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High resolution energy-loss spectroscopy
32
Ultramicroscopy28 (1989) 32-39 North-Holland, Amsterdam
HIGH RESOLUTION ENERGY-LOSS SPECTROSCOPY P.E. BATSON IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598, USA
Received at Editorial Office 3 November 1988; presented at Workshop March 1988
In principle, Electron Energy-LossSpectroscopyin the Scanning Transmission Electron Microscopecan obtain information related to the electronic structure of single defect structures in semiconductors. The instrumental requirements necessary to accomplish this are discussed. Examples include: Si and GaAs interband excitations, graphite EXELFS analysis, and surface and defect induced structure at the SiL2.3 edge.
1. Introduction During the past several years, I have attempted to obtain a high energy resolution capability appropriate for addressing the many questions that routinely arise in the semiconductor field. For instance, " H o w does a single dislocation affect the electrical properties of a nearby metal-semiconductor interface?" In principle, the very local change in the near-neighbor environment of atoms near the defect will produce defect electronic states which may be quite different from the bulk crystal states. These states should have a physical size of the order of 5 to 20.4,, or a few times bigger than the actual structural distortion. Since this size is close to the probe size which is readily obtainable in the m o d e m field-emission-equipped Scanning Transmission Electron Microscope (STEM), it seemed possible that scattering conditions might be achieved that would allow the defect scattering to dominate the results. Thus, we would obtain a tool to probe directly the electronic structure of individual defects. In electron microscopy, Electron Energy-Loss Spectroscopy (EELS) has been treated as an accessory to the primary task of obtaining structural images. The idea was to obtain chemical and elemental information to complete the structural information. For the problem of determining electrical properties, we need to make the energy loss
analysis be the primary task. Then, a correlation of the obtained electrical properties with the physical structure can be attempted to increase our understanding of the origin of the electronic properties of materials. In principle, electronic structure information can be obtained in various ways. We may obtain the Electron Loss Near-Edge Fine Structure [1] (ELNES) at a core edge. This allows a look at the conduction band states in the materials. Or we may examine the direct interband scattering in the low energy loss region to gain information about both the valence and conduction band states [2]. The instrumental requirements for this work are discussed and some recent examples of experimental results show what is currently feasible. The main problem in accomplishing this task is to obtain an adequate energy resolution. The energy scale of interest here is 0 - 5 eV rather than the 10-1000 eV range useful in obtaining elemental compositions. In order to define spectral shapes within this energy range, the energy resolution of the system should be 0.1 eV or better. This resolution should be obtainable with relatively large angle collection conditions. The reason becomes immediately apparent when we consider the U n c e r t a i n t y Principle requirements for the momentum spread of a small defect. The description in reciprocal space of a 5 ~, size defect must include spatial frequencies out to 2~r/5 = 1.2 ,~-1
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.E. Batson / High resolution energy-loss spectroscopy
or a scattering angle of about 8 mR for 100 keY electrons. In practice, the STEM instrument uses a probe convergence that is about twice as large as this, further increasing the collection needs. Thus, in order to collect a reasonable fraction of the scattered signal, the spectrometer system must be able to deliver 0.1 eV resolution or better with a collection half-angle of 10-15 mR. As the energy window is made smaller, the measured intensity is of course made smaller. Typical spectrometer resolutions of 2 eV or higher used for elemental analyses thus deliver 10 to 50 times more intensity per spectral point than does the 0.1 eV resolution experiment. The instrument at IBM has been designed to try to obtain an energy resolution which is adequate to address the defect electronic structure questions. It utilizes the VG Microscopes HB501 as a probe-forming optical bench, and expands from there to implement a high quality EELS system, with the intention of creating mainly a spectroscopic tool that has the capability of spatial resolution.
2. The instnnnent Various aspects of the instrument have been described in detail elsewhere. They include the electron optics [3], parallel detection [4] and computer control [5]. I describe briefly here the characteristics which make the system useful for obtaining electronic information. As pictured in fig. 1, the system includes the VG HB501 STEM, a Wien filter electron spectrometer situated within a high voltage electrode to allow spectroscopic analysis at low electron velocities, and a detector chamber which includes a Princeton Applied Research diode array for parallel detection while retaining the standard VG slit mechanisms for single electron counting. All components are interconnected and controlled by a Series/1 minicomputer. There is also a link to the mainframe environment for spectral processing. The most important characteristic of this system is its use of the high voltage environment for housing the spectrometer. To allow an energy
33
Fig. 1. Overall view of the high energy resolution EELS-STEM system. The Wien filter is mounted within the horizontal chamber above the microscope. Single slit counting and diode array parallel detection are located above the Wien filter chamber.
resolution of order 0.1 eV, the spectrometer stability must be several times better than that - about 10-20 meV. Electron spectroscopy at the primary beam energy of 100-200 keV would thus require power supply stabilities in both the spectrometer and the microscope of 10 -7 . While these ate possible, they are not readily available, and in any case would probably constitute the limiting factor in the routine use of the spectroscopy. Housing the spectrometer within a high voltage environment can correct this problem. In the present instrument electrons ate decelerated to 50-100 eV for analysis by an electrode which is held near the microscope high voltage. This eliminates the high voltage instabilities, and reduces the stability requirements for the spectrometer to about 10 -4 . It cannot be emphasized too strongly that this gain more than offsets the inconvenience of the physical design required for the high voltage environment. More to the point, it appears that at present this is the only way to achieve a resolution that will allow us to address questions concerning electronic structure.
