Scanning tunneling microscope combined with a scanning electron ...

Published in Rev. Sci. Instrum. 57 (2) 221-224 (1986)

Scanning tunneling microscope combined with a scanning electron microscope Ch. Gerber,a) G. Binnig,b) H. Fuchs,c) O. Marti, and H. Rohrer

IBM Zurich Research Laboratory, 8803 Rüschlikon, Switzerland

We have developed a small scanning tunneling microscope (STM) to be incorporated into a scanning electron microscope (SEM). Vibration isolation and damping is achieved solely with Viton dampers. As a stand-alone unit, a tunnel-gap stability of about 1 A is reached at atmospheric air pressure without additional sound protection. Stability improves by at least an order of magnitude when incorporated into a SEM.

Introduction Scanning tunneling microscopy (STM) is a novel surface-analytical method for real-space imaging of surface structures and composition on an atomic scale.1-3 The surface properties are sensed by the tunnel current flowing from or to a fine metal tip scanned over the surface. The width of the tunnel gap is typically in the 4 to 20-Å range. The three main components of the STM are the fine (< 1 µm) and coarse (> 1 µm) controls of the position of the tunnel tip and the vibration-isolation and damping stages.1-4A schematic of a possible STM configuration is shown in Fig. 1 with the fine X-Y-Z motion on the tip side, and the rough motion (X and Y only) on the sample side. One of the central instrumental problems is the stability of the width of the tunnel gap. This requires careful protection of the tunnel junction against vibrations, in particular against those which can excite a mechanical eigenmode in the mechanical path between tip and sample. The external vibrations are taken care of by the suspension system, and the internal vibrations created by the fine X-Y-Z drive of the tip itself are kept in frequency below the lowest mechanical eigenfrequency by an electrical low pass in the tip-position control loop. The latter limits the imaging speed; high speeds thus require a rigid sample-tip connection. Vibration protection is therefore a compromise between rigidity for high imaging speed and flexibility for convenient rough positioning of the sample. At present, the louse-type STM4 with the convenience of


rough positioning is most widely used. Rigid STM's with high mechanical eigenfrequencies5,6 use concepts of the squeezable tunnel junction.7 The louse-type STM requires good isolation from external vibrations. The vibration-isolation and damping system using coil springs and eddy-current damping become relatively large, i.e., linear dimensions of some tens of centimeters. Generally, performance improved with size. The use of the STM together with other surface-analytical tools thus required large UHV systems. This is not only expensive but also impractical. In particular, incorporation of a STM into a conventional SEM is best done with a small-size STM. Such a combination makes use of both the ultrahigh resolution and versatility of STM and the established virtues of SEM. This was the main motivation to develop a "pocket-size" STM. Before turning to the pocket-size STM, we briefly summarize the vibration-isolation and damping systems of our earlier STM's. In the first one, we used superconducting levitation as vibration isolation.1 The gap-width stability was sufficient to resolve monoatomic steps on CaIrSn4.2 The second4 and third8 generation STM's used a double-stage spring system. The spring systems differed mainly in size (e.g., in the third generation, damping magnets further away from the tunnel unit for in-situ LEED and Auger analysis), the tunnel units in the materials used (e.g., machinable ceramics in the second generation, quartz and Pyrex glass in the third generation). The 7 x 7 reconstruction of the Si(111) was imaged with the second generation.9 The third generation was first used for the Au(100) investigation.10 In UHV and with some additional viton dampers squeezed into the coil springs, the gap-width stability was considerably better than 0.1 A. With this instrument, local11 and scanning12 tunneling spectroscopy were also successfully performed. With the same version, the Madrid group pioneered STM applications in biology13 and surface metrology14 at ambient air pressure. We should like to mention that the gap-width stability is not simply a matter of the mechanical construction of the STM and the vibration-isolation system. The noise of an STM trace also reflects the stability of the surface under investigation (which again depends on many other factors like applied voltage, contamination state, environment) and that of the tip [e.g., rapid changes of tip by hopping of the atom(s) providing the tunnel current, mechanical vibration of whiskerlike tips]. Thus, quotations of gap-width stabilities are a measure of the noise of the STM traces in a given experiment and include all the above factors.

