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Energy-loss microscopy of ZrO2
Micron, 1975, Vol. 6: 65-72. Pergamon Press. Printed in Great Britain.
Energy-loss microscopy of ZrO2 R. A. PLOC
Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada and F. P. OTTENSMEYER
Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada Manuscript received February 12, 1975 A prism-mirror-prism energyfilter installed in a transmission electron microscope was used to examine oxide films formed on zirconium. Bright and dark-field images were obtained using electrons which were elastically and inelastically scattered. Reproducible energy losses of 13.8, 23.9 and 37.9eV were characteristic of the ZrOz films examined.
An added bonus of the PMP filter was the production of an energy-loss spectrum. By ensuring that the astigmatic cross-over point for each energy was located at the level of a pair of knife edges, it was possible to select any range of electron energies desired for use in image formation. The construction of the PMP transmission electron microscope has offered the possibility of high resolution, high contrast images in thick or thin biological or materials science specimens.
INTRODUCTION
Most modern commercial, transmission electron microscopes (TEM) are capable of at least 0.3nm lattice plane resolution. In spite of this, in thick biological and non-biological specimens the best point to point resolution is often 3 or 4nm. Such seemingly poor performance is the result of the high chromatic aberration coefficient of the objective lens. Energy losses suffered by the incident high energy electrons passing through substances have the effect of limiting maximum useful specimen thickness since the greater the thickness the more severe the image degradation. Crewe and colleagues (see Crewe, 1974) at the University of Chicago, have attempted to solve the problem of resolution loss by developing the Scanning Transmission Electron Microscope (STEM). In 1964 (Henry (1964a, b) described an electron-optical prism-mirrorprism (PMP) energy filter which has since been incorporated into a conventional T E M (for a list of references leading to various applications in materials science, see Jouffrey, 1972). The purpose of the filter system was, on the one hand, to study energy loss spectra of materials and on the other, to improve imaging characteristics by eliminating the effect of electron energy losses. In a similar instrument Henkelman and Ottensmeyer (1974a) have demonstrated the realization of this goal in biological applications.
MATERIALS A N D M E T H O D S
The instrument used in this study has been described in detail by Henkelman and Ottensmeyer (1974a, b). Briefly, the P M P filter was installed between the intermediate and projector lenses of a Siemens 1A transmission electron microscope by extending the column 33cm and placing a further intermediate lens of long focal length below the filter. The instrument was operated at 60kV and displayed a resolution of 0.8-1.0nm before and after incorporation of the energy filter. The theoretical aberration of the filter over a field of view of ll.tm was less than 0.16nm. However, in the design of this instrument the adjustments necessary for materials science applications had not been given priority, therefore, after a number of rather cumbersome manipulations the acceptable field of view was frequently less than 65
66
R.A. Ploc and F. 17. Ottensmeyer
optimal. This nevertheless did not affect our conclusions. The zirconia samples examined have been described by Ploc (1970a, b). In this study ctZrO 2 films, 45, 70 and 143nm thick grown at 300°C in 760 Torr of dry oxygen, have been removed tYom their substrates and examined in transmission. An anodized Zircaloy-2* oxide film 57.5nm thick (oxalic acid, constant current mode--6.25mA/cm 2 to a final 25V) was also examined. The thermally formed films consisted of crystallites of monoclinic a Z r O 2 ranging in size from the resolution limit of the microscope to a few tens of nanometers in diameter situated in a field of apparently 'amorphous' oxide. In general, the oxide structure is highly defective being distorted and containing crystal lattice defects due to non-stoichiometry. The anodically formed oxide possessed larger crystallites, generally 40-60nm dia, a tetragonal (or cubic) crystal structure and approximately 3nm dia pores within the 'amorphous' areas.
