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    Scanning probe microscopy (SPM) is a family of mechanical probe microscopes that measures surface morphology in real space with a resolution down to the atomic level. SPM was originated from scanning tunneling microscopy (STM), in which the electrical current caused by the tunneling of electrons through the tip and the sample is used as the feedback parameter to maintain a separation between them. This technique, invented in 1981, was a totally new one that can image atom arrangement on a surface in real space for the first time. It is so invaluable to science and technology related to surface phenomena that the inventors of STM shared the Nobel Prize in Physics with the inventor of the electron microscopy in 1986.

    Because STM requires that both the tip and the sample be conductive, it cannot handle samples that are not a good conductor.  In 1986, atomic force microscopy (AFM) was developed to measure the surface morphology of materials that are not a good conductor. AFM has since been developed very rapidly and has found much more applications than STM in many fields. Almost all kind of materials can be measured using AFM. Besides surface morphology mapping, SPM has been developed in the past two decades to measure a variety of surface properties, such as chemical, electrical, magnetic, and mechanical properties. The diversity of SPM is based on the fact that the probe tip is in contact or close to the sample surface so that various interactions between the tip and the sample become accessible.

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 1.  STM

    The principle of the STM may be simple: tunneling of electrons between two electrodes under an electric filed. However, to develop the concept of electron tunneling into a technology of imaging atomic resolution on a surface was not simple. To measure the tunneling current, the distance between the two electrodes must be close to each other on the order of 1 nm. Surface cleanness and vibration-free system are essential to measuring the tunneling current accurately. Shown here is an STM image obtained on a highly ordered pyrolytic graphite (HOPG) substrate.
   
    Quantum mechanics predicts an exponential dependence of tunneling current with the distance between the two electrodes. An observation of this dependence between a W-tip and Pt surface by G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel in 1981 was marked as the invention of STM. Atomic resolution measured on a Si(111) 7x7 surface in 1982 might be considered the breakthrough of the STM instrumentation. Since then, there have appeared a huge number of papers on STM.

    STM, as a research approach, has been mainly used to measure atomic resolution or electronic structure of solid surfaces in UHV. A video from IBM demonstrates the ability of manipulating individual atoms.

How STM works?
    Shown here is a diagram depicting how STM works. By approaching the tip with a specified bias and current, the tip will be held at a certain distance from the sample surface so that the specified current (set point) is realized.  By scanning the tip across the sample under this condition, the system compares the measured current I and the set point current I_s and uses the error signal I-Is as the feedback parameter to apply an appropriate voltage to the z-piezo to adjust the tip-sample distance so as to diminish the error signal (i.e., I-I_s ~ 0), thus providing the height profile of the "topography" of the surface.  This is the constant current mode.  The other operation (constant height) mode is to keep the tip-sample distance while recording the current, which apparently requires the scanned area to be flat. 

    In order to understand the tunneling of electrons between the tip and the sample, we need to have a model for electrons at a potential well and observe how they can tunnel through the potential barrier.  

   
    Shown here is the wavefunction of the electrons in the well and its spread into the barrier and tunnel to the other side of the barrier. The wavelike behavior of the electrons is governed by the Schrödinger equation shown below:

By way of continuation principle of the wavefunctions and their derivatives for electrons at the three regions (the energy well, barrier and the other side of the well), the probability of find electrons on the other side of the barrier is

which is proportional to the tunnel current. Therefore, it is shown that the tunneling current increases exponentially when tip-sample distance decreases. With an average work function of ~4 eV for metals, the tunnelling current is proportional to e^-2w with w in Å.

    Here is a rough estimation of how tunnelling current changes with tip-sample distance. When tip-sample distance changes by 1 Å, the tunneling current will change 7.3-10 times.  A consequence of this sensitivity is the physics behind STM.  Suppose that an STM tip is terminated by a single atom (radius ~1.5 Å). The next atom is therefore ~2.6 Å away from the terminus atom, whose contribution is e^-5.2=0.6% of the current contributed by the terminus atom. This high sensitivity of current over distance makes STM an imaging tool having the atomic resolution (of course with good tips and samples).