34
P,E. Batson / High resolution energy-lossspectroscopy
The particular choice of spectrometer is separate from the decision to employ deceleration. The Wien filter was used in this instrument for several reasons. First, it has a straight optic axis. This allows a more precise mechanical alignment during construction. Also, it allows passage of the electron beam at partial excitation for alignment during operation. Thus it is possible to separate alignment of pre- a~d post-spectrometer optics from alignment of the spectrometer itself. The Wien filter also provides first-order focussing without precise machining of magnetic pole pieces. Finally, it operates within a fairly wide range of excitation to allow compensation for variation in optical properties of other parts of the system. Therefore, details of the deceleration and. acceleration fields did not have/to ~ e x p l o r e d to obtain a working device. It was eiiough to estimate the first order focussing properties using tabulated values of focal lengths and principal planes for generalized situations. This system was designed to operate at a 200 eV analysis energy. It works better at 100 eV, and that is where most routine use is now concentrated. Two quadrupole lenses, located between the spectrometer and the specimen, transfer the electron source at the specimen to a point within about 1 cm of the deceleration electrode. The quadrupole design was chosen over a simple cylindrical lens due to the need to match the Wien filter optics, which are different in the two perpendicular planes which define the filter optic axis. The double quadrupole design is the minimum required to allow some independent control of the focal length in the two perpendicular planes. In retrospect, a cylindrical lens paired with a weak quadrupole corrector may be a better choice, but this was not explored for the present design. Diode array parallel detection has been added fairly recently to enhance the usability of the system. As will be come apparent below, it was not uncommon to spend as long as an hour integrating intensity to obtain one 100-point spectrum at a resolution of 0.3 eV with a single slit. Constraints of specimen contamination and damage do not allow this a m o u n t of time. Parallel detection reduces this time to 20-50 s and thus allows us to obtain many spectra within a reasona-
ble time to facilitate systematic studies. The present setup uses the diode array in a fairly complicated manner to make the operation of the array transparent to the operator. Programming automatically corrects for variations in background, channel-to-channel gain, and diode binwidth to give a result which may be directly compared with a serial single slit result obtained from the same area. This allows a high degree of confidence in the result. At the present time, when the array is still fairly new, this is a welcome operating characteristic. Computer control has been incorporated wherever possible to reduce the use of discrete electronics, and to simplify operation. The system in use here is fairly old. However, since it was based on a multi-tasking operating system for maximum flexibility, it was possible to incorporate new instrumental capabilities without changing the operation of prior programming. Thus addition of the parallel array was possible without sacrificing the serial operation. This is a principal that will become more and more important as small computer operating systems become more sophisticated and standardized. If prior work is to be built upon, rather than swept away and lost, extreme care must be exercised in the initial design and implementation of experimental control systems. In the end, this work has achieved a practical spectrometer resolution of 150 meV with parallel collection at a spectrometer collection half-angle of 12.5 mR. The accuracy and stability of the system is + 25 meV over an energy loss range of 0-1000 eV.
3. The electron source With a spectrometer resolution limit of 0.1-0.2 eV, we find that the spectral resolution is limited to 0.25-0.35 eV by the intrinsic width of the field emission source. This may be decreased by reducing the work function of the field emitter. A Zr-treated emitter is currently available on an experimental basis from FEI Company. This consists of a tungsten field emitter having a ring of Zr metal around the tip shank. During operation, the
35
P.E. Batson / High resolution energy-loss spectroscopy
.~' ff~k'W . . . .
-71 \ix :IL^Z
' ?.H ~ ; / 5 e V " Vacuur • \, , , -1 0 2 3 Energy Loss (eV)
Fig. 2. A comparison of the standard W field emission, distribution with a low-work-function Zr-treated W field emitter.
The distributions are described with the simple tunneling theory. Direct interband scattering results for Si and Ga.As are shown for comparison. It is hoped that similar low-work-function emitters may be useful for high resolution work in the absence of beam monochromatizationequipment.
Zr migrates to the tip region, reacts with oxygen and lowers the W surface work function from 4.5 eV to 2.5 eV. As explained in detail elsewhere [6], a current density which is comparable to the standard W tip can be achieved by lowering the extraction field. This gives a narrower energy distribution. An example of a typical comparison is shown in fig. 2. Two results are achieved. First, the spectral resolution can be pushed to 0.2 eV from 0.35 eV. Secondly, as can be noticed in the figure, the background intensity from the tunneling distribution can be dramatically reduced in the 0.5-1.0 eV energy loss region. Fig. 2 also includes results for the direct interband scattering intensities in Si and G a A s to make the point that the energy loss scattering in this energy range is very weak. It is so weak that there is a severe background problem from the exponential tail of the field emission tunneling distribution. Simple calculations confirm this finding [7]. Thus, the background reduction is the most important result obtained on going to the Zr-treated source. If the tip radius is properly controlled, this Zr-treated tip can be incorporated in the standard STEM gun without modifications to the extraction voltage supplies. A good spatial resolution has also recently been demonstrated. Two questions remain. First, lowering the work function leads to increased susceptibility to contaminants on the tip surface. It is therefore not yet clear
whether the current stability will be adequate. Secondly, some thought needs to be given to the tip extraction geometry because field emission can occur more readily at undesired locations. In the end, it m a y be necessary to provide a shield of the tip shank to prevent field emission from the built up Zr ring.
4. W i d e energy scans Fig. 3 shows a inelastic scattering results over a wide energy range for graphite. These results are a composite of a set of experiments which were designed to obtain the Extended Energy-Loss Fine Structure (EXELFS) which is present above the carbon K edge. The data were obtained with a S T E M objective aperture subtending a half-angle of 16 m R at the specimen. The spectrometer collection aperture was chosen to match this. The parallel detector was set up to give a 0.6 eV wide spectral slit. The spectral resolution was dominated by this slit size. Between 275 and 525 eV, each spectral point was integrated for about 0.65 s in each of a total of twenty 20 s experiments. The sum therefore reflects the results of 13 s per point, obtained in 400 s of recording time. This produced about 6 × 10 6 detector counts at the graphite ls---, o* peak, corresponding to about 750,000 detected
~> Graphite
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Fig. 3. Energy loss spectrum for graphite suitable for EXELFS analysis. The core loss itself is obtained with very high accuracy. The low loss region is accurate enough to perform a multiple scattering analysis. The thickness of the sample in this case was about 0.5 t/?t.
