STM combined with SEM without SEM capability limitations

Ultramicroscopy 42-44 (1992) 1558-1563 North-Holland

STM combined with SEM without SEM capability limitations A l e x a n d e r O. G o l u b o k a n d V l a d i m i r A . T i m o f e e v Institute for Analytical Instrumentation, Russian Academy of Sciences, 26 Ogorodnikova, St. Petersburg 198103, Russia

Received 12 August 1991

An advanced originally designed STM unit combined with a high-resolution SEM to obtain simultaneous STM and SEM images has been developed. High-quality STM parameters have been achieved as a result of the compact size of the unit mounted on the SEM goniometer without any additional vibration isolation. The basis of the STM unit is a 3-coordinate piezowalker which provides coarse and fine tip-to-specimen approach and lateral specimen movement in a 1.5 x 1.5 x 2.5 mm3 range at any SEM goniometer position. The shortest specimen-objective lens distance is 4 ram. Some results of simultaneous studies obtained with a Hitachi S-2500 SEM are presented.

1. Introduction A scanning tunneling microscope (STM) combined with a scanning electron microscope (SEM) is of great interest, for. several reasons. O n the one hand such a combination expands the range of surface investigations. O n the other hand such a combination has provision for both S E M and STM contrast-mechanism studies. Besides the local tunneling spectroscopy mode, the S T M offers many additional possibilities for the investigation of the local electron properties of the surface. We believe such a hybrid microscope may be very useful for n a n o t e c h n o l o g y applications as a result of the possibility of specimen surface modification by the tunneling tip at the n a n o m e t e r scale. Beginning with G e r b e r et al. [1], m a n y groups worked in the field of S E M - S T M combination [2-8]. T h e aim of this work is the further develo p m e n t of this S T M application. W e tried to design a c o m p a c t S T M unit which could be combined with every S E M without any modifications. T h e r e f o r e we did not use any mechanical drivers and inputs into the S E M ' s v a c u u m chamber. O u r main aim was to a c c o m m o d a t e the S T M on the S E M goniometer instead of the specimen holder, without limitations for any provided specimen transference and any additional isolation against

vibration. We tried to minimize the distance between the specimen and the S E M ' s pole piece. T h e last task was to provide the specimen movement relative to the tunneling tip for tunneling current capturing and choosing the scanning area without any additional mechanical drivers, as has been said above.

2. The S E M - S T M design Fig. 1 schematically shows the S T M unit design. T h e unit is placed on the S E M ' s goniometer (1) and may be transferred relative to the electron b e a m axis as a usual S E M specimen holder (X, Y, Z, R and T axes). The singularity of the unit is the tunneling tip (2) which is directed towards the inside of the piezoscanner (3). It permits us to reduce the distance between the pole piece (4) and the specimen (5). T h e main part of the tunneling unit is the 3-coordinate inertial piezodriver m o u n t e d inside the scanner with the STM's specimen (5) on it. T h e operation of the inertial piezodriver is based on the slipping of the carriers (6) (during Z m o v e m e n t ) and (7) (during X and Y movement) relative to the travelling supports (8) and (9), respectively. T h e support (8) is m o u n t e d on the piezotube (10). By

0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

A.O. Golubok, V.A. Timofeev /STMcombined with SEM

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Fig. 1. Sketch of the STM unit. (1) SEM goniometer, (2) tunneling tip, (3) piezoscanner, (4) objective lens, (5) specimen, (6) Z-axis slipping carrier, (7) X-Y slipping carrier, (8) Z-axis travelling support, (9) X - Y travelling support, (10) piezotube, (11) rotation excluder, (12) spring.

applying the controlling voltage to the different electrodes of the tube, the slipping carriers (6) and (7) can move along the X, Y and Z axes. For example, we shall briefly describe the process of up movement along the Z axis. In the first phase of the step the piezotube (10) is activated by a sharp electrical impulse to provide its fast shortening. This process leads to slipping of carrier (6) relative to travelling support (8). During the second phase the applied voltage slowly decreases down to the zero level and the piezotube (10) returns to the initial state together with support (8). The accelerations during this phase are small and the slipping of carrier (6) does not occur. As a result, the slipping carrier (6) is placed higher relative to the initial state Z 0 by a step A Z. Now, we described the process of the Y+ movement. In the first phase of the step the piezotube (10) is activated by sharp electrical impulse to provide its fast bending in the Y- direction. This process leads to slipping of carrier (7) relative to travel-