 2.  AFM

          Examples of AFM images obtained on four different samples

     To obtain similar spatial resolution as in STM for insulating surfaces, AFM was invented in 1985 by G. Binnig, C.F. Quate and Ch. Gerber as shown in their 1986 paper. The first AFM used an STM to detect the deflection of the cantilever in order to measure the contact force between the AFM tip and the sample surface. Most AFM systems, however, use optical detection scheme.

    A sharp tip (apex radius ~10 nm) formed on a soft cantilever is used to probe the interaction (force) between the tip and sample surface. The interactive force may be described by the Lennard-Jones potential which deals with the interaction between two atoms: w(r) = -A/r⁶ + B/r¹², where r is the separation of the two bodies, A and B are interaction constants. Then the interactive force is F = -dw(r)/dr= -6A/r⁷ + 12B/r¹³. Following a text book, A and B are known to be10^-77Jm⁶ and 10^-134Jm¹², respectively. A calculation for the interaction force between the two atoms is shown to the right. Around a separation distance of 0.4 nm between the two atoms, a small attractive force is seen. When the separation distance gets smaller and smaller the repulsive force increases steeply.

     In practice, the attractive force between the AFM tip and a sample surface can be much larger than what is described above for the tow-atom system. This is because, at least, the size of AFM tips (radius ~10 nm) is much larger than an atom. Moreover, much longer-range forces such as capillary forces and electrostatic forces exist. 

     The principle of AFM is shown below. The AFM operates by keeping constant the interaction between the tip and sample surface through a feedback system that adjusts the distance between the tip and the sample surface. Depending on the interaction between the tip and sample surface, which is used as the feedback signal, there are two different imaging modes: contact mode and dynamic force mode.  The interactive force between the tip and the surface is detected by measuring the deflection of the cantilever using a laser diode to radiate the cantilever and a photodiode to detect the reflected laser beam. The quadrant photodiode is able to measure both the deflection and torsion of the cantilever, which are used to measure the normal force exerted on the tip, as well as the friction force sensed by the tip when it scans across the sample surface.   

    While scanning the tip across the sample surface (x, y), the system adjusts the distance (z, which is thus the measure of the height of the sample surface features) between the tip and the sample surface to maintain a constant contact force (contact mode) or oscillation amplitude (dynamic force mode). A 3-D image is thus constructed by the lateral dimension the tip scans and the height the system measures.  Shown below is an example of AFM image obtained on a stamp.  The 3D rendering of the AFM image is in a false color scale.

 2.1  Contact Mode AFM

    In contact mode AFM, the tip is mechanically in contact with the sample surface.  This is shown in the figure to the left, in which the tip is shown in free space and in contact with the sample surface.  When the tip is pushed to the sample surface, the cantilever bends up.  If the bending of the cantilever is z, then the force exerted on the sample surface is k_cz, where k_c is the spring constant of the cantilever.  The deflection of the cantilever, z, is determined with a laser beam shining on the free end of the cantilever (i.e., over the tip)  and a position sensitive photodetector that receives the reflected laser beam from the cantilever.

    This applied force k_cz can be evaluated from a force-distance curve which is measured when the tip is brought to and then retracted from the sample surface, as shown below. Inserts in the figure show the interaction between the tip and the sample surface, which is detected by the deflection of the cantilever. There is no interaction between the tip and surface when the tip is far away from the surface (a). When the tip is brought close enough to the surface there is an attractive force between them. Usually, the gradient of the attractive force is much larger than the spring constant of cantilever used in contact mode AFM (< 1 N/m), so that the tip is snapped to the surface to make a contact between the tip and surface (b). Further extending the tip results in a loading (repulsive) force to the surface (c). A force in the repulsive force region is usually used as the feedback parameter for the AFM system to obtain surface morphology. Forces of a couple of nN may be used in contact mode AFM. In the retracting cycle (d and e), because of the adhesion established after the contact between the tip and the surface, the tip will not detach from the surface until the force used to pull it exceeds the adhesion force between them (f). This pull-off force serves as a measure of the adhesion force between the tip and the sample.

