The working distance in the SEM is the distance at which the beam is focussed, normally the distance from the final pole piece of the lens to the sample when the image is in focus. It is variable by moving the stage up and down (Z-height) and by focussing the specimen at that height. The actual value is operator and instrument dependant.
The best image resolution will be found at the shortest working distances, depending upon your detector efficiency. In my SEM that is 1.5mm witha semi-in-lens detector, however the disadvantage of a short working distance is a small depth of focus. If you increase the working distance - my maximum is 48mm you obtain the greatest depth of focus, but the resolution will be poorer and the signal strength weaker - this can be compensated for by increasing spot size.
The efficiency of certain detectors will be working distance dependent. For example if the working distance is reduced then the sample can rise above some out-lens detectors such as the standard Everhart-Thornley and the collection efficiency will deteriorate. Conversely for some semi-in-lens detectors as you increase the working distance the efficiency will drop off.
There is also the apparent illumintion to take into account with ET detectors with the sample near their minimum efficiency working distance. It can make very topographical samples appear with high contrast due to 'shadowing'. By increasing the working distance slightly it has the effect of reducing the shadowing and giving a more even grey-tone appearance to a topographic sample, though you lose some resolution.
The minimum obtainable magnification is also dependant on working distance. At long working distances it is possible to reach lower maginfications, conversely at short working distances the minimum magnification is increased.
Scan rotation will vary according to working distance due to the spiral of the beam so if your sample is aligned E-W at a short working distance you will notice that the image will rotate slightly at longer working distances.
Correct setting of the working distance is critical for EDX applications as the X-ray detecor has to be at the focal point for maximum collection. This will be determined by the geometry of your final lens and the takeoff angle of the detector - I have EDX working distances of 15mm and 20mm on two different SEMs for example. As you move away from the critical working distance for the detector the X-ray counts will reduce.
In SEM, it can be 4 to 10mm depending on the function. Lower the distance and higher the spot size will be getting more of counts for EDS analysis. For imaging 9-10mm is preferred
Hello, Abbas.
SEM images are used for determining the morphology of materials and some times with using high magnification, in measuring the materials sizes (in about...). However, TEM is a microscope with different mechanism and used for determining the exact sizes of materials and some times morphology of them when they are not present on the surface of the analyzed sample.
The working distance in the SEM is the distance at which the beam is focussed, normally the distance from the final pole piece of the lens to the sample when the image is in focus. It is variable by moving the stage up and down (Z-height) and by focussing the specimen at that height. The actual value is operator and instrument dependant.
The best image resolution will be found at the shortest working distances, depending upon your detector efficiency. In my SEM that is 1.5mm witha semi-in-lens detector, however the disadvantage of a short working distance is a small depth of focus. If you increase the working distance - my maximum is 48mm you obtain the greatest depth of focus, but the resolution will be poorer and the signal strength weaker - this can be compensated for by increasing spot size.
The efficiency of certain detectors will be working distance dependent. For example if the working distance is reduced then the sample can rise above some out-lens detectors such as the standard Everhart-Thornley and the collection efficiency will deteriorate. Conversely for some semi-in-lens detectors as you increase the working distance the efficiency will drop off.
There is also the apparent illumintion to take into account with ET detectors with the sample near their minimum efficiency working distance. It can make very topographical samples appear with high contrast due to 'shadowing'. By increasing the working distance slightly it has the effect of reducing the shadowing and giving a more even grey-tone appearance to a topographic sample, though you lose some resolution.
The minimum obtainable magnification is also dependant on working distance. At long working distances it is possible to reach lower maginfications, conversely at short working distances the minimum magnification is increased.
Scan rotation will vary according to working distance due to the spiral of the beam so if your sample is aligned E-W at a short working distance you will notice that the image will rotate slightly at longer working distances.
Correct setting of the working distance is critical for EDX applications as the X-ray detecor has to be at the focal point for maximum collection. This will be determined by the geometry of your final lens and the takeoff angle of the detector - I have EDX working distances of 15mm and 20mm on two different SEMs for example. As you move away from the critical working distance for the detector the X-ray counts will reduce.
