Friction, lubrication, adhesion, and fracture processes are controlled by the mechanical response of two contact surface materials. In contrast to conventional tribology where frictional interaction of solid surfaces is dominated by the properties of bulk materials, under a light load and small masses, the physical and chemical properties of the surfaces are important in microtribology and nanotribology. It has been reported that the friction and wear behaviour of ceramics and ceramic coatings are influenced by the environment in which components operate. There is thus great interest in being able to measure directly the elastic and inelastic responses of material on this scale.

Thin film lubrication has been widely used in precision translations, high density memory drives and integrated silicon micromechanisms to improve performances through the reduction of friction and wear at the contact surfaces. In recent years the electrodeposition

of conductive polymers onto well defined surfaces has attracted more attention in microengineering and nanotechnology. A distinctive advantage of this technique is that the deposition process is well-controlled and works on irregular surfaces. The choice of monomer, counter-ion, solvent, and growth potential determines the film morphology and can lead to smooth, fibrillar or spherical microstructures which may enable the optimisation of a polymer coating to a particular tribological requirement. However, it has been found that films produced under a nominal same condition gives variable performance in friction which adds complexity in data interpretation and the control of the film production.

Experimental evidence shows that the film morphology such formed varies in the molecule size and shape and the distribution which may affect its mechanical and tribological properties. Therefore, to characterise such a surface, it is necessary to correlate the three aspects of surface: morphology, mechanical response and frictional behaviour by direct measurements on the same surface point.

In addition to the materials interest, scanning probe microscopes also rely on the mechanical response of small areas of surfaces, particularly contact mode AFMs (atomic force microscopes). The elastic deformation of the tip-sample system during AFM measurements falsifies the topography and causes incorrect values for interaction force as well as the loss of true atomic resolution.

Scanning probe microscopy (SPM) is a global term for a multitude of microscopy modes, all based on the same principle. Scanning microscopes image by"touch".

Description of the Prior Art Scanning microscopy uses a cantilever arm mounted to the tip of which is a triangular probe whose apex is only tens of nanometres wide. A laser beam is used to monitor movement of the end of the cantilever arm as the probe is moved across a surface being imaged. The movement of the end of the cantilever arm is translated by a computer into three-dimensional information.

Scanning probe microscopy (SPM) such as scanning tunnelling microscope (STM) and atomic force microscope (AFM) is a global term for imaging a surface via a sharp tip to reveal surface structure down to atomic levels. Since their inventions, variants of STM/AFM have been developed to extend the measurement of surface topography to other surface related phenomena. Attempts have been made successfully by using an AFM or its equivalent force probe microscopes to monitor surface hardness or elastic modulus, or interfacial force, or frictional force together with the surface topography, but none of them were able to measure the above three functions together. Most of AFMs and the variants to date only control and monitor displacements and forces are inferred from a nominal known spring constant of a cantilever. This can be a problem when the tip-sample interaction is significant, thus the contact stiffness dominates the spring behaviour of the cantilever which leads to the inferred force value worthless. Modifications have been made by some researchers by introducing force instead of displacement into the feedback control loop of AFM, but their performances are limited due to the fact that these modifications are based on the commercial AFMs which were designed primarily for surface imaging. The main limitation of these commercial instruments is the limit range for force application, which will not cope with the increasing demand for normal load up to mN region to cover a wide range of materials for investigation of friction, indentation, wear and fatigue.

Brief Summary of the Invention The present invention seeks to provide an improved scanning probe microscope.

Accordingly, the present invention provides a scanning probe microscope for measuring the hardness of a surface of a sample object at a location on said surface, comprising: a stage means for mounting the object; a probe mounted relative to said stage means; first drive means for moving the probe and the stage means relative to one another along a

first, z axis and in an x-y plane orthogonal to said first, z axis; wherein the probe has: a body; stylus means for contacting the object surface; mounting means resiliently mounting the stylus to the body so as to allow movement of the stylus relative to the body along the z axis; actuator means for moving the stylus means relative to the body along the z axis towards the stage means; and position monitoring means for monitoring the position of the stylus relative to the body along the z axis and providing a position signal representative thereof; and wherein the microscope further comprises: and a control circuit including a feedback circuit connecting the first drive means and the position monitoring means and operable for controlling the first drive means to bring the stylus and the probe into a preselected relationship along the z axis in response to initial contact of the stylus with the object surface; and said control circuit further comprises second drive means for actuating said actuator means to apply a preselected force to the stylus means subsequent to said initial contact, and signal means for receiving said position signal and providing a hardness signal representative of the local hardness of the object surface at the point of contact of the stylus with the surface.

