Conventional SEM

Specimen at high vacuum – requires sample fixation and dehydration or freezing.

Charging is minimized by coating sample with metal or carbon or lowering the operating kV.

Variable Pressure Scanning Electron Microscope

-Vacuum in the sample chamber can range from high vacuum (< 10-6 Pascals) up to 3,000 Pa.

-Environmental- 3,000 Pa/ Variable – 750 Pa

- Gas in the sample chamber allows uncoated and unfixed samples to be imaged.

-Detectors used at higher pressures are backscatter or special secondary detectors.

- Moisture on the sample can be controlled by cooling/heating stage and water injection system.

LV SEM – High Vacuum Mode

LV SEM - Low Vacuum Mode

SEMElectron/Specimen Interactions

When the electron beam strikes a sample, both photon and electron signals are emitted.

Incident Beam

Specimen

X-rays

- composition info

Auger electrons- Surface sensitive compositional

Backscattered electrons- Atomic number and topographical

Cathodoluminescence

- Electrical

Secondary electrons

- Topographical

Specimen CurrentElectrical

Secondary Electron Detector

Backscatter electron detector

Signal Detection in Variable Pressure Mode

Charging is minimized

Peltier stage

Heats to 50 CCools to - 25 C

Control of Water vapor and temperature

Applications

Live centipede

Bacteria on rock

Fresh moss withliquid water

Pattern produced in silica gel

Live Drosophila larva