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A high performance Wehnelt grid for transmission electron microscopes
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Micron, 1973,4:121-135
A high performance Wehnelt grid for transmission electron microscopes B. V. JOHANSEN Methodology Department, National Institute of Public Health, Geitmyrsveien 75, Oslo 1, Norway.
Manuscript received July 25, 1972: Revised April 11, 1973
A modified Wehnelt grid using conventional 'hair-pin' filaments has been designed for the electron gun of a transmission electron microscope. Unlike the standard cylindrical grid, the modified grid has a conical shape. The modified grid gives a significant increase in brightness of illumination compared with the standard form using a 'hair-pin' filament and similar brightness, coherence and resolution when compared with a grid fitted with a pointed filament. Further advantages of the modified design are that the filament life is considerably increased and there is less effect on the specimen in terms of contamination or etching. Un nouveau genre de cylindre Wehnelt utilisant desfilaments en U d'usage courant a gt~ construit pour le canon a dlectrons d'un microscope glectronique d transmission. Contrairement gtla grille du module standard, la grille modif~e est de forme conique. La grille modif~e produit une plus grande intensit~ d' illumination que la grille standard comportant des filaments en U et une rgsolution, coMrence et intensit~ analogue ~ celle d'une grille munie d'une source ponctueUe. Par ailleurs la durge de service des filaments est consid~rablement augmentge et la contamination du specimen est rgduite quand les nouvelles grilles sont utilisges. Ein modifizierter Wehneltzylinder mit herkOmmlicher Haarnadelkathode wurde fiir den Elektronenstrahlenerzeuger eines Transmissionselektronenmikroskops konstruiert. Im Gegensatz zu herkOmmlichen Wehneltzylindern ist dieser kegelf6rmig. Das modifizierte Gitter erzielt eine signifikant hOhere Beleuchtungsstiirke als die normale Haarnadelmontage und iihnliche Beleuchtungsstiirke, Kohiirenz und AuflOsung wie die Spitzenkathode. Vorteile dieser Modifikation sind auch die erheblich verliingerte Lebensdauer der Kathode und die herabgesetzte Verunreinigung und Jftzung der Probe.
INTRODUCTION In studies being carried out in this laboratory on methods to improve operating conditions for high resolution electron microscopy of biological specimens, we have recently obtained encouraging results with a modified design of the Wehnelt grid using standard 'hair-pin' filaments. The modified design is based on developments from the J E O L scanning electron microscope grid assembly and the work of Steigerwald (1949) and features a casing which is conical in shape. Pointed filaments give a significant improvement in brightness and coherence when compared with 'hair-pin' filaments. They were first introduced into transmission electron microscopy by Hibi (1956) who obtained an increased number of Fresnel fringes in magnesium oxide crystals, a result which had only previously been obtained by the use of anode apertures a few micrometres in diameter (Boersch, 1943; Haine and Mulvey, 1952). Similar filaments had, in fact, been used earlier by Mfiller (1937) in the field emission microscope, but unlike those used by Hibi, the tip was not ground to a point. Hibi achieved this by hand-grinding. It was, therefore, a considerable step forward when Sakaki and M611enstedt (1956), using a modification of the electrolytic etching technique first described by Niemeck and Ruppin (1954), introduced pointed filaments with radii of less than l~m. Subsequently, and in order to make a better 9
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below were carried out.
Brightness of illumination. The current density per unit solid angle or 'brightness of illumination' (A c m - 2 sterad-1), is a parameter of the electron gun and the illuminating lens efficiency, v. Borries and Ruska (1939) showed the following relationship for the brightness of illumination, R: R
jo . . . . . . . . .
(1)
where Jo is the current density in the specimen plane (A cm-z) and 132 is the solid angle (sterad). In this investigation the current density was measured with a Faraday cage connected to a pico-ammeter just above the screen at x 100,000 magnification. The current density in the specimen plane, Jo is given by the following equation: jo -- jF .M'2 . . . . . . . . . (2) where jF is the current density in the Faraday cage, and M ' is the magnification in the registration plane which was 73% of the total magnification of the microscope. For known values of the diameter of the aperture ~bc of the second condenser, the illuminating spot size in the specimen plane ~o and the distance between the latter and the aperture plane L of the second condenser, the angle t3 is given by: tan/3 =
¢o-¢o
2L
.........
(3)
For small angles tan fl ~ / 3 . In all experiments reported, /3 = 4.3 × 10 -4 rad. (¢o-~ 200~m; ¢o = 2~m; L 230mm = 2.3 × 105Bm). Coherence and resolution. Analysis of coherence and resolution was carried out on graphitized carbon specimens (lattice spacing d = 0.34nm) supported by a holey carbon film approximately 30nm thick. The spatial distribution of the phase granulation in the support film was analysed with an optical diffractometer according to Thon (1966) and Johansen (1972c). Contamination versus etching. The test specimen was a holey carbon film of uniform thickness. Contamination was considered to have occurred if the diameter of the holes was reduced and conversely, etching to have taken place if the diameter was increased. The experiments were performed on holes located within the same square of the grid for all three Wehnelt systems. This was very important because thickness variations lead to changes in the thermal conductivity of the carbon film and hence changes in the contamination and etching conditions. The influence of any possible contaminants within the specimen cartridge was reduced to a negligible amount by allowing only the central part of the grid to be illuminated. In order to determine whether contamination or etching had occurred, a hole in the carbon film, approximately 0.3~m in diameter, was illuminated with a focused beam and with identical brightness for all three Wehnelt systems (R ~ 1.7 × 1 0 6 A c m - 2 s t e r a d - 1 ) . With each system two micrographs were taken at an interval of 6min and at an initial magnification of × 100,000. For accurate measurements the negatives were magnified five times. Cathode life. The total running time of each cathode was measured with an hourcounter connected to the cathode circuit.
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RESULTS
Brightness of illumination The optimum operating distance between the cathode tip and the front of the modified Wehnelt grid was determined by changing the distance in steps of 0.2mm using the eight adjustment screws and measuring the brightness of illumination at approximately the same beam current (10~A) at each new position. The screen brightness, measured with the Faraday cage, was plotted in a co-ordinate system where the abscissae 25
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0,2
0,4
0,6
0,8
1,0
WEHNELT FRONTTO CATHODETiP DISTANCE
(~)
4
Text Figure 3. A modified Wehnelt grid made for the Siemens Elmiskop IA featuring the same pointed angle (137 °) and cathode position as for the JEM 100B electron microscope. Text Figure 4. Graph showing the relation between the brightness of illumination and the position of the cathode tip. The optimum position of the cathode tip is 0.8mm behind the Wehnelt grid front. Beam current, I 0~A. represented the Wehnelt-cathode distance and the ordinate the brightness of illumination, R. Text Figure 4 shows that m a x i m u m brightness occurs when the cathode is positioned 0.8mm behind the front of the grid. This cathode position was used for all later experiments. The optimum position of the cathodes for the two other Wehnelt systems was confirmed using 0.2mm spacer rings in the one fitted with a 'hair-pin' filament and by adjusting the front cap position of the pointed cathode in the other. The brightness of the illumination, R was measured for all three grid systems using a beam current of 10~A and an illuminating aperture, fl = 4.3 x 10 -4 rad. The R-values for the standard Wehnelt grid with a 'hair-pin' filament, the one with a pointed cathode and the modified grid were 2.9 × 10~; 1.7 × l0 B and 1.9 x 106Acm-2sterad-1 respectively (see Text Figure 5).
Coherence and resolution In optical theory, the terms coherent and incoherent are often used to characterize
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radiation. Two beams are said to be coherent if, when combined, they can produce interference patterns in the apparatus under consideration. On the other hand, the two beams are incoherent if they are incapable to produce interference. In general the effect of a fixed illuminating aperture/3 is evaluated by means of the coherent conditions of an extended light source (Born and Wolf, 1965; Langer and Hoppe, 1966-67). Specimen details with smaller structure distances than: d = A (rr/3) -a . . . . . . . . .
(4)
are then illuminated with an effective coherent beam. According to equation (4), 100kV electrons (A -- 0.0037nm) give coherent illumination for details approximately 0.6nm apart, using an illuminating aperture/3 -- 2 × 10 -a rad. and likewise approximately 6nm for /3 =: 2 × 10 .4 rad. The illuminating conditions used during these 107
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5 6 Text Figure 5. Histograms demonstrating the brightness of illumination obtained for the three Wehnelt grids using a beam current of 10btA. The dotted line indicates the maximum theoretical brightness according to the Langmuir-equation (equation 5 in text). See Text Figure 7 (facing) for key.
