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Submitted on 1 Jan 1984
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SINGLE ATOM DETECTABILITY OF A ToF ATOM-PROBE
T. Sakurai, T. Hashizume, A. Jimbo
To cite this version:
T. Sakurai, T. Hashizume, A. Jimbo. SINGLE ATOM DETECTABILITY OF A ToF ATOM-PROBE.
Journal de Physique Colloques, 1984, 45 (C9), pp.C9-343-C9-347. �10.1051/jphyscol:1984957�. �jpa-
00224443�
JOURNAL DE PHYSIQUE
Colloque C9, supplément au n"12, Tome <f5, décembre 1984 page C9-343
SINGLE ATOM DETECTABILITY OF A ToF ATOM-PROBE
T. Sakurai, T. Hashizume and A. Jimbo
The Institute for Solid State Physios, The University of Tokyo, Minato-ku, Tokyo, Japan
Résumé - A l'aide de notre nouvelle sonde à atomes à focalisation qui est capable de détecter tous les signaux entrant dans le trou-sonde avec 100 % d'efficacité de détection, nous avons essayé d'évaluer l'erreur de visée d'une sonde à atomes à temps de vol. Nos résultats préliminaires suggèrent que l'erreur de visée est surtout due au glissement du disque d'ionisation à par- tir de la position réelle de l'atome. Sur les plans d'indice faible, il est possible.de détecter chaque atome individuel en compensant ce déplacement.
Cependant, dans le cas de certains plans d'indice élevé, il est plutôt diffi- cile de prédire la différence de trajectoire ionique entre les ions issus du gaz image et les ions métalliques évaporés et ainsi l'efficacité de détection est inévitablement basse.
Abstract - Making use of our new focusing type ToF atom-probe which is capable of detecting all the signals entering into the probe hole with 100% detection efficiency, we have attempted to evaluate the aiming-error of a ToF atom-probe. Our preliminary data suggests that the aiming-error is primarily due to the shift of the ionization disc from the actual atom position. In the low index planes it is possible to detect every single atom by compensating this shift. However in the case of some of the high index planes, it is rather difficult to predict the difference in ion trajectory between imaging gas ion and evaporated metal ion and thus the detection efficiency is unavoidably low.
We have last year reported the successful construction of an focusing-type ToF atom-probe. This instrument has several unique features not found in many ToF atom-probes currently in operation (Fig.l).(l) For example an auxiliary chamber incorporated between the main FIM chamber and the Poschenrieder electro- static focusing lens houses a 2" diameter chevron channelplate-screen assembly to directly observe the image within a probe-hole. This capability enables us to correlate the signal detected at the detection chamber with the change of the image inside a probe-hole.*' > This chevron channelplate also acts as a signal detector of the straight-type ToF atom-probe with a 68 cm drift-flight. The availability of both straight-type and focusing-type have been found almost indispensable for the study of semiconductors and some alloys in which orderly field evaporation may not take place and thus a wide range of energy deficits are expected. This will be discussed in another paper reported in the present meeting.(3) The precise viewing of the probing area is useful also for aligning the ion- optics and optimizing the lens focusing conditions. Fig.2 shows an example of the focusing power of our instrument. A portion of the (111) area of the <110> oriented tip was positioned to the probe-hole. Four atoms are imaged by the chevron channl- plate in the auxiliary chamber. These atoms can also be viewed at the detection chamber at a reduced size. The H e
+ion beam can be focused into a spot of
approximatelv 1 mm diameter at the best focusing condition after traveling 3000 mm.
With this focusing power, a channeltron can be successfully employed, replacing a conventional chevron channelplate as a signal detector, to achieve 100% detection efficiency. Fig.3 compares the detection efficiency of channeltron and channel-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984957
C9-344 JOURNAL DE PHYSIQUE
plate. The superior performance of the channeltron is evident by the doubling of the efficiency. The problem of negative ion feedback inherited to the chennl- tron amplifier can be succumbed in the case of the focusing-type atom-probe. Its high mass resolution (m/~m;2000) enables us to discriminate noises due to negative ion feedback because they are always accompanied with true signals and have some odd mass-to-charge ratios.
