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Submitted on 1 Jan 1987
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COMBINED FIELD ION AND SCANNING TUNNELING MICROSCOPE
T. Sakurai, T. Hashizurne, I. Kamiya, Y. Hasegawa, A. Sakai, A. Kobayashi, J. Matsui, S. Takahashi, E. Kono, H. Watanabe
To cite this version:
T. Sakurai, T. Hashizurne, I. Kamiya, Y. Hasegawa, A. Sakai, et al.. COMBINED FIELD ION AND SCANNING TUNNELING MICROSCOPE. Journal de Physique Colloques, 1987, 48 (C6), pp.C6- 79-C6-84. �10.1051/jphyscol:1987613�. �jpa-00226816�
COMBINED FIELD ION A N D SCANNING TUNNELING MICROSCOPE
T. Sakurai, T. Hashizurne, I. Kamiya, Y. Hasegawa, A. Sakai, A. Kobayashi, J. ~atsui*, S. ~akahashi*, E. ~ono* and H. Watanabe*
The Institute for Solid State Physics 7-22-1 Roppongi, Minato-ku, Tokyo, Japan 'NEC Corporation, Miyarnae-ku, Kawasaki, Japan
Abstract
Realizing the importance of characterizing a STM probe tip on an atomic scale, we have constructed a new instrument which combines a field ion microscope and scanning tunneling microscope. A complete STM set-up, which is similar to the one developed by Demuth, is mounted on an 8" O.D. flange and a FIM set-up is mounted on a 10"
O.D. flange. FI images of the STM probe tip are observed using a 2"
O.D. chevron channelplate-image intensifier screen assembly with liquid nitrogen cooling. A field necessary for field ionization of He imaging gas is provided through a negative electrode placed near the probe tip so that the high voltage system is completely separated from the STM system. A preliminary data is presented for the
graphite (0001) plane in vacuum.
A scanning tunneling microscope (STM) has been enjoying much publicity and popularity since its invention by Binning and Rohrer in 1981. The excitement of this microscope lies in the expectation that it can resolve the geometric and electronic structures of almost any surfaces on truly an atomic scale without assuming the periodicity of a crystal. However, it is generally assumed that the STM probe tip must consist effectively of a single or few atoms when a true atomic resolution is to be attained. Little information is available so far on this critical issue of the structure of the STM tip. Indeed, there has been only one successful work in this respect, reported by Kuk and Silverman./l/ They have inspected the surface of the
tungsten-made STM tip by a field ion microscope (FIM) in-situ of the scanning tunneling microscope before and after the STM scans and showed a preliminary data on the relationship between the STM resolution and tip surface geometry.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987613
C6-80 JOURNAL DE PHYSIQUE
Realizing the importance of their work and urgent needs of
detailed information on the STM resolution, we have constructed a new instrument which combines the STM and FIM. Fig. 1 is the schematic of this new instrument. The main chamber is a 20 cm diameter cylinder with 63 cm length and houses two identical STM-FIM set-ups at the symmetric positions. The set-up on the right hand side is used as a noise reference to help us evaluate the noise level of the STM. The entire STM set-up is mounted on the 8" O.D. horizontally placed flange and the FIM set-up is placed on the 10" O.D. vertically positioned flange together with a sputtering ion gun. The system is pumped by a 400 11s Riber Ion Pump and a 330 l/s Seiko magnetic levitation turbo-molecular pum (TMP) backed by another TMP. The operation pressure is 2 r 10-lg Torr. A specially designed Ti
sublimation pum may be used occasionally to reduce residual hydrogen down to low Torr. The entire system is mounted on a high-leg air-suspended vibration isolation table.
Fig. 2 is the photograph of the STM-FIM set-up. The STM drive consists of the coarse mechanical drive for the STM specimen and fine electro-mechanical drive for the STM probe tip. The coarse one is very silimar to the so-called "grasshopper" type (with a stopper located nearhy the STM tip) used by Demuth group./2/ The STM specimen is mounted on the head of the grasshopper. The external rotary-linear motion feedthrough is used to manipulate a Mitsutoyo
n sub1 Lmat Lon pump
channel plate-screen negat Lve electrod
turbo molecular pump Lon pump
Fig. 1 . Schematic of the STM-FIM system
the head of the grasshopper is flipped out exposing the STM tip and piezo cylinder.
micrometer spindle, which pushes the bottom of the grasshopper. This coarse drive is operated manually by monitoring the movement through a 400x optical microscope. The fine drive utilizes two types of electro-mechanical drives: piezoelectric one and electrostrictive one./3/ The multi-layered electrostrictive drive has 10 micron stroke in the z-direction (perpendicular to the specimen surface).
