Optimisation and applications of the Cambridge University 600 kV high resolution electron microscope

Ultramicroscopy 9 (1982) 203-214 203 North-Holland Publishing Company

OPTIMISATION AND APPLICATIONS OF TH E CAMBRIDGE UNIVERSITY 600 kV HI GH RESOLUTION ELECTRON MICROSCOPE

David J. SMITH, R.A. CAMPS, V.E. COSSLETT, L.A. FREEMAN and W.O. SAXTON

High Resolution Electron Microscope, University of Cambridge, Free School Lane, Cambridge, UK

and

W.C. NIXON, H. AHMED, C.J.D. CATTO, J.R.A. CLEAVER, K.C.A. SMITH and A.E. TIMBS

University Engineering Department, Trumpington Street, Cambridge, UK

Received 14 June 1982 (presented at Workshop January 1982)

The Cambridge University 600 kV high resolution electron microscope is currently operating with a directly interpretable image resolution of close to 2 A. Following a brief outline of salient features of this H R E M which have proven crucial to optimising its performance, details are given of methods whereby such high resolution levels are now obtainable on a routine basis. Some recent applications are also described. The success of these studies confirms that electron microscopy at the atomic resolution level will have an increasing impact in many areas of materials research.

1. Introduction

It has long been known that ~the high voltage electron microscope (HVEM) has the potential for providing image resolution on the level of atomic dimensions [1]. However, it was comparatively re- cently that these potentialities came close to being realised [2-5], and then only as a direct result of improvements in engineering design and construction [6,7] to ensure that factors such as mechanical and electrical stabilities were no longer limiting. In this short review we first summarise (section 2) some of the novel features of the Cambridge Uni- versity 600 kV high resolution electron microscope (HREM) [8,9] which account for much of its success in providing high resolution levels routinely. In section 3, we describe other local improvements and modifications which assist in ensuring rehable operation for extended periods, whilst in section 4 we outline the relevance of image processing/simulation to high resolution imaging. This is followed in section 5 by details, necessarily brief, of some more recent studies which

utilise the capabilities of the HREM. Finally, in section 6, we consider likely future prospects for both instrumental developments and the study of materials at atomic resolution.

2. Design features

To ensure that the performance of the electron microscope approaches closely the theoretical limits, it is vital that all likely perturbing factors be reduced to the point where their influence is only marginal at worst. Whilst this state has been re- ached in recent commercially available 100 and 200 keV instruments, as indicated by their ability to transfer specimen information well beyond the Scherzer value [10,11], it is also clear that high resolution HVEMs are restricted, at best, to resolution figures around this limit (although, ad- mittedly, without any of the oscillations of the contrast transfer function which prevent such images from lower voltage instruments from being directly interpretable). Moreover, in some HVEMs,

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204 D.J. Smith et al. / Optimisation and applications of Cambridge University 600 k V HREM

operation is restricted to magnifications of typically around 200,000 times [12] because of limited gun brightness. This makes astigmatism dorrection a tedious process [13] - direct visual correction not being possible - and necessitates the recording of through-focal series in order to be certain of ob- taining images near the opt imum defocus position. Somewhat surprisingly, perhaps, given their greatly increased mass, it also appears that HVEMs are more, rather than less, susceptible to mechanical vibration of the entire microscope assembly, and night-time operation, when the effects of machin- ery, passing traffic etc. will be minimised, is re- ported as commonplace. Finally, the technical problems associated with stabilising the high voltage supply of an electron microscope increase substantially as the voltage increases to the extent that this (in)stability has been pinpointed as being the prime resolution-limiting factor of at least two

Fig. 1. HREM column and operator (R.A.C.).

high resolution HVEMs [3,5]. During the design, construction and commissioning of the Cambridge 600 kV HREM, shown in fig. 1, close attention has been given to the requirements of mechanical and electrical stability, as well as to providing suffi- cient electron source brightness. Further details can be found elsewhere (e.g., refs. [6,8,9,14,15] and references listed therein).