Taking advantage of this superior detection efficiency, we have investigated the problem of aiming-error of ToF atom-probe. Although many atom-probe users recognize the problem of aiming error, there have been virtually no detailed investigations on this subject. This is partly because no atom-probes until now were able to achieve 100% detection efficiency. We believe that our focusing- type atom-probe is most suited for this investigation since it does detect every ion without failure once it enters into the probe-hole. Mo-0.3 at% Re tips were chosen in this project. Field ion images of this kind of material exhibit rondomly dispersed bright spots superimposed with a reguler Mo image. Since the number density of those bright spots is roughly proportional to bulk Re concentration, it was speculated that the bright image spots correspond to Re atoms on the surface. This was, however, never confirmed successfully. Indeed our attempt in 1979 using the conventional ToF atom-probe was turned out to be negative .against this speculation. (4) No Re signals were detected out of 2500 attemps of evaporating and detecting those bright rondom spots. We now believe that the instrument we were using then was not capable of performing this kind of stringent tests requiring very high detection efficiency. Indeed we now have clear evidence that those bright spots does represent Re atoms on the surface.
We can detect Re atoms with almost 100% if the image spots are positioned inside the probe-hole properly with respect to the center of the net plane. Fig.4 shows that the tip used in the present work does contain approximately 0.36 at% Re in the bulk. After this bulk analysis, attemps were made to identify rondom bright spots on the (110) plane. When the spot is placed everytime at the center of the probe-hole, the result turned out to be marginal, Re atoms were detected only
*70% of the attempts. During these attempts we have realized th2t there appear systematic aiming errors indicating the trajectory of field evaporated surface ions differs from that of field ionized gas ions by a significant amount. The example at the various sites within the (110) net plane is given in Fig.5. A couple of remarks can be made when bright spots located near the center of the (110) plane were tried, the detection efficiency is rather low and no reliable identification is possible. Whenever the image spot is placed toward the outer edge of the probe-hole, not at the center, the detection efficiency increased markedly. When the bright spot is place outward approximately 213 off from the center of the probe-hole, 100 percent detection was achieved. This suggests that the ionization disk at the edge site atom of the (110) plane is shifted outward with respect to the center of the plane. The estimation of the plane size from the FI image alone thus results in the smaller number of atoms within a probe-hole/
This conclusion is consistent with the experimental observation that apparent detection efficiency at the (110) plane is over 100%. Our observation also suggests that two step mechanism of field evaporation involving surface diffusion prior to actual evaporation is very unlikely in the (110) plane. When we tried the same test at the (211) plane, the result is different. One example is shown in Fig.6. In this case the image spot must be placed outward along the trough in the direction of <Ill> of the (211) plane in order to be detected reliably. This can also be understood realizing the slight difference in ion trajectory between imaging gas atom and surface metal atom.
However a disturbing result was obtained from the (361) plane which is
surrounded by the (110), (121) and (010) plane. Detection and identification of
Re atom was very difficult whenever the bright spot is positioned within the
probe-hole. We believe that rather poor symmetry of this area prevents to
establish the favorable evaporation direction, similar to the case of the atoms
near the center of the (110) plane. In conclusion, it is possible to set the
probe-hole properly to detect surface atoms with 100% detection efficiency by
correcting the aiming error in the planes with high symmetry. We have shown the
usefulness and power of 100% detection efficiency of our focusing type ToF atom-
probe. This report is preliminary and we plan to continue this work to document
the aiming error problem of atom-probe FIM in detail.
References
(1) T.Sakurai et al. Proceedings of the 30th IFES, p.150 (1983), Philadelphia,USA (2) T.Sakurai et al. Appl.Phys.Lett., 44, 38,(1984)
(3)'~.Sakurai et al, Proceedings of the 31th IFES (4) T.Sakurai et al, Japn.J.Appl.Phys., 2, L167 (1980)
TOP VIEW I
i
TIP HOLDER
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Fig.l A schematic of our focusing-type ToF atom-probe equipped with a 68 cm straight-type mode.
Fig.Z ~ e + FI images of the (111)
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C H R N N E L P L R T E
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Fig.3 Comparison of the
ko C H R N N E L T R O N detection efficiency between
channeltron and channelplate
LJat the signal detector.
0CL
0
N e G R S 1 0 - ' T o r r
V, = 1 5 . 7 - 1 5 . 9 K V
K
NUMBER OF H . V . P U L S E S
Fig.4 Bulk composition of Mo-0.3 at% Re shows Re atoms are dispersed uniformly at the 0.36 at% concentration
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