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ling support (9). During the second phase the applied voltage slowly decreases and the piezotube (10) returns to the initial state together with the support (9). Slipping of the carrier (7) does not occur. As a result, the slipping carrier (7) with the specimen (5) on it moves relative to the initial state by a step A y in the Y+ direction. The movement along Y-, X + and X - directions occurs in a similar way. A special mechanism (11) to exclude rotation of carrier (7) in the X - Y plane is also provided. For upwards X movement while the STM unit is tilted around the Y axis the slipping carrier (7) is pulled to the travelling support (9) by spring (12). The step value along all axes is regulated by controlling the voltage pulses. There are two modes for operating the piezodriver: fast and slow. In the fast mode the steps follow one by one at a frequency of 50 Hz. This mode is used for long specimen (more than 100/~m) displacements far away from the tip. In the slow mode the piezoscanner approaches the tip towards the specimen surface, checking after each step for tunneling current. If the current is absent the next step occurs after the piezoscanner has returned to the initial state. If a tunneling current is present the next step does not occur. The step frequency is 8 Hz in this mode of operation. The slow mode is used for approach of the tip to the specimen when the specimen is near the tip. It should be noted that our unit is symmetrical relative to the Z axis for reducing thermal drift along the X and Y axes. The piezotube (10) and the piezoscanner (3) were made of equal materials and have equal length and side thicknesses. This leads to a reduction of drift along the Z axis. In spite of its small dimensions the coarse approach range along the Z axis is 2.5 mm. The displacement of the specimen along the X and Y axes for selecting the scanned area is 1.5 x 1.5 m m 2. The X axis movement can operate at a maximum specimen tilt value along the Y axis of about 20 °. The largest specimen is a disk that is 14 m m in diameter. The piezoscanner is easy to change. The scanned area varies from 3 x 3 izm 2 up to 5 x 5 /~m 2 for different scanners. The Z displacement range is 1-1.5 /~m. The resonant frequency of the tip-to-specimen assembly is 5.6

A.O. Golubok, V.A. Timofeet: /STMcombined with SEM

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Fig. 2. Piezodriver step value diagrams. (a) Z-movement step histogram, (b) X-movement step histogram.

kHz. The shortest distance between specimen and pole piece is 4 mm.

3. Results and discussion

The most important and complicated system of STM is the the approach of the tip to the sample that should allow tunneling current capturing. It is known that for reliable current capturing without micro contacts during approach it is necessary to have the specimen displacement step size along the Z axis several times smaller than the piezoscanner displacement range. For our coarse approach system we have measured single steps (histogram fig. 2a). The step values were measured by STM using the equation AZ=Zn+

I -Zn,

where Z , is the value of piezoscanner displacement at the tunneling current capturing point ( I t = 1 nA, Ut = 0.5 V) after n steps of the coarse approach system. Fig. 2a shows that the average step size at a control pulse of 90 is AZ = 38 nm and the greatest step value is AZma× = 53.8 rim. That is still 30 times smaller than the value of tip Z displacement and warrants capturing of the tunneling current without microcontacts. Fig. 2b shows the histogram of the X-step value distribution for the horizontal unit position at a control pulse height of 200 V. Similar results were ob-

Fig. 3. Images of a diffraction grid on a GaAs surface, covered by an Au film. (a) SEM image, the tunneling tip is visible; (b) equal square STM image; (c) high-magnification STM image.

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Fig. 5 (a) An SEM image of the STM tip. (b) An STM image of the tunneling tip.

Fig. 4. Images of a mosquito eye covered by Au film. (a) SEM overview; (b) SEM high-magnification image, the tunneling tip is visible; (c) high-resolution STM image.

tained for Y movement. In the case of X - Y specimen movement for SEM stage tilts around the Y axis the asymmetry of step values due to gravitation acting appears. This asymmetry may be eliminated by changing the control pulse height. Thus using this 3-dimensional piezodriver we have an ability to investigate in SEM and STM modes every point of specimen inside a range of 1.5 X 1.5 x 2.5 mm 3 simultaneously. Fig. 3 shows the simultaneously obtained SEM (fig. 3a) and STM (figs. 3b and 3c) images of a diffraction grid manufactured on the GaAs sur-

