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A NEW MICROTIP FOR STM
Y. Akama, E. Nishimura, A. Sakai, N. Sano, T. Sakurai
To cite this version:
Y. Akama, E. Nishimura, A. Sakai, N. Sano, T. Sakurai. A NEW MICROTIP FOR STM. Journal de
Physique Colloques, 1989, 50 (C8), pp.C8-211-C8-216. �10.1051/jphyscol:1989836�. �jpa-00229934�
COLLOQUE DE PHYSIQUE
Colloque C8, suppl6ment au n o 11, Tome 50, novembre 1989
A NEW MICROTIP FOR STM
Y. AKAMA, E. NISHIMURA, A. SAKAI, N. SANO* and T. SAKURAI'
Foshiba ULSI Research Center 1 Komukai Toshiba-cho, Kawasaki 210, Japan The Institute for Solid State Physics, The University of Tokyo 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan
An electron-beam deposition in SEM has been successfully applied to make a new type of STM microtip of submicran diameter.
This microtip has a straight shank and enables us to obtain low- distorted STM images of microfabricated patterns. Analyses of the microtip by TEM, Auger and atom-probe indicate that the microtip is amorphous and mainly consists of carbon, oxygen and hydrogen.
1. INTRODUCTION
Scanning tunneling microscopy (STM) is now becoming a standard tool for atomic-scale investigation of surface structures /l/. Atom- resolved images of various semiconductor and metal surfaces have been obtained with a sharp tunneling tip which is essentially the same as the one used in FIM and FEM. Since the tunneling current flows at the atomic cluster on the top surface of the tip, the shank portion usually plays no role in STM measurements. However, it is not the case in the topographic measurements of practical surfaces by STM /2/.
For example, the microfabricated pattern has many narrow and deep grooves and their sizes are typically 1 um or less. Even a sharply electropolished tip cannot probe the bottom surface of these grooves since the diameter of its tapered shank exceeds the groove width before the apex reaches the bottom. The STM images of such grooves, as a result, are heavily distorted and do not represent the true geometry of the grooves. A fine and straight tip is thus necessary for the application of STM for topographic measurements.
In order to overcome this difficulty, a new tip-preparation method has been developed which produces a new type of STM tip just suited for the investigation of deep structures.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989836
2. TIP PREPARATION
A fine microtip is obtained by virture of the electron-beam deposition inside the SEM. An electropolished metal tip is brought into the SEM with its axis parallel to the beam axis. When the
electron beam (beam voltage 30 kV, emission current 100 pA) is focused onto the top surface of the tip, the residual gas molecules are
dissociated and deposited onto the beam spot, resulting in a needle- like structure as illustrated in Fig 1.
The diameter of this microtip depends on the focusing, and submicron-sized microtips can be produced with careful focusing.
Figure 2 shows an example of the microtip grown on top of an electro- polished Pt-Ir tip. The shank diameter is approximately 0.1 pm and the tip length is 4 pm. A remarkable feature of this new tip is its straight shank : it is not tapered and its diameter does not change with tip.length. This microtip thus satisfies necessary conditions
for the STM tip to investigate deep holes and grooves.
It should be noted that the electron-beam deposition in SEM or TEM is not a new technique and has been sometimes employed for the contamination lithography 3 Also, a fine tungsten rod of 15 nm diameter has been fabricated by the deposition in TEM /4/. These are, however, mainly aimed at writing fine patterns on a substrate and not at preparing STM tips.
3 . TIP STRUCTURE AND CHEMICAL COMPOSITION
3-1. TEM investigations
Figure 3 shows a TEM micrograph of the microtip and a diffraction pattern from it. It is noted that the diffraction pattern only shows a halo pattern which clearly indicates that the microtip is amorphous.
Also the tip surface is seen to be smooth and free from irregular protrusions which tend to hinder stable STM imaging.
3-2. Auger measurements
As the size of the microtip is too small for usual Auger
measurement, the same electron-beam deposition was used to put a thick line of tip material on a Si substrate, and the chemical composition of this line was analyzed by Auger spectroscopy. The result is shown in Fig. 4. Apart from the Si peaks from the substrate, carbon and oxygen are detected. This result appears to be reasonable since the constituents of residual gases, which makes the microtip, should be mainly water vapour and hydrocarbons.