The present invention also provides a method of measuring the local hardness of an object

surface using a scanning probe microscope having a probe comprising: a body; stylus means for contacting the object surface; mounting means resiliently mounting the stylus to the body so as to allow movement of the stylus relative to the body along the z axis; actuator means for moving the stylus means relative to the body along the z axis towards the stage means; and position monitoring means for monitoring the position of the stylus relative to the body along the z axis and providing a position signal representative thereof ; wherein the method comprises the steps of : a) monitoring the position of the stylus relative to the probe body in the z axis; b) moving the probe towards the surface along the z axis until a change in the position of the stylus relative to the body indicates contact of the stylus at a selected point on the surface; c) moving the probe along the z axis relative to the stylus whilst maintaining contact of the stylus with the object surface until a preselected relationship is established between the positions of the stylus and the body along the z axis to set a datum position for the stylus at said selected contact point; d) increasing by a preselected amount the force applied to the stylus along the z axis to move the stylus into the object surface; e) monitoring the movement of the stylus in the z direction relative to the datum and

generating a position signal in dependence thereon; f) and processing said position signal to provide a hardness signal representative of the hardness of the object surface at said selected surface contact point.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevation of a probe of a preferred embodiment of nanotribological probe microscope (NPM) according to the present invention; Figure 2 is a sectional plan view along the line 2-2 of Figure 1; Figure 3 is a block circuit diagram of the preferred form of NPM according to the present invention; Figure 4 is a flow chart illustrating the operational steps of the microscope of Figure 3.

Referring firstly to Figures 1 and 2 a probe 10 of a preferred form of nanotribological probe microscope (NPM) has a body 11 and a diamond stylus 12 (Berkovich tip) mounted on a silica rod 14 which is a few millimetres long. The rod is fixed to the centre of a thin cross- shaped cantilever or beam 16 so as to be suspended by the beam 16 which is secured to the body at each of its four ends 16a, b, c, d and acts as a flexible spring. This allows vertical movement of the stylus 12, i. e. along the axis of the rod 14 and also allows some torsional movement of the beam 16. The stylus is thus compliantly coupled to the body. Secured to the beam 16, on the opposite side to the stylus 12, is a permanent magnet 18. This is surrounded by a coil 20. The coil 20 and magnet 18 form a force actuator 32 which enables the application of a force to the stylus 12 by passing current through the coil 20. The magnitude of the force applied to a sample surface 34 (Figure 3) via the stylus can be adjusted by controlling the current in the coil.

Four electrodes 22 are positioned each below a respective arm of the beam 16. These electrodes are conveniently deposited on a top mounting plate 24 which has an etched depth of about 50 am for the four electrodes 22. The cross-beam 16 is made from 25 um or 50 um thick foil of Cu/Be and may be used as a common electrode to provide four capacitive sensors 25 with the electrodes 22. Alternatively, as shown a thin silica wafer 26 may be secured to the beam 16 to form the common electrode. The wafer 26 is more rigid than the beam 16 and can be better aligned parallel with the electrodes 22.

The four individual capacitive sensors 25 are configured as two pairs of capacitive sensors.

One pair serves as a height/force sensor for measuring the vertical movement of the stylus and hence the topography of the sample 34 and the axial force applied to the stylus or the force normal to the surface 34 at the stylus 12. This pair of capacitative sensors can thus be used to measure the force which is due to the deflection of the beam 16 and is added to or subtracted from the normal force at the stylus 12. The second pair of the capacitive sensors 25 is a friction sensor operated in a differential mode for measuring the torsional motion of the beam 16 and thus the frictional force at the stylus 12.

The NPM is operated in a similar manner to a normal contact mode AFM but under a controlled loading force by the combination of the magnet 18/coil 20 and the beam 16. It measures surface topography up to 10 Rm with subnanometre resolution, force application in a range from 10-8 N to 10-3 N or 10-2 N for elastic modulus mapping and lateral force measurement in a similar force range resolving to around 10 nN for frictional measurement.

Because the probe comprises mainly the stylus 12, two pairs of capacitive sensors 25 and the force actuator of the coil 20 and magnet 18 it can be made small and compact in size and can be employed in any commercial x-y-z stage for three dimensional measurements or x-y positioner for two dimensional measurements.

The x-y movements of the probe 10 can be produced by the NPM via a PZT stage 28, in which case the whole probe is assembled in a solid shielding form to improve thermal stability, with only the stylus exposed to the sample 34.

For a general version, the NPM has the probe 10 and a z-positioner with the x-y movement provided separately.

