2Transmission Electron Microscope Optical instrument in that it uses a lens to form an imageScanning Electron MicroscopeNot an optical instrument (no image forming lens) but uses electron optics. Probe forming-Signal detecting device.
3Electron OpticsRefraction, or bending of a beam of illumination is caused when the wavelength enters a medium of a different optical density.
4Electron Optics In light optics this is accomplished when a wavelength of light moves from air into glassIn EM there is only a vacuum with an opticaldensity of 1.0 whereas glass is much higher
5Electron Optics In electron optics the beam cannot enter a conventional lens of a different optical density.Instead a “force” must be applied that has thesame effect of causing the beam of illuminationto bend.
6Electron Optics In electron optics the beam cannot enter a conventional lens of a different optical density.Instead a “force” must be applied that has thesame effect of causing the beam of illuminationto bend.Electromagnetic Force or Electrostatic Force
7Classical optics: The refractive index changes abruptly at a surface and is constant between thesurfaces. The refraction of light at surfaces separatingmedia of different refractive indices makes it possibleto construct imaging lenses. Glass surfaces can beshaped.2) Electron optics: Here, changes in the refractiveindex are gradual so rays are continuous curves ratherthan broken straight lines. Refraction of electronsmust be accomplished by fields in space aroundcharged electrodes or solenoids, and these fields canassume only certain distributions consistent withfield theory.
8Converging (positive) lens: bends rays toward the axis. It has a positive focal length. Forms a realinverted image of an object placed to the left of thefirst focal point and an erect virtual image of anobject placed between the first focal point and thelens.
9Diverging (negative) lens: bends the light rays away from the axis. It has a negative focal length.An object placed anywhere to the left of a diverginglens results in an erect virtual image. It is notpossible to construct a negative magnetic lensalthough negative electrostatic lenses can be made
10Electron Optics Electrostatic lens Must have very clean and high vacuumenvironment to avoid arcing across plates
17The two forcevectors, one in thedirection of theelectron trajectoryand the otherperpendicular toit, causes theelectrons to movethrough the magneticfield in a helicalmanner.
18The strength of the magnetic field is determined by the number of wraps of the wire and the amount of current passing through the wire. A value of zero current (weak lens) would have an infinitely long focal length while a large amount of current (strong lens) would have a short focal length.
19A TEM image is made up of nonscattered electrons (which strike the screen) and scattered electrons which do not and therefore appear as a dark area on the screen
20Some of the scattered electrons will only be partially scattered and thus will reach the screen in an inappropriate position giving a false signal and thus contributing to a degradation of the image. These forward scattered electrons can be eliminated by placing an aperture beneath the specimen.
21The design of an electromagnetic lens results in a very strong lens with a very short focal length thus requiring that the specimen lie within the lens itself along with an aperture to stop the highly scattered electrons
22Upper Pole PieceSpecimenApertureLower Pole PieceBoth the specimen rod and the aperture rod assembly have to be inserted into the lens. They are made of nonmagnetic metals such as copper, brass, and platinum
23While a small opening objective aperture has the advantage of stopping scattered electrons and thus increasing image contrast it also dramatically reduces the half angle of illumination for the projection lenses and thus decreases image resolution
24Lens Defects Since the focal length f of a lens is dependent on the strength of the lens, if follows that different wavelengths will be focused to different positions. Chromatic aberration of a lens is seen as fringes around the image due to a “zone” of focus.
25Lens DefectsIn light optics wavelengths of higher energy (blue) are bent more strongly and have a shorter focal lengthIn the electron microscope the exact opposite is true in that higher energywavelengths are less effected and have alonger focal length
26Lens DefectsIn light optics chromatic aberration can be corrected by combining a converging lens with a diverging lens. This is known as a “doublet” lens
27Lens DefectsA few manufacturers have combined an electromagnetic (converging) lens with an electrostatic (diverging) lens to create an achromatic lensLEO Gemini Lens
28The simplest way to correct for chromatic aberration is to use illumination of a single wavelength! This is accomplished in an EM by having a very stable acceleration voltage. If the e velocity is stable the illumination source is monochromatic
29The problem arises when electrons are differentially scattered within the specimen slowing some more than others and thus producing poly-chromatic illumination from a monochromatic beam.
30The effects of chromatic aberration are most profound at the edges of the lens so by placing an aperture immediately after the specimen chromatic aberration is reduced along with increasing contrast
31Lens DefectsThe fact that wavelengths enter and leave the lens field at different angles results in a defect known as spherical aberration. The result is similar to that of chromatic aberration in that wavelengths are brought to different focal points
32Spherical aberrations are worst at the periphery of a lens so again a small opening aperture that cuts off the most offensive part of the lens is the best way to reduce the effects of spherical aberration
33DiffractionDiffraction occurs when a wavefront encounters an edge of an object. This results in the establishment of new wavefronts
34DiffractionWhen this occurs at the edges of an aperture the diffracted waves tend to spread out the focus ratherthan concentrate them. This results in a decrease in resolution, the effect becoming more pronounced with ever smaller apertures.
35Apertures Disadvantages Advantages Decrease resolution due to effects of diffractionDecrease resolution by reducing half angle of illuminationDecrease illumination by blocking scattered electronsAdvantagesIncrease contrast by blocking scattered electronsDecrease effects of chromatic and spherical aberration by cutting off edges of a lens
36If a lens is not completely symmetrical objects will be focussed to different focal planes resulting in an astigmatic image
37The result is a distorted image The result is a distorted image. This can best be prevented by having as near to perfect a lens as possible but other defects such as dirton an aperture etc. can cause an astigmatism
38Astigmatism in light optics is corrected by making a lens with a corresponding defect to correct for the defect in another lensIn EM it is corrected using a stigmatorWhich is a ring of electromagnets positioned around the beam to “push” and “pull” the beam to make it more perfectly circular