Focussing and working distance

The focal point of the electron beam is the point in space below the objective lens where the beam converges into a tight spot, and then begins to diverge again, or spread out. Your image is "in focus" when the focal point of the beam is coincident with the features of interest on your sample surface. If the focal point is above or below the surface of your sample, the image will be out of focus, or blurry.

Focussing Best Practice

Focussing is a skill, and it will require patience and practise to become efficient. Here are a few tips to help you get started:

1. Go to the lowest magnification first, e.g. ×70
2. Move to a region on your sample where you know there should be clear features, such as an edge, scratch, or defect
3. If you can see features but they are blurry, try turning the Coarse focus knob one way or the other and try to get the best image you can initially
4. Once you can see some coarse details, find a region you want to focus on and centre this in the image (double-click), increase your magnification until the feature is blurry again
5. Activate the reduced-area scan, and then use the Fine focus dial to try to improve the image
   1. Just turn the dial one way or the other, with a big turn, and determine if it gets better or worse: if it gets better keep going, if it gets worse, turn the other way!
   2. When the image is improving keep turning the dial in the same way until you get the best image and then keep going until it gets worse again, then back-track and find the best image
6. If needed, increase the magnification more and repeat the process until you cannot improve the image sharpness any more

Always focus at a magnification higher than what you intend to capture. This will ensure you have the sharpest image. For example if you want to capture an image at ×200, focus using the reduced scan at a magnification of ×1000, then reduce the magnification back down to capture the image.

Important Note!

Focus refers to the electron beam. While it has an impact on the quality of image you obtain, it does not change anything on the detectors. Thus your focus is independent of the detector that you are using. Once you are in focus with one detected signal, you are also in focus with all other detectable signals. So changing detector does not require re-focussing.

Working Distance

Working distance (WD) is the distance, usually measured in millimetres, between the objective lens and the focal plane. It is often used to also describe the sample height, or the distance between the sample surface and the objective lens. This can become confusing, but generally we fix the working distance to a minimum value to optimise for resolution, and then focus the beam to this point.

If you are just using the SEM for imaging, there is usually no need to change the working distance after your sample has been mounted. You may just notice that the WD value changes as you change your focus.

On the FlexSEM the minimum WD is 5 mm, and more often will be ~5-6 mm after you have mounted your samples.

If the surface of your sample varies considerably in height, for example large particles on a flat surface, you may find that when you are focussed on one position in the image, other areas may appear blurry. This is because SEM generally has a shallow depth of field at a short working distance. This is simply because when the WD is short, the beam must converge quickly to obtain focus over a short distance, meaning that it also diverges quickly after that focal point.

To improve depth of field, you can increase the working distance by lowering the sample height, down to 10 mm for example. But at a longer working distance you sacrifice both image resolution and signal to noise (as generally you will be moving further away from detectors).

Because the sample is further away from the objective lens at a long WD, the field of view, or the area that you can image at a given magnification, will also increase.

Optimising for SEM vs EDS

The optimal WD conditions are different for SEM and EDS on the FlexSEM:

• SEM imaging - optimal working distance is 5-6 mm
• EDS analysis - optimal working distance is 10 mm

If you want to get the best of the instrument in both modes you will need to keep this optimisation in mind while acquiring different sets of data. However, if you want to capture both SEM and EDS, and your feature sizes do not require the best resolution from the SEM, then optimise for EDS and perform all your work at WD = 10 mm.

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