Role of sample height
Sample height, or working distance (WD), refers to the distance between the bottom of the SEM column and the top of the sample. Within the sample chamber the sample stage can be wound up closer to the end of the column (a short working distance) or dropped down lower (a long working distance).
The shorter the working distance, the smaller the diameter of the beam is at the sample surface. So, when possible, the WD is kept at 10mm or smaller for high resolution imaging. The disadvantage is that focal depth is drastically reduced at small WD. This can be offset by using a smaller objective aperture and putting up with the reduction in electrons that comes with this choice (grainier image).
Depth of field
In many SEMs an external working distance (Z) control is used to either raise or lower the specimen. This value is often mistaken for an accurate WD. However, the true working distance (WD) is measured electronically as the point of focus on the sample surface to the SEM column above. There are three reasons why the value of the external Z control (mechanical control) and the WD provided on the image screen are different.
- The ‘on screen’ value of the WD is only an accurate measurement if the electron beam is focused accurately onto the specimen surface. An under-focused or over-focused image will provide a false WD value as well as a blurry image.
- The value of the external Z and even a true WD from an accurately focused specimen will be different because both measurements may be taken from different points on the specimen holder.
- Specimens that are not uniformly flat will have a different true WD for different topographical features.
The WD impacts on the depth of field and resolution of the SEM image. As the WD is increased the beam divergence angle (alpha) is decreased which provides a greater depth of field. The "trade-off" for an increased WD is that the electron beam must travel a greater distance from the gun and therefore has a larger spot size on the specimen.
|WD||5 to 30mm|
|Depth of field||Shallow ----------------- Deep|
|Resolution||High ----------------------- Low|
Depth of field refers to the zone in which the specimen appears acceptably in focus to the eye. This "range" over which the image appears to be in focus is typically several thousand times greater in an SEM than in the light microscope and results in the almost 3 dimensional appearances of many SEM micrographs.