VoLUME 56, NUMBER

PHYSICAL REVIEW LETTERS

9

3 MAR. cH 1986

Atomic Force Microscope
G. Binnig"~

and C.

F. Quate'

Edward L Gi.nzton Laboratory, Stanford University,

'

Stanford, California 94305

and

Ch.

Gerber"

IBM San Jose Research Laboratory, San Jose, California 95193
I, Received 5 December
1985)
The scanning tunneling microscope is proposed as a method to measure forces as small as 10
N. As one application for this concept, we introduce a new type of microscope capable of investigating surfaces of insulators on an atomic scale. The atomic force microscope is a combination of the principles of the scanning tunneling microscope and the stylus profilometer. It incorporates a does not damage the surface. Our preliminary results in air demonstrate a lateral resoluprobe that 0
0
tion of 30 A and a vertical resolution less than 1 A.
PACS numbers:

68.35.Gy

%e are concerned

in this paper with the measurement of ultrasmall forces on particles as small as single atoms. %e propose to do this by monitoring the elastic deformation of various types of springs with the scanning tunneling microscope (STM). ' It has been a

common practice to use the displacement of springs as a measure of force, and previous methods have relied on electrostatic fields, magnetostatic fields, optical waves, and x rays. Jones~ has reviewed the devices that use variable capacitances and he reports that displacements of 10 4 A can be measured. SQUIDs3 are superconducting elements that measure the expulsion of magnetic fields in variable-inductance devices. They are used in gravity gradiometers to measure displacements of 10 6 A. Tabor and co-workers in their work with van der Waals forces have used optical interference methods to measure displacements of 1 A.
With an x-ray interferometer constructed from a single crystal of silicon, Deslattes' has also measured displacements of 10
A which is about 1'/0 of the nuclear diameter. We are proposing a new system wherein the STM is used to measure the motion of a cantilever beam with an ultrasmall mass. The force required to move this beam through measurable distances (10 A) can be as small as 10 ' N. The masses involved in the other techniques are too large to reach this value. This level of sensitivity clearly penetrates the regime of interatomic forces between single atoms and opens the door to a variety of applications. The atomic force microscope (AFM) is a new tool designed to exploit this level of sensitivity.
It will be used to investigate both conductors and insulators on an atomic scale. %e envision a general-purpose device that will measure any type of force; not only the interatomic forces, but electromagnetic forces as well.
%ith the STM, 6 the atomic surface structure of conductors is well resolved.
For bulk insulators7 an equivalent method is missing although the stylus profi-

lometer (SP)8 9 has been developed into a powerful microscopic technique. Teague et al. ' have used the
SP to record three-dimensional images of surfaces with a lateral resolution of 1000 A and a vertical resolution of 10 A. A related technique is the scanning capacitance microscope described by Matey and Blanc.
They report a lateral resolution of 5000 A and a vertical resolution of 2 A.
The SP has much in common with the STM. The tip in the STM and the stylus in the SP are both used to scan the surface, sense the variations of the sample, and generate three-dimensional images. The stylus in the profilometer is carried by a cantilever beam and it rides on the sample surface. This means that a rough surface can be plastically deformed.
The radius of this stylus is about 1 p, m, and the loading force extends from 10 to 10 N. '
The spring in the AFM is a critical component. %e need the maximum deflection for a given force. This requires a spring that is as soft as possible. At the same time a stiff spring with high resonant frequency is necessary in order to minimize the sensitivity to vibrational noise from the building near 100 Hz. The resonant frequency, fo, of the spring system is given where k is the spring conby f0= (I/2sr)(k/nto)', stant and ttto is the effective mass that loads the spring.
This relation suggests a simple way out of our dilemma. As we decrease k to soften the spring we must also decrease mo to keep the ratio k/mo large. The limiting case, illustrated in Fig. 1, is but a single atom adsorbed at site A in the gap of an STM. It has its own mass and an effective k that comes from the coupling to neighboring atoms.
The mass of the spring in manmade structures can be quite small but eventually microfabrication'4 will be employed to fabricate a spring with a mass less than
10 '0 kg and a resonant frequency greater than 2 kHz.
Displacements of 10
A can be measured with the
STM when the tunneling gap is modulated. The force

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PHYSICAL REVIEW LETTERS

VoLUME 56, NUMBER 9

3 M~RcH 1986

E3

SCANNERS,
F FEEDBACK

A

AFM

T ATOM

C

FE E DBACK

ST&

D

F

X

BLOCK (ALUMINUM)

Z

Q
Y

.25 mm

A: AFM SAMPLF

FIG. 1. Description of the principle operation of an STM as well as that of an AFM. The tip follows contour B, in one case to keep the tunneling current constant (STM) and in the other to maintain constant force between tip and sample
(AFM, sample, and tip either insulating or conducting).
The STM itself may probe forces when a periodic force on the adatom 3 varies its position in the gap and modulates the tunneling current in the STM. The force can come from an ac voltage on the tip, or from an externally applied magnetic field for adatoms with a magnetic moment.

required to produce these displacements is 2&& 10 ' N and this is reduced by 2 orders of magnitude when a cantilever with a 0 of 100 is driven at its resonant frequency.
AFM images are obtained by measurement of the force on a sharp tip (insulating or not) created by the proximity to the surface of the sample. This force is kept small and at a constant level with a feedback mechanism. %hen the tip is moved sideways it will follow the surface contours such as the trace 8 in Fig.

