Ultramicroscopy 48 (1993) 315-320

North-Holland

Scanning tunneling microscope for ultra-high vacuum with a high-precision coarse positioning

J. MCndez, J. G6mez-Herrero, Huang Wen Hao ’ and A.M. Bat-6 Departamento de Fisica de la Materia Condensada, C-III, Unicersidad Auto’noma de Madrid, E-28049 Madrid, Spain

Received 26 August 1992

We have designed and built a new type of scanning tunneling microscope for UHV operation, which uses a separable

inchworm approach mechanism, so it removes the effects of inchworm piezos during tunneling. We have adopted V-shape

guides in order to precisely define the coarse movement. The repeatability of the approach is demonstrated. Atomic

resolution is also obtained in both air and UHV ambiance. We also show its potential application as a nanofabrication

device. The microscope includes cleaning, sharpening and characterization of tips.

1. Introduction

Scanning tunneling microscopy (STM) is be- coming a regular technique in a large number of laboratories. In particular, up to now there are more than ten ultrahigh-vacuum STM’s commer- cially available. Nevertheless, adapting these in- struments for particular purposes is not easy. As a consequence many groups have developed their own “home-made” devices [1,2]. In this paper we present a STM design which works in a standard UHV Varian chamber with the dedicated aim of fabrication, manipulation and characterization of microstructures. Therefore, in addition to the re- quirements of reliability and mechanical stability we have carefully looked for a precise control of (i) the tip-sample distance and relative horizon- tal position, and (ii) the tip size and shape. The features of the instrument are: The coarse ap- proach is made by means of an inchworm, in order that this function is reliable and precise. The inchworm pushes a carriage which contains the sample holder. Once the tunneling position is reached, the carriage and consequently the sam-

’ Permanent address: University of Science and Technology of China, Hefei, People’s Republic of China.

ple can be uncoupled from the inchworm. The movement of the carriage is guided by V-shape trails in order to have high precision. The tip can be heated at high temperature while a high volt- age is applied, in order to clean and remake it in case of strong damage. With this instrument we currently obtain atomic-resolution images in air and in UHV. We have also verified the high accuracy of the coarse movement which allows us to recover the sample image position after succes- sive approaches.

2. Instrument design

Since UHV involves long bake-up treatment and many possible sources of troubles we have kept in mind the necessity of a reliable and dependable device. Our experience in STM has shown us that one of the main sources of trouble in a microscope is the coarse positioner, which moves the relative position of tip versus sample in steps exceeding the range of the piezodrive (typi- cally a fraction of a micrometer). Taking this fact into account we have chosen a commercial inch- worm (UHV 1OOC) [3] that moves linearly in steps of nanometers in a range of centimeters; how-

0304.3991/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

Fig. I. Schematics of the microscope. (a) The inchworm pushes the carriage and brings the sample holder close to the tip, Vibration

isolation is provided by a combination of springs and plates. (h) The use of gauge blocks and V-shape guides provides precise

movement.

ever, we have adapted the piezo-step motor in a different manner from that usually used (fig. la). Instead of mounting the sample directly on the inchworm, it has been placed on top of a mobile

carriage. At the back of this carriage an engage- ment system has been attached. This system per- mits us not only to pull or push the carriage, but also to leave the carriage disengaged from the

Fig, 2. Repeatability of the coarse-positioning system. Two successive images of silicon showing the same carbon contamination and

a mark produced by a current pulse. In addition to a slight shift the images show the distortion caused by the pieroelectric creep. The importance of these images is that the second one has been taken just after withdrawing the tip 1 cm from the sample. The

error in the angle in this process is below 0.008”.

