Hitachi SU8230 Cold Field Emission SEM Yale West Campus Materials - PowerPoint PPT Presentation

Hitachi SU8230 Cold Field Emission SEM Yale West Campus Materials Characterization Core (MCC) ywcmatsci.yale.edu

Core Policies • DO NOT let other people use the facility under your account. • DO NOT try to fix parts or software issues by yourself! • DO NOT surf web using instrument computer! • Follow checklist and SOP! DO NOT explore program! • Facility usage time at least twice a month, OR receive training again (two practice sessions within one week). • No trainings on monthly users Materials Characterization Core (MCC) Yale West Campus 2/20 ywcmatsci.yale.edu

SEM: Basic Theory Electron source Condensor lens 1 Objective aperture Condensor lens 2 Deflection coils Objective lens Sample Materials Characterization Core (MCC) Yale West Campus 3/20 ywcmatsci.yale.edu

SEM: Electron Sources Tungsten wire Cold Field Emission (CFE) LaB 6 single crystal Field Assisted Thermionic Source - Schottky Extraction Voltage Acc Voltage Brightness: 10 5 A/cm 2 sr Brightness: 1000 x Brightness: 10 x Brightness: 500 x Beam size = 30 - 50 Å Beam size = 50 - 100 kÅ Beam size = 50- 100 kÅ Beam size = 100 - 250 Å Operation temperature: 300 K Operation temperature: 3000 K Operation temperature: 2500 K Operation temperature: 2500 K Vacuum: 10 -11 Torr Vacuum: 10 -5 Torr Vacuum: 10 -7 Torr Vacuum: 10 -9 Torr Lifetime: > 10000 hrs Lifetime: 300 hrs Lifetime: 500 - 1000 hrs Lifetime: > 4000 hrs Materials Characterization Core (MCC) Yale West Campus 4/20 ywcmatsci.yale.edu

Demagnification Optics • Demagnification  image resolution • Resolution  image intensity Beam size at condenser lens focus plane 𝑒 B = 𝑒 G 𝑞 1 𝑟 1 d G : Beam size exiting the gun p 1 : Object distance of condenser lens q 1 : Image distance of condenser lens Beam size on specimen surface at objective lens focus plane 𝑒 p = 𝑒 B 𝑞 2 𝑋𝐸 p 2 : Object distance of objective lens q 2 : Image distance of objective lens WD : Working Distance between the bottom of the objective lens and sample surface Materials Characterization Core (MCC) Yale West Campus 5/20 ywcmatsci.yale.edu

Accelerating voltage ( V acc )  Increasing accelerating voltage   less spherical aberration  smaller probe diameter and better resolution  Increase beam penetration  obscure surface detail  Increase the probe current at the specimen. A minimum probe current is necessary to obtain an image with good contrast and a high signal to noise ratio.  Potentially increase charge-up and damage in specimens that are non-conductive and beam sensitive. SEM images of vanadium oxide nanotubes at different acc voltages Penetration depth V acc Image courtesy http://www.microscopy.ethz.ch/ Materials Characterization Core (MCC) Yale West Campus 6/20 ywcmatsci.yale.edu

Factors Affecting SE Emission: Working Distance (WD) Working Distance : the distance between the bottom of the objective lens and the specimen Increasing WD  • increased depth of focus • Increased probe size  lower resolution • increased effects of stray magnetic fields  lower resolution • increased aberrations due to the need for a weaker lens to focus. 200 um aperture and 10 mm WD. 200 um aperture and 38 mm WD Materials Characterization Core (MCC) Yale West Campus 7/20 ywcmatsci.yale.edu

SEM: Electron-Specimen Interactions Secondary electrons (SE < 50 eV)   Electron beam Topographical information Back-scattered electrons (BSE)   AE (1-5 nm) SE (5 – 50 nm) Composition (atomic number) and topographical information EDX BSE (~300 nm)  Characteristic X-ray (EDX)  Composition (1-3 µm) information (Energy Dispersive X-ray CL X-ray Spectroscopy) Continuous X-ray (1-3 µm) Auger electrons (AE)   Surface sensitive composition information  Cathodoluminescence (CL)  Electric states information  Fluorescence  Phosphorescence  Continuous X-ray (Bremsstrahlung)  Insulator charging Imaging resolution  Interaction volume Sample Materials Characterization Core (MCC) Yale West Campus 8/20 ywcmatsci.yale.edu

Schematic Electron Energy Spectrum SE forms a large low-energy  peak < 50 eV  BSE Shallow depth of SE production  topography information  Small interaction volume  high imaging Counts Elastic resolution, comparable to reflection e-beam size Auger Electron (AE)  AE produces relatively small peaks on the BSE distribution 50 eV 2000 eV Kinetic Energy (eV) Goldstein et al. 1981 Materials Characterization Core (MCC) Yale West Campus 9/20 ywcmatsci.yale.edu

