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Atomic Force Microscopy (AFM)Edit

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The relation between torsional spring constant and lateral spring constant is in doubt. Please check ("Normal and torsional spring constants of atomic force microscope cantilevers" Green, Christopher P. and Lioe, Hadi and Cleveland, Jason P. and Proksch, Roger and Mulvaney, Paul and Sader, John E., Review of Scientific Instruments, 75, 1988-1996 (2004), DOI:http://dx.doi.org/10.1063/1.1753100) and ("Lateral force calibration in atomic force microscopy: A new lateral force calibration method and general guidelines for optimization" Cannara, Rachel J. and Eglin, Michael and Carpick, Robert W., Review of Scientific Instruments, 77, 053701 (2006), DOI:http://dx.doi.org/10.1063/1.2198768) for details.

 
Typical AFM setup. The deflection of a microfabricated cantilever with a sharp tip is measured be reflecting a laser beam off the backside of the cantilever while it is scanning over the surface of the sample.

ResourcesEdit

Methods in AFMEdit

 

A wealth of techniques are used in AFM to measure the topography and investigate the surface forces on the nanoscale:

For imaging sample topography:

  • Contact mode, where the tip is in contact with the substrate. Gives high resolution but can damage fragile surfaces.
  • Tapping / intermittent contact mode (ICM), where the tip is oscillating and taps the surface.
  • Non-contact mode (NCM), where the tip is oscillating and not touching the sample.

For measuring surface properties (and imaging them):

  • Lateral force microscopy (LFM), when the tip is scanned sideways it will tilt and this can be measured by the photodetector. This method is used to measure friction forces on the nanoscale.
  • Force Modulation Microscopy. Rapidly moving the tip up and down while pressing it into the sample makes it possible to measure the hardness of the surface and characterize it mechanically.
  • Electrical force microscopy. If there are varying amount of charges present on the surface, the cantilever will deflect as it is attracted and repelled. kelvin probe microscopy will normally be more sensitive than measuring s static deflection.
    • Kelvin probe microscopy. By applying an oscillating voltage to an oscillating cantilever in non-contact mode and measuring the charge induced oscillations, a map can be made of the surface charge distribution.
    • Dual scan method - an other kelvin probe method described below.
  • Magnetic Force Microscopy. If the cantilever has been magnetized it will deflect depending on the magnetization of the sample.
  • Force-spectroscopy or force-distance curves. Moving the cantilever up and down to make contact and press into the sample, one can measure the force as function of distance.
  • Nanoindentation. When pressing the cantilever hard into a sample it can leave an imprint and in the force distance curve while doing indentation can tell about the yield stress, elastic plastic deformation dynamics.
  • Liquid sample AFM. By immersing the cantilever in a liquid one can also image wet samples. It can be difficult to achieve good laser alignment the first time.
  • Electrochemical AFM.
  • Scanning gate AFM
  • Nanolithography
    • Dip-pen lithography


Reviews of Atomic Force MicroscopyEdit

 
SEM image of a typical AFM cantilever
  • Force measurements with the atomic force microscope: Technique, interpretation and applications. Surface Science Reports 59 (2005) 1–152, by Hans-Jurgen Butt, Brunero Cappella,and Michael Kappl. 152 pages extensive review of forces and interactions in various environments and how to measure and control these with AFM.

Cantilever MechanicsEdit

Cantilever has width w, thickness t, length L and the tip height from the cantilever middle to to the tip is h.

 
The typical geometry of an AFM cantilever. Length l, thickness t, width w, and tip height h is measured form the middle of the beam

When the cantilever is bent by a point force in the z-direction   at the tip will deflect distance z(x) from the unloaded position along the x-axis as [1]

 

with cantilever length  , Youngs modulus  , and moment of inertia  .

The tip deflection is

 

giving a spring constant   from

 

so

 

The angle of the cantilever in the x-z plane at the tip  , which is what gives the laser beam deflection will be

 

 

 

 
The difference between a hinged and fixed beam's angle of deflection and the cantilever tip. The fixed beam will give a larger deflection signal

giving the relation

 

between the tip deflection distance and tip deflection angle. This is a factor 3/2 bigger than the result we would expect if the beam was stiff and hinged at the base, showing us that we get a bigger deflection of the laser beam when the beam is bending than when its straight.