   
    A very soft cantilever with a spring constant of ~ 0.1 N/m is usually used in contact mode AFM. A photograph of such a cantilever is shown in the optical picture below. The cantilever is so soft that it will be pulled onto the surface because the gradient (~ 10 N/m) of attractive force between them is usually much larger than the spring constant of such soft cantilevers.

    After a mechanical contact between the tip and the sample surface, a repulsive force establishes between them. This force is used as the feedback parameter (by maintaining a constant force through adjustment of the sample height while the tip scans the surface) to obtain AFM images.

    Because the tip is mechanically contacted with surface in the contact mode AFM, many surface properties such as friction force distribution and mechanic properties can be measured simultaneously with the topographic image. Also, nano-lithography on some materials is also available by controlling the applied forces. A working knowledge on force-distance curve is essential for understanding and interpreting the imaging mechanism of contact mode AFM (especially when things go wrong).

 2.1.1  Force-Distance Curves

    Force-distance curves are obtained by extending the tip to the surface to make a contact between the tip and the sample surface followed by retracting it from the surface. The original point for the distance may be defined as the mechanical contact between the tip and surface in the extending cycle. Extending the tip beyond that point results in a loading force applied to the surface. The slope of this loading force is a measure of the Young's modulus of the sample, possibly convoluted with the spring constant of the cantilever. As a result, a cantilever whose spring constant is comparable with the surface stiffness should be used to measure the Young's modulus of the sample.

    In the retracting cycle, because of the adhesion properties between the tip and surface, the tip will not depart from the surface until the force used to pull the tip from the surface exceeds the adhesion force between them. This pull-off force can be considered as a measure of the adhesion force between the tip and surface. Adhesion force can be related to surface energies of the tip and the sample, as well as their interfacial energy. Shown below is an example of measuring adhesion force at different regions on a biaxially-oriented polypropylene (BOPP) film. The striped (mechanically scratched) areas have higher adhesion force than the normal surface; we will see later that this is also reflected in the friction force images.
   
    Click here to see adhesion force increase for UV/ozone treated polypropylene films. Adhesion force can be related to surface energies of the tip and sample surfaces, as well as their interfacial energy. If there were liquid-like contaminants on the sample surface, the capillary force should be considered. Recently, the measurement force-distance curves has been demonstrated to be able to record events of (a) the breaking of a single molecular bond (single molecule force spectroscopy) and (b) the folding and unfolding of proteins by confining the molecules between the AFM tip and the sample.

 2.1.2  Lateral Force Microscopy (LFM)

    Lateral force microscopy (LFM) is based on measuring the torsional movement of the cantilever when the tip scans the surface, which is illustrated below. Lateral force detection in AFM is usually used to image different friction forces on a surface. As indicated in the figure, the difference in the bi-directional lateral force images corresponds to the friction force image. Friction force imaging can be used to identify regions having different hydrophilicity on the basis of their different interactions with the AFM tip. Here is an example showing higher friction force observed on the scratched area on a BOPP film, which is due to higher surface energy on the scratched area. Force-distance curves obtained on the normal and striped (scratched) areas are shown above, revealing that the friction force contrast seen is related to the adhesion force.

    Friction force can also be also used to detect chemical functional groups on a surface with the tip modified by a specific chemical functional group (chemical force microscopy), such as OH and COOH. For example, a commercially available silicon tip is terminated by its native oxide SiO₂.  This hydrophilic tip can be used to distinguish the chemical amphiphilicity of octadecylphosphonic acid (OPA) molecular layers. That is, a film surface terminated by the hydrophilic headgroup shows larger friction force than that terminated by the hydrophobic tails. Combined with height measurement, one can identify whether an amphiphilic molecular layer is an even-number multilayer (terminated by the headgroup) or an odd-number multilayer (terminated by the tail).           
    On the other hand, the lateral force imaging may reveal local topographic changes via an enhancement of the torsional movement of the cantilever when the tip scans across edges of the surface features. Click here to see an LFM image on a 15-min-UV/ozone-treated polypropylene film, where the droplets are distinguished from the surrounding areas. This technique has proven useful in detecting different phases on a surface whose height range is large, which may be encountered in practice.