A TEM can be modified into a scanning transmission electron microscope (STEM) by the addition of a system that rasters the beam across the sample to form the image, combined with suitable detectors. Scanning coils are used to deflect the beam, such as by an electrostatic shift of the beam, where the beam is then collected using a current detector such as a Faraday cup, which acts as a direct electron counter. By correlating the electron count to the position of the scanning beam (known as the "probe"), the transmitted component of the beam may be measured. The non-transmitted components may be obtained either by beam tilting or by the use of annular dark field detectors....
Scanning electron microscopy is used for inspecting topographies of specimens at very high magnifications using a piece of equipment called the scanning electron microscope. SEM magnifications can go to more than 300,000 X but most semiconductor manufacturing applications require magnifications of less than 3,000 X only. SEM inspection is often used in the analysis of:
die/package cracks and fracture surfaces, bond failures, and physical defects on the die or package surface.During SEM inspection, a beam of electrons is focused on a spot volume of the specimen, resulting in the transfer of energy to the spot. These bombarding electrons, also referred to as primary electrons, dislodge electrons from the specimen itself. The dislodged electrons, also known as secondary electrons, are attracted and collected by a positively biased grid or detector, and then translated into a signal.
To produce the SEM image, the electron beam is swept across the area being inspected, producing many such signals. These signals are then amplified, analyzed, and translated into images of the topography being inspected
Note: The original poster has changed the question to What is the WD label in SEM image?
It is simply a measure of the distance, in mm, from the lens to the sample when the image is in focus.
Working Distance (WD) – In general, the shorter the WD, the higher the attainable resolution. If possible, a WD of 3mm of even 2mm should be used. The depth of field (vertical range in which sample features remain in focus), however, decreases with decreasing the WD. Hence, rough samples may need significantly larger WDs. Also, the accelerating voltage and emission current should be decreased for shorter WD, in order to minimize the resulting local electric field. WD also limits sample tilt.
Dear Abbas
To get good quality SEM image, the Working distance of the lens to the sample should be 6.0 - 6.7 mm. It is optimized distance for taking SEM image of thin films. But in case of bulk sample it can be 6 mm to 7mm.
Hi everyone,
Could anyone tell me if there is any reference for this statement please:
"In SEM, it can be 4 to 10mm depending on the function. Lower the distance and higher the spot size will be getting more of counts for EDS analysis. For imaging 9-10mm is preferred"
Regards
That statement is mostly nonsense. The only true part is that higher spot size will give give counts for EDS, the rest is erroneous.
Thanks for the reply Ian.
If I am performing EDS analysis, what would be the ideal WD to validate the analysis?
Thanks
Muhammad, this will depend solely on your hardware, and it will differ from system to system. So I cannot tell you what your value should be. For each SEM model and EDX detector there will be a recommended WD for analytical applications. You must obtain this information from the manufacturer of the detector, or whoever installed the detector.
The reason for this is that it will depend on the take-off angle of the detector and the design of the final pole piece in the SEM. The WD needs to be set such that it is "focussed" at the point where the incident beam strikes the specimen. Too high or too low and the take-off angle will be incorrect and the counts will plummet. Some software can correct for a slight variation in WD and calculate the resultant take-off angle, but even in these cases the counts will be low. The effect of incorrect WD is poorer counting statistics.
You ask about validating the analysis. This would need to be done using a certified reference material. Make sure that your results are close to what is on the certificate. Only then should you start to analyse your own materials. I have said many times that it is very easy to produce absolute garbage by EDX, with high precision!
An image in one of the SEM micrograph shows "S3400 15.0KV 67.3mm x 6 SE". Can anyone say me what does it mean ? How much magnification should I consider? shall i consider 6X as the magnification factor??
Dear Binsu Babu,
Can you tell me which company make SEM are you using to obtain the images?
- Badges
- Science method