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College of Nanoscale Science & Engineering By: Elroy Tatem Advisors: Dr. Cherrice Traver Dr. Bradley Thiel (U Albany) Modeling of Dynamic Secondary Electron Contrasts in SEM specimens
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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
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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
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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.
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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.
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College of Nanoscale Science & Engineering Charging Effects Sample/ Surface interaction Secondary emission energy vs. Initial beam energy
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College of Nanoscale Science & Engineering Charging Effects dielectric (SiO 2 ) Cu pads Cu pad close-up showing SiO 2 surface structure
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College of Nanoscale Science & Engineering Charge Density Charge density as a function of time is comparable to F
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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
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College of Nanoscale Science & Engineering Circuit Model RC Circuit Constant multiplier Common emitter amplifier Signal multiplier amplifier
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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
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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)
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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
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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
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College of Nanoscale Science & Engineering Results: Excel Model Curren t (a) Magnifi cation (x) Area (cm 2 ) Frame Rate (s) EoEo dwell time σbσb δ(E) initial KnbinVo(eV)pi 6.00E- 07 2.00E+ 01 1.00E+ 02 0.5 1.88E- 07 5.00E- 05 1.20E- 06 0.5 6.25E+ 02 0.72 1.00E- 08 2.00E+ 04 3.1415 93 ηε Frame s 6.40E+ 02 3.210 Charge graph σ(t) δ(E)potential build up 1.2435 2E-07 0.0082 84339 59462 24.118 δ integrated per frame 8.4451 3E-08 1.2892 2E-07 1.5444 9E-07 1.6991 E-07 1.7996 E-07 1.8691 E-07 1.9198 4E-07 1.958E -07 1.99E- 07 2.01E- 07 04.16E-07 2.69E- 07 1.2E-144.16E-07 2.69E- 07 3.6E-144.155E-07 2.68E- 07 7.2E-144.155E-07 2.68E- 07 1.2E-134.155E-07 2.68E- 07 1.8E-134.155E-07 2.68E- 07 2.5E-134.155E-07 2.68E- 07 3.4E-134.155E-07 2.68E- 07 4.4E-134.155E-07 2.68E- 07 5.4E-134.155E-07 2.68E- 07 6.7E-134.155E-07 2.68E- 07 8E-134.155E-07 2.68E- 07
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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
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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
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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.
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College of Nanoscale Science & Engineering Questions?
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