 
AFM cantilever, with deflection angles and detector setup. The Z-deflection from the sample Z topography is giving a deflection in the xz-plane and measured by the top-bottom detector pair. Lateral forces on the cantilever give both torsion (yz-plane deflection and Left/ritgh detector signal) and a lateral deflection in the xy-plane that cannot be measured by the detector.

The AFM detector signalEdit

The cantilever can bend in several ways, which is detected by the quadrant photo detector that most AFMs are equipped with. Normal topography signal is given by 'normal' deflection of the cantilever tip in the x-z direction,   and detected by the left-right (or A-B) detector coupling quadrants as  .

Lateral forces applied to the tip will bend the cantilever in the x-y and x-z plane too. Lateral deflection cannot be detected by the quadrant detector since it doesn't change laser beam deflection, and deflection is also rather small, as we shall see. Lateral forces also twist the cantilever tip producing torsional deflection in the y-z direction,   which in turn produces the lateral force signal from the top-bottom detector measuring  

For deflection in the z-direction, 'normal' spring constant relating the force and deflection   is

 

Expressed in the angle of deflection,   there is an angular spring constant

 

with  .

 
AFM cantilever and the forces acting between the tip and the sample.

Contact, Tapping, and Non-contact ModeEdit

If an oscillator experiences an attractive force pulling it away from it rest position, the resonance frequency will drop (at snap-in it will be zero). A repulsive force squeezing it will increase the resonance frequency.

 
The repulsive and attractive force regimes as the AFM tip approaches the sample.

If an AFM tip is moved to contact with a sample, the resonance frequency is first decreasing slightly due to attractive forces and then increasing due to the repulsive forces. Eventually the repulsive force become so high we cannot oscillate it and we have achieved contact.

Contact mode: Because the tip is in contact, the forces are considerably higher than in non-contact mode and fragile samples as well as the tip can easily be damaged. The close contact on the other hand makes the resolution good and scan speeds can be high.

The varying resonance frequency is the cantilever moves between the attractive end repulsive regions of the force distance curve can be used to measure the cantilever position and to keep it in the attractive or repulsive part of the force distance curve.

 
The tip oscillation frequency for tapping and non-contact mode AFM are to either side of the tip resonance frequency. The green signal is the oscillation amplitude while the yellow is the phase

Non-contact mode: If we oscillate the cantilever at a higher frequency than its free resonance and use the feedback loop to maintain a oscillation amplitude setpoint slightly lower than that of the free oscillation, it will move the tip down until the attractive forces lower the resonance frequency and makes the oscillation amplitude drop to the setpoint level.

Tapping mode: if we oscillate the cantilever at a lower frequency that its free oscillation, moving it towards the sample will first make it oscillate at a lower frequency which will make the stage move closer to try and raise the oscillation amplitude, and at eventually as it reaches repulsive forces will settle where resonance frequency cannot increase more without giving too high an amplitude.

Typical AFM cantilever properties
Use for k (N/m) f (kHz)
Non contact (NC) 10-100 100-300
Intermittent contact (IC) 1-10 20-100
Contact 0.1-1 1-50

Contact ModeEdit

Tapping ModeEdit

Tapping-mode (also called intermittent contact mode) is the most widely used operating mode in which the cantilever tip can experience both attractive and repulsive forces intermittently. In this mode, the cantilever is oscillated at or near its free resonant frequency. Hence, the force sensitivity of the measurement is increased by the quality factor of the cantilever. In tapping-mode operation, the amplitude of the cantilever vibration is used in feedback circuitry, i.e., the oscillation amplitude is kept constant during imaging. Therefore it is also referred as amplitude modulation AFM (AM-AFM). The primary advantage of tapping mode is that the lateral forces between the tip and the sample can be eliminated, which greatly improves the image resolution. Tapping mode experiments are done generally in air or liquid. Amplitude modulation is not suitable for vacuum environment since the Q-factor of the cantilever is very high (up to 105) and this means a very slow feedback response.

Non-contact ModeEdit

Lateral Force MicroscopyEdit

If the sample is scanned sideways in the y direction, the frictional forces will apply a torque on the cantilever, bending it sideways and this can be used to measure the frictional forces. The lateral force gives both a lateral and torsional deflection of the tip. Only the torsional can be detected in the photodetector.