36
P.E. Batson / High resolution energy-loss spectroscopy
electrons, or about 1.2 x 105 electrons s -1 eV -1. This agrees favorably with the expected rate of 105 expected using a beam current of 2 nA into a 1 nm diameter probe incident onto a 50 nm thick piece of graphite having a core loss differential scattering cross section [8] of 2 x 10 -22 cm 2 eV -~ atom -~. These results can be directly compared with those obtained almost 10 years ago [10] with serial recording from a straight-edge magnetic sector spectrometer. There, only about 300 counts s -1 were obtained in 20 min recording times. Clearly we are now able to realize the promise that was apparent but unrealized at that time. Between 200 and 1000 eV, a short acquisition using a 2 eV window was performed to establish the overall behavior of the data. In the low loss region, somewhat better data has been obtained to allow a multiple scattering analysis of the core loss. An overlap of data between 200 and 250 eV allows normalization of the two sets of data. Examination of a serial scan in the low energy loss region indicated a need to correct the parallel output gain in the presence of the very high incident beam flux. Therefore, the very low energy loss region ( - 1 to 5 eV) has been obtained with a single slit serial scan. Finally, a scan without the specimen for the background intensity was obtained from 200 to 1000 eV for the purpose of subtraction. This spectrometer design suffers from a high background intensity due to scattering of the main beam within the spectrometer when scanning tO high energy loss. It becomes a problem above about 500 eV energy loss and should be estimated and subtracted. This background contribution is smoothly varying and increases from about 10% of the measured signal at 300 eV to 90% of the signal at 1000 eV. The data in fig. 3 have had this contribution removed by comparison with the background measured with the specimen absent. The acquisition times that are necessary with this system may seem long relative to those being achieved with the magnetic sectors in systems using thermionic sources. For a spot size of 500 nm and an energy resolution of 1 eV, we may use a thermionic electron source to give us a current of about 500 nA, With no requirement for high energy resolution, the parallel detector can be run
at a much higher efficiency. The above graphite example, for instance, gives a gain over single channel counting of only × 13 because the total spectrum width is only 9 eV at the detector, and we need to cover a 250 eV wide scan. At 1 eV resolution, a parallel array can be configured to cover this entire range to get an enhancement over a 0.6 eV wide single slit of about x 400. Thus a low dispersion system will give an advantage of about 8 x 103 over the high resolution system for the above experiment, giving a results which has a statistical accuracy similar to fig. 3 in 10 ms. Thus, it seems that the conventional, low resolution, systems may give better results for the wide energy scans required for EXELFS analyses. On the other hand, it could be remarked that the present setup gives remarkably good results for a system which has been optimized to obtain sub-1 eV resolution from 0.5 nm sized areas. If both the high energy and spatial resolution are relaxed, much larger signals are possible. The VG HB501 system with the magnetic gun lens is capable of supplying 100 nA into a 10 nm spot size. With a redesigned Wien filter - perhaps twice as large, operating at a pass energy of 5 kV, with an energy resolution of 1 eV - it should be possible to obtain much higher collection efficiency and much lower background than are currently present in the IBM system. This operation should not require more complicated lenses because the energy dispersion will naturally decrease as the operating voltage is increased. If the design were done carefully, both high and low energy resolution should be possible in the same system.
5. EXELFS analysis This description illustrates what must routinely be recorded to allow a reasonably confident interpretation of the energy loss data. With it, it has been possible to take the above data, remove multiple inelastic scattering from the core loss, strip the slowly varying background, and perform a creditable EXELFS analysis. This analysis is summarized in fig. 4. There, I show (1) the data multiplied by AE 28 to approximate the result of a photon absorption experiment, (2) the kx(k) EX-
P.E. Batson / High resolution energy-lossspectroscopy C K Edge
(a)
6. Acquisition of precise shapes
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Fig. 4. Results of the EXELFS analysis for the data in fig. 3. This experiment was performed with the incident and collected beams closely aligned with the [10001 zone axis. Therefore some diffraction effects m a y be involved.
ELFS function and transform window, and (3) the radial distribution function. The 1.4, 2.8, and 3.4 .A neighbor distances are present, but the 2.4 .A distance is not strong. Differences between this result and others previously published [9-11] do not appear to be a result of inaccuracies in the analysis, but may be due to diffraction effects. This experiment was performed with both the incident and collected beams aligned with the [1000] zone axis of the graphite to explore the sensitivity of the EXELFS to diffraction a n d / o r angular integration conditions. Changes on the EXELFS structure do appear to occur for different diffraction conditions. These changes are being examined to help understand details of the physics of the inelastic scattering. In particular, it is hoped that the experiments may help us to understand whether the many plane waves making up the convergent beam in the STEM can interact coherently in the process of an inelastic scattering event. (See for instance the recent literature [12-15].)
Fig. 5 shows a low energy loss result for MgO, plotted together with a detailed look at the MgL~, 3 edge. The two scans were acquired in similar times with similar collection conditions using the parallel collection system. The result illustrates again that it is u s e f u l t o first characterize the scattering over a fairly wide energy range. Then a particular feature of interest can be obtained with a very high accuracy. It is probably important in passing to emphasize that the energy scales here are accurate to + 25 meV. This is automatic and depends only on the accuracy of the spectrometer power supplies, which need calibration about twice a year. The results in fig. 5 illustrate that a statistical accuracy of better than 0.1% is obtainable in the 0-200 eV range from a sample that is of order 50 n m thick. However, the intention in this work - ultimately - is to make a correlation with high resolution structural microscopy. Therefore, high resolution energy loss must be compatible with the high resolution samples. These are seldom as thick as 50 nm, and so they present several problems. First, the energy loss signal will be correspondingly smaller as the number of atoms within the probe are reduced. Second, radiation damage becomes a consideration, even with the semi-conductors, as the thermal conductance is reduced. Third, as the thickness, or lateral dimensions of the speci-
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30 40 50 60 70 80 Energy Loss (eV) Fig. 5. Low energy loss results for M gO with a detailed look at the onset of the M g L2. 3 edge. T h e inset is not magnifications of regions of the wider range scan, b u r a separate scan using a 50 meV step size. The spectral resolutioh was about 0.3 eV using a collection half-angle of 8 m R .
