AU3076692A - Piezoelectric bimorph cantilevers for surface analysis instruments - Google Patents

Description

TITLE: "PIEZOELECTRIC BIMORPH CANTILEVERS FOR SURFACE ANALYSIS INSTRUMENTS"

Technical Field

This invention concerns apparatus for investigating surfaces. More particularly, it concerns sensors for use in instruments such as atomic force microscopes and surface force apparatus. The sensors incorporate piezoelectric bimorphs.

Background to the Invention An instrument known as "surface force apparatus" was developed by J.N. Israelachvili more than a decade ago. His surface force apparatus is described in the paper by J.N. Israelachvili and G.E. Adams which was published in the Journal of the Chemical Society, Faraday Transactions I. volume 74, page 975, 1978. Another type of surface force apparatus was developed in the Research School of Physical Sciences (as it then was) at The Australian National University, Canberra, Australia. That instrument has been marketed by Anutech Pty Limited since 1982. A new form of the surface force apparatus is described in the paper by J.L. Parker, H.K. Christenson and B.W. Ninham which was published in the Review of Scientific Instruments, volume 60, page 3135, 1989. However, despite the increased versatility and performance of the newer instruments, the basic force measurement procedure has changed little since its inception. Precise measurements of the force between two curved surfaces (usually arcuate with a radius of 2 cm, with convex surfaces adjacent to each other) and the separation of the surfaces, is measured. The force is determined from the deflection of a spring (typically having a spring constant, k, of lOONm^"1) on which one of the surfaces is mounted. Force resolutions of 10'⁷N are readily obtained with this technique. The separation between the surfaces is measured using an optical interferometry technique known as FECO ("fringes of equal chromatic order"), with a resolution of about 0.2 nanometre. A limitation on this technique is that the surfaces employed in the apparatus must be thin (typically from 2 to 4 micrometres thick) and optically transparent.

Atomic force microscopes operate using similar principles, but instead of having two curved macroscopic surfaces, one of the surfaces is usually a planar macroscopic surface and the other is replaced by an atomically sharp tip which is mounted (with the tip pointing to and closely spaced from the macroscopic surface) at the end of a weak spring. The spring typically has a spring constant, k, of 1 N "¹, but with modern etched silicon nitride cantilevers, spring constants as low as 0.1 Nm"¹ can be obtained. The spring deflection (due to changes in atomic forces) is measured using a laser beam reflected from the back of the tip mounting, and is held constant by controlling the distance between the tip and the macroscopic surface (usually by step pulses from a feedback loop, applied to a DC motor or to a piezoelectric device) while the surface is subjected to a planar raster scan before the tip. A three- dimensional image of the surface is then mapped out.

The force resolution of the atomic force microscope depends upon the type of spring on which the atomically sharp tip is mounted. Using the etched silicon nitride springs mentioned above, detection of the deflection of the spring to 0.1 nm gives a force resolution of 10"^1:lN.

One of the problems associated with the use of the surface force apparatus and the atomic force microscope is that only relative forces can be measured, for although the small springs upon which one surface (in the case of the surface force apparatus) or the atomically sharp tip (in the atomic force microscope) is mounted are notionally identical, they almost invariably have different spring constants (even when etched from silicon nitride). Calibration of the instrument, therefore, is required each time a new spring is used. In addition, a small, but potentially significant, departure from a true reading is obtained each time the spring-mounted surface or tip moves through an arc due to flexure of the spring.

Disclosure of the Present Invention

It is an object of the present invention to provide an improved spring arrangement for use in equipment such as a surface force apparatus or an atomic force microscope, which enables (with supporting electronics) force and displacements to be measured directly, and which enables surfaces of optically opaque materials to be investigated. A subsidiary object of the present invention is to provide a spring arrangement which ensures that a spring-mounted surface or tip retains its orientation relative to a second surface when the spring is deflected. These objectives are achieved by using, for the conventional spring in such equipment, a piezoelectric bimorph, which is two elongate slabs of a piezoelectric material, fastened together with their polarisation directions facing each other. The fastening together is effected at two locations. One of these locations is at the end of the bimorph sandwich, which is designed to be clamped within the instrument. The other fastening together is at the location on the bimorph at which the surface or tip is to be mounted, which may be at the other end of the bimorph but which may be a variable location, in which case the clamping arrangement will be a slidable clamp.

