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A method of alignment for convergent beam diffraction in TEM mode for a JEM-100C electron microscope
Uhramicroscopy 7 (1982) 343-350 North-Holland Publishing Company
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A METHOD OF ALIGNMENT FOR CONVERGENT BEAM DIFFRACTION IN TEM MODE FOR A JEM-100C ELECTRON MICROSCOPE M.J. WITCOMB Electron Microscope Unit. Universit)' of the Wit.'atersrond, Johannesburg 2000, Republic i~f South Africa Received 26 November 1981
The standard methods of aligning the microscope for convergent beam diffraction are shown to result in a 0.2-0.5 ~tm deflection of the beam when diffraction is ,selected. Two simple alignment methods are described, one of which has resulted in a deflection of less than 0.02 gm.
1. Introduction Selected area diffraction cannot be utilised for sample areas less than 0.5 #m since the regions forming the main and diffracted beams do not overlap if the selected area at the specimen plane is less than 0.5 ttm [!]. Errors in area selection can be anything up to a few micrometres [2]*. An advantage of convergent beam diffraction over selected area diffraction is that it offers a means of obtaining more localised crystallographic information. Essentially, the minimum area analysed is determined by the size of the electron probe focused onto the sample. For our JEM-100C in TEM mode, minimum analysed area obtainable by selected area diffraction is 0.6 ~m compared to about 0.1 #m for convergent beam diffraction. In TEM mode convergent beam electron diffraction when adjusting the intermediate/diffraction lenses to form the diffraction pattern, the spot may. no longer be focussed on the specimen while the'illuminated region may shift. The focussing problem is easily solved since the shadow image in the diffraction spot or disc is eliminated by slight adjustment of the second condenser or objective lens. Steeds [3] notes that the shifted illumination case is detected by the diffraction pattern being of the wrong format, for example, no extra spots * [Editor's note: See also W.D. Riecke. Optik 18 (1961) 278.]
appear in the diffraction pattern if the beam is centred on a precipitate. The specimen stage controis can be adjusted until extra spots appear providing that they are intense enough to be seen on the fluorescent screen. When determining the crystal structure of a second phase particle enclosed in a matrix material the volume contributing to the diffraction pattern should be localised to the volume of material containing only the particle or to the column of material containing the most precipitate and least matrix material. In addition, there should be sufficient angular resolution in the diffraction pattern such that the diffraction spots from the particle and matrix can be distinguished and preferably do not overlap. This is particularly important in cases such as platinum carbide precipitates in quenched platinum where lattice spacings and crystal structure are similar. The carbide platelets, which vary in size up to about 0.25 #m in diameter and are believed to consist of one layer of carbon atoms [4], are such that convergent beam diffraction is the appropriate technique to utilise in order to extract the necessary crystallographic information required to construct the reciprocal lattice structure. Convergent beam diffraction in TEM mode was chosen in preference to that in STEM mode for these platelets in order to obtain greater angular resolution and because the defects tended to slip out of the foil during STEM diffraction. This
0304-3991/82/0000-0000/$02.75 © 1982 North-Holland
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M.J. II ~tc~nnh / Method of alignment for conrergent beam diJ'J)'action in TI'M
paper describes a reliable alignment method for convergent beam diffraction and is particularly important in cases where the precipitate is so thin andS/or small, such as platinum carbide precipitates, that the extra diffraction spots cannot be seen on the final viewing screen. Once the microscope has been aligned by the present method, the beam is focused on the precipitate, the diffraction mode button is pressed and the precipitate convergent beam diffraction pattern is obtained. No additional adjustments are required.
2. Alignment methods, results and discussion The side-entry JEM-100C. which does not have free mode operation of its lenses, was aligned according to the JEOL operators manual. The microscope was operated at 100 kV with a standard (non-pointed) tungsten filament saturated in the normal way. The beam current was 30 p.A, this being the difference between the indicated emission current and dark current. A 100 p.m variable condenser aperture and spot size 4 (maximum excitation of first condenser lens,) were used. The spot size at the sample was approximately 0.12 p.m when using the STEM condenser and objective polepieces. (A smaller spot size may be possible on other JEM-100C microscopes: our microscope shows considerable noise in the beam picked up from the STEM circuitry). These conditions yielded a degree of probe convergence, 2 a i. equal to 1.24× 10 3 radians where a i is the semi-angle between the probe and the optic axis. The beam was focussed onto the precipitate in magnification (MAG) mode and the diffraction pattern obtained by pressing the diffraction (SA DIFF) button. Diffraction patterns obtained by this method failed to yield spots other than those from the matrix. However, following exactly the same procedure on a Philips EM400 with a LaB6 filament yielded extra diffraction spots. Subsequently, contamination spot tests on a carbon film in the JEM-100C revealed that when the diffraction mode button was pressed the beam was deflected up to 0.5 p.m from its original position on the sample. (The contamination spots are really cones of contamination viewed vertically.)
