Probe for an atomic force microscope

#This application is a U.S. national phase application of PCT/GB2004/003065, filed Jul. 15, 2004, which claims the benefit of United Kingdom Application Nos. GB 0316577, filed Jul. 15, 2003, GB 0323229, filed Oct. 3, 2003, and GB 0408234, filed Apr. 13, 2004, the entire contents of which are incorporated herein by reference.

#This invention relates to the field of atomic force microscopes, to the probes employed in such microscopes and to a method of operating such microscopes. In particular, it relates to an atomic force microscope that does not employ conventional feedback control of probe height.

#The atomic force microscope (AFM), or scanning force microscope (SFM), was invented in 1986 by Binnig, Quate and Gerber. Like all other scanning probe microscopes, the AFM is based on the principle of mechanically scanning a nanometric probe over a sample surface in order to acquire an “interaction map” of the sample. The interaction force in this case is simply the molecular interaction between the sample and the tip of a sharp probe attached to a cantilever spring. When the probe tip is brought into close proximity with the sample, the cantilever bends in response to the interaction force. Images are collected by scanning the sample relative to the probe and measuring the deflection of the cantilever as a function of lateral position. An optical lever technique is usually used to measure this bending. Since the cantilever obeys Hooke's Law for small displacements, the interaction force between the tip and the sample can be deduced.

#Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings.

#FIG. 1 is a diagrammatic illustration of the forces involved as a cantilevered probe makes contact with a sample surface in a prior art atomic force microscope.

#FIG. 2 shows a schematic implementation of an atomic force microscope that includes a probe, which is in accordance with a first embodiment of the present invention.

#With reference to FIG. 2 there is shown a schematic implementation of an AFM, indicated generally by 10, that utilises a first embodiment of a probe constructed in accordance with an aspect of the present invention. The AFM apparatus 10 shown comprises a plate 12 adapted to receive a sample 14, and which is mounted on one prong of a tuning fork 16. The tuning fork 16 is connected to a piezoelectric transducer 18 and a coarse driving means 20. The piezoelectric transducer 18 is used to drive the sample 14 (together with the plate 12 and fork 16) in three dimensions: x, y and z directions. As is conventional in the field, the z axis of a Cartesian coordinate system will be taken to be that perpendicular to a plane occupied by the sample 14. That is, the interaction force is dependent both on the xy position of a probe 22 over the sample 14 (the pixel it is imaging), and also on its height above it. A tuning fork control (not shown) is arranged to apply a sinusoidal voltage to the tuning fork 16 and so to excite a resonant or near-resonant vibration within the xy plane. Optionally, the plate 12 and tuning fork 16 may be supported on a vibration isolating mount 32 so as to isolate the vibration of the tuning fork 16 from the remainder of the microscope. However, at the image frequencies contemplated with a microscope employing this probe, external noise is less of a problem than for lower image frequencies and so the vibration isolating mount may be dispensed with. The probe 22 is a low-mass AFM probe and, during a scan, an interaction force is developed between the probe tip 22a and the sample surface. A probe detection mechanism 28 is arranged to measure the displacement of the probe tip 22a or the bending of the beam 22b supporting the tip, which is indicative of interaction force strength. Data collected by the probe detection mechanism 28 is analysed and output to a display 30.

#In general, prior art cantilever probes are fabricated from silicon or silicon nitride, which allows them to be produced readily using mature silicon microfabrication technology. Unlike prior art cantilever probes however, the probe 22 according to this invention has a polymer coating 22c applied to the supporting beam 22b of the probe. This coating 22c, as will be explained in more detail later, serves to dissipate energy that would otherwise be mechanically stored in the probe through the excitation of oscillatory modes and thereby lowers the Q factor of the supporting beam for one or more of its vibrating modes in comparison to the same beam without the presence of the coating 22c.

#In taking images using the apparatus 10, the sample 14 is first brought into contact with the probe 22 using the coarse driving means 20. Fine height and initial start position adjustments are made with the piezo driver 18 whilst the probe detection mechanism 28 measures the bending of the probe as a result of the probe 22—sample 14 interaction force. Once the measured bending reaches a desired level, the sample surface is scanned beneath the probe 22. In scanning the sample 14 under the probe 22, the tuning fork 16 is set to vibrate into and out of the plane of the Figure (y axis). This oscillates the stage on which the sample is mounted. At the same time, the piezo 18 translates the sample 14 in a perpendicular (x) direction. Sample oscillation is with a relatively large amplitude, of the order of a few microns. During the course of a scan, readings are continually taken by the probe detection mechanism 28, which, as is standard in the art, may be based on an optical lever technique: probe bend is measured using laser light reflected from the probe. The output signal from the probe detection mechanism 28 is fed directly to a processor and display 30.