Since a piezoelectric bimorph is electrically equivalent to a capacitor, the charge on which varies with the dimensions of the device, and any deflection of the bimorph will cause a charge to be developed on the sandwich, then observation of the charge on the bimorph will enable the force applied to it, to cause its deflection, to be calculated.

Thus, according to the present invention, there is provided a cantilever arrangement for use in instruments such as surface force apparatus and atomic force microscopes, said cantilever arrangement comprising a piezoelectric bimorph consisting of two elongate slabs of a piezoelectric material, said piezoelectric material having a polarisation direction, said slabs being mounted with their polarisation directions facing each other; said slabs being clamped at one end of said bimorph; a further clamp being applied to said bimorph at a location remote from said one end; said further clamp being adapted to support a surface or atomically sharp tip or the like; said bimorph having connection points thereon for monitoring equipment adapted to monitor any electrical charge developed on the bimorph.

The present invention also encompasses instruments which include such a cantilever arrangement.

Embodiments of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings Figure 1 shows, schematically, a cantilevered piezoelectric bimorph with a charge monitoring arrangement connected to the bimorph.

Figure 2 illustrates the cantilever arrangement of the present invention, installed in an assembly for mounting in a surface force apparatus.

Figure 3 is a partly schematic, partly block diagram illustration of a surface force apparatus incorporating the present invention.

Detailed Description of Illustrated Embodiments A piezoelectric bimorph constructed to replace conventional springs in surface analysis instruments and used as a sensor, is the essential feature of the present invention. As noted above, a piezoelectric bimorph consists of two slabs of piezo material fastened together with their polarisation directions facing each other. If one end of this piezoelectric bimorph sandwich (shown diagrammatically in Figure 1) is deflected by applying a force to it, then a compressive strain is produced in one slab and an expansion is produced in the other. Specifically, for a piezoelectric bimorph of length L and thickness T and charge constant d₃₁ the charge Q developed for a given applied force F can be calculated from the relation:

^ 2 T*^™1

It is well known that piezoelectric devices, when used as actuators, can show a large amount of hysteresis and creep in their motion. The same may be expected for a piezo bimorph when used as a displacement sensor. However there is one essential difference between the control and measurement of displacement with these devices that allows exploitation. In one case the potential across the device is controlled and in the other a charge is measured. Piezoelectric devices are electrically equivalent to a capacitor and so the charge (Q) is a product of capacitance (C) and voltage (V) (Q=CV). But the dimensions of the device also depend on V and so a change in V produces a change in the capacitance, which in turn changes Q. Displacement is linear with changing charge. By contrast, when the voltage is controlled, hysteresis results. For a sensor, the charge Q is the quantity being measured and the output is linear with deflection. Accurate measurement of charge is difficult. In order to read the charge developed on the bimorph, a resistor is placed in parallel with the piezo and when a charge is produced a current flows (see Figure 1). The charge on the bimorph will flow to earth through the resistor and as a result the bimorph becomes discharged. The bimorph used has a capacitance of InF and with a normal voltmeter (resistance 100 MΩ) the bimorph discharges exponentially with a time constant (τ) of 0.1s. Electrometer technology has been employed to increase the resistance of the voltmeter from 10¹⁰ to 10¹² ohms to give discharge decay times up to 1000 sees.

If the voltage drop across the resistor is υ, y the deflection of the bimorph and α its proportionality constant then the voltage is described by the differential equation.

dt τ dt

the solution of which is

where

V(t)= Y.

V(t) is directly proportional to the deflection of the spring. The force can be measured by continuous solution of the equation for V(t), with a limit in accuracy determined soley by the accuracy with which the integral can be performed and the accuracy of τ . Since the surface investigation equipment referred to above is frequently used with the surfaces immersed in a liquid, the bimorph will usually be protected by a sheath. The sheath is preferably of "TEFLON" (trade mark) material.

