Faraday Cage

Introduction

The interaction between the specimen and electrons can cause damage in the structure under investigation. For this reason it is important to measure the electron dose. To measure the electron dose a Faraday cage is installed in SA plane of the microscope. This SA Faraday cage can be used over the whole SA magnification range to easily and accurately determine the electron dose per surface area. Under proper calibration and with the help of the exposure meter and the CCD camera, the electron dose can be measured in any magnification range.

Alignment and Operation

The faraday cage is mounted in an isolated SA diaphragm holder (marked #1), together with three standard SA apertures ( #2: 50micron; #3: 200micron; #4:800micron ). The aperture on the Faraday cage is of equal size as the #2 SA aperture (50micron). Measurement with the Faraday cage is done as follows:

1. Remove the black plastic cover on the SA aperture selector by sliding it out.
   
2. Unplug the grounding adopter on the Faraday cage connector.
   
3. Plug in and turn on the Keithley ammeter, switch the Keithley ammeter to automatic mode.
   
4. Move SA apterure out of the beam and set the system for given application. Skip to 8 if the Faraday cage is aligned.
   
5. Focus the electron beam with the intensity button (C2) and center the beam (Shift-X,Y).
   
6. Insert the SA apterture, select #2 and center the aperture to see the focused beam.
   
7. Switch to Faraday cage aperture (#1) and center the Faraday cage with the X and Y movement (Maximum readout of Keithley ammeter).
   
8. If it is desire to know the electron dose at any particular moment bring in the Faraday cage and read the Keilthley ammeter. In this case ensure that the SA aperure is homogeneously illuminated.
   
9. To convert the meter readout to e/A2, use the formula given in the following.

The Faraday cage measures the total electron current through this aperture. If the illumination of the SA aperture is homegeneous and covers the whole aperture, the electron dose is simply the total current divided by the total specimen surface that is imaged onto this aperture. The magnification of the specimen onto the SA Faraday cage plane is 36x. The surface of the specimen (A) that is imaged into the SA aperture is 1.515 x 106 nm2.

The resulting electron dose E.D. for 50micron Faraday aperture and TWIN lens is:

ED = 41.20 I (e/A2sec)

where I is the Faraday current measured in nanoampere (nA). For accurate measurements the following condition should be satisfied:

1. The SA-Faraday cage is homegenously illuminated, that is a circle of more than 15cm diameter at the main flurescent screen is illuminated at the maganifications of 60kx or lower.
2. The electron beam should not extend beyond the SA Faraday aperture diaphragm (dismeter 3mm), otherwise the Keithley ammeter will pick up the leaking current from other parts of the SA holder. This means that the fluorescent screen should not be completely illuminated at the maganification of 3000x or lower.

Calibration for Exposure Meter and CCD Camera

The measurement of electron dose with Faraday cage is not always handy. The maganification range and illumination area are also limited. The exposure meter and CCD camera can be used as an alternative means to measure electron dose. To convert exposure meter reading and CCD camera counts to electron dose, the following calibration results are given.

CCD counts = 10.5 x electron

To see the raw data and calibration results.

The Faraday current can be calculated from the exposure meter reading by an empirical equation:

Faraday Current (nA) = 2.2 x Emulsion / Expo.

That is the Faraday current in nanoampere equals 2.2 times the emulsion setting for the camera and divided by the exposure meter reading from the main screen. The raw data is listed in the table.