J. M&dez et al. / STM for UHV with high-precision coarse positioning 317

inchworm shaft. So, when the experiment is car- ried on, it remains in a stand-alone position in- creasing stiffness and reducing thermal drift. In addition the sample is not perturbed by the high voltages needed to drive the inchworm, since all voltages are connected to ground. To ensure a high accuracy in the carriage movement a guide mechanism has been designed. It consists of a groove made with two very well defined cylinders and a flat surface taken from a gauge block. Three sapphire balls have been glued underneath the mobile; one of the balls slides on the gauge block while the two others move along the groove (fig. lb). This is important in order to be sure that the mobile makes a translation movement with no rotation. This gives a high accuracy to the coarse movement, as we show in fig. 2. The two images have been subsequently taken after with- drawing the sample 1 cm from tunneling position.

HIGH VOLTAGE WIRE

lmm TUNGSTEN WIRE

The displacement between the two images is less than 240 nm. This means an angle error below 0.008”. The sample holder is just placed on top of the carriage. Two lateral levers ended by sap- phire balls and attached to the carriage have been adapted in order to ensure a reliable sam- ple-holder position as well as a stiff tunnel gap. The sample holder is made of a 20 mm tantalum cylindrical tube, so that the sample can be heated at high temperature.

The tip holder, made of titanium, is attached to the piezoelectric tube through an insulator Vespel piece [l] and a grounded ring. The piezo- tube allows us to scan the tip on a 1.5 pm x 1.5 pm area. The calibration of the tube has been made first by using a highly oriented pyrolytic graphite in air and with a silicon (111) surface when the microscope is in UHV. Electrical con- nections to all the piezo sectors have been done

A- METALLIC PLATE

HOLDER

CURRENT WIRE

BELLOWS-

TRANSFT1oN ROTATION

MANIPULATOR

WOBBLE STICK i//

L8" FLANGE

BELLOWS

Fig. 3. Schematics of the tip-treatment device. The tip is grounded through the wobble stick. By means of a rotational manipulator, a 1 mm thick tungsten wire makes contact with the tip. Then a current flows and heats the tip up to 1300 K. By applying a high

voltage to a metallic plate in front of the tip during heating, tip sharpening occurs,

by a 0.006 inch diameter Kapton-coated wire soldered to each electrode [4]. Since current is very sensitive to environment noise, a 1 mm coax- ial cable has been used [51. To avoid vibrations due to the bigger mass of the cable, this has been fixed to the stainless-steel plate, using again a 0.006 inch wire for the last 1 cm. The wire has been welded to the piezo using a commercial UHV-compatible alloy.

In general, the state of the tip is one of the most important conditions to obtain atomic reso- lution. Although it is possible by electrochemical etching to obtain sharp W tips with reasonable control and reproducibility, it is more operative, as experience shows, to have several tips avail- able. So, our system has a carrousel which allows us to interchange four different tips. It is at- tached to the back part of the microscope, just opposite to the scanning tube. The replacement system uses a wobble-stick to transfer tips be- tween the back part and the piezo. Nevertheless, it is even better to have the possibility to clean and characterize the tips in UHV. For that pur- pose, we have mounted a device which allows us to heat the tip up to 2000 K, by mechanical and electrical contact to a W wire (fig. 3). In addition, a high voltage can bc applied between the tip and the metal plate. This system allows us to charac- terize the tip state by measuring Fowler- Nordheim plots [6].

The microscope is mounted on a standard 8 inch Conflat flange. To avoid vibrations as much as possible a 3-stage system has been adopted. The first 2 stages are inside the chamber, and they consist of 4 soft springs that support 3 stain- less-steel plates separated by rubber pieces [7]. While the spring provides a low-frequency cut-off ( - 3 Hz) the rubber provides some dump and a frequency of - 50 Hz. The third stage consists of a bundle of three 0.75 bungee cords. During normal operation the chamber remains on top of a hydraulic jack but for tunnel operation the jack is lowered down and the chamber is then hung by the cords. The final cut-off frequency is 1 Hz, improving the stability of the microscope.

The chamber has a special sample park. This park consists of two stainless-steel V-blocks. Usu- ally one of the sample holders is in the manipula-

tor while the other remains on one of the V- blocks, so exchanging samples is an easy and reliable operation.