Text Figure 6. Histograms of mean cathode life for the three types of grids. The values obtained for the standard and pointed cathodes are based on log-book records over a two year period with the JEM 100B microscope. The figures given for the modified grid are based on a two months' duration test. See Text Figure 7 (facing) for key. experiments was fl ---- 4.3 × 10-4 rad. All three Wehnelt grids, therefore, should satisfy the requirements for 'effective coherent illumination'. Using the standard Wehneh grid the brightness of illumination was increased by opening the illuminating aperture to fl -- 10-3 rad. in order to approach the screen brightness of the two other Wehnelt systems without increasing the beam current. With reference to equation (4) the beam coherence should now be reduced. The influence of the larger illuminating aperture on defocused high resolution micrographs of graphitized carbon was analysed with an optical diffractometer. The reciprocal spatial frequencies ('phase contrast resolution') of the substrate phase granulation appear as concentric rings in the optical diffractogram and increase in numbers with further defocusing of the micrograph (Thon, 1965, t966). Knowing the exact incre-
127 A HIGH PERFORMANCE WEHNELT GRID
ments of the objective lens focal verniers made it possible to take micrographs at the same degree of defocus for all three Wehnelt grids. Text Figures 8-10 are micrographs taken at 230nm underfocus using the three Wehnelt systems and with the optical
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~STANDARDGRID ~POINTEDCATHODE GRID
~NEW GRID
Text Figure 7. Contamination and etching conditions for the three Wehnelt grid systems applying the same brightness of illumination, R -~ 1.7 x 10CA era-2 sterad-~. In order to obtain the same brightness for the standard Wehnelt grid as for the two others, the beam current was raised from 10~tA to 60gA. transforms as inserts. The diffractograms for the pointed cathode (Text Figure 9) and the modified Wehnelt grid (Text Figure 10) contain the same number of concentric rings, but the one from the standard Wehnelt grid with a larger illuminating aperture fl -----10 -8 rad (Text Figure 8) contains only one continuous ring in addition to the one closest to the zero-order diffraction spot. The outer ring in the optical transforms shown
128 ,IOHANSEN 0.3nm. in Text Figures 9 a n d I 0 COrresponds to reciDra,~ 1 ,,_
J I
. . . . oao COherenceOa~ ~,~gltion to the one ~ , ~ ' . cinsert) of the ~tl~o<..~ ull/u.se ° "~c~ as poor reo~--:' ,~*t,~esr to the zer,~ ~_r_, "~c.g_ranula. Text .Figure 9. Same specimen as Tex • . o~,mtlon. X 1,500,000 "~u~uer diffraction
•
illuminating aperture
t F1
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resolution is e ore, of.fl ~_ 4.3 l n - 4 gure 8 imaged ,~,:~L . in the . . . . . :_ ao,~y recognized T ~ "~' rad. at a o ,_'~,t- a pointed fr .... r.-~,,~ous tigure a,~,4 ,L" ""c. carbon fil,~ ~L defocu~ of 23Onto ~r,_ cathode and an •~,tuncles). Both C O h e r , , ~ meoptical transfo'r~/J~ase granulation is -'" ~ ne 0.34nm lattice . . . . c ann resolution ~ m a{so reveals mor~ m o r e pronounced t Contamination versus etching "-~ gooa. X 1,500,000 ~ -rags (reciprocal sptahaa~
a n dT hthe e m lens i c r o s cCUrrents o p e was on allowed a n d with to warm up for m o r e t h a n 2 h r with the h i g h v o l t a g e T h e W e h n e l t grids were tested with b o t h anti'contamination t I 0 - 6 A c m - . a s t e r a d - ~. T h e b e a m the Same brightness o f m.~ra'ps COmpletely filled. Wehnelt grid were 8.5 a ~ 0 A, c ur r r pectivel e n t s for the p o i n t e d c a t h"o"duem lannadt a o n : R ~ 1.7 ~< c u r r e n t h a d to be i n c r e a s e tl,~ ~ . A es Y, b u t us/-,- ,L . for the modified " vvtz~x in o r d e r to ^ L . • ~-~ ~ e s t a n d a r d ~rid -~ u o t a l n the s a m o r.~zJ_ ~ me beam tion pasr ofor systems. oAs s h o w nm in s t a n d a r of d lllumina. Wehnelt grid d u c ethe d st w p eoc iomt he ne r etching f 0.5nm i n - T~e x t F i g u r e ' " 7, the ~'"gntness m o d i f i e d Wehnelt grid a contamin~,in,~ ^,. ~, . . a n d the p o i n t e d c a t h o d e a n d . . . . . . , , - u . a a a n d O.05nm m m - ~ , res • pectively.
the
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Cathode life time The results presented in Text Figure 6 for 'hair-pin' and pointed cathodes are mean
Text Figure 10. Same specimen and imaging conditions as in Text Figure 9, but using the modified Wehnelt grid system. The micrograph and the optical transform show the same good coherence and resolution as when using the pointed cathode. × 1,500,000. values based on log-book records over a 2yr period with the J E M 100B microscope. The mean fife-time of a 'hair-pin' type cathode is 43hr and 17hr for that of a pointed cathode. When standard 'hair-pin' cathodes were used in the modified Wehnelt grid, the average life of the filaments was 105hr. The latter data are based on a 2 month duration test of the modified Wehnelt system.
The modified Wehnelt grid used in the Siemens Elmiskop 1A In order to further examine the promising results obtained with the modified grid on t h e J E M 100B, a similar grid was made for the Siemens Elmiskop 1A. As shown in Text Figure 3, the grid made for the Siemens Elmiskop 1A has the same pointed angle and aperture diameter as the one used previously. The distance between the cathode tip and the Wehnelt grid front is also the same. However, it was not possible to carry out the same experimental programme as before because there was no Faraday cage or specimen anti-contamination device attached to the microscope. The light measuring device, however, gave a very good estimate of the screen brightness. From this, the increase of brightness with the modified grid was calculated to be 6 to 8 times that of a standard Wehnelt grid system provided with an identical illuminating aperture. The
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.1( ) H A N S E N
duration tests on the cathodes have only just started, but it seems that is not less than 1.5 times that of the standard grid system. DISCUSSION Brightness of illumination In the nineteen-fifties and early sixties, several authors investigated the influence of the shape of the "conventional' Wehnelt grid and the inter-electrode distances on the brightness of illumination in a three electrode electron gun (Haine and Einstein, 1952; Boersch and Born, 1960; Maruse, 1960). As a result, the so-called re-entrant Wehnelt grid seems to be the one generally recognized for 'hair-pin' cathodes, although the re-entrant angle varies among brands of microscopes ( J E M 100B, 145°; Elmiskop 1A, 90°). For pointed cathodes, however, the flat Wehnelt front seems most favourable as far as the brightness of illumination is concerned (Maruse and Sakaki, 1958; Hibi et al., 1962). Fernandez-Moran (1966) also reported on improved image quality for biological specimens using a flat front Wehnelt grid featuring a disposable 10-30~m thick molybdenum aperture with a 0.5mm central hole. The higher brightness of illumination as compared with results obtained with other pointed cathodes, is mainly due to the smaller Wehnelt aperture diameter. The centring of the cathode tip in such a small Wehnelt aperture takes a lot of skill and precision, and if it is not performed properly, the brightness of illumination and cathode life-time will be reduced drastically. In this laboratory, we have found that the initial slow increase in temperature applied to a pointed cathode reduces stresses in the tungsten wire but often moves the cathode tip off centre. Recentring is then quite essential if optimum efficiency of the electron gun is to be maintained. The maximum brightness of illumination of an electron gun source is given by the Langmuir equation: jk'e'Vo Rtlae°r' - r r ' k ' T . . . . . . . . . (5) where jk is the specific emission at the cathode; e, is the electron charge; Vo, is the accelerating potential; k, is the Boltzman's constant and T, is the cathode temperature. With tungsten heated to 2700-2800°K, where high emissivities can be attained with reasonable cathode life-time, the maximum theoretical brightness according to equation (5) is R = 4.6 × 105 A cm -2 sterad -1 for 100kV electrons. Due to the small angular aperture of the system, the actual energy flux is small. The coaxial system of the cathode, the Wehnelt grid and the anode forms an electrostatic lens which produces a diverging electron beam. The above study revealed that the brightness of illumination of the standard Wehnelt grid approached the theoretical values in good agreement with results obtained by other workers (Haine and Einstein, 1952; Boersch and Born, 1960). Investigations carried out by Maruse and Sakaki (1958), Drechsler et al. (1960) and Sakaki and Maruse (1960) on pointed cathode systems, emphasized that the increase in brightness beyond the theoretical maximum is due to thermionic field (T-F) enhancement around the cathode tip. The modified electron gun configuration reported on in this paper has details similar to the one described by Steigerwald (1949 ; Text Figure 11). The Steigerwald-cathode,