    Shown below is an example of friction force microscopy study of multilayers of an amphiphilic molecule, octadecylphosphonic acid, formed on a Si wafer.  Topographic image (scan area: 14.0 μm ⨉ 8.8 μm) in (a) shows formation of bilayer and odd-numbered multilayers.  Numbers shown in (a) indicate the number of molecular layers in the multilayers.  The friction force image in (b) shows that the bilayer and the exposed Si wafer surface have a similar friction force, which is larger than that of the multilayers.  The friction force contrast observed is a reflection of the fact that the bilayer is terminated by the polar OPA headgroup having a stronger interaction with the Si tip covered by its native oxide of SiO₂, while the odd-numbered layers are terminated by the nonpolar methyl (CH₃) groups.  

 2.1.3  Force Modulation

    In addition to the topographic feature, one can probe local elastic properties of materials through a mechanical interaction between the surface and the tip. As shown below, this can be done by oscillating the sample height while measuring the response of the cantilever with the lock-in amplifier technique. This technique actually measures the slope of the force-distance curves at the repulsive force region.  Elasticity difference on a surface can be distinguished by using this technique.

    Let h be the piezo movement, p the deflection of the cantilever and d the penetration of the tip into sample, then h=p+d.  The cantilever's spring constant is k_c and the force F exerted on the tip can be calculated from p: F=k_cp.  Hertzian model connects the penetration d, force F, the radius of the probe R and Young's moduli of the probe and the sample: d=F^2/3(D²/R)^1/3,  where D is the reduced Young's modulus from the Young's moduli of the probe E_t and the sample E_s, related by Poisson's ratio σ: D=3(1-σ²)(1/E_s+1/E_t)/4.  Obviously, for polymer samples, E_s<<E_t, thus D=3(1-σ²)/(4E_s).  The relationship between h and F can be written as follows:

The experimental method measures dF/dh, which is a measure of the Young's modulus of the sample.

    The oscillation of the sample height may be realized by applying sinusoidal voltage from a function generator to the z-direction of the piezo (PZT) scanner on which the sample is fixed. A sinusoidal voltage is applied to the piezo scanner to oscillate the sample height with a peak-to-peak amplitude of about 1 nm. The response of the cantilever to this oscillation is detected with a lock-in amplifier and is used to obtain images relevant to local elasticity of sample surface. The oscillation of the sample height would not influence topographic images as far as its frequency is higher than the cutoff frequency of the feedback loop. Therefore, both topography and elasticity distribution images can be obtained simultaneously. An example of elasticity mapping of PS and PS/PEO blend film spin-coated on a mica substrate using a cantilever with a spring constant of 0.75 N/m is shown in the figure to the left.

        

    The mechanism for the force modulation is described using force-distance curves (a) obtained on the mica and the PS film and the simultaneously obtained response (b) of the cantilever to an oscillation of the sample height with an amplitude of 1 nm at 5 kHz. The spring constant of the cantilever used was 18 N/m and the approaching and retracting speed for the tip was 3 nm/s. The difference seen for the cantilever response (b) is due to the different slope of the force-distance (a) curves on the different materials, which is a reflection of the difference in Young's moduli for the mica (200 GPa) and the PS (5GPa).   

 2.1.4  Locally Modifying Surfaces

    Surface may be modified by applying large forces through the tip to the surface during scanning. This technique has applications in creating nanometer-scale structures on a surface. For example, on the crystallized polyethylene oxide (PEO) thin films, both the surface structure and elasticity were modified locally by the AFM tip. A close look at the modification of the PEO surface is shown here.

 2.2  Dynamic Force Mode AFM Techniques

    Dynamic force (tapping or non-contact) mode AFM, in which a cantilever oscillated around its resonant frequency is used to probe surface features, was developed initially to eliminate surface degradation encountered in contact mode AFM, especially for soft materials.
    In order to understand the interaction between the oscillating tip and the sample surface, one needs to take a look at the equation of motion for a forced (driving force: F₀cosωt) vibrating cantilever in free space, where there is no tip-sample interaction.  In such a case, the dynamics of the cantilever (i.e., its position z as a function of time t) can be modeled as a point mass attached to a spring as follows:

With the two definitions of ω₀² = k/m and c=mω₀/Q , where ω₀ is the angular resonant frequency of the cantilever and Q the quality factor, the above equation of motion can be rewritten as:

This second order, non-linear differential equation has a steady-state solution of

with A being the amplitude of the oscillation and φ the phase lag relative to the driver, which are shown in the illustration below with a simulation.