RESULTS ~
DISCUSSION
Energy loss spectra The insert in Fig. 1 is a typical energy loss spectrum taken from a thin (50rim) tetragonal or monoclinic ZrO2 film. Three distinct energyloss peaks are visible with a continuous loss spectrum extending to several kilovolts. Several densitometer traces were made on each of a number of plates and compared to a 'standard' 30nm thick aluminium loss-spectrum to yield the indicated scale. Energy resolution was approximately 2.75-3.0eV (0.5 peak width of zero-loss band), though peak positions were located more accurately. The three peaks corresponded to losses of 13.8, 23.9 and 37.9eV respectively. The largest distinct toss is believed to result from a combination of the first two excitations; respectively a surface and a bulk plasmon-loss. According to Raether (1965), for a spherical crystal boundary (less than a given critical radius) the surface plasmon, cos, is cop/~/g where cop is the bulk plasmon frequency a result in agreement with our value of 13.8eV. The energy loss traces of Fig. 1 were obtained *Zirconium alloy containing 1.2-1.7 wt % Sn, 0.07-0.20 wt ~oo Fe, 0.05-0.15 wt % Cr, 0.03-0.08 wt % Ni with a total Fe, Cr, Ni between 0.18 and 0.38 wt %.
from photographically recorded sepctra i as in the insert) using a Joyce-Loebl automatic recording microdensitometer, model M K I I | c. As different amounts of optical filtering and electronic damping were used in the densitometer, the spectra from the two film thicknesses cannot be directly compared. However, as was expected, the surface plasmon intensity did not significantly increase with specimen thickness. For the stated losses, the plasmon wavelengths correspond to 270, 153.5 and 97nm respectively. The principal operating reflections were a combination of the (111)m and (111)m in the thermal and the (111) in the anodically formed films. The two-beam extinction distances at 60keV, without temperature correction, are 42.2, 34.7 and 31.8nm, respectively.
Images Our initial observations of 'loss-images' have been directed towards answering the following questions: (1) does exclusion of inelastically scattered electrons from the 'total' (elastic and inelastically scattered) image increase resolution and contrast ? (2) do the 'inelastic' (inelastically scattered l electrons carry significant information? (3) how localized are the 'inelastic' events? As mentioned earlier the resolution of the microscope has not been affected by the P M P system. Achromatic images from the 'inelastic' electrons can be obtained by setting the knife edges to accept a narrow energy spread around the no-loss peak and increasing the accelerating voltage by an amount equal to the desired energy loss. Only electrons which have under gone this energy loss then pass through the slit to form the image. Bright field. Figures 2a and 2b are bright field (BF) electron micrographs taken from a 45nm thick, thermally formed ctZrO 2 film. The 'total' image utilizes all electrons passing through the objective aperture; the 'elastic' image, only those elastically scattered (or not scattered at all) and the 'total inelastic' image, all the electrons differing in energy from those in the beam incident on the specimen. There are at least three criteria that might be used to distinguish resolution differences in these figures. First, the minimum distinguishable particle size; second, the minimum reproducible distance between two points and thirdly, edge sharpness. Each of these definitions, however,
Energy-Loss Microscopy of ZrO2
ACCELERATING
VOLTAGE (incident electrons)
=
67
60keV
Fig. I. Insert showing an energy loss spectrum from a 50nm thick thermally formed ctZrO 2 film. The peaks in the densitometer traces (from spectra for the stated oxide thicknesses), correspond to energy losses of 0, 13.8, 23.9 and 37.9eV are inadequate since loss of resolution not only stems from inability to focus but also from further sources such as the non-localized nature of the inelastic event and from the displacement and superpositioning of focused images. Ploc (1970c) discussed the latter problem and Parsons and Hoelke (1974) demonstrated its significance. In areas which yield non-uniform quantities of 'elastic' and 'inelastic' electrons criterion one is an unreliable test of resolution. For instance, in an 'elastic' BF image, when a crystallite is bent away from the Bragg diffracting position
the number of electrons passing through the aperture increases. In the 'total' image, the loss of electrons due to Bragg diffraction may be more than compensated for by those inelastically scattered and the black region (deficiency of image forming electrons) decreases in size thereby making the crystallite appear smaller than the 'elastic' counterpart. Criterion two is a standard test but normally requires special samples and as mentioned earlier, is probably not reliable due to the multifarious nature of loss of resolution. Be-