Figure 2 shows a peizoelectric bimorph mounted in an assembly for inclusion in the Mark IV surface force apparatus produced by Anutech Pty Limited. The bimorph 10 consists of two slabs of "VERNITRON' (trade mark) piezoelectric material, each 25.1 mm long, 3 mm wide, and 0.15 mm thick. The bimorph 10 is mounted within a TEFLON sheath 11. One end of the bimorph is clamped by a clamp 12. The other end of the bimorph is clamped with a moveable slider 14 which allows the sensitivity and the spring constant of the bimorph to be varied at will.

One of the surfaces 20 being investigated is supported on a surface holder adaptor 15, which is connected rigidly to the moveable slider 14.

The electrical connection of the displacement transducer (see Figure 3) to the bimorph is via a connector from which wires enter the shaft 13 through an upper opening 13A. Those wires pass down through the shaft 13, through the inner passage 13B in the centre of the shaft, and are connected to the bimorph. Thus both the bimorph and the wiring are completely isolated from the solution which bathes the surfaces being investigated.

Figure 3 illustrates the essential components of the Mark IV surface force equipment marketed by Anutech Pty Limited. Since persons of skill in this art will be familiar with that equipment or its predecessor models (the original model being designed by Dr Israelachvili of The Australian National University), no further explanation of the operation of that equipment - or of the atomic surface microscope which functions in a similar manner - is necessary. However, those persons who require further information are referred to the paper by Dr Parker entitled "A Novel Method for Measuring the Force between Two Surfaces in a Surface Force Apparatus", which was published in Langmuir, volume 8, page 551, 1992, and which is essentially the same disclosure as the specification of Australian provisional patent application No PK 9797, filed on 26 November 1991. The contents of that paper by J L Parker and the specification of Australian patent application No PK 9797 are incorporated into the present specification by this reference thereto.

Industrial applicability

Surface force measuring and surface mapping instruments are becoming increasingly used in well-equipped industrial laboratories. Applications of this technology, and thus of the present invention, include the following (this list is not intended to be exhaustive):

. Mining and mineral processing (for example, in coal/mineral slurry separation); . Surface coating techniques (for example, paints, thin films, polymers, resins); . Agricultural chemicals (pesticide, herbicide and fertilizers applications); . Reproduction chemicals i.e. (used in photographic films, photocopies/laser printer toners); . Adhesives (in glues, tapes);

. Lubricants/Bearings (particularly oils, oil additives and substitutes); Semiconductor properties (in deposition layers);

Paper making technology; and

Computer and compact disc development.

Claims (7)

1. A cantilever arrangement for use in instruments such as surface force apparatus and atomic force microscopes, said cantilever arrangement comprising a piezoelectric bimorph consisting of two elongate slabs of a piezoelectric material, said piezoelectric material having a polarisation direction, said slabs being mounted with their polarisation directions facing each other; said slabs being clamped at one end of said bimorph; a further clamp being applied to said bimorph at a location remote from said one end; said further clamp being adapted to support a surface or atomically sharp tip or the like; said bimorph having connection points thereon for electrical connection to monitoring equipment adapted to monitor any electrical charge developed on the bimorph.

2. A cantilever arrangement as defined in claim 1, in which said bimorph is enclosed within a protective sheath.

3. A cantilever arrangement as defined in claim 2, in which said sheath is made from TEFLON (trade mark) material.

4. A cantilever arrangement as defined in claim 1, claim 2 or claim 3, in which said further clamp comprises a moveable slider, the position of said moveable slider on said bimorph being variable, to vary the sensitivity and the spring constant of said bimorph.

5. A cantilever arrangement as defined in claim 4, in which a surface holder adaptor or an atomically sharp tip holder adaptor is connected rigidly to said moveable slider.

6. A cantilever arrangement as defined in claim 4, mounted in an assembly for inclusion in a surface force apparatus or in an atomic force microscope.

7. A surface force apparatus or an atomic force microscope including a cantilever arrangement as defined in any preceding claim, or as hereinbefore described with reference to the accompanying drawings, or as described in the paper published in Langmuir, volume 8, pages 551 to 556, 1992.