The electron source for the 600 kV H R E M is an indirectly heated LaB 6 type, after Ahmed and Broers [16], run off separate high-efficiency power supplies [17], and is, to our knowledge, the only one of its type in use on any HREM. Operation is normally in the low power mode wherein most of the heating of the LaB 6 rod is by bombardment current [18] with lifetimes typically around 200 hours. Using a separate gun test-rig to simulate H R E M operating conditions, we have shown that brightness levels of close to 1 X 10 6 m / c m 2. sr at a gun operating voltage of 20 kV are obtainable throughout the entire lifetime of a rod, after an initial "running-in" period of perhaps 4 -8 h [19]. Observations with the H R E M confirm that these gun brightnesses are applicable, after relativistic correction, to actual microscopy conditions - current densities at the specimen in excess of 10 A / c m 2 are obtainable at 500 kV whilst maintain- ing beam divergence angles of around (3-4) x 10 4 rad. Spatial coherence (i.e. finite beam divergence) does not then limit resolution when operating at magnifications sufficiently high that astigmatism can be corrected visually from the fluorescent screen [20]. The LaB 6 electron gun is thus a major factor in optimising the effectiveness of operating time with the 600 kV HREM.

A further possible limitation of microscope performance related to the electron source is the anomalous energy spread (Boersch effect [21]) which depends on the total emission current. This chromatic effect is obviously of less consequence for higher voltage operation. Nevertheless, it is still reassuring that our measurements using a retard- ing-field energy analyzer on the gun test-rig men- tioned above show that the change in energy spread with current for LaB 6 is very similar to that of tungsten, with a value of 2.4 eV (FWHM) at 45 ~A being typical [19]. Under normal operating conditions at 500 kV or more it would thus not be

D.J. Smith et al. / Optimisation and applications of Cambridge University 600 k V HREM 205

expected that the beam spread should contribute any serious degradation to resolution.

The high voltage generator and electron acceler- ation for the 600 kV HREM were supplied by Emil Haefely and Co. Basel [22]. The initial electrical stabilities in terms of both drift and ripple did not, however, meet highest resolution requirements, and considerable local attention to cable layout, as well as modifications to the feedback circuitry, were necessary before these were adequate [23]. Recurrent problems have also been experienced with poor contacts developing in the resistors composing the bleeder chain (this chain being used to provide voltage levels for the accelerator stages). Monitoring of thecur ren t down this chain provides a direct indication of any further resistor failure developing as well as giving a use- ful, though indirect, indication of the high voltage stability.

The suspension system for the entire microscope column and accelerator consists of three wall- mounted pneumatic cylinders with rolling rubber diaphragms incorporating self-levelling facilities. Measurements indicate resonant frequencies of less than 1 Hz with typical ambient vibrations being attenuated by about 100 times (see fig. 2). A further measure of its effectiveness is shown by the fact that micrographs at the present resolution limit of the microscope can be recorded at any

time of the day or night despite the microscope's location in central Cambridge. It is interesting, however, that, despite its low coupling to building vibrations, it is very sensitive to acoustic noise which thus needs to be minimised in the microscope's vicinity.

The remaining major factor which might poten- tially restrict the performance of the electron microscope is the stability of its specimen stage. A stage conforming to the stringent requirements of microscopy at the atomic resolution level has been supplied for the 600 kV HREM by Mr. J.H. Lucas (Rickling, Essex). It rests on the lower polepiece for mechanical and thermal stability, with a side- entry loading mechanism which is detached during microscopy. Two orthogonal tilts of ±30 ° are available with a goniometer holder without any loss of positional stability relative to that obtainable with a fixed non-tilting holder.

A good indication of the overall mechanical stability of an electron microscope, as well as the

10U¢

III

~ O I

tl

Fig. 2. Vibration trace (A) wall supported, (B) compressed air, (C) impulse to column. Fig. 3. Ni 0.88 ,~ lattice fringes.

206 D.J. Smith et al. / Optimisation and applications of Cambridge University 600 k V HREM

Fig. 4. Specimen airlock with search coil (A) and transducer (a).

coherence of its incident illumination, is provided by the standard "lattice resolution" test. Half- spacing lattice fringes of 0.88 ,A from a sample of nickel have been recorded with the HREM at 575 kV (see fig. 3) and similar half-spacing fringes are often recorded in other materials.

Fig. 5. SiC with incoherent ( 112.)-type interfaces (arrowed).

3. Modifications and improvements

Despite being designed to the highest stan- dards, the resolving power of the 600 kV HREM was initially well short of atomic levels - which was perhaps to be expected given its effectively "prototype" nature and the need to integrate its various components into a high performance instrument. A variety of unexpected factors had first to be located and then their effects eliminated, or at least minimised, before the resolution closely approached theoretical limits. Such factors in- cluded excessive turbulence in the water supply plumbing, magnetic material in the specimen stage assembly and the defective bleeder resistors re- ferred to earlier. It later became apparent that the performance was often variable from day to day, even during the same session and yet with operating conditions ostensibly identical. The cause of these variations was eventually established to be external AC magnetic fields originating from items of electrical equipment with faulty mains supply wiring, in buildings around the HREM. Locating

these faults was far from straightforward, although helped considerably by the use of a "stray field detector" (shown in fig. 4) in the form of a search coil, of effective area 5 m E, which is also portable. Steps have now been taken to provide improved magnetic shielding in regions of the column apparently susceptible to disturbance by the stray fields; it is our established practice to monitor continuously the levels of these fields. Our experi- ences have also demonstrated the value of continu- ous monitoring of microscope vibrations, by way of the transducer shown in fig. 4, as well as of the bleeder resistor current. Should any of these factors become excessive, in terms of reaching levels where their influence on image quality is known to be detrimental, then microscopy is normally halted until the particular fault condition is remedied.