    From the simulation shown above, the cantilever can only be oscillated when the driver frequency is close to the resonant frequency of the cantilever.  It is also clear that there is a sharp change of the phase lag at the resonant frequency.  In dynamic force mode AFM, the tip-sample interactive force alters the resonant frequency, which in turn changes the oscillation amplitude.  In practice, as shown in the figure below, the amplitude of the oscillation decreases with decreasing tip-sample distance.  Because the tip-sample interactive force is complicated, there is hardly an analytical solution for the equation of motion with this force involved.  However, simulation seems to reflect the experimental amplitude change as a function of the tip-sample distance [see A. San Paulo and R. Garcia, Surf. Sci. 471, 71 (2001)].  

    For dynamic force mode AFM, silicon cantilevers with a spring constant of 5 ~ 40 N/m are used. A typical 40 N/m cantilever is 125 μm long, 30 μm wide and 3.7 μm thick. A reduced amplitude is used as the feedback parameter and the change in oscillation amplitude of a cantilever versus tip-sample distance is shown below, which was obtained on a BOPP film. Interactions between the tip and the surface at different tip-sample distance are indicated by inserts a-c. 

    The figure above shows that when the tip is far away from the sample surface (a), the oscillation amplitude of the cantilever is constant, representing the free space situation where there is no interaction between the tip and the surface. The amplitude decreases when the tip approaches close enough to the sample surface so that it "feels" attractive and/or repulsive forces (b). The cantilever stops oscillating when the tip is brought in to mechanically contact the surface (c). Dynamic force mode AFM works by scanning the tip across the sample surface and adjusting the distance between the two through maintaining a constant reduced amplitude (e.g., 50% of that in free space). This adjustment of the tip-sample separation allows construction of the topographic image. There are many modes measuring surface properties based on this dynamic force mode AFM, as described in the following.    

    The AFM images shown here clearly illustrate the formation of mounds on UV/ozone treated BOPP film, while the original surface is characterized by fiber-like network structure (the scan area is 2 μm ⨉ 2 μm and the height range is ~ 25 nm) and an increase in adhesion force. This increase in adhesion force indicates an increase in surface energy due to the oxidation of the modified polymer film. 

     When the oscillation amplitude is large (say, >2 nm), the tip actually taps the surface, which is why it is called dynamic force or tapping mode AFM. In practice of measuring larger scale area, larger amplitude is usually used because it results in more stable imaging conditions. Most dynamic force mode AFMs, as described above, are amplitude-modulation AFM.  If the oscillation amplitude is very small (say, a couple of nm), then the dynamic force mode could be called non-contact mode because at such small amplitude, the tip would not need to tap the surface to sense the interaction between them. Operation of non-contact mode AFM is achieved in UHV for obtaining atomic resolution through frequency-modulation techniques.

 2.2.1   Phase Shift Imaging

    The phase shift angle of an oscillating cantilever is sensitive to tip-surface interactions, which is basically material specific (e.g., surface chemistry and viscoelasticity). Therefore, the phase shift contrast in tapping mode AFM can be used to distinguish different surface compositions on a surface (see the schematic below). There are many surface properties that may have an effect on the phase shift contrast. They can be differences in friction, viscoelasticity, adhesion, material, etc. Phase imaging usually gives clear contrast on a surface if there are detectable differences in surface properties as described above. So, the explanation of a phase shift image should be careful and usually depends on other observations and background knowledge on the sample. It should be noted that phase imaging is a very valuable approach for SPM researchers because they probably find and in fact are finding some new phenomena during their searching answers and explanation to the phase shift measurement. Applications include visualizing phase separation in polymer blends, fillers in polymer nanocomposites and surface chemistry. Shown below is topographic (left) and phase shift image (right) for a toner particle of a carbon black matrix with polymer fillers (scan area: 3.5 μm ⨉ 3.5 μm).