One of the most important parts of the microscope alignment with respect to convergent beam diffraction purposes involves the accurate setting up of the beana displacement compensation coils. When properly aligned, the illumination spot should not shift when the focus knobs are turned (objective lens beam displacement compensation) or when the magnification is changed (intermediate lens beam displacement compensation). In the instruction manuals of newer instruments the beam compensators are required to be adjusted by the former method because the beana compensator is only made up of one set of coils. This method is quick and easy, taking about 3 rain. In the present study, without using the anticontaminator device, a feature on a carbon film was centred and focused at × 50,000, and then the probe was focused onto the feature so that the probe was about 0.2 #m in diameter. The voltage/current centres and condenser alignment circuit, the latter t a k e n to m e a n the compensator/corrector/wobbler circuit in the present paper, were aligned at X 50,000. The medium focus was turned fully counter-clockwise and the probe recentred using the alignment translate controls. Then the medium focus was turned fully clockwise and the probe recentred with the beam compensators. This procedure was repeated until no further adjustment was necessary. The condenser alignment circuit in particular was checked repeatedly during this procedure. Subsequent to this alignment, a 0.12 p.m diameter probe was focused onto the carbon film at X 50,000 in MAG mode. After 5 s. making sure the probe remained in the same spot on the film. diffraction at a camera length of 76 cm was obtained. After another 5 s, magnification mode was reinstituted and the probe position checked. The sequence was repeated until about 15 s of irradiation of the film in both magnification and diffraction mode operation had been achieved. The magnification mode contamination spot A and corresponding diffraction mode contamination spot A' (referred to henceforth as the magnification/diffraction contamination spots) are shown in fig. 1. While not coincident, the contamination spots were relatively close, being separated by 0.2 p.m. Early JEM-100C microscopes had two sets of beam compensator
M.J. Witcomb / Method of alignment for concergent beam diffraction in TEM
B
C O-
0°9
o B'
A
A'
0.3pro Fig. I. Contaminationmagnification/diffractionspot pairs on a carbon film resultingfromdifferentbeam compensatoralignments: AA' after the objective lens beam displacementcompensation method. BB' (X25.000/xI00,000) and CC' (x 100.000/x 160.000) after the intermediate lens beam displacement compensationmethod.
screws, one set for the objective lens and one for the intermediate lens compensation. While our microscope has only one set of screws, the manual contains instructions for both sets of alignment. Thus the intermediate lens adjustment was tried for spot coincidence. The voltage/current centres and condenser alignment circuit were aligned at x 50,000. A feature on the carbon film was centred and focused at × 100,000, and the 0.12/tm diameter probe was centred with the alignment translator controls. At X25,000 the probe was recentred with the beam compensators. This procedure was repeated until no further adjustment was necessary. The condenser alignment circuit was checked repeatedly during this procedure. The magnification/diffractibn contamination spot separation obtained by this method was 0.5 ~m, see spots B and B' in fig. !. The instruction manual notes that this alignment can also be carried out between × 100,000 and × 160,000. For a converging alignment, it was necessary for the alignment translators to be used at the lower magnification while the beam compensators were required at the higher magnification. After this alignment, the magnification/diffraction contamination spots, C and C' were separated by approximately 0.2 ttm, see fig. i. For precise alignment, both intermediate lens
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alignment methods took roughly 30 min. Clearly the three alignment methods are unacceptable, yielding varying magnification/diffraction contamination spot separations as well as differing spot orientation relationships. The obvious solution to the alignment problem, seemed to be to make use of the contamination spots as part of an alignment method *. This procedure will be termed the "contamination spot method". Each beam compensator was separately turned clockwise and anti-clockwise and the directional change of the diffraction contamination spot position with respect to the magnification contamination spot was noted. The probe was found to be deflected in a shallow arc rather than a straight line. A series of magnification/diffraction contamination spots was then made on a carbon film adjusting both beam compensators until coincident spots were obtained. This procedure took about 40 min and eight compensator adjustments. A spot size of 0.12 #m was utilised, while all the contamination spots were made )<50,000 and a diffraction camera length of 76 cm. Between each adjustment the current/voltage centre and condenser alignment circuit were adjusted. After this alignment, only one circular contamination spot could be seen so that the spot separation was certainly less than 20 nm. Successful convergent beam diffraction patterns of precipitates smaller than 0.1 #m diameter were then taken with the )<50,000/76 cm combination over a four-month period during which time no change in alignment was observed to have occurred. An example of such work on platinum carbides is shown in fig. 2 and 3. A faulted l / 3 a [100] loop (carbide precipitate) is shown edge-on to the beam in fig. 2, while fig. 3 is the corresponding convergent beam diffraction pattern. The predominant streaking testifies to the accuracy of the microscope alignment as does the contamination on the sample. (A theory together with supporting experimental evidence for the generation of contamination rings and spots on samples has been reported by Fourie [5,6].) The contamination ring was the result of about 10 min exposure to the probe on a rather dirty part of the specimen. The probe was elon* [Editor's note: See K.H. Mtiller,Optik 33. (1971) 296.]
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M.J. ||l'i/('(in|h / Method t~f alignment fi~r contrer~ent beam difl'ra~'thm in TI'M
"i
Fig. 2. Faulted I/3a [100] loop on edge to the beam. Note that after taking the convergent beam diffraction pattern the magnification/diffraction contamination spot pair is centred on the loop.
gated using the condenser stigmator controls in order to sample as much of the precipitate as possible. Since the probe was centred on the precipitate, the alignment accuracy for the diffraction pattern was within about 15 rim. Subsequently, it seemed that a quicker method of setting up and checking the alignment might be to observe the shadow image of a sample feature in the central convergent beam diffraction disc as the second condenser lens was adjusted. This will be referred to as the "diffraction spot method". As the second condenser lens is adjusted towards probe focus, the image in the diffraction spot increases in magnification and disappears at in-
o
0
0
o O
O
C O
O Fig. 3. Convergent beam [110] diifraction pattern from the edge-on precipitate in fig. 2. Streaks are from the precipitate, .,,pots are derived Irom the platinum matrix.