The fine-drive STM tip scanning is performed by a cylinder-type piezoelectric device (9 mm O.D. and 8 mm I.D. x 20 mm length) reported by ~innig./4/
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The grasshopper type specimen motion segment and the piezo-drive make up our scanning tunneling microscope body. The STM body is mounted on the five-layer vibration isolation plates ( 6 0 mm x 80 mm square). Each plate is connected by three-types of springs at four corners to shield out the mechanical noise. A 125 mm diameter A1 disk which supports the isolation plates is suspended by three
springs. The mechanical lever seen at the left side of the isolation plates is used to clamp the entire STM set-up to the bottom 8" O.D.
flange. This lever is used to de-couple and to float the STM body after the coarse approach of the specimen is completed using the 400x optical microscope.
The FIM set-up consists of a negative electrode and 2" O.D.
chevron channelplate-screen assembly. They are mounted on a single station which is coupled with the external linear motion mechanical feedthrough with 100 mm stroke. Only after the head of the grasshopper is flipped out completely, the FIM mode can be brought toward the STM section and the center of the negative electrode can be positioned right above the STM tip. Helium imaging gas is used for the FI imaging of an ordinary tungsten STM tip. The negative
electrode is operational up to -15 kV while the STM tip mounted on the inner wall of the piezo ceramics is kept below 500 V to protect the piezo-drive. Field evaporation is used for the cleaning and characterization of the STM tip. The STM specimen is cleaned by sputtering and annealing. All the electric connections are made using a 0.07 mm diameter Ni wire with polyimide coating. The system has been designed such a way that each of the STM-FIM set-ups can be assembled completely separately and the entire alignment can also be tested and adjusted outside the vacuum chamber.
The instrument has been tested using Si and graphite so far./5/
In this paper a few data are shown for graphite surfaces scanned in the UHV condition. Figs. 3 and 4 are the STM images of the (0001)
Fig. 3 . STM image of the (0001 ) plane of graphite.
Mechanical contact took place when the STM tip passed the high points of the surface.
plane of graphite. Fig. 5 is'the FIM image of the (110) plane of the tungsten STM tip used to obtain the STM image of Fig. 4. It is interesting to note that it was possible in the case of the graphite surface to take the STM data with a direct mechanical contact between the tip and specimen. Indeed Fig. 3 was taken under such a condition in the so-called STS (scanning tunneling spectroscopy) mode, or constant z-position mode (no feedback to control the tunneling current). The STM tip has made direct contact at the high points of the surface. Upon making contact, the vacuum junction was converted into ohmic junction and thus the monitoring tunneling current became suddenly saturated. The corrugation is, in this case, estimated to become "over 10 A", if this current changes are treated as if that of the vacuum junction current. Large corrugation in air reported in many publications on the STM study of graphite appears to be resulted by this direct contact junction with the oxide film over the tungsten STM tip.
When the gap distance was kept larger so not to make direct mechanical contact, ordinary vacuum junction could be maintained and the STM image in Fig. 4 was obtained. This data is unprocessed one and no data reduction was performed to illustrate the noise level of our STM-FIM device.
In conclusion, the new instrument combining the STM and FIM has been constructed and preliminary data have been obtained.
C6-84 JOURNAL DE PHYSIQUE
Fig. 5. FIM images of the STM tip before and after the experiment.
References
/ I / Y. Kuk & P. Silverman, Appl. Phys. Lett. 48, 1597 (1986).
/ 2 / R. M. Tromp, R. J. Hamers, & J. E. Demuth, Phys. Rev. Lett. 55, 1303 (1985).
/ 3 / 0. Nishikawa, M. Tomitori & A. Minakuchi, Surf. Sci. 181, 210 (1987).
/ 4 / G. Binnig & D. P. E. Smith, Rev. Sci. Instrum. 57, 1688 (1986).
/ 5 / T. Sakurai et al. to be published.
