What Happens When A Probe Fails

There are many parameters that determine the performance of an AFM probe. These include the tapping frequency, spring constant, and probe radius. The spring constant is particularly important to control probe normal forces to mitigate shear forces or to provide the required normal force to deform surfaces in a specific range of modulus. Probe stiffness is also important in image quality when imaging soft materials.

The probe radius is the primary design parameter related to resolution. The process of tip qualification to measure the probe radius using a sample with sharp features has been discussed in a previous blog entry. An effective probe radius can be also be measured using controlled deformations while measuring the modulus of a material with known modulus. These methods of measuring probe radius are required for mechanical measurements as well as certifying image quality.

The degradation of the probe radius will impact image resolution as the surface features are convoluted with a larger than anticipated probe radius. A "sharp" probe might have a radius of 3-5 nm, so a blunting to 30-50 nm will result in the dramatic loss of image quality. Probes coated with conducting material, such as platinum or aluminum, will have additional artifacts in such modes as TUNA or conductive AFM in the current channels due to the loss of conducting contact area with the sample.

In this post visible apex AFM probes were imaged in a field emission SEM. This probe, the Bruker OTESPA, is called a visible apex probe because the functional part of the probe is at the very end of the cantilever, allowing one of locate objects of interest in the optical microscope of the AFM very easily. The probes were imaged with a 2 kV beam using the through lens detector and field immersion. The larger top image shows a good probe. Even at 2 kV parts of the probe appear to be "transparent" as SE1 secondary electrons are generated near the edges of the probe, while SE2 secondary electrons are generated from the thicker body of the probe.

It is common for probes to fail not only by being blunted or worn-- from their radius being drastically increased-- but from picking up objects. The smaller images show three examples of failed OTESPA probes. The probes showed lower resolution and in all cases either self-similar structures or features with companions. These SEM images show objects attached to the probe. In the second image a blunt object is stuck near the end of the probe, and the shape of this object where it contacts the surface would be convoluted with the sample features. In the third image a sharp object is attached just behind the probe. This can act as an companion probe that contacts features and probes companions or doubles to sample features. The last image shows deposition of material across much of the probe, and the attachment of a significantly large foreign object. An object of this size will not only degrade image quality by make the probe very unstable as it interacts with the surface.

The point of this post is to show that things really do go wrong with a probe when image quality degrades.

Noise Floor & Near Atomic Resolution

The Dimension 3000 is a scanned tip SPM which allows for great flexibility: imaging in liquids as well as imaging with magnetic and non-ambient temperature stages. What is lost is lateral stability of the scanner due to the large lateral scan range of 100 μm. The vertical noise floor is still quite good at ~ 0.75 Å.

To test the noise floor one images freshly cleaved mica. The image is a 25 nm scan of V4 mica in contact mode with nearly zero normal force-- i.e. no deflection. Image drift has been minimized by scanning for an extended period to eliminate any piezo hysteresis and to allow the sample mounting to fully relax. The Z-limit has been set to its minimum value for maximum Z-piezo digital resolution. A sharpened Veeco SiN DNP-S probe with 0.12 N/m spring constant was used.

Note that some period structure is visible through the noise. 2D Fourier transform of the image shows significant noise bands in the vertical direction-- i.e. the slow scan direction-- which prevents spectral filtering and recovering of a high resolution atomic level image. The roughness, Ra, is 0.95 Å which is a bit above the spec'd noise floor of 0.75 Å.

This image is a 500 nm AFM image taken with an Veeco OTESP Si tapping tip. Again the X-Y hysteresis and sample drift were minimized by scanning for some time. The Z-limit was minimized for maximum Z-piezo digital resolution. Note the R_a of 0.45 Å which is a typical noise floor measurement for this system.

Tip Qualification

Tip quality is essential to quality AFM images. In a very general sense, an AFM image is the convolution of the tip morphology with that of the surface being imaged. Ideally an AFM probe has a radius of curvature (ROC) < 10 nm which allows for high resolution imaging of small features. When the ROC is increased due to wearing of the tip or the adhesion of a foreign object to the tip, the imaging resolution is degraded. In some cases, image artifacts may arise. The most common artifacts are self similar features. In some cases all the image structures are triangular in shape due to the fracturing of the AFM tip pyramid or ovoid in shape due to the wearing of the tip or the attachment of a foreign object to the tip. While a radical change in tip performance is readily visible, small changes often go unnoticed and these changes of more difficult to identify on certain types of samples. As such a standard surface morphology is desirable. An ideal standard should be capable of providing quantitative information about the tip quality.

The image above is a titanium surface that has very sharp crystallites emerging from the surface. This is an ideal tip qualification standard as these sharp edges can be used to deconvolute the shape of the AFM tip. By imaging such a sample the sharp edges of the crystal habits can be identified and their shape profile then used to estimate the rip ROC. This process is known as tip qualification, and can be performed just before and after a series of images to certify they are of the highest quality.

This image shows the actual tip qualification. The top left image shows a green "+" at each tip qualification point-- a sharp edge on the titanium crystallites. The bottom left image shows an estimate of the tip shape. The top right image shows the cross section of the tip 10 nm from the tip apex while the bottom right image shows it 20 nm from the tip apex. The diameters of these cross sections are estimated, and user defined upper limits are used to determine if the tip is "good", "suspect" or "bad". This tip is considered "suspect" because the diameter 20 nm from the tip apex is greater than the threshold value of 60 nm.