38
P.E. Batson / High resolution energy-loss spectroscopy
men become comparable to the unit cell size of the material, the bandstructure within the region is expected to be modified. In fig. 6, I show results for the Si L2,3 core edge taken from various positions close to the edge of a Si sample. A background due to valence electronic scattering has been subtracted to show the absorption edge. At positions greater than 10 nm from the edge, the results are indistinguishable from results obtained in a continuous specimen. Approaching to within 2 nm, some small changes in shape appear. Within 1 nm, gross changes are present, including intensity in front of the edge which may be a result of scattering involving surface states. Structure at the Si edge is loosely relatable to the conduction band density of states in the Si [16]. The disappearance of this structure is therefore probably an indication of the necessary modification of the conduction band as the edge of the sample is approached. The appearance of additional structure is the welcome signature of localized electronic states at the surface. In fig. 7, I show two results from rather thicker Si. They compare the result from the unmodified Si bulk with the result from a dislocation bounding the edge of a stacking fault. This sample has been the subject of other investigations [17] designed to investigate the detailed structure of these bounding dislocations. In particular, the sample has been subjected to a thermal history designed to create non-equilibrium stacking faults. It is also known to be cathodoluminescent as a result of this
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97 98 99 100 101 102 103 104 Energy Loss (eV)
Fig. 7. Comparison of the Si L2, 3 edge from the bulk with that obtained in an area containing a dislocation which b o u n d s a stacking fault. Extra scattering intensity below the edge probably reflects the presence of e m p t y electronic states in the Si gap. Also extra intensity appears near 102.3 eV. This scattering m a y also be caused by states in the gap near the BfiUouin zone center. However, more information is needed for identification.
treatment. The energy loss spectra show clear differences between the bulk and the dislocation. There are two differences: additional intensity appears below the absorption edge at the dislocation, possibly signifying new final states within the gap, and additional intensity appears at 102.3 eV. Further work will explore the evolution of these features as defects are approached. It may thus be possible to locate particular structural features of an extended defect which give rise to well defined modifications of the energy loss spectra.
6. Conclusions i
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Fig. 6. Results for the Si L2. 3 edge at three different locations near the edge of a Si sample. At distances greater than 10 rim, the results are identical to those obtained from the bulk. A s the probe approaches the surface, features which reflect the Si bandstructure begin to be washed out.
It is now possible to obtain experimental results which are of good enough quality to begin answering some detailed questions involving the probe-specimen interaction. Does our description of the energy loss process need to be modified in the presence of coherent sources? How do constricted volumes modify the energy loss results? What are the impact parameter issues when scattering via excitation of a localized defect state? As these issues become better understood, we will be in a good position to begin the task of the detailed comparison of electronic structure with atomic positions so that a fundamental theoretical understanding may be achieved.
P.E. Batson / High resolution energy-loss spectroscopy Acknowledgements I w a n t to acknowledge m a n y c o n v e r s a t i o n s with J. Silcox o n spectrometer design. I wish to t h a n k also H. A l e x a n d e r a n d J.C.H. Spence for the o p p o r t u n i t y to e x a m i n e the Si sample involved i n their work. I a m also i n d e b t e d to the referee for p o i n t i n g out the gains which a r e possible with the n e w e r field emission g u n systems.
References [1] C.H. Spence, Ultramieroscopy 18 (1985) 165. [2] P.E. Batson, K.L. Kavanagh, J.M. Woodall and J.W. Mayer, Phys. Rev. Letters 57 (1986) 43. [3] P.E. Batson, Rev. So. Instr. 57 (1986) 43. [4] P.E. Batson, Rev. Sci. Instr. 59 (1988) 1132. [5] P.E. Batson and G. Trafas, Ultramieroscopy 8 (1982) 293.
39
[6] P.E. Batson, in: Proc. 45th Annual EMSA Meeting Baltimore, MD, 1987, Ed. G.W. Bailey (San Francisco Press, San Francisco, 1987) p. 132. [7] P.E. Batson, in: Scanning Microscopy Suppl. 1 (Scanning Microscopy Intern., Chicago, 1987) p. 189. [8] 1LD. Leapman, P. Rez and D.F. Mayers, J. Chem. Phys. 72 (1980) 1232. [9] B.M. Kincaid, A.E. Meixner and P.M. Platzman, Phys. Rev. Letters 40 (1978) 1296. [10] P.E. Batson and A.J. Craven, Phys. Rev. Letters 42 (1979) 893. [11] M.M. Disko, O.L Krivanek and P. Rez, Phys. Rev. B25 (1982) 4252. [12] V.W. Maslen and C.J. Rossouw, Phil. Mag. A47 (1983) 119. [13] D.K. Saldin and P. Rez, Phil Mag. B55 (1987) 481. [14] V.W. Maslen, Phil. Mag. B55 (1983) 491. [151 D.K. Saldin, Phil. Mag. B56 (1987) 515. [16] P.E. Batson, Ultramicroscopy 22 (1987) 89. [17] H. Alexander, J.C.H. Spence, D. Shindo and N. Long, Phil. Mag. A53 (1986) 627.
