Ultramicroscopy 1 (1975) 89-96 © North-Holland Publishing Company
A METHOD FOR THE IMPROVEMENT OF THE VISIBILITY OF TRANSMISSION ELECTRON MICROSCOPE IMAGES W.M. STOBBS Cavendish Laboratory, University of Cambridge, Cambridge, UK and
U. VALDP~ GNSM-CNR and Istituto di Fisica, University of Bologna, Bologna, Italy Received 4 April 1975
A method is presented of improving the visibility of transmission electron microscope images in any situation in which a high resolution in only one chosen direction is of interest. The technique is based on the use of slot shaped objective apertures. Such apertures are of reduced area relative to a circular aperture giving the same all round resolution. The background intensity due to inelastically scattered electrons is thus reduced. The aperture device developed is described, while the value of the method is demonstrated by its application to the observation of dislocations. Further possible applications are indicated. resolution will be limited by the spherical aberration of the objective lens, unless this is compensated for by defocusing the lens as when taking micrographs showing a given lattice spacing. As the aperture is made smaller the in-focus resolution improves until it becomes diffraction limited. For an objective lens of spherical aberration coefficient C s = 1.9 mm, diffraction becomes the limiting aberration for an aperture, placed symmetrically about the beam, when its size is such that the half angle, a, subtended by it is less than about 6.6 × 10 -3 rad. This angle corresponds, in a Jeol JEM 100B electron microscope operated at 100 kV, to a linear aperture dimension of about 50 tam and to a diffraction limited resolution of about 0.27 nm (or about 0.5 of the Cu 220 Bragg angle at
1. Introduction The problem of increasing the signal-to-noise ratio in transmission electron microscope images has been approached in a number of different ways. The techniques used are of two types. Within the microscope the electron beam might be energy filtered [1] or dark field techniques may be used [2,3]. Alternatively the contrast in the final micrographs may be improved by various optical processes (see for example ref. [4]). In our case we consider ways in which the contrast can be improved using specially shaped objective apertures.
IO0 kV). 2. The effect of objective aperture size
In the case of thick specimens an increasing proportion of the electrons passing through the foil suffer energy losses and are scattered to large angles from the Bragg beams (see for example ref. [5]). If one could use energy filtering the contribution of these electrons to the image could be excluded. A partial solution is however to use an objective aperture of small size, thereby reducing the background intensity
The size and shape of the objective aperture used in the electron microscope govern the range of scattering angle of the electrons which are used for the formation of the image. In the case of thin specimens, for which the probability of electrons suffering inelastic scattering is small, and for large aperture sizes, the 89
90
I¢.M. Stobbs. U. Valdr'e / Improvement o f visibility o f TEM images
in the image, but potentially reducing the resolution obtainable by approaching the diffraction limitation. In fact, since modern electron microscopes are often run using the prefield of the objective lens to give a convergent beam at the specimen to increase the brightness, diffraction limitations do not become important for defect imaging, using a strongly excited reflection, before the objective aperture is reduced in size to such an extent that it obscures part of the elastically scattered spot. In the Jeol JEM 100B, unless the second condenser is defocused to give a more parallel beam at the specimen, this limitation on the objective aperture size corresponds typically to it subtending a half angle of about 1.5 × 10 -3 tad, roughly one tenth of the copper 220 Bragg angle, the diffraction resolution limit then still being only about 1.2 nm. However there are some circumstances in which the resolution required can be of the order of or better than 1.5 nm (see for example ref. [6]) and yet a conventional circular objective aperture, using parallel illumination would make the image contrast too low because of the contribution of wide angle inelastically scattered electrons. To clarify the principle of the method, which we describe here, of using a variable shaped objective aperture it will be halpful to consider a particular example. It is well known (see for example ref. [7]) that computer calculations for the images of dislocations taken under weak beam conditions [3] suggest that such images should be obtainable using foils of much greater thickness than is in fact experimentally practicable. The major reason for this is that the visibility of the image of the defect is limited in thick foils by contributions to the background from inelastically scattered electrons. Thus, when using the weak beam technique, particularly when the angular spread of the incident beam is limited, the smallest objective aperture is used for the resolution required [6]. Holyever, with the improved stage stabilities of modern microscopes, defect strain field contrast can be examined in dark field at resolutions approaching or better than 1.0 nm using increased deviation parameters and long exposure times. If a circular objective aperture is used, of a suitable size to retain this resolution in the image, the visibility of the weak beam images in the thicker regions becomes too l'ow. The only images obtainable are then from defects in foils of such a small
thickness that the proximity of the free surface makes the analysis of the contrast extremely difficult [7].