I. The "Pocket-Size" STM The essential new part of the pocket-size STM is the vibration-damping and isolation system. In the second- and third-generation STM, we used Viton connectors between coil springs and frames.4 We


found that replacing the coil springs by rods did not dramatically deteriorate the vibration-isolation properties, at least not for small amplitudes.15 This led to a vibration-isolation system without any coil springs and damping magnets as depicted in Fig. 2. It consists of a stack of stainless-steel plates with three (or more) viton dampers between each pair of steel plates. The viton dampers were simply cut from a viton O-ring, and are about 5 mm long and 2 mm in diameter. The electrical leads, copper wires of 0.05 mm diameter, are conducted from plate to plate via slit viton dampers. Coil or other springs between first and second plates are optional. They help somewhat to take care of large vibrational amplitudes. Figure 3 shows the performance of the pocket-size STM without springs, operated at ambient air pressure and resting on an ordinary wooden laboratory table without any electrical shielding of the wires and any sound protection. The gap-width oscillations in the few angstrom range are due to the lowest mechanical mode of the system excited mainly by sound. They appear to be the main source of the gap-width noise. The overall performance is about equivalent to that of the double-stage coil-spring suspended STM's. Excellent gap-width stability is obtained by additional protection from large-amplitude vibrations. This can be achieved by prefiltering of vibrations with, e.g., a coil-spring system or by incorporating the STM into a large system like an SEM, where vibrations have mostly been taken care of by other means.

II. STM SEM Combination This combination is intended to (a) guide the tip of the STM, (b) study rough deformation or changes of surface or tip induced intentionally or unintentionally during operation, and (c) to correlate STM and SEM/SAM (scanning Auger microscopy) data. Point (a) is important to locate distinct gross features (below the resolution limit of optical microscopes) on a surface for further detailed STM investigation. Item (b) is of quite general interest to STM, be it to find well-defined and nondestructive procedures for approach of tip and sample, to study the effects of unintentional tip sample contacts, or to learn about tip-induced material deposition or etching. An example for (c) is the correlation of a specific feature observed by STM with gross features observed in SEM/SAM. Both techniques should benefit from each other quite generally. The main requirements for such a combination were to perform SEM on tip and sample in situ, i.e., in the STM configuration, easy and rapid sample transfer, and UHV compatibility. (Also in situ tip exchange is desirable. This problem is under consideration.) Figure 4 shows the schematic of the incorporation of the pocket-size STM into an HB-100 SEM/SAM of vacuum generators.


The STM sits on an X-Y-Z manipulator. No springs are used, neither between STM and manipulator nor the optional spring stage of the STM itself. The manipulator serves both for alignment of the tip with the SEM electron beam and the slit of the Auger analyzer, as well as for disconnecting the STM from the SEM. The latter is simply achieved by lowering the STM by about 1.5 cm. The SEM can then be operated in the normal mode with the original X-Y-Z drive and sample holder. Simultaneous operation of STM and SEM is also possible provided the set current for the STM is, say, twice as large as the beam current of SEM. The latter is of the order of 1 nA, the STM current usually a few nA. Figures 5 and 6 show two STM graphs of pyrolytic graphite. Figure 5 is taken on a relatively dirty graphite sample, the "noise" seen is therefore attributed to surface and tip contaminations and instabilities. With a clean tip and clean cleaved sample, however, the noise performance is excellent. Figure 6 is taken with a tunnel current of 2 nA and a voltage of 50 mV. The minima correspond to the centers of the hexagonal carbon rings, the sequence of shallow and deep minima is due to a slightly skewed scanning direction. A detailed physical interpretation of this surface will be given elsewhere.16 Thus, the subangstrom features are real also and the gap stability is estimated to be better than a tenth of an angstrom. SEM on a sample mounted on the STM is at present limited by the vibrations of the whole STM with respect to the SEM lenses, which are of the order of 100-200 Å. The major factor for these large vibration amplitudes is attributed to the long chamber (which was constructed for a different type of STM) and consequently the long connector (see Fig. 4), i.e., long distance of about 25 cm between STM manipulator and SEM lens. Their shortening should greatly improve the performance of SEM. We conclude that a springless STM, with viton dampers only, operates with a stability of a few angstroms at ambient-air pressure and at least an order of magnitude better when protected from sound and large amplitude vibrations. The small size makes the pocket-size STM well suited for combination with other UHV analytical tools or electron microscopy.


References

a)

Present address: IBM San Jose Research Laboratory, 5600 Cottle Road, San Jose, CA 95193.

b)

Present address: IBM San Jose Research Laboratory, 5600 Cottle Road, San Jose, CA 95193, and W. W. Hansen Laboratories of Physics, Stanford University, Palo Alto, CA 94304.

c)

Present address: I 542 - ZKL/I, BASF Aktiengesellschaft, D 6700 Ludwigshafen, West Germany. 1

G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Appl. Phys. Lett. 40, 178 (1982); Physica 109 & HOB, 2075 (1982).

