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

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 Sample/ Surface interaction Secondary emission energy vs. Initial beam energy

College of Nanoscale Science & Engineering Charging Effects dielectric (SiO 2 ) Cu pads Cu pad close-up showing SiO 2 surface structure

College of Nanoscale Science & Engineering Charge Density  Charge density as a function of time is comparable to 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  Constant multiplier  Common emitter amplifier  Signal multiplier amplifier

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 Curren t (a) Magnifi cation (x) Area (cm 2 ) Frame Rate (s) EoEo dwell time σbσb δ(E) initial KnbinVo(eV)pi 6.00E E E E E E E E E ηε Frame s 6.40E Charge graph σ(t) δ(E)potential build up E δ integrated per frame E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E- 07 8E E E- 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?