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sometimes called the 'telefocus' cathode, because it was originally designed for microscopes without a condenser lens system, produces an electron beam converging towards the specimen plane. Braucks (1958) showed that it was possible to change the location of the electron beam cross-over at the optical axis by varying the distance between the conical and the control electrode. In the Wehnelt system described here, the control electrode has been removed from the Steigerwald-cathode. Furthermore, the cathode position and the Wehnelt aperture diameter are different. CATHODE
I
CONTROl. ELECTRODE
m
ANOOE
'
11
Text Figure 1I. A schematic drawing (After Braucks, 1958) of the Steigerwald 'telefocus' cathode, showing the equipotential distribution in the inter-electrode space. By moving the cathode away from the control electrode a beam cross-over is obtainable closer to the electron gun, which also allows the specimen to be illuminated with a diverging electron beam. The general performance of a Wehnelt grid system is affected by several factors such as geometry, space charge spreading of the beam and initial velocities of the electrons. The initial velocities of the electrons are dependent only on the temperature of the tungsten wire and are accounted for in the Langmuir-equation (equation 5), which is taken as standard. The other factors are more or less interdependent and make full analysis of the beam difficult. An investigation carried out by Haine and Einstein (1952) showed that below a temperature of 2700°K the beam was not affected by the space charge in any part of the bias range. It is, therefore, assumed that the increased brightness beyond the theoretical value, using the conical Wehnelt grid, is due to a better electron gun geometry. The combination of the pointed Wehnelt grid and the cathode tip 0.8mm behind the grid front, seems to produce equipotential surfaces resulting in a more favourable electrostatic lens. As a result, the smaller effective source diameter and angular divergence
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of the electron beam makes it possible to achieve optimal imaging condition of the source by the condenser lenses. It also seems possible that the effect of the spherical aberration in the electrostatic lens is reduced due to the 'lens combination' formed by the equipotential field distribution in the inter-electrode space of the geometry of the gun. The results obtained on the brightness of illumination using the above electron gun configuration are very encouraging, but, in the author's opinion, can still be improved upon. Further experiments are in progress in order to study, in more detail, how the performance is influenced by the conical angle as well as by the Wehnelt aperture diameter and the inter-electrode distances at the lowest possible beam current. Coherence and resolution Questions on coherence arise whenever it is necessary to compute the resultant intensity due to super-position of radiation arising from separate locations. If the radiation arising from two parts of an object is coherent, the intensity at the image plane is an interference pattern for which the two parts must be added with due regard to relative phase. If two incoherent sources are observed, the intensity at the image plane is the sum of the intensities for each one taken separately. Since the maximum size of the illuminating aperture that can be used for coherent illumination is rarely fulfilled in practical electron microscopy (Hanszen, 1971), the illumination is said to be partially coherent when the illuminating aperture is small compared with the objective aperture. Using the M611enstedt electron interference microscope, Hibi (1962) and Hibi and Takahashi (1969) reported on good beam coherence for pointed cathodes. The degree of coherence was determined from the visibility curve of electron interference fringes obtained in the case of exact focus and also after variation of other physical parameters. In a theoretical investigation published by Lenz (1965), however, it was concluded that bi-prism interference fringes were not suitable either for the determination of the contrast transfer function or for the point resolution of an electron microscope. The far less complicated instrument used in this investigation, the optical diffractometer, seem to have proved its efficiency both in determining the electron beam coherence and the point resolution in carbon substrate phase granulation. With the modified Wehnelt grid it is possible to further extend the beam coherence compared to that of a standard grid system because of sufficient brightness at smaller illuminating apeltures. With condenser apertures of 100 to 200~m in diameter, a transverse coherence of tens of nanometers can be achieved. With reference to the light optical transfer theory, the illumination is incoherent when the illuminating and the objective aperture are at least equal. In the micrograph shown in Text Figure 8, the size of the illuminating and the objective apertures do not quite fulfil the requirements for incoherent illumination, but they are approaching the same value. Since incoherent illumination suppresses the phase contrast contribution of a mixed object, this sometimes disadvantageous effect can be used most favourably. In low resolution material, the amplitude contrast can be enhanced by combining incoherent illumination with a small objective aperture (20~m diameter) and a reduction of the accelerating potential. Using this combination in connection with the modified Wehnelt grid, a much greater level of brightness is obtained than before at lower accelerating potentials (20-40kV). The increased brightness at these potentials makes it possible to correct the astigmatism leading to a marked improvement in the
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quality of the image.
Contamination versus etching An analysis of the specimen contamination mechanism and factors controlling the contamination rate was presented by Ennos (1953) and Heide (1963). Due to these investigations it is now generally recognized that the main cause of contamination is the residual gas atmosphere surrounding the specimen. Liquid nitrogen traps are now available for most microscopes and serve as an effective means of reducing contamination. Experience gained in this laboratory, as well as results reported by Yada (1968), however, revealed electron beam etching in specimens under certain illuminating conditions even when effective anti-contamination traps were employed. Experiments with the standard Wehnelt grid resulted in specimen contamination whereas etching was obtained with the two other grid systems even though the same test specimen, microscope vacuum and brightness of illumination (R----1.7 ×106 A cm -2 sterad-1) were used. However, when using the standard Wehnelt grid, the beam current was increased from approximately 10~A to 60V2k (as for the two other grids) in order to obtain the same degree of brightness. This supports the observations of Hart, Kassner and Maurin (1970) who reported that there seems to be another important source of contamination. It is suggested, therefore, that molecules contributing to the contamination are 'channelled' down the beam path, possibly from as far away as the electron gun area. This hypothesis requires further investigation and falls outside the scope of the present study. In order to operate the microscope according to optimum anti-contamination conditions, it is necessary to work at the equilibrium between contamination and etching. The results obtained with the modified Wehnelt grid (etching of only 0.05nm min-1) seem very advantageous in this respect. Cathode life-time The modified Wehnelt grid, using 'hair-pin' cathodes, is space charge limited, which means that the saturation level may easily be recognized and unnecessary overloading thereby avoided. Pointed cathodes do not have a saturation point, which can easily lead to overheating of the cathode and reduce its life. The extended life of the cathode experienced with the modified Wehnelt grid system seems to be related to a more favourable geometry around the cathode tip. The distance from the cathode tip and to the Wehnelt grid aperture is larger than in the standard and the pointed cathode grids. This makes tungsten deposits in the aperture less critical to micro-discharges as in the other two cases. ACKNOWLEDGEMENTS I am grateful to J E O L (Scandinavia) for providing details about the Wehnelt grid used in their Scanning Electron Microscope. I am also indebted to Mr. Knut Antonsen for making the Wehnelt grids to the very high level of precision which was required. I am also grateful to Dr. L. O. Froholm for valuable discussions throughout the experimental work and for his suggestions and criticisms when writing this paper. REFERENCES BOERSCH, H., 1943. Fresnelsche Beugung im Elektronenmikroskop. Phys. Z-, 44:202-211. 10
