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Ultramicroscopy 60 (1995) 11-16

Ultramicroscopy

Letter

Wear of the atomic force microscope tip under light load, studied by atomic force microscopy Andrew

Khurshudov,

Koji Kato

Laboratory of Tribology, Faculty of Engineering, Tohoku University, Sendai 980-77, Japan

Received 10 February 1995; in final form 16 May 1995

Abstract The purpose of this paper was to study the wear process of the non-conductive microfabricated silicon nitride (Si,N,) AFM tip which occurs during apparently wearless light-loaded scanning of the smooth silicon surface. AFM tip shape and wear were imaged using an AFM tip of the same size and geometry. Study included the observation of initial and worn surfaces, detecting of probing tip destruction and worn area size calculations. Scanning of the flat silicon surface with relatively low nominal load of 10 nN resulted in wear of the tip without significant drop in the quality of AFM image, possibly, because of formation of rough surface with asperities, acting as a probing tip. Wear mechanisms of the tip were analyzed. Low-cycle fatigue process, resulting in surface fracture, was found the most probable wear mechanism of the SisN, AFM tip.

1. Introduction

During topography measurements, an AFM tip moves across the scanned area and repeatedly contacts the same surface regions without apparent damage of its atomic structure. Images which apparently resolve atoms can be recorded at loads of lop7 N in spite of the fact that this load exceeds the strength of chemical bonds and should break it [l]. There are some hypotheses explaining this fact [2,3], but there is no data about changes (wear> on the probing tip surface. The purpose of this paper is to study the wear process and mechanisms of the non-conductive microfabricated S&N, AFM tip which occur during apparently wearless light-loaded scanning of the smooth silicon surface. 0304-3991/95/$09.50 0 1995 Elsevier SSDI 0304-3991(9.5)00071-2

Science

There is a number of papers dealing with indirect [4,51 and direct [6,7] measurement of the STM tip shape by means of the STM itself. Until recently, no attempts have been made on imaging of the non-conductive AFM tip. Study of the diamond tips of relatively large apparent radius (about 2 pm> was done in Ref. [8] using different tungsten and diamond AFM tips. Results showed a strong effect of tip convolutions since both the sample and the tip had approximately the same shape and size. Progressive effect of the wear of the relatively soft tungsten tip on the image quality was found, but possibility to measure AFM tip shape by this method was shown. Possibility to image the tip using microspheres has been shown in Ref. [9]. Authors of the papers [lO,ll] have shown that the AFM tip can be clearly imaged

B.V. All rights reserved

A. Khur-shudou, K. Knto / Ultmnicroscopy 60 (1995) II -16

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using needle-like samples. This paper presents data about the direct observation of non-conductive microfabricated AFM tip shape and wear using a tip of the same shape and size.

2. Experimental

details

Microfabricated Si,N, cantilevers used in this study had (according to the AFM supplier) pyramidal shape tips (Fig. 1) with a radius of lo-20 nm [12]. Atomic force microscope SP13700 series (Seiko Instruments Inc., Japan) used in this work has a four-quadrant photodetector to measure the laser beam deflection during the deformation of the V-type golden-coated cantilever. Cantilevers were 200 pm long, 400 nm thick and spring constant was 0.02 N/m. Cantilevers made an angle of - 13” to the horizontal. Firstly, a force curve was obtained as the dependence of the sample-tip interaction force on the separation [131. An AFM image (512 x 512 pixels) of the selected tip was obtained using the same type of tip under constant force mode. Normal load was 0.087 nN, scanning frequency was 0.5-1.0 Hz. After initial imaging, the tested tip was subjected to wearing. It was set into an AFM tip holder and used for topography measurements of the (111) silicon wafer surface under the con-

stant force mode. Silicon wafer was cleaned in ultrasonic bath by acetone. Peak-to-valley roughness of the silicon surface was about 1 nm, R, = 0.1 nm. Normal force selected for this test was 10 nN, scanning frequency was 1 Hz. Scanned area was of 5 X 5 pm2 size, one scanning cycle included 512 scanning lines, total number of scanning cycles was 5, the total sliding distance was 25.6 mm. During this test we controlled the obtained image to find some fast shape changing of the tip (fracture, for instance) by sharp changing of the picture contrast, but did not observe it. Comparison of the pictures after first and fifth cycles did not show noticeable difference. After completing this scanning, the tested tip was observed again by the AFM tip (the same as before) with the same imaging parameters.

3. Results Maximum adhesive force (pull-off force), obtained from the force curve (Fig. 21, was equal to N 18.5 nN with a small scatter. Attractive force is typically much smaller than adhesive [13] and was equal to N 2 nN only. Fig. 3 presents the typical images of the tested tip before (a) and after (b) wearing. Reciprocal sliding direction during wear test is from left to

Apex angle = 700 Tip radius = 1O-20 nm

Golden coating

3 pm

II

I

/

a I

+

Silicon nitride t-l 4pm

-@It+ 400 nm

Fig. 1. Design of the microfabricated silicon nitride tip.