68
R.A. Ploc and F. P. Ottensmeyer
Fig. 2a. Bright field electron micrographs of a 45rim thick aZrO~ film using all (total) the transmitted electrons; only those elastically scattered (elastic) and all the inelastically scattered electrons (total inelastic) transmitted through a 301am diameter objective aperture located on the central axis of the TEM. The 'total' image corresponds to the superimpositioning of the 'elastic' and 'total inelastic' and is that image normally obtained in the conventional transmission electron microscope. Arrows show identical areas. cause of the great abundance of crystallites, densitometer contours of their edges were taken as a relative measure of the increase in information obtained by energy filtering. Figure 3 shows the result of such traces between two recognizable points in dark field (DF) images. Indeed, the 'elastic' image displayed the sharpest rise and 0.5 peak widths for a given feature and hence, best contrast and resolution. Inclusion of the 'inelastic' electrons degraded resolution by increasing the background and smearing fine detail. Figure 4 shows the dramatic increase in image quality that can be achieved by eliminating the 'loss' electrons from the image. T h e specimen is an anodieally formed Z r O 2 film 45nm thick (as described by Ploc, 1970a) which contains 3nm diameter pores. The 'plasmon loss' image of Figs. 2 and 5 support the contention that the 13.8 and 23.9eV losses are due to surface and bulk plasmon losses respectively. For instance, the '13.SeV loss' image displays information associated
with the surface of the crystallites (size) and never bulk information such as the 35nm spaced Moird fringes in Fig. 2b. Dark field. Figure 5 shows six DF micrographs from an area near that shown in Fig. 2, In this instance only those electrons which have gone through the (I 11)m/(111)m Bragg reflection are used to image the crystallites. The markers indicate identical areas while within the triangle and near the middle of the left edge of the 'elastic' image 2nm Moird fringes were resolvable in the original prints. In the case of these 'loss' micrographs the possibility of electrons experiencing a plasmon loss then being diffracted and contributing to a high resolution DF image cannot be ruled out (i.e. ~ (111),, = 34.7nm). Densitometer traces of the energy-loss spectrum of the (111),,/(il 1)m beam revealed a much smaller spread in peak heights (see Fig. 6), than in the BF case (see Fig. 1). In Fig. 5, the plasmon-loss electrons appear to be confined to the oxide crystallites while the higher non-
Energy-Loss Microscopy of ZrO ~
69
Fig. 2b. As for Fig. 2a except the image has been formed by using incident electrons which have lost the stated amounts of energy. The circles show identical areas and the arrow in the 23.9eV loss image, 3.5nm fringes. plasmon losses, though possessing information, produce a much higher background intensity. The probability of the 65eV loss electrons being due to multiple excitations of the same energy plasmon is small; for instance, for )~p (plasmon wavelength) = 150nm (energy loss of 25eV) and for a crystal thickness of 45nm, the probability for the 1st plasmon excitation is 24 and 3 ~o for the second. TOTAL
In the conventional high resolution DF images, large scattering angles have been used to reduce the effect of the inelastic scattering on the image. Such procedures resulted in prolonged exposures which in turn emphasized instrument and specimen instabilities. Use of the P M P filtering system eliminated such problems by allowing the use of relatively small scattering angles. As expected, Fig. 5 demon13-$ eV
Loss
ELASTIC nm
37-9 eV L ~ s
65eV
Loss
Fig. 3. Densitometer traces between the same distinguishable points in dark field (DF) images such as shown in Fig. 5. The densitometer spot was a small fraction of the size of the image points.
70
R.A. Ploc and F. P. Ottensmeyer
Fig. 4. 'Total' and 'elastic' images of an anodicatly ZrO2 film containing 2-3nm diameter pores, visible in the latter only. strates that the 'total' DF image is equivalent to the 'elastic' image degraded by a diffuse background containing weak aberrated images. CONCLUSIONS
T h e use of p r i s m - m i r r o r - p r i s m filter in a transmission electron microscope yielded high contrast electron images of highly defective materials with at least 1-2nm resolution (resolution limit of the present microscope used in this work). Images formed with plasmon-loss electrons have revealed that they possess significant information but cannot be directly used to study the non-localized nature of inelastic scattering. Oxide films grown on zirconium substrates produced three reproducible plasmon energy losses; namely, 13.8, 23.9 and 37.9eV.
