation with continuous change of ellipticity. A variable line space (VLS) plane grating monochromator beamline will provide soft x-ray in the spectral range from 100eV to 1500eV. Electrostatic lens Electrostatic mirror Magnetic separator Diagnostic last image Transmission @50nm Fig.3 PEEM3 endstation on the ALS 11.0.1 section floor. PEEM3 microscope will be installed to a soft X-ray EPU beamline. A bendable K-B mirror will provide 3 to 50 micron spot size on sample. PEEM3 Relative Flux density 5% Bending 7.3.1.1 1 Fig. 4 Spherical and chromatic aberration region covered by PEEM3 electrode mirror and the values required to correct the aberrations of the objective lens for different object potentials and working distance. 5nm >90% EPU 11.0.2 >1000 Fig.5. Comparison of resolution versus transmission of PEEM2 and PEEM3. The acceleration potential is 20kV and the working distance is 2mm. In our model to determine the resolution of PEEM3, the secondary electron distribution is used. We create a statistical ensemble of electrons with initial energy and angle spread and track the electron beam distribution weighted with the probability anywhere in the system. The resolution is defined as 68% in intensity of the point spread function. The diffraction effect is calculated for each energy electron and summed up incoherently to yield diffraction Airy pattern. Operating at 20kV and 2mm working distance, the point resolution for 100% transmission reaches 50nm with the mirror corrector, a significant reduction from that of 440nm without correction. The best resolution can be achieved is 5nm at 2% transmission.