    Phase shift is sensitive to tip-sample interaction. This is the reason why phase shift image is a powerful technique in differentiating component materials on a sample.  According to Cleveland et al. and Garcia et al., phase shift angle f is related to tip-sample energy dissipation (E_dis) process (which in turn reveals difference in material properties) and hydrodynamic damping in the medium (E_med).  In amplitude-modulated AFM with a cantilever oscillated at frequency ω (ω₀ being the angular resonance frequency) having a free amplitude A₀ and setpoint A, the phase lag φ is

    Shown below is an example of applying phase shift imaging to visualize organelles of a sectioned rat brain.  The rat brain is not fixed so that the surface chemistry is preserved.  AFM topographic and phase shift images shown below are obtained for neurons in the CA1 area of the hippocampus of the rat brain section (thickness: 30 μm). The time-of-flight secondary ion mass spectrometry (ToF-SIMS) negative secondary ion images of CH¯, CN¯ and PO₂¯ are overlapped.  The ion images were obtained with Bi₃⁺ primary ion beam with the high spatial resolution (burst alignment) mode.  The white lines inserted in those images guide the eye to organelles imaged by both the phase shift and the ToF-SIMS.

    The topography hardly displays information on organelles.  However, the phase shift image reveals clearly the organelles having darker contrast, indicating that these subcellular features are much softer than the surrounding areas.  Because organelles are rich in phospholipids, which are oily, they ought to dissipate more energy from the oscillating AFM probe.  On the other hand, the brighter areas may be dried proteins and salts, which are hard materials, leaving a much less energy dissipation from the oscillating probe.  Indeed, the ion images support these assignments of the observed phase shift contrast: PO₂¯ is representative for phospholipids and DNAs and CN¯ for proteins.

 2.2.2   Magnetic Force Microscopy (MFM)

    Magnetic domains can be visualized in AFM when a magnetized tip is used. A local topographic data in each scan line are first obtained with the dynamic force mode AFM. Then the tip is lifted up in a certain distance (say 50 nm) and repeats scanning the same line. The magnetic force occurred between the tip and surface results in a change in the magnitude (or phase) of the oscillating cantilever, from which the magnetic force distribution is visualized.  This technique is used in mapping magnetic force distributions on recorded magnetic media (data storage) and micromagnetic structure on magnetic materials. For more information, you may want to read this paper.

 2.2.3   Electric Force Microscopy (EFM)

          Similar to MFM, EFM uses a conductive tip to probe the difference of electric filed gradient distribution on a surface. This technique can be used in failure analyses of integrated circuit (IC).

 2.2.4   Scanning Surface Potential Microscopy (SSPM)

    This technique maps the local surface potential distribution together with the topographic image by keeping a certain separation between the sample surface and the conductive tip biased with a sinusoidal voltage is applied. The principle of the SSPM is shown in the figure on the right. If there is a difference in the electric potential between the tip and sample surface, an oscillating electromagnetic force appears between the tip and sample surface at the frequency equal to that of the applied sinusoidal voltage.  This force oscillates the cantilever, which is used as the feedback parameter: the system tries to stop this oscillation by applying a dc voltage to the tip so as to make the potential difference between the tip and sample surfaces vanish. The applied dc voltage to the tip is thus equal to the surface potential of the spot the tip locates, which thus constructs the surface potential mapping together with the topographic image.

    As shown in the figure to the left, gold films deposited on a glass substrate were used to confirm the SSPM.  The potential difference between the two gold films was made by biasing them.  During scanning, the bias was changed so that different potential differences between the two gold films were recorded. 

    Pd deposited on a semiconductor is an example for generating a contact potential between the metal and the semiconductor surface. Here is a result showing the contact potential difference between the metal and the semiconductor surface. Another examples of measuring surface potential distribution are a Pd (110) surface and a thin film giving a clear distribution of the surface potential.