Henkelman, R. M. and Ottensmeyer, F. P., t974b. An electrostatic mirror. 07. Phys E (07. Sc#nt. Instrum.), 7: 176-178. Henry, L., 1964a. Filtrage magn6tique des vitesses en microscopic 61ectronique. Ph.D. Thesis, University of Paris, France. Henry, L., 1964b. Filtrage magn6tique des vitesses en microscopie ~lectronique. Bull. Soc. Frans. Mindr. Crist., 88: 172-198. Jouffrey, B., 1972. On some problems of energy losses. In: Proc. 5th European Congress on Electron Microscopy, Manchester, England, Institute of Physics (London), 190-195. Parsons, J. R. and Hoelke, C. W., 1974. Electron microscopy of plasmons. In : Proc. 8th International Congress on Electron Microscopy, Canberra, Australia, Sanders, J. V. and Goodchild, D. J. (eds.), Australian Academy of Science, Canberra, A.C.T., 392-393. Ploc, R. A., 1970a. Anodic zirconia films. In: Proc, 7th International Congress on Electron Microscopy,
Acknowledgements--Thanks are given to M. A. Miller
for his technical help in obtaining the densitometer traces and to R. M. Henkelman for Fig. 4. REFERENCES
Crewe, A. V., 1974. The high resolution scanning transmission electron microscope. In: Proc. 32nd Annual Meeting EMSA, Arceneaux, C. J. (ed.), Claitors Publishing Division, Baton Rouge, Louisiana, U.S.A., 316-319. Henkelman, R. M. and Ottensmeyer, F. P., 1974a. An electron microscope energy filter for biology. 07. Microscopy, 102: 79-94.
Grenoble, France, Favard, P. (ed.), Soci~t~ Frangaise de Microscopic l~lectronique, Paris, France, 2: 371-372. Ploc, R. A., 1970b. Physical changes in thin ZrO2 films with thickening. Ibid, 373-374. Ploc, R. A., 1970c. Aberrations in the transmission electron microscope. In: Proc. 28th Annual:Meeting EMSA, Arceneaux, C.J. (ed.), Claitors Publishing Division, Baton Rouge, Louisiana, U.S.A., 352353. Raether, H., 1965. Solid state excitations by electrons. In: Springer Tracts in Modern Physics, Hoehler, G. (ed.), Springer-Verlag New York Inc., 38 t 84-157.
Energy-Loss Microscopy of ZrO2
Fig. 5. High resolution (11 lm/(T11),, dark field micrographs from an area near that shown in Fig. 2. The energy cut was measured to be 5.6eV.
71
72
R . A . Ploc and F. P. Ottensmeyer
Fig. 5 (contd). H i g h resolution (111),,/(111),, dark field micrographs from an area near thai shown in Fig. 2. The energy cut was measured to 5.6eV.
i
•
1.111} Illll
11
Fig. 6
Energy-loss microscopy of ZrO2 R. A. PLOC
Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada and F. P. OTTENSMEYER
Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada Manuscript received February 12, 1975 A prism-mirror-prism energyfilter installed in a transmission electron microscope was used to examine oxide films formed on zirconium. Bright and dark-field images were obtained using electrons which were elastically and inelastically scattered. Reproducible energy losses of 13.8, 23.9 and 37.9eV were characteristic of the ZrOz films examined.
An added bonus of the PMP filter was the production of an energy-loss spectrum. By ensuring that the astigmatic cross-over point for each energy was located at the level of a pair of knife edges, it was possible to select any range of electron energies desired for use in image formation. The construction of the PMP transmission electron microscope has offered the possibility of high resolution, high contrast images in thick or thin biological or materials science specimens.
INTRODUCTION
Most modern commercial, transmission electron microscopes (TEM) are capable of at least 0.3nm lattice plane resolution. In spite of this, in thick biological and non-biological specimens the best point to point resolution is often 3 or 4nm. Such seemingly poor performance is the result of the high chromatic aberration coefficient of the objective lens. Energy losses suffered by the incident high energy electrons passing through substances have the effect of limiting maximum useful specimen thickness since the greater the thickness the more severe the image degradation. Crewe and colleagues (see Crewe, 1974) at the University of Chicago, have attempted to solve the problem of resolution loss by developing the Scanning Transmission Electron Microscope (STEM). In 1964 (Henry (1964a, b) described an electron-optical prism-mirrorprism (PMP) energy filter which has since been incorporated into a conventional T E M (for a list of references leading to various applications in materials science, see Jouffrey, 1972). The purpose of the filter system was, on the one hand, to study energy loss spectra of materials and on the other, to improve imaging characteristics by eliminating the effect of electron energy losses. In a similar instrument Henkelman and Ottensmeyer (1974a) have demonstrated the realization of this goal in biological applications.