For sideways/lateral bending, the lateral spring constant is corresponding to the normal spring constant but with width and thickness interchanged

 

and a similar eq's for angular deflection as above. With thickness typically much smaller than the width for AFM\ cantilevers, the lateral spring constant is 2-3 orders of magnitude higher than  

For a sideways, lateral force   on the cantilever we will have a sideways deflection determined by

 

If the lateral force is applied to the AFM tip,  , it will give a lateral deflection but also apply a torque   twisting the beam

 

Twisting an angle   gives a torional tip deflection of  

The relation for the torsional spring constant is (please check this equation)

 

with

 

and then

 

From above we have

 

The factor   is typically   - so about 1 but larger or smaller depending on whether its a contact or non-contact cantilever.

Friction Loop ScanEdit

 
Typical signal from a scan with the AFM in lateral force mode - a friction loop scan. At the turning points the tip sticks to the surface and the signal has a linear slope with the detector sensitivity. When the lateral tip-sample force exceeds the static friction force between the sample and substrate, the tip will start to slide with the dynamic friction force and s steady signal.

For optimal torsional sensitivity - but the following is not always correct since it depends highly on the contact forces you need etc: For high torsional sensitivity,  . Since we are in contact mode AFM, L must be large and t thin for a low  . So better torsional sensitivity means wider cantilevers and definitely large tip heights.

Coupled Lateral and Torsional deflection in the cantileverEdit

But how much will a cantilever bend laterally and how much will it twist when applied a lateral force at the tip? An applied lateral force will move two degrees of freedom with a Hookes law behaviour - the torsion and lateral motion. The applied force   is also an applied  

The effective spring constant for pushing the tip in the y direction is then

 

with   and

 

and the deflection made in the torsional spring   is

 

and this approaches   when  so the cantilever is more prone to tilting than lateral deflection. The lateral deflection can be found from

 

The torsional deflection angle is then

 

as anticipated from the assumption that the torsional and lateral springs are coupled in series. So when a constant force is applied, the detector signal is a measurement of the force.

Question: during a friction loop scan, the tip is fixed by static friction on the surface and its a constant deflection, both torsional and lateral deflection must be included to find the actual deflected distance of the tip before its pulled free from to slide the surface and the lateral deflection could influence the beginning of the friction loop curve?

Measuring the cantilever dimensionsEdit

The vibration frequency of the fundamental mode of the cantilever is an easily measurable quantity in the AFM and can be used to evaluate a cantilever is within specifications. Its given by

 

Easily measurable quantities in AFM: length L, resonance freq f, tip length  width  ,

Not so easy: thickness t, cross section (often there are inclined sidewalls), force konst   tip length from middle of the cantilever since we dont know the thickness.

Noise Sources in AFMEdit

Thermal noiseEdit

 

 


for a 1 N/m cantilever this amounts to  Å. So a 1 Å noise level requires   N/m which is not a very low spring constant for a contact mode cantilever.

So thermal noise can become a problem in some AFM cantilevers at room temperature!!

Electrical Force MicroscopyEdit

The Kelvin Probe Microscopy Method and Dual Scan Method can be used to map out the electrical fields on surfaces with an AFM.

Kelvin Probe Microscopy MethodEdit

 
The principle of Kelvin probe microscopy (KPM). The lock-in amplifier generates a signal on the tip and the electrostatic tip-surface interaction is readout by the laser and the lock-in amplifier adjusts accordingly.

In the Kelvin probe microscopy (KPM) method a voltage is applied between the AFM tip and the surface. Both a DC and AC voltage is applied to the tip so the total potential difference   between the tip and surface is:

 

where   is the local surface potential,   is the position of the tip,   is the DC signal on the tip,   is the amplitude of the AC signal, and   is the frequency of the AC signal.

The frequency of the AC signal is much lower than the resonance frequency of the cantilever (a factor of 10) so the two signals can be separated by a lock-in amplifier. Via the electrostatic forces the setup measures the surface potential. If one assumes that the electrostatic force ( ) between the tip and surface is given by [2]

 

where   is the capacitance and   is the distance between the tip and the surface.

If a parallel plate capacitor is assumed

 ,

where   is the area of the tip. The derivative of capacitance is

 .

Combining the force ( ) and   yields:

 

 

Using Pythagorean identities

 

and de Moivre's formula

 

We find

 

 

 

This inserted in the equation for the force ( ) gives [3]:

 

 

 

where

 ,

 , and  .

The frequency   is set by an external oscillator and can therefore be locked by the lock-in amplifier. The signal detected by the lock-in amplifier (the   part) is minimized by constant varying  . When this signal approaches zero, this corresponds to   i.e. mapping   vs. the sample surface   gives  .