 3.  A Simple Method to Check AFM Tip Performance Using a Polymer Film

 3.1  BOPP film surface for tip radius evaluation

    An AFM image of a surface is constructed through the detection of interactions between the tip apex and the surface features. An interaction, whether it be a contact force, an oscillation amplitude or others, is the feedback signal used to adjust the proximity of the tip and the surface features. Because of this imaging mechanism, an AFM image is, in practice, a convolution of the tip geometry and the surface features. Based on the actual geometry, as shown below, the tip apex or the surface feature, whichever is sharper, acts as the effective probe.


    In practice, there could be a large-sized contaminant on the tip apex, making sharper surface features the effective probe tip. Therefore, images collected using a contaminated or damaged tip can be dominated by the geometry of the AFM tip itself (i.e., self-imaging of the tip) if the surface features are sharper than the tip. Interpretation of such images can easily be misleading if the tip effect is not taken into account. To ensure that the tip is "good" enough for imaging a surface, one needs reference samples that have known surface features, suitable for checking the tip performance. Introduced here is a simple and effective method of evaluating tip performance by imaging a BOPP film, which is characterized by nanometer-scale sized fibers. The BOPP film surface is appropriate for use as a reference because a contaminated tip will not detect the fiber-like network structure, as shown in the figure below. Imaging the very fine fiber-like structure of the BOPP film surface is a good criterion for the tip performance. Many other samples with known surface features can also be used to characterize the geometry of AFM probes.

    Because the polymer film is soft compared to the silicon tip (Young's modulus for polypropylene is 1-2 GPa, while for silicon it is 132-190 GPa), the polymer will not damage the tip when the tip is pushed into the polymer. As shown in the figure below, this property can be used to clean a contaminated tip, i.e., by pushing the contaminated tip into the polymer, contaminants could be removed from the tip apex. Another important property of the BOPP is that the polymer film is highly hydrophobic and has a very low surface energy of ~ 30 mJ/m² (The surface energy for Si is ~ 1400 mJ/m²; and the surface tension of water is 72 mN/m). These properties prevent contaminants from accumulating on the surface and hence prevent the contamination of the tip in the evaluation process. This method of using BOPP to check AFM tips AND to clean contaminated tips was highlighted in April 1, 2001 issue of Analytical Chemistry.

 3.2  Applications to blind tip reconstruction

    Considerable efforts have been expended to mathematically extract the geometry of the tip based solely on an algorithm derived from a given image, which is known as blind reconstruction. This methodology is based on the assumption that protrusions in the AFM image represent the self-image of the tip, which is equivalent to the statement that sharper features on the sample surface act as the probe to image the AFM tip. This method has proven useful and successful in estimating tip geometry from an existing image, when appropriate samples were chosen (i.e., some surface features on the sample are sharper than the tip). Once the tip geometry is known, the tip effect may be subtracted from the original image through the mathematical operation of erosion, also known as deconvolution. Dilation is another mathematical operation, which adds the tip effect to an existing AFM image by "scanning" the known tip across the "surface" of the image. This transformation appears useful in simulating tip effect for a given image, because the mechanism of AFM can be regarded as the dilation between the tip geometry and surface features of the sample.

    We have found that a BOPP film is suitable for checking tip performance and for cleaning contaminated tips, thus making it possible to collect images of the same area of a BOPP film surface before and after the tip was cleaned. Therefore, the difference between the two different images is solely due to the contamination of the tip. We took advantage of our ability to collect AFM images of the same area using the same tip, in one instance, contaminated and, in the other, after being cleaned. Commercial software SPIP (Metrology Image ApS, Denmark) was used to estimate the tip geometry using its "tip characterization module", in which the blind reconstruction algorithm is implemented. First we used blind reconstruction on the image collected using the contaminated tip. Blind tip reconstruction allows one to extract the geometry of the tip from a given image. Once we had estimated the geometry of the contaminated tip, we used it to simulate the tip effect using the image collected with the cleaned tip. By comparing the simulation result with the image collected with the contaminated tip we showed that the blind reconstruction routine works well. 

