Image is Too Noisy?

            1          Spot size is too small?

2          GUN Emission current is too low?

3          Variable aperture is too small?

4          Specimen position is not ideal - too far from the detector or the desired surface is tilted away from the detector?  Tilt towards the detector?

5          High voltage is too low?

Resolution is Poor?

1          Spot size too large?

2          Working distance too long?

3          Final Aperture too large?

4          Emission current too low or alignment poor?

5          High voltage too low?

6          Image does not contain high resolution information?

7          Too much backscatter in the image (kV too high or specimen tilted towards the detector)?

 Presentation of the Image is Poor?

1          Spot size incorrect, too small or too large?

2          Would tilt help present the information?

3          Have you the ideal WD?

4          Have you the correct kV for the job?

5          Have you the correct aperture (small) to obtain a good depth of field, do you  need to move the specimen further away from the final lens?

Are you seeing a mixed SE/BSE image when you need a more pure BSE or SE image?  Lower the kV for the latter raise the kV for the former.

 Photographic Procedures

Unlike conventional photography where the camera is adjusted to match the light available, in the SEM the image is adjusted to suit a fixed camera setting.  This is achieved by displaying the Wave Form and adjusting the signal and image intensity to obtain a specific trace position on the screen with a specific amount of AC in the trace.  If the camera has been correctly adjusted, using the f stop and base brightness and contrast levels, the trace which produces a good quality image should be about 2 cms high (AC) and placed in the centre of the CRT.

At your working magnification, tune the spot size and signal control, until the image quality is satisfactory. 

i) Low magnification - use spot size and Contrast to obtain a TV quality image.

ii) Higher magnification - adjust for a higher level of NOISE

1.                  Move to double your working magnification, and correct the focus and astigmatism.  Remember to use the stigmators as fine focus controls, the procedure - focus - stigmate X - stigmate Y - repeat, is ideal.  Do not try to look at the whole of the image, evaluate a very small area, looking for maximum contrast as an indication of focus.  Doubling the magnification ensures that focus and astigmatism corrections are sufficient for the working level, allowing a margin for error. 

2.                  If the image is not good enough, checked by using a slow scan at double the working mag, use a smaller spot size (image dims or becomes more noisy) and repeat the focus and astigmatism settings. 

3.                  Re check the image quality at double the photo mag and at a slowish scan if ok move on if not go back to 3.

4.                  Drop to the photo mag and set the brightness and contrast with a line scan, wave form (WFM) or LSP.

5.                  Set up the WFM for a signal level that is suitable for the film speed being used.  A starting point would be a 1.5cm high trace on the centre of the CRT.  If the CRT is marked set the contrast and brightness to fall between the marks.

6.                  Too much contrast - lower the height of the trace peak to peak, and centre it between the marks. 

7.                  Too little contrast - increase the height peak to peak and centre between the marks.

8.                  Too dim - raise the level with the brightness but keep the contrast the same.  Reverse the procedure if the image is too bright.