High Aspect Ratio AFM Probes

The spatial resolution of the surface imaged by the atomic force microscope depends on two properties of the tip: the radius of the tip and the aspect ratio. The aspect ratio refers to the ratio of the height and width of the AFM tip. The standard AFM probe made of silicon has a conical, pyramidal or tetrahedral tip. The conical tip can be manufactured to have a relatively high aspect ratio. However, these conventional probes cannot accurately solve the morphology of highly non-planar features, such as deep trenches and narrow trenches that are common in semiconductor device processing. This is because the height and width of the probe are shorter than the height of the side wall and wider than the interval between the side walls of the structure. Therefore, the tip apex cannot reach the bottom of the groove. In addition, these traditional probes cannot image high-resolution particles, cells, or columns because the tip cannot reach the feature bottom corner near the steep edge, so the shape of the structure cannot be accurately drawn. Therefore, the width of the tip should be reduced, and its length should be increased, ie the tip should have a higher aspect ratio to fit inside the trench and produce high-resolution images of these types of features.

Since the height of these features usually does not exceed 1 µm, only a part of the tip needs to have a high aspect ratio, that is, it extends from the apex of the tip to slightly longer than the sidewall height of the trench or tall structure under study. The high aspect ratio of the tip can be achieved in several different ways. First, silicon can be chemically or plasma etched, or ion beam milled to remove part of the tip sidewall. This can reduce the taper angle of the tip. Second, high aspect ratio materials can be grown on or attached to the apex of the tip, such as carbon nanotubes or silicon nanowires. Carbon nanotubes also have high strength, so they can also improve the robustness of the soldering iro