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BOERSCH, H. and BORN, G., 1960. Messungen an Elektronenstrahlerzeugern. In: Proc. 4th. International Congress on Electron Microscopy, Berlin, 1958, Mbllenstedt, G., Niehrs, H. and Ruska, E. (eds.), Springer Verlag, l: 35-39. BORN, M. and WOLF, E., 1965. Principles of Optics. Pergamon Press, London and New York. BORRIES, B.v. and RUSKA, E., 1939. Versuche, Rechenungen und Ergebnisse zur Frage des Auflbsungsvermbgens beim Ubermikroskop. Z" techn. Phys., 20: 225-235. BRAUCKS, F. W., 1958. Untersuchungen an der Fernfokuskathode nach Steigerwald. Optik., 15 : 242-260. DRECHSLER,M., COSSLETT,V. E. and NIXON,W. C., 1960. The point cathode as an electron source. In : Proc. 4th International Congress on Electron Microscopy, Berlin, 1958, M611enstedt, G., Niehrs, H. and Ruska, E. (eds.), Springer Verlag, 1: 13-20. ENNOS, A. E., 1953. The origin of specimen contamination in the electron microscope. J. Phys. D. (Br. J. appl. Phy.), 4: 101-106. FERNANDEZ-MORAN, H., 1966. Application of improved point cathode sources to high resolution electron microscopy. In : Proc. 6th International Congress on Electron Microscopy, Kyoto, Ryzoi Uyeda (ed.), Maruzen Co. Ltd., Tokyo, Japan, 1: 27-28. HAINE, M. E. and EINSTEIN,P. A., 1952. Characteristics of the hot cathode electron microscope gun. J. Phys. D. (Br. J. appl. Phy.), 3: 40-46. HAINE, M. E. and MULVEY, T., 1952. The formation of the diffraction image in the (labor diffraction microscope. J. opt. Soc. Amer., 42: 763-773. HANSZEN, K.-J., 1962. Beziehungen Zwischen den geometrischoptischen und den elektrischen Eigenschaften von Haarnadel- und Spitzenkathoden. In : Proc. 5th International Congress on Electron Microscopy, Philadelphia, Breese, S. S. (ed.), Academic Press, New York, 1: KK-11. HANSZEN, K.-J., 1971. Optical transfer theory of the electron microscope. In: Advances in Optical and Electron Microscopy. Barer, R. and Cosslett, V. E. (eds.), Academic Press, New York and London, 4: 1-84. HART, R. K., KASSNER,T. F. and MAURIN,J. K., 1970. The contamination of surfaces during high-energy electron irradiation. Phil. Mag., 21: 453-467. HEIDE, H. G., 1963. Die Objektverschmutzung im Elektronenmikroskop und das Problem der Strahlensch~idigung durch Kohlenstoffabbau. Z. angew. Phys., 15:116-128. HIBI, T., 1956. Point filaments and its applications. In: Proc. International Congress on Electron Microscopy, London, 1954, Royal Microscopical Society, 636-638. HIBI, T., 1962. Point cathode and resolution of electron microscope. In: Proc. 5th International Congress on Electron Microscopy, Philadelphia, Breese, S. S. (ed.), Academic Press, New York, l: K K - I . HIBI, T. and TAKAHASHI,S., 1969. Relation between coherence of electron beam and contrast of electron image. Z. angew. Phys., 27: 132-138. Him, T. and YADA, K., 1964. Point cathodes and resolution of electron microscope (II). The effect of the operating condition of the point cathode and that of a stigmator. J. Electronmicroscopy, 13: 94-100. HIBI, Z., YADA, K. and TAKAHASHI, S., 1962. Point cathode and resolution of electron microscope. J. Electronmicroscopy, 11 : 244-252. JOHANSEN, B. V., 1972a. High resolution stability of an electron microscope measured with an optical diffractometer. In: Proc. Scandinavian Societyfor Electron Microscopy, Gothenburg, 1971. JOHANSEN, B. V., 1972b. A new Wehnelt design for high resolution electron microscopy. In: Proc. Scandinavian Societyfor Electron Microscopy, Aarhus, 1972, (in press). JOHANSEN, B. V., 1972c. An optical diffractometer of simplified design for the analysis of high resolution electron micrographs. Micron, 3: 256-270. gANGER, R. and HOPPE, W., 1966/67. Die Erh6hung von Aufl6sung und Kontrast im Elektronenmikroskop mit Zonenkorrekturplatten. Optik, 24: 470-489. LENZ, F., 1965. Kann man Biprisma-Interferenzstreifen zur Messung des elektronenmikroskopischen Aufl6sungsverm6gens verwenden ? Optik, 22: 270-288. MARUSE, S., 1960. Vergleichende Untersuchungen der Eigenschaften von Spitzenkathoden
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und Haarnadelkathoden in Strahlerzeugungssystemen mit negativer Wehneltblende. In: Proc. European Regional Conference on Electron Microscopy, Delft, Houwink, A. L. and Spit, B.J. (eds.), 1: 73-77. MARUSE, S. and SAKAKI,Y., 1958. l~ber einige elektronenoptische Eigenschaften der Spitzenkathode. Optik, 15: 485-490. MOLLER, E. W., 1937. Elektronenmikroskopische Beobachtungen yon Feldkathoden. Z. Phys., 106: 541-550. NIEMECI~, F. W. and RuPPIN, D., 1954. Elektrolytische Atzung von Kathodenspitzen fiir das Feldelektronenmikroskop. Z. angew. Phys., 6: 1-3. SAKAKI, Y. and M6LLENSTEDT, G., 1956. Beitrag zur Verwendung yon Spitzenkathoden in der Elektronenoptik. Optik, 13: 193-200. SAKAKI, Y. and MARUSE, S., 1960. Ober die Arbeitsweise und die elektronenoptischen Eigenschaften der Spitzenkathode. In : Proc. 4th International Congress on Electron Microscopy, Berlin, 1958, M611enstedt, G., Niehrs, H. and Ruska, E. (eds.), Springer Verlag, 1: 9-13. SKINNER, L. M., 1969. An accurate method for measuring the magnification of an electron microscope. 07. Phys. E. (ft. scient. Instrum.), 2: 206-208. STEIOERWALD,K. H., 1949. Ein neuartiges Strahlerzeugungs-System ftir Elektronenmikroskope. Optik, 5: 469-478. SWIFT, D. W. and NIXON, W. C., 1960. The behaviour of a point cathode in a triode electron gun. In: Proc. European Regional Conference on Electron Microscopy, Delft, Houwink, A. L. and Spit, B.J. (eds.) 1: 69-72. THON, F., 1965. Elektronenmikroskopische Untersuchungen an diinnen Kohlefolien. Z. Naturforsch., 20a: 154-155. TnON, F., 1966. Zur Defokussierungsabh~ngigkeit des Phasenkontrastes bei der elektronenoptischen Abbildung. Z. Naturforsch., 21a: 476-478. YADA, K., 1968. Influence of illumination beam on the image contrast. J. Electronmicroscopy, 17: 162-163. YADA, K. and HIBI, T., 1966. The contrast of electron images in the case of using pointed cathodes. In : Proc. 6th International Congress on Electron Microscopy, Kyoto, Ryozi Uyeda (ed.), Maruzen Co. Ltd., Tokyo, Japan, 1: 25-26.
Micron, 1973, 4:137-138
137 BOOK REVIEWS
Stereology 3. E. R. W E I B E L , G. M E E K , B. RALF, P. E C H L I N and R. ROSS (Eds.), Published for The Royal Microscopical Society by Blackwells, Oxford, 1972 (Hardback, 393 pages). Price: £7.00. Stereology is an important technique which will provide three-dimensional information and interpretation from certain two-dimensional images. The subject of stereology is relatively new when applied to microscopy, the latter being an essentially two-dimensional system as the final images are recorded on a photographic film or viewed on a suitable screen. The Third International Congress on Stereology was held in Berne, Switzerland, during 1971 and the above volume has been published covering the invited contributions together with a selection of other papers. There are eleven invited papers by distinguished workers in the field of stereology which are concerned with subjects ranging from the quantitative analysis of the structure, function of cells and organelles to analysing images from mineral specimens, scanning electron micrographs and automatic image processing for stereology, etc. The other nineteen contributions also deal with a wide selection of papers covering the theory and applications of stereology to microscopy. Although the volume includes a range of biological and non-biological contributions and contains a fair amount of mathematical analysis, it will be of considerable value to microscopists generally and especially to workers concerned with three-dimensional image construction. The book also contains an extensive and valuable list of published works at the end of each contribution. The production of the book is of a very high standard and includes a large number of micrographs together with clear line drawings of a size that are easy to follow in relation to the text. This volume on stereology is highly recommended to electron microscopists generally as the field of three-dimensional electron image interpretation and image reconstruction is expanding rapidly. R. W. Horne
Metallurgical Stereographic Projections. J. S. S M A I L L . A d a m Hilger, London, pp. 262 + viii, 1972. Price: £7.00. The Stereographic Projection is a convenient method for representing three-dimensional crystallographic data in two dimensions. One has the option of using projections or tables of interplanar or interzonal angles in work which demands a three-dimensional appreciation of the problem. Most people seem to prefer the use of projections since they provide a measure of the angles involved together with a pictorial distribution of the poles relative to one another. The Stereographic Projection (SP), or its unfortunate abbreviation, Stereogram, has become more widely used since the advent of electron microscopy and single crystal technology. Earlier treatises on the subject, references to which are provided in the book, were largely concerned with the application of SPs in X-ray diffraction studies. The author intends his book for the metallurgist and materials scientist. It can effectively be split into three main parts. The first part, covering Chapters 1-3, deals with the crystallographic notations used and also generates the equations to be used later. The Miller and Miller-Bravais systems of indexing hexagonal crystals are b o t h mentioned but unfortunately are not treated in any depth, and this m a y lead to confusion. This section tends to look mathematical but this should not deter would-be purchasers because the mathematics are simple and easily followed with the aid of lucid diagrams. The third chapter concerns the use of the SP and one gets the impression, quite wrongly, that its application in X - r a y and E M is about to be discussed. In fact, the author concentrates on the latter aspect apart from certain general applications; clear instructions on how to use the SP in, for example, trace analysis, lattice rotation, certain
Micron, 1973,4:121-135
A high performance Wehnelt grid for transmission electron microscopes B. V. JOHANSEN Methodology Department, National Institute of Public Health, Geitmyrsveien 75, Oslo 1, Norway.