finite magnification (probe focus) with no spatial information then being present in the diffraction spot. The diffraction pattern is then in an exact reciprocal relationship with the image plane. Continued same direction adjustment of the second condenser lens causes an inversion at probe focus after which the feature reappears in the spot and decreases in size. Thus it might be expected that perfect alignment would be when it is possible to adjust the beam compensators so that a shadow image of a feature on the sample such as a recognisable edge or small contamination spot comes up and down centrally in the diffraction disc with varying condenser focus. As a test of this postulate, the microscope was completely misaligned, then realigned as per the JEOL instruction manual but using the diffraction spot method to set up the beam compensators. The beam compensator alignment took 5-10 min and was routine but tedious. It was found necessary to check the current/voltage centre and especially the condenser alignment circuit particularly after each beam compensator adjustment during the early part of the alignment. Again the alignment was carried out at X50,000/76 cm. It was found after several attempts that the method could yield almost coinciding magnification/diffractioncontamination spots. However it was necessary for the alignment to be absolutely precise with the condenser aperture accurately centred and for a contamination spot test to be carried out to confirm the alignment. The best alignment achieved was a spot separation of 40 nm. It should be emphasised that the movement of the feature in the diffraction disc is extremely sensitive to the slightest displacement of the probe on the sample, whether it be as a result of the contamination build up and charging on the feature during the alignment process a n d / o r through beam instability. As has been stressed earlier, the alignment and diffraction were all carried out in the present study at the same magnification (×50,000), camera length (76 cm) and accelerating voltage (100 kV). An evaluation has been made of the parameters that may affect the alignment if the microscope is aligned under the above conditions, but convergent beam diffraction patterns obtained under different circumstances:
M.J. Witcomb / Method of alignment for convergent beam diffraction it~ TEM
(I) Accelerating voltage: no detectable difference was noted when taking diffraction patterns at 60 and 100 kV. (2) Current/voltage centre adjustment: no significant difference was found when either of these alignment methods was used to set up the image rotation centre. (3) Objective lens setting: there are two combinations of the objective course and medium settings for a given feature focus. The alignment was found to be independent of the combination used. Varying the stage height (Z) control between the two extreme positions which correspondingly necessitated greatly varying the objective lens current had no effect on the alignment. (4) Camera length: the camera lengths closest to the one used in the alignment. 46 and 120 cm, caused negligible spot separation. The extreme camera lengths of 20 and 360 cm were found to be unacceptable, the latter camera length resulting in a spot separation of 0.15 #m. The spot deviations did not show any obvious relationship to the rotation variations between the different camera lengths. (5) Condenser alignment circuit adjustment: this must be fairly precise. Unfortunately on our microscope it is subject to random drift. Fig. 4 shows the effect of short-term drift. For this micrograph, the circuit was adjusted and immediately the series AA' at 76 cm and BB' at 360 cm was taken. At the completion of this series the condenser alignment circuit was checked and the Y alignment compensator/corrector was found to have slightly altered. It was readjusted and the sequence CC' at 76 cm and DD' at 360 cm was taken. Obviously such a wobbler drift or alignment inaccuracy can be disastrous.' (6) Magnification: Fig. 5 shows a sequence of contamination spots taken at different magnifications but with the condenser alignment circuit always adjusted at ×50,000/76 cm. The contamination spot method of alignment was used at ×50,000 (note, it can be seen that the alignment was not quite exact, a circular coincident spot being achieved at × 66,000 instead of × 50,000). There is obviously a limitation on the usable magnifications for undertaking convergent beam diffraction patterns with the contamination and diffraction
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66
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l;ig. 4. Condenser alignment circuit adjustment magnification/diffraction contamination spots as a function of camera length and circuit drift. AA' and CC' at 76 cm without and with drift. BB' and DD' at 360 cm without and with drift respectively.
alignment procedures. The deviation from spot coincidence corresponds directly to the post sample lens programming sequence. Between × 33,000 and × 50,000 the image rotation abruptly changes direction and rotates through a fairly large angle of 53 °. Between × 100,000 and X 130,000 the image suddenly changes direction by 61°. Magnification changes between these transition points cause only small ( ~ 10°), same direction rotations and generally result in only a rotational difference between the spots. Thus for larger precipitates, original alignment of the beam compensators at say × 20,000 would yield a greater choice of working magnifications, seven magnification settings instead of three.
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Fig. 5. Effect of different magnifications on the magnification/diffraction contamination spot pairs. The original alignment was by the diffraction spot method × 50.000/76 cm.
M..I. ll'itc,md, / Method ~ ahgmm'm for com'crgem &'am dtfl'ruction in TI'M
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(7) Condenser alignment circuit/magnification combinations: When the magnification is changed, the alignment of the condenser alignment circuit alters. Two studies have been made using the beam compensator alignment conditions of ×50,000/76 cm. The first investigation involved setting the condenser alignment circuit and irradiating the carbon film at the same magnification. A similar set of contamination spots at each magnification was obtained as in fig. 5, the spot separations being within 10% of those measured in that figure. The second study was to set the condenser alignment circuit at different magnifications but to generate contamination spots at × 50,000. Over the magnification range × 8,300 to × 160,000 spot coincidence was excellent, see fig. 6, but below × 8,300 the condenser alignment circuit markedly changes, and this results in significantly diverging spots. This appears to be related to the large difference in condenser alignment between the lower and higher magnifications. A few further comments are pertinent to the taking of convergent beam diffraction patterns. As mentioned earlier, due to the size of the defects in the quenched platinum, the defects yielded such an extremely weak signal that the precipitate spots were not visible on the viewing screen. Extended exposure times were thus essential, 120-300 s being typical. These exposures place stringent require-
0
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Fig. 6. Effect of irradiating tile carbon film with a fl~cused probe at × 50,000 but setting the condenser align=neat circuit at different magnifications. The original alignment was by the contamination spot method at x 50.(X)0/46 cm.