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A.O. Golubok, V.A. Timofeev / STM combined with SEM

face and covered by an Au film. It is important to note that STM controlling impulses do not influence the SEM operation due to sufficient screening. Fig. 3c shows the high-resolution STM image. The fine structure of the surface is well observed. This structure seems to appear during GaAs monocrystal surface etching. It is necessary to note that the 2 nm high steps which are well distinguished on the STM image are poorly observed on the SEM image. Fig. 4 introduces the simultaneously obtained SEM (figs. 4a and 4b) and STM (fig. 4c) images of a mosquito's eye. Fig. 4 demonstrates the benefit of a large SEM scanning area and the STM's high resolution capability combined in one device. To demonstrate the efficacy of our 3-coordinate piezodriver, we have observed by S E M - S T M the tip which was normally used in STM. The observed tip was mounted on the specimen holder by its apex towards the STM tip. Fig. 5 introduces the SEM (fig. 5a) and STM (fig. 5b) images of the tip. On this STM image the fine structure of the tip is also well observed contrary to the SEM image. Insofar as the tip size is approximately about 2 / x m we think that fig. 5 is a good demonstration of the piezodriver capabilities. For checking the scanning tunneling spectroscopy mode, I - V curves were recorded from the diffraction grid. Typical curves are represented in fig. 6. In this mode the feedback loop breaks up for an interval At = 50 ms in definite points of the scanning area and the tunneling voltage changes from the starting up to the finishing level. For the curves in fig. 6 the starting voltage is - 8 0 0 mV and the finishing one is +800 inV. These curves confirm the sufficient vibration stability of the tunneling unit. Curve 6a was obtained while the electron b e a m of the SEM fell on the specimen and Curve 6b while it was switched off. The distinct zero shift of curve 6a is caused by the influence of the electron beam. It is clear that this zero shift may be simply compensated if required. The main problem of a hybrid S T M - S E M mentioned by all authors [2-8] is the hydrocarbon film deposition on the surface of investigated specimens under the influence of the electron

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beam. To reduce this effect one should decrease the beam current and the time of SEM-mode operation and increase the tunneling voltage. The level of hydrocarbon impurities in the Hitachi S-2500 chamber permits simultaneous S E M - S T M observation of the specimen surface over several minutes. This time depends strongly on the surface type and state. This effect can limit detailed investigations. For minimization of contamination effects one should operate under oil-free vacuum conditions or in an ultrahigh vacuum as was done in ref. [1]. All parts of the unit shown in fig. 1 are made of nonmagnetic stainless hard steel, except for the piezotubes (3) and (10) and the tip holder, which is made of titanium. Thus our STM is completely ultrahigh-vacuum-compatible and may be combined with U H V SEM's.

4. Conclusion

A compact combined S E M - S T M device with a s p e c i m e n - p o l e piece distance of 4 m m is described. The STM may be accommodated instead of a SEM specimen holder on the SEM's goniometer, and displaced, tilted and rotated. The

A.O. Golubok, V..A. Timofeev / STM combined with SEM

b u i l t - i n 3 - c o o r d i n a t e inertial piezodriver does n o t r e q u i r e any a d d i t i o n a l m e c h a n i c a l i n p u t s in the S E M ' s c h a m b e r . T h e suggested u n i t is c o m p a t i b l e with nearly every c o m m e r c i a l SEM. T h e results of s i m u l t a n e o u s observations by S E M a n d S T M of the same s p e c i m e n regions d e m o n s t r a t e the wide capabilities of this hybrid microscope.

References [1] Gh. Gerber, G. Binnig, H. Fuchs et al., Rev. Sci. Instr. 57 (1986) 221.

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[2] M. Anders, M. Miick and C. Heiden, Ultramicroscopy 25 (1988) 123. [3] T. Ichinokawa, Y. Miyazaki and Y. Koga, Ultramicroscopy 23 (1987) 115. [4] R. Wiesendanger, D. Anselmetti, L. Eng et al., Ultramicroscopy 25 (1988) 129. [5] L. Vazquez, A. Bartolome, R. Garcia et al., Rev. Sci. Instr. 59 (1988) 1286. [6] J.M. G6mez-Rodriguez, L. Vfizquez, A. Bartolome et al., Ultramicroscopy 30 (1989) 355. [7] M. Kuwabara, W. Lo, J.C.H. Spence, J. Vac. Sci. Technol. A 7 (1989) 2745. [8] A.P. Volodin, G.A. Stepanjan, M.S. Khaikin et al., Prib. Tekh. Eksp. 5 (1989) 185.