3 - 3 . Atom-probe analyses
FIM observations and atom-probe analyses of the microtip were carried out with the high performance ToF atom-probe in The Institute for Solid State Physics /5/. In these measurements, the microtip grown on sharply electropolished Pt-Rh tip was used as a sample.
Although the FIM and the atom-probe are thought to be the best tools for analyzing the microtip, the F1 image of the microtip could not be obtained possibly because of the low conductivity of the amorphous microtip material : the microtip perhaps breaks off before its F1 image can be observed. Therefore, as in the case of Auger measurements, atom-probe analyses were performed not on the microtip itself but on the microtip material uniformly deposited on the apex part of the Pt-Rh tip. Mass spectrum shown in Fig. 5 is the
atom-probe data taken in UHV and in the voltage pulse mode. Only the ions originated mainly from the deposited layer was used to make the mass histogram so that it shows only a small number of metal ions.
The.main mass peak is hydrogen, and fhere are also some peaks in the mass range 10-20 which can be identified as carbon and water. This atom-probe data is thus quite consistent with the Auger measurements and confirms that the microtip material consists of carbon, oxygen and hydrogen (which cannot be detected by Auger analysis).
4. STM EXPERIMENTS
In order to test the applicability of using the microtip as an STM tip, several STM experiments were carried out with microtips grown on Pt-Ir tips. The STM unit used in these experiments is of the conventional type, having a tubular piezo driver for tip scanning.
All measurements were made in air.
First of all, it should be noted that the microtip has sufficient conductivity to make STM measurements possible : the tip resistance may be higher than that of.conventiona1 metal tips but still lower than the tunneling resistance. Since the STM image may be obtained even without the microtip, one has to check sometimes the existence of the microtip after STM experiments. From the SEM observations of the tip before and after scanning, the microtip was found to survive unless the tip touches the sample surface. The microtip has thus enough toughness to repeat STM scanning. Also no effects of tip vibration was observed in STM experiments with the microtip
As described in section 2 , the microtip has a straight and submicron-diameter shank. In order to utilize this unique feature of the microtip, an array of rectangular grooves was monitored by an STM with and without a microtip. Figure 6(a) shows an STM image of a groove 0.8 pm wide and 0.4 pm deep, obtained with an electropolished Pt-Ir tip. This image shows a smooth corrugation and only small dips at the groove positions. Obviously, the tip does not reach the bottom of the groove due to its tapered shank and the image becomes completely different from the real surface geometry. On the other hand, when the same pattern is scanned with the microtip, the image shown in Fig. 6(b) is obtained. In this case, the bottom of the groove is imaged and the shape of the pattern is correctly reproduced.
Comparison of Figs. 6(a) and 6(b) clearly demonstrates the advantage of using the microtip in topographic measurements.
It should be added that the atom-resolved imaging of graphite can be achieved with the microtip. Therefore, the spatial resolution of this new STM tip is no less than that of the conventional tips used in high-resolution STM imaging.
5. SUMMARY
A new STM microtip has been prepared by utilizing the electron- beam deposition of residual gases in SEM. The chemical constituents of this microtip are carbon, oxygen, and hydrogen, and the tip structure was found to be amorphous. The microtip has a long and straight shank with submicron diameter and was found to be capable of obtaining the correct STM images of deep micropatterns which are inaccessible for previous STM tips.
ACKNOWLEDGMENTS
The authors would like to thank K.Sugihara, H.Okano, for helpful discussions. The authors also would like to thank 1.Higashikawa for his contribution in tip preparation and K.Taira for taking the data in Fig.6.
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I I I I I I
FIG. 1. Schematic of the microtip formation.
F I G . 2 . SEM micrograph of t h e micrbtip g r o w n o n a Pt-Ir t i p . T h e diameter a n d t h e length of t h a shank a r e 0.1 p m a n d 4 p m , respectively.
KINETIC ENERGY (eV)
FIG.3. TEM micrograph and t h e FIG.4. Auger spectrum from t h e dlffractlon pattern from microtlp materlal deposited
t h e mlcrotlp. o n a S1 wafer.
FIG.5. Atom-probe mass histogram of the microtip material deposited on a Pt-Rh tip.
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3FIG.6. STM image of the line-and-space pattern (0.8 p m wide and 0.4,pm deep) ( a ) obtained with a conventional STM tip and ( b ) obtained with a microtip.