Samples can be placed and removed from the microscope by means of a standard manipulator providing X-Y-Z translation, 360” rotation and tilt capability. At the end of the manipulator a cylinder has been added. To remove the sample holder from the microscope, this tube is intro- duced into the inner hole of the sample holder and then by actuating with the tilt mechanism a lever is lowered down against the sample holder. For sample heating an electron bombardment heating filament has been attached inside the

cylinder. For 1 mm thick Si samples the heating of the sample is reasonably uniform.

3. Electronics

A NanoScope II electronics controls all the piezo movements of the STM tip. But in order to drive the inchworm a parallel computer is used. To perform this task, the tunnel current at the output of the preamplifier is split into two differ- ent paths. one going to the NanoScope electron- ics and the other to an A/D converter in the PC. This computer provides. via a DAC, a step-ramp checking the tunnel current at every single step. Since inchworm steps can be as low as I nm, it is possible to reach the tunneling position with that high precision.

4. Experimental results

In this section we describe the performance of our instrument by showing some representative images.

Fig. 4 is a constant-height image showing atomic resolution on a highly oriented pyrolytic graphite sample. As has been pointed out, this sample is used to calibrate the X-Y piezo move- ments.

Fig. 5 is a topographic image of the Sit 1 I I )7 x 7 reconstruction. This image was obtained with a pressure of 4 x lo- “’ mbar. In order to cool down the sample as soon as possible, the sample

J. M&dez et al. / STM for UHV with high-precision coarse positioning 319

Fig. 4. Highly oriented pyrolytic graphite. Constant-height

image taken at 0.27 V and with a scanning speed of 625 rim/s..

holder was placed on a cold finger just after annealing at 1200°C. The atomic corrugation is

found to be - 0.1 nm, in good agreement with

other authors [8]. Finally, we show in fig. 6 the application of the

microscope for surface modification: fig. 6a shows a protrusion made on the silicon surface in UHV

Fig. 5. Silicon (ill), 7X7 reconstruction. The bias was 1.7 V

sample positive and the speed was 350 rim/s..

by increasing the current up to 1 nA while the tip was 200 nm away from the surface and the bias voltage was 100 V; and fig. 6b is a square mark on the silicon surface made during scanning.

In summary, we have built a STM working in UHV as a tool for precise nanofabrication. The

Fig. 6. Surface modification. (a) Protrusion created on the silicon surface by increasing the current up to 1 nA at 200 nm away from

surface and with bias of 100 V. (b) Square mark on silicon done when scanning 200 nm X200 nm, at low speed, due to the contamination of the surface.

320 J. M&de2 et al. / STM for UHV with high-precision coarse positioning

coarse-approach positioning device is by means of References

an inchworm which pushes a carriage containing the sample holder sliding on a V-shape guide. The microscope includes tip-treatment possibili- ties. The repeatability of the positioning and the atomic resolution of the images are demon- strated, as well as the fabrication of nanostruc- tures.

Acknowledgement

Financial support through CICYT project PB89-0167 is gratefully acknowledged.

111

121

[31

141

D.M. Zeglinski, D.F. Ogletree, T.P. Beebe, R.Q. Hwang,

G.A. Somojai and M.B. Salmeron, Rev. Sci. Instr. 61

(1990) 3769.

J.E. Demuth, R.J. Hamers, R.M. Tromp and M.E.

Welland, IBM J. Res. Develop. 30 (1986) 396.

Model UHV 1OOC; Burleigh Instruments Inc., Burleigh

Park, Fishers, NY 14453, USA.

California Fine Wire Co., Grover City, CA, USA.

EutecRod 157; Eutectic Co., Flushing, NY, USA.

J. Mendez, M. Luna and A.M. Barb, Surf. Sci. 226 (1992)

294.

Ch. Gerber, G. Binnig, H. Fuchs, 0. Marti and H. Rohrer,

Rev. Sci. Instr. 57 (1986) 221.

R.S. Becker, J.A. Golovchenko, E.G. McRae and R.S.

Swartzentruber, Phys. Rev. Lett. 55 (1985) 2028.