MATERIALS A N D M E T H O D S
The instrument used in this study has been described in detail by Henkelman and Ottensmeyer (1974a, b). Briefly, the P M P filter was installed between the intermediate and projector lenses of a Siemens 1A transmission electron microscope by extending the column 33cm and placing a further intermediate lens of long focal length below the filter. The instrument was operated at 60kV and displayed a resolution of 0.8-1.0nm before and after incorporation of the energy filter. The theoretical aberration of the filter over a field of view of ll.tm was less than 0.16nm. However, in the design of this instrument the adjustments necessary for materials science applications had not been given priority, therefore, after a number of rather cumbersome manipulations the acceptable field of view was frequently less than 65
66
R.A. Ploc and F. 17. Ottensmeyer
optimal. This nevertheless did not affect our conclusions. The zirconia samples examined have been described by Ploc (1970a, b). In this study ctZrO 2 films, 45, 70 and 143nm thick grown at 300°C in 760 Torr of dry oxygen, have been removed tYom their substrates and examined in transmission. An anodized Zircaloy-2* oxide film 57.5nm thick (oxalic acid, constant current mode--6.25mA/cm 2 to a final 25V) was also examined. The thermally formed films consisted of crystallites of monoclinic a Z r O 2 ranging in size from the resolution limit of the microscope to a few tens of nanometers in diameter situated in a field of apparently 'amorphous' oxide. In general, the oxide structure is highly defective being distorted and containing crystal lattice defects due to non-stoichiometry. The anodically formed oxide possessed larger crystallites, generally 40-60nm dia, a tetragonal (or cubic) crystal structure and approximately 3nm dia pores within the 'amorphous' areas.
RESULTS ~
DISCUSSION
Energy loss spectra The insert in Fig. 1 is a typical energy loss spectrum taken from a thin (50rim) tetragonal or monoclinic ZrO2 film. Three distinct energyloss peaks are visible with a continuous loss spectrum extending to several kilovolts. Several densitometer traces were made on each of a number of plates and compared to a 'standard' 30nm thick aluminium loss-spectrum to yield the indicated scale. Energy resolution was approximately 2.75-3.0eV (0.5 peak width of zero-loss band), though peak positions were located more accurately. The three peaks corresponded to losses of 13.8, 23.9 and 37.9eV respectively. The largest distinct toss is believed to result from a combination of the first two excitations; respectively a surface and a bulk plasmon-loss. According to Raether (1965), for a spherical crystal boundary (less than a given critical radius) the surface plasmon, cos, is cop/~/g where cop is the bulk plasmon frequency a result in agreement with our value of 13.8eV. The energy loss traces of Fig. 1 were obtained *Zirconium alloy containing 1.2-1.7 wt % Sn, 0.07-0.20 wt ~oo Fe, 0.05-0.15 wt % Cr, 0.03-0.08 wt % Ni with a total Fe, Cr, Ni between 0.18 and 0.38 wt %.
from photographically recorded sepctra i as in the insert) using a Joyce-Loebl automatic recording microdensitometer, model M K I I | c. As different amounts of optical filtering and electronic damping were used in the densitometer, the spectra from the two film thicknesses cannot be directly compared. However, as was expected, the surface plasmon intensity did not significantly increase with specimen thickness. For the stated losses, the plasmon wavelengths correspond to 270, 153.5 and 97nm respectively. The principal operating reflections were a combination of the (111)m and (111)m in the thermal and the (111) in the anodically formed films. The two-beam extinction distances at 60keV, without temperature correction, are 42.2, 34.7 and 31.8nm, respectively.