Referring to Figure 3, the probe 10 of the preferred form of NPM is connected to a z- positioner 30 which can be a commercial piezoelectric translator. The probe has the force actuator 32 which is the magnet/coil actuator 18,20 of Figure 1, capacitive sensors 25 for measurement of the normal force/height and frictional force, and stylus 12 with a Berkovich tip for interacting with the sample surface 34. The sample surface 34 is, in turn, moved in x and y directions in an x-y plane by an x-y stage 36 which is controlled through two channels of a DAC (Digital to Analogue Converter) 38 by a control circuit 39.

There are two operational modes for the measurements of height or profile, frictional force and elastic modulus of the sample surface 34. One mode is a general scanning mode for measurements of profile and friction. In this mode a constant force F along the z axis towards the sample set by the combination of the force actuator 32 and the deflection of the beam 16 is applied to the stylus 12. The second mode of operation is a force ramping mode for elastic modulus/hardness measurement. In this mode, the force applied to the sample surface 34 by the stylus 12 is linearly increased from an initial value to a suitable value and then linearly decreased to the initial value. At each scanning point on the sample surface the two modes are operated sequentially.

In the first mode, the force interaction between the stylus 12 and the sample surface 34 which is caused by the height variation of the sample surface and the lateral force applied to the tip are detected by the height and friction capacitive sensors 25. The height signal from the sensors 25 is measured by a capacitive AC bridge 40, applied to an amplifier 42 and active filter amplifier 44, and then to one channel, ADC1, of an Analogue to Digital Converter (ADC) 46. The digitised height signal is then compared with a set or reference value Vset in the comparator 47 and an error or difference signal is sent to a control computation unit 48. The output from the control unit 48 is then passed through one channel, DAC1, of a Digital to Analogue Converter (DAC) 50, to a high voltage drive unit 52 which actuates the z-positioner 30 to move the probe in such a way as to maintain the interaction force at the

stylus 12 at the set or reference value i. e. the force F is maintained constant. The output signal from the control unit 48 is an indication of the variation of the height of the sample at the point of contact of the stylus relative to a datum level.

To measure the frictional force, the x-y stage 36 is moved in the x-y plane to"drag"the stylus across the sample surface. The capacitive friction sensors 25 are connected in a differential AC bridge 54 forming a differential capacitive sensor which responds only to the lateral tilt and not the vertical movement of the stylus. The differential AC bridge 54 provides an output signal as a measurement representative of the friction. This is again amplified and filtered by an amplifier 56 and active filter amplifier 58 and then passed to a computer 60 via a second channel ADC2 of the ADC 46 for further processing. The scanning mode is much similar to a general operation of STM (Scanning Tunnelling Probe Microscope)/AFM except that in the NPM the setting force F can be adjusted by the force actuator 32 over a fairly large range up to 10-2 N for different applications.

In the force ramping mode, a ramp signal from a ramp signal generator 62 is sent, via DAC2, a second channel of DAC 50, and a current drive 64, to the force actuator 32 to apply an additional force to the sample surface at the stylus 12. This additional force is increased by the ramp signal from the generator 62 which may be increased linearly or in a stepped manner.. The deformation or penetration of the sample surface 34 is monitored by the force/height sensors 25 which monitor changes in the vertical position of the stylus relative to the probe body. The signal from the force/height sensors 25 is applied through the AC bridge 40, amplifier 42, filter/amplifier 44. and ADC1, to the comparator 47, whose output applies a compensating or error signal through the control unit 48 and high voltage drive unit 52 to the z-positioner 30. As a result, the probe is moved in the z axis towards or away from the sample surface in order to maintain the deflection of the beam 16 due to the application of the force F at its initial value. Thus, the penetration or deformation of the sample surface is determined only by the change in F as a result of the application of the ramp signal and the contribution of the beam deflection to the force applied to the stylus is reduced to negligible proportions or eliminated.

The output signal from the control computation unit (or the comparator 47) is a measure of the deformation or penetration and thus the hardness of the sample surface.

However, in the force ramping mode it is possible to switch off the feedback. With there being no feedback control signal to the z-positioner 30, the force applied to the stylus 12 and thus the sample surface is produced by a combination of the beam deflection and the force actuator 32. This mode of operation is suitable for the application of small forces. The output signal from the bridge 40, the amplifier 42, the filter 44 and the ADC 46 is a measure of the deformation/penetration.

The procedure for measuring the height and elastic modulus/hardness of the sample surface 34 at each point and the frictional force is illustrated in the form of a flow chart in Figure 4.