Ultramicroscopy28 (1989) 32-39 North-Holland, Amsterdam
HIGH RESOLUTION ENERGY-LOSS SPECTROSCOPY P.E. BATSON IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598, USA
Received at Editorial Office 3 November 1988; presented at Workshop March 1988
In principle, Electron Energy-LossSpectroscopyin the Scanning Transmission Electron Microscopecan obtain information related to the electronic structure of single defect structures in semiconductors. The instrumental requirements necessary to accomplish this are discussed. Examples include: Si and GaAs interband excitations, graphite EXELFS analysis, and surface and defect induced structure at the SiL2.3 edge.
1. Introduction During the past several years, I have attempted to obtain a high energy resolution capability appropriate for addressing the many questions that routinely arise in the semiconductor field. For instance, " H o w does a single dislocation affect the electrical properties of a nearby metal-semiconductor interface?" In principle, the very local change in the near-neighbor environment of atoms near the defect will produce defect electronic states which may be quite different from the bulk crystal states. These states should have a physical size of the order of 5 to 20.4,, or a few times bigger than the actual structural distortion. Since this size is close to the probe size which is readily obtainable in the m o d e m field-emission-equipped Scanning Transmission Electron Microscope (STEM), it seemed possible that scattering conditions might be achieved that would allow the defect scattering to dominate the results. Thus, we would obtain a tool to probe directly the electronic structure of individual defects. In electron microscopy, Electron Energy-Loss Spectroscopy (EELS) has been treated as an accessory to the primary task of obtaining structural images. The idea was to obtain chemical and elemental information to complete the structural information. For the problem of determining electrical properties, we need to make the energy loss
analysis be the primary task. Then, a correlation of the obtained electrical properties with the physical structure can be attempted to increase our understanding of the origin of the electronic properties of materials. In principle, electronic structure information can be obtained in various ways. We may obtain the Electron Loss Near-Edge Fine Structure [1] (ELNES) at a core edge. This allows a look at the conduction band states in the materials. Or we may examine the direct interband scattering in the low energy loss region to gain information about both the valence and conduction band states [2]. The instrumental requirements for this work are discussed and some recent examples of experimental results show what is currently feasible. The main problem in accomplishing this task is to obtain an adequate energy resolution. The energy scale of interest here is 0 - 5 eV rather than the 10-1000 eV range useful in obtaining elemental compositions. In order to define spectral shapes within this energy range, the energy resolution of the system should be 0.1 eV or better. This resolution should be obtainable with relatively large angle collection conditions. The reason becomes immediately apparent when we consider the U n c e r t a i n t y Principle requirements for the momentum spread of a small defect. The description in reciprocal space of a 5 ~, size defect must include spatial frequencies out to 2~r/5 = 1.2 ,~-1
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.E. Batson / High resolution energy-loss spectroscopy
or a scattering angle of about 8 mR for 100 keY electrons. In practice, the STEM instrument uses a probe convergence that is about twice as large as this, further increasing the collection needs. Thus, in order to collect a reasonable fraction of the scattered signal, the spectrometer system must be able to deliver 0.1 eV resolution or better with a collection half-angle of 10-15 mR. As the energy window is made smaller, the measured intensity is of course made smaller. Typical spectrometer resolutions of 2 eV or higher used for elemental analyses thus deliver 10 to 50 times more intensity per spectral point than does the 0.1 eV resolution experiment. The instrument at IBM has been designed to try to obtain an energy resolution which is adequate to address the defect electronic structure questions. It utilizes the VG Microscopes HB501 as a probe-forming optical bench, and expands from there to implement a high quality EELS system, with the intention of creating mainly a spectroscopic tool that has the capability of spatial resolution.
2. The instnnnent Various aspects of the instrument have been described in detail elsewhere. They include the electron optics [3], parallel detection [4] and computer control [5]. I describe briefly here the characteristics which make the system useful for obtaining electronic information. As pictured in fig. 1, the system includes the VG HB501 STEM, a Wien filter electron spectrometer situated within a high voltage electrode to allow spectroscopic analysis at low electron velocities, and a detector chamber which includes a Princeton Applied Research diode array for parallel detection while retaining the standard VG slit mechanisms for single electron counting. All components are interconnected and controlled by a Series/1 minicomputer. There is also a link to the mainframe environment for spectral processing. The most important characteristic of this system is its use of the high voltage environment for housing the spectrometer. To allow an energy
33
Fig. 1. Overall view of the high energy resolution EELS-STEM system. The Wien filter is mounted within the horizontal chamber above the microscope. Single slit counting and diode array parallel detection are located above the Wien filter chamber.
resolution of order 0.1 eV, the spectrometer stability must be several times better than that - about 10-20 meV. Electron spectroscopy at the primary beam energy of 100-200 keV would thus require power supply stabilities in both the spectrometer and the microscope of 10 -7 . While these ate possible, they are not readily available, and in any case would probably constitute the limiting factor in the routine use of the spectroscopy. Housing the spectrometer within a high voltage environment can correct this problem. In the present instrument electrons ate decelerated to 50-100 eV for analysis by an electrode which is held near the microscope high voltage. This eliminates the high voltage instabilities, and reduces the stability requirements for the spectrometer to about 10 -4 . It cannot be emphasized too strongly that this gain more than offsets the inconvenience of the physical design required for the high voltage environment. More to the point, it appears that at present this is the only way to achieve a resolution that will allow us to address questions concerning electronic structure.
