Contact Information:

   Telephone

   978 689-3977

   Email  Support

Causes of Poor Resolution 

There are times when you are operating the sem, that the image quality (resolution) just doesn’t appear as sharp as normal. There are a number of possible causes for this. Below we will discuss a few of the more common ones. 

1.      One of the most common causes of poor resolution is contamination of the objective, or some other aperture in the optics column. After a sem runs for 400 ~500 hours, contamination will start to build up on the platinum, or moly aperture discs. At first, there will be light brownish coloring around the aperture hole. It will eventually turn dark black.

The problem most commonly seen with this contamination is an inability to correct the astigmatism in the image. The stigmator controls will be at their limits, with astigmatism still present.

Fortunately, this problem is easily corrected by replacing the apertures with new ones, or in the case of platinum, the apertures can be flamed in a platinum boat.

2.      In the case of a Lab6 beam source, a bad source can cause both poor resolution and signal. As the tip ages, the sharpness of the point will diminish. This will broaden the beam, and reduce the resolution of the sem. The life expectancy of a Lab6 source will vary depending upon operating conditions, but 1500~2000 hours is typical.

There are also some other conditions that can appear when the tip has a problem, such as ghosted images, and drifting alignment. 

3.      Another cause of poor resolution can be a misaligned instrument. It is very important that the sem be properly aligned to obtain high resolution images. Starting with the saturation and alignment of the filament, and working down the column, everything needs to be set correctly.

It is especially important, with sems, that have an adjustable objective aperture strip, that the strip is centered correctly. This is done by adjusting the aperture position while varying the focus of the image. The image should not appear to move when the focus is changed. Some sems have an objective lens wobbler to assist with this adjustment. The observed image should go in and out of focus without moving across the viewing screen.

4.      The correct operating conditions are also extremely important, in obtaining the best resolution. In general the best resolution will be obtained with a higher gun accelerating voltage (20~30kv). It is also important to place the specimen at the shortest working distance that is possible, depending upon restrictions from tilting, or an x-ray detector.

Next, the correct aperture, and spot size must be selected. The smaller the beam diameter,  the  best possible resolution. There are some limits, however, as to how small the beam can be. If the spot size is set too small, the image will appear very noisy, and it may be difficult to see what you are focusing on. There is a tradeoff, between resolution and signal level, that is sample and magnification dependant. It may take some experimenting, to find the right combination.

5.      Poor vacuum: If the instrument has a good tip, and the apertures are clean, yet the image sharpness is not good, it could be that the sem vacuum has degraded. If possible it may be a good idea to put a cold cathode gauge on the sem, to be able to read the vacuum level. Typically, the specimen chamber should be in the low 10~6 Torr range. The vacuum in the gun will vary with different sems, depending upon whether or not, it has a Tungsten, Lab6, or Field Emitter Tip.