ments on the specimen stage stability. In the present work, however, since the stage was somewhat unstable due to user damage, repeated 12 s multiple exposures were utilised. The method is extremely tedious and tiring. The only advantage is that one is certain that the probe does remain on the precipitate during the majority of the exposure time. As contamination and charge build up on the analysed area, the probe tends to wander off the feature of interest. In addition some form of hysteresis does appear to be present initially when switching between different modes of operation of the JEM-100C. Usually the first several times that the diffraction button is pressed causes a significant displacement of the probe. Thus the magnification/diffraction sequence was always carried out 4-5 times or until stability was achieved before a particular area of the carbon film was exposed for each pair of spots. In addition, if by accident the buttons next to MAG (LOW MAG and SCAN) were pressed instead of MAG when returning from diffraction mode operation the probe could be displaced some micrometres. Area charging was at least partially responsible for some probe ;displacement since moving to a different neighbouring area in the course of a multiple exposure operation, during which time the probe had remained stationary on the sample, caused the probe to be displaced some distance. The convergent beam diffraction patterns taken of precipitates in platinum were recorded on Kodak electron image film type 4463 and processed in concentrated Kodak D-19 developer at 20°C for times up to 20 min. It should be noted that this film has now been withdrawn from the market by Kodak and replaced by type 4489. Unfortunately the latter is only half the speed of the former. A good general background on convergent beam diffraction has been given in two review articles by Lehmpfuhl [7] and Steeds [3]. A method of setting up convergent beam diffraction in a JEM-100B using free-mode operation of the lenses has been described by Goodman [8]. Olsen and Goodman [9] have discussed the importance of the source image used for convergent beam diffraction and high resolution imaging. With regard to probe deflection, Tanaka [10] has reported for a JEM100CX, utilising a modified condenser-objective
MJ. Witcomh / Method of alignmentfor concergent I,eam diffraction in TEM
lens, that the diffraction pattern obtained by the selected area diffraction (SA DIFF) function is not exactly the same as defined by the selected area magnification (SA MAG) function. (On the JEM-100C the deflection occurs whether MAG or SA MAG functions are used, for example, see fig. 5.) Tanaka states that the change of the stray magnetic field from the intermediate lens caused by switching over from SA MAG to SA DIFF affects the effective field intensity of the objective lens and gives rise to the deflection of the incident beam when it passes through the specimen chamber, that is, there is cross-talk between the imaging lenses. The result is to defocus the image or enlarge the probe size as well as deflecting the probe. Even though the microscope was equipped with a special iron pipe around the optic axis in the specimen chamber to avoid the influence of external magnetic fields, the probe was deflected although it was only by 15 nm. Utilising present standard microscope manual alignment procedures for the JEM-100C, the probe deflection is at least an order of magnitude greater than this unless procedures as outlined earlier are applied.
3. Conclusions Using a probe size of 0.12 tim, an alignment is easily possible whereby the minimum separation of the contamination spots obtained by convergent beam in MAG and SA DIFF modes is less than 25 nm when using the contamination or diffraction spot method of alignment of the beam compensators. A marginally better alignment accuracy of 15 nm was achievable using the contamination spot method. Nattirally, a smaller probe size than that used here would permit a greater accuracy. The final alignment steps were always carried out using short exposure times of the probe on the carbon film in order to keep the contamination spots as small as possible while allowing them to be seen. When the best coincident spot was achieved (by the contamination spot method), a feature on the sample was found not to come up quite centrally in the diffraction disc when repeatedly checking the alignment with the diffraction spot method. While the most satisfactory method to take con-
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vergent beam diffraction patterns is under the same conditions as the original alignment, some latitude appears possible in the condensor alignment adjustment provided the magnification used when taking a diffraction pattern is set to be the same as or close to (see fig. 5) the original alignment magnification value. The variation permissible should be checked for each instrument; the limits will be a function both of the size of the probe and the feature to be analysed. The final accuracy of the alignment will be dependent on the positional stability of the probe on the feature on the sample, the influence on the probe of the switching processes, hysteresis and circuit drift as well as the size of the probe utilised or obtainable.
Acknowledgements The initial platinum carbide convergent beam diffraction work was carried out on the Philips EM400 while the author was on leave at the Materials and Molecular Research Division, Lawrence Berkeley Laboratory and the Department of Materials Science and Mineral Engineering, University of California, Berkeley. Professor G. Thomas and Dr. K.H. Westmacott are thanked for their hospitality during that visit and Dr. U. Dahmen is gratefully acknowledged for heat-treating the platinum sample used in this paper. Financial support is gratefully acknowledged from the Council of the University of the Witwatersrand, CSIR, Pretoria and LBLUC. References [I] J.B. Warren, Microdiffraction, in: Introduction to Analytical Electron Microscopy, Fxts.J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum, New York, 19"/9) p. 369. [2| M.H. Loretto and R.E. Smallman, Defect Analysis in Electron Microscopy (Chapman and tiall, London, 1975) p.10 [3] J.W. Steeds, Convergent Beam Electron Diffraction, in: Introduction to Analytical Electron Microscopy, Eds. J.J. Hrcn, J.l. Goldstein and D.C. Joy (Plenum, New York, 1979) p. 387. [4] K.H. Westmacott and M.I. Perez, J. Nucl. Mater. 83 (1979) 231. [51 J.T, Fourie, in: Scanning Electron Microscopy/1979/ll, Ed. O. Johari (AMF O'Hare, IL) p. 87.
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M.J. Witcomh / Method of alignment for ('ont,ergent beam diffra(.tion in TEM
[6] J.T. Fourie, in: Scanning Electron Microscopy/1981, Ed. O. Johari (AMF O'Hare, IL), in press. [7] G. Lehmpfuhl, in: Proc. 9th Intern. Congr. on Electron Microscopy, Vol. I11 (Microscopical Soc. of Canada. Toronto, Ontario, 1978) p. 304.
[8] P. Goodman, in: Scanning Electron Microscopy/I980/l, Ed. O. Johari ( A M F O'Hare, IL) p. 53. [91 A. Oben and P. Goodman, Ultramicroscopy 6 ( 1981 ) I01. [I0] M. Tanaka, J E O L Ncw,~ 16E-3 (1978) 13.