Images Our initial observations of 'loss-images' have been directed towards answering the following questions: (1) does exclusion of inelastically scattered electrons from the 'total' (elastic and inelastically scattered) image increase resolution and contrast ? (2) do the 'inelastic' (inelastically scattered l electrons carry significant information? (3) how localized are the 'inelastic' events? As mentioned earlier the resolution of the microscope has not been affected by the P M P system. Achromatic images from the 'inelastic' electrons can be obtained by setting the knife edges to accept a narrow energy spread around the no-loss peak and increasing the accelerating voltage by an amount equal to the desired energy loss. Only electrons which have under gone this energy loss then pass through the slit to form the image. Bright field. Figures 2a and 2b are bright field (BF) electron micrographs taken from a 45nm thick, thermally formed ctZrO 2 film. The 'total' image utilizes all electrons passing through the objective aperture; the 'elastic' image, only those elastically scattered (or not scattered at all) and the 'total inelastic' image, all the electrons differing in energy from those in the beam incident on the specimen. There are at least three criteria that might be used to distinguish resolution differences in these figures. First, the minimum distinguishable particle size; second, the minimum reproducible distance between two points and thirdly, edge sharpness. Each of these definitions, however,
Energy-Loss Microscopy of ZrO2
ACCELERATING
VOLTAGE (incident electrons)
=
67
60keV
Fig. I. Insert showing an energy loss spectrum from a 50nm thick thermally formed ctZrO 2 film. The peaks in the densitometer traces (from spectra for the stated oxide thicknesses), correspond to energy losses of 0, 13.8, 23.9 and 37.9eV are inadequate since loss of resolution not only stems from inability to focus but also from further sources such as the non-localized nature of the inelastic event and from the displacement and superpositioning of focused images. Ploc (1970c) discussed the latter problem and Parsons and Hoelke (1974) demonstrated its significance. In areas which yield non-uniform quantities of 'elastic' and 'inelastic' electrons criterion one is an unreliable test of resolution. For instance, in an 'elastic' BF image, when a crystallite is bent away from the Bragg diffracting position
the number of electrons passing through the aperture increases. In the 'total' image, the loss of electrons due to Bragg diffraction may be more than compensated for by those inelastically scattered and the black region (deficiency of image forming electrons) decreases in size thereby making the crystallite appear smaller than the 'elastic' counterpart. Criterion two is a standard test but normally requires special samples and as mentioned earlier, is probably not reliable due to the multifarious nature of loss of resolution. Be-
68
R.A. Ploc and F. P. Ottensmeyer
Fig. 2a. Bright field electron micrographs of a 45rim thick aZrO~ film using all (total) the transmitted electrons; only those elastically scattered (elastic) and all the inelastically scattered electrons (total inelastic) transmitted through a 301am diameter objective aperture located on the central axis of the TEM. The 'total' image corresponds to the superimpositioning of the 'elastic' and 'total inelastic' and is that image normally obtained in the conventional transmission electron microscope. Arrows show identical areas. cause of the great abundance of crystallites, densitometer contours of their edges were taken as a relative measure of the increase in information obtained by energy filtering. Figure 3 shows the result of such traces between two recognizable points in dark field (DF) images. Indeed, the 'elastic' image displayed the sharpest rise and 0.5 peak widths for a given feature and hence, best contrast and resolution. Inclusion of the 'inelastic' electrons degraded resolution by increasing the background and smearing fine detail. Figure 4 shows the dramatic increase in image quality that can be achieved by eliminating the 'loss' electrons from the image. T h e specimen is an anodieally formed Z r O 2 film 45nm thick (as described by Ploc, 1970a) which contains 3nm diameter pores. The 'plasmon loss' image of Figs. 2 and 5 support the contention that the 13.8 and 23.9eV losses are due to surface and bulk plasmon losses respectively. For instance, the '13.SeV loss' image displays information associated
with the surface of the crystallites (size) and never bulk information such as the 35nm spaced Moird fringes in Fig. 2b. Dark field. Figure 5 shows six DF micrographs from an area near that shown in Fig. 2, In this instance only those electrons which have gone through the (I 11)m/(111)m Bragg reflection are used to image the crystallites. The markers indicate identical areas while within the triangle and near the middle of the left edge of the 'elastic' image 2nm Moird fringes were resolvable in the original prints. In the case of these 'loss' micrographs the possibility of electrons experiencing a plasmon loss then being diffracted and contributing to a high resolution DF image cannot be ruled out (i.e. ~ (111),, = 34.7nm). Densitometer traces of the energy-loss spectrum of the (111),,/(il 1)m beam revealed a much smaller spread in peak heights (see Fig. 6), than in the BF case (see Fig. 1). In Fig. 5, the plasmon-loss electrons appear to be confined to the oxide crystallites while the higher non-
Energy-Loss Microscopy of ZrO ~
69
Fig. 2b. As for Fig. 2a except the image has been formed by using incident electrons which have lost the stated amounts of energy. The circles show identical areas and the arrow in the 23.9eV loss image, 3.5nm fringes. plasmon losses, though possessing information, produce a much higher background intensity. The probability of the 65eV loss electrons being due to multiple excitations of the same energy plasmon is small; for instance, for )~p (plasmon wavelength) = 150nm (energy loss of 25eV) and for a crystal thickness of 45nm, the probability for the 1st plasmon excitation is 24 and 3 ~o for the second. TOTAL
In the conventional high resolution DF images, large scattering angles have been used to reduce the effect of the inelastic scattering on the image. Such procedures resulted in prolonged exposures which in turn emphasized instrument and specimen instabilities. Use of the P M P filtering system eliminated such problems by allowing the use of relatively small scattering angles. As expected, Fig. 5 demon13-$ eV
Loss
ELASTIC nm
37-9 eV L ~ s
65eV
Loss
Fig. 3. Densitometer traces between the same distinguishable points in dark field (DF) images such as shown in Fig. 5. The densitometer spot was a small fraction of the size of the image points.