34
P,E. Batson / High resolution energy-lossspectroscopy
The particular choice of spectrometer is separate from the decision to employ deceleration. The Wien filter was used in this instrument for several reasons. First, it has a straight optic axis. This allows a more precise mechanical alignment during construction. Also, it allows passage of the electron beam at partial excitation for alignment during operation. Thus it is possible to separate alignment of pre- a~d post-spectrometer optics from alignment of the spectrometer itself. The Wien filter also provides first-order focussing without precise machining of magnetic pole pieces. Finally, it operates within a fairly wide range of excitation to allow compensation for variation in optical properties of other parts of the system. Therefore, details of the deceleration and. acceleration fields did not have/to ~ e x p l o r e d to obtain a working device. It was eiiough to estimate the first order focussing properties using tabulated values of focal lengths and principal planes for generalized situations. This system was designed to operate at a 200 eV analysis energy. It works better at 100 eV, and that is where most routine use is now concentrated. Two quadrupole lenses, located between the spectrometer and the specimen, transfer the electron source at the specimen to a point within about 1 cm of the deceleration electrode. The quadrupole design was chosen over a simple cylindrical lens due to the need to match the Wien filter optics, which are different in the two perpendicular planes which define the filter optic axis. The double quadrupole design is the minimum required to allow some independent control of the focal length in the two perpendicular planes. In retrospect, a cylindrical lens paired with a weak quadrupole corrector may be a better choice, but this was not explored for the present design. Diode array parallel detection has been added fairly recently to enhance the usability of the system. As will be come apparent below, it was not uncommon to spend as long as an hour integrating intensity to obtain one 100-point spectrum at a resolution of 0.3 eV with a single slit. Constraints of specimen contamination and damage do not allow this a m o u n t of time. Parallel detection reduces this time to 20-50 s and thus allows us to obtain many spectra within a reasona-
ble time to facilitate systematic studies. The present setup uses the diode array in a fairly complicated manner to make the operation of the array transparent to the operator. Programming automatically corrects for variations in background, channel-to-channel gain, and diode binwidth to give a result which may be directly compared with a serial single slit result obtained from the same area. This allows a high degree of confidence in the result. At the present time, when the array is still fairly new, this is a welcome operating characteristic. Computer control has been incorporated wherever possible to reduce the use of discrete electronics, and to simplify operation. The system in use here is fairly old. However, since it was based on a multi-tasking operating system for maximum flexibility, it was possible to incorporate new instrumental capabilities without changing the operation of prior programming. Thus addition of the parallel array was possible without sacrificing the serial operation. This is a principal that will become more and more important as small computer operating systems become more sophisticated and standardized. If prior work is to be built upon, rather than swept away and lost, extreme care must be exercised in the initial design and implementation of experimental control systems. In the end, this work has achieved a practical spectrometer resolution of 150 meV with parallel collection at a spectrometer collection half-angle of 12.5 mR. The accuracy and stability of the system is + 25 meV over an energy loss range of 0-1000 eV.
3. The electron source With a spectrometer resolution limit of 0.1-0.2 eV, we find that the spectral resolution is limited to 0.25-0.35 eV by the intrinsic width of the field emission source. This may be decreased by reducing the work function of the field emitter. A Zr-treated emitter is currently available on an experimental basis from FEI Company. This consists of a tungsten field emitter having a ring of Zr metal around the tip shank. During operation, the
35
P.E. Batson / High resolution energy-loss spectroscopy
.~' ff~k'W . . . .
-71 \ix :IL^Z
' ?.H ~ ; / 5 e V " Vacuur • \, , , -1 0 2 3 Energy Loss (eV)
Fig. 2. A comparison of the standard W field emission, distribution with a low-work-function Zr-treated W field emitter.
The distributions are described with the simple tunneling theory. Direct interband scattering results for Si and Ga.As are shown for comparison. It is hoped that similar low-work-function emitters may be useful for high resolution work in the absence of beam monochromatizationequipment.
Zr migrates to the tip region, reacts with oxygen and lowers the W surface work function from 4.5 eV to 2.5 eV. As explained in detail elsewhere [6], a current density which is comparable to the standard W tip can be achieved by lowering the extraction field. This gives a narrower energy distribution. An example of a typical comparison is shown in fig. 2. Two results are achieved. First, the spectral resolution can be pushed to 0.2 eV from 0.35 eV. Secondly, as can be noticed in the figure, the background intensity from the tunneling distribution can be dramatically reduced in the 0.5-1.0 eV energy loss region. Fig. 2 also includes results for the direct interband scattering intensities in Si and G a A s to make the point that the energy loss scattering in this energy range is very weak. It is so weak that there is a severe background problem from the exponential tail of the field emission tunneling distribution. Simple calculations confirm this finding [7]. Thus, the background reduction is the most important result obtained on going to the Zr-treated source. If the tip radius is properly controlled, this Zr-treated tip can be incorporated in the standard STEM gun without modifications to the extraction voltage supplies. A good spatial resolution has also recently been demonstrated. Two questions remain. First, lowering the work function leads to increased susceptibility to contaminants on the tip surface. It is therefore not yet clear
whether the current stability will be adequate. Secondly, some thought needs to be given to the tip extraction geometry because field emission can occur more readily at undesired locations. In the end, it m a y be necessary to provide a shield of the tip shank to prevent field emission from the built up Zr ring.
4. W i d e energy scans Fig. 3 shows a inelastic scattering results over a wide energy range for graphite. These results are a composite of a set of experiments which were designed to obtain the Extended Energy-Loss Fine Structure (EXELFS) which is present above the carbon K edge. The data were obtained with a S T E M objective aperture subtending a half-angle of 16 m R at the specimen. The spectrometer collection aperture was chosen to match this. The parallel detector was set up to give a 0.6 eV wide spectral slit. The spectral resolution was dominated by this slit size. Between 275 and 525 eV, each spectral point was integrated for about 0.65 s in each of a total of twenty 20 s experiments. The sum therefore reflects the results of 13 s per point, obtained in 400 s of recording time. This produced about 6 × 10 6 detector counts at the graphite ls---, o* peak, corresponding to about 750,000 detected
~> Graphite
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~ Plasmon
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200
'
'
'
400
500
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300
600
Energy Loss (eV)
Fig. 3. Energy loss spectrum for graphite suitable for EXELFS analysis. The core loss itself is obtained with very high accuracy. The low loss region is accurate enough to perform a multiple scattering analysis. The thickness of the sample in this case was about 0.5 t/?t.