3. The variable objective aperture device Provided that a high resolution is required in only one chosen direction in a particular micrograph (as in usually the case when examining, for example, the separation of partial dislocations) a simple solution to the problem raised in section 2 is to use a rectangular slit aperture. This should be of a sufficient length to retain the required one dimensional resolution, but of smaller area than the conventional circular aperture so as to reduce the number of inelastically scattered electrons contributing to the image background. Clearly it is necessary that the size, shape, orientation and position of such an aperture can be changed in the back focal plane of the objective. While Riecke [8] among others have designed condenser apertures of variable shape the added requirements of variable shape and orientation make the design problem difficult given the limited space available within the objective lens. A schematic drawing of the prototype system we have used on the Jeol JEM 100B electron microscope is shown in fig. 1. The principle of the design is to superpose rectangular apertures* held on each of two independently rotatable coaxial platforms placed at the head of an aperture blade. The apertures are positioned in the platforms in such a manner that the surfaces defining the final slot shape are in contact. The platforms can be rotated independently by the action of the wires V, W shown schematically in the figure. The wires for each platform pass through the shaft of the aperture drive system and are anchored at one end via a spring, while the other end terminates in a micrometer drive. The special objective aperture drive which has been developed for the system [9] allows for large x , y and z movement of the aperture blade. These large movements are clearly necessary (see fig. 1) to facilitate the positioning of a given aperture, in a given orientation, on the axis of the microscope. We have found that a suitable aperture size for weak beam work using the Jeol JEM 100B or C is I00/am × 40 lam. When these apertures are super* Supplied by Polaron Equipment Ltd., Watford, Herts., U.K.
W.M. Stobbs, U. Valdrb/ Improvement of visibility of TEM images
91
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Fig. l. Schematic scale drawing of the objective aperture blade, b) is a section of a) through XX. The platforms P and Q, each carrying two apertures A, are independently rotated by the wires V and W. S indicates one return spring and H one of the hooks for clamping wires.
posed orthogonally, comparison may be made of images formed with effective aperture sizes continuously variable from 40/am × 40/am to 40/am X 2/am. The 2/am figure is limited by two factors, the dimension of the Bragg spot used and irregularities on the edge of the apertures. To a limit determined by recording times the first difficulty can be partially overcome by spreading the illumination, while irregularities on the edges of the apertures can be reduced by electro-polishing. In practice, for weak beam work and the type of aperture available at present, we have found that a convenient aperture size is 40/am X 15/am when using the pole piece designed for the top entry goniometer stage in the Jeol JEM 100B.
4. Use of the device Initial experiments with our prototype device have demonstrated its practical usefulness and suggested further improvements to its design, which will be described elsewhere [9]. Our preliminary observations have been on the improvement in the visibility of weak beam images of dislocations. Fig. 2 and fig. 3 should be compared. Both micrographs show the same area of the primary slip plane of a foil of a fatigued copper single crystal. Both micrographs were taken under identical weak beam axial dark field imaging and recording conditions (in particular for both
W3g ~ 0 at E) except that for fig. 2 a square objective aperture was used and for fig. 3 a slot aperture. The square aperture was of half angle ~2 × 10 -3 rad and the slot aperture, while being of the same length as that used for fig. 2, subtended ~7 × 10 -4 rad at right angles to g. A realistic comparison of the appearance of the micrographs may be obtained from figs. 2a and 3a which were printed in an identical fashion. The print times for figs. 2b and 3b were adjusted to give the best contrast possible in both micrographs. Most of the dislocations visible in the wall structure [10] are of primary Burgers vector and g was chosen in such a way that g • b for these dislocations was 2. For the micrograph shown in fig. 3 the aperture was orientated so that the long side was roughly parallel to b. In the JEM 100B the smaller of the above apertures allows a maximum resolution of about 0.9 nm in the direction of the long side of the aperture and about 2.5 nm at right angles to this. The orientation of the aperture, as described for fig. 3 thus maximises the possible resolution across a primary edge dislocation. Of course there is little detail in a weak beam image, with W3g >~0 in copper, in the range between 0.9 nm and 2.5 nm. Our point here however is to demonstrate the increase in visibility gained without loss in resolution in the regions between 100 nm and 150 nm in thickness as at E in fig. 3. Figs. 4 and 5 are enlargements of the area E in figs. 2 and 3 respectively. While the images of the dislocations making up the dipole are clearly resolved in
92
W.M. Stobbs, U. Vald~e / Improvement of visibility of TEM images
Fig. 2. Weak beam micrograph of dislocations in a (111) copper foil taken at 100 kV, using g = 202, with a 60 s exposure. The beam used for this and the following figures was passed axially through the objective lens with the relevant objective aperture placed symmetrically around it. The dislocations are o f Burgers vector b = ½a []01 l, so that g" b = 2. W3g >~0 near E where the thickness is about 130 nm. The objective aperture used was square. The printing condition for 2a is the same as that for fig. 3a, the printing condition for 2b is adjusted for improved contrast.