4 pm

A. Khurshudou, K. Kate / Ultramicroscopy 60 (1995) 11-16

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right on the picture. In the center of Fig. 3a the image of a sharpened part of probing tip can be seen. The fact that this observation was possible may be explained by small distance between scanning lines equal to - 3 nm, much smaller than the tip diameter (20-40 nm). Fig. 3b presents image of the tip after wearing, made with the same probing tip as in the case of Fig. 3a. Comparison of Fig. 3a and Fig. 3b shows some flat worn area on the top of the tip after scanning compared to initial relatively perfect sharp pyramid. In the case of unworn tip, crossing edges of the pyramid can be reliably seen. After wear, the whole central part of the tip was destroyed.

4. Discussion Fig. 2. The force curve for the interaction of Si,N, AFM tip and silicon wafer surface. Five measurements at different places are shown.

Fig. 3. Images of the tested tip before (a) and after (b) wearing direction during wear test is from left to right on the pictures.

On the basis of the presented data, tip wear can be confirmed. But in order to understand its

(2

X 2

pm’,

1 Hz, 512

X

512 pixels, 0.087 nN). Reciprocal

sliding

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A. Khurshrtdou, K. Knto / Ultramicroscopy 60 (1995) II -16

mechanism, contact conditions and the worn surface should be analyzed. 4.1. Contact condition According to Maugis’ approach [14] to DMT (Derjaguin-Muller-Toporov) theory of elastic contact with adhesion [15], the adhesive force in the interface is equal to the pull-off force. Ac-

Fig. 4. Cross-sections

cordi~gly, we may assume. an additional normal load of the order of 18.5 nN even for almost zero nominal load of 0.087 nN during topographical scanning. This additional load is twice bigger than the force provided by cantilever bending. Maximum Hertzian contact pressure for the sphere/plane contact is Qm,, = [6PE * ‘/ Q-~R~]‘.~~,where P is total normal load 1161.For R=lO nm, E*=83 GPa, P=lO nN we have

of the initial (a) and worn (b)

tip.

A. Khurshudou,

K. &to / Uffrnmicroscopy

Q max= 5.1 GPa, for P = 28.5 nN (including adhesion) we have Q,,, = 7.2 GPa. If R = 20 nm, maximum contact pressure will be about 3.2-4.6 GPa. Critical value of maximum Hertzian contact pressure at which plastic flow will start is Q,,,, = 0.6H, where H is the material hardness 1161.For silicon (H= 8.5 GPa [171), Q,,,, =: 5.1 GPa, for silicon nitride (H = 20 GPa [lb]) Q,,,.+ = 12.0 GPa. On the basis of this data we can conclude that the silicon wafer surface was deformed elastoplastically, while the silicon nitride tip was deformed elastically only. We did not observe silicon surface wear or deformation; possibly, because the scale of these processes was of the same order as the silicon surface roughness. 4.2. Worn su@zce analysis Fig. 4 presents the cross-sections of the initial (a) and worn (b) tip. The pyramid sides slope was found equal to N 50-52” (apex angle of 76-SO?. Central part of the worn tip contains many peaks, maximum surface roughness of the worn area is about - 200 nm - much higher than the maximum roughness ‘of the unworn surface. This high measured roughness may be partly’ a result of the tip-tip convolution, but the fact that it is much bigger than that for the initial surface seems to be correct. The center line of the worn surface is not horizontal, and makes an angle of 14-25” to the horizontal. Most likely, this is the effect of the initial angle between cantilever and silicon wafer (- 13”) plus some possible additional cantilever bending. The radius of the initial contact area between tip and silicon wafer was calculated to be l-l.7 nm, depending on normal load. Analysis of Fig. 3b allowed calculation of the square of the worn tip surface equal to 0.11 pm2 and to evaluate average contact pressure: Q = 0.09-0.25 MPa, depending on load. This pressure is rather small, but, according to Fig. 4b, real contact occurs between the silicon surface and sharp asperities, formed by the wear process on the tip surface. The radii of these asperities seems to be comparable with that for the initial peak and are of the order of ten nanometers. It means a presence of