36
P.E. Batson / High resolution energy-loss spectroscopy
electrons, or about 1.2 x 105 electrons s -1 eV -1. This agrees favorably with the expected rate of 105 expected using a beam current of 2 nA into a 1 nm diameter probe incident onto a 50 nm thick piece of graphite having a core loss differential scattering cross section [8] of 2 x 10 -22 cm 2 eV -~ atom -~. These results can be directly compared with those obtained almost 10 years ago [10] with serial recording from a straight-edge magnetic sector spectrometer. There, only about 300 counts s -1 were obtained in 20 min recording times. Clearly we are now able to realize the promise that was apparent but unrealized at that time. Between 200 and 1000 eV, a short acquisition using a 2 eV window was performed to establish the overall behavior of the data. In the low loss region, somewhat better data has been obtained to allow a multiple scattering analysis of the core loss. An overlap of data between 200 and 250 eV allows normalization of the two sets of data. Examination of a serial scan in the low energy loss region indicated a need to correct the parallel output gain in the presence of the very high incident beam flux. Therefore, the very low energy loss region ( - 1 to 5 eV) has been obtained with a single slit serial scan. Finally, a scan without the specimen for the background intensity was obtained from 200 to 1000 eV for the purpose of subtraction. This spectrometer design suffers from a high background intensity due to scattering of the main beam within the spectrometer when scanning tO high energy loss. It becomes a problem above about 500 eV energy loss and should be estimated and subtracted. This background contribution is smoothly varying and increases from about 10% of the measured signal at 300 eV to 90% of the signal at 1000 eV. The data in fig. 3 have had this contribution removed by comparison with the background measured with the specimen absent. The acquisition times that are necessary with this system may seem long relative to those being achieved with the magnetic sectors in systems using thermionic sources. For a spot size of 500 nm and an energy resolution of 1 eV, we may use a thermionic electron source to give us a current of about 500 nA, With no requirement for high energy resolution, the parallel detector can be run
at a much higher efficiency. The above graphite example, for instance, gives a gain over single channel counting of only × 13 because the total spectrum width is only 9 eV at the detector, and we need to cover a 250 eV wide scan. At 1 eV resolution, a parallel array can be configured to cover this entire range to get an enhancement over a 0.6 eV wide single slit of about x 400. Thus a low dispersion system will give an advantage of about 8 x 103 over the high resolution system for the above experiment, giving a results which has a statistical accuracy similar to fig. 3 in 10 ms. Thus, it seems that the conventional, low resolution, systems may give better results for the wide energy scans required for EXELFS analyses. On the other hand, it could be remarked that the present setup gives remarkably good results for a system which has been optimized to obtain sub-1 eV resolution from 0.5 nm sized areas. If both the high energy and spatial resolution are relaxed, much larger signals are possible. The VG HB501 system with the magnetic gun lens is capable of supplying 100 nA into a 10 nm spot size. With a redesigned Wien filter - perhaps twice as large, operating at a pass energy of 5 kV, with an energy resolution of 1 eV - it should be possible to obtain much higher collection efficiency and much lower background than are currently present in the IBM system. This operation should not require more complicated lenses because the energy dispersion will naturally decrease as the operating voltage is increased. If the design were done carefully, both high and low energy resolution should be possible in the same system.
5. EXELFS analysis This description illustrates what must routinely be recorded to allow a reasonably confident interpretation of the energy loss data. With it, it has been possible to take the above data, remove multiple inelastic scattering from the core loss, strip the slowly varying background, and perform a creditable EXELFS analysis. This analysis is summarized in fig. 4. There, I show (1) the data multiplied by AE 28 to approximate the result of a photon absorption experiment, (2) the kx(k) EX-
P.E. Batson / High resolution energy-lossspectroscopy C K Edge
(a)
6. Acquisition of precise shapes
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250 300 350 400 450 500 550 ~
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37
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Fig. 4. Results of the EXELFS analysis for the data in fig. 3. This experiment was performed with the incident and collected beams closely aligned with the [10001 zone axis. Therefore some diffraction effects m a y be involved.
ELFS function and transform window, and (3) the radial distribution function. The 1.4, 2.8, and 3.4 .A neighbor distances are present, but the 2.4 .A distance is not strong. Differences between this result and others previously published [9-11] do not appear to be a result of inaccuracies in the analysis, but may be due to diffraction effects. This experiment was performed with both the incident and collected beams aligned with the [1000] zone axis of the graphite to explore the sensitivity of the EXELFS to diffraction a n d / o r angular integration conditions. Changes on the EXELFS structure do appear to occur for different diffraction conditions. These changes are being examined to help understand details of the physics of the inelastic scattering. In particular, it is hoped that the experiments may help us to understand whether the many plane waves making up the convergent beam in the STEM can interact coherently in the process of an inelastic scattering event. (See for instance the recent literature [12-15].)
