HOW TO CHOOSE BY: AFM TECHNIQUE
AFM was introduced as a contact mode technique, in which the quasistatic deflections of the cantilever caused by tip-sample interactions were used for a feedback-control surface imaging. The tip engagement is followed by rastering it over a sample surface in a way that the tip-sample force was kept at the set-point level by adjusting the vertical sample (or tip) position. In this way, a piezoelectric scanner pivots the tip precisely along the surface profile.
In further AFM developments, oscillatory modes were introduced, in part, to avoid shearing sample deformation in contact mode. In these modes, a piezoelement positioned close to the probe is used to excite the cantilever oscillation at its resonant frequency. As the oscillating probe approaches a sample and comes into intermittent contact with it, the oscillation parameters such as amplitude, frequency, phase, quality factors are changing. Amplitude modulation and frequency modulation, in which respectively the cantilever amplitude or frequency (phase) are chosen for feedback during scanning, are the main AFM oscillatory modes.
The contact and oscillatory modes have a large number of related techniques that were developed in response to different characterization needs. Besides surface imaging performed in the contact and oscillatory modes, there are spectroscopic modes based on measurements of deflection, amplitude, or phase changes as the probe approaches a sample and retracts from it. These curves (often named 'force curves') can be measured at a particular location or when obtained at multiple locations can be combined into maps (also known as 'force volume').
Initially, AFM probes were made by gluing a diamond shard to a cantilever cut out of metallic foil or by tapering a Fe, Ni or W wire. This tedious preparation has been substituted later by batch production of the probes using semiconductor technologies. In the first commercial AFM probes the cantilever and tip consisted of a thin Si3N4 film on a glass substrate. The tip has a square pyramid shape with a nominal radius of curvature at the tip apex ~ 20 nm. According to the preparation technology these probes can be made thin, which defines relatively small spring constants in the 0.01 - 0.6 N/m range. These probes are regularly used for imaging in contact mode and applied to soft samples.
The tip shape and radius at the apex are important parameters that define the range of applications and the quality of the probe. Large surface corrugations limit lateral image resolution substantially and bring the tip shape into the play. For imaging of critical-dimension structures such as deep and narrow trenches, specially etched probes (for example, with a FIB technology) or those made of carbon nanotubes or Hi'Res-C spikes should be used. High-resolution imaging of flat samples depends primarily on the tip apex.
Monolithic silicon probes, which are etched from a Si wafer, are most appropriate for ambient and vacuum AFM studies. Their cantilevers have a rectangular shape with the following parameters: width - 30 - 60 nm, length - 100 - 400 nm, thickness 1 - 8 µm. Spring constants of commercial probes vary in the 0.1 N/m to 600 N/m range. Typical dimensions of Si tips are: height 8 - 20 µm, opening angle of ca. 30 - 40 degrees, apex radius 10 nm. They have a pyramidal shape, which in the ideal case should be triangular near the apex.
Si probes are sharper than Si3N4 ones, yet they have limitations in stiffness when imaging soft samples is of interest. An appropriate solution for high-resolution imaging of such objects can be obtained by making hybrid probes consisting of Si3N4 cantilevers and Si tips. Unfortunately, such probes are rare.
In addition to sharp probes, which are applied for high-resolution imaging, there is sometimes a need for probes with large apex dimensions. Probes with rounded apex shape with a diameter in the 50 - 100 nm range are in demand for nanomechanical measurements and also for low-wear imaging.
Characterization of AFM probes is rather important issue because variations in the tip shape and apex size are not uncommon. There are direct and indirect ways of characterization of the probes. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provide a direct visualization of the tip shape and apex dimensions. The indirect experimental procedure is based on imaging special test structures such as nanoporous Al. The analysis of the images obtained on such test samples helps determining the shape of the tip and its apex size. The test samples should be used with extreme caution by performing probe evaluation in the low-force regime in order to avoid tip damage.
Al backside coating improves the reflection of the laser beam. In some cases, a researcher sacrifices reflectivity in order to avoid a possible bending of the cantilever in experiments at different temperatures. There is also a chance that the coating of the cantilever backside brings some additional material to the probe apex thus making it duller.
For measurements of the electric or magnetic properties of samples the cantilever coatings play the most essential role. These studies require coated probes with different stiffness as well as with apexes of various sizes. The Si probes in our catalog can be purchased with a number of coatings.
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HQ:CSC probes for contact mode
Probes with stable reflective coating
In aggressive liquid media
Probes with chemically stable coating
HQ:NSC probes for noncontact mode
High-resolution in UHV
Hi'Res-C probes for noncontact mode
Robust samples in air
HQ:NSC probes with high spring constant
Soft, weakly-adhering, or fragile, samples in air
HQ:NSC probes with medium spring constant
Near-liquid samples in air
HQ:NSC probes with medium spring constant
High-resolution in air
Hi'Res-C probes with medium spring constant
Under liquid (water)
Probes with resonant frequency 50-70 kHz having stable reflective coating
Under aggressive liquid medium
Probes with resonant frequency 50-70 kHz having chemically inert coating
Tip shape and radius characterization
Samples with hard sharp pyramidal nanostructures