Lens Aberrations: Astigmatism  The SEM electromagnetic lenses can not be machined to perfect symmetry.  A lack of symmetry  an oblong beam: a disk of minimum confusion  stronger focusing plane  narrower beam diameter  weaker focusing plane  wider diameter  Astigmatism correction  Apply current differentially to stigmator coils  circular beam Materials Characterization Core (MCC) Yale West Campus 10/20 ywcmatsci.yale.edu

SE Detector: Everhart-Thornley (E-T) Detector  E-T detector: low-secondary 1-2 kV electrons are attracted by +200 V on grid and accelerated onto scintillator by +10 kV bias; Photocathode Faraday Cage  The light produced by -50 to +200 V scintillator (phosphor surface) passes along light pipe to E-beam external photomultiplier (PM) Output (0.5 – 30 kV) which converts light to electric signal. Optical  Dynodes Back scattered electrons also waveguide SE<50 eV detected but less efficiently Scintillator because they have higher Electron Multiplier +10kV BSE energy and are not significantly deflected by grid Sample potential. Materials Characterization Core (MCC) Yale West Campus 11/20 ywcmatsci.yale.edu

Schematic of SU8200: Optics and detection system • SE detectors: • SE(L): SE lower detector • SE(U): SE upper detector • HA(T): HA-BSE top detector • Control/filtering electrode • Conversion electrode • Hi-Pass Top Filter Materials Characterization Core (MCC) Yale West Campus 12/20 ywcmatsci.yale.edu

SE(L) in normal modes ( V acc : 0.5 ～ 30 kV) Sample: photocatalyst V acc : 3 kV Signal : SE(L) Sample courtesy of : Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato SE + BSE signal • SE(L) (secondary electron Lower detector) • Signal amount is relatively low, but will increase when WD is longer. • Highly topographical information  shadowing effect • Less sensitive to specimen charge-up • Signal of the Lower detector is less sensitive to charging artifacts • Low resolution comparing to upper detector Materials Characterization Core (MCC) Yale West Campus 13/20 ywcmatsci.yale.edu

SE(U) in normal modes ( V acc : 0.5 ～ 30 kV) Sample: photocatalyst V acc : 3 kV Signal : SE(U) Sample courtesy of : Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato • SE(U) (secondary electrons detected with the Upper detector through the objective lens) • Large signal amount, high detection efficiency • High resolution at the topmost surface information • High edge contrast • Sensitive to specimen charge-up • BSE not detected. Materials Characterization Core (MCC) Yale West Campus 14/20 ywcmatsci.yale.edu

LA-BSE in normal modes ( V acc : 0.5 ～ 30 kV) Sample: photocatalyst V acc : 3 kV Signal : LA-BSE(U) Sample courtesy of : Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato • LA-BSE  SE at the control electrode and detected with the Upper detector. • Amount of SE controlled by variable negative electrode voltage. • Compositional + Topographic information Mixture of SE and LA-BSE image • Less sensitive to specimen charging-up Materials Characterization Core (MCC) Yale West Campus 15/20 ywcmatsci.yale.edu

HA-BSE in normal modes ( V acc : 0.5 ～ 30 kV) Mixed particles of electrode BaCO 3 /TiO 2 V acc : 1.5 kV Signal: HA-BSE • HA-BSE (High-Angle Backscattered Electron) • HA-BSE  SE at the conversion electrode and detected with Top detector HA(T). • Small signal amount • Rich Compositional information • Less topographic information • Less sensitive to specimen charge-up Materials Characterization Core (MCC) Yale West Campus 16/20 ywcmatsci.yale.edu

Beam Deceleration (Landing voltage 10 V ～ 2 kV) Al electrolytic capacitor y electron V acc : 500 V Magnification: 100kx • A negative voltage (deceleration voltage, V rtd up to 3.5 kV) applied to the specimen, thereby slowing down the primary electron beam to the desired landing energy. • Landing voltage (10 V – 2 kV): V lnd = V acc – V rtd ; V rtd : Deceleration voltage Courtesy of St. Jude Medical, CRMD-U.S.A. • Resolution improved in deceleration mode V acc : 500 V Magnification: 100kx Materials Characterization Core (MCC) Yale West Campus 17/20 ywcmatsci.yale.edu

Scanning Transmission Electron Microscope (STEM) Mode SE detector Photon guide STEM detector BF-STEM image Internal DF-STEM image surface information information • A STEM image providing internal specimen Specimen: Carbon nanotubes V acc : 30 kV Magnification : 250kx information can be obtained simultaneously with the secondary electron image. • The optional Bright Field STEM Aperture Unit is often utilized to generate enhanced contrast differentiation on materials of similar density. Materials Characterization Core (MCC) Yale West Campus 18/20 ywcmatsci.yale.edu