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A METHOD OF ALIGNMENT FOR CONVERGENT BEAM DIFFRACTION IN TEM MODE FOR A JEM-100C ELECTRON MICROSCOPE M.J. WITCOMB Electron Microscope Unit. Universit)' of the Wit.'atersrond, Johannesburg 2000, Republic i~f South Africa Received 26 November 1981
The standard methods of aligning the microscope for convergent beam diffraction are shown to result in a 0.2-0.5 ~tm deflection of the beam when diffraction is ,selected. Two simple alignment methods are described, one of which has resulted in a deflection of less than 0.02 gm.
1. Introduction Selected area diffraction cannot be utilised for sample areas less than 0.5 #m since the regions forming the main and diffracted beams do not overlap if the selected area at the specimen plane is less than 0.5 ttm [!]. Errors in area selection can be anything up to a few micrometres [2]*. An advantage of convergent beam diffraction over selected area diffraction is that it offers a means of obtaining more localised crystallographic information. Essentially, the minimum area analysed is determined by the size of the electron probe focused onto the sample. For our JEM-100C in TEM mode, minimum analysed area obtainable by selected area diffraction is 0.6 ~m compared to about 0.1 #m for convergent beam diffraction. In TEM mode convergent beam electron diffraction when adjusting the intermediate/diffraction lenses to form the diffraction pattern, the spot may. no longer be focussed on the specimen while the'illuminated region may shift. The focussing problem is easily solved since the shadow image in the diffraction spot or disc is eliminated by slight adjustment of the second condenser or objective lens. Steeds [3] notes that the shifted illumination case is detected by the diffraction pattern being of the wrong format, for example, no extra spots * [Editor's note: See also W.D. Riecke. Optik 18 (1961) 278.]
appear in the diffraction pattern if the beam is centred on a precipitate. The specimen stage controis can be adjusted until extra spots appear providing that they are intense enough to be seen on the fluorescent screen. When determining the crystal structure of a second phase particle enclosed in a matrix material the volume contributing to the diffraction pattern should be localised to the volume of material containing only the particle or to the column of material containing the most precipitate and least matrix material. In addition, there should be sufficient angular resolution in the diffraction pattern such that the diffraction spots from the particle and matrix can be distinguished and preferably do not overlap. This is particularly important in cases such as platinum carbide precipitates in quenched platinum where lattice spacings and crystal structure are similar. The carbide platelets, which vary in size up to about 0.25 #m in diameter and are believed to consist of one layer of carbon atoms [4], are such that convergent beam diffraction is the appropriate technique to utilise in order to extract the necessary crystallographic information required to construct the reciprocal lattice structure. Convergent beam diffraction in TEM mode was chosen in preference to that in STEM mode for these platelets in order to obtain greater angular resolution and because the defects tended to slip out of the foil during STEM diffraction. This
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M.J. II ~tc~nnh / Method of alignment for conrergent beam diJ'J)'action in TI'M
paper describes a reliable alignment method for convergent beam diffraction and is particularly important in cases where the precipitate is so thin andS/or small, such as platinum carbide precipitates, that the extra diffraction spots cannot be seen on the final viewing screen. Once the microscope has been aligned by the present method, the beam is focused on the precipitate, the diffraction mode button is pressed and the precipitate convergent beam diffraction pattern is obtained. No additional adjustments are required.
2. Alignment methods, results and discussion The side-entry JEM-100C. which does not have free mode operation of its lenses, was aligned according to the JEOL operators manual. The microscope was operated at 100 kV with a standard (non-pointed) tungsten filament saturated in the normal way. The beam current was 30 p.A, this being the difference between the indicated emission current and dark current. A 100 p.m variable condenser aperture and spot size 4 (maximum excitation of first condenser lens,) were used. The spot size at the sample was approximately 0.12 p.m when using the STEM condenser and objective polepieces. (A smaller spot size may be possible on other JEM-100C microscopes: our microscope shows considerable noise in the beam picked up from the STEM circuitry). These conditions yielded a degree of probe convergence, 2 a i. equal to 1.24× 10 3 radians where a i is the semi-angle between the probe and the optic axis. The beam was focussed onto the precipitate in magnification (MAG) mode and the diffraction pattern obtained by pressing the diffraction (SA DIFF) button. Diffraction patterns obtained by this method failed to yield spots other than those from the matrix. However, following exactly the same procedure on a Philips EM400 with a LaB6 filament yielded extra diffraction spots. Subsequently, contamination spot tests on a carbon film in the JEM-100C revealed that when the diffraction mode button was pressed the beam was deflected up to 0.5 p.m from its original position on the sample. (The contamination spots are really cones of contamination viewed vertically.)
One of the most important parts of the microscope alignment with respect to convergent beam diffraction purposes involves the accurate setting up of the beana displacement compensation coils. When properly aligned, the illumination spot should not shift when the focus knobs are turned (objective lens beam displacement compensation) or when the magnification is changed (intermediate lens beam displacement compensation). In the instruction manuals of newer instruments the beam compensators are required to be adjusted by the former method because the beana compensator is only made up of one set of coils. This method is quick and easy, taking about 3 rain. In the present study, without using the anticontaminator device, a feature on a carbon film was centred and focused at × 50,000, and then the probe was focused onto the feature so that the probe was about 0.2 #m in diameter. The voltage/current centres and condenser alignment circuit, the latter t a k e n to m e a n the compensator/corrector/wobbler circuit in the present paper, were aligned at X 50,000. The medium focus was turned fully counter-clockwise and the probe recentred using the alignment translate controls. Then the medium focus was turned fully clockwise and the probe recentred with the beam compensators. This procedure was repeated until no further adjustment was necessary. The condenser alignment circuit in particular was checked repeatedly during this procedure. Subsequent to this alignment, a 0.12 p.m diameter probe was focused onto the carbon film at X 50,000 in MAG mode. After 5 s. making sure the probe remained in the same spot on the film. diffraction at a camera length of 76 cm was obtained. After another 5 s, magnification mode was reinstituted and the probe position checked. The sequence was repeated until about 15 s of irradiation of the film in both magnification and diffraction mode operation had been achieved. The magnification mode contamination spot A and corresponding diffraction mode contamination spot A' (referred to henceforth as the magnification/diffraction contamination spots) are shown in fig. 1. While not coincident, the contamination spots were relatively close, being separated by 0.2 p.m. Early JEM-100C microscopes had two sets of beam compensator
M.J. Witcomb / Method of alignment for concergent beam diffraction in TEM
B
C O-
0°9
o B'
A
A'
0.3pro Fig. I. Contaminationmagnification/diffractionspot pairs on a carbon film resultingfromdifferentbeam compensatoralignments: AA' after the objective lens beam displacementcompensation method. BB' (X25.000/xI00,000) and CC' (x 100.000/x 160.000) after the intermediate lens beam displacement compensationmethod.