On beginning the measurement, the system of NPM is initialised by moving the probe to a preselected position above the sample surface. A preselected signal is applied to the current drive 64 to apply a constant current to the coils 20 of the force actuator 32 and apply the constant setting force F to the stylus 12. The scanning or movement range of the x-y stage, the ramping mode (with or without feedback) and the value and range of the ramping signal is also programmed into the microprocessor 60. A preselected set or reference value is applied to the comparator 47 to generate an error signal which is used to activate the drive unit 52 and thus the z-positioner 30. The latter moves the probe and thus the stylus towards and into contact with the sample surface at the scanning location or point. The z position 30 continues to move the probe towards the surface, applying force to the stylus until the error signal is reduced to zero or a selected level. The setting force F is thus applied by the stylus to the sample surface.

The output signal Vc of the control unit 48 represents the vertical position of the probe (and thus the height of the sample surface at that location) and is here set as a datum level.

Changes in this signal as the stylus is repositioned at other locations on the sample surface can be monitored and stored as a relative height measurement in the microprocessor 60 to provide a map of the surface profile or topography. The results can be displayed on the display 70.

The ramping signal V, is then applied by the ramp signal generator 62 to the force actuator 32 to apply a ramping force to the sample surface 34 by the stylus 12.

If the feedback loop is switched off then the signal Vp generated through the bridge 40, amplifier 42, filter/amplifier 44 and ADC 1 is stored by the microprocessor, together with the ramping voltage V,. These signals are processed and displayed by the display 70 to give an indication of the elastic modulus/hardness of the sample at the scanning location.

If the feedback loop is on then the output signal of the height capacitive sensor is applied through the bridge 40, amplifier 42, filter/amplifier 44 and ADC 1 to the comparator 47 for comparison with the set value and the height of the probe is adjusted by the z-positioner 30 in accordance with the comparison. The height of the probe will continue to be adjusted by <BR> <BR> <BR> the z-positioner 30 until Vp is equal to the set value V5et. Then the control signal Vc for the z-positioner 30 and the ramping signal V, are stored and processed by the microprocessor 60 for display on the display 70.

The x-y stage is also moved in a preselected direction at a set speed in the x-y plane with the effect that the stylus 12 is"dragged"across the sample surface 34. The twisting of the beam 16 and thus the deflection of the stylus 12 is measured by the differential bridge 54 of the frictional force measurement circuit which generates a signal VF representative of the measured frictional force. This is stored by the microprocessor 60 and can be displayed on the display 70.

The probe is moved to successive scanning points on the sample surface and the above described measurements are repeated at each scanning point to build up maps of the properties across the sample surface. These include a topography map, frictional force map and hardness/Young's modulus map.

It is also possible to use the force ramping mode to measure adhesion of the sample surface and also shear stress.

In the normal course of a measurement process, the adhesion and shear stress would be measured before measurement of the hardness of the sample surface.

To measure adhesion, the probe and thus the stylus 12 is moved by the z-positioner relative to the sample surface along the z axis towards the sample surface as described above whilst the force applied to the stylus is monitored. The momentary deflection of the stylus caused by contact with the surface is detected. The position of the probe at this contact point is registered as a datum representing the surface and the probe is then withdrawn by the z- positioner 30. The adhesion or resistance to withdrawal of the stylus from the surface is measured by the height sensors 25 as the beam 16 deflects. The level of the output signal from the height sensors is an indication of the magnitude of the adhesion at the surface.

The stylus can be biassed towards the sample surface by the force actuator 32 before contacting the surface or can be held at a neutral position relative to the body by the beam 16 in the absence of any force applied by the force actuator 32.

Once contact is detected, the feedback loop can be arranged to cause the z-positioner 30 to withdraw the probe until the stylus is again at its preselected bias position or at its neutral position relative to the body whilst maintaining contact. At this point Vp = Vset and the error signal has returned to zero or its original value.

In order to measure the shear stress, a small oscillation signal is applied either to the probe or to the sample or to both to cause the stylus to oscillate or"dither"in the x-y plane on the surface of the sample. The deflection of the stylus in x-y is in the nanometer level typically between 1 and 10 nanometers. The deflection of the stylus in the x-y plane is measured by the frictional force measurement circuit to provide an indication of the shear stress.

It is possible to use the NPM as an end-point detector to measure surface adhesion forces and static attraction forces over a range of materials. In addition, the NPM can be used as a micro/nano engraving tool to scratch a surface following a required pattern. It will be appreciated that whilst the probe is described as being moved in the x-y plane and z axis, the movement is relative to the sample and the probe could be held stationary whilst the sample is moved, or a combination of both.