Lens aberration correction

The majority of STEM instruments are simply conventional TEMs with the addition of scanning coils. As a result, these non-dedicated STEMs are capable of TEM/ STEM, as well as SEM imaging for thicker samples. The development of HRTEMs and dedicated STEMs with lens aberration correction have now pushed the... 

The first corrected electron-optical SEM was developed by Zach [10]. Eor low-voltage SEM (LVSEM, down to 500 eV electron energy instead of the conventional energies of up to 30 keV) the spot size is extremely large without aberration correction. Combining and correction and a electrostatic objective lens, Zach showed that a substantial improvement in spot size and resolution is possible. The achievable resolution in a LVSEM is now of the order of 1-2 mn. More recently, Krivanek and colleagues succeeded in building a corrected STEM [53,M]. 

The simplest microscope is the light microscope it consists of an objective lens and an eyepiece. Microscope objectives and eyepieces usually consist of complex lens systems of two or more lenses to correct for lens aberrations (Figure 4.6). The objective lens forms a real intermediate image, which is then magnified by the eyepiece. The objective lens and eyepiece are maintained at a fixed... 

There are a number of ways to compensate for and/or reduce lens aberrations. For example, combining lenses with different shapes and refractive indices corrects chromatic and spherical aberrations. Selecting single wavelength illumination by the use of filters helps eliminate chromatic aberrations. We expect that the extent to which lens aberrations have been corrected is reflected in the cost of the lens. It is a reason that we see huge price variation in microscopes. 

Classification of the objective lens is based on its aberration correction capabilities, mainly chromatic aberration. The following lenses are shown from low to high capability. 

A very important reeent advanee in eleetron microscope development is in the correction of lens aberration. From the very start of electron microscopy development, the importance of lens aberration as a limiting factor of the spatial resolution of the electron microscope has been recognised [3.41, 3.42]. Ways to correct these aberrations were proposed a long time ago and put into actual microscopes with partial success at least about 30 years ago [3.43]. The main idea is to compensate the positive spherieal aberration of the round lens with the negative aberration of multipole lenses. 

FIGURE 3-21. Compound lens for correction of chromatic aberration. 

A simple lens constructed from a single piece of glass exhibits several defects or aberrations that reduce its performance. These aberrations are minimized in the design of modern lenses, which are constructed using multiple lens elements of different materials and carefully calculated curvatures, the details of which need not concern the user. For the aberration corrections to be effective, however, it is important that the lenses be used as specified in their design. 

N. Seong, G. Yeo, H. Cho, and J. Moon, "Measimement of lens aberration by using in-situ interferometer and classification of lens for correct application," Proceedings ofSPlE, vol. 4000, pp. 30-39,2000. 

A nice article that describes the importance of oxygen diffusion for silicone-based soft contact lenses may be found online http //www.clspectrum.com/article.aspx7article=12953 Note if the lens contains a hydrophobic surface, it will disrupt the tear flow that results in the deposition of an albumin film on the lens. Not only will this reduce the effectiveness of the lens to correct optical aberrations, but will also cause infection/irritation. Eor more details regarding the lens surface and eye complications, see Rao, J. B., Saini, J. S. Complications of Content Lenses in Contact Lenses. Aquavella, J. V., Rao, G. N., eds. Lippincott Williams and Wilkins Philadelphia, PA, 1987. 

Aberration correction using exit wave restoration or commercial aberration correcting lenses generally only compensate the coherent objective lens aberrations up to either third or fifth order spherical aberration. For these microscopes the information limit resolution is no longer determined by the coherent objective lens aberrations but by chromatic aberrations coupled to the stability of the electron source, lens current and power supply. Correction of the remaining chromatic aberration is seen by many as the next step in the future of high-resolution electron microscopy. ... 

Fig. 6.4 shows the improvement in image quality that results. The corrected lens can operate at a greater working distance, and this together with a greater beam current may make a corrected SEM useful in analytical work. As with the STEM, the depth of field is much reduced in the aberration-corrected microscope, to the point where the instrument forms thin optical sections that are in focus. Multiple exposures can be combined (in through focus... 

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