There are a number of ways, easily imple- mented, whereby the ease of operation for atomic resolution microscopy at high voltage can be im-

D.J. Smith et al. / Optimisation and applications of Cambridge University 600 k V HREM 207

proved [20,24]: most involve improvements to the viewing conditions so that correction of image astigmatism and selection of objective lens focus can be carried out directly in real time at high magnification. Replacement of the central portion of the fluorescent screen with a thin foil of a heavy metal such as platinum leads to a doubling of image brightness without loss of resolution by increasing the amount of electron back-scattering in the screen material [25]. The phosphor has been carefully optimised with the emphasis on image resolution rather than brightness (we use 10 m g / c m 2 of P22). The viewing binoculars have also been properly optimised for efficient light collec- tion. These steps, coupled with the use of the high-brightness LaB 6 gun, have enhanced image observation with the 600 kV HREM to such an extent that lattice fringes to as fine as 2 ~, spacings can be regularly seen on the fluorescent screen at electron-optical magnifications of 800,000 times or more. Substantial improvements to operating efficiency obviously result.

An alternative approach to the problems of image observation is to employ some sort of image pick-up and display system. A system based on an Image Isocon camera tube (type P8041) has been developed for the 600 kV HREM [26]. It has been applied to the observation of beam-sensitive specimens [27], and the observation and recording of dynamic events [28], as well as allowing the possibility of on-line digital computer processing [29]. It also allows high resolution observations at light levels well below those useable for direct operator viewing. Such a system thus offers another option for greatly improved operating efficiency.

4 . I m a g e p r o c e s s i n g / s i m u l a t i o n

In high resolution studies, where the imaging process depends strongly on the contrast transfer characteristics of the objective lens, the final image is a very sensitive function of the objective lens defocus, as well as depending on specimen thickness. For small-unit-cell materials, of particular relevance to materials science, the situation is also complicated by the periodic recurrence with focus of so-called Fourier images [30]. Intuitive interpre-

tation of high resolution images becomes possible over very restricted ranges of thickness and focus, and normally only to resolutions corresponding to the first zero of the contrast transfer function at the optimum, or Scherzer, defocus. Even then, any detailed investigation of structural defects should be accompanied by complete image simulations, both to establish the precise imaging conditions and also to verify the image interpretation. Access to a computer of adequate size and speed, as well as resident staff expert in image processing and simulation, should thus be regarded as an integral part of any high resolution facility.

A number of essential functions should be per- formed by the image processing facilities. These include: - CTF measurement from micrograph diffractograms (for diagnosing instrumental limitations, as- sisting interpretation, and determining effective resolution); - CTF correction by image filtering (with particular reference to amorphous materials), individually or (preferably) in series; - detecting and quantifying order in images of amorphous materials (e.g., "phase randomisation" tests); - averaging noisy images from periodic structures; - multi-slice image simulation. With direct connection of image processing facilities to a high resolution instrument, via a suitable image pick-up system, the possibilities for beam and image manipulation also include automatic focussing and stigmating [29]. In principle, at least, such accurate control over the imaging conditions could have a marked influence on the cost-effectiveness of the actual microscopy.

Finally, it is appropriate to mention briefly here our recent study on the relative influences of beam and crystal misalignment on high resolution electron microscopy [31]. It has already been pointed out that considerable image degradation might be expected to occur as a result of beam tilts substantially less than those crystal tilts normally consid- ered as limiting for high resolution microscopy [32]. Moreover, diffractograms from micrographs of amorphous support films - commonly used in assessment of imaging conditions - cannot provide any a posteriori indications of either magni-

208 D.J. Smith et al. / Optimisation and applications of Cambridge University 600 kV HREM

tude or direction of any beam misalignment when this is small (~< 2 mrad) since its primary effect is to phase-shift the CTF. Misalignment thus ad- versely affects the phases of diffracted beams when observing crystalline materials, displacing detail laterally in the images, but the modulus of the CTFs - revealed in diffractograms from amorphous materials - remains unaltered. Several possibilities exist for correction of residual beam misalignment [31]; we have adopted the most straightforward of these and find it adequate for most purposes. It involves tilting the beam in each of four orthogonai directions in turn, and adjust- ing the mean position until the image texture at all four tilted positions appears identical. Without this, or some other equally precise method for beam alignment, then efforts to extract quantita- tive information from high resolution electron micrographs concerning atomic positions will be highly unreliable - certainly, good agreement between image simulation and electron micrographs, down to the finest details, will not be possible.

tailed study at high voltage and high resolution in those materials which thus becomes feasible opens up new and unexpected areas to microscopical investigation.

