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 Effectsdielectric

(SiO2)

Cu pads

Cu pad

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

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 ModelCurrent (a)

Magnification (x)

Area (cm2)

Frame Rate (s)

Eodwell time

σbδ(E) initial

K n bin Vo(eV) pi

6.00E-07

2.00E+01

1.00E+02

0.51.88E-

075.00E-

051.20E-

060.5

6.25E+02

0.721.00E-

082.00E+

043.1415

93

η εFrames

6.40E+02

3.2 10

Charge graph σ(t)

δ(E) potential build up

1.24352E-07

0.008284339

5946224.118

δ integrated per frame

8.44513E-08

1.28922E-07

1.54449E-07

1.6991E-07

1.7996E-07

1.8691E-07

1.91984E-07

1.958E-07

1.99E-07

2.01E-07

0 4.16E-072.69E-

07

1.2E-14 4.16E-072.69E-

07

3.6E-14 4.155E-072.68E-

07

7.2E-14 4.155E-072.68E-

07

1.2E-13 4.155E-072.68E-

07

1.8E-13 4.155E-072.68E-

07

2.5E-13 4.155E-072.68E-

07

3.4E-13 4.155E-072.68E-

07

4.4E-13 4.155E-072.68E-

07

5.4E-13 4.155E-072.68E-

07

6.7E-13 4.155E-072.68E-

07

8E-13 4.155E-072.68E-

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?