70
R.A. Ploc and F. P. Ottensmeyer
Fig. 4. 'Total' and 'elastic' images of an anodicatly ZrO2 film containing 2-3nm diameter pores, visible in the latter only. strates that the 'total' DF image is equivalent to the 'elastic' image degraded by a diffuse background containing weak aberrated images. CONCLUSIONS
T h e use of p r i s m - m i r r o r - p r i s m filter in a transmission electron microscope yielded high contrast electron images of highly defective materials with at least 1-2nm resolution (resolution limit of the present microscope used in this work). Images formed with plasmon-loss electrons have revealed that they possess significant information but cannot be directly used to study the non-localized nature of inelastic scattering. Oxide films grown on zirconium substrates produced three reproducible plasmon energy losses; namely, 13.8, 23.9 and 37.9eV.
Henkelman, R. M. and Ottensmeyer, F. P., t974b. An electrostatic mirror. 07. Phys E (07. Sc#nt. Instrum.), 7: 176-178. Henry, L., 1964a. Filtrage magn6tique des vitesses en microscopic 61ectronique. Ph.D. Thesis, University of Paris, France. Henry, L., 1964b. Filtrage magn6tique des vitesses en microscopie ~lectronique. Bull. Soc. Frans. Mindr. Crist., 88: 172-198. Jouffrey, B., 1972. On some problems of energy losses. In: Proc. 5th European Congress on Electron Microscopy, Manchester, England, Institute of Physics (London), 190-195. Parsons, J. R. and Hoelke, C. W., 1974. Electron microscopy of plasmons. In : Proc. 8th International Congress on Electron Microscopy, Canberra, Australia, Sanders, J. V. and Goodchild, D. J. (eds.), Australian Academy of Science, Canberra, A.C.T., 392-393. Ploc, R. A., 1970a. Anodic zirconia films. In: Proc, 7th International Congress on Electron Microscopy,
Acknowledgements--Thanks are given to M. A. Miller
for his technical help in obtaining the densitometer traces and to R. M. Henkelman for Fig. 4. REFERENCES
Crewe, A. V., 1974. The high resolution scanning transmission electron microscope. In: Proc. 32nd Annual Meeting EMSA, Arceneaux, C. J. (ed.), Claitors Publishing Division, Baton Rouge, Louisiana, U.S.A., 316-319. Henkelman, R. M. and Ottensmeyer, F. P., 1974a. An electron microscope energy filter for biology. 07. Microscopy, 102: 79-94.
Grenoble, France, Favard, P. (ed.), Soci~t~ Frangaise de Microscopic l~lectronique, Paris, France, 2: 371-372. Ploc, R. A., 1970b. Physical changes in thin ZrO2 films with thickening. Ibid, 373-374. Ploc, R. A., 1970c. Aberrations in the transmission electron microscope. In: Proc. 28th Annual:Meeting EMSA, Arceneaux, C.J. (ed.), Claitors Publishing Division, Baton Rouge, Louisiana, U.S.A., 352353. Raether, H., 1965. Solid state excitations by electrons. In: Springer Tracts in Modern Physics, Hoehler, G. (ed.), Springer-Verlag New York Inc., 38 t 84-157.
Energy-Loss Microscopy of ZrO2
Fig. 5. High resolution (11 lm/(T11),, dark field micrographs from an area near that shown in Fig. 2. The energy cut was measured to be 5.6eV.
71
72
R . A . Ploc and F. P. Ottensmeyer
Fig. 5 (contd). H i g h resolution (111),,/(111),, dark field micrographs from an area near thai shown in Fig. 2. The energy cut was measured to 5.6eV.
i
•
1.111} Illll
11
Fig. 6