Specimens from each of the areas above have been examined with the 600 kV HREM. Some are described briefly below and shown in figs. 5-12. Other significant results obtained include: - direct observation, for the first time, of separate tetrahedral sites in the pyroxenoid silicates wol- lastonite and rhondonite [33]; - direct observation of regions of local order in an amorphous (as evidenced by its electron diffraction pattern) metallic glass, namely palladium- silicon [34]; - direct observation of dislocations in small ( - 10-15 nm) multiply-twinned particles of silver and gold [35]; - direct observation of atomic arrangements around both perfect and imperfect twin boundaries in copper [36];

5 . A p p l i c a t i o n s

The achievement of atomic resolution in the HVEM makes a wide range of materials amenable to direct study, especially close-packed metals and binary oxides. The detailed information about atomic arrangements in the major crystallographic orientations - where projected atomic separations are typically in the range 2-2.5 A - which then becomes available should, in turn, provide funda- mental insights into macroscopic behaviour. Areas where significant impact might be expected include: - ceramics; - catalysts, particularly small metal particles; - semi-conductors (defects and interfaces); - corrosion and oxidation/epitaxial growth; - oxide and metallic glasses; - i n t e r f a c e s / b o u n d a r i e s in metals, alloys and oxides.

Furthermore, many beam-sensitive specimens which prove difficult, if not impossible, to image at 100 kV become markedly more stable under electron irradiation at elevated voltage. The de- Fig. 6. Model Pt/7-AI203 catalyst. Bi-twin arrowed.

D.J. Smith et al. / Optimisation and applications of Cambridge University 600 kV HREM 209

- direct characterisation of elemental metal and oxide particles in intercalated graphite catalysts [37]; - direct observation at the molecular level of the aromatic hydrocarbon quaterrylene [38].

Ceramics: Elucidation of the nature of the de- fect content of silicon carbide and other important industrial ceramics such as the Si -A1-O-Ns, in- cluding the nature of grain boundary phases, is essential for understanding their behavour during fabrication and deformation. With interlayer spacings of around 2.5 .~, structure images are, however, difficult to obtain. Fig. 5 shows incoherent (112}-type interfaces between the cubic r-phase (left) and a-phase polytypes in silicon carbide. Image simulations are necessary for a complete picture of the stacking rearrangements at these boundaries [39].

Catalysts (small metal particles): Many industrial catalysts consist of small metal particles supported on some refractory ceramic. Char- acterisation of particulate structure using direct lattice imaging in the HREM can reveal features which may be of catalytic importance [37,40]; see fig. 6, which shows a model catalyst consisting of Pt particles supported on amorphous y-A1203 . Bi- twinning is established in as much as 30% of the Pt particles (example arrowed).

Semiconductors: The performance of semicon- ductor devices obviously depends on the nature of any defects present. High resolution lattice and structure imaging enables these to be investigated at the atomic level [41], thereby providing insight into macroscopic electrical properties. Fig. 7 shows a number of dislocation loops and an extrinsic stacking fault in the material CdTe - note that fault motion under the influence of the electron beam has been recorded on video tape using the image pick-up system [28]. Fig. 8 shows a cross-

Fig. 7. CdTe showing dislocation loops and extrinsic stacking fault. Fig. 8. Cross-section of silicon-on-sapphire interface.

210 D.J. Smith et al. / Optimisation and applications of Cambridge University 600 kV HREM

is tOO small to be studied by conventional diffraction techniques.