Fig. 5 shows a low energy loss result for MgO, plotted together with a detailed look at the MgL~, 3 edge. The two scans were acquired in similar times with similar collection conditions using the parallel collection system. The result illustrates again that it is u s e f u l t o first characterize the scattering over a fairly wide energy range. Then a particular feature of interest can be obtained with a very high accuracy. It is probably important in passing to emphasize that the energy scales here are accurate to + 25 meV. This is automatic and depends only on the accuracy of the spectrometer power supplies, which need calibration about twice a year. The results in fig. 5 illustrate that a statistical accuracy of better than 0.1% is obtainable in the 0-200 eV range from a sample that is of order 50 n m thick. However, the intention in this work - ultimately - is to make a correlation with high resolution structural microscopy. Therefore, high resolution energy loss must be compatible with the high resolution samples. These are seldom as thick as 50 nm, and so they present several problems. First, the energy loss signal will be correspondingly smaller as the number of atoms within the probe are reduced. Second, radiation damage becomes a consideration, even with the semi-conductors, as the thermal conductance is reduced. Third, as the thickness, or lateral dimensions of the speci-
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30 40 50 60 70 80 Energy Loss (eV) Fig. 5. Low energy loss results for M gO with a detailed look at the onset of the M g L2. 3 edge. T h e inset is not magnifications of regions of the wider range scan, b u r a separate scan using a 50 meV step size. The spectral resolutioh was about 0.3 eV using a collection half-angle of 8 m R .
38
P.E. Batson / High resolution energy-loss spectroscopy
men become comparable to the unit cell size of the material, the bandstructure within the region is expected to be modified. In fig. 6, I show results for the Si L2,3 core edge taken from various positions close to the edge of a Si sample. A background due to valence electronic scattering has been subtracted to show the absorption edge. At positions greater than 10 nm from the edge, the results are indistinguishable from results obtained in a continuous specimen. Approaching to within 2 nm, some small changes in shape appear. Within 1 nm, gross changes are present, including intensity in front of the edge which may be a result of scattering involving surface states. Structure at the Si edge is loosely relatable to the conduction band density of states in the Si [16]. The disappearance of this structure is therefore probably an indication of the necessary modification of the conduction band as the edge of the sample is approached. The appearance of additional structure is the welcome signature of localized electronic states at the surface. In fig. 7, I show two results from rather thicker Si. They compare the result from the unmodified Si bulk with the result from a dislocation bounding the edge of a stacking fault. This sample has been the subject of other investigations [17] designed to investigate the detailed structure of these bounding dislocations. In particular, the sample has been subjected to a thermal history designed to create non-equilibrium stacking faults. It is also known to be cathodoluminescent as a result of this
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97 98 99 100 101 102 103 104 Energy Loss (eV)
Fig. 7. Comparison of the Si L2, 3 edge from the bulk with that obtained in an area containing a dislocation which b o u n d s a stacking fault. Extra scattering intensity below the edge probably reflects the presence of e m p t y electronic states in the Si gap. Also extra intensity appears near 102.3 eV. This scattering m a y also be caused by states in the gap near the BfiUouin zone center. However, more information is needed for identification.
treatment. The energy loss spectra show clear differences between the bulk and the dislocation. There are two differences: additional intensity appears below the absorption edge at the dislocation, possibly signifying new final states within the gap, and additional intensity appears at 102.3 eV. Further work will explore the evolution of these features as defects are approached. It may thus be possible to locate particular structural features of an extended defect which give rise to well defined modifications of the energy loss spectra.
6. Conclusions i
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Fig. 6. Results for the Si L2. 3 edge at three different locations near the edge of a Si sample. At distances greater than 10 rim, the results are identical to those obtained from the bulk. A s the probe approaches the surface, features which reflect the Si bandstructure begin to be washed out.
It is now possible to obtain experimental results which are of good enough quality to begin answering some detailed questions involving the probe-specimen interaction. Does our description of the energy loss process need to be modified in the presence of coherent sources? How do constricted volumes modify the energy loss results? What are the impact parameter issues when scattering via excitation of a localized defect state? As these issues become better understood, we will be in a good position to begin the task of the detailed comparison of electronic structure with atomic positions so that a fundamental theoretical understanding may be achieved.
P.E. Batson / High resolution energy-loss spectroscopy Acknowledgements I w a n t to acknowledge m a n y c o n v e r s a t i o n s with J. Silcox o n spectrometer design. I wish to t h a n k also H. A l e x a n d e r a n d J.C.H. Spence for the o p p o r t u n i t y to e x a m i n e the Si sample involved i n their work. I a m also i n d e b t e d to the referee for p o i n t i n g out the gains which a r e possible with the n e w e r field emission g u n systems.
References [1] C.H. Spence, Ultramieroscopy 18 (1985) 165. [2] P.E. Batson, K.L. Kavanagh, J.M. Woodall and J.W. Mayer, Phys. Rev. Letters 57 (1986) 43. [3] P.E. Batson, Rev. So. Instr. 57 (1986) 43. [4] P.E. Batson, Rev. Sci. Instr. 59 (1988) 1132. [5] P.E. Batson and G. Trafas, Ultramieroscopy 8 (1982) 293.
39
[6] P.E. Batson, in: Proc. 45th Annual EMSA Meeting Baltimore, MD, 1987, Ed. G.W. Bailey (San Francisco Press, San Francisco, 1987) p. 132. [7] P.E. Batson, in: Scanning Microscopy Suppl. 1 (Scanning Microscopy Intern., Chicago, 1987) p. 189. [8] 1LD. Leapman, P. Rez and D.F. Mayers, J. Chem. Phys. 72 (1980) 1232. [9] B.M. Kincaid, A.E. Meixner and P.M. Platzman, Phys. Rev. Letters 40 (1978) 1296. [10] P.E. Batson and A.J. Craven, Phys. Rev. Letters 42 (1979) 893. [11] M.M. Disko, O.L Krivanek and P. Rez, Phys. Rev. B25 (1982) 4252. [12] V.W. Maslen and C.J. Rossouw, Phil. Mag. A47 (1983) 119. [13] D.K. Saldin and P. Rez, Phil Mag. B55 (1987) 481. [14] V.W. Maslen, Phil. Mag. B55 (1983) 491. [151 D.K. Saldin, Phil. Mag. B56 (1987) 515. [16] P.E. Batson, Ultramicroscopy 22 (1987) 89. [17] H. Alexander, J.C.H. Spence, D. Shindo and N. Long, Phil. Mag. A53 (1986) 627.