Iq.M. Stobbs, U. Valdrb / hnprovement o f visibility o f TEM images
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Fig. 3. Weak beam micrograph of the area shown in fig. 2 taken under indentical imaging and recording conditions except that the aperture used was rectangular. The line F marks the long direction of the aperture. The printing condition for 3a was tile same as that for fig. 2a, while that for 3b is adjusted.
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Fig. 4. Enlargement of the area E in fig. 2, square aperture. Printing condition for best contrast.
Fig. 5. Enlargement of the area E in fig. 3, rectangular aperture. Printing condition for best contrast.
fig. 5 their contrast is much lower in fig. 4 for which the background intensity was much higher. The contrast in fig. 5 is characteristic of a dipole the images being, as would be expected, much sharper than those of the partial dislocations comprising a single dislocation (see figs. 6 and 7). In fig. 4, taken from the micrograph with the larger square aperture, two dislocation images are in fact just resolvable, though they are best seen on the original plate. While the average spacing of the dislocation images in fig. 5 is about 3.5 nm, the spacing of the weaker contrast images in fig. 4 is only about 2.7 nm. Although there are slight changes in specimen orientation along the length of the dipole, this difference in the average spacing can be explained realistically only if the inelastic contribution to the dipole image is strongly peaked at its centre. There is also an improvement in defect visibility in the thinner regions as may be seen directly by examination of figs. 2 and 3 or figs. 6 and 7 which are enlargements of area H, in figs. 2 and 3 respectively. In fig. 7, the dislocation in near edge orientation at (a) may clearly be seen to be dissociated whereas in fig. 6, showing the same area, the weaker imaged partial dislocation is not visible against the increased background. It will also be noted that thickness fringes are visible to a greater foil thickness in the lower right hand corner of fig. 7, with the objective aperture of smaller area, than in fig. 6 with the large square aperture.
Preliminary results on the comparison of microdensitometer traces of dislocation images, such as those shown in the above figures, as a function of foil thickness and objective aperture dimension, have shown the high intensity of the incoherent background arising from wide angle inelastically scattered electrons. For example microdensitometer comparisons of the dipole in figs. 4 and 5 show that the background intensity away from the dipole image is reduced by a factor of ~0.5 when the aperture area was reduced by a factor of ~0.3. Using the definition for the visibility as the difference of the intensity of the image and background divided by their sum, the dipole visibility was improved from 0.10 to 0.22 when the smaller rectangular aperture was used. Gai and Howie [11] have observed similarly high incoherent backgrounds in weak beam images of GaP as well as in Cu and Ni. They also show that such high background intensities are unlikely to be explained either by thermal diffuse scattering or by disorder scattering from surface contamination and suggest that a major contribution might arise from scattering due to electronic excitation. The use of the variable rectangular aperture provides a simple experimental means of analysing the formation of the image as a function of scattering angle. There are however many more applications for the device. Obviously the technique could be extended to facilitate the observation of the dissociation of dislo-
I¢.M. Stobbs, U. Valdrb / Improvement of visibility of TEM images
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5. Conclusions
Fig. 6. Enlargement of the area H in fig. 2, square aperture. Printing condition for best contrast. cations in materials of a higher stacking fault energy than copper. More interestingly the device could be used to investigate image formation using streaks in the diffraction pattern. Such streaks might be caused by platelet types of defects. However, in some cases, extremely weak streaking in the diffraction p.attern can be caused by a low frequency perturbation of the lattice associated with a charge density wave as, for example, in 2 H - T a S 2 [12], and micrographs formed by using such a streak would be of considerable interest.