Interfaces/boundaries: As well as being relevant in studies of ceramics and semiconductors, interfaces are of profound importance in determining the macroscopic behaviour of many other materials of interest to geologists, solid state chemists and materials scientists. Grain boundaries prove difficult to analyze from electron micrographs because they are generally non-planar [44], but even twin boundaries are far from straightforward, because of the possibility of relaxation, and hence atomic displacement, or of segregated impurities, as well as the problems of ensuring sufficiently accurate crystal and beam alignments for imaging thicker crystal regions. There is much to be said for using other "indirect" imaging methods to provide further complementary information [45,46]. Planar defects, generally crystallographic shear planes (CSPs), can also accommodate non- stoichiometry. In thiS regard, it is interesting that

Fig. 9. Gold oxide islands on Au-Ag alloy surface.

sectional view of an epitaxially grown silicon-sapphire interface together with regions of apparently ordered intersecting defects which are believed to take up the strain involved in the lattice mismatch at the interface [42].

Corrosion and oxidation/epitaxial growth: The study of epitaxial growth as well as surface corrosion and oxidation can be greatly facilitated by observations on the atomic scale. An illustrative example is provided by our recent study of oxide formation during the corrosion of silver-gold alloys in nitric acid [43]. High resolution images, as shown in fig. 9, directly confirmed the presence of domains consisting of an oxide phase epitaxed with the alloy substrate. These results indicated that direct lattice imaging, combined with optical diffraction, should be invaluable, particularly for other systems where the amount of surface phase Fig. 10. CSP in TiOi.9985 with disorder apparent.


shows an image of a sodium-f l ' " -a lumina crystal observed in a sample from a solid electrolyte bat- tery [49]. Even at 200 kV, this material, and related f l" and fl .... polytypes, appears to be sensitive to electron irradiation which produces collapse of the so-called Na-conduction planes. Beam stability is crucial here since it is the actual presence (and structure) of such partially collapsed planes in the original material which is of central interest.

It is obviously impracticable to attempt image simulations in all instances to establish figures for "instrumental" and structural resolutions, although it is readily apparent from most of the images above that the 600 kV H R E M is regularly producing micrographs with information on the 2 A scale or better. Recent diffractograms from images of amorphous materials recorded at 500 kV show detail to just on 2 ,~ with axial illumination and to 1.6 ,~ with tilted illumination (the dif-

Fig. 11. Stacking fault in (1120) projection of dolomite.

our recent observations of CSPs in a slowly cooled sample of reduced rutile, TiOi.9985 , have established the presence of extensive lateral and longi- tudinal disorder among many of the CSPs - an example is shown in fig. 10 - with the obvious conclusion that a better understanding of reduc- t ion /deformat ion mechanisms might emerge from further detailed high resolution studies [47].

Beam-sens i t i ve specimens: An unexpected bonus of our high resolution HVEM studies came With the realisation that some beam-sensitive materials, such as carbonates and sulphides, which give - at best - poor quality lattice fringes at 100 kV, prove at least an order of magnitude more stable at 500 kV. Indeed, their radiation stability is such that structure images, closely approaching the 2 .~ level in resolution, are obtainable. Fig. 11 shows an image of dolomite, CaMg(CO3), which is a sedi- mentary carbonate. The detailed atomic arrangement along the stacking fault and its termination can be deduced directly from the image [48]. Fig. 12 Fig. 12. Na-fl"'-alumina with collapsed conduction plane.

212 D.J. Smith et a L / Optimisation and applications of Cambridge University 600 k V HREM

ference suggesting that vertical focal spread is the factor limiting performance). Moreover, image simulations to match recent images of the shape- memory alloy CuZnAI confirm that diffracted beams at 1.96 and 2.04 ,~, make strong contribu- tions to the image [44], even where the crystal is thin, although an image recorded at the extended Scherzer position (cross-over at about 2.0 A) does not correctly repesent a projection of the structure, and it is necessary to image at the "second broad band" to get an image (with reversed contrast) which shows directly the atomic arrangement in perfect crystal regions at least.

6. Future prospects

A wide diversity of materials have now been imaged with the 600 kV HREM, and although many of the existing micrographs remain to be evaluated it is possible to conclude that this microscope, and others with comparable performance, will have a significant impact on our understanding of materials at the atomic level. How- ever, it is also worth pointing out that the electron-optical design of nearly all currently operating high voltage HREMs is far from optimised [20], and substantial improvements are possible. Moreover, the use of on-line, interactive, image processing systems to provide auto-stigmating and focussing [29] should enhance still further the quantity and quality of micrographs recorded at atomic resolution levels.

Acknowledgements

The Cambridge University 600 kV High Reso- lution Electron Microscope was built as a joint project between the Cavendish Laboratory and the Department of Engineering, with major financial support from the SRC. We greatly appreciate the many valuable collaborations with friends and col- leagues which have contributed to the success of the HREM project. Continuing support from the SERC is also gratefully acknowledged.

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Optimisation and applications of the Cambridge University 600 kV high resolution electron microscope