Test structures

Any SPM image represents not only the sample itself but also the shape of a cantilever tip used to obtain that image. Moreover, the shape of the tip can change during scanning, thus causing additional distortions of the image. For correct interpretation of the image data, it is useful to determine the tip shape before scanning and check if it has changed afterwards.

MikroMasch produces gratings designed specifically for tip characterisation – TGT01. They comprise an array of tips with a full cone angle of less than 20 degrees and a radius of curvature of less than 10 nm. The tips are arranged in a square grid. The pitch is 2.12 µm (3 µm diagonal length). The tip height is about 0.6–0.8 µm. At least one tip can be found in a 3 x 3 µm scan.

The main advantages of this structure are:

  • strict symmetry of the tip faces;
  • small cone angle (less than 20 degrees);
  • small curvature radius of the tips (less than 10 nm).

We used this structure to examine the most common SPM cantilever tips: Si3N4 pyramidal, sharpened Si3N4 pyramidal, and Si conical. Corresponding images are presented in Fig.1-3. The scans have been obtained with Solver P4-SPM-MDT microscope in tapping mode It should be noted that the scanning tip gives an inverted image of itself, and the image is a mirror transformation of the real tip shape (compare SPM images and SEM photos of the same tips in Fig.1-3). Likewise, the distortion of scanned images caused by the finite size of the tip is also a mirror transformation of the tip features.

Fig.1 SPM and SEM images of Si3N4 pyramidal tip.

The SPM and SEM images of a Si3N4 pyramidal tip are shown in Fig.1. The SPM image shows the characteristic features of the tip: flat top area with a size of about 0.1 µm and surface contamination. The tip shape is completely reproduced. The tip in the SPM images has an inclination of about 20 degrees. This is the angle between the cantilever plane and the sample plane.

Fig.2 SPM and SEM images of a sharpened Si3N4 pyramidal tip.

The images of a Si3N4 sharpened pyramidal tip are shown in Fig.2. A cross-section of the SPM image permits to measure the sum of the curvature radii of the scanning tip and the characterizing tip.

 
Fig.3 SPM image and cross-section of ULTRASHARPTM silicon tip

The image of a silicon conical tip of ULTRASHARP cantilever is shown in Fig. 3. The sum of the curvature radii of the scanning tip and the characterising tip is equal to 150 angstroms, which corresponds to a curvature radius of both tips of less than 100 angstroms.

As mentioned above, tip destruction and contamination can lead to the distortion of the obtained image. For example, in studying E. coli we observed a dramatic decrease in image quality after scanning in contact mode. We used TGT01 to compare the tip shape before and after scanning (Fig.4-5).

Fig.4 Image of a tip before scanning.
Fig.5 Image of the same tip after scanning a bio sample in contact mode.

When a particular shape is repeated throughout an image, rotate the grating and repeat the scan. To determine the aspect ratio of the tip, you can also use TGX01 (Fig. 6).

Fig.6a Image of TGX01 taken by ULTRASHARP cantilever (the cone angle at the apex is less than 20°). The step height is 1500 nm. The tip reaches the grating bottom.
Fig.7b Image of TGX01 taken by a pyramidal tip (the angle at the apex is about 70°). The measured step height is 800 nm. The tip does not reach the grating bottom due to a larger aspect ratio.

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