2

G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett. 49, 57 (1982).

3

For references, see G. Binnig and H. Rohrer, Physica 127B, 37 (1984); see also Proceedings of the STM workshop of the IBM Europe Institute, July 1-5, 1985 in Lech, Austria, to be published in the IBM J. Res. Dev. A more detailed account of the technique is given by G. F. A. van de Walle, J. W, Gerritsen, H. van Kempen, and P. Wyder, Rev. Sci. Instrum. 56, 1573 (1985).

4

G. Binnig and H. Rohrer, Helv. Phys. Acta 55, 726 (1982).

5

R. V. Coleman, B. Drake, P. K. Hansma, and G. Slough, Phys. Rev. Lett. 55,394 (1985).

6

J. Demuth, IBM Techn. Discl. Bull, (in press); and in Proceedings of the STM workshop of the IBM Europe Institute, July 1-5, 1985 in Lech, Austria, to be published in the IBM J. Res. Dev.

7

J. Moreland, S. Alexander, M. Cox, R. Sonnenfeld, and P. K. Hansma, Appl. Phys. Lett. 43, 387 (1983); J. Moreland and P. K. Hansma, Rev. Sci. lnstrum. 55, 399 (1984).

8

G. Binnig and H. Rohrer, Sci. Am. 253, 50 (1985).

9

G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett. 50, 120 (1983).

10

G. Binnig, H. Rohrer, Ch. Gerber, and E. Stoll, Surf. Sci. 144, 321 (1984).

Published in Rev. Sci. Instrum. 57 (2) 221-224 (1986) 11

G. Binnig, K. H. Frank, H. Fuchs, N. Garcia, B. Reihl, H. Rohrer, F. Salvan, and A. R. Williams, Phys. Rev. Lett. 55, 991 (1985).

12

G. Binnig, H. Fuchs, J. Kübler, F. Salvan, and A. R. Williams (to be published).

13

A. M. Baro, R. Miranda, J. Alaman, N. Garcia, G. Binnig, H. Rohrer, Ch.Gerber, and J. L. Carrascosa, Nature 315, 253 (1985).

14

A. M. Baró, R. Miranda, N. Garcia, H. Rohrer, Ch. Gerber, R. Garcia Cantu, and J. L. Peña, Metrologia (in press); R. Miranda, N. Garcia, A. M. Baro, R. Garcia, J. L. Peña, and H. Rohrer, Appl. Phys. Lett. 47, 367 (1985).

15

G. Binnig, Ch. Gerber, and O. Marti, IBM Tech. Discl. Bull. 27, 3137 (1984).

16

G. Binnig, H. Fuchs, Ch. Gerber, H. Rohrer, E. Stoll, and E. Tosatti (to be published).


Figure captions FIG. 1. Schematic of a STM using a piezotripod, {X- Y-Z) for fine positioning of the tunnel tip T and a piezomotor L for rough positioning of the sample S. Vibration isolation is achieved with coil spring V and eddy-current damping (not shown). FIG. 2. "Pocket-size" STM. Vibration-isolation and damping is achieved by a stack of stainless-steel plates with pieces of Viton O-rings (not shown) in between. The top plate carries the "louse" (1: top metal plate with sample holder with only one of the three screws tightened, 2: piezobody, 3: anod-ized aluminum feet) and theX-F-Zpiezotripod. The tip is fixed on a titanium holder with hole 4 for escape on the Auger electrons. The sample holder is simply slid into the hole provided. The current lead 5 to the tip has to be relatively rigid. The wiring is led from stage to stage by viton dampers. The first spring stage is optional. FIG. 3. STM graph of a gold surface taken at ambient-air pressure with the STM resting directly on a wooden laboratory bench without sound protection. FIG. 4. Schematic of STM in an HB-100 SEM/SAM. The connector (length not to scale) is fixed to the XY-Z manipulator and carries the STM. The original SEM sample holder with X- Y-Z manipulator, below the aperture selector, is not shown. FIG. 5. STM graph of graphite taken with STM inside SEM. FIG. 6. STM graph of a clean graphite surface. All features seen on a scan are real. The irregularities in distance from scan to scan are artifacts of plotting the STM graph. Scan speed is 2.5 s/scan, cutoff frequency of the servoloop about 300 Hz. The dots indicate the positions of the hexagon centers of the graphite honeycomb structure.


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