GB2238161A - Attractive atomic force microscope - Google Patents

Abstract

In an Atomic Force Microscope in which a cantilevered member 12, Fig. 2b, is used to investigate the profile of a sample surface by interacting with the interatomic forces of the sample surface, the motion of the atomic tip of member 12 is monitored by a scanning tunneling microforce tip 10 which is mounted between the cantilevered member 12 and the sample. By this arrangement it is possible to resolve the attractive portion of the inter-atomic force between the specimen and the atomic tip. Fig. 7 shows an embodiment in which both the atomic tip 12 and the tunneling tip 10 are of triangular configuration. <IMAGE>

Description

A?TRACTTVE ATOMIC FORCE MICROSCOPY This invention relates to the atomic force microscopy, in particular to a variant of atomic force microscopy that will allow the attractive portion of the inter-atomic potential to be measured.

BACKGROUND Atomic force microscopy (AFM) is a well known method of mapping non-conducting surfaces at sub-atomic resolution, described by Binning in US patent 4,724,318 and by Binning, Quate and Gerber (Physical Review Letters, 56, 930-933 (1986)). It consists of measuring the deflection of a lever with an atomically sharp tip as that tip interacts with the interatomic forces of a surface. The forces involved are small (10 exp -13 to 10 exp -8 Newtons), and the minute deflection (of the order of Angstroms) is measured using a tunneling tip. A tunneling tip is a well known part of a scanning tunneling microscope (STM), as described by Binning et al in US patent 4,343,993, and is capable of measuring sub-atomic distances.

The plot of the Lennard-Jones potential energy verses interatomic separation for Ar-Ar interaction in figure 1, shows both a repulsive 2 and an attractive 1 region.

The AFMs have all been used in the repulsive mode - i.e.

they have been operated with the atomic tip being repelled by the sample, as in region 2. With the conventional AFM arrangement, in which the tunneling tip is placed in tandem, behind the atomic force tip as shown in figure 2a, one can only measure the repulsive section of the interatomic force. In order to observe the attractive section of the interatomic force, an alternative arrangement of the tunneling tip is required. It must be on the specimen side of the atomic force tip, as shown in figure 2b, The advantages of this invention of an AFM capable of being operated in the attractive mode, include a smaller specimen tip force, a greater specimen to tip working distance and the fact that the tip is less likely to move the specimen it is observing.

When the tip starts to drag a specimen along, any adhesion by the specimen to the substrate will result in a decreased specimen-tip interaction, and a freeing of the tip. (In the repulsive mode, any specimen adherence results in an initially increased specimen-tip interaction, making specimen motion relative to the supporting substrate more likely). This reduced force on the specimen could be of importance in imaging large or biological molecules, especially those which only attach to the substrate with weak Van der Waal's forces or hydrogen bonds.

The reason the conventional AFM is not capable of resolving the attractive potential can be seen by considering figures 3 and 4. Figure 3 shows the tunneling current as a function of the separation between the tunneling tip and the atomic force lever (d). This is given by the well known relationship: I = C exp(-sqrt(d)) --(1) where I is the tunneling current, C and are material dependant constants and d is the separation between the atomic tip and the specimen surface. A plot of the tunneling current verses the separation between the atomic force lever 12 and the specimen 14 is shown in figure 4, for the case of the the conventional arrangement of figure 2a.It can be seen that when the AFM is arranged in this tandem configuration, the separation, d, must be kept as large as possible when the specimen is being approached in order to minimize the possibility of a collision between tunneling tip 10 and the atomic force lever 12. This is done by keeping the separation d to that which will just detect tunneling, ie about 7 Angstroms.

As can be seen from figure 4, when the initial attractive force is encountered 1, a very small decrease in the current, 3, occurs because of the flat nature of the response curve at this point, 6.

This small decrease will be difficult to detect in the presence of noise. When the repulsive force is encountered 2, the current rises 4 , and as it rises moves into a steeper part of the response curve 5 a large current is quickly observed.

When the AFM is set up in the manner shown in figure 2b the current verses approach distance, s, is as shown in figure 5.

The current now shows a shape rise, 7, as the initial attractive force, 1, is felt, before dropping, 8, when the repulsive force, 2, is encountered. This increase, 7, should be readily detectable even in the presence of thermal noise and will enable the user to control the tip on this attractive edge of the inter-atomic potential.

An embodiment of the invention will now be described with reference to the accompanying diagrams in which: Figure 2b is a schematic representation of the arrangement of the tunneling and atomic force tips; Figure 6 is an elevation of the tip regions; Figure 7 is a plan view of the tip regions; and Figure 8 is a schematic view of the attractive atomic force microscope.

The microscope consists of an atomic force detecting, atomically sharp tip 12, which is the end of a thin triangular lever 15. The lever 15 and tip 12 are made by etching down from a supporting silicon piece 18. Typical dimensions for the lever 15 would be 100 microns long, and 3 microns thick. The supporting silicon wafer 18 is adhesively attached to an invar block 20 which is in turn attached to a cylindrical piezoelectric tube 22. The atomically sharp tunneling tip 10 is supported on an Invar block 10, which is triangular in section in both sagital and meridian section, for greater stability. This support block 10 is attached to the Invar support 20 via a second piezoelectric tube 24. The function of the piezoelectric tube 24 is to control the tunneling tip 12 with respect to the conducting gold electrode 30 coated on the underside of the silicon lever 15. The function of the piezoelectric tube 22 is to control the distance between the surface being studied 14 and the atomic force sensing tip 12.

Materials other than silicon, Invar and gold can obviously be used, where there thermal and conduction properties allow.

The piezoelectric tube 22 is attached to an Invar or steel housing 24, which rests on some suitable precision relocation bearings 26, attached to the specimen housing. The microscope could be inverted from the view shown in figure 8, so that the specimen block was easily removable. The entire device is controlled by feedback and display electronics 28, and rests on a suitable anti-vibration mounting 32. This anti-vibration mounting 32 may consist of alternating layers of steel and viton rings.

Claims (5)

1. ATTRACTIVE ATOMIC FORCE MICROSCOPE

   CLAIMS 1. An Atomic Force Microscope adapted so as to be capable of resolving the attractive region of the interatomic force between the atomic tip and the sample, by having a tunnelling tip monitor the motion of the lever holding the atomic tip from the sample side.
2. 2. An Atomic Force Microscope adapted as in claim 1, in which the tunneling tip is attached to or is part of an invar or other low thermal expansion coefficient material block having triangular cross section in both sagital and meridionai direction for increased stability.
3. 3. An Atomic Force Microscope adapted as in claims 1 or 2, in which the atomic force tip and the monitoring tunneling tip have separate piezo motion control blocks.
4. 4. An Atomic Force Microscope adapted as in claim 1, 2 or 3, in which a conducting layer is applied to the specimen side of the atomic force tip to facilitate tunneling tip monitoring of the atomic tip.
5. 5. A attractive atomic force microscope substantially as described herein with reference to Figures 1-8 of the accompanying drawing.