Manuscript received July 25, 1972: Revised April 11, 1973
A modified Wehnelt grid using conventional 'hair-pin' filaments has been designed for the electron gun of a transmission electron microscope. Unlike the standard cylindrical grid, the modified grid has a conical shape. The modified grid gives a significant increase in brightness of illumination compared with the standard form using a 'hair-pin' filament and similar brightness, coherence and resolution when compared with a grid fitted with a pointed filament. Further advantages of the modified design are that the filament life is considerably increased and there is less effect on the specimen in terms of contamination or etching. Un nouveau genre de cylindre Wehnelt utilisant desfilaments en U d'usage courant a gt~ construit pour le canon a dlectrons d'un microscope glectronique d transmission. Contrairement gtla grille du module standard, la grille modif~e est de forme conique. La grille modif~e produit une plus grande intensit~ d' illumination que la grille standard comportant des filaments en U et une rgsolution, coMrence et intensit~ analogue ~ celle d'une grille munie d'une source ponctueUe. Par ailleurs la durge de service des filaments est consid~rablement augmentge et la contamination du specimen est rgduite quand les nouvelles grilles sont utilisges. Ein modifizierter Wehneltzylinder mit herkOmmlicher Haarnadelkathode wurde fiir den Elektronenstrahlenerzeuger eines Transmissionselektronenmikroskops konstruiert. Im Gegensatz zu herkOmmlichen Wehneltzylindern ist dieser kegelf6rmig. Das modifizierte Gitter erzielt eine signifikant hOhere Beleuchtungsstiirke als die normale Haarnadelmontage und iihnliche Beleuchtungsstiirke, Kohiirenz und AuflOsung wie die Spitzenkathode. Vorteile dieser Modifikation sind auch die erheblich verliingerte Lebensdauer der Kathode und die herabgesetzte Verunreinigung und Jftzung der Probe.
INTRODUCTION In studies being carried out in this laboratory on methods to improve operating conditions for high resolution electron microscopy of biological specimens, we have recently obtained encouraging results with a modified design of the Wehnelt grid using standard 'hair-pin' filaments. The modified design is based on developments from the J E O L scanning electron microscope grid assembly and the work of Steigerwald (1949) and features a casing which is conical in shape. Pointed filaments give a significant improvement in brightness and coherence when compared with 'hair-pin' filaments. They were first introduced into transmission electron microscopy by Hibi (1956) who obtained an increased number of Fresnel fringes in magnesium oxide crystals, a result which had only previously been obtained by the use of anode apertures a few micrometres in diameter (Boersch, 1943; Haine and Mulvey, 1952). Similar filaments had, in fact, been used earlier by Mfiller (1937) in the field emission microscope, but unlike those used by Hibi, the tip was not ground to a point. Hibi achieved this by hand-grinding. It was, therefore, a considerable step forward when Sakaki and M611enstedt (1956), using a modification of the electrolytic etching technique first described by Niemeck and Ruppin (1954), introduced pointed filaments with radii of less than l~m. Subsequently, and in order to make a better 9
124
JOHANSEN
below were carried out.
Brightness of illumination. The current density per unit solid angle or 'brightness of illumination' (A c m - 2 sterad-1), is a parameter of the electron gun and the illuminating lens efficiency, v. Borries and Ruska (1939) showed the following relationship for the brightness of illumination, R: R
jo . . . . . . . . .
(1)
where Jo is the current density in the specimen plane (A cm-z) and 132 is the solid angle (sterad). In this investigation the current density was measured with a Faraday cage connected to a pico-ammeter just above the screen at x 100,000 magnification. The current density in the specimen plane, Jo is given by the following equation: jo -- jF .M'2 . . . . . . . . . (2) where jF is the current density in the Faraday cage, and M ' is the magnification in the registration plane which was 73% of the total magnification of the microscope. For known values of the diameter of the aperture ~bc of the second condenser, the illuminating spot size in the specimen plane ~o and the distance between the latter and the aperture plane L of the second condenser, the angle t3 is given by: tan/3 =
¢o-¢o
2L
.........
(3)
For small angles tan fl ~ / 3 . In all experiments reported, /3 = 4.3 × 10 -4 rad. (¢o-~ 200~m; ¢o = 2~m; L 230mm = 2.3 × 105Bm). Coherence and resolution. Analysis of coherence and resolution was carried out on graphitized carbon specimens (lattice spacing d = 0.34nm) supported by a holey carbon film approximately 30nm thick. The spatial distribution of the phase granulation in the support film was analysed with an optical diffractometer according to Thon (1966) and Johansen (1972c). Contamination versus etching. The test specimen was a holey carbon film of uniform thickness. Contamination was considered to have occurred if the diameter of the holes was reduced and conversely, etching to have taken place if the diameter was increased. The experiments were performed on holes located within the same square of the grid for all three Wehnelt systems. This was very important because thickness variations lead to changes in the thermal conductivity of the carbon film and hence changes in the contamination and etching conditions. The influence of any possible contaminants within the specimen cartridge was reduced to a negligible amount by allowing only the central part of the grid to be illuminated. In order to determine whether contamination or etching had occurred, a hole in the carbon film, approximately 0.3~m in diameter, was illuminated with a focused beam and with identical brightness for all three Wehnelt systems (R ~ 1.7 × 1 0 6 A c m - 2 s t e r a d - 1 ) . With each system two micrographs were taken at an interval of 6min and at an initial magnification of × 100,000. For accurate measurements the negatives were magnified five times. Cathode life. The total running time of each cathode was measured with an hourcounter connected to the cathode circuit.
A HIGH PERFORMANCE WEHNELT GRID
125
RESULTS
Brightness of illumination The optimum operating distance between the cathode tip and the front of the modified Wehnelt grid was determined by changing the distance in steps of 0.2mm using the eight adjustment screws and measuring the brightness of illumination at approximately the same beam current (10~A) at each new position. The screen brightness, measured with the Faraday cage, was plotted in a co-ordinate system where the abscissae 25
i0
1 105 |
|
It
|
!
0,2
0,4
0,6
0,8
1,0
WEHNELT FRONTTO CATHODETiP DISTANCE
(~)
4
Text Figure 3. A modified Wehnelt grid made for the Siemens Elmiskop IA featuring the same pointed angle (137 °) and cathode position as for the JEM 100B electron microscope. Text Figure 4. Graph showing the relation between the brightness of illumination and the position of the cathode tip. The optimum position of the cathode tip is 0.8mm behind the Wehnelt grid front. Beam current, I 0~A. represented the Wehnelt-cathode distance and the ordinate the brightness of illumination, R. Text Figure 4 shows that m a x i m u m brightness occurs when the cathode is positioned 0.8mm behind the front of the grid. This cathode position was used for all later experiments. The optimum position of the cathodes for the two other Wehnelt systems was confirmed using 0.2mm spacer rings in the one fitted with a 'hair-pin' filament and by adjusting the front cap position of the pointed cathode in the other. The brightness of the illumination, R was measured for all three grid systems using a beam current of 10~A and an illuminating aperture, fl = 4.3 x 10 -4 rad. The R-values for the standard Wehnelt grid with a 'hair-pin' filament, the one with a pointed cathode and the modified grid were 2.9 × 10~; 1.7 × l0 B and 1.9 x 106Acm-2sterad-1 respectively (see Text Figure 5).
Coherence and resolution In optical theory, the terms coherent and incoherent are often used to characterize
126
JOHANSEN
radiation. Two beams are said to be coherent if, when combined, they can produce interference patterns in the apparatus under consideration. On the other hand, the two beams are incoherent if they are incapable to produce interference. In general the effect of a fixed illuminating aperture/3 is evaluated by means of the coherent conditions of an extended light source (Born and Wolf, 1965; Langer and Hoppe, 1966-67). Specimen details with smaller structure distances than: d = A (rr/3) -a . . . . . . . . .
(4)
are then illuminated with an effective coherent beam. According to equation (4), 100kV electrons (A -- 0.0037nm) give coherent illumination for details approximately 0.6nm apart, using an illuminating aperture/3 -- 2 × 10 -a rad. and likewise approximately 6nm for /3 =: 2 × 10 .4 rad. The illuminating conditions used during these 107
i000 co
g
7 5
o I
i00
~z
106 10
,~
10 s
D
'>555?
"\\~
K"
N
5 6 Text Figure 5. Histograms demonstrating the brightness of illumination obtained for the three Wehnelt grids using a beam current of 10btA. The dotted line indicates the maximum theoretical brightness according to the Langmuir-equation (equation 5 in text). See Text Figure 7 (facing) for key.