1.
The experimental setup is shown in Fig. 2. The cantilever with the attached stylus is sandwiched between the AFM sample and the tunneling tip. It is fixed to a small piezoelectric element called the modulating piezo which is used to drive the cantilever beam at its resonant frequency.
The STM tip is also mounted on a piezoelectric element and this serves to maintain the tunneling current at a constant level. The AFM sample is connected to a three-dimensional piezoelectric drive, i.e. , the x,y, z scanner. A feedback loop is used to keep the force acting on the stylus at a constant level. Viton spacers are used to damp the mechanical vibrations at high frequencies and to decouple the lever, the STM tip, and the AFM sample. The tip is brought in close proximity to the sample by mechanical squeezing of the Viton layers. High-frequency (
100 Hz) filtering of building vibrations is done as in the pocket-size STM' with a stack of metal plates separated by Viton.
We have operated the AFM in four different modes which relate to the connections of the two feedback circuits, one on the STM and the other on the tip. All four of these modes worked in principle. They each served to maintain a constant force, fo, between the sample and the diamond stylus while the stylus followed the contours of the surface.

)

B: AFM

DIAMOND

TIP

C: STM TII (Au)
LI:

MODULATING

TIP

.8 mm

CANTILEVER,
STM SAMPLE

E:

DIAMOND

F: VITON

PIEZO

LEVER

{Au- FOIL)~

FIG. 2. Experimental setup, The lever is not to scale in
(a). its dimensions are given in (h). The STM and AFM piezoelectric drives are facing each other, sandwiching diamond tip that is glued to the lever.

the

In the first mode we modulated the sample in the z direction at its resonant frequency (5.8 kHz). The the force between the sample and the diamond stylus deflects the levsmall force that we want to measure er holding the stylus. In turn, this modulates the tunneling current which is used to control the AFMfeedback circuit and maintain the force fo at a constant level. In the second and third modes, the lever carrying the diamond stylus is driven at its resonant frequency in the z direction with an amplitude of 0. 1 to 10 A. The force, fo, between sample and stylus changes the resonant frequency of the lever. This changes both the amplitude and phase of the ac modulation of the tunneling current. Either of these can be used as a signal to drive the feedback circuits.
In the fourth mode we used one feedback circuit. It was connected to the AFM and it was controlled by the tunneling current in the STM. This system maintained the tunneling gap at a constant level by changing the force on the stylus.
The fourth mode was further improved by reconnection of both feedback circuits in such a way that the
AFM sample and the STM tip were driven in opposite directions with a factor n less in amplitude for the
STM tip. The value of a ranged from 10 to 1000.
In contrast to previous methods, the absolute value of fo, the force on the stylus, was not well defined except at the beginning of the measurement. The deformation of the spring, 4z, is we11 calibrated at the starting point, but as the measurement proceeds each component of the system moves in an unknown way because of thermal drifts. These change the initial calibration. we know that the threeAdditionally, dimensional motion of the AFM sample must produce modest amounts of change in Az so as to compensate for the simultaneous motion of the stylus as it follows

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PHYSICAL R EVIEW LETTER

MBER 9
VOLUME 56, NUM

e,

m of the surface. . Therefore, even ln the in ifts the force fa will vary certaIn rang e that Is ep b thth'"u h
0f the surface and the a ue
The fourth mode p roved to e b 1e. %'e used it to re cord the resu its shown in FIgs. s an the topograp h y for two differen t
Al 03) surface. Th areas o e x axis are displaced from each other by m r' h 1o d therma I dif
'
'
'
he vertical dash ed lines of Fig.. 3 in d t th m th variation in the y direction o topog e s that can e these the noise amplitu de on the traces we c can estimate lvc a perio d i c corrugation below j, A when t e d 100 A. re inferith th th d a senc

t ese which might be related to wa water films coverlllg the surfaces o t h e AFM. Th fi s tructure on the rihto g onl be observed whe the app I'Ie d force exceeded the hen threshold.
For smaller forces the structure was t e sm te

MARCH

1986

%e suspect that this level is determined ev ed e su n netrate the film. n the force that Is necessary to pen the f'&rs t three methodds we use d s ma11 forces we 11

s m eared out. y below the threshold

an d there we lose the fine struc-

ture. rovements overr the handmade crease t e sed h ere should increase the resolution to t he e we will be a e atomic microfabrication techniques s'4 will a liow us to reduce ow . unl 't by several or ers of magnitu d e. %hen the instrue in an ultra Ig -v er w ere be well characterized we
'
now ll experience t a at least 2 orders o f m agnitude. With eas ' ' these optimum conditions t h e thermally induced vi rations er at room t e mperature will limit ill t e to 10
N. If h s cooled below 300 m we estimate that the lower limit mK a ion ~

