Set up Low Dose Mode                

zhy, jpg, &vdb 2010

Low dose mode uses 3 states  to store microscope settings for different functions:

Search- for the selection of areas to image

Focus- for focusing away from the imaging area

Exposure- for actual imaging

1. Start Low Dose Mode

a) click on Low Dose (yellow) on the Low Dose OCX

b) reload saved settings if desired

        1. expand Low Dose flap out

        2. click Settings page

        3. click Load and find a file with saved settings (if present)

2. Set up magnification and spot sizes for each of the 3 states: Search, Focus and Exposure:

a) Search state settings:

1. Search state is normally used with low SA magnification, ~2000 - 4000x

2. use a large spot size number (i.e. number 8 or 9=weaker beam) to prevent dosing

3. large under focus = better contrast (i.e. -50 um) to enhance contrast for visibility to find best areas of interest.

b) Exposure state settings:

1. Exposure state is the settings (magnification, spot size) used to collect data from an area of interest.

2. In Exposure, the beam should always be expanded until it is in parallel range

3. calculate desired dose, expand the beam & set the exposure time to get this dose

4. For film, the beam must be spread to at least 2.4 microns for full coverage

c) Focus state settings:

1. set focusing distance 3-5 um depending on grid and sample parametes

2. set focus angle (0-360deg)- 110deg is close to the Titan’s tilt axis

3. for TEM low dose, the mag. should be equal to or greater than the data collection mag. A mag. of 96,000 works well. For the FEI tomography package, it is recommended that one use the same magnification for both Exposure and Focus.

4. use a smaller spot size (4-6) and brighter beam (condensed) for focus

3. Align any Exposure and Search image shift

        a) find and center a feature in Exposure state using stage shift (joy stick) x and y

        b) switch to the Search state

        c) find the same feature

1. if it is no longer centered, expand flap out of Low Dose OCX,

2. under Options, click search shift box

3. Adjust MFs to center the feature in the Search State--now the multifunction knobs (MFs) are linked to image shift

4. Check Beam shift and Normalize Lens Currents

a) Cycle through the three states (in the order of 1-Search, 2-Focus 3-Exposure) in the same fashion twice. Center beam in each state

b) For Search or Focus states if the expanded beam is not centered, use beam trackball or MFs (defaulted to beam shift) to center the beam.

c) For Exposure state, use Direct Alignment beam shift to center the beam (or use Direct Alignment/ tomo beam shift when running FEI Tomo).

5. To Save good settings for next session

a) expand the flapout window of Low Dose

b) under settings, click on save as …

c) save the file in a folder on the Z drive where you can find it

d) this saves all basic settings for all 3 states

e) Next session, skip most of set up by loading the settings file

f) You will still need to check beam centering and image shifts