    Comparison of tip geometry from the blind reconstruction method and from scanning electron microscopy (SEM) images has been made by Dongmo et al. [J. Vac. Sci. & Technol. B 14, 1552 (1996)]. We developed a simpler way to test blind reconstruction: comparison of AFM images collected in the same area of the BOPP by clean and contaminated tips. If the estimation of the contaminated tip geometry is reasonable, then one expects to be able to use the estimated tip geometry to dilate the image collected using the clean tip to obtain an image resembling one collected using the contaminated tip. Conversely, one can also determine if the deconvolution works by eroding the image collected with the contaminated tip using the estimated tip geometry to see whether the result resembles the image collected using the clean tip.

    Because an AFM image is a convolution of the surface features and the tip geometry, if neither of them is known, there is no way to know, on an unknown sample, if the image is dominated by the surface features or the tip effect. When the tip is much sharper than the surface features, it will collect an image reflecting the "true" surface features. This is the reason why a reference sample is essential to check the tip performance. It is important to note that a tip could be easily contaminated or damaged depending on the chemical and mechanical properties of the sample surface. Using electron microscopes one can evaluate the outlines of the tip shape from specific directions, but it is difficult, if not impossible, to capture the three-dimensional geometry of the tip. In combination with blind reconstruction, using BOPP film to check the tip performance provides a simple and effective protocol to test the estimation of the tip geometry of a contaminated tip. One can do this by comparing the image collected using the contaminated tip with the image generated by dilating the image collected using the clean tip.

    On the other hand, when the tip is much larger than the surface features, using such a tip to scan the surface will result in an image that is merely a reflection of the geometry of the tip apex itself. In this case, it is evident that the information about the surface features is physically lost. Therefore, the erosion operation will not lead to the recovery of the "true" surface features, though the mathematic operation may result in an image which is likely closer to the "true" surface features. The degree of the recovery by the erosion operation is dependent on how severely the tip is contaminated. One can imagine that different surface features can have similar images if a large tip is used: they are dominated by the tip effect.

H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Atomic force microscopy study of biaxially-oriented polypropylene films", J. Mater. Eng. Perform., 13, pp.451-460 (2004).

H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Use of biaxially-oriented polypropylene film for evaluating and cleaning contaminated atomic force microscopy probe tips: An application to blind tip reconstruction", Rev. Sci. Instrum. 73, pp.3831-3836 (2002).

H.-Y. Nie and N.S. McIntyre, "A simple and effective method of evaluating atomic force microscopy tip performance", Langmuir 17, pp.432-436 (2001).

 4.  A General Method to Clean Contaminated Tips using UV/ozone Treatment

     The tip can be contaminated during scanning some surfaces or just left in air as recognized by the unstable and degraded images obtained by the tip. When the tip was in this condition, we took out the tip for 5-minute treatment in UV/ozone. After that, the imaging condition became stable and images were improved largely. Therefore, the UV/ozone treatment is effective to clean the tip and hence opened a way of recycle-using probe tips.
   
    The wavelength of UV light from a mercury lamp is mainly 253.7 nm; with a much lower percentage at 184.9 nm. Photons with those two wavelengths are effective for cleaning organic contaminants.

    Although ozone can be generated by irradiating oxygen (air) with short wavelength light (184.9 nm; photon energy at this wavelength is 6.70 eV or 154.59 kcal/mol), a separate ozone source (such as an ozone generator) is required to provide enough ozone concentration to clean the contaminants more rapidly. What is really doing the cleaning job in short time is the atomic oxygen, which is produced by the decomposition of ozone in the presence of UV light (253.7 nm; photon energy at this wavelength is 4.89 eV or 112.66 kcal/mol). This atomic oxygen oxidizes organic contaminants to form volatile molecules. Meanwhile the UV light also has an effect to excite the contaminant molecules to make them more reactive with ozone and/or atomic oxygen. Ozone itself is reactive with organic contaminants, therefore, ozone alone is also able to clean organic contamination but will take much longer time than UV/ozone combined.

 5.  Concluding remarks

    SPM techniques are a promising tool and a platform for nanoscience and nanotechnology. More and more researchers in many different fields are using SPM. Some want to develop a technology based on SPM to fabricate nanoscale devices. Others are discovering knowledge in physics, chemistry, biology and materials science on nano and/or mesoscale. SPM promises to provide us new and exciting discovers in surface science, physics, chemistry, materials science and biological technology. 

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