Text Figure 6. Histograms of mean cathode life for the three types of grids. The values obtained for the standard and pointed cathodes are based on log-book records over a two year period with the JEM 100B microscope. The figures given for the modified grid are based on a two months' duration test. See Text Figure 7 (facing) for key. experiments was fl ---- 4.3 × 10-4 rad. All three Wehnelt grids, therefore, should satisfy the requirements for 'effective coherent illumination'. Using the standard Wehneh grid the brightness of illumination was increased by opening the illuminating aperture to fl -- 10-3 rad. in order to approach the screen brightness of the two other Wehnelt systems without increasing the beam current. With reference to equation (4) the beam coherence should now be reduced. The influence of the larger illuminating aperture on defocused high resolution micrographs of graphitized carbon was analysed with an optical diffractometer. The reciprocal spatial frequencies ('phase contrast resolution') of the substrate phase granulation appear as concentric rings in the optical diffractogram and increase in numbers with further defocusing of the micrograph (Thon, 1965, t966). Knowing the exact incre-
127 A HIGH PERFORMANCE WEHNELT GRID
ments of the objective lens focal verniers made it possible to take micrographs at the same degree of defocus for all three Wehnelt grids. Text Figures 8-10 are micrographs taken at 230nm underfocus using the three Wehnelt systems and with the optical
~o
Z :2:
0.1
+ 0.01
-0,1 4~C Z
• mu
-I
W" Z
-10
~STANDARDGRID ~POINTEDCATHODE GRID
~NEW GRID
Text Figure 7. Contamination and etching conditions for the three Wehnelt grid systems applying the same brightness of illumination, R -~ 1.7 x 10CA era-2 sterad-~. In order to obtain the same brightness for the standard Wehnelt grid as for the two others, the beam current was raised from 10~tA to 60gA. transforms as inserts. The diffractograms for the pointed cathode (Text Figure 9) and the modified Wehnelt grid (Text Figure 10) contain the same number of concentric rings, but the one from the standard Wehnelt grid with a larger illuminating aperture fl -----10 -8 rad (Text Figure 8) contains only one continuous ring in addition to the one closest to the zero-order diffraction spot. The outer ring in the optical transforms shown
128 ,IOHANSEN 0.3nm. in Text Figures 9 a n d I 0 COrresponds to reciDra,~ 1 ,,_
J I
. . . . oao COherenceOa~ ~,~gltion to the one ~ , ~ ' . cinsert) of the ~tl~o<..~ ull/u.se ° "~c~ as poor reo~--:' ,~*t,~esr to the zer,~ ~_r_, "~c.g_ranula. Text .Figure 9. Same specimen as Tex • . o~,mtlon. X 1,500,000 "~u~uer diffraction
•
illuminating aperture
t F1
"
resolution is e ore, of.fl ~_ 4.3 l n - 4 gure 8 imaged ,~,:~L . in the . . . . . :_ ao,~y recognized T ~ "~' rad. at a o ,_'~,t- a pointed fr .... r.-~,,~ous tigure a,~,4 ,L" ""c. carbon fil,~ ~L defocu~ of 23Onto ~r,_ cathode and an •~,tuncles). Both C O h e r , , ~ meoptical transfo'r~/J~ase granulation is -'" ~ ne 0.34nm lattice . . . . c ann resolution ~ m a{so reveals mor~ m o r e pronounced t Contamination versus etching "-~ gooa. X 1,500,000 ~ -rags (reciprocal sptahaa~
a n dT hthe e m lens i c r o s cCUrrents o p e was on allowed a n d with to warm up for m o r e t h a n 2 h r with the h i g h v o l t a g e T h e W e h n e l t grids were tested with b o t h anti'contamination t I 0 - 6 A c m - . a s t e r a d - ~. T h e b e a m the Same brightness o f m.~ra'ps COmpletely filled. Wehnelt grid were 8.5 a ~ 0 A, c ur r r pectivel e n t s for the p o i n t e d c a t h"o"duem lannadt a o n : R ~ 1.7 ~< c u r r e n t h a d to be i n c r e a s e tl,~ ~ . A es Y, b u t us/-,- ,L . for the modified " vvtz~x in o r d e r to ^ L . • ~-~ ~ e s t a n d a r d ~rid -~ u o t a l n the s a m o r.~zJ_ ~ me beam tion pasr ofor systems. oAs s h o w nm in s t a n d a r of d lllumina. Wehnelt grid d u c ethe d st w p eoc iomt he ne r etching f 0.5nm i n - T~e x t F i g u r e ' " 7, the ~'"gntness m o d i f i e d Wehnelt grid a contamin~,in,~ ^,. ~, . . a n d the p o i n t e d c a t h o d e a n d . . . . . . , , - u . a a a n d O.05nm m m - ~ , res • pectively.
the
A H I G H PERFORMANCE WEHNELT GRID
129
Cathode life time The results presented in Text Figure 6 for 'hair-pin' and pointed cathodes are mean
Text Figure 10. Same specimen and imaging conditions as in Text Figure 9, but using the modified Wehnelt grid system. The micrograph and the optical transform show the same good coherence and resolution as when using the pointed cathode. × 1,500,000. values based on log-book records over a 2yr period with the J E M 100B microscope. The mean fife-time of a 'hair-pin' type cathode is 43hr and 17hr for that of a pointed cathode. When standard 'hair-pin' cathodes were used in the modified Wehnelt grid, the average life of the filaments was 105hr. The latter data are based on a 2 month duration test of the modified Wehnelt system.
The modified Wehnelt grid used in the Siemens Elmiskop 1A In order to further examine the promising results obtained with the modified grid on t h e J E M 100B, a similar grid was made for the Siemens Elmiskop 1A. As shown in Text Figure 3, the grid made for the Siemens Elmiskop 1A has the same pointed angle and aperture diameter as the one used previously. The distance between the cathode tip and the Wehnelt grid front is also the same. However, it was not possible to carry out the same experimental programme as before because there was no Faraday cage or specimen anti-contamination device attached to the microscope. The light measuring device, however, gave a very good estimate of the screen brightness. From this, the increase of brightness with the modified grid was calculated to be 6 to 8 times that of a standard Wehnelt grid system provided with an identical illuminating aperture. The
130
.1( ) H A N S E N
duration tests on the cathodes have only just started, but it seems that is not less than 1.5 times that of the standard grid system. DISCUSSION Brightness of illumination In the nineteen-fifties and early sixties, several authors investigated the influence of the shape of the "conventional' Wehnelt grid and the inter-electrode distances on the brightness of illumination in a three electrode electron gun (Haine and Einstein, 1952; Boersch and Born, 1960; Maruse, 1960). As a result, the so-called re-entrant Wehnelt grid seems to be the one generally recognized for 'hair-pin' cathodes, although the re-entrant angle varies among brands of microscopes ( J E M 100B, 145°; Elmiskop 1A, 90°). For pointed cathodes, however, the flat Wehnelt front seems most favourable as far as the brightness of illumination is concerned (Maruse and Sakaki, 1958; Hibi et al., 1962). Fernandez-Moran (1966) also reported on improved image quality for biological specimens using a flat front Wehnelt grid featuring a disposable 10-30~m thick molybdenum aperture with a 0.5mm central hole. The higher brightness of illumination as compared with results obtained with other pointed cathodes, is mainly due to the smaller Wehnelt aperture diameter. The centring of the cathode tip in such a small Wehnelt aperture takes a lot of skill and precision, and if it is not performed properly, the brightness of illumination and cathode life-time will be reduced drastically. In this laboratory, we have found that the initial slow increase in temperature applied to a pointed cathode reduces stresses in the tungsten wire but often moves the cathode tip off centre. Recentring is then quite essential if optimum efficiency of the electron gun is to be maintained. The maximum brightness of illumination of an electron gun source is given by the Langmuir equation: jk'e'Vo Rtlae°r' - r r ' k ' T . . . . . . . . . (5) where jk is the specific emission at the cathode; e, is the electron charge; Vo, is the accelerating potential; k, is the Boltzman's constant and T, is the cathode temperature. With tungsten heated to 2700-2800°K, where high emissivities can be attained with reasonable cathode life-time, the maximum theoretical brightness according to equation (5) is R = 4.6 × 105 A cm -2 sterad -1 for 100kV electrons. Due to the small angular aperture of the system, the actual energy flux is small. The coaxial system of the cathode, the Wehnelt grid and the anode forms an electrostatic lens which produces a diverging electron beam. The above study revealed that the brightness of illumination of the standard Wehnelt grid approached the theoretical values in good agreement with results obtained by other workers (Haine and Einstein, 1952; Boersch and Born, 1960). Investigations carried out by Maruse and Sakaki (1958), Drechsler et al. (1960) and Sakaki and Maruse (1960) on pointed cathode systems, emphasized that the increase in brightness beyond the theoretical maximum is due to thermionic field (T-F) enhancement around the cathode tip. The modified electron gun configuration reported on in this paper has details similar to the one described by Steigerwald (1949 ; Text Figure 11). The Steigerwald-cathode,
A HIGH PERFORMANCE WEHNELT GRID
131