screws, one set for the objective lens and one for the intermediate lens compensation. While our microscope has only one set of screws, the manual contains instructions for both sets of alignment. Thus the intermediate lens adjustment was tried for spot coincidence. The voltage/current centres and condenser alignment circuit were aligned at x 50,000. A feature on the carbon film was centred and focused at × 100,000, and the 0.12/tm diameter probe was centred with the alignment translator controls. At X25,000 the probe was recentred with the beam compensators. This procedure was repeated until no further adjustment was necessary. The condenser alignment circuit was checked repeatedly during this procedure. The magnification/diffractibn contamination spot separation obtained by this method was 0.5 ~m, see spots B and B' in fig. !. The instruction manual notes that this alignment can also be carried out between × 100,000 and × 160,000. For a converging alignment, it was necessary for the alignment translators to be used at the lower magnification while the beam compensators were required at the higher magnification. After this alignment, the magnification/diffraction contamination spots, C and C' were separated by approximately 0.2 ttm, see fig. i. For precise alignment, both intermediate lens
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alignment methods took roughly 30 min. Clearly the three alignment methods are unacceptable, yielding varying magnification/diffraction contamination spot separations as well as differing spot orientation relationships. The obvious solution to the alignment problem, seemed to be to make use of the contamination spots as part of an alignment method *. This procedure will be termed the "contamination spot method". Each beam compensator was separately turned clockwise and anti-clockwise and the directional change of the diffraction contamination spot position with respect to the magnification contamination spot was noted. The probe was found to be deflected in a shallow arc rather than a straight line. A series of magnification/diffraction contamination spots was then made on a carbon film adjusting both beam compensators until coincident spots were obtained. This procedure took about 40 min and eight compensator adjustments. A spot size of 0.12 #m was utilised, while all the contamination spots were made )<50,000 and a diffraction camera length of 76 cm. Between each adjustment the current/voltage centre and condenser alignment circuit were adjusted. After this alignment, only one circular contamination spot could be seen so that the spot separation was certainly less than 20 nm. Successful convergent beam diffraction patterns of precipitates smaller than 0.1 #m diameter were then taken with the )<50,000/76 cm combination over a four-month period during which time no change in alignment was observed to have occurred. An example of such work on platinum carbides is shown in fig. 2 and 3. A faulted l / 3 a [100] loop (carbide precipitate) is shown edge-on to the beam in fig. 2, while fig. 3 is the corresponding convergent beam diffraction pattern. The predominant streaking testifies to the accuracy of the microscope alignment as does the contamination on the sample. (A theory together with supporting experimental evidence for the generation of contamination rings and spots on samples has been reported by Fourie [5,6].) The contamination ring was the result of about 10 min exposure to the probe on a rather dirty part of the specimen. The probe was elon* [Editor's note: See K.H. Mtiller,Optik 33. (1971) 296.]
346
M.J. ||l'i/('(in|h / Method t~f alignment fi~r contrer~ent beam difl'ra~'thm in TI'M
"i
Fig. 2. Faulted I/3a [100] loop on edge to the beam. Note that after taking the convergent beam diffraction pattern the magnification/diffraction contamination spot pair is centred on the loop.
gated using the condenser stigmator controls in order to sample as much of the precipitate as possible. Since the probe was centred on the precipitate, the alignment accuracy for the diffraction pattern was within about 15 rim. Subsequently, it seemed that a quicker method of setting up and checking the alignment might be to observe the shadow image of a sample feature in the central convergent beam diffraction disc as the second condenser lens was adjusted. This will be referred to as the "diffraction spot method". As the second condenser lens is adjusted towards probe focus, the image in the diffraction spot increases in magnification and disappears at in-
o
0
0
o O
O
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O Fig. 3. Convergent beam [110] diifraction pattern from the edge-on precipitate in fig. 2. Streaks are from the precipitate, .,,pots are derived Irom the platinum matrix.
