Atomic-Scale Friction Measurements Using

Friction Force Microscopy : General Principles and New Measurement

Techniques

Presented By

Udit Kumar

(163110065)

MEMS, IIT Bombay

Bhushan, B. and Ruan, J ASME J. Tribol, 116, pp.378-388.(1994)

Overview

• AFM – An introduction

• General components and its functions

• AFM Imaging modes: contact vs. non-contact

• Two contact scanning modes: constant height and constant force

• Lateral/Friction Force Microscopy (LFM/FFM)

• A Review of atomic-scale friction measurement

• Author’s setup :a new friction measurement technique

• Friction measurement methods

• A comparison between different methods

• Calibration

• Results, summary & conclusions

AFM – An introduction

• Atomic force microscopy (AFM) was developed when people tried to extend STM technique to investigate the electrically non-conductive materials, like proteins.

• In 1986, Binnig and Quatedemonstrated for the first time the ideas of AFM, which used an ultra-small probe tip at the end of a cantilever (Phys. Rev. Letters, 1986, Vol. 56, p 930).

• In 1987, Wickramsinghe et al. developed an AFM setup with a vibrating cantilever technique (J. Appl. Phys. 1987, Vol. 61, p 4723), which used the light-lever mechanism.

AFM Imaging modes: contact vs. non-contact

• Contact mode (left): the deflection of cantilever is kept constant.

• Non-contact mode (right): the tip is oscillated at the resonance frequency and the amplitude of the oscillation is kept constant.

• Tapping mode: somewhere between the contact and non-contact mode.

Two contact scanning modes: constant height and constant force

Constant Height (of Scanner)

• In this mode, the spatial variation of the cantilever deflection is used directly to generate the topographic data set because the height of the scanner is fixed as it scans.

• Constant-height mode is often used for taking atomic-scale images of atomically flat surfaces

Constant Force:

• In this mode, the deflection of the cantilever can be used as input to a feedback circuit that moves the scanner up and down in z, responding to the topography by keeping the cantilever deflection constant

• With the cantilever deflection held constant, the total force applied to the sample is constant

Lateral/Friction Force Microscopy (LFM/FFM)

A Review of Atomic-Scale Friction MeasurementTechniques : 1

• Sample mounted on three orthogonal PZT (piezoelectric tubes)

• Tungsten wire used as cantilever as well as tip (after etching)

• Two laser beams were used independently to measure cantilever deflection

• Measured the friction of Ti tip sliding against graphite. Schematic of the FFM by Mate et al. (1987)

Normal and friction forces are measured by measuring the cantilever deflection in both normaland lateral directions by optical interference

A Review of Atomic-Scale Friction MeasurementTechniques : 2

• Diamond tip held by parallel leaf spring was used.

• A U-shape electromagnetic was set to pull the tip assembly to overcome the friction force.

• The friction force was measured by measuring the current that was required to hold the tip stationary.

Schematic of FFM by Kaneko (1988). (a) The over ail set up showing the tip assembly (sample support assembly not shown) and the associated instrumentation, (6) parallel spring unit for supporting and loading the sample

A Review of Atomic-Scale Friction MeasurementTechniques : 3• A PZT tube scanner capable of 3-D

movement was used for moving and loading the sample against the tip.

• Diamond tip was used which was ground to the shape of a three-sided pyramid.

• Applied normal force was obtained by the tube scanner displacement and the stiffness of the single-leaf spring

• Friction force is measured by measuring the required voltage difference between the electrodes.

Schematic of FFM by Kaneko et al. (1991). Sample is mounteddirectly on the PZT as opposed on a parallel spring unit

Author’s Setup :A New Friction Measurement Technique

(a) Schematic of FFM used in this study; (b) Schematic of the PECVD Si3N4 cantilever beam with tips used

Author’s Setup :A New Friction Measurement Technique

(a) Schematics defining the x and y directions relative to the cantilever, and showing the sample traveling direction in two different measurement methods discussed in the text, (b) deformation of the tip/cantilever shown as a result of sliding in the x and y directions.

• Tip is brought in contact with the surface, features make it deflect.

• The differential signal from the vertical two quadrants (top and bottom) of the photodiode detector [(T - B)/(T + B)] provides a sensitive measure of the cantilever vertical deflection, which is called “AFM" signal.

• Thus the vertical motion of the tube scanner relates directly to the topography of the sample surface. This mode of operation is referred as "height" mode.

• The laser beam will be reflected out of the plane defined by the incident beam and the vertically reflected beam from an untwisted cantilever. This produces an intensity difference of the laser beam received between the left and right ("L" and "R") quadrants of the detector.

Author’s Setup :A New Friction Measurement Technique

Friction Measurement Methods : Method 1 Height Mode With Parallel Scans

• Scanning in +Y & -Y direction known as parallel scan, in +X & -X direction is perpendicular.

• We note that topographic measurements are made using "height" mode at any scanning angle.

• The friction force between the sample and the tip will also cause a cantilever deflection.

W0 -> Stationary sample

Wf -> Friction force

• The PZT vertical position is affected by the surface topographic profile of the sample in addition to the friction force being applied at the tip, this difference has to be taken point by point at the same location on the sample surface

Schematic illustration of the height difference of the PZT tubescanner as the sample is scanned in y and - y directions

Method 2—"Aux“ Mode With Perpendicular Scanand Suggested Modifications

• The sample is scanned perpendicular to the long axis of the cantilever beam (+X & -X).

• The differential signal between the left and right detectors is denoted as FFM signal [(L -R)/(L + R)]. This signal can be related to the degree of twisting, hence to the magnitude of friction force.

• the presence of an adhesive force at the tip-sample interface, the slope of the friction data (FFM signal vs. normal load) needs to be taken for an accurate value of coefficient of friction.

A Comparison Between Methods

Method 1• The method of using "height"

mode and parallel scan is very simple to use.

• This method can provide 3-D friction profiles and the corresponding topographic profiles.

• the PZT scanner displays a hysteresis when the traveling direction of the sample is reversed

• The measurement of average• friction can serve as an internal

means of friction force calibration.

Method 2• More desirable approach• The subtraction of FFMF signal

from FFM for the two scans does not introduce error to local friction force data.

• In this method, since topography data are not affected by friction, accurate topography data can be measured simultaneously with friction data and better localized relationship between the two can be established.

Calibrations

• We need to have accurate spring constant (Kc) of cantilever

• Properties of Si3N4 cantilever could be different from bulk

• Can be measured using a stainless steel spring-sheet of known stiffness as sample

• To convert the FMMT signal into friction force we need information on the detectors such as the quantum efficiency of the detector, the laser power, the instrument's gain, etc. in order to be able convert the signal into the degree of twisting.

Results

Summery & Conclusions

• Method 1 is to scan the sample along the long axis of the cantilever with the AFM operated in "height" mode

• Method 2, samples are scanned in the direction perpendicular to the long axis of the cantilever with the FFM operated in "aux" mode.

• Method 1 can be used as an internal means of instrumental calibration for method 2. Method 2 can be used easily to measure local friction force profiles in either scan directions as well as surface topographic profiles simultaneously.