sometimes called the 'telefocus' cathode, because it was originally designed for microscopes without a condenser lens system, produces an electron beam converging towards the specimen plane. Braucks (1958) showed that it was possible to change the location of the electron beam cross-over at the optical axis by varying the distance between the conical and the control electrode. In the Wehnelt system described here, the control electrode has been removed from the Steigerwald-cathode. Furthermore, the cathode position and the Wehnelt aperture diameter are different. CATHODE
I
CONTROl. ELECTRODE
m
ANOOE
'
11
Text Figure 1I. A schematic drawing (After Braucks, 1958) of the Steigerwald 'telefocus' cathode, showing the equipotential distribution in the inter-electrode space. By moving the cathode away from the control electrode a beam cross-over is obtainable closer to the electron gun, which also allows the specimen to be illuminated with a diverging electron beam. The general performance of a Wehnelt grid system is affected by several factors such as geometry, space charge spreading of the beam and initial velocities of the electrons. The initial velocities of the electrons are dependent only on the temperature of the tungsten wire and are accounted for in the Langmuir-equation (equation 5), which is taken as standard. The other factors are more or less interdependent and make full analysis of the beam difficult. An investigation carried out by Haine and Einstein (1952) showed that below a temperature of 2700°K the beam was not affected by the space charge in any part of the bias range. It is, therefore, assumed that the increased brightness beyond the theoretical value, using the conical Wehnelt grid, is due to a better electron gun geometry. The combination of the pointed Wehnelt grid and the cathode tip 0.8mm behind the grid front, seems to produce equipotential surfaces resulting in a more favourable electrostatic lens. As a result, the smaller effective source diameter and angular divergence
132
J OHANSEN
of the electron beam makes it possible to achieve optimal imaging condition of the source by the condenser lenses. It also seems possible that the effect of the spherical aberration in the electrostatic lens is reduced due to the 'lens combination' formed by the equipotential field distribution in the inter-electrode space of the geometry of the gun. The results obtained on the brightness of illumination using the above electron gun configuration are very encouraging, but, in the author's opinion, can still be improved upon. Further experiments are in progress in order to study, in more detail, how the performance is influenced by the conical angle as well as by the Wehnelt aperture diameter and the inter-electrode distances at the lowest possible beam current. Coherence and resolution Questions on coherence arise whenever it is necessary to compute the resultant intensity due to super-position of radiation arising from separate locations. If the radiation arising from two parts of an object is coherent, the intensity at the image plane is an interference pattern for which the two parts must be added with due regard to relative phase. If two incoherent sources are observed, the intensity at the image plane is the sum of the intensities for each one taken separately. Since the maximum size of the illuminating aperture that can be used for coherent illumination is rarely fulfilled in practical electron microscopy (Hanszen, 1971), the illumination is said to be partially coherent when the illuminating aperture is small compared with the objective aperture. Using the M611enstedt electron interference microscope, Hibi (1962) and Hibi and Takahashi (1969) reported on good beam coherence for pointed cathodes. The degree of coherence was determined from the visibility curve of electron interference fringes obtained in the case of exact focus and also after variation of other physical parameters. In a theoretical investigation published by Lenz (1965), however, it was concluded that bi-prism interference fringes were not suitable either for the determination of the contrast transfer function or for the point resolution of an electron microscope. The far less complicated instrument used in this investigation, the optical diffractometer, seem to have proved its efficiency both in determining the electron beam coherence and the point resolution in carbon substrate phase granulation. With the modified Wehnelt grid it is possible to further extend the beam coherence compared to that of a standard grid system because of sufficient brightness at smaller illuminating apeltures. With condenser apertures of 100 to 200~m in diameter, a transverse coherence of tens of nanometers can be achieved. With reference to the light optical transfer theory, the illumination is incoherent when the illuminating and the objective aperture are at least equal. In the micrograph shown in Text Figure 8, the size of the illuminating and the objective apertures do not quite fulfil the requirements for incoherent illumination, but they are approaching the same value. Since incoherent illumination suppresses the phase contrast contribution of a mixed object, this sometimes disadvantageous effect can be used most favourably. In low resolution material, the amplitude contrast can be enhanced by combining incoherent illumination with a small objective aperture (20~m diameter) and a reduction of the accelerating potential. Using this combination in connection with the modified Wehnelt grid, a much greater level of brightness is obtained than before at lower accelerating potentials (20-40kV). The increased brightness at these potentials makes it possible to correct the astigmatism leading to a marked improvement in the
A HIGH PERFORMANCE WEHNELT GRID
133
quality of the image.
Contamination versus etching An analysis of the specimen contamination mechanism and factors controlling the contamination rate was presented by Ennos (1953) and Heide (1963). Due to these investigations it is now generally recognized that the main cause of contamination is the residual gas atmosphere surrounding the specimen. Liquid nitrogen traps are now available for most microscopes and serve as an effective means of reducing contamination. Experience gained in this laboratory, as well as results reported by Yada (1968), however, revealed electron beam etching in specimens under certain illuminating conditions even when effective anti-contamination traps were employed. Experiments with the standard Wehnelt grid resulted in specimen contamination whereas etching was obtained with the two other grid systems even though the same test specimen, microscope vacuum and brightness of illumination (R----1.7 ×106 A cm -2 sterad-1) were used. However, when using the standard Wehnelt grid, the beam current was increased from approximately 10~A to 60V2k (as for the two other grids) in order to obtain the same degree of brightness. This supports the observations of Hart, Kassner and Maurin (1970) who reported that there seems to be another important source of contamination. It is suggested, therefore, that molecules contributing to the contamination are 'channelled' down the beam path, possibly from as far away as the electron gun area. This hypothesis requires further investigation and falls outside the scope of the present study. In order to operate the microscope according to optimum anti-contamination conditions, it is necessary to work at the equilibrium between contamination and etching. The results obtained with the modified Wehnelt grid (etching of only 0.05nm min-1) seem very advantageous in this respect. Cathode life-time The modified Wehnelt grid, using 'hair-pin' cathodes, is space charge limited, which means that the saturation level may easily be recognized and unnecessary overloading thereby avoided. Pointed cathodes do not have a saturation point, which can easily lead to overheating of the cathode and reduce its life. The extended life of the cathode experienced with the modified Wehnelt grid system seems to be related to a more favourable geometry around the cathode tip. The distance from the cathode tip and to the Wehnelt grid aperture is larger than in the standard and the pointed cathode grids. This makes tungsten deposits in the aperture less critical to micro-discharges as in the other two cases. ACKNOWLEDGEMENTS I am grateful to J E O L (Scandinavia) for providing details about the Wehnelt grid used in their Scanning Electron Microscope. I am also indebted to Mr. Knut Antonsen for making the Wehnelt grids to the very high level of precision which was required. I am also grateful to Dr. L. O. Froholm for valuable discussions throughout the experimental work and for his suggestions and criticisms when writing this paper. REFERENCES BOERSCH, H., 1943. Fresnelsche Beugung im Elektronenmikroskop. Phys. Z-, 44:202-211. 10