finite magnification (probe focus) with no spatial information then being present in the diffraction spot. The diffraction pattern is then in an exact reciprocal relationship with the image plane. Continued same direction adjustment of the second condenser lens causes an inversion at probe focus after which the feature reappears in the spot and decreases in size. Thus it might be expected that perfect alignment would be when it is possible to adjust the beam compensators so that a shadow image of a feature on the sample such as a recognisable edge or small contamination spot comes up and down centrally in the diffraction disc with varying condenser focus. As a test of this postulate, the microscope was completely misaligned, then realigned as per the JEOL instruction manual but using the diffraction spot method to set up the beam compensators. The beam compensator alignment took 5-10 min and was routine but tedious. It was found necessary to check the current/voltage centre and especially the condenser alignment circuit particularly after each beam compensator adjustment during the early part of the alignment. Again the alignment was carried out at X50,000/76 cm. It was found after several attempts that the method could yield almost coinciding magnification/diffractioncontamination spots. However it was necessary for the alignment to be absolutely precise with the condenser aperture accurately centred and for a contamination spot test to be carried out to confirm the alignment. The best alignment achieved was a spot separation of 40 nm. It should be emphasised that the movement of the feature in the diffraction disc is extremely sensitive to the slightest displacement of the probe on the sample, whether it be as a result of the contamination build up and charging on the feature during the alignment process a n d / o r through beam instability. As has been stressed earlier, the alignment and diffraction were all carried out in the present study at the same magnification (×50,000), camera length (76 cm) and accelerating voltage (100 kV). An evaluation has been made of the parameters that may affect the alignment if the microscope is aligned under the above conditions, but convergent beam diffraction patterns obtained under different circumstances:
M.J. Witcomb / Method of alignment for convergent beam diffraction it~ TEM
(I) Accelerating voltage: no detectable difference was noted when taking diffraction patterns at 60 and 100 kV. (2) Current/voltage centre adjustment: no significant difference was found when either of these alignment methods was used to set up the image rotation centre. (3) Objective lens setting: there are two combinations of the objective course and medium settings for a given feature focus. The alignment was found to be independent of the combination used. Varying the stage height (Z) control between the two extreme positions which correspondingly necessitated greatly varying the objective lens current had no effect on the alignment. (4) Camera length: the camera lengths closest to the one used in the alignment. 46 and 120 cm, caused negligible spot separation. The extreme camera lengths of 20 and 360 cm were found to be unacceptable, the latter camera length resulting in a spot separation of 0.15 #m. The spot deviations did not show any obvious relationship to the rotation variations between the different camera lengths. (5) Condenser alignment circuit adjustment: this must be fairly precise. Unfortunately on our microscope it is subject to random drift. Fig. 4 shows the effect of short-term drift. For this micrograph, the circuit was adjusted and immediately the series AA' at 76 cm and BB' at 360 cm was taken. At the completion of this series the condenser alignment circuit was checked and the Y alignment compensator/corrector was found to have slightly altered. It was readjusted and the sequence CC' at 76 cm and DD' at 360 cm was taken. Obviously such a wobbler drift or alignment inaccuracy can be disastrous.' (6) Magnification: Fig. 5 shows a sequence of contamination spots taken at different magnifications but with the condenser alignment circuit always adjusted at ×50,000/76 cm. The contamination spot method of alignment was used at ×50,000 (note, it can be seen that the alignment was not quite exact, a circular coincident spot being achieved at × 66,000 instead of × 50,000). There is obviously a limitation on the usable magnifications for undertaking convergent beam diffraction patterns with the contamination and diffraction
AAS 76cm
-i
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347
66
DS
360e 0.3pm
l;ig. 4. Condenser alignment circuit adjustment magnification/diffraction contamination spots as a function of camera length and circuit drift. AA' and CC' at 76 cm without and with drift. BB' and DD' at 360 cm without and with drift respectively.
alignment procedures. The deviation from spot coincidence corresponds directly to the post sample lens programming sequence. Between × 33,000 and × 50,000 the image rotation abruptly changes direction and rotates through a fairly large angle of 53 °. Between × 100,000 and X 130,000 the image suddenly changes direction by 61°. Magnification changes between these transition points cause only small ( ~ 10°), same direction rotations and generally result in only a rotational difference between the spots. Thus for larger precipitates, original alignment of the beam compensators at say × 20,000 would yield a greater choice of working magnifications, seven magnification settings instead of three.
11• 160000
e3oo • 10000 • 16000 • $ 20000 0 • 26000 0 •
011 130000 •
I00000
• 66000
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Fig. 5. Effect of different magnifications on the magnification/diffraction contamination spot pairs. The original alignment was by the diffraction spot method × 50.000/76 cm.
M..I. ll'itc,md, / Method ~ ahgmm'm for com'crgem &'am dtfl'ruction in TI'M
34g
(7) Condenser alignment circuit/magnification combinations: When the magnification is changed, the alignment of the condenser alignment circuit alters. Two studies have been made using the beam compensator alignment conditions of ×50,000/76 cm. The first investigation involved setting the condenser alignment circuit and irradiating the carbon film at the same magnification. A similar set of contamination spots at each magnification was obtained as in fig. 5, the spot separations being within 10% of those measured in that figure. The second study was to set the condenser alignment circuit at different magnifications but to generate contamination spots at × 50,000. Over the magnification range × 8,300 to × 160,000 spot coincidence was excellent, see fig. 6, but below × 8,300 the condenser alignment circuit markedly changes, and this results in significantly diverging spots. This appears to be related to the large difference in condenser alignment between the lower and higher magnifications. A few further comments are pertinent to the taking of convergent beam diffraction patterns. As mentioned earlier, due to the size of the defects in the quenched platinum, the defects yielded such an extremely weak signal that the precipitate spots were not visible on the viewing screen. Extended exposure times were thus essential, 120-300 s being typical. These exposures place stringent require-
0
8300
160000 0 50000 0
.)
