Working distance in SEM and its effects on image contrast

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Working Distance in SEM and its Effects on Image Contrast

In Scanning Electron Microscopy (SEM), the working distance is the distance between specimen and lower pole piece of scanning electron microscope (SEM). The SEM contrast strongly depends on the working distance. The lower pole piece is the objective lens, which is the final lens of the electron column. For instance, at short working distance, the major contribution to the signal collected using the lateral detector is that of SE3 electrons, which are backscattered by the pole piece as shown in Figure 4843a. The corresponding contrast is a Z contrast.

Source of Secondary Electrons in SEM

Figure 4843a. Source of Secondary Electrons in SEM. BS1 and BS2 are backscattering electrons; SE1 - SE4 are secondary electrons.

The resolution and depth of field in a SEM are influenced by the working distance (WD). The depth of field (DOF) depends on both the working distance and the electron beam convergence angle (α). The approximate equation for DOF is given by:

-------------------------------------------------------- [4843a]

Where:

is the depth of field.
is the semi-angle of the electron beam convergence

Figure 4843b shows the DOF dependence on working distance for different beam convergence angles.

DOF dependence on working distance for different beam convergence angles

Figure 4843b. DOF dependence on working distance for different beam convergence angles (α).

The working distance can be selected depending on the application, the type of sample, and the resolution required, but here are the typical ranges:

Short Working Distance (1–10 mm):
- Advantages: Higher resolution and better signal collection from secondary and backscattered electrons.
- Common Uses: High-resolution imaging (especially when using high magnifications).
- Limitations: Lower depth of field, meaning less of the sample will be in focus at once.

Medium Working Distance (10–20 mm):
- Advantages: Provides a balance between resolution and depth of field.
- Common Uses: General imaging at moderate magnifications, and energy dispersive X-ray spectroscopy (EDS), which requires more space for the X-ray detector.

Long Working Distance (>20 mm):
- Advantages: Higher depth of field, which allows more of the sample to be in focus at the same time.
- Common Uses: Imaging large, uneven samples or when lower resolution is acceptable. It's also used when performing other analytical techniques (e.g., EDS) that need more space for the detectors.

Therefore, choosing the working distance depends on the trade-off between resolution and depth of field. A shorter working distance enhances resolution but reduces the depth of field, while a longer working distance increases the depth of field but decreases resolution.

Practical Electron Microscopy and Database

An Online Book, Second Edition by Dr. Yougui Liao (2006)

Working Distance in SEM and its Effects on Image Contrast