We have presented a simple and novel method of improving image visibility when only unidirectionally high resolution is required. We have shown that it is possible to make and use an objective aperture drive device for the electron microscope allowing in situ continuous variation of aperture size, orientation and shape. Our preliminary results on the effect of the inelastic background on weak beam images of dislocations in copper using the device, show that the image visibility is markedly improved (by at least a factor of two) on reducing one dimension of the rectangular objective aperture, the high resolution being retained in a direction in the image plane corresponding to that of the unchanged dimension of the aperture in the diffraction plane. It is also demonstrated that the inelastic scattering is peaked at the centre of the dislocation image to such an extent that the weak beam image separation, at least of a dipole, can vary by as much as 30% on changing aperture size. Clearly care would be needed in the comparison of such experimental images with computed profiles which neglected the contribution of large angle inelastic scattering. The device can be useful in any materials science or biological application where a high resolution is required in any single chosen direction in a particular micrograph.
Acknowledgements We are indebted to the Science Research Council (W.M.S.) and to the Consiglio Nazionale delle Ricerche (U.V.) for financial support. We are grateful to Dr. A. Howie for his interest and for discussions that led to the idea of using rectangular apertures. Our thanks are also due to Mr. F.P. Marks for his skillful construction of the aperture system.
References
Fig. 7. Enlargement of the area H in fig. 3, rectangular aperture. Printing condition for best contrast.
[ 1] R.M. Henkehnan and F.P. Ottensmeyer, J. of Microscopy 102 (1974) 79. [2] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley
96
[3] [4]
[5] [6]
W.M. Stobbs, U. Valdrb / Improvement of visibility of TEM images and M.J. Whelan, Electron Microscopy of Thin Crystals (London: Butterworths, 1965). D.J.H. Cockayne, I.L.F. Ray and M.J. Whelan, Phil. Mag. 20 (1969) 1265. R.W. Home and R. Markham, in Practical Methods in Electron Microscopy, A.M. Glauert Ed. (North Holland, Amsterdam, 1972), Vol. 1, p. 325. T. Groves, Ultramicroscopy 1 (1975) 15. W.M. Stobbs, in: Electron Microscopy and Materials Science, E. Ruedl and U. Valdr~ Eds. (Luxemburg: The Commission of the European Comminities, 1975), Vol. 2.
[7] W.M. Stobbs and C.H. Sworn, Phil. Mag., 24 (1971) 1365. [8] D. Riecke, in: Electron Microscopy and Materials Science, E. Ruedl and U. Valdr~ Eds. (Luxemburg: The Commission of the European Communities, 1975), Vol. 1. [9] W.M. Stobbs and U. Valdr~, accepted for publication in J. Phys. E. (1975). [10] P.J. Woods, Phil. Mag. 28 (1973) 155. [11] P.L. Gai and A. Howie, Phil. Mag. 31 (1975) 519. [12] J.P. Chevalier and W.M. Stobbs, Phil. Mag. 31 (1975) 733.
A METHOD FOR THE IMPROVEMENT OF THE VISIBILITY OF TRANSMISSION ELECTRON MICROSCOPE IMAGES W.M. STOBBS Cavendish Laboratory, University of Cambridge, Cambridge, UK and
U. VALDP~ GNSM-CNR and Istituto di Fisica, University of Bologna, Bologna, Italy Received 4 April 1975
A method is presented of improving the visibility of transmission electron microscope images in any situation in which a high resolution in only one chosen direction is of interest. The technique is based on the use of slot shaped objective apertures. Such apertures are of reduced area relative to a circular aperture giving the same all round resolution. The background intensity due to inelastically scattered electrons is thus reduced. The aperture device developed is described, while the value of the method is demonstrated by its application to the observation of dislocations. Further possible applications are indicated. resolution will be limited by the spherical aberration of the objective lens, unless this is compensated for by defocusing the lens as when taking micrographs showing a given lattice spacing. As the aperture is made smaller the in-focus resolution improves until it becomes diffraction limited. For an objective lens of spherical aberration coefficient C s = 1.9 mm, diffraction becomes the limiting aberration for an aperture, placed symmetrically about the beam, when its size is such that the half angle, a, subtended by it is less than about 6.6 × 10 -3 rad. This angle corresponds, in a Jeol JEM 100B electron microscope operated at 100 kV, to a linear aperture dimension of about 50 tam and to a diffraction limited resolution of about 0.27 nm (or about 0.5 of the Cu 220 Bragg angle at
1. Introduction The problem of increasing the signal-to-noise ratio in transmission electron microscope images has been approached in a number of different ways. The techniques used are of two types. Within the microscope the electron beam might be energy filtered [1] or dark field techniques may be used [2,3]. Alternatively the contrast in the final micrographs may be improved by various optical processes (see for example ref. [4]). In our case we consider ways in which the contrast can be improved using specially shaped objective apertures.