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BOERSCH, H. and BORN, G., 1960. Messungen an Elektronenstrahlerzeugern. In: Proc. 4th. International Congress on Electron Microscopy, Berlin, 1958, Mbllenstedt, G., Niehrs, H. and Ruska, E. (eds.), Springer Verlag, l: 35-39. BORN, M. and WOLF, E., 1965. Principles of Optics. Pergamon Press, London and New York. BORRIES, B.v. and RUSKA, E., 1939. Versuche, Rechenungen und Ergebnisse zur Frage des Auflbsungsvermbgens beim Ubermikroskop. Z" techn. Phys., 20: 225-235. BRAUCKS, F. W., 1958. Untersuchungen an der Fernfokuskathode nach Steigerwald. Optik., 15 : 242-260. DRECHSLER,M., COSSLETT,V. E. and NIXON,W. C., 1960. The point cathode as an electron source. In : Proc. 4th International Congress on Electron Microscopy, Berlin, 1958, M611enstedt, G., Niehrs, H. and Ruska, E. (eds.), Springer Verlag, 1: 13-20. ENNOS, A. E., 1953. The origin of specimen contamination in the electron microscope. J. Phys. D. (Br. J. appl. Phy.), 4: 101-106. FERNANDEZ-MORAN, H., 1966. Application of improved point cathode sources to high resolution electron microscopy. In : Proc. 6th International Congress on Electron Microscopy, Kyoto, Ryzoi Uyeda (ed.), Maruzen Co. Ltd., Tokyo, Japan, 1: 27-28. HAINE, M. E. and EINSTEIN,P. A., 1952. Characteristics of the hot cathode electron microscope gun. J. Phys. D. (Br. J. appl. Phy.), 3: 40-46. HAINE, M. E. and MULVEY, T., 1952. The formation of the diffraction image in the (labor diffraction microscope. J. opt. Soc. Amer., 42: 763-773. HANSZEN, K.-J., 1962. Beziehungen Zwischen den geometrischoptischen und den elektrischen Eigenschaften von Haarnadel- und Spitzenkathoden. In : Proc. 5th International Congress on Electron Microscopy, Philadelphia, Breese, S. S. (ed.), Academic Press, New York, 1: KK-11. HANSZEN, K.-J., 1971. Optical transfer theory of the electron microscope. In: Advances in Optical and Electron Microscopy. Barer, R. and Cosslett, V. E. (eds.), Academic Press, New York and London, 4: 1-84. HART, R. K., KASSNER,T. F. and MAURIN,J. K., 1970. The contamination of surfaces during high-energy electron irradiation. Phil. Mag., 21: 453-467. HEIDE, H. G., 1963. Die Objektverschmutzung im Elektronenmikroskop und das Problem der Strahlensch~idigung durch Kohlenstoffabbau. Z. angew. Phys., 15:116-128. HIBI, T., 1956. Point filaments and its applications. In: Proc. International Congress on Electron Microscopy, London, 1954, Royal Microscopical Society, 636-638. HIBI, T., 1962. Point cathode and resolution of electron microscope. In: Proc. 5th International Congress on Electron Microscopy, Philadelphia, Breese, S. S. (ed.), Academic Press, New York, l: K K - I . HIBI, T. and TAKAHASHI,S., 1969. Relation between coherence of electron beam and contrast of electron image. Z. angew. Phys., 27: 132-138. Him, T. and YADA, K., 1964. Point cathodes and resolution of electron microscope (II). The effect of the operating condition of the point cathode and that of a stigmator. J. Electronmicroscopy, 13: 94-100. HIBI, Z., YADA, K. and TAKAHASHI, S., 1962. Point cathode and resolution of electron microscope. J. Electronmicroscopy, 11 : 244-252. JOHANSEN, B. V., 1972a. High resolution stability of an electron microscope measured with an optical diffractometer. In: Proc. Scandinavian Societyfor Electron Microscopy, Gothenburg, 1971. JOHANSEN, B. V., 1972b. A new Wehnelt design for high resolution electron microscopy. In: Proc. Scandinavian Societyfor Electron Microscopy, Aarhus, 1972, (in press). JOHANSEN, B. V., 1972c. An optical diffractometer of simplified design for the analysis of high resolution electron micrographs. Micron, 3: 256-270. gANGER, R. and HOPPE, W., 1966/67. Die Erh6hung von Aufl6sung und Kontrast im Elektronenmikroskop mit Zonenkorrekturplatten. Optik, 24: 470-489. LENZ, F., 1965. Kann man Biprisma-Interferenzstreifen zur Messung des elektronenmikroskopischen Aufl6sungsverm6gens verwenden ? Optik, 22: 270-288. MARUSE, S., 1960. Vergleichende Untersuchungen der Eigenschaften von Spitzenkathoden
A HIGH PERFORMANCEWEHNELT GRID
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und Haarnadelkathoden in Strahlerzeugungssystemen mit negativer Wehneltblende. In: Proc. European Regional Conference on Electron Microscopy, Delft, Houwink, A. L. and Spit, B.J. (eds.), 1: 73-77. MARUSE, S. and SAKAKI,Y., 1958. l~ber einige elektronenoptische Eigenschaften der Spitzenkathode. Optik, 15: 485-490. MOLLER, E. W., 1937. Elektronenmikroskopische Beobachtungen yon Feldkathoden. Z. Phys., 106: 541-550. NIEMECI~, F. W. and RuPPIN, D., 1954. Elektrolytische Atzung von Kathodenspitzen fiir das Feldelektronenmikroskop. Z. angew. Phys., 6: 1-3. SAKAKI, Y. and M6LLENSTEDT, G., 1956. Beitrag zur Verwendung yon Spitzenkathoden in der Elektronenoptik. Optik, 13: 193-200. SAKAKI, Y. and MARUSE, S., 1960. Ober die Arbeitsweise und die elektronenoptischen Eigenschaften der Spitzenkathode. In : Proc. 4th International Congress on Electron Microscopy, Berlin, 1958, M611enstedt, G., Niehrs, H. and Ruska, E. (eds.), Springer Verlag, 1: 9-13. SKINNER, L. M., 1969. An accurate method for measuring the magnification of an electron microscope. 07. Phys. E. (ft. scient. Instrum.), 2: 206-208. STEIOERWALD,K. H., 1949. Ein neuartiges Strahlerzeugungs-System ftir Elektronenmikroskope. Optik, 5: 469-478. SWIFT, D. W. and NIXON, W. C., 1960. The behaviour of a point cathode in a triode electron gun. In: Proc. European Regional Conference on Electron Microscopy, Delft, Houwink, A. L. and Spit, B.J. (eds.) 1: 69-72. THON, F., 1965. Elektronenmikroskopische Untersuchungen an diinnen Kohlefolien. Z. Naturforsch., 20a: 154-155. TnON, F., 1966. Zur Defokussierungsabh~ngigkeit des Phasenkontrastes bei der elektronenoptischen Abbildung. Z. Naturforsch., 21a: 476-478. YADA, K., 1968. Influence of illumination beam on the image contrast. J. Electronmicroscopy, 17: 162-163. YADA, K. and HIBI, T., 1966. The contrast of electron images in the case of using pointed cathodes. In : Proc. 6th International Congress on Electron Microscopy, Kyoto, Ryozi Uyeda (ed.), Maruzen Co. Ltd., Tokyo, Japan, 1: 25-26.
Micron, 1973, 4:137-138
137 BOOK REVIEWS
Stereology 3. E. R. W E I B E L , G. M E E K , B. RALF, P. E C H L I N and R. ROSS (Eds.), Published for The Royal Microscopical Society by Blackwells, Oxford, 1972 (Hardback, 393 pages). Price: £7.00. Stereology is an important technique which will provide three-dimensional information and interpretation from certain two-dimensional images. The subject of stereology is relatively new when applied to microscopy, the latter being an essentially two-dimensional system as the final images are recorded on a photographic film or viewed on a suitable screen. The Third International Congress on Stereology was held in Berne, Switzerland, during 1971 and the above volume has been published covering the invited contributions together with a selection of other papers. There are eleven invited papers by distinguished workers in the field of stereology which are concerned with subjects ranging from the quantitative analysis of the structure, function of cells and organelles to analysing images from mineral specimens, scanning electron micrographs and automatic image processing for stereology, etc. The other nineteen contributions also deal with a wide selection of papers covering the theory and applications of stereology to microscopy. Although the volume includes a range of biological and non-biological contributions and contains a fair amount of mathematical analysis, it will be of considerable value to microscopists generally and especially to workers concerned with three-dimensional image construction. The book also contains an extensive and valuable list of published works at the end of each contribution. The production of the book is of a very high standard and includes a large number of micrographs together with clear line drawings of a size that are easy to follow in relation to the text. This volume on stereology is highly recommended to electron microscopists generally as the field of three-dimensional electron image interpretation and image reconstruction is expanding rapidly. R. W. Horne
Metallurgical Stereographic Projections. J. S. S M A I L L . A d a m Hilger, London, pp. 262 + viii, 1972. Price: £7.00. The Stereographic Projection is a convenient method for representing three-dimensional crystallographic data in two dimensions. One has the option of using projections or tables of interplanar or interzonal angles in work which demands a three-dimensional appreciation of the problem. Most people seem to prefer the use of projections since they provide a measure of the angles involved together with a pictorial distribution of the poles relative to one another. The Stereographic Projection (SP), or its unfortunate abbreviation, Stereogram, has become more widely used since the advent of electron microscopy and single crystal technology. Earlier treatises on the subject, references to which are provided in the book, were largely concerned with the application of SPs in X-ray diffraction studies. The author intends his book for the metallurgist and materials scientist. It can effectively be split into three main parts. The first part, covering Chapters 1-3, deals with the crystallographic notations used and also generates the equations to be used later. The Miller and Miller-Bravais systems of indexing hexagonal crystals are b o t h mentioned but unfortunately are not treated in any depth, and this m a y lead to confusion. This section tends to look mathematical but this should not deter would-be purchasers because the mathematics are simple and easily followed with the aid of lucid diagrams. The third chapter concerns the use of the SP and one gets the impression, quite wrongly, that its application in X - r a y and E M is about to be discussed. In fact, the author concentrates on the latter aspect apart from certain general applications; clear instructions on how to use the SP in, for example, trace analysis, lattice rotation, certain