6600 0.3pm
Fig. 6. Effect of irradiating tile carbon film with a fl~cused probe at × 50,000 but setting the condenser align=neat circuit at different magnifications. The original alignment was by the contamination spot method at x 50.(X)0/46 cm.
ments on the specimen stage stability. In the present work, however, since the stage was somewhat unstable due to user damage, repeated 12 s multiple exposures were utilised. The method is extremely tedious and tiring. The only advantage is that one is certain that the probe does remain on the precipitate during the majority of the exposure time. As contamination and charge build up on the analysed area, the probe tends to wander off the feature of interest. In addition some form of hysteresis does appear to be present initially when switching between different modes of operation of the JEM-100C. Usually the first several times that the diffraction button is pressed causes a significant displacement of the probe. Thus the magnification/diffraction sequence was always carried out 4-5 times or until stability was achieved before a particular area of the carbon film was exposed for each pair of spots. In addition, if by accident the buttons next to MAG (LOW MAG and SCAN) were pressed instead of MAG when returning from diffraction mode operation the probe could be displaced some micrometres. Area charging was at least partially responsible for some probe ;displacement since moving to a different neighbouring area in the course of a multiple exposure operation, during which time the probe had remained stationary on the sample, caused the probe to be displaced some distance. The convergent beam diffraction patterns taken of precipitates in platinum were recorded on Kodak electron image film type 4463 and processed in concentrated Kodak D-19 developer at 20°C for times up to 20 min. It should be noted that this film has now been withdrawn from the market by Kodak and replaced by type 4489. Unfortunately the latter is only half the speed of the former. A good general background on convergent beam diffraction has been given in two review articles by Lehmpfuhl [7] and Steeds [3]. A method of setting up convergent beam diffraction in a JEM-100B using free-mode operation of the lenses has been described by Goodman [8]. Olsen and Goodman [9] have discussed the importance of the source image used for convergent beam diffraction and high resolution imaging. With regard to probe deflection, Tanaka [10] has reported for a JEM100CX, utilising a modified condenser-objective
MJ. Witcomh / Method of alignmentfor concergent I,eam diffraction in TEM
lens, that the diffraction pattern obtained by the selected area diffraction (SA DIFF) function is not exactly the same as defined by the selected area magnification (SA MAG) function. (On the JEM-100C the deflection occurs whether MAG or SA MAG functions are used, for example, see fig. 5.) Tanaka states that the change of the stray magnetic field from the intermediate lens caused by switching over from SA MAG to SA DIFF affects the effective field intensity of the objective lens and gives rise to the deflection of the incident beam when it passes through the specimen chamber, that is, there is cross-talk between the imaging lenses. The result is to defocus the image or enlarge the probe size as well as deflecting the probe. Even though the microscope was equipped with a special iron pipe around the optic axis in the specimen chamber to avoid the influence of external magnetic fields, the probe was deflected although it was only by 15 nm. Utilising present standard microscope manual alignment procedures for the JEM-100C, the probe deflection is at least an order of magnitude greater than this unless procedures as outlined earlier are applied.
3. Conclusions Using a probe size of 0.12 tim, an alignment is easily possible whereby the minimum separation of the contamination spots obtained by convergent beam in MAG and SA DIFF modes is less than 25 nm when using the contamination or diffraction spot method of alignment of the beam compensators. A marginally better alignment accuracy of 15 nm was achievable using the contamination spot method. Nattirally, a smaller probe size than that used here would permit a greater accuracy. The final alignment steps were always carried out using short exposure times of the probe on the carbon film in order to keep the contamination spots as small as possible while allowing them to be seen. When the best coincident spot was achieved (by the contamination spot method), a feature on the sample was found not to come up quite centrally in the diffraction disc when repeatedly checking the alignment with the diffraction spot method. While the most satisfactory method to take con-
349
vergent beam diffraction patterns is under the same conditions as the original alignment, some latitude appears possible in the condensor alignment adjustment provided the magnification used when taking a diffraction pattern is set to be the same as or close to (see fig. 5) the original alignment magnification value. The variation permissible should be checked for each instrument; the limits will be a function both of the size of the probe and the feature to be analysed. The final accuracy of the alignment will be dependent on the positional stability of the probe on the feature on the sample, the influence on the probe of the switching processes, hysteresis and circuit drift as well as the size of the probe utilised or obtainable.
Acknowledgements The initial platinum carbide convergent beam diffraction work was carried out on the Philips EM400 while the author was on leave at the Materials and Molecular Research Division, Lawrence Berkeley Laboratory and the Department of Materials Science and Mineral Engineering, University of California, Berkeley. Professor G. Thomas and Dr. K.H. Westmacott are thanked for their hospitality during that visit and Dr. U. Dahmen is gratefully acknowledged for heat-treating the platinum sample used in this paper. Financial support is gratefully acknowledged from the Council of the University of the Witwatersrand, CSIR, Pretoria and LBLUC. References [I] J.B. Warren, Microdiffraction, in: Introduction to Analytical Electron Microscopy, Fxts.J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum, New York, 19"/9) p. 369. [2| M.H. Loretto and R.E. Smallman, Defect Analysis in Electron Microscopy (Chapman and tiall, London, 1975) p.10 [3] J.W. Steeds, Convergent Beam Electron Diffraction, in: Introduction to Analytical Electron Microscopy, Eds. J.J. Hrcn, J.l. Goldstein and D.C. Joy (Plenum, New York, 1979) p. 387. [4] K.H. Westmacott and M.I. Perez, J. Nucl. Mater. 83 (1979) 231. [51 J.T, Fourie, in: Scanning Electron Microscopy/1979/ll, Ed. O. Johari (AMF O'Hare, IL) p. 87.
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M.J. Witcomh / Method of alignment for ('ont,ergent beam diffra(.tion in TEM
[6] J.T. Fourie, in: Scanning Electron Microscopy/1981, Ed. O. Johari (AMF O'Hare, IL), in press. [7] G. Lehmpfuhl, in: Proc. 9th Intern. Congr. on Electron Microscopy, Vol. I11 (Microscopical Soc. of Canada. Toronto, Ontario, 1978) p. 304.
[8] P. Goodman, in: Scanning Electron Microscopy/I980/l, Ed. O. Johari ( A M F O'Hare, IL) p. 53. [91 A. Oben and P. Goodman, Ultramicroscopy 6 ( 1981 ) I01. [I0] M. Tanaka, J E O L Ncw,~ 16E-3 (1978) 13.