IO0 kV). 2. The effect of objective aperture size
In the case of thick specimens an increasing proportion of the electrons passing through the foil suffer energy losses and are scattered to large angles from the Bragg beams (see for example ref. [5]). If one could use energy filtering the contribution of these electrons to the image could be excluded. A partial solution is however to use an objective aperture of small size, thereby reducing the background intensity
The size and shape of the objective aperture used in the electron microscope govern the range of scattering angle of the electrons which are used for the formation of the image. In the case of thin specimens, for which the probability of electrons suffering inelastic scattering is small, and for large aperture sizes, the 89
90
I¢.M. Stobbs. U. Valdr'e / Improvement o f visibility o f TEM images
in the image, but potentially reducing the resolution obtainable by approaching the diffraction limitation. In fact, since modern electron microscopes are often run using the prefield of the objective lens to give a convergent beam at the specimen to increase the brightness, diffraction limitations do not become important for defect imaging, using a strongly excited reflection, before the objective aperture is reduced in size to such an extent that it obscures part of the elastically scattered spot. In the Jeol JEM 100B, unless the second condenser is defocused to give a more parallel beam at the specimen, this limitation on the objective aperture size corresponds typically to it subtending a half angle of about 1.5 × 10 -3 tad, roughly one tenth of the copper 220 Bragg angle, the diffraction resolution limit then still being only about 1.2 nm. However there are some circumstances in which the resolution required can be of the order of or better than 1.5 nm (see for example ref. [6]) and yet a conventional circular objective aperture, using parallel illumination would make the image contrast too low because of the contribution of wide angle inelastically scattered electrons. To clarify the principle of the method, which we describe here, of using a variable shaped objective aperture it will be halpful to consider a particular example. It is well known (see for example ref. [7]) that computer calculations for the images of dislocations taken under weak beam conditions [3] suggest that such images should be obtainable using foils of much greater thickness than is in fact experimentally practicable. The major reason for this is that the visibility of the image of the defect is limited in thick foils by contributions to the background from inelastically scattered electrons. Thus, when using the weak beam technique, particularly when the angular spread of the incident beam is limited, the smallest objective aperture is used for the resolution required [6]. Holyever, with the improved stage stabilities of modern microscopes, defect strain field contrast can be examined in dark field at resolutions approaching or better than 1.0 nm using increased deviation parameters and long exposure times. If a circular objective aperture is used, of a suitable size to retain this resolution in the image, the visibility of the weak beam images in the thicker regions becomes too l'ow. The only images obtainable are then from defects in foils of such a small
thickness that the proximity of the free surface makes the analysis of the contrast extremely difficult [7].
3. The variable objective aperture device Provided that a high resolution is required in only one chosen direction in a particular micrograph (as in usually the case when examining, for example, the separation of partial dislocations) a simple solution to the problem raised in section 2 is to use a rectangular slit aperture. This should be of a sufficient length to retain the required one dimensional resolution, but of smaller area than the conventional circular aperture so as to reduce the number of inelastically scattered electrons contributing to the image background. Clearly it is necessary that the size, shape, orientation and position of such an aperture can be changed in the back focal plane of the objective. While Riecke [8] among others have designed condenser apertures of variable shape the added requirements of variable shape and orientation make the design problem difficult given the limited space available within the objective lens. A schematic drawing of the prototype system we have used on the Jeol JEM 100B electron microscope is shown in fig. 1. The principle of the design is to superpose rectangular apertures* held on each of two independently rotatable coaxial platforms placed at the head of an aperture blade. The apertures are positioned in the platforms in such a manner that the surfaces defining the final slot shape are in contact. The platforms can be rotated independently by the action of the wires V, W shown schematically in the figure. The wires for each platform pass through the shaft of the aperture drive system and are anchored at one end via a spring, while the other end terminates in a micrometer drive. The special objective aperture drive which has been developed for the system [9] allows for large x , y and z movement of the aperture blade. These large movements are clearly necessary (see fig. 1) to facilitate the positioning of a given aperture, in a given orientation, on the axis of the microscope. We have found that a suitable aperture size for weak beam work using the Jeol JEM 100B or C is I00/am × 40 lam. When these apertures are super* Supplied by Polaron Equipment Ltd., Watford, Herts., U.K.