Dual Scan MethodEdit

 
The principle of the Dual Scan (DS) method where first a topography line scan is made, then the tip is lifted a distance d and another line scan is made with the source drain voltage turned on.

In the Dual Scan (DS, or sometimes called lift-mode method) one first makes a line scan with no potential on either the AFM tip or the sample in either tapping or non-contact mode. Next, the AFM tip is raised several tens of nanometers (30-70 nm) above the surface. A new line scan is made at this height, but this time with a potential on the sample also in non-contact mode. This is repeated over the desired scan area until the whole area has been scanned. For imaging the surface potential, the phase of the cantilever vibration is mapped out. The principle is shown in the figure where d is the distance between the tip and the surface in the second scan. The phase shift is dependent on the force ( ) acting on the tip [4]

 

where   is the quality factor of the cantilever,   the spring constant, and   the distance between the tip and the surface.

For small phase shifts the phase can be written as

 

The derivative of the force can be written as:

 

where   is the surface potential and   is the capacitance between the tip and the surface [5] . The second derivative of the capacitance is

 

Combining equations for the phase and the derivative of the force yields the phase dependence of the phase shift

 

To find the surface potential, one must estimate the other parts of equation for the phase. The spring constant ( ) can be determined if the dimensions (a regular cantilever) and material of the cantilever are known:

 

where   is the Young modulus,   is the width of the cantilever,   the height, and   is the length. The quality factor ( ) of the cantilever can be found by measuring the shape of resonance peak. The second derivative of the capacitance can be estimated by assuming that the tip of the AFM is a plate with radius   so the derivative of the capacitance is given by:

 

where   is the vacuum permittivity. This way of estimating the other parts of the equation for the phase is quite accurate according to [6]. One can also estimate the values by measuring them at a surface with a known potential and at different known heights and then one simply calculate backwards for that particular AFM tip.

DiscussionEdit

Both the DS and KPM methods have their strengths and weaknesses. The DS method is easier to operate, since it has fewer interlinked parameters, needing to be adjusted. The KPM method is faster, as it does not require two scans (an image with the DS method with a resolution of 512   512 pixels and a scan rate of   Hz takes about half an hour). The DS method will normally obtain much better lateral resolution in the potential image compared to the KPM method. This is due to the fact that the signal depends on the second derivative of the capacitance, which in turn depends on the distance in   compared to the KPM method where the dependence is only  . This rapidly reduces the problem of the tip sidewall interaction. On the other hand, the KPM method has better sensitivity because it operates much closer to the surface.

ReferencesEdit

  • A. Bachtold, M. S. Fuhrer, S. Plyasunov, M. Forero, E. H. Anderson, A. Zettl, and P. L. McEuen; Physical Review Letters 84(26), 6082-6085 (2000).
  • G. Koley and M. G. Spencer; Applied Physics Letters 79(4), 545-547 (2001).
  • T. S. Jespersen and J. Nygård; Nano Letters 5(9), 1838-1841 (2005).
  • V. Vincenzo, and M Palma, and P. Samorí; Advanced Materials 18, 145-164 (2006).
  • Veeco, "Electrostatic Force Microscopy for AutoProbe CP Research Operating Instructions", 2001 (Manual).
  • D. Ziegler and A. Stemmer; Nanotechnology 22, 075501 (2011).

Suppliers of AFM systemsEdit

Suppliers of cantilevers for AFMEdit

Overview of properties of various cantilevers

Software for AFM image analysisEdit

ReferencesEdit

See also notes on editing this book about how to add references Nanotechnology/About#How_to_contribute.

  1. Senturia `Micromechanics'
  2. Veeco, "Electrostatic Force Microscopy for AutoProbe CP Research Operating Instructions", 2001 (Manual)
  3. V. Vincenzo, and M Palma, and P. Samorí; Advanced Materials 18, 145-164 (2006)
  4. T. S. Jespersen and J. Nygård; Nano Letters 5(9), 1838-1841 (2005)
  5. T. S. Jespersen and J. Nygård; Nano Letters 5(9), 1838-1841 (2005)
  6. T. S. Jespersen and J. Nygård; Nano Letters 5(9), 1838-1841 (2005)
  7. http://en.nanoscopy.ru
  8. Massimo Sandal, Fabrizio Benedetti, Alberto Gomez-Casado, Marco Brucale, Bruno Samorì (2009). "Hooke: an open software platform for force spectroscopy". Bioinformatics 25 (11): 1428-1430.