tatem elroy presentation by KQp8n3l0

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									   Modeling of Dynamic Secondary
Electron Contrasts in SEM specimens

                By: Elroy Tatem

                     Advisors:
                Dr. Cherrice Traver
           Dr. Bradley Thiel (U Albany)




                              College of Nanoscale Science & Engineering
                        What is an SEM

   Electromagnetic fields act as
    lenses which direct and focus
    a beam of electrons

   These electrons excite the
    surface of the sample and
    cause it to emit electrons

   The electrons are detected by
    built in circuitry and sent to
    the monitor



                                     College of Nanoscale Science & Engineering
              What is an SEM (continued)

   Specimens have to be specially prepared. Specimens must be
    coated in a conductive substance, which makes characterization of
    insulators, semiconductors, and living samples difficult

   Specimens can be viewed without this preparation in newer SEMs
    and ESEMs, which use low vacuum and ion gas to counteract the
    effects of charging




                                       College of Nanoscale Science & Engineering
                          Project Goals

   Improve current circuit model for
    charging in poorly conducting
    specimens in an SEM

   Quantify the effects of charging in
    poorly conducting specimens in an
    SEM

   Model the charging phenomenon in a
    Microsoft™ EXCEL® program.




                                          College of Nanoscale Science & Engineering
                     Charging Effects
   “Artifacts”

    • Show up as unwanted contrasts in the image produced by the
      SEM

    • Can be random or have a pattern

    • Sometimes repeatable

    • Caused by excessive negative charge build up on a sample.




                                        College of Nanoscale Science & Engineering
                              Charging Effects


                                                           Secondary emission energy
                                                                         vs.
                                                               Initial beam energy




Sample/ Surface interaction




                                        College of Nanoscale Science & Engineering
                        Charging Effects
dielectric
 (SiO2)




                    Cu pad

                                Cu pads




       close-up
       showing
     SiO2 surface
       structure
                                    College of Nanoscale Science & Engineering
                        Charge Density
   Charge density as a function of time



                              is comparable to F




                             F


                             F

                                           College of Nanoscale Science & Engineering
                          Circuit Model
   The first draft was made such that it would retain its RC properties

   The output should be dampened depending on how much charge
    has collected on the sample surface




                                         College of Nanoscale Science & Engineering
                          Circuit Model
   RC Circuit
                         Common emitter amplifier



                                           Signal multiplier amplifier




   Constant multiplier




                                       College of Nanoscale Science & Engineering
                         Circuit Model
   The second circuit discarded the MOSFET multiplier as it would
    have required a voltage- current transformation

   The second multipliers are controlled by a potentiometer which
    simulates the ion flux




                                       College of Nanoscale Science & Engineering
                            Excel Program
   The program is able to model the phenomenon by allowing the user
    to input specific microscope and specimen parameters

   Inputs
     • Current
     • Magnification
     • Frame Rate
     • Dwell Time
     • Area
     • Initial beam intensity
     • Resistivity/permittivity (bulk)




                                         College of Nanoscale Science & Engineering
                           Excel Program
   The program returns valuable information to the user

   Outputs
     • ∑σi(t) - Charge surface density per unit of time

     • δ(E) - Ratio of input current to output current (ISE/IBE)

     • ∫δ(E) – Area under charging curve




                                            College of Nanoscale Science & Engineering
                   Results: Circuit Model
   The potentiometer models the way that the newer
    ESEMs use ions to affect the charging that takes place.




                                                 Red = RC model output
                                                 Orange = Controlled
                                                  charge output




                                        College of Nanoscale Science & Engineering
                                 Results: Excel Model
          Magnifi                 Frame
Curren                 Area                          dwell             δ(E)
          cation                  Rate      Eo                σb                  K         n           bin        Vo(eV)   pi
t (a)                  (cm2)                         time              initial
          (x)                     (s)                                                                         2.69E-
                                                                                        0       4.16E-07          07
6.00E-     2.00E+      1.00E+               1.88E-   5.00E-   1.20E-              6.25E+                 1.00E-    2.00E+   3.1415
                                      0.5                                   0.5                  0.72
    07         01          02                   07       05       06                  02                      2.69E- 04
                                                                                                             08                 93
                                                                                  1.2E-14       4.16E-07          07
                                                                                                              2.68E-
                                                                                  3.6E-14   4.155E-07             07
                                  Frame
η         ε
                                  s                                                                           2.68E-
                                                                                  7.2E-14   4.155E-07             07
6.40E+
                 3.2                   10
    02                                                                                                        2.68E-
                                                                                  1.2E-13   4.155E-07             07
                                                                                                              2.68E-
                                                                                  1.8E-13   4.155E-07             07

                                                                                                              2.68E-
Charge                                                                            2.5E-13   4.155E-07             07
graph     δ(E)         potential build up                                                                     2.68E-
σ(t)                                                                              3.4E-13   4.155E-07             07
1.2435     0.0082       59462                                                                                 2.68E-
 2E-07      84339      24.118                                                     4.4E-13   4.155E-07             07
                                                                                                              2.68E-
                                                                                  5.4E-13   4.155E-07             07
δ integrated per                                                                                              2.68E-
frame                                                                             6.7E-13   4.155E-07             07
8.4451     1.2892      1.5444     1.6991    1.7996   1.8691   1.9198   1.958E     1.99E-    2.01E-            2.68E-
 3E-08      2E-07       9E-07       E-07      E-07     E-07    4E-07       -07        07
                                                                                   8E-13        07
                                                                                            4.155E-07             07

                                                                       College of Nanoscale Science & Engineering
                      Results: Excel Program
        The curve is extended between the charging time and just before the
         discharging takes place to emphasize the charging curve




       The value of ∫δ(E) reaches a maximum value which restricts any
        excess charging on the sample




                                            College of Nanoscale Science & Engineering
                            Future Plans
   Improve model
     • Replace the potentiometer with an equivalent circuit
     • Calculate specific values for inputs
     • Test inputs against

   Make program more useable
     • Cosmetic additions

   Other platforms




                                          College of Nanoscale Science & Engineering
                           References
   SEM Movie – Oxford instruments
   Transistor Image – CNSE Metrology Dept
   Charge Density Pictures – Charging Effects in Scanning Electron
    Microscopy – Shaffner
   Excel - Microsoft Corporation
   Multisim - Electronics Workbench Corporation.




                                       College of Nanoscale Science & Engineering
Questions?




      College of Nanoscale Science & Engineering

								
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