W.M. Stobbs, U. Valdrb/ Improvement of visibility of TEM images
91
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Fig. l. Schematic scale drawing of the objective aperture blade, b) is a section of a) through XX. The platforms P and Q, each carrying two apertures A, are independently rotated by the wires V and W. S indicates one return spring and H one of the hooks for clamping wires.
posed orthogonally, comparison may be made of images formed with effective aperture sizes continuously variable from 40/am × 40/am to 40/am X 2/am. The 2/am figure is limited by two factors, the dimension of the Bragg spot used and irregularities on the edge of the apertures. To a limit determined by recording times the first difficulty can be partially overcome by spreading the illumination, while irregularities on the edges of the apertures can be reduced by electro-polishing. In practice, for weak beam work and the type of aperture available at present, we have found that a convenient aperture size is 40/am X 15/am when using the pole piece designed for the top entry goniometer stage in the Jeol JEM 100B.
4. Use of the device Initial experiments with our prototype device have demonstrated its practical usefulness and suggested further improvements to its design, which will be described elsewhere [9]. Our preliminary observations have been on the improvement in the visibility of weak beam images of dislocations. Fig. 2 and fig. 3 should be compared. Both micrographs show the same area of the primary slip plane of a foil of a fatigued copper single crystal. Both micrographs were taken under identical weak beam axial dark field imaging and recording conditions (in particular for both
W3g ~ 0 at E) except that for fig. 2 a square objective aperture was used and for fig. 3 a slot aperture. The square aperture was of half angle ~2 × 10 -3 rad and the slot aperture, while being of the same length as that used for fig. 2, subtended ~7 × 10 -4 rad at right angles to g. A realistic comparison of the appearance of the micrographs may be obtained from figs. 2a and 3a which were printed in an identical fashion. The print times for figs. 2b and 3b were adjusted to give the best contrast possible in both micrographs. Most of the dislocations visible in the wall structure [10] are of primary Burgers vector and g was chosen in such a way that g • b for these dislocations was 2. For the micrograph shown in fig. 3 the aperture was orientated so that the long side was roughly parallel to b. In the JEM 100B the smaller of the above apertures allows a maximum resolution of about 0.9 nm in the direction of the long side of the aperture and about 2.5 nm at right angles to this. The orientation of the aperture, as described for fig. 3 thus maximises the possible resolution across a primary edge dislocation. Of course there is little detail in a weak beam image, with W3g >~0 in copper, in the range between 0.9 nm and 2.5 nm. Our point here however is to demonstrate the increase in visibility gained without loss in resolution in the regions between 100 nm and 150 nm in thickness as at E in fig. 3. Figs. 4 and 5 are enlargements of the area E in figs. 2 and 3 respectively. While the images of the dislocations making up the dipole are clearly resolved in
92
W.M. Stobbs, U. Vald~e / Improvement of visibility of TEM images
Fig. 2. Weak beam micrograph of dislocations in a (111) copper foil taken at 100 kV, using g = 202, with a 60 s exposure. The beam used for this and the following figures was passed axially through the objective lens with the relevant objective aperture placed symmetrically around it. The dislocations are o f Burgers vector b = ½a []01 l, so that g" b = 2. W3g >~0 near E where the thickness is about 130 nm. The objective aperture used was square. The printing condition for 2a is the same as that for fig. 3a, the printing condition for 2b is adjusted for improved contrast.
Iq.M. Stobbs, U. Valdrb / hnprovement o f visibility o f TEM images
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Fig. 3. Weak beam micrograph of the area shown in fig. 2 taken under indentical imaging and recording conditions except that the aperture used was rectangular. The line F marks the long direction of the aperture. The printing condition for 3a was tile same as that for fig. 2a, while that for 3b is adjusted.
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I¢.M. Stobbs, U. Valdrb / Improvement of visibility of TEM images
Fig. 4. Enlargement of the area E in fig. 2, square aperture. Printing condition for best contrast.