60 (1995) I1 -16

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high contact stress during the time of scanning. The presence ‘of these sharp asperities on the worn surface may explain the fact that no drop in imaging quality was observed during scanning one or some of these asperities acted as a probing tip. Observation of the worn surface (see Fig. 3b) shows plastic smearing in the direction from left to right (direction of reciprocal sliding). According to contact conditions analysis we should not expect plastic deformation of silicon nitride. Even if it occurs, this type of deformation must produce a relatively smooth surface, not as rough as we have. Possibly, this surface pattern directed from left to right, is an image of wear particles of both silicon and Si,N,, adhered to the tip surface because of, for example, electrostatic attraction and oriented from left to right during the tip imaging after wearing. 4.3. Wear mechanism We may rule out possible abrasive wear mechanism of the tip because silicon nitride is much harder than silicon. In general, two mechanisms may produce this kind of rough surface: adhesion and fatigue: Adhesive wear is usually accompanied by the relatively weaker material transfer - material whose cohesive strength is smaller than. the strength of interfacial junction. Plastic or elastic/plastic contact conditions with friction may destroy the surface layers (oxides, absorbates, etc.) and activate strong adhesion. In our case, silicon may be deformed plastically, and be directly transferred onto the tip surface, because of silicon’s lower strength than silicon nitride. On the other hand, the tip is also subjected to high normal and tangential two-directional stresses below the plastic limit. Low-cycle fatigue process may result in defects accumulation in silicon nitride with cracking to follow. It will also produce a rough surface. Both wear mechanisms are possible in this case; but, taking into account the fact that our tests were carried out in air and only the silicon surface was cleaned before test, we may suggest some contaminants layer in the interface of the

tip and plane. Continuous contamination of the surface during test should significantly reduce the effect of adhesion. The most probable mechanism is therefore low-cycle fatigue of the surface of silicon nitride tip resulted in material fracture and rough surface formation. Adhesion could contribute to the fracture process producing tensile stresses during sliding. Concerning the particles observed on the tip surface (Fig. 3b), they are, most likely, a mixture of Si,N, and SiO, - wear products of both tip and surface layer of the silicon.

5. Conclusions ~icromachined non-conductive Al?M tip shape and wear were imaged using the AFM tip of the same size and geometry. The suggested method allowed observation of initial and worn surfaces, detecting probing tip destruction and worn area size and roughness calculations. (ii) Scanning of the flat silicon surface with relatively low nominal load of 10 nN resulted in wear of the tip without noticeable drop in the quality of AFM image, possibly, because of formation of rough surface with asperities, acting as a probing tip. (iii) Wear mechanisms of the tip were analyzed. Low-cycle fatigue process, resulting in surface fracture, was found the most probable wear mechanic of the S&N, AFM tip.

Referenees 111 G.M. McClelland and J.N. Glosli, Friction at the atomic scale, in: Fundamentals of Friction: Macroscopic and Microscopic Processes, Eds. I.L. Singer and H.M. Pollock (Khrwer, Dordrecht, 1992) p. 405. 121J.B. Pethica, Phys. Rev. Lett. 57 (1986) 3235. [3] H.J. Mamin, E. Ganz, D.W. Abraham, R.E. Thomson and J. Clarke, Phys. Rev. B 34 (1986) 9015. 141 G.F.A. van de WaIIe, H. van Kempen and P. Wyder, Surf. Sci. 167 (1986) L219. [5] G. Reiss, J. Vancea, H. Wittmann, J. Zweck and H. Hoffmann, J. Appl. Phys. 67 (1990) 1156. 161 R. Garcia Cantu and M.A. Huerta Garnica, J. Phys. 50 (Suppi. 11) (1989) C8-235. [7] R. Molier, A. Essfinger and M. Rauscher, J. Vat. Sci. Technol. A 8 (1990) 434. 181 L. Hellemans, K. Waeyaert, F. Hennau, L. Stockman, I. Heyvaert and C. van Haesendonck, J. Vat. Sci. Technol. B 9 (1991) 1309. 191 P. Mark&via and MC. Goh, Langmuir 10 (1994) 5. [IO] L. Monteiius and J.O. Tegenfeldt, Appl. Phys. Lett. 62 (1993) 2628. [l l] L. Montelius, J.O. Tegenfeldt and P. van Heeren, J. Vat. Sci. Technol. B 12 (1994) 2222. [12] E. Meyer and H. Heinzelmann, Scanning force microscopy @FM), in: Scanning Tunnelling Microscopy II, Eds. R. Wiesendanger and H.-J. Giintherodt, Vol. 28 of Springer Series in Surface Sciences (Springer, Berlin, 1992) p. 99. [13] N.A. Burnham, D.D. Domingues, R.L. Mowery and R.J. Colton, Phys. Rev. Lett. 64 (1990) 1931. [14] J. Maugis, J. Colloid Interf. Sci. 150 (1992) 243. [15] B.V. Detjaguin, V.N. Muller and Y.P. Toporov, J. Colloid Interf. Sci. 53 (1975) 314. [16] B. Bhushan, Tribology and Mechanics of Magnetic Storage Devices (Springer, New York, 1990). [17] WA. Glaeser, Materials for Tribdogy, Vol. 20 of Tribology Series (Elsevier, Amsterdam, 1992) p. 133. 1183B. Bhushan and B.K. Gupta, Handbook of Tribology ~M~raw-HiII, New York, 1991) p_ 4.54.