~

"I

es interesting when we compare it w ' to t e in teratomic forces.
In th t ls w1 I the binding energy is rials that are h e ld together with the
If we arbitrarily equate wea t h ee nergy to a force acting throug h a
A we find that a binding energy o f 1 e V is equivalen in in f rces therefore
—
m 0 ' N for ionic bonds to 0 li

dt

M some o
The limiting

f

f

of06

tM

vit sensitivi y of our instrum

30
ZO

IQ

traces for another area of the ceramic ther le. The curves grouped under A were recorde d with ed *

-p

. 3. The AFM trace s on

a ceramic

(Alq03) sample.

The vertical scale tran a f tip of 10-" /A. For the l
3x10 8 N. The stabili tyo f the regulated orce
10
10
N. Th e successive traces are disp ace trac along the y axis.

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the lowest to the is near

ig

set

g

i

drift

duced below this level. The force fo was rese x10 Nfort e ra value near 5x

VOLUME 56, NUMBER

PHYSICAL REVIEW LETTERS

9

than these values. Therefore, we should be able to measure all of the important forces that exist between the sample and adatorns on the stylus.
Finally, we want to point out that these forces also exist in the tunneling microscope itself and that they can have a strong influence on the data collected with the STM.
The STM could be used as a force microscope in the mode described here by simply mounting the STM tip on a cantilever beam.
We are pleased to thank J. Pethica for his inspiring talk on the problems of the tip at the STM workshop in Oberlech, Austria, which although some months later probably triggered the idea for the AFM. We also want to express our appreciation to H. Rohrer and
D. Pohl for those stimulating discussions. This work was supported in part by a grant from the IBM Corporation and in part by the Defense Advanced

"'

—

—

Research Projects Agency.

'~Also at IBM San Jose Research Laboratory, San Jose,
Cal. 95193, and on leave from IBM Research Laboratory,
Zurich, Switzerland.
~~~Also at Xerox Palo Alto Research Center, Palo Alto,
Cal. 94304.
&'~on leave from IBM Research Laboratory,
Zurich,
Switzerland.
'G. Binnig and H. Rohrer, Sci. Am. 253, 50 (1985).
2R. V. Jones, Proc. IEEE 17, 1185 (1970).
3E. R. Mapoles, Development of Sauperconducting Gravity
Gradiomerer for a Test of the Inverse Square Lavv (University
Microfilm International, Ann Arbor, Michigan, 1981), p. 4;
J. Clark, Physics (Amsterdam) 126BAC, 441 (1984).
,

3 M&RCH 1986

4D. Tabor and R. H. S. Winterton, Proc. Roy. Soc. London, Ser. A 312, 435 (1979); J. N. Israelachvili and D. Tabor, in Progress in Surface and Membrane Sciencee, dited by
J. F. Danielli, M. D. Rosenberg, and D. A. Cadenhead
(Academic, New York, 1973), Vol. 7. sR. D. Deslattes, Appl. Phys. Lett. 15, 386 (1968).
66. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys.
Rev. Lett. 49, 57 (1982), and 50, 120 (1983).
7Thin insulating films can be studied with the STM as illustrated in G. Binnig, H. Fuchs, J. Kubler, F. Salvan, and
A. R. Williams, to be published.
SJ. B. P. W'illiamson, Proc. Inst. Mech. Eng. 182, 21

(1967).
P. G. %ierer, and J. M. Bennett. Appl.
(1984).
~OE. C. Teague, F. E. Scire, S. M. Backer, and S. %'. Jensen, Wear 83, 1 (1982); see also P. A. Engel and D. B.
Millis, Wear 75, 423 (1982).
"J.R. Matey and J. Blanc, J. Appl. Phys. 57, 1437 (1985).
'zE. J. Davis and K. J. Stout, Wear 83, 49 (1982).
' T. Vorburger, private communication.
'4K. E. Peterson, Proc. IEEE 70, 420 (1982).
~sB. H. Flowers and E. Mendoza, Properties of Mauer (Wiley, London, 1970), Chap. 3, pp. 22-55.
' H. Krupp, W. Schnabel, and G. Walter, J. Colloid Interface Sci. 39, 421 (1972).
'7J. H. Coombs and J. B. Pethica, IBM J. Res. Dev. (to be
9K. H. Guenther,

Optics 23, 3820

published). These authors point out that the forces between the sample and the tip in the STM, especially when the tip presses against an oxide layer, can be strong enough to alter the calibration of the z piezo,
E, C. Teague, Room Temperature Gold- Vacuum-Gold TunMicrofilms International,
(University
neling Experiments
Ann Arbor, Michigan, 1978), Chap. 4, pp. 141-148, where he discussed the van der Waals forces of attraction between two gold spheres used in vacuum tunneling.

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