Fig. 5. Enlargement of the area E in fig. 3, rectangular aperture. Printing condition for best contrast.
fig. 5 their contrast is much lower in fig. 4 for which the background intensity was much higher. The contrast in fig. 5 is characteristic of a dipole the images being, as would be expected, much sharper than those of the partial dislocations comprising a single dislocation (see figs. 6 and 7). In fig. 4, taken from the micrograph with the larger square aperture, two dislocation images are in fact just resolvable, though they are best seen on the original plate. While the average spacing of the dislocation images in fig. 5 is about 3.5 nm, the spacing of the weaker contrast images in fig. 4 is only about 2.7 nm. Although there are slight changes in specimen orientation along the length of the dipole, this difference in the average spacing can be explained realistically only if the inelastic contribution to the dipole image is strongly peaked at its centre. There is also an improvement in defect visibility in the thinner regions as may be seen directly by examination of figs. 2 and 3 or figs. 6 and 7 which are enlargements of area H, in figs. 2 and 3 respectively. In fig. 7, the dislocation in near edge orientation at (a) may clearly be seen to be dissociated whereas in fig. 6, showing the same area, the weaker imaged partial dislocation is not visible against the increased background. It will also be noted that thickness fringes are visible to a greater foil thickness in the lower right hand corner of fig. 7, with the objective aperture of smaller area, than in fig. 6 with the large square aperture.
Preliminary results on the comparison of microdensitometer traces of dislocation images, such as those shown in the above figures, as a function of foil thickness and objective aperture dimension, have shown the high intensity of the incoherent background arising from wide angle inelastically scattered electrons. For example microdensitometer comparisons of the dipole in figs. 4 and 5 show that the background intensity away from the dipole image is reduced by a factor of ~0.5 when the aperture area was reduced by a factor of ~0.3. Using the definition for the visibility as the difference of the intensity of the image and background divided by their sum, the dipole visibility was improved from 0.10 to 0.22 when the smaller rectangular aperture was used. Gai and Howie [11] have observed similarly high incoherent backgrounds in weak beam images of GaP as well as in Cu and Ni. They also show that such high background intensities are unlikely to be explained either by thermal diffuse scattering or by disorder scattering from surface contamination and suggest that a major contribution might arise from scattering due to electronic excitation. The use of the variable rectangular aperture provides a simple experimental means of analysing the formation of the image as a function of scattering angle. There are however many more applications for the device. Obviously the technique could be extended to facilitate the observation of the dissociation of dislo-
I¢.M. Stobbs, U. Valdrb / Improvement of visibility of TEM images
95
5. Conclusions
Fig. 6. Enlargement of the area H in fig. 2, square aperture. Printing condition for best contrast. cations in materials of a higher stacking fault energy than copper. More interestingly the device could be used to investigate image formation using streaks in the diffraction pattern. Such streaks might be caused by platelet types of defects. However, in some cases, extremely weak streaking in the diffraction p.attern can be caused by a low frequency perturbation of the lattice associated with a charge density wave as, for example, in 2 H - T a S 2 [12], and micrographs formed by using such a streak would be of considerable interest.
We have presented a simple and novel method of improving image visibility when only unidirectionally high resolution is required. We have shown that it is possible to make and use an objective aperture drive device for the electron microscope allowing in situ continuous variation of aperture size, orientation and shape. Our preliminary results on the effect of the inelastic background on weak beam images of dislocations in copper using the device, show that the image visibility is markedly improved (by at least a factor of two) on reducing one dimension of the rectangular objective aperture, the high resolution being retained in a direction in the image plane corresponding to that of the unchanged dimension of the aperture in the diffraction plane. It is also demonstrated that the inelastic scattering is peaked at the centre of the dislocation image to such an extent that the weak beam image separation, at least of a dipole, can vary by as much as 30% on changing aperture size. Clearly care would be needed in the comparison of such experimental images with computed profiles which neglected the contribution of large angle inelastic scattering. The device can be useful in any materials science or biological application where a high resolution is required in any single chosen direction in a particular micrograph.
Acknowledgements We are indebted to the Science Research Council (W.M.S.) and to the Consiglio Nazionale delle Ricerche (U.V.) for financial support. We are grateful to Dr. A. Howie for his interest and for discussions that led to the idea of using rectangular apertures. Our thanks are also due to Mr. F.P. Marks for his skillful construction of the aperture system.
References
Fig. 7. Enlargement of the area